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.

Engineering intraporous solvent environments: effects of aqueous-organic solvent mixtures on competition between zeolite-catalyzed epoxidation and H2O2 decomposition pathways

“…The conditions used for rate measurements in this study avoid internal mass transfer constraints, as indicated by the linear dependence of turnover rates on alkene concentration (Section 3.1), 57 which is consistent with expectations based on prior studies of reactions of H 2 O 2 with alkenes in Ti substituted MFI, *BEA, and FAU at similar conditions. 29,33,58 A mass-transfer limited material would show a sub-linear dependence on alkene concentration because the mean concentration of alkenes throughout the pores would not depend linearly on the uid phase alkene concentration.…”

“…Interactions between solvents and reactants at solid-liquid interfaces provide energetic contributions that strongly inuence zeolite-catalyzed epoxidation reactions, [27][28][29][30][31][32][33][34] among other chemistries. For example, the identity of the organic solvent affects alkene epoxidation turnover rates.…”

“…Turnover rates for epoxidations of C 6 -C 18 linear alkenes in CH 3 CN with aqueous H 2 O 2 in hydrophilic Ti-BEA surpass rates in hydrophobic Ti-BEA by factors of 40 to 450. 27,33,34 The most hydrophilic zeolites possess multiple (SiOH) x nests per unit cell that stabilize hydrogen bonded networks of H 2 O molecules within their pores, while the most hydrophobic zeolites contain far less than one (SiOH) x per unit cell and contain less H 2 O. 42,43 During epoxidation turnovers, the formation of the alkene epoxidation transition state (or the adsorption of an epoxide) disrupts and displaces hydrogen bonded H 2 O and CH 3 CN molecules from pores and brings entropic gains that stabilize the transition state to increase rates.…”

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.…”

A triphenylamine-based Pt(ii) metallacage via coordination-driven self-assembly for nonlinear optical power limiting

“…Coordination-chemistry-driven self-assembly has emerged in the last decade as a straightforward synthetic approach and has provided increasingly intricate and functional supramolecular coordination complexes (SCCs), [1][2][3][4] including both metalorganic frameworks (MOFs) [5][6][7] and metal-organic polyhedra (MOPs). 8,9 The chemical and structural diversity, tunability, flexibility, and permanent porosity of these materials make them valuable for a wide range of applications in guest encapsulation, [10][11][12] catalysis, 13,14 sensors, [15][16][17] bioimaging agents, 18,19 and so on. Another advantage unique to SCCs is the availability of sufficient pore space to accommodate functional groups on the building blocks without interfering with their linkage or alignment.…”