XAFS study on active iron sites in MCM-41 as a catalyst for liquid phase oxidation

Iron-containing mesoporous molecular sieves (Fe-MCM-41) synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods have been used as catalysts for the liquid phase oxidation of bulky organic compounds, i.e., anthracene and trans-stilbene, with diluted hydrogen peroxide. In the case of Fe-MCM-41-DHT, analysis of Fe K-edge XAFS revealed that isolated and tetrahedrally coordinated Fe species exist in the framework of MCM-41. On the other hand, the Fe-MCM-41-TIE mainly contains small iron oxide clusters, which have octahedrally coordinated Fe species. The tetrahedrally coordinated Fe species in Fe-MCM-41-DHT showed both high activity and efficiency of hydrogen peroxide in the liquid phase oxidations, whereas the iron oxide clusters in the Fe-MCM-41-TIE showed low activity. Moreover, the iron cations incorporated inside the framework of MCM-41 do not leach during the reaction, whereas the small iron oxide clusters leach out into the liquid phase and do not contribute to the catalytic reaction.