Propylene epoxidation on supported silver catalysts: Insights into reaction mechanism and effect of promoters and butadiene co-feed
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abstract
The gas-phase epoxidation of propylene to propylene oxide (PO) remains one of the most challenging chemical reactions, even though the epoxidation of ethylene by O 2 over Ag/a-Al 2O 3 catalyst has been the preferred industrial synthesis route for decades. The low selectivity for direct propylene epoxidation over silver catalysts is attributed to the vulnerability of the allylic hydrogen to abstraction by oxygen. Epoxidation of propylene and PO decomposition have been studied on unpromoted and Cs-promoted Ag/a-Al 2O 3 catalyst. Increases in promoter levels of Cs to 1000 ppm resulted in increased selectivity for PO with a simultaneous decrease in conversion. The conversion of propylene and selectivity towards PO decreased with increase in O 2/propylene ratio. Thus the highest conversion (0.95%) and highest selectivity (10.9%) were obtained under propylene-rich conditions (O 2/C 3H 6= 0.4, T= 573 K). On the 1000 ppm Cs promoted catalyst, co-feeding 1,3-butadiene with propylene results in increase in selectivity towards PO from 10.9 % to 36.6% with an increase in conversion from 0.95% to 2.5%. Since relatively little attention has been paid to the network of reactions that may follow oxygen addition to propylene on silver, we have also studied propylene oxide reactions on unpromoted and cesium-promoted silver. A more clear picture of the mechanism for propylene epoxidation on silver emerges from these experiments and shows that the reaction network that connects propylene with propylene oxide and other C3 oxygenated products can be explained in terms of reactions of surface oxametallacycle intermediates.
