Oxidative dehydrogenation of ethane on γ-Al 2O 3 supported vanadyl and iron vanadyl phosphates : Physico-chemical characterisation and catalytic activity

Abstract

γ-Al 2O 3 supported vanadyl and iron vanadyl phosphates (VOPO 4 and Fe 0.23(VO) 0.77PO 4) calcined at 550 or 650 °C have been investigated as catalysts for the oxidative dehydrogenation (ODH) of ethane to ethylene in the temperature range 450–650 °C in a fixed bed reactor operating under atmospheric pressure. Catalysts have been characterised by X-ray diffraction (XRD), BET surface area measurements, X-ray photoelectron spectroscopy (XPS), NH 3 temperature programmed desorption (TPD) and temperature programmed reduction (TPR). A good dispersion of the active phase has been obtained. The presence of various vanadium species (vanadyl phosphate, V 4+ and V 3+ ions, V 5+ and V 4+ oxides) have been detected on the catalysts surface with a significantly different distribution between supported vanadyl and iron vanadyl phosphate, V 4+ ion being the prevailing species on VOPO 4/Al 2O 3 (VOP/Al). The different surface species show different acidity and reducibility. Supported vanadyl phosphate catalysts are more active than iron modified samples and catalysts calcined at 650 °C give better catalytic performances than those calcined at 550 °C. Catalytic tests promote the formation of V 3+, whose fraction increases with the reaction temperature. The better catalytic properties of supported vanadyl phosphate have been attributed to the presence of surface V 4+ ions.