The chemical role of rhenium in the epoxidation of ethylene and butadiene
Abstract
Supported silver catalysts promoted with Cs and Cl are known to be active for ethylene and butadiene epoxidation. Re has recently been shown to promote ethylene epoxidation (EO), but the details of its chemical role have not been fully determined. Therefore, we sought to understand the chemical role of Re in the steady state epoxidation of ethylene and butadiene by systematically varying the Re content of catalysts in the absence of Cl. A systematic study of the effect of catalyst calcinations temperature was also carried out. Catalysts were prepared by wet impregnation, calcination at 250, 350, and 500°C, and subsequent reduction. The catalyst performance was found to decrease in activity and increase in selectivity with calcination temperature. Increasing the calcination temperature removed nitrogen at 350°C and carbon at 500°C from the catalyst, thereby enhancing Cs and Re mobility on the catalyst surface. The removal of nitrogen compounds at 350°C gave the largest improvement in performance. A maximum in EO rate at 258ppm Re with a corresponding decrease in the CO2 rate was seen with increasing Re loading for catalysts calcined at 500°C. A maximum in oxygen order and a minimum in ethylene order were also observed at 258ppm Re. A slight decrease in the EO apparent activation energy, but no trend in the CO2 energy, was observed with increasing Re loading. Clearly, the primary role of Re is to block low selectivity sites, but at higher promotion levels, Re also blocks EO sites. Results of catalysts calcined at 250 and 350°C also support this conclusion. A dual site mechanism with a bimolecular and unimolecular rate-determining step for CO2 and EO, respectively, is consistent with rate data for catalysts calcined at 500°C. The role of Re was primarily to increase the activation barrier in the rate limiting step for CO2 formation, but a secondary increase in the enthalpy of ethylene and oxygen adsorption was also observed. The coverages of oxygen and oxygenated ethyl intermediates were found to increase with Re loading while a complementary decrease was observed in ethylene on epoxidation sites. The combustion sites were dominated by oxygen containing species, whose coverage did not vary with Re promotion level.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
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