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

Fe-MCM-41 for selective epoxidation of styrene with hydrogen peroxide

Fe-MCM-41 prepared by direct hydrothermal synthesis catalyzes the epoxidation of styrene with diluted H2O2. Th

Vanadium-containing MCM-41 for partial oxidation of lower alkanes

Vanadium-containing mesoporous molecular sieves synthesized by both template-ion exchange (TIE) and direct hydrothermal (DHT) methods have been studied for partial oxidation of lower alkanes. UV-vis and in situ laser Raman spectroscopic studies suggest that the former synthetic method can provide tetrahedrally coordinated vanadium species mainly dispersed on the wall surface of MCM-41, while the latter method leads to vanadium sites mainly incorporated into the framework of MCM-41. H-2-TPR measurements show that the vanadium species in the TIE samples can be reduced at lower temperatures than those in the DHT samples. NH3-TPD investigations suggest that weak acid sites mainly exist over MCM-41 along with a small amount of medium ones. The introduction of vanadium by the TIE method increased the amount of weak acid sites, while both weak and medium acid sites of MCM-41 are decreased with introducing vanadium up to a certain content by the DHT method. In the oxidations of ethane and propane, the alkane conversions increase remarkably with increasing vanadium content, and moderate selectivities to ethylene and propylene are obtained over the TIE catalysts. The same catalysts, however, are not selective for the oxidative dehydrogenation of isobutane. On the other hand, propylene and isobutene are obtained with high selectivity over the DHT catalysts with vanadium content exceeding 1 wt% in the oxidations of propane and isobutane, respectively. Acrolein and methacrolein can also be formed respectively with considerable selectivity over the DHT catalysts with lower vanadium content. It is likely that the medium acid sites that remained in these samples play roles in the formation of oxygenates through the adsorption of alkenes or allylic intermediates. (C) 2001 Academic Press