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

Tungstate‐zirconium oxide catalysts (WO x /ZrO 2 ) 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 ZrO 2 support and thus provide an excellent surface dispersion of WO x species. As a result, FSP‐made WO x /ZrO 2 (FSP‐WO x /ZrO 2 ) has a much higher surface concentration of three‐dimensional Zr‐WO x clusters than corresponding materials prepared by conventional impregnation (IM‐WO x /ZrO 2 ). The coordination of W‐OH to the unsaturated Zr 4+ sites in these clusters results in a remarkable decrease of the concentration of Lewis acid sites (LAS) on the surface of ZrO 2 and promotes the formation of bridging W−O(H)‐Zr hydroxyl groups acting as BAS. FSP‐WO x /ZrO 2 possesses ~80 % of BAS and a B/L ratio of around 4, while IM‐WO x /ZrO 2 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‐WO x /ZrO 2 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.