Insights into Complex Reaction Processes Governing Hydrocarbon Conversion through Microkinetic Modeling

Marsden, Grant Thomas

doi:https://doi.org/10.21985/n2-pc1g-ba76

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Insights into Complex Reaction Processes Governing Hydrocarbon Conversion through Microkinetic Modeling

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Shale gas is a critical energy resource that is comprised primarily of light gases that are expensive to transport. Because these gases are geographically spread-out and there is insufficient capacity to transport them to centralized processing facilities, they must often be flared, leading to great sources of resource waste and direct pollution. To ensure efficient use of these valuable resources, there has been significant drive to develop new methods to fine-tune product selectivity for fuel production by advancing fundamental knowledge on zeolite catalysts and oligomerization mechanisms. This doctoral work has aimed to build and apply computational frameworks to capture complex effects within reaction processes for fuel conversion chemistry. Despite the industrial importance of zeolite catalysts, there remains great uncertainty around key functional properties, such as the impact of the proximity of active sites within a zeolite. In this thesis, a framework was developed to capture this effect through microkinetic modeling, a unifying approach that combines knowledge from quantum mechanics at the atomic scale with process scale reactor equations. Using a model system of methanol dehydration in a zeolite catalyst known as chabazite, density functional theory calculations were utilized to evaluate the energies of chemical states along the reaction pathway in the presence of both isolated and paired active sites. These energies were then incorporated into a microkinetic model, the results of which were compared to experimental data, which enabled analysis of reaction flux, speciation, and the degree to which each elementary reaction step controlled the overall product formation rate. This led to the insight that paired active sites are able to enhance methanol dehydration by stabilizing a methanol trimer species that was also reactive. Expanding this framework involved further increasing both topological and chemical complexity. To advance chemical complexity, the thermal oligomerization of ethylene was analyzed as a key reaction for the energy transition. This reaction has more than a century of study, yet lacked up to this point significant study using quantum chemical calculations, leading to a lack of mechanistic understanding. A microkinetic model that incorporated density functional theory calculations, adjusted within bounds of uncertainty, was developed, unraveling the drivers of initiation for this reaction, how the primary initiation mode shifts with conversion, and revealing the origin of odd-numbered carbon species in the product distribution from the β-scission of radical species that were formed from intramolecular hydrogen shift reactions. To expand the topological complexity of this modeling framework, methanol dehydration over orthorhombic MFI, a zeolite with 12 crystallographically unique T-sites, was studied. Along with proximity to other T-sites, the location of a T-site within the structure of a zeolite is a major factor for determining catalytic activity. The microkinetic model developed in Chapter 2 was expanded to account for systems of N distinct sites and was used to capture the effects of each independent T-site of MFI on methanol dehydration to dimethyl ether. Flux, surface speciation, and degree of rate control analyses were applied to this model to identify a new critical species, methanol tetramer, which differed from the results of methanol dehydration on chabazite. Altogether, this work creates a platform to predictively model the impact of industrially relevant catalytic properties on the process scale from atomic scale knowledge, enabling the rational design of next-generation catalytic materials.

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