Selective oxidation of p-substituted toluenes to the corresponding benzaldehydes over (VO)(2)P2O7: an in situ FTIR and EPR study

The adsorption and oxidation of p-chlorotoluene (PCT), p-methoxytoluene (PM T), and toluene on vanadyl pyrophosphate catalyst (VPP) were studied by in situ FTIR and EPR spectroscopy. Various,amounts of strongly adsorbed benzal dehydes and cyclic anhydride species were observed by FTIR in dependence on the different educts after oxidation experiments. The extent of spin-spin exchange pertubation and, thus, the loss of the EPR signal intensity caused by substrate adsorption and interaction is influenced by the nature of the aromatic compound. The strength of reactant and product adsorption on the catalyst surface was found to be an important selectivity-limiting factor i n the aldehyde formation. The benzaldehyde adsorption is enhanced by additi onal interaction of the carbonyl group with Bronsted acid hydroxyl groups g enerated during oxidation reaction, which facilitates deeper oxidation. The co-adsorption of pyridine is one possibility to suppress the strong aldehy de adsorption and to improve the selectivities. Yields of benzaldehydes and selectivities at constant conversion increase in the order PMT < toluene < PCT. Strong product adsorption favoured by electron donating p-substituent s causes total oxidation leading to lower aldehyde selectivities. Both, the acid/basic characters of the reactants and products and their steric prope rties play an important role for adsorption/desorption processes. (C) 2000 Elsevier Science B.V. All rights reserved.