Influence of phosphorus and vanadium additives in the development of surface acid catalytic properties of mesoporous alumina

Abstract

Alumino-phosphoro-vanadate catalysts of the general formula Al₁₀₀P_xV_y, where x,y = 0, 5, 10 and 20, have been prepared by co-precipitation of calculated amounts of Al(NO₃)₃·9H₂O, H₃PO₄ and V₂O₅ dissolved in NH₄OH, with ammonia solution at pH = 9.5. The resulting solids, after characterization by N₂-porosimetry and XRD measurements, were checked for their surface acidity by ammonia–TPD and were also tested for their catalytic activity towards isopropanol decomposition. It was found that the addition of phosphorus increases the acid sites per square metre of the Al₁₀₀P_xV_y solids and the same effect is also observed by the addition of vanadium. The catalytic activity of the solids with y < 20, expressed as molecules decomposed per surface acid site per second, shows maxima for catalysts with higher cumulative addition of phosphorus and vanadium, at those P plus V concentrations that result in amorphous solids while the onset of crystallization results in reducing the activity. The opposite behaviour is observed for catalysts with 20% V where the crystal phase of V₂O₅ is apparent in XRD right from the beginning but becomes gradually amorphous at high phosphorus addition at x = 20.