Selectivity Control for Cellulose to Diols: Dancing on Eggs

Feb 1, 2017 - Selectivity Control for Cellulose to Diols: Dancing on Eggs. Mingyuan Zheng, Jifeng Pang, Ruiyan Sun, Aiqin Wang, and Tao Zhang*. State ...
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Selectivity Control for Cellulose to Diols: Dancing on Eggs Mingyuan Zheng, Jifeng Pang, Ruiyan Sun, Aiqin Wang, and Tao Zhang* State Key Laboratory of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ABSTRACT: Catalytic conversion of cellulose to ethylene glycol (EG) or 1,2propylene glycol (1,2-PG) represents an attractive approach in the valorization of biomass, due to the high atom economy of the reaction process and large market demand of the diol products. The one-pot catalytic conversion of cellulose is a complex reaction network, comprising hydrolysis, retro-aldol condensation, hydrogenation, isomerization, dehydrogenation, thermal side reactions, etc. In addition to EG and 1,2-PG, a variety of byproducts such as sorbitol, mannitol, erythritol, 1,2butanediol, and glycerol may be coproduced. The key point for obtaining high selectivity for EG or 1,2-PG lies in effective control of the major reaction steps in the reaction network proceeding at matching rates. In this Perspective, we depict the general reaction route for glycol production from cellulose and summarize the active elements for the retro-aldol condensation reaction, which is the determinant step for the formation of C2 and C3 intermediates. In situ or operando methods for the catalyst characterization are discussed. Then the reaction kinetics for one representative example, i.e. the tungstenic catalyst, is summarized briefly and approaches to control the product selectivity are suggested. After an overview of the progress and challenges in catalytic conversion of lignocellulose for applications, we present an outlook for cellulose conversion to diols from the aspects of catalyst development, reaction mechanism study, and practical applications. KEYWORDS: biomass, cellulose, glucose, retro-aldol condensation, isomerization, ethylene glycol, 1,2-propylene glycol, hydrogenation

1. INTRODUCTION Ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) are high-value chemicals with a very large market demand. More than 23 million tons of EG and 1.8 million tons of 1,2-PG were produced in 2015, widely used for the synthesis of polyesters, antifreeze, and fine chemicals. The market capacity is expected to further grow annually at a rate of 5% in the following 10−20 years with the growing global demands for clothes, packaged items, and unsaturated resin materials.1,2 Currently, EG and 1,2-PG are produced mainly from petro-ethylene and petropropylene via epoxy compounds. Using renewable biomass to synthesize clean fuels and bulk chemicals such as EG and 1,2PG will lessen the dependence on fossil resources and contribute to the sustainable development of the world economy, and this accordingly has attracted significant attention worldwide in the past decade.3−8 Moreover, since C, H, and O elements from the feedstock are largely reserved in the polyol products, the process possesses high atom economy, which is keenly pursued in green chemistry.9 The beginning of diol synthesis from biomass, specifically from cellulose and sugars, can be dated back to 1933 or even earlier.10 For many decades, the yields or selectivity of the target diols lingered at low levels, normally 99 23.8 100 100 100 100 92.6 >97 94.8 100 98.8

SEG/%

S1,2‑PG/%

Slac/% 64

22.0 57.6 22.9

13.5 9.2 32.2

46.6 32.6 21.7 31.1

17.1 16.8 19.2 12.1

64.8b

1.9

19.1

34.4

60

89.6 54

100 99 98

40 44

ref 84 42 88 88 56 36 36 53 36 89 28 90 91 84

a

conversn, SEG, S1,2‑PG, and Slac represent cellulose conversion and selectivity of EG, 1,2-PG, and lactic acid or ester, respectively. bThe selectivity includes EG and ethylene glycol monoether.

methanol it is more active for an epimerization reaction to form mannose.85,86 The bulky SnO2 on the external surface of the zeolite crystal is inert for glucose isomerization in aqueous solution but active in methanol.86 With the aid of Naexchanged silanol groups, Sn4+ centers in Sn-Beta catalyst provide Lewis acid sites for activating the carbonyl group at C-1 and hydroxyl group at C-2 of glucose and realize glucose epimerization to mannose in methanol following a 1,2intramolecular carbon shift mechanism: i.e., the Bilik reaction.77 In contrast, silanol groups adjacent to the Sn sites impose cooperative effects on glucose isomerization to fructose.76 Taarning and co-workers found that, over the Lewis acidic SnBeta zeolite, sugars are selectively transformed to methyl lactate in yields of 43−64%. An RAC reaction mechanism was proposed for the sugar degradation to C3 products.84 Deng et al. studied the performance of Pt-SnOx/Al2O3 catalyst in cellulose conversion.42 They found that, as the Sn/ Pt ratios exceeded 1.5, the Pt-SnOx/Al2O3 catalysts gave C2 and especially C3 products (e.g., acetol) as dominant products but not hexitols. Glucose isomerization to fructose and a sugar RAC reaction were supposed to happen on the segregated SnOx species, which were deemed to be in the form of Sn(OH)2. In a subsequent study, 53.9% selectivity to acetol was obtained in cellulose conversion over SnOx-modified Ni/Al2O3 catalysts.87 SnOx supported on Al2O3 was found to possess more strongly basic sites than other oxides, such as ZnOx, CeOx, and AlOx on Al2O3. This accounts for the higher activity in isomerization and RAC reactions. More recently, Sun et al. designed a Sn-based selectivityswitchable catalyst for cellulose conversion to EG and 1,2-PG.88 A superior activity toward EG (57.6% yield) with up to 86.6% total polyol yield was obtained over composite catalysts of Ni/ AC and metallic Sn powders, while a 22.9% yield of EG and 32.2% yield of 1,2-PG were produced in the case of Ni/AC and SnO catalysts. The valence state of Sn had a crucial effect on reaction selectivity. The active site for the high selectivity of EG was ascribed to the Sn species in NiSn alloy, which was formed in situ from metallic Ni and Sn powders. Sn in the NiSn alloy showed a slightly positive valence, as evidenced by Mössbauer characterization, which might account for the activity for catalyzing the RAC reaction of glucose to GA. Differently, SnO

