Lactates from Biomass and Their Catalytic

Review pubs.acs.org/CR

Production of Lactic Acid/Lactates from Biomass and Their Catalytic Transformations to Commodities Paï vi Mak̈ i-Arvela,† Irina L. Simakova,‡ Tapio Salmi,† and Dmitry Yu. Murzin*,† †

Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Biskopsgatan 8, 20500 Turku, Finland Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russia

‡

S Supporting Information *

3.2.6. Engineering: Comparison of Performance between the Flow and Batch Modes 3.2.7. Brief Summary 3.3. Dehydration of Lactic Acid and Alkyl Lactates to Acrylic Acid 3.3.1. Feedstock 3.3.2. Effect of the Catalyst Type 3.3.3. Reaction Conditions 3.3.4. Mechanism 3.3.5. Stability of the Catalyst in the Transformation of Lactic Acid to Acrylic Acid 3.3.6. Brief Summary 3.4. Decarbonylation and Decarboxylation of Lactic Acid 3.5. Esterification of Lactic Acid 3.5.1. Effect of the Feed 3.5.2. Effect of the Catalyst Type and Catalyst Regeneration 3.5.3. Effect of the Reaction Conditions in the Esterification of Lactic Acid 3.5.4. Engineering Aspects in the Esterification of Lactic Acid 3.5.5. Brief Summary 3.6. Condensation of Lactic Acid to 2,3-Pentanedione 4. Conclusions Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Abbreviations References

CONTENTS 1. Introduction 2. Production of Lactic Acid and Lactates from Biomass 2.1. Production of Lactic Acid 2.1.1. Lactic Acid from Different Feedstocks 2.1.2. Catalysts for Production of Lactic Acid 2.1.3. Effect of the Reaction Conditions on Production of Lactic Acid 2.2. Production of Alkyl Lactates 2.2.1. Production of Alkyl Lactates from Different Feedstocks 2.2.2. Catalysts for Production of Alkyl Lactates 2.2.3. Effect of the Reaction Conditions 3. Catalytic Transformations of Lactic Acid to Commodity Chemicals 3.1. Hydrogenation of Lactic Acid 3.1.1. Feedstock 3.1.2. Catalyst 3.1.3. Reaction Conditions 3.1.4. Kinetics and Mechanism of Lactic Acid Hydrogenation 3.1.5. Engineering Aspects: Hydrogenation in Trickle-Bed and Batch Reactors 3.1.6. Scale-Up 3.1.7. Brief Summary 3.2. Oxidative Dehydrogenation of Lactic Acid 3.2.1. Reaction Network 3.2.2. Feedstock 3.2.3. Catalyst 3.2.4. Effect of the Reaction Conditions 3.2.5. Mechanism of Oxidative Dehydrogenation of Lactic Acid © XXXX American Chemical Society

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1. INTRODUCTION Lactic acid is an important commodity chemical, an intermediate for producing alkyl lactates, propylene glycol, propylene oxide, acrylic acid, and poly(lactic acid).1 Lactic acid has found applications in food, pharmaceuticals, and cosmetics.1 In addition, lactic

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Figure 1. Lactic acid and derivatives. Adapted from Gallezot.55

acid-based biopolymers are of high interest.1 The lactic acid market has been estimated to be 3.3 × 105 tons by 2015,2 showing that the demand is continuously increasing. Conventionally, lactic acid is produced via fermentation starting from carbohydrates.1 This process, however, produces low-purity lactic acid and suffers from low productivity. Conventional production of lactic acid has been reported in detail by Datta et al.,1 also covering different engineering aspects and technologies for purification of lactic acid produced via fermentation. These include, for example, electrodialysis. In general, it can be stated that there is a need to develop new catalytic processes for synthesis of lactic acid not only from carbohydrates, but also from lignocellulosic material, which is cheaper and more abundant compared to carbohydrates. The research in catalytic production of lactic acid3−16 or alkyl lactates6,9−11,17−53 has been very intensive during recent years. Since lactic acid can be made from different reagents, such as lignocellulosic material, cellulose, carbohydrates, sugars, trioses, glycolaldehyde, and glycerol, and the reaction conditions are relatively demanding, it can be easily observed that these reactions are usually complex, involving several types of transformations, such as aldol condensation, retro-aldol condensation, dehydration, 1,2-hydride shift, etc.41 Furthermore, usually bifunctional, heterogeneous catalysts with tuned catalyst properties and high hydrothermal stabilities are needed. Thus, the research related to lactic acid is still very intensive and ongoing. There are a few reviews already devoted to lactic acid or its derivatives.1,54−57 Synthesis of different lactate derivatives has been briefly reviewed by Gallezot,55 also covering several other reactions, for example, sugar isomerization and dehydration. Analogously, in the works of Taarning et al.57 and Corma et al.,54 the scope is much broader, thus limiting the detailed discussion of different catalyst properties and conditions for production of lactic acid and its derivatives. In addition, production of polymers and solvents from lactic acid has been reported by Datta et al.1 Moreover, a recent review by Dusselier et al.58 summarized

fermentation and catalytic routes for production of lactic acid and its further transformations without however presenting technological and modeling aspects. Since the industrial synthesis of lactide has also recently been reviewed by Dusselier et al.,58 it is not included in this work. Briefly, lactide synthesis can be summarized as follows: it is principally performed in two steps; i.e., polylactide prepolymer is generated in the first step via dehydration followed by thermal depolymerization in the second step, giving lactide. Besides consideration of lactic acid as a platform for commodity chemicals, the interest in lactic acid arose sharply with the development of biodegradable polylactides (PLAs) produced by polymerization of the dilactide derived by self-esterification of lactic acid. The synthesis and technological development of PLA were described in detail by Degree et al.,59 and a recent overview covering many aspects of PLA production, market applications, and physicochemical properties, including biodegradability, has been published.60 Therefore, other important lactic acid reactions will be of interest in this review. The aim of this review is to describe the research in the production of lactic acid from biomass and its transformation to commodity chemicals mainly since 2002. The following reactions will be discussed in detail: catalytic production of lactic acid or alkyl lactates using homogeneous and heterogeneous catalysts (section 2), catalytic transformation of lactic acid or alkyl lactates via hydrogenation to 1,2-propanediol (propylene glycol),61−98 dehydrogenation to pyruvic acid,99−120 dehydration to acrylic acid,121−139 decarbonylation and decarboxylation to acetaldehyde,140,141 condensation of lactic acid to 2,3-pentanedione,142−153 and esterification (Figure 1) (section 3),17,18,20,26,29−31,35,36,42 excluding, however, polymerization. The main emphasis is on elucidation of catalyst properties, the stability of catalysts and their reuse, and reaction conditions and mechanisms in the production of both lactic acid and its derivatives in the presence of homogeneous and heterogeneous catalysts. Furthermore, kinetic modeling and reactor technologies are also discussed in catalytic hydrogenation of lactic acid as well as in its esterification. B

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2. PRODUCTION OF LACTIC ACID AND LACTATES FROM BIOMASS Lactic acid is commercially produced via fermentation of biomass,33 starting typically, for example, from corn-derived glucose.48 Fermentation processes suffer from low productivity along with costly separation and several purification steps (Figure 2).1,40

processes can utilize either harsh subcritical conditions and metal salts4 or milder conditions and heterogeneous catalysts.10 Catalytic production of lactic acid from biomass has recently been a highly interesting research topic. Lactic acid and alkyl lactates can be catalytically produced from lignocellulosic raw material,4 cellulose,8 and sugars,3,5,10,11 as well as from trioses, which are intermediates formed from sugars. If water is used as a solvent, lactic acid is formed, whereas in alcohol solvents lactates are generated. The formation of lactic acid or alkyl lactates from different feedstocks involves several reaction steps, which are described in section 2.1.1 and section 2.2.1, respectively. In addition to these biobased materials, lactic acid can also be produced directly from glycerol,13,14 which is a byproduct from production of biodiesel. Alternatively, a twostep method can be utilized, where glycerol is catalytically oxidized to dihydroxyacetone153,154 followed by its dehydration to pyruvic aldehyde and further reaction with water to lactic acid7,11 or in an alcohol as a solvent to alkyl lactate.7,42,52,53 Productions of both lactic acid and alkyl lactates from biomass have been separately reviewed in this work by elucidating several parameters, such as the feedstock, the catalyst type and its reuse, regeneration, and stability, and the effect of the reaction conditions. 2.1. Production of Lactic Acid

2.1.1. Lactic Acid from Different Feedstocks. Lactic acid has been catalytically produced using metal salts either under subcritical conditions4 or at lower temperatures.11 Furthermore, strong alkali was used as a catalyst under subcritical conditions in the production of lactic acid.3 Several feedstocks have been applied, for example, lignocellulosic feedstock,4 cellulose,10 disaccharides, e.g., sucrose,5,10 monosaccharidic sugars,3,5,9,11,48 trioses,3−7,12,52 C2-containing glycolaldehyde,3 and glycerol.3,14 Trioses, dihydroxyacetone and glyceraldehydes, are formed via retro-aldol condensation from sugars, whereas glycolaldehyde is formed from the degradation of glucose to C2 and C4 aldoses (Figure 3).48 Glycerol is a particularly interesting feedstock, being a byproduct from biodiesel production with currently low value. Typically, lactic acid production in high yields is more demanding from the lignocellulosic feedstock than from, e.g., sugars.

Figure 2. Conventional method for producing lactic acid. Adapted from Datta et al.1

A detailed description of the conventional lactic acid process has been given by Datta et al.1 There is a need to develop new processes for lactic acid production, since the concentration of lactic acid should be higher in catalytic processes than in traditional fermentation processes, thus allowing more efficient and economic separations. Moreover, lignocellulose is cheap and is a more abundant raw material than carbohydrates. These

Figure 3. Retro-aldol condensation of glucose to glycol aldehyde and aldotetrose and fructose to dihydroxyacetone and glyceraldehyde. Adapted from Holm et al.48 C

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Table 1. Catalytic Production of Lactic Acid entry

reactant

catalyst

conditions

conversion (%)

glucose

Ni(II) Cr(III) Zn(II) Cr(III) ZrW AlW C-SO3H Ni(II) Co(II)

300 300 300 300 190 190 190 300 300

°C, °C, °C, °C, °C, °C, °C, °C, °C,

120 s 120 s 120 s 120 s 24 h, 5 MPa of He 24 h, 5 MPa of He 24 h, 5 MPa of He 120 s 120 s

45 32 20 50 42 47 29 53 74

fructose sorbose mannose sucrose

140 300 180 300 300 300 160

°C, °C, °C, °C, °C, °C, °C,

6h 10 min 2h 10 min 10 min 10 min 20 h

98

7 8 9 10

AlCl3 NaOH Al-3Zr ZnSO4 ZnSO4 ZnSO4 Sn-β

11

dihydroxyacetone

ZnSO4 Ca(OH)2 ZSM-5 Sn-β Sn-Si-CSM H-USY AlCl3 Ca(OH)2 ZSM-5 ZnSO4 AlCl3 Sn-β H-USY NaOH Cr(III) CrCl3 Pt/CaCO3 Ir/CaCO3 Ru/C NaOH

300 °C, 25 MPa 25 °C 140 °C 80 °C

1 2 3 4 5

wheat bran rice husk sawdust maize cellulose

6

12

glyceraldehyde

13

pyruvaldehyde

14

glycerol

15

glycolaldehyde

125 °C, 140 °C, 25 °C 140 °C 300 °C, 140 °C, 125 °C, 125 °C, 300 °C, 300 °C, 140 °C, 200 °C, 180 °C, 200 °C, 300 °C,

92

100 90 92 >99 100 100 90

24 h 1.5 h

25 MPa 1.5 h 24 h 48 h 10 min 120 s 30 min 4 MPa of H2, 18 h, boric acid, pH 12 30 bar, pH 13, 1 M NaOH, 48 h 4 MPa of hydrogen, 0.01 M NaOH 10 min

100 >99

100 45 96 20

selectivity for/yield of LA (%) 6 21 27 19 yield yield yield 12 12 yield 18 yield 37 yield yield yield yield yield yield 59 91 yield 83 71 89 59 91 yield 89 90 63 yield yield yield 54 78 65 yield

19 28 4

42 24 48 42 40 28 42 86

90

75

40 52 98

28

ref 4 4 4 4 8 8 8 4 4 5 11 3 9 5 5 5 10 5 5 15 16 52 6 7 11 15 16 5 11 52 7 3 4 11 14 12 13 3

oligomers in the first step (Figure 4). This implies that at 190 °C the pKw of water is low, and thus, due to autohydrolysis, water acts as a Brønsted acid.155 Thereafter, solid Lewis acid sites interact with C3 hydroxyl in the oligomeric glucose unit, leading to dehydroxylation. In the final step the Lewis acid is cleaved away. Since high reaction temperatures and pressures are needed for the cellulose transformation to lactic acid, the catalyst stability is crucial. Tungstated alumina and zirconia catalysts and their properties and stability as well as recycling are discussed in section 2.1.2.2 and section 2.1.2.3, respectively. Transformation of carbohydrates to lactic acid has not been intensively studied due to the challenges of using water as a solvent, opposite the case for production of alkyl lactates in alcohol solvents (see section 2.2.1).5,11 The main challenge of using water as a solvent is the formation of undesired byproducts and relatively low yields.10 Several carbohydrates and sugars have been transformed to lactic acid5,6,11 using either harsh conditions with subcritical water5 or milder conditions11 and homogeneous metal salts as catalysts. Lactic acid yields for various sugars at 300 °C varied between 40% and 48% using ZnSO4 as a catalyst (Table 1, entries 7−9)

Lactic acid has been produced directly from lignocellulosic biomass under subcritical water in the presence of homogeneous metal ions.4 Under subcritical temperatures in the presence of metal ions, organic material is dissolved, since water is ionized and acts as a Brønsted acid. Typically, reaction times are very short, being only, for example, 120 s. The maximum selectivity for lactic acid is only ca. 27% at 20% conversion starting from sawdust and using Zn(II) as a homogeneous catalyst at 300 °C under nitrogen.4 On the other hand, with rice husk as a feedstock, the yield of lactic acid at 300 °C and 120 s in the presence of Cr(III) was only 7%.4 Cellulose transformations to lactic acid have been investigated using several types of heterogeneous catalysts, zeolites, sulfonated carbon, sulfated zirconia, tungstated alumina, and heteropolyacids.10 The highest yields have been achieved with tungstated alumina, being 28% at 47% conversion at 190 °C under 5 MPa of helium (Table 1, entry 5), followed by tungstated zirconia.8 Since cellulose transformations involve several steps, it is a relatively demanding reaction. The reaction mechanism for formation of lactic acid from cellulose, proposed by Holm et al.,10 consists of cellulose autoprotolysis to soluble D

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Figure 4. Proposed mechanism for production of lactic acid from cellulose over a solid Lewis acid. Reprinted with permission from ref 8. Copyright 2011 Elsevier.

however, be noted that when the same experiments were started with trioses, much higher selectivities for lactic acid were obtained (Figure 5) (see below).5 As a comparison, glucose was also converted to various products in water using different metal salts as catalysts at a lower temperature of 140 °C.11 Typically, high conversions above 95% were achieved after 6 h with CrCl2, CrSO4, AlCl3, and Al2(SO4)3, but the yields of lactic acid remained maximally 18% with AlCl3.11 Other products with relatively high amounts were formic and levulinic acids, as well as mannose and HMF. On the other hand, the main product with ZnCl2 as a catalyst was HMF. Especially with ZnSO4, humins, which were not quantified, were also formed. Lactic acid has also been produced from trioses either under hydrothermal conditions with the aid of metal salts4 or alkali3 or alternatively using heterogeneous catalysts at lower temperature, such as 140 °C.11 Trioses are intermediates in the production of lactic acid from biomass formed during retroaldol condensation of sugars. Several compounds can, however, be formed depending on the starting sugar, for example, trioses and C2 and C4 compounds. Thus, 1,3-dihydroxyacetone and glyceraldehyde are formed from glucose via its isomerization to fructose followed by retro-aldol condensation of fructose over Lewis acid sites to the above-mentioned trioses (Figure 7).6,57 On the basis of a very recent study,48 it was, however, proposed that not only trioses but also C2 and C4 compounds were formed. It was reported that retro-aldol condensation of fructose gives dihydroxyacetone and glyceraldehyde, whereas when starting from glucose, C2 and C4 aldoses, glycolaldehyde, and aldotetrose are formed (Figure 3).48 In addition to the two abovementioned trioses, namely, dihydroxyacetone and glyceraldehyde, pyruvaldehyde is also a triose, being formed as a product via dehydration of trioses. Hydrothermal conditions in the presence of metal ions have been applied in the transformation of trioses to lactic acid.3,4 In the transformation of glyceraldehyde, the yield of lactic acid increased with increasing amounts of NaOH and temperature from 200 to 300 °C.3 The maximum yield of lactic acid was

Figure 5. Selectivities for lactic acid starting from different raw materials using ZnSO4. Reprinted with permission from ref 5. Copyright 2005 Elsevier.

Figure 6. Glucose transformation to lactic acid using ZnSO4 at 260 °C under 25 MPa of pressure. Reprinted with permission from ref 5. Copyright 2005 Elsevier.

(Figures 5 and 6). These experiments were performed in a continuous reactor with different residence times. The results revealed that an optimum residence time was about 120 s (Figure 6). In addition to lactic acid, trioses and (hydroxymethyl)furfural (HMF) were among the products. The selectivity for lactic acid increased with increasing reaction temperature from 200 to 360 °C, simultaneously decreasing the residence time.5 It should, E

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efficient for production of lactic acid from trioses, such as Sn-β,52 Sn-Si-CSM carbon mesoporous silica hybrid material,6 and H-USY.7 Very high selectivity for lactic acid (83%) at 92% conversion of dihydroxyacetone was achieved at 110 °C after 6 h over Sn-CSM catalyst, which is a carbon−silica composite material.6 This material is composed of siliceous MCM-41 filled with a polyaromatic hydrocarbon network. The optimum carbon content was found to be 18.3 wt %. This hybrid material has mild Brønsted acid sites combined with Lewis acid sites. Oxidized carbons have weak Brønsted acid sites,157 such as phenol, carboxylic groups, etc.6 The details of the catalyst optimization are reported in section 2.1.2.2. In addition, recycling of SnCSM catalyst has been reported (section 2.1.2.3). Furthermore, H-USY catalysts have been shown to be relatively suitable for production of lactic acid from dihydroxyacetone. When the latter was transformed in aqueous solution using H-USY zeolite as a catalyst at 125 °C, the main product was lactic acid, 71 wt %. Furthermore, other products, accounting for 18 wt %, were generated together with 3 wt % glyceraldehyde, which is formed via isomerization from dihydroxyacetone. Trioses are not stable in water, in contrast to methanol; thus, the dehydration− alkylation route producing alkyl lactates is more quantitative than the dehydration−hydration route, which gives lactic acid.7 A kinetic graph showing formation of lactic acid from dihydroxyacetone is given in Figure S1 (Supporting Information).7 Lactic acid formation from glyceraldehyde, which is an isomer of dihydroxyacetone, is slower than from dihydroxyacetone as the substrate,7 when the yields of lactic acid within the same reaction time are compared (Table 1, entries 11 and 12). This in turn also decreased the selectivity of lactic acid formed from glyceraldehyde compared to that achieved from dihydroxyacetone.7 Glycolaldehyde, a C2 aldose, is a very reactive compound. Thus, its reactions to both lactic acid and alkyl lactates suffered from relatively low selectivities. Glycolaldehyde has been applied as a reactant for production of lactic acid under a high temperature of 300 °C using NaOH as a catalyst.3 The reaction time was, however, relatively long, 10 min, at that high temperature, thus also leading to a low yield of 28%. Direct synthesis of lactic acid from glycerol, which is a byproduct from biodiesel manunfacture, has been recently investigated using alkali-metal-supported catalysts.3,14 Biodiesel is made via transesterification of vegetable oils, and it has been reported that, in production of 10 kg of biodiesel, 1 kg of crude glycerol is also formed.158 In addition, it has also been shown that boric acid has beneficial effects on the lactic acid yield and on the transformation rate of glycerol14 due to its complexation properties. Furthermore, the reaction atmosphere and pH are very important for production of lactic acid.3 In addition to direct synthesis, a two-step method involving selective oxidation of glycerol to dihydroxyacetone50,159−161 followed by its transformation to lactic acid has also been suggested. Lactic acid was prepared via glycerol hydrogenolysis using Pt/CaCO3 as a catalyst together with boric acid. The latter acts as a Lewis acid activating hydroxyl group, thus increasing the transformation rate of glycerol. Furthermore, boric acid suppressed the degradation rate of glycerol.14 The reaction scheme is depicted in Figure 9, displaying metal-catalyzed dehydrogenation followed by isomerization and finally Cannizarro reaction to lactic acid.14 A parallel undesired reaction instead of the Cannizarro reaction is hydrogenation to 1,2-propanediol. The highest selectivity for lactic acid was 51% at the 86% conversion level at 200 °C under 2 MPa of nitrogen in the presence of boric acid.14 The effect of the reaction conditions in glycerol

Figure 7. Reaction scheme for production of dihydroxyacetone and glyceraldehyde from glucose via isomerization followed by retro-aldol condensation. Adapted from Taarning et al.57

close to 40% after 10 min using 0.75 M NaOH at 300 °C,3 which was higher than that obtained from glucose, which was 23%. Thus, it was pointed out that several side reactions occurred during the transformation of sugar to lactic acid under strongly alkaline, subcritical conditions.3 High yields of lactic acid were obtained also from pyruvaldehyde under hydrothermal conditions in the presence of metal ions.4 For example, the yield of lactic acid was 52% at 300 °C after 120 s in the presence of Cr(III) ions.4 Pyruvaldehyde is an intermediate formed from glucose, and lower reaction rates are expected with lignocellulosic material than with pure model compounds due to the presence of lignin. It was also proposed that inversion of the polarity of Lewis acid to Lewis base for metal ions could occur, and the latter one is able to make a complex between pyruvaldehyde and metal ion.4,5 Thereafter, the rehydration in this complex leads to formation of lactic acid and the free catalyst (Figure 8).5

Figure 8. Proposed reaction mechanism for production of lactic acid from pyruvaldehyde in the presence of a Zn(II) complex. Reprinted with permission from ref 5. Copyright 2005 Elsevier.

Homogeneous metal salts have also been applied as catalysts for transforming trioses to lactic acid at lower temperatures (140 °C)11 than under hydrothermal conditions reported by Kong et al.4 The results revealed that certain metal cations are very selective for production of lactic acid, for example, AlCl3 and CrCl3, giving about 90% selectivities with nearly complete conversion within 30 min using relatively low initial concentrations of dihydroxyacetone or pyruvaldehyde, respectively (Table 1, entries 11 and 13). It should, however, be pointed out here that with higher initial triose concentrations selectivities were not very high and thus the reaction conditions should be optimized (see section 2.1.3.4). Heterogeneous catalysts are more attractive compared to homogeneous ones from the industrial point of view. There are several heterogeneous catalysts which have been reported to be F

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most promising catalyst for glucose transformation was AlCl3, giving 18% selectivity at 98% conversion, whereas ZnSO4 gave a yield of only 3% lactic acid at 140 °C within 6 h,11 and the main products from glucose at this low temperature were fructose and mannose as well as HMF. The main difference in the reaction conditions is the autohydrolysis of water, which makes the reaction conditions acidic at high temperatures. The reaction mechanism occurring in the presence of metal ions is claimed to originate from inversion of the polarity of Lewis acid Zn(II) to a Lewis base.4,5 The formation of lactic acid occurs under alkaline conditions via a Cannizzaro-type disproportionation of an aldehyde, pyruvaldehyde, which is an intermediate product from decomposition of biomass, followed by hydration to lactic acid. 2.1.2.2. Heterogeneous Catalysts. The reaction of lactic acid production is a complex one, involving dehydration, isomerization, and retro-aldol condensations, and thus, catalyst properties should be optimized for the reaction.15 When different types of solid acid catalysts were systematically tested in this reaction, the main result was that both the rate and the product distribution were very different depending on the type of acid sites in the catalyst8 as demonstrated below. Zeolites have been investigated as catalysts for biomass transformation to lactic acid.7,8,10 One of the most important results is that the rate of transformation of semicrystalline polymeric cellulose over Brønsted acidic H-USY was similar to that of the blank experiment.8 On the other hand, triose transformation to lactic acid was successfully demonstrated applying H-USY-6 as a catalyst at 177 °C.7 This catalyst exhibited 6.6-fold more Lewis acidic sites than H-USY-30 with a SiO2 to Al2O3 ratio of 30. The yield of lactic acid with the former catalyst was 71% compared to 47% obtained with the latter catalyst7 when nearly complete conversion of dihydroxyacetone was achieved at 125 °C during 4 h.7 The second best catalyst for production of lactic acid from dihydroxyacetone was H-β with a SiO2 to Al2O3 ratio of 12.5, giving 63% lactic acid.7 ZSM-5 gave only moderate selectivity for lactic acid.7 Sn modifications of zeolites by Lewis acids have been beneficial for enhancing selectivities for lactic acid.10,47 Sn-β exhibited moderate selectivities for lactic acid in transformations of sucrose10 and glucose10 to lactic acid. In addition to lactic acid, about 6% levulinic acid and 2% HMF were also formed from sucrose with this catalyst at 160 °C.10 Tungstated alumina and zirconia were promising Lewis acidic catalysts for producing lactic acid from cellulose.8 For example, cellulose conversion was extensively enhanced over tungstated alumina and zirconia catalysts, being 47% and 42%, respectively, within 24 h at 190 °C. The most promising catalyst was tungstated alumina, being able to produce lactic acid with 59% selectivity at 47% conversion of cellulose at 190 °C within 24 h under 5 MPa of helium.8 It should here, however, be pointed out that tungstated alumina catalyst was not active at 150 °C, indicating that Brønsted acidity created by autohydrolysis of water is also needed for hydrolysis of cellulose (see section 2.1.3.1). Cesium salts of 12-tungstophoshoric acid are Brønsted acidic, water-tolerable solid catalysts.8 Cellulose transformation was investigated with Brønsted acidic catalyst Cs2HPW12O40, but no rate enhancement compared to the blank experiment was achieved.8 A solid Lewis acid site can be coordinated via a hydroxyl group to soluble oligosaccharide in the high-electron-dense position O-2, since the ether linkage is sterically limited for the interaction. Thereafter, protolysis of the ether linkage occurs followed by dehydroxylation of the C−O

Figure 9. Reaction scheme for production of lactic acid from glycerol.14

transformations is discussed in section 2.1.3.3 and section 2.1.3.4. 2.1.2. Catalysts for Production of Lactic Acid. Several homogeneous and heterogeneous catalysts have been tested for production of lactic acid from biomass, such as alkali,3 metal ions,4,5,11 tin chlorides, 10 SnO2,10,52 acidic resins,52 zeolites,7,8,10 metal-modified zeolites,52,97 mesoporous materials,47 sulfonated carbon,8 tungstated alumina,8 mixed oxides,9 cesium salts of 12-tungstophosporic acid,8 and carbon−silica hybrid material.6 In addition, catalysts for transformation of glycerol to lactic acid are Rh/C,3 Ru/C,13 Ir/C,13 Ir/CaCO3,13 and Pt/C.13 In this section a comparison of different types of catalysts is given together with a discussion on the process feasibility, whereas in section 2.1.2.3 catalyst stability and reuse are addressed. 2.1.2.1. Homogeneous Metal Salts. Homogeneous catalysts used in the transformation of biomass to lactic acid are, for example, alkali3,15 and metal ions,4,5,11 requiring typically high temperatures and short residence times.3−5,11 High temperatures involve the use of either sub- or supercritical conditions.5 Supercritical water exhibits a critical temperature and pressure of 374 °C and 21 MPa, respectively.5 Under these conditions water has catalytic properties, since it is easily ionized and forms hydroxide ions.4 The role of water at high reaction temperatures is discussed in section 2.1.3.1. It has, however, been pointed out that alkali under hydrothermal conditions causes serious corrosion problems,4 which are challenges in scaling up the process. The use of large amounts of alkali under hydrothermal conditions is not very desirable, since these conditions are corrosive and require nearly stoichiometric amounts of alkali.10 It has, however, recently been demonstrated that the production of lactic acid from dihydroxyacetone (DHA) was performed at room temperature using an excess of Ca(OH)2 with a 2/1 molar ratio.15 This study gave 59% selectivity for lactic acid at complete conversion of DHA. From the industrial point of view it was interesting to note that it was possible to utilize DHA in the fermentation broth and convert it to lactic acid in the presence of Ca(OH)2.15 It is known that sugar transformations with alkali have not been very selective. Metal ions have been applied as catalysts under sub- or supercritical conditions in transforming biomass to lactic acid4,5 as well as at lower temperatures, such as 140 °C.11 Ni(II) was the best catalyst among tested Cr, Ni, Zn, and Co salts in cellulose transformation to lactic acid, being able to produce the latter with a selectivity of 13% at 7% conversion at 300 °C within 120 s,4 whereas the best catalyst starting from glucose was Cr(III). Relatively high yields of lactic acid varying in the range of 40−48% in sugar transformations were also achieved with ZnSO4 at 300 °C.5 At lower temperature, 140 °C, the G

