(7d) Benzene Alkylation with Light Alkenes on Acidic Mordenite: The Effects of Acid Size Locations and Confinements on Reaction Mechanisms

Benzene/toluene alkylations with light alkenes are important reactions to produce cumene, ethylbenzene and ethyltoluene. Acidic zeolites are widely used in the petrochemical industry due to their high reactivity, non-corrosivity, and environmentally friendly nature compared to traditional liquid acids. Yet, reaction pathways of benzene alkylation on solid acids have remained uncertain. Moreover, the role of microporous structure in zeolites in reactivity, selectivity, and stability for benzene alkylation pathways has remained unclear. This work aims to probe mechanistic details of alkylation pathways and the impacts of microporous structures in zeolites on the reactivity, selectivity, and stability of benzene alkylation catalysis by combining such methods. We use toluene alkylation with ethylene and propylene as model reactions in order to avoid the use of carcinogenic benzene reactants; toluene alkylation catalysis also offers additional information of C-C formation locations via unique isomer formations, while still exhibiting similar reaction scheme as benzene alkylation. Acidic MOR zeolite is chosen as our model catalyst due to its industrial relevance. MOR zeolite also contains H⁺ sites located at two distinct porous environments, allowing us to study the effects of pore confinement on the catalytic activity, selectivity, and stability by selectively exchanging H⁺ sites in 8 membered ring (MR) with Na⁺. In doing so, we show that such selective titrations significantly improve the stability of toluene alkylation by suppressing alkene dimerization. Kinetic, spectroscopic, and theoretical methods were combined to show that the H⁺ sites in 12 MR are saturated with Ï€-bonded toluene, which reacts with ethylene to form ethyltoluene products. In contrast, H⁺ sites in 8 MR are inaccessible to toluene reactants and are predominantly used for alkene dimerization reactions, which occur via alkoxide intermediates. These results provide insight into how the microporous structure can be tuned to fit the transition state of the desired product leading to improved selectivity.