One-Pot Conversion of Cellulose to Ethylene ... - ACS Publications

One-Pot Conversion of Cellulose to Ethylene Glycol with Multifunctional Tungsten-Based Catalysts AIQIN WANG AND TAO ZHANG* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China RECEIVED ON JULY 23, 2012

CONSPECTUS

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ith diminishing fossil resources and increasing concerns about environmental issues, searching for alternative fuels has gained interest in recent years. Cellulose, as the most abundant nonfood biomass on earth, is a promising renewable feedstock for production of fuels and chemicals. In principle, the ample hydroxyl groups in the structure of cellulose make it an ideal feedstock for the production of industrially important polyols such as ethylene glycol (EG), according to the atom economy rule. However, effectively depolymerizing cellulose under mild conditions presents a challenge, due to the intra- and intermolecular hydrogen bonding network. In addition, control of product selectivity is complicated by the thermal instabilities of cellulosederived sugars. A one-pot catalytic process that combines hydrolysis of cellulose and hydrogenation/hydrogenolysis of cellulose-derived sugars proves to be an efficient way toward the selective production of polyols from cellulose. In this Account, we describe our efforts toward the one-pot catalytic conversion of cellulose to EG, a typical petroleumdependent bulk chemical widely applied in the polyester industry whose annual consumption reaches about 20 million metric tons. This reaction opens a novel route for the sustainable production of bulk chemicals from biomass and will greatly decrease the dependence on petroleum resources and the associated CO2 emission. It has attracted much attention from both industrial and academic societies since we first described the reaction in 2008. The mechanism involves a cascade reaction. First, acid catalyzes the hydrolysis of cellulose to water-soluble oligosaccharides and glucose (R1). Then, oligosaccharides and glucose undergo C
C bond cleavage to form glycolaldehyde with catalysis of tungsten species (R2). Finally, hydrogenation of glycolaldehyde by a transition metal catalyst produces the end product EG (R3). Due to the instabilities of glycolaldehyde and cellulose-derived sugars, the reaction rates should be r1 , r2 , r3 in order to achieve a high yield of EG. Tuning the molar ratio of tungsten to transition metal and changing the reaction temperature successfully optimizes this reaction. No matter what tungsten compounds are used in the beginning reaction, tungsten bronze (HxWO3) is always formed. It is then partially dissolved in hot water and acts as the active species to homogeneously catalyze C
C bond cleavage of cellulose-derived sugars. Upon cooling and exposure to air, the dissolved HxWO3 is transformed to insoluble tungsten acid and precipitated from the solution to facilitate the separation and recovery of the catalyst. On the basis of this temperature-dependent phase-transfer behavior, we have developed a highly active, selective, and reusable catalyst composed of tungsten acid and Ru/C. Our work has unearthed new understanding of this reaction, including how different catalysts perform and the underlying mechanism. It has also guided researchers to the rational design of catalysts for other reactions involved in cellulose conversion.

1. Introduction

abundant biomass on earth. It is estimated that the world-

Cellulose is a biopolymer of D-glucose linked by β-1,4-glycosidic bonds with a polymerization degree up to 10 000. It constitutes, together with hemicellulose and lignin, the main component of cell walls of plants. Cellulose is the most

wide production of lignocellulose amounts to 170  109 t/year,

www.pubs.acs.org/accounts 10.1021/ar3002156 & XXXX American Chemical Society

of which 35
50 wt % is cellulose.1 Such a tremendous, nonfood, and renewable resource is being considered as the most promising alternative to fossil resources for the Vol. XXX, No. XX

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One-Pot Conversion of Cellulose to Ethylene Glycol Wang and Zhang

The importance of end product EG and the unprecedentedly high efficiency of this catalytic reaction have evoked intense interest from both academia and industry. In this Account, based on our own results and research from other groups, we try to give a full image of the one-pot conversion of cellulose to EG.

