Furfural: A Promising Platform Compound for ... - ACS Publications

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Furfural: A Promising Platform Compound for Sustainable Production of C4 and C5 Chemicals Xiaodan Li, Pei Jia, and Tiefeng Wang* Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Furfural is a promising renewable platform compound derived from lignocellulosic biomass that can be further converted to biofuels and biochemicals. The highly functionalized molecular structure of furfural makes it a desired raw material for the sustainable production of value-added chemicals containing oxygen atoms. The conversion of furfural to C4 and C5 chemicals by various catalytic processes is reviewed. The C5 chemicals are mainly produced through sequential steps of selective hydrogenation and/or hydrogenolysis, while most of the C4 chemicals are synthesized with selective oxidation as the first step. This review divides the chemical products from furfural into several groups according to their carbon numbers and synthesis routes, with emphasis on the catalysts and reaction mechanisms. The applications of these chemicals and their traditional production from fossil feedstocks have also been added as background information. Additionally, recent advances in the development of heterogeneous catalysts for furfural production are briefly reviewed. KEYWORDS: furfural, platform compound, C4 chemical, C5 chemical, reaction pathway, catalyst contents of water and oxygenates.14 Further processes, such as Fischer−Tropsch synthesis, the water-gas shift reaction, methanol synthesis, catalytic aromatization, and hydrodeoxygenation, are necessary to upgrade the primary biofuels to the desired range by reducing the oxygenated contents and raising the energy contents.6,15−17 In contrast to gasification and pyrolysis, acid-catalyzed hydrolysis is a more complicated process that deconstructs lignocellulose into a series of C5 −C 6 sugars,1,5,18 which can be further rearranged with partial removal of their oxygen atoms to form various renewable platform compounds such as furfural, 5-hydroxymethylfurfural (HMF), lactic acid, and levulinic acid.1,19 Further catalytic conversion of these functionalized platform compounds allows the production of a wide range of chemicals and fuels. Although the sequential conversion process via platform compounds is more complex and expensive than the one-step processes (i.e., gasification and pyrolysis), it is the most promising way to produce high-quality biofuels and valuable chemicals. Furfural has been selected as one of the top 30 biomassderived platform compounds by the U.S. Department of Energy on the basis of several indicators such as the raw material, estimated processing cost, technical complexity, and market potential.19 Furfural is mainly produced by hydrolysis and dehydration of xylan, which exists in large quantities in

1. INTRODUCTION Humanity is now faced with the challenges of the increasing demand for fuels and chemicals driven by global population growth and diminishing fossil resources, energy security due to political issues, and environmental problems caused by CO2 emissions and the resulting global warming.1,2 To solve these, various forms of renewable energy resources have been explored to develop sustainable processes. Biomass, a renewable nonfossil carbon energy source, is regarded as an ideal alternative to traditional fossil resources because it is environmentally friendly and abundant. In recent decades, great interest has been devoted to the production of biofuels and biochemicals using nonedible lignocellulosic biomass, which is abundant in agricultural residues and waste streams.3,4 The use of lignocellulosic biomass avoids the food-versus-fuel debate and can potentially significantly reduce CO2 emissions. It is one of the most promising options for the green and sustainable production of fuels and chemicals. Lignocellulosic biomass mainly contains cellulose, hemicellulose, and lignin, which constitute 40−50%, 25−35%, and 15−20% of lignocellulose, respectively.1 The main approaches for the conversion of lignocellulosic biomass are gasification, pyrolysis, and hydrolysis, as summarized in Figure 1.5−9 When the simplicity and cost of the process are preferred versus the selectivity for target products, lignocellulosic biomass can be converted to syngas by gasification and bio-oil by pyrolysis.10−13 However, the biofuels directly obtained from gasification and pyrolysis are of poor quality because of their high acidity and high © 2016 American Chemical Society

Received: June 30, 2016 Revised: September 14, 2016 Published: September 28, 2016 7621

DOI: 10.1021/acscatal.6b01838 ACS Catal. 2016, 6, 7621−7640

Review

ACS Catalysis

Figure 1. Main approaches for lignocellulosic biomass conversion.

Figure 2. Catalytic conversion of furfural to fuel components and chemicals.

hemicellulose. The conversion of xylose/xylan into furfural is a well-explored process that was first industrialized in 1921 by the Quaker Oats Company.20 However, the current industrial production of furfural still uses traditional and inefficient technologies that are limited by a low yield of furfural, the use of corrosive homogeneous acid catalysts, high energy consumption, and severe pollution. To overcome these problems, many efforts have been devoted to enhancing furfural production by using heterogeneous acid catalysts and improving the reaction and separation efficiencies. The approaches to improve the reaction and separation efficiencies include solvent extraction in a

biphasic process, the introduction of ionic liquids as additives, and the supercritical carbon dioxide extraction technique,21−27 which are beyond the scope of the current review; readers are referred to related reviews on this topic.23,24,28 The recent progress in furfural production using heterogeneous catalysts will be briefly summarized in the current review, considering that heterogeneous catalysts are promising for enhancing furfural production, thus offering opportunities to explore more valuable downstream products for the furfural industry. Furfural can be converted by several catalytic processes, such as selective hydrogenation, oxidation, hydrogenolysis, and 7622


Review

ACS Catalysis Table 1. Heterogeneous Catalysts for Furfural Production from Xylose

a

entry

solvent

catalysta

temperature (°C)

furfural yield (%)

ref

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 29 30 31 32 33 34

H2O/MIBK H2O/toluene H2O/MIBK H2O/toluene H2O/toluene H2O DMSO H2O/toluene H2O H2O H2O H2O/toluene H2O/toluene H2O/toluene DMSO H2O/butanol H2O/MIBK H2O/toluene H2O H2O/toluene H2O H2O/toluene H2O/toluene H2O/toluene DMSO H2O/toluene DMSO H2O/toluene H2O/toluene DMSO DMSO H2O/butanol H2O/toluene H2O

HY-faujasite (15) HY-faujasite (15) H-mordenite (12) H-mordenite (12) HY-ferrierite (20) HY-ferrierite (20) HY-ferrierite (20) H-mordenite (13) H-ZSM-5 Sn-beta + HCl Sn-beta + Amberlyst SAPO-5 SAPO-11 SAPO-44 MCM-41 MCM-41 MCM-41-SO3H MCM-41-SO3H H-MCM-22 (24) H-MCM-22 (24) ITQ-2 (24) ITQ-2 (24) Nu-6(2) ITQ-18 H3PW12O40/MCM-41 H3PW12O40/MCM-41 Cs3PW12O40/MCM-41 Cs3PW12O40/MCM-41 SBA-15-SO3H Nafion 117 Amberlyst-15 SO42−/ZrO2−TiO2 SnO2 SGO

170 170 170 170 140 140 140 260 200 110 110 170 170 170 140 170 140 140 170 170 170 170 170 170 140 160 140 160 160 150 170 170 100 200

29 42 20 34 35 13 23 98 46 14 10 65 58 63 45 44 51 76 52 70 54 66 23 43 52 48 45 33 69 60 78 48 31 62

58 58 58 58 59 59 59 60 61 62 62 63 63 64 66 66 67 67 71 71 71 71 70 70 69 68 69 68 65 73 72 75 74 76

The values in parentheses are the Si/Al ratios.

decarboxylation,29−33 to a range of C4 and C5 molecules, which are important building blocks for both the production of liquid hydrocarbon fuels and fuel additives and the synthesis of valuable chemicals.1,19,34−36 During the past decades, the production of biofuels from furfural has received extensive attention. In a typical example of the transformation of furfural into fuels and fuel additives, furfural can be selectively hydrogenated to potential fuel components such as 2-methylfuran (2-MF) and 2-methyltetrahydrofuran (2-MTHF), which can be further upgraded to conventional fuels with an optional combination of aldol condensation, etherification, and hydrodeoxygenation.37−39 In addition to fuels and fuel additives, furfural can be converted to a variety of valuable C4 and C5 chemicals, such as valerolactone, pentanediols, cyclopentanone, dicarboxylic acids, butanediol, and butyrolactone.34 Most of the C5 chemicals are produced through sequential steps of selective hydrogenation and/or hydrogenolysis, while the C4 chemicals are mainly synthesized with selective oxidation as the first step. Figure 2 outlines the important fuel and chemical products derived from furfural. According to their applications, carbon numbers, and synthesis routes starting from furfural, these products are classified into three groups: (1) fuel components, (2) C5 chemicals, and (3) C4 chemicals.

