Kinetic hydrogen isotope effects in ethylene oxidation on silver catalysts

Abstract

Kinetic hydrogen isotope effects (KHIEs), (k)D/(k)H, for ethylene oxide formation, ethylene oxide combustion, and carbon dioxide formation in ethylene oxidation on silver catalysts were determined using rate constants of empirical reaction rate equations. These rate constants were obtained on pure Ag and K2SO4-Ag catalysts at 413–473 K and NaCl-Ag catalyst at 518–576 K keeping steady active states in the fixed bed flow reactions. On the Ag and K2SO4-Ag catalysts resulting in ethylene oxide selectivity values of 30.9–63.3%, KHIEs for total formation of carbon dioxide (0.43–0.61) were in disagreement with theoretical values of 0.25–0.30 for CH bond breaking reaction and also with the value of 0.73 for intramolecular hydrogen transfer reaction, while KHIE for ethylene oxide combustion coincided with the theoretical value 0.73. Consequently, the KHIE for total formation of carbon dioxide was considered to be fixed by the constructive ratio of the direct ethylene combustion route and the ethylene oxide combustion route, which have the CH bond breaking step and the intramolecular hydrogen transfer step as a rate-determining step, respectively. On the NaCl-Ag catalyst indicating ethylene oxide selectivity of 80.0–88.8%, KHIEs for carbon dioxide formation were 0.67–0.80, but no ethylene oxide combustion occurred; these results suggested that the rate-determining step of the direct ethylene combustion route was the intramolecular hydrogen transfer step. KHIEs for ethylene oxide formation on the Ag and K2SO4-Ag catalysts decreased from 1.99 to 1.19 with increase in ethylene oxide selectivity, while those on the NaCl-Ag catalyst were 0.86–1.14, of which the average value was almost 1.0. The KHIEs larger than 1.0 for ethylene oxide formation were explained as mainly due to an ensemble isotope effect caused by remarkable reduction in decomposition rates of combustion intermediates of ethylene and ethylene oxide on replacing of H by D. Additionally, three cycle mechanisms for the ethylene oxidation on silver catalysts were discussed.