Preparation And Evaluation Of WO₃/SiO₂ Catalyst For Olefin Metathesis To Propylene

WO₃/SiO₂ is the only industrialized catalyst for olefin metathesis to propylene.It is less costly,easier to prepared,more anti-poisoning and harder to carbon deposition compared with Mo-or Re-based metathesis catalysts.However,the higher reaction temperature it required and its lower activity at low temperatures cause a widespread improvement of WO₃/SiO₂ catalyst.In this paper,we started our study with the selection of silica support.Then,the effects of catalyst particle size,preparation method,WO3 loadings and additives were researched in details.Furthermore,the stability of the WO₃/SiO₂ mixed with MgO catalyst was tested.We hope our work will offer a reference for the further optimization of WO₃/SiO₂ catalyst.Firstly,we chose several kinds of commercial silica gel as suppots to prepare WO₃/SiO₂ catalysts using impregnation method.It was found that the overall olefin metathesis performance of each catalyst is closer.Owing to the lower acidity,the catalyst which was prepared with high purity silica exhibits a higher propylene selectivity.Other factors that may affect the WO₃/SiO₂ metathesis performance during the preparation procedure were investigated,such as support-crushing method,catalyst particle size,impregnation method and WO3 loadings.The results show that support-crushing method has little effect on catalyst metathesis performance.The catalysts of different particle sizes within a certain range have samilar metathesis performance,incipient wetness should be the best choice for preparing WO₃/SiO₂ because the process is simple and the catalyst possesses good performance.The optimal WO3 loading is 6wt%⁸wt%.Then we investigated the influence of different additives on WO₃/SiO₂ catalyst and studied metathesis performance of bimetallic catalyst.It was found that with MgO,SiO2 and ZrO2 added,activity of WO₃/SiO₂ could remained at a high level.The reaction system had the best activity,selectivity and stability when WO₃/SiO₂ physically mixed with the modified MgO.The metathesis performance of bimetallic catalyst is worse than WO₃/SiO₂ catalyst mixed with MgO catalytic systems.Finally,we did stability tests on the catalytic system of WO₃/SiO₂ physically mixed with modified MgO.When mixed C4 and ethene were used as the feedstock,the n-butene conversion rate fell by 1.3% and the propylene selectivity remained unchanged within 72 h.In order to eliminate the influence of the complicated composition and impurities in mixed C4,cis-2-butene and ethene were also used as the feedstock.We found the reaction system showed an excellent stability,and the cis-2-butene conversion rate and propylene selectivity maintained at a stable level within 144 h.It should be noted that no matter which kind of feedstock was used,the 1-butene yield increased gradually during the reaction,except in the reactions those catalyzed by industrial WO₃/SiO₂ catalyst and WO₃/SiO₂ catalyst prepared from high purity silica.It is presumably due to some impurities contained in the silica support which will increase the isomerization activity of WO₃/SiO₂ catalyst.