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. 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 . These changes reflect an increase in apparent activation free energies (ΔG‡) that stem from large entropy differences caused disrupting hydrogen-bonded clusters of water within pores. Differences in ΔG‡ across this series of materials show excess free energy contributions (GE) associated with hydrogen bonding between silanol groups, adventitious water molecules, 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. Under anhydrous conditions, the enthalpies and entropies of 1OO adsorption remain constant across the series of materials, however, the trace water leads to smaller entropy losses and lower heats of adsorption for 1OO in hydrophilic structures. 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.

Catalytic and thermodynamic effects of hydrogen bonding on epoxidations within Ti-substituted zeolites

2019 Spring National Meeting

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