Hydrophobic voids within titanium silicates have long been considered necessary to achieve high rates and selectivities for alkene epoxidations with H2O2. The catalytic consequences of silanol groups and their stabilization of hydrogen-bonded networks of water (H2O), however, have not been demonstrated in ways that lead to a clear understanding of their importance. We compare turnover rates for 1-octene epoxidation and H2O2 decomposition over a series of Ti-substituted zeoliteBEA (Ti-BEA) that encompasses a wide range of densities of silanol nests ((SiOH)4). The most hydrophilic Ti-BEA gives epoxidation turnover rates that are 100 times larger than those in defect-free Ti-BEA, yet rates of H2O2 decomposition are similar for all (SiOH)4 densities. These differences cause the most hydrophilic Ti-BEA to also give the highest selectivities, which defies conventional wisdom. Spectroscopic, thermodynamic, and kinetic evidence indicate that these catalytic differences are not due to changes in the electronic affinity of the active site, the electronic structure of Ti-OOH intermediates, or the mechanism for epoxidation. Comparisons of apparent activation enthalpies and entropies show that differences in epoxidation rates and selectivities reflect favorable entropy gains produced when epoxidation transition states disrupt hydrogen-bonded H2O clusters anchored to (SiOH)4 near active sites. Transition states for H2O2 decomposition hydrogen bond with H2O in ways similar to Ti-OOH reactive species, such that decomposition becomes insensitive to the presence of (SiOH)4. Collectively, these findings clarify how molecular interactions between reactive species, hydrogen-bonded solvent networks, and polar surfaces can influence rates and selectivities for epoxidation (and other reactions) in zeolite catalysts.
ASJC Scopus subject areas
- Colloid and Surface Chemistry