A computational investigation into the confinement induced thermo-kinetic effects of crystalline porous materials

Abstract The reactivity and selectivity of a chemical reaction may be significantly enhanced by the confining effects of hierarchically structured, heterogeneous, porous frameworks. While the composition of such frameworks span organic, metal-organic, and inorganic building units, their underlying feature is the presence of unique microporous topologies, which have a propensity for stabilizing adsorbates mainly through long-range dispersive interactions. Upon adsorption, the molecule is confined to the space of the framework's pore, where it sacrifices its degree of freedom (entropy) in favor of stabilizing interactions (enthalpy) with the framework structure. Under isothermal-isobaric conditions, the balance between entropic loss and enthalpic gain is dictated by the extent of confinement, which regulates the spontaneity, or thermodynamic favourability of the process. Excessive entropic loss may promulgate frameworks to act as molecular sieves, where prohibitively large species that cannot fit within their voids are excluded from adsorption and/or the reaction space. Likewise, insufficient confinement may result in the adsorbate merely equilibriating with the bulk gas phase, lacking the stability necessary to catalyze a particular process. Optimal confinement occurs when the geometry of the adsorbate matches the framework topology, acquiring enough stability to promote their respective reactions without limiting the product's rate of desorption, the reactants rate of adsorption, or limiting the formation of reactive intermediates. The continued discovery of new frameworks, and the hypothetical existence of hundreds of millions of others, make it conceivable and even likely that the optimal framework for many catalytic processes has yet to be discovered. Confinement driven heterogeneous catalysis is therefore a multifaceted problem; and despite our detailed understanding on an individual basis, our mechanistic understanding and ability to make empirical predictions remains limited.In this thesis, we elucidate the phenomena of confinement by: 1) presenting its geometric relationship with the adsorption entropy, and the ability to make empirical predictions through interpretable linear correlations, 2) measuring confinement from a kinetic perspective on a zeolite promoted reaction system, and evaluating factors for framework optimization, 3) identifying readily quantifiable, geometric features of the adsorbate/framework system for rapid prediction of the adsorption entropy within zeolites.