Recent Advances in Thermo-, Photo-, and Electro-catalytic Glycerol

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Review

Recent Advances in Thermo-, Photo-, and Electro-catalytic Glycerol Oxidation Georgios Dodekatos, Stefan Schünemann, and Harun Tüysüz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01317 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Catalysis

Recent Advances in Thermo-, Photo-, and Electro-catalytic Glycerol Oxidation Georgios Dodekatos, Stefan Schünemann, Harun Tüysüz* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany *[email protected]

Abstract Glycerol is a highly versatile molecule due to its three hydroxyl groups and can be transformed to a plethora of different value-added fine chemicals and products. It is an important byproduct in biodiesel production and, hence, produced in high amounts, which resulted in a high surplus flooding the market over the last decades. Thus, glycerol is regarded as a potential platform chemical and many research efforts were devoted to find active catalysts to transform glycerol to various products via different catalytic processes. The selective oxidation reaction is one of the most promising reaction pathways to produce valuable fine chemicals used in the chemical and pharmaceutical industry. This review describes the recent developments in selective glycerol oxidation to value-added products over heterogeneous catalysts. Particular emphasis is not only put on newly developed catalysts based on supported noble metal nanoparticles but also on catalysts containing non-precious metals. The idea of using cost-efficient non-noble metals for glycerol oxidation is appealing from an economic point of view. Numerous parameters can influence the catalytic performance of the materials, which can be tuned by various synthetic approaches. The reasons for enhancements in activity are critically examined and put into perspective between the various studies. Moreover, during the last decade, many research groups also reported photocatalytic and, more scarcely, electrocatalytic pathways for glycerol oxidation, which are also described in detail herein and have otherwise found little attention in other reviews. Keywords: glycerol, selective oxidation, biomass, catalytic conversion, photocatalysis, electrocatalysis

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1. Introduction The majority of fuels and important commodity chemicals are produced from fossil fuel resources, such as coal, natural gas, and petroleum. Today’s society depends on these fossil resources to maintain and further expand its prosperity. However, the amounts of available petroleum, natural gas, and coal decrease with the consequence that the resources will eventually deplete – whereas it is still under debate when this point will be reached, in particularly for petroleum. In addition, environmental concerns are connected with the combustion of fossil fuels since increased CO2 emissions propel the climate change and increase global temperatures. In 2014 the transportation sector accounted for approximately 65% of the global oil consumption and produced approximately 34% of the global energyrelated CO2 emissions according to the International Energy Agency.1 Nowadays, there is a large political interest and effort to mitigate global warming that is mainly caused by the CO2 emission. In 2015 at the Paris Climate Accord, 195 members declared to agree to reduce the global temperature rise in this century below two degrees Celsius compared to pre-industrial values.2 Hence, due to the aforementioned upcoming shortages and detrimental effects on the climate, biofuels, i. e. fuels based on renewable materials (biomass), are regarded as a promising alternative to fossil fuels to fulfill the energy demand and potentially decrease CO2 emissions. Furthermore, a well-established biofuel production could decrease a country´s dependence on imported petroleum and also create a new market and opportunities from which society could benefit. Nonetheless, it has to be emphasized that it is still under debate if biofuels can hold their promise to make the society less dependent on fossil fuels and to contribute to prevent the climate crisis.3-4 Despite the aforementioned skepticism, a steady increase in biodiesel production can be observed and the forecast until 2025 by the OECD-FAO (Food and Agriculture Organization of the United Nations) also evidences that the biodiesel production will remain high, although the yearly increase will level off (Figure 1).5 However, the demand for biofuels is governed by mainly three factors. Firstly domestic policies (e. g. blending mandates) have a considerable influence on the economic feasibility of the biofuel production. Secondly, biofuel prices are closely linked to the prices of the used feedstocks (e. g. the demand for biodiesel is closely related to the price of vegetable oil). Thirdly, the demand for biofuels also depends on the current market price for fossil fuels. High prices for fossil fuels will make biofuels more cost competitive, which leads to a higher demand for biofuels.


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Figure 1. World biodiesel production and biodiesel price (producer price Germany net of biodiesel tariff and energy tax) development and forecast. Based on data from OECD/FAO (2016), OECD-FAO Agricultural Outlook 2016-2025, OECD Publishing, Paris.5

Biodiesel is produced via acid or base catalyzed transesterification of fats and oils (triglycerides) with methanol yielding glycerol and fatty acid methyl esters (FAMEs, biodiesel), as shown in Scheme 1. One mole of triglyceride results in three moles of the corresponding FAME and one mole of glycerol. In other terms, 1 ton of biodiesel production yields roughly 100 kg of pure glycerol (110 kg of crude glycerol).6 The cost of biodiesel production is governed by the cost of the feedstock and that of the production itself. The price for the different feedstocks depends on the region in which the biodiesel is produced. For example in North and Middle Europe, by far the most popular oil used in biodiesel production is rapeseed oil, while in North America it is soybean oil, and in Asia it is palm oil.7 The production cost of biodiesel is, amongst other factors, significantly governed by the cost of its separation from the by-product glycerol.8-9


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Scheme 1. Transesterification process shown for triglyceride and methanol forming biodiesel and glycerol.

The fast development of the biodiesel industry during the last decades resulted in the decoupling of the glycerol production and demand, thus in a high surplus of glycerol which is known as glycerol glut.10 Since 1995 there has been an oversupply of glycerol in the world market11 and, as a consequence, the supply of glycerol is entirely independent of its demand. In 2008, biodiesel became the primary glycerol source leaving the fatty acid industry as primary source behind.12 This resulted in a drastic drop of the glycerol price and, consequently, in the treatment of glycerol as waste stream by many biodiesel plants – burning it without any further value.12-13 Moreover, crude glycerol has to be purified in order to be used as feedstock for the established applications.7,

14

Some of the refinery processes are

filtration, chemical additions, and fractional vacuum distillation, which cannot be performed by small- and medium-scale plants due to the high costs.12, 15-16 New processes of utilizing glycerol have to be developed in order to substantially increase the demand and the price of crude glycerol and, hence, make the biodiesel production more economically feasible. Preferably, high value products have to be produced in order to be less dependent on the glycerol market price fluctuations and achieve higher margins. Intensive research is conducted to find ways of glycerol valorization by catalytic transformations into chemicals. The focus lies on the development of new technologies and catalytic routes to monomers or intermediates for the production of commodity chemicals to expand the market beyond the currently limited applications of glycerol. Fortunately, glycerol is a highly functionalized molecule with three hydroxyl groups, which makes it a suitable candidate for the conversion into more valuable compounds. The different reaction processes range from glycerol reforming (thermo-17-20, electro-21, and photocatalytic22-24), over biocatalytic fermentation7, 6,

esterification

7

19-21

,

hydrogenolysis21,

16,

21,

32-34

25-27

hydrogenation28,

,

6,

, and etherification

21,

33,

dehydration7,

35

6

7,

, to oligomerization

polymerization , carbonylation , and oxidation (Scheme 2 and Table 1). 31

21,

37-51

36

29-31

,

and

The variety of

value-added products range from acrylic acid , glyceric acid and dihydroxyacetone, over


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lactic acid52-53 to 1,2-propanediol and 1,3-propanediol25-26, 54-58. These various processes are intensively investigated by researchers around the world.

Scheme 2. Different processes of the catalytic conversion of glycerol into useful chemicals.

The oxidation process is one promising route to obtain highly desired products. The range of products which can be obtained and their potential applications are listed in Error! Reference source not found.. Apart from formic acid which is produced in high amounts by producers like BASF51, any other product has a higher economic value than glycerol.59 Especially,

glyceric

acid,

dihydroxyacetone,

tartronic

acid,

glyceraldehyde,

and

hydroxypyruvic acid are considered as high-value chemicals. Dihydroxyacetone is conventionally produced by a fermentation process via the microorganism Gluconobacter oxydans in a fed batch reactor.43, 60-61 This product is economically the most interesting due to its applications in cosmetics, where it is used as ingredient in sunless skin tanning lotions. For further information on biocatalytic conversions of glycerol, which are out of the scope of this review, the reader is referred to review articles specialized on this topic.62-64 As in the case of dihydroxyacetone, glyceric acid is also conventionally produced by a fermentation process.50 If it can be produced at low cost, wide uses will be expected, including as raw material for


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chemical products such as bioplastics, pharmaceuticals for alcohol metabolism acceleration or disease treatment, and cosmetics.65-66 Glycolic acid is used in personal care products. Furthermore, it can be used as versatile cleaning agent, for instance, as degreasing agent and for rust removal or metal cleaning in general. It is mainly manufactured from acid-catalyzed formaldehyde carbonylation.40 Although formic acid has in direct comparison no economic beneficial value compared to glycerol, the oxidation from glycerol to formic acid is still investigated.67-71 This is due to the fact that formic acid can be used as hydrogen storage carrier to catalytically release H2 on demand.72-77 However, the direct reforming of glycerol to H2 might be more feasible than the formation of formic acid. Table 1. Glycerol and its typical oxidation products and their applications. Structure

Application -Synthon in organic chemistry48, starting material in D, L-serin synthesis48, tanning agent in cosmetics, monomer in polymeric biomaterials6 Application in medicine: metabolite in the glycolysis cycle and an intermediate in the synthesis of aminoacids49-50; used for treatment of skin disorders6

Oxygen scavenger6, pharma-ceuticals in the treatment of osteoporosis and obesity43

Complexing agent48, precursor in organic synthesis48, anti-HIV agents10

Degreasing agent, rust removal, skin care products48, chemical peels performed in dermatology48, textile dyeing and leather tanning agent40 Used in leather industry in Asia, agriculture in Europe, formic acid salts used for environmentally friendly runway de-icing, fuel in fuel cells51


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Glycerol is not the only bio-renewable platform for the synthesis of fine chemicals. Lignocellulosic biomass, which makes up for 95 % of the total plant biomass, is by far the most abundant bio-renewable resource and thus holds great promise for the conversion to generate value-added products.78 Additionally, and in contrast to glycerol lignocellulosic biomass does not directly compete with the food demand, which makes lignocellulosic biomass more sustainable in the long term. In contrast, the production of glycerol from biomass is heavily dependent on feedstocks from vegetable oils, due to the transesterification process in the biodiesel production. On the other hand, the complex polymeric structure of lignocellulosic biomass makes its breakdown to a targeted chemical in high yields rather challenging and requires several reaction and separation steps as pointed out in several reviews.78-81 In this context, the synthesis of value-added chemicals from glycerol is more straightforward. As long as the production of biodiesel increases, and the supply of glycerol remains decoupled from its demand glycerol conversion to value-added products is interesting from an economic point of view. It is obvious that if efficient technologies and catalytic processes can be developed for the selective oxidation of glycerol, biodiesel production can be performed with higher economic profit. Moreover, the spectrum of applications for the different value-added products can be extended if new reaction pathways can be discovered for producing these chemicals at a lower cost. In this context, the use of heterogeneous catalysts with O2 as oxidant might pave the way for environmentally friendly and costefficient processes to produce these compounds. Noble metal catalysts based on Au, Pt, and Pd and alloys thereof were the main research focus for oxidation of oxygenates in the last decades.45

50, 80

Especially, in the oxidation of

biomass-derived compounds it is apparent that the mayor type of catalysts were supported noble metals. Also in the case of aqueous-phase glycerol oxidation, noble metal catalysts are the materials of choice and researchers extensively studied how various reaction parameters but also material properties can improve the catalytic performance.43,

47

Besides the highly

investigated noble metal catalysts, also non-precious metals were explored as active catalysts for glycerol oxidation during the last decade, which is in line with the general efforts to utilize non-noble catalysts for various reactions.82 Although the number of publications is low compared to that of noble metal catalysts, it has an academic and economic appeal to find active catalysts based on non-precious metals. Avoiding noble metals for glycerol oxidation would result in a more cost-efficient reaction process since, as pointed out by Kimura, Dumeignil and co-workers, the catalyst price still represents 95% of the production costs for


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dihydroxyacetone, tartronic acid and mesoxalic acid even after 10 times of reuse, due to the presence of noble metals.43 Another approach for glycerol oxidation is photocatalysis. The selective transformation of organic compounds via photocatalysis emerged during the last decades as a promising concept for obtaining desired products and simultaneously moving toward a sustainable chemistry.83-86 Also glycerol was investigated as a suitable chemical to be oxidized to highvalue products. However, this comparably young research field still has room for further development but also has the potential to pave the way to unprecedented catalytic performances. Furthermore, the electrocatalytic pathway for glycerol oxidation is under investigation by many researchers and, similarly to photocatalysis, utilizes generated holes and electrons to drive the reaction. This concept has the potential to elegantly combine H2 generation at the cathode and the formation of value-added products at the anode by applying a suitable potential.87 Likewise to the thermocatalytic process, noble metals are selected to investigate the electrocatalytic oxidation process. Nonetheless, also in this field, researchers devoted their efforts to find suitable non-noble catalysts.88-89 The electrocatalytic pathway can produce value-added products from glycerol economically. Indeed, Huber, Han and co-workers inferred that the electrocatalytic oxidation of glycerol resulted in a more cost-efficient production of glyceric acid compared to the thermocatalytic reaction.90 Also here, the alternative approach holds great promise for a cost-efficient valorization of glycerol. In this review, the recent advances in glycerol oxidation which emerged during the last years are presented. For earlier contributions to this field, we kindly refer the reader to the following other reviews.6, 16, 43, 45, 47, 91-92 The focus lies recent advances of strategies in the synthesis of catalysts to improve the catalyst’s performance in terms of activity, selectivity, and stability. In addition, special emphasis is put on photocatalytic pathways,the utilization of non-precious metal containing catalysts, and electrocatalytic methods for glycerol oxidation, which have not been summarized in the literature up to now; these topics are thoroughly elucidated in this article. In the following sections, effects of diverse parameters like catalyst preparation method, textural parameters, particle size and shape of the catalyst, catalyst-support interactions and the reaction environments and conditions on the conversion and the selectivity of glycerol oxidation will be evaluated. It should be kept in mind that the goal of this review is not to cover all published glycerol oxidation catalysts; only some recent examples will be discussed. ACS Paragon Plus Environment