Several excellent reviews of the conversion of furfural have been published, and most of them focused on the biofuel applications of furfural derivatives.1,6,8,9,29,30,33,35,36,40−44 For instance, Resasco et al.6 reviewed the production routes of different fuel components from furfural, while Lange and coworkers36 critically reviewed the fuel properties of the typical fuel components derived from furfural on the basis of their energy density, polarity, boiling point, and ignition characteristics. The current review focuses on the catalytic conversion of furfural to value-added chemicals. First, the recent progress on furfural production using heterogeneous catalysts is reviewed. A general comparison is made between the production of biofuels and biochemicals from furfural. Then the state-of-the-art advances in catalytic conversion of furfural to value-added C4 and C5 chemicals are reviewed, with emphasis on catalysts and reaction mechanisms. Finally, the review provides perspectives on the discussed C4 and C5 chemicals based on their application potential and competitiveness with production from fossil feedstocks.

2. FURFURAL PRODUCTION WITH HETEROGENEOUS CATALYSTS In the traditional processes, furfural is primarily produced from xylose/xylan in the presence of a homogeneous acid catalyst such 7623


Review

ACS Catalysis as HCOOH, CH3COOH, HCl, H2SO4, HNO3, or H3PO4.45−49 For instance, the typical process of Quaker Oats with dilute sulfuric acid as the catalyst has an ∼50% yield of furfural from xylan, and similar yields have been reported for most industrial processes.50,51 The moderate yield of furfural is mainly due to the formation of solid humins via undesired polymerization reactions.52,53 The use of homogeneous acid catalysts has the problems of difficult separation and reuse, severe corrosion, rigorous operating conditions, and high environmental and safety risks.54 Many efforts have been devoted to the development of heterogeneous catalysts to increase the yield of furfural and make the production process cleaner and greener. Table 1 summarizes the significant achievements in furfural production using heterogeneous acid catalysts. The most studied solid catalysts are zeolites, which have tunable acidities, excellent thermal and chemical stabilities, and shape selectivity.55−57 In the early stage, a variety of H-zeolites (e.g., HY-faujasite, H-mordenite, and H-ferrierite) were studied for dehydration of xylose using different solvents such as H2O, H2O/methyl isobutyl ketone (MIBK), H2O/toluene, and dimethyl sulfoxide (DMSO) in a batch reactor at 140−170 °C, but the yields of furfural, which heavily depended on the acidic and structural properties of the zeolite, were mostly lower than 40%.58,59 Employing a fixed-bed reactor, Lessard et al.60 increased the reaction temperature to 260 °C and found that the H-mordenite catalyst pretreated with H3PO4 gave a high furfural yield of 98% for the dehydration of xylose in H2O/ toluene. O’Neill et al.61 studied the dehydration of xylose in water using H-ZSM-5 as the catalyst, and a 46% yield of furfural was obtained at 200 °C. In addition to H-zeolites, Sn-beta zeolites and microporous silicoaluminophosphates (SAPOs) have also been extensively studied for the dehydration of xylose to furfural. For instance, combining Sn-beta zeolite with Amberlyst-15 or HCl, Choudhary et al.62 investigated the isomerization of xylose to xylulose and its subsequent dehydration to furfural in the aqueous phase. A series of SAPO catalysts, such as SAPO-5, SAPO-11, SAPO-40, and SAPO-44, have been tested for the dehydration of xylan/xylose to furfural in water/organic biphasic systems, and the yields of furfural reached 40−65%.63,64 Recently, modified mesoporous silicas with higher specific surface area and porosity, such as SBA-15 and MCM-41, have received more attention. Shi et al.65 reported the dehydration of xylose to furfural in the presence of SBA-15-SO3H, and a 68.3% yield of furfural was obtained in H2O/toluene. Unmodified MCM-41 exhibited a furfural yield lower than 40% in either DMSO or H2O/1-butanol, while MCM-41-SO3H had furfural yields of 51% and 76% in H2O/MIBK and H2O/toluene, respectively.66,67 When the polyoxometalates H3PW12O40 and Cs3PW12O40 were supported on MCM-41, the yields of furfural were 48% and 33%, respectively, in H2O/toluene and increased to 52% and 45%, respectively, in DMSO.68,69 The use of conventional microporous zeolites and mesoporous silicas as catalysts for furfural production from xylose/xylan is mainly limited by the internal diffusion. To overcome this problem, Valente and co-workers70,71 prepared a series of delaminated zeolites denoted as ITQ-2, ITQ-6, and ITQ-18 by swelling and ultrasonication of the zeolite precursors of MCM-22, ferrierite, and Nu-6(2). The delaminated zeolites possessed higher specific surface areas and porosities than the precursors, which favored the internal diffusion and desorption of xylose and furfural. Xylose can also be dehydrated to produce furfural over sulfonated metal oxides, sulfonated graphene oxide (SGO), and

ion-exchange resins. When ion-exchange resins such as Nafion SAC-13, Nafion 117, Amberlyst-15, and Amberlyst-70 were used as catalysts, moderate yields of furfural in the range of 40−70% were obtained.72,73 Suzuki et al.74 prepared a broad range of metal oxides (e.g.,TiO2, ZrO2, SnO2, and Al2O3) sulfonated with sulfuric acid and found that SnO2 gave the highest xylose conversion and furfural yield. Zhang et al.75 also proved that the mixed metal oxide catalyst SO42−/ZrO2−TiO2 performed better than the selected zeolite catalysts. Lam et al.76 reported that SGO gave a 61% yield of furfural from xylose. Related research found that the bare metal oxides and graphene oxide showed almost no activity for xylose dehydration to furfural, and thus, modification with sulfonated groups is necessary to enhance the catalyst acidity. Different reaction mechanisms for the production of furfural from xylose have been proposed,33,77−85 but most studies of the use of heterogeneous catalysts are based on the cyclodehydration mechanism involving the stepwise liberation of three molecules of water.33,46,81,84 This mechanism is usually employed for Brønsted acid catalysts, while some modifications are made to the steps leading to furfural over Lewis acid catalysts. As shown in Figure 3, the isomerization of xylose to xylulose turns out to be

Figure 3. Experimentally verified cyclodehydration mechanism for the dehydration of xylose to furfural over Brønsted acid and Lewis acid catalysts. Adapted with permission from refs 57 and 62. Copyright 2013 Elsevier and 2011 American Chemical Society, respectively.

