The synergistic effect of benzyl benzoate on the selective oxidation of toluene to benzaldehyde

•Noncarboxylic acid solvents are applied for the liquid-phase oxidation of toluene.•Performances of the ester solvents and acetic acid are compared.•The reaction mechanism and the influence of intermediate products are revealed. To resolve the defects of acetic acid solvents for the liquid-phase oxidation of toluene to benzaldehyde with oxygen, a low-corrosive and green method was developed by introducing benzyl benzoate into the reaction. It was found that compared with the case of acetic acid, benzyl benzoate showed gratifying effect on the oxidation of toluene to benzaldehyde. The reaction was more active in the initial stage of oxidation, and the generated benzaldehyde was protected from further oxidation due to the synergistic effect of benzyl benzoate along with toluene-derived products benzyl alcohol and benzyl bromide. Benzyl benzoate inhibited the esterification of benzyl alcohol, while benzyl bromide promoted the formation of benzyl alcohol. Therefore, considerable benzyl alcohol concentration could be maintained and high benzaldehyde selectivity could be achieved under high toluene conversion.