(706d) Effects of Proton Location in Acidic Mordenite Zeolites Toward Activity and Selectivity of Toluene Alkylation

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.