the dominant pathway in the presence of Lewis acid sites, which has been verified by several experimental studies.62,86,87 For instance, Choudhary et al.62 reported that the Lewis acid zeolite mainly converted xylose to xylulose with almost no production of furfural and that the intermediate xylulose could be quickly transformed into furfural in the presence of Brønsted acid catalysts such as HCl and Amberlyst-15. As summarized in Table 2, the heterogeneous acid catalysts for furfural production from xylose mainly include different types of zeolites, ion-exchange resins, and sulfonated metal oxides. The properties of a heterogeneous catalyst, such as acidity (acid type, strength, and amount), structure (pore size and surface area), hydrophobicity, and stability, can significantly affect the catalytic performance. For acidity, Lewis acid sites can shift the reaction pathway to the xylose−xylulose−furfural route, which is faster than the direct xylose−furfural route catalyzed by Brønsted acid sites, while the presence of Brønsted acid sites is required to catalyze the dehydration of xylulose to furfural and improve the furfural yield.62 Therefore, the L/B (acid site) ratio directly affects the dehydration rate and furfural yield. The optimum L/B ratio is usually in the range of 30−80%. Higher L/B ratios lead to the formation of more carbonaceous byproducts due to high conversion rates, while lower L/B ratios enhance the undesired polymerization reactions.57,62,63,88 Increasing the acid content and strength properly can increase the reaction rate, but toostrong acidity also accelerates the undesired secondary reactions.72 The acidity of zeolite catalysts can be adjusted by 7624


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ACS Catalysis

Table 2. Properties and Catalytic Performances of Different Types of Heterogeneous Catalysts for Production of Furfural from Xylose entry

a

catalyst

aciditya

pore size (nm)

SBETb (m2/g)

furfural yield (%)

modification

300−500 500−800 300−700 800−1200 50−100 300−500 50−400 >50

20−50 10−50 40−70 40−80 20−70

adjusted composition combined with Brønsted acids adjusted composition decorated with polyoxometalates increased surface area

40−80 20−70

combined with Lewis acids sulfonated

1 2 3 4 5

microporous H-zeolites microporous Sn-beta zeolites microporous SAPOs mesoporous zeolites delaminated zeolites

L+B L L+B L+B L+B

0.3−1.5 0.6−0.8 0.3−0.7 3.0−5.0 −

6 7

ion-exchange resins and Nafion metal oxides

B L+B

>10.0 −

L denotes Lewis acid sites, and B denotes Brønsted acid sites. bBrunauer−Emmett−Teller specific surface area.

Table 3. Gas-Phase Hydrogenation of Furfural to FA over Non-chromium Catalysts entry a

1 2 3 4 5 6 7 8 9 10 a

catalyst

WHSV (h−1)

H2/furfural molar ratio

T (°C)

conv. (%)

FA yield (%)

ref

Cu2Cr2O5 Cu/SiO2 Cu/SiO2 Cu/ZnO Cu/MgO Cu−Co/SiO2 Cu−Ca/SiO2 Pd−Cu/zeolite Ni−Mg−Al Ni−Co−Al

52 2.3 0.5 0.5 4.8 3.1 0.33 (LHSV) 7.7 15.1 15.1

25 25 17 17 2.5 6 5.1 0.08 25 25

200 290 140 220 180 200 130 300 155 155

22 77 98 95 98 65 100 58 95 99

20 63 73 31 96 64 99 58 65 70

91 97 95 95 94 96 112 93 98 98

This Cu−Cr catalyst serves as a reference for the non-chromium catalysts.

Table 4. Liquid-Phase Hydrogenation of Furfural to FA over Non-chromium Catalysts entry

catalyst

solvent

H2 pressure (MPa)

T (°C)

conv. (%)

FA yield (%)

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Ru/UiO-66 Pd/SiO2 Pd−Cu/MgO Pt/C Pt−Sn/SiO2 Ir−ReOx/SiO2 Cu/Al2O3 Fe(NiFe)O4−SiO2 Cu−Fe oxides Cu−Co/SBA-15 Co/SBA-15 Ni−Fe−B Ni−Mo−B/Al2O3 CuNiMgAl oxides

H2O octane H2O butanol isopropanol H2O H2O heptane octane isopropanol ethanol ethanol methanol ethanol

0.5 − 0.6 8 1 6 2 2 6 2 2 1 5 1

20 − 110 175 100 50 90 90 200 170 150 100 80 200

95 75 100 99 100 100 81 94 87 99 92 100 99 93

94 53 99 50 96 97 81 93 84 80 88 100 90 83

101 104 105 102 103 113 99 100 106 107 108 109 110 111

another important property, as the strongly hydrophobic nature of the catalyst surface will lead to furfural-yield-loss reactions.57 The stability is also a key factor, as leaching, blocking, or poisoning of the acid sites considerably decreases the conversion rate and furfural yield and can even lead to deactivation of the catalyst. For example, a decrease in acid concentration and accumulation of byproducts can significantly contribute to the deactivation of both zeolites and metal oxide catalysts, as verified by NH3 temperature-programmed desorption (TPD) characterization and thermal treatments of the used catalysts.67,89 The use of efficient heterogeneous catalysts can reduce operating costs and energy consumption and promote the industrial production of furfural.

changing the composition ratio (e.g., the Si/Al ratio), varying the preparation conditions, and modification by metals or acid groups.58,59,63−65,67 One issue is the fact that the strong acidity of strong ion-exchange resin catalysts results in decreased furfural yields due to undesired side reactions, while most of the metal oxide catalysts need to be decorated with sulfonated groups to improve the acid content.57,72,74 For structure, the zeolite catalysts have the advantages of shape selectivity and tunable pore structure. The pore size of a catalyst should be close to the molecular sizes of xylose (6.8 Å) and furfural (5.7 Å) because a small pore size will inhibit the diffusion of xylose and furfural and a large pore size will facilitate the further rearrangement of furfural to large molecules.61 A high specific surface area can provide more active sites accessible to xylose. The surface areas of mesoporous zeolites and delaminated zeolites are usually higher than those of other solid catalysts.66,70,71 Hydrophobicity is 7625


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ACS Catalysis

Figure 4. Mechanisms of furfural hydrogenation to FA over (a) Cu and (b) group VIII metals based on DFT calculations. Adapted with permission from refs 35, 97, and117. Copyright 2016 Royal Society of Chemistry, 2010 Elsevier, and 2011 Elsevier, respectively.

3. CONVERSION OF FURFURAL TO C5 CHEMICALS In this section, we focus on the conversion of furfural to C5 chemicals, including furfuryl alcohol, tetrahydrofurfuryl alcohol, levulinic acid, γ-valerolactone, pentanediol, and cyclopentanone. Selective hydrogenation is a fundamental reaction for the reductive conversion of furfural to C5 chemicals, and its optional combination with hydrogenolysis or rearrangement can lead to a wider range of C5 molecules. 3.1. Furfuryl Alcohol and Tetrahydrofurfuryl Alcohol. Furfuryl alcohol (FA) is the most important chemical derived from furfural, and its production consumes approximately 65% of the overall furfural produced.90 The industrial production of FA is performed by selective hydrogenation of furfural in the gas or liquid phase using Cu-based catalysts, with the Cu−Cr catalysts being the most widely used.90−93 However, chromium in the Cu−Cr catalysts causes serious environmental problems due to its high toxicity. Therefore, many non-chromium catalysts have been developed for furfural hydrogenation in both the gas and liquid phases, as summarized in Tables 3 and 4, respectively.91−113 For the gas-phase hydrogenation of furfural, most of the selected catalysts gave FA yields of over 70%, while for the liquid-phase hydrogenation, a larger range of catalysts produced FA with yields of over 90%. Catalyst deactivation has been observed for both gas- and liquid-phase hydrogenations of furfural, especially with the use of Cu-based catalysts.91,92 The formation of coke, strong adsorption of reaction species, a change in the oxidation state of active sites, and sintering of metal particles have been proposed as possible explanations for catalyst deactivation.6,36 Recently, catalyst stability was successfully improved using novel catalyst preparation methods such as atomic layer deposition (ALD) and encapsulation in metal− organic frameworks, thus reducing catalyst deactivation.114,115