mechanism was proposed to interpret the product formation and product distribution. In contrast, for phosphotungstic acid, a tungstenic catalyst, merely a 6.2% yield of glycolic acid was afforded under the same conditions. These results suggest that Mo catalysts are able to catalyze the RAC reaction under suitable conditions. Moreover, they are possibly more active than the W catalyst for the RAC reaction. Therefore, an intriguing question is why is molybdenum incapable of giving a high yield of EG in cellulose conversion although it has high activity for the RAC reaction? The reason might be related to the reaction atmosphere and the valence of molybdenum. It is reported that molybdenum catalysts showed valence-sensitive catalytic performance in the Bilik reaction. When Mo stays at VI valence, a high activity can be obtained.78,80 For the cellulose conversion which is performed in a hydrogen-rich environment at high temperatures, the valence of Mo will be reduced to a lower one which might lead to the disappearance of RAC activity. Cu/CuCrOx was reported to be effective for cellulose conversion (see Table 1), which gave 1,2-PG and EG in yields of 36.3% and 7.6%, respectively.24 The metallic Cu in the catalyst took charge of hydrogenation, but the role of Cr oxide in the catalytic reaction was not identified. Considering the product distribution and the fact that Cr belongs to the same group of the element W, it can be conjectured that CrOx is active for sugar RAC reactions. In addition, consistent with the results in the literature,55,81,82 Cr species could catalyze glucose isomerization to fructose, which could account for 1,2-PG formation as the dominant product over EG in the cellulose conversion over Cu/CuCrOx catalysts. 3.2.2. Tin Group Catalysts. Tin group catalysts include Snand Pb-based catalysts. Tin is a versatile active component for catalytic conversion of sugars. It can catalyze glucose isomerization to fructose, glucose epimerization to mannose, sugar RAC reactions to GA and trioses, and triose conversion to lactates.76,83−86 The catalytic performance of tin is highly dependent on the valence, assembling state, microenvironment around tin sites, and reaction medium. Davis and co-workers found that Sn-Beta catalyst with Sn4+ incorporated in the zeolite frameworks is highly active for glucose isomerization to fructose in water at 383−413 K, but in 1944

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Figure 3. Theoretical calculations for the La(III)−OH-catalyzed conversion of glucose to glycolaldehyde. Calculated Gibbs energy profiles (in kcal/ mol) and optimized structures of the species involved in the catalytic process, including intermediates and transition states, are given.36 Reproduced with the permission from ref 36. Copyright 2015, American Chemical Society.

the cellulose conversion over lanthanum(III) catalysts follows a dual-route reaction mechanism, as evidenced by density functional theory (DFT) calculations and experimental analyses. In the major route, glucose is selectively cracked into C2 molecules by lanthanum(III), which is similar to that over a tungstenic catalyst or NiSn alloy catalysts.15,88 In the minor route, sugars derived from cellulose hydrolysis are first hydrogenated into hexitols and then degraded into EG and 1,2PG through hydrogenolysis by cooperative catalysis of basic sites provided by lanthanum(III) oxide and hydrogenation sites of metallic nickel. This means that, during the reaction, both sugars and sugar alcohols can be transformed into C2−C3 glycols by the nickel−lanthanum(III) catalyst, which was not observed over tungstenic- and tin-based catalysts. Moreover, the nickel−lanthanum(III) catalyst showed a remarkably high level of activity toward the degradation of cellulose. In one instance, the TON of cellulose conversion to EG reached 339 at a very low concentration (0.2 mmol/L) of lanthanum(III), the amount of which is merely one-sixth of that of tungstenic catalyst required to give a similarly high EG yield.37 By aid of computational calculations, the reaction mechanism of the core reaction step, i.e. RAC reaction of glucose by catalysis of lanthanum(III), was depicted at a molecular level (Figure 3). During the process of C2−C3 bond cleavage in a glucose molecule, the epimerization reaction first takes place as a consequence of an La(III)−OH interaction with the oxygen atoms in CO, OH, and OH groups on C1, C2, and C3, respectively. Then, a 2,3-hydride shift reaction occurs, leading to the breaking of the C2−C3 bond in glucose to form equimolar GA and erythrose. The formed erythrose is further cracked into 2 mol of GA by the catalysis of La(III)−OH without epimerization. The role of La(III) in the catalytic cleavage of the C2−C3 bond was supported by an analysis of the reaction solution by tandem mass spectrometry. The epimerization reaction predicted by the calculation was also confirmed by 13C nuclear magnetic resonance (NMR) experiments. In addition, comparative calculations showed that the Ea value for glucose RAC reaction is lower than that for fructose, which could account for the higher yield of EG in comparison to 1,2-PG in the reaction. The approach adopted in

was active for both the RAC reaction and isomerization of glucose to fructose, which is consistent with the result in other studies.87 Metallic Sn granules and SnO2 were inactive for EG formation. In all, Sn catalysts have shown versatile functions for sugar conversion. By using Sn(II) or alloyed Sn species as catalysts, the product distribution between EG and 1,2-PG can be tuned to some extent. Recently, Wang et al. reported the catalytic conversion of cellulose into lactic acid in the presence of dilute lead(II) ions.56 The lactic acid yield reached >60% from microcrystalline cellulose and several lignocellulosic raw biomasses at 463 K. Theoretical and experimental studies suggested that lead(II) ions in combination with water catalyzed cascading steps for lactic acid formation, including the isomerization of glucose into fructose, RAC of fructose to trioses. and the selective conversion of trioses into lactic acid. Therefore, it can be expected that when it is combined with a hydrogenation catalyst, the lead(II) catalyst could also be effective for cellulose degradation to EG and 1,2-PG. However, preventing lead(II) ions from reducing into inactive metallic lead under the high pressure of H2 would be the major challenge for this catalyst design. 3.2.3. Lanthanide Catalysts. The group of lanthanide catalysts comprises La-, Ce-, and Y-based catalysts, which have been proven to be effective for cellulose conversion to EG and 1,2-PG (Table 2).36,53 Er(OTf)3 was found effective for lactic acid production from cellulose, demonstrating a high selectivity for C3−C4 bond cleavage in C6 sugars and the possibility for 1,2-PG production over Er-based catalysts.89 Girard et al. studied cellulose conversion in the presence of soluble Ce salts and solid Pt/BaZrO3 catalyst and obtained ca. 40% EG and 1,2-PG selectivity. They supposed that homogeneous Ce ions catalyzed the sugar RAC reaction, similarly to La cations reported by Sun et al.36,53 Sun et al. conducted an in-depth study on nickel−lanthanum(III) catalysts by combining metallic nickel and a variety of lanthanum compounds.36 Over an optimal catalyst, 10% Ni− 0.5% Ir/La2O3, the overall yield of EG and 1,2-PG reached 63.7%. In comparison to tungsten- and tin-based catalysts, the lanthanide catalyst shows several attractive features. First of all, 1945

