Browsing by Subject "methanol-to-hydrocarbons"

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    Analysis of Steady-State and Transient Chemical Rates in Molecular Interconversion
    (2020-07) Foley, Brandon
    This dissertation focuses on relating macroscopic observable quantities to the fundamental elementary step reactions that mediate the chemical transformation of reactants to products in composite reactions in the context of steady-state and transient rates of chemical reactions, non-catalytic and catalytic reactions, and product-forming or catalyst-deactivating reactions. With a few notable exceptions, the formation and scission of chemical bonds as a reaction progresses is not directly observable. Instead, chemists rely on making macroscopic measurements of rate as functions of chemical activities (or concentrations) and temperature to elucidate the mechanism of unseen events occurring at the molecular scale. Analytical descriptions that explicitly relate the properties of steady-state and transient reaction rates and steady-state fractional coverages of intermediates to the underlying elementary-step reactions that comprise a reaction mechanism are developed in this research. Specifically, it is demonstrated that all measurable properties of reaction rates (e.g., reaction orders, activation energies, and fractional coverages on catalytic active sites) are explicit functions of sensitivities, which are quantitative measures of the rate-control of elementary steps in composite reactions. These relationships are utilized to elucidate the mechanism of the acid-catalyzed condensation of formaldehyde with benzene to form diphenylmethane (DPM) on HZSM-5 zeolite catalysts, a reaction implicated as causing deactivation during methanol-to-hydrocarbons catalysis. Explicit relationships between deactivation "rate" and the deactivation mechanism in heterogeneously catalyzed reactions are also explored. Metrics to assess the rate, yield, and selectivity of site-loss are defined for catalyzed systems by treating active sites as consumable reactants in deactivation reactions. These metrics enable the elucidation of deactivation mechanisms by measuring site-loss rate, yield, and selectivity as functions of reactant concentrations and temperature in methods analogous to those used to elucidate mechanisms of forming products that are observable in the reactor effluent. These methodologies are demonstrated in the elucidation of formaldehyde-mediated deactivation mechanisms during methanol-to-hydrocarbons catalysis on HZSM-5 zeolite catalysts.
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    A mechanistic understanding of light olefins selectivity in methanol-to-hydrocarbons conversion on MFI
    (2016-11) Khare, Rachit
    Methanol-to-hydrocarbons (MTH) conversion is the final processing step in converting alternative feedstock such as coal, natural gas, and biomass, to hydrocarbon fuels and petrochemicals. Methanol reacts on acidic zeolite catalysts via an indirect “hydrocarbon-pool” mechanism to form a wide variety of hydrocarbons including light olefins, gasoline-range hydrocarbons, and aromatics. The hydrocarbon-pool mechanism involves two reaction cycles simultaneously operating inside the zeolite pores: an olefins-based reaction cycle and an aromatics-based reaction cycle. The observed product distribution in MTH can be rationalized as an effect of the relative rates of propagation of the aromatics-based and the olefins-based reaction cycles. Quantifying the relative propagation of these two catalytic cycles and understanding how these cycles contribute to the overall product distribution under varying reaction conditions, varying feed composition, and on different zeolite topologies or morphologies, is critical for developing structure-function relationships for MTH catalysts. In this work, the effects of independently varying (i) the feed composition (by co-feeding hydrocarbons or oxygenates), (ii) the concentration of active sites (by changing the chemical composition of the zeolite), and (iii) the diffusion characteristics of the zeolite (by changing the crystallite size or silylating the external surface), on the relative rates of propagation of the aromatics- and olefins-based cycles, and consequentially on the observed MTH product selectivity are presented.