Although the liquid-phase hydrogenation allows a higher yield of FA than the gas-phase hydrogenation, the gas-phase process is preferred in industry. The high operating cost and expensive equipment limit the large-scale liquid-phase hydrogenation of furfural in a batch reactor under high H2 pressure.6 The selective hydrogenation of furfural over a variety of environmentally acceptable metal catalysts (Cu, Pd, Pt, Rh, Ru, Ni, Co, and Zn) has been extensively studied using both experimental and theoretical methods. Different intermediates adopting various adsorption modes as well as different reaction pathways have been proposed.35,97,116−118 Among these reaction mechanisms, one for Cu-based catalysts and one for group VIII metal catalysts are the most widely accepted, and they are compared in Figure 4 on the basis of density functional theory (DFT) calculation results. As shown in Figure 4a, the Cu metal surface has a very weak affinity for CC bonds, and therefore, furfural prefers the η1(O)-aldehyde binding mode, with its carbonyl group directly bonded to the surface via a lone pair of electrons of oxygen, and consequently favors hydrogenation of the CO bond.97 The further hydrogenation of the η1(O)absorbed species to FA can proceed via either an alkoxide intermediate (pathway 1) or a hydroxyalkyl intermediate (pathway 2). The lower activation barrier for pathway 2 indicates that the first H atom prefers to attack at the O atom rather than the C atom of the carbonyl group. This reaction mechanism has been supported by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) studies through the identification of the species adsorbed on the catalyst surfaces.97 In contrast, on group VIII metal surfaces, the most stable adsorption mode of furfural is the η2(C,O)-aldehyde configuration with both the C and O atoms of the carbonyl group bonded to the metal surface (Figure 4b).116,117 The further hydrogenation of the absorbed species 7626


Review

ACS Catalysis

Figure 5. Hydrogenation of furfural to FA and THFA.

occurs preferentially with a hydroxyalkyl intermediate to produce FA. However, when the temperature increases to a certain value, the preferred binding mode turns to the η1(C)-acyl configuration, and the thermodynamically favored decarboxylation to produce furan may occur, as will be discussed later in section 4.1. In addition, complete saturation of the carboxyl group of furfural can produce another important C5 molecule, 2-methylfuran (2MF). 2-MF has been identified as an important biofuel component, and its theoretical and experimental studies have already been reviewed;36,38,40,44,119,120 therefore, we chose to exclude this molecule in the current work. In addition to DFT calculations, parallel experimental studies, including highresolution electron energy loss spectroscopy (HREELS) and TPD measurements, also support this reaction mechanism.121,122 The good correlation between the DFT calculations and experimental studies show that DFT predictions are very helpful to give reasonable and intuitive explanations of reaction mechanisms regarding the intrinsic properties of metals, although the gaps between the DFT predictions and catalytic reactions cannot be ignored. The hydrogenation of CC bonds in the furan ring of furfural can produce tetrahydrofurfural and tetrahydrofurfuryl alcohol (THFA). As the furan ring is much more stable than the carboxyl group, metals with strong affinities for CC bonds favoring parallel adsorption of the furan ring to the metal surface are employed for the production of THFA. Currently, THFA can be produced by hydrogenation of either FA or furfural in the presence of Pd-, Ni-, and Ru-based catalysts in both the gas and liquid phases. For example, Nakagawa et al.123 reported the gasphase hydrogenation of furfural to THFA using Ni/SiO2 catalysts with a Ni particle size of 70%). Further hydrogenation of LA can produce GVL, which has wide applications in the fields of solvents, polymer synthesis, and fuel and fine chemical production. LA can be hydrogenated to GVL through two different approaches: hydrogenation with external hydrogen under high pressure or catalytic transfer hydrogenation (CTH) with formic acid or an alcohol as the hydrogen donor. In early studies, a series of Ru-based homogeneous catalysts (e.g., RuCl2, RuCl3, or Ru(acac)3 coordinated with PBu3, TPPTS, P(Oct)3, or PPh) were used to convert LA to GVL under moderate conditions,138,139 but the use of Ru-based homogeneous catalysts had the problems of high cost, complicated preparation processes, and difficult separation and reuse. To overcome these limitations, various supported metals, such as Pt, Pd, Au, Ru, Ni, and Co, have been extensively studied for the reduction of LA to GVL. For example, Upare et al.140 investigated the selective hydrogenation of LA to GVL over a series of carbon-supported noble metal catalysts and found that Ru was the most active and selective. Galletti et al.141 reported the hydrogenation of LA to GVL using a commercial supported Ru catalyst combined with a heterogeneous acid cocatalyst, Amberlyst-70, and obtained a 99% yield of GVL under mild conditions. A recent review by Gilkey and Xu142 includes the 7630


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ACS Catalysis Table 6. Conversion of Furfural, FA, and THFA to Pentanediols selectivity (%) Entry

substrate

Catalyst

solvent

conv. (%)

1,5-PeD

1,2-PeD

1,4-PeD

1-PeOH

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

THFA THFA THFA THFA THFA THFA THFA THFA THFA THFA furfural furfural furfural furfural furfural furfural furfural THFA THFA FA

Rh/SiO2 ReOx/SiO2 Rh−ReOx/SiO2 Ru/C Rh−MoOx/SiO2 Rh/SiO2 + sCO2 Rh/MCM-1+ sCO2 Rh/C + sCO2 Pd/MCM-41 + sCO2 Rh−WOx/SiO2 Ir−ReOx/SiO2 Ru−Ir−ReOx/SiO2 Pt−Ir−ReOx/SiO2 Rh−Ir−ReOx/SiO2 Pd−Ir−ReOx/SiO2 Pt/Co2AlO4 Pt/hydrotalcite Ir−MoOx/SiO2 Pd−MoOx/SiO2 Ru/MnO2

water water water water water water water water water water water water water water water water water 2-PrOH 2-PrOH water

5.7 0.1 96.2 5.0 50.1 30.4 80.5 34.8 50.5 30.1 99.9 99.9 99.9 99.9 99.9 − 99.8 75 13 89.2

18.0 0.0 80.1 15.1 95.5 78.2 91.2 29.8 12.6 85.0 0.2 0.5 0.3 1.2 62.4 25.6 1.5 65 70 0

61.7 31.2 0.0 26.1 0.0 11.4 0.0 42.3 77.4 − 0.0 0.6 0.5 0.2 1.4 4.5 74.1 0 0 41.1

