The Shape of Water in Zeolites and Its Impact on Epoxidation Catalysis

Abstract

Solvent structures that surround active sites reorganize during catalysis and influence the stability of surface intermediates. Within the pores of a zeolite, H<sub>2</sub>O molecules form hydrogen-bonded structures that differ significantly from bulk H<sub>2</sub>O. Spectroscopic measurements and molecular dynamics simulations show that H<sub>2</sub>O molecules form bulk-like three-dimensional structures within 1.3 nm cages, while H<sub>2</sub>O molecules coalesce into oligomeric one-dimensional chains distributed throughout zeolite frameworks when the pore diameter is smaller than 0.65 nm. The differences between the motifs of these solvent structures provide opportunities to manipulate enthalpy-entropy compensation relationships and significantly increase rates of catalytic turnover events. Here, we explain how the reorganization of these pore size-dependent H<sub>2</sub>O structures during alkene epoxidation catalysis gives rise to entropy gains that increase turnover rates by up to 400-fold. Collectively, this work shows how solvent molecules form discrete structures with highly correlated motion within microporous environments, and that the reorganization of these structures may be controlled to confer stability to reactive intermediates.

Unusual water-assisted NO adsorption over Pd/FER calcined at high temperatures: The effect of cation migration

“…[10][11][12][13][14] Their unique structures confine molecules into small spaces and alter the reaction pathway by modulating transition states as seen in several examples. [15][16][17] Zeolites, in general, strongly interact with water, due to the hydrophilic nature of these materials. 18 Clustered water molecules are always present adjacent to Brønsted acid sites under ambient conditions and solvate charge balancing cations and reacting molecules.…”