The mechanism of ethylene epoxidation

Abstract

The mechanism of the epoxidation of ethylene with oxygen over silver catalysts has been examined. This study was prompted by the notion that if one wishes to increase the selectivity of epoxide formation one should know the phase of adsorbed oxygen that reacts to give epoxide. Experiments were undertaken (a) to determine whether adsorbed atomic oxygen is incorporated into ethylene to give epoxide, and (b) to establish the relative rates of reaction of molecular oxygen impinging from the gas phase and of preadsorbed atomic oxygen. Experiments were performed in a high-vacuum recirculation system designed to allow volumetric absorption studies as well as kinetic studies using a mass spectrometer to be made. The catalyst consisted of silver powder precovered with chemisorbed 16O such that it gave epoxide upon reaction with ethylene. It was contacted with ethylene/ 18O 2 gas mixtures of different ratios. It is shown that under conditions where oxygen isotope equilibration in the gas phase is slow, ethylene initially reacts more rapidly with preadsorbed oxygen atoms than with molecular oxygen adsorbed in the precursor state ( Po 2 < 20 Torr; 1 Torr = 133.3 N m −2) if O ads Ag s ∼ 1. If in the begining only subsurface oxygen atoms are present, this subsurface oxygen and gas-phase oxygen are incorporated into ethylene at about equal rates. It is concluded that ethylene reacts with atomic oxygen to give epoxide either directly, or indirectly, after recombination of the oxygen atoms.