Groups IV-VI metal atoms dispersed on silica and substituted into zeolites catalyze epoxidations of alkenes. Among these catalysts, epoxidation turnover rates and selectivities are sensitive to differences in the density of silanol nests near active sites, where these differences may be introduced with zeolite synthesis protocols or post-synthetic modifications. However, a molecularly detailed understanding of the causes for these phenomena at the solid-liquid interface remains elusive. Here, we use a series of Ti-BEA catalysts that contain a wide range of silanol nest densities to demonstrate the influence of molecular interactions within zeolite pores that originate from the interactions of solvent molecules with silanol defects. Measured heats of adsorption of CD3CN on Ti sites and the ligand to metal charge transfer energies of reactive Ti-OOH intermediates remain constant across these Ti-BEA structures, which suggests that the electronic structures of these active sites are equivalent. Consequently, differences in catalysis arise from direct interactions with silanol groups mediated by solvent molecules. As the densities of silanol defects decrease in Ti-BEA, epoxidation turnover rates for 1-octene (with aqueous H2O2 in acetonitrile) decrease by 100-fold yet the rates are similar in the absence of hydrogen bonding solvents. These changes reflect differences in excess free energy contributions (GE) associated with hydrogen bonding between silanol groups, hydrogen bonding networks mediated by solvents, and transition state complexes. Values for GE agree closely with measurements of these excess quantities from liquid-phase calorimetry of 1-octene oxide (1OO) adsorption into the series of Ti-BEA catalysts. In situ infrared spectroscopy further reveals that the extent of hydrogen bonding among solvent molecules is more sensitive to the adsorption of reactants in the most hydrophilic structures, which signals the direct interactions of adsorbates and transition states with solvent structures proposed to be responsible for differences in GE. These structure function relationships are sensitive also the identity of the framework, the hydroperoxide oxidant, and the hydrogen bonding nature of the prevalent solvent molecule. Collectively, these data and interpretations give insight to the interactions between catalyst surfaces, solvating molecules, and reactive species which we may use to increase catalytic rates and selectivities within zeolites.

Role of the solvent in mediating interactions between active sites and the extended surface of zeolites during epoxidation reactions

2019 Fall National Meeting

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