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ray absorption spectroscopy (XAS).96−99 The examples shown below would give general inspiration for investigating biomass conversion to diols through in situ or operando ways. As mentioned previously, two types of catalysts are involved in the biomass conversion to diols, taking charge of RAC and hydrogenation reactions, respectively. XAS analysis is a valid method for characterizing heterogeneous hydrogenation catalysts. Chia et al. studied the structure of the bimetallic hydrogenation catalyst RhRe/C using XAS under in situ (i.e., freshly reduced) and aqueous operando conditions for 2(hydroxymethyl)tetrahydropyran hydrogenolysis.96 From in situ X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra, they confirmed that Re and Rh species are reduced to metallic nanoparticles with a Rh-rich core and a Re-rich shell structure. The operando XAS analysis further demonstrated that the Re does not form Re−OH to a measurable extent in the presence of liquid water and flowing H2 at 393 K. They conjectured that the superficial Re atoms on RhRe/C activate water molecules to generate acidity through interaction with the O atom in water and further catalyze the C−O bond cleavage. Ketchie et al. monitored the metal particle structure and size evolution during the reaction at 473 K under aqueous phase conditions by in situ XAS.97 Different from Al2O3- and SiO2-supported Ru catalysts, the Ru particles on carbon and titania supports were found to be stable due to the stability of the supports. O’Neill et al. probed the deactivation mechanism of Cu/γ-Al2O3 as a hydrogenation catalyst.98 The copper nanoparticles on the catalyst sintered throughout the time on stream reaction in the liquid phase and gradually lost catalytic activity. Lobo et al. conducted an in-depth study on the surface properties of supported Pt catalysts for 1-propanol re-forming in the presence of liquid water at high temperature (503−533 K) and pressure (69 bar).99 The coverage of reactant species on the catalyst surface was derived from the linear combination of operando XANES spectra. The Pt surface was deemed to be covered with high concentrations of water (96% coverage of water at 503 K and 30 bar) and CO (43% coverage of CO at room temperature) under the reaction conditions. In addition to in situ probing of heterogeneous catalysts, XAS analysis is also capable of investigating transition-metal cations complexing with biomass.100−102 Gardea-Torresday et al. characterized Cr(VI) binding and reduction to Cr(III) by the agricultural byproducts of Avena using XAS analysis.101 More studies on the binding of copper(II), zinc(II), chromium(III), and chromium(VI) to hops biomass were conducted by Parsons et al.102 From a comparative study between the EXAFS spectra of biomass binding cations and the various transitionmetal salts, the molecular geometries of cations binding to hops were proposed. In situ ATR-FTIR analysis is a sensitive and time-resolved method to measure the reactant and product signals in aqueous solutions.94,103 When it is combined with XAS analysis, more exact information can be obtained. Vieira et al. studied the mechanism of chromium removal by chitosan-based sorbents using ATR-FTIR and XAS analysis.103 They found that amino groups were responsible for the adsorption and about 70% of adsorbed chromate ions remained in the Cr(VI) oxidation state when the pristine chitosan was used as a sorbent. Differently, in the case of glutaraldehyde-cross-linked membranes, carbonyl groups and imino bonds were involved in the adsorption mechanism, and ca. 44% of chromate anions were reduced to the Cr(III) oxidation state by free aldehyde groups in the

this study could serve as a model for the study of reaction mechanisms over other catalyst systems (Mo, W, Pb, Sn, etc.) which are effective for RAC reactions or epimerization reactions of sugars. 3.2.4. Other Transition-Metal Catalysts. NbPOx-supported metal catalysts were used for breaking C−C bonds of sugars via a RAC reaction in cellulose conversion in methanol solutions.28 EG and ethylene glycol monoether (EGME) were obtained at a combined yield of 64.1% over a Ru-Ni/NbOPO4 catalyst after a 20 h reaction at 493 K under 3 MPa of H2. Methanol played an important role in protecting the CO bond in the glucose intermediate from hydrogenation through acetalization, thus leading to the production of EG and EGME. Otherwise, the main product was isosorbide, given that the reaction was conducted in aqueous solutions. The use of methanol can remarkably decrease the energy consumption for EG separation by distillation in comparison to that using water as the solvent. On the other hand, the reaction efficiency in the methanol solution is rather low (20 h for 3 wt % ball-milled cellulose conversion) and hence needs further improvement for practical application. Recently, Tang et al. found that vanadyl (VO2+) cations can catalyze ball-milled cellulose degradation to form lactic acid under anaerobic conditions (54% yield, 453 K for 2 h). The reaction involves isomerization of glucose to fructose, RAC of fructose to form trioses, and the isomerization of trioses. In contrast, under aerobic conditions, formic acid was the dominant product which resulted from oxidative cleavage of C−C bonds in the intermediates by the redox conversion of VO2+/VO2+.90 Evidently, vanadium element could be active for the RAC reaction, and the catalytic performance has some relationship with the valence of vanadium. In another work, a first-principles investigation showed that vanadium would show higher activity than Mo and W elements in the 1,2-carbon shift reaction of the sugar epimerization.72 Again, the active component for catalytic epimerization shows a high activity for the RAC reaction. Amphoteric ZnO supported Ni catalysts were used for cellulose conversion to give 1,2-PG (34.4% yield) and EG (19.1% yield).91 The high selectivity to 1,2-PG was attributed to the basic properties of the ZnO support. Additionally, we wish to ascribe elements such as Ti and Zr as potential catalytic components for cellulose conversion to EG and 1,2-PG. According to the studies by Holm et al., Ti-Beta and Zr-Beta catalysts were able to transform glucose, fructose, and sucrose into methyl lactate in 30−40% yields in methanol, similar to the behavior of Sn-Beta catalyst.84 3.2.5. In Situ and/or Operando Catalyst Characterizations. Cellulose conversion to diols is typically conducted at temperatures over 453 K because of the high activation energies for cellulose hydrolysis (130−180 kJ/mol)92 and sugar RAC reactions (∼140 kJ/mol).44,93 The pressure from solvent vapor and hydrogen in the reactor is over 5 MPa during the reaction, which sets a pressure gap for in situ or operando characterization of the interaction between sugar reactants and catalytically active species. Another challenge for the operando characterization comes from the liquid state of the reaction environment, in which the characteristic signals of reactant molecules are significantly attenuated or overlapped by those of the solvent. A few in situ approaches have been used to study catalysts under real reaction conditions, including attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR),94 in situ far-infrared (FIR) spectra,95 and operando X1946

