Study On Selective Oxidation Of Toluene Over Heterogenous Catalysts

Toluene is a mass product in petroleum chemistry industry and has only low economic value, but its oxidation products, such as benzyl alcohol, benzaldehyde and benzoic acid, are important chemical products and intermediates with high added value. Among these, benzaldehyde has the highest economic value and market demand because of its wide application in Pharmaceuticals, pesticides, fragrance and dyes industries. At present, benzaldehyde in our country is mainly synthesized by the chlorination of toluene followed by hydrolysis. This method causes serious environmental pollution, and the product contains chlorine impurities, which restricts its application in fragrance and pharmaceutical industries. Benzaldehyde can also be produced by liquid phase oxidation of toluene, whereas it is mainly obtained as by-product with only low yield. Therefore, the research of heterogeneous catalysts for toluene oxidation with good catalytic performance and high selectivity under mild conditions attracts the academic and industrial world.To obtain heterogeneous catalysts with high catalytic performance, lots of literature on liquid catalytic oxidation of toluene at home and abroad is reviewed in this paper, mainly summarizing the application of heterogeneous catalysts of Co and novel nano-gold catalysts in the liquid phase oxidation of toluene. The idea is suggested for research of liquid phase oxidation of toluene over Au/Co3O4、Co-Y and Au/Mont catalysts. The catalysts were prepared by coprecipitation, ion-exchange and deposition-precipitation methods respectively, and characterized by ICP-AES、XRD and TEM. The influnences of gold loadings, the prepared methods, reaction time, reaction temperature, reaction time and other influence factors on the catalytic performance were investigated using acetic acid as reaction solvent and oxygen as oxidant.(1) AU/CO3O4catalysts were prepared by coprecipitation method. Under the reaction conditions of toluene0.1mol, acetic acid10mL, catalyst0.06g,130℃1.0MPa,6h,3.70%conversion of toluene and43.25%selectivity to benzaldehyde were obtained over3%AU/Co3O4catalyst. This suggested that AU/Co3O4catalysts obtained by coprecipitation method exhibited good catalytic performance in toluene oxidation. According to the XRD result, the loading of Au improved the degree of crystallinity of Co3O4.(2) To decrease the amount of Co and improve the catalyst stability, different Co-Y catalysts were prepared by ion-exchange method and their catalytic performance were tested in this paper. According to the ICP-AES result, the maximum ion exchange capacity about4%could be reached after ion-exchange for2h. Under the conditions of130℃,1.0MPa and10h,4.20%conversion of toluene and44.92%selectivity to benzaldehyde were obtained over Co-Y catalyst. Besides, the results showed that the loading of Au might cause the decrease of catalytic performance of NH4Y and Co-Y.(3) Au/Mont catalytic were prepared on new catalytic material of Mont by deposition-precipitation method, and it showed good catalytic performance in the liquid phase oxidation of toluene. Under the conditions of140℃,1.0MPa and10h,3.36%conversion of toluene and46.26%selectivity to benzaldehyde were obtained over1.0%Au/Mont. According to the ICP-AES result, however, the actual loading efficiency of Au/Mont catalysts were very low (2%-5%), it was probably because the isoelectric point of Mont support was very low, which was unfavourable to the precipitation of Au(OH)x on the surface of the support.Totally, the three kind of catalysts reported in this paper not only solve the difficulty of separation of homogeneous catalysts, but also exhibited comparative catalytic performance in toluene conversion, especially in benzaldehyde selectivity with other heterogeneous catalysts of Co reported before under mild reaction conditions.The results above provide a good basis for further research of toluene oxidation and also expand the application of traditional Co catalysts and new Au catalysts in the liquid phase oxidation of toluene.