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Unfortunately, more extended catalyst characterization in addition to specific surface area determination was not made. Ru/C was, however, a relatively selective catalyst in glycerol hydrogenolysis at 200 °C under 4 MPa of hydrogen in the presence of NaOH, giving 65% selectivity for lactic acid at 20% conversion.13 The effect of alkali was relatively large, since at higher alkali concentration only 28% selectivity for lactic acid over Ru/C was achieved. 2.1.2.3. Catalyst Deactivation, Stability, Reuse, and Regeneration. Cellulose transformation to lactic acid was successfully demonstrated with AlW catalyst (see section 2.1.1).8 This catalyst was also the most stable one under hydrothermal conditions. This catalyst was reused after treatment at 550 °C for 3 h in air. The results were very promising, since cellulose conversion remained relatively the same in three consecutive experiments.8 The lactic acid yield only decreased slightly from 28% at 47% conversion to 24% at 45% conversion. Furthermore, the γ-alumina phase remained the same in the fresh and the spent tungstated alumina catalysts according to XRD.8 In the transformation of sugars to lactic acid, more carbon deposits are accumulated on the catalyst surface than in the case of alcohol as a solvent.10 Furthermore, irreversible catalyst deactivation can occur in water, dependent on the reactant structure.7 In the transformation of PA and dihydroxyacetone to lactic acid studied over H-USY catalyst at 160 °C, the amount of carbon accumulated on the catalyst was measured.7 The results revealed that when PA was used as a reactant, carbon accumulation was more prominent than in the case of dihydroxyacetone as a reactant.7 The specific surface area and acidity decreased by 75% and 46%, respectively, in the case of dihydroxyacetone as a reactant in water as a solvent at 177 °C, whereas the corresponding decreases in methanol were 15% and 19% when the experiment with methanol was carried out at 157 °C.7 These results clearly indicated that the catalyst is more stable in methanol than in water as a solvent. The main reason for catalyst coking was claimed to be decomposition of pyruvaldehyde, leading to both reversible and irreversible structural damage of zeolite. The long-term stability of H-USY catalyst was tested, showing that the catalyst deactivated continuously and the concentration of pyruvaldehyde, an intermediate product, started to increase after a 25 h time-on-stream.6,7,10 Relatively harsh conditions have been applied in direct transformation of glycerol to lactic acid, namely, pH 12, 200 °C, and 2 MPa of helium.14 Recycling of Pt/CaCO3 catalyst, which was applied in the transformation of glycerol to lactic acid at pH 10.5 at 175 °C, showed that the catalyst activity had declined, but its selectivity for lactic acid remained constant. The analysis of the used catalyst showed that the surface of the support was corroded and Pt particles were sintered.14 2.1.3. Effect of the Reaction Conditions on Production of Lactic Acid. The optimization of the reaction conditions is very important to achieve high activity and selectivity. Several effects, such as reaction temperature, initial reactant concentration, and pH, can be crucial for catalyst performance and stability. Especially in the transformation of trioses, the initial reactant concentration has a large effect on the product selectivity. Typically, the preferred reaction conditions are low initial reactant concentration and high reaction temperature,11 which are summarized below. 2.1.3.1. Effect of the Reaction Temperature. The effect of temperature has been studied in the transformation of trioses,7 sugars,5 and cellulose8 as feedstocks both over homogeneous11 and over heterogeneous7,8 catalysts. The stability of catalysts in

linkage at O-2. Lactic acid is formed during dehydroxylation and cleavage of the Lewis acid site. Brønsted acidic sulfated carbon has been investigated as a catalyst in cellulose transformation, but the yield of lactic acid was only 4% (Table 1, entry 5).8 From the mechanistic point of view, it is also important that C-SO3H was able to produce levulinic acid.8 Both H-USY and C-SO3H were not very stable under hydrothermal conditions (see section 2.1.2.3.). Glycerol transformation for production of lactic acid has been demonstrated over several heterogeneous catalysts, such as Ir/C and Pt/CaCO314 and Ir/CaCO3 and Ru/C.13 Typically, the highest yields of lactic acid have been achieved in the presence of an inert gas3 and under alkaline conditions.3 Especially an alkaline CaCO3 has been preferred over active carbon.3 When the performances of Ir/C and Ir/CaCO3 were compared in glycerol transformation, it was observed that, at 180 °C in 1 M NaOH under 3 MPa of helium, the former catalyst exhibited 45-fold higher initial activity, where selectivity for lactic acid was 51% at 25% conversion for Ir/C and 86% at 21% conversion for Ir/CaCO3.14 Since the reaction network in glycerol transformation involves several steps, a bifunctional catalyst is needed to facilitate dehydrogenation of glycerol to glyceraldehyde on the metal surface in the first step followed by dehydration of glyceraldehydes, leading to formation of an enol in equilibrium with pyruvaldehyde, which in turn can either be hydrogenated in the presence of hydrogen to 1,2-propanediol or alternatively under helium undergo Cannizzarro reaction to form sodium lactate (Figure 10).14 Alkaline Pt/CaCO3 was also

Figure 10. Conversion and product distribution in the hydrogenolysis of glycerol over Ru/C catalyst at 200 °C under 4 MPa of hydrogen in the presence of 0.8 M NaOH: (◆) glycerol conversion; yield of (*) formic acid, (o) lactic acid, (Δ) propylene glycol, (□) ethylene glycol, (+) methanol, and (×) CO2. Reprinted with permission from ref 13. Copyright 2007 Elsevier.

shown to be an efficient catalyst for glycerol transformation to lactic acid, giving 54% selectivity for lactic acid at 45% conversion at pH 12 in the presence of borate esters at 200 °C.14 In addition to Pt/CaCO3, Rh/Al2O3 also gave high selectivity for lactic acid, 69%, in the presence of borate. The conversion, however, was much lower, about 30%, compared with that for Pt/CaCO3, which was 45%.14 On the other hand, much lower selectivities were achieved with Pt/C, Pt/SiO2, and Ru/C. H

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The final pH of the solution decreased with increasing temperature, being at 100 °C close to 13, whereas at 180 °C it was 12.65. 2.1.3.2. Effect of the Initial Reactant Concentration. The initial dihydroxyacetone concentration has a crucial effect on the selectivity,11 whereas the conversion remained the same when the initial dihydroxyacetone concentration was changed in the range of 0.1−1 M at 140 °C with AlCl3 catalyst. Typically, with increasing initial dihydroxyacetone concentration, the selectivity for lactic acid dropped and the formation of brown insoluble humins increased.11 Analogously, in the transformation of dihydroxyacetone over H-USY catalyst to lactic acid at 130 °C, the triose conversion increased with increasing initial concentration of dihydroxyacetone, but the rate of lactic acid formation remained, however, the same or was lowered at higher dihydroxyacetone concentrations.7 After prolonged reaction times it was observed that trioses are fully converted in water at 125 °C even in the absence of any catalyst mainly to products other than lactic acid.7 2.1.3.3. Effect of pH in Glycerol Transformation to Lactic Acid. Glycerol transformation has been performed with heterogeneous catalysts, and typically lactic acid selectivity increased with increasing pH.3,14 Lactic acid selectivity in glycerol transformation over Pt/CaCO3 increased with increasing pH and boric acid concentration, being the highest, about 54%, at close to pH 12 when the BA concentration was 100 mM and the reaction was performed under 4 MPa of hydrogen,14 whereas at pH 5.6 only 7% selectivity for lactic acid was achieved. The main product under low pH was 1,2-propanediol, which formed with about 60% selectivity.14 2.1.3.4. Effect of the Reaction Atmosphere in Glycerol Transformation to Lactic Acid. The effect of the gas atmosphere was very prominent when either hydrogen or helium was used as the reaction atmosphere in glycerol transformation over Pt/CaCO3.14 When this reaction was performed under nitrogen compared to hydrogen, the conversion was much higher; i.e., after 18 h at 200 °C glycerol conversion was 83%, whereas in the presence of 4 MPa of hydrogen and otherwise under similar conditions it was only 44%. In the former case, only traces of 1,2-propanediol were formed in the absence of hydrogen, but lactic acid selectivity remained about the same, being 55%. In addition, glycerol degradation became prominent. Analogous results showing higher reaction rates in helium than in hydrogen were obtained when comparative studies of the effect of helium and hydrogen on glycerol transformation over Ir/C at 180 °C in the presence of NaOH were performed.12

water at relatively high temperature can also be challenging, although they were stable in methanol in the presence of methyl lactate.7 On the other hand, if water is used as a solvent, heterogeneous catalysts undergo irreversible catalyst modifications, leading to catalyst deactivation.7 Biomass production to lactic acid occurs over metal ions either under sub- or supercritical conditions5 or at lower temperatures with heterogeneous catalysts.7 The benefit is that water itself acts as a catalyst and a solvent, whereas the main drawback is the equipment cost.5 In cellulose transformation to lactic acid, one of the most interesting results was that a Lewis acidic AlW catalyst was not active at 150 °C,8 whereas this catalyst was efficient at 190 °C. Thus, it was concluded that Brønsted acidity formed via autohydrolysis of water at 190 °C was needed in combination with tungstated alumina to produce high yields of lactic acid. The pKw of water decreases with increasing temperature, facilitating autohydrolysis. It is known that teh pKw of water at 25 °C is 14, whereas it is 11.4 at 175 °C.156 In addition, it should be pointed out here that in the production of lactic acid in water free carboxylic acids are also formed, additionally increasing the Brønsted acidity, whereas in methanol they are absent.10 The effect of temperature in the transformation of sugars to lactic acid has been studied with metal salts.5 Under hydrothermal conditions fructose was transformed to lactic acid using ZnSO4 as a catalyst. Both the temperature and residence time were changed simultaneously in a flow reactor. The results showed that selectivity for lactic acid increased from 8% at 200 °C with a residence time of 180 s to 48% at 360 °C with a residence time of 10 s.5 Selectivity, analogously to fructose, although slightly lower, was achieved for glucose; i.e., at 200 °C and 180 s a selectivity of 2% was achieved, whereas at 300 °C and a 30 s residence time it was 32%.5 The effect of temperature in triose transformation to lactic acid has been investigated in a few studies.7,11 When homogeneous metal salts are used as catalysts, the effect of the temperature increase on the reaction rate was very prominent, since the time for reaching 90% conversion for dihydroxyacetone decreased from 110 min at 120 °C to 10 min at 180 °C with AlCl3 at an initial concentration of 0.5 M.11 Analogously to homogeneous catalysts, a beneficial temperature effect has also been found for heterogeneous catalysts.7 The increase of the reaction temperature enhanced the transformation of dihydroxyacetone to lactic acid in the temperature range of 116−190 °C over H-USY catalyst, since the initial formation rate of lactic acid increased by a factor of 62 when the reaction temperature was increased from 116 to 190 °C and an initial dihydroxyacetone concentration of 0.31 M was used.7 It should, however, be pointed out here that a higher initial dihydroxyacetone concentration together with a higher reaction temperature is not beneficial for formation of lactic acid (see section 2.1.3.4). The effect of temperature on direct transformation of glycerol to lactic acid was investigated.12,14 The selectivity for lactic acid was the highest with Pt/CaCO3 catalyst in the presence of boric acid at 170 °C, being 60%,14 whereas at lower temperature lower selectivity was achieved due to formation of 1,2-propanediol. In the case of Ir/C in the absence of boric acid, the conversion of glycerol increased with increasing reaction temperature from 8% at 100 °C to 98% at 200 °C. At the same time the highest selectivity for lactic acid, 77%, was achieved under a helium atmosphere at 180 °C, at which glycerol was also stable in the absence of any catalyst.12

2.2. Production of Alkyl Lactates

2.2.1. Production of Alkyl Lactates from Different Feedstocks. Alkyl lactates have been produced using both homogeneous43,44 and heterogeneous10,40,48,51,53,57 catalysts. The reaction involves dehydration of triose to pyruvaldehyde followed by its esterification to pyruvaldehyde hemiacetal, which thereafter can form either methyl lactate or alternatively the dimethyl acetal of pyruvaldehyde (Figure 11).57 The selection of the feedstock as the starting reagent for synthesis of alkyl lactates is crucial from the economic point of view. Glucose and sucrose, a disaccharide composed of glucose and fructose, are less expensive and more abundant than fructose.10 On the other hand, in comparing the yields or selectivities achieved for different feedstocks, it can be seen that lactate yields are relatively low from sugars compared to those obtained I

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was relatively selective for methyl lactate over the same catalyst.10,47 Furthermore, comparative selectivities for methyl lactate were achieved from glucose and fructose as raw materials (Table 2, entries 3 and 4), whereas mannose, arabinose, galactose, and xylose gave slightly lower selectivities than achieved from glucose and fructose. Furthermore, several other sugars have been studied as raw materials for production of alkyl lactates. These are, for example, lactose and maltose, but their conversion at 160 °C after 16 h remained at 61%, giving only 16% and 18% selectivities for methyl lactate.48 The reason for slightly lower methyl lactate yields achieved from xylose compared to glucose was stated to originate from the fact that less triose is formed from a pentose, xylose, than from glucose,48 since xylose undergoes retro-aldol condensation to a triose and glycolaldehyde. This compound in turn can form a dimethyl acetal, thus lowering the yield of methyl lactate. Furthermore, a ketohexose, such as fructose, decomposes to dihydroxyacetone and glyceraldehydes, whereas an aldohexose, glucose, produces aldotetrose and glycolaldehyde, and in comparative experiments more methyl lactate was achieved compared to that produced from glucose. It was, however, also confirmed that trimerization of glycolaldehyde (C2) produces C6 monosaccharides, which can be esterified to methyl lactate. The esterification of glycolaldehyde was separately investigated, showing that the main product over Sn-β catalyst at 160 °C

Figure 11. Reaction scheme for production of methyl lactate from glyceraldehyde and dihydroxyketone. Adapted from Taarning et al.57

from the intermediate products, such as dihydroxyacetone or glyceraldehyde (Table 2). There is, however, one exception: methyl lactate yields from sucrose are very high compared to those achieved from glucose and fructose.10 In addition, methyl lactate production from cellobiose has been demonstrated.48 The highest yields of and selectivities for alkyl lactates are summarized here, whereas the details of different catalyst properties are described in section 2.2.2 and the effect of the reaction conditions is discussed in section 2.2.3. Methyl lactate yields and selectivities vary with the feedstock. For example, only 13% methyl lactate was achieved from cellobiose with Sn-β catalyst,48 whereas transformation of sucrose Table 2. Catalytic Synthesis of Biomass to Alkyl Lactates entry

reactant

catalyst

1 2

cellobiose sucrose

Sn-β SnCl4·5H2O Sn-β

3

glucose

4 5 6 7 8 9

fructose mannose arabinose galactose xylose dihydroxyacetone

Al-3Zr Sn-β Sn-β Sn-β Sn-β Sn-β Sn-β CrCl3·6H2O AlCl3·6H2O SnCl2 SnBr2 SnI2 Sn-Si-CSM-773 Sn-MCM-41 USY USY Ti-Sil-HPB-60 La-NaY Sn-β Sn-montmorillonite H-USY SnCl2−H2O AlCl3 H-USY SnCl2/hydroxyapatite USY SnCl2 SnCl2 Sn-β Sn-β

10 11

12 13

dihydroxyacetone glyceraldehyde

puryvalaldehyde glycolaldehyde

conditions

conversion (%)

160 °C, 44 h, methanol 160 °C, 20 h, methanol 160 °C, 16 h, methanol 160 °C, 20 h, methanol 180 °C, 1.7 MPa, 2 h 160 °C, 16 h, methanol 160 °C, 16 h, methanol 160 °C, 16 h, methanol 160 °C, 16 h, methanol 160 °C, 16 h, methanol 160 °C, 16 h, methanol 90 °C, 3 h, methanol 90 °C, 3 h, methanol 90 °C, 3 h, methanol 90 °C, 3 h, methanol 90 °C, 3 h, methanol 90 °C, 1 atm of N2, 6 h, ethanol 90 °C, 2 h, ethanol 110 °C, 4 h, ethanol 90 °C, 6 h, ethanol 90 °C, 6 h, ethanol 90 °C, 6 h, ethanol 80 °C, 24 h, methanol 150 °C, 15 h, methanol 115 °C, 24 h, methanol 110 °C, 6 h, 1-butanol 140 °C, 180 min 115 °C, 48 h, methanol 110 °C, 6 h, 1-butanol 110 °C, 1 h, ethanol 90 °C, 3 h, methanol 90 °C, 3 h, methanol 160 °C, 16 h, methanol 160 °C, 20 h, methanol

62 >99 92 98 92 98 98 96 96 95 99

J

100 100 81 30 92 >99 >99 100 >99

selectivity for/yield of LA (%) 13 64 57 45 34 51 54 47 39 45 42 yield yield yield yield yield 100 98 81 yield 98 53 99 97 96 yield 88 99 yield 83 yield yield yield yield

50 62 89 83 71

65

88

63 88 85 16 16

ref 48 10 48 10 9 48 48 48 48 48 48 41 41 41 41 41 6 162 51 49 40 50 52 53 7 42 11 7 42 51 41 41 48 163

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was methyl vinylglycolate48 formed via aldol condensation to C4. Alkyl lactate was also produced from glycolaldehyde, containing two C atoms.48 This reaction was not, however, very selective, since the main product is methyl vinylglycolate.48 Thus, the yield of methyl lactate was very low in comparison to the results achieved with dihydroxyacetone52 and glyceraldehydes.7 Methyl vinylglycolate is formed in a consecutive step followed by aldol condensation of 2 mol of glycolaldehyde to tetrose, which in turn dehydrates and esterifies with methanol, forming methyl vinylglycolate.48 In addition, glycolaldehyde can also undergo consecutive aldol condensation, forming ketohexose, which in turn reacts also to trace amounts of (hydroxymethyl)furfural. 2.2.2. Catalysts for Production of Alkyl Lactates. Several homogeneous and heterogeneous catalysts have been tested for production of lactic acid or its esters from biomass, such as tin chlorides,10 SnO2,10,52,53 acidic resins,52 zeolites,7,10,40,48−51 metal-modified zeolites,47,48,50,52,163,164 aluminophosphates,50 micro- and mesoporous titanosilicates,40 mesoporous materials,40,47,53,162 Al-pillared clays,50 montmorillonite clay,53 and hydroxyapatite mineral.42 2.2.2.1. Homogeneous Catalysts. Alkyl lactates have also been prepared from trioses using homogeneous catalysts.41,42 Relatively efficient homogeneous catalysts are, for example, tin chloride and aluminum chloride (Table 2, entries 9 and 10)41,42 as well as CrCl3.41 The main drawback using homogeneous catalysts is their difficult separation from the products, although it has been demonstrated that it is possible to reuse tin chloride at least five times repeatably in the transformation of dihydroxyacetone to methyl lactate.41 Additionally, the product can also be contaminated with metals.40 Some of these metals are also toxic and corrosive.51 Thus, it is preferred to use heterogeneous catalysts for synthesis of alkyl lactates. Tin halides are active and selective homogeneous catalysts for transforming dihydroxyacetone41 and sucrose10 to alkyl lactates. They are, however, corrosive and difficult to reuse,51 although repeated uses of tin chlorides have been reported (see section 2.2.2.3).41 The reaction mechanism involving formation of a cyclic intermediate composed of Sn−O−C and Sn−OH bonds followed by its isomerization to enediol in the presence of alcohol has been proposed for transforming dihydroxyacetone and glyceraldehyde to alkyl lactates with SnCln.41 In the consecutive step pyruvic aldehyde was proposed to form a specific intermediate with SnCln, being able to undergo a 1,2-hydride shift from the terminal carbon and forming alkyl lactate.41 Noncatalytic reaction of pyruvic aldehyde with methanol gave only a very low yield of alkyl lactate, confirming the need for a catalyst for this reaction. 2.2.2.2. Heterogeneous Catalysts. Research in finding suitable heterogeneous catalysts for transforming trioses to alkyl lactates has been very intensive during recent years.6,7,48,50,51,53,57 As a result the most suitable catalysts for this purpose have been found to be zeolites,7,50,51 metal-modified zeolites,50,54 metalmodified mesoporous materials,40,162 Sn-montmorillonite,53 and Sn-containing carbon−silica hybrid material.6 The heterogeneous catalysts affording the highest selectivities for alkyl lactates with different substrates are presented here together with the desired catalyst properties. High yields of and selectivity for alkyl lactates have been demonstrated over Sn−carbon−silica hybrid catalysts starting from an intermediate product, dihydroxyacetone (Table 2, entry 9),6 whereas much lower yields were reported for fructose

and even less for glucose. Dihydroxyacetone is an intermediate being formed already via retro-aldol transformation from fructose. An additional step is required, when starting from glucose, namely, its isomerization to fructose, which explains the lower yield of alkyl lactate compared to that starting from fructose. In addition, Sn-MCM-41,162 Sn-β,52 Ti-SIL-HPB-60,38 and Sn-montmorillonite53 were also active and selective catalysts for production of alkyl lactates from trioses. Typically, reaction temperatures vary in the range of 90−150 °C. The production of alkyl lactates has been shown to be quantitative and very selective. Depending on the catalyst properties, a few different reaction mechanisms for formation of alkyl lactates over heterogeneous metal-modified catalysts were proposed, such as Meerwein− Ponndorf−Verley reduction52 as well as keto−enol isomerization.53 It should, however, be pointed out here that these two mechanisms also involve different types of active sites on the catalyst surface as follows. Alkyl lactates have been reported to be formed via Meerwein−Ponndorf−Verley-type reduction by tin ions over β zeolite catalysts,50 whereas over Snmontmorillonite they were claimed to be formed via keto−enol isomerization over Brønsted acidic catalyst.53 It was explained that over Sn-montmorillonite dihydroxyacetone isomerizes first to glyceraldehyde (GLA), which thereafter dehydrates to pyruvaldehyde. Hemiacetal is formed from the alkylation of pyruvaldehyde, and its keto−enol isomerization leads to formation of methyl lactate. A competitive parallel route is formation of pyruvaldehyde dialkyl acetal, which in turn can dehydrate and react with 2 mol of methanol, giving 1,1,2,2tetramethoxypropane.53 Clays are also suitable support materials exhibiting acidity, and their properties can be tuned by addition of metal. Only moderate yields of lactic acid from dihydroxyacetone and glyceraldehyde were achieved with H-montmorillonite clay.7 Zeolites have been intensively used as catalysts in the transformation of trioses to alkyl lactates.50,51,53 The location of Al is crucial for determining the type of acidity, being either Brønsted or Lewis acidity. When the amount of framework aluminum was determined by solid-state NMR for several zeolites, such as H-β, H-ZSM-23,50 H-ZSM-5, NH4-Y, and USY, and plotted as a function of the ethyl lactate yield, it was observed that the yield of ethyl lactate decreased with increasing percentage of Al in the framework. The extraframework aluminum species, for example, Al3+, Al(OH)2+, Al(OH)2+, Al(OH)3, AlO(OH), and Al2O3, exhibit on the other hand Lewis acidity.53 High yields of lactates were formed with the catalysts exhibiting low amounts of Al in the framework (Figure 12).51 Extraframework aluminum was also emphasized to be important for La-Y zeolite, which was calcined at 800 °C. During this treatment, strong Brønsted acid sites were reduced and Lewis acid sites were created.51 The reason is that La3+ ions can react with water on the surface and form strong LaOH Brønsted acid sites. Dehydroxylation performed at high temperature leads to creation of Lewis acid sites. This catalyst gave 53% selectivity for and a 49% yield of ethyl lactate within 6 h during dihydroxyacetone transformation at 90 °C. NaY zeolite exhibits strong Brønsted acid sites, which are depicted at 60 ppm in Hammond et al.27Al MAS NMR as tetrahedral framework aluminum.50 When Li+ ion was exchanged, and the catalyst was dealuminated at 250 °C, the peak at 60 ppm decreased and extraframework octahedral aluminum species appeared at 0 ppm.50 When additionally dealumination was performed at 800 °C, even pentacoordinated aluminum species K

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Carbon−silica hybrid materials with tunable acidity have been prepared and tested in transformation of sugars to alkyl lactates.6 One promising catalyst for transformation of sugars to alkyl lactates was carbon−silica hybrid material which was synthesized as follows: Si-MCM-41 material was prepared, and Lewis acidity was introduced by liquid-phase grafting. Thereafter, a carbon precursor solution was introduced into the pores of the Sn-Si-MCM-41 via the incipient wetness method. Carbon is partially pyrolyzed under an inert atmosphere. The amount of weak Brønsted acid sites can be tuned by oxidizing the hybrid material at the desired temperature, affording formation of oxygen-rich functional groups. The pyrolysis temperature was varied in the temperature range of 400−900 °C.6 The selection of the pyrolysis temperature determines which type of surface groups are generated on the carbon surface. If the pyrolysis temperature is about 500 °C, cyclic ether groups, which do not have Brønsted acidity, are formed. The optimum carbon content of 18 wt %, giving the highest yield of alkyl lactate starting from dihydroxyacetone, was obtained after pyrolysis at 500 °C and oxidation at 300 °C. Lower yields of alkyl lactates were achieved with these types of catalysts using fructose or glucose as the starting reagent. Furthermore, if the catalyst contained only Brønsted acidity, such as carbon−silica micromesoporous hybrid material Si-CSM (see below), but no Lewis acidity, alkyl lactates could be produced with high selectivity, albeit with very low yields. It has been pointed out that grafting of Sn(IV) creates only a few residual Brønsted acidic silanol groups, whereas direct incorporation of Sn into silica is preferred due to generation of enough weak Brønsted acid sites.6 On the other hand, if strong Brønsted acids are present in the catalysts, pyruvaldehyde acetals are formed.6 Aluminum and gallium oxides were not very active in the transformation of dihydroxyacetone to alkyl lactates due to their low acid site strength.162 This was explained by the fact that Al3+ and Ga3+ ions are located on the surface of solid oxides. The Brønsted acidity of aluminum and gallium oxides was also low due to the hydroxylation of surface oxides.162 Analogously, TiO2 was very selective, but the yield of ethyl lactate starting from dihydroxyacetone remained very low.40 Aluminum−zirconium mixed oxide afforded only moderate selectivity (37%)9 for lactic acid. Mixed oxides with a moderate base strength determined by the CO2 desorption method were claimed to be beneficial for production of lactic acid,9 whereas Al2O3 also containing strong basic sites as well as ZrO2 containing only mild basic sites gave lower yields of lactic acid.9 Unfortunately, the nature of basic sites being either Lewis or Brønsted was not investigated by Zeng et al.9 by, e.g., pyridine desorption or solid-state NMR. Furthermore, moderate yields of lactic acid have been achieved with sulfated zirconia.7 Sn-montmorillonite with Brønsted acidity gave a 97% yield of methyl lactate at over 99% conversion at 150 °C.53 It should be pointed out here that if the reaction was performed with Brønsted acidic catalyst at 90 °C, large amounts of pyruvaldehyde acetal and 1,1,2,2-tetramethoxypropane (TMP) were formed. On the other hand, at a higher temperature, 150 °C, in the beginning of the reaction acetals and TMP were also formed. Since methyl lactate is thermodynamically stable, these side reactions were, however, reversible, thus yielding high selectivity for lactate at high temperature after prolonged reaction times.53 As a comparison catalysts with only Lewis acidity, such as Cu-montmorillonite, were investigated in the transformation of dihydroxyacetone to methyl lactate at 150 °C,53

Figure 12. Yield of ethyl lactate as a function of the amount of framework aluminum in the transformation of dihydroxyacetone at 90 °C after 6 h. Adapted from Pescarmona et al.51

appeared at 40 ppm. These two catalysts, namely, LiNaY 250 °C and LiNaY 800 °C, gave respectively methyl lactate yields of 28% and 49% from dihydroxyacetone at 90 °C in 6 h, thus showing the importance of extraframework aluminum in the catalyst. Acetals were, however, also formed with these catalysts in relatively large amounts, namely, with yields of 20% and 43%, respectively. Extraframework aluminum species were also beneficial for ethyl lactate selectivity, which was remarkably enhanced at 90 °C in 6 h in the transformation of dihydroxyacetone over USY zeolites, treated with steam followed by their mild acid treatment. Maximally, a 58% yield of ethyl lactate was achieved.49 As a conclusion it was stated that dealumination of zeolite was required to obtain extraframework aluminum species, which are beneficial for production of lactic acid or alkyl lactates.49 Crystalline Sn-β was very active already at 40 °C when dihydroxyacetone was transformed to methyl lactate with more than 90% yield.47 This catalyst exhibited isomorphously substituted as well as hydrolyzed tin sites according to FT-IR characterization showing the corresponding peaks in the desorption of deuterated acetonitrile (Figure S2, Supporting Information).47 Furthermore, as a comparison, no methyl lactate was formed from dihydroxyacetone in a temperature range of 40−120 °C from dihydroxyacetone over Si-BEA, indicating that Lewis acidity is needed.47 In the case of a disaccharide, sucrose, as a reactant, a 59% yield of methyl sucrate was achieved at 160 °C with Sn-β catalyst. On the other hand, Sn-MFI zeolite exhibits narrow pores. The FTIR characterization revealed that Sn was in isomorphous substitution in MFI, while no hydrolyzed tin sites with strong Lewis acid sites were observed. Thus, Sn-MFI was only moderately active at 80 and 120 °C, giving yields of methyl lactate of 70% and 77%, respectively. Furthermore, in sucrose transformation diffusional limitations occurred, thus lowering the yield of methyl sucrate at 160 °C to 24%.47 In addition, titanosilicate-type TS-1 zeolite exhibiting an MFI-type structure was reported to be very selective (98%) in the transformation of dihydroxyacetone, however, giving only a 13% yield in 6 h at 90 °C.40 Due to the complexity of the reaction, several types of acid− base properties of the catalyst are needed. The dehydration step is promoted by Brønsted acids,10,51 whereas isomerization and retro-aldol condensation are promoted by basic conditions.9 Furthermore, Lewis acids catalyze formation of alkyl lactates.51 Thus, tuning of the catalyst properties is needed. L

