Heterogeneous Catalytic Transfer Hydrogenation as an Effective

Review pubs.acs.org/acscatalysis

Heterogeneous Catalytic Transfer Hydrogenation as an Effective Pathway in Biomass Upgrading Matthew J. Gilkey and Bingjun Xu* Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States

ABSTRACT: Reducing oxygen content in biomass-derived feedstocks via hydrodeoxygenation (HDO) is a key step in their upgrading to fuels and valuable chemicals. Organic molecules, e.g., alcohols and formic acid, can donate hydrogen to reduce the substrate in a process called catalytic transfer hydrogenation (CTH). Although it is practiced far less frequently than molecularhydrogen-based HDO processes, CTH has been proven to be an efficient and selective strategy in biomass upgrading in the last two decades. In this paper, we present a selective review of recent progress made in the upgrade of biomass-derived feedstocks through heterogeneous CTH, with a focus on the mechanistic interpretation. Hydrogenation and cleavage of CO and C−O bonds, respectively, are the two main categories of reactions discussed, owing to their importance in the HDO of biomass-derived feedstocks. On acid−base catalysts, Lewis acid−base pair sites, rather than a single acid or base site, mediate hydrogenation of carbonyl groups with alcohols as the hydrogen donor. While acid−base catalysts typically only catalyze the hydrogenation of carbonyl groups with alcohols as the hydrogen donor, metal-based catalysts are able to mediate both hydrogenation and hydrogenolysis reactions with either alcohols or formic acid. Several model reactions involving platform chemicals in biomass upgrading, e.g., 5-hydroxymethylfurfural, levulinic acid, and glycerol, are used in the discussion to illustrate general trends. Because alcohols are typically both the hydrogen donor and the solvent, the donor and solvent effects are intertwined. Therefore, solvent effects are discussed primarily in the context of sugar isomerization and reactions with formic acid as the hydrogen donor, in which the solvent and hydrogen donor are two separate species. Current challenges and opportunities of future research to develop CTH into a competitive and complementary strategy of the conventional molecular-hydrogen-based processes are also discussed. KEYWORDS: catalytic transfer hydrogenation, hydrogen donor, hydride transfer, hydrodeoxygenation, biomass, lignocellulose

1. INTRODUCTION

weight. While the oxygen content of lignin monomers is 30−40 wt %, the exact structure of lignin is still not well established.1 Dehydration and hydrodeoxygenation (HDO) are key processes to partially or fully remove the oxygen-containing functional groups in feedstocks. Dehydration removes oxygen atoms by eliminating water, which does not involve changes in the oxidation state of carbon atoms and typically proceeds via acid−base chemistry. In contrast, HDO lowers the oxygen content of the molecule through reduction with a hydrogen donor. Most HDO processes employ molecular hydrogen as

As the only abundant source of non-food-based renewable carbon, the efficient upgrading of lignocellulosic biomass to fuels and chemicals has the potential of substantially reducing the anthropogenic carbon footprint if implemented on a global scale. The main components of lignocellulosic biomass, i.e., lignin, cellulose, and hemicellulose, and their derived platform compounds possess many desired chemical linkages, e.g., furanic and phenyl rings, which provide an excellent starting point. However, the oxygen content of biomass-derived feedstocks is often significantly higher than that of the desired products, especially for fuels or fuel additives (Scheme 1): C5 (C5H10O5) and C6 (C6H12O6) sugars derived from hemicellulose and cellulose contain more than 50% oxygen by © 2016 American Chemical Society

Received: September 26, 2015 Revised: January 12, 2016 Published: January 19, 2016 1420

DOI: 10.1021/acscatal.5b02171 ACS Catal. 2016, 6, 1420−1436

Review

ACS Catalysis

reduces the complexity and cost of the experimental setup. Furthermore, the lower hydrogenating capability of most organic hydrogen donors, in comparison to molecular hydrogen, enhances the degree of control in selective hydrogenation and/or hydrogenolysis, when partially hydrogenated molecules are targeted. Meanwhile, substantial modifications of the existing molecular-hydrogen-based hydrogenation facilities are likely needed when organic hydrogen donors are employed. For example, additional separation and recycling units are needed to separate the unconverted hydrogen donor from the reaction mixture, which can then be recycled into the feed. The spent organic hydrogen donor can also be separated and integrated into the other steps of biomass upgrading: e.g., aldehydes and ketones produced when alcohols are used as hydrogen donors could be used in carbon chain growth reactions through aldol condensation.2 Another option is to rehydrogenate the spent hydrogen donor; however, molecular hydrogen is likely needed. Some reactions involve two sequential steps: hydrogen production from organic molecules, e.g., aqueous-phase reforming (APR), followed by hydrogen-based HDO. These reactions are not typically viewed as CTH even though all steps occur in the same reactor. CTH provides extra dimensions in the design and optimization of HDO processes. For example, organic hydrogen donors could transfer hydrogen through different mechanistic pathways to the substrate from molecular hydrogen, e.g. intermolecular hydride transfer, which could be exploited for selectivity control. Further, the competitive adsorption of organic hydrogen donors with reactants and reaction intermediates could also have a significant effect on the product distribution. Although CTH was introduced more than a century ago by the seminal report of Pd black catalyzed disproportionation of methyl terephthalate by Knoevenagel,3 it has been largely overshadowed by the success of molecular-hydrogen-based HDO processes until the last few decades. The development of both homogeneous and heterogeneous catalysts has made significant progress in broadening the scope of CTH chemistry and in addressing the drawbacks of low reaction rates and yields

Scheme 1. Removing Oxygen-Containing Functional Groups Is Key in the Upgrading of Lignocellulosic Biomass to Fuel and Chemicals

the hydrogen donor, owing to its wide availability and easy activation on many metal surfaces. However, liquid-phase hydrogen donors could be advantageous because high-pressure hydrogen gas presents a considerable safety hazard. Moreover, the highly oxygenated biomass-derived molecules generally have high boiling points and are prone to decomposition upon vaporization, and thus many biomass-upgrading processes are conducted in the liquid phase in the presence of solvents. The low solubility of molecular hydrogen in most solvents makes high H2 pressure necessary to achieve desired conversions and yields. In addition to the safety concerns, handling highpressure hydrogen gas incurs a hefty infrastructure cost on the industrial scale, which poses an economic barrier for developing a sustainable biomass-upgrading industry, especially in the initial stages. Organic molecules offer a renewable alternative to molecular hydrogen, acting as hydrogen donors in the reduction of chemical bonds with the assistance of catalysts through a pathway known as catalytic transfer hydrogenation (CTH). The employment of liquid organic hydrogen donors alleviates the safety concern of handling high-pressure, flammable hydrogen gas, enhances the solubility of the hydrogen donor in liquid-phase reactions, and substantially

Scheme 2. Common Mechanisms for Homogeneous CTH Reactions: (a) Direct Hydrogen Transfer; (b) Monohydride Mechanism; (c) Dihydride Mechanisma

a

Both (b) and (c) belong to the metal hydride route. 1421


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Scheme 3. Common Mechanisms for Heterogeneous CTH Reactions: (a) Direct Hydrogen Transfer; (b) Metal Hydride Route

