Recent Advances in Thermo-, Photo-, and Electrocatalytic Glycerol

Review Cite This: ACS Catal. 2018, 8, 6301−6333

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Recent Advances in Thermo‑, Photo‑, and Electrocatalytic Glycerol Oxidation Georgios Dodekatos, Stefan Schünemann, and Harun Tüysüz*

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Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany ABSTRACT: Glycerol is a highly versatile molecule because of 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 placed not only on newly developed catalysts based on supported noble-metal nanoparticles but also on catalysts containing nonprecious 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 among the various studies. Moreover, during the past 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

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 continue to decrease, with the consequence that the resources will eventually deplete; however, it is still under debate when this point will be reached, in particular for petroleum. In addition, environmental concerns are connected with the combustion of fossil fuels because increased CO2 emissions propel climate change and increased global temperatures. In 2014, the transportation sector accounted for approximately 65% of the global oil consumption and produced approximately 34% of the global-energy-related 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 with preindustrial values.2 Hence, because of the aforementioned upcoming shortages and detrimental effects on the climate, biofuels, that is, 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 wellestablished biofuel production could decrease a country’s © XXXX American Chemical Society

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 shows 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. First, domestic policies (e.g., blending mandates) have a considerable influence on the economic feasibility of the biofuel production. Second, 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). Third, 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. Biodiesel is produced via acid- or base-catalyzed transesterification of fats and oils (triglycerides) with methanol Received: April 4, 2018 Revised: May 25, 2018 Published: May 30, 2018 6301

DOI: 10.1021/acscatal.8b01317 ACS Catal. 2018, 8, 6301−6333

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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

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 fermentation,7,19−21 hydrogenolysis,21,25−27 hydrogenation,28 dehydration,7,21,29−31 esterification,6,16,21,32−34 and etherification,6,21,33,35 to oligomerization7,36 and polymerization,7 carbonylation,6 and oxidation (Scheme 2 and Table 1).37−51 The variety of value-added

yielding glycerol and fatty acid methyl esters (FAMEs, biodiesel), as shown in Scheme 1. One mole of triglyceride

Scheme 2. Different Processes of the Catalytic Conversion of Glycerol into Useful Chemicals

Scheme 1. Transesterification Process Shown for Triglyceride and Methanol Forming Biodiesel and Glycerol

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, among other factors, significantly governed by the cost of its separation from the byproduct glycerol.8,9 The fast development of the biodiesel industry during the last decades resulted in the decoupling of the glycerol production and demand; thus, the result is 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 on its demand. In 2008, biodiesel became the primary glycerol source; previously, the fatty acid industry served as the primary source.12 This resulted in a drastic drop of the glycerol price and, consequently, in the treatment of glycerol as a 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 because of 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

products range from acrylic acid,31 glyceric acid, and dihydroxyacetone, over lactic acid52,53 to 1,2-propanediol and 1,3-propanediol.25,26,54−58 These various processes are intensively investigated by researchers around the world. 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 Table 1. Apart from formic acid which is produced in high amounts by producers like BASF,51 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 oxidans 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 6302


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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 valueadded 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 cost-efficient 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 major 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 and material properties can improve the catalytic performance.43,47 Besides the highly investigated noble-metal catalysts, nonprecious metals were also explored as active catalysts for glycerol oxidation during the past 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 with that of noblemetal catalysts, it has an academic and economic appeal to find active catalysts based on nonprecious metals. Avoiding noble metals for glycerol oxidation would result in a more costefficient reaction process because , as pointed out by Kimura, Dumeignil, and co-workers, the catalyst price still represents 95% of the production costs for dihydroxyacetone, tartronic acid, and mesoxalic acid even after 10 times of reuse, because of the presence of noble metals.43

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 a versatile cleaning agent, for instance, as a degreasing agent and for rust removal or metal cleaning in general. It is mainly manufactured from acidcatalyzed formaldehyde carbonylation.40 Although formic acid has in direct comparison no economic beneficial value compared with 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. Glycerol is not the only biorenewable 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 biorenewable 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 because of 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 6303


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Scheme 3. Simplified General Reaction Scheme for Glycerol Oxidation over Noble-Metal Catalysts Adapted from Ref 106

reaction environments, and conditions of 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.

