Thermal behaviour, redox properties, and catalytic performance of alkali-promoted V 2O 5 catalysts in the selective oxidation of p-methoxytoluene: a comparative study

Abstract

Alkali-promoted V 2O 5 catalysts M–V 2O 5 (M=Li, K, Cs) were synthesised by impregnation of V 2O 5 with alkali sulphate solution. Pure V 2O 5 was used for comparison. X-ray diffraction, spectroscopic (FTIR), and thermoanalytical methods (STA/MS) have been used to characterise the phase composition, the adsorption properties, and the reducibility of the catalysts. The catalytic performance was proved using the oxidation of p-methoxytoluene (PMT) to p-methoxybenzaldehyde (PMBA) as test reaction. The surface acidity is lowered, but the reducibility is enhanced with increasing size and basic properties of the alkali cation. This leads to an increased adduct (PMT) adsorption and decreased product (PMBA) adsorption in the order V 2O 5<Li–V 2O 5⪡K–V 2O 5<Cs–V 2O 5. Consequently, the catalytic performance is improved in the same way. The formation of bronze phases at relative low temperatures in the case of K– and Cs–V 2O 5 stabilise V 4+ oxidation state and improve the redox properties and consequently the catalytic results. The admixture of the non-reactive pyridine enhances the aldehyde selectivity by further lowering of the surface acidity. Additionally, pyridinium cations generated during catalytic reaction and incorporated into the formed alkali bronze phases stabilise these structures.