Aromatic alkylation is an important reaction to produce ethylbenzene, cumene, and ethyltoluene. This work investigates the mechanistic details of toluene alkylation with ethylene on acidic mordenite (H-MOR) zeolite, chosen due to their importance in the chemical industry. H-MOR possesses protons distributed in two distinct microporous environments: eight-membered ring (8-MR) and twelve-membered ring (12-MR) micropores. By investigating reaction kinetics before and after titration of protons in 8-MR with Na+, we aim to elucidate the impact of proton location on reactivity and selectivity. Our findings demonstrate that alkylation rates on protons exclusively within the uniform 12-MR environments increase linearly with ethylene pressure, while remaining independent of toluene pressure. Through in-situ infrared analysis, we confirm that π-bonded toluene is the most abundant surface intermediate under all relevant conditions. Dimerization rates remained undetectable at all conditions, even at high C2H4/C7H8 ratios (> 9; 503-523 K), because the large 12-MR micropore environment selectively stabilizes the C-C coupling transition states for toluene-ethylene reactions without stabilizing that for ethylene-ethylene reactions. However, the inclusion of protons in the smaller 8-MR environments leads to a dramatic shift in rates and selectivities. Alkylation rates per proton are approximately 100 times higher in 8-MR compared to 12-MR, indicating enhanced activity for toluene alkylation. In-situ infrared analysis shows that these protons in 8-MR are saturated with ethoxides via their interactions with ethylene due to their inaccessibility to large toluene molecules. Notably, detectable dimerization rates suggest a shift towards the kinetically relevant C-C coupling step between ethoxide and ethylene, which increases with higher ethylene pressures. These findings offer molecular-level insights into the alkylation process within confined spaces, elucidating the nuanced effects of proton location and confinement effects on reaction kinetics.
