U. Bentrup et al., Selective oxidation of p-substituted toluenes to the corresponding benzaldehydes over (VO)(2)P2O7: an in situ FTIR and EPR study, J MOL CAT A, 162(1-2), 2000, pp. 383-391
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
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