Boosting the formation of Brønsted acids on flame-made WOx/ZrO2 for glucose conversion

Tungstate-zirconium oxide catalysts (WOx/ZrO2) with much higher concentrations 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 single-step 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 5-hydroxylmethylfurfural (HMF). The catalytic study demonstrated that the B/L ratio plays a crucial role in glucose conversion, virtually independent of the total acidity of the catalysts. 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 %, comparable to some homogenous catalysts.