Proximal Brønsted Acid Sites in Zeolites Cooperate to Stabilize Transition States via Hydrogen Bonding and Multi-Ion-Pair Interactions

Brønsted acid sites, when separated across molecular length scales, can cooperate via hydrogen-bonding and multi-ion-pair interactions to stabilize reactive intermediates and transition states during catalytic turnovers. Here, we study mechanisms underlying such inter-site cooperation using high symmetry chabazite (CHA) framework zeolite catalysts, which provide a model platform to precisely synthesize and quantify acid sites in distinct spatially isolated and proximal site arrangements. Specifically, we explore the mechanisms by which proximal H + sites interact at low and high temperatures using two different kinetic probe reactions. At low temperatures (<450 K), methanol dehydration to dimethyl ether occurs on H + sites covered by methanol (0.2−52 kPa CH 3 OH), with experimentally measured turnover rates (per H +, 415 K) that are >8× higher, and activation enthalpies that are 25 kJ mol −1 lower, on proximal than isolated H + sites. Density functional theory (DFT) calculations on H + sites covered by adsorbed methanol provide evidence that proximal H + sites facilitate hydrogen-bonding interactions via co-adsorbates that enthalpically stabilize the anionic conjugate bases of carbocations in dehydration transition states. Similarly, measured turnover rates (per H +, 748 K) are >7× higher during high-temperature (>700 K) protolytic alkane cracking and dehydrogenation on proximal than isolated H + sites, and DFT-optimized structures show evidence of hydrogen-bonding stabilization of carbocationic transition states at proximal acid sites. However, experimentally measured activation entropies (by ∼25 J mol −1 K −1 ) and activation enthalpies (by ∼10 kJ mol −1 ) are higher at proximal than isolated H + sites, suggesting that proximal sites entropically stabilize (and enthalpically destabilize) transition states, the opposite behavior to DFT predictions based on static (0 K) potential energy minimizations. Molecular dynamics simulations indicate that greater entropic freedom is conferred to guest cations hosted by anionic lattice sites nearby other H + sites due to multi-ion-pair interactions with conjugate lattice anions. These findings provide mechanistic insights into how proximal acid sites cooperate via hydrogen-bonding interactions to enthalpically stabilize ion-pair transition states at low temperatures and via multi-ion-pair electrostatic interactions to entropically stabilize ion-pair transition states at high temperatures.