Stability and distribution of cesium in Cs-promoted silver catalysts used for butadiene epoxidation

Abstract

Highly Cs-promoted Ag catalysts, such as those used for C 4H 6 epoxidation, undergo deactivation with respect to activity and selectivity for EpB formation upon storage in air. This loss in performance during storage in air is due to moisture-induced agglomeration of the Cs salt promoter. The agglomeration of the Cs promoter is reversible; calcination in air at temperatures between 200 and 300 °C efficiently redistributes the Cs over the Ag surface. For optimal performance, the Cs promoter must be evenly distributed on the Ag surface. Surface analysis by XPS confirms that calcination of such a deactivated catalyst between 150 and 300 °C increases the surface concentrations of both Cs and Cl (when CsCl is used as the promoter salt) while the concentration of surface Ag concurrently decreases. The strong positive correlation between the Cs/Ag surface ratio and the catalytic activity and selectivity supports the conclusion that an even distribution of Cs on the Ag surface is required for optimal performance of promoted catalysts for C 4H 6 epoxidation. The basis of the Cs-promoter effect is clearly electronic in nature. The fact that optimal distribution of Cs on the Ag surface increases both selectivity to EpB and activity for C 4H 6 conversion makes it very difficult to conclude that the role of Cs is only to block surface sites that lead to combustion of EpB to CO 2 and H 2O. This conclusion is also consistent with the earlier results of Monnier who showed that successful promoters for EpB catalysts were limited to Tl +, Cs +, and Rb +, but not K +. Within this series of promoters, the only noteworthy differences were not the ionic radii, but the Pauling polarizabilities, of these cations (the largest of any ions in the Periodic Table), which support the idea of electronic effects being responsible for Cs-promoted Ag catalysts for olefin epoxidation. Loss of activity upon storage for Cs-promoted Ag catalysts used for C 2H 4 epoxidation has not been reported. Catalysts used for C 4H 6 epoxidation contain much higher levels of Cs promoters (600–1400 ppm Cs) compared to catalysts used for C 2H 4 epoxidation (200–400 ppm Cs). Results in this paper show that catalyst deactivation due to storage in air is a strong function of Cs loading; deactivation for C 4H 6 epoxidation becomes negligible below approximately 560 ppm Cs, thus explaining why such deactivation has not been reported for catalysts typically used for C 2H 4 epoxidation.