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 high-value products. However, this comparably young research field still has room for further development and 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 coworkers inferred that the electrocatalytic oxidation of glycerol resulted in a more cost-efficient production of glyceric acid compared to the thermocatalytic reaction.90 Furthermore, 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 here is to describe 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 nonpreciousmetal-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

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 showed in 2002 that also glycerol can also 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 toward 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 6304


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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 poly(vinyl 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 to avoid 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 carbonbased 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 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 where 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 stabilizer-removed 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

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 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 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 a 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. 2.1. Glycerol Oxidation over Au-Containing Catalysts. This section comprises the effect of the variation of the synthesis parameters of sol-immobilization, 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 Prati,96−98 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 because 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 with 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 Aucontaining 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. 6305


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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 of 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.

postsynthesis. However, a direct comparison of the catalytic performances is not possible because of 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 (Figure 3) 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. 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 (Table 2). 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 because 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

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 because the surfactant is required to stabilize the sols. Nonetheless, this method was very recently introduced by Hutchings and co-workers.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 Figure 2a,b, the deposited Au NPs were spherical, and the stabilizer-free prepared catalysts showed slightly larger particle sizes compared with the catalyst prepared with PVP. 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 because 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 2c). 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 co-workers 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 seems that the preparation of catalysts in the total absence of protecting agents provides new catalysts with altered properties compared with materials where the protecting agent is removed 6306


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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 have 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 with 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 glyceric acid to the H2O2 formation over carbonnanofiber-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 structuresensitive with the Au (111) surface promoting the C−C cleavage because of 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

Figure 3. High-resolution HAADF STEM images of single Au atom and Au2 clusters supported on TiO2 prepared in water at 1 °C via solimmobilization. Reproduced with permission from ref 125. Copyright 2015 American Chemical Society.

Table 2. Selected Catalytic Data from Ref 125 for Glycerol Oxidation over Au/TiO2 Catalysts Prepared under Different Conditionsa selectivity/% Au/TiO2

b

TOF/h

H2O/1 °C H2O/25 °C H2O/50 °C 50 EtOH/50 °C EtOH/50 °C

−1

915 663 341 202 314

GLA

TA

73 74 76 76 78

12 12 8 11 7

a

Reaction conditions: glycerol/Au = 1000:1, NaOH/glycerol = 4:1, 50 °C, 0.3 MPa. TOF determined after 15 min reaction time. bThe first column indicates the solvent (water, ethanol) or solvent mixture (50 vol % ethanol/50 vol % water) used for the catalyst preparation via solimmobilization.

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 reaction can

Table 3. Typical Results for Glycerol Oxidation over Au-Containing Catalysts Reported in the Last Years basea

T/°C

PO2/MPa

reaction time/h

conversion/%

selectivityb/%

researchers and year

Au/SiO2

base-free

80

-c

24

100

99 AA

Kapkowski 2014

Au/CNSd

2:1

60

0.5

4

57

63 GLA

Gil 2014

Au/HY

4:1

60

0.3

9

98

80 TA

Cai 2014

Au/CuO

base-free

50

0.2

4

98

82 DHA

Liu 2014

Au/MgOAl2O3

base-free

80

1

3

12

82 DHA

Xu 2015

Au/Fe3O4

base-free

100

1

14

43

61 GLA

Behr 2015

Au/N-CNFe

4:1

50

0.3

1.5

91

64 GLA

Villa 2016

Au/C

4:1

50

0.3

6

92

71 GLA

Dimitratos 2016

catalyst

comment

ref

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 showed the highest yields toward DHA among other supported Au catalysts 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

146 145

144 141

142

143

147 132

a

NaOH to glycerol ratio if not otherwise denoted. bAA: acetic acid, GLA: glyceric acid, TA: tartronic acid, DHA: dihydroxyacetone. cH2O2 was used as oxidant; dcarbon nanospheres; ecarbon nanofibers. 6307


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Figure 4. Plots of (a) kcat and (b) initial TOF values with Pd surface coverage. Reaction conditions: 0.2 g of catalyst, 60 °C, 0.1 M glycerol, NaOH/ glycerol = 4:1, 120 mL min−1 O2 flow. Reproduced with permission from ref 167. Copyright 2014 Royal Society of Chemistry (CC BY-NC 3.0).

