Direct catalytic conversion of cellulose to liquid straight-chain alkanes

Abstract

High yields of liquid straight-chain alkanes were obtained directly from cellulosic feedstock in a one-pot biphasic catalytic system. The catalytic reaction proceeds at elevated temperatures under hydrogen pressure in the presence of tungstosilicic acid, dissolved in the aqueous phase, and modified Ru/C, suspended in the organic phase. Tungstosilicic acid is primarily responsible for cellulose hydrolysis and dehydration steps, while the modified Ru/C selectively hydrogenates intermediates en route to the liquid alkanes. Under optimal conditions, microcrystalline cellulose is converted to 82% n-decane-soluble products, mainly n-hexane, within a few hours, with a minimum formation of gaseous and char products. The dominant route to the liquid alkanes proceeds via 5-hydroxymethylfurfural (HMF), whereas the more common pathway via sorbitol appears to be less efficient. High liquid alkane yields were possible through (i) selective conversion of cellulose to glucose and further to HMF by gradually heating the reactor, (ii) a proper hydrothermal modification of commercial Ru/C to tune its chemoselectivity to furan hydrogenation rather than glucose hydrogenation, and (iii) the use of a biphasic reaction system with optimal partitioning of the intermediates and catalytic reactions. The catalytic system is capable of converting subsequent batches of fresh cellulose, enabling accumulation of the liquid alkanes in the organic phase during subsequent runs. Its robustness is illustrated in the conversion of the raw (soft)wood sawdust.

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© 2015 The Royal Society of Chemistry. Received 16th May 2014; Accepted 9th September 2014; First published online 09 Sep 2014. This work was carried out within the EU FP7 project BIOCORE that is supported by the European Commission through the Seventh Framework Program for Research and Technical development under contract no FP7-241566. B.O.d.B. acknowledges the European Commission for his doctoral fellowship. Mia Tielen is acknowledged for her technical assistance. M.D. acknowledges the Research Foundation – Flanders (FWO) for postdoctoral funding and the Belgian American Educational Foundation (BAEF) for his honorary fellowship. B.F.S acknowledges IAP financing (Belspo). The authors would like to kindly thank Jeroen Snelders and Christophe M. Courtin (Laboratory of Food Chemistry and Biochemistry & Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium) for their support in the determination of the composition of the softwood sample.

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