Selective low temperature oxidation of methane with hydrogen peroxide by quasi-heterogeneous catalysis
Methane is our most abundant hydrocarbon. However, this natural resource is not being effectively utilised. Currently, its primary industrial use is the manufacture of methanol through the intermediate formation of synthesis gas\ud (H2 and CO). This is an energy intensive process requiring temperatures of up to 850 ⁰C and pressures of up to 100 atm. Clearly, a direct method of converting methane to methanol under mild conditions would provide many advantages.\ud Au-Pd/TiO2 has already been shown to be an effective catalyst for the oxidation of methane with H2O2 at low temperature (50 ⁰C). In this work, the intrinsic activity of the Au-Pd nanoparticles and the role of the support in this reaction are investigated. This is achieved by comparing, in detail, the activity of Au-Pd/TiO2 with that of unsupported Au-Pd nanoparticles. The unsupported nanoparticles are not only found to be active for this reaction but in fact display superior activity compared to the supported nanoparticles. The basis for this difference in activity is investigated and shown to be caused by an increase in H2O2 decomposition which occurs when the particles are supported thus reducing oxidant availability for the methane oxidation reaction. Further investigations\ud into the intrinsic activity of the unsupported Au-Pd nanoparticles, and the factors affecting it, are then carried out. A study was also carried out into the use of\ud methyl hydroperoxide (a methane oxidation product) in combination with H2O2 to activate methane. The first stage of this study was to establish a method of producing methyl hydroperoxide and to this end several copper catalysts were prepared and tested. Cu/ZSM-5 was selected for this purpose and was used thereafter to produce methyl hydroperoxide for subsequent reactions with methane without the presence of a solid catalyst. The reaction conditions for the methyl hydroperoxide system were optimised and some preliminary\ud investigations of the reaction mechanism were carried out.
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