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small amounts of weak Brønsted acid sites. Tin does not change the charge balance in the silica framework, compared to aluminum or gallium, thus forming Lewis acid sites, whereas the latter ones form Brønsted acidity. Lewis acidity catalyzed a 1,2-hydride shift162 and formation of alkyl lactate.51 There is also, however, somewhat contradictory information from the performance of Sn-MCM-41, since this catalyst was not selective in transforming dihydroxyacetone to methyl lactate at 120 °C.53 It should, however, be pointed out here that Sn-MCM-41 catalyst was not characterized by Wang et al.53 Compared to Sn modification of MCM-41, causing weak Brønsted acidity, strong Brønsted acidity has been observed in Al- and Ga-modified MCM-41 and zeolites. In a comparison of the catalytic activities of alumina and gallia, it was however noticed that the latter one was more active due to its higher density of Lewis acid sites and higher strength of its Brønsted acid sites.162 In addition, Ti-MCM-41 was selective for ethyl lactate in dihydroxyacetone transformation at 90 °C, giving 83.7% selectivity in 6 h.40 It should, however, be pointed out here that it gave slightly lower activity and selectivity compared to the novel titanosilicate beads, which also exhibited higher acidity. As a comparison Si-MCM-41 was also tested, and it was totally inactive due to the lack of Lewis acidity.40 2.2.2.3. Catalyst Deactivation, Stability, and Reuse. Catalyst stability, reuse, and deactivation tests for production of alkyl lactates have been performed in sugar10 and triose7 transformations. Catalyst calcinations are also beneficial for recovery of the catalyst activity.10,162Leaching of the active metal can be challenging in some cases.47 Several Sn-, Zr-, and Ti-β zeolite catalysts were reused in sucrose transformation in methanol after catalyst calcination,10 and the results showed that these catalysts could be applied successfully in six consecutive experiments. The structure of β zeolites remained unchanged, and their specific surface areas decreased only slightly after 100 h of use. It was, however, observed that in the fixed-bed operation a gradual deactivation occurred,10 and thus, the importance of the catalyst calcinations was emphasized. In general, it should be stated that catalyst deactivation was less prominent in the case of methanol as a solvent compared to water (see section 2.2.3.1).6,7,10,51 Several heterogeneous catalysts have been recycled in biomass transformation to alkyl lactate, such as Sn-MCM-41,162 Sn-β,47,48 and Ti-SIL-HPB.40 In some cases, especially under hydrothermal conditions, the zeolites were not stable.7 Furthermore, metal leaching has been reported to occur.47 Solvent selection and temperature affect the catalyst stability.7 Typically, esterification of biomass-derived molecules is performed under relatively low temperatures, facilitating successful catalyst recycling.7,10,40 Furthermore, alkaline reaction conditions are more harsh for catalyst stability in glycerol transformations to lactic acid.14 A comparative study of the effects of the solvent and temperature was performed in the transformation of dihydroxyacetone over H-USY-6 in batch and in continuous operations either in water or in methanol, giving respectively lactic acid and methyl lactate.7 The results revealed that the reaction conditions are crucial for the catalyst stability, especially if the reactions are conducted in water at high temperature.7 The activity decline of H-USY-6 catalyst was nearly 3-fold more prominent in water (177 °C) than in methanol (157 °C) after a 48 h time-on-stream in dihydroxyacetone transformation.7 The origin of the zeolite deactivation in water was stated to be the decrease of the specific surface area and the crystallinity loss of

giving 12% methyl lactate. Thus, it was concluded that strong Brønsted acidity is essential for producing methyl lactate.51 Al-pillared beidellite clay showed 56% selectivity at 30% conversion of dihydroxyacetone to methyl lactate at 90 °C in 6 h.50 Substantial amounts of acetal were also formed due to the presence of relatively strong Brønsted acid sites. Highly dispersed SnCl2 supported on hydroxyapatite (Ca10(PO4)6(OH)2) mineral was an efficient catalyst for production of different alkyl lactates from trioses,42 despite the fact that the specific surface area of calcined SnCl2/hydroxyapatite (HAP) was only 43 m2/g. For example, an 88% yield of butyl lactate was achieved with 31 wt % SnCl2/HAP in 6 h at 110 °C starting from dihydroxyacetone.42 Catalyst recycling was not, however, very successful (see section 2.2.2.3). Brønsted acidic catalysts only, such as ion exchange resins, are not able to produce alkyl lactates starting from dihydroxyacetone,6 and thus, it was concluded that bifunctional catalysts containing both Brønsted and Lewis acid sites are preferred in the synthesis of alkyl lactates from biomass.6 Mesoporous amorphous aluminosilicates, which were prepared without calcinations, exhibited a low degree of crystallinity and mild Brønsted acidity.50 Maximally, a 22% yield of ethyl lactate was achieved in dihydroxyacetone transformation at 90 °C in 6 h, but the selectivity was extremely high, 96%.50 Mild Brønsted acidity was claimed to be needed in the initial dehydration step during dihydroxyacetone transformation to pyruvaldehyde, whereas acetalization could be avoided due to the lack of strong Brønsted acid sites. Micromesoporous titanosilica beads, which exhibited very high acidity, were also tested in dihydroxyacetone transformation to ethyl lactate.40 Mesopores were created by using Amberlite resin as a template. Titanosilicate contains mainly Lewis acidity, with only weak Brønsted acid sites being present as confirmed by 27Si NMR.165 Furthermore, Ti species were confirmed to be in tetrahedral coordination by UV−vis corresponding to Ti(OSi)4 and Ti(OH)(OSi)3. This type of new titanosilicate catalyst was very initially active and selective in dihydroxyacetone transformation to ethyl lactate, giving a selectivity of 97.9% at 31% conversion.38 The studied catalyst, however, deactivated within 5 h of reaction time; after that, no further conversion occurred. The reasons for the low conversion were not discussed by Lin et al.40 Several mesoporous metal-modified catalysts have been applied in the production of alkyl lactates.40,47,162 In a comparative study the performance of Sn-SBA-15 and Sn-MCM-41 with two different Si/Al ratios, 50 and 200, was investigated in the transformation of dihydroxyacetone to methyl lactate at different temperatures.47 At 40 °C the yields of methyl lactate were below 20%, increasing to ca.70% with elevated temperature. The highest yield of methyl lactate, about 98%, was achieved at 120 °C with Sn-MCM-41 with a Si/Al ratio of 50.47 On the other hand, when these catalysts were studied in sucrose esterification, at 160 °C, it was observed that the yield of methyl sucrate was around 28%, independent of the Si/Al ratio as well as the support structure.47 These mesoporous materials exhibited mild acidity and an amorphous structure. The best catalyst in their study was, however, Sn-β, which exhibited strong Lewis acid sites and a crystalline structure (see above). Sn-MCM-41 was reported to be a promising catalyst for production of ethyl lactate from dihydroxyacetone.162 It was stated that this catalyst exhibited tin in tetrahedral coordination in a silica framework, giving strong Lewis acidity, which was confirmed by pyridine desorption. In addition, tin gives rise to M

dx.doi.org/10.1021/cr400203v | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

to ethyl lactate were achieved with SnCl4·5H2O catalyst in dihydroxyacetone transformation at 90 °C in 2 h.41 The specific reasons for this result were not proposed. An alcohol structure can also affect the selectivity for alkyl lactates. One interesting result from the effect of the solvent was that with a longer chain alcohol as a solvent the selectivity for alkyl lactate increased over Sn-HAP catalyst.42 The reason for this was claimed to be the formation of less byproducts due to the steric effect of 2-propanol or 1-butanol. Analogously to these results, less acetals and more lactate were formed in 2-propanol than with 1-propanol, when a comparative study was performed in the transformation of dihydroxyacetone to lactates over USY zeolite at 110 °C.51 The selectivity for lactates with these alcohols was 83% and 69%, respectively. In addition to shorter chain alcohols, such as methanol, ethanol, 2-propanol, and 1-butanol,42 longer alcohols were also successfully used as solvents in dihydroxyacetone transformation over Sn-Si-CSM catalyst at 90 °C during 6 h. The yield of lactate increased in the following order: 54% (tetradecanol) < 53% (dodecanol) < 67% (decanol) < 83% (octanol).6 2.2.3.2. Effect of Temperature. The effect of temperature has been studied in the transformation of sugars48 as well as trioses7,53 and glycolaldehyde48 as feedstocks in alcohol48 over heterogeneous catalysts.7,48,53 The effect of temperature in the transformation of sugars to alkyl lactates48 has been studied with heterogeneous catalysts. In the synthesis of alkyl lactates, usually higher reaction rates and selectivities for lactates can be achieved compared to those in the case of water as a solvent.10 Selectivity for methyl lactate increased at higher reaction temperatures with xylose as a reactant and Sn-β catalyst. In xylose transformations to methyl lactate the corresponding increase of methyl lactate selectivity was from 26% at 100 °C to 43% at 160 °C.48 There is, however, one exception, transformations of glucose at higher temperature, since glucose can also undergo other reactions under high temperatures, leading to formation of products other than alkyl lactates.50 For example, the selectivity for methyl lactate in glucose transformation increased from 34% at 100 °C to 56% at 140 °C, whereas at 160 °C a slightly lower selectivity of 52% was achieved. The yield of methyl vinylglycolate increased with increasing temperature,48 as it is formed due to the fragmentation of glucose to glycolaldehyde and aldotetrose followed by aldol condensation of glycolaldehyde.48 The effect of temperature in the transformation of trioses using heterogeneous catalysts has been investigated in a few studies.40,48,53 The initial formation rates of ethyl lactate increased when TiSil-HPB-60 catalyst was used in the temperature range of 75−105 °C40 in dihydroxyacetone transformations at an initial concentration of 0.4 M. Namely, the ethyl lactate yield and selectivity increased from 15% to 47% and from 79% to 97%, respectively, with elevation of temperature.40 In addition, with Sn-montmorillonite catalyst in the transformation of dihydroxyacetone highly diluted in methanol, the methyl lactate yields also increased with increasing reaction temperature from 90 to 150 °C.53 Glycolaldehyde transformation to methyl lactate was not very successful at high temperatures despite the fact that an increase of the reaction temperature from 100 to 160 °C also improved the yield of methyl lactate from 4% to 16% when glycolaldehyde was transformed in the presence of Sn-β catalyst.48 At the same time, however, even higher yields than those for methyl lactate were also achieved for the undesired product, methyl vinylglycolate, being 13% and 27%, respectively,

the catalyst.7 Furthermore, it was observed that lactic acid destroys the zeolite structure, whereas in methanol its structure remains unaltered. Recycling of catalysts has been successfully demonstrated under relatively low temperatures in alcohols. This was the case also when TiSIL-HPB was studied in the transformation of dihydroxyacetone to ethyl lactate at 90 °C in consecutive 6 h experiments. The spent catalyst was only washed at room temperature in ethanol, and both the conversion level and selectivity remained constant, being 30% and 99%, respectively.40 It should, however, be pointed out here that the reaction temperature was relatively low. Recycling of Sn-β was successfully performed when it was used as a catalyst in sucrose esterification at 160 °C.10 The catalyst was calcined after each experiment to remove carbon deposits on the catalyst surface.10 Mesoporous Sn-MCM-41 was successfully recycled in the transformation of dihydroxyacetone to ethyl lactate at 90 °C when the catalyst was calcined at 500 °C for 2 h between the consecutive experiments.162 Relatively high amounts of Sn, 5.9 wt %, have been leached out from Sn-β catalyst during sucrose transformations in methanol at 160 °C for 3 h.47 Thereafter the catalyst was filtrated, and more sucrose was added to the filtrate. Furthermore, conversion of sucrose was, however, negligible, indicating that no homogeneous catalysis occurred. Catalyst calcination after its use is beneficial to recover its activity and selectivity.10,51 In the transformation of dihydroxyacetone to ethyl lactate at 90 °C, recycling tests of USY catalyst were performed. The results showed that the catalytic performance was equally good when the catalyst was either washed only with ethanol or calcined at 450 °C.51 When no calcination was applied, lower catalyst activity was achieved, as was the case when SnCl2/HAP was recycled in the production of alkyl lactates from dihydroxyacetone by only washing and drying at 110 °C.42 The origin for catalyst deactivation was not, however, discussed. 2.2.3. Effect of the Reaction Conditions. The optimization of the reaction conditions is very important to achieve high activity and selectivity. Several effects, such as solvent selection, reaction temperature, initial reactant concentration, and pH, can be crucial for catalyst performance and stability. Especially in the transformation of trioses, the initial reactant concentration has a large effect on the product selectivity. Typically, the preferred reaction conditions are a low initial reactant concentration and high reaction temperature,11 which are summarized below. 2.2.3.1. Effect of the Solvent. Several studies have been done using different alcohols as solvents for production of alkyl lactates.6,10,42,51 Typically, the yield of and selectivity for alkyl lactate decreased with increasing carbon chain length in alcohol when H-USY was used as a catalyst in dihydroxyacetone transformation.51 For example, in sucrose transformations over Sn-β catalyst at 160 °C during 20 h, the yield of alkyl lactate decreased as follows: 66% (methanol) > 37% (EtOH) > 23% (2-propanol).10 It should also be pointed out here that in addition to alkyl lactates small amounts of alkyl vinylglycolates were formed. This product is a result of retro-aldol condensation of hexose to glycolaldehyde and erythrose, leading in a consecutive step to the formation of alkyl vinylglycolate.48 There are also, however, exceptions to the trend of the yield of and selectivity for alkyl lactates decreasing with increasing alcohol chain length.41 Higher yields of butyl lactate compared N

dx.doi.org/10.1021/cr400203v | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

3. CATALYTIC TRANSFORMATIONS OF LACTIC ACID TO COMMODITY CHEMICALS Lactic acid is an attractive raw material for its conversion to various chemicals. The primary classes of these reactions are hydrogenation, dehydrogenation, dehydration, condensation, esterification, polymerization, and substitution at the alcohol group. Lactic acid serves as a highly attractive building block for the synthesis of such chemicals as acrylic acid, pyruvic acid, 2,3-pentanedione, lactic acid esters, and 1,2-propanediol (1,2-PDO) (Figure 1).1,55,61,82

indicating that the transformation of glycolaldehyde is not very selective at higher reaction temperatures.48 The reaction mechanism for transformation of glycolaldehyde is discussed in section 2.2.1. In addition, Gibbs free energies were estimated for all reaction compounds in the transformation of dihydroxyacetone to methyl lactate using density functional theory calculations,53 namely, for glyceraldehyde, dihydroxyacetone, pyruvaldehyde, pyruvaldehyde hemiacetal, pyruvaldehyde acetal, and 1,1,2,2-tetramethyoxypropane. The results revealed that methyl lactate exhibited the smallest ΔG value of −108.9 kJ/mol relative to dihydroxyacetone, indicating that the former is thermodynamically the most stable compound, whereas the corresponding ΔG value for glyceraldehyde is 20.9 kJ/mol. 2.2.3.3. Effect of the Initial Reactant Concentration. The effect of the initial reactant concentration on the formation of alkyl lactates has been very scarcely investigated.53 In general, it can be stated that at high concentrations of, e.g., trioses, selectivity for alkyl lactate is not very high. Transformations of dihydroxyacetone to methyl lactate were investigated over Sn-montmorillonite catalyst at 150 °C using an initial concentration of DHA in the range of 0.25−25 mol/L. The results revealed that the highest selectivity for alkyl lactates was achieved with an initial concentration of 2.5 mol/L, being 77% at a nearly complete conversion. If a 10-fold higher initial DHA concentration was used, the methyl lactate selectivity was only 36%. The authors also proposed a reaction network with two quantified side products, namely, pyruvaldehyde dimethyl acetal (PADA) and 1,1,2,2-tetramethoxypropane (TMP). The latter was formed from PADA in a consecutive step via addition of two methanol molecules and removal of one water molecule. In addition to the identified side products, other side products were also formed. The reaction network from DHA to methyl lactate was composed of DHA isomerization to glyceraldehyde, its dehydration to pyruvaldehyde, and further reaction to pyruvaldehyde hemiacetal (PAMH). Thereafter, PAMH reacted further either to methyl lactate or in parallel to PADA as also depicted in Figure 11 and consecutively to TMP. The formation of methyl lactate (ML) is through a keto−enol isomerization of PAMH.53 Biomass transformation to lactic acid or alkyl lactates has been summarized in this section. The transformation of biomass to lactic acid in water is more challenging than production of alkyl lactates in methanol due to high stability of the latter. Furthermore, several side products, such as levulinic and formic acids and 5-(hydroxymethyl)furfural, are formed in biomass transformation to lactic acid, since the reaction network includes dehydration, isomerization, and retro-aldol condensations. Moreover, trioses are not stable in water at relatively high temperatures, thus decreasing the yield of lactic acid. The extra cost in production of alkyl lactates is related to recycling of methanol. The highest yields of lactic acid have been obtained starting from trioses at relatively low temperatures, 80−90 °C, with either zeolites or metal-modified zeolites. It should also be pointed out that zeolites are typically not stable under hydrothermal conditions. In the case of alkyl lactates, very high yields and selectivities have been achieved starting from dihydroxyacetone using Sn-β, Sn-montmorillonite, and H-USY as catalysts in the temperature range of 90−115 °C or alternatively a hybrid Sn-Si-CSM carbon-containing mesoporous material, which possesses weak Bronsted acids together with Lewis acid sites. In addition, glycerol, which is the byproduct from biodiesel synthesis, can be upgraded to lactic acid with 78% selectivity over Ir-modified alkaline CaCO3 at 180 °C in the presence of sodium hydroxide.

3.1. Hydrogenation of Lactic Acid

A catalytic hydrogenation of lactic acid to propylene glycol provides an eco-friendly alternative to the petroleum-based process for 1,2-PDO synthesis. Moreover, this method is carbon efficient, since it implies hydrogenation of the −COOH moiety to −COH without any loss of carbon atoms. 1,2-PDO is a desired commodity chemical used as a deicing fluid, as an antifreeze, for the production of unsaturated polyester resins, and in the production of drugs and cosmetics.54,61,82,93 The formation of 1,2-PDO from lactic acid requires hydrogenation of the carboxyl group to an alcohol without removal of the α-hydroxyl group, while hydrogenolysis of the α-hydroxyl group results in the formation of propionic acid. Hydrogenation of lactic acids and esters is often performed under severe reaction conditions due to the low reactivity of the carboxylic acid group with hydrogen.79 Table 3 shows that Table 3. Equilibrium Constants for Vapor-Phase Hydrogenation Reactions of Lactic Acid at 0.1 MPa80 equilibrium constant (KP) reaction

150 °C

200 °C

250 °C

CH3CHOHCOOH + 2H2 → CH3CHOHCH2OH + H2O CH3CHOHCOOH + 2H2 ← CH3CH2COOH + H2O

5.00 × 100

1.26 × 100

4.00 × 10−1

1.01 × 1010

1.30 × 109

1.61 × 108

thermodynamic considerations strongly favor the formation of propionic acid compared to 1,2-PDO. These thermodynamic values indicate that higher conversions of lactic acid to 1,2-PDO are possible at lower temperatures, lower lactic acid concentrations, and higher hydrogen partial pressures.80 The values were determined from the thermodynamic data given by Yaws.177 Broadbent et al.62 reported for the first time catalytic hydrogenation of free lactic acid (Figure 13) over an unsupported Re

Figure 13. Hydrogenation of lactic acid to 1,2-propanediol (1,2-PDO). Adapted from Broadbent et al.62

black catalyst at 150 °C and 27 MPa of hydrogen pressure, achieving yields of propylene glycol as high as 80%. Hydrogenation of ethyl lactate to propylene glycol was done at 150− 250 °C and high hydrogen pressures, 20−30 MPa, over copper/ chromium oxide and Raney Ni catalysts with an 80% yield of propylene glycol.63 Hydrogenation of lactic acid to propylene glycol over a Ru-containing catalyst in the liquid phase was carried out at 14.5 MPa and 150 °C, leading to noticeable lactic acid conversion and selectivity for propylene glycol.77 O

dx.doi.org/10.1021/cr400203v | Chem. Rev. XXXX, XXX, XXX−XXX

5% Ru/C 50.9 wt % slurry in water

5% Ru/C

4

5

P

5% Ru/C

60% Ni/Al2O3 CuCrOx Raney Ni 3% Ru/TiO2 5% Ru/Al2O3 Ru/C (PMC, Inc.) Ru/C 5% Ag/SiO2 5% Co/SiO2 5% Cu/SiO2 5% Ni/SiO2 5% Pt/SiO2 5% Ru/SiO2 MgO−poly[(γaminopropyl) siloxane]−Ru complex (MgO−NH2−Ru),

7

8

10

9

5% Ru/C

6

3

2

5% Ru/C, 50.9 wt % slurry in water 0.5−5.0% Ru/C 0.64−1% Ru/TiO2 2% Ru/TiO2 5.0% Ru/TiO2 1−3% Ru/CeO2 5% Ru/C

catalyst/mass/slurry concn (wt %)

1

entry

dispersion 34% dispersion 13% 9.2 25.5 34.9 13.3 4.5 18.6

850 730 116.1 135.2 194.2 117.6 139.8 138.1 liquida

dispersion 40%

49

liquidb

liquid

a

dispersion 13%

liquidb

780

liquida

dispersion 12 ± 2%/granular (15−30 mesh) activated carbon

liquidb

disperssion 13%

dispersion 13%/−/150 μm

liquidb

liquida

−/−/ 1.4 mm

liquida

700a 30 30 30 253a

860

dispersion 8.8%/−/150 μm 2 nm 2.3 nm 2.3 nm 5 nm 90

150

70−150

8.3

3.2

3.4−10.3

0.27 M

1M

0.55 M 1.15 M 3.6 M 0.55 M calcium lactatea 0.55 M

0.8−1.5 M, WHSV = 0.2−1.3 h−1

14.5

14.5

14.5

1.4−10.0

240

5

110 −170 6.0−9.0

150

150

150

80 −150

5 wt % (0.56 M) 130 −150 6.8−13.6 10 wt % (1.15 M)

1 M, WHSVc = 0.95 h−1

no data

0.05−5.0 M

particle size (nm)/amt of surface metallic Cu0 per gram/grain size H2 pressure (μm) lactic acid quantity temp (°C) (MPa)

liquida

liquid- or vapor-phase reaction

716

specific surface area (m2 g−1)

Table 4. Catalytic Hydrogenation of Lactic Acid to Propylene Glycol

solvent

products

1,2-PDO, ethanol, 1- or 2-propanol, methane, ethane, propane 1,2-PDO, ethanol (0.022 M), 1-propanol (0.013 M), 2-propanol (0.009 M), methane, ethane, propane 1,2-PDO, ethanol, 1-propanol, traces of 2-propanol

no data

1,2-PDO, ethanol, 1-PrOH 1,2-PDO, nd

92 70 40 0

ca. 55−57

98

100 during 7 h

water

1,2-PDO, ethanol, ∼0 1-propanol, traces of ∼3 2-propanol ∼18 ∼37 ∼60 ∼95 ∼98 water 1,2-PDO, acetol ca. 2 ca. 1 ca. 0 ca. 30 ca. 20 ca. 35 5 mL of water 1,2-PDO 100

water

water

water

water

water

50 mL of water nd

97

ca. 81 (at 100 °C)

no data no data no data no data no data no data no data n.m.d n.m. n.m. n.m. n.m. n.m. 100

∼92 ∼92 ∼92 0

98

90

67

77

77

77

76

64

78

ref

TOF ≈ 14−31 h−1 TOF ≈ 79 h−1 TOF ≈ 44 h−1 TOF ≈ 14 h−1 TOF ≈ 3−14 h−1 yield ca. 55% in trickle-bed reactor, yield ca. 10% in batch reactor 75−95

80−99

max selectivity for max conversion at 1,2-PDO optimal reaction (%)/TOF (h−1)/ conditions (%) yield (%)

Chemical Reviews Review

dx.doi.org/10.1021/cr400203v | Chem. Rev. XXXX, XXX, XXX−XXX

Later on it was found that lactic acid hydrogenation in the vapor phase over a Cu/SiO2 catalyst can lead to high conversion and selectivity for propylene glycol even at atmospheric hydrogen pressure.80,91,92 The data on utilization of various feedstocks and catalysts are summarized in Tables 4 and 5. In section 3.1.1 we consider mainly the feedstock, while the subsequent section 3.1.2 is devoted to a detailed description of the catalysts. It is apparently clear that the development of active catalysts for hydrogenation of lactic acid and lactates to 1,2-PDO under mild reaction conditions is of great interest. 3.1.1. Feedstock. Direct hydrogenolysis of lactic acid seems to be attractive, but suffers from several drawbacks, such as catalyst deactivation due to polymerization of lactic acid and formation of undesirable propionic acid. In addition, the fouling and plugging problems are severe under the reaction conditions. To avoid the above-mentioned problems and to perform the hydrogenation process more effectively, carboxylic acids are usually converted into more readily reducible esters. The advantages of using lactic acid esters in the gas-phase hydrogenolysis are their chemical nonaggressiveness and relatively low boiling points, a possibility of increasing the substrate concentration while retaining a high degree of conversion, and an increase of the selectivity and stability of the catalyst due to the inhibition of the side reactions in the presence of hydrogen.61 Lactate hydrogenolysis over silica-supported copper catalysts with different copper loadings in the vapor phase provides higher propylene glycol yields at atmospheric hydrogen pressure80,93 compared to those obtained with other catalysts. Taking into account that lactate is an attractive and available feedstock since commercial production of neat lactic acid is based on alkyl lactate ester distillation, lactate hydrogenolysis can be considered as a more promising method for propylene glycol synthesis compared to lactic acid hydrogenation.1,61,85 Note that lactate salts generated in fermentation cannot be directly hydrogenated and must be acidulated to the free lactic acid prior to hydrogenation.77 3.1.1.1. Effect of the Alkyl Substituent. A comparison of the literature data of the hydrogenolysis of lactic acid esters with different alkyl substituents is presented in Table 6.93 The effect of the substituent in alkyl lactate on conversion and selectivity for propylene glycol over copper catalysts was studied by Simonov et al.93 and reported in a review by Murzin and Simakova.61 A comparison of the reactivity of methyl and butyl lactates at the same weight hourly space velocity (WHSV) in terms of converted moles shows that the nature of the alkoxyl group does not substantially affect the ratio of the products (Figure 14). It was discovered that more profound transformations of both methyl and butyl lactate occur when the copper content is raised to 45.5 wt %, and the use of butyl lactate is preferential due to higher conversion at temperatures above 180 °C. In addition, methanol produced during methyl lactate hydrogenolysis undergoes undesirable transformations in the presence of copper-containing catalysts, leading to formation of methyl formate, dimethyl ether, and carbon monoxide and limiting the extent of its repeated use in the esterification of lactic acid. Butanol, unlike methanol, probably does not have side dehydrogenation reactions. The authors concluded that any of the volatile lactic acid esters can therefore be used to synthesize propylene glycol, but the use of methyl lactate is undesirable due to its side catalytic transformations.93 Yang et al. studied178 the effect of steric hindrance of the substrates by changing from a methyl to a isopropyl group in

a The main phase was not specified by the authors, and assignment of the reaction phase is somewhat arbitrary as besides the main phase the other phase can also exist. bThe main reaction phase is mentioned on the basis of the assignment of the original papers. cWeight hourly space velocity. dNot measured.

88 87 1,2-PDO, 1-propanol 4.0, 10 mL of H2/CO = 2 water 240 13

0.1 mol % Au/m-ZrO2

liquida

3−5/12.0/0.63−1.6 mm 3−8/11.0 m2 g−1/0.63− 1.6 mm 2 nm 5 wt %

CuZn/SiO2 ca. 250 (13%Cu) Cu/SiO2 (14%Cu) ca. 520 Cu/SiO2 (45.% Cu)

CuCr2O4 (28%Cu) 12

100

50 65 45 97

7

14

92 24 3 water 5 × 10 × 10/6.0/0.63− 1 wt %, WHSV = 200 1.6 mm 0.08 h−1 3−7/7.0/0.63−1.6 mm vaporb

140− 220 0.1−0.72 −/157 μmol of Cu0/g cat/- 85 wt % SiO2 Cab-O-Sil (EH-5, Cabot Corp.) vaporb 380 10% Cu/SiO2 11

0.096mmolofRu/g

catalyst/mass/slurry concn (wt %) entry

0.1

water

solvent

products

1,2-PDO, propionic acid, 1-PrOH, 2-hydroxy propionaldehyde 1,2-PDO, 1- or 2-propanol, 2-hydroxypropanal

100

54−88

max selectivity for max conversion at 1,2-PDO optimal reaction (%)/TOF (h−1)/ conditions (%) yield (%) particle size (nm)/amt of surface metallic Cu0 per gram/grain size H2 pressure (μm) lactic acid quantity temp (°C) (MPa) liquid- or vapor-phase reaction specific surface area (m2 g−1)

Table 4. continued

Review

80

ref

Chemical Reviews

Q

dx.doi.org/10.1021/cr400203v | Chem. Rev. XXXX, XXX, XXX−XXX

[RuHCl(CO)(HN (CH2CH2PPh2)2), 0.98 wt % Ru

6

R

10

9

8

4.7% Ru−B/γ-Al2O3 4.9% Ru−4% Sn−B/γ-Al2O3 4.1% Ru−7% Sn−B/γ-Al2O3 4.2% Ru−12% Sn−B/γ-Al2O3 4.0% Ru−37% Sn−B/γ-Al2O3 4.7% Ru−B/γ-Al2O3 4.1% Ru−7% Zn−B/γ-Al2O3 4.5% Ru−4% Co−B/γ-Al2O3 4.0% Ru−37% Fe−B/γ-Al2O3 4.2% Ru−12% Sn−B/γ-Al2O3 Ru−B/SBA-15 (Sn 0%) Ru−B/Sn-SBA-15−80 Ru−B/Sn-SBA-15−70 Ru−B/Sn-SBA-15−50 Ru−B/Sn-SBA-15−30 Ru−B/Sn-SBA-15−10 Ru−B/Sn-SBA-15−10imp 10% Cu/SiO2 10% Co/SiO2 10% Ni/SiO2 10% Fe/SiO2 1.8% Pd/SiO2 10% Ru/SiO2 10% Co/active carbon

1% Ru/CSP 2% Ru/CSP Ru/charcoal

5

7

2.0 mL

liquida

5% Ru/TiO2, 5% Ru/SiO2, 5% Ru/γ-Al2O3, 5% Ru/NaY, 5% Ru/C

4

577 599 598 615 607 628 614 356 332 354 357 411 372

164.7 164.9 164.9 165.4 168.5 164.7

vaporb

liquidb

liquidb

liquidb

liquida

liquida

1.5 mL

liquida

Ru−Sn−B/γ-Al2O3

3

55 348 171 325 1107

1.5 mL

liquida

Ru−B/γ-Al2O3, boron loadings 7.9 and 8.5 wt %

56 26 11 9 7 56 14.3 13.1 9.1 9.4 19.8 16.2 14.7 11.2 10.3 14.2 15.8 22.7 (CuO) 17.9 (Co3O4) 21.4 (NiO) 18.3 (Fe2O3) 8.6 (PdO) 11.9 (RuO3)

8.2/−/− 8.5/−/− 11.7/−/− 15.2/−/− 7.8/−/− 3/−/0.5

150

150

150

30−40

150−180

90−150

90−150

90− 150

90 −150

1.0−7.0

5.5

5

5

5

5.0

5.0

4.0

4.0

4.0

H2 pressure temp (°C) (MPa)

WHSVc = 90−180 0.1250.25 h−1 and H2/ethyl lactate of 50110

5 mL

5 mL

10 mmol of methyl (R)lactate, 99.6% ee 5 mL

0.5 mL

1.5 mL

2

5−10 nm

liquida

catalyst

lactate quantity

Ru−B/TiO2; boron loadings 7.9 and 8.5 wt %

particle size (nm)/amt of surf metallic Cu0 per g/grain size (μm)

1

liq- or vapphase react

entry

spec surf area (m2/g)

Table 5. Catalytic Hydrogenation of Ethyl Lactate to Propylene Glycol solvent

products

1,2-PDO, no data about side products

1,2-PDO, 1-propanol, PA, LA 87 84 75 52 45 94

1,2-PDO, 1-propanol, 98 PA, LA, C2H6 in the gas phase 1,2-PDO, 1-propanol, 78.7 PA, LA

3 mL of water, 1,2-PDO, PA, LA, MeOH, EtOH, methane, ethane i-PrOH, n-BuOH, dioxane NaOMe (2 M in (R)-1,2-PDO, 99.2% ee MeOH, 0.5 mmol) + MeOH (5.5 mL) 30 mL of n-heptane 1,2-PDO, 1- or 78.7 2-propanol, 38.2 2-hydroxypropyl 90.7 lactate 86.2 30.7 30 mL of n-heptane 1,2-PDO, 1- or 78.7 2-propanol, 28.5 2-hydroxypropyl 51.0 lactate 87.3 86.2 30 mL of n-heptane 1,2-PDO, 1- or 15.4 2-propanol, 26.7 2-hydroxypropyl 34.0 lactate 47.7 68.4 39.9 19.7 solvent-free 1,2-PDO, methanol, 100 ethanol, 1- or 100 2- propanol, acetol, 3.3 ethyl propionate, 1.1 ethyl acetate 8.2 10.1 29.3

1.5 mL of water, EtOH, i-PrOH, n-hexane, dioxane 1.5 of mL water, EtOH, i-PrOH, n-hexane, dioxane 1.5 mL of water, EtOH, i-PrOH, n-hexane, dioxane 4 mL of water, EtOH, i-PrOH, n-hexane, dioxane