direct hydrogen transfer pathway, the α-H is transferred from the α-C of the alcohol to the carbonyl carbon in a concerted step via a six-membered-ring intermediate, without forming a metal hydride. This is also commonly referred to as the Meerwein−Ponndorf−Verley (MPV) mechanism. The use of deuterated hydrogen donors proves to be a helpful diagnostic tool in differentiating the underlying reaction pathways. If the α-C is deuterated in the hydrogen donor, as in Scheme 2, the isotopic purity of D will be preserved on the α-C of the newly formed alcohol. It should be noted that the presence of a base, which could be a ligand, a solvent molecule, or a dissolved basic species, can extract the hydroxyl proton in the hydrogen donor enhances the reactivity. A strong parallel can be drawn in heterogeneous CTH reactions regarding the direct hydrogen transfer pathway: solid catalysts with an electron-deficient Lewis acid site and a neighboring base site can also catalyze CTH reactions following the MPV mechanism (Scheme 3a). The formation of metal hydride is the signature of the metal hydride route (Scheme 2b,c), which could be grouped into two subcategories: i.e., the monohydride and dihydride mechanisms. In the monohydride route, only the hydrogen on the αC of the hydrogen donor, e.g., alcohols and formic acid, is transferred to the metal. In contrast, the dihydride mechanism entails that both the hydrogen atom in the hydroxyl group and that bonded to the α-C are transferred to the metal. The key difference between these two pathways is whether the hydrogen atoms in O−H and C−H maintain their identity in the product. To demonstrate this difference, the α-C of isopropyl alcohol is deuterated while the hydroxyl is not in Scheme 2. In the monohydride mechanism, only the D is transferred to the metal, which is subsequently added to the carbonyl carbon. Thus, the D is always bonded to the α-C in the product. In contrast, upon formation of the dihydride, the H and D become equivalent and are equally likely to end up bonding to the O or C. Interested readers are referred to a recent review article for a detailed discussion of further subcategorization of the monohydride mechanism into inner-sphere and outer-sphere hydrogen transfers.24 Adsorbed hydrogen atoms are important reaction intermediates in heterogeneous CTH reactions on metal surfaces, which resemble the dihydride mechanism because adsorbed hydrogen atoms are typically considered as chemically equivalent. However, this assumption could at least

that have long plagued CTH in its early years. CTH is effective in reducing functional groups by adding hydrogen to either unsaturated bonds (hydrogenation), e.g., CC,4,5 CC,6,7 CO,8−10 NO,11−13 NN,14 and CN,13 or single bonds leading to bond cleavage (hydrogenolysis), e.g., C−O,15−18 C− N,7 C−S,19 and C−X (halogen).20 This paper aims to selectively review recent progress in the upgrade of biomass-derived feedstocks through CTH and discuss challenges and opportunities in establishing CTH as an efficient, selective, and potentially economically viable pathway in the production of biofuels and renewable chemicals. Owing to the vast amount of literature on the general topic of CTH, the scope of this article is limited to heterogeneously catalyzed CTH reactions relevant to the conversion of biomass-derived molecules, though mechanistic insights from homogeneous systems will be leveraged to rationalize experimental observations. Brieger et al.3 and Johnstone et al.21 have reviewed earlier CTH work in the context of both homogeneous and heterogeneous catalysis. There are also several excellent review articles dedicated to more recent work on homogeneously catalyzed CTH reactions.22−24 We will focus our effort on two categories of reactions, i.e., hydrogenation of carbonyl groups and hydrogenolysis of C−O bonds, owing to their central importance in the field of biomass research.

2. GENERAL STRATEGIES FOR HYDROGEN ADDITION To facilitate the discussion of the effect of catalysts, hydrogen donors, and solvents on reaction rates and product distributions in the following sections, we will first introduce several common mechanisms of CTH. The identification of active sites on solid catalysts in CTH is challenging, in that the surface structure of the catalyst under reaction conditions is typically dynamic in nature. The presence of solvents, reactants, and products in bulk liquid phase could mask the features from adsorbed reaction intermediates in many conventional in situ spectroscopies. In this regard, mechanistic insights into CTH reactions catalyzed by structurally well-defined homogeneous catalysts will be instrumental. There are two main mechanisms for homogeneous CTH reactionsdirect hydrogen transfer (Scheme 2a) and the metal hydride route (Scheme 2b,c).22,24 When an alcohol acts as the hydrogen donor in reducing a carbonyl group following the 1422


Review

ACS Catalysis

Scheme 4. Possible Mechanisms of CTH Reactions via Direct Hydrogen Transfer on (a) General Acid−Base Pair, (b) Sn-βZeolite (Open Sn Site), (c, d) MgO, and (e) Hydrous Zirconia

electron-deficient metal center, is to bond with the electron-rich oxygen in the hydroxyl and carbonyl groups in the hydrogen donor and acceptor, respectively, whereas the adjacent base site attracts the proton in the hydroxyl and weakens the O−H bond. The stronger the interaction between the hydroxyl oxygen and the Lewis acid site, the more acidic the hydroxyl hydrogen in the hydrogen donor becomes, which facilitates the abstraction of the hydrogen by the base site. Conversely, strongly basic sites can effectively abstract the hydrogen from the hydroxyl group of the alcohol, leading to the formation of an alkoxide adsorbed on the adjacent Lewis acid site, thus promoting the hydride transfer (Scheme 4a). Thus, both strong acids and strong bases facilitate CTH, but it is unlikely that strong Lewis acid and base sites can coexist on the same catalyst. Although either the acid or base property of a catalyst could dominate, an acid−base pair is needed to complete the catalytic cycle. Alcohols are frequently used as both the hydrogen donor and the solvent in the acid−base pair catalyzed CTH reactions. Their low cost, renewable nature, and unique ability to participate in intra- and intermolecular hydride transfers make alcohols an attractive option for HDO reactions. Secondary alcohols are more efficient hydrogen donors than primary alcohols, which can be rationalized by the enhanced stabilization effect on the carbocation from two alkyl groups

be partially attributed to the difficulty in experimentally differentiating hydrogen adsorbed on different types of sites on the metal particles. Adsorbed hydrogen atoms can be produced by either the dissociative adsorption of molecular hydrogen or dehydrogenation of organic hydrogen donors. The assumption that all adsorbed hydrogen atoms are chemically equivalent leads to the hypothesis that the reaction mechanisms and product distributions when using either hydrogen gas or an organic hydrogen donor should in principle be equivalent. Important differences in the surface coverage of atomic hydrogen and other adsorbed species do exist, and their effect will be discussed in the next several sections.

3. SOLID CATALYSTS FOR CTH 3.1. Acid−Base Catalysts. Acid−base pair sites can facilitate the CTH between an alcohol and a carbonyl group, mainly through direct hydrogen transfer, since the formation of hydrides on most acid and base sites is difficult. In addition, there is generally no change in the oxidation state of the active sites during the course of the reaction in acid−base chemistry. Although both Lewis acidic and basic materials have been reported to be active in mediating CTH processes following the MPV mechanism, it is important to recognize that both Lewis acid and base sites are necessary to catalyze the reaction (Scheme 4a). The role of the Lewis acid site, typically an 1423


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ACS Catalysis Scheme 5. General Pathway for Upgrading Cellulose-Derived Feedstocks to Fuel and Chemicals

(secondary carbocation) during the hydride transfer,21 in comparison to a single alkyl group in primary carbocations. Tertiary alcohols cannot serve as hydrogen donors, due to the lack of an α-H. Self-etherification of alcohols is a common side reaction, especially in the presence of acid sites; however, it is frequently neglected in the literature because the conversion, yield, and carbon balance are calculated on the basis of the substrate of the CTH reaction. Quantification of products formed from the hydrogen donor provides information on the total hydrogen that is produced from the hydrogen donor versus hydrogen that participates in the hydrogenation reactions. In other words, the hydrogen balance provides quantitative information on the efficiency of an organic hydrogen donor in the reaction. Lewis acid sites on solid catalysts can effectively mediate direct hydrogen transfers, in strong parallel with homogeneous CTH systems.24,25 In this regard, zeolites containing tetravalent metal dopants, e.g., Ti, Sn, Zr, and Hf, have shown remarkable activity toward intra- and intermolecular hydride transfer following the MPV mechanism. Replacing a small fraction of tetrahedrally coordinated framework Si atoms with tetravalent dopants, e.g., Ti, Sn, Zr, and Hf, does not disrupt the charge neutrality of the zeolite framework but introduces Lewis acid sites at the heteroatoms.26−28 In particular, Sn-containing Beta zeolite, or Sn-Beta, is remarkably active and selective in catalyzing the isomerization of glucose to fructose (Scheme 5).29,30 It is a key step in converting cellulose-derived feedstock to a versatile platform chemical, 5-hydroxymethylfurfural (HMF), which can only be produced from dehydration of fructose (five-membered ring) but not from glucose or mannose (six-membered ring). Isomerization of glucose to fructose is proposed to occur on the Lewis acidic Sn site, which bonds to the electron-rich oxygen atoms of the carbonyl and hydroxyl groups on the C1 and C2, respectively, and facilitates intramolecular hydride transfer from C2 to C1 (Scheme 4b).31 Furthermore, Davis and co-workers identified that the active sites are “open Sn sites”, i.e., Sn atoms that bond to bridging oxygen atoms and a hydroxyl group, rather than the “closed Sn sites”, in which Sn atoms bond to four bridging oxygen atoms.32−34 An open Sn site can be created by hydrolyzing a Sn−O−Si bond of a closed Sn site, leading to a stannanol (Sn− OH) and a silanol group (Scheme 6a). Ion exchange with Na+ leads to the formation of Si−O−Na+ and likely Sn−O−Na+ (Na/Sn ratios are greater than unity in Na-exchanged Sn-Beta), which are potential base sites for extracting the proton from a hydroxyl during the hydride transfer.34 The absence of isomerization activity of Na-exchanged Sn-Beta in NaCl/H2O, which prevents the loss of Na+ during the isomerization reaction, provides concrete evidence supporting the hypothesis that open Sn sites are the active sites in the isomerization of glucose, because closed Sn sites should be unaffected by ion exchange (Scheme 6b). Density functional theory (DFT) based

