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

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


Ithisuphalap, K. - Presenter, The City College of New York, The City University of New York
Monroe, H., NREL
Kwon, S., Colorado School of Mines
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 π-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 π-bonded C7H8 and the micropore are responsible for its favorable formation compared to ethoxide from C2H4. This π-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.