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2. Glycerol Oxidation over Noble Metals Typical catalysts employed for glycerol oxidation in order to obtain value-added products are the three noble metals Pt, Pd, Au, and their corresponding alloys. Very early studies were performed by Kimura et al. in 1993 with Pt catalysts supported on charcoal for glycerol oxidation in a batch reactor or in a fixed-bed reactor.93-94 The authors reported that Pt catalysts promoted by Bi exhibited improved selectivities toward dihydroxyacetone at the expense of glyceric acid. Garcia et al. confirmed these observations in their studies in 1995 and additionally used Pd/C catalysts for glycerol oxidation with high selectivities toward glyceric acid under basic conditions.95 The fact that Au catalysts are suitable for polyol oxidation reactions – and in particular for glycerol oxidation – was identified in a later stage by other research groups. In 1998 and later, the seminal works of Rossi and Prati showed that nanosized Au catalysts supported on carbon or alumina are capable of diol oxidation.96-98 It was further mentioned that Au catalysts, unlike Pt catalysts, did not suffer from oxygen poisoning, giving the advantage that higher oxygen pressures could be applied during the reaction process. The research group of Hutchings extended this concept and could show in 2002 that also glycerol can be oxidized to various products by Au catalysts.37-39 The main factors that influence the activity and selectivity of the Au catalyzed glycerol oxidation are the NaOH/glycerol ratio, the reaction temperature, and the oxygen pressure. For example, Au catalysts under base-free conditions yield dihydroxyacetone as the major product. The presence of a base changes the selectivity towards higher oxidized reaction products like glyceric acid. On the other hand, Pt-based catalysts are also active under acidic conditions and suffer from oxygen poisoning, such that the catalytic activity of Pt-based catalysts is often reported to depend significantly on the oxygen pressure. These two examples already show that the effects of the different reaction parameters on the catalyst’s performance are crucial for the performance of a catalyst. However, the effect of these parameters has exhaustively been discussed in previous research articles and reviews.43, 47, 91, 99

Fundamental studies on detailed reaction mechanisms and the surface chemistry of oxygenates on heterogeneous metal catalysts have been reviewed previously100-103

and,

hence, are not in the focus of this review. Nonetheless, in the field of aqueous-phase glycerol oxidation, we particularly want to refer to the work of Davis and co-workers who revealed the role of O2, H2O2, and the base NaOH in glycerol oxidation over Pt and Au catalysts with the aid of isotope studies and DFT calculations.104 In these intriguing studies, the authors ACS Paragon Plus Environment

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demonstrated that O2 is not incorporated in the oxidation products but rather removes electrons (in form of hydrides) from the Au surface to close the catalytic cycle. The regeneration of the catalyst surface produces H2O2. The base NaOH is required in order to form glycerolate, either in aqueous solution prior to adsorption on the Au surface or on the Au surface via adsorbed hydroxyl species. Hence, Au catalysts generally require basic conditions in order to effectively oxidize glycerol.38, 41, 105 The general reaction pathway for glycerol oxidation over noble metal catalysts is depicted in Scheme 3. In spite of the detailed studies obtained for ethanol as monofunctional alcohol, glycerol with its three hydroxyl groups enables the formation of various acids. General concepts of a consecutive reaction pathway to form for instance tartronic acid are reported in the literature; although also studies exist where it is inferred that tartronic acid is a primary product, formed directly at the catalyst´s surface. Indeed, by surveying different reports, slight alterations of the proposed reaction pathway can be noticed and it seems that a definite reaction pathway still has to be published. Thus, the given reaction pathway in Scheme 3 should only be regarded as a general guide where detailed mechanistic pathways are not described. Nonetheless, it should allow the reader to get a first impression of how the various products are formed.

Scheme 3: Simplified general reaction scheme for glycerol oxidation over noble metal catalysts adapted from ref.106


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2.1 Glycerol Oxidation over Au-Containing Catalysts

This section comprises the effect of the variation of the synthesis parameters of solimmobilization, the effect of Au particle size, and the effect of support on the catalytic performance for glycerol oxidation over mono- and bimetallic Au catalysts. Furthermore, the recent advances in working under neutral pH for glycerol oxidation are described. The first studies showing that supported monometallic Au catalysts are capable of polyol oxidation under basic conditions were conducted by Rossi and Prati96-98, followed by the studies of Hutchings et al.37-39 who showed that Au catalysts are also active for glycerol oxidation. The reports were in that sense groundbreaking since these groups demonstrated that Au nanoparticles (NPs) can be used as efficient catalysts for glycerol oxidation with improved resistances to oxygen and (by-)product poisoning compared to supported Pt98, 107-110 and Pd catalysts.96, 108, 111 On the other hand, basic aqueous conditions had to be applied, whereas Pd and Pt catalysts are also active under neutral or acidic conditions. Many reviews can be found in the literature, where the glycerol oxidation over Au-containing catalysts is described45, 91-92, 100, 112-115

with one of the latest being published by the pioneers in this field, Prati and

Hutchings, in 2015.47 Since then, further improvements in catalyst preparation and understanding of the structure-activity relations have been achieved and are described herein. The nature of the catalyst and its preparation method plays an important role in catalytic reactions, and many different synthetic methodologies have been explored for glycerol oxidation. The sol-immobilization method is one of the most frequently employed preparation procedures for these materials to obtain small Au NPs deposited on various supports. In general, this method requires the usage of capping agents (polymers such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), or molecules like cetyltrimethylammonium bromide (CTAB), tetrakis(hydroxymethyl)phosphonium chloride (THPC), and citrate) which protect the Au NPs from agglomeration and ensure spherical nanoparticles.116 Hence, the interaction between the capping agent, the metal NP and glycerol can substantially affect the catalytic performance (beneficially or detrimentally) and many researchers have studied both the influence of different capping agents on the catalytic performance117-121 and also the efficient removal116, 121-123 of the capping agent avoiding the alteration of the Au NP morphology. Prati and co-workers investigated the effect of the protecting agent (PVA) removal of Au NPs supported on different carbon based materials on glycerol oxidation.123 PVA was removed by thoroughly washing the catalyst with distilled water at room temperature or at 60 °C. Interestingly, the presence or absence of PVA on Au NPs (with similar mean particle ACS Paragon Plus Environment

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diameters) supported on activated carbon had no effect on the activity and selectivity of the glycerol oxidation. This, however, could not be observed for Au NPs supported on graphite were the PVA removal resulted in an increase in Au mean particle diameter and, hence, altered catalytic activity and selectivity. These findings are in contradiction to the results obtained by the same group in an earlier study with Au/TiO2.119 The authors demonstrated in their study that the presence of PVA – depending on the amount – has an influence and either lowers (high amounts of PVA) or improves (low amounts of PVA) the activity of the Au/TiO2 catalyst toward glycerol oxidation. The increase in activity in the presence of PVA was also observed for Au/TiO2 samples prepared by the deposition-precipitation method where small amounts of PVA were subsequently added into the reaction solution for glycerol oxidation. Furthermore, the presence of PVA directed the selectivity toward glyceric acid. Supported PVA-free Au NPs, on the other hand, promoted C-C cleavage and, hence, the production of C2 and C1 compounds. It was proposed that the protecting PVA layer interacts with glycerol and leads to different adsorption modes of glycerol on the active sites. However, the true role of the protecting agent is difficult to identify. This is, on the one hand, owed to the fact that any attempt to effectively remove the stabilizing agent will have an influence on the metal nanoparticle itself and, hence, make the comparison between stabilizer-capped and stabilizerremoved catalysts difficult. Furthermore, comparing catalysts prepared by a method different to the sol-immobilization, which ensures that no stabilizing agent is adsorbed on the metal surface (like the deposition-precipitation method), might ignore crucial factors like altered particle size distributions. On the other hand, stabilizing agents do not reside on the metal surface during the catalytic oxidation but dissolve in the aqueous phase. Hence, the surface composition changes during the reaction making an unambiguous connection between the stabilizer and the effect on the catalysis difficult. A recent review is solely devoted to the effect of the stabilizing agent on the catalytic performance for noble metal catalysts emphasizing the significance in this research field.121 Instead of removing the protecting agent after the immobilization of the Au sol on the support, another approach lies in the preparation of the Au sol in the absence of the protecting agent prior to the immobilization process. A successful synthesis of uniform Au NPs might not be expected by this altered preparation method since the surfactant is required to stabilize the sols. Nonetheless, this method was very recently introduced by Hutchings and coworkers.124 The authors employed TiO2 supported Au and AuPd catalysts for glycerol and benzyl alcohol oxidation and investigated the effect of the stabilizing agent. As illustrated in


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Figure 2 a and b, the deposited Au NPs were spherical and the stabilizer-free prepared catalysts showed slightly larger particle sizes compared to the catalyst prepared with PVP.

Figure 2. a) HAADF STEM micrographs of 1 wt% Au/TiO2 prepared by the sol-immobilization method in the absence of a stabilizing agent during the sol formation. b) Particle size distributions of 1 wt% Au/TiO2 samples prepared with PVP or stabilizer-free (SF). c) Catalytic data for glycerol oxidation over 1 wt% Au/TiO2 prepared by sol-immobilization with PVP (black squares), PVA (red triangles), and stabilizer-free (SF, green dots). Solid lines denote the glycerol conversion; dashed lines denote the glyceric acid selectivity; dotted lines denote tartronic acid selectivity. Reaction conditions: 110 mg catalyst, 0.3 M glycerol, 60 °C, 0.3 MPa O2, NaOH/glycerol = 2:1, glycerol/Au = 500:1. Reproduced with permission from ref

124

. Copyright 2017 Wiley-

VCH.

For the catalyst synthesis, it was important that the colloidal metal NPs without stabilizing polymer needed to be deposited onto a support within 30 min since the particles were unstable in solution for extended times. Interestingly, the catalysts prepared by the sol-immobilization technique without the use of any stabilizing polymers were active for the oxidation reactions and showed similar catalytic performances as the catalysts prepared with PVA or PVP (Figure 2 c). The authors, hence, showed that a stabilizing agent is not required for the preparation of supported Au NPs by sol-immobilization. As mentioned before, it is expected that the stabilizing agent plays a pivotal role in particle morphology and, hence, catalytic activity. Indeed, certain effects were observed by altering either the stabilizing agent or the used amount.121 Therefore, it is surprising that the authors observed that the catalysts prepared with or without stabilizing agent showed similar catalytic behaviors with only slight changes in selectivity. However, in a later study the authors found altered selectivities for PVAstabilized and PVA-free Au and AuPd NPs supported on Al2O3 and MgO.106 Prati and coworkers obtained different catalytic results for Au/TiO2 samples where the protecting agent was removed after the catalyst synthesis (see above).119 In this light, it is seems that the preparation of catalysts in the total absence of protecting agents provides new catalysts with ACS Paragon Plus Environment

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altered properties compared to materials where the protecting agent is removed postsynthesis. However, a direct comparison of the catalytic performances is not possible due to the different reaction conditions applied in both reports. The sol-immobilization method allows the facile tuning of physicochemical properties of the metal NPs by variation of both the nature and concentration of the capping agent.116, 121 Wells and co-workers demonstrated that further synthesis parameters have an influence on the catalyst properties. They investigated the influence of the catalyst preparation conditions (temperature and water/ethanol solvent ratio) on the structural and catalytic properties of Au/TiO2 catalyst for glycerol oxidation.125 Au/TiO2 materials were prepared via the solimmobilization method with PVA as capping agent. For each solvent system, an increase in the Au mean particle diameter occurred by increasing the temperature during the catalyst preparation. Furthermore, switching to pure ethanol or water/ethanol mixtures yielded larger Au NPs. More importantly, they observed that differently prepared Au/TiO2 samples with similar Au mean particle diameter exhibited different catalytic performances. They assigned this to the presence of isolated ultrasmall Au clusters (1 to 5 atoms) detected by HAADFSTEM analysis (Error! Reference source not found.) for the most active catalyst (with an Au mean particle diameter of 2 nm, determined by TEM analysis) which was prepared in water at 1 °C.

Table 2. Selected catalytic data from ref.

125

for

glycerol oxidation over Au/TiO2 catalysts prepared under different conditions.a Au/TiO2b

TOF / h-1

H2O / 1 °C

915

73

12

H2O / 25 °C

663

74

12

H2O / 50 °C

341

76

8

202

76

11

314

78

7

50 EtOH / 50 °C EtOH / 50 °C Figure 3. High resolution HAADF STEM images of single Au atom and Au2 clusters supported on TiO2

prepared

in

water

at

1 °C

via

sol-

Selectivity / % GLA TA

a

Reaction conditions: glycerol/Au = 1000:1,

NaOH/glycerol = 4:1, 50 °C, 0.3 MPa. TOF determined after 15 min reaction time. b

The first column indicates the solvent (water, ethanol) or

immobilization. Reproduced with permission from

solvent mixture (50 vol% ethanol/ 50 vol% water) used for the

ref.125.

catalyst preparation via sol- immobilization.

Copyright

2015

American

Chemical

Society.


