Iron-catalyzed propylene epoxidation by nitrous oxide: Toward understanding the nature of active iron sites with modified Fe-MFI and Fe-MCM-41 catalysts

Alkali metal salt (KCl)-modified Fe-MFI and Fe-MCM-41 containing iron species in different locations have been studied for the epoxidation of propylene with nitrous oxide. In the allylic oxidation of propylene without modification, the samples with iron species in the framework positions showed slightly higher activity for the formation of acrolein and allyl alcohol. After modification with KCl, propylene oxide (PO) became the main product. Significant differences in PO formation activity were observed between the samples containing iron in different locations. For the Fe-MFI and the Fe-MCM-41 prepared by the direct hydrothermal (DHT) method, in which iron was located mainly in the framework position, much lower activity was obtained after KCl modification. In contrast, the Fe-MFI after steam treatment and the Fe-MCM-41 prepared by template ion-exchange (TIE) and conventional impregnation methods, which contained extra-framework iron species (e.g., FeO_x clusters), exhibited higher PO formation activity after modification. A PO selectivity of 80% could be achieved at a propylene conversion of 3.3% over the KCl-modified Fe-MCM-41 prepared by the TIE or the impregnation method at 598 K (time on stream, 70 min). The modification with KCl increased the dispersion of the extra-framework iron species. Consequently, highly dispersed extra-framework iron species, probably in tetrahedral coordination, are proposed for the epoxidation of propylene with nitrous oxide.