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. 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 past decade great effort has been devoted to exploit and understand the expected improved properties of alloyed catalysts for glycerol oxidation,151−160 among other reactions.150,161−165 Moreover, a variety of different preparation methods emerged,166 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 while 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% yield compared with 16 and 22% for Au/C and Pd/C catalysts, respectively. Generally, AuPd catalysts exhibited improved resistance against deactivation compared with 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

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 afterward. It has been shown that combining bi- or trimetallic catalysts (Au, Pd, Pt) with acidic or basic supports (e.g., H-mordenite, 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 pHneutral 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 of 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. As the 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 6308


Review

Yang 2017 53 GLA 60

NaOH to glycerol ratio if not otherwise denoted. LA: lactic acid, GLA: glyceric acid, TA: tartronic acid, DHA: dihydroxyacetone. thermally expanded graphene oxide.

184 a

0.3 60 base-free AuPt/TEGO

c

100 100 100 60 4:1 4:1 4:1 2:1 AuPt/LaMnO3 AuPt/LaCrO3 AuPt/LaMnO3 AuCu/CeZrOx

0.3 0.3 0.3 0.6

b

c

181

bimetallic AuCu catalysts show superior catalytic activity compared with the monometallic Au catalysts; AuCu catalysts slightly increase glycolic acid selectivity different Au/Pt ratios investigated; Au leaching and AuPt NP sintering was observed 4

Evans 2016

6 6 24 5

75 95 100 80

70 GLA 86 LA 88 TA 79 GLA

Kaminski 2017

175

177 Villa 2015 63 DHA 80 4 80 base-free Bi-AuPt/AC

0.3

0.3 180 base-free AuPt/USY-600

ref

182 2

80

60 LA

Purushothaman 2014

AuPt/CeO2 shows superior catalytic performance compared with other supports and the monometallic counterparts investigation of various zeolite-supported AuPt catalysts for glycerol oxidation to LA or GLA 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 reaction times showed a remarkably high selectivity toward tartronic acid

comment researchers and year

Purushothaman 2014 80 LA 99 0.5 0.5 100 4:1

selectivityb/% conversion/% reaction time/h T/°C PO2/MPa basea catalyst

Table 4. Typical Results for Glycerol Oxidation over Au-Containing Bimetallic Catalysts Reported in the Last Years 6309

AuPt/CeO2

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 behavior was also reported over alloyed AuPd NPs168 and over Au on Pd catalysts.169 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 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 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/MCM-41 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

183

ACS Catalysis


a

base-free

base-free

3:1

base-free

base-free

base-free

base-free

base-free

PT/N-MWCNTc

Pt/MCNd

Pt/IERe

Pt/N-CNT

Pt/AC Pt/AMCf

Pt/AC

Pt/TiO2

Pt/NG-MWCNT

6310

60 60

60

150

60

90 25

60

50

60

60

60

60

T/°C

0.6 0.5

0.5

10 mL min−1 0.1 20 mL min−1 150 mL min−1 0.5

0.15a

0.3

0.5

0.5

0.1

PO2/MPa

3 3

3

18

6

4

8

4

3

24

4

reaction time/h

67 58

64

78

55

100 68

76

91

63

55

91

86

conversion/%

74 GLA 85 GLA

81.0 GLA

70 LA

35.3 GLA

69 LA 78 GLA

56 GLA

63 GLA

59 GLA

54.9 GLA

55 GLA

63 LA

selectivityb/%

Yang 2017 Sun 2017

Zhang 2017

Komanoya 2016

Lei 2016

Zhang 2016 Tan 2016

Chen 2015

Gross 2015

Wang 2015

Zhang 2015

Long 2015

Chornaja 2015


Pt/AC prepared by polyol method is catalytically superior compared with Pt/AC prepared by wet impregnation method TiO2 showed the best results among other supports investigated (Nb2O5, ZrO2, Al2O3, MgO, SnO2, SiO2, AC); Pt-PVP NPs were added into the reaction solution in the presence of the metal oxide; Pt NP immobilization occurred during glycerol oxidation MWCNTs-pillared N-doped graphene (NG) superior support for Pt NPs compared to bare MWCNTs which results in improved catalytic performance N-doped carbon film coated active carbon with high surface area was used as support Pt NPs encapsulated with a carbon film show superior catalytic performances compared to the naked supported Pt NPs