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ACS Catalysis

Another sample prepared in a water/ethanol mixture with a similar Au mean particle diameter (1.8 nm) but significantly lower TOFs contained no ultrasmall clusters and only Au clusters below 1 nm were present. Generally, high selectivities toward glyceric acid were obtained for all catalysts (Error! Reference source not found.). The authors inferred that the variations in the population of ultrasmall Au clusters, in combination with other solvent/PVA effects, are responsible for the contrasting catalytic properties. These findings are in that sense intriguing since the catalytic activity of supported Au NPs is correlated with the average nanoparticle size. Generally, smaller Au NPs result in higher TOFs for glycerol oxidation and the particle size also influences the selectivity.38-39, 43, 98, 126-132 However, many studies conducted did not employ HAADF-STEM or similar high resolution studies to determine small Au clusters and, hence, neglect this parameter for correlating the particle size with the activity of the catalyst. On the other hand, the authors observed a significant drop in TOF for the most active Au/TiO2 sample for the first recycle experiment implying an aggregation of the Au clusters and Au NPs, the latter one being confirmed by TEM analysis.125 This implies that the importance of Au nanoclusters for this kind of reactions can be neglected. In other words, if studies show that meaningful correlations between the Au mean particle size and catalytic performance can be established and additionally can prove good recyclability of the material, it should be reasonable to assume that nanoclusters do not significantly contribute to the investigated catalytic system. As aforementioned, the Au particle size plays an important role in the catalytic performance for glycerol oxidation and many researchers reported Au size-dependent catalytic activities and selectivities in their studies.38, 98, 126-131, 133-134 Generally, smaller particles (2-6 nm) exhibit a higher activity than larger particles (10-30 nm); however, the selectivity toward glyceric acid is higher for large Au particles. The higher activity was ascribed to the higher surface exposition of the metal with smaller Au particles compared to large particles. Also the amount of particle edges is increased, which are considered as potentially active phases.43 Ketchie et al. correlated the formation of H2O2 with the selectivity toward glycolic acid for glycerol oxidation over carbon or titania supported Au catalysts.135 More importantly, the same group could show in a further study that the size of the Au particles determines the formation rate of H2O2.130 The lower selectivity of small Au particles toward glyceric acid was attributed to the higher formation rate of H2O2 during glycerol oxidation. Hence, the H2O2 formation plays a pivotal role in the selectivity. Wang et al. also ascribed the change in selectivity toward


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glyceric acid to the H2O2 formation over carbon nanofiber supported Au NPs.136 Higher amounts of H2O2 led to C-C cleavage and C2 and C1 product formation as previously suggested by Ketchie et al.130 They furthermore inferred that the selectivity toward glyceric acid is structure-sensitive with the Au (111) surface promoting the C-C cleavage due to the higher formation rate of H2O2. D’Agostino et al. demonstrated that the Au particle size also affects the surface affinity of glycerol and, hence, the reactivity.137 Au/TiO2 catalysts exhibited smaller Au particle sizes with lower Au loading, which in turn resulted in higher catalytic activities for glycerol oxidation. 1H NMR T1/T2 relaxation time measurements revealed that glycerol had a higher surface affinity relative to water for the catalyst with low Au loading and, hence, smaller Au particle sizes. This emphasizes the fact that still further effects have to be taken into account in order to explain the observed catalytic performances for many employed catalysts. As mentioned in the beginning, the pH value of the reaction solution influences the glycerol conversion and selectivity significantly. At this point, it should also be mentioned that it was recently found that NaOH alone i. e. in absence of a heterogeneous catalyst acts as a homogeneous catalyst for the selective glycerol oxidation at prolonged reaction times and low conversions.44,

138

The addition of a base is crucial for Au catalysts to be able to

effectively oxidize glycerol. On the other hand, the drawback of using a base is that the products are present as salts and further processing is required afterwards. It has been shown that combining bi- or trimetallic catalysts (Au, Pd, Pt) with acidic or basic supports (e.g., Hmordenite, Mg(OH2)) enables Au-containing catalysts to work with high catalytic activities under neutral conditions.46,

105, 139-140

However, studies based on monometallic Au catalysts

under pH-neutral conditions are scarcely available.141-143 In this light, it is notable that Liu et al. demonstrated that Au/CuO catalysts work with remarkable catalytic performances and high selectivities toward dihydroxyacetone for glycerol oxidation under base-free conditions (Table 3).141,

144-147

The difficulty to achieve high activities under neutral conditions over

monometallic Au catalysts is evidenced by the fact that among the different supports investigated (Al2O3, TiO2, CuO, NiO, and ZrO2), only CuO exhibited a decent catalytic performance. After further optimization of the reaction parameters, it was possible to reach dihydroxyacetone yields of up to 80% after 4 h reaction time at 50 °C. Further isotopic studies and reactions performed with different substrates revealed that dihydroxyacetone is a primary product and that Au/CuO showed a high selectivity toward the oxidation of secondary alcohols.


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As above selected examples confirmed, the efficiency of the Au based catalysts for glycerol oxidation is influenced by many chemical and physical parameters, which needs to be taken into account during the material preparation and catalytic screening steps. The performance of a range of Au containing catalysts along with other parameters is summarized in Table 3. In this section the influence of synthesis parameters on the structure and catalytic activity of exclusively Au containing catalysts was discussed with a special focus on the effect of the capping agent during sol-immobilization synthesis, which was discussed in detail. The next section concerns the synthesis of bimetallic Au containing catalysts and its effect on the catalytic performance in glycerol oxidation reactions.


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Table 3. Typical results for glycerol oxidation over Au-containing catalysts reported in the last years.

Catalyst

Basea

T / °C

PO2 / MPa

Reaction time / h

Au/SiO2

basefree 2:1

80

-c

60

Au/N-CNFe

4:1 basefree basefree basefree 4:1

Au/C

4:1

Au/CNSd Au/HY Au/CuO Au/MgOAl2O3 Au/Fe3O4

Selectivityb %

24

Conv ersion /% 100

/

Researchers and Year

99 AA

0.5

4

57

63 GLA

Kapkowski 2014 Gil 2014

60 50

0.3 0.2

9 4

98 98

80 TA 82 DHA

Cai 2014 Liu 2014

80

1

3

12

82 DHA

Xu 2015

100

1

14

43

61 GLA

Behr 2015

50

0.3

1.5

91

64 GLA

Villa 2016

50

0.3

6

92

71 GLA

Comment

Materials show remarkably high selectivity toward acetic acid but poor recyclability Investigation of the effect of the nitrogen content in the support on catalyst properties and catalytic performance HY zeolite was the most promising support for high TA selectivities Au/CuO among other supported Au catalysts showed the highest yields toward DHA Investigation of the acid-base property of the support on the catalytic activity and selectivity Preparation of catalysts which are magnetically separable from the reaction solution Correlation of the location of Au NPs on the CNF with the observed selectivity Effect of capping agent on structure and catalytic activity investigated

Dimitratos 2016 a NaOH to glycerol ratio if not otherwise denoted. b AA: acetic acid, GLA: glyceric acid, TA: tartronic acid, DHA: dihydroxyacetone; c H2O2 was used as oxidant;d carbon

nanospheres; e carbon nanofibers.


Ref.

146

145

144 141

142

143

147

132

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ACS Catalysis

2.2 Bimetallic Au-Containing Catalysts

This section discusses the effect of alloying Au with other noble-metals on the catalytic activity and selectivity, as well as the activity of these materials under pH-neutral conditions. The use of bimetallic noble metal NPs has proven to be a promising method in order to circumvent inherent drawbacks of monometallic catalysts and, hence, improve catalytic performances in terms of activity, selectivity, and stability.45,

128, 140, 148-150

During the last

decade great effort has been devoted to exploit and understand the expected improved properties of alloyed catalysts for glycerol oxidation151-160, among other reactions.150,

161-165

Moreover, a variety of different preparation methods emerged166 which can be expected to alter the catalytic performance with otherwise same compositions of the bimetallic catalysts. In a very early study in 2007, for instance, Ketchie et al. could show that bimetallic AuPd/C catalysts improved the selectivity toward glyceric acid by maintaining a similar activity compared to Au/C catalysts.154 The authors proposed that the Pd sites accelerated the H2O2 decomposition, which is assumed to be the reason for C-C cleavage and consequently formation of C2 products. Later on, Wong and co-workers further studied the structure-activity relationship in more detail by using AuPd/C catalysts with variable Pd surface coverages for glycerol oxidation.167 A volcano-shape catalytic activity dependence was observed for the materials with the best performance at 80% surface coverage of Pd (Figure 4). The decoration of Pd on Au resulted in increased activities with similar selectivities to glyceric acid – leading to 42% yields compared to 16 and 22% for Au/C and Pd/C catalysts, respectively. Generally, AuPd catalysts exhibited improved resistance against deactivation compared to Pd catalysts. However, an increase in Pd surface coverage resulted in poor deactivation resistances. Nonetheless, the authors concluded that further effects have to be taken into account to explain the decreased activity with higher Pd coverages (> 80%). X-ray absorption near edge structure (XANES) analysis of AuPd/C samples showed that fewer oxidized Pd ensembles were observed than for the Pd/C sample. The authors hypothesized that Au stabilizes the metallic state of surface Pd atoms and that this might be a reason for the activity enhancement observed in other AuPdcatalyzed reactions. The volcano-like behavior between the Pd content and catalytic activity was also reported for benzyl alcohol oxidation by the research group of Hutchings.165 They formed alloyed AuPd NPs where no segregated phases are present as in the studies of Wong et al.167 It seems that the volcano-like Pd content-activity relationship is not restricted to a segregated nanostructure of the catalyst – indeed, for glycerol oxidation, the volcano-like


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behavior was also reported over alloyed AuPd NPs168 and over Au on Pd catalysts169. Furthermore, Hutchings et al. pointed out that for the water-gas shift reaction, CO oxidation, and formic acid decomposition performed in the gas phase the beneficial volcano-plot behavior was not observed indicating that the reaction mechanism plays a pivotal role in order to determine the activity enhancing effects of bimetallic catalysts.165

Figure 4. Plots of (a) kcat and (b) initial TOF values with Pd surface coverage. Reaction conditions: 0.2 g catalyst, 60 °C, 0.1 M glycerol, NaOH/glycerol = 4:1, 120 mL min-1 O2 flow. Reproduced from ref.

167

with

permission from the Royal Society of Chemistry (CC BY-NC 3.0).

Various kinds of supports have been investigated for supported noble metal catalysts and it was shown that the support influences the catalytic activity and selectivity for glycerol oxidation by affecting the dispersion, size, morphology, stability and electron density of the noble metal NPs. Besides this, the support can have a more direct influence on the reactants in solution (educts, products or intermediates) by other properties (e.g., hydrophobicity, acidic or basic sites etc.), which were investigated by several groups.131, 170-174 Evans et al. used LaBO3 perovskite supports (where B = Cr, Mn, Fe, Co or Ni) for bimetallic AuPt nanocatalysts for glycerol oxidation.175 They could show that by changing the B site the catalytic activity and selectivity of the materials changed remarkably as a result of the altered oxygen adsorption capacity. For instance, by using AuPt/LaMnO3 catalysts, they observed a selectivity toward glyceric acid of 70%, whereas the AuPt/LaCrO3 catalyst exhibited a selectivity of 86% toward lactic acid under the same reactions conditions (Table 4). As mentioned in the section before, it is highly desired to work under base-free conditions for glycerol oxidation. Villa et al. used AuPt catalysts supported on a range of different acidic (H-mordenite, SiO2, MCM-41, and sulfated ZrO2) and basic (NiO and MgO) oxides in order ACS Paragon Plus Environment

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ACS Catalysis

to elucidate the role of the acid nature, strength, and density on the activity and selectivity for base-free glycerol oxidation.176 The materials were characterized and the trend of total amounts

of

acid

sites

was

AuPt/sulfated ZrO2 > AuPt/H-mordenite

> AuPt/SiO2 > AuPt/MCM-41. A reversed trend was observed for the catalytic activity so that it was concluded that the higher the total amount of acid sites, the lower the catalytic activity. Basic supports promoted the activity but also increased the C-C bond cleavage. In the case of acidic supports, higher selectivities to C3 products were obtained. Interestingly, AuPt/MCM41 with the lowest amount of acidic sites observed in the series exhibited high selectivities to glyceraldehyde (46% at 30% conversion). The suppression of the further oxidation to glyceric acid marks an important step in order to produce this labile intermediate. Another approach performed by Villa et al. was the use of Bi as promotor for AuPt/AC catalysts to drive glycerol oxidation under base-free conditions.177 The promotor Bi is known to shift the selectivity toward dihydroxyacetone93,

95, 178-179

and was already earlier used to

promote bimetallic catalysts for glycerol oxidation.180 The addition of Bi (0.1 wt% loading) improved the catalytic activity of AuPt/AC (Table 4) and Pt/AC and yielded higher amounts of dihydroxyacetone at the expense of glyceric acid. Selectivities of 63% to dihydroxyacetone at 80% conversion were reached after 4 h reaction time. More importantly, the authors showed that Au stabilizes Bi and prevents Bi leaching into the solution, which resulted in more stable catalysts with better performances in recycling tests. These and other results are summarized in Table 4.181-184 As this section illustrated, alloying Au with other noble-metal particles is a promising pathway to obtain more active and selective catalysts as compared to monometallic Au catalysts. More importantly, bimetallic Au catalysts are, in contrast to monometallic Au containing catalysts, also active for the selective oxidation of glycerol in pH-neutral media.