82

−/5.4 h−1

178

85

65

65

89

TON up to 4000 87 with optical purity, >99% ee 50.7 53.5 91.5 85.1 71.2 50.7 82.9 60.3 80.1 85.1 75.4 97.6 98.0 98.4 98.9 75.1 79.8 98.5 97.4 96.0 40.2 92.6 95.7 98.4

ref

83, 84

82

50.7/2.9 h−1

88/−/− 93/−/− 81/−/− 75/−/− 81/−/− 96

82

95

max convers at max selectiv for optimal react 1,2-PDO (%)/TOF condit (%) (h‑1)/yield (%)

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S

catalyst

10% Co/Al2O3 10.8% Cu/SiO2 11.5% Cu/SiO2 11.9% Cu/SiO2 86% Cu/SiO2 76% Cu/SiO2 34% Cu/SiO2 Cu/SiO2 (45.5% Cu) prep by H2 red. of (CuH)4[Si4O10] (OH)8·nH2O Co/SiO2 (precipitation gel (PG)) Co/SiO2 (deposition−precipitation (DP)) 17.6% CoZn/SiO2 18.0% CoFe/SiO2 18.5% CoCu/SiO2 16.9% CoSn/SiO2 19.6% Co/SiO2

156 169 170 171 125

liquidb

liquidb

125

9.0 9.5 7.0 6.0 12.1

n.m.d

12.1

vaporb

191

12/46/− 3/113/− 15/54/− 20/284/− 15/326/− 6/213/− 3−8/11.0 m2 g−1 metallic Cu0/−

particle size (nm)/amt of surf metallic Cu0 per g/grain size (μm)

vaporb

liq- or vapphase react

525 219 275 75 105 195 ca. 520

spec surf area (m2/g) 180

2 g of ethyl lactate

160

8

8

0.1

2.5

H2 pressure temp (°C) (MPa)

methyl lactate, 90−150 0.4−1.2 h−1; butyl lactate, 0.4−1.2 h−1 2 g of ethyl 160 lactate

WHSV = 0.369 h−1

lactate quantity

18 g of ethanol

18 g of ethanol

water, methanol, solvent-free

1,4-dioxane

solvent

1,2-PDO, 1-propanol, methanol, ethyl propionate

1,2-PDO, 1-propanol, methanol, ethyl propionate

1,2-PDO, acetol

products

50.9 9.0 71.6 0.6 90.4

73.6 30.3 86.5 69.3 95.8

ca. 76, Tred ≈ 750 °C

ca. 95, Tred ≈ 350 °C 94

95

93

ca. 95, Tred ≈ 350 °C ca. 72, Tred ≈ 750 °C

86

96.3 99 98 98 98 98 98 ∼94 (0.4 h−1) ∼80 (0.4 h−1) ∼62 (0.4 h−1)

2.3 15 34 19 100 100 74 ∼100 (0.4 h−1) ∼91 (0.4 h−1) ∼62 (0.4 h−1)

max convers at max selectiv for optimal react 1,2-PDO (%)/TOF condit (%) (h‑1)/yield (%)

ref

The main phase was not specified by the authors, and assignment of the reaction phase is somewhat arbitrary as besides the main phase the other phase can also exist. bThe main reaction phase is mentioned on the basis of the assignment of the original papers. cWeight hourly space velocity. dNot measured.

a

14

13

12

11

entry

Table 5. continued

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A novel method of lactic acid hydrogenation by syngas over Au/ZrO2 was discovered by Lei et al.88 All catalysts reported in the literature for lactic acid hydrogenation and lactate hydrogenolysis are summarized and presented respectively in Tables 4 and Table 5. Ru-based catalysts have attracted a lot of attention in hydrogenating lactic acid and ethyl lactate due to their excellent intrinsic activities (Table 4, entries 1−10, and Table 5, entries 1−9). Zhang et al.77 reported higher activity of Ru catalysts supported on carbon, alumina, and titania (Table 4, entries 6−8) compared to Raney Ni, copper chromite, Pd/C, and Ni/ Al2O3 in lactic acid hydrogenation (Figure 15).77 Primo et al.64

Table 6. Conversion of Lactic Acid Esters and Selectivity for 1,2-PDO on Various Catalysts ((A) Conversion and (B) Selectivity)a CuCr2O4b lactate substrate methyl butyl ethyl

A 91

B

Ru− Sn/γ-Al2O3c A

B

CuO−ZnOd A

B

89 91.5

90.7

37.4

Cu/SiO2e A

B

98.8 96.4

78.3 80.8

97.7

Adapted from Simonov et al. Autoclave, τreactor = 2 h, 225 °C, 15−20 MPa. cAutoclave, τreactor = 10 h, 150 °C. dFlow mode; the flow rate is 1.06 h−1, −39 °C, 1.6 MPa. eThe flow rate is 0.35 h−1, −73 °C, 0.1 MPa. a

93 b

Figure 15. Results of catalyst screening for conversion of lactic acid to propylene glycol. Feed: 100 mL of 0.55 M (5 wt %) lactic acid in water. Reaction conditions: T = 150 °C, P = 14.5 MPa of H2, catalyst loading 1.0 g. Reprinted with permission from ref 77. Copyright 2001 Elsevier.

showed that, by using a bifunctional ruthenium-supported catalyst, Ru/TiO2 (Table 4, entry 2), where the support activates the carbonyl group and small ruthenium particles on the support (average crystal size 2.0 nm) efficiently dissociate H2, it is possible to increase the activity of the conventional 5 wt % Ru/C (Aldrich) catalyst 3-fold while preserving the selectivity above 95%.64 Utilization of Ru-based catalysts usually requires introduction of some additives (boron, tin or iron, etc.) (Table 5, entries 1−3 and 7−9). Several research groups have studied the promoting effect of tin on the ruthenium catalysts.65,66,81,89 It was found that addition of tin to Ru-based catalysts significantly enhanced the selectivity for the corresponding alcohol (Table 5, entries 3 and 7−9). Luo et al.89 reported that Ru−B/γ-Al2O3, prepared by impregnation of γ-Al 2 O 3 with potassium borohydride reductant solution followed by introduction of RuCl3 (along with SnCl2 in some catalysts), exhibits 79% conversion and 51% selectivity for propylene glycol (Table 5, entry 7). Byproducts such as lactic acid, 2-hydroxypropyl lactate, 1-propanol, and isopropyl alcohol were also formed in different amounts. Addition of tin (Sn/Ru atomic ratio of 7%) substantially increases the activity and selectivity for glycol, exhibiting a selectivity of 91.5% at ethyl lactate conversion of 90.7% (Table 5, entry 7). The same authors65 studied the effect of the addition of other promoters (Co, Fe, Zn) to Ru−B/ γ-Al2O3 on the activity and selectivity for 1,2-propanediol (Table 5, entry 8). It was found that the addition of Sn and Fe improves the activity and selectivity of the Ru−B/γ-Al2O3 catalyst, whereas for Co- and Zn-promoted Ru−B/γ-Al2O3 catalysts a decrease in conversion along with an increase in selectivity for glycol was observed. Ru−B/TiO2 catalyst showed an excellent catalytic performance for the hydrogenation of ethyl lactate to 1,2-PDO under mild conditions (90 °C and

Figure 14. Conversion of methyl and butyl lactate and propylene glycol selectivity as a function of temperature. Reaction conditions: WHSV(methyl lactate) = 0.8 h−1, WHSV(butyl lactate) = 1.2 h−1, hydrogen flow rate 10 L h−1, alkyl lactate/H2 = 1/107 (molar ratio). Reprinted with permission from ref 93. Copyright 2012 Elsevier.

alkyl propionate hydrogenation over Ru/CSP (glucose-based carbon spheres). It was found that increasing the bulkiness of the alkyl group decreased the conversion of alkyl propionate due to difficulty of the substrate adsorption on the catalyst. Simultaneously, it slowed the hydrogenolysis of the alkyl group and increased the selectivity for the desired propanol, providing a selective attack of hydride species on the carbonyl group.178 3.1.1.2. Effect of Impurities. The effect of the residual biogenic impurities present in lactic acid obtained by glucose fermentation on the kinetics of hydrogenation to 1,2-PDO over Ru/C catalysts was examined.97 While refined lactic acid exhibited a stable conversion, the catalytic activity showed a steep decline with partially refined lactic acid feedstock. Various model impurities were added to pure lactic acid to study the reasons for catalyst deactivation. This illustrates the need for improved fermentation and purification technologies to produce platform molecules with purity suitable for further catalytic processing to chemicals.55 3.1.2. Catalyst. 3.1.2.1. Overview of Hydrogenation Catalysts. Lactic acid or lactates can be converted to propylene glycol in the liquid phase over Ru catalysts with or without any promoters64−67,77,78,83−86,89,90,96,98,178 as well as over Co/SiO2 or over Cu catalysts in the vapor phase.68,80,86,91−93 Pd, Ni, and Fe on SiO2 as catalysts were studied by Huang et al.,85 and Co/SiO2 was investigated by Huang et al.85 and Xue et al.94,95 T

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selectivity for 1,2-PDO was observed with increasing current in the range of 10−100 mA. Besides Ru-based catalysts, Cu-based catalysts were also extensively studied. Cortright et al.80 reported the vapor-phase hydrogenation of lactic acid over silica-supported copper. The reactions were performed at total pressures between 0.10 and 0.72 MPa and temperatures between 140 and 220 °C. Under 0.72 MPa of presure lactic acid gives predominantly propylene glycol (88% selectivity at 100% conversion) without any significant catalyst deactivation (Table 4, entry 11). The catalytic properties of copper-containing catalysts (copper chromite, copper−zinc hydroxysilicate, and copper hydroxysilicate) were studied by Simonov et al.91−93 (Table 4, entry 12; Table 5, entry 12), Simakova et al.,69 and Demeshkina et al.70 and summarized by Murzin and Simakova61 The highest activity was exhibited by copper on silica catalysts (Table 7) that had a precursor with a chrysocola-type structure (Figure S2, Supporting Information).

4.0 MPa H2) without any additives. The activity of the catalyst was greatly improved in aqueous solution. The conversion of ethyl lactate was up to 98%, and the selectivity for 1,2-PDO was above 95% (10 h reaction time) (Table 5, entry 1).82 Feng et al.83 made an attempt to hydrogenate ethyl lactate over Ru catalysts supported on TiO2, SiO2, γ-Al2O3, NaY, and active carbon without addition of any promoters to simplify the catalyst preparation and thereby the cost of the process and found Ru/SiO2 to be an efficient catalyst (Table 5, entry 4).83,84 Vapor-phase hydrogenation reactions of ethyl lactate by various transition metals (Co, Cu, Ru, Pd, Ni, and Fe) deposited on SiO2 as catalysts were studied by Huang et al.85 Among them, Co/SiO2 and Cu/SiO2 showed very high 1,2-PDO selectivities of 98.5% and 97.4% at complete conversion of ethyl lactate (Table 5, entry 10).85 Catalytic hydrogenation of lactic acid to propylene glycol was performed over various metals, 5 wt % Ag, Co, Cu, Ni, Pt, and Ru, supported on silica prepared by an incipient wetness impregnation method. Three metals (Ag, Co, Cu) did not demonstrate any catalytic activity for hydrogenation of lactic acid, while Ni, Pt, and Ru supported on silica displayed rather high activities. Among the tested catalysts, 5 wt % Ru/SiO2 exhibited the highest activity for hydrogenation of lactic acid (35% after 7 h) (Table 4, entry 9) (Figure 16).67 Note that all these catalytic systems worked only under high-pressure (above 4−5 MPa) conditions.

Table 7. Catalytic Properties of the Copper-Containing Catalystsa selectivity (%) catalyst

conversion (%)

1,2-PDO

propionic acid

Cu−Cr Cu−Zn−Si Cu−Si-14 Cu−Si-45

3 7 45 97

24 14 50 65

76 80 47 33

Reaction conditions: mass of catalyst 0.5 g, temperature 200 °C, H2 pressure 0.1 MPa, WHSV = 0.08 h−1.91 a

The high activity of copper on silica catalysts was presumably due to the formation of stable, highly dispersed particles of metallic copper during preliminary reduction. In the presence of this catalyst, high selectivity (∼75%) was attained in the formation of propylene glycol at a close to 100% conversion of lactic acid under optimum conditions (200 °C and 0.1 MPa) (Table 4, entry 12), substantially exceeding 7.3% conversion of lactic acid (Table 4, entry 11) attained at a hydrogen pressure of 1 bar reported by Cortright et al.80 A low conversion of lactic acid, when copper chromite and copperzinc hydroxosilicate were used as catalysts, was explained91 by a very slow copper cation reduction with molecular hydrogen, which is necessary to restore active metallic copper particles responsible for catalysis. High activity of the Cu−Si catalysts having a chrysocolla mineral type precursor can be associated with irreversible reduction of copper cations in the silicate composition with the removal of oxygen anions in the form of water (Figure S7, Supporting Information). Silicasupported copper catalysts with different copper loadings were tested in vapor-phase hydrogenolysis of methyl and butyl lactates at atmospheric hydrogen pressure.93 The catalyst with a copper loading of 45.5 wt.% afforded 98% methyl lactate conversion and selectivity for propylene glycol of 78% at 200 °C (Table 5, entry 12). The main byproduct was hydroxyacetone in the solvent-free conditions, while in the presence of water and methanol side reactions of hydrolysis and dehydration occurred, leading to a selectivity decline.93 The formation of 1,2-PDO in ethyl lactate hydrogenation can be enhanced significantly under mild reaction conditions by using a highly loaded novel Cu−silica nanocomposite catalyst with a Cu content up to 86% (Table 5, entry 11).86 Higher activity compared to that reported by Cortright et al.80 was

Figure 16. Lactic acid conversion over various silica-supported metal catalysts. Reactant: 50 mL of 1 M lactic acid solution. Reaction conditions: time 7 h, T = 150 °C, P = 8 MPa, catalyst 0.5 g. Reprinted with permission from ref 67. Copyright 2011 Springer.

Mao et al.90 carried out hydrogenation of several acids over a magnesia-supported poly[(γ-aminopropyl)siloxane]− ruthenium complex (MgO−NH2−Ru) with a Ru content of 0.97 wt % and found this catalyst to be capable of catalyzing hydrogenation of lactic acid (as well as acetic, propionic, and isobutyric acids) with 100% yield of 1,2-PDO (Table 4, entry 10). The catalyst was very stable and could be used repeatedly without any noticeable changes in its catalytic activity. Dalavoy et al.81 described a novel process of electrocatalytic hydrogenolysis (ECH) in an aqueous electrolyte at ambient pressure and 70 °C, leading, however, to lactaldehyde (2-hydroxypropanaldehyde) as the major product. A reticulated vitreous carbon (RVC) electrode serves to agglomerate, support, and supply current to a 5% Ru/C powder catalyst, e.g. the same catalyst as used in the classical hydrogenations. The ECH conditions are milder (ambient pressure, 70 °C vs 10.3 MPa of H2, 180 °C) in comparison with those of chemical hydrogenation. More surprisingly, the major electrohydrogenation product is lactaldehyde (LAL), with small quantities of 1,2-PDO also formed. An increase in the product yields and a shift in U

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Figure 17. Comparison of the catalytic activity of the copper−silica nanocomposite with that of the conventional Cu-loaded silica catalyst. Reprinted with permission from ref 86. Copyright 2013 Elsevier.

Figure 18. Catalytic activity of the catalysts as a function of the reduction temperature in liquid-phase hydrogenolysis of ethyl lactate: (A) PG Co/ SiO2, (B) DP Co/SiO2, (■) conversion, (○) 1,2-PDO selectivity, (●) ethyl propionate selectivity. Reaction conditions: hydrogen pressure 6.0 MPa, mass of ethyl lactate 2 g, mass of ethanol as solvent 18 g, mass of prereduced catalyst 1 g, 10 h. Adapted from Xue et al.94

ascribed to well-dispersed Cu nanoparticles in the nanosized silica matrix, leading to the formation of a nanocomposite. Typically, hydrogenation was carried out at 180 °C and 2.5 MPa with an ethyl lactate flow rate of 0.185 g/h (WHSV = 0.369 h−1 (Table 5, entry 7) (Figure 17).86 Vapor-phase hydrogenolysis of ethyl lactate to 1,2-propanediol was performed over a series of Cu−ZnO catalysts with different Cu/Zn ratios prepared by the coprecipitation method in a fixed-bed reactor. The results showed that the catalyst with a Cu/Zn ratio of 2/1 has high activity. Ethyl lactate conversion of 99.2% and 92.5% selectivity for 1,2-PDO were achieved at 220 °C and 4.0 MPa. The relationship between the catalyst structure and its performance was preliminarily studied.68 Liquid-phase ethyl lactate hydrogenolysis at 160 °C and 8.0 MPa of initial H2 pressure was carried out to investigate the catalytic properties of Co/SiO2 depending on the reduction temperature and the preparation method (precipitation gel (PG) and deposition−precipitation (DP)) (Figure 18).94 Even at a reduction temperature of 250 °C, which is significantly lower than that corresponding to temperature-programmed reduction (TPR), the PG catalyst showed conversion of 7% with 82% selectivity for 1,2-PDO, while almost no reaction took place over the DP catalyst at this reduction temperature

(Table 5, entry 13). With a reduction temperature increase, conversion over a PG type of catalyst increases substantially to 99% at 450 °C, thereafter decreasing slightly to 95% at a reduction temperature of 750 °C. On the contrary, the conversion of ethyl lactate for the DP catalyst increased monotonously to 72% with an increase of the reduction temperature from 77 to 750 °C.94 Xue et al.95 also investigated the effect of metal additives (Zn, Fe, Cu, and Sn) on the properties and catalytic performance of Co/SiO2 catalyst (prepared by a coprecipitation method) in the liquid-phase hydrogenation of ethyl lactate at 160 °C and 8.0 MPa of initial H2 pressure (Table 5, entry 14). Both ethyl lactate conversion and 1,2-PDO selectivity decreased upon incorporation of the additives (Table 8). Ethyl lactate conversion was related more to the distribution of the cobalt species and the reducibility of the catalyst than to the size and surface area of the catalyst particles.95 Recently, the gold supported on monoclinic zirconia (m) (Au/m-ZrO2) catalyst was applied by Lei et al.88 for the conversion of lactic acid using instead of pure H2 a H2-rich syngas with a H2/CO = 2 ratio, typical for methanol or Fischer−Tropsch synthesis. The catalyst with a Au loading of 1 mol % afforded 100% lactic acid conversion and selectivity for 1,2-PDO of 87% at 240 °C (Table 4, entry 13).88 V

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Table 8. Effect of Additives on the Performance of Co/SiO2 and CoM/SiO2 in the Hydrogenation of Ethyl Lactate and the Properties of the Catalystsa selectivity (%) catalyst

SBET (m2/g)

particle size (XRD) (nm)

conversion (%)

1,2-PDO

ethyl propionate

1-propanol

methanol

CoZn CoFe CoCu CoSn Co

156 169 170 171 125

9.0 9.5 7.0 6.0 12.1

50.9 9.0 71.6 0.6 90.4

63.6 30.3 86.5 69.3 95.8

13.7 65.6 3.0 11.0 0.4

12.3 2.4 9.9 5.9 3.1

0.3 1.7 0.7 13.8 0.7

Reaction conditions: mass of catalyst 1 g, mass of ethyl lactate 2 g, mass of ethanol 18 g, temperature 160 °C, H2 pressure 8.0 MPa, reaction time 10 h.95 a

Figure 19. Enantioselective hydrogenation of alkyl lactate with Ru-MACHO to form (R)-1,2-PDO. Reprinted from ref 87. Copyright 2012 American Chemical Society.

Besides heterogeneous catalytic hydrogenations of lactic acid and its derivatives homogeneous catalysts have also been reported with the aim to obtain enantio-enriched products. Processes for the hydrogenation of esters have been developed for (R)-1,2-PDO in the presence of the complex readily synthesized from [RuHCl(CO)(PPh3)3] and HN(CH2CH2PPh2)2 (Table 5, entry 6)87 The complex has been coined RuMACHO because of its structure, which resembles a brawny athlete holding ruthenium (Figure 19). Methyl lactate in methanol solution was reduced at 30 °C and gave turnover numbers (TONs) up to 4000 (Figure 19). The optical purity of the (R)-1,2-PDO (>99% ee) made by the hydrogenation of methyl (R)-lactate was higher than that via the asymmetric hydrogenation of hydroxyacetone over Ru-SEGPHOS (98.5% ee)71 Earlier for Ru catalysts it was demonstrated that hydrogenation of optically active L-(+)-lactic acid took place at temperatures below 80 °C without racemization.72 Similar results were obtained by Zhang et al.,73 where the process was performed at higher temperatures (130−300 °C) but with the hydrogen pressure reduced to 3.3 MPa.61 3.1.2.2. Effect of the Active Component. Several groups studied lactic acid hydrogenation67 and ethyl lactate hydrogenolysis85 with different metals on the same support, thus eliminating the effect of the latter on interpretation of catalytic data. Huang et al.85 studied a series of SiO2-supported Fe, Co, Ni, Ru, and Pd metal catalysts for vapor-phase hydrogenolysis of ethyl lactate to 1,2-propanediol in a fixed-bed reactor (Table 5, entry 10). The screening was performed at 180 °C, 5.0 MPa, and a H2/ethyl lactate molar ratio of 110, at which complete conversion of ethyl lactate is thermodynamically possible. The conversion of ethyl lactate over SiO2-supported metal catalysts follows the order 10% Cu/SiO2 = 10% Co/SiO2 ≫10% Ru/ SiO2 > 1.8% Pd/SiO2 > 10% Ni/SiO2 > 10% Fe/SiO2 (Table 9). Jang et al.67 compared different metals such as Ag (average size 9.2 nm), Pt (4.5 nm), Cu (34.9 nm), Co (25.5 nm), Ru (18.6 nm), and Ni (13.3 nm) supported on SiO2 in lactic acid hydrogenation (Table 4, entry 8). It was demonstrated that Ru and Ni, with particles larger than Ag particles, exhibited higher activity (Figure 16). The authors argued that inactive Cu/SiO2

Table 9. Catalytic Activity and Properties of SiO2-Supported Metal Catalystsa catalyst

conversion (%)

10% Cu/SiO2 10% Co/SiO2 10% Ru/SiO2 1.8% Pd/SiO2 10% Ni/SiO2 10% Fe/SiO2

100.0 100.0 10.1 8.2 3.3 1.1

selectivity for 1,2-PDO SBET (%) (m2/g) 98.5 97.4 95.7 92.6 96.0 40.2

356 332 372 411 354 357

average oxide particle size (XRD) (nm) 22.7 17.9 11.9 8.6 21.4 18.3

(CuO) (Co3O4) (RuO2) (PdO) (NiO) (Fe2O3)

Reaction conditions: 180 °C, 5.0 MPa, WHSV = 0.125 h−1, H2/ethyl lactate = 110 (molar ratio). Adapted from Huang et al.85 a

had the highest surface area (194 m2/g), and the other catalysts exhibited lower surface areas in the range of 110−140 m2/g. The parent silica support had a surface area of 200 m2/g; thus, pore blocking by metal particles happened during the impregnation step. No correlation was established between the surface area and the catalytic activity.67 3.1.2.3. Effect of the Metal Particle Size. In the case of hydrogenation catalyzed by ruthenium, only a few reports discuss the fact that small ruthenium nanoparticles are potentially more active than the catalysts with larger particles and that the agglomeration of the metal gives an activity decline. In the work of Primo et al.64 the crystallite size of the ruthenium on TiO2 was changed by sintering Ru in 0.64% Ru/TiO2 at high temperature (400 °C), as well as by increasing the ruthenium content (Table 4, entry 2). When the ratio between the initial rate and the surface ruthenium atoms is plotted versus the surface metal atoms, for the Ru/TiO2 catalysts with different metal crystallite sizes, a constant value is obtained as can be seen in Figure S8 (Supporting Information). Huang et al.85 observed that rather than cobalt dispersion per se the cobalt loading and preparation methods significantly affect the activity of Co/SiO2. The average reaction rate correlated with the amount of the bulklike Co3O4-phase precursor, indicating that the metallic cobalt from the bulklike Co3O4 precursor is more active for lactate hydrogenolysis (Figure 20). W

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The reaction includes a number of steps in the catalytic cycle, such as catalyst adsorption, regeneration of the catalyst active component, etc. Therefore, analysis of the cluster size effect is not straightforward, since it is difficult to assign the observed dependence of the reaction rate on metal dispersion to a particular step in the cycle, as it might change with the cluster size alterations. 3.1.2.4. Effect of the Support. Two variables are known to determine differences in activities observed between the same metal supported on different supports. One is the crystallite size of the metal, while the other is the nature of the support. The catalytic behaviors of Ru/C and Ru/TiO2 catalysts with similar Ru particle sizes (∼2.3 nm) were compared in lactic acid hydrogenation (as well as for levulinic, succinic, and itaconic acids) (Table 4, entry 2).64 For 5% Ru/C (Aldrich), which was reported to be a very active and selective catalyst,74,77 or selfmade 0.6−2% Ru/C catalysts, high conversions and selectivities (Figure 21) were obtained with a TOF of 14 h−1 (calculated

Figure 20. Correlation between the specific activity and the percentage of the bulklike Co3O4 phase in Co/SiO2 with different metal loadings and preparation methods in vapor-phase hydrogenolysis of ethyl lactate at a hydrogen pressure of 2.5 MPa and temperature of 160 °C. Reprinted with permission from ref 85. Copyright 2008 Elsevier.

The increase of catalytic activity with additive insertion was also associated with decreasing active component size, affecting the reaction rate. Ruthenium particles highly dispersed on TiO2 in the size range of 5−10 nm in Ru−B/TiO2 exhibited excellent activity.89 Transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicated that the metal particles are an amorphous alloy formed between Ru and B, which is consistent with the results reported by Luo et al. (Table 5, entry 7).89 The XRD pattern of Ru−B/TiO2 exhibited no ruthenium particles, and this also indicated that ruthenium particles are amorphous and highly dispersed on TiO2, resulting in high hydrogenation activity.89 An opposite additive effect on the active metal size was found by Xue et al. (Table 5, entry 14).95 for Co/SiO2 promoted by Zn, Fe, Cu, and Sn (Table 8). Incorporation of additives decreased the size of the cobalt particles, which resulted in accelerated formation of cobalt species that interacted strongly with the support, that is, cobalt phyllosilicate. The results of ethyl lactate to 1,2-PDO catalytic hydrogenation showed that incorporation of the metal additives profoundly inhibited the catalyst activity, especially when Fe or Sn was incorporated. In addition, the incorporation of the additives did not improve the selectivity for 1,2-PDO. The poorer activity of the bimetallic catalysts likely resulted from an insufficient prereduction owing to the formation of a difficult-to-reduce cobalt phyllosilicate species. It can be concluded that although metal and nonmetal additives are widely used in general to improve the activity, selectivity, and stability of catalysts, the formation of difficult-to-reduce species in this particular case needs to be taken into account.95 In a study by Simonov et al.91 of lactic acid hydrogenation, the catalytic activity of prereduced Cu-containing oxide compounds of different compositions and structures, copper chromite CuCr2O4 (28 wt % Cu), copperzinc hydroxosilicate (Cu0.3Zn0.7)3[Si4O10](OH)2·nH2O (13 wt % Cu), and copper hydroxosilicate (CuH)4[Si4O10](OH)8·nH2O (14 wt % Cu) (Table 7), correlated strongly with the specific surface area of metallic copper rather than with dispersion and especially with the Cu loading due to the crucial difference in the character of Cu interactions with hydrogen.91

Figure 21. 1,2-Propanediol yield versus reaction time: (■) 0.64% Ru/ TiO2, (▲) 1% Ru/TiO2, (●) 5% Ru/C, (◆) 0.64% Ru/TiO2 calcined. Reaction conditions: 0.4 mol % Ru, 150 °C, 3.2 MPa of H2 pressure. Reprinted with permission from ref 64. Copyright 2011 Royal Society of Chemistry.

from the initial reaction rates as the number of 1,2-PDO molecules formed per hour and per Ru atom). However, with 0.64% Ru/TiO2 containing small ruthenium nanoparticles (2.0 nm) supported on TiO2 (anatase, Aldrich), the TOF was larger (51 h−1); moreover, lactic acid is converted to 1,2-PDO under milder reaction conditions.64 In fact, the average particle size for Ru/C at different loadings from 0.5 to 5 wt % is almost constant (average 2.3 nm), while due to the much smaller specific surface area of TiO2 the average particle size of 5 wt % Ru/TiO2 is 5 nm. When the TOF values for the titania-based ruthenium catalysts (30 m2/g) were compared with those of other supports such as active carbon (700 m2/g) or CeO2 (253 m2/g), it was observed that the TOF values for the former one are clearly higher, at least for samples with a Ru loading of ≤2 wt % (Figure 22). This indicates that, besides a possible influence on the Ru crystallite size, the support also plays a direct role. The authors showed by H/D exchange experiments that 1% Ru/TiO2, 2% Ru/CeO2, and 5% Ru/C catalysts have similar hydrogen dissociation activities (ca. 50% at 25 °C), suggesting that hydrogen activation does not limit the hydrogenation activities on Ru catalysts due to the high efficiency of the metal to activate hydrogen. However, different abilities of TiO2 and CeO2 to activate a carboxylic group were found for adsorption of propanoic acid (taken as a model instead of lactic acid) from the gas phase. Carbon was unsuitable for studying propionic acid adsorption due to a lack of transparency and experimental X

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increasing activity (Figure 23), indicating that the hydrogenation of ethyl lactate is more active on small metal particles over the oxide supports. Nevertheless, Ru/C is not very active with a small Ru particle size, which is probably attributable to different surface features of the active carbon. SiO2 was considered to be a superior support for the hydrogenation of ethyl lactate to 1,2-PDO,62 exhibiting satisfying activity and the highest selectivity for 1,2-PDO with an 82.1% yield of 1,2-PDO.83 Figure 23 displays, however, even higher conversion for the same reaction time in the case of Ru/TiO2. Because of different structures and surface characteristics of the tested supports, TiO2, SiO2, γ-Al2O3, NaY, and active carbon, the reaction activities and product selectivities are different from each other. The authors suggested that the support material is likely to influence the reaction paths in the presence of metal catalysts and consequently influences the catalytic performance; however, elucidation of the exact role of the support needs further studies.83 A comparative study of the support influence was also performed for cobalt catalysts. Table 10 summarizes the reaction

Figure 22. Influence of the support and Ru loading on the TOF value in lactic acid hydrogenation: (●) active carbon, (■) TiO2, (▲) CeO2. Reaction conditions: 150 °C, 4 MPa of hydrogen pressure. Reprinted with permission from ref 64. Copyright 2011 Royal Society of Chemistry.

limitations of IR spectroscopy. The latter showed that the carboxylate group is less prone to hydrogenation on CeO2 compared with the carbonyl group activation on Ru/TiO2. Therefore, Primo et al.64 related the high activity of Ru/TiO2 to the synergy between the ability of small ruthenium crystallites to activate the H2 and the role of the TiO2 to adsorb and activate the acid, probably on the periphery of the metal crystallites. In situ Fourier transform infrared spectroscopy has been employed to investigate the adsorption and thermal decomposition of lactic acid unfortunately only on TiO2.179,180 It was shown that lactic acid dissociates on TiO2 by dehydrogenation to form CH3CH(OH)COO (lactate) and CH3CH(O)COOH (2-oxypropionic acid). At a temperature above 150 °C, these species are transformed to propionate, involving H transfer, with minor acetate formation. In the presence of O2, however, acetate and CO2 are formed, due to decarboxylation and H abstraction from the CH group. It is possible that different supports affect the lactic acid dissociative adsorption in different ways.179 In the work of Feng et al.83 the catalytic behavior of Ru/TiO2, Ru/SiO2, Ru/γ-Al2O3, Ru/NaY, and Ru/C in ethyl lactate hydrogenolysis was studied (Table 5, entry 4). It was shown that the catalytic performance is dependent on the nature of the support as seen in Figure 23. The average Ru particle sizes (in the range of 8−15 nm) decreased in the order Ru/NaY > Ru/ γ-Al2O3 > Ru/SiO2 > Ru/TiO2, which is consistent with their

Table 10. Effect of the Support in the Hydrogenolysis of Ethyl Lactate over 10 wt % Co-Based Catalystsa selectivity (%) support

conversion (%)

1,2-PDO

1-propanol

2-propanol

othersb

SiO2 AC Al2O3 Al2O3c

65.3 29.3 2.3 8.1

98.3 98.4 96.3 95.8

0.7 0.4 1.7 2.0

0.4 0.3 1.3 1.3

0.6 0.9 0.7 0.9

Reaction conditions: 160 °C, 2.5 MPa, WHSV = 0.258 h−1, H2/ethyl lactate = 50/1 (molar ratio).85 b“Others” means products mainly containing acetol and ethyl propionate. cReduction under H2 flow at 550 °C for 8 h. a

performance over 10 wt % cobalt supported on SiO2, Al2O3, and active carbon (AC) in ethyl lactate hydrogenolysis.85 With respect to the support, the following activity sequence holds: SiO2 > AC ≫Al2O3. In addition, a complementary experiment was performed at a reduction temperature of 550 °C (Table 10), increasing the conversion by 5.8%. However, the conversion is still much lower than that on 10% Co/SiO2 and 10% Co/AC. A low activity of 10% Co/Al2O3 was mainly ascribed to the low amount of bulklike Co3O4 in comparison with Co/SiO2, exhibiting a superior performance.88 Thus, further studies were focused on cobalt supported on SiO2. A closer inspection shows that the percentage of bulklike Co3O4 on 10% Co/Al2O3 is much less than that on 10% Co/SiO2. On the basis of the hypothesis of a structure-sensitive reaction, the low activity of 10% Co/ Al2O3 should be partially ascribed to its low percentage of bulklike Co3O4. 3.1.2.5. Effect of the Catalyst Loading. To find the minimum catalyst amount required to achieve good performance in the hydrogenation of ethyl lactate, the effect of the catalyst amount was investigated. As the amount of the catalyst increases, more reactive sites are available for the reaction; hence, the conversion of ethyl lactate increases. Nevertheless, too high a Ru/SiO2 amount promotes the excessive hydrogenolysis reaction of 1,2-PDO, leading to the formation of byproducts such as 1-propanol and 2-propanol. This can be responsible for the 1,2-PDO selectivity drop at a higher dosage of Ru/SiO2. Thus, when 100 mg of Ru/SiO2 was used, the yield of 1,2-PDO was high, reaching 82.1% (Table 5, entry 4).