catalyst preparation by the extractive-pyrolytic method; different calcination temperatures were applied and correlated with the catalytic performance magnetic Fe3O4 particles encapsulated in polypyrrole (PPy) and decorated with Pt NPs were employed as easily separable catalysts N-doped MWCNT supported Pt catalyst showed superior catalytic performance and stability compared with the MWCNT supported Pt catalyst MCN materials showed with increasing N content an increase of weak basic sites and a decrease in particle size of supported Pt catalyst resulting in improved catalytic activity investigation of the effect of the counterion of the IER on the Pt/IER catalyst preparation and catalytic performance Pt/NCNTs with different N-content are investigated for glycerol oxidation and compared to results with metal oxide supported Pt catalysts Investigation on the role of base type correlation of oxygen functional groups on the support with catalytic performance

comments

198 205

200

195

196

199 201

202

194

190

204

197

192

ref

NaOH to glycerol ratio if not otherwise denoted. bLA: lactic acid, GLA: glyceric acid. cmultiwall carbon nanotubes. dmesoporous carbon nitride. eion-exchange resin. factivated mesoporous carbon.

base-free base-free

base-free

Pt/Fe3O4@PPy

Pt/N2.5C-XC-72 Pt@C/MWCNT

5:1

basea

Pt/Y2O3

catalyst

Table 5. Typical Results for Glycerol Oxidation over Pt-Containing Catalysts Reported in the Last Years

ACS Catalysis Review


Review

ACS Catalysis

base-free conditions (Figure 5). They could show that nitrogen doping alters the electronic structure and surface basicity of the

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. 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 1993,93,94 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 the 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 Ptcontaining 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. Chen et al. used nitrogen-doped carbon nanotube (N-CNT) supported Pt catalysts for the efficient glycerol oxidation under

Figure 5. TEM dark-field micrograph and STEM-EDX elemental mappings of 1% Pt/N-CNT. Reproduced with permission from ref 202. Copyright 2015 Royal Society of Chemistry (CC BY 3.0).

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 2-fold increase in TOF.202 Furthermore, smaller Pt NPs could be obtained on N-doped CNTs compared with 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 toward glyceraldehyde upon N-doping.203 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 with the results by Chen et al.202 and Ning et al.,203 N-doping resulted in improved Pt dispersion with smaller 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. The same group demonstrated in a recent study that MWCNT-supported Pt NPs encapsulated with a carbon film (Figure 6a−d) exhibited a superior catalytic performance and stability for ethanol (Figure 6e) and glycerol oxidation compared with 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 6311


Review

ACS Catalysis

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 of aqueous solution of ethanol (0.2 g mL−1), 30 mg of catalyst, 2 MPa O2, 130 °C, 2 h. Reproduced with permission from ref 205. Copyright 2017 Elsevier.

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 toward glycerol oxidation. Unlike Aucontaining 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 toward 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. 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 preloaded 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 preloaded PtBi/N-CNT catalysts. Xiao et al. performed a comparative study with bimetallic PtBi catalysts over various supports (AC, ZSM-5, MCM-41, Bidoped 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 with 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 Ptcontaining catalysts were shown to improve the catalytic activity as well as selectivity compared with their monometallic counterparts, which is ascribed to different effects of the promotors. 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 with 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 Because of 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 6312


Review

NaOH to glycerol ratio if not otherwise denoted. bGLAD: glyceraldehyde, DHA: dihydroxyacetone, LA: lactic acid, GLA: glyceric acid. cmultiwall carbon nanotubes. dcarbon nanotubes. a

