(691f) A Kinetic, Spectroscopic, and Theoretical Study on Toluene Alkylation with Ethylene on Acidic Mordenite

Aromatic alkylation with light alkene on acidic zeolites provides important pathways to produce ethylbenzene, cumene, and ethyltoluene products. This work provides mechanistic details of toluene (C₇H₈) alkylation with ethylene (C₂H₄) on acidic mordenite by combining kinetic, spectroscopic, and theoretical methods. All protons in a small eight-membered ring (8-MR) are selectively exchanged with Na⁺, allowing the kinetic inquiry on protons solely in 12-MR environments. Alkylation products were the only products detected, whereas C₂H₄ dimerization/oligomerization products were not detected at all conditions tested. Alkylation rates remained independent of C₇H₈ pressure but increased linearly with C₂H₄pressure, suggesting Ï€-bonded C₇H₈ as the most abundant surface intermediates. Such a conclusion is consistent with a larger adsorption equilibrium constant for C₇H₈ than for C₂H₄ from infrared and theoretical results. Dispersion-corrected density functional theory (DFT) results show that favorable van der Waals interactions between Ï€-bonded C₇H₈ and the micropore are responsible for its favorable formation compared to ethoxide from C₂H₄. This Ï€-bonded C₇H₈ reacts with co-adsorbed C₂H₄ in a concerted manner to form ethyltoluene (C₉H₁₂) products. DFT-derived kinetically relevant transition state involves a concurrent transfer of H⁺ to the C₂H₄ moiety and the C-C formation with C₇H₈. The preferential formation of o- and p-C₉H₁₂ is expected due to inductive and resonance effects. Yet, measured isomer selectivity between o-, m-, and p-C₉H₁₂ reached 3:1:1 even at near-zero conversion (< 0.05%), indicating the formation of all isomers as primary products. Such selectivity is consistent with DFT predictions. The extent of H⁺ transfer to C₂H₄ at the C-C coupling transition state is greater in forming m-C₉H₁₂ than o-C₉H₁₂, making it a better electrophile and thus compensating for less nucleophilic nature at the m-position. These results provide molecular-level pictures of the alkylation process occurring within confined spaces that have remained unclear and even debatable in the literature.