Ab initio molecular dynamics simulations of lanthanide coordination structures in water and in faujasite

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Shiery, Richard

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2022

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Dissertation

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Atomic and molecular resolution can provide unique insights into the ambiguous mechanisms by which lanthanum increases the hydrothermal stability of faujasite in cracking catalysts, as well as in cation exchange in faujasite. The structures of the lanthanide aqua ions were resolved with density functional theory calculations and ab initio molecular dynamics (AIMD) simulations within ~0.05 Å of experimental results. Reaction energies were quantified by predicting the first hydrolysis constant of lanthanide aqua ions within ~1.1 pKa units using AIMD with rare event simulation techniques and electronic structure calculations. The capture of structural and reaction trends in the lanthanide aqua ion served as a benchmark for implementing similar methods in lanthanum-exchanged faujasite. In faujasite, AIMD simulation identified the preferred binding site of lanthanum. AIMD with a rare event simulation technique was used to quantity the free energy of faujasite aluminum tetrahedra deprotonation, with and without lanthanum exchanged in faujasite. The presence of lanthanum makes faujasite deprotonation energetically more favorable, thus making faujasite less hydrophilic. The local structure of water confined in faujasite was simulated with AIMD. The model faujasite structure was modified to produce a series of systems to study the influence of confinement, hydrophilicity, and cation exchanged on the local structure of water as quantified from radial distribution functions. Increases in hydrophilicity in hydrogen-exchanged faujasite disrupts the confined water structure. While lanthanum ions compensate for a larger magnitude of charge, equivalent number of sodium ions have a higher probability of occupying sites interacting with the supercage and thus disrupt the local structure of water more significantly than lanthanum.

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