The effect of alkali metal promotion on vanadium-containing catalysts in the vapour phase oxidation of methyl aromatics to the corresponding aldehydes

The comparison of vanadium phosphate catalysed ammoxidation with the oxidation of methyl aromatics to the corresponding nitriles and aldehydes showed significant differences in catalytic performance. The ammoxidation proceeds with nearly complete conversion and high nitrile selectivities, whereas the partial oxidation of the same feed to aldehydes under similar conditions leads to poor activity and selectivity only. This result is supposed to be due to the lack of a sufficient basicity in the reaction system; catalyst surface acidity causes strong reactant-solid interaction that is the reason for consecutive overoxidation, Therefore, potassium-containing vanadium phosphates and more basic alkali cation-containing vanadia solids (M/V2O5, M = Li, Na, K, Rb, Cs) were synthesised and used as catalysts in the vapour phase oxidation of toluene (TO), p-methoxytoluene (MTO) and p-chlorotoluene (CTO) to the corresponding aldehydes. Moreover, the effect of the addition of a non-oxidisable base (e.g. pyridine) to the reaction mixture was studied. Parent catalyst samples and used specimens were characterised by X-ray diffractometry and infrared spectroscopy. Vanadates and bronze phases of the more bulky alkali metal cations are generated during catalyst synthesis and catalytic reaction that may lead to new surface arrangements with vanadia. The catalytic results showed that alkali metal-containing vanadia catalysts revealed significant higher performance compared to vanadium phosphate solids. The increasing alkali cation size and basicity in the order from Li to Cs leads to an increase in the aldehyde selectivity. The effect of acid-base properties of catalyst bulk and surface in connection with reactant nucleophilicity and the impact of electronic effects of the substituents on the catalytic behaviour are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.