Scheme 6. (a) Formation of a Closed Sn Site through Hydrolysis of an Open Sn Sitea and (b) No Formation of Na+ on Closed Sn Sites

a

Na+ exchange with an open Sn site occurs on the silanol group and potentially also on the stannanol group.

calculations confirm that the open Sn site mediated pathway for glucose isomerization is favored over the closed Sn site.35 Recent computational results suggest that the bidentate interaction of oxygen atoms on C1 and C2 in glucose in the sixmembered-ring intermediate may be less energetically favored than pathways with glucose bonded to Sn in a monodentate configuration.36,37 Three potential pathways have been proposed that involve hydrogen-bonding-assisted C1−C2 hydride transfer. (1) Rai et al. identified a pathway involving the neighboring silanol formed with the open Sn site via the hydrolysis of a Si−O−Sn bond (Scheme 7a). Glucose initially loses a proton to the Sn−OH group and bonds to the Sn center through C2−O, which is followed by concerted C1−C2 hydride transfer and silanol-mediated proton transfer.37 (2) An adsorbed water molecule mediates the concerted C1−C2 hydride transfer and proton transfer from water to the carbonyl oxygen (Scheme 7b).36 (3) A vicinal silanol group mediates the concerted C1−C2 hydride and proton transfer from the silanol group to the carbonyl oxygen (Scheme 7c).36 Conrad et al. attempted to mimic the closed Sn sites by grafting Sn(OTMS)3O− and Sn(O-TMS)2(O−)2 species (TMS = trimethylsilane) on SiO2 and found that the turnover number of those sites is lower than that of Sn-Beta by more than a factor of 6.38 This result shows that Sn(−OSi)4 sites are reasonably active in the glucose isomerization reaction. Meanwhile, the lack of pore structure and hydrophobic environment for the grafted Sn(−OSi)4 site could lead to significant differences in its interaction with the reactant in comparison to that on the Sn sites in Sn-Beta, which introduces uncertainty in the interpretation of the activity difference. The high solubility of glucose in water makes water an excellent solvent for the isomerization reaction; however, the presence of bulk water severely deactivates Lewis acid sites by strong adsorption. Defect-free Sn-Beta, synthesized in the fluoride medium, has hydrophobic pores and excellent water 1424


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Scheme 7. Proposed Mechanisms of Glucose Isomerization on an Open Sn Site of Sn-Beta on the Basis of DFT Calculations: (a) Neighboring Silanol-Assisted Pathway; (b) Adsorbed Water-Assisted Pathway; (c) Vicinal Silanol-Assisted Pathway

ization,28,30,39 most likely because the rates of both reactions are limited by the hydride transfer step. Metal oxides, e.g. ZrO2,43 ZrO(OH)2,44 MgO,43 etc., have also shown high activity in performing the MPV reaction to GVL. In situ generated HCl and ZrO(OH)2 catalysts from a salt precursor (ZrOCl2·8H2O) form GVL with high selectivity (84.5%) in 2-BuOH;44 however, the role of in situ generated HCl is unclear. One possibility is that HCl, a strong acid, catalyzes the esterification reaction of LA to levulinates, from which the reaction proceeds much more quickly over amphoteric oxides such as ZrO2. This is consistent with the work of Chia et al., which showed that levulinates, rather than LA, reacted more quickly to form GVL over ZrO2.43 A higher concentration of basic sites in amphoteric oxides induces strong binding of acidic functional groups on LA, thereby suppressing the rate of reaction. Thus, controlling the ratio of acid and base sites is crucial in optimizing the rate of hydrogenation. Lewis acid mediated CTH over metal oxides can also be used to selectively produce 2,5-bis(hydroxymethyl)furan (∼94%) from HMF using ZrO(OH)x, where surface hydroxyl groups act as active sites for alkoxide formation, thereby facilitating the MPV mechanism.45 Zeolites have also been employed to reduce furfurals into furfuryl alcohols.9,46−48 Vlachos et al. coupled hydrogenation to 2,5-bis(alkoxymethyl)furans with etherification reactions with an alcohol to produce high-molecularweight ethers,9 which proceeds via the sequential CTH and etherification steps on Lewis acid sites. Similarly, furfural can be hydrogenated to furfuryl alcohol via zeolite-mediated CTH using a variety of Sn/Al combinations.46,47 Moreover, Sn/Alcontaining Beta zeolites can further catalyze tandem reactions (hydrogenation and hydrolysis) to form levulinates and, subsequently, GVL in one pot.46,47 Base-mediated CTH reactions were proposed to proceed through a two-step process (Scheme 4c):8,49−53 the alcohol (hydrogen donor) dissociates to form the corresponding alkoxide on a weakly acidic metal cation, e.g., Mg2+, while the hydroxyl proton resides on the basic O2− anion. The α-H of the alkoxide and the proton adsorbed on the base site are then transferred to the carbon and oxygen atoms in the carbonyl group, respectively. This pathway is quite different from the typical MPV mechanism because the six-membered-ring intermediate does not involve the metal site. An alternative

resistance, as evidenced by its more than 2-fold higher glucose conversion in comparison to that on the relatively hydrophilic Sn-MCM-41.30 However, Sn-Beta is more active than Ti-Beta in converting glucose to fructose, though both catalysts are hydrophobic.30 The activation barrier of glucose isomerization on tetravalent-metal-doped Beta zeolites has been correlated with the polarizability of the heteroatom and the basicity of the neighboring oxygen atom, with the activation barrier increasing in the sequence Sn ≈ Zr < Nb < Ti ≈ Ge < V.39 A similar intramolecular hydride transfer mechanism is responsible for the isomerization of pentoses40,41 and glyceraldehyde40 on SnBeta. Lewis acid catalyzed CTH can be integrated into cascade reactions in the production of γ-valerolactone (GVL) and 2,5bis(alkoxymethyl)furans. GVL, a versatile platform chemical, can be produced in a two-step cascade reaction starting with levulinates on Ti-, Sn-, Zr-, and Hf-Beta.28,42 Levulinate is initially hydrogenated to the corresponding 4-hydroxypentanoate with an alcohol as the hydrogen donor, which is followed by lactonization to GVL (Scheme 8). The lower activation energy on Sn-, Zr-, and Hf-Beta in comparison to that on TiBeta in GVL formation parallels that in glucose isomerScheme 8. Reaction Mechanism for γ-Valerolactone Production via CTH of Levulinate on Tetravalent-MetalDoped Beta Zeolites

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ACS Catalysis Table 1. Metal-Mediated CTH Results with a Variety of Substrates Using Alcohols as the Hydrogen Sourcea substrate

desired product

catalyst

solvent

temp (K)

time (h)

conversn (%)

yield (%)

ref

EL LA LA LA furfural furfural furfural furfural HMF HMF HMF HMF glycerol glycerol glycerol

GVL GVL GVL GVL 2-MF 2-MF 2-MF 2-MF DMF DMF DMF DMF 1,2-PDO 1,2-PDO 1,2-PDO

Ru(OH)x/TiO2 Raney Ni Raney Ni Ru/C Pd/Fe2O3 Cu/Fe2O3 Ru/RuOx/C Ru/RuOx/C Cu-PMO Cu-PMO Pd/Fe2O3 Ru/RuOx/C Cu:Al Ni−Cu/Al2O3 Pd/Fe2O3

