(714a) Ab-Initio characterization of Water Clusters at Acid Sites in Microporous Zeolites and Their Influence on Ethanol Dehydration Kinetics

Microporous zeolites synthesized with framework-substituted cations stabilize unique water networks that influence the structure and stability of solvated ethanol dehydration transition states. To probe the role of solvation in micropores, it is necessary to capture the bonding interactions and structure of model reactants and transition states. We evaluated the stability of water at Lewis acid sites in zeolite Beta with ab initio molecular dynamics (AIMD) where a densely clustered water phase competes with gas-like water molecules. The speciation of Sn Lewis acids affects the size and structure of this clustered phase. The identification of stable clustered phases at Sn acid sites presents an avenue for generating computational solvent models in microporous materials that are in equilibrium with the extended solvent environment and are representative of experimental reaction conditions. These techniques assist in uncovering the detailed molecular picture of complex solvated reactions.

The generation of acid site solvent models was applied to modeling ethanol dehydration in H-Al-Beta at conditions near intrapore condensation of water. Clusters of six water molecules were found to be stable at reaction temperatures (373 K). To study the coadsorption of ethanol and water at active sites to form reactive intermediates, free energies of adsorption were calculated from AIMD including the translational entropy of adsorbates. We found that ethanol adsorption displaces one water from a water-alkanol cluster to form a solvated ethanol monomer (C₂H₅OH)(H⁺)(H₂O)₅. The solvated bimolecular dehydration transition state was then simulated with metadynamics-biased AIMD. The solvated transition state is stabilized at the periphery of the water cluster, with the non-polar ethyl groups excluded from the solvation environment. This insight explains why the penalty to solvate the transition state is not incurred until water coverages surpass the size of stable clusters in experiments. We can then predict how different micropore geometries control solvation during catalysis.