Research Of VPO/SiO₂and VCrO Catalysts For Ammoxidation Of Chlorine-substituted Toluenes

Aromatic nitriles are important fine chemical intermediates, which are widely used in various fields of national economy. Gas phase ammoxidation was considered to be one of the best way to produce aromatic nitriles. The key to industrialize ammoxidation is the development of catalyst. In this work, VPO and VCrO ammoxidation catalysts were studied, using chlorinated toluene as ammoxidaiton substrates, and have got some conclusions as follows:(a) It was known from the experimental data of ammoxidation reactions, use of support and addition of a moderate amount of phosphorus could improve the catalytic activity of VPO catalysts. The results of characterization of the catalysts showed that use of support and addition of a moderate amount of phosphorus could decrease the reduction temperature of V(Ⅴ), and cause positive effect on the oxidative dehydrogenation step of ammoxidation reaction; the use of support could dramatically decrease the oxidation temperature of V(Ⅲ) and V(Ⅳ), and thus was conducive to the regeneration process of active center of the catalysts. The addition of phosphorus in an appropriate scope had the effect of improving the acidity VPO catalysts. The acidity of the catalyst reached the highest value (1.010mmol/g) when V/P was1/1.6.The higher the acidity of VPO catalysts, the more amount of ammonia can be absorbed, and thus more nitrogen insertion species can be provided for ammoxidation reaction, as a result, the selectivity of the product becomes higher.(b) The specific surface and activity of aerosil supported VPO catalysts can be improved by use of alcohol as the dipping solvent instead of water (VPO/CW)in the preparation stage. The yields of p-chlorobenzonitrile,3,4-dichlorobenzonitrile,2,4-dichlorobenzonitrile and2,6-dichlorobenzonitrilewere84.02%,82.28%,73.22%, and70.42%use CW116as catalyst, and the yields improved to91.22%,87.35%, 80.89%and78.83%use CA116as catalyst.(c) Sphere-like V-Cr-O catalyst was successfully synthesized by a facile one pot hydro thermal approach for the first time. The diameter of the as-obtained VCrO catalyst was4μm. The sphere-like structure was conducive to the diffuse of reaction materials; the specific surface was as high as117m2/g, and thus can provide many active centers; the oxidation temperature as well as reduction temperature of V in the VCrO catalyst were much lower than in VPO catalysts; TPD results revealed that the VCrO catalyst exhibited more strong acid centers than VPO catalysts. It was found that it had excellent performance in the ammoxidation reactions, specially, the catalytic temperature was decreased to about593K, which was much lower than the previous reports (653K～693K).