Acid strength and solvation in catalysis by MFI zeolites and effects of the identity, concentration and location of framework heteroatoms

Abstract

The effects of heteroatom identity (Al3+, Ga3+, Fe3+, or B3+), concentration and location on catalysis by MFI zeolites are examined and interpreted mechanistically using methanol dehydration rate constants and density functional theory estimates of acid strength (deprotonation energies, DPE). In doing so, we shed light on the concomitant effects of confinement and acid strength on catalytic reactivity. Rate constants (per H+ from pyridine titrations during catalysis) in the first-order and zero-order kinetic regimes decreased exponentially as the DPE of MFI with different heteroatoms increased. These trends reflect a decrease in the stability of ion-pair transition states relative to the relevant precursors (H-bonded methanol and methanol dimers, respectively, for these two regimes) with decreasing acid strength and resemble those in mesoporous solid acids (e.g., polyoxometalates). Confinement effects, weaker in mesoporous solids, give larger rate constants on MFI than on POM clusters with similar DPE. Such reactivity enhancements reflect the effects of MFI voids that solvate transition states preferentially over smaller precursors via van der Waals interactions with the confining voids. Both dehydration rate constants on MFI with 0.7–2.4 H+ per unit cell volume (5.2nm3) are independent of Al or H+ densities, indicating that neither H+ location nor acid strength depend on acid site concentration. Higher site densities (3.6 H+ per unit cell) lead to larger first-order rate constants, but do not influence their zero-order analogs. These data reflect, and in turn provide evidence for, the initial siting of H+ in less constrained channel intersections and their ultimate placement within the more solvating environments of the channels themselves. Thus, the higher reactivity of Al-rich samples, often attributed to the stronger acid sites, arises instead from the ubiquitous role of zeolites as inorganic solvents for the relevant transition state, taken together with H+ siting that depends on Al density. We find that heteroatom composition, but not Al density, influences acid strength in MFI, consistent with experiment and theoretical estimates of DPE, and that methanol dehydration rate constants, properly interpreted, provide relevant insights into the combined effects of acid strength and confinement on the reactivity of solid Brønsted acids.