The influence of surface coverage on the catalytic activity of silica-supported heteropolyacids

Abstract

A series of heteropolyacid (HPA)—silica composites were prepared from H3PMo12O40 (HPM) and H4PVMo11O40 (HPVM) by supporting on two types silica supports: Aerosil–Degussa and Romsil–Ramnicu Valcea with different active phase concentrations. The structure and texture of these HPA/silica composites were studied by XRD, FT-IR spectroscopy and nitrogen physisorption at 77 K. The effect of the support on the thermal stability was investigated by TG-DTG and DTA. By the deposition of heteropolyacids on silica type supports, an important decrease of thermal stability was observed on Romsil types and a small decrease on Aerosil types. By comparing with initial supports, the specific surface areas of HPAs deposited on all supports decreased with the active phase concentration. The catalytic properties of the prepared samples were studied by using a reactant pulse method at different temperatures. The ethanol conversion proceeds by two main pathways: an oxide hydrogenation reaction on redox catalytic centers with acetaldehyde formation, and a dehydration reaction on acidic centers with ethylene and diethyl ether formation. The reaction rates of ethanol conversion and acetaldehyde formation increased on supported HPA compared to bulk HPA. The favorable effect of HPAs deposition on silica supports for the oxidehydrogenation pathway to acetaldehyde results from the examination of apparent activation energies Ea and reaction rate values. Evidence of decreasing Ea was obtained for acetaldehyde formation in the case of silica Romsil-supported HPM (39.6 kJ/mol) comparatively with bulk HPM (56.6 kJ/mol). For all reaction temperatures, the reaction rates of acetaldehyde formation are higher for supported samples than unsupported ones.