Partial methane oxidation: insights from first principles and micro-kinetics calculations
Sammanfattning: Partial methane oxidation is a much-desired reaction with some intriguing challenges. Not only is there a need to activate methane and oxygen, but there is also a need to control the selectivity and prevent over-oxidation to thermodynamically more stable products, like carbon dioxide and water. In fact, this is so difficult that at many oil extraction sites, methane, which inevitably accompanies the welled crude oil, is today flared since gas-phase methane is too inconvenient to store and transport. In nature, there are enzymes that can partially oxidize methane to methanol at ambient pressure and temperature, although at a very slow rate. An often studied class of material with the potential of being an inorganic analogue to these enzymes are zeolites. Zeolites are a porous class of material that can readily be synthesized and that have been shown to convert methane to methanol at ambient conditions with a high selectivity, but with a low conversion. Unravelling the bottlenecks of this, as of yet, inefficient reaction, calls for an atomistic understanding of what is in fact controlling activity and selectivity of the catalysts at hand. In this thesis, zeolites and chemically related structures, zeotypes, are studied using first-principles calculations combined with micro-kinetic modelling. As a first step, a candidate for the active site in these materials, the [Cu-O-Cu]2+ motif, which is found primarily in the ZSM-5 zeolite, and its relevance for Cu, Ni, Co, Fe, Ag, and Au is investigated. Using a straightforward first-principles based micro-kinetic model, we find that this motif is only relevant for copper. Vibrational IR-spectra and temperature programmed desorption spectra are also calculated for monomer and dimer copper motifs in the ZSM-5 and SSZ-13 zeolites, and the results support experimental conclusions. When studying the continued reaction of methanol to dimethyl ether, large-scale trends in activity correlated to the acidity of the acid sites in three zeolite framework types have been determined, indicating that tuning acidity will change the selectivity between methanol and dimethyl ether. The partial oxidation of methane is also studied using molybdenum sulfide clusters, Mo6S8 . These small clusters enables studies of a wider reaction network, similar to the one in zeolites, where the oxygenated species are replaced by their sulfur-contaning counterparts. In this way, it allows investigation of the activity and selectivity towards methanethiol and dimethyl sulfide using different reaction mechanisms and different promoters. The reaction of methane with H2S is used when cleaning sour natural gas, which is why, in this case, H2S is used as the oxidant instead of oxygen. Our results show that the presence of some promoters on the sulfide clusters affect activity, while others affect selectivity. Furthermore, the results show that diffusion is important to include in the kinetic model.
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