4:1 PtFe/CeO2

60

base-free PtSn/AC

60

3:1 PtCu/AC

90

base-free PtSb/N-CNT

60

base-free base-free PtCu/CNT PtBi/N-CNT

60 60

difficult by XPS analysis; hence, δ+ is used to denote a nonmetallic state). It was shown that both preparation methods yield similar catalytic performances (Figure 7a), 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 because 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 stable catalysts for glycerol oxidation compared with the catalysts prepared by sol-immobilization (Figure 7b). 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 with 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 with 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 demonstrated that the secondary hydroxyl group of vicinal diols is preferably oxidized over PdAg catalysts, which increases the selectivity toward 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 because no postcharacterization 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

208 Jin 2016 71 GLA 57 2

Dou 2016 55 GLA 91 2

69.3 LA 80 4

6

51

38 DHA

Zhang 2016

Ribeiro 2016 Ning 2016 57 GLAD 56 DHA 41 36 30 6

211

207

variation of Cu content in PtCu/AC catalysts; 0.5%Cu-1.0%Pt/AC showed the best promotional effect various bimetallic PtM/AC (M = Mn, Fe, Co, Ni, Cu, Zn, Au) catalysts were investigated, where PtSn/AC showed the best performance PtFe alloy catalysts showed increased catalytic performances compared with the monometallic Pt/CeO2 catalyst

212 209

206

PtSb catalysts show superior catalytic activity and selectivity toward DHA compared with PtBi and monometallic Pt catalysts improvement of catalytic performance with PtCu catalysts under base-free conditions effect of Bi and Sb promotors on catalytic performance; investigation of in situ-generated PtBi and PtSb catalysts for glycerol oxidation Nie 2012 63 DHA 66 2 60

150 mL min−1 0.3 150 mL min−1 150 mL min−1 100 mL min−1 15 mL min−1 0.1 d

PtSb/MWCNT

c

base-free

researchers and year selectivityb/% reaction time/h conversion/% PO2/MPa T/°C basea catalyst

Table 6. Typical Results for Glycerol Oxidation over Pt-Containing Bimetallic Catalysts Reported in the Last Years

comments

ref

ACS Catalysis

6313


5:1

base-free

2:1

4:1

2:1

initial pH 11 constant pH 11 4:1

4:1

Pd/Al2O3

Pd/CNSc

Pd/CNTd

Pd/AC

Pd/HTc

PdPb/Al2O3

Ag95−Pt5/CeO2

Ag/Al2O3

60

60

50

45

90

60

60

80

60

50

160

T/°C

0.5

5 vol % H2O2 200 mL min−1 0.5

0.8

0.6

0.3

150 mL min−1

0.1

0.3

1

PO2/MPa

5

3

3

1

3

0.5

7

5

7.5

3

2

54

85

95

100

70

100

90

21.4

100

98

46

reaction time/h conversion/%

51 GLCA

57 GLCA Zaid 2017

Skrynska 2016

Vajicek 2016

Faroppa 2016

59 DHAf 68 GLA

Abd Hamid 2016

Namdeo 2016

Ribeiro 2016

Yan 2016

Chornaja 2015

Chan-Thaw 2015

Liu 2013


80 GLA

44 GLA

61 GLA

27.6 MOXA

76 GLA

81 GLA

48 LA

selectivityb/%

catalyst activity depends on the nature of the Al2O3 support with basic alumina supports yielding the most active catalysts. other promotors (Au and Pd) and higher amounts of Pt did not improve activity

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 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 differences were observed for the Pt/CNT catalyst in terms of activity and selectivity AC showed the best catalytic performance in terms of activity and selectivity among all other investigated supports (Al2O3, TiO2, SiO2) catalytic activity was attributed to the basic sites of the hydrotalcite (HTc) support and well-dispersed Pd NPs bimetallic PdPb catalysts exhibited a remarkable increase in catalytic performance compared with the monometallic Pd catalyst Bi promotor strongly enhanced the catalytic performance

comment

223

222

215

219

213

214

212

217

192

218

216

ref

NaOH to glycerol ratio if not otherwise denoted. bLA: lactic acid, GLA: glyceric acid, MOXA: mesoxalic acid, DHA: dihydroxyacetone; GLCA: glycolic acid. ccarbon nanospheres. dcarbon nanotubes. e Dowex anion exchange resin. fdetermined at 85% conversion.

a

4:1

Pd/CTF

PdBi/CDe

base-free

basea

Pd/TiO2

catalyst

Table 7. Typical Results for Glycerol Oxidation over Pd-Containing Mono- and Bimetallic Catalysts Reported in the Last Years

ACS Catalysis Review

6314


Review

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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 ref 218. Copyright 2015 Wiley-VCH.

