Study On The Direct Catalytic Oxidation Of Toluene To Benzaldehyde

Benzaldehyde is an important intermediate in the production of perfumery,pharmaceuticals, dyestuff and agrochemicals. Traditionally, benzaldehyde isfabricated by the following two important processes, that is, it is fabricated from thechlorination of toluene followed by hydrolysis, or as a by-product in the liquid phaseoxidation of toluene to benzoic acid. Generally, the product obtained from theformer process contains chlorinated impurities and does not meet food and drugsgrade specification. The latter process requires rather high temperature and highpressure, and gives low yields of benzaldehyde. The vapor phase oxidation oftoluene is an attractive method for synthesizing benzaldehyde, however, successfulresults are scarce. Thus, alternative catalyst and processes that can overcome orameliorate these disadvantages would be attractive.A series of Mo-V catalysts were prepared by wet impregnation method and usedfor the catalytic oxidation of toluene. Benzaldehyde was successfully synthesizeddirectly from toluene under mild reaction conditions (low temperature andatmosphere pressure) using batch reaction method. The results of control experimentand blank experiment show that the target reaction is a heterogeneously catalyzedone.Using Mo and V as the active components,γ-Al₂O₃ supported catalyst showsbetter activity than the CeO₂, TiO₂ and SiO₂ supported ones for the catalyticoxidation of toluene. A series of Mo-V/γ-Al₂O₃ catalysts with different metalloadings were prepared with different impregnation procedures. It is found that the catalytic performances are closely related to the relative contents of the activespecies and the crystalline species formed in the catalyst. As the main catalyticcomponent, V species shows certain activity for the oxidation of toluene, and theco-existence of Mo and V in the catalyst facilitates the formation of benzaldehydewhich may be resulted from the synergetic effect of the co-existed Mo and V. Thehighest selectivity (79.5%) to benzaldehyde was obtained on the catalyst with aMo/V molar ratio of 0.7. The optimal conditions obtained in the present work are asthe following: reaction time, 3 hours; reaction temperature, about 343 K, underwhich a benzaldehyde of 13.2% with a selectivity of 79.5% is obtained.The catalysts prepared in the present work are characterized by TPR, XPS andXRD techniques. The results show that the highly dispersed amorphous V species isfavorable for benzaldehyde formation, and the presence of crystalline V₂O₅ speciesdecreases the selectivity to benzaldehyde, while AlVMoO₇ species is effective forthe activation of C-H bond of benzene ring, thus, it favors the production of cresols.