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Figure 4. Time-resolved in situ ATR-IR spectra during the hydrogenolysis of 5 wt % glycerol in water over 9Pt8WAl at 473 K and 45 bar of H2 after water spectrum subtraction in the complete investigated region (A) and in the ν(CH) (3000−2700 cm−1) and ν(CO) (1150−950 cm−1) regions (B).104 Reproduced with permission from ref 104. Copyright 2017, Elsevier.

A great trouble frequently encountered in spectroscopic characterizations is the disturbance from unreactive species interacting with the catalytic sites: namely, the spectator for the reaction. Concentration-modulation excitation spectroscopy (cMES) is effective for selective detection of key reaction intermediates under reaction conditions.105−107 The real active species and spectators during the reaction can be differentiated by using c-MES for any spectroscopic method such as timeresolved XANES, EXAFS, and FTIR, if only some components of the perturbed catalytic system respond to the concentration modulation. Evidently, this method is suitable to be used for in situ study of the reaction mechanism of cellulose and sugar conversions, in which many side reactions and byproducts are readily present. As shown above, there have been a number of techniques developed for in situ characterization under high-pressure and hydrothermal conditions. On the other hand, the sensitivity and accuracy of these methods are generally lower than those for the reactions proceeding at ambient pressures and gas−solid interphase. In addition to the exploration of more smart in situ or operando characterization methods, using theoretical calculations is a feasible way to circumvent the disturbance of reaction environments and probe the reaction mechanism at a molecular level. Both homogeneous and heterogeneous catalytic systems for biomass conversions have been investigated by DFT calculations, including the glucose RAC reaction by La(III),36 glucose activation by Cr2+ dimers,80 etheric C−O breakage by La(OTf)3,100 and secondary C−O bond hydrogenolysis over Rh−Re catalysts.101 The pivotal precondition for the calculations is building a sound reaction model on the basis of a deep understanding of the reaction mechanism and catalyst structure from the experimental results. Very recently, Li et al. investigated the reaction mechanism of glucose isomerization over WO3·H2O catalyst using DFT calculations.108 They concluded that glucose isomerized to fructose at undercoordinated W6+ sites, via a H shift from C2 to C1 involving a cooperative action of Lewis acidic tungsten sites with neighboring proton donors.108 On the other hand, in some literature reports, glucose was readily transformed to mannose but not to fructose over tungstenic catalyst.71,109−111 The discrepancy between the reaction results in these studies might be related to the different reaction conditions or subtle differences in the catalyst preparation. For instance,

sorbent. Weckhuysen’s group recently studied cellulose hydrolysis in the presence of a solid acid catalyst under elevated temperatures and autogenous pressures using in situ ATR-IR spectroscopy.94 The differences in vibrational characteristics of the most relevant species, including glucose, glucose oligomers, fructose, hydroxymethylfurfural, and several acids, are distinguishable even in the presence of water under elevated temperatures and pressures. Correlating to the analysis of highperformance liquid chromatography, they found that the cellulose hydrolysis proceeded first through the disruption of the glycosidic linkages of cellulose to form smaller cellulose molecules, which were further degraded to monomeric glucose after glycosidic linkage breaking. This work may give inspiration for studying the reaction process of cellulose conversion to EG over tungstenic catalysts. As mentioned above, in addition to glucose conversion, the direct degradation of cellulose and oligosaccharides from the reducing-end sugar via an RAC reaction is expected to contribute to EG in the cellulose conversion to EG.29 An ATR-IR monitoring analysis during the reaction might provide more direct evidence to identify which is the dominant reaction intermediate for cellulose conversion to EG. A very recent study further demonstrated the feasibility of using in situ ATR-IR for a reaction under hydrothermal and high-pressure conditions.104 Glycerol hydrogenolysis over Pt/ WOx/Al2O3 catalyst was characterized at 473 K and 45 bar of H2, which are very close to the conditions used for cellulose conversion to EG. As shown in Figure 4, many characteristic IR peaks assigned to the reactant and products were observed even in the presence of water solvent. By the aid of evidence obtained from ATR-IR and conditional experiments, a triple role of tungsten oxide in the reaction was disclosed. In addition to infrared spectroscopy, in situ FIR spectroscopy was used for characterizing transition-metal chloride coordination with sugars in ionic liquids. Liu et al. studied glucose conversion in the presence of CrCl3, VCl3, FeCl3, and PtCl2 in 1-butyl-3-methylimidazolium chloride.95 According to the spectra (wavenumbers 250−550 cm −1 ) of metal ions interacting with glucose, products, and various model compounds, the relative bond strengths and the numbers of ligands were proposed, and the distinguished performance of CrCl3 for glucose transformation to hydroxymethylfufural was interpreted. 1947

