Distinct Catalytic Reactivity of Sn Substituted in Framework Locations and at Defect Grain Boundaries in Sn-Zeolites

Abstract

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.

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

“…The kinetics of bimolecular ethanol dehydration to diethyl ether were measured in a differential packed-bed reactor system described previously. 60,132 The pressures of C 2 H 5 OH (2 Â 10 À3 to 10 kPa) and H 2 O (0-75 kPa) were varied non-systematically over the course of the experiment. No deactivation was detected over the course of kinetic measurements ($7-21 days), veried by periodically returning to a reference condition (5 kPa C 2 H 5 OH, 50 kPa H 2 O).…”

Systematic Modification of UiO‐66 Metal‐Organic Frameworks for Glucose Conversion into 5‐Hydroxymethyl Furfural in Water

“…[31] Partial hydrolysis of a fully coordinated tetrahedral {SnO 4 } sites is believed to generate open sites, where Sn is present as {(SiO) 3 Sn(OH)} with an adjacent silanol group (Si-OH). [32] Both open and closed sites can coordinate two molecules of water, changing the tetrahedral coordination into octahedral, and these can subsequently be replaced by glucose that coordinates directly to the Lewisacidic silicon. The adjacent silanol groups are significant in stabilising the transition state that allows isomerisation to fructose.…”

“…Studies on the nature of the active Sn sites in zeolite beta, which has been used for glucose conversion, widely agree that the reactions take place on open metal sites [31] . Partial hydrolysis of a fully coordinated tetrahedral {SnO 4 } sites is believed to generate open sites, where Sn is present as {(SiO) 3 Sn(OH)} with an adjacent silanol group (Si‐OH) [32] . Both open and closed sites can coordinate two molecules of water, changing the tetrahedral coordination into octahedral, and these can subsequently be replaced by glucose that coordinates directly to the Lewis‐acidic silicon.…”

Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid catalysis

“…A related question concerns the length scales over which solvent networks can influence the thermodynamics of IS and TS. Long-range interactions may be sensitive to crystal-scale structural properties such as the degree of crystallinity or faulting, 93 or the presence of less ordered 94,95 or hierarchical pore structures. The design of porous materials to direct the structure and arrangement of solvent and charge networks offers promise to exert more precise control over reactivity through differences in the stabilization of kinetically relevant IS and TS moieties.…”