− − − − − − − − − − 17.0 20.3 22.5 23.4 6.7 7.1 − − − −

6.2 5.1 15.9 7.8 3.8 8.2 8.8 20.0 10.0 6.0 0.1 0.2 0.3 0.3 11.2 − − 20 0 −

150 150 150 150 152 157 157 157 157 156 113 113 113 113 113 161 162 160 160 163

Au/ZrO2 and ZSM-5 hybrid catalyst was used, a minority of GVL was formed from LA with 5-methyl-2(5H)-furanone as the intermediate. This Au/ZrO2 and ZSM-5 hybrid catalyst was stable and kept active after four runs. The production of LA and GVL from furfural is summarized in Figure 8. It can be seen that the three reactions, namely, hydrogenation of furfural, hydrolysis of FA, and reduction of LA, need different catalysts and reaction conditions. On the basis of the mechanistic understanding of each reaction, one-pot catalytic processes have already been developed for the production of LA and GVL, in which the design of efficient multifunctional catalysts is the key point. Although LA and GVL are expensive specialty chemicals in small demand, they are used in diverse areas. As listed in Table 5, both LA and GVL can be transformed into many value-added chemicals and fuel components, which can be widely used in the fields of cosmetics, agrochemicals, polycarbonates, plastics, fragrances, and transportation.1,5,19,35,132,146 3.3. Pentanediol and Cyclopentanone. Ring opening is another important reaction for the conversion of furfural and its derivatives (i.e., THFA) over supported metal catalysts. The predominant products obtained from this reaction are pentanediols (Table 6), which could be promising biofuel components, monomers of polyesters, and valuable intermediates for the production of fine chemicals.36,147 The petroleumbased routes for pentanediol production usually start from acrolein, pentanediolic acid, pentene, and cyclopentene,148,149 but they are less competitive than the biomass-based routes because of the harsh reaction conditions, complicated processes, and expensive raw materials. As shown in Figure 9, pentanediols can be produced through the one-pot conversion of furfural or hydrogenolysis of THFA. Tomishige and co-workers113,150−156 have done extensive research on this subject. They reported the hydrogenolysis of THFA to 1,2-pentanediol (1,2-PeD) over a Rh/SiO2 catalyst in aqueous solution.150 In contrast, Chatterjee et al.157 found that Rh/SiO2 gave a higher selectivity for 1,5-PeD than 1,2-PeD in supercritical CO2 (sCO2). When Rh/SiO2 was modified with Mo or Re, the Rh nanoparticles were partially

Figure 9. Conversion of furfural to PeDs and CPO.

covered with metal oxide species (MoOx and ReOx),153 which favored the hydrogenolysis of THFA to 1,5-PeD rather than 1,2PeD (Figure 10). Tomishige and co-workers150,152 reported that the Rh−MoOx/SiO2 and Rh−ReOx/SiO2 catalysts gave 1,5-PeD in 85% and 86% yield, respectively, with a much higher hydrogenolysis activity than the monometallic Rh catalyst. However, the mechanism of the hydrogenolysis of THFA over Rh-ReOx/SiO2 was controversial. Koso et al.154 proposed a C−O hydrogenolysis mechanism based on substrate reactivity, kinetic analysis, and deuterium labeling experiments. As shown in Figure 10c, THFA was first adsorbed on ReOx clusters to form alkoxide species; then H2 was activated on the Rh metal surface to produce hydride species and protons, and the hydride species reacted with the substrate bound on the interface between Rh and ReOx; finally, the reaction of alkoxides with protons released 1,5-PeD. Chia et al.158 proposed another mechanism based on the results of surface science experiments and DFT calculations, indicating that the acidic Re−OH formed on Rh metal particles 7631


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of the furan ring over a Pt/Co2AlO4 catalyst under mild conditions. In contrast, Mizugaki et al.162 found that Pt nanoparticles supported on hydrotalcite catalyzed the direct transformation of furfural to 1,2-PeD in a high yield of 73% under additive-free conditions. Zhang et al.163 reported the hydrogenolysis of FA to 1,2-PeD over a series of MnOx-supported Ru, Pt, Pd, and Rh catalysts, and the yield of 1,2-PeD was 42.1% over the Ru/MnOx catalyst. In addition to 1,5-PeD and 1,2-PeD, another pentanediol, 1,4-PeD, can also be synthesized from furfural over hydroxyapatite-supported Pt−Mo bimetallic catalysts via LA, where high LA conversion (100%) and 1,4PeD yield (70%) were achieved at 130 °C.164 Another valuable C5 chemical derived from furfural is cyclopentanone (CPO), which is a versatile compound used for the synthesis of fuels, fungicides, pharmaceuticals, rubber chemicals, and flavor and fragrance chemicals.165,166 By means of petroleum-based routes, CPO can be prepared by vapor-phase catalytic cyclization of 1,6-hexanediol or adipic esters, liquidphase oxidation of cyclopentene by nitrous oxide, and hydrogenation of phenol.167−169 However, all of these petroleumbased processes are less developed because of the high cost of the feedstocks and low yields of CPO. The most studied catalysts for the transformation of furfural to CPO are Cu-based catalysts, which give moderate CPO yields in the range of 60−80%. For example, Yang et al.170 investigated the conversion of furfural to CPO over Ni−Cu/SBA-15 bimetallic catalysts in aqueous media under a H2 atmosphere and obtained a 62% yield of CPO, while Li et al.171 obtained a 67% yield of CPO from furfural over Cu− Co bimetallic catalysts. Hronec et al.172 reported the conversion of furfural to CPO with a 92.1 mol % yield over carbon-supported Pd−Cu catalysts, suggesting that the distribution of Pd0 and Cu+ was responsible for the catalyst activity and selectivity. Very recently, Wang et al.173 prepared a CuNi0.5@C bimetallic catalyst using a Cu-based metal−organic framework as the precursor, and this catalyst had a high CPO yield of 96.9%. Other Cu-based catalysts such as Cu−Zn−Al and Cu−Ni−Al were also investigated for the production of CPO from furfural.174,175 In addition to Cu-based catalysts, several noble metals show good activity for the conversion of furfural to CPO. Hronec and co-workers176,177 found that in the presence of 5% Pt/C catalyst, a 76.5% yield of CPO was obtained in water at 160 °C and 8.0 MPa H2. They also studied the effect of the solvent and the metal type (Ni, Pt, Pd, or Ru) on the rearrangement of the furan ring to CPO, showing that Pt was more selective to produce CPO than other metals and that the acid−base properties of the solvent as well as the furfural concentration influenced the product distribution.177 Fang et al.178 designed a novel catalyst with Ru nanoparticles supported on an acidic MOF material (Ru/MIL101) for highly active and selective conversion of furfural to CPO in aqueous media. Complete conversion of furfural with a CPO selectivity higher than 96% was achieved within 2.5 h at 160 °C and 4.0 MPa H2. Zhang et al.179 reported the transformation of

Figure 10. Hydrolysis of THFA to 1,5-PeD over Rh-based catalysts: (a) model structure of Rh−MoOx/SiO2; (b) model structure of Rh−ReOx/ SiO2; (c) mechanism of THFA hydrogenolysis via hydride-mediated carbanion formation; (d) mechanism of THFA hydrogenolysis via oxocarbenium formation by concerted protonation/hydride transfer steps. Adapted with permission from refs 35, 153, 154, and 158. Copyright 2016 Royal Society of Chemistry, 2011 Elsevier, 2011 Elsevier, and 2011 American Chemical Society, respectively.

gave a proton to the tetrahydrofuran ring and the α-hydrogen of the alcohol was transferred to the β-position, thus breaking the C−O bond to open the ring (Figure 10d). Similarly, Guan et al.159 performed DFT calculations to investigate the role of MoO3 on the Rh catalyst in the hydrogenolysis of THFA to 1,5PeD and employed the mechanism proposed by Chia et al.158 In addition to Rh-based catalysts, Wang et al.160 reported the use of an Ir−Mo/SiO2 catalyst for the conversion of THFA to 1,5-PeD, and a 1,5-PeD yield of ∼40% was obtained. The direct conversion of furfural to pentanediols over Rhbased catalysts is difficult because these hydrogenolysis catalysts have low activity in hydrogenation of the furan ring. The addition of a third metal shows a promoting effect. Liu et al.113 reported the one-pot selective conversion of furfural to 1,5-PeD over a Pdadded Ir−ReOx/SiO2 bifunctional catalyst with THFA as the intermediate and obtained a maximum 1,5-PeD yield of 71.4%. A similar result was obtained using a remodified Rh−Ir alloy catalyst.155 Additionally, Xu et al.161 developed a novel process for the direct conversion of furfural to 1,5-PeD by hydrogenolysis

