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

We have prepared a series of well-characterized acid catalysts, including Zr–P, SiO2–Al2O3, WOX/ZrO2, γ-Al2O3, and HY zeolite and tested them for aqueous-phase dehydration of xylose. We have characterized the concentration of both Brønsted 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 Brønsted 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 Brønsted 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 Brønsted 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 Brønsted to Lewis acid sites. Furthermore, gas-phase characterization of acid sites can be used to predict catalytic activity in the aqueous phase.