Novel Approach for Conversion of Xylose to Ethanol

Kripa K. Rao1, Silpa Chelikani2, Ronald Fournier2, Patricia Relue2, and Sasidhar Varanasi1. (1) Chemical and Environmental Engineering, University of Toledo, 2805 Bancroft Street, 3048 Nitschke Hall, Toledo, OH 43606, (2) BioEngineering, University of Toledo, 2805 Bancroft Street, 3048 Nitschke Hall, Toledo, OH 43606

Economic production of ethanol from plant biomass could be significantly increased if the feedstock for the fermentation is completely utilized. Out of the Simple sugars recovered from the lignocellulose, D-glucose can be readily converted into ethanol by bakers or brewers yeasts (Saccharomyces Cerevisiae). However xylose obtained by the hydrolysis of hemicellulosic portions is not fermentable by many of the same species of yeasts. Alternatively xylose can be converted into its ketose isomer, xylulose by the enzyme xylose isomerase and this isomer can be further converted into ethanol. The economics of this two-step approach can be significantly improved by conducting the isomerisation reaction of xylose and fermentation of the resulting xylulose to ethanol in a single unit operation. This approach has the additional advantage that it would enable one to overcome the unfavorable equilibrium of the isomeraization reaction, by continuous removal of the xylulose via the fementaion process. A major obstacle; however to this approach is that the pH at which xylose isomerase displays its optimal activity (pH of 7-8.0) is much different from the pH at which the fermentation is carried out (pH of 4.0-5.0). Herein, we propose a novel scheme to provide a means by which the isomerization and fermentation can take place simultaneously in a single vessel. This technique would involve the immobilization of xylose isomerase in a porous polymer pellet with an additional layer of urease enzyme immobilized on it. These bilayered pellets are dispersed in a fermentation broth, which contains prespecified amounts of urea in addition to the other necessary ingredients. It will be shown how this system will achieve the objective and the resulting kinetics of the combined isomerization-fermentation process will presented.