Epoxidation of styrene by anhydrous H₂O₂ over TS-1 and γ-Al₂O₃ catalysts: effect of reaction water, poisoning of acid sites and presence of base in the reaction mixture

Abstract

The styrene conversion and product (viz. styrene oxide, phenyl acetaldehyde, benzaldehyde) selectivity in the liquid-phase epoxidation of styrene by H₂O₂ (H₂O₂/styrene = 2) over TS-1 (Si/Ti = 80) and γ-Al₂O₃ are strongly influenced by the presence of water and/or base (viz. urea and pyridine) in the reaction mixture. The TS-1 showed high styrene conversion activity but no epoxide selectivity in the absence of any base. When anhydrous H₂O₂ (24% H₂O₂ in ethyl acetate), with the continuous removal of the reaction water (using the DeanStark trap), was used instead of 50% aqueous H₂O₂, both the conversion and epoxide yield are increased drastically for the γ-Al₂O₃, whereas for the TS-1, the increase in the conversion was quite small and there was also no improvement in the epoxide selectivity and/or yield. However, when urea or pyridine was added in the reaction mixture, the epoxide selectivity for both the catalysts was increased depending on the concentration of the base added; the increase in the selectivity was very large for the TS-1 but small for the γ-Al₂O₃. Poisoning of the acid sites of the γ-Al₂O₃ by the chemisorbed ammonia or pyridine (at 100 °C) caused a small decrease in the conversion, but it also caused a large decrease in the epoxide selectivity. However, the pyridine poisoning of the TS-1 caused a little beneficial effect, a small increase in the epoxide selectivity. The ammonia poisoning of the TS-1, however, resulted in a small decrease in the conversion with no improvement in the epoxide selectivity. As compared to the TS-1, the γ-Al₂O₃ catalyst showed a much better performance in the epoxidation by anhydrous H₂O₂ with the continuous removal of the reaction water. However, the reaction water, if not removed continuously, is detrimental to the γ-Al₂O₃, causing a large decrease in the catalytic activity and selectivity for styrene oxide but an increase in the selectivity for benzaldehyde.