(493c) Toluene Alkylation with Light Alkenes on Acidic Mordenite: The Effects of Acid Size Locations and Confinements on Reaction Mechanisms

Alkylation of aromatics with light alkenes provides reaction pathways to produce industrially important chemicals, including cumene, ethylbenzene and ethyltoluene. Acidic zeolites are widely used as catalysts although mechanistic details of this reaction have remained uncertain in the literature. This work uses toluene alkylation with ethylene as a model reaction to understand how this reaction and undesired ethylene dimerization occur on protons confined within small eight-membered ring (8-MR) and large 12-MR channels in MOR. Our in-situ infrared and theoretical analyses show that protons in 8-MR channels are inaccessible to large toluene reactants, which may infer that these protons are inactive for alkylation. To test this hypothesis, we prepared MOR samples with protons only in 12-MR by selectively titrating protons in 8-MR. In contrast to our hypothesis, measured alkylation rates (per H⁺) are higher for samples with protons in both 8-MR and 12-MR than those with protons only in 12-MR. Our density functional theory (DFT) calculations show that ethoxides, formed on the protons in 8-MR side channels, can “reach” toluene in vicinal 12-MR, forming a C-C bond to form ethyltoluene product. Alkylations on protons in 12-MR followed first order in ethylene pressure and zero-order in toluene pressure, inferring Ï€-bonded toluene as the most abundant surface intermediates (MASI); such a conclusion is consistent with our infrared and DFT results. In contrast, the reaction remained zero-order in ethylene pressure for protons in 8-MR due to the size-exclusion of toluene; ethoxides are MASI on those sites at relevant conditions. These different kinetic responses, in turn, require careful interpretation of rate data for rigorous rate and selectivity comparisons among protons in different environments. These results provide an illustrative example of “pore mouth catalysis”, where the large reactants transform into the products without directly “accessing” the active sites.