2-propanol 2-propanol 2-propanol 2-propanol 2-propanol 2-propanol 2-propanol 2-BuOH MeOH MeOH 2-propanol 2-propanol 2-propanol 2-propanol 2-propanol

363 353 room temp 353 453 453 453 453 260 320 453 463 493 493 453

24 9 9 9 7.5 7.5 10 10 3 0.75 24 6 3 24 24

>99 >99 >99 >99 >99 37 95 >99 >99 >99 >99 >99 70 60 100

89 >99 >99 93 13 0 61 74 48 32 71 80 38 39 94

107 118 118 79 17 17 10 78 74 74 17 9 102 18 18

Abbreviations: EL = ethyl levulinate, GVL = γ-valerolactone, LA = levulinic acid, 2-MF = 2-methylfuran, HMF = 5-hydroxymethylfurfural, DMF = 2,5-dimethylfuran, 1,2-PDL = 1,2-propanediol.

a

of hydrous zirconia is weaker than that of MgO,67 the dissociation of the alcohol to an alkoxide is likely not favored. The condensation reaction between Zr−OH and the alcohol is key to the irreversible formation of the adsorbed alkoxide upon desorption of water, which helps initiate the catalytic cycle. Acid/base-catalyzed CTH processes are remarkably efficient and selective in reducing carbonyl groups. However, they are ineffective in cleaving C−O single bonds, which could be attributed to (1) the inability to activate C−O bonds via the Lewis acid mediated MPV mechanism and (2) the inability to produce active hydrogen species, e.g., Had, on metal oxides from organic hydrogen donors to complete the catalytic cycle. The cleavage of the C−O bond is an indispensible step in the upgrading of cellulose-, hemicellulose-, and lignin-derived feedstocks (Scheme 1), which can be accomplished by metalmediated CTH processes via the metal hydride mechanism discussed in the next section. 3.2. Metal Catalysts. The ability to efficiently activate H− H, CO, C−O, and C−H bonds makes metalsparticularly noble metalsthe most widely used hydrogenation and hydrogenolysis catalysts. The most common form of metal catalysts is metal (nano)particles on a high-surface-area support. Although some support materials, e.g., activated carbon,68−70 are inert, many supports contain acid or base sites, e.g., Al2O3,68,71 zeolites,72,73 and porous metal oxides.74 Unsupported porous metal catalysts may also contain heteroatoms due to the preparation procedure, e.g. the residual Al species in Raney Ni produced from the alkali corrosion of Ni−Al alloy.75 The coexistence of multiple types of active sites poses challenges in achieving a molecular level mechanistic understanding due to the structural complexity. At the same time, it provides flexibility in tailoring catalysts for reactions wherein different reaction steps require different types of active sites. The dissociative adsorption of H2 to adsorbed atomic hydrogen is generally the first mechanistic step in metalmediated HDO processes with hydrogen gas, whereas metalcatalyzed CTH processes start by activating the organic hydrogen donor. It is generally assumed that the mechanistic pathways of HDO processes with H2 and organic hydrogen donors converge after adsorbed atomic hydrogen is formed (Scheme 3b); however, formation of negatively charged hydride species has been proposed as a surface intermediate

mechanism similar to that of Lewis acid catalysts (MPV mechanism) could be drawn, which is consistent with all existing results (Scheme 4d). In this case, the reaction is likely driven primarily by the basic sites through the abstraction of the proton in the hydroxyl group, rather than the interaction of reactants with the weak acid sites. However, as in the acidmediated case, the presence of both acid and base sites is necessary to achieve high catalytic performance.54,55 The ratelimiting step in base-mediated CTH reactions remains unclear. Reduced electron density on the carbonyl group facilitates hydride transfer, as evidenced by the enhanced reaction rate when electron-withdrawing groups neighboring the carbonyl group are present,52,56 which is an indication that the hydride transfer is a kinetically relevant step. This hypothesis can be tested by measuring the kinetic isotopic effect with an undeuterated and a deuterated α-C in the alcohol. MgO is the most frequently used basic CTH catalyst,49,53,57−61 but it deactivates rapidly. Szöllösi et al. reported that treating the MgO catalyst with chloromethanes significantly enhances catalyst stability by blocking the Lewis acid site by adsorbed Cl.53,60,61 Moreover, Mg/Al mixed oxides,50,52,55 including hydrotalcites54 and potassium phosphate,56 are also active in catalyzing the MPV reactions. Hydrous zirconia has been shown to be a very effective MPV catalyst,62−65 and its catalytic performance is sensitive to the preparation method and calcination temperature. As an amphoteric oxide, ZrO2 possesses acid−base pair sites, though neither is very strong. The high activity of hydrous zirconia has been attributed to the presence of surface hydroxyl groups, which diminish in number as the calcination temperature exceeds 300 °C.66 The surface hydroxyl group reacts with the alcohol (hydrogen donor) to form an adsorbed alkoxide and water (Scheme 4e).62,63 The α-H of the alkoxide is then transferred to the carbonyl carbon of the hydrogen acceptor via the formation of the six-membered-ring intermediate, similar to that in the classic MPV mechanism. The aldehyde or ketone formed from the dehydrogenation of the alcohol (R1R2CO) desorbs from the Zr site, leaving behind the alkoxide (R3R4CHO−) derived from the hydrogen acceptor. In the final step, the hydrogenated product (R3R4CHOH) is formed via the displacement of the alkoxide by an incoming alcohol molecule, thus completing the catalytic cycle. Since the basicity 1426


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ACS Catalysis with organic hydrogen donors,16,76 indicating more substantial mechanistic differences may exist. In the following section, metal-catalyzed CTH processes are broken down first by the type of hydrogen donor and then by substrates and reactions. 3.2.1. Alcohols as Hydrogen Donors. Alcohols are widely used hydrogen donors for metal-catalyzed CTH reactions (Table 1), with many parallels with acid−base chemistry discussed in section 3.1. Secondary alcohols generally show higher activity than primary alcohols in dehydrogenation over metal surfaces, facilitating hydrogen transfer to the substrate. This can be readily attributed to the enhanced stabilizing effect of two, rather than one, alkyl groups via inductive electron donation to the α-C of the alcohol in the dehydrogenation process.8,17,77,78 In particular, Vlachos and co-workers recently systematically studied the effect of the structure of alcohols on the conversion of furfural to 2-methylfuran (2-MF) on Ru/C.77 Aside from confirming secondary alcohols being superior to primary alcohols, this work shows that a longer alkyl chain in the alcohol is beneficial for CTH activity, which holds for both primary and secondary alcohols. However, this effect diminishes when the side chain contains more than two carbon atoms: i.e., the CTH activity of alcohols increases in the sequence ethanol < 1-propanol ≈ 1-butanol < 2-propanol < 2butanol ≈ 2-pentanol. The reduced enhancement effect for longer side chains could be attributed to the diminished added stabilizing effect, site blocking caused by the larger footprint of the adsorbed alcohol, or a combination of the two. Interestingly, methanol has been employed as a hydrogen donor in the hydrogenolysis of glycerol.18 When methanol dehydrogenates into formaldehyde and H, the formaldehyde can react with water to form formic acid. The decomposition of formic acid to CO2 and H is more energetically favorable than methanol dehydrogenation; thus, CO2 was the only product aside from hydrogen when methanol was used as the hydrogen donor on Ni−Cu/Al2O3 at 220 °C. Thus, two molecules of hydrogen are produced for each methanol molecule, twice as much as that from 2-propanol. Comparable glycerol conversions and product distributions were observed with 2propanol and methanol as the hydrogen donors.18 Moreover, supercritical methanol has been demonstrated as an effective environment for conversion of HMF to DMF.74 3.2.1.1. HDO of Oxygenated Furanics. HDO of oxygenated furanic compounds, e.g. HMF and furfural, to reduced furans, e.g. DMF and 2-MF, involves both hydrogenation of the carbonyl bond and hydrogenolysis of the C−O bond (Scheme 9). The acid−base pair mediated CTH cannot accomplish the latter, and therefore, 2,5-bis(alkoxymethyl)furans produced from the etherification between 2,5-bis(hydroxymethyl)furfural (BHMF) and the alcohol is the final product.28 In contrast, high yields of DMF and 2-MF can be achieved on metal catalysts from HMF and furfural, respectively. Co-feeding furfural and 1,4-butanediol (1,4-BDO) on a Cu−Ni catalyst produces 2-MF and γ-butyrolactone (γ-BL) with close to 100% yield in the vapor phase at temperatures above 190 °C.79 In addition, similar conversions and product distributions were observed when the dehydrogenation of 1,4-BDO and the HDO of furfural with hydrogen gas were carried out separately, indicating that the dehydrogenation and HDO pathways are independent. Hermans and co-workers investigated the CTH of furfural with 2-propanol over Fe2O3-supported metals (Pd, Ni, and Cu) in the liquid phase, showing that Pd was far more active toward the HDO reaction than Cu and Ni.17 Moreover, only Pd/Fe2O3 is capable of catalyzing hydrogenolysis of

