Abstract
The adsorption and oxidation of p-chlorotoluene (PCT), p-methoxytoluene (PMT), and toluene on vanadyl pyrophosphate catalyst (VPP) were studied by in situ FTIR and EPR spectroscopy. Various amounts of strongly adsorbed benzaldehydes 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 in the aldehyde formation. The benzaldehyde adsorption is enhanced by additional interaction of the carbonyl group with Brønsted acid hydroxyl groups generated during oxidation reaction, which facilitates deeper oxidation. The co-adsorption of pyridine is one possibility to suppress the strong aldehyde 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-substituents causes total oxidation leading to lower aldehyde selectivities. Both, the acid/basic characters of the reactants and products and their steric properties play an important role for adsorption/desorption processes.
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