Isomerization of xylose over zeolites
Abstract
To improve the utilization of hemicellulose, xylose, the major hydrolysate of hemicellulose, is isomerized to xylulose, a yeast fermentable sugar, by zeolite catalysts. Xylose, lyxose and xylulose are pentose isomers which are interconvertible via their acyclic shapes under general acid-base catalysis. Zeolites, by providing locally high acidity in the solid phase, may catalyze pentose transformations at a physiological bulk pH. In the channels and cages of properly chosen zeolites, a better selectivity to isomerization product may be achieved. Several zeolites with desirable shape selectivity and acidity have shown the capability of converting xylose to xylulose in aqueous solutions. Among the catalysts examined, HY zeolite showed the highest catalytic activity and structural stability. Xylose reactions at different sugar concentrations and zeolite loadings have been studied in the temperature range between 80 and 160$\sp\circ$C. The identified reactions of xylose include the isomerization to xylulose and lyxose, and the dehydration to furfural. The low yield of furfural in these zeolite catalyzed reactions relative to that in a homogeneous acid catalyzed reaction indicates the possibility of shape selectivity inside zeolite particles. The reaction kinetics for the interconversion among xylose, xylulose, and lyxose has been studied. The transformations among these three pentoses follow reversible, first order expressions. The reaction rate constants indicate that the interconversion between xylulose and lyxose is faster than that between xylose and xylulose, and that the reactions between xylose and lyxose are the slowest. The equilibrium constants derived from these kinetic parameters agree with those reported in the literature for xylose/xylulose isomerization and for xylose/lyxose interconversion. Ethanol fermentation is carried out using the reaction broth as a carbon source. Yeast is able to utilize the xylulose in the reaction broth directly to produce ethanol without pretreatment.
Degree
Ph.D.
Advisors
Tsao, Purdue University.
Subject Area
Chemical engineering|Agricultural chemicals|Agricultural engineering
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