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The Ea values for GA and glucose hydrogenation were found to be very similar (42.6 kJ/mol vs 49.6 kJ/mol) over the Ru/AC catalyst. However, due to the discrepancy in the preexponential factor, the rate of GA hydrogenation is ca. 4 times faster than that of the glucose hydrogenation. An experimental study identified that the presence of GA significantly inhibits glucose hydrogenation, which only starts to occur when GA has been almost consumed in the reaction. Very recently, the overall reaction kinetics of glucose conversion to EG over W−Ru catalysts in a (semi)continuous reaction system was identified.113 On the basis of the simplification of reaction network and kinetic data measured for six major reactions, yields of EG, hexitols, and gas were described as functions of the reaction temperature, the concentration of glucose, and the feeding rate. The simulated results matched the experimental data over a range of reaction conditions, verifying the model and method for the study. This work could provide valuable reference for industrial production of bio-EG from sugars, with the reaction selectivity economically controlled in a time and feedstock-efficient way. The remaining task for the kinetic study on cellulose conversion to EG is to identify the kinetics of cellulose hydrolysis in the presence of tungstenic catalysts and integrate it with that of glucose conversion into a complete expression. In addition, to depict the kinetics of 1,2-PG formation, the isomerization of glucose to fructose, the fructose RAC reaction, and C3 intermediate dehydration should also be considered, which remarkably increase the difficulty in obtaining constructive results for cellulose conversion. From the kinetic study of pivotal reaction steps in cellulose conversion to EG, one may find the essential reasons the cellulose conversion has to be conducted at high temperatures over 373 K. On one hand, this should be attributed to the high Ea value (120−180 kJ/mol) for hydrolysis of the β-1,4glycosidic bond in cellulose.114 The crystalline structure and hydrogen bonds extensively existing in the glucan network further increase the difficulty in cellulose depolymerization. On the other hand, the RAC reaction of glucose to form glycol aldehyde also needs a high Ea value (e.g., 140 kJ/mol over tungstenic catalysts) and only proceeds at temperatures over 453 K at notable rates. For the hydrogenation of sugar and reaction intermediates, because the Ea value is much lower (50 kJ/mol),44,112 this reaction step does not need harsh reaction conditions. Therefore, to realize cellulose conversion at a low temperature such as 373 K, it is necessary to decrease the activation energies for these two critical reactions. Shuai et al. developed a cellulase-mimetic solid catalyst, i.e., sulfonated chloromethyl polystyrene resin (CP-SO3H), for

WOx(5%)/Al2O3 had 67.9% selectivity for glucose isomerization to fructose.68 However, over 50% WO3/Al2O3 + Ru/C catalysts, EG was obtained with 45% selectivity in cellulose conversion, indicating that sugar was directly degraded without isomerization to fructose.30 These results demonstrate that one must be very careful to establish a catalytic model prior to theoretical calculations. The combination of experimental characterizations and theoretical calculations will give more convincing explanations for understanding reaction mechanisms.

4. REACTION KINETICS AND ITS APPLICATION Extensive studies have been performed in developing novel catalysts and understanding the reaction mechanism for cellulose conversion to EG and 1,2-PG. However, a few of them focused on the essential reaction kinetics because of the harsh reaction conditions and the complexity of the reaction network catalyzed by multifunctional active components in the catalysts. Zhang et al. studied the kinetics of RAC reaction of glucose to GA and hydrogenation of GA to EG in the presence of ammonium metatungstate (AMT) and 1% Ru/C catalysts, respectively.93,112 As shown in Scheme 2, the RAC reaction is a pseudo-first-order reaction with respect to glucose, with an apparent activation energy in the range of 141.3−148.8 kJ/ mol.44,93 In contrast, the Ea value of sugar hydrogenation is much lower: ca. 50 kJ/mol.44,112 Therefore, in comparison to the sugar hydrogenation, RAC is much more sensitive to the reaction temperature, and a high temperature favors GA and EG formation. The pre-exponential factor in the Arrhenius equation for the RAC reaction is as high as 5.39 × 1013, consistent with the features of a homogeneous catalytic reaction. GA readily undergoes side reactions with a low Ea value of 52.7 kJ/mol and reaction orders of 2−2.5.44,93 Accordingly, keeping the GA concentration at a low level during the reaction is necessary to obtain high selectivity for EG. Zhao et al. conducted catalytic conversion of highconcentration glucose to EG using AMT and Ru/C catalysts.44 By employment of a semicontinuous reaction system, the GA concentration was kept at a low level and EG was obtained in a high yield of 60%. The reaction selectivity closely depended on the reaction temperature and the feeding rate of the feedstock, which was well interpreted by the results of a reaction kinetics study.44 Similarly, Ooms et al. realized a high yield of EG from glucose over a nickel−tungsten carbide catalyst in a semicontinuous reactor.64 For the kinetics of hydrogenation reactions, a Langmuir− Hinshelwood−Hougen−Watson (LHHW) model was used.112 1948

