Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than within hydrophilic micropores; however, relative contributions of competitive water adsorption at active sites and preferential stabilization of isomerization transition states have remained unclear. Here, we employ a suite of experimental and theoretical techniques to elucidate the effects of coadsorbed water on glucose isomerization reaction coordinate free energy landscapes. Transmission IR spectra provide evidence that water forms extended hydrogen-bonding networks within hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase glucose isomerization turnover rates measured on Ti-Beta zeolites transition from first-order to zero-order dependence on glucose thermodynamic activity, as Lewis acidic Ti sites transition from water-covered to glucose-covered, consistent with intermediates identified from modulation excitation spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically higher (by 3–12×, 368–383 K) when Ti sites are confined within hydrophobic micropores. Apparent activation enthalpies and entropies reveal that glucose and water competitive adsorption at Ti sites depend weakly on confining environment polarity, while Gibbs free energies of hydride-shift isomerization transition states are lower when confined within hydrophobic micropores. DFT calculations suggest that interactions between intraporous water and isomerization transition states increase effective transition state sizes through second-shell solvation spheres, reducing primary solvation sphere flexibility. These findings clarify the effects of hydrophobic pockets on the stability of coadsorbed water and isomerization transition states and suggest design strategies that modify micropore polarity to influence turnover rates in liquid water.

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Water networks confined within zeolites solvate clustered reactive intermediates and must rearrange to accommodate transition states that differ in size and polarity, with thermodynamic penalties that depend on the shape of the confining environment.

Palladium-exchanged zeolites are candidate materials for passive NO x adsorption in automotive exhaust aftertreatment, where mononuclear Pd cations behave as precursors to the purported NO x adsorption sites. Yet, the structures of zeolite lattice binding sites capable of stabilizing mononuclear Pd 2+ ions, and the mechanisms that interconvert agglomerated PdO and Pd domains into mononuclear Pd 2+ ions during Pd redispersion treatments, remain incompletely understood. Here, we use a suite of spectroscopic methods and quantitative site titration techniques to characterize mononuclear and agglomerated Pd species on zeolites with varying material properties and treatment history. Aqueous-phase methods to introduce Pd onto NH 4 -form zeolites initially form mononuclear [Pd(NH 3 ) 4 ] 2+ complexes, but subsequent thermal treatments (573−723 K; air) lead to in situ formation of H 2 that first reduces Pd 2+ to metallic Pd domains, which are then oxidized by air to PdO domains. Progressive treatment of Pd-zeolites in air to higher temperatures (723−1023 K) converts larger fractions of agglomerated PdO to mononuclear Pd 2+ , as quantified by H 2 temperature programmed reduction, because higher temperature treatments facilitate Pd redispersion toward deeper locations within chabazite (CHA) crystallites, which is corroborated by complementary titrimetric and spectroscopic data. Pd-CHA zeolites synthesized with similar bulk Pd and framework Al content, but varying framework Al arrangement, provide evidence that six-membered rings (6-MR) hosting paired Al sites (Al−O−(Si−O) x −Al, x = 1, 2) stabilize Pd 2+ ions and that otherwise isolated Al sites can stabilize [PdOH]+ species, identifiable by an IR OH stretch at 3660 cm −1 . These findings clarify the underlying chemical processes and gas environments that cause Pd agglomeration in zeolites and their subsequent redispersion to mononuclear Pd 2+ ions, which prefer binding at 6-MR paired Al sites in CHA, and indicate that higher temperature air treatments lead to more uniform Pd spatial distributions throughout zeolite crystallites.

Measurements of turnover rates of gas-phase bimolecular ethanol dehydration to diethyl ether (404–438 K) on a suite of hydrophobic and hydrophilic Sn-zeolites (Sn-Beta, Sn-BEC, Sn-MFI) of varying Sn content, together with quantitative titration of active Sn sites by pyridine during catalysis, identify two types of Sn sites with reactivity differing by more than an order of magnitude (>20×). Apparent activation entropies to form bimolecular dehydration transition states from predominantly ethanol monomer-covered sites are less negative (ΔΔS app ⧧ = 48 ± 22 J mol–1 K–1) at the more reactive subset of Sn sites, which are present in amounts equivalent to 17–26% of the Sn sites quantified by the peak centered at 2308 cm–1 in CD3CN IR spectra (Sn2308) but not correlated with that at 2316 cm–1 (Sn2316). Synthetic and postsynthetic treatments to prepare Sn-zeolites containing Sn sites hosted within diverse local coordination environments suggest that Sn2316 sites are not associated with Sn bound to residual fluoride anions or Sn sited at external crystallite surfaces, amorphous domains, or among the diverse T-site locations contained within CHA, MFI, BEC, and STT frameworks. Treating Sn-Beta in HF or NH4F solutions, which dissolve zeolitic domains preferentially at defect grain boundaries, decreased the number of Sn2316 sites but not Sn2308 sites. These data indicate that Sn2316 sites are preferentially located at stacking faults in zeolite Beta, which provide tetrahedral coordination environments for Sn in defect-open configurations ((HO)–Sn–(OSi)3) with proximal Si–OH groups that do not permit condensation to tetrahedral closed configurations (Sn–(OSi)4). A computational model was developed for stacking fault defect-open Sn sites, which predict apparent activation free energies for bimolecular ethanol dehydration that are 65–74 kJ mol–1 higher (at 404 K) than those at framework-closed Sn sites that are capable of stabilizing transition states via Sn site opening and closing as part of the catalytic cycle, consistent with the lower experimentally measured ethanol dehydration reactivity for Sn2316 sites. In contrast, defect-open sites possess Si–OH groups that preferentially stabilize hydride shift transition states involved in glucose–fructose isomerization catalytic cycles. These findings highlight the ability of a given zeolite framework to confer structural diversity to nominally site-isolated Lewis acid centers, thus generating configurations with distinct reactivity for different chemical transformations.

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Ab‐initio molecular dynamics simulations and transmission infrared spectroscopy are employed to characterize the structure of water networks in defect‐functionalized microporous zeolites. Thermodynamically stable phases of clustered water molecules are localized at some of the defects in zeolite Beta, which include catalytic sites such as framework Lewis acidic Sn atoms in closed and hydrolyzed‐open forms, as well as silanol nests. These water clusters compete with ideal gas‐like structures at low water densities and pore‐filling phases at higher water densities, with the equilibrium phase determined by the water chemical potential. The physical characteristics of these phases are determined by the defect identity, with the local binding and orientation of hydroxyl moieties around the defects playing a central role. The results suggest general principles for how the structure of polar solvents in microporous solid acids is influenced by local defect functionalization, and the thermodynamic stability of the condensed phases surrounding such sites, in turn, implies that the catalysis of Lewis acids will be influenced by local water ordering.

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites.T urnover rates (per H +)f or methanol and ethanol dehydration increase with the fraction of H + sites sharing sixmembered rings of chabazite (CHA) zeolites.D ensity functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density.C ertain Al distributions yield rigid arrangements of anionic charge that stabilizec ationic intermediates and transition states via H-bonding to decrease barriers.T his is ak ey feature of acid catalysis in zeolites olvents,w hichl ack the isotropyo fl iquid solvents.T he sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.