Study of Cs-promoted, alpha-alumina-supported silver, ethylene-epoxidation catalysts .2. Effects of aging

The aging effects of an alpha-alumina-supported Ag, ethylene epoxidation catalyst have been examined as a function of reaction time in a Berty backmix reactor. The catalyst was prepared by impregnation of commercial alpha-alumina pellets (1.3 m(2)/g) in a Ag2O/lactic acid solution and contained 13.87 wt% Ag and 420 ppm Cs by weight. The fresh and used catalysts have been characterized using scanning electron microscopy (SEM), ion scattering spectroscopy, Anger electron spectroscopy, and X-ray photoelectron spectroscopy, With aging the fresh catalyst becomes more active (percentage of conversion to ethylene oxide increases), and then the activity drops with increasing reaction time. Large changes are observed in the spectra obtained from these surfaces which are due primarily to morphological changes caused by sintering as observed in the SEM micrographs. The surface Cl content is increased at the support surface by adsorption of ethyl chloride, added in small amounts to the feed gas as a moderator, from the gas phase. The surface also becomes enriched in Na due to migration of the Na binder material to the surface where it associates with the Cl to from NaCl. The SEM micrographs indicate that both the Ag and alumina crystallites grow with age and that the large Ag crystallites do not interact with the aged alumina surface suggesting that the formation of NaCl plays a role in the sintering process. (C) 1996 Academic Press, Inc.