Design of solid acid catalysts for aqueous-phase dehydration of carbohydrates: The role of Lewis and Bronsted acid sites

We have prepared a series of well-characterized acid catalysts, including Zr-P, SiO2-Al2O3, WOx/ZrO2, gamma-Al2O3, and HY zeolite and tested them for aqueous-phase dehydration of xylose. We have characterized the concentration of both Bronsted and Lewis acid sites in these catalysts with TPD and FT-IR spectroscopy using gas-phase NH3 and compared the catalytic activity and selectivity with that of homogeneous catalysts for the dehydration of aqueous solutions of xylose. The catalyst selectivity is a function of the Bronsted to Lewis acid site ratio for both the heterogeneous and homogeneous reactions. Lewis acid sites decrease furfural selectivity by catalyzing a side reaction between xylose and furfural to form humins (insoluble degradation products). At 20% xylose conversion, catalysts with high Bronsted to Lewis acid ratios, such as Zr-P, exhibit furfural selectivities as much as 30 times higher than catalysts with higher Lewis acid site concentrations. Dehydration reactions using ion-exchange polymer resins with high Bronsted acid site concentrations showed similar selectivities to Zr-P and HCl. Using HY zeolite revealed a low furfural selectivity due to strong irreversible adsorption of the furfural in the pores, causing an increase in the rate of humin formation. Thus, to design more efficient aqueous-phase dehydration catalysts, it is desirable to have a high ratio of Bronsted to Lewis acid sites. Furthermore, gas-phase characterization of acid sites can be used to predict catalytic activity in the aqueous phase. (C) 2011 Elsevier Inc. All rights reserved.