Figure 23. Catalytic performances of several ruthenium catalysts for the hydrogenation of ethyl lactate: conversion of ethyl lactate (white), selectivity for 1,2-PDO (gray). Reaction conditions: 150 °C, 5 MPa, 100 mg. Reprinted with permission from ref 83. Copyright 2011 Elsevier. Y

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Table 11. Physical Properties of Cu/SiO2 Catalysts Prepared by the Precipitation Method and Their Catalytic Activitya catalyst 86% 76% 34% 11%

Cu/SiO2 Cu/SiO2 Cu/SiO2 Cu/SiO2

conversion (%)

selectivity for 1,2-PDO (%)

100 100 74 9

98 99 98 98

TOF

Cu particle size (TEM) (nm)

5.4 1.3

20 15 6 3

Cu surface areab (m2/g 11.7 13.4 8.8 4.7

cat)

SBET (m2/g) 75 105 195 275

Reaction conditions: 180 °C, 2.5 MPa, ethyl lactate WHSV = 0.369 h−1 (5% EL in dioxane), H2/EL = 125 (mol), time-on-stream 30 h. Adapted from Kasinathan et al.86 bCalculated from dissociative N2O adsorption (N2O + 2Cu → Cu2O + N2); it is assumed that a reduced copper surface has a surface density of 1.46 × 1019 Cu atoms/m2.

a

catalyst surface along with chrysocolla phase above a 45.5% Cu loading. Along with increasing copper loading, simultaneously the support surface area also increases, which might lead to additional activity in some alkyl lactate transformation steps involving catalyst acidic sites. Therefore, the highest catalytic activity of 45.5 wt % Cu/SiO2 catalyst (Figure 25) seems to be

This was comparable to that of the boron-added ruthenium catalyst.83 The effect of the Cu loading on a silica support was investigated to enhance the productivity of ethyl lactate hydrogenation by increasing the number of Cu active sites since the conversion of ethyl lactate over 11% Cu/SiO2 was only 12% (Table 11). At the same time the mean Cu particle size gradually increased from 3 to 20 nm as the Cu loading increased from 11% to 86% (Table 5, entry 11).86 It is interesting that the catalytic activity was not proportional to the Cu surface area in the range of Cu loading from 11% to 34% (Table 11). In other words, 34% Cu/SiO2 gave a 4.2 times higher TOF than 11% Cu/SiO2, which indicates that the Cu sites are not homogeneous.86 Huang et al.85 summarized the characterization results with different metal loadings and preparation methods of Co/SiO2, showing that increasing the cobalt loading from 3% to 15% resulted in an increase of the mean particle size of both the Co0 phase from 10.6 to 14.0 nm and the Co3O4 phase from 14.1 to 18.7 nm as determined by XRD. It was likely that the amount of the metallic cobalt from bulklike Co3O4-phase precursors influences the activity in ester hydrogenolysis. As shown in Figure 20, a quasi-linear correlation was observed between the average reaction rate and the percentage of the bulklike Co3O4phase precursors, suggesting that the metallic cobalt from the bulklike Co3O4-phase precursor is more active than the cobalt surface species on the support.85 To increase the activity of Cu−Si catalysts, the copper loading in the precursor was varied from 14 to 53 wt % in the study of Simonov et al.92 Figure 24 shows that conversion of

Figure 25. Effect of the copper content (wt %) (on curves) on the activity of Cu/SiO2 catalyst upon hydrogenolysis. Reaction conditions: hydrogen flow rate 10 L h−1, injection rate of methyl lactate 0.4 mL h−1, methyl lactate/H2 = 1/107 (molar ratio). Reprinted with permission from ref 93. Copyright 2012 Elsevier.

associated with both high Cu metal dispersion and a high specific surface area of the catalyst.61,69,91−93 3.1.2.6. Effect of the Catalyst Prereduction. All tested catalysts were reduced before the reaction with different temperature profiles depending on the active component, support nature, and catalyst preparation method. Base metal catalysts required reduction in situ or preliminary passivation before exposure to air for ex situ utilization. Depending on the active metal, different reduction conditions were found for Co/SiO2, Cu/SiO2, Ru/SiO2, Pd/SiO2, Ni/SiO2, and Fe/SiO2.85 Xue et al.94 showed that the catalytic properties of Co/SiO2 prepared by the PG and DP methods in liquid-phase ethyl lactate hydrogenolysis were strongly dependent on the reduction temperature. With the latter increasing, the conversion over the PG type of catalyst increases substantially from 7% at 250 °C to 99% at 450 °C and then decreases slightly to 95% at s reduction temperature of 750 °C (Figure 18 A). Differently, the conversion of ethyl lactate for the DP catalyst increased monotonously from 5% to 72% with an increase of the reduction temperature from 77 to 750 °C (Figure 18 B). It should be noted that both catalysts showed high selectivity (>72%) for 1,2-PDO in the range of reduction temperature investigated, suggesting high selectivity of Co/SiO2 catalysts in the liquidphase hydrogenolysis of ethyl lactate independent of the

Figure 24. Effect of the copper loading on the conversion of lactic acid (■), selectivity for 1,2-PDO (▲), and selectivity for propionic acid (●). Reaction conditions: Cu/SiO2, T = 200 °C, P(H2) = 0.1 MPa, WHSV = 0.08 h−1. Adapted from Simonov et al.92

lactic acid monotonically increases with an increase of the copper content, while the selectivity for 1,2-PDO reaches the maximum at a copper content of about 45.5 wt % corresponding to the structure of chrysocola mineral (Figure S2, Supporting Information).92 Bulk copper oxide particles were formed on the Z

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preparation method (Figure 18). The TPR results demonstrated that Co3O4 is mainly present in the calcined PG precursor, while the main cobalt phase in the catalyst prepared by the DP method is cobalt phyllosilicate. Because of different reducibilities of the cobalt phase, the activities of PG and DP catalysts differ strongly. The reason is that metallic cobalt is found to be the active species during the hydrogenolysis of ethyl lactate.94 Xue et al.95 studied Co/SiO2 bimetallic catalysts and showed the TPR profiles were completely distinct from the profile of the monometallic sample. The electronic properties and surface distribution of Co were significantly changed by incorporation of the additives. The particle size of the cobalt species was diminished, which strengthened the interactions between the cobalt particles and the silica support and decreased the reducibility of the catalyst. The presence of more than two reduction peaks for the former profiles indicated the existence of more than one cobalt oxide species in the bimetallic systems. These peaks could be attributed to cobalt in various oxidation states and to various interactions of cobalt with the support surface, decreasing ethyl lactate conversion and 1,2-PDO selectivity.95 3.1.2.7. Catalyst Deactivation and Stability. The stability of 10% Cu/SiO2 catalyst for the vapor-phase conversion of lactic acid was studied for a 20 wt % lactic acid aqueous solution at 200 °C and a total pressure of 0.30 MPa. The WHSV for these measurements was 0.03 h−1, and the hydrogen flow rate was adjusted in a way that the molar ratio in the feed of lactic acid, water, and hydrogen was equal to 1/20/400. Figure 26 shows

Figure 27. Stability of 45.5 wt % Cu/SiO2 catalyst during butyl lactate hydrogenolysis: (■) conversion, (o) selectivity. Reaction conditions: WHSV = 0.2 h−1, hydrogen supply velocity 10 L h−1, 180 °C. Reprinted with permission from ref 93. Copyright 2012 Elsevier.

during hydrogenolysis of lactates to propylene glycol compared to hydrogenation of the initial nonesterificated lactic acid.61,93 An 85 h stability test was performed over the optimal 10% Co/SiO2 catalyst (prepared via the rotary evaporation drying method) with no loss of activity or selectivity under the reaction conditions of 160 °C, 2.5 MPa, 0.25 h−1, and H2/ethyl lactate molar ratio of 50 (Table 5, entry 6).85 Investigation of the effect of impurities in lactic acid on catalytic behavior with the time-on-stream (TOS) showed that sulfide-containing amino acids completely and irreversibly poison the catalyst, whereas the nonsulfurous amino acid alanine only partially and reversibly diminishes lactic acid conversion via competitive adsorption on the metal surface. Addition of the model protein compounds leads to a partial Ru/C catalyst deactivation via plugging of pores in the activated carbon support. Removal of the impurity from the feed allows only partial recovery of the original catalyst activity.97 Ru/CSP catalyst could be reused up to six times in ethyl lactate hydrogenation to 1,2-PDO with a slight decrease in conversion from 94% to 87% and selectivity from 95% to 86%.178 The stability of catalysts during recycling was studied by Mao with Ru-MACHO.90 After each reaction the catalyst was separated from the solution, washed with ethanol (10 mL), dried, and thereafter added to the reaction mixture to catalyze the reaction for the subsequent run. The catalyst showed no decline of activity during three repeated cycles. 3.1.3. Reaction Conditions. The product distribution can depend on the reaction conditions used. To estimate the optimal reaction conditions, the influence of temperature, hydrogen pressure, solvent, and initial reactant concentration on lactic acid and lactate conversion as well as on selectivity for 1,2-PDO was studied in almost all papers related to this topic. In general, the authors reported that the temperature increase seems to provoke side product formation and thereby decreases selectivity for 1,2-PDO, while lactic acid and lactate conversion increases with temperature. A hydrogen pressure increase gives a rise in both conversion and selectivity, providing an equilibrium shift of hydroxyacetone to 1,2-PDO in the case of lactates. The optimal temperature and hydrogen pressure are varied depending on the selected catalyst as well as the liquid- or vapor-phase mode of operation. The most representative findings from the different papers are given below. 3.1.3.1. Effect of the Solvent. Different solvents were reported to be used for dissolution of lactic acid and lactates: water,78,90,93,178 methanol,93,178 ethanol,82,84,1781,4-dioxane,82,86,178 n-hexane,82 2-propanol,82,178 and 1-butanol.178 Solvent-free conditions were also tested.85,93

Figure 26. Stability of 10 wt % Cu/SiO2: (●) lactic acid conversion (%), (Δ) selectivity for 1,2-PDO (%), ( ◇ ) selectivity for 2-hydroxypropionaldehyde (%), (+) selectivity for propionic acid (%). Reaction conditions: 200 °C, 0.3 MPa, WHSV = 0.03 h−1, lactic acid/water/H2 = 1/20/400 (molar ratio). Reprinted with permission from ref 80. Copyright 2002 Elsevier.

no evidence of deactivation over the Cu/SiO2 catalyst in 22 days of operation at the stated conditions: the conversion of lactic acid was essentially 100%, and the selectivity for 1,2-PDO was higher than 80%.80 Cu/SiO2 catalyst was stable with a long time-on-stream, as was demonstrated in lactate hydrogenolysis (Figure 27).93 Interestingly, the catalyst stability depended on the temperature at which the catalyst was reduced. Preliminary catalyst reduction at 300 °C provided higher stability and activity than reduction at 380 °C, while the selectivity was the same with the time-on-stream and independent of the reduction temperature for all catalysts. Note the high stability of Cu/SiO2 catalyst AA

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Table 12. Effect of Solvents on Ethyl Lactate Hydrogenationa selectivity (%)

a

solvent

conversion (%)

1,2-PDO

1-propanol

propionic acid

lactic acid

TOF (h−1)

EtOH i-PrOH n-hexane dioxane H2O H2Ob

28.7 56.7 23.1 20.3 95.1 98.0

73.8 90.0 83.4 89.5 88.8 96.5

4.2 2.7 3.7 3.5 3.2 0.8

7.1 1.4 2.8 2.4 2.4 1.6

14.7 5.8 10.2 4.7 5.4 1.0

4.6 10.9 4.2 3.9 18.4 6.9

Reaction conditions: 150 °C, 4.0 MPa, 0.079 mmol of catalyst, 0.5 mL of ethyl lactate, 1.5 mL of solvent, 1500 rpm, 4 h.82 bT = 90 °C, time 12 h.

Chen et al.78 studied hydrogenation of acid mixtures and combinations of acids with product alcohols at several concentration ratios in aqueous solutions at 70−150 °C, 3.4− 10.3 MPa of hydrogen pressure, and 0.05−5 M acid feed concentrations. Adding a second acid significantly decreases the conversion of a given acid (Figure S3, Supporting Information). This inhibitory effect increases with the concentration of the added acid increase, indicating that lactic and propionic acids compete for active sites on the catalyst surface. The presence of 1,2-PDO in the starting solution has a minor effect on the hydrogenation rates of either acid, indicating that 1,2-PDO adsorption on the catalyst surface is weak (Figure S4, Supporting Information).78 The effect of solvents on the hydrogenation of ethyl lactate is shown in Table 12.82 In all reaction solutions, the detectable byproducts were 1-propanol (n-PP), propionic acid (PA), and lactic acid (LA). According to the results in Table 12, the conversions in alcohols are higher than in n-hexane and dioxane. The catalytic activity of the catalyst in different solvents increases in the order dioxane < n-hexane < ethanol < 2-propanol. Interestingly, an excellent catalytic performance of RuB/TiO2 was measured with water as a solvent. A conversion of substrate equal to 95.1% and selectivity of 88.8% for 1,2-PDO were obtained. The amounts of byproducts n-PP and PA in water were close to those in other solvents; LA yields were obviously lower than in ethanol and hexane. The result indicated that the hydrogenation of ethyl lactate to 1,2-PDO was favorable in water and the presence of water did not promote substrate hydrolysis. Furthermore, the formation of LA in n-hexane and dioxane further proved that the production of LA was from the hydrogenolysis of the C−O bond in ethyl lactate.82 Fan et al.82 suggested that in aqueous solutions the hydrophilicity of TiO2 results in the formation of a water film on the surface of the catalyst. Competitive adsorption of water on the surface of the catalyst and the probable hydrogen bond between 1,2-PDO and the solvent weaken the adsorption of 1,2-PDO or shorten the time spent by 1,2-PDO on the surface of the catalyst. As a result, the activity of the catalyst and the selectivity for 1,2-PDO are improved. In addition, the formation of a hydrogen bond between water and ethyl lactate on the surface of the catalyst probably polarizes the CO bond of the epi carboxyl group in a way that the attack of activated H2 to the carboxyl group of the substrate is facilitated.82 Yang et al.178 observed a strong transesterification reaction between ethyl lactate and solvent alcohols when the hydrogenation was performed over Ru/CSP in methanol, 2-propanol, or 1-butanol. Contrary to the transesterification product, the desired 1,2-PDO was found to a minor extent. The best catalytic result was obtained in water as a solvent. In this case, 93.7% of ethyl lactate was converted with selectivity of

95.4% for 1,2-PDO (Table 5, entry 5), which was better than that reported for hydrogenation of ethyl lactate in n-heptane over Ru−Sn bimetallic catalysts (Table 5 entries 7 and 9).68,89 To reveal the role of solvent during hydrogenolysis, neat methyl lactate was dissolved in water and in methanol.93 The resulting solutions with a methyl lactate concentration of 17 wt % underwent hydrogenolysis at the optimal conditions (Table 13). In the presence of water the selectivity for Table 13. Effect of Solvent on the Formation of Products in Methyl Lactate Hydrogenation over Cu/SiO2 Catalyst at 200 °C, WHSV = 0.18 h−1, and a Hydrogen Flow Rate of 10 L h−1 93 solvent

hydroxyacetone concn (%)

water methanol none

18.8 6.4 21.2

propanoic acid concn (%)

propylene glycol concn (%)

concn of other byproducts (%)

8.4 1.2

59.7 74.7 73.9

6.3 16.6 4.8

propylene glycol decreased because of intensive side hydrolysis with the subsequent formation of lactic and propanoic acids. In the presence of methanol selectivity was slightly increased, but methyl 2-methoxypropionate and methanol conversion products were detected in the reaction mixture. Methanol dehydrogenation to methyl formate over copper-containing catalysts and methanol dehydration to dimethyl ester over solid-acid catalysts are well-known. The reactive ability of methanol leads to product contamination and irreversible loss of methanol, which can be otherwise used in lactic acid esterification. Although methanol is attractive, being inexpensive and easily available on an industrial scale, Simonov et al.93 recommended to avoid using methanol, because of its high reactivity in the presence of a copper−silica catalyst. Eventually, in the presence of solvents hydrogenolysis of methyl lactate becomes less selective and more energyconsuming due to additional heating and vaporization of the solvent compared with the neat methyl lactate case. Thus, the authors93 concluded that hydrogenolysis of neat alkyl lactate seems to be more selective and preferable than that of soluted alkyl lactate. 3.1.3.2. Effect of the Hydrogen Pressure. As illustrated in Figure 28, the hydrogen pressure obviously influences the catalytic performance of Ru/SiO2. The conversion of ethyl lactate increased monotonously with increasing hydrogen pressure. Ethyl lactate can be almost completely converted at 7 MPa. As the hydrogen pressure was elevated from 3 to 7 MPa, the selectivity for 1,2-PDO initially exhibited an increase, ultimately reaching a maximum at 5 MPa, thereafter starting to decrease. Thus, high selectivity for 1,2-PDO requires an AB

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Figure 28. Effect of the hydrogen pressure on the (■) ethyl lactate conversion, (●) selectivity for 1,2-PDO, and (▲) yield of 1,2-PDO. Reaction conditions: 160 °C, 100 mg of catalyst. Adapted from Feng et al.83 Figure 30. Solubility of hydrogen in water and in 10% aqueous lactic acid solution. In water: (---) 100 °C,137 () 150 °C,137 (■) 100 °C, (●) 150 °C. In 10 wt % (1.15 M) lactic acid solution: (□) 100 °C, (Δ) 130 °C, (o) 150 °C, (◇) 170 °C.98

optimal hydrogen pressure, found to be 5 MPa (Figure 28). This hydrogen pressure is not too high and can be reliably controlled in industry.83 Conversion of lactic acid and selectivity of propylene glycol over Ru/SiO2 increases with pressure, with the highest yield achieved at 130 °C. Such behavior is probably due to the increase of hydrogen solubility in aqueous solutions with a reaction pressure increase (Figure 29).67

to 96% at 0.5 MPa and then to 100% along with almost 100% selectivity for 1,2-PDO at pressures higher than 1.5 MPa. It is notable that the 76% Cu/SiO2 catalyst was very active with a 1,2-PDO selectivity of 92% even at atmospheric pressure when compared with the low Cu-loaded catalyst 10% Cu/SiO2. For the latter one the ethyl lactate conversion was linearly proportional to the reaction pressure in the range between atmospheric pressure and 5 MPa.75,86 Huang et al.85 observed that high conversion of ethyl lactate on 10% Co/SiO2 can be obtained at relatively low pressure; for example, complete conversion was achieved at 1.0 MPa and 180 °C. The authors showed that the ethyl lactate conversion increased gradually from 10.7% to 100% when the reaction pressure increased from 0.3 to 7.0 MPa. The selectivity for 1,2PDO was over 96% at 1.0−7.0 MPa, and only a slight decrease was observed when the pressure was decreased to 0.3 MPa.85 3.1.3.3. Effect of Temperature. Figure 31 shows the effect of the reaction temperature on the conversion of ethyl lactate,

Figure 29. Catalytic activity of 5% Ru/SiO2 with a time-on-stream depending on the hydrogen pressure: (■) yield of 1,2-PDO, (●) selectivity for 1,2-PDO, (▲) ethyl lactate conversion (130 °C, 7 h). Reprinted with permission from ref 67. Copyright 2011 Springer.

Zhang et al.121 measured the solubility of hydrogen in water and in 10 wt % (1.15 M) lactic acid solution at several temperatures (Figure 30) and compared the results with data reported by Stephen et al.175 It was demonstrated that in 10% lactic acid solution the solubility of hydrogen has the same trend with temperature and pressure as in water, but the solubility in lactic acid solution is about 10% lower than that in pure water at the same temperature.98 Cortright et al.80 showed the effects of the total pressure at 200 °C on the conversion of lactic acid and the selectivity for different products over 10 wt % Cu/SiO2. The conversion of lactic acid increased from 74% to 100% and the selectivity for 1,2-PDO increased from 54% to 88% as the total pressure was increased from 0.10 to 0.72 MPa. For the latter value complete conversion of lactic acid was observed, with selectivities for 1,2-PDO and 2-hydroxypropionaldehyde of 88% and 3%, respectively. Kasinathan et al.86 varied the H2 pressure in the reactor from atmospheric to 4.5 MPa at a constant reaction temperature of 180 °C and a WHSV of 0.369 h−1. The ethyl lactate conversion at atmospheric pressure was only 32%. It was increased rapidly

Figure 31. Effect of the reaction temperature on (■) the conversion of ethyl lactate, (●) selectivity for 1,2-PDO, and (▲) yield of 1,2-PDO. Reaction conditions: 5 MPa, 100 mg of Ru/SiO2 catalyst. Adapted from Feng et al.83

selectivity for 1,2-PDO, and yield of 1,2-PDO over Ru/SiO2.83 As expected, the conversion of ethyl lactate increases remarkably with increasing reaction temperature. As the reaction temperature was elevated from 110 to 170 °C, there was a steady increase in the ethyl lactate conversion from 37.3% to 94.4%. The selectivity for 1,2-PDO changed slightly when the reaction temperature increased from 110 to 170 °C, AC

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dramatically decreasing at 170 °C. This indicates that too high a reaction temperature favors the formation of lactic acid and byproducts, such as 1-propanol. The optimal tradeoff between the conversion and the selectivity is 160 °C; at this temperature, the highest yield of 1,2-PDO (82.1%) was obtained. Similar results were also observed in the earlier studies of these authors employing Ru/TiO2,180 but a better catalytic performance was achieved over Ru/SiO2 by Feng et al.83 Jang et al.67 also observed that a high temperature (150 °C) intensifies secondary reactions to byproducts at a hydrogen pressure of 8 MPa. Thus, there exists an optimal reaction temperature for maximizing the 1,2-PDO yield. The decrease of selectivity is mainly due to the decrease in hydrogen pressure with reaction time as the reaction was done in a batch reactor. The other main products were propionic acid and 2-hydroxypropionaldehyde. The authors did not detect C1 or C2 compounds and propanol during the reaction (Figure 32).67

Figure 33. Effect of temperature on methyl lactate conversion (■) and selectivity for 1,2-PDO (o) over 45.5% Cu/SiO2. Reaction conditions: liquid supply velocity 0.2 mL h−1, methyl lactate/H2 = 1/214 (molar ratio). Reprinted with permission from ref 93. Copyright 2012 Elsevier.

reaction temperature increases from 140 to 180 °C. The 1,2PDO selectivity is high (>98%) in the range of 140−180 °C, but it diminishes from 98.5% to 70.0% as the temperature increases from 180 to 273 °C due to the increase of the undesired 1- or 2-propanol formation as a result of excessive hydrogenolysis.85 Interestingly, the optimal reaction temperature for the hydrogenation of propionic acid to propanol (a side reaction of lactic acid hydrogenation) over Ru catalyst was found to be 240 °C, and the reaction did not occur at 160 °C (Figure S5, Supporting Information).90 The effect of WHSV on the reaction product composition depending on temperature was studied in neat methyl lactate hydrogenolysis.93 It was found that the concentration of methyl lactate decreases and the concentrations of the main products increase with decreasing WHSV (Figure 34).93 Simonov et al.93

Figure 32. Catalytic activity of 5% Ru/SiO2 with a time-on-stream: (■) yield of 1,2-PDO, (●) selectivity for 1,2-PDO, (▲) ethyl lactate conversion (8 MPa, 7 h). Reprinted with permission from ref 67. Copyright 2011 Springer.

Cortright et al.80 studied the effect of temperature on the conversion of lactic acid and the selectivity for different products over 10 wt % Cu/SiO2 at 0.1 MPa. They showed that the conversion of lactic acid decreases from 60% to 8% and the selectivity for 1,2-PDO increases from 77% to 91% as the temperature is decreased from 220 to 140 °C. The apparent activation energies were determined to be 37 and 66 kJ/mol for the production of 1,2-PDO and propionic acid, respectively.80 Simonov et al.93 have found that an increase of temperature leads to an increase of methyl and butyl lactate conversion as the overall reaction rate is increased, but selectivity for the desired propylene glycol decreases due to an increase of hydroxyacetone formation (Figure 33). The maximum propylene glycol yield of 76% obtained during methyl lactate hydrogenolysis was achieved at a methyl lactate conversion of 98% and selectivity of 78% at 200 °C (Figure 33). The reaction temperature was varied from 160 to 240 °C at a constant reaction pressure of 2.5 MPa and a WHSV of 0.369 h−1.86 The conversion of ethyl lactate was increased from 91% to 100% as the reaction temperature was increased from 160 to 180 °C. However, at temperatures higher than 180 °C, the 1,2-PDO selectivity decreased with temperature, while the conversion of ethyl lactate was almost 100%. It was stated that the decrease of 1,2-PDO selectivity was mainly due to dehydration, giving 1-propanol as a side product.86 Huang et al.85 reported the effect of temperature on the reaction performance over 10% Co/SiO2. The conversion of ethyl lactate increases from 3.3% to almost 100% when the

Figure 34. Dependence of methyl lactate conversion on WHSV at temperatures of 170, 200, and 220 °C and a hydrogen flow rate of 10 L h−1. Reprinted with permission from ref 93. Copyright 2012 Elsevier.

noted that propylene glycol and hydroxyacetone selectivities do not depend on WHSV, i.e., on methyl lactate conversion (Figure 35). 3.1.3.4. Effect of the Initial Reactant Concentration. The initial LA and PA hydrogenation rates at four different feed concentrations (approximately 0.1, 0.5, 2, and 5 M) and otherwise identical reaction conditions were reported by Chen et al.78 (Figure 36). The nonlinear dependence of the initial AD

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LA + S ⇄ LA·S (fast)

(1)

H 2 + S ⇄ H 2 ·S (fast)

(2)

H 2 ·S + LA → P1·S + S (slow)

(3)

P1·S + H 2 ·S ⇄ PG·S (fast)

(4)

PG·S ⇄ PG + SLA· S (fast)

(5)

The rate-limiting step was considered to be irreversible, and the adsorption of water was neglected. The total catalyst site density CT was considered constant, and all sites were considered equivalent. Several variations of the above L−H model were evaluated, including those with dissociative hydrogen adsorption or adsorption of only one species (lactic acid or hydrogen), but all of them gave negative rate or adsorption constants. Only the above model involving molecular adsorption of lactic acid and hydrogen, a surface reaction of lactic acid with hydrogen as the rate-limiting step, and finally desorption of 1,2-PDO gave all positive kinetic and adsorption constants with the correct dependence on temperature.98 Calculations based on experiments and literature correlations show that gas−liquid, liquid−solid, and intraparticle mass-transfer resistances can be neglected in the batch hydrogenation of lactic acid to propylene glycol (PGL) at the conditions implemented. The intrinsic reaction rate data at 130 and 150 °C were fit to an L−H-type rate expression. The fit of the data was good considering the existence of side reactions and the range of reaction parameters implemented. The kinetic model proposed by the authors serves as a useful tool for further research and design of a process to produce propylene glycol from fermentation-derived lactic acid.98 Another variant of the L−H model was applied by Chen et al.78 in the study of aqueous-phase lactic and propionic acid hydrogenation over Ru/C in a stirred batch reactor. A two-site L−H kinetic model with a single set of rate and adsorption constants fitted the conversion kinetics of both individual and mixed acid hydrogenations. The following set of elementary reactions was used to model both lactic acid and propionic acid hydrogenation:

Figure 35. Dependence of propylene glycol selectivity in methyl lactate hydrogenolysis as a function of WHSV at 170, 200, and 220 °C and a hydrogen flow rate of 10 L h−1. Reprinted with permission from ref 93. Copyright 2012 Elsevier.

