Implications of Transition State Confinement within Small Voids for Acid Catalysis

Abstract

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. First-order 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.

Transition-state scaling relations in zeolite catalysis: influence of framework topology and acid-site reactivity

“…11 In this work we quantify dispersion interactions in two ways. 11 In this work we quantify dispersion interactions in two ways.…”

“…[1][2][3][4] In solid acid catalysis involving zeolites, catalytic activity and selectivity are determined by acid strength as well as framework topology. [8][9][10][11][12][13] The reaction rates generally increase with acid strength and confinement, but it remains unclear how to specifically tailor framework and acidity of zeolites to improve catalytic activity. [8][9][10][11][12][13] The reaction rates generally increase with acid strength and confinement, but it remains unclear how to specifically tailor framework and acidity of zeolites to improve catalytic activity.…”

“…[20][21][22][23] When using zeolites and other solid Brønsted acids as catalysts it is a central challenge to quantify the acidity of the material. 7,11,21,23,25,26 However, recent works indicate that DPE is an incomplete descriptor of the acid-site reactivity that determines catalytic activity of solid acids. 7,11,21,23,25,26 However, recent works indicate that DPE is an incomplete descriptor of the acid-site reactivity that determines catalytic activity of solid acids.…”

Kinetic, Spectroscopic, and Theoretical Assessment of Associative and Dissociative Methanol Dehydration Routes in Zeolites

“…[3a, 4,10] Zero-order rate constants are similar on MFI samples with 0.05-10 mm crystals and 0.7-3.6 H + per unit cell (0.012-0.019 DME (H + s) À1 , 433 K); [4] thus, diffusional constraints did not influence measured rates, which reflect the intrinsic reactivity of Brønsted acid sites within MFI. [4] These pressure effects on turnover rates reflect the prevalence of monomers at low pressures and of dimers at higher pressures; they cannot be described by dissociative (sequential) routes because these would lead to zero-order or negative-order reactions when monomers or dimers are predominant species, respectively [Eq.…”

“…(1)) prevail on MFI at typical conditions of dehydration catalysis, and by inference, on other zeolites (FAU, SFH, BEA, MTW, MOR, MFI, MTT) in light of their similar kinetic behavior. [10] Direct routes also prevail on the stronger acid sites of polyoxometalates (deprotonation energies of 1080-1143 kJ mol À1[3a] vs. 1171-1205 kJ mol À1 on crystalline aluminosilicates [4,11] ), because changes in acid strength influence the two free energy terms in Equation (3) to similar extents.…”