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Table 4. Typical results for glycerol oxidation over Au-containing bimetallic catalysts reported in the last years. Catalyst

Basea

AuPt/CeO2

4:1

100

0.5

0.5

Conv ersion /% 99

AuPt/USY600

basefree

180

0.3

2

80

60 LA

Bi-AuPt/AC

basefree 4:1 4:1 4:1 2:1

80

0.3

4

80

63 DHA

100 100 100 60

0.3 0.3 0.3 0.6

6 6 24 5

75 95 100 80

70 GLA 86 LA 88 TA 79 GLA

AuPt/LaMnO3 AuPt/LaCrO3 AuPt/LaMnO3 AuCu/CeZrOx

T / °C

PO2 / MPa

Reaction time / h

Selectivity /%

Researcher s and Year

80 LA

Purushoth aman 2014

AuPt/CeO2 shows superior catalytic performance compared to other supports and the monometallic counterparts

183

Purushoth aman 2014 Villa 2015

Investigation of various zeolite supported AuPt catalysts for glycerol oxidation to LA or GLA

182

Addition of Bi as promotor shifted the selectivity toward DHA for AuPt/AC; furthermore, Au stabilizes Bi and prevents Bi leaching into the solution Variation in the B site of the perovskite support LaBO3 resulted in an altered reaction pathway and, consequently, in changed selectivities. Prolonged reactions times showed a remarkably high selectivity toward tartronic acid. Bimetallic AuCu catalyst show superior catalytic activity compared to the monometallic Au catalyst. AuCu catalysts slightly increase glycolic acid selectivity. Different Au/Pt ratios investigated; Au leaching and AuPt NP sintering was observed

177

b

Evans 2016

Comment

Kaminski 2017 c AuPt/TEGO base- 60 0.3 4 60 53 GLA Yang free 2017 a NaOH to glycerol ratio if not otherwise denoted; b LA: lactic acid, GLA: glyceric acid, TA: tartronic acid, DHA: dihydroxyacetone; c thermally expanded graphene oxide.


Ref.

175

181

184

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ACS Catalysis

2.3 Glycerol Oxidation over Pt Containing Catalysts

Although supported Pt NPs were one of the first noble metal catalysts investigated for glycerol oxidation initiated by Kimura et al. in 199393-94, there are still great efforts committed by several research groups in order to improve this system.185-193 Unlike for Au catalysts, Pt catalysts can be used under acidic conditions. Generally, the main challenges lie in the achievement of higher resistances against oxygen poisoning and an improved selectivity toward dihydroxyacetone. These issues are subject of current research and are further discussed in the following. Some recent results are summarized in Table 5.194-200 Similar to gold catalysts, the size and shape of Pt and its interaction with the support are the driving forces for altered catalytic activities. Li et al. investigated the effect of size and shape of Pt NPs supported on silica for the selective oxidation of glycerol.49 They demonstrated that higher Pt loadings led to larger Pt NPs which then again resulted in higher conversions and higher selectivities for the oxidation of the terminal alcohol group. Furthermore, cuboctahedral NPs exhibited higher turnover frequencies than tetrahedral NPs with, however, no big influence on the selectivity. A more thorough, kinetic study, conducted by the same authors, indicated that for Pt/SiO2 catalysts the changes in activity and selectivity were a function of conversion and reversible deactivation of the catalyst occurred due to poisoning of produced dihydroxyacetone.110 They hypothesized that dihydroxyacetone poisons specific sites of the Pt catalysts which results in the observed changes in selectivity with increasing conversion. The implication of this study is that the temporal evolution of the reaction has to be taken into account when comparing the activity and selectivity for glycerol oxidation reactions. As discussed for mono- and bimetallic Au containing catalysts, the role of the support was also investigated for Pt-containing catalysts. Tan et al. demonstrated that Pt catalysts supported on KOH-activated mesoporous carbon (AMC) were able to catalyze the glycerol oxidation (78% selectivity toward glyceric acid at 68% conversion) at room temperature and under base-free conditions (Table 5).201 The activation procedure resulted in surface oxygen functional groups which were pivotal for a high activity of the catalyst; however, carboxylic groups were detrimental for the catalytic performance and only functional groups like phenol, ether, or carbonyl/quinone groups improved the activity. Hence, a heat treatment at 600 °C was necessary to remove the carboxylic groups and obtain active catalysts. The enhancement was attributed to the increased hydrophilicity and basicity of the carbon surface and by improved stabilization of the Pt NPs. ACS Paragon Plus Environment

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Chen et al. used nitrogen-doped carbon nanotube (N-CNT) supported Pt catalysts for the efficient glycerol oxidation under base-free conditions (Figure 5). They could show that nitrogen doping alters the electronic structure and surface basicity of the N-CNTs. Pt/N-CNTs outperformed the pristine Pt/CNTs catalysts for glycerol oxidation in terms of glycerol conversion and glyceric acid selectivity showing an almost twofold increase in TOF.202 Furthermore, smaller Pt NPs could be obtained on N-doped CNTs compared to the neat CNTs. The improved catalytic performance through N-doping was ascribed to: i) electron transfer from N to Pt; ii) accelerated activation of molecular oxygen; iii) increase in surface defect sites which promotes the adsorption of glycerol and oxygen; and iv) the increased surface basicity allowing a more facile activation of the –OH groups in glycerol. The authors also pointed out that the catalysts can be reused for several times, but a regeneration step under H2 is required due to oxygen poisoning (PtOx formation) of the catalyst. Similar results were found by Ning et al. in the same year, who however observed improved selectivities towards glyceraldehyde upon N-doping. 203

Figure 5. TEM dark-field micrograph and STEM-EDX elemental mappings of 1% Pt/N-CNT. Reproduced from ref. 202 with permission from the Royal Society of Chemistry (CC BY 3.0).

Zhang et al. reported the use of N-doped multiwall carbon nanotubes as supports for Pt NPs for glycerol oxidation under base-free conditions.204 In good accordance to the results by Chen et al.202 and Ning et al.203, N-doping resulted in improved Pt dispersion with smaller


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ACS Catalysis

mean particle sizes and in electron-enriched Pt NPs. Moreover, the workgroup demonstrated that Pt/N-MWCNTs had an improved recyclability compared to Pt/MWCNTs.

Figure 6. HRTEM micrographs of encapsulated Pt@C-MWCNTs (a and b) and naked Pt/MWCNTs (c and d). e) Recycling of Pt@C-MWCNTs for ethanol oxidation. Reaction conditions: 20 mL aqueous solution of ethanol (0.2 g mL-1), catalyst 30 mg, 2 MPa O2, 130 °C, 2 h. Reproduced with permission from ref205. Copyright 2017 Elsevier.

The same group demonstrated in a recent study that MWCNT supported Pt NPs encapsulated with a carbon film (Figure 6 a-d) exhibited a superior catalytic performance and stability for ethanol (Figure 6 e) and glycerol oxidation compared to the naked Pt NPs supported on MWCNT (Table 5; entry Pt@C/MWCNT).205 Pt/MWCNT was prepared by a polyol method and the carbon film encapsulation was achieved by H2 treatment at 600 °C for 1 h. The authors concluded that the encapsulation prevented Pt NP sintering, leaching and overoxidation during the reaction, which was confirmed by ICP-AES, TEM, XPS and TPD studies. They furthermore hypothesized that strong adsorption of oxidation products inhibiting glycerol oxidation could also be prevented by carbon film encapsulation. This section discussed the effect of Pt size and shape on the catalytic activity towards glycerol oxidation. Unlike Au containing catalysts, Pt containing catalysts are also active under pH-neutral and acidic conditions. Current approaches to solve the main issues in Pt catalyzed glycerol oxidation, which are Pt poisoning by oxygen and the only moderate selectivities towards dihydroxyacetone, were discussed in this section. As the next section will show, these problems can also be addressed by the employment of Pt containing bimetallic catalysts.


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Table 5. Typical results for glycerol oxidation over Pt-containing catalysts reported in the last years. Catalyst

Basea

T / °C

PO2 MPa

Pt/Y2O3

5:1

60

0.1

/

Reaction time / h 4

Conv ersion /% 86

Selectivityb / %


Comments

63 LA

Chorna Catalyst preparation by the extractive-pyrolytic method; different calcination ja 2015 temperatures were applied and correlated with the catalytic performance Pt/Fe3O4@ base-free 60 0.5 24 91 55 GLA Long Magnetic Fe3O4 particles encapsulated in polypyrrole (PPy) and decorated with Pt NPs PPy 2015 were employed as easily separable catalysts PT/Nbase-free 60 0.5 3 55 54.9 GLA Zhang N-doped MWCNT supported Pt catalyst showed superior catalytic performance and MWCNTc 2015 stability compared to the MWCNT supported Pt catalyst. Pt/MCNd base-free 60 0.3 4 63 59 GLA Wang MCN materials showed with increasing N content an increase of weak basic sites and a 2015 decrease in particle size of supported Pt catalyst resulting in improved catalytic activity Pt/IERe 3:1 50 0.15a 8 91 63 GLA Gross Investigation of the effect of the counter ion of the IER on the Pt/IER catalyst 2015 preparation and catalytic performance Pt/N-CNT base-free 60 10 mL 4 76 56 GLA Chen Pt/NCNTs with different N-content are investigated for glycerol oxidation and min-1 2015 compared to results with metal oxide supported Pt catalysts Pt/AC 90 0.1 100 69 LA Zhang Investigation on the role of base type 2016 Pt/AMCf base-free 25 20 mL 68 78 GLA Tan Correlation of oxygen functional groups on the support with catalytic performance min-1 2016 Pt/AC base-free 60 150 mL 6 55 35.3 GLA Lei Pt/AC prepared by polyol method is catalytically superior compared to Pt/AC prepared -1 min 2016 by wet impregnation method Pt/TiO2 base-free 150 0.5 18 78 70 LA Koman TiO2 showed the best results among other supports investigated (Nb2O5, ZrO2, Al2O3, oya MgO, SnO2, SiO2, AC). Pt-PVP NPs were added into the reaction solution in the 2016 presence of the metal oxide. Pt NP immobilization occurred during glycerol oxidation Pt/NGbase-free 60 0.5 3 64 81.0 GLA Zhang MWCNTs-pillared N-doped graphene (NG) superior support for Pt NPs compared to MWCNT 2017 bare MWCNTs which results in improved catalytic performance Pt/N2.5Cbase-free 60 0.6 3 67 74 GLA Yang N-doped carbon film coated active carbon with high surface area was used as support XC-72 2017 Pt@C/M base-free 60 0.5 3 58 85 GLA Sun Pt NPs encapsulated with a carbon film show superior catalytic performances WCNT 2017 compared to the naked supported Pt NPs a NaOH to glycerol ratio if not otherwise denoted; b LA: lactic acid, GLA: glyceric acid; c multiwall carbon nanotubes; d mesoporous carbon nitride; e ion exchange resin; f activated mesoporous carbon.


Ref.

192 197 204 190 194 202 199 201 196 195

200 198 205

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ACS Catalysis

2.4 Bimetallic Pt-Containing Catalysts

As in the case for Au-containing catalysts, using promotors or alloys for Pt catalysts significantly enhances the catalytic performance and can shift the selectivity to the desired products.206-207 Thus, many researchers devoted their studies to modified Pt NPs and typical results regarding the activity and selectivity of these materials are presented in this section and listed in Table 6. Jin et al. prepared bimetallic PtFe nanoclusters supported on CeO2 and investigated these materials for glycerol oxidation (Table 6).208 They showed via kinetic studies that the presence of Fe species in the Pt catalysts lowers the activation barriers for both primary and secondary oxidation reactions resulting in increased activities and selectivities for glycerol oxidation. The authors ascribed the improved catalytic performance to the lattice mismatch between Pt and Fe resulting in high index surfaces. Also after careful optimization of the reaction parameters high selectivities toward tartronic acid were achieved over PtFe catalysts, which were not observed for Pt catalysts. In 2016, Ning et al. explored the promoting effect of Bi and Sb on Pt catalysts supported on N-CNTs for glycerol oxidation with high selectivities toward dihydroxyacetone.209 They demonstrated that Bi and Sb act as site blockers on the surface of Pt functioning as geometrical promoter. More importantly, it was revealed that having Bi or Sb present in the reaction solution (rather than having Bi pre-loaded on the catalyst) results in improved catalytic performances. This was an indication that through adsorption of Bi on Pt/N-CNT similarly active sites could be generated as in the case of pre-loaded PtBi/N-CNT catalysts. Xiao et al. performed a comparative study with bimetallic PtBi catalysts over various supports (AC, ZSM-5, MCM-41, Bi-doped MCM-41).210 PtBi/MCM-41 was found to exhibit the highest dihydroxyacetone yield. Their results were further supported by DFT computations to obtain a deeper insight into the reaction mechanism. Both experimental and calculated results revealed that Bi should be located at the surface and not in the bulk of the Pt metal in order to achieve the desired effect. Dou et al. employed PtSn/AC catalysts with different Pt/Sn ratios for glycerol oxidation.211 The NPs were prepared by a polyol method and deposited by sol-immobilization on activated carbon. Pt9Sn1/AC showed the best catalytic performance among the catalysts investigated with an improved glycerol conversion (50% yield toward glyceric acid at 91% conversion) compared to the monometallic Pt/AC sample (69% conversion) under the same reaction conditions (60 °C, 15 mL min-1 O2 flow, 8 h). As Table 6 indicates, bimetallic Pt containing


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catalysts were shown to improve the catalytic activity as well as selectivity compared to their monometallic counterparts, which is ascribed to different effects of the promotors.


