(706e) Consequences of Confinement within and across Zeolite Frameworks on Relative Rates and Selectivities of Aromatic Methylation Reactions

Aromatic methylation reactions are catalyzed by solid Brønsted acids, wherein controlling selectivity among similarly-sized regio-isomer products is desired. On unconfined acids, kinetically-controlled selectivity for toluene methylation to xylene isomer products (ortho-, meta-, para-) is dictated by the relative stability of arenium transition states, governed by resonance and inductive effects. Within microporous zeolites, xylene formation transition state free energies depend sensitively on confining void size, leading to higher selectivities to p-xylene (403 K) on TON (>70%; ~0.55 nm voids) than on BEA (~30%; ~0.67 nm voids), and to similar p-xylene selectivities at H⁺ sites confined within smaller channels (~0.55 nm) than larger intersection voids (~0.70 nm) of MFI.¹ Herein, we generalize these concepts by considering arene reactants (benzene, toluene, para-xylene) and, thus, transition states of varying sizes confined within differently sized voids (TON, MFI, BEA, FAU, MCM-41). Reaction kinetics measured at low temperatures (403–433 K) and aromatic pressures (<10 kPa) minimize contributions of dealkylation, transalkylation, and disproportionation and show a transition from a first-to-zero order dependence in aromatic concentration as H⁺ sites transition from covered by methylating species to covered by co-adsorbed aromatic reactants.² First-order rate constants reflect transition state free energies relative to gaseous aromatics, permitting separation of transition state stability from the co-adsorbed state. First-order rate constants show a non-monotonic dependence on the fit between the transition state and confining void; values are maximized when sizes of transition states and confining voids are similar but decrease by more than an order of magnitude as surfaces become too large or too small to confine and stabilize transition states effectively. Quantifying confinement effects across transition state and void sizes provides a complementary approach to studies of a given transition state within varying frameworks, providing guidance for zeolite synthesis strategies to alter active site distributions to increase desired selectivities among regio-isomers.³