Metathesis and isomerization of n-butene and ethylene over WO3/SiO2 and MgO catalysts: Thermodynamic and experimental analysis

Abstract

The rapid increase in the demand for propylene and its derivatives requires urgently developing propylene-producing technology. To better understand the complicated reactions on W-based metathesis catalyst for propylene production and further to clarify the involved reaction mechanism, thermodynamic calculation was first performed to analyze the gas mixture under different conditions theoretically. The analyzed reaction system is close to the ideal gas under typical reaction conditions. The detailed investigation of reactions under different conditions was then carried out using WO3/SiO2 and MgO catalysts in a fixed-bed tubular micro reactor. Experimental results together with calculation demonstrate that temperature plays the most important role in regulating metathesis reaction compared with other factors such as pressure, ethylene content and weight hourly space velocity (WHSV). Also, ethylene content in the feedstock affects a lot on metathesis process. With an optimized ratio of ethylene/n-butene, cross-metathesis of 2-butene and ethylene is more likely to take place than that of 1-butene and 2-butene, and trans-2-butene has a generally better activity to react with ethylene than cis-2-butene due to the steric effect. On the other hand, isomerization is more independent on temperature compared with metathesis reaction. The consumption of 2-butene in metathesis will promote isomerization of 1-butene to 2-butene. Some extreme conditions such as higher pressures, larger ethylene contents and lower temperatures will provide opportunities for polymerization of ethylene. Based on the reaction analysis under different conditions, the reaction routes are summarized and a supposing mechanism for the formation of metallacyclobutane intermediate from trans/cis-2-butene is proposed.