Katalitska aktivacija metana za pretvorbo v višje ogljikovodike z mikrokinetičnim opisom : doktorska disertacija

slovenian

doktorska dela

2022

activation   catalysis   methane   micro-kinetics   packed-bed reactor

Univerza v Mariboru, Fakulteta za kemijo in kemijsko tehnologijo

While the amount of obtained natural gas is increasing, especially at remote, inaccessible oil and gas veils, most of the methane remains underutilized. Due to its low price and the fact that oil reserves are limited, it is desirable to make a better use of methane. For the valorization of methane into higher value-added compounds, the most desirable route is as direct as possible. Two methods were investigated in this thesis. The first, most direct method was non-oxidative conversion of methane to hydrocarbons with two carbon atoms. To achieve this, in addition to methane, we only need a suitable catalyst. The second, more indirect method was bromination of methane to form methyl bromide, which can be converted to C2Hx hydrocarbons by coupling. Microkinetic models help to understand the reactions that occur heterogeneously (on the catalyst surface) and homogeneously (in the fluid). The reaction mechanism of such a model consists only of elementary reactions. If the microkinetic model and mechanism are accurate, the model is valid in any range of operating conditions. The main issues with the first method of methane valorization are mainly low stability (rapid deactivation) and selectivity of active catalysts.We attempted to prepare stable catalysts with zeolite supports loaded with different metals.The objective was to prepare a stable catalyst that would be suitable for modeling of microkinetics, and then develop a microkinetic model for the selected catalyst within a packed-bed reactor.The catalysts were prepared in various ways including; ion exchange, wet impregnation and, incipient wetness impregnation. All prepared catalysts suffered from some form of deactivation due to coke deposition. Catalysts prepared by ion exchange demonstrated the lowest activity. Catalysts prepared by wet impregnation method showed better activity, especially bimetallic Mo-Fe catalysts on HZSM-5. It was determined that the addition of molybdenum to iron improves the activity of the catalyst but negatively affects its stability. For molybdenum catalysts, the metal dispersion on the support was better than for iron catalysts. This was the result of MoO3 sublimation and migration into the interior of the pores, where molybdenum can also be ion exchanged with protons at Bronsted acid sites within the zeolite. Amongst the iron catalysts, the metal species on the surface of the zeolite were deposited in the form of nanoparticles. As a result of these nanoparticles, the activity of molybdenum-containing catalysts was improved, however, the pores of the zeolite clogged more rapidly during the reaction leading to faster deactivation relative to the iron catalysts. Even the most stable iron catalysts were not stable enough to be suitable for the development of an accurate and reliable microkinetic model. We also prepared a known catalyst for which a microkinetic model has not been developed yet. This catalyst contained individual platinum atoms acting as active sites on a cerium oxide support. This catalyst was stable during the reaction, although we also observed some coke formation. With this carefully selected catalyst, subsequent reactions were carried out in a packed-bed reactor under various reaction conditions and the obtained results were used in a microkinetic model that included all elemental reactions at relevant active sites. After the development of the microkinetic model, we did a sensitivity analysis. Thus, we identified the most important reactions. It turned out that the most important and difficult step was the activation of methane molecules, i.e. its binding to the active site and cleavage of the C-H bond. Intermediate reactions after coupling of the two CH3 radicals have a negligible effect to the overall reaction of combining methane into C2Hx hydrocarbons. Selectivity for ethylene, ethane and acetylene is most affected by adsorption and desorption reactions of products and methane.In the second part we investigated the bromina

07.08.2023