Supported H4SiW12O40/Al2O3 solid acid catalysts for dehydration of glycerol to acrolein: Evolution of catalyst structure and performance with calcination temperature

Abstract

Al2O3 supported silicotungstic acid (H4SiW12O40/Al2O3) samples were prepared by impregnation and calcined at 350, 450, 550, 650°C to study the structural evolution of the H4SiW12O40 heteropolyacid and its effect on the catalytic performance during glycerol conversion to acrolein. For comparison, a WOx/Al2O3 catalyst was also prepared by impregnation with ammonium paratungstate. The catalysts were extensively characterized using BET, NH3-TPD, FT-IR, XRD, Raman, UV–vis DR and XAFS. The evolution of catalyst structure and catalytic performance was carefully resolved and analyzed. XAFS analysis reveals that the oxygen coordination number in the first shell with short WO bond distance increases as the calcination temperature is increased, suggesting the transform of W-species on surface. Slight degradation of Keggin structure of H4SiW12O40 supported on Al2O3 is observed upon calcination at 350 and 450°C, though [SiW12O40]4− anions with Keggin structure are still the dominating species. The total decomposition of Keggin unit occurs after calcination at 550 and 650°C. Isolated WO6 species and small amount of WO3 micro-crystallites are formed as a result of the decomposition. The glycerol conversion increases with acid center concentration under the specified reaction condition. As well, selectivity to acrolein increases with Brønsted/Lewis acid ratio, suggesting the crucial role of Brønsted acid sites for acrolein formation.