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

Tungstate-zirconium oxide catalysts (WO_x/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 WO_x species. As a result, FSP-made WO_x/ZrO₂ (FSP-WO_x/ZrO₂) has a much higher surface concentration of three-dimensional Zr-WO_x clusters than corresponding materials prepared by conventional impregnation (IM-WO_x/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-WO_x/ZrO₂ possesses ~80 % of BAS and a B/L ratio of around 4, while IM-WO_x/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-WO_x/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.