2. Hydrolytic Conversion of Cellulose over Solid Catalysts Cellulose is intrinsically recalcitrant owing to the abundant FIGURE 1. Production of EG and polyesters from biomass-based and fossil-based resources. PET and PEF are abbreviations of poly(ethylene teraphthalate) and poly(ethylene furandicarboxylate), respectively.

sustainable production of fuels and chemicals. Since cellulose has a relatively high O/C ratio, while fuels usually possess a much lower O/C ratio, excess oxygen must be removed when cellulose is transformed into fuels, which would be achieved at the expense of C or H and thereby has 2,3

a lower efficiency from the viewpoint of atom economy. On the other hand, transforming cellulose into oxygenates, such as polyols, proves to be a highly atom-economic reaction because most of the oxygen-functional groups in

intra- and inter-molecular hydrogen bonds that protect the β-1,4-glycosidic bonds from attack by foreign molecules (Scheme 1, left panel). For instance, it is insoluble in most solvents, including water. Only with concentrated mineral acids or supercritical water can cellulose be depolymerized to a substantial degree. However, these methods suffer from environmental problems or harsh operation conditions. In contrast, hydrolytic conversion of cellulose with a solid catalyst proves an environmentally benign and milder reaction.10,11 Depending on the catalyst function and the reaction temperature, the major product varies from glucose to polyols (Scheme 1, right panel). When the reaction proceeds below

the cellulose are preserved in the target products.4,5 Polyols, such as ethylene glycol (EG), propylene glycol (1,2-PG and 1,3-PG), glycerol, xylitol, sorbitol, and mannitol, are important chemicals widely used as monomers in the plastic industry, functional additives in the food industry, and intermediates in the pharmaceutical industry. Among

423 K with an acid catalyst, hydrolysis of cellulose pre-

them, EG has the largest market with an annual consumption of 20 million metric tons predominantly for the manufacture of poly(ethylene teraphthalate) (PET) fibers and bottles.6 Currently, EG is being produced from petroleum-

ball-milling pretreatment of cellulose. On the other hand,

derived ethylene via multiple steps of cracking, epoxidation, and hydration (Figure 1). In 2008, we for the first time discovered a nonpetroleum route for the production of EG from

sorbitol is produced as the main product.10,16
18 This cas-

renewable cellulose using a nickel-promoted tungsten carbide catalyst.7 In comparison with the petroleum-dependent multistep process, the biomass route presents prominent advantages of a one-pot process and renewable feedstock (Figure 1). This novel reaction route may lead to the production of bio-EG and eventually to the production of bio-PET. Moreover, because poly(ethylene furandicarboxylate) (PEF) is being considered as an alternative to petroleum-based PET,8,9 we can also envisage the production of bioplastic PEF from cellulose-derived EG and furandicarboxylate, which thereby will greatly reduce the dependence on petroleum resources and the emission of greenhouse gases. B ’ ACCOUNTS OF CHEMICAL RESEARCH

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dominates with glucose as the primary product.12
15 Nevertheless, microcrystalline cellulose is poorly accessible and less reactive with solid acids at temperatures below 423 K, which requires a long reaction time (12
48 h), a high catalyst/cellulose ratio (1
10), and an energy-intensive when the reaction proceeds under the presence of pressurized H2 and a hydrogenation catalyst (e.g., supported Ru, Pt, Ir, etc.), hydrolytic hydrogenation of cellulose occurs and cade reaction circumvents the problem of metastable glucose and is therefore allowed to proceed at a relatively high temperature (453
523 K) without compromising the selectivity; thus a high reaction rate can be accomplished. The metal sites are not only for the hydrogenation of glucose but also promote the hydrolysis of cellulose via heterolytic dissociation of H2,11 while the acid sites originate from acidic groups on the support surface17 or simply from hot water.16 When the reaction proceeds in the presence of a tungstenbased catalyst at temperatures above 503 K (e.g., 518 K), hydrolytic hydrogenolysis of cellulose takes place and EG instead of sorbitol becomes the major product.7 This reaction involves not only hydrolysis of cellulose but also the cleavage of C
C bonds and therefore requires a multifunctional catalyst.

One-Pot Conversion of Cellulose to Ethylene Glycol Wang and Zhang

SCHEME 1. Cellulose Structure (left) and Its Hydrolytic Conversion to Different Products Depending on the Catalyst and Reaction Condition (right)

In the earlier studies about hydrolytic hydrogenolysis of sugars, base catalysts such as CaO were used to function as the C
C bond cleavage agent.19 Nevertheless, base catalysts could cause isomerization of glucose to fructose resulting in glycerol rather than EG as the major product. To the best of our knowledge, tungsten-based compounds are the most effective agent to promote the C
C bond cleavage reaction of cellulose for the production of EG.

