Epoxidation of ethylene over AgNaCl catalysts

Abstract

Epoxidation of ethylene over sodium chloride-doped and granular sodium chloride-supported silver catalysts was performed at atmospheric pressure using conventional flow reactors. Although the stationary activity was somewhat low, each type of catalyst has shown high selectivity, 84–87%, under an ethylene-rich atmosphere. The selectivity was almost independent of reaction temperature, contact time, and catalyst composition. The total reaction rate rose exponentially with temperature until an apparent activation energy of 75.3 kJ mol −1 was obtained. Under oxygen-rich atmospheres, marked drops in activity were observed. In exposing the deactivated catalysts to a stream of hydrogen, carbon dioxide was desorbed. The strongly enhanced adsorption of carbon dioxide promoted the formation of silver chloride. Silver particles on the granular sodium chloride were homogeneously dispersed and ~200 nm in size. In addition, the crystallite size of silver was 43–50 nm, which was not altered after the reaction or even after heating at 673 K in oxygen gas. In a pulse reaction using an ethylene-rich gas mixture, acetaldehydes other than ethylene oxide were formed, and, as the catalyst surface had been oxidized by irreversible adsorption of oxygen, the formation of carbon dioxide was promoted and that of acetaldehyde decreased.