Kinetic, Spectroscopic, and Theoretical Study of Toluene Alkylation with Ethylene on Acidic Mordenite Zeolite

This work combines kinetic, spectroscopic, and theoretical methods to understand mechanistic details of toluene alkylation with ethylene on acidic MOR zeolite, chosen due to its industrial relevance in producing ethyltoluene. In doing so, we show that the protons in the small eight-membered-ring (8-MR) micropores are inaccessible to large toluene molecules, which were selectively titrated with Na+ prior to kinetic measurements. Kinetic data, taken together with in situ infrared spectra and density functional theory (DFT) calculations, show that all protons in the 12-MR are saturated with π-bonded toluene, favoring thermodynamic stability due to extensive dispersive interactions with the porous structure. The π-bonded toluene reacts with ethylene in a concerted manner, where the protonation of ethylene and C–C coupling occurs concurrently. The C–C formation at the ortho position takes precedence over meta and para locations, although the free energies of C–C coupling transition states show marginal differences. Consequently, all three isomers are formed as primary products, with an ortho, meta, and para ratio of 3:1:1 (<0.2% conversion; 503 K). Undesired ethylene dimerization products remained undetectable on partially Na+-exchanged MOR samples under all conditions, consistent with larger DFT-derived barriers for C–C coupling between two ethylene molecules than between toluene and ethylene, reflecting the role of 12-MR micropores that selectively stabilize the latter transition state. These results provide mechanistic insights into aromatic alkylation and the role of micropores on rates and selectivities, which will ultimately inform design strategies for solid acid catalysts with improved catalytic performance.