(445b) Effects of Acid Site Proximity and Confinement in Zeolites on Methanol Dehydration Reaction Mechanisms Prevalent during Low-Temperature Catalysis

Chabazite (CHA) zeolites synthesized using mixtures of
organic N,N,N-trimethyl-1-adamantylammonium and
inorganic sodium cations contain different arrangements of framework Al atoms (Al−O(−Si−O)x−Al)
in isolated (x ≥ 3) or paired (x = 1, 2) configurations, in relative amounts
that depend on the total amount of Na⁺ retained on crystalline
products [1]. Turnover rates of methanol dehydration to dimethyl ether (415 K,
per proton), a well-studied probe of acid strength and confinement effects in
zeolites [2], increase systematically as the fraction of paired protons
increases in small-pore, 8-membered ring (8-MR) zeolites (e.g., CHA, AEI), a phenomenon
not observed in medium-pore MFI zeolites. Increased turnover rates correlate
with the appearance of surface methoxy species observed by in situ IR spectroscopy (415 K, 0.1-22 kPa CH₃OH), in
amounts that increase with paired acid site content on 8-MR zeolites, but are not
present in medium-pore zeolites [3]. Methanol dehydration activation enthalpies
and entropies measured in first-order and zero-order kinetic regimes (383-423
K) on CHA zeolites of varying Al arrangement indicate that free energy barriers
are lower at paired than at isolated protons. Density functional theory (DFT)
indicates that paired protons in CHA zeolites preferentially stabilize dissociative
dehydration pathways involving surface methoxy intermediates, relative to
associative dehydration pathways. Confinement within small-pore zeolites also
causes inhibition of turnover rates (415 K, per proton) at high methanol
pressures (>10 kPa) because extraneous physisorbed
methanol within microporous voids appears to destabilize dehydration transition
states via solvent crowding effects. DFT provides additional mechanistic insight
into the sensitivity of methanol dehydration reaction coordinates to proton
proximity in small-pore, but not medium- or large-pore, zeolites at low
temperatures (<433 K). These findings highlight the catalytic diversity of paired
and isolated Brønsted acid site ensembles for methanol conversion catalysis,
even for single T-site frameworks (e.g., CHA), caused by differences in the stabilities
of reactive intermediates and transition states, which can alter the prevalent
reaction mechanism.