Boosting the Formation of Brønsted Acids on Flame-made WOx/ZrO2 for Glucose Conversion.

ABSTRACT

WOx/ZrO2 with a higher concentration of Brønsted acid sites (BAS) and a bigger ratio of Brønsted to Lewis acid sites (B/L) than achievable by conventional impregnation (IM) were synthesized using flame spray pyrolysis (FSP). The rapid quenching and short residence time inherent to FSP prevent the accumulation of W atoms on the ZrO2 support and thus provide an excellent surface dispersion of WOx species. As a result, FSP-made WOx/ZrO2 (FSP-WOx/ZrO2) has a much higher surface concentration of three-dimensional Zr-WOx clusters than corresponding materials prepared by conventional impregnation (IM-WOx/ZrO2). The coordination of W-OH to the unsaturated Zr4+ sites in these clusters results in a remarkable decrease of the concentration of Lewis acid sites (LAS) on the surface of ZrO2 and promotes the formation of bridging W-O(H)-Zr hydroxyl groups acting as BAS. FSP-WOx/ZrO2 possesses ~80% of BAS and a B/L ratio of around 4, while IM-WOx/ZrO2 exhibits ~50% BAS and a B/L ratio of around 1. These catalysts were evaluated in the dehydration of glucose to HMF. The catalytic study demonstrated that B/L ratio plays a crucial role in glucose conversion. The best catalyst, FSP-WOx/ZrO2 with a W/Zr ratio of 1/10 affords nearly 100% glucose conversion and an HMF selectivity of 56-69%.