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Three kinds of bulk iron phosphates were prepared via different methods and employed as model catalysts in the oxy-bromination of methane (OBM). NaFe(PO) was obtained via a fluoride route for the first time that showed even superior catalytic performance than FePO. Phase evolution from NaFe(PO) to NaFe(PO) after the OBM was confirmed both by X-ray diffraction and Fe Mössbauer spectroscopy. Temperature-programmed reduction of hydrogen and desorption of CHBr revealed that the redox capacity of iron phosphates were responsible for the generation of bromine radicals on catalyst surfaces while CHBr-involved reactions likely belonged to gas-phase reactions. Graphical Abstract: Model iron phosphate catalysts were synthesized by three different methods. It was found that the NaFe(PO) catalyst prepared by a fluoride route showed superior OBM performance than the other two catalysts both with the phase of FePO, especially for producing higher CHBr selectivity.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Iron phosphate catalysts possess a unique selectivity to oxidative dehydrogenation. Unlike ordinary catalysts consisting of molybdenum and/or vanadium, they have no double-bond oxygen species (M&z.dbnd6;O) that are considered to be responsible for the oxygen insertion function and, as a result, for the degradation by C–C bond fission. The defect of iron phosphate catalysts is a lack of oxidation activity. Addition of a very small amount of molybdenum into iron phosphate enhances markedly the oxidation activity without modifying the high selectivity originated from iron phosphate. The marked increase in the activity is ascribable to an increase in the reducibility of iron phosphate, because the reoxidizability of iron phosphate is high enough. It is proposed that the reaction over a Mo-doped iron phosphate is promoted by redox cycle of iron phosphate but not by that of molybdenum phosphate. [Copyright &y& Elsevier]

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 °C), while formaldehyde is mainly produced along with carbon oxides with oxygen at relatively higher temperatures (400–600 °C). 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. [Copyright &y& Elsevier]