Study On The Catalytic Performance Of Micron Vanadium-Molybdenum Composite Oxide For Ammoxidation Of Chlorinated Toluene

Aromatic nitrile has a wide range of uses and is used in the synthesis of plastics,coatings,dyes,etc.They are important intermediates in organic reactions and are prepared via a variety of strategies,in which the ammonia oxidation route can synthesize aromatic nitrile in one step.The core technology for preparing aromatic nitriles by ammoxidation is the development of highly selective and active catalysts.Most of the catalysts used in ammoxidation of aromatic hydrocarbon are supported catalysts,which are usually prepared by impregnation method or spray drying method.Silica gel is one of the most frequently used carrier.In the industry,Catalysts prepared using impregnation method mainly used in a fluidized bed reactor.The active site in the catalysts will loss due to mutual friction in the process of the reaction,leasing to the deactivation of catalysts.The nano-silica sol was used as the carrier to prepare micron-scale catalyst by the agglomeration of nano-particles at high temperature.Catalyst granules were formed by mixing support with catalyst together.The active sites are evenly distributed inside and outside of catalysts.The supported catalysts show good fluidized quality,which is beneficial for improving the activity of catalyst and prolonging the service life of catalyst.In this paper,to explore the effect of supports on the catalystic performance,both spray drying method and solvothermal method are used to prepare vanadium-molybdenum catalyst system.The details are as follows:Using V₂O₅and（NH₄）₆Mo₇O₂₄·4H₂O as raw materials,a single vanadium and two-component vanadium-molybdenum composite oxide catalyst with a Mo/V ratio of0 to 1 was prepared by spray drying.The catalyst was analyzed by various characterization methods and applied to the ammoxidation reaction of p-chlorotoluene,and the influence of Mo/V ratio on the structure and catalytic performance of the catalyst was investigated.The characterization shows that the catalyst is micron-sized spherical particles with a diameter of about 10-50μm,and the particle size gradually decreases with the increase of Mo/V ratio;The addition of molybdenum component can form an active V₂Mo O₈ crystal phase with vanadium,which can inhibit the deep oxidation of raw materials when catalyzing the ammoxidation reaction of p-chlorotoluene,and improve the yield and selectivity of p-chlorobenzonitrile;When Mo/V is 0.5,a single monoclinic V₂Mo O₈ crystal phase is formed.At this time,the catalyst has the best catalytic performance of ammoxidation,the conversion rate of p-chlorotoluene is close to 100%.The selectivity and the yield of p-chlorobenzonitrile are both above 80%.When the Mo/V ratio is less than 0.5,the catalyst will not only form a V₂Mo O₈ crystal phase,but also a V₂O₅ crystal phase with strong oxidability,which will cause deep oxidation of some raw materials and reduce the selectivity of p-chlorobenzonitrile.When the Mo/V ratio is greater than 0.5,the more oxidative Mo O₃crystal phase will be formed,and the product yield and reaction selectivity will also be reduced.Using V₂O₅ and（NH₄）₆Mo₇O₂₄·4H₂O as raw materials,the catalysts with Mo/V ratio of 0.5 and metal oxide content of 20%,30%,40%,50%,60%,70%and 100%were prepared by spray drying method.The catalyst was analyzed by various characterization methods and applied to the ammoxidation reaction of different chlorinated toluene,and the influence of metal oxide content on the structure and catalytic performance of the catalyst was investigated.The results show that the catalyst is in the form of micron spherical particles,and the particle size gradually decreases with the increase of the metal oxide content;when the metal oxide content is lower than50%,the catalyst will form a single monoclinic V₂Mo O₈ crystal phase with higher purity;When the content is 40-50%,the composite oxide has the highest crystal phase strength and a relatively large specific surface area.The catalytic ammoxidation reaction of p-chlorotoluene has relatively high activity,the conversion rate of p-chlorotoluene is close to 100%,and the yield and selectivity of p-chlorobenzonitrile are both above 80%;When the metal oxide content is higher than 50,the catalyst will form a mixed crystal phase of V₂Mo O₈ and V₂O₅.And as the oxide content increases,the catalytic activity gradually decreases.The catalyst Cat-M40 with a metal oxide content of 40%was screened to be used in the ammoxidation reaction of different chlorotoluenes.It was found that the catalyst can catalyze p-chlorotoluene,2,4-dichlorotoluene and 3,4-dichlorotoluene with high activity.The activity is the highest when catalyzing the ammoxidation of p-chlorotoluene,while the ammoxidation activity of o-chlorotoluene and m-chlorotoluene is relatively low.The chlorine-containing substituents at the ortho and meta positions will reduce the catalytic activity.Using V₂O₅ and（NH₄）₆Mo₇O₂₄·4H₂O as raw materials,a solvothermal method was used to prepare a vanadium-molybdenum composite oxide catalyst with a Mo/V ratio of 0.5.The catalyst was analyzed by various characterization methods and applied to the p-chlorotoluene ammoxidation reaction,and the effects of reaction time and calcination temperature on the structure and performance of the catalyst were investigated.The results showed that when the reaction time was 48h and the calcination temperature was 550℃,the catalyst Cat-48h formed a single monoclinic V₂Mo O₈ crystal phase.The catalyst is a stack of nanosheets to form a layered structure,and macroscopically,it has a micron-level rectangular structure with a length of 800nm and a thickness of 200 nm.When the catalyst was used in the ammoxidation reaction of p-chlorotoluene,Cat-48h catalyst showed the best activity,and the molar yield and selectivity of p-chlorobenzonitrile are both above 83%.The study also found that when the reaction time is short,in addition to the relatively high active V₂Mo O₈ crystal phase,there is also a strong oxidizing Mo O₃ crystal phase in the catalyst,resulting in a corresponding decrease in product yield and selectivity.