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ACS Catalysis

Table 6. Typical results for glycerol oxidation over Pt-containing bimetallic catalysts reported in the last years. Catalyst

Basea

PtSb/MWC NTc PtCu/CNTd

base-free

60

base-free

60

base-free

60

PtBi/N-CNT PtSb/NCNT PtCu/AC

T / °C

base-free

60

3:1

90

PO2 MPa

/

150 mL min-1 0.3 150 mL min-1 150 mL min-1 100 mL min-1 15 mL min0.1

Selectivityb %

Reactio n time / h 2

Conv ersion /% 66

63 DHA

30

41

57 GLAD

6 6 4

/

Research Comments Ref. ers and Year Nie PtSb catalysts show superior catalytic activity and selectivity toward DHA compared to 206 2012 PtBi and monometallic Pt catalysts 212 Ribeir Improvement of catalytic performance with PtCu catalysts under base-free conditions o 2016

36 56 DHA 51 80

38 DHA

Ning 2016

Effect of Bi and Sb promotors on catalytic performance; Investigation of in situ generated PtBi and PtSb catalysts for glycerol oxidation

69.3 LA

Zhang Variation of Cu content in PtCu/AC catalysts; 0.5%Cu-1.0%Pt/AC showed the best 2016 promotional effect PtSn/AC base-free 60 2 91 55 GLA Dou Various bimetallic PtM/AC (M = Mn, Fe, Co, Ni, Cu, Zn, Au) catalysts were 2016 investigated, where PtSn/AC showed the best performance PtFe/CeO2 4:1 60 2 57 71 GLA Jin PtFe alloy catalysts showed increased catalytic performances compared to the 2016 monometallic Pt/CeO2 catalyst a NaOH to glycerol ratio if not otherwise denoted; b GLAD: glyceraldehyde, DHA: dihydroxyacetone, LA: lactic acid, GLA: glyceric acid; c multiwall carbon nanotubes; d carbon nanotubes.


209

207

211

208

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2.5 Glycerol Oxidation over Pd and Ag Containing Mono- and Bimetallic Catalysts

Pd containing catalysts were also employed for glycerol oxidation in the aqueous phase from early on.38 Although the number of reports for monometallic supported Pd catalysts is low in comparison to Au and Pt catalysts, there is still ongoing research to improve these catalysts for glycerol oxidation. In general, basic conditions are also required for Pd and Ag containing catalysts to obtain high activity for glycerol oxidation as in the case of Au, although small glycerol conversions can be obtained under neutral conditions.43, 45 Due to the low number of reports emerged during the last years, both the mono- and bimetallic Pd and Ag containing catalysts are summarized in this section with some examples listed in Table 7.213-217 Chan-Thaw et al. used nitrogen-containing covalent triazine frameworks as supports for Pd catalysts.218 Pd was deposited on or within the support by a sol-immobilization or an impregnation method. The impregnation technique resulted in the formation of PdHx catalysts which contained solely oxidized Pdδ+ states (assignment of the true oxidation state was difficult by XPS analysis; hence,

δ+

is used to denote a non-metallic state). It was shown that

both preparation method yield similar catalytic performances (Figure 7 a), although no metallic Pd states were present in the impregnated catalyst. The authors inferred that metallic Pd states were in situ formed during the glycerol oxidation since also Pd2+/CTF catalysts were to some extent active for the reaction. During nine recycling runs, Pd0 was formed for the impregnated catalyst increasing the selectivity toward glyceric acid from 62 to 80%. Moreover, the confinement of Pd NPs within the framework of the support led to stabile catalysts for glycerol oxidation compared to the catalysts prepared by sol-immobilization (Figure 7 b).


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Figure 7. a) Reaction profiles for glycerol oxidation over Pt/CTF catalysts and b) recycling runs for 4 h reactions. Reaction conditions: 0.3 M glycerol, NaOH/glycerol = 4:1, 0.3 MPa O2, 50 °C. Reproduced with permission from ref218. Copyright 2015 Wiley-VCH.

Ribeiro et al. demonstrated that Pd/CNT catalysts exhibited a similar behavior as Pt/CNT catalysts for glycerol oxidation under basic conditions.212 Both catalysts were capable to reach conversions close to 90% with selectivities to glyceric acid of 60-70% after 5 h reaction time (Table 7). The effect of the reaction conditions (temperature, oxygen pressure, basicity) was thoroughly studied for both catalysts by the authors and it could generally be observed that Pt/CNT produced slightly more glycolic or formic acid at the expense of tartronic acid than Pd/CNT. Moreover, both catalysts showed a good stability over 4 consecutive runs. Faroppa et al. modified γ-Al2O3 supported Pd catalysts with Pb and observed a remarkable increase in glycerol conversion with high selectivities toward dihydroxyacetone.219 The reactions were conducted under basic conditions at 45 °C and with H2O2 as oxidant instead of O2. The bimetallic PdPb alloy increased the glycerol conversion from 19 to 100% compared to the monometallic Pd/Al2O3 catalyst. The authors revealed that a Pb/Pd atomic ratio of 0.5 resulted in the optimum catalytic performance (Table 7). They concluded that Pb modified the regioselectivity of the reaction and led to the preferential oxidation of the secondary hydroxy group of glycerol. Hirasawa et al. used a PdAg alloy (Pd/Ag = 1) catalyst for base-free glycerol oxidation.220 They obtained high selectivities to dihydroxyacetone (85%) at 52% glycerol conversion and superior catalytic performance of the PdAg alloy catalyst compared to the monometallic Pd and Ag catalysts with negligible low conversions. They suggested that Ag prevents the deactivation of Pd. The authors later on reported the mechanistic details for PdAg catalysts for glycerol oxidation221 and extended their studies to various other substrates and ACS Paragon Plus Environment

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demonstrated that the secondary hydroxyl group of vicinal diols is preferably oxidized over PdAg catalysts, which increases the selectivity towards dihydroxyacetone. Recently Skrzynska et al. found that also alumina supported monometallic Ag itself acts as a heterogeneous catalyst for the selective oxidation of glycerol under basic conditions yielding mainly the C2 product glycolic acid with a selectivity of 57 % at 85 % conversion.222 In their study, the authors prepared Al2O3 supports via different methods to obtain different phases and basicities of the Al2O3 support. As their results indicate, basic Al2O3 performs best in terms of conversion and glycolic acid selectivity. Nonetheless, it is questionable whether the alumina support remains stable under the basic reaction conditions (1.2 M aqueous NaOH) and elevated temperatures, especially since no post-characterization or recycling data of the catalysts are presented. The same group later studied the promotion effect of Ag based catalyst by noble metals (Au, Pd, and Pt) supported on CeO2 and found that, in contrast to Au and Pd, 5 wt% of Pt (relative to Ag) improved the catalytic activity to form glycolic acid with a selectivity of 51 % at a conversion of 54 % (Table 7).223 In this section, recent results of rarely studied Pd or Ag containing catalysts for the oxidation of glycerol are summarized. As in the case of Au, Pd containing catalysts are also active under basic conditions and show similar selectivities as Pt containing catalysts under comparable reaction conditions. As already illustrated by the results of the previous sections, alloying of Pd with various noble and non-noble metals also increases the catalytic activity and selectivity in the selective oxidation of glycerol.


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Table 7. Typical results for glycerol oxidation over Pd-containing mono- and bimetallic catalysts reported in the last years. Catalyst

Basea

T / °C

PO2 MPa

Selectivityb / %


2

Conv ersion /% 46

Pd/TiO2

160

1

Pd/CTF

basefree 4:1

48 LA

Liu 2013

50

0.3

3

98

81 GLA

ChanThaw 2015

Pd/Al2O3

5:1

60

0.1

7.5

100

76 GLA

basefree

80

150 mL min-1

5

21.4

27.6 MOXA

Chornaja 2015 Yan 2016

Pd/CNSc

Pd/CNTd

2:1

60

0.3

7

90

61 GLA

Pd/AC

4:1

60

0.6

0.5

Pd/HTc

2:1

90

0.8

3

PdPb/Al2O3

initial 45 pH 11 constant 50 pH 11 4:1 60

5 vol% H2O2 200 mL min-1 0.5

1

PdBi/CDe Ag/Al2O3

/

Reaction time / h

3 3

Ag954:1 60 0.5 5 Pt5/CeO2 a NaOH to glycerol ratio if not otherwise denoted;

b

Comment

Ref.

Addition of AlCl3 for performance improvement and compared to other Lewis acids; Pd/TiO2 showed poor recyclability PdHx NPs confined within N-containing covalent triazine framework (CTF) are active for glycerol oxidation. Pd0 is formed during recycling tests.

216

Catalyst preparation by the extractive-pyrolytic method; different calcination temperatures were applied and correlated with the catalytic performance Investigation of the dependence of surface basicity of functionalized CNS support on catalytic performance for aldol condensation, benzyl alcohol oxidation, and glycerol oxidation No major difference were observed to Pt/CNT catalyst in terms of activity and selectivity AC showed among all other investigated supports (Al2O3, TiO2, SiO2) the best catalytic performance in terms of activity and selectivity Catalytic activity was attributed to the basic sites of the hydrotalcite (HTc) support and well-dispersed Pd NPs

192

Ribeiro 2016 100 44 GLA Namdeo 2016 70 80 GLA Abd Hamid 2016 Faroppa Bimetallic PdPb catalysts exhibited a remarkable increase in catalytic performance 100 59 DHAf 2016 compared to the monometallic Pd catalyst 95 68 GLA Vajicek Bi promotor strongly enhanced the catalytic performance 2016 85 57 GLCA Skrynska Catalyst activity depends on nature of the Al2O3 support with basic alumina supports 2016 yielding the most active catalysts. 54 51 GLCA Zaid Other promotors (Au and Pd) and higher amounts of Pt did not improve activity 2017 LA: lactic acid, GLA: glyceric acid, MOXA: mesoxalic acid, DHA: dihydroxyacetone; GLCA: glycolic acid c carbon

nanospheres; d carbon nanotubes; e Dowex® anion exchange resin; f determined at 85% conversion.


218

217

212

214

213

219

215

222

223

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3. Glycerol Oxidation over Non-precious Metals In the previous section, it was thoroughly elaborated how noble metal containing catalysts are capable of oxidizing glycerol to value-added products. Nonetheless, it remains of great concern to reduce the use of noble metals for various kinds of reactions and be able to perform such organic transformations with less noble and earth-abundant metals. Although some efforts have been devoted to employ non-precious homogeneous catalysts for the selective oxidation of glycerol67-68, 224-230, the reports presented in this section will focus on heterogeneous catalysts. A comprehensive overview over the use of non-noble metals as catalysts for the selective oxidation of glycerol is given. The described catalysts range from various transition metals (Fe, Cu, Cr, Co, Ni) incorporated into silicalites, hydrotalcites, heteropolyacids, and zeolites. Among the introduced catalysts, emphasis is put on Cocontaining catalysts as a promising alternative to noble metal catalysts. Typical catalysts and results obtained for glycerol oxidation are summarized in Table 8. Some efforts have been dedicated to finding active materials for glycerol oxidation based on CuNi-Al layered double hydroxides231 and Mg-Al layered double hydroxides (LDHs) in combination with various transition metals (Cr, Mn, Fe, Co, Cu).232-237 Zhou et al. incorporated various metals (Ni, Zn, Cu, Co, etc.) into Mg-Al LDHs and investigated the effect of the material composition on the catalytic performance for glycerol oxidation.232 The study revealed that Cu containing LDHs (CuAlMg) showed the most promising catalytic results with higher yields toward glyceric acid compared to the other transition metals. Indeed, uncalcined CuAlMg catalysts exhibited 36% and 51% selectivities toward glyceric acid and formic acid at 86% conversion (2:1 NaOH:glycerol, 60 °C, 3 h reaction time, O2 bubbling through the suspension). In contrast, by using Ni, Zn, or Co containing catalysts, the conversion is drastically diminished and formic acid or glycolic acid is formed with high selectivity. Calcination of CuAlMg resulted in a further remarkable boost in activity and selectivity for glycerol oxidation (70% selectivity toward glyceric acid at 97% conversion), which was not observed for the other metal containing LDHs. It has to be emphasized that upon calcination the LDH structure is destroyed and poorly ordered mixed oxides composed of MgO, Al2O3, and copper oxide are formed. However, this composite mixture containing Cu seemed to be a promising material for glycerol oxidation avoiding noble metals. The authors concluded that Cu(OH)2 present in the LDH structure is less active than the Cu2+ species present in the calcined mixed oxides.


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Wang et al. reported in a series of studies the employment of sulphonato-salen-Cr3+ complexes intercalated into Mg-Al LDHs (CrAlMg) as catalysts for the selective oxidation of glycerol under neutral or basic conditions.233-236 In the first study in 2012, the authors demonstrated that a selectivity toward dihydroxyacetone of 44% at a glycerol conversion of 73% was achieved over CrAlMg catalysts with 3% H2O2 as oxidant under base-free conditions (60 °C, 4 h reaction time, atmospheric pressure).233 It is noteworthy to mention that the best performance of the catalyst was observed to be at neutral pH with a decrease in conversion and selectivity toward dihydroxyacetone with lower or higher pH value. In addition, the catalyst was recycled up to 5 times without any significant loss in activity or selectivity and negligible Cr leaching. In a follow-up study, they studied the effect of the elemental composition of the LDH hosts for the intercalated Cr3+ complexes on the catalytic performance.234 They concluded that among the studied metals (Cu, Ni, Mn) Cu showed the most promising catalytic results (59% selectivity toward dihydroxyacetone at 86% conversion, 3% H2O2, 60 °C, 6 h reaction time, atmospheric pressure). However, it has to be pointed out that in this study the Cu containing CrAlMg was not compared to the Cu-free CrAlMg and only metal modified CrAlMg were compared with each other. Exchanging the metal cation Cr3+ in the complex with different other metals (Mn, Fe, Co, and Cu) did not yield improved catalytic performances implying that the sulphonato-salen-Cr3+ complex was the best performing one.236 Under basic conditions (8:1 NaOH:glycerol) with O2 instead of H2O2 as oxidant on the other hand, it was evident that the LDH hosted Cu complex showed superior catalytic performances compared to the other catalysts with a high selectivity toward glyceric acid (86% at 83% conversion).235 Also here, high recyclability and negligible Cu leaching were evidenced. Very recently, Jin et al. reported that Co catalysts supported on Mg-Al LDH structures showed high glycerol conversion with remarkable selectivities to tartronic acid.237 In their study, they applied different synthesis procedures for their materials, namely a coprecipitation and a sol-gel method, and studied the effect on the catalytic performance. In the co-precipitation method, the Co nitrate was simultaneously introduced with Al and Mg nitrates to precipitate, whereas in the sol-gel method the Co nitrate was added after the formation of the Mg-Al hydroxide gel. The reaction profile for Co0.15/Mg3Al-sol-gel is depicted in Figure 8 a) where it is shown that a full conversion of glycerol can be achieved after 18 h reaction time with a tartronic acid selectivity of 58%. Furthermore, the catalysts exhibited a decent stability as corroborated by the recycling experiments shown in Figure 8 b.