Scheme 9. Reaction Pathway for Converting HMF to DMF via Sequential Hydrogen of the Carbonyl Group and Hydrogenolysis of the C−O Bonda

a

Abbreviations: HMF = 5-hydroxymethylfurfural, MFUR = 5methylfurfural, MFA = 5-methyl-2-furyl(methanol), BHMF = 2,5(bishydroxymethyl)furan.

furfuryl alcohol to 2-MF. Significant amounts of ring-saturated (tetrahydromethylfuran, 36%) and decarbonylated (furan, 24%) products were formed along with 2-MF (26%) on Pd/ Fe2O3 under continuous flow conditions. Pd/Fe2O3 shows much higher CTH activity than Pd/C, which is attributed to the oxophilic nature of Fe in facilitating the activation of the O−H bond in 2-propanol. HDO of HMF to DMF is mechanistically similar to that of furfural but requires one more hydrogenolysis step. A 72% yield of DMF from HMF was achieved on Pd/Fe2O3 under conditions similar to those for furfural.17 The lack of unprotected α-carbon makes BHMF and DMF less susceptible to dimer or oligomer formation via alkylation,77 which leads to the higher yield of DMF than 2-MF. Riisager and co-workers recorded ∼32% yield toward DMF when using supercritical methanol as a hydrogen source (>99% conversion of HMF) over Cu-doped hydrotalcite, with significant overhydrogenation to 2,5-dimethyltetrahydrofuran.74 At high temperatures (>300 °C), fast dehydrogenation of MeOH to H2 and CO2 is likely to create a hydrogen-rich environment that favors the saturation of the furan ring. High yields for 2-MF (∼70%) and DMF (∼80%) were obtained on Ru-based catalysts from furfural and HMF, respectively, with secondary alcohols.10,77 This could in part be attributed to the oxophilic nature of Ru that leads to strong bonding to oxygen-containing species, such as furfural and furfuryl alcohol, which accelerates the HDO of furfural and suppresses side reactions such as the hydrogenation of the furan ring. In a subsequent study, Jae et al. discovered that mildly oxidized Ru/C catalysts (referred to as Ru/RuOx/C), which contain both metallic (Ru) and Lewis acidic (RuOx) sites, are most active and selective in the HDO of HMF to DMF.69 The benefit of the coexistence of metal and Lewis acid sites is evidenced by the meager DMF yields on fully reduced Ru/C (∼30%) and RuO2 (∼10%), in contrast with the ∼70% yield on the physical mixture of Ru/C and RuO2.9 Higher DMF yield (>80%) on Ru/RuOx/C, which is prepared by mildly oxidizing fully reduced Ru/C under a diluted O2 atmosphere, in comparison to that on a physical mixture of Ru/C and RuO2 emphasizes the importance of atomically mixed metal and Lewis acid sites. Isotopic labeling investigations with detailed mass fragmentation pattern analysis reveal that the hydrogenation of the carbonyl group in furfural proceeds through a Lewis acid mediated MPV mechanism, as evidenced by the transfer of the deuterium atom from the α-carbon of 2propanol-d8 to the α-carbon of the formed furfuryl alcohol (Scheme 10). In addition, the incorporation of deuterium into 1427


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Scheme 10. Proposed Mechanism for 2-MF Formation from Furfural over Ru/RuOx/C: (a) Hydrogenation of Furfural; (b) Hydrogenolysis of Furfuryl Alcohol

Scheme 11. Proposed Pathways for 1,2-PDO Formation from Glycerol, Proceeding by (a) Initial Dehydrogenation and (b) Initial Dehydration

1,2-PDO is currently produced industrially from propylene oxide, which is derived from petroleum-based propylene.83 Hydrogen can be produced from glycerol from APR on Pt- and Ni-based catalysts;83−86 thus, with a tailored combination of metal and acid−base sites, hydrogen generated in situ can be used for the hydrogenolysis of glycerol.83,87−90 In this approach, hydrogen production via APR and hydrogenolysis could be viewed as sequential processes, and the latter typically occurs in a molecular-hydrogen-rich environment.83 Therefore, it is unclear whether CTH is a major pathway and is beyond the scope of this article. Glycerol hydrogenolysis is proposed to occur through several possible pathways. Many studies indicate that the initial step in glycerol hydrogenolysis to 1,2-PDO is dehydrogenation of the terminal alcohol group to form glyceraldehyde (Scheme 11a),91−96 though this step is likely reversible in reducing environments. Glyceraldehyde can then be dehydrated and tautomerized, via an enol intermediate, to form pyruvaldehyde, which is hydrogenated to 1,2-PDO (Scheme 11a). The formation of glyceraldehyde through dehydrogenation at high hydrogen pressure is still a topic of debate.83,91,97 It has been proposed that this mechanism holds true when the rate of dehydration from glyceraldehyde is higher than that of dehydration from glycerol,94 which is accelerated in the presence of base.95,96 It should be noted that, in the absence

the furan ring in the product (2-MF) shows that the hydrogenolysis of furfuryl alcohol occurs, at least in part, via the activation of the furan ring,80 a phenomenon known to occur facilely over Ru surfaces.81 Furan rings adsorb strongly with Ru surfaces, which promotes the hydrogenolysis of the C− O bond in the hydroxymethyl group80 and the C−O bond within the furan ring. Coadsorption of different solvent molecules appears to play a significant role in the ability to activate the ring.81 Furthermore, it is hypothesized that the Lewis acidic RuOx contributes to the hydrogenolysis by accepting the −OH group from the furfuryl alcohol. These findings were further confirmed by the significant kinetic isotope effects observed in both hydrogenation and hydrogenolysis steps, indicating that hydrogen (or deuterium) transfer to the substrate is rate limiting. The synergistic effect of the metal and Lewis acid sites on Ru/RuOx/C is reinforced by the enhanced 2-MF yield on fully reduced Ru/C with homogeneous Lewis acids: e.g., AlCl3.82 Etherification occurs among the hydrogen donor molecules and between furfuryl alcohol and the hydrogen donor in the presence of Lewis acids; however, it is a reversible process.10 3.2.1.2. Hydrogenolysis of Glycerol to 1,2-Propanediol. Hydrogenolysis of glycerol, a byproduct of biodiesel production, is a promising renewable pathway to synthesize 1,2-propanediol (1,2-PDO), an important polymer precursor. 1428


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Scheme 12. Proposed Reaction Pathways of Glycerol on Ni−Cu/Al2O3: (a) Hydrogenolysis to 1,2-PDO; (b) Dehydration to Acetol

Scheme 13. Proposed Pathways of LA Hydrogenation to GVL and Further Hydrogenated Products