Figure 11. Proposed reaction pathway for the conversion of furfural to CPO with FA, 2-cyclopentenone, and 4-hydroxy-2-cyclopentenone verified as three key intermediates. Adapted with permission from refs 170 and 172. Copyright 2013 Royal Society of Chemistry and 2015 Elsevier, respectively. 7632


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environmental problems due to the use of toxic catalysts like Cr and Pb,181 while the oxidation of butadiene or GBL was always accompanied by many side reactions resulting in a low yield of furan. Currently, the commercial production of furan is mainly based on decarbonylation of furfural via cleavage of the C−C bond. Furfural can be decarbonylated to give furan in both the gas and liquid phases. Different catalysts have been investigated for this reaction, including supported noble metal catalysts (e.g., Pd, Rh, and Pt)31,119 and mixed metal oxides based on non-noble metals (e.g., Zn−Fe, Zn−Fe−Mn, Zn−Cr, and Zn−Cr−Mn).182−185 Pd has been identified to be active and selective for furfural decarbonylation, especially at high temperature and H2 pressure.119 The experimental and theoretical studies show that when the temperature increases to a certain extent, the preferred binding mode of furfural changes from the η2(C,O)-aldehyde configuration to the η1(C)-acyl configuration, leading to the formation of furan via decarbonylation, and increasing surface H coverage can also promote the decarbonylation of furfural.35,116 Although the noble metals are known to be effective catalysts, they are expensive and limited in abundance. In comparison, the use of non-noble metal mixed oxides can greatly reduce the cost, whereas it requires more rigorous operating conditions, which usually cause catalyst deactivation by the formation of some heavy products. The further hydrogenation of CC bonds in the furan ring can produce THF, which has been widely used as a solvent and intermediate for chemical production. One of the earliest processes for THF production was proposed by Quaker Oats using furfural as feedstock, in which furfural was first decarbonylated to form furan and then the furan was hydrogenated to THF using noble metal catalysts at high temperature and pressure.81 However, this process has already been phased out because of the low THF yield, severe environmental pollution, and high energy consumption. To date, no other biomass-based commercial processes have been effectively developed for THF production, and THF is mainly produced through petroleum-based routes by the hydrogenation of maleic anhydride and dehydration of 1,4-butanediol (BDO).186,187 Although the processes for producing furan and THF have already been commercialized and studied for decades, they still have the problems of complex procedures, rigorous operating conditions, high catalyst costs, and low product yields. The limited values and profits of these two traditional chemicals are limiting further studies of their production. 4.2. Furanones. Furanone has three structural isomers, namely, 2(5H)-furanone, 2(3H)-furanone, and 3(2H)-furanone.188 Among these three furanones, we mainly focus on 2(5H)-furanone because it is more active and more widely used in the synthesis area.188 Furanone can be synthesized by deoxygenation of substituted butanoic acids at high temperature, transformation of hydroxybutyrolactones with acids or amines, hydrolysis of 2-methoxyfuran, and cyclocarbonylation of terminal alkynols in the presence of Pd catalysts.189−192 However, the above methods are deficient because they involve expensive reagents, complicated processes, extreme reaction conditions, and low furanone yields. Currently, 2(5H)-furanone is mainly produced by oxidation of furfural using hydrogen peroxide as the oxidant. Badovskaya and co-workers193 reported the oxidation of furfural to 2(5H)-furanone in aqueous media with a yield of 25% by autocatalysis, and the yield was not significantly improved in the presence of Mo(VI) or Cr(VI).194,195 Cao et al.196 found that when the autocatalytic

furfural to CPO using Au/TiO2 catalysts, and the maximum yield of CPO was nearly 100%. They found that the use of anatase TiO2 with only mild Lewis acid sites as the support was essential to prevent undesired side reactions and attain high CPO selectivity. Mechanistic studies of the conversion of furfural to CPO are still in the early stages, and most of the reported catalysts are supported metal catalysts employing a mechanism similar to that shown in Figure 11.172,177 First, furfural is selectively hydrogenated to FA over the metal catalyst, and the FA then spontaneously rearranges to 4-hydroxy-2-cyclopentenone (HCP) in water. The HCP intermediate is subsequently converted into 2-cyclopentenone and finally hydrogenated to form CPO. This mechanism was proposed on the basis of a series of control experiments identifying the three intermediates (i.e., FA, HCP, and 2-cyclopentenone). Further studies are still needed to investigate the specific interactions between the intermediates and active sites as well as the relationship between the binding modes of these species and the reaction pathway. The typical catalysts and reaction conditions for the production of PeDs and CPO from furfural are summarized in Figure 9. Hydrogenolyses of different C−O bonds of the fivemembered ring of furfural, FA, or THFA can generate different PeDs, and a variety of Rh-, Ir-, Ru-, and Pt-based catalysts have been investigated. Most of the research has focused on the hydrogenolysis of THFA to give 1,5-PeD using modified Rh and Ir catalysts such as Rh−ReOx, Rh−MoOx, and Ir−ReOx, with which yields of 40−75% have been obtained. These studies also indicated that optimizing the ratio of metal oxide promoters to Rh or Ir can enhance the selectivity for 1,5-PeD because the formation of the metal−metal bond between metal particles and partially reduced promoters is considered to be a key factor affecting the product distribution. When PeDs are directly produced from furfural, the addition of a third noble metal is usually needed to promote the hydrogenation of furfural to THFA. Most of the Rh-based catalysts undergo deactivation, possibly as a result of leaching or sintering of the metal particles, while the Pt-based and Pd-added catalysts are slightly more stable than the Rh counterpart.113,150,152,161,162 However, the deactivation mechanism is still controversial, as the extent of deactivation usually depends on the type of catalyst and the reaction conditions. Unlike the production of PeDs from furfural, which is heavily dependent on noble metals, the production of CPO mainly uses Cu-based catalysts, and yields of 60−100% can be obtained. It was also proposed that one of the important factors influencing the furan ring rearrangement to give CPO is the stabilization of a carbocation transition state by strong binding on the metal surface and additional interactions with coadsorbed water and furfural or FA.177

4. CONVERSION OF FURFURAL TO C4 CHEMICALS On one hand, furfural can be decarbonylated to afford furan, which can be further hydrogenated to tetrahydrofuran (THF). On the other hand, the selective oxidation of furfural can produce a number of C4 oxygenated products, such as acid anhydrides, dicarboxylic acids (succinic, malic, and fumaric acids), and furanones. This section will focus on the above C4 chemicals derived from furfural. 4.1. Furan and Tetrahydrofuran. Furan is the simplest fivemembered heterocyclic compound containing oxygen. In the early stage, several approaches for furan production were proposed, such as oxidation of butadiene or γ-butyrolactone (GBL), decarboxylation of furoic acid, and decarbonylation of furfural.180 The decarboxylation of furoic acid had severe 7633


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Figure 12. Proposed reaction mechanisms for furfural oxidation. Pathway A: furfural to furanones or MA over acid catalysts using H2O2 as the oxidant. Pathway B: furfural to SA over sulfuric acid or sulfonated resins using H2O2 as the oxidant. Pathways C and D: furfural oxidation to MAN over metal catalysts using O2 as the oxidant. Adapted with permission from refs 35 and 198. Copyright 2016 Royal Society of Chemistry and 2016 Elsevier, respectively.