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The sensitivity of EG selectivity to the feedstock concentration was ascribed to the pseudo-second-order GA condensation vs the pseudo-first-order hydrogenation.44,93,112 Since the different reaction steps in the reaction network have large discrepancies in Ea values, tuning the reaction temperature could change the relative rates of different reactions and affect the reaction selectivity. However, because cellulose hydrolysis usually needs a high reaction temperature (>463 K),117 it is not easy to observe the effects of temperature on the reaction selectivity in the cellulose conversion to EG. Significant effects of temperature on the reaction selectivity have been observed on glucose conversion. For instance, the EG yield was merely 14.6% at 453 K and increased to 60.0% at 518 K, whereas the corresponding hexitol yield decreased from 62.4% to 6.8%.44 In addition to the reaction temperature, the reaction solvent and atmosphere also play roles in product selectivity control. It was found that sugars have a tendency to polymerize in aqueous solution, whereas the side reactions can be suppressed in methanol solution to get a higher yield of target products.118 A similar effect was also observed in cellulose conversion to EG. As shown in Wang’s work, in methanol solutions, EG and ethylene glycol monoether were obtained at combined yields of 64.1% over a Ru-Ni/NbOPO4 catalyst.28 However, in aqueous solutions, the main product was changed to isosorbide over the same catalyst. The acidities and coordination environments of transition-metal sites in methanol and aqueous solutions are possibly different, which results in a large change in reaction selectivity. Molybdenum catalysts were reported to be effective for glycolic acid synthesis from cellulose under an oxygen atmosphere.70 A posthydrogenation of glycolic acid could afford EG product. Undoubtedly, more hydrogen would be consumed in comparison with that for the direct conversion of cellulose in hydrogenation. For cellulose conversion to 1,2-PG, the key point in obtaining high selectivity lies in effectively isomerizing glucose to fructose before glucose undergoes the RAC reaction. Among various transition-metal elements, Sn(II) is rather attractive due to its dual activities for isomerization and RAC reactions. A selectivity of acetol up to 53.9% (the precursor of 1,2-PG) was obtained over a Ni-SnOx/Al2O3 catalyst.87 Keeping tin species at the +2 valence state in an H2-rich and hydrothermal environment would be an important precondition for an effective catalyst. Sun et al. found that tin(II) species are active for sugar isomerization and RAC reactions. Differently, the tin in Ni-Sn alloy is merely active for the RAC reaction but inactive for sugar isomerization reactions. By tuning the valence state of tin, the reaction selectivity can be tuned between EG and 1,2PG to some extent.88 Taking advantage of interactions between active sites and the support is an alternative way to tune the reaction selectivity from EG to 1,2-PG. As mentioned above, when tungsten oxide was highly dispersed on Al2O3 in the forms of WO4 and oligomeric WOx, Lewis acidic sites were generated on PdWOx(5)/Al2O3 catalyst, which catalyzed glucose isomerization and gave a high selectivity of 1,2-PG of up to 60.8% in glucose conversion.68 In contrast, 45% selectivity of EG vs 10% selectivity of 1,2-PG were obtained over 50 wt % WO3/Al2O3 + Ru/C catalysts, where tungsten oxide was loaded on alumina with a low dispersion.30 Zeolites and MOF materials can provide high surface areas and multifunctional sites for reactions when they are used as supports for catalysts. However, there have been nearly no

microcrystalline cellulose hydrolysis and realized 93% glucose yield after 10 h at 393 K.115 The Ea value was decreased by half to 83 kJ mol−1, in comparison to that when sulfuric acid was used (170 kJ mol−1). The low Ea value of CP-SO3H was attributed to the ability to adsorb/attract cellulose and to disrupt the hydrogen bonds of cellulose. There is a synergistic effect between the cellulose-binding sites (−Cl) and catalytic sites (−SO3H). Similar synergy was also observed between phenol or carboxylic acid groups and −SO3H on the sulfonated carbon catalyst, over which the Ea value was 110 kJ mol−1.116 These studies provide clues for further improving the catalytic activity for cellulose hydrolysis by decreasing the Ea value, as for cellulose with an Ea value as low as 3−50 kJ mol−1. For the RAC reaction, there has been no report that its Ea value can be decreased remarkably by employing synergistic effects of multifunctional groups on a catalyst. However, similar to the study on cellulase simulation for cellulose hydrolysis, more valid catalyst systems might be explored for RAC reactions by mimicking aldolase, which degrades sugars to dihydroxyacetone and glyceraldehyde in organisms at ambient temperature. Therefore, integration of chemo-catalysis and biocatalysis would create more unexpected progress in cellulose conversion to diols or other valuable chemicals under milder conditions.

5. SELECTIVITY MODULATION On the basis of a deep understanding of the catalytic functions and the relationship of major reaction steps in the reaction network, the reaction selectivity of cellulose conversion to EG and 1,2-PG can be modulated by several approaches: (1) using suitable active components for catalysts, (2) tuning the ratio of different active components or changing the catalyst amount, (3) tuning the reaction temperature, (4) using a suitable reaction solvent or atmosphere, (5) taking advantage of the interaction between active sites and the catalyst support. In order to obtain a high yield of EG but not 1,2-PG, the isomerization reaction of glucose to fructose should be avoided. Those catalytic components providing active sites for sugar isomerization, such as tin, lead, zinc, and chromium species as well as bases should not be introduced to the reaction system. Moreover, since the sugar alcohol products are inert for RAC reactions over the transition-metal species, the rates of hydrogenation and RAC reactions of sugars should be kept at a suitable balance. As mentioned in section 3.2.1, once the rate of hydrogenation significantly overwhelms that of the RAC reaction, hexitols would be the dominant product at the expense of EG yield. On the other side, an overly fast RAC reaction with a slow rate of hydrogenation would also lead to a low yield of EG due to the significant side reactions of GA and sugars.44 The hydrolysis reaction of cellulose generally does not affect the EG selectivity, unless its rate remarkably surpasses those of RAC and hydrogenation reactions. Such conditions would cause significant side reactions due to the instability of glucose at high temperatures. In fact, a reasonably slow rate of cellulose hydrolysis contributes to a high yield of EG. This is because the in situ released sugars will give a low concentration of GA intermediate through the RAC reaction and consequently depress the side reactions of GA and sugars. Zhao et al. studied glucose conversion at different feeding rates (or different concentrations of glucose in the reactor) in a semicontinuous batch reactor.44 It was found that at a 10 mL/min feeding rate of glucose, merely a 17.7% yield of EG was obtained, in contrast to the 53.4% yield obtained at a feeding rate of 0.667 mL/min. 1949

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Figure 5. (a) Plot of global yield of EG and 1,2-PG as a function of lignin percent in corn stalks. Pretreatment methods: (A) ammonia and H2O2; (B) butanediol; (C) NaOH; (D) H2O2; (E) ammonia; (F) 50% ethanol; (G) hot water; (H) hot lime water; (I) SC-CO2; (J) none (raw corn stalk). (b) Conversion of Miscanthus at high concentrations: (A) raw; (B) epidermal protectors removed by organic solvents; (C) epidermal protectors and lignin removed by organic solvents and alkali.60 Reproduced with the permission from ref 60. Copyright 2011, American Chemical Society.

