Small phosphorus-modified molybdenum oxide clusters encapsulated inside the mesoporous channels of SBA-15 exhibit excellent catalytic performance for the partial oxidation of methane to formaldehyde with oxygen. A formaldehyde selectivity of 90% has been obtained at a single-pass methane conversion of 5.8%.

Iron phosphate supported on MCM-41 has been studied for partial oxidation of methane with both oxygen and nitrous oxide. Characterizations with XRD, Raman spectroscopy, XPS, and H2-TPR suggest that the supported iron phosphate species with loading amounts lower than 40 wt% are located and dispersed in the mesopores of MCM-41. Such iron phosphate species can be reduced more readily than the unsupported iron phosphate at lower temperatures. Methane is selectively converted to methanol, formaldehyde, and dimethyl ether over the supported and the unsupported iron phosphate with nitrous oxide at milder temperatures (300-500℃), while formaldehyde is mainly produced along with carbon oxides with oxygen at relatively higher temperatures (400-600℃). The supporting of iron phosphate onto MCM-41 with loading amounts of ca. 20-40 wt% increases both methane conversion and overall selectivity to useful oxygenates with either oxygen or nitrous oxide. Kinetic studies indicate that the activation of oxygen occurs rapidly, while the activation of nitrous oxide proceeds at a comparable rate with the conversion of methane by the active oxygen species over both the supported and the unsupported catalysts. The supported catalyst, however, enhances the activation of nitrous oxide and thus remarkably inhibits the carbon deposition occurring over the unsupported iron phosphate.

Iron-containing mesoporous molecular sieves (Fe-MCM-41) synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods have been characterized and used as catalysts for the liquid-phase epoxidation of styrene with diluted H2O2. The characterizations with XRD, diffuse reflectance UV-vis, ESR, andEXAFSsuggest that iron cations are highly dispersed and tetrahedrally coordinated with oxygen in the DHT samples with iron content lower than ca. 0.9-1.1 wt% (Si/Fe=105-86). This coordination environment is similar to that in a ferrisilicate zeolite with MFI structure and contributes to the increase in relative strong acidic sites, as indicated by NH3-TPD studies. On the other hand, the TIE samples mainly contain small iron oxide clusters. The conversion of styrene over the DHT catalyst increases significantly with increasing iron content up to 1.1 wt%. The selectivity to styrene oxide and the efficiency of H2O2 for the epoxidation can be improved by adding H2O2 slowly to keep the H2O2 concentration low during the reaction. As compared with the DHT catalyst, the TIE catalyst shows a much poorer performance for the epoxidation of styrene. It is suggested that the atomically isolated iron sites account for the epoxidation reaction with hydrogen peroxide. 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.

SBA-15-supported iron phosphate (FePO4) has been characterized with XRD, TEM, Raman spectroscopy and H2-TPR, and studied for the partial oxidation of CH4 with O2. The characterizations suggest that the FePO4 species with loading amount lower than 40 wt.% are encapsulated within the mesoporous channels of SBA-15, and these species possibly exist as small FePO4 clusters with the local coordination environment of iron being similar to that in the crystalline FePO4. However, these encapsulated FePO4 clusters could be reduced at remarkably lower temperatures than the crystalline FePO4. The SBA-15-supported FePO4 catalysts exhibit higher CH4 conversion and HCHO selectivity than the unsupported and the MCM-41-supported ones in the partial oxidation of CH4 with O2. The catalyst with a loading amount of 5 wt.% shows the highest HCHO selectivity at a given CH4 conversion and the highest HCHO formation rate based on the amount of FePO4 in the catalyst. It is likely that the improved catalytic performances of the SBA-15-supported samples are related to the enhanced redox properties of FePO4 species, the large porous diameter and the high inertness of SBA-15.

Magnetite iron oxide (Fe3O4) has been found to be an efficient heterogeneous catalyst for the epoxidation of alkenes by molecular oxygen in the absence of a sacrificial reductant among various transition metal oxides. The reaction probably proceeds via a radical mechanism.