Metathesis of 1-hexene over heterogeneous tungsten-based catalysts.
Olefin (alkene) metathesis can be used for the redistribution of carbon bonds to produce preferred higher range carbon number olefins (C10-C16) from low value medium chain olefins such as 1-hexene. In this study, the performance of various heterogeneous, tungsten based catalysts was investigated for the gas-phase linear cross metathesis of 1-hexene. The tested catalysts included tungsten trioxide on alumina, at various specific loadings, 8wt% tungsten trioxide on silica as well as 8wt% tungsten trioxide on silica, at various specific loadings of a potassium promoter. An existing lab-scale, fixed bed metathesis reactor system was used to conduct the required experimental work under steady state conditions. The experimental plan used for conducting the investigation of the alumina-supported catalyst was a combination of the One-Variable-At-a-Time (OVAT) approach as well as the factorial design method. The domain of the range under investigation was 8-20% for the specific loading of tungsten trioxide, 420-500oC for reaction temperature and 30-80 mol% for the feed composition of 1-hexene with constant space time. A half factorial experimental design was used for the WO3/SiO2 and potassium doped WO3/SiO2 catalysts. The potassium loadings of the 8wt% WO3/SiO2 catalyst were between 0.05-0.5wt%. Reaction temperatures of between 420-460°C were used together with 1-hexene feed compositions of between 60-80mol% and space times of 200-400g.min.mol-1. The WO3/Al2O3 catalyst was found to be an inferior catalyst for the metathesis of 1-hexene at all combinations of specific loadings and operating conditions tested. The highest yields obtained for both the detergent range olefins (C10-C16) and primary metathesis product decene (C10) were less than 1.5%. Changes made to the calcination temperature, calcination time and pH of the impregnation solution during the catalyst preparation stages had no significant effect on the yields produced. The WO3/Al2O3 catalyst was found to behave more as an isomerization catalyst rather than one for the metathesis reaction. The optimum reaction conditions determined when investigating the 8wt% WO3/SiO2 catalyst were found to be a reaction temperature of 460°C, a feed gas composition of 60mol% 1-hexene and a space time of 400g.min.mol-1. The conversion of 1-hexene, the yield of the detergent range olefins and the yield of decene at the above mentioned reaction conditions were 82%, 8.30% and 5.92% respectively Doping of the WO3/SiO2 catalyst with potassium was found to be successful in reducing the amount of isomerization and increasing the yields of both the detergent range olefins and decene by approximately 1.5% and 2% respectively when doping with 0.1 and 0.5wt% potassium. The experimental runs conducted were at the optimum reaction conditions obtained using the 8wt% WO3/SiO2 catalyst. The highest selectivity of the detergent range olefins (24.64%) and decene (23.62%) was obtained when using the 0.5wt% K doped WO3/SiO2 catalyst. At the optimised reaction conditions, the 0.5wt% potassium loading on WO3/SiO2 performed the best.