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 metalcontaining 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. 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 with the other catalysts with a

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 Agcontaining 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.

3. GLYCEROL OXIDATION OVER NONPRECIOUS 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 fewer noble and more earth-abundant metals. Although some efforts have been devoted to employ nonprecious homogeneous catalysts for the selective oxidation of glycerol,67,68,224−230 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 Co-containing catalysts as a promising alternative to noblemetal 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 with 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 6315


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NaOH to glycerol ratio if not otherwise denoted. bGLA: glyceric acid, DHA: dihydroxyacetone, TA: tartronic acid, LA: lactic acid, GLCA: glycolic acid. chydrotalcite. d3% H2O2 was used as oxidant; heteropolyacid.

Tao 2016 Dodekatos 2017 91 LA 41 GLCA 94 80 5 3 60 90 base-free 4:1 AlPMo12O40 HPAe Cu/CoO

70 6.8:1 CoMgAl HT

60 60 base-free 4:1 CrAlMg HT CuNiAl HTc

high selectivity toward glyceric acid (86% at 83% conversion).235 Also here, high recyclability and negligible Cu leaching were demonstrated. 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 coprecipitation 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 8a, 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 8b. The catalysts prepared by the sol−gel method showed superior catalytic performances compared with the materials prepared by the coprecipitation 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 coprecipitation 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 9a−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 9e, 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. On the basis of these promising results, we investigated in a following study the role of the posttreatment 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 coprecipitation method was used to prepare nonordered CuCo-based 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 9f). Generally, high selectivities toward glycolic acid and formic acid were observed. Moreover, the asprepared material without any post-treatment 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

e

a

243 241

237

Co incorporated into the HT via different synthesis methods; materials were calcined and H2 treated prior to catalytic use HPA catalysts demonstrated to be tolerant to crude glycerol for glycerol oxidation in situ generation of catalytically active phases for glycerol oxidation; synergy between Cu and Co species required Jin 2016 58 TA 100 18

233 231 sulphonato-salen-Cr complex was hosted in the HT amino-functionalized hydrotalcites showed superior catalytic performance Wang 2012 Wu 2013 73 68 4 4

44 DHA 76 GLA

232 hydrotalcite catalysts showed improved catalytic performance after calcination Zhou 2011 71 GLA 97 3

const. O2 flow -d 60 mL min−1 const. O2 flow 0.1 0.1 60 4:1 CuAlMg HT

comment researchers and year selectivityb/% reaction time/h conversion/% PO2/MPa T/°C basea catalyst

Table 8. Typical Catalysts and Results for Glycerol Oxidation over Non-Precious Metal Catalysts

c

ref

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Figure 8. (a) Conversion, selectivity, and carbon mass balance profiles for glycerol oxidation over Co0.15/Mg3Al-sol−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.

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 panel 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 nonordered CuCo-based catalysts prepared by different post-treatments: 2CuCo-ap, as-prepared sample by a coprecipitation method; 2CuCo-cal, asprepared 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.

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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 depicts commercially obtained support. Reaction conditions: 30 mg of catalyst, 15 mL of 0.05 M aq glycerol solution, 4:1 NaOH:glycerol, 90 °C, 0.1 MPa O2, 3 h. Reproduced with permission from ref 244. Copyright 2017 Royal Society of Chemistry (CC BY 3.0).