conversion (see Figure 5a).60 A basic solution of Ca(OH)2 is also capable of removing lignin from feedstock. However, Ca2+ retained in the pretreated cellulose causes tungstenic catalyst deactivation due to the formation of insoluble CaWO4. A similar kind of deactivation was also observed in the presence of Fe3+.58 Pretreating cellulosic materials with mineral acid solutions was proposed to remove inorganic metal ions so as to obtain high yields of hydrogenolysis products.121 Ethanol/water mixtures are applicable to the fractionation of raw biomass such as barley straw.63 After partial removal of lignin from feedstock, EG was obtained at a yield of ca. 35% over Ru-W/AC catalysts. However, such organosolv pretreatment is quite energy-consuming, typically using 50/50 ethanol/ water at 473 K for 1 h, and is not appropriate for use on a large scale. Very recently, Shuai et al. explored a novel method to fractionate lignocellulosic biomass into lignin, hemicellulose, and cellulose at unprecedentedly high yields of 76−90 mol % by using a small amount of formaldehyde in the acidic solvent.122 Formaldehyde effectively depressed the lignin repolymerization during the biomass depolymerization. This work should stimulate more ideas for developing effective processes for biomass delignification and multicomponent fractionation, which will contribute to the development of viable processes for biomass conversion, including EG and 1,2PG production. Comprehensive use of the fractionated biomass components can enhance the viability of biomass conversion. Therefore, the chemo-catalytic process may be integrated with the biorefinery industry to give a variety of chemicals and bio-based materials. For instance, when the DLEG process is combined with an acetone−butanol−ethanol (ABE) fermentation process, cellulose is transformed to EG and 1,2-PG, hemicellulose is used for ABE production, and lignin can be recycled for synthesis of green polymers.16 The largest economic and environmental benefits will be obtained from the integration of disciplinary processes. Another important issue for bio-EG production is energy savings. To this end, high-concentration cellulosic feedstock should be used so that EG and 1,2-PG products can be obtained and separated from a concentrated solution with less energy consumption. On the other hand, because of the low density and the solid state of cellulose in water, the cellulose concentration has to be limited below 28 wt % (cellulose:water = 28:100), in view of the mass transfer and diffusion effects at currently available stirring efficiency. In the case of no negative

reports for these supported catalysts giving high performance in cellulose conversion to EG and 1,2-PG. The overly small pore size or susceptibility to hydrothermal conditions could be responsible for this consequence. On the other hand, Sn-Beta, Ti-Beta, and Zr-Beta catalysts have been found to be active in sugar conversion to lactate acid in methanol, which involved sugar isomerization, RAC, etc.84 Zhang et al. greatly enhanced the hydrothermal stability of zeolite by functionalizing the silanol defects with organosilanes.119 In another study, a MOF material was used as a precursor for synthesis of carbon nanofiber supported nickel−tungsten bimetallic catalysts, by taking advantage of its high surface area and the uniform dispersion of metal species in it.120 Therefore, it can be conjectured that there would be many opportunities for zeolite and MOF-based materials applications in cellulose conversion to diols, to give enhanced activity and tunable reaction selectivity.

6. CATALYTIC CONVERSION OF LIGNOCELLULOSE AND POTENTIAL APPLICATION In view of its practical availability and cost of feedstock, lignocellulose should be used instead of pure cellulose for largescale implementation of bio-EG and bio-1,2-PG production. In addition to the cellulose component, lignocellulosic raw biomass contains hemicellulose, lignin, some amount of water-soluble species, ash, and metal compounds, which affect the results of the direct conversion of lignocelluose to glycols. More issues should be studied and addressed for developing a variable biodiol production process, including but not limited to effective pretreatments of lignocellulose, effects of inorganic impurities contained in the feedstock, high concentration feedstock conversion, catalyst stability and regeneration, and diol product separation. As a typical abundant agriculture waste, corn stalk was studied in catalytic conversion over a Ni-W2C/AC catalyst.60 The reactivity of the raw biomass is significantly lower than that of pure cellulose, owing to the wrapping effect of lignin on cellulose. Hemicellulose is readily degraded into EG and 1,2PG. However, lignin is rather stable and is only partially degraded to phenolic products during the lignocellulose conversion.61 When lignin was removed from corn stalk to less than 5% through pretreatments, e.g. using aqueous mixtures of ammonia and hydrogen peroxide, the overall yield of EG and 1,2-PG was improved to 48% in the catalytic 1950

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have shown that the presence of a small amount of miscellaneous diols in EG does not notably deteriorate the EG performance in the applications. The bio-EG at >98% purity can be used as easily as petro-EG for poly(ethylene terephthalate) synthesis.124 Therefore, establishing a new quality standard for bio-EG product might be a wise strategy to promote bio-EG commercialization.

effect from lignin, e.g., by using pure cellulose feedstock or Miscanthus (see Figure 5b) treated with base solutions, the EG yield can be remained at a high level (40−65%) as the cellulose concentration is increased to 10−28%.16,62 In comparison to insoluble cellulose, sugars have remarkable advantages in terms of reaction efficiency and operational convenience in catalytic conversion. Glucose can be converted to EG at high concentrations (up to 50%) with >60% yields by tungstenic catalysts in a (semi)continuous reaction system.44,64,113 Referring to this process, it is possible to convert the solid cellulose at higher concentrations (>28 wt %) in a continuous reaction mode. For instance, lignocellulosic feedstock may be continuously squeezed into the stirred tank reactor at a wet or even dry state by a specific feeding apparatus, such as a screw pump, and then gradually liquefied in the reaction solution to produce EG by catalysis. The major challenge in this proposed process is to find or develop a reliable feeding system for the solid cellulosic feedstock, used particularly under high H2 pressures. An alternative strategy for cellulose conversion at high concentrations in a high-efficiency continuous mode is to saccharify cellulose into soluble oligosaccharides. Hilgert et al. explored a mechanocatalytic solid-state depolymerization method for cellulose conversion.123 After loading of 10 wt % sulfuric acid and ball-milling for 2 h, cellulose was fully transformed to water-soluble oligosaccharides, which was much easier to hydrogenate to hexitols in comparison with the untreated cellulose. Evidently, cellulose saccharification is of paramount significance for biomass conversion. It not only provides sugars as a versatile platform for synthesis of various chemicals but also contributes to the development of high-efficiency chemical engineering processes. For cellulose conversion to diols which is conducted under hydrothermal conditions, the catalyst stability is usually determined by the hydrogenation catalyst. For instance, when tungstenic catalyst is used, the tungsten acid catalyst is chemically stable and can be recycled from the distillation residues after EG separation via precipitation or calcination. A skeletal catalyst, such as Raney Ni, is rather robust under hydrothermal conditions and has been widely used in glucose hydrogenation to sorbitol. It is a suitable hydrogenation catalyst for cellulose conversion in view of its much lower price in comparison to noble-metal catalysts and stable performance in recycling experiments.43 For the lignocellulose conversion at high concentrations, some side products such as unsaturated or lipidic compounds derived from lignin degradation preferentially absorb on the hydrogenation catalyst and depress EG production.62 Therefore, in addition to delignification pretreatments on lignocellulose feedstock, periodic cleansing of the fouled hydrogenation catalyst will be useful for keeping the catalyst at high activity. Last but not least, an issue that needs mentioning is the separation of diols. Currently, a distillation or multieffect distillation process is widely used in the petro-EG industry for separating EG from heavier byproducts of diethylene glycol and triethylene glycol. In the biomass-derived diol products, EG, 1,2-PG, and a small amount of 1,2-butanediol generally coexist in the aqueous solution. These diols have very close boiling points (3−10 K) and need more energy to be separated by conventional distillation. Thus, developing effective methods, such as membrane techniques or selective removal by reactions, to separate the target product with less energy consumption is highly desirable. On the other hand, some preliminary studies

