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

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
(ΔΔSapp⧧ = 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.