Figure 36. Concentration dependence of the initial hydrogenation rate: (▲) LA, (□) PA. Reaction conditions: 130 °C, 7.0 MPa, 0.5 g of catalyst, 50 mL of aqueous solution, 1000 rpm. Reprinted from ref 78. Copyright 2007 American Chemical Society.

rate on the initial concentration indicates that the hydrogenation reactions are not of first order with respect to LA or PA. The authors78 suggested that a Langmuir−Hinshelwood type of rate expression described the reaction kinetics well. The effect of the lactate feed was studied by Kasinathan et al.86 in ethyl lactate hydrogenation at the optimized reaction temperature of 180 °C and pressure of 2.5 MPa. It was found that when the WHSV was increased from 0.369 to 0.738 h−1, the ethyl lactate conversion and 1,2-PDO selectivity were hardly affected, still being higher than 98%. However, a further increase in the WHSV to 1.476 h−1 resulted in a significant decrease in ethyl lactate conversion, while 1,2-PDO selectivity was not affected substantially.86 3.1.4. Kinetics and Mechanism of Lactic Acid Hydrogenation. Several groups have studied the mechanism of lactic acid hydrogenations and kinetic regularities applying the Langmuir−Hinshelwood (L-H) type of kinetics.78,83,85,91,93,95,98 Lactic acid hydrogenation is a relatively simple process having only a few byproducts. The kinetics of aqueous-phase hydrogenation of lactic acid to propylene glycol over a 5 wt % Ru/carbon catalyst has been measured in a stirred batch reactor.98 A simple L−H model in which molecular hydrogen and lactic acid adsorb on the catalyst surface and subsequently react to form 1,2-PDO was thus implemented as a conceptual description of the reaction, where S is a vacant catalytic site:98

acid + S1 ⇄ acid · S1 (fast)

(6)

H 2 + 2S2 ⇄ 2H·S2 (fast)

(7)

2H·S2 + acid · S1 → intermediate· S1 + 2S2 (slow)

(8)

intermediate· S1 + 2H· S2 ⇄ alcohol · S1 + 2S2 (fast)

(9)

alcohol· S1 ⇄ alcohol + S1 (fast)

(10)

In this two-site L−H kinetic model, the acid adsorbs on one type of surface catalytic site (S1) and hydrogen dissociatively adsorbs on a second type of site (S2), contrary to the one-site model suggested by Zhang et al.98 The authors assumed the irreversible surface reaction of the adsorbed acid to be the rate-controlling step; all other steps were assumed to be rapid and close to equilibrium, and the adsorption of water was neglected. Several variations of the above L−H model were examined, including those with molecular hydrogen adsorption or the same-site adsorption of the acid and hydrogen. Those models gave negative values of adsorption constants or a poor fit of data at high acid concentrations. Only the two-site model gave positive values for all kinetic constants and reliably predicted the acid hydrogenation kinetics over a wide range of concentrations. AE

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Figure 37. Possible reaction scheme for hydrogenation of ethyl lactate. Adapted from Feng et al.83 Figure 39. Scheme for the transformation of alkyl lactates in the presence of Cu/SiO2.61 Reprinted with permission from ref 61. Copyright 2011 Springer.

A possible reaction scheme of ethyl lactate conversion,83 given in Figure 37, was proposed on the basis of the products formed83,119 and the work of Luo et al.89 The desired product 1,2-PDO was formed mainly via the direct hydrogenation of ethyl lactate over the ruthenium catalysts. This suggestion of the direct hydrogenation path is reasonable since the selectivity for lactic acid is very low over a wide range of conversions.65,89 Byproducts included 1-propanol, 2-propanol, lactic acid, and 2-hydroxypropyl lactate. 1-Propanol and 2-propanol were obtained by the hydrogenolysis of 1,2-PDO, which was clearly verified by Amada et al.198 in their studies on the hydrogenolysis of 1,2-PDO to propanols. Using water as the solvent, ethyl lactate could be converted to lactic acid by hydrolysis. Trace amounts of 2-hydroxypropyl lactate were detected in the products, probably formed by transesterification of 1,2-PDO with ethyl lactate or by esterification of 1,2-PDO with lactic acid.83,89 While converting lactic acid or its esters to 1,2-PDO, Huang at el.85 detected several byproducts, e.g., 2-hydroxypropionaldehyde and transesterification products (Figure 38).

lactic or propionic aldehydes were not identified. The main reaction path included the adsorption of lactate on metallic copper with the acid-promoted protonation of the C−O bond of the carboxylic group followed by the hydrogenation of the CO bond to form hemiacetal, which in parallel is converted to propylene glycol and hydroxyacetone, providing thermodynamic equilibrium between them. The side path where alkyl lactate is converted first to alkyl acrylate followed by polymerization blocking the active catalyst sites is completely suppressed.93 The same authors earlier proposed an analogous mechanism for lactic acid hydrogenation over Cu/SiO2.92 The scheme, unlike that proposed for lactate hydrogenolysis, included a side path of lactic acid dehydration on acidic sites of the silica surface to form acrylic acid followed by the hydrogenation of the CC bond over metallic copper to yield propionic acid. The formation of a few other byproducts is shown in Figure 40

Figure 38. Reaction scheme of 1,2-propanediol preparation from lactic acid or lactate. Reprinted with permission from ref 85. Copyright 2008 Elsevier. Figure 40. LA transformation routes over a Cu/SiO2 catalyst. Adapted from Simonov et al.92

Simonov et al.93 considered participation of surface protons of Cu/SiO2 in alkyl lactate transformation, suggesting that OH groups on the surface of SiO2 can promote the hydrogenation step by protonation of carbonyl oxygen followed by hydrogen addition to form an intermediate hemiacetal. On the basis of the kinetic data and the identification of intermediates by GC/MS and 1H NMR, a lactate transformation network over reduced silica-supported copper was proposed (Figure 39). The presence of hydroxyacetone was confirmed by 1H NMR, while

as well. Thus, Simonov et al.93 demonstrated that in the case of lactate hydrogenolysis compared to lactic acid hydrogenation active catalytic sites remained available for reactants and the catalyst was stable with a long time-on-stream, as illustrated in Figure 27. On the basis of the product distribution, Xue et al.95 proposed that 1,2-PDO in the liquid-phase hydrogenolysis of AF

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bench-scale batch reactors.76 The authors showed that it is possible to model reactor behavior under these conditions by including wetting and mass transport, but the results must be viewed carefully in light of the challenges of predicting fractional wetting and mass transport coefficients for different physical systems. Nevertheless, the use of such models, based on intrinsic kinetics developed in batch reactors, appears to be a step forward in understanding trickle-bed behavior and in facilitating reasonable scale-up of trickle reactor systems.76 Comparison of propylene glycol yields vs time during LA hydrogenation in batch and trickle-bed reactors is given in Figures 42 and 43.76

ethyl lactate is formed mainly via direct hydrogenation of ethyl lactate over the promoted Co/SiO2 catalysts rather than by a stepwise process (e.g., the hydrolysis of ethyl lactate followed by hydrogenation of lactic acid), as was indicated by the lower amounts of lactic acid in the product distribution over a wide range of conversions. Xue et al.95 suggested that hydrogenation and dehydration reactions are competitive during the liquidphase hydrogenation of ethyl lactate (Figure 41).

Figure 41. Proposed reaction pathway for liquid-phase hydrogenolysis of ethyl lactate over Co/SiO2. Adapted from Xue et al.95

Figure 42. Propylene glycol yield (90 °C, 8.3 MPa of H2) vs time in the trickle-bed reactor (WHSV given in kilograms of LA per kilogram of catalyst per hour): (◆) run 13, WHSV = 0.95; (■) run 14, WHSV = 0.47; (▲) run 9, WHSV = 2.0; (●) run 10, WHSV = 1.0; (Δ) run 1, WHSV = 2.0; (□) run 2, WHSV = 1.0. Reprinted from ref 76. Copyright 2011 American Chemical Society.

3.1.5. Engineering Aspects: Hydrogenation in TrickleBed and Batch Reactors. Xi et al.76 examined the Ru/C (reported previously by Zhang et al.98) catalyzed hydrogenolysis of lactic acid to propylene glycol in trickle-bed and batch reactors to better understand the influence of mass transfer and partial wetting and to identify operating conditions where intrinsic kinetic rates can be obtained. The carbonsupported ruthenium catalyst used in this work is similar but not identical to that applied by Zhang et al.98 At high liquid flow rates and low conversions in the trickle-bed reactor, propylene glycol formation rates agree well with the intrinsic rates obtained in a stirred batch reactor, with the rate independent of the feed flow rate or bed configuration in the trickle-bed reactor. Experimental data at 130 and 150 °C from a stirred batch reactor were fitted to a mechanistically based L−H rate expression with the surface reaction as the rate-limiting step: −rLA =

Figure 43. Propylene glycol yield vs time in a 50 mL solution in a batch reactor at 90 °C and 8.3 MPa of H2: (▲) 0.5 g of catalyst, (■) 0.25 g of catalyst, (◆) 0.125 g of catalyst. Reprinted from ref 76. Copyright 2011 American Chemical Society.

kC LAPH2 (1 + K H2PH2 + KLAC LA )2

(11)

Table 14. Reaction Rates of 1,2-PDO Formation in TrickleBed and Batch Reactorsa

The reaction rate coefficients k, KH2, and KLA were determined at each temperature by a least-squares fit with the MathCad program at diffierent catalyst loadings (0.5, 1.0, and 1.5 g in 100 g of 1.15 M LA solution) and diffierent pressures (6.8, 10.2, and 13.6 MPa). The authors have developed a detailed trickle-bed reactor model for lactic acid hydrogenolysis that accounts for interphase mass transfer, temperature gradients, and catalyst particle partial wetting. That model was applied in its original and simplified forms to hydrogenolysis of LA, incorporating the intrinsic kinetic expression shown in eq 11 to describe the reaction rate. Application of a mass transport model to the trickle-bed reactor at lower flow rates allows the rates to be predicted outside the intrinsic kinetic regime. These results provide a guidance for proper operation of laboratory tricklebed reactors and make it possible to predict performance in a trickle-bed reactor on the basis of experiments conducted in

RPG,G (kmol/m3 of catalyst/s) catalyst bed mass (g) 9.1 9.1 + fines batch a

flow rate 200 mL/h

flow rate 100 mL/h

−4

(5.1 ± 0.4) × 10−4 (4.9 ± 0.4) × 10−4

(5.5 ± 0.4) × 10 (5.1 ± 0.4) × 10−4 (4.6 ± 0.3) × 10−4

Reaction conditions: 90 °C, 8.3 MPa, 1 M lactic acid.76

The similarity between batch and trickle-bed rates (Table 14) is further evidence that the catalyst in the utilized laboratoryscale trickle-bed reactor is fully wetted, representing one of the few cases where trickle-bed reactors are purposefully operated in a “differential” model to achieve rates similar to those obtained in batch reactors.76 AG

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3.1.6. Scale-Up. In principle, practically all abovementioned catalytic processes can be scaled up since the catalyst preparation is based on simple catalyst synthesis methods and accessible chemicals. However, only Kuriyama et al. studied a large-scale methyl lactate hydrogenation for the synthesis of nonracemic 1,2-PDO, a useful chiral building block for pharmaceuticals that has been produced via the asymmetric hydrogenation of hydroxyacetone since 1992 at Takasago.71 The authors showed that the Ru-SEGPHOS complex is a good catalyst for this reaction, providing more than 99% enantiomeric excess (ee) required for pharmaceutical intermediates (Figure 44).87A large-scale methyl lactate hydrogenation was

catalysts were also extensively studied, demonstrating their high activity and selectivity, especially in the case of Cu/SiO2 catalysts. The optimal temperature and hydrogen pressure were varied depending on the selected catalyst as well as on the operation mode (liquid- or vapor-phase). Liquid-phase hydrogenation requires very high hydrogen pressure and relatively expensive platinum group metals, while the vapor-phase mode could be very effective even at low hydrogen pressure in the presence of base metals. Lactate hydrogenolysis can be considered as a more promising method compared to lactic acid hydrogenation due to higher reactivity of the ester group at milder reaction conditions and absence of corrosion. Hydrogenolysis of neat alkyl lactate seems to be more selective and preferable than that of dissolved alkyl lactate. Any of the volatile lactates can be used for synthesis of 1,2-PDO, especially those generating an alcohol that can be reused for lactate synthesis. Application of alcohols that are converted to the side products during the reaction should be avoided. A detrimental effect of feedstock impurities illustrated a need to improve fermentation and purification technologies to produce lactate feedstock with purity suitable for catalytic hydrogenolysis/hydrogenation. 3.2. Oxidative Dehydrogenation of Lactic Acid

Very few attempts have been reported concerning the lactic acid direct oxidative dehydrogenation process. It seemed difficult to obtain high yields of pyruvic acid directly from lactic acid using chemical catalysts because of the side formation of CO2 through C−C bond fission. Lactic acid in the form of alkyl lactates can be dehydrogenated to the corresponding pyruvates more easily. Even if it does not require, prior to oxidative dehydrogenation, esterification of lactic acid and finally hydrolysis of the produced alkyl pyruvate, the transformation of lactate to its esters and subsequent separation are preferable. Along with alkyl lactates, sodium lactate was also reported as a promising starting material since it does not require adjustments of its solution to pH 8 during the reaction.101,102 In this section studies from 2002 onward are reviewed, including a few seminal earlier works. Pyruvates are widely used as dietary and weight-control supplements, nutraceuticals, and antioxidants. They are also important starting materials widely applied in the chemical, pharmaceutical, and agrochemical industries possessing both reactive ketonic and carboxyl groups. For example, pyruvate is used as a promising raw material for the synthesis of pharmaceutical precursors such as L-tyrosine, N-acetyl-D-neuraminic acid, and (R)-phenylacetylcarbinol. Novel biotechnological systems that can yield pyruvate were summarized recently in several reviews166−170 with emphasis on the enzymatic synthesis of pyruvate from a cheaper substrate, lactate,172 and via glucose fermentation.173 The classical chemical route that produces pyruvic acid by dehydration and decarboxylation of tartaric acid is currently applied on a technical scale.174 This process is simple to realize, however, not being cost-effective because of the energy-intensive pyrolysis of tartaric acid. Thus, catalytic transformations of a cheaper substrate, lactate, have been considered as a more attractive method (Figure 46). Among the catalysts related to the oxidative dehydrogenation of lactic acid and lactates to pyruvic acid and pyruvates, respectively, iron orthophosphate,103,104 Pd-doped iron phosphate,104 iron phosphate doped with Te6+, Mo6+, W6+, U6+, V5+, Sb3+, Co2+, Ni2+, Pb2+, and Sn2+,99 TeO2−MoO3 with crystalline α-Te2MoO7 as the active phase,105 Pd/C and Pd/C doped with Te,101 and homogeneous vanadium oxytrichloride (VOCl3)106 have been reported.

Figure 44. Comparison of 1,2-PDO processes. Reprinted from ref 87. Copyright 2012 American Chemical Society.

Figure 45. Large-scale hydrogenation of methyl (R)-lactate. Reaction conditions: substrate (2200 kg, 21 133 mol), Ru-MACHO (6.4 kg, 10.6 mol), NaOMe (28% in MeOH, 51.0 kg, 256.2 mol), MeOH (6369.2 kg), H2 (4.0−4.2 MPa), 26−28 °C, 12 h. Reprinted from ref 87. Copyright 2012 American Chemical Society.

carried out with 0.05 mol % Ru-MACHO on a multiton scale per batch (Figure 45). After distillation, 1477 kg of (R)-1,2PDO was produced with 99.2% ee from 2200 kg of methyl (R)-lactate with 99.6% ee.87 3.1.7. Brief Summary. Lactic acid and lactate hydrogenation to 1,2-PDO was generalized in this section. The most promising is heterogeneous catalytic hydrogenation of lactic acid and its derivatives over metals with high intrinsic hydrogenation ability. Homogeneous catalysts were also reported with the aim to synthesize enantio-enriched products. Utilization of Ru-based catalysts usually requires introduction of some additives (boron, tin, iron, etc.). These catalysts are characterized by high activity and selectivity at high hydrogen pressure (above 4−5 MPa). Besides Ru-based catalysts, Cu- and Co-based AH

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reaction.99,100,103,104 Utilization of ethyl lactate solutions in acetonitrile106 and in toluene105 has also been reported. The effect of an alkyl substituent was studied by Yasukawa et al.106 The oxidative dehydration of various lactate esters was carried out using the optimum oxidizing agent O2 and VOCl3 catalyst at room temperature and 0.1 MPa for 40 min, yielding 49% methyl pyruvate, 56% ethyl pyruvate, 72% isopropyl pyruvate, and 59% n-butylpyruvate depending on the lactate type. In the case of lactic acid as a starting material, the pyruvic acid yield at room temperature for 20 min determined by HPLC was 28% at complete conversion of lactic acid. This result indicates that the rates of side reactions such as decarboxylation are faster than or at least comparable to the rates of the main reaction.106 3.2.3. Catalyst. 3.2.3.1. Effect of the Catalyst Type. Corma et al.54 reviewed the literature related to lactate to pyruvate catalytic conversions prior to 2002. Some of these studies are also worth mentioning in this review. Vapor-phase oxidation using V2O5-based mixed oxide catalysts has been described when lactic acid conversion above 95% is obtained, with selectivity for pyruvates of about 90%.107,108 Lead-modified palladium on carbon,109,110 Pd/Pt-based catalysts,111 and phosphates or polyphosphates of Mo and V supported on silica112 have been reported to be effective for the selective gasphase oxidation of lactates to pyruvates. Hayashi et al.113−115 described oxidation of ethyl lactate to pyruvate in the vapor phase over various oxides. The screening showed a binary oxide, TeO2−MO3, to be an active catalyst, affording pyruvate with high selectivity (over 90% selectivity at ethyl lactate conversion of about 80% at 300 °C). Several studies suggested crystalline α-Te2MoO7 as the active phase.105,106 Telluromolybdates (MOTeO2MoO3, where M = Co, Mn, or Zn) also showed excellent activities in the selective oxidation of ethyl lactate to pyruvate at 250−300 °C.117 The direct oxidation of lactic acid to pyruvic acid is much more difficult. Most of the catalysts presented above catalyze the oxidative C−C bond fission, converting the major part of lactic acid to acetaldehyde and CO2 rather than to pyruvic acid. Indeed, very few attempts have been reported concerning the oxidative dehydrogenation of lactic acid. Thus, Ai et al.100,118 performed the direct oxidation of lactic acid to pyruvic acid at 227 °C over iron phosphates with a P/Fe atomic ratio of 1.2 (FePO4, Fe2P2O7, and Fe3(P2O7)2). During the reaction, FePO4 was transformed to another not identified phase (coined the M-phase) consisting of both Fe2+ and Fe3+ ions. The best performance was obtained with the catalyst composed of the M-phase, achieving 60% conversion with 62% selectivity for pyruvic acid.100 In a later work Ai et al.99 introduced an iron phosphate catalyst doped with a small amount of molybdenum for the oxidative dehydrogenation of lactic acid to pyruvic acid, leading to selectivity enhancement from 72% to 87% with an increase in the yield from 40% to ca. 70% compared to neat iron phosphate. In another work Ai et al.104 found that palladium-doped iron phosphate (0.8 wt % Pd) catalyst is 10 times more active than the neat iron phosphate (0 wt % Pd) catalyst, providing conversion of 44% and selectivity for pyruvic acid of 80% at 10-fold lower loading compared to 52% and 73% for neat iron phosphate. Sugiyama et al.101 reported oxidative dehydrogenation of sodium lactate to sodium pyruvate with a maximum yield of 68.2% in an aqueous phase proceeding favorably using Pd/C as such and doped with Te at 85 °C with no adjustments in the solution pH under pressurized oxygen.

Figure 46. Reaction scheme for pyruvate production. Reprinted from ref 106. Copyright 2011 American Chemical Society.

The effects of additives,99,101,104 catalyst preparation methods,103 calcination temperature,103 treatments by water,100,103 iron reduction and reoxidation ability,103 the oxidizing agent’s nature,106 and pressure106 on the catalytic performance have been studied. Enhancement of pyruvate formation by application of a micro flow reactor with homogeneous vanadium catalysts106 as well as by increasing the ratio between the dead volume and lactate solution in a batch reactor at high air pressure without pH adjustment was shown by Sugiyama et al.101 Oxidative dehydrogenation of lactic acid and/or its esters was examined in both the gas and liquid phases. Although in the former case high yields of pyruvate from lactate were achieved, besides the requirements for evaporation at high temperature, a reaction temperature above 200 °C was needed, increasing the operational costs. In the liquid-phase reaction with solid catalysts such as Pd−metal alloys supported on activated carbon, oxidative dehydrogenation of lactate took place under milder conditions at temperatures of less than 90 °C. This mode of operaton required, however, expensive precious metal catalysts.101 An efficient lactic acid conversion to pyruvate at room temperature was achieved with homogeneous V catalysts using a micro flow system.101 From the viewpoint of effective production of pyruvate, a reaction at low temperature with an inexpensive catalyst would be desirable. 3.2.1. Reaction Network. The general reaction network is summarized in Figure 47.105 The major reaction is lactic acid

Figure 47. Ethyl lactate oxidative dehydrogenation reaction network. Reprinted with permission from ref 105. Copyright 1997 Elsevier.

oxidative dehydrogenation at the secondary hydroxyl group. Since the product pyruvic acid in its free-acid form is prone to decomposition, the substrate is preferably supplied as ethyl ester to protect the carboxyl moiety. Esterification is beneficial for vapor-phase flow operation in making acids more volatile. Hydrolysis of ethyl lactate gives free pyruvic acid with further decarboxylation to acetaldehyde. Ethanol, which is another fragment of ester hydrolysis, could be either oxidized to acetaldehyde or dehydrated to ethylene at temperature above 350 °C. 3.2.2. Feedstock. Aqueous solutions of lactic acid and lactates (generally methyl or ethyl esters) were mainly applied with continuous adjustment of the solution pH ≈ 8 during the AI

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Yasukawa et al.106 considered oxidative dehydrogenation of lactate ester catalyzed by different vanadium species under homogeneous conditions as a promising one because the reaction temperature is below 100 °C. The results of the oxidation reaction catalyzed by various vanadium compounds with oxidizing ability are shown in Table 15. Almost no reaction proceeds under

Table 16. Catalytic Activity of Iron Phosphate Catalysts Prepared by Different Methodsa preparation method A B C

Table 15. Oxidation of Methyl Lactate to Methyl Pyruvate Using Different Various Vanadium Compounds as Catalysts106,a catalyst

amt of V species (mmol)

conversion of LAb (%)

yield of 2ab (%)

3d,e 4f f,g 5 6 7f 8 9

VOCl3 VO(acac)2 VO(acac)2 VO(acac)2 VO(acac)2 VO(acac)2 VO(acac)2 V2O5 H4PMO11VO40

0.13 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.1

50 8 36 26 31 9 19 9 30

10f 11h

none none

31c 5 36 19 22 5 17 trace not detected trace trace

entry 1 2

2 3

a

surface area (m2/g) 54 7.7 7.2

amt of catalyst used (g)

yieldb (mol %)

yieldb/area (mol %/m2)

5 1 5 5

27.0 11.3 4.2 8.8

0.21 0.11 0.25

Adapted from Ai et al.103 bYield of pyruvic acid obtained at 220 °C.

a

Conditions: mass of catalyst 1.0 g, O2 pressure 0.1 MPa, room temperature, reaction time 20 min. bGC analysis. cIsolated yield. d Carried out under N2. em-CPBA (2 mmol) added. fm-CPBA (1 mmol) added. gAfter 45 min. hTetrabutyl hydroperoxide (2 mmol) added. Figure 48. Product distributions obtained from the reaction of sample A catalyst in oxidative dehydrogenation of lactic acid: (o) pyruvic acid, (Δ) citraconic anhydride, (▼) propionic acid, (□) acetic acid, (●) COx. Reaction conditions: lactic acid (aqueous solution) feed rate 20 mmol/h, air feed rate 500 mmol/h, water feed rate 950 mmol/h, mass of catalyst 5 g, 220 °C. Reprinted with permission from ref 103. Copyright 1999 Elsevier.

pure oxygen in the absence of vanadium species, as shown in Table 15 (entries 2−10 and 2−11). Typical oxidizing agents such as V2O5 and molybdovanadophosphoric acid (H4PMo11VO40) could not catalyze oxidation of lactate (entries 2−8 and 2−9) because no alkoxyvanadium complexes were formed with lactate by these vanadium species. In the case of H4PMo11VO40, decomposition of lactate mainly occurred due to the strong acidic property of the catalyst without any byproducts formed via oxidative decomposition. Pure oxygen proved the most suitable in these screenings, since the pyruvate formation is higher with VOCl3 in this case than using other catalysts. In the presence of a stoichiometric amount (2 mmol) of m-chloroperbenzoic acid (m-CPBA), pyruvate was formed in comparatively high yields (35%, entries 2 and 3). Moreover, the addition of a minor amount (0.1 mmol) of m-CPBA (entries 2−4 and 2−5) increased the productivity of ethyl pyruvate catalyzed by VO(acac)2. These results indicate that structural changes of the vanadium compound from the initial species to the active one is necessary for starting the catalytic cycle. When VO(acac)2 was used as the catalyst, m-CPBA was a more effective oxidizing agent than dissolved molecular oxygen in terms of the structural change of vanadium species discussed above.106 3.2.3.2. Effect of the Catalyst Preparation Method. Ai et al.103 studied lactic acid oxidative dehydrogenation at 200− 240 °C over one of the most basic iron phosphates, FePO4, coming from different suppliers (methods A and B (Table 16)) and prepared from Fe(NO3)3 in method C (Table 16). The product distribution at feed rates of lactic acid, air, and water of ca. 20, 500, and 950 mmol/h, respectively, is presented in Figure 48. Figure 49 shows that the source of starting materials as well as the preparation methods insignificantly affected the specific activity (activity per unit surface area).

Figure 49. Effects of the preparation methods on the selectivity in oxidative dehydrogenation of lactic acid: (o, Δ, □) pyruvic acid, (●, ■, ▲) citraconic anhydride; (o, ●) sample A, (Δ, ▲) sample B, (□, ■) sample C. Reaction conditions: lactic acid (aqueous solution) feed rate 20 mmol/h, air feed rate 500 mmol/h, water feed rate 950 mmol/h, mass of catalyst 5 g, 220 °C. Reprinted with permission from ref 103. Copyright 1999 Elsevier. AJ

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Table 17. Effects of Doping of Metal Ions (Mn+) on the Formation of Pyruvic Acid from Lactic Acid99 Fe/Mn+/P atomic ratio

metal ion n+

M none none Te6+ Mo6+ W6+ U6+ V5+ Sb3+ Co2+ Ni2+ Pb2+ Sn2+ a

amt of catalyst (g)

Xia

1/0/1.05 1/0/1.2

10 10

27.3 23.4 22.1 22.1 17.6 13.3 9.0 9.0 9.0 9.0

1/0.2/1.2 1/0.2/1.2 1/0.1/1.3 1/0.2/1.2 1/0.4/1.4 1/0.2/1.4 1/0.2/1.4 1/0.2/1.4 1/0.2/1.4 1/0.2/1.4

10 2.5 10 10 10 10 10 10 10 10

temp (°C)

conversion (%)

yield (%)

selectivity (%)

215 215 225 240 200 210 240 210 220 195 220 235 235

50.8 40.0 54.2 35.0 32.0 30.4 37.5 41.6 40.0 57.7 48.4 49.5 32.0

36.5 29.5 37.2 30.0 27.7 12.2 23.6 32.3 28.0 19.5 33.5 34.0 18.0

72 73 69 86 87 40 63 78 70 34 69 68 55

Electronegativity of the metal ion.135

3.2.3.3. Effect of the Calcination Temperature. The effects of the calcination temperature on the structure and the catalytic activity of FePO4 were studied by Ai et al.103 By raising this temperature, the surface area of sample A decreases markedly, while the areas of samples B and C, which are relatively low, remain unchanged. As for the structures of the samples, the variations are different depending on the variations in the preparation method. Sample A remains amorphous up to 460 °C, while above 480 °C it is transformed to a quartz phase without the formation of an intermediate, such as a tridymite phase. Samples B and C remain in the tridymite phase up to 480 °C, being transformed to the quartz phase at a temperature above 500 °C.103 The catalytic activities of samples B and C are not affected by a rise in the calcination temperature. In the case of sample A, both the surface area and activity decreased with a rise in the temperature, though the specific activity also did not change with the calcination temperature independent of the variation in the structure of FePO4. 3.2.3.4. Effect of Reduction and Reoxidation Activities. The effects of reduction and reoxidation on the catalytic activities were studied for each iron phosphate sample A, B, and C.176 During reduction at a temperature of 400 °C for 6 h, all three iron orthophosphate samples were transformed to iron pyrophosphate (Fe2P2O7) consisting of Fe2+ ions. The surface area marginally increased in the cases of samples B and C, while it decreased in the case of sample A. On the other hand, by reoxidation in air at a temperature of 400 °C for 6 h, all of the reduced samples were transformed to a new phase, designated by the authors as the Y-phase or so-called α-Fe3(P2O7)2. The variations in the catalyst structure by reduction and reoxidation are expressed schematically as follows:

(Xi). The value of Xi is defined as Xi = X0(1 + 2n),119 where n is the charge and X0 is the electronegativity of the neutral atom (n = 0) given by Pauling. A high oxidation activity is obtained in a wide Mo/Fe atomic ratio range from 0.01 to 1.0. This is about 10 times more than the ratio obtained from the original iron phosphate without Mo6+. The selectivity for pyruvic acid increases by doping of only 0.1 mol % Mo6+ (Mo/Fe atomic ratio 0.001). The highest selectivity is obtained at a Mo/Fe atomic ratio ranging from 0.01 to 0.03. The selectivity is ca. 86% at a conversion of 60% and ca. 77% at a conversion of 90%. The one-pass yield reaches 70%. The catalytic activity and selectivity are not affected if the source of Mo6+ is changed ((NH4)6Mo7O24·4H2O, H3PMo12O40, and 80% MoO3). Effects similar to those for Mo6+ doping are not observed in the cases of doping by V5+ or W6+ (Table 18), indicating the uniqueness of Mo6+.99

