The Shape of Water in Zeolites and the Impact on Alkene Epoxidations

Molecular interactions at the solid-liquid interface greatly influence the stabilities of surface-bound intermediates, which have implications for catalysis and adsorption. Within zeolites, solvent molecules possess structures that reorganize to accommodate the formation of surface intermediates, which reflect changes in the free energies of adsorption, reaction, and activation. In epoxidation catalysis, the presence of silanol nests ((SiOH)₄) within zeolites facilitate the formation of extended hydrogen-bonded structures that surround active sites and introduce differences in the stability of transition states that impact rates and selectivities. Here, we examine how these interactions affect alkene epoxidations in Ti-containing zeolites with distinct topologies (MFI, BEA, FAU) and (SiOH)₄ densities and compare kinetic parameters to differences in the coordination and distribution of solvent molecules within pores.

Turnover rates for 1-hexene, 1-octene, and 1-decene epoxidations are greater in Ti-zeolite catalysts that contain significant densities of hydrogen-bonded SiOH ((SiOH)_x; e.g., silanol nests) than within defect-free analogues. These rates reflect differences in activation enthalpies and entropies between a given Ti-zeolite and those quantities for the most hydrophobic form of that framework: these differences are defined as excess enthalpies and entropies (H^‡,ε, S^‡,ε) of activation, respectively. Changes in H^‡,ε and S^‡,ε primarily reflect changes in the enthalpy and entropy of the epoxidation transition states, which depend sensitively on their solvating environment. In situ vibrational spectroscopy of intraporous H₂O and comparisons with molecular dynamics simulations that quantify hydrogen bonding interactions and the structure of solvent molecules within zeolite pores show that water congregates near (SiOH)_x and forms dynamic structures (e.g., two-dimensional chains) that change with progress along the reaction coordinate. The disruption of H₂O structures is reflected in S^‡,ε, whose magnitude depends on the extent of H₂O perturbation and the topology of the surrounding pore. This research was supported by the DOE Office of Basic Energy Sciences, under grant DE-SC0020224.