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Figure 8. a) Conversion, selectivity and carbon mass balance profiles for glycerol oxidation over Co0.15/Mg3Alsol-gel catalyst. b) Recycling experiments for glycerol oxidation over Co0.15/Mg3Al-sol-gel catalyst (reaction time: 6 h). Reaction conditions: 70 °C, 0.22 M aqueous glycerol solution, 6.8:1 glycerol:NaOH, O2 bubbling. Reprinted with permission from ref. 237 Copyright 2016 American Chemical Society.

The catalysts prepared by the sol-gel method showed superior catalytic performances compared to the materials prepared by the co-precipitation method. The authors claimed that the reason can be found in the improved interaction of the Co species with surface hydroxides, which was not possible for the Co species incorporated into the framework by the co-precipitation method. However, it has to be emphasized that the materials prior to catalytic testing were calcined under air and activated under H2 which should destroy the LDH structure. On the contrary, the characterization of the catalysts and conclusions drawn from this were made before the materials were calcined and activated. Hence, some ambiguity remains in determining the reason for the high activity. However, these materials showed a remarkable catalytic performance for glycerol oxidation. This stresses the fact that specifically tailored non-noble metal catalysts are capable of resembling catalytic properties of noble metal catalysts with, in some cases, remarkable selectivities toward desired oxidation products, which are difficult to obtain. Our group demonstrated that catalysts containing Cu and Co are active for glycerol oxidation and that a specific synergy between both metals is required to obtain high catalytic performances.238 The most active materials in this study consisted of a mesoporous ordered CoO framework with embedded metallic Cu nanoparticles (Figure 9 a-d), which were prepared from copper cobalt spinel oxides by a mild reduction technique with ethanol vapor developed in our group.239-240 As can be seen in Figure 9 e), the reduction step increases the catalytic performance of the materials remarkably and the catalyst with a Co/Cu ratio of 2 to 1 exhibited the highest glycerol conversion. Based on these promising results, we investigated ACS Paragon Plus Environment

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in a following study the role of the post-treatment on the catalytic performance of these materials and identified the active phases of CuCo-based catalysts for glycerol oxidation.241 For this purpose, a facile co-precipitation method was used to prepare non-ordered CuCobased catalysts which experienced different post-treatments. We demonstrated that the mild reduction step with ethanol vapor resulted in the highest catalytic performance for glycerol oxidation also for these materials (Figure 9 f). Generally, high selectivities toward glycolic acid and formic acid were observed. Moreover, the as-prepared material without any posttreatment showed similar high catalytic performances as the material treated with the mild reduction step. To further explain this observation, XRD studies were conducted for fresh and spent catalysts and it was shown that the possible active phases were CoO(OH) and CuO in intimate contact with each other. These phases were formed during the oxidation reaction for both, the as-prepared and the mildly reduced, materials. On the other hand, these phases were not detected for the calcined sample and the sample reduced under H2 atmosphere, which resulted in poor catalytic performances (Figure 9 f). Also in this case as for the ordered mesoporous samples (Figure 9 e), it could be shown that a Co/Cu ratio of 2 to 1 exhibited the highest performance for glycerol oxidation (Figure 9 g). Furthermore, the single phases CoO(OH) and CuO showed a negligible catalytic activity, emphasizing the necessity of both phases in close contact for high catalytic performances. Whereas our study showed that Co and Cu species must both be present in close contact to obtain an active catalyst, Palacio et al. reported a ceria supported Co3O4 catalyst with good selectivity (79.8 %) at a high glycerol conversion (85.7 %) towards lactic acid. The different findings can be rationalized by the high reaction temperature of 250 °C that the authors used in their study.242


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Table 8. Typical catalysts and results for glycerol oxidation over non-precious metal catalysts. Basea

Catalyst

T /

PO2

°C

MPa

/

Reaction

Conv

Selectivityb /

Researchers

time / h

ersion

%

and Year

Comment

Ref.

/% CuAlMg

4:1

60

HTc

const.

3

97

71 GLA

Zhou 2011

Hydrotalcite catalysts showed improved catalytic performance after calcination

232

4

73

44 DHA

Wang

Sulphonato-salen-Cr complex was hosted in the HT

233

O2 flow

CrAlMg HT

base-

60

-d

free c

CuNiAl HT

4:1

2012

60

60 mL

4

68

76 GLA

Wu 2013

Amino functionalized hydrotalcites showed superior catalytic performance

231

18

100

58 TA

Jin 2016

Co incorporated into the HT via different synthesis methods. Materials were

237

min-1

CoMgAl

6.8:1

70

HT

O2 flow

AlPMo12O40

base-

HPAe

free

Cu/CoO

4:1

a

const.

60

0.1

calcined and H2 treated prior to catalytic use. 5

94

91 LA

Tao 2016

HPA catalysts demonstrated to be tolerant to crude glycerol for glycerol

243

oxidation. 90

0.1

3

80

41 GLCA

Dodekatos

In-situ generation of catalytically active phases for glycerol oxidation. Synergy

2017

between Cu and Co species required.

b

NaOH to glycerol ratio if not otherwise denoted; GLA: glyceric acid, DHA: dihydroxyacetone, TA: tartronic acid, LA: lactic acid, GLCA: glycolic acid; c hydrotalcite; d 3%

H2O2 was used as oxidant; e heteropolyacid.


241

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Figure 9. a) SEM image and b) cross-sectional SEM image of a Cu/CoO catalyst (Co/Cu=8). c) EDX mapping of cobalt (blue) and copper (yellow) of the region in b). d) Schematic illustration for the formation of ordered mesoporous Cu/CoO by mild reduction from the CuxCoyO4 spinel oxide. e) Conversions for glycerol oxidation over the ordered mesoporous oxide spinels and reduced Cu/CoO catalysts with different Co/Cu ratios. Reprinted with permission from ref

238

. Copyright 2015 Wiley-VCH. f) Conversions for glycerol oxidation over non-

ordered CuCo-based catalysts prepared by different post-treatments: 2CuCo-ap, as-prepared sample by a coprecipitation method; 2CuCo-cal, as-prepared sample calcined under air; 2CuCo-H2, calcined sample further reduced under H2/Ar flow; 2CuCo-Et, calcined sample further reduced under ethanol/N2 flow. g) Conversion profiles for glycerol oxidation over CuCo-ap and CuCo-Et with different Co/Cu ratios. Reprinted with permission from ref 241. Copyright 2016 Wiley-VCH.

Later on, we employed ordered mesoporous Cu-containing Al2O3 catalysts for glycerol oxidation.244 Cu was incorporated into the γ-Al2O3 crystal structure with different loadings and, in contrast to our previous results with Cu/CoO catalysts241, no reduction step was required to obtain active catalysts. The catalyst with the best performance contained 5 wt% Cu (Figure 10 a); any further increase in Cu resulted in a decrease in catalytic activity caused by the reduced surface area and deteriorated pore structure with higher loadings (Figure 10 b). ACS Paragon Plus Environment

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Furthermore, the for redox reactions catalytically less active tetrahedral sites in Al2O3 were occupied by the Cu atoms with higher loadings, whereas at low loadings the octahedral sites were preferentially incorporated by Cu. Moreover, the effect of different co-solvents in the reaction solution was investigated and we showed that the addition of ethanol or n-propanol of up to 50 vol% resulted in remarkable improved catalytic performances of the catalysts. This phenomenon was tentatively attributed to the change in the solvent polarity of the reaction medium and is still under investigation in our group.

Figure 10. a) TEM micrograph of mesoporous Cu-Al2O3 with 5 wt% Cu loading. b) Catalytic data for glycerol oxidation over various Cu-Al2O3 catalysts. Digits denote the Cu weight loading; -sf stands for surfactant free and –Comm for commercially obtained support. Reaction conditions: 30 mg catalyst, 15 mL of 0.05 M aq. glycerol solution, 4:1 NaOH:glycerol, 90 °C, 0.1 MPa O2, 3 h. Reproduced from ref.

244

with permission from the Royal

Society of Chemistry (CC BY 3.0).

A continuous flow reactor setup is, compared to the mostly studied batch reactor, more practicable for the large scale implementation of glycerol oxidation processes, since the operation cost of a continuous process is lower than that of batch reactors. However, most studies focused on the academic aspects of glycerol oxidation and batch reactions. In that sense, Pérez-Ramírez’s research group followed a rather economical approach for glycerol valorization and explored the continuous gas-phase oxidation of glycerol to dihydroxyacetone over tailored Fe-containing (alumino) silicalites that can be synthesized in kg-scale via a hydrothermal synthesis.245-246 Fe-silicalite prepared by isomorphous substitution of iron in the all-silica framework followed by steam activation at 873 K (denoted as FeS-s873) exhibited very mild acidity and highly dispersed iron species in form of isolated cations or small FeOx clusters in extraframework positions (Figure 11 a). This catalyst exhibited a dihydroxyacetone yield of 90% which was stable over a timeframe of 24 h (Figure 11 b, sample FeS-s873).245


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These results suggest that a major challenge in the implementation of non-noble metal catalysts in industrial processes, which is their stability, can be circumvented by this catalyst. The steam treatment (30 vol% H2O/N2 flow at 773-1173 K for 5 h) was applied in order to modify the acidity of the ferrosilicate and to vary the morphology and redox properties of the iron sites. In contrast, impregnated or hydrothermally prepared and steam activated aluminum-containing catalysts (i.e., MFI zeolites, denoted as FeZ) featured strong acidity and/or a high iron clustering degree. This promoted competitive dehydration or oxidation reactions and resulted in poor dihydroxyacetone yields (Figure 11 c).

Figure 11. a) Iron species formed under a progressively harsher steam treatment of isomorphously substituted iron silicalite. Harsher conditions or the use of Fe-containing MFI zeolite as materials generally resulted in bigger FeOx or Fe2O3 particle formation. Reprinted with permission from ref.

246

. Copyright 2015 Royal Society

of Chemistry. Catalytic performances of b) FeZ (Fe-containing alumino silicates) and c) FeS (Fe-containing silicalites) for glycerol oxidation to dihydroxyacetone in the gas phase. The suffix s773 to s1173 denotes the steam treatment at different temperatures in K (see main text). Harsher steam treatment resulted in lower amounts of small FeOx clusters, which decreased the catalytic performance. In the case of FeZ, already mild steam treatments at 873 K led to Fe2O3 formation, which decreased the selectivity toward dihydroxyacetone. Reaction conditions: 350 °C, 0.012 cm³ min-1 glycerol flow, O2/glycerol = 3.5, GHSV = 52900 h-1. Samples were collected between 1 and 2 h on stream. Reprinted with permission from ref. 245. Copyright 2015 American Chemical Society.


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The authors emphasized that the Fe-sites have to be present in the form of isolated cations or as small and well-dispersed FeOx clusters (Figure 11 a) in order to ensure a high selectivity toward dihydroxyacetone. Forming bigger Fe2O3 particles resulted in higher selectivities toward pyruvaldehyde and pyruvic acid at the expense of dihydroxyacetone. Moreover, high Brønsted acidity significantly favored coke formation which further hindered high catalytic performances. The most active catalyst (FeS-s873) showed a decent stability in a 24 h run with only a slight decrease in dihydroxyacetone selectivity, which suggested that iron sintering might have occurred. Interestingly, these materials were also able to outperform noble metal catalysts (Pt/Al2O3 and Pd/Al2O3) emphasizing the impact of non-precious metal catalysts for glycerol oxidation in the gas phase. Most non-noble metal catalysts are cheaper than their noble-metal based counterparts. However, also the operating cost, which includes the lifetime of the catalyst, needs to be considered and will greatly vary between different catalysts. A life-cycle analysis of the process revealed that all environmental indicators are improved (e. g. reduced CO2 emissions by 45 to 50 %) and that the operating cost is halved, as compared to the state of the art biocatalytic production of dihydroxyacetone.246 These studies by Pérez-Ramírez’s research group are not only interesting from an academic viewpoint but also extensively demonstrate the potential of heterogeneous catalysis for the replacement of the currently used biocatalytic process for the synthesis of dihydroxyacetone. Tao et al. reported the use of the heteropolyacids (HPA) supported on carbon (microtubes derived from willow catkins) and SiO2 as catalysts for the base-free oxidation of glycerol to lactic acid under mild conditions.247 The use of HPAs for conversion of glycerol to lactic acid was recently patented by the authors.248 Molecular HPAs (H3PW12O40, H3PMo12O40, and H5PMo10V2O40) in homogeneous phase were screened for lactic acid formation and H3PMo12O40 exhibited the highest yield. The catalytic performance of molecular HPAs appeared to correlate with their redox potential and Brønsted acidity. In a further step, carbon supported H3PMo12O40 were employed for the oxidation reaction. Prior to the HPA immobilization, the carbon surface was functionalized with ethanediamine and 1bromodecane to form a lipid-like layer, in which the HPA was incorporated. An improved catalytic performance resulting in 94% lactic acid selectivity at 98% conversion was observed. No leaching of HPMo was detected under the applied reaction conditions and HPMo/C could be recycled 5 times without significant loss of activity. In their latest publication regarding glycerol conversion to lactic acid, they extended their studies of supported H3PMo12O40 catalysts utilizing graphene oxide as support.249 Moreover, Tao et al. ACS Paragon Plus Environment

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also studied the use of AlPMo12O40 and CrPMo12O40 (polyoxometalates) as heterogeneous catalysts for glycerol oxidation to lactic acid under base-free conditions.243 AlPMo12O40 exhibited 91% selectivity toward lactic acid at 94% conversion under mild reaction conditions (60 °C, base-free, 5 h reaction time). The material could be reused more than 12 times without decrease in activity. Furthermore, they could show that AgxH3−xPMo12O40 is highly active for glycerol oxidation.250 The silver content could be varied and it was found that Ag3PMo12O40 was the most active catalyst with 93% selectivity toward lactic acid at 99% conversion under mild base-free conditions (60 °C, 5 h reaction time) and could be reused up to 12 times without any leaching of the catalyst. Noticeably, all heteropolyacids and polyoxometalates so far introduced by Tao et al. for selective glycerol oxidation to lactic acid also showed a decent activity for solvent-free oxidation of glycerol and good stability and activity for the oxidation of crude glycerol.243, 247, 250 Conclusively, the diverse reports on non-noble metal catalysts for the selective oxidation of glycerol which were recently published show activities and selectivities which are comparable to that of noble metal catalysts. Especially Cu and Co containing materials show promising results and may facilitate the industrial implementation of glycerol oxidation processes for the production of value-added chemicals, by reducing the catalyst price which significantly contributes to the production cost of the oxidation products.

