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

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

V-MCM-41 for selective oxidation of propane to propene and acrolein

The V-MCM-41 with lower V content catalyzes the oxidation of propane to acrolein with a yield of 3.3% and selectivity of 20%, while that with higher V content mainly produces propene

Synthesis of V-MCM-41 by template-ion exchange method and its catalytic properties in propane oxidative dehydrogenation

Vanadium has been introduced to MCM-41 without collapse of the mesoporous structure by exchanging VO2+ ions in the aqueous solution with the template cations in the uncalcined MCM-41. This template-ion exchange (TIE) method provides tetrahedrally coordinated vanadyl species dispersed in the channel of MCM-41. Such synthesized V-MCM-41 shows higher catalytic activity in the oxidative dehydrogenation of propane than that prepared by direct hydrothermal method

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

Carbon fibers prepared by pyrolysis of methane over Ni/MCM-41 catalyst

The pyrolysis of CH4 to H-2 and carbon fibers over MCM-41 supported Ni catalyst has been investigated at different temperatures, The initial conversion of CH4 at 873 K is higher than it is at 773 K, but the deactivation at 873 K occurs rapidly. The investigations with SEM and TEM techniques indicate that the pyrolysis of CH4 generates mainly solid carbon fibers at 773 K but mostly hollow ones at 873 K on Ni/MCM-41. Ni metal particles have hardly been found on the tip of or inside the carbon fibers. Raman spectroscopic studies suggest that the carbon fibers formed at 873 K are more graphitic than those obtained at 773 K. The temperature-programmed reactions with O-2 (TPO) and H-2 (TPH) reveal that the solid carbon fibers formed at 773 K show higher reactivity towards O-2 and H-2 than the hollow ones formed at 873 K. (C) 2002 Elsevier Science Inc. All rights reserved

Cr-MCM-41 for selective dehydrogenation of lower alkanes with carbon dioxide

Cr-MCM-41 synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods is studied for dehydrogenation of lower alkanes including C2H6, C3H8 and i-C4H10 with CO2. Both methods lead to Cr species highly dispersed on the wall surface of MCM-41 and exhibited similar catalytic behaviors. Selectivity higher than 90% to each alkene has been achieved, and the presence of CO2 enhances the conversion of alkane

Leaching features of Fe-MCM-41 during epoxidation of alkene with hydrogen peroxide

Fe-MCM-41 prepared by direct hydrothermal (DHT) synthesis is effective for epoxidation of alkenes with H2O2, and shows unique leaching behaviors. The Fe content remaining in the catalyst after reaction is 0.8 wt%, close to that capable of entering the framework of MCM-41. This value is almost independent of the Fe content in the fresh catalyst and is not changed after repeated reactions. The Fe cations in the framework are responsible for the epoxidation and stable towards leaching