3. Performance of Various Tungsten-Based Catalysts Activity and Selectivity. The hydrolytic hydrogenolysis of cellulose was performed in water with pressurized H2 (6
10 MPa H2) at 503
523 K. The reaction could be completed within 0.5
4 h depending on the feedstock concentration and the reaction temperature. Figure 2 compares the performances of various tungsten-based catalysts at the typical reaction condition of 518 K, 6 MPa H2, 1 wt % cellulose, and 0.5 h. Under these reaction conditions, the conversions of cellulose are almost complete over all the tungsten-based catalysts, indicative of their high activities for degradation of cellulose. However, the yields of EG vary greatly with the catalyst employed, reflecting the great difference in the selectivity to EG. According to their functions, these tungsten-based catalysts can be classified into two groups. Tungsten carbide (WCx) and tungsten phosphide (WP) are themselves multifunctional catalysts (group A, Figure 2),

which means that they are able to catalyze the hydrolysis of cellulose and the subsequent hydrogenolysis of sugars without promotion of a transition metal. The acid sites can arise both from hot water and from the surface tungsten oxides or phosphates,16,17 while the hydrogenation sites originate from the platinum-like electronic properties of WCx or WP.20 It is this multifunctional property of tungsten carbide and phosphide catalysts that results in a moderate yield of EG (∼30%) without the presence of a transition metal.7,21,22 In addition to EG and other polyols illustrated in Figure 2, a large fraction of unsaturated products (unidentified) and a trace amount of gases (less than 1%, including methane, ethane, CO2, etc.) are also formed, thereby reducing the selectivity for EG. However, the addition of a transition metal indeed leads to a further increase of the EG yield, by 10
40% depending on the transition metal type and the preparation method. For instance, Ni
W2C/AC affords an EG yield of 61%, which almost doubles that from the W2C/AC.7 When Ni is introduced by a postimpregnation method (denoted as Ni
(W2C/AC)), the EG yield is further increased to 73%.23 Meanwhile, the unsaturated byproducts decrease greatly, making the carbon balance approach 95%. To be noted, transition metals like Ni, Ru, Pt, Pd, and Rh are themselves not effective for the C
C cleavage; therefore they only give a very low yield of EG (60%) can be achieved. Of industrial importance, the combination of tungsten acid and Ru/C gave a high yield of EG and excellent reusability. Although great progress has been made in the one-pot conversion of cellulose to EG, the efficient transformation of biomass to important bulk chemicals is still in its infancy. Future research should be encouraged in the following aspects: (i) development of powerful in situ characterization techniques adaptable to high-temperature and high-pressure liquid media, which will contribute to the mechanistic understanding at a molecular level;47 (ii) design of cheaper but more efficient and robust catalysts for biomass conversion based on great advancement in material science and theoretical calculations;25,48 (iii) development of a cost-effective pretreatment method to remove selectively lignin and minerals from raw biomass, which is a prerequisite for process commercialization;45,49,50 and (iv) development of advanced technology for separation and purification of EG from a spectrum of side products and water and to intensify the process for efficient use of energy.51 This work is supported by the National Science Foundation of China (Grants 20773124, 20903089, and 21176235) and 973 program of China (Grant 2009CB226102). We are grateful for the discussions with Drs. Mingyuan Zheng, Na Ji, Changzhi Li, Jifeng Pang, and all the graduate students.

BIOGRAPHICAL INFORMATION Aiqin Wang joined Prof. Tao Zhang's group after she received a Ph.D. degree in 2001 from Dalian Institute of Chemical Physics (DICP) under the supervision of Profs. Dongbai Liang and Tao Zhang. In 2003, she moved to National Taiwan University as a postdoctoral fellow in Prof. Chung-Yuan Mou's group for synthesis

and catalysis of gold-based alloy nanoparticles until 2005. Then, she went back to DICP and joined Prof. Tao Zhang's group where she was promoted to a full professor in 2009. Her current research interests involve catalysis by nanogold and gold alloy particles, catalytic conversion of biomass, and design and synthesis of new catalytic materials.

