Cooperative Effects between Hydrophilic Pores and Solvents: Catalytic Consequences of Hydrogen Bonding on Alkene Epoxidation in Zeolites

Abstract

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 zeolite *BEA (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.

Structure and solvation of confined water and water–ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis

“…Different densities of Brønsted acid H + or Si-OH groups within zeolites stabilize water networks of distinct clustered and extended hydrogen-bonded structures, [43][44][45]75 which have been reported to inuence reactivity in numerous ways, including different competitive adsorption of solvent(s) and reactants, 64,65 entropic gains when water networks are disrupted by transition states, 28,76 and entropic losses when transition states are disrupted by water networks. 34 Thus, interrogation of H-zeolite samples with varying H + and Si-OH density is required to disentangle their separate effects on intrapore solvent structure, prior to relating systematically varying solvent structures to the rate and equilibrium constants of a probe reaction catalyzed by H + sites, such as bimolecular ethanol dehydration to diethyl ether (DEE).…”

“…Here, the apparent activation volume is positive because transition states include two adsorbed C 2 H 5 OH molecules while the MARI species only includes one. The disruption of extended hydrogen-bonded networks by transition states has also been implicated in aqueous-phase glucose-fructose isomerization 34 and 1-octene epoxidation catalysis 28 in Ti-Beta zeolites, where zeolites of different hydrophobicity enabled including or excluding H 2 O networks or clusters, but not systematic variation of the intrapore structure of these networks between these two limiting conditions. Here, we have demonstrated that gas-phase kinetic measurements and spectroscopic characterizations with co-fed solvent molecules under conditions that lead to intrapore condensation of extended liquid-like networks enable quantifying the thermodynamic nonidealities introduced by the solvation of kinetically relevant intermediates and transition states within conning environments, which are inaccessible to measurements performed in the liquid phase.…”

“…24 Inorganic, crystalline, microporous frameworks offer independent synthetic control of the density, chemical identity, and coordination of catalytic active sites, and the structure and polarity of the environments that conne them, which make zeolites and zeotype-molecular sieves a versatile platform to study fundamental principles underlying the structure and catalytic behavior of conned solvents. Lewis acid zeolites containing framework Ti 4+ or Sn 4+ centers have been reported recently to show signicant differences in reactivity for aqueous-phase olen epoxidation [25][26][27][28][29] and sugar isomerization [30][31][32][33][34] in response to changes in active site structure and the stabilization or exclusion of extended water networks within microporous environments. The interplay between the solvated structures and reactivity of Brønsted acidic H + sites associated with framework Al 3+ in siliceous zeolites (Si-O(H + )-Al), however, are less well-understood because of the greater diversity of H + , reactant, and transition state complexes engendered by solvating water within conning voids.…”

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

“…Titanium substituted zeolites with the FAU (1.3 nm supercages), BEA (0.65 nm pores), MFI (0.55 nm pores) frameworks and siliceous CDO (0.45 nm pores) were synthesized through post-synthetic modification 5,39,40 or hydrothermal synthesis 19 to prepare materials with a range of controlled topologies (e.g., 2D pore networks, 3D supercages), mean pore dimensions (0.45 -1.3 nm), and densities of (SiOH) x . Here, we make comparisons primarily among materials that contain the greatest and lowest densities of (SiOH) x (i.e., the most hydrophilic and hydrophobic samples).…”

Hydrogen‐Bonding Triggered Assembly to Configure Hollow Carbon Nanosheets for Highly Efficient Tri‐Iodide Reduction

“…Φ NMR is calculated as the ratio of the peak area for the Q 3 to the sum of Q 4 and Q 3 sites, and it can quantitatively measure the variation of isolated SiOH on the surface of S-1. [30] From bottom to up in (a) and (b), the reaction time is 0, 1, 5, 30, 60, and 120 min in turns. Obviously, the red shift of the isolated SiOH can be observed while its amount remains unchanged following the reaction time.…”