HREELS. The results showed that furfural was first decarbonylated to give furan, which was further oxidized to produce 2(5H)furanone at high O coverage and high temperature. This surface science study provides possibilities for the aerobic oxidation of furfural to furanones over heterogeneous metal catalysts, which has rarely been reported.200 The main utility of 2(5H)-furanone at present is limited to its use as an intermediate for the synthesis of biologically active natural products and surfactants,188,201 whereas the potential for further conversion of 2(5H)-furanone to value-added chemicals in larger demand still remains unexplored. In view of the chemical structure, 2(5H)-furanone has only one more additional CC bond than GBL, while the further saturation and ring opening of 2(5H)-furanone can produce BDO. Thus, 2(5H)-furanone is a promising platform compound for the production of GBL and BDO, which can be widely used as monomers, solvents, and perfumes.202 In our previous work, selective hydrogenation of 2(5H)-furanone over a series of SiO2supported group VIII monometallic catalysts was studied.203 This work showed that Pd and Rh exhibited better catalytic performance for the hydrogenation of 2(5H)-furanone to GBL than other selected monometallic catalysts such as Pt, Ru, Ni, Co, and Cu. However, further studies are still needed to design more active and selective catalysts. The selective hydrogenation and ring opening of 2(5H)-furanone will offer novel reaction pathways with high atom economy for the production of C4 lactones and diols from furfural. 4.3. Acid Anhydrides and Dicarboxylic Acids. Oxidation is one of the most important and versatile reactions for the conversion of furfural to C4 chemicals, especially acid anhydrides and dicarboxylic acids.204 In this section, processes for the production of maleic anhydride (MAN), maleic acid (MA), and SA from furfural are reviewed. Currently, the commercial processes for MAN, MA, and SA production are all based on

furfural oxidation was carried out in a biphasic system using dichloroethane as the solvent, a 37% yield of 2(5H)-furanone was obtained, but the reaction time was longer than 10 h. Poskonin197 reported the synthesis of 2(5H)-furanone from furfural in water using niobium(V) acetate tetrahydrate as the catalyst, but more than 80 h was needed to obtain a 60% yield. The most common approach for 2(5H)-furanone synthesis is to oxidize furfural in a water/dichloromethane biphasic system using formic acid as the catalyst and sodium sulfate or potassium carbonate as the additive. Although the oxidation of furfural to 2(5H)-furanone has been studied for decades, the yield of 2(5H)-furanone is still moderate (ranging from 30% to 50%) and a long reaction time and the use of chlorinated solvents are needed. In our recent work, furfural was oxidized to 2(5H)furanone in 60−62% yield in an aqueous/organic biphasic system using 1,2-dichloroethane or ethyl acetate as the solvent and formic acid as the catalyst.198 The use of ethyl acetate allows the production of 2(5H)-furanone using greener and less toxic conditions. In addition, studies of the solvent effect for oxidation of furfural to 2(5H)-furanone showed that in homogeneous systems with methanol, isopropanol, THF, or GBL as the solvent, the yield of 2(5H)-furanone decreased and the yield of succinic acid (SA) increased with increasing dielectric constant of the solvent. A reaction mechanism for furfural oxidation to furanones using homogeneous acid catalysts and hydrogen peroxide as the oxidant was proposed on the basis of experimental results. As shown in Figure 12 (mechanism A), furfural is first transformed into 2-formyloxyfuran via Baeyer− Villiger rearrangement, and then 2-formyloxyfuran is hydrolyzed to 2-hydroxyfuran and its tautomers, namely, 2(5H)-furanone and 2(3H)-furanone.193,196,198 2(3H)-Furanone can be isomerized to 2(5H)-furanone in the presence of Et3N.196 In addition, Williams et al.199 studied the oxidation of furfural on oxygen-precovered Pd surfaces (O/Pd(111)) using TPD and 7634


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ACS Catalysis Table 7. Oxidation of Furfural To Give Acid Anhydrides and Dicarboxylic Acids entry

catalyst

[O]

solvent

reaction time (h)

T (°C)

main product

yield (%)

ref

1 2 3 4 5 6 7 8 9 10 11 12

Amberlyst-15 Nafion-NR50 p-TsOH HCl VOx/Al2O3 Mo4VO14 Mo4VO14 Mo4VO14 Cu(NO3)2 + H3PW12O40 Cu(NO3)2 + H3PMo12O40 H3PMo12O40 TS-1

H2O2 H2O2 H2O2 H2O2 O2 O2 O2 O2 O2 O2 O2 H2O2

water water water water vapor water AcOH AcAN water water biphasic water

24 24 24 24 − 16 16 16 14 14 14 10

80 80 80 80 320 120 120 120 98 98 110 50

SA SA SA SA MAN MA MAN MAN MA MA MA MA

74 41 72 49 73 15 65 47 12 49 38 78

217 217 217 217 200 210 210 210 212 212 214 215

Figure 13. Oxidation of furfural to afford acid anhydrides and dicarboxylic acids.

identified as the rate-determining step. At high reaction temperatures (200−300 °C), some heavy products caused by undesired polymerizations cover the active sites and finally lead to catalyst deactivation. Several approaches, such as introducing water and increasing the oxygen concentration, were proposed to solve this problem,211 but their effects were limited. Recently, the aqueous-phase oxidation of furfural using either oxygen or hydrogen peroxide as the oxidant has received more attention. Yin and co-workers212,213 reported that the combination of copper nitrates with phosphomolybdic acids could selectively convert furfural to MA in 49.2% yield or MAN in 54.0% yield in a liquid medium using O2 as the oxidant. Guo and Yin214 further studied the oxidation of furfural with O2 using phosphomolybdic acid catalysts in an aqueous/organic biphasic system and obtained a 34.5% yield of MA and 68.6% selectivity for MA under the optimized conditions. Alonso-Fagúndez et al.215 reported the aqueous oxidation of furfural to give MA in a high yield of 78% using hydrogen peroxide as the oxidant and titanium silicalite (TS-1) as the catalyst. Choudhary et al.216,217 investigated the aqueous oxidation of furfural to give SA with hydrogen peroxide as the oxidant using various heterogeneous catalysts, and SA yields of 74.2% and 41% were achieved over Amberlyst-15 and Nafion-NR50, respectively. The proposed reaction mechanism indicated that π−π interactions between the tolyl ring of the ion-exchange catalyst and the furan ring in furfural favored the transformation of furfural to the 2(3H)-

fossil feedstocks. Typically, MAN can be produced by oxidation of benzene or butane, and the route starting from butane is preferred for economic and environmental reasons as well as its stoichiometric advantage (no loss of carbon atoms).205−208 Subsequent hydrolysis of MAN produces MA, while SA is mainly produced by selective hydrogenation of MAN or MA. With the increasing depletion of fossil resources, more interest has been devoted to the production of acid anhydrides and dicarboxylic acids using biomass as raw materials. For example, biobased C5 and C6 sugars can be used to produce SA by fermentation,209 but this is beyond the scope of this review. Another promising option is to produce acid anhydrides and dicarboxylic acids by the catalytic conversion of biomass-derived platform compounds such as furfural. Typical studies of the oxidation of furfural to give acid anhydrides and dicarboxylic acids are summarized in Table 7. The gas-phase oxidation of furfural to MAN with air or oxygen over vanadium oxide-based catalysts has been studied for decades, and a wide range of MAN yields have been reported (15−90%). For example, Alonso-Fagúndez et al.200 converted furfural to MAN at 320 °C using VOx/Al2O3 as the catalyst, and a 73% yield of MAN was obtained. Li et al.210 studied the aerobic oxidation of furfural over a Mo4VO14 catalyst, and a 65% yield of MAN was achieved. When vanadium oxide-based catalysts are used, the active sites are reduced by furfural and reoxidized by oxygen, and the reoxidation of the reduced catalyst has been 7635


