Abstract
Brønsted acid sites in zeolites are typically described as single sites. Theoretical investigations of proximate acid sites have so far only found or considered indirect effects, where the additional acid sites influence the reactivity but do not directly participate in the reaction. Here, we investigate a case where a second acid site directly takes part in the reaction mechanism, and this leads to a significant lowering of the reaction barrier. This is shown for the side-chain mechanism of the aromatic cycle, which is believed to be the main source of ethylene in the methanol-to-olefins process. We investigate this mechanism based on the heptamethylbenzenium cation in H-SSZ-13 using quantum chemical calculations. We find that cooperative effects arising from the presence of a second acid site in the same cavity lower the barrier of the rate-determining step by about 40 kJ/mol, making this mechanism plausible. For the paring mechanism, the second site only has an indirect influence and leads to a destabilization of the transition state on the order of 15 kJ/mol compared to the single site case. The barriers for both the side-chain mechanism and the paring mechanism are found to be in the range of 150–170 kJ/mol. This is compatible with the experimentally observed formation of ethylene and propylene in comparable amounts. These findings suggest that higher propylene selectivity could be obtained for H-SSZ-13, if the majority of acid sites does not share a cavity with a second acid site. Based on simulations of random Al-siting, this is unlikely for typical Si/Al ratios < 100.
Published Version
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