1,2-PDO with hydrogen gas, dehydration of glycerol to acetol is a major pathway in CTH with 2-propanol.18,101 Higher yields for 1,2-PDO were obtained on Ni−Cu/Al2O3 with 2-propanol, which is still ∼20% lower than those with high-pressure hydrogen (45 bar) under otherwise identical conditions.71 Since hydrogenolysis and dehydration are parallel reaction pathways (Scheme 12),71 the higher yield of acetol could be attributed to the lower atomic hydrogen coverage with the organic hydrogen donor (vs high-pressure hydrogen). Moreover, Pd/Fe2O3 has also shown to be an effective catalyst for glycerol hydrogenolysis to 1,2-PDO.102 Similar to the case of furfural HDO,17 the presence of the Fe2O3 support is key to the catalytic performance, which most likely facilitates the activation of the alcohol. 3.2.1.3. Levulinic Acid to γ-Valerolactone. The conversion of levulinic acid (LA) and levulinates to GVL can be mediated not only by acid−base pairs, as discussed in the previous section, but also by metal catalysts. Mechanistic studies by Bond et al. using molecular hydrogen, rather than CTH, suggest that there are two possible pathways toward GVL over metal catalysts: e.g., Ru/C.103 One possibility is that LA or levulinates are first hydrogenated to form 4-hydroxypentanoic (HPA) acid or a corresponding ester; the resulting HPA can then undergo lactonization or ring closure to form GVL as the final product (Scheme 13). Alternatively, LA or levulinates can first undergo lactonization or ring closure to angelicalactones (α- or β-angelicalactone), followed by hydrogenation to GVL, as proposed by Hasan et al. over Pd/C in the presence of base.104 Bond et al. explored these two pathways in the

of hydrogen, glycerol can be converted to lactic acid through a similar mechanism over an iron pincer catalyst.98 However, instead of hydrogenation of pyruvaldehyde, an intramolecular Cannizzaro reaction occurs in the presence of a base to form lactic acid (or the corresponding lactate salt). Alternatively, glycerol hydrogenolysis can begin through Brønsted acid catalyzed dehydration (Scheme 11b), rather than dehydrogenation, forming acetol as the only intermediate. 97,99,100 Tomishige et al. showed that this pathway is accelerated in the presence of acid sites, e.g. Amberlyst.100 Acetol can then be hydrogenated over a metal, e.g. Ru, Cu, etc., to form 1,2-PDO. To eliminate the need for acids or bases, the use of bimetallic or bifunctional catalysts, e.g. Ni−Cu,15,18,71 has been employed, forming 1,2-PDO from glycerol through direct hydrogenolysis routes. 2-Propanol is an efficient hydrogen donor on coprecipitated Cu−Al catalysts for the hydrogenolysis of glycerol. Low conversion (99 trace 100 29 98 43

132 132 132 127 128 126 131 129 127 130 127 130 18 70 143

n.r. >99 98 99 99 >99 >99 n.r. 100 34 99 97

a

Abbreviations: n.r. = not reported, HMF = 5-hydroxymethylfurfural, DMF = 2,5-dimethylfuran, THF = tetrahydrofuran, DMSO = dimethyl sulfoxide, GVL = γ-valerolactone, BL = butyl levulinate, LA = levulinic acid, 1-HB = (1-hydroxyethyl)benzene, HDL = 1,6-hexanediol.

is possible that the partial oxidation of Ru provides Lewis acidic sites that facilitate the lactonization step. 3.2.2. Formic Acid and Formate as Hydrogen Donors. Formic acid (FA) can be formed renewably from lignocellulosic biomass109 or from electrochemical reduction of CO2,110 which makes FA an environmentally friendly source for both highpurity hydrogen production22,111,112 and a hydrogen donor for CTH reactions (Table 2). Two surface species have been proposed to directly participate in hydrogen transfer (Scheme 14): (a) adsorbed hydrogen and (b) adsorbed formate species.

presence and absence of H2, subsequently concluding that angelicalactones are observed as a result of proton-catalyzed ring closure directly from LA only in the absence of molecular H2. In the presence of H2, the rate of reaction toward HPA is orders of magnitude higher than the rate toward angelicalactones. Thus, in similarity to acid−base catalysis, LA hydrogenation proceeds via HPA as an intermediate. Use of molecular hydrogen over metal catalysts, however, can lead to a substantial level of overhydrogenation of GVL to 2methyltetrahydrofuran (2-MTHF) or linear alcohols such as 1,4-pentanediol and 2-butanol (Scheme 13).105 Thus, CTH promises better control of the degree of hydrogenation by maintaining low effective H2 pressure on the catalyst. While metals easily reduce CO bonds, lactonization is an inherent challenge over metal catalysts. This is evidenced by the work of Hasan, where Pd/C alone was not capable of forming any GVL without the addition of base (KOH or NaOH) and a maximum yield of 83% yield of GVL was obtained using 2propanol as the hydrogen source in the presence of KOH under microwave conditions.104 Metals supported on acidic supports catalyze the cascade reaction without the need for homogeneous acid or base.78,106,107 Fujitani et al. reported a GVL yield of up to 89% from levulinic esters on TiO2supported Ru(OH)x catalysts over 24 h with 2-propanol as the hydrogen source.106 A higher GVL yield was obtained on Ru/C in comparison to Ru(OH)x at lower reaction temperature (80 vs 90 °C) and shorter time (9 vs 24 h), indicating that metallic Ru sites could be more active than Ru(OH)x sites. Remarkably, a close to 100% yield of GVL was obtained on Raney Ni at room temperature in 9 h, in comparison to 27% on Ru/C under otherwise identical conditions.108 In contrast, Ni supported on TiO2, SiO2, CeO2, and C is inactive. The initial hydrogenation step to HPA occurs easily over oxophilic metal sites such as Ru; however, the lactonization step proceeds much more slowly on metal surfaces.107 The high GVL yields obtained on Raney Ni, especially at low temperatures (Table 1), could be attributed to the residual Al species in the Raney Ni from the alkali corrosion process of Ni−Al alloy,75 which is consistent with the low activity for supported Ni catalysts. In light of the reaction mechanism for the HDO of furanic compounds on Ru/RuOx/C and the oxophilic nature of Ru, it

Scheme 14. Proposed Surface Adsorbed Hydrogen Donor in CTH Reaction with FA: (a) Adsorbed Hydrogen Atom; (b) Adsorbed Formate

Adsorbed atomic hydrogen formed via stepwise hydrogen transfer from FA to the metal surface113 appears to be a natural choice; however, it fails to explain some results obtained from isotopic labeling studies on Pd/C,114,115 one of the most widely used CTH catalysts with FA. Although the carboxylic acid hydrogen is more acidic than the formyl hydrogen in FA, they become indistinguishable upon deposition on the metal surface (Scheme 14a) and thus should exhibit equivalent reactivity. However, in the CTH of methyl phenylpropiolate, HCOOD and DCOOH led to primarily undeuterated and deuterated products, respectively (Scheme 15a),114 which suggests that the formyl hydrogen, rather than the carboxylic acid hydrogen, is transferred to the substrate in CTH. Moreover, isomerization of enol ether compound 1 (Scheme 15b) with both DCOOH and DCOOD resulted in the D incorporation at the carbon adjacent to the methoxy group, whereas no D incorporation was observed with HCOOD, which further supports the hypothesis that the formyl hydrogen is the active species in 1430


Review

ACS Catalysis

Scheme 15. (a) Hydrogenation of Methyl Phenylpropiolate on Pd/C with DCOOH or HCOOD, (b) Isomerization of Compound 1 on Pd/C in with Deuterated FAs, (c) Proposed Mechanism for the Isomerization of Compound 1 on Pd/C, and (d) Potential Reaction Mechanism for (i) Hydrogenolysis of C−O Bonds, (ii) Hydrogenolysis of CO Bonds, and (iii) Hydrogenation of CC Bonds with FA via CTH

