Selective oxidation of toluene to benzaldehyde: Investigation of structure-reactivity relationships by in situ-methods

(VO) 2 P 2 O 7 and potassium-doped V 2 O 5 catalysts have been studied in the selective oxidation of toluene to benzaldehyde by in situ -EPR, -FTIR, -XRD, -UV/VIS and -XPS. In (VO) 2 P 2 O 7 , Bronsted surface sites formed under reaction conditions favour strong product adsorption and, thus, total oxidation. They can be blocked by adding pyridine, which improves catalytic performance. K-V 2 O 5 catalysts are markedly reduced during reaction. Both V 5+ and V 4+ species are likely to be active in the catalytic redox cycle. Crystalline K 0.5 V 2 O 5 formed on the catalyst surface under feed probably lowers the catalytic performance due to structural reasons.