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

The effects of heteroatom identity (Al 3+ ,G a 3+ ,F e 3+ ,o r B 3+ ), 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 (5.2 nm 3 ) are independent of Al or H + densities, indicating that neither H