Dehydration of glycerol to acrolein over Wells–Dawson and Keggin type phosphotungstic acids supported on MCM-41 catalysts

Abstract

Gas phase dehydration of glycerol to acrolein was studied using the Wells–Dawson and Keggin type phosphotungstic acids supported on MCM-41 catalysts. The samples were characterized using XRD, FTIR, pyridine-FTIR, UV–Vis, N2 adsorption, NH3-TPD, ICP, SEM and elemental analysis. FT-IR and UV–Vis spectra results confirmed the presence of Wells–Dawson and Keggin type phosphotungstic acids on the support. XRD results suggested that the active phases were highly dispersed on MCM-41. N2 adsorption results showed that the mesoporous structure of MCM-41 was retained after loading of phosphotungstic acids. H3PW12O40/MCM-41 showed higher total acid sites than H6P2W18O62/MCM-41, but the number of Brønsted acid sites on H6P2W18O62/MCM-41 was higher than that of H3PW12O40/MCM-41. Glycerol conversion increased with the total amount of acid sites. The acid type also influenced catalytic performance. The selectivity to acrolein increased with increasing the ratio of Brønsted and Lewis acid, and lower value of Brønsted/Lewis acid favored the production of hydroxyacetone. H6P2W18O62/MCM-41 owned more Brønsted acid sites, the dehydration reaction was more likely to occur via the Brønsted acid sites dehydration route, thus generating more acrolein and formaldehyde. Meanwhile, H3PW12O40/MCM-41 exhibited more Lewis acid sites, the dehydration reaction was more likely proceeding via the Lewis acid sites dehydration route and generating more hydroxyacetone and acetaldehyde. Furthermore, we concluded that the reasons for decrease of catalytic performance in glycerol conversion were leaching of HPW and coke deposition.