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One-pot conversion of cellulose to ethylene glycol with multifunctional tungsten-based catalysts.

Accounts of chemical research (2013-02-21)
Aiqin Wang, Tao Zhang
ABSTRACT

With 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 cellulose-derived 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 petroleum-dependent 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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Ethylene glycol, anhydrous, 99.8%
Sigma-Aldrich
Ethylene glycol, ReagentPlus®, ≥99%
Sigma-Aldrich
Cellulose, microcrystalline, powder
Sigma-Aldrich
Cellulose, fibers, (medium)
Sigma-Aldrich
Cellulose, microcrystalline, powder, 20 μm
Supelco
Ethylene glycol, Pharmaceutical Secondary Standard; Certified Reference Material
Supelco
Avicel® PH-101, ~50 μm particle size
Sigma-Aldrich
Sigmacell Cellulose, Type 20, 20 μm
Sigma-Aldrich
α-Cellulose, powder
Sigma-Aldrich
Avicel® PH-101, tested according to Ph. Eur.
Sigma-Aldrich
Ethylene glycol, spectrophotometric grade, ≥99%
Supelco
Ethylene glycol, analytical standard
USP
Ethylene glycol, United States Pharmacopeia (USP) Reference Standard
Sigma-Aldrich
Cellulose, colloidal, microcrystalline
Sigma-Aldrich
Ethylene glycol, BioUltra, ≥99.5% (GC)
Sigma-Aldrich
Sigmacell Cellulose, Type 101, Highly purified, fibers
Supelco
Cellulose, acid washed, powder, for column chromatography
Sigma-Aldrich
Sigmacell Cellulose, Type 50, 50 μm
Supelco
Cellulose, powder, for column chromatography
Sigma-Aldrich
Tungsten, powder, 10 μm, ≥99.99% trace metals basis
Sigma-Aldrich
α-Cellulose, BioReagent, suitable for insect cell culture
Sigma-Aldrich
Tungsten, powder, 12 μm, 99.9% trace metals basis
Sigma-Aldrich
Ethylene glycol 5 M solution
Sigma-Aldrich
Tungsten, wire, diam. 0.25 mm, ≥99.9% trace metals basis
Supelco
Cellulose, DS-0, powder, suitable for thin layer chromatography (TLC)
Sigma-Aldrich
Tungsten, foil, thickness 0.05 mm, ≥99.9% trace metals basis
Supelco
Cellulose, DFS-0, microcrystalline, suitable for thin layer chromatography (TLC)
Sigma-Aldrich
Tungsten, foil, thickness 0.127 mm, ≥99.9% trace metals basis
Sigma-Aldrich
Tungsten, foil, thickness 0.25 mm, ≥99.9% trace metals basis
Sigma-Aldrich
Tungsten, foil, thickness 0.5 mm, ≥99.9% trace metals basis