FePO4 (reduction) → Fe2P2O7 (reoxidation) → Y‐phase

No changes in the XRD patterns were observed for doping by Mo6+ up to 7.5 mol % (Mo/Fe atomic ratio ML > EL. However, the product selectivity increased in the opposite way. The highest AA selectivity of 79% was found for EL, followed by ML and LA. On the basis of the results presented in Table 7, the authors proposed that ML is a better substrate to obtain high selectivities for AA and MA compared to other feedstock. 3.3.2. Effect of the Catalyst Type. Recently, various heterogeneous catalysts were reported for lactic acid to acrylic acid dehydration, such as calcium hydroxyapatite,123 NaY zeolite122,124 and its various modifications, KF/NaY, KCl/NaY, KBr/NaY, and KI/NaY,122 NaY zeolites modified with sodium and potassium phosphates,121 KNaY zeolite,129 NaY modified by La, Ce, Sm, and Eu,130 La3+ and Ba2+,75 alkaline-earth metals (Mg, Ca, Sr, and Ba),131 K+, Ba2+, and La3+,132 La-NaY,133 and CaSO4 with copper, sodium, and potassium phosphate promoters.134 The highest acrylic acid yield was 63.7% achieved at 330 °C and an 88 s contact time from 26 wt % lactic acid over an optimal CaSO4 catalyst with copper, sodium, and potassium phosphate promoters where the ratio of components m(CaSO 4 )/m(CuSO 4 )/m(Na 2 HPO 4 )/m(KH 2 PO 4 ) was 150.0/13.8/2.5/1.2.134

Table 22. Conversion of Different Substrates Using Ca3(PO4)2−Ca2(P2O7) (50/50, wt %) Composite Catalyst at 390 °Ca product selectivity (%) substrate

conversion (%)

acrylic acid

alkyl acrylate

acetaldehyde

propionic acid

others

EL ML LA

57 91 100

79 75 54

5 5 0

13 14 14

2 4 14

1 2 18

Reaction conditions: mass of catalyst 4.0 g, substrate concentration 50 wt % in water, flow rate 2.1 mL/h, TOS = 27 h. “Others” contain 0.5−2.0% CO, which is roughly proportional to acetaldyhyde formation, and the rest are methyl 3-methoxypropionate, acetol, D-lactide, and hydrogen.114

a

AP

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of acetaldehyde via decarbonylation. Furthermore, sodium lactate formation occurred when Na2HPO4/NaY was used as a catalyst for production of acrylic acid.121 Zhang et al.121,125 reported high activity of silica-supported sodium phosphates for the selective dehydration of ML to MA and AA. The NaH2PO4/SiO2 showed a higher selectivity for MA and AA (52% at 99.5% ML conversion) at 380 °C than Na2HPO4/SiO2 (15.6% selectivity for MA and AA at 88.6% ML conversion) and Na3PO4/SiO2 (18.9% selectivity for MA and AA at 99.4% ML conversion). Relatively poor selectivity for AA and MA for Na2HPO4/SiO2 and Na3PO4/SiO2 was probably related to the strong acidity of these catalysts, which favored decarbonylation and dehydration, giving large amounts of acetaldehyde. For the best catalyst, NaH2PO4/SiO2, an optimum loading of NaH2PO4 from 1.0 to 2.1 mmol/g was found, providing high overall selectivity for MA and AA. When the loading of NaH2PO4 was above 2.1 mmol/g, multilayer crystalline polyphosphates such as (NaPO3)n with a high degree of condensation were formed and the density of the terminal POH groups responsible for the catalytic conversion of ML to MA and AA was reduced.125 A high content of basic sites was able to promote formation of acrylic acid with KI/NaY catalyst.122 Part of KI was decomposed during calcination to I2 and K2O.122 Furthermore, it was claimed that when the electronegativity of an extraframework anion is diminished, the basic strength of framework oxygen is increased. There was a correlation between the basic strength of the catalyst, which increased in the order Cl < Br < I, and the selectivity for acrylic acid.122 A combination of calcium sulfate as the main catalyst and other metal sulfates as promoters was used for the conversion of methyl lactate to acrylates, with the results shown in Figure 59.137 The highest combined yield of acrylic acid and

The results on the effect of unsupported and calcined alkaline-earth-metal phosphates of Ca3(PO4)2, CaHPO4, and Ca(H2PO4)2 in the vapor-phase dehydration of ML are presented in Table 23. It was found that Ca3(PO4)2 gave the highest conversion of ML (78.5%) and overall selectivity for AA and MA (67.8%), which was associated with the higher surface area of Ca3(PO4)2 compared to other phosphates.127 To verify the effect of the silica support nature on the dehydration of ML, three Ca3(PO4)2−SiO2 catalysts (80/20, wt %) with different silica sources (silicate, colloidal silica, and fumed silica) were tested under the same reaction conditions. All the catalysts gave AA, acetaldehyde, and MA as the major products along with the other products such as methyl 3-methoxypropionate, acetol, and D-lactide. One of the catalysts gave the highest conversion of ML (73.6%) and combined product selectivity for AA and MA (77.1%), which was assigned to the higher surface area, desired acid−base strength, and pore volume of this particular silicate. The results on the effect of the catalyst composition of this catalyst, Ca3(PO4)2−SiO2, on the dehydration of ML along with the reaction conditions are presented in Table 24. When the Ca3(PO4)2 content in Ca3(PO4)2−SiO2 was increased from 70 to 95 wt %, the conversion of ML was increased from 58% to 85.8%, while for 100 wt % it remained at 78.5%.127 The catalytic performance of potassium-exchanged NaY in comparison with NaY in the formation of MA was studied.129 Although the conversion of ML (83.0%) over K-exchanged NaY was slightly lower than that over NaY (88.6%) at 330 °C, the selectivity (45.7%) over KNaY was much higher compared to that over NaY (32.1%). As a result, the yield over KNaY was improved. Better catalytic performance of KNaY catalyst compared with NaY suggested that this more basic zeolite favors the transformation of ML to MA.129 Wang et al.130 demonstrated that NaY zeolites modified by a rare-earth metal improved the catalytic dehydration performance of lactic acid to acrylic acid and speculated about the possible reasons. On the basis of some preliminary characterization results, they concluded that the improvement was due to both the physical structure factors (such as pore size, pore volume, and surface area) and chemical factors (such as the specific location of rare-earth-metal ions and lower surface acidity density). Wang et al. reported130 that, compared with unmodified NaY zeolite, catalysts with different La3+ contents all showed higher selectivity for AA, and the selectivity for acetaldehyde (AD) decreased. It can be expected that the addition of La3+ to NaY zeolites would restrain the production of AD to some extent. NaY with 2% La3+ was shown to be the optimum catalyst, and the highest AA yield of 56.3%, with the lowest amount of unknown compounds (29.8%), was achieved.130 Yan et al.131,132 investigated other differently substituted zeolites. The yields of acrylic acid were found to increase as follows: 2% Mg/NaY < 2% Ca/NaY < 2% Sr/NaY < 2% Ba/ NaY. This order is consistent with the basicity of the respective cation clusters and the amount of medium basic sites in the catalysts. The yield of acetaldehyde is increased in the sequence 2% Ba/NaY > NaY > 2% Ca/NaY > 2% Sr/NaY > 2% Mg/ NaY, in agreement with the amounts of medium acidic sites. Moreover, the sequence for the yields of acrylic acid over the Ba/NaY zeolites containing various amounts of Ba was similar to that for the amount of medium basic sites. Higher amounts of Ba can inhibit the formation of acetaldehyde. Among the studied catalysts, 2% Ba/NaY gave the highest yield of acrylic

Figure 59. Effect of promoters on the combined yield of acrylic acid and methyl acrylate in methyl lactate dehydration. The temperature was between 250 and 420 °C and was not specified exactly in the reference. Reprinted with permission from ref 137. Copyright 2008 Chemical Industry Press.

methyl acrylate was obtained over the catalyst combination of calcium sulfate and cupric sulfate at 400 °C. In many cases, the catalytic activity of metal sulfates was determined by the strength of the surface acid sites. The maximum acid strength of solid sulfate salts was in accordance with the order Fe3+ > Al3+ > Cu2+ > Ni2+ > Mn2+ > Mg2+. The moderate acid strength of cupric sulfate could thus be responsible for its good catalytic performance.137 AQ

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Table 23. Dehydration of Methyl Lactate with Different Catalystsa product selectivity catalyst

ML conversion (%)

acrylic acid

acetaldehyde

methyl acrylate

propionic acid

others

Ca3(PO4)2 CaHPO4 Ca(H2PO4)2

78.5 54.2 5

62.5 41.6 33.6

30.2 46.9 62.6

5.3 6.8 0.8

1.4 1.5 1.5

0.6 3.2 1.5

Reaction conditions: mass of catalyst 1.3 g, [ML] = 50 wt % in water feed, flow rate 0.7 mL/h (LHSV = 0.35 h−1), N2 flow 5 mL/min, 370 °C, TOS = 20 h. “Others” means methyl 3-methoxypropionate, acetol, and D-lactide. Adapted from Lee et al.127 a

Table 24. Catalytic Activity in Methyl Lactate Dehydration Using Different Ca3(PO4)2/SiO2 Ratiosa product selectivity (%) catalyst Ca3(PO4)2/SiO2 ratio

methyl lactate conversion (%)

acrylic acid

acetaldehyde

methyl acrylate

propionic acid

others

70/30 80/20 90/10 95/5 100/0

58.0 73.6 73.5 85.8 78.5

46.0 70.5 69.6 65.6 62.5

44.1 20.7 22.7 24.5 30.2

3.9 6.4 5.8 7.9 5.3

1.8 1.1 0.9 0.9 1.4

4.2 1.3 1.0 2.1 0.6

Reaction conditions: mass of catalyst 1.3 g, [ML] = 50 wt % in water feed, flow rate 0.7 mL/h (LHSV = 0.35 h−1), N2 flow 5 mL/min, 370 °C, TOS = 20 h. “Others” means methyl 3-methoxypropionate, acetol, and D-lactide.127 a

acid at up to 44.6% because it contained the highest amount of medium basic sites and a suitable Ba2+ cluster character.131 3.3.2.2. Effect of the Catalyst Preparation Method. Lamodified NaY zeolites have been prepared through impregnation and in situ synthesis methods.133 Catalytic dehydration of lactic acid showed that the La/NaY catalyst made by the impregnation method exhibited higher selectivity for acrylic acid than the La-NaY prepared by the other method. The characterization results indicated that the enhanced catalytic performance could be attributed to the difference in La3+ ion location caused by the different preparation protocols, which influence the electric field distribution of zeolites cages, the reactant adsorption, chemical activation, and the reaction path.133 Yan et al.75 studied the modified NaY zeolites with different synthesis procedures. La/NaY showed higher selectivity for acrylic acid compared with the parent NaY zeolites, in line with the results obtained by Yu et al.133 3.3.3. Reaction Conditions. 3.3.3.1. Effect of the Carrier Gas. Traditionally, dehydration was carried out in N2. Zhang et al.134 studied the effect of the carrier gas and introduced carbon dioxide as well as nitrogen. Both N2 and CO2 can act as a diluent to aid the feedstock evaporation and facilitate the transportation in a tubular reactor, thus inhibiting coke formation. The authors found a 63.7% molar yield of acrylic acid, obtained at 330 °C with CO2 as a carrier gas, much higher than 46.1% with N2. Zhang et al. attributed the enhancement in acrylic acid molar yield with CO2 as a carrier gas to the fact that an excessive CO2 presence inhibited decarbonylation/ decarboxylation (Figure 59), thus improving dehydration selectivity.137 3.3.3.2. Effect of Temperature. Dehydration of LA as well as alkyl lactate is an endothermic reaction; thus, from the viewpoint of thermodynamics higher temperature is favorable for gaining a high reaction rate and conversion of methyl lactate.137 Conversion of lactic acid also increased with increasing reaction temperature.123 On the other hand, selectivity for acrylic acid exhibited a maximum at 375−390 °C, depending on the catalyst and reaction conditions since application of high temperature leads to the enhancement of side reactions and secondary transformations of acrylates. For example, it was shown that at 400 °C the selectivity for acetaldehyde slightly

Figure 60. Conversion ratio and yields versus temperature for different products with 60% methyl lactate as the feedstock: (▼) conversion ratio of methyl lactate, (▲) combined yield of acrylic acid and methyl acrylate, (●) yield of acrylic acid, (■) yield of methyl acrylate. Adapted from Zhang et al.137

increased, while at lower temperatures, below 300 °C, 2,3-pentanedione was formed (see section 3.6).123 It can be seen from Figure 60 that conversion of methyl lactate increases steadily from only 35.5% at 250 °C to 100% at 500 °C; however, the highest combined yield of acrylates is observed at 400 °C with a decline of the acrylate yield at temperatures above 400 °C.137 The influence of temperature on Ca3(PO4)2−SiO2 (80/20, wt %) catalyzed dehydration of ML is presented in Table 25.127 The conversion of ML increased with a temperature increase from 350 to 400 °C; however, the total selectivity for AA and MA decreased due to decarbonylation and dehydration of ML (Figure 60).137 The optimum temperature was 400 °C, wherein the conversion of ML was 73.6% and the combined selectivity for AA and MA was ca. 77%. The effect of the reaction temperature on MA synthesis was investigated over KNaY.129 It was observed that the yields of MA increased insignificantly from 34.4% to 37.9% with reaction temperature in the range of 280−340 °C at a constant LHSV of 0.4 h−1. At 340 °C, 83.0% ML conversion with 45.7% selectivity for MA was achieved. When the reaction temperature exceeded 340 °C, more byproducts were formed, and the yield of MA slightly decreased to 31.7% at 370 °C.129 AR

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Table 25. Dehydration of Methyl Lactate at Different Temperatures with Ca3(PO4)2−SiO2 Catalysta product selectivity (%) temp (°C)

ML conversion (%)

acrylic acid

acetaldehyde

methyl acrylate

propionic acid

others

350 370 390 400

64.3 73.6 84.6 87.2

70.9 70.5 64.5 59.0

23.5 20.7 23.4 24.1

1.9 6.4 7.3 11.2

1.7 1.1 2.9 3.8

2.0 1.3 1.9 1.9

a

Reaction conditions: mass of catalyst 1.3 g, [ML] = 50 wt % in water feed, flow rate 0.7 mL/h (LHSV = 0.35 h−1), N2 flow 5 mL/min, 370 °C, TOS = 20 h. “Others” means methyl 3-methoxypropionate, acetol, and D-lactide. Adapted from Lee et al.127

Figure 61. Yield of acrylic acid versus reaction temperature at different calcination temperatures: (■) 300 °C, (●) 370 °C, (▲) 400 °C, (▼) 430 °C, (◆) 460 °C. Adapted from Zhang et al.137

Table 27. Production of Acrylic Acid from Lactic Acid at Various Lactic Acid Concentrations in Aqueous Solutiona

Zhang et al.137 estimated the activation energies and rate constants for primary pathways of methyl lactate transformation, excluding polymerization and hydrolysis of methyl lactate (Table 26). The dependence of the AA molar yield in CO2 on the LA dehydration temperature from 250 to 420 °C over CaSO4/ CuSO4 treated at different calcination temperatures (300− 460 °C) is shown in Figure 61.137 The highest yield of acrylic acid was observed at 330 °C over the catalyst calcinated at 430 °C, which was attributed to the moderate solid acidity of this catalyst. The optimal reaction temperature is lower than that (400 °C) for methyl lactate dehydration in N2.137 The catalyst calcination temperature has a minor effect on the molar yield dependence of propanoic acid and acetaldehyde on the reaction temperature.134 3.3.3.3. Effect of the Lactic Acid Concentration. Lactic acid and methyl lactate feedstock with different concentrations could be obtained from biomass-originated substrates (section 2.1 and section 2.2) and thus applied to acrylic acid synthesis. From a practical viewpoint, higher initial concentrations of the substrate are preferable due to process energy savings; however, they could be unfavorable for high productivity of acrylic acid. Therefore, the effect of the lactic acid and lactate concentrations on conversion and selectivity in this process was thoroughly considered in the literature. Zhang et al. reported that a decrease of LA concentration from 40 to 15 wt % can increase LA conversion from 75.4% to 100%; meanwhile AA selectivity first increases from 65.3% to 72.3% and then gradually decreases to 48.0%, indicating that there is an optimal LA concentration in solution, namely, 25 wt %, at which the highest AA yield of 58.4% can be obtained (Table 27).121 Zhang et al.134 evaluated the effect of the lactic acid concentration on dehydration product yields at 330 °C (Table 28). The catalyst was calcinated at 430 °C for 3 h; the flow rates of lactic acid and carrier CO2 were 0.1 and 20 mL/min, respectively. A lower feed concentration is favorable for the formation of products, especially acrylic acid.134

yield (mol %) of acetaldehyde yield (mol %) of 2,3pentanedione yield (mol %) of acetic acid yield (mol %) of propanoic acid yield (mol %) of acrylic acid conversion of LA (%) selectivity for 2,3propanedione (%) selectivity for acrylic acid (%)

40.0 wt % lactic acid

34.0 wt % lactic acid

25.0 wt % lactic acid

15.0 wt % lactic acid

2.7

4.2

5.0

11.0

3.4

4.6

5.1

7.1

0.3

0.4

0.5

2.2

0.7

1.0

1.5

1.4

49.2

56.6

58.4

48.0

75.4 4.5

78.3 5.9

85.3 6.0

100.0 7.1

65.3

72.3

68.5

48.0

Reaction conditions: catalyst 14 wt % Na2HPO4/NaY, 340 °C, N2 carrier gas flow 30 mL/min. Adapted from Zhang et al.121 a

The same authors137 studied the effect of the methyl lactate concentration on dehydration product yields over CaSO4/ CuSO4 at 400 °C with N2 as a carrier gas (Figure 62). The highest combined yield of acrylates of 63.9% obtained from 60% (by mass) methyl lactate was higher compared to that obtained from neat methyl lactate. Therefore, the effect of water should be taken into account. High yields of acrylic acid and methyl acrylate obtained from 60% methyl lactate showed that the acid sites were not poisoned by water in this case. In addition to acting as a heat exchanger, it was generally thought that water could accelerate the surface renewal of catalysts, thus inhibiting coke formation. However, since water is one of the products; the enhancement in acrylate yield could be inhibited. Moreover, the presence of water led to methyl lactate hydrolysis, which accounted for conversion with lower yields in acrylic acid and methyl acrylate at low feed concentrations as shown in Figure 62.137

Table 26. Activation Energy and Rate Constants for Different Reactions in the Formation of Methyl Lactatea

a

reaction

EA (kJ/mol)

k400 °C (s−1)

CH3CH(OH)COOCH3 → CH2CHCOOCH3 + H2O CH2CHCOOCH3 + H2O → CH2CHCOOH + CH3OH CH3CH(OH)COOCH3 → CH3CHO + CO2 + CH4 + H2O

38.1 64.1 65.2

0.185 1.103 0.0055

Adapted from Zhang et al.137 AS

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Table 28. Product Distribution over Lactic Acid Dehydration in CO2 at 300 °Ca

acetaldehyde concn (mol %) acetic acid concn (mol %) propanoic acid concn (mol %) acrylic acid concn (mol %) concn of unknown products (mol %) final concn of acrylic acid (wt %) conversion of lactic acid (%) a

5 wt % lactic acid

15 wt % lactic acid

25 wt % lactic acid

35 wt % lactic acid

45 wt % lactic acid

55 wt % lactic acid

65 wt % lactic acid

23.4 0 30.4 43.9 2.3

29.4 2.0 18.4 45.9 4.3

19.6 1.6 10.4 63.4 5.0

20.8 0.9 8.3 34.5 35.5

21.5 0.9 6.6 27.4 43.6

24.9 0.7 6.7 23.4 44.3

25.6 0.6 7.8 22.1 43.9

1.8 82

6.2 90

12.9 81

8.1 67

8.4 68

9.1 71

10.2 71

Adapted from Zhang et al.134

WHSV of 3 h−1 with a corresponding contact time of 1.196 s gave the highest selectivity for acrylic acid (60%) at complete conversion, whereas with a longer residence time both conversion of lactic acid and selectivity for acrylic acid decreased over hydroxyapatite as a catalyst.123 The correlations of products yield with contact times at temperatures of 330, 360, and 400 °C were studied by Zhang et al. (Table 29).134 The highest acrylic acid yield of 63.7% was achieved with a rather long contact time of 88 s at 330 °C. Compared with the results at 330 °C, the corresponding contact times that gave the highest acrylic acid yields at 360 and 400 °C were 29 and 12 s, respectively, which were remarkably shorter. The authors explained this as the occurrence of secondary reactions of acrylic acid, e.g., decarbonylation/ decarboxylation and reduction of acrylic acid to propanoic acid, which happened more easily at elevated temperatures. This can be responsible for the steady increase in the molar yields of propanoic acid and acetaldehyde at prolonged contact times for all reaction temperatures studied. Other side reactions such as dimerization or self-polymerization of less volatile lactic acid also led to a decrease in the acrylic acid yield. The nonvolatile polymers with low molecular weight form coke on the catalyst surface when decomposed during prolonged heating.134 The same authors137 reported the effect of the contact time on methyl lactate dehydration, reaching basically the same conclusions as for lactic acid dehydration. It can be seen from Figure 64 that the highest combined yield of acrylates (63.9%) is reached with a contact time of 7.7 s at 400 °C. At longer contact time, an obvious decrease in the yields of acrylates due to side reactions is observed, although the conversion increases remarkably. The authors137 suggested that a rather long contact time gave rise to side reactions such as decarbonylation/ decarboxylation, polymerization, and reduction. Alternatively, because of methyl lactate dehydration, hydrolysis of methyl lactate to lactic acid can probably occur at high temperature; thus, low molecular weight lactic acid polymers will deposit on the catalyst surface and decompose after prolonged heating, giving rise to coke formation.137 3.3.4. Mechanism. The main reaction pathways of ML transformations are shown in Figure 65. Dehydration of ML gives MA and H 2 O, dehydration and deesterification (hydrolysis) of ML leads to AA and methanol, decarbonylation of ML results in AD, methanol, and CO, and decarboxylation of ML forms AD, methane, and CO2. A series of quantum chemical calculations with MP2 and B3LYP methods were carried out for probing the most possible reaction pathways of ML over sodium tripolyphosphate, which was used as a model catalyst for sodium polyphosphate

Figure 62. Conversion of methyl lactate and yields versus methyl lactate concentration at 400 °C for different products: (▼) conversion of methyl lactate, (▲) yield of acrylic acid, (■) yield of methyl acrylate, (●) yield of propanoic acid. Adapted from Zhang et al.137

A study on the effect of water on the catalyst performance137 demonstrated that simultaneous destruction and formation of acid sites, caused by hydration and dehydration, probably resulted in some sort of steady state, effectively maintaining the acidity and inhibiting coke formation (Figure 63). Compared

Figure 63. Conversion ratio and yields versus time for different feedstocks at 400 °C: (▲) conversion ratio of 60% methyl lactate, (▼) combined yields of methyl acrylate and acrylic acid with 60% methyl lactate as the feedstock, (■) conversion ratio of pure methyl lactate, (●) combined yields of methyl acrylate and acrylic acid with pure methyl lactate as the feedstock. Adapted from Zhang et al.137

with neat methyl lactate, a higher conversion and combined yield of acrylic acid and methyl acrylate, as well as a prolonged catalyst life, were observed with 60% methyl lactate as the feedstock.137 3.3.3.4. Effect of the Residence Time. The contact time is an important factor since it influences conversion and selectivity. Thus, many side reactions such as polymerization of acrylates, decarbonylation/decarboxylation, and reduction may occur at prolonged contact times. A relatively short residence time and AT

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Table 29. Product Yields with Different Contact Times in CO2 at 330 °Ca

acetaldehyde concn (mol %) acetic acid concn (mol %) propanoic acid concn (mol %) acrylic acid concn (mol %) concn of unknown products (mol %) final concn of acrylic acid (wt %) a

5 wt % lactic acid

15 wt % lactic acid

25 wt % lactic acid

35 wt % lactic acid

45 wt % lactic acid

55 wt % lactic acid

65 wt % lactic acid

23.4 0 30.4 43.9 2.3

29.4 2.0 0.4 45.9 4.3

19.6 1.6 10.4 63.4 5

21.5 0.9 8.3 34.5 35.5

21.5 0.9 6.6 27.4 43.6

24.9 0.7 6.7 23.4 44.3

25.6 0.6 7.8 22.1 43.9

1.8

6.2

12.9

8.1

8.4

9.1

10.2

Adapted from Zhang et al.134

Figure 64. Conversion ratio and yields versus contact time with methyl lactate as the feed at 400 °C: (●) conversion of methyl lactate, (■) combined yields of methyl acrylate and acrylic acid. Reprinted with permission from ref 137. Copyright 2008 Chemical Industry Press.

Figure 66. Reaction pathways of lactic acid dehydration in hightemperature water. Reprinted with permission from ref 135. Copyright 2009 Elsevier.

A simplified reaction network of lactic acid transformations in water at high temperature is given in Figure 66.135 It consists of a dehydration reaction pathway for lactic acid to acrylic acid (k1) and a combined decarboxylation and decarbonylation reaction pathway for lactic acid to acetaldehyde (k2). The reaction mechanism of dehydration is shown in Figure 67.135 In this mechanism, water directly takes part in

Figure 65. Schematic reaction mechanism of the gas-phase dehydration of methyl lactate over sodium tripolyphosphate: (a) stepwise mechanism, (b) concerted mechanism. Reprinted with permission from ref 139. Copyright 2010 Elsevier.

Figure 67. Proposed water-catalyzed dehydration reaction mechanism for the formation of acrylic acid from lactic acid. Reprinted with permission from ref 135. Copyright 2009 Elsevier.

supported on silica.139 The calculated results indicate that transformations of ML are mainly through the direct decomposition of ML to acrylic acid and methanol and the decarbonylation of ML to AD, methanol, and CO. Both of these routes proceed via stepwise mechanisms and start from the same reaction intermediate (Figure 65 a). In contrast, dehydration of ML to MA and H2O is less important as compared to the above-mentioned reactions. The main route for the formation of MA from ML is via esterification reaction of AA with methanol, and this reaction can only proceed to a limited extent due to its higher activation barrier than that of the reverse reaction. Over sodium tripolyphosphate, the predominant consumption route for LA is decarbonylation, forming AD, H2O, and methanol. The values of the activation barriers also indicate that over sodium polyphosphate selectivity of ML transformation to AA is higher than that of LA transformation to AA.139

the transition state of the dehydration reaction by forming a sixmembered ring transition state. Increasing water concentration therefore increases the dehydration reaction rate. 3.3.5. Stability of the Catalyst in the Transformation of Lactic Acid to Acrylic Acid. Calcium hydroxyapatite was a very stable catalyst in the transformation of lactic acid to acrylic acid, since the conversion remained constant after 300 h and the selectivity decreased only slightly from 60% to 57%.123 In this case the initial concentration of lactic acid was also relatively high, 50 wt %. Only slight deactivation of K/NaY catalyst from 100% to 96.4% conversion within 22 h occurred in lactic acid transformation to acrylic acid at 325 °C, whereas in the absence of K the deactivation of NaY was more prominent and the conversion decreased to 90.2%.123 The stability of Na2HPO4/NaY catalyst was studied with time-on-stream at 340 °C during 30 h with LHSV = 2.7 h−1 and an LA concentration of 34 wt %.80 LA conversion declines AU

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On the other hand, the overabundance of water in the initial solution, which is also generated during dehydration, could result in methyl lactate hydrolysis, inhibiting an enhancement of the acrylate yield. In addition, a rather long contact time gave rise to side reactions such as decarbonylation/decarboxylation, polymerization, and reduction. 3.4. Decarbonylation and Decarboxylation of Lactic Acid

Lactic acid decarbonylation has been scarcely investigated.124,140,141 The reaction occurs via formation of acetaldehyde, water, and carbon monoxide, whereas CO2 is formed instead of CO via decarboxylation.85 The best catalytic performance for production of acetaldehyde has been achieved over silicotungstic acid,140 whereas in the case of NaA zeolite as a catalyst acetaldehyde is the most prominent side product, with acrylic acid being the main product.124 Silica-supported heteropolyacids have been applied as catalysts for production of acetaldehyde from lactic acid at 275 °C in a fixed-bed reactor.140 Keggin-type heteropolyacid catalysts exhibit very strong acidity, and thus, they are potential catalysts for decarbonylation. Several types of supported heteropolyacids, for example, H4SiW12O40/silica CARiACT and H4SiW12O40/SBA-15, are very active and selective for production of acetaldehyde, giving 80% yield at 92% conversion and 83% yield at 85% conversion, respectively (Figure 69). On the

Figure 68. Test of catalyst stability over 14 wt % Na2HPO4/NaY: (a) fresh catalyst, (b) reactivated catalyst. Reaction conditions: 340 °C, 34 wt % lactic acid, LHSV = 2.7 h−1, N2 gas carrier flow rate 30 mL/min. Reprinted from ref 121. Copyright 2011 American Chemical Society.

noticeably within the first 3 h and essentially remains unchanged afterward. The AA selectivity, however, decreases continuously within a period of 30 h (Figure 68). Such catalyst deactivation is thought to be a result of LA polymerization on the catalyst. To verify this point, the deactivated catalyst was treated in an air flow (30 mL/min) at 500 °C for 3 h and tested in a second run. The reactivated catalyst showed 100% recovery in LA conversion as well as better performance in terms of less AA selectivity decline with time. Such behavior was related to higher dispersion of surface phosphate species, stronger interactions between sodium lactate and NaY, and a decrease in the amount of polylactate species. The influence of dispersion could be explained by a better stabilizing effect of the welldispersed surface phosphate species on the carboxylate group of LA and AA.121 3.3.6. Brief Summary. Section 3.3 considers dehydration of LA via the α-hydroxyl group to produce acrylic acid as the target product. The desired catalyst properties for selective synthesis of acrylic acid from lactic acid are reported to be quite different, requiring both high acidity and high basicity. The best yield in convertion of LA to AA was 68% using a mixture of Na2SO4 and CaSO4 as a catalyst in a fixed-bed reactor. The role of calcium and sodium was ascribed to protection of the carboxylic group via formation of lactate salt, thus inhibiting a side decarbonylation/decarboxylation reaction. In the case of ML transformation to AA, the best result was observed over a composite catalyst, Ca3(PO4)2−Ca2(P2O7), with a moderate acid−base strength, giving 91% conversion of ML and combined selectivity for MA and AA of 80% at 390 °C. The enhancement in acrylic acid yield can be provided by an excessive CO2 presence, inhibiting decarbonylation/decarboxylation and hereby improving dehydration selectivity. Dehydration of LA as well as alkyl lactate is an endothermic reaction, and hereby the thermodynamics favors higher reaction rates for ML conversion at higher temperature. On the other hand, application of high temperatures leads to enhancement of side reactions and secondary transformations of acrylates. Generally, higher initial concentrations of the substrate are preferred from a practical viewpoint due to energy savings; however, in LA dehydration presence of water in diluted feedstock could accelerate the surface renewal of catalysts, thus inhibiting coke formation.

