Implications of Transition State Confinement within Small Voids for Acid Catalysis

The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the strength) of the protons that act as Brønsted acids. Firstorder rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton locations; first-order rate constants also depended exponentially on these enthalpies. In contrast, zero-order rate constants, which reflect the stability of transition states relative to protonated CH3OH dimers similar in size, depended weakly on dispersive stabilization, whether measured from experiment or simulations, because dispersive forces influence species similar in size to the same extent. These results, taken together, demonstrate the preeminent effects of confinement on zeolite reactivity and the manner by which the local voids around protons held within diverse intracrystalline environments give rise to the unique behaviors that have made zeolites ubiquitous in the practice of catalysis. Enthalpic stabilization of relevant transition states prevail over entropic losses caused by confinement at low temperatures in a manner reminiscent of how catalytic pockets and solvents do so in catalysis by molecules or enzymes.