Enhanced xylan conversion to xylitol in a bio- and chemocatalytic one-pot process

A novel hybrid process for the conversion of xylan via hydrolytic hydrogenation achieved a xylitol yield of up to 65 %. As part of this study, the hypothesis of decreased product (xylose) inhibition of enzymes during xylan hydrolysis due to rapid conversion of xylose to xylitol was investigated experimentally. We found that xylose inhibition can play an important part in the reaction kinetics of xylan hydrolysis with commercially available enzymes. For example, at 10 wt.-% xylose concentration, the activity of enzymes dropped by ca. 80 %. In contrast, the final desired product xylitol showed only a slight inhibitory effect of about 20 % activity loss of the enzymes. The one-pot process of hydrolytic hydrogenation requires enzyme stability at higher-than-optimal temperatures and pressures, to ensure their activity at harsher process conditions needed for the chemo-catalytic hydrogenation. Therefore enzyme activity was tested at elevated temperatures and pressures. The tests were carried out with two enzyme mixtures (Ecopulp TX-800 A and Alkozym CPX). Both showed no activity loss over 60 min exposure time at pressures of up to 40 bar. However, their thermal resistance differs strongly, with Alkozym already deactivating at 50 ?C, while Ecopulp remained mostly active at up to 80 ?C. The novel sequential process (4 consecutive steps) achieved a xylitol yield of up to 65 % with almost 80 % xylan conversion. Our findings show both limitations and potential benefits of hybrid processes which try to exploit both biological and chemical catalysis. This type of combined catalysis for reaction cascades could further accelerate the ongoing research in process development for catalytic biorefineries.