Iron-catalyzed gas-phase epoxidation of propylene by N2O via halide-assisted oxygen transfer Original Research Article

The gas-phase epoxidation of propylene using nitrous oxide as an oxidant over bulk and supported iron-containing catalysts was studied. The presence of silica and an alkaline metal in both types of catalysts was found to be indispensable. Bulk catalysts formed in potassium–iron mixed silicates were studied, with respect to their crystalline structure, oxidation state and performance in propylene epoxidation. Over these catalysts a 69% selectivity to propylene oxide at 1.6% propylene conversion was reached. The presence of separated iron oxide phase was shown to promote the undesired allylic oxidation of propylene. Supported, halide-modified Fe-containing catalysts exhibit a superposition of two epoxidation routes. One, independent of air-pretreatment, shows similarity with that observed at bulky potassium–iron silicates; and the second, more selective, induced by air-pretreatment. The second mechanism, dominant in the first 30 min on stream, results in a maximum of 45.7% propylene conversion at 70% selectivity to propylene oxide. An improvement of the epoxidation activity was obtained, comprising reaction–reactivation switching steps. A reaction mechanism involving KCl as an oxygen transmitter and a metastable form of iron moieties is suggested.