Consequences of confinement within zeolites on rate constants reflect the stabilization of a transition state relative to MARI species, typically varied by examining a reaction within frameworks comprising differing void sizes.
1 Herein, we study arene methylation on porous materials of varying voids (TON, *BEA, AFI, FAU, MCM-41) and a single material (MFI) synthesized with different structure-directing agents (SDAs)—tetrapropylammonium (TPA) or TPA+ethylenediamine (EDA)—to bias Al distributions towards intersection (~0.70 nm) or smaller channel voids (0.55 nm), respectively. Methylation of different arenes form transition states of varying size and structure (e.g., those forming toluene, xylenes (
meta-,
ortho-,
para-), and 1,2,4-trimethylbenzene), resulting in finer variations in relative guest-host sizes. We unify relative effects of confinement on intermediate-transition state pairs after accounting for differences in intrinsic electronic stability and accounting for void (D
void) and transition state size (D
critical). The (de)stabilization ascribed to confinement across zeolitic materials and arenium size is captured by fitting a single van der Waals intermolecular potential to the average distance between the arenium transition state and the confining void. This correlation permits interpretation of differences observed among MFI materials, wherein
para-xylene selectivity is much higher on sites within the channels (>80%) than within intersections (~30%).
2 MFI with varied acid strength introduced by substituting different trivalent heteroatoms (Al
3+, Fe
3+, Ga
3+, B
3+), prepared using TPA or TPA+EDA confirm that rate constants decrease exponentially with increasing deprotonation energy (DPE), while rate constant ratios (i.e., xylenes selectivity) correlate with the SDA used, providing evidence that generalizes the influence of SDA-induced heteroatom siting. First- and zero-order methylation rate constants depend similarly on confinement and acid strength despite arene reference states in the gas phase and adsorbed within zeolite voids,
3 respectively, suggesting that equilibrium constants for net reactions that adsorb toluene and remove DME-derived species from surface methoxy groups depend weakly on confinement.
