Selective Oxidation Of Benzylic Carbon-hydrogen Bond To Aromatic Aldehydes Over Cobalt Oxide Catalysts Using Molecular Oxygen In Liquid Phase

Aromatic aldehydes are important chemical intermediates which can be widely applied in pharmaceuticals,pesticides,spices,foods and other fields.The traditional producing process for aromatic aldehyde by chlorohydrolysis has serious environmental pollution and chlorine-contaimination of the products,and the resulting aromatic aldehydes can’t be used in the field of pharmaceuticals and foods.Therefore,it is of great significance to directly oxidize benzylic carbon-hydrogen bond to aromatic aldehydes by molecular oxygen with negligible pollution and chlorine-free products.In this thesis,cobalt oxide catalysts were prepared by impregnation,and the structure,morphology,chemical state and other properties of the prepared catalysts were characterized.The catalysts were tested for the selective oxidation of the benzylic carbon-hydrogen bond with molecular hydrogen,the structure-reactivity relationship was invetigated,and the mechanism of selective oxidation of benzylic carbon-hydrogen bond was probed.The research work is as follows:1.Catalytic oxidation of m-phenoxytoluene to m-phenoxybenzaldehyde using molecular oxygen in liquid phaseCoOx/SNS catalysts were prepared by excessive impregnation using self-made silica nanospheres as support.The effects of different Co precursors and excessive impregnation times on the composition,structure,morphology and chemical state of the catalysts were investigated.The performances of the catalysts were investigated by the selective oxidation of m-phenoxytoluene to m-phenoxybenzaldehyde with molecular oxygen,and the relationship between their physicochemical properties and catalytic performaces was explored.The catalyst with cobalt acetate as the precursor exhibits the best catalytic activity for the selective oxidation of m-phenoxytoluene and also has better selectivity for m-phenoxybenzaldehyde.The cobalt content in the catalyst increases while increasing the times of excessive impregnations,but the bigger particle size and the decreased dispersion degree of the cobalt oxide were resulted.The prepared CoOx/SiO2 catalyst with twice of excessive impregnation presented a optimized loading with a relatively small particle size and high dispersion degree,and exhibited a m-phenoxytoluene conversion of 14.2%,a m-phenoxybenzaldehyde selectivity of 46.5%,and a m-phenoxybenzaldehyde yield of 6.6%under optimized reaction conditions.2.Catalytic oxidation of toluene to benzaldehyde using molecular oxygen in liquid phase at room temperatureThe supported transition metal oxide catalysts were prepared by the incipient wetness impregnation.The effects of the active components,supports,loadings and solvents on the selective oxidation of toluene were investigated in liquid phase.The physicochemical properties,catalytic performances and structure-activity relationship of the catalyst were investigated by the means of the characterizations of the composition,structure,morphology,and chemical state of the catalysts and its test for selective oxidation of toluene with molecular oxygen in liquid phase.It was found that the Co catalyst exhibited the highest activity for selectivity oxidation of toluene among the catalysts with the oxide of Co,Mn,and Ce as active components,respectively.Compared with activated carbon(AC)and Al2O3 supported CoOX catalysts,the catalyst with Co oxide on the SiO2 support possessed the highest dispersion degree,the smallest particle size and the best catalytic activity.The CoOx/SiO2 catalyst at a loading of 2 mmol/g exhibited a toluene conversion of 91.2%,a selectivity to benzaldehyde of 68.8%,a selectivity to benzyl alcohol of 8.4%,and a selectivity to benzoic acid of 20.5%under optimized conditions.The solvent fluorinated alcohol was found to act against the transformation of benzaldehyde to benzoic acid.3.Catalytic oxidation of toluene to benzaldehyde over cobalt oxide catalysts with different chemical statesThe bulk and spported cobalt oxide catalysts were prepared by calcination of their precursors in air or nitrogen atmosphere,respectively.The physicochemical properties of the catalysts and their performance of selective oxidation of toluene were investigated.The bulk catalysts with cobalt acetate as the precursor calcined in air and nitrogen atmosphere were Co3O4 and a mixture of CoO+Co3O4,respectively,and the latter presented higher catalytic performance in the selective oxidation of toluene to benzaldehyde.The supported catalysts prepared using cobalt nitrate as the precursor resulted in pure Co3O4 phase regardless of the calcination atmosphere(air or nitrogen),and the conversion of toluene was not higher than 6%.The supported CoOx catalyst with cobalt acetate as the precursor presented much higher catalytic activity than their counterparts prepared using cobalt nitrate as the precursor.The catalyst calcined in nitrogen atmosphere contained more CoO species than those prepared in air,and its cobalt oxide particles gave smaller size,better dispersion degree,and higher catalytic activity.The CoOx/SiO2 catalyst at a loading of 6 mmol/g prepared,which was prepared using cobalt acetate as precursor in nitrogen atmosphere,realized a toluene conversion of 97.5%,a selectivity to benzaldehyde of 62.3%under the experimental conditions.This indicates that the CoO species facilitates the conversion of toluene and the selective production of benzaldehyde.In conclusion,cobalt oxide catalysts are active for selective oxidation of benzylic carbon-hydrogen bond to aromatic aldehydes using molecular oxygen in liquid phase.As for the selective oxidation of m-phenoxytoluene to m-phenoxybenzaldehyde,the CoOx/SNS catalysts prepared using cobalt acetate as a precursor presented a smaller crystallite size and a higher dispersion,and the exhibited a higher catalytic activity.The content and the dispersion degree of cobalt oxide was found an apparent effect on its catalytic performance in selective oxidation of m-phenoxytoluene.In the case of selective oxidation of toluene to benzaldehyde,the CoOx/SiO2 catalyst prepared using cobalt acetate and silica as the precursor and the support,respectively,had best catalytic activity.Fluorinated alcohol as a solvent can inhibit over oxidation of benzaldehyde to benzoic acid.Calcination under nitrogen atmosphere is beneficial to increasing the dispersion of cobalt oxide,and leading to more cobalt monoxide species and the higher conversion,which indicates that the increase of Co2+ content in the catalyst can promote the selective oxidation of toluene to benzaldehyde.