Nano-vanadium-chromium Composite Oxides And Their Catalytic Performances For Ammoxidation Of Chlorotoluenes

In recent years,with the improvement of people’s living standards,chemical products have been used in many aspects of people’s lives.As an important organic synthesis intermediate,aromatic nitriles are widely used in dye,medicine,pesticide,perfume,and materials industries.Ammoxidation is the primary method for synthesizing aromatic nitriles,and the key is to develop green and environmentally friendly catalysts with high conversion rate,good selectivity,long service life,simple synthesis procedure,which can be widely applied to different substrates.In this paper,nano-vanadium-chromium composite oxides with different vanadium-chromium ratios and various crystal structures were synthesized by hydrothermal method,and then used to catalyze the ammoxidation of chlorotoluenes.The main results are as follows:1.The composite oxides with a vanadium and chromium atomic ratio of 2:1 were synthesized by a simple hydrothermal method with V₂O₅ and Cr O₃（1:1 molar ratio）as raw materials and tartaric acid as a reducing agent.The oxides were characterized by XRD,XPS,SEM,TEM,TG,BET,FI-IR,and other methods.It was found that the precursor formed a flower-like structure after hydrothermal reaction for 48 h,and then converted to single Cr₂V₄O₁₃ crystal phase（Cr V₂-H48-600）after being calcined at600°C,of which hollow flower-ball structure with average diameter of 559 nm was formed by self-assembly of nanosheets with a length of 100-500 nm,a width of 20-100 nm and a thickness of 10-50 nm.The vanadium was pentavalent and chromium was trivalent in Cr V₂-H48-600,and the ratio of vanadium and chromium on surface was consistent with the compound composition.Further more,the reduction temperature of pentavalent vanadium in Cr V₂-H48-600 was significantly reduced.Cr V₂-H48-600 was used to catalyze the ammoxidation of o-chlorotoluene to prepare o-chlorobenzonitrile.Under the conditions of reaction temperature 683 K,ammonia ratio 4,air ratio 20,and space velocity 261 h^-1,the conversion of o-chlorotoluene reached 85.4%,the molar yield and the selectivity of o-chlorobenzonitrile reached 74.9%,and 87.6%,respectively.When catalyzing the ammoxidation of different chlorotoluenes,the catalytic activity order was:p-chlorotoluene>3,4-dichlorotoluene>o-chlorotoluene>2,6-dichlorotoluene,among which the molar yields for the ammoxidation of p-chlorotoluene and 3,4-dichlorotoluene were higher than 85%,and the selectivities were more than 90%.The vanadium-chromium composite oxide showed good catalytic performances for the ammoxidation of chlorotoluenes.Furthrt more,the ammoxidation activities of chlorotoluenes were affected by the synergistic effect of the steric hindrance and electronic effect,with steric hindrance being the dominant factor.2.The 1:1 composite oxide precursors of vanadium and chromium were synthesized through hydrothermal reaction at 180°C with V₂O₅ and Cr O₃ as raw materials,and ethanol,tartaric acid,citric acid or oxalic acid as reducing agents.The precursors were calcined in air to obtain vanadium-chromium composite oxide,which were then characterized by XRD,XPS,SEM,TEM,BET,FI-IR,and other methods.It was found that when ethanol was used as the reducing agent,the composite oxide was a mixed-crystal phase and the peak intensity was low.While using tartaric acid,the resulting composite oxide was a pure monoclinic Cr VO₄ crystal phase,and when oxalic acid and citric acid were used,orthorhombic Cr VO₄ were obtained.Further more,the structure and morphology of the composite oxide obtained with tartaric acid were more regular with a particle size of 300 nm,and as the hydrothermal reaction time was extended,the crystal phase structures were unchanged,but the particle sizes gradually increased.The calcination temperature also affected the diffraction intensity of the composite oxides.A pure composite oxide crystal phase was obtained at 700 ^oC with the highest peak intensity.The performances of these composite oxides as catalysts to catalyze the ammoxidation of o-chlorotoluene were studied.The