Analysis of Steady-State and Transient Chemical Rates in Molecular Interconversion

Abstract

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.