Effects of Cesium and Chlorine on Butadiene Adsorption on Ag/α-Al2O3 Catalysts

Abstract

Integral heats of adsorption, Qad, were measured at 300 K for butadiene on unpromoted and promoted Ag/α–Al2O3 catalysts with average Ag particle sizes in the range of 250–350 nm. The promoted catalysts mimic those used commercially and were prepared using either CsNO3 to study the effect of cesium or CsCl to study the combined effect of cesium and chlorine, and the Cs content was maintained at either 400 ppm or 1200 ppm. Prior to measurement of either the butadiene uptake or the corresponding energy change, the catalysts were subjected to a pretreatment involving calcination at 523 K for 2 h followed by reduction in H2 at 473 K for 1 h. The butadiene measurements were made on either a clean reduced or an O-covered Ag surface, with the latter containing either a monolayer or a half-monolayer of oxygen after adsorption at 443 or 300 K, respectively. Butadiene adsorption on reduced Ag surfaces was almost all reversible, but the very low irreversible uptakes gave Qad values ranging from 130 to 250 kcal/mol, thus indicating that more than molecular adsorption was occurring. For the unpromoted catalyst, the amount of irreversibly adsorbed butadiene increased as the extent of the surface precovered by oxygen increased, and apparent Qad values for irreversibly adsorbed butadiene on the O-covered surfaces were much lower compared to those for the reduced surface, with the lowest Qad value of 13 kcal/mol occurring for the surface that was half covered by oxygen at 300 K. The amount of irreversibly adsorbed butadiene on O-covered surfaces decreased only slightly and was the same when the catalyst was promoted with 400 ppm of cesium via either CsNO3 or CsCl; however, Qad increased to 84 kcal/mol when CsNO3 was used, but decreased to 33 kcal/mol with CsCl, compared to 64 kcal/mol for the unpromoted sample. Both the amount and the Qad value for irreversibly adsorbed butadiene on O-covered Ag surfaces dropped when the amount of Cs was increased to 1200 ppm, and with the CsCl-promoted catalyst almost all the butadiene was reversibly adsorbed. Adsorption of 1-butene on the unpromoted Ag catalyst indicated lower uptakes compared to butadiene, but much higher heats of adsorption. This implies that certain aspects of the bonding of 1-butene and butadiene on the supported catalyst are different. Differences in the adsorption of 1-butene and butadiene on unsupported Ag were less apparent, possibly due to the low uptakes of butadiene on the low-surface-area Ag powder. Both the high Qad values and the frequent exothermic tail during calorimetric measurements appear to be associated with butadiene dissociation or dimerization reactions on the Ag surface.