Figure 69. Yields of acetaldehyde and propanoic acid and conversion of lactic acid over different 20 wt % HPA supported on SBA-15 catalysts at 275 °C in a fixed-bed reactor using a 20 wt % aqueous solution of lactic acid as a feed. Adapted from Katryniok et al.140

other hand, H4PVMo11O40 and H3PMo12O40 have shown a lower conversion than the above-mentioned catalyst under the same reaction conditions. The reason for this is the lower acidity of molybdenum-containing catalysts compared to tungsten-based ones. Lower acid site density was also observed for CARiACT silica due to its lower specific surface area compared to that of SBA-15. The applied supports, CARiACT silica and SBA-15, exhibited specific surface areas of 272 and 541 m2/gcat, respectively. It should also be pointed out here that both catalysts kept their original activity and selectivity during 5 h time-on-stream experiments. The properties of supported HPA/SBA-15 are of interest due to its promising catalytic performance. When a 20 wt % loading of HPA was used, the specific surface area of the catalyst was 20% lower than that of the bare support. Furthermore, it was confirmed by solid-state NMR that silanol groups were covered by silicotungstic acid. The surface coverage of silanol groups AV

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calculated by comparing their intensities for bare and HPA (heteropolyacids) supported on SBA-15 was about 17%. From a mechanistic point of view, an interesting observation was that molybdenum-containing catalysts preferred decarboxylation, whereas the decarbonylation pathway was prominent for tungsten-containing catalysts. When vanadium was added to the heteropolyacid, its redox character increased, enhancing at the same time the formation of propanoic acid. From a practical point of view, the preferable catalyst recommended for the process would be silicotungstic acid supported on CARiACT silica due to its lower price. In summary, very few studies have dealt with decarboxylation and decarbonylation of lactic acid to acetaldehyde. Preferable feedstock for this reaction is lactic acid, and the best catalyst for production of acetaldehyde seems to be silicotungstic acid (Keggin-type heteropolyacid catalysts). Several types of supported heteropolyacids, for example, H4SiW12O40/silica CARiACT and H4SiW12O40/SBA-15, demonstrated high activity and selectivity for production of acetaldehyde, giving 80% yield at 92% conversion and 83% yield at 85% conversion, respectively.

Figure 70. Effect of the molar ratio of alcohol to lactic acid on the final conversion of lactic acid: (▲, solid line) ethanol, catalyst Amberlyst 15,28 (△, solid line), ethanol, catalyst Amberlyst 15,17 (●, dotted line) 2-propanol, catalyst Amberlyst 15, (+, dashed line) 1-butanol, catalyst ion-exchange resin,36 (×, dashed−dotted line) isobutanol, catalyst ionexchange resin.36 The reaction temperature was 80 °C.

3.5. Esterification of Lactic Acid

Esterification of lactic acid can be performed using either homogeneous or heterogeneous catalysts as well as with the aid of enzymes.181 The emphasis in this review is, however, on the use of homogeneous and heterogeneous chemical catalysts. Typical homogeneous catalysts are hydrochloric and sulfuric acid,33,37 p-toluenesulfonic acid,21 and chlorosulfonic acid.35 In addition, phosphoric acid and methanesulfonic acid have also been applied.37 They are, however, difficult to separate from the reaction solutions and cause corrosion problems.30 Several types of heterogeneous catalysts have been used, for example, ion-exchange resins29,33 and clay- and resin-supported heteropolyacids.24 The main parameters which have been studied are the molar ratio of alcohols to lactic acid,17,18,30,36 temperature,17,18,20,26,29−31,35,36,42 initial lactic acid concentration,30 and catalyst loading.18,24,29−31,35,36,42 In the case of solid catalysts, catalyst reuse has also been investigated.18,24,31,45 Furthermore, the effect of an additional solvent has been studied.26 Since esterification is limited by equilibrium, several publications can be found on shifting it by means of evaporation,19−21,23,28,38,182 continuous operation,37,183 reactive distillation,29,34,39 or using biphasic solvent systems.43 3.5.1. Effect of the Feed. Esterification of lactic acid has been investigated with different alcohols, methanol,26,34,43 ethanol,20,23,24,28,30,32,33,42,46,190 2-propanol,35 isobutanol,33,36 1-butanol,36and benzyl alcohol,33 using either homogeneous or heterogeneous catalysts. The lactic acid conversion typically decreases with increasing alcohol chain length, which is clearly visible from the comparative results using ethanol, 2-propanol, 1-butanol, and isobutanol as alcohols. The corresponding lactic acid conversions with a 1/1 alcohol to lactic acid molar ratio at 80 °C using an ion-exchange resin as a catalyst are 50%,17 20%,35 17%36 and 16%,36 respectively. The results using other molar ratios are shown in Figure 70. 3.5.2. Effect of the Catalyst Type and Catalyst Regeneration. 3.5.2.1. Effect of the Catalyst Type. Mostly heterogeneous catalysts have been applied in the esterification of lactic acid, such as ion-exchange resins,24,31,33,36 but in addition other polymeric sulfonic acid catalysts have been applied26 together with Preyssler acid18 and HNbMoO6.33 Ion-exchange

resins are typically made of cross-linked poly(vinylbenzene),19 and their maximum allowed operating temperature is recommended to be 120 °C.20 Ion-exchange resins have been extensively used as catalysts in the esterification of lactic acid with different alcohols (Table 30).31 Typically, they exhibit a relatively large specific surface area, except perfluorinated sulfonated resin Nafion 50R (Table 31). Amberlyst 15 is a macroreticular polymer of styrene and divinylbenzene, whereas Dowex 50WX2 is a gel type. Typically this catalyst swells in hydrophilic reaction medium. Some of the sulfonic acid groups can be dissolved. Water has been confirmed to be strongly adsorbed on ion-exchange resins, since FTIR measurements for the spent ion-exchange resin catalyst showed that water is more strongly adsorbed compared to lactic acid or a CO group containing ester (Figure S6, Supporting Information).199 During esterification, typically the reaction rate becomes lower, since the adsorbed water prevents other molecules from adsorbing on the active sites.30 Furthermore, water can solvate, ionize, or dissociate acidic protons of the sulfonic group.30 Comparative studies have been done in the esterification of lactic acid using different ion-exchange resins, such as Amberlyst 15, Amberlyst 36, Dowex 50WX8-200, and Dowex 50WX2-200 (Figure 71).31 In lactic acid esterification both macroreticular and gel-type resin have been applied as catalysts. Ion-exchange resins swell in polar media, and their thermal stability is limited (Table 31). Macroreticular resins, such as Amberlyst 15, typically have large specific surface areas compared to the gel-type resins (Figure S7, Supporting Information), for example, S-100 (Table 31). An image of gel-type resin Dowex 50WX2 is shown in Figure S8 (Supporting Information).184 Gel resins have micropores and very small specific surface areas.185 A schematic image of microand macroporous resin swelling is shown in Figure S7.186 Typically, there are some differences in the reaction rates, but equilibrium conversions are expected to be the same. For example, Amberlyst 15 exhibited a higher rate compared to Amberlyst 36,31 which might be explained by the fact that the AW

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Table 30. Data from the Esterification of Lactic Acid with Different Alcohols and Catalysts entry

catalyst

conditions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Amberlyst 15 Amberlyst 36 dimethyl acrylate sulfite p-toluenesulfonic acid Amberlyst XN-1010 Amberlyst 15 Amberlyst 15 Amberlyst 15 002 NKC Preyssler acid H3PW12O40·H2O H3PMo12O40·H2O Lewatit-S100 H0.9Nb0.9Mo1.1O6 Amberlyst 15 Nafion NR50 HNbMoO6 Amberlyst 15 Amberlyst 15 TiO2−ZrO2 TiO2−Al2O3 ion-exchange resin 12-tungstophosphoric acid ion-exchange resin Nafion NR50 HNbMoO6 Amberlyst 15

42−80 °C

a

85 °C 62−90 °C 50−90 °C 60−88 °C 60−88 °C 70−85 °C 70 °C 70 °C 70 °C 50−80 °C 70 °C 70 °C 70 °C 45−96 °C 170 °C 170 °C 60−90 °C 120 °C 60−90 °C 70 °C 70 °C 70 °C

alcohol

Keq

alcohol to lactic acid molar ratio

Ea (kJ/mol)

methanol

3

48.52

methanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol 2-propanol 1-butanol 1-butanol 1-butanol 1-butanol 1-butanol 1-butanol 1-butanol butanol 2-butanol benzyl alcohol benzyl alcohol benzyl alcohol

3

49.08

1.3

TOF (h−1)

2.88 48 49.98 47 51.58 52.26 47.11

3 3 1 1 1 11.4 1.5 2 2 2 1.5 12.7 12.2 3

20.5a 22.00 16.9b 18.5b 4.8b 53.4 26.2c 16.6c 8.63 (80 °C) 8.86 (80 °C)

3 2 2 2

54.34 Y, 30.6 65.57 16.2b 11.7b 4.6b

ref 30 30 26 21 20 17 30 182 199 199 18 24 24 24 32 35 33 33 33 29 45 44 36 195 36 33 33 33

After 2 h. bAfter 5 h. cAfter 1 h.

Table 31. Properties of the Heterogeneous Catalysts Applied in the Esterification of Lactic Acid catalyst

specific surface area (m2/gcat)

Amberlyst NX-1010 Amberlyst 15 Amberlyst 36 Weblyst D80 Weblyst D009 20% SO3−poly(EGDMA-VTAZ) S-100 gel type HPA-S-100 Nafion NR50 H0.9Nb0.9Mo1.1O6

540 77 33 25−40 25−40 220 0.5 2 0.02 12

pore size (nm) 56 24 200−400 200−400 3.4

particle size (mm) 0.2−1.2 0.4−0.6 0.3−1.25 0.3−1.25 2.5−3.0

max temp (°C) 120 100 150 120 130

280

pore size of the former is larger (Table 31).187 Nafion NR50 catalyst exhibited higher TOF than Amberlyst 15 in the esterification of lactic acid with 1-butanol and benzylalcohol (Table 30, entries 17 and 19 and entries 26 and 28).33 It has high acidity with pKA = −6,188 whereas the pKa measured in water for Amberlyst 15 is 0.70.189 The conversions after 5 h were, however, slightly higher with Amberlyst 15 than with Nafion 50R for the same amount of catalyst, although the latter one has only about 19% Amberlyst 15 acid capacity (Table 31). Ion-exchange resin catalysts have also been reused in the esterification of lactic acid (see section 3.5.2.2). Ion-exchange resin supported heteropolyacid catalysts were more active than only the resin.24 Heteropolyacids with a Keggin structure were bonded to the resin surface via a hydrate linkage.24 The ion-exchange resin S-100-H+ with three different loadings of tungstophosphoric acid, namely, 2, 10, and 20 wt %,

acid concn (mmol of H+/gcat)

ref

4.7 5.4 4.9 5.0 4.39 3.3 4.5−4.7 0.89 1.55

20 46 187 34 36 26 24 24 196 32

was applied in the esterification of lactic acid with ethanol at 70 °C. The reaction rates increased with the HPA loading.24 When S-100-Na was used as a support, the supported HPW and HPMo catalysts were inactive, indicating that protons are needed. Teh HPW Keggin structure has less accessible protonated oxygen than HPMo, and higher reaction rates were observed for the latter catalyst, HPMo-S-100, which is also reusable (Figure 72) despite slight deactivation.24 Furthermore, the solubility of HPMo is lower than that of HPW in the reaction solution. An optimum loading of HPA was 2 wt %, and the proton efficiency did not increase with increasing HPA loading. The possible explanation might be the interaction of HPA with the resin, which limits its swelling capacity.186 Although ion-exchange resins have been extensively used in esterification reactions, they suffer from some drawbacks, such as deactivation due to strong adsorption of water.26 AX

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Figure 71. Lactic acid conversion in its esterification with methanol at 80 °C using a methanol to lactic acid ratio of 3 and different ionexchange resins as catalysts. Reprinted from ref 31. Copyright 2002 American Chemical Society.

Figure 73. (a) Amounts of acidic and basic sites as a function of the concentration (mol %) of TiO2 in TiO2−ZrO2. (b) Turnover frequency in the esterification of lactic acid with 1-butanol at 170 °C after 1 h of reaction. Reprinted with permission from ref 45. Copyright 2011 Elsevier.

Figure 72. Reuse of HPMo heteropolyacid supported on Lewatit S-100 catalyst in the esterification of lactic acid with ethanol at 70 °C with a molar ratio of alcohol to lactic acid of 1/1: (■) fresh catalyst, (□) two cycles, (●) three cycles, (o) four cycles. Adapted from Engin et al.24

sites,44,45 and their amounts vary with increasing amount of TiO2 (Figure 73a). For TiO2−Al2O3 catalysts the highest catalytic activity was achieved for TiO2−Al2O3 with a Ti/Al ratio of 1/1, being 94% at 170 °C. The corresponding TOF values are given in Table 30, entries 21 and 22.45 This catalyst of an amorphous structure exhibited the largest specific surface area of 268 m2/gcat. It was also pointed out that typically TiO2− Al2O3 oxides have Lewis acidity with Ti/Al < 1 and Brønsted acidity with Ti/Al = 1. During esterification reaction, Lewis acids are converted to Brønsted acids in the presence of water at high temperature.172 The latter acids are proposed to be the active catalytic species during esterification of lactic acid according to the following reaction mechanism:167

Furthermore, the thermal stability of ion-exchange resins is limited (Table 31), and some sulfonic acid groups can be dissolved.167 An alternative catalyst to ion-exchange resins is sulfonic acid-functionalized poly(ethylene glycol dimethacrylate−1-vinyl-1,2,4-triazole) (poly(EGDMA−VTAZ(−SO3H))). It was tested with two different concentrations of sulfonic acid groups, namely, 10 and 20 wt %, as a catalyst in the esterification of lactic acid with methanol at 60 °C using a molar ratio of 3/1 of methanol to lactic acid.26 A schematic picture of this catalyst, which exhibits a high specific surface area (Table 31, entry 6) is shown in Figure S9 (Supporting Information). The sulfur contents and acid capacities of 10 and 20 wt % (poly(EGDMA−VTAZ(−SO3H))) were 9.59 and 12.82 wt % and respectively 3.05 and 4.39 mmol/gcat.25 The results were somewhat unexpected, since a higher esterification rate was achieved with 10 wt % catalyst compared to 20 wt % catalyst. The explanation is in more pronounced deactivation of 20 wt % catalyst due to its hydrophilic nature. Mixed oxides, which have enhanced thermal stability compared to ion-exchange resins, have been used as catalysts in the esterification of lactic acid with 1-butanol. For example, TiO2−ZrO245 and TiO2−Al2O344 have both acidic and basic

+H +

CH3CH(OH)COOH ←→ ⎯ CH3CH(OH)C+(OH)2 +C4 H 9OH

←⎯⎯⎯⎯⎯⎯⎯→ CH3CH(OH)C(OH)(OH 2+)(OC4H 9) −H3+O

←⎯⎯⎯→ CH3CH(OH)COOC4H 9

(13)

A carbenium ion formed during a proton transfer to lactic acid oxygen is attacked by oxygen in 1-butanol, releasing H3+O and forming an ester. For TiO2−ZrO2 maximum amounts of acidic and basic sites were determined for a Ti/Zr ratio of 1/1 (Figure 73a). AY

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It should also be pointed out here that the specific surface area followed an analogous trend, with the acidic and basic site density exhibiting a maximum value with 50 mol % TiO2 in ZrO2.45 XRD measurements revealed that the mixed oxides TiO2−ZrO2 were amorphous whereas pure oxides were crystalline.45 Furthermore, it was observed that teh turnover frequency increased with increasing acid site density for TiO2− ZrO2 catalysts (Figure 73b).45 The highest reaction rates and yields of butyl lactate were achieved for TiO2−ZrO2 with a Ti/ Zr ratio of 3/1 and for TiO2−Al2O3 with a Ti/Al ratio of 1/1, being 94% at 170 °C.45 Due to the high reaction temperature, which could be applied because of high mixed oxide thermal stability, a higher equilibrium yield of butyl lactate could also be achieved. These catalysts have also been regenerated and reused (see section 2.1.2.3 and section 2.2.2.3). Layered nonstoichiometric oxides are active catalysts for esterification of lactic acid with ethanol,32,33 1-butanol, isobutanol, and benzyl alcohol.33 For example, the catalyst with the highest activity in the esterification of lactic acid with ethanol was H0.9Nb0.9Mo1.1O6, which exhibited 13.2 Å basal spacing and 6.1 Å interlayer spacing.32 Another mixed oxide, HNbMoO6, was tested in the esterification of both propionic acid and lactic acid. The result was very interesting, showing that this mixed oxide was unable to intercalate carboxylic acids, whereas it is very active as a catalyst for lactic acid esterification. It was concluded that the hydroxyl group in lactic acid is crucial, enabling intercalation of it inside the mixed oxide,33 and that such intercalation is thus related to catalyst activity. When the yields of different lactates in the esterification of lactic acid with three different alcohols, ethanol, 1-butanol, and benzyl alcohol, were compared at 70 °C using a ratio of alcohol to ester of 5, they declined as HNbMoO6 > Amberlyst 15 > Nafion NR50,33 showing that the mixed oxide was highly active in the studied reactions. Furthermore, yields of different lactate esters declined over HNbMoO6 with the alcohol order 1-butanol > ethanol > isobutanol > benzyl alcohol, showing that there is an optimum size of the alcohol molecule that is favorable in using mixed oxide as a catalyst.33 3.5.2.2. Regeneration and Reuse of Different Heterogeneous Catalysts. Ion-exchange resins are typically washed prior to their reuse.30 With washing, catalyst poisons can be removed.31 Furthermore, ion-exchange resins are dried at 99 °C, as their desulfonation starts at 120 °C.29 It has been shown that no catalyst deactivation occurred for Amberlyst 15 in the esterification of lactic acid with ethanol at 70 °C in three consecutive experiments (Figure 74).30 Heteropolyacids supported on an ion-exchange resin, such as Lewatit-S100, were successfully reused in five consecutive esterification experiments at 70 °C using a 1/1 molar ratio of lactic acid to ethanol, although some deactivation took place (Figure 72).24 This deactivation was caused by either washing out some of the heteropolyacid or dissolution of sulfonic acid groups. Furthermore, it was shown that the deactivation of H3P12O4·H2O was less prominent than that of H3PMo12O40·H2O. On the other hand, Preyssler catalyst, composed of Keggin anion [PW12O40]3−, was stable and reusable in lactic acid esterification. Furthermore, its structure was shown by FTIR measurements to remain unchanged.18 Mixed oxides, such as TiO2−ZrO2, can be regenerated via calcination in air after an esterification reaction, and their activity can be restored after regeneration.45 Analogously, TiO2−Al2O3 with a 1/1 atomic ratio of Ti/Al could also be

Figure 74. Reuse of Amberlyst 15 ion-exchange resin in lactic acid esterification at 70 °C with a molar ratio of ethanol to lactic acid of 1.82. Reprinted from ref 30. Copyright 2008 American Chemical Society.

reused up to six times in lactic acid esterification at 140 °C with 1-butanol in the absence of any significant deactivation.44 3.5.3. Effect of the Reaction Conditions in the Esterification of Lactic Acid. 3.5.3.1. Effect of the Alcohol to Lactic Acid Ratio. An excess of alcohol shiftd the equilibrium in the esterification of lactic acid toward the product side; thus, higher yields of esters can be achieved.17,30,35,36 On the other hand, product separation costs are then increased.20 The results of several esterification studies using different molar ratios and different alcohols are summarized in Figure 70. Typically, an increase of lactic acid conversion is maximally about 30% when the amount of alcohol is doubled. The structure of the alcohol also determines the conversion of lactic acid, as can be clearly seen in Figure 70; the longer the alcohol chain, the lower the lactic acid conversion (see section 3.5.1). There are, however, some exceptions illustrating conversion of lactic acid with different alcohols is too low compared to the results shown in Figure 70. In the esterification of methanol the curve (not shown here) displaying the conversion of lactic acid as a function of the methanol to lactic acid molar ratio at 80 °C using Amberlyst 15 falls between the curves of isobutanol and 1-butanol in Figure 70.31 Furthermore, low conversions were also observed for Preyssler catalyst in the esterification of lactic acid with ethanol (Figure 75).18 When a large excess of alcohol is used, the equilibrium conversion can be close to 100%. This was the case, for example, in esterification of lactic acid with

Figure 75. Effect of the alcohol to acid molar ratio on acid conversion in the esterification of lactic acid with ethanol at 85 °C using supported Preyssler catalyst: alcohol to acid molar ratio 1 (■), 2 (□), 4 (▲), 6 (○), and 8 (×). Adapted from ten Dam et al.14 AZ

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1-butanol using Amberlyst 15 as a catalyst at 120 °C. The conversion of lactic acid with a molar ratio of 12 for 1-butanol to lactic acid was 99% after 5 h, giving a yield of butyl lactate of 89%.45 3.5.3.2. Effect of the Initial Concentration of Lactic Acid. High initial concentrations of lactic acid would be preferable for industrial production of lactic acid esters. There are still some drawbacks of using high lactic acid concentrations, since lactic acid easily formd dimers, trimers (Figure 76), and even

ratio of lactic acid to ethanol42 and 80 wt % lactic acid as a reagent without the mention of anything about the presence of oligomers. In the above-mentioned example, 36 wt % lactic acid was present in the initial mixture, indicating that HPLC analysis of dimers would have been beneficial. Furthermore, it shows that all published data from lactic acid esterification should be critically interpreted. Esterification of concentrated lactic acid solution involves esterification of dimers and trimers as well. The reaction network becomes more complicated and the reaction proceeds more slowly due to consecutive hydrolysis of dimers and trimers. Thereafter a monomeric ester is formed. A typical kinetic plot from the esterification of lactic acid with ethanol is depicted in Figure 76.30 The final mixture contains in addition to ethyl lactate also traces of dimeric and trimeric esters. 3.5.3.3. Effect of Temperature. The effect of temperature in lactic acid esterification has been extensively investigated.17,18,20,26,29−31,35,36,42 The heat of the esterification reaction is small, and therefore, the effect of temperature on the equilibrium constant is quite minor.31 Lactic acid is a relatively strong acid with a pKa of 3.85,34 and thus, it can also be esterified in the absence of any additional catalyst. The activation energy for self-catalyzed esterification of lactic acid with methanol has been determined to be 56 kJ/mol, whereas for ion-exchange resin catalyzed esterification it is 49 kJ/mol (Table 30, entries 1, 3, and 6).31 Furthermore, the activation energies reported for esterifications with methanol and ethanol are about the same, whereas they increase when 1-butanol or isobutanol is used as the alcohol (Table 30, entries 20 and 25). This trend has, however, one exception: the reported activation energy in lactic acid esterification with isopropanol is very low (Table 30, entry 16).35 The equilibrium conversion is typically not affected very much by a change in temperature.42 There are also several publications from which it is difficult to assess the effect of temperature on the equilibrium yield since experiments have been stopped too early before the constant conversion level has been achieved.31,36 In some cases, however, the equilibrium conversion of lactic acid increased with increasing temperature, for example, in the esterification of lactic acid with ethanol using a molar ratio of alcohol to lactic acid of 1/1 with Preyssler catalyst (Figure 77).14 The temperature dependence of the equilibrium constant for esterification of lactic acid with ethanol using Amberlyst 15 as a catalyst and varying the molar ratio of ethanol to lactic acid and temperature in the ranges of 1−2.84 and 50−89 °C, respectively, was modeled, giving the following equation for the equilibrium constant:30

Figure 76. Molar fractions of oligomers formed in the esterification of lactic acid with ethanol at 90 °C. The molar ratio of ethanol to lactic acid was 3. Reprinted from ref 30. Copyright 2008 American Chemical Society.

Table 32. Composition of a Lactic Acid Solution with Its Different Nominal Concentrations in Water17 nominal loading 20 wt % lactic acid 23 wt % (66 mol %) feed component L1 L2 L3 L4 77 wt % (94.4 mol %) H2O monomer 2.6 M equivalent

20 wt % lactic acid 20 wt % lactic acid 46 wt % (15 mol %) 58 wt % (43 mol %) 3 wt % (0.5 mol %) 22 wt % (9 mol %) 6 wt % (2 mol %) 2 wt % (0.4 mol %) 51 wt % (84 mol %) 12 wt % (45 mol %) 6M 10.8 M

tetramers.17,30,54 The amounts of oligomers for different initial concentrations of lactic acid are shown in Table 32. For 20 wt % lactic acid solution no dimers are present,17,31,36,54 whereas for 88 wt % lactic acid−water solution the fraction of dimers, trimers, and tetramers is already 11.4 mol %. Esterification of lactic acid with an 88 wt % concentration has been demonstrated, since it is still miscible in water.17 High lactic acid concentrations give challenges in the analysis of the reaction mixture. Typically, the presence or absence of oligomers is quantified by HPLC,17,23,29,182,183 whereas lactic acid and monomeric ester are analyzed by GC. In some cases, if no HPLC method was available, the presence of dimeric esters was calculated after hydrolysis and titration, giving the total acidity.24 In addition to the above-mentioned GC data for diluted solutions, no extra analyses are needed.32,33 GC can qualitatively show the presence of some heavy compounds, which might be dimers.44 In some cases, however, only GC has been applied as an analytical method using, for example, a 1/3

ln K = 2.9625 − 515.13/[T (K)]

(14)

In the studied temperature range eq 14 gives the lowest and the highest values of K as 3.99 and 4.68. Concentrated lactic acid contains in addition lactic acid dimers and trimers, which can also form esters. Activation energies have also been determined for monomeric, dimeric, and trimeric esters,182 and their corresponding values were 47, 72, and 93 kJ/mol, respectively, when esterification of lactic acid was performed with ethanol and Amberlyst 15 as a catalyst. Esters can, however, be hydrolyzed, and only small amounts of dimeric and trimeric esters are present in the final mixture (Figure 76). 3.5.4. Engineering Aspects in the Esterification of Lactic Acid. 3.5.4.1. Shifting the Equilibrium To Achieve High Yields of Esters. Several different methods in addition to use of an alcohol excess, which was described in section 3.4, BA

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Figure 78. Comparison of lactic acid conversion between conventional reactive distillation and reactive distillation using water side draw as a function of the molar ratio of methanol to lactic acid. Reprinted with permission from ref 34. Copyright 2009 Elsevier. Figure 77. Esterification of lactic acid with ethanol using a 1/1 molar ratio of ethanol to lactic acid over Preyssler catalyst: (▲) 70 °C, (○) 75 °C, (□) 80 °C, (■) 85 °C. Adapted from ref 18.

modified with FeCl3. Butanol−water azeotrope was continuously removed from the top of the column. Thereafter, the light butanol fraction was recycled into the reactor, whereas the heavy water phase was removed.39 The reaction mixture was distilled after the reaction at a reduced pressure, with butanol taken as the top fraction at 80 °C and 27 kPa and butyl lactate obtained at 105 °C under 6.4 kPa of pressure. In permeation processes, typically hydrophilic membranes are applied, for example, cationic polyelectrolytes21 and zeolites.182 Since the latter are acid sensitive, those which are less acid sensitive, for example, zeolite T, have been applied in lactic acid esterification with vapor permeation. Some of the studies combining reaction with permeation have been performed using concentrated lactic acid but ignoring the formation of oligomers,22,30 which can affect the interpretation of the results, since total acidity measurements by titration cannot separate monomer and oligomer concentrations. A comparative work using both diluted lactic acid and 50 wt % lactic acid as reactants has been published23 taking into account formation of oligomers by analyzing them with HPLC. Esterification of lactic acid combined with vapor permeation has been intensively studied, and the main characteristics of the processes are summarized in Table 33.23,28,46 Membranes should exhibit high chemical and thermal stability as well as mechanical strength.48 Zeolite polyelectrolyte membranes have been applied in lactic acid esterification with pervaporation.46 It has been found that it is easier to use heterogeneous catalysts than homogeneous ones, since the latter easily attack the membranes and are difficult to separate from the product mixture.46 The formation of oligomers was studied in the esterification−pervaporation of lactic acid with ethanol using a molar ratio of ethanol to lactic acid of 2.23 The reaction was performed at 75 °C, whereas the permeation occurred at a slightly lower temperature, 69 °C. A stainless steel permeation cell was used as a membrane, and a lowered pressure of 0.1 Pa was applied on the permeate side. The results revealed that when comparing the process using either 20 or 50 wt % lactic acid in the feed, less water was removed from the latter one due to its lower water concentration in the reaction mixture.23 The kinetics for formation of ethyl lactate together with other products in the esterification−pervaporation process is shown in Figure 79.23 A zeolitic membrane, NaA, supported on carbon/zirconia was applied in the esterification−pervaporation of lactic acid

have been applied in shifting the equilibrium to the product side in the esterification of lactic acid. The idea in combining the reaction with separation is to increase productivity and decrease the production costs.23 These methods are a combination of the reaction with vapor permeation,21,46 reactive distillation,29,39 reactive distillation combined with a side draw,34 and the use of biphasic solutions together with Brønsted acidic ionic liquids.43 Although reactive distillation has been applied for esterification of lactic acid in the production of methyl,34 ethyl,39 and butyl lactates,29 due to the low volatility of lactic acid, the process has some challenges. These processes should, however, be developed separately, since different alcohols and esters have different boiling points, and in some cases even azeotropes are formed, for example, 1-butanol and water.29 Reactive distillation facilitates higher yields of ester using only a smaller excess of alcohol.34 Reactive distillation of lactic acid with methanol has been performed in a column containing, for example, a Sulzer Katapak element in the lower reactive section filled with Amberlyst 15.34 The stripping section is located above the reactive section. Reactive distillation combined with a side draw, in which water, having an intermediate boiling point compared to other compounds (the boiling points of methanol, methyl lactate, and lactic acid are 64.7, 144, and 224 °C, respectively), is removed, facilitates the use of a smaller excess of alcohol to give higher yields of methyl lactate (Figure 78).34 In butyl lactate synthesis using reactive distillation with Amberlyst 15 as a catalyst with a butanol to lactic acid molar ratio of 2, the temperature is increased from 90 to 140 °C during the course of the reaction. The initial concentration of lactic acid was 30 wt %, and a conversion of 92% was achieved within 5 h.29 In this case, lactic acid was fed at the top, whereas 1-butanol was fed from the bottom. The distillate contained 94 wt % water together with 6 wt % butanol, whereas the bottom fraction was composed of 60 wt % butyl lactate, 39 wt % butanol, and traces of water. It was also pointed out that the extent of oligomerization was minor in the proposed process. Reactive distillation was also applied in the esterification of ammonium lactate with butanol with a molar ratio of butanol to NH4LA of 3/1 in the presence of an ion-exchange resin BB

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Table 33. Permeation Membranes Applied in the Esterification−Permeation of Lactic Acid molar ratio of alcohol to lactic acid, catalyst ethanol to lactic acid 1.2, Amberlyst XN-1010 ethanol to lactic acid 2, p-toluenesulfonic acid ethanol to lactic acid 2, Amberlyst 15 ethanol to lactic acid 2, p-toluenesulfonic acid ethanol to lactic acid 3, Amberlyst 15 ethanol to lactic acid 2.4, Amberlyst 15

conditions 1.37 kPa vacuum 70 °C membrane temperature 100 Pa 10 Pa 80 °C,