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A comparative study on chemical conversion of cellulose between the batch-type and

Cellulose 9: 301–311, 2002.

©2002Kluwer Academic Publishers. Printed in the Netherlands.

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A comparative study on chemical conversion of cellulose between the batch-type and ?ow-typesystems in supercritical water N

Katsunobu Ehara and Shiro Saka*

Department of Socio-Environmental Energy Science, Graduate School of Energy Science, Kyoto University, 606-8501Sakyo-ku, Kyoto, Japan; *Authorfor correspondence (e-mail:saka@energy.kyoto-u.ac.jp;phone:+81-75-753-4738;fax:+81-75-753-4738)

Received 29October 2001; accepted in revised form 4March 2002

Key words:Chemical conversion, Hydrolysis, Sacchari?cation,Supercritical water Abstract

Microcrystalline cellulose (avicel)was treated in supercritical water using batch-type and ?ow-typesystems. The ?ow-typesystem made it possible to shorten the heating, treating and cooling times, compared with the batch-type system. As a result, the ?ow-typesystem was able to liquefy avicel without producing any supercritical water-insoluble residue. Although hydrolyzed products such as glucose and fructose, and pyrolyzed products such as levoglucosan, 5-hydroxymethyl furfural, erythrose, methylglyoxal, glycolaldehyde and dihydroxyacetone were found in common from the water-soluble portion treated by both systems, the ?ow-typesystem gave a water-soluble portion with more hydrolyzed and less pyrolyzed products, together with water-soluble oligosaccharides consisting of cellobiose to cellododecaose and their decomposed products at their reducing end of glucose, such as [?-D -glucopyranosyl]1–11?-D -levoglucosan, [?-D -glucopyranosyl]1–11?-D -erythrose and [?-D -glucopyrano-syl]1–11?-D -glycolaldehyde. In addition, the precipitates of polysaccharides were recovered after 12h setting of the water-soluble portion. These results indicated that the ?ow-typesystem can hydrolyze cellulose with mini-mizing pyrolyzed products. On the other hand, the batch-type system resulted in a higher yield of the pyrolyzed products due to the longer treatment, but a higher yield of glucose due possibly to the higher pressure and con-comitantly higher ionic product of water. Based on these lines of evidence, the process to increase the yield of the sugar is discussed under supercritical water treatment. Introduction

Fossil resources, which increase the carbon dioxide emission in the environments through their combus-tion, are considered to become exhausted sooner or later. Therefore, biomass resources will become more important in the future as alternatives to the fossil re-sources. For the conversion of biomass resources into useful chemicals and biomass-energy, sacchari?cationof cellulosic resources followed by fermentation is one of the directions. Furthermore, glucose which can be converted to chemicals such as ethanol, ethylene and 5-hydroxymethyl furfural (5-HMF)is a key prod-Presented at the 8th Annual Meeting of the Cellu-lose Society of Japan which was held on July 12–13,2001.

A comparative study on chemical conversion of cellulose between the batch-type and

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uct. For this purpose, two major methods exist for hydrolysis of carbohydrates, by acid catalyst (Parisi1989) and enzymatic sacchari?cation(Changand Holtzapple 2000). Although these processes have been re?nedand considerably developed, they have at least the following drawbacks; the former has a corrosion problem of the reactor by acid, while the latter involves the use of enzyme expensive for its preparation (Goldstein1980).

For chemical conversion of cellulose, on the other hand, a new approach has been developed recently with non-catalytic subcritical (Sakakiet al. 1996a, 1996b; Walsum et al. 1996) or supercritical water (>374°C,>22.1MPa) treatment (Sakakiet al. 1996b; Sasaki et al. 1998). In our laboratory, it was conse-quently reported that woody biomass resources can be

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