Study On The Ammoxidation Of Chlorotoluene Catalyzed By Micron Vanadium-Chromium Composite Oxides Catalyst

Aromatic nitriles are fine chemicals widely used in synthetic pesticides,medicine,plastics,dyes and paints and so on.They are usually produced through ammoxidation reaction,which is the most economical and convenient industrial process for aromatic nitriles.The key for such process is to develop a catalyst with active components compatible with the substrate.At present,the catalysts for the ammoxidation of aromatic hydrocarbons are mostly supported catalysts（commonly on silica gel）,which can be prepared by impregnation or spray drying.By using impregnation method,the active component is dispersed on the surface of the carrier.When applied in the fluidized bed,the active component may be lost due to friction,leading to the decrease of the service life.On the other hand,ammoxidation reaction is a strongly exothermic;while the catalyst prepared by impregnation method has a large particle size and poor fluidization quality,leading to local overheating,decrease of selectivity and fast loss of active components.In the spray drying method,nano-silica sol is used as the carrier,and micron-sized catalyst is produced by agglomeration of nanoparticles at high temperature.The carrier and the active component form the catalyst particles through"mixing",and the active component is evenly distributed inside and out,forming fine particles with good fluidization quality,thus may overcome the aforementioned disadvantages of impregnation method,improve activity,increase the load and extend the service life of the catalyst.In this paper,vanadium-chromium composite oxide catalyst was prepared by spray drying method.The main contents are as follows:Large particle size silica sol（L30）,acidic silica sol（H30）,special-purpose silica sol carrier（S30）,ammonium silica sol（NH₃30）and sodium silica sol（Na30）were selected as the catalyst carrier,and five supported VCrO catalysts with Cr/V ratio of 1:1 and metal oxide content of 50%（wt）were prepared by spray drying.The catalysts were characterized by XRD,XPS,BET,FT-IR and TPR,and used for the ammoxidation of p-chlorotoluene.The results showed that the different substrates had certain influence on the crystal phase,the metal oxidation state,the dispersion degree of active components,the specific surface area,and the reduction of vanadium species,thus affected the activity and the selectivity.For the catalysts prepared by H30,S30 and NH₃30,only monoclinic CrVO₄ was formed,which was the active phase of the catalytic reaction.When used for the ammoxidation reaction of p-chlorotoluene,all of them showed excellent catalytic performance,with the yield of p-chlorobenzene nitrile and selectivity over 74%and 75%,respectively.However,for the catalysts prepared by L30 and Na30,the activity was quite low,due to the poor dispersion of vanadium on the surface,the small specific surface area and pore volume,and the difficulty in reducing vanadium species.In addition,the catalyst prepared by Na30 also formed some orthorhombic CrVO₄ species with no catalytic activity,further reducing the catalytic activity.Catalysts with metal oxide content of 50%（wt）while of different Cr/V ratios were prepared by spray drying method using silicon sol（S30）as the carrier.The catalysts were characterized by XRD,XPS,FT-IR and TPR,and were used to catalyze the ammoxidation reaction of p-chlorotoluene.The results showed that the Cr/V ratio greatly influenced the crystal phase of the catalyst and the catalytic performance.The addition of Cr inhibited the formation of the more oxidative V2O5 crystal phase,promoted the formation of highly active CrVO₄ species,and improved the dispersion of the active component.With the increase of Cr/V ratio,the ratio of V⁴⁺/V⁵⁺increased,the activation energy required for the reduction of vanadium species decreased,thus the catalytic activity may be improved.When the Cr/V ratio was 1:1,the catalyst showed the best catalytic performance,with the conversion of p-chlorotoluene of 98.11%,the molar yield of p-chlorobenzene nitrile and selectivity of 78.22%and 79.73%,respectively.When Cr/V ratio was less than 1,mixed phases of monoclinic-CrVO4,orthorhombic-CrVO4 and monoclinic-Cr2V4O13 were formed,resulting in low catalytic activity.A Cr/V ratio greater than 1 led to the formation of hexagonal-Cr₂O₃ crystalline phases with strong oxidizing ability resulted in excessive oxidation and decrease of selectivity.Catalysts with Cr/V ratio of 1:1 and different metal oxide contents were prepared using silica sol（S30）as the carrier by spray drying method.The catalysts were characterized by XRD,XPS,TPR,BET,FT-IR,and were used to catalyze the ammoxidation reaction of p-chlorotoluene.The results showed that only monoclinic crystal CrVO₄ was formed with the metal oxide content within the range of 30%⁷0%.The increase of metal oxide content reduced the average valence state of vanadium,increased the ratio of V⁴⁺/V⁵⁺,and reduced the activation energy,which was conducive to the redox cycle in the catalytic reaction and improved the activity of the catalyst.When applied to the ammoxidation reaction of p-chlorotoluene,the catalyst with a metal oxide content of 60%showed the best catalytic performance.Under the optimal reaction conditions,the conversion of p-chlorotoluene was99.76%,the molar yield of the product and selectivity was 87.44%and 87.65%,respectively.At the same time,it was found that the catalysts with 60%metal oxide content also showed excellent catalytic performance for toluene,o-chlorotoluene and m-chlorotoluene.The sequence of the reactivity of Toluene and O-,M-,P-chlorotoluene was as follows:P-CT>O-CT≈Toluene>M-CT,suggesting that reactivity was influenced by both the electronic effect and steric effect.