4. Photocatalytic Glycerol Oxidation Many investigations about the selective transformation of various compounds have been conducted during the last decades in order to utilize the abundantly available solar energy. Among those, solar water splitting for hydrogen generation251-252, CO2 reduction253-255, and pollutant degradation256-257 have been intensively studied. On the other hand, reports for the selective oxidation of glycerol over semiconductor photocatalysts are scarcely available and only emerged during the last decade258-271, although selective transformations of organic compounds via photocatalysis have gained great interest for a vast amount of other substrates.84,

272-274

Generally, in heterogeneous photocatalysis, the photon energy of the

impinging (solar) light is absorbed by a semiconductor and transformed into charge carriers (positively charged holes and negatively charged electrons). The migration of the charge carries to the photocatalyst surface allows their subsequent transfer to adsorbed species in order to drive the chemical reaction. Hence, solar energy can be transformed into chemical energy. For a detailed description of the photocatalytic process, the reader is kindly referred to other reviews.275-278 Herein, the progress of the utilization of photocatalysts for glycerol ACS Paragon Plus Environment

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oxidation is elucidated ranging from typical TiO2 semiconductors to other semiconductor materials and also plasmonic photocatalysts. Maurino et al. were the first to report the photocatalytic oxidation of glycerol under aerated and ambient conditions.258 They used P25 and Merck powders as TiO2 photocatalysts and furthermore investigated the influence of surface modification by fluoride anions. The main products observed were dihydroxyacetone and glyceraldehyde. A peculiar behavior was observed for the photocatalytic reactions performed over P25 with different glycerol concentrations. For low glycerol concentrations, an increase in the initial reaction rate was observed with a strong decline at higher concentrations. This was attributed to different reaction sites on P25. Fluorination of the photocatalyst surface could block these active sites so that the high reaction rate was not observed at low glycerol concentrations but showed a fivefold increase at the high concentration regime. This was attributed to the fact that fluorides block the strong adsorbing active sites of P25 which were responsible for the high activity at low substrate concentration but also for the low activity at high substrate concentrations due to the facilitated back reaction. In a further study, the same group emphasized the fact that glycerol is asuitable probe molecule to investigate different oxidation mechanisms over TiO2 photocatalysts due to its variety in product formation.261 The observed photocatalytic results were rationalized based on different modes of surface complexation of glycerol and whether the hydroxyl radical or direct hole transfer mechanism was predominant for glycerol oxidation. Augugliaro et al. studied the selective photocatalytic oxidation of glycerol over commercial and home-made TiO2 samples.259 The reactions were performed in batch photoreactors under aerobic conditions and UV-vis irradiation (125 W medium pressure Hg lamp or six 15 W fluoroscent lamps). The authors showed that, under certain reaction conditions, commercial P25 was the superior photocatalyst compared to the home-made anatase or rutile samples or the commercial Sigma-Aldrich TiO2. Observed products in the reaction solution were glyceraldehyde, dihydroxyacetone, and formic acid (total selectivity around 30%). Calculations based on the total organic compound content revealed that significant amounts of CO2 were produced (10 to 30% selectivity) during the reaction. Interestingly, unknown products analyzed by mass spectrometry with m/z 176 and 268 were also formed. The authors proposed that glycerol oligomerized with glyceric acid, an expected intermediate compound which was not detected via HPLC, to form these products with high molecular weight. Zhou et al. presented an integrated process of photocatalytic nitrobenzene reduction to aniline combined with the selective oxidation of glycerol over Pd/TiO2 ACS Paragon Plus Environment

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photocatalysts under deareated conditions and UV-vis irradiation.267 P25 was the best performing TiO2 support compared to anatase and rutile. Furthermore, they could show that glycerol was more effective as scavenger compared to methanol, ethanol, and isopropanol and yielded high nitrobenzene conversions and selectivities toward aniline. The main oxidation product was hydroacetic acid with glyceraldehyde, dihydroxyacetone, and formic acid as sideproducts. Although the selectivity to desired products like dihydroxyacetone and glyceraldehyde can be regarded as low, it is still interesting that TiO2 with UV irradiation can partially oxidize glycerol without total decomposition to CO2. Although the selectivity towards the desired products like dihydroxyacetone and glyceraldehyde can be regarded as low, it is still interesting that TiO2 with UV irradiation can partially oxidize glycerol without its total decomposition to CO2. The aforementioned example gives evidence that TiO2 photocatalysts are to some extent capable for the selective oxidation of glycerol.

Figure 12. a) SEM micrograph of Bi2WO6 photocatalyst (scale bar is 1 µm). b) Conversions, yields, and selectivities for photocatalytic glycerol oxidation to dihydroxyacetone over Bi2WO6 under visible light irradiation. Reaction conditions: 8 mg photocatalyst, 0.3 M aqueous glycerol solution (initial pH ca. 6.7), suspension saturated with O2, 300 W Xe arc lamp with UV cut-on filter (λ > 420 nm). c) Glycerol conversions for control experiments with different radical scavengers under otherwise similar conditions as in b) (0.06 M aqueous glycerol solution) indicating that positive valence band holes are pivotal for the reaction to proceed. Reprinted with permission from ref. 263 Copyright 2013 Royal Society of Chemistry.

Despite the fact that TiO2 materials are the most thoroughly studied semiconductor photocatalysts, it is important to identify other active semiconductor materials for photocatalytic reactions. For example, shifting the semiconductor’s absorption into the visible light region by changing the semiconductor not only allows for the use of a higher percentage of the solar light spectrum but, in general, also enables the formation of holes with lower oxidation potential – and, thus, potentially preventing overoxidation. Xu and co-workers were the first to report the use of Bi2WO6 as a selective photocatalyst for dihydroxyacetone


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formation from glycerol under visible light irradiation.263 The Bi2WO6 materials exhibited a flower-like morphology with good crystallinity as shown in Figure 12 a). Dihydroxyacetone was generally formed in high yields (80 to 87%) after 5 h reaction time over different Bi2WO6 photocatalysts prepared by hydrothermal treatment with variaton in reaction time. Also reactions perfomed with higher concentrations of glycerol resulted in high yields of dihydroxyacetone ( Figure 12 b). The authors concluded that positive valence band holes together with dissolved O2 or activated superoxide radicals are required for the reaction to proceed based on control experiments with various radical scavengers ( Figure 12 c). This study is one important example that research should not only focus on

TiO2-based photocatalysts for selective transformation reactions, but also explore other semiconductors. In a follow-up study in collaboration with the research group of Pagliaro, they could show that silica-entrapped Bi2WO6 photocatalysts exhibited a strong improvement in photocatalytic activity.265 The authors proposed several advantages achieved by the encapsulation of the photocatalyst: the transparent, stabilizing silica matrix allowed improved light penetration, promoted the efficiency of the photogenerated electron–hole separation and enhanced the adsorption capacity toward glycerol. Comparison of non-entrapped Bi2WO6 and silica entrapped Bi2WO6 (10 wt% loading) photocatalysts showed a threefold increase in glycerol conversion after 4 h irradiation time by using the silica-entrapped sample with only one tenth of the active photocatalyst.265 Muhler and co-workers modified Bi2WO6 nanoplatelets with MoOx, deposited via chemical vapor deposition (CVD), and investigated the photocatalytic activity of these materials for oxygen evolution and selective glycerol oxidation.271 The authors could show that under UV-vis irradiation the unmodified Bi2WO6 exhibited no photocatalytic activity for glycerol oxidation. This is in contrast to the results reported by Xu et al.263 and might be due to the different preparation methods of the materials. Nonetheless, modification of Bi2WO6 with MoOx with different numbers of theoretical monolayers ranging from 1 to 8 increased the activity for glycerol oxidation to dihydroxyacetone. Decoration of Pt NPs on MoOx/Bi2WO6 also showed a remarkable increase in activity, whereas the photocatalytic performance in consequence was independent of the MoOx content. Moreover, unmodified Pt/Bi2WO6 showed the highest increase in dihydroxyacetone yield implying that MoOx modification is detrimental for the photocatalytic performance of Pt/Bi2WO6. Mechanistic details for the explanation of this behavior have yet to be reported and are being investigated by the group. Molinari et al. used Na4W10O32 photocatalysts for the selective oxidation of glycerol under UV-vis irradiation.262 Photoexcitation of dissolved Na4W10O32 lead to the formation of hydroxyl radicals with strong oxidation potentials which effectively could oxidize glycerol ACS Paragon Plus Environment

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but with low selectivities to value-added products and a high tendency to over-oxidation. Entrapment of Na4W10O32 inside a silica matrix by a sol-gel method resulted in a heterogeneous photocatalyst with improved selectivities toward glyceraldehyde (60%) and dihydroxyacetone (6%). It was shown by EPR spectroscopy that the heterogenization of the catalyst does not influence its ability of hydroxyl radical formation. Hence, it was concluded that the differences in selectivity critically depend on textural features that allow the tuning of photocatalytic properties of Na4W10O32 through the control of surface interactions with substrates and intermediates.

Another approach to drive the photochemical transformation of glycerol to value-added products is the use of plasmonic photocatalysts, which, in general, have garnered great attention in the field of photocatalysis273, 279-280 and other applications281. The principle is based on the utilization of the plasmonic properties of (mostly) noble metal nanoparticles, which results in visible light absorption and an energy transfer to the semiconductor. In particular, our group demonstrated that it is possible to enhance the catalytic performance of Au/TiO2 catalysts for glycerol oxidation in the aqueous phase under neutral conditions by additional illumination of the reaction solution with visible light.282 In this context, the visible light irradiation during the reaction process acts as an aid to facilitate the thermocatalytic glycerol oxidation to obtain value-added products, such as glyceric acid, glycolic acid and dihydroxyacetone.

a) shows that the visible light irradiation can improve the glycerol conversion severalfold compared to the reactions conducted in the dark. High selectivities toward dihydroxyacetone were achieved ( Figure 13 b). As shown in Figure 13 c) and d), we proposed a reaction mechanism and a glycerol oxidation pathway for the photocatalytic process. It is expected that the visible light irradiation generates hot electrons in the Au nanoparticle, which then transfer to the conduction band of the TiO2. The remaining positive holes in the Au nanoparticle can participate in the oxidiation of glycerol under milder oxidation conditions compared to holes with high oxidation potentials generated in the valence band of semiconductors. The electrons are transferred to adsorbed molecular oxygen forming the superoxide radical and, subsequently, hydrogen peroxide. In fact, we could show that the increased hydrogen peroxide concentration for irradiated reactions compared to the reactions performed in the dark is pivotal in order to observe an enhancement in activity. The proposed reaction pathway of glycerol oxidation presented in Figure 13

Figure 13

d) is similar to the generally proposed reaction pathways found in the literature for

the thermocatalytic reaction47, which is expected due to the superimposition of the thermocatalytic pathway with that of the photocatalytic reaction. ACS Paragon Plus Environment

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Figure 13. Conversions a) and selectivities b) of (photo-)catalytic glycerol oxidation over Au/TiO2 with different Au loadings. Reaction conditions: 1 g L-1 catalyst, 5 mL 0.05 M aqueous glycerol solution, initial pH = 7, 90 °C, 5 bar pure O2, 5 h reaction time, visible light irradiation (λ > 420 nm) if denoted. c) Proposed mechanism for surface plasmon-assisted glycerol oxidation reaction under visible light irradiation. d) Proposed reaction pathway for surface plasmon-assisted and thermally catalyzed glycerol oxidation. Reprinted with permission from ref. 282. Copyright 2016 Royal Society of Chemistry.

The studies were extended for monometallic Au and bimetallic AuCu catalysts supported on different mesoporous SiO2 supports.283 We could show that monodispersed mesoporous silica spheres (MS) as catalyst support exhibited the best results in comparison to SBA-15, KIT-6, and MCM-41. Furthermore, bimetallic AuCu catalysts supported on MS (Figure 14 a and b) could further increase the performance by a factor of 2.5 compared to Au catalysts (Figure 14 c) and still maintain high selectivities toward dihydroxyacetone. Hence, it was also demonstrated therein that photocatalytic studies should not only be restricted to TiO2 photocatalysts.


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Figure 14. a) Bright-field and b) dark-field STEM micrograph of AuCu/MS with a molar ratio of 5 Au : 1 Cu. c) Glycerol conversion of AuCu/MS. MS: Monodispersed mesoporous silica spheres. Reaction conditions: : 1 g L-1 catalyst, 5 mL 0.05 M aqueous glycerol solution, initial pH = 7, 90 °C, 5 bar pure O2, 5 h reaction time, visible light irradiation (λ > 420 nm) if denoted. Reprinted with permission from ref.