7. SUMMARY AND OUTLOOK Great progress has been achieved in the synthesis of EG and 1,2-PG from cellulosic biomass by using a variety of highly efficient catalysts, most of which belong to the transition metals of groups 3−6. Elements such as W, Mo, Cr, Sn, Pb, La, Ce, Y, V, Nb, Zn, Ti, and Zr are found to be catalytically active for the degradation of sugars via RAC reactions to form C2 or C3 intermediates. Effective coupling of cellulose hydrolysis, glucose degradation, and hydrogenation of the intermediates could give the EG product. An additional isomerization reaction of glucose to fructose is indispensable for 1,2-PG formation. Some correlations seem to exist between the RAC reaction and epimerization reaction. More studies on the catalysts effective for epimerization of sugars would possibly be helpful for developing novel catalysts for RAC reactions. In addition, similar to the development of heterogeneous catalysts by mimicking cellulase for cellulose hydrolysis, some highperformance heterogeneous catalysts for RAC reactions might be designed by using synergy between multifunctional active sites in the catalyst. With a decrease in activation energies for cellulose hydrolysis and RAC reactions, high reaction activities would be available under milder conditions, similar to cellulase and aldolase working in organisms. The cross-disciplinary development would promote the catalytic conversion of cellulose to EG, 1,2-PG, and other important chemicals in more efficient and environmentally benign ways. Among the various catalysts currently available, tungstenic catalysts are the most selective to obtain EG from cellulose. They are also effective for 1,2-PG production if the catalytic property of the tungsten species is modulated suitably to generate Lewis acid sites: for instance, by an interaction between tungsten and the support. Although there is a lack of molecular level in-depth understanding of the mechanism of sugar degradation over tungsten species, the knowledge of the reaction network and reaction kinetics has provided valuable information for tuning the reaction selectivity among EG, 1,2PG, erythritol, and hexitols. The main strategy to control reaction selectivity can be proposed from four aspects. They are using suitable active components, taking advantage of the discrepancy among the kinetics of reaction steps, modulating the electronic property of active sites or introducing additional active components to the catalyst to orient the reaction toward the desired route, and changing the reaction solvent or atmosphere. The principle in obtaining a high yield of target diol is effectively coupling the major reactions and keeping their rates matching each other. On the basis of the study of reaction kinetics, behaviors of glucose conversion to EG under a range of reaction conditions in the presence of tungstenic catalysts have been simulated in a (semi)continuous reaction mode. For cellulose conversion to diols, the kinetics of cellulose hydrolysis needs to be integrated with that of sugar conversions, so that the overall reaction performance can be described and reaction selectivity can be modulated accurately. To get generalized rules in cellulose degradation to C2 and C3 diols, it is necessary to understand the mechanism of 1951

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catalytic cracking of the C−C bond in sugars over various transition metals from both the universal and the specific properties of different elements. Theoretical calculations would be a powerful means for depicting the detailed interactions between sugar and metal sites at a molecular level. Comprehensive use of in situ and operando characterizations under hydrothermal conditions would provide more strong evidence to support the mechanism proposed. The great challenge for in situ and operando methods lies in detection of intermediate signals at active sites and differentiating them from the background under condensed liquid conditions, particularly at high temperatures and high pressures. ATR-IR and XAS have been used for monitoring the catalyst structure evolution, complexation of cations and biomass, and cellulose degradation. More characterization methods such as in situ or operando Raman spectroscopy may be explored, by referring to the operando ATR-FTIR method. Concentration-modulation excitation spectroscopy is a highly attractive method and would play an important role in identification of the authentic active species and the spectators during the cellulose conversion. Lignocellulose can be effectively converted to EG and 1,2-PG at a reasonable yield (40−65%) and high concentration (10− 28%) if the lignin component is removed with suitable pretreatments, typically using basic solutions. To promote the practical application of biomass conversion to EG and 1,2-PG, more efforts should be exerted from the following aspects. They are (1) feedstock delignification with high efficiency and less environmental impact, (2) full utilization of biomass fractions by integrating disciplinary processes involving chemo-catalysis, biorefinery, and material sciences, (3) lignocellulose saccharification to soluble sugars, which is of great significance for efficient operation in chemical engineering processes, (4) the design of specific apparatus and reactors for feeding and conversion of concentrated solid cellulose, and (5) energysaving methods for separating products with complex composition and close boiling points. With progress in the techniques and enhanced impetus for developing a sustainable economy, the industrial production of EG and 1,2-PG from lignocellulose would be realized first in biomass-rich areas and countries, and the knowledge accumulated in the extensive fundamental studies will give valuable guidance to tune the reaction selectivity and meet the market demand.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.Z.: [email protected]. ORCID

Tao Zhang: 0000-0001-9470-7215 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21376239, 21306191, and 21690084), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), and the Department of Science and Technology of Liaoning province under contract of 2015020086-101. 1952

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DOI: 10.1021/acscatal.6b03469 ACS Catal. 2017, 7, 1939−1954