performances of monoclinic Cr VO₄ were better than that of orthorhombic Cr VO₄.Under the conditions of temperature 410 ^oC,space velocity 261 h^-1,and the molar ratio of chlorotoluene to ammonia and air 1:4:20,the conversion of o-chlorotoluene was 94.1%,the molar yield of o-chlorobenzonitrile reached 81.7%,and the selectivity was 86.8%.The Cr-V composite oxides showed a high level of reactivity among the binary composite oxides.3.The 1:1 composite oxide precursors of vanadium and chromium were synthesized by hydrothermal method with ethanol,tartaric acid,citric acid or oxalic acid as reducing agents,then calcined in a nitrogen atmosphere to obtain vanadium-chromium composite oxides.The oxide structures were characterized by XRD,XPS,SEM,BET,FI-IR,and other methods.It was found that the air and nitrogen atmospheres during calcination had great influences on the crystal phase structure of the composite oxides.The reducing agents had little effect on the crystal phase-type under nitrogen atmosphere,but influenced the diffraction intensity and purity of the crystal phase.When tartaric acid,citric acid or oxalic acid was used as reducing agents,Cr VO₃ crystal phases were obtained in all cases,but pure Cr VO₃ composite oxide crystal phases were obtained with the highest diffraction intensity only for tartaric acid.Further more,when using tartaric acid as the reducing agent,both the calcination temperature and hydrothermal reaction time had great impact on the crystal phase of the composite oxides.The crystal structures of the composite oxides were different at different calcination temperatures,and the pure Cr VO₃ crystal phase with high intensity was obtained when calcination at 700 ^oC.The hydrothermal reaction time also had great influences on the crystal structures.By extending the reaction time,the crystal phases of the composite oxides changed from a mixed crystal phase mainly containing Cr VO₃→a pure Cr VO₃ crystal phase→a mixed crystal with mainly orthorhombic Cr VO₄ Phase→orthogonal and monoclinic Cr VO₄mixed crystal phase→pure monoclinic Cr VO₄ crystal phase.The composite oxides obtained at hydrothermal reaction time of 12,24,36,48 and 72 hours,hereby denoted as CV-T12-700N,CV-T24-700N,CV-T36-700N,CV-T48-700N,and CV-T72-700N respectively,were used as catalysts for the ammoxidation of o-chlorotoluene.It was found that the catalytic activity gradually increased with the extension of the hydrothermal reaction time.The conversion,the yield and selectivity at a hydrothermal reaction time of 12 and 24h were relatively low,whereas the catalytic activities of the oxides at a hydrothermal reaction time from 36 to 72h were relatively high.Among these,the most active CV-T72-700N achieved a conversion of 92.2%for o-chlorotuluene,and the yield and selectivity of o-chlorobenzonitrile were 81.2%and 88.1%,respectively.It showed that the activity of the Cr VO₄ crystal phase was significantly higher than that of the Cr VO₃ crystal phase,and the pentavalent vanadium in the composite oxide was more conducive to the catalytic reaction cycle than the trivalent vanadium.The composite oxide obtained at a hydrothermal reaction time of 12h showed the lowest catalytic activity,which was mainly due to the formation of VO₂ and the Cr VO₃ crystal phase,both of which were of low activity.The composite oxide CV-T48-700N obtained at a hydrothermal reaction time of 48h had lower reactivity than CV-T36-700N and CV-T72-700N,because the CV-T48-700N particles fused to a coral-like structure,leading to significant increase of the particle size and decrease of the specific surface area,hence a decrease of the number of exposed active sites and the activity.Through this research,it was found that the two-component vanadium-chromium composite oxides synthesized by the hydrothermal method had high reactivity for the ammoxidation of chlorotoluenes.The synthesis conditions greatly influenced the crystal structure and morphology of the oxides,thereby affecting the reaction activity.This work may provide the theoretical basis for the research and development of ammoxidation catalysts for chlorotoluenes.