283

.Copyright 2015 American

Chemical Society.

It has to be emphasized that in this section, up to now, only the photocatalytic oxidation of glycerol under aerated conditions is presented. Many studies describe photocatalytic glycerol reforming under anaerobic conditions where glycerol acts as sacrificial electron donor in order to facilitate H2 generation.23,

284-303

The fate of glycerol – as for other substrates

employed for photocatalytic reforming295, 304-314 – in this photocatalytic process is eventually CO2 and comparably little attention has been paid on gaining value from intermediate product formation during glycerol reforming. A high activity toward photocatalytic H2 generation with a concomitant selective oxidation of the alcohol to value-added products might further improve the economic feasibility of the process.315 Indeed, some studies focused on the intermediate formation and further analyzed the reaction products for photocatalytic H2 generation with glycerol as sacrificial agent, which, however, is out of the scope of this review artile to be elucidated in detail.289, 316-322. As the presented works in this section illustrate, the photocatalytic pathway is a novel, yet promising strategy towards the oxidation of glycerol under milder reaction conditions with, compared to the conventional thermocatalytic pathway, altered selectivities. As the examples show, not only TiO2, but also other semiconductor materials with more suitable band edge energies, as well as plasmonic photocatalysts show promising results and should be investigated for tehir potential in the selective oxidation of glycerol more intensively in the future.


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5. Electrocatalytic Glycerol Oxidation The electrocatalytic pathway for glycerol conversion recently emerged as a very promising approach for achieving high selectivities to different glycerol oxidation products and for driving the selectivity to a selected product by variation of the applied bias or current density. Unlike thermocatalytic glycerol oxidation reactions that are typically performed at elevated oxygen pressures and high temperatures, electrocatalytic glycerol oxidation is typically performed under atmospheric pressure and moderate temperatures. This makes the electrocatalytic glycerol oxidation also appealing in terms of energy efficiency and economics as reported by Kim et al..90 In their combined experimental and economic study, the authors found that the electrocatalytic process with a Pt/C catalyst can reduce the minimum selling price of the main product glyceric acid from 4.91 $/kg to 2.30 $/kg. Initial research on the electrochemical selective oxidation of glycerol was started by the group of Pagliaro in 2006, who reported the TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) mediated glycerol oxidation to selectively form dihydroxyacetone with a yield of 25 % after a reaction time of 20 h.323 In 2012 Koper and coworkers demonstrated the selective glycerol oxidation over a carbon supported Pt electrode with 100 % selectivity to dihydroxyacetone. Besides the addition of Sb, which blocks certain Pt sites that are active for primary alcohol oxidation, as it is also the case for the thermocatalytic oxidation of glycerol (see chapter 2.2), the applied current is pivotal for achieving high dihydroxyacetone selectivities. However, the major drawback of the author’s findings was that the quantitative dihydroxyacetone selectivity can only be achieved at very low conversions below 1 % and drops significantly at higher conversions such that the overall dihydroxyacetone yield does not exceed 1 %. In a later study, the same group studied the effect of other adatoms (Pb, Sn, In) on the activity and selectivity of carbon supported Pt electrodes.324 The studied promotors were found to increase the activity of the electrocatalyst but - unlike Sb – did not promote the selectivity towards dihydroxyacetone. Since then, electrocatalytic glycerol oxidation over the same noble metals that were used for the thermocatalytic conversion of glycerol, namely Pt, Au, Pd, and alloys thereof were reported.325-333 High yields and selectivities towards dihydroxyacetone were reported by Lee et al., who employed a PtSb/C electrocatalyst in acidic solutions.334 By this a dihydroxyacetone yield of 61 % at a glycerol conversion of 90 % was achieved. The authors further demonstrated that the selectivity can be tuned by the applied potential, where higher anode potentials promote C-C cleavage and thus to the formation of C2 and C1 products. Furthermore, Sb and Bi, the same structural promotors for Pt that increase the ACS Paragon Plus Environment

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dihydroxyacetone selectivity in thermocatalytic glycerol oxidation were shown to be crucial for suppressing the oxidation of the primary alcohols, which yields glyceraldehyde in the electrocatalytic reaction, as it is also the case for the thermocatalytic reaction. As mentioned in section 2.4, monometallic Ag catalysts are a more low-priced alternative to costly noble metals such ach Au, Pt, and Pd in thermocatalysis to yield glycolic acid. In this regard, Suzuki et al. reported a bulk silver electrode for the electro-oxidation of glycerol in basic media with different NaOH and glycerol concentrations to yield a mixture of formic acid, glycolic acid, and glyceric acid with unspecified selectivities and yields.335 Further studies regarding the selectivity, activity, and stability of silver electrocatalysts will have to be conducted to evaluate the potential of this material for the selective electro-oxidation of glycerol. All of the aforementioned examples demonstrate the electrocatalytic selective glycerol oxidation over noble metal catalysts. Lam et al. were the first to report an earth abundant Cobased electrocatalyst for the selective oxidation of glycerol.336 The catalyst was synthesized via the deposition of a Co-1,2-bis(diphenylphosphino)ethane (Co-DPPE) on FTO. However, no analytics of the deposited Co-DPPE during or after the electrocatalysis was conducted such that the state of the catalyst and the nature of the active species remain unclear. In this system, the product selectivity can be switched between lactic acid and glyceric acid, where lactic acid is the main reaction product at 60 °C and low current density ( 1.8 mA cm-2), whereas glyceric acid is produced at 20 °C and higher current densities (8.8. mA cm-2) (Figure 15). As the authors emphasize, the catalyst is also robust against 10 vol% of methanol, which is a typical impurity of glycerol streams from biodiesel production.

Figure 15: Selectivity towards glyceric acid (S-G.A.), lactic acid (S-L.A.), carbon balance (C Bal.), and Faradaic efficiency (% Fara.) at a) 60 °C and b) 20 °C. Reaction conditions: 0.25 M glycerol in 1 M aqueous NaOH, stirring speed: 200 rpm. Reprinted with permission from ref.336 Copyright 2017 Royal Society of Chemistry.


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6. Concluding Remarks and Prospects The increasing global production of biodiesel resulted in a concomitant increase in glycerol production over the last decades. Glycerol is nowadays regarded as a possible platform chemical and finds hundreds of applications as food and feed ingredients, in toothpaste, cosmetics, pharmaceuticals, tobacco, explosives, etc.6,

9, 21, 48, 80

It also finds its use in the

industrial production of epichlorohydrin337, methanol338, and propylene glycol21, 339, as well as in the production of 1,3-propanediol and dihydroxyacetone by biotechnological conversion.340 Moreover, the production of acrolein from glycerol has reached the pilot level.340 Nonetheless, the aforementioned applications of glycerol are not sufficient to deal with the vast amounts of glycerol flooding the market. This scenario has prompted an important research to explore various valorization routes in order to convert this highly functionalized polyol to various compounds, which can find further uses in industry (Scheme 2). Besides the production of propanediol and acrolein which are required as bulk chemicals for the polymer production, the production of high-value fine chemicals via the selective oxidation reaction shows a great economic appeal. Furthermore, the production of expensive compounds ensures that processes possibly established in the future are more resistant against any glycerol price fluctuations. However, in order to achieve this, further research efforts have to be devoted in the research area of the selective oxidation of glycerol. As described in this review, during the last years great progress has been achieved in the catalyst preparation and further insights have been gained in structure-activity relations, which are pivotal for a further development of better catalysts and better control of activity/selectivity parameters. Nevertheless, there is still significant room to further improve our knowledge and gain detailed insights in the reaction pathways of glycerol to the various oxidation products. Also this knowledge will allow the further improvement of the reaction process to obtain high selectivities toward the desired products by impeding the undesired reaction pathways. In this light, the selectivity toward the target products, such as glyceric acid, dihydroxyacetone, and tartronic acid, has to be significantly improved in order to make the catalysts attractive for industry. Furthermore, the catalyst stability also requires further improvement in order to ensure cost-efficient life cycles and, thus, attractive economic assessments. It has also to be pointed out that cost-efficient production routes of the aforementioned oxidation products can open new markets for the products in the industry, which were restricted due to currently high production costs. In this context, non-precious metal ACS Paragon Plus Environment

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containing catalysts have emerged as promising alternative to noble metal catalysts, which are still in the very early stages of investigation. Generally, these catalysts cannot compete with noble metal catalysts regarding their catalytic activity. Nonetheless, the herein described materials hold promise to be valuable catalysts if the selectivities toward single products can be further tuned. Any cost assessment of the promising materials has then to be conducted individually based on the catalyst stability and the target product, but, by simple comparison of the metal prices, non-precious metal containing catalysts already seem to be advantageous. In this context, it is worthy to mention that also studies demonstrating that the use of any metal catalyst can be avoided emerged230, 341, which is a further step in the direction of a costefficient reaction process. The economic aspect of glycerol oxidation should in general be kept in mind when exploring new active catalysts. However, fundamental research always allows investigating new reaction pathways to ensure a gain in knowledge and give the chance to discover unprecedented reaction pathways. The photocatalytic pathway has emerged during the last decades as intriguing reaction concept to transform various organic compounds to desired products. In the case of glycerol, the research is still in its infancy but it has been shown in this review that photocatalysts with encouraging catalytic performances were already obtained. Nevertheless, further research efforts have to be devoted in this field in order to understand the reaction mechanisms and optimize the activity/selectivity parameters. The combination of selective glycerol oxidation with hydrogen generation via photocatalysis evidences that practical applications can be achieved ensuring a more sustainable future. Finally, an important aspect in glycerol oxidation for an economically feasible reaction process is the use of crude glycerol, which contains impurities like methanol, NaOH, and other salts. Finding active and stable catalysts which can catalyze the reaction in crude glycerol will be a huge step forward in applying these catalysts in industry. However, this aspect is only scarcely investigated and only a few reports can be found in the literature, which tackle this problem. Circumventing the purification process of crude glycerol and its direct usage to produce valuable compounds might be a promising way to make the biodiesel production more economically viable. Besides the fundamental aspects, where great progress has been achieved over the last years, this last challenge should not be neglected by the researchers in this field.


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Acknowledgements This work was supported by the MAXNET Energy consortium of Max Planck Society, the Cluster

of

Excellence

RESOLV

(EXC

1069)

funded

by

the

Deutsche

Forschungsgemeinschaft (DFG) and Fonds der Chemischen Industrie (FCI).

References 1. Key World Energy Statistics; OECD/IEA: Paris, France, 2016. 2. UN Climate Change Newsroom http://newsroom.unfccc.int/unfccc-newsroom/finale-cop21/ (accessed Jul 10, 2017). 3. Serrano-Ruiz, J. C.; Luque, R.; Sepulveda-Escribano, A. Transformations of Biomass-Derived Platform Molecules: From High Added-Value Chemicals to Fuels Via Aqueous-Phase Processing. Chem. Soc. Rev. 2011, 40, 5266-5281. 4. Michel, H. The Nonsense of Biofuels. Angew. Chem. Int. Ed. 2012, 51, 2516-2518. 5. Oecd-Fao Agricultural Outlook 2016; OECD Publishing: Paris, 2016. 6. Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F. Improved Utilisation of Renewable Resources: New Important Derivatives of Glycerol. Green Chem. 2008, 10, 13-30. 7. Hejna, A.; Kosmela, P.; Formela, K.; Piszczyk, L.; Haponiuk, J. T. Potential Applications of Crude Glycerol in Polymer Technology Current State and Perspectives. Renew. Sust. Energ. Rev. 2016, 66, 449-475. 8. Fan, X.; Burton, R. Recent Development of Biodiesel Feedstocks and the Application of Glycerol: A Review. Opel Fuels Energ. Sci. J. 2009, 2, 100-109. 9. Ayoub, M.; Abdullah, A. Z. Critical Review on the Current Scenario and Significance of Crude Glycerol Resulting from Biodiesel Industry Towards More Sustainable Renewable Energy Industry. Renew. Sust. Energ. Rev. 2012, 16, 2671-2686. 10. Johnson, D. T.; Taconi, K. A. The Glycerin Glut: Options for the Value-Added Conversion of Crude Glycerol Resulting from Biodiesel Production. Environ. Prog. 2007, 26, 338-348. 11. Christoph, R.; Schmidt, B.; Steinberner, U.; Dilla, W.; Karinen, R., Glycerol. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2012; pp 6781. 12. Gholami, Z.; Abdullah, A. Z.; Lee, K.-T. Dealing with the Surplus of Glycerol Production from Biodiesel Industry through Catalytic Upgrading to Polyglycerols and Other Value-Added Products. Renew. Sust. Energ. Rev. 2014, 39, 327-341. 13. Katryniok, B.; Paul, S.; Belliere-Baca, V.; Rey, P.; Dumeignil, F. Glycerol Dehydration to Acrolein in the Context of New Uses of Glycerol. Green Chem. 2010, 12, 2079-2098. 14. Tan, H. W.; Abdul Aziz, A. R.; Aroua, M. K. Glycerol Production and Its Applications as a Raw Material: A Review. Renew. Sust. Energ. Rev. 2013, 27, 118-127. 15. Ciriminna, R.; Della Pina, C.; Rossi, M.; Pagliaro, M. Understanding the Glycerol Market. Eur. J. Lipid Sci. Technol. 2014, 116, 1432-1439. 16. Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chemoselective Catalytic Conversion of Glycerol as a Biorenewable Source to Valuable Commodity Chemicals. Chem. Soc. Rev. 2008, 37, 527549. 17. Silva, J. M.; Soria, M. A.; Madeira, L. M. Challenges and Strategies for Optimization of Glycerol Steam Reforming Process. Renew. Sust. Energ. Rev. 2015, 42, 1187-1213.


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Recent Advances in Thermo-, Photo-, and Electro-catalytic Glycerol

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