The selective oxidation of toluene to benzaldehyde is a key challenge. It is due to inherent reactivity of the benzylic C H bond in benzaldehyde, which undergo further oxidation to benzoic acid or complete mineralization under oxidative conditions. Therefore, the major scientific challenge is to control oxygen activation, reaction pathways, and catalyst surface chemistry that favours partial oxidation. Addressing this challenge requires the understanding of reaction strategies adopted and its mechanism, catalytic systems including structure, oxidation state, oxygen species, and reaction conditions. In the present manuscript, the selective oxidation of toluene to benzaldehyde using liquid, vapour, and gas phases is reviewed. The use of homogeneous catalysts such as Schiff bases, metal acetates, and Metal tetra-phenyl porphyrin for liquid phase oxidation of toluene has the advantage of high selectivity for aldehyde, and moderate temperature requirement. However, recovery and deactivation of the catalyst, as well as the use of protic solvents for the overoxidation of benzaldehyde, are drawbacks. The heterogeneous catalysts, such as metal oxide, mixed metal oxide, metal-organic framework, carbon-supported, zeolite-supported, Schiff base supported catalysts for liquid phase oxidation of toluene require higher operating temperature and solvent amount. However, recovery of the catalyst is an advantage. Vapour and gas phase reaction, except photocatalyst, is reported to a larger extent for the removal of benzaldehyde from effluent steam by complete oxidation of it into carbon dioxide. Nano metal oxides as a photocatalyst for the vapour phase oxidation of toluene give higher selectivity and require lower operating conditions. However, lower toluene conversion, costly synthesis of the catalyst, and the use of toxic organic solvents are drawbacks. The use of novel techniques involving microwave or ultrasound in liquid phase toluene oxidation incorporated with heterogeneous metal oxides improves the yield and selectivity of benzaldehyde at the same time, reducing waste generation and energy requirement.
Nayak et al. (Sun,) studied this question.