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.

a high glycerol conversion (85.7%) toward lactic acid. The different findings can be rationalized by the high reaction temperature of 250 °C that the authors used in their study.242 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 catalysts,241 no reduction step was required to obtain active catalysts. The catalyst with the best performance contained 5 wt % Cu (Figure 10a); 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 10b). Furthermore, for the 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

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 9f). Also in this case as for the ordered mesoporous samples (Figure 9e), it could be shown that a Co/Cu ratio of 2 to 1 exhibited the highest performance for glycerol oxidation (Figure 9g). 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 6318


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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 catalysts were employed for the oxidation reaction. Prior to the HPA immobilization, the carbon surface was functionalized with ethanediamine and 1-bromodecane 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. 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.

different cosolvents 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. A continuous flow reactor setup is, compared with the mostly studied batch reactor, more practical for the large-scale implementation of glycerol oxidation processes because 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-Ramı ́rez’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 Fesilicalite 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 the form of isolated cations or small FeOx clusters in extraframework positions (Figure 11a). This catalyst exhibited a dihydroxyacetone yield of 90%, which was stable over a time frame of 24 h (Figure 11b, sample FeSs873).245 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 11c). 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 11a) 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 nonprecious 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 with the state of the art biocatalytic production of dihydroxyacetone.246 These studies by Pérez-Ramı ́rez’s research group are not only interesting from an academic viewpoint but also extensively demonstrate the potential of

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 generation,251,252 CO2 reduction,253−255 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 have only emerged during the past decade,258−271 although selective transformations of organic compounds via photocatalysis have 6319


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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 of 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 panel 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.

reactors under aerobic conditions and UV−vis irradiation (125 W medium pressure Hg lamp or six 15 W fluorescent lamps). The authors showed that, under certain reaction conditions, commercial P25 was the superior photocatalyst compared to the homemade 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 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 side-products. 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 toward 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. 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

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 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 5-fold 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 a suitable probe molecule to investigate different oxidation mechanisms over TiO2 photocatalysts because of its variety in product formation.261 The observed photocatalytic results were rationalized on the basis of 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 homemade TiO2 samples.259 The reactions were performed in batch photo6320


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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 of 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.

overoxidation. Xu and co-workers were the first to report the use of Bi2WO6 as a selective photocatalyst for dihydroxyacetone formation from glycerol under visible light irradiation.263 The Bi2WO6 materials exhibited a flower-like morphology with good crystallinity as shown in Figure 12a. Dihydroxyacetone was generally formed in high yields (80 to 87%) after 5 h of 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 12b). The authors concluded that positive valence band holes together with dissolved O2 or activated superoxide radicals are required for the reaction to proceed on the basis of control experiments with various radical scavengers (Figure 12c). 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 nonentrapped Bi2WO6 and silica entrapped Bi2WO6 (10 wt % loading) photocatalysts showed a 3-fold increase in glycerol conversion after 4 h irradiation time by using the silicaentrapped 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 but with low selectivities to value-added products and a high tendency to overoxidation. 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 applications.281 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 6321


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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 of 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.

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 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 toward the oxidation of glycerol under milder reaction conditions and with altered selectivities, compared with the conventional thermocatalytic pathway. 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 their potential in the selective oxidation of glycerol more intensively in the future.

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. Figure 13a shows that the visible light irradiation can improve the glycerol conversion severalfold compared with the reactions conducted in the dark. High selectivities toward dihydroxyacetone were achieved (Figure 13b). As shown in Figure 13c,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 13d is similar to the generally proposed reaction pathways found in the literature for the thermocatalytic reaction,47 which is expected due to the superimposition of the thermocatalytic pathway with that of the photocatalytic reaction. 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 14a,b) could further increase the performance by a factor of 2.5 compared with Au catalysts (Figure 14c) and still maintain high selectivities toward dihydroxyacetone. Hence, it was also demonstrated therein that photocatalytic studies should not only be restricted to TiO2 photocatalysts. 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

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 6322


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Figure 15. Selectivity toward 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.

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 Co-based 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.

(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 co-workers 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 toward 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 toward dihydroxyacetone were reported by Lee et al., who employed a PtSb/C electrocatalyst in acidic solutions,334 and a 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 the formation of C2 and C1 products. Furthermore, Sb and Bi, the same structural promotors for Pt that increase the 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 as 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.

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 epichlorohydrin,337 methanol,338 and propylene glycol,21,339 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, 6323


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challenge should not be neglected by the researchers in this field.

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, nonprecious-metal-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, nonprecious-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 emerged,230,341 which is a further step in the direction of a cost-efficient 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 evidence 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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Harun Tüysüz: 0000-0001-8552-7028 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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).

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