Tao Zhang received his Ph.D. degree in 1989 from Dalian Institute of Chemical Physics (DICP) under the supervision of Profs. Liwu Lin and Jingling Zang. After one year at the University of Birmingham as a postdoctoral fellow with Prof Frank Berry, he joined DICP again in 1990 where he was promoted to a full professor in 1995. He is currently the director of DICP. He also serves as the Associate Editor-in-Chief of Chinese Journal of Catalysis, an Editorial Board Member of Applied Catlysis B, and an Advisory Board Member of ChemPhysChem. His research interests are focused on the catalytic conversion of biomass, design and synthesis of single-atom catalysts, and environmental catalysis. FOOTNOTES *E-mail address: [email protected]. The authors declare no competing financial interest. REFERENCES 1 Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev. 2006, 106, 4044–4098. 2 St€ocker, M. Biofuels and Biomass-to-Liquid Fuels in the Biorefinery: Catalytic Conversion of Lignocellulosic Biomass Using Porous Materials. Angew. Chem., Int. Ed. 2008, 47, 9200– 9211. 3 Rinaldi, R.; Sch€uth, F. Design of Solid Catalysts for the Conversion of Biomass. Energy Environ. Sci. 2009, 2, 610–626. 4 Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411–2502. 5 Ruppert, A. M.; Weinberg, K.; Palkovits, R. Hydrogenolysis Goes Bio: From Carbohydrates and Sugar Alcohols to Platform Chemicals. Angew. Chem., Int. Ed. 2012, 51, 2564–2601. 6 Yue, H.; Zhao, Y.; Ma, X.; Gong, J. Ethylene Glycol: Properties, Synthesis, and Applications. Chem. Soc. Rev. 2012, 41, 4218–4244. 7 Ji, N.; Zhang, T.; Zheng, M.; Wang, A.; Wang, H.; Wang, X.; Chen, J. G. Direct Catalytic Conversion of Cellulose into Ethylene Glycol Using Nickel-Promoted Tungsten Carbide Catalysts. Angew. Chem., Int. Ed. 2008, 47, 8510–8513. 8 Eerhart, A. J. J. E.; Faaij, A. P. C.; Patel, M. K. Replacing Fossil Based PET with Biobased PEF; Process Analysis, Energy and GHG Balance. Energy Environ. Sci. 2012, 5, 6407– 6422. 9 Ma, J.; Pang, Y.; Wang, M.; Xu, J.; Ma, H.; Nie, X. The Copolymerization Reactivity of Diols with 2,5-Furandicarboxylic Acid for Furan-Based Copolyester Materials. J. Mater. Chem. 2012, 22, 3457–3461. 10 Fukuoka, A.; Dhepe, P. L. Catalytic Conversion of Cellulose into Sugar Alcohols. Angew. Chem., Int. Ed. 2006, 45, 5161–5163. 11 Dhepe, P. L.; Fukuoka, A. Cellulose Conversion under Heterogeneous Catalysis. ChemSusChem 2008, 1, 969–975. 12 Suganuma, S.; Nakajima, K.; Kitano, M.; Yamaguchi, D.; Kato, H.; Hayashi, S.; Hara, M. Hydrolysis of Cellulose by Amorphous Carbon Bearing SO3H, COOH, and OH Groups. J. Am. Chem. Soc. 2008, 130, 12787–12793. 13 Pang, J.; Wang, A.; Zheng, M.; Zhang, T. Hydrolysis of Cellulose into Glucose over Carbons Sulfonated at Elevated Temperatures. Chem. Commun. 2010, 46, 6935–6937. 14 Lai, D. M.; Deng, L.; Li, J.; Liao, B.; Guo, Q. X.; Fu, Y. Hydrolysis of Cellulose into Glucose by Magnetic Solid Acid. ChemSusChem 2011, 4, 55–58. 15 Komanoya, T.; Kobayashi, H.; Hara, K.; Chun, W. J.; Fukuoka, A. Catalysis and Characterization of Carbon-Supported Ruthenium for Cellulose Hydrolysis. Appl. Catal. A: Gen. 2011, 407, 188–194. 16 Luo, C.; Wang, S.; Liu, H. Cellulose Conversion into Polyols Catalyzed by Reversibly Formed Acids and Supported Ruthenium Clusters in Hot Water. Angew. Chem., Int. Ed. 2007, 46, 7636–7639. 17 Ding, L.; Wang, A.; Zheng, M.; Zhang, T. Selective Transformation of Cellulose into Sorbitol by Using a Bifunctional Nickel Phosphide Catalyst. ChemSusChem 2010, 3, 818–821.

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