hydrogen transfer. The fact that H/D exchange only occurs at the electron-deficient carbon bonded to the methoxy group indicates that the transferred hydrogen is a negatively charged hydride species. Adsorbed atomic hydrogen does not have any appreciable preference for the carbon atom next to the methoxy or the phenyl group,114 as the Pd−H bond was proved to be amphipolar.116 On the basis of these observations, the isomerization reaction is proposed to proceed via an intermolecular hydride transfer mechanism with a sixmembered-ring intermediate, similar to the MPV mechanism (Scheme 15c): the electron-deficient carbon atom in the CC bond coordinates with the formyl hydrogen (D in Scheme 15c), while the electron-rich carbon bonds with the Pd atom. The hydride transfer is accompanied by the release of CO2 and the reduction of the CC bond to the C−C bond. Cleavage of C−H and C−Pd bonds occurs after the rotation of the newly formed C−C bond, converting the substrate from a trans to a cis configuration. Several recent works proposed the negatively charged adsorbed hydride species as an intermediate in CTH reactions,70,76,117 which could occur following similar pathways (Scheme 15d): (i) the formyl hydrogen transfers to the electron-deficient carbon next to the ether bond, leading to the hydrogenolysis of the ether bond, (ii) the formyl hydrogen transfers to the electron-deficient carbonyl carbon, leading to the hydrogenation of the CO bond, (iii) the formyl hydrogen transfers to the electron-deficient carbon in the CC bond, e.g., β-C in α,β-unsaturated carbonyl systems,115

leading to the hydrogenation of the CC bond. The proposed mechanisms outlined in Scheme 15d could be verified by isotopic labeling studies similar to those conducted by Spencer and co-workers.114,115 The hypothesis that the adsorbed formate directly participates in the hydride transfer mechanism is consistent with the fact that the addition of bases, e.g., triethylamine (Et3N), are frequently added in the CTH system with FA,70,114,115,117−119 which likely promote the formation of the adsorbed formate species (Scheme 15d). Moreover, the hydrogen-donating ability of formates was shown to be dependent on the ionic radius of the countercations, with all organic and metal cations showing better CTH activity than protons.76,118,120 Formate with larger metal cations showed higher activity for the hydrogenolysis of benzyl acetate, which was attributed to the ease of separating the ions due to the longer initial distance of the charge centers.76 However, FA exhibits higher activity than formate salts in the CTH of αmethylbenzyl alcohol (MBA) on Pd/C, which could be attributed to the role the proton plays in the dehydration step.121 3.2.2.1. Levulinic Acid to γ-Valerolactone. In contrast to the case of alcohols as hydrogen donors, many metal catalysts can efficiently mediate the reaction cascade from LA to GVL with FA,122 though many of the same principles apply. Fe(0) carbonyls have been shown to be an effective class of catalysts for hydrogenation of organic molecules and are a promising 1431


Review

ACS Catalysis Scheme 16. Proposed Transesterification-Assisted Hydrolysis Mechanism in the Conversion of HMF to DMF

propanol, MeOH, and H2 (Table 3). As a polyol, glycerol itself can serve as a hydrogen source via APR and provide molecular

class of catalysts due to economic feasibility, commercial availability, and facile synthesis.123 Burtoloso et al. used Fe3(CO)12 to produce GVL very selectively from LA. However, similarly to the work of Hasan et al. using Pd/C catalysts and alcohols as the hydrogen source, a base (pyridine or imidazole) was needed in addition to Fe3(CO)12 to obtain a maximum yield of 92% at 180 °C; without added base, only 12% yield was observed under identical conditions.123 While Au-based catalysts are generally unable to activate alcohols as hydrogen donors, they are remarkably efficient in activating FA for both hydrogen generation124 and the conversion of LA to GVL via CTH with quantitative yields.125,126 One-pot synthesis of GVL from glucose, fructose, sucrose, starch, and cellulose with 30− 60% yields has been achieved on Au/ZrO2 catalysts.126 Rubased catalysts are also active in reducing LA and C6 sugars to GVL,118,119,127,128 though the activity is slightly lower than Au/ ZrO2 under similar conditions.126 While Ru supported on inert materials such as activated carbon are active in converting LA to GVL, Au/C and Au/SiO2 show almost no activity.126 The difference could be attributed to the Lewis acid sites present on Ru for the lactonization step due to the almost unavoidable presence of the RuOx phase, while on Au-based catalysts, acid sites must be introduced by the support. Interestingly, ZrO2supported Cu129 and Ag−Ni130 catalysts also show very high activity toward GVL production, albeit at higher temperatures. 3.2.2.2. HDO of Oxygenated Furanics. Like alcohols, FA has been successfully implemented in the upgrading of oxygenated furanics, i.e. furfural and HMF, to their reduced forms, i.e. 2-MF and DMF, respectively. In particular, Thananatthanachon and Rauchfuss explored the conversion of HMF to DMF, acquiring 95% yield to the HDO product with FA in a mixture of THF (or DMSO) and H2SO4 over Pd/ C.131 In combination with FA’s ability to dehydrate fructose to HMF, the same group demonstrated the possibility of a onepot synthesis of DMF directly from fructose. Fructose is typically dehydrated in the aqueous phase in the presence of Brønsted acids, an environment that accelerates the humin formation from HMF.132,133 By utilization of FA as the hydrogen source in organic solvents, fructose can be dehydrated and subsequently reduced to DMF in one pot,131 eliminating the need to use biphasic systems for HMF isolation.134 Esterification is proposed to be a key step prior to the hydrogenolysis of the C−O bond in the HDO of HMF to DMF (Scheme 16) on both Pd/C and Ru/C.131,135 The cleavage of the C−OH bond adjacent to the furan ring is shown to occur after the formation of the in situ generated formate esters: e.g., the formation of FMF and FMMF (Scheme 14). Three distinct roles of formic acid in this process were identified: (1) hydrogen donor, (2) acid catalyst, and (3) deoxygenation agent for furanylmethanols.131 3.2.2.3. Hydrogenolysis of C−OH and Ether Bonds. FA is also an effective hydrogen donor for the production of 1,2PDO via the hydrogenolysis of glycerol. Güemez et al. compared the activity and product distribution on the Ni− Cu/Al2O3 catalyst among four different hydrogen donors under otherwise similar reaction conditions (220 °C, 10 h):18 FA, 2-

Table 3. Comparison of H-Donors in Glycerol Hydrogenolysisa,18

substrate

H donor

H donor

conversn (%)

S1,2‑PDO (%)

Sacetol (%)

FA 2-propanol MeOH H2 (45 bar) none

34 28 26 35 16

86 77 51 91 49

7 15 44 2 39

conversn (%)

SH2 (%)

100 46 43 N/A

100 8 100 N/A

a

Reaction conditions: 45 bar of N2 or H2 pressure, 493 K for 10 h, 20 mL of 20 wt % glycerol aqueous solution, 0.5 g of Ni−Cu catalyst, 0.02 mL/min donor solution feed rate.

hydrogen for the hydrogenolysis reaction. However, the low efficiency of this process is evidenced by the low glycerol conversion (16%) and selectivity for 1,2-PDO (49%), which is likely limited by a slow hydrogen production rate. Higher glycerol conversions were observed in the presence of hydrogen donors, with FA and high-pressure hydrogen (45 bar) being superior to the alcohols for both glycerol conversion and 1,2PDO selectivity. The dehydrogenation of alcohols is likely limiting the rate of glycerol conversion, due to the high activation barrier of dehydrogenating a primary alcohol (MeOH) and the competing dehydration and hydrogenation pathways for 2-propanol to propene and propane, respectively. 1,2-PDO selectivities and yields are comparable with highpressure hydrogen and FA, indicating that CTH can be a competitive route with a suitable choice of hydrogen donor. Hydrogenolysis of the C−O bond in cyclic ethers has attracted considerable interest, owing to the prospect of producing linear alcohols, alkenes, and alkanes from biomassderived oxygenated furanic compounds: e.g., furfural and HMF.136,137 Ebitani et al. showed that FA is an effective hydrogen donor in the production of 1,6-hexanediol (HDL) from HMF over Pd/ZrP, achieving a maximum HDL yield of 43% from HMF in ethanol. This is a promising result because ring-opening chemistry is typically affiliated with high H2 partial pressures (>3 MPa).138−140 Mechanisms previously proposed for furan ring opening suggest that a protonation step is needed for appreciable selectivity in ring opening.141,142 The strongly acidic ZrP is likely a major cause of the enhanced ring-opening activity, because less acidic supports used in the work, i.e. Nb2O3 and ZSM-5, demonstrated a considerable drop in HDL yield. 3.2.2.4. Organic Hydrogen Donors in the Depolymerization and HDO of Lignin Feedstocks. Depolymerization of lignin and HDO of lignin-derived monomeric and dimeric phenolics both need to cleave C−O bonds. Although high1432


