Effect of Acetic Anhydride on the Oxidation of Toluene to Benzaldehyde with Metal/Bromide Catalysts

Acetic anhydride (AA) was used as specific additive for liquid-phase oxidation of toluene in acetic acid to weaken the corrosivity of reaction system and increase the yield of benzaldehyde by using Co/Mn/Br catalyst system and 1,1,2,2-tetrabromoethane as bromine sources. The effect of AA on the liquid-phase oxidation of toluene to benzaldehyde was carefully studied. It was found that adding a certain amount of AA in acetic acid caused different reaction process from the reaction in pure acetic acid. Noticeably, addition of AA could increase both the selectivity of benzaldehyde and conversion of toluene during 2–8 h, whereas in pure acetic acid, benzaldehyde selectivity gradually dropped with a rise in toluene conversion. AA could greatly decrease water content in the reaction system, possibly reduce the corrosivity of substrates, and increase the toluene conversion. However, too much AA could cause the deactivation of the catalyst. By optimizing conditions, the maximum selectivity and yield of benzaldehyde reached 67.6 and 26.1%, respectively. On the basis of the experiment, a possible mechanism for the effect of AA was presented. Adding AA into acetic acid could improve the selective oxidation of methyl aromatics to aldehyde.