Research On Design And Preparation Of V-based Catalysts For Selective Oxidation Of Toluene And Dimethylbenzenes

Toluene and dimethylbenzenes are important chemical raw materials with wide source and low price.Their effective utilization and transformation to high value-added products are important directions of chemical production.Toluene and xylene can be converted into corresponding aromatic aldehydes by selective oxidation of C-H bond.Aromatic aldehydes are a kind of important chemical intermediates,which are widely used and in short supply in China.The common preparation methods of aromatic aldehydes include catalytic oxidation,chemical oxidation,electrochemical oxidation and so on.Among them,the oxidation of toluene and dimethylbenzenes to obtain corresponding aromatic aldehydes is a green and chlorine free production process with easy continuous production,simple process,low cost and better application prospects.However,this reaction has the disadvantages of low aromatic conversion,low selectivity of aromatic aldehydes,complex products composition and so on.Therefore,the key to this reaction is to seek efficient catalysts and optimize reaction conditions.In this paper,a series of catalysts were prepared by using Al₂O₃-Zr O₂ as the carrier and V element as the main catalytic element.By adding different auxiliary elements into the catalyst as the second component and the third component,changing the proportion between different components,changing the type of carrier and the preparation method of the catalyst,a series of catalysts were prepared and used in the vapor phase oxidation of toluene.By changing the reaction temperature,air and toluene flow rate,it was found that the V-Ag-Ce/Ti O₂（V:Ag:Ce=4:2:1）catalyst prepared by over impregnation method showed the best catalytic performance when the air flow rate was 150 m L/min,the toluene flow rate was 0.35 m L/min,and the reaction temperature was 525℃.The corresponding maximum yield of benzaldehyde was 25.3%,the toluene conversion and the benzaldehyde selectivity reached27.4%and 92.2%respectively.At the same time,the V-Ag-Ce/Ti O₂ catalysts with different V:Ag:Ce proportions were used in the gas-phase oxidation of o-xylene,m-xylene and p-xylene.It was obtained that the catalysts had good catalytic effect on the gas-phase oxidation of toluene and dimethylbenzenes,and the catalysts with the best catalytic performance were V-Ag-Ce/Ti O₂（V:Ag:Ce=4:2:1）catalyst all the same.The N₂ adsorption desorption test showed that the distribution of active components was more uniform and the catalytic effect was better when the specific surface area of the catalyst was larger.The NH₃-TPD test showed that when the total acid amount on the catalyst surface is about 0.1 mmol/g,the weak acid amount is between 0.01～0.02 mmol/g,the medium strong acid amount is between 0.02～0.04 mmol/g,and the strong acid amount is less than 0.06mmol/g,it is more favorable for the gas-phase selective oxidation of toluene and xylene to prepare the corresponding aromatic aldehydes.The XPS showed that there was electron transfer in the catalyst,and the yield of benzaldehyde will be higher when the oxygen vacancy accounts for about 40%of all oxygen species.Finally,the possible reaction mechanism of V-Ag-Ce/Ti O₂ catalyst for gas-phase oxidation of toluene was proposed:Firstly,the acid site adsorbs toluene to the catalyst surface;Secondly,the oxygen vacancy activates the gaseous oxygen into lattice oxygen and reacts with toluene on the catalyst surface;Finally,the product desorbs from the catalyst surface.