Catalytic reactions at solid-liquid interfaces possess the complexities of catalysis at gas-solid interfaces but are further complicated by interactions of solvent molecules. The presence of solvents introduces specific interactions among solvent molecules, catalyst surfaces, and reactive species that can change reaction rates and selectivities by orders of magnitude. Here, we show that liquid-phase alkene epoxidation rates and selectivities at framework Ti-atoms within zeolites (e.g., MFI, BEA, and FAU) sense differences in the density of silanol nests near active sites and the spatial constraints provided by pore walls. We combine measured epoxidation turnover rates, in situ infrared spectroscopy, and isothermal titration calorimetry to examine the non-covalent intermolecular interactions within zeolite pores that impact catalysis. Activation barriers for epoxidation depend on the geometric constraints of the zeolite structure, the density of silanol groups in pores, the structure of the alkene reagent, and the hydrogen-bonding nature of the solvent solution. These factors determine excess free energy contributions (GE) associated with hydrogen bonding between silanol groups, hydrogen bonding networks mediated by solvents, and transition state complexes that lead to more than 100-fold differences in epoxidation turnover rates. Values for GE agree closely with measurements of these excess quantities from liquid-phase calorimetry of epoxide adsorption and water adsorption into solvent filled pores of Ti-zeolite catalysts. In situ infrared spectroscopy shows that the extent of hydrogen bonding among solvent molecules is more sensitive to the adsorption of reactants in the most hydrophilic structures. Examination of enthalpy-entropy compensation relationships for adsorption and activation demonstrate that changes in the equilibrium solvent structures, which depend on zeolite topology and polarity, provide opportunities to engineer solvent-surface interactions that yield large entropy gains at small enthalpic costs. Collectively, these data and interpretations give insight to the interactions between catalyst surfaces, adventitious solvating molecules, and reactive species on which we may build structure-function relationships to increase catalytic rates and selectivities within zeolites.

Roles of cooperative interactions among surfaces, solvents, and reactive intermediates for catalytic epoxidations within Ti-zeolites

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.