Kameh Tajvidi

Hydrogenolysis of Cellulose over Cu-Based Catalysts—Analysis of the Reaction Network

A series of polyols, carbohydrates, and cellulose were tested in the aqueous, CuO/ZnO/Al2O3-catalyzed hydrogenolysis reaction at 245 °C and 50 bar H2. The compositions of liquid-phase products were analyzed; based on these results a unified reaction mechanism is proposed that accounts for the observed product distribution. Elementary transformations such as dehydration, dehydrogenation/hydrogenation, Lobry de Bruyn-van Ekenstein isomerization and retro-aldol cleavage were identified as most important for controlling the selectivity of simple polyols and carbohydrates. For cellulose the product distribution is considerably different than for glucose or sorbitol, indicating a change in the reaction pathway. Therefore, next to the traditional hydrolysis of the glycosidic bond, an additional depolymerization mechanism involving only the reducing ends of cellulose oligomers is proposed to account for this observation.

Copper-Based Catalysts for Efficient Valorization of Cellulose

The combination of diminishing fossil fuel resources and an increasing energy demand has increased the focus on the possible transformations of renewable feedstocks. Biomass is a promising alternative source of carbon for implementing a potentially fossil-free chemical industry. When employed in fuel production, biomass reduces the carbon dioxide footprint of combustion. This has led to an increased use of biofuels such as biodiesel and bioethanol in recent years; however, their production is based on food crops, which induces competition between food and fuel production. This challenge can be remedied by using lignocellulose as feedstock. Lignocellulose consists of cellulose, hemicellulose, and lignin, with cellulose as the major constituent in amounts of up to 50 %. Therefore, efficient chemical transformations of cellulose into platform chemicals will play a potentially crucial role in the transition to biorefinery schemes. Acid treatment of cellulose depolymerizes this abundant biopolymer to glucose, and depending on the reaction conditions dehydration to products such as 5-hydroxymethylfurfural (HMF) occurs. 4] Combining hydrolysis with hydrogenation enables the direct transformation of cellulose into highly valuable intermediates such as C6 sugar alcohols and short-chain alcohols. The first studies on hydrolytic hydrogenation were performed in the 1960s by Sharkov, using noble-metal catalysts together with diluted mineral acids. Recently, Fukuoka et al. reported the aqueous-phase conversion of cellulose over Pt/ Al2O3 at 190 8C within 24 h, reaching up to 30 % yield of hexitols. In addition, Lu et al. demonstrated the hydrolytic hydrogenation of cellulose over Ru clusters at higher temperature in significantly shorter times, reaching yields of C6 polyols of up to 40 %. Previously, we investigated the conversion of cellulose by combining diluted mineral acids and noble-metal catalysts, such as Ru, Pt, and Pd, supported on activated carbon. Interestingly, rather different product distributions were observed over these catalysts : C5–C6 polyols were the main products in the case of Ru, while Pd and Pt yielded short-chain alcohols and gaseous products. Optimization of the reaction conditions and acid-to-metal ratio allowed the selective formation of C6 sugar alcohols, [9] or up to 80 % yields of C4–C6 sugar alcohols at only 160 8C. Nevertheless, when considering large-scale applications the high costs and limited availability of noble metals encourages the development of noble-metal-free catalysts. Recently, Zhang et al. presented first studies on cellulose degradation over nickel-promoted tungsten carbide, yielding up to 75 % ethylene glycol. However, the recyclability of the catalysts remained challenging. Simple Cu-based catalysts present a promising alternative, as they are known for their high activity toward hydrodeoxygenation of C O bonds and have already reached high activity in the hydrogenolysis of glycerol. Moreover, Gallezot et al. investigated Cu-based catalysts in the hydrogenolysis of sugar alcohols such as sorbitol with 63 % selectivity to deoxyhexitols. Interestingly, in a recent study a Cudoped porous metal oxide was utilized to transform wood biomass into liquid and gaseous products. Liquefaction occurred in supercritical methanol at temperatures above 300 8C and pressures of 160–220 bar, resulting in a mixture of various aliphatic alcohols and CO2. In contrast, we present herein the controlled hydrolytic hydrogenation of cellulose over Cu-based catalysts into C1–C3 compounds that are already utilized in today’s chemical industry, including methanol, ethylene glycol (EG), 1,2and 1,3-propanediol (PD), as well as glycerol. First investigations concentrated on CuO/ZnO/Al2O3, a catalyst usually utilized in industrial methanol synthesis. At a reaction temperature of 245 8C, using water as solvent, C C and C O bond cleavage was facilitated, resulting in C1–C3 compounds as main products. In line, reactions at this temperature over Ruand Pt/Al2O3 delivered significant amounts of the described C1–C3 compounds. Interestingly, a comparable product distribution could be reached by applying simple Cu-based materials, allowing direct conversion of cellulose (Figure 1 a and Supporting Information Table S1). These results could be further optimized by increasing the metal content of the catalyst, reaching overall yields of liquid-phase products of up to 95 %, with 67.4 % C1–C3 compounds of which 15.4 % 1,2-PD and 27.1 % methanol, respectively (Figure 1 b). In addition, the high reaction temperature initiates not only the formation of C1–C3 compounds but also dehydroxylation to products such as 1,2,6-hexanetriol (1,2,6-HT) or 1,2-butanediol (1,2-BD; Supporting Information Table S2). Interestingly, these results can be transferred to spruce as feedstock, achieving 87.6 % conversion and 38.6 % yield emphasizing EG and 1,2-PD as main products (Supporting Information Table S3). Based on these results CuO/ZnO/Al2O3 catalysts can be classified as promising alternatives for supported-noble-metal catalysts in the direct transformation of cellulose. From an industrial and environmental point of view, catalyst recyclability is of major importance considering continuous production processes. To facilitate recycling experiments, cellobiose was used as substrate in a first step (Supporting Information Figure S1). The investigated Cu-based catalyst could be [a] Prof. Dr. R. Palkovits Institut f r Technische und Makromolekulare Chemie RWTH Aachen University Worringerweg 1, 52074 Aachen (Germany) Fax: (+ 49) 241 8022177 E-mail : palkovits@itmc.rwth-aachen.de [b] K. Tajvidi, K. Pupovac, M. K krek Max-Planck-Institut f r Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470, M lheim an der Ruhr (Germany) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201200482.