文摘
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 Brxf8;nsted 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 Brxf8;nsted 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 Brxf8;nsted 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 Brxf8;nsted 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 Brxf8;nsted to Lewis acid sites. Furthermore, gas-phase characterization of acid sites can be used to predict catalytic activity in the aqueous phase.