HIGH SURFACE AREA ALPHA-ALUMINAS .3. OXIDATION OF ETHYLENE, ETHYLENE OXIDE, AND ACETALDEHYDE OVER SILVER DISPERSED ON HIGH SURFACE AREA ALPHA-ALUMINA

摘要

Silver nanocrystallites dispersed on high surface area (HSA) alpha-alumina were prepared, and the activity and selectivity to ethylene oxide (EO) during ethylene oxidation over these catalysts were determined in the absence of Cl promoters. Cesium was also added to some of the samples to neutralize any surface acidity. Over all the Ag/(HSA) alpha-Al2O3 catalysts, the selectivity to EO was negligible regardless of the addition of cesium while the apparent activation energy for carbon dioxide formation of 110 kJ mol(-1) over the unpromoted samples was higher than that of 80 kJ mol(-1) over a low surface area (LSA) alpha-alumina-supported silver catalyst. When this latter catalyst with selectivity to EO was mixed with (HSA) alpha-Al2O3, only carbon dioxide and water were obtained as a final product. EO isomerization and oxidation as well as acetaldehyde oxidation were also studied, and both silver and the support were found to be active in these reactions; hence these (HSA) alpha-aluminas are poor supports for ethylene epoxidation. In contrast to a promoting effect obtained at low concentrations in commercial catalysts, the higher loadings of cesium on these Ag/(HSA) alpha-Al2O3 catalysts increased the turnover frequency of ethylene oxidation as well as the rates of EO and acetaldehyde oxidation, while the apparent activation energy for ethylene combustion decreased to 80 kJ mol(-1) in the presence of cesium. [References: 27]