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Ab Initio Methods for Modeling the Thermodynamics of Molecules Adsorbed in Zeolites


Zeolites are crystalline microporous solids composed of corner-sharing, tetrahedrally- coordinated silicate (SiO4) units. The isomorphic substitution of a framework Si atom by an Al atom is charge-compensated by a proton, introducing Brønsted-acidic bridging-hydroxy groups. These proton-exchanged zeolites are used in a large number of processes, including hydrocarbon cracking, isomerization, and alkylation, and conversion of petroleum to trans- portation fuel. In addition to catalytic processes, zeolites can be used as adsorbents for carbon capture, molecular sieving, and pollution control technologies. It is therefore of great interest to predict the impact of zeolite structure and composition on its functional proper- ties to screen new catalysts and improve existing catalysts. Quantum chemical calculations can provide molecular-scale information on zeolite-adsorbate interactions, as well as model the energetic changes and dynamics of important reactions that occur within the channels and pores of zeolite catalysts. However, the application of quantum chemical calculations for the study of chemical reactions occurring in zeolites is made difficult by the lack of reliable methods to generalize the theory beyond zero Kelvin. Modeling adsorption and desorption free energies is particularly troublesome, relying on a subtle balance between enthalpic and entropic terms. While the enthalpic term is becoming ever more accurate through density functional development, the much more temperature-sensitive entropic term remains gener- ally underquantified by frequently-assumed harmonic approximations. The consequence is an inability to computationally replicate experimental observations, such as rate coefficients or equilibrium constants, with chemical accuracy. This work is concerned with a re-examination of harmonic approximations, including quantifying its failures in reaction kinetics of aldol condensation on isolated metal sites, exploring alternative approximation methods for anhar- monic intramolecular motions, and developing new methods for approximating anharmonic external molecular motions for species adsorbed in zeolites.

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