Propane partial oxidation over M 3+-substituted vanadyl phosphates dispersed on titania and silica

Abstract

Mono-substituted M 3+ compounds of vanadyl phosphate dihydrate VOPO 4·2H 2O, with formula [M(H 2O)] x VO 1− x PO 4· nH 2O (M=Al, Fe, Cr, x=0.15–0.20, n=2–2.40), and a di-substituted compound with formula Fe 0.08Cr 0.08(H 2O) 0.16VO 0.84PO 4·2.9H 2O, either pure or supported, were characterised by XRD, EDAX, TG/DTA, physisorption and chemisorption measurements, and tested as catalysts in the partial oxidation of propane. Incorporation of the M 3+ cation into VOPO 4 produces a marked increase in surface area, pore volume, and reducibility, with subsequent enhancement of the catalytic activity. Upon adsorption of these compounds on titania, a homogeneous distribution of highly dispersed species is obtained, whereas on silica small conglomerates of crystalline phases of VOP and FeVOP are formed, presumably by polymerisation on the acidic surface sites of the support. The titania-supported samples exhibit higher catalytic activity and better selectivity to partial oxidation products (acetic acid and propene), compared to silica-based materials; these effects are attributed to the higher dispersion and reducibility of the surface species. Propane oxidation over the supported materials undergoes transition to the ignited state, in which surface temperatures up to 900 K are attained, and homogenous reactions yield mainly propene and CO.