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Figure 14. Value-added chemicals derived from furfural.

polyester resins, vinyl copolymers, surface coatings, lubricant additives, plastics, agrochemicals, and pharmaceuticals.219−222 The applications of MA, SA, and MAN are listed in Table 5. It should be noted that furanones, MAN, MA, and SA can all be used as feedstocks in the production of GBL, BDO, and THF, which have very wide applications. For example, GBL can be used for the synthesis of pyrrolidones and their polymers, such as N-methylpyrrolidone (NMP), N-vinylpyrrolidone (NVP), and poly(vinylpyrrolidone) (PVP).207,223 The catalytic conversion of acid anhydrides and dicarboxylic acids to diols and lactones has been extensively studied during the past decades, and excellent reviews have been reported on this topic.204,221,224,225

furanone intermediate, which was further selectively converted to SA.216 The examples discussed above show that the aqueousphase oxidation of furfural using hydrogen peroxide as the oxidant allows the production of MA and SA under moderate conditions with no obvious catalyst deactivation. However, in addition to SA and MA, other undesired dicarboxylic acids such as malic acid may also be produced, resulting in difficulties related to product separation. Different reaction mechanisms have been proposed for oxidation of furfural to diacids or acid anhydrides with either hydrogen peroxide or oxygen as the oxidant,198,204,213−216,218 and the most widely accepted ones are shown in Figure 12. There mainly exist two reaction mechanisms for furfural oxidation using hydrogen peroxide as the oxidant. In mechanism A, furfural is first oxidized to 2(5H)-furanone and 2(3H)-furanone via Baeyer−Villiger rearrangement as described in section 4.2, and these intermediates can be further oxidized to MA and SA, respectively.198,216 In mechanism B, the furan ring of furfural is first opened to form a dienol, which is ketonized to give a diketo aldehyde; the diketo aldehyde is then oxidized to SA using hydrogen peroxide.217 When oxygen is used as the oxidant, two reaction mechanisms are proposed.35,213,218 Both start with oxidation of furfural to give furoic acid and its subsequent decarbonylation to afford furan. In mechanism C, an oxygen atom is added to bridge C1 and C4 of the furan ring, leading to the formation of MA or MAN via ring opening. In contrast, in mechanism D the formed furan is oxidized to introduce oxygen atoms at positions 2 and 5 of the furan ring to form 5-hydroxy2(5H)-furanone, which is then transformed into MAN. Understanding the reaction mechanisms can guide the design of efficient heterogeneous catalysts for the selective oxidation of furfural to C4 acid anhydrides and dicarboxylic acids. The typical catalysts and reaction conditions for oxidation of furfural to afford furanones, MAN, MA, and SA are summarized in Figure 13. Acid anhydrides and dicarboxylic acids are important raw materials for the manufacture of unsaturated

5. OUTLOOK AND CHALLENGES On the basis of the research reported in the literature, we summarize the C4 and C5 chemicals produced from furfural in Figure 14. We also describe the current progress, existing challenges, and future opportunities related to furfural production and conversion in the following paragraphs. Furf ural production using heterogeneous catalysts. The industrial potential for sustainable production of C4 and C5 chemicals from furfural strongly depends on the affordable and sustainable production of furfural. Several types of heterogeneous acid catalysts, especially modified zeolites, have been developed to replace the corrosive and less-selective homogeneous acid catalysts. However, most of these studies are still at the stage of laboratory research, and the main drawbacks of employing these heterogeneous catalysts in commercial processes are the high cost and catalyst deactivation. The reaction mechanisms and solvent effects for the conversion of xylan/xylose to furfural over heterogeneous acid catalysts require more studies. Although the catalytic mechanisms of Lewis acid and Brønsted acid sites have been experimentally investigated for dehydration of xylose to give furfural, further experiments are needed to identify the active sites and investigate the reaction mechanisms of undesired side 7636


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using fossil feedstocks. However, the following challenges must be overcome: (a) polymerization of furfural is severe, especially for aerobic oxidation of furfural; (b) control of the selectivity is difficult for the aqueous oxidation of furfural with hydrogen peroxide; (c) the lack of highly selective catalysts leads to low yields of the target products; and (d) separation and purification of the target products are complex. Designing efficient heterogeneous catalysts is the key issue to overcome the above problems. Catalyst designs should be based on reaction mechanisms, which have attracted more attention in recent years. However, most of them are based on experimental studies, and few theoretical investigations have been reported. Actually, pure theoretical calculations or simulations are less effective for the investigation of the reactions that increase the O/C ratio. More efforts are needed to combine kinetic studies, theoretical calculations, and experimental verifications.

reactions such as polymerizations. In addition to catalyst design, selecting a proper solvent, employing ionic liquids as additives, and adopting supercritical carbon dioxide extraction to efficiently separate the produced furfural from the reaction system to avoid its further polymerization are also promising. The combination of new reaction and separation techniques with the use of heterogeneous catalysts will bring opportunities to produce furfural at a competitive cost. Furf ural conversion to biochemicals versus biof uels. When furfural is converted to fuel components, selective deoxygenation is required to remove its functional groups containing O atoms to meet the demand of fuels. On the contrary, the conversion of furfural to value-added chemicals containing O atoms can make better use of this highly functionalized biomass-derived molecule. In addition, the use of furfural as a functionalized renewable platform compound allows the sustainable production of value-added chemicals through fewer steps than required when starting from fossil feedstocks. However, selectivity control is a significant challenge because many of the target products are often intermediates of other reactions or degrade under the reaction conditions. Advanced in situ technologies can be used to identify the reaction intermediates, and DFT calculations can provide a better understanding of the reaction mechanisms. Besides, kinetics studies can help discover the whole reaction network, including both the target and side reactions. Production of C5 chemicals f rom f urf ural. Although FA and THFA are conventional products with limited profits, the direct hydrogenation of furfural to FA or THFA is a basic reaction for further production of biofuels and biochemicals. Thus, many experimental and theoretical studies have been conducted to design efficient hydrogenation catalysts and provide insights into the hydrogenation mechanisms. The conversion of furfural to LA, GVL, PeDs, or CPO is extremely promising because it provides a green and sustainable route for the production of value-added chemicals with high atom economy. At present, most of the relevant studies have mainly focused on proposing novel conversion routes or employing new catalysts in the cascade reactions, while the final yields of products are moderate and the reaction mechanisms are still controversial. Because the conversion of furfural to value-added C5 chemicals usually involves more than one reaction step, the design of highly selective and active multifunctional catalysts is a key issue. This should be facilitated by the identification of active sites and a good understanding of the reaction mechanisms at a molecular level. DFT calculations have provided deep insights into the reaction mechanisms, especially for the conversion of furfural into furan-based C5 chemicals, but most of them lack experimental support. In situ methods such as Fourier transform infrared spectroscopy can be employed to overcome the gaps between theoretical models and real catalysts. For cascade reactions to convert furfural to C5 chemicals over multifunctional catalysts, matching each step of the reaction with the corresponding active sites is the key issue. To achieve this, it is desired to prepare catalysts with different active sites selectively covered or left alone by novel catalyst synthesis methods such as ALD. Production of C4 chemicals f rom furf ural. Compared with the decarbonylation products of furfural (furan and THF), the oxidation products with increasing molar ratio of O to C atoms (furanones, acid anhydrides, and dicarboxylic acids) are much more attractive. The production of C4 lactones, diols, acid anhydrides, and dicarboxylic acids from furfural provides a competitive alternative to the commercial production routes

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-10-62794132. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21676155 and 21476122) and the Program for New Century Excellent Talents in University (NCET-12-0297).

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