Contrasting confinement effects at aluminosilicate protons within mesopores versus external surfaces in reactions of polyaromatics

The concept of confinement in zeolite catalysis is invoked to describe dispersive host–guest interactions as facilitated by micropores of molecular dimension. This study extends confinement effects to mesoporous aluminosilicate catalysts in reactions involving (poly)aromatic reactant molecules and transition states that approach relevant mesopore length scales. In alkylation reactions between 1,3,5-trimethylbenzene and dibenzyl ether, which cannot access protons in medium- and small-pore zeolitic micropores, congruent enthalpic and entropic energy barriers were extracted from zeroth order rate constants for MFI and Al-MCM-41. These equivalent reactivities on zeolite crystal surfaces and Al-MCM-41 mesopores for reactants that are “small” relative to proton environments additionally indicate that these protons are of similar acid strength. Contrastingly, nonidentical rates in self-etherification of 1-pyrenemethanol (PM), a tetracyclic primary alcohol, were revealed to result from PM monomer and dimer MASI identities in MFI and Al-MCM-41, respectively, which yielded a difference in their apparent activation energies. The discrepancy in monomer and dimer site coverages was attributed to disfavored dimer formation in Al-MCM-41, as dimers approached Al-MCM-41 pore diameters, whereas MFI external surfaces lacked this confinement-induced restriction. These findings illustrate how rates (and in turn, selectivities) dictated by stabilizing van der Waals in zeolites can be sensitive to such forces in systems as the one presented herein where guest species are comparable in size to their mesoporous containers, potentially affording an additional lever to consider in catalyst design and selection for bulky molecule transformations.