Structure and solvation of confined water and water–ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis† †Electronic supplementary information (ESI) available. See DOI: 10.1039/d0sc02589e
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. Aqueous-phase reactions within microporous Brønst...
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| Published in: | Chemical science (Cambridge) 2020-06, Vol.11 (27), p.7102-7122 |
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| Main Authors: | , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | 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.
Aqueous-phase reactions within microporous Brønsted acids occur at active centers comprised of water-reactant-clustered hydronium ions, solvated within extended hydrogen-bonded water networks that tend to stabilize reactive intermediates and transition states differently. The effects of these diverse clustered and networked structures were disentangled here by measuring turnover rates of gas-phase ethanol dehydration to diethyl ether (DEE) on H-form zeolites as water pressure was increased to the point of intrapore condensation, causing protons to become solvated in larger clusters that subsequently become solvated by extended hydrogen-bonded water networks, according to
in situ
IR spectra. Measured first-order rate constants in ethanol quantify the stability of S
N
2 transition states that eliminate DEE relative to (C
2
H
5
OH)(H
+
)(H
2
O)
n
clusters of increasing molecularity, whose structures were respectively determined using metadynamics and
ab initio
molecular dynamics simulations. At low water pressures (2–10 kPa H
2
O), rate inhibition by water (–1 reaction order) reflects the need to displace one water by ethanol in the cluster en route to the DEE-formation transition state, which resides at the periphery of water–ethanol clusters. At higher water pressures (10–75 kPa H
2
O), water–ethanol clusters reach their maximum stable size ((C
2
H
5
OH)(H
+
)(H
2
O)
4–5
), and water begins to form extended hydrogen-bonded networks; concomitantly, rate inhibition by water (up to –3 reaction order) becomes stronger than expected from the molecularity of the reaction, reflecting the more extensive disruption of hydrogen bonds at DEE-formation transition states that contain an additional solvated non-polar ethyl group compared to the relevant reactant cluster, as described by non-ideal thermodynamic formalisms of reaction rates. Microporous voids of different hydrophilic binding site density (Beta; varying H
+
and Si–OH density) and different size and shape (Beta, MFI, TON, CHA, AEI, FAU), influence the relative extents to which intermediates and transition states disrupt their confined water networks, which manifest as different kinetic orders of inhibition at high water pressures. The confinement of water within |
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| ISSN: | 2041-6520 2041-6539 |