| Benzaldehyde is a colorless oily liquid with bitter almond flavor.It has been widely used in the manufacture of perfume,medicine,dyes,spices,plasticizers,flame retardants,etc.As a natural product,it can be easily extracted from different plants,such as almonds,peaches and almonds.At present,the world’s demand for benzaldehyde has exceeded 90,000tons/year,and natural sources have been difficult to meet market demand.At present,there are four mainstream routes for industrial synthesis of benzaldehyde,which are toluene chlorination hydrolysis,photocatalytic oxidation,toluene liquid phase oxidation and toluene gas phase oxidation.The gas phase oxidation reaction of toluene is very representative.The process is simple and can be operated continuously.The reaction conditions are mild,the product is easy to separate,and the product does not contain toxic chlorides.The main difficulty of this process is the selection and development of high-performance catalysts.Our group had successfully developed a composite oxide catalyst for the selective oxidation of isobutene to methacrolein,and the yield of methacrolein can reach more than 85%.In order to further expand its application field,this paper modified the above catalysts for toluene gas phase selective oxidation reaction with the same reaction mechanism as isobutene oxidation.The specific research content:Based on the isomorphous substitution method,the Mo element in the active center of the composite oxide catalyst was regulated,and the catalysts with different V/Mo atomic ratios and different W/Mo atomic ratios were prepared respectively,and the organic acid was used to further optimize the catalyst performance.At the same time,the optimum process conditions of the reaction catalyst were explored.（1）When V/Mo=1:10,the conversion of toluene reached 86.2%,but the peroxidation phenomenon was serious.The effect of p H change on the activity of the catalyst was also obvious.The catalyst prepared under strong acidic conditions has poor solubility of polymolybdate,and V2O5 was enriched on the surface of the catalyst.At this time,the conversion of toluene reached a maximum of 90.8%.（2）With the increase of W/Mo atomic ratio,the conversion and selectivity of toluene showed an upward trend,and when W/Mo=2:5,the selectivity of benzaldehyde reached a maximum of 12.4%.It may be because W was beneficial to improve the dispersion of metal ions on the surface,which has a positive effect on the recrystallization of the catalyst,and the catalyst structure was more stable.The effect of p H on the W element modified catalyst is also obvious.The catalyst prepared under strong acidic conditions did not grow sufficiently,resulting in poor catalyst activity.The conversion of toluene was proportional to the reaction temperature,and the selectivity of benzaldehyde reached a maximum of 12.4%at 380 oC.The increase of benzene water ratio promoted the toluene conversion to increase first and then stabilize.When the space velocity increases,the toluene conversion rate remains stable,which indicated that the increase of space velocity has little effect on the toluene conversion rate under the condition of sufficient oxygen.The optimum space velocity should be controlled at 1200 h-1,and the ratio of oxygen to nitrogen should be above1:4.（3）The addition of complexing agent can affect the crystal form of catalyst and produce new active phase.The addition of complexing agent can also improve the dispersion of metal ions on the catalyst surface,so that the metal ions on the catalyst surface were more evenly dispersed,which was more conducive to the recrystallization of the catalyst.After calcination,the volatilization of the complexing agent in the form of gas will also affect the pore structure of the catalyst.The large pore size catalyst obtained by tartaric acid modification can make toluene pass through the internal pores of the catalyst quickly,which was conducive to the rapid separation of products and the prevention of excessive oxidation.The selectivity of tartaric acid modified catalyst to benzaldehyde can reach 60%. |