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Scheme 17. Proposed Mechanism for Sorbose Formation from Glucose via a C1−C5 Intramolecular Hydride Transfer on TiBetaa

a

Adapted with permission from ref 146. Copyright ©2015 American Chemical Society.

elegantly demonstrated that sorbose is produced through a C1− C5 intramolecular hydride transfer (Scheme 17), while a C1−C2 hydride transfer leads to the formation of fructose (Scheme 4b). More interestingly, Sn-Beta almost exclusively catalyzes the C1−C2 hydride transfer of glucose to fructose in water, while a 15% selectivity for mannose, formed via a C1−C2 intramolecular carbon shift of glucose, was observed when the reaction was carried out in methanol. Moreover, almost a factor of 4 increase in glucose conversion on Sn-Beta was observed in methanol, in comparison to that in water. Increased solvent uptake into the hydrophobic pores in methanol, which results in better stabilization of the transition state, has been proposed as a possible factor contributing to the enhanced glucose conversion in methanol.147 The choice of solvent has a substantial impact on the catalytic activity and product distribution when formic acid or formate serves as the hydrogen donor; however, the mechanism through which solvents exert their influence is far from clear. In the hydrogenolysis from α-methylbenzyl alcohol to ethylbenzene with FA in EtOH on supported Pd catalysts, the presence of 20 vol % water as a cosolvent increases both the conversion of α-methylbenzyl alcohol and selectivity for ethylbenzene.121 Enhanced ionizability of FA is speculated to be the reason for the beneficial effect of the presence of water as a cosolvent. However, the selectivity of ethylbenzene decreases when the volumetric fraction of water exceeds 20%, and the dehydrogenation pathway to acetophenone becomes more prominent. The product distribution of CTH of HMF with FA as the hydrogen donor on supported Pd catalysts could be drastically different with different solvents:131,142 the formate ester of HMF (FMF in Scheme 16) is converted quantitatively to DMF in tetrahydrofuran on Pd/C with added sulfuric acid, while ring-opening products, e.g., HDL and 2,5-hexanedione, account for almost half of the products on Pd/ZrP in EtOH. Since both reaction systems have protons (homogeneous and heterogeneous) and Pd sites, the role solvents play could be an important factor leading to the difference in product

pressure hydrogen is by far the most frequently used hydrogen donor in these processes, Song et al. showed that active hydrogen species formed from methanol, ethanol, and ethylene glycol on Ni/C was more effective than high-pressure hydrogen (5 MPa) in the depolymerization of birch sawdust.143 In addition, alcohols can stabilize the species formed upon cleaving C−O bonds. Furthermore, CTH of supercritical methanol on Cu-doped porous metal oxides has also been reported to be effective in lignin depolymerization through the organosolv process.144 More recently, CTH of vanillin, a derivative from lignin upgrade, with FA has been reported on a supported Pd/Ag bimetallic catalyst, which involves the reduction of both CO and C−O bonds.145 However, applications of CTH in lignin upgrading are still relatively rare.

4. SOLVENT EFFECTS In homogeneous CTH processes, solvent molecules often coordinate with the metal center of the soluble catalyst, thus potentially restricting the access of the reactants to the metal center and altering the electronic structure. Similar siteblocking effects are expected for heterogeneous CTH reactions when solvent molecules interact more strongly with the catalyst surface than the reactant molecules do. However, the effects of the solvent and the hydrogen donor are frequently intertwined when an alcohol is used as the hydrogen donor, because it typically also serves as the solvent. In this regard, Lewis acid catalyzed glucose isomerization via intramolecular hydride transfer provides the rare opportunity of independently varying the solvent, since the hydrogen donor and acceptor are two functional groups in the substrate. Davis and co-workers reported that fructose was the favored product when the isomerization of glucose on Ti-Beta was carried out in water, while sorbose became the major product when the reaction was conducted in methanol under otherwise identical conditions.146 In addition, a 15−125% increase in the turnover number was observed in methanol in comparison to water as the solvent. A combined isotopic labeling, 13C NMR, and kinetic study 1433


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conditions is highly desirable, because the signals from bulk solvents often overwhelm the spectral features from the interface. Furthermore, a detailed knowledge of the similarity and differences in mechanisms of CTH and hydrogen gas based HDO reactions will aid the choice of a suitable approach for target reactions. 5.4. Mechanisms through Which Solvents Interact with Catalytic Sites and Affect Reaction Rates. Properties of active sites at the solid−liquid interface could be drastically different from those measured with ex situ vapor-phase techniques: e.g., CO2/NH3-TPD and CO chemisorption. In situ characterization tools and computational methods are needed to accurately probe how the presence of solvent affects the interaction between reactants and active sites. The influence of the coordination of solvent molecules on the active sites should be correlated with rates and product distributions to identify predictive descriptors for the solvent choices. 5.5. Upgrading Lignin-Derived Feedstocks. While CTH has been increasingly regarded as an alternative in the upgrade of (hemi)cellulose-based biomass to molecular-hydrogen-based routes, its applications in the HDO of lignin-derived feedstocks remain relatively unexplored. CTH has the potential to selectively activate C−O bonds in the depolymerization of lignin and HDO of phenolic monomers and dimers.

distributions. However, more rigorous comparative studies are needed to elucidate the solvent effect.

5. PERSPECTIVES The wide range of reactions and intricate mechanistic pathways discussed in sections 2−4 have demonstrated the breadth and depth of CTH as an effective approach to upgrade biomassderived feedstocks. The employment of organic hydrogen donors, e.g., secondary alcohols and FA, introduces alternative mechanistic pathways other than those based on hydrogen gas, which adds extra dimensions in manipulating the product distribution. Moreover, the prospect of using renewable organic hydrogen donors could further reduce the carbon footprint of fuel and chemicals produced from biomass by removing the dependence on natural-gas-derived hydrogen. In order for CTH to make a real impact in biomass conversion, it is critical that CTH-based processes be either competitive with or complementary to the conventional high-pressure hydrogen gas based HDO processes, which will stay dominant in the foreseeable future. The versatility of hydrogen donors, solvents, and mechanistic pathways of CTH afford the possibility of developing highly efficient and selective processes; meanwhile, a better understanding of the following five aspects through continued research efforts is needed to fully realize CTH’s potential. 5.1. Key Descriptors for Metal Sites’ Ability to Activate Organic Hydrogen Donors in the Absence of Coexisting Acid or Base Sites. Activation of organic hydrogen donors on metallic sites typically involves adsorption, C−H/O−H bond cleavage, and metal−H bond formation , and metal−H bond cleavage on reaction with the substrate. The ability of metal to facilitate each of these steps will affect the overall efficiency of CTH reactions. Thus, descriptors such as adsorption energy of common hydrogen donors, e.g., alcohols and FA, hydrogen binding energy, and oxophilicity have the potential to predict metal effectiveness in CTH. 5.2. Synergy between Metal and Acid−Base Sites. The roles of metal and acid−base sites in CTH need to be elucidated. A detailed knowledge of the nature (metal, Brønsted or Lewis acid, or base sites), strength, and density of sites is essential to establish structure−activity relationships. Most heterogeneous CTH reactions are carried out in the liquid phase, and the properties of catalytic sites in the presence of solvents would be drastically different from those in the absence of solvents. Therefore, characterization capable of taking solvent effects on catalytic sites into account will be highly desirable. In addition, the systematic employment of catalysts with tailored site compositions and well-defined structures, e.g., size-controlled metal nanoparticles supported on Lewis/ Brønsted acidic supports, in diagnostic model reactions will be helpful in elucidating the synergistic effects among different types of sites. 5.3. Molecular Level Reaction Mechanism. Mechanisms of CTH reactions are typically the fusion of two catalytic cycles, i.e. the activation of the organic hydrogen donor and the reactant, which can be either sequential or concerted steps. Combined kinetic, in situ spectroscopic, isotopic labeling, and computational investigations are needed to identify the active hydrogen species involved in the hydrogen transfer step and achieve a comprehensive understanding of the reaction pathway on the molecular level. In particular, the development of novel interfacial sensitive spectroscopic tools capable of identifying reaction intermediates at solid/liquid interfaces under reaction

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for B.X.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001004.

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