Cooperative Effects of Active Sites in the MTO Process: A Computational Study of the Aromatic Cycle in H-SSZ-13

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.

Selective Light Hydrocarbon Production from CO2 Hydrogenation over Na/ZnFe2O4 and CHA-Zeolite Hybrid Catalysts

“…Interestingly, ZFO + SSZ-13 demonstrates a notable preference for C 3 hydrocarbons, achieving a selectivity of 25.7%, while ZFO + SAPO-34 exhibits a balanced selectivity across the C 2 –C 4 range. The dominance of C 3 products suggests that ethylene-to-propane (propylene) conversion reaction occurs within the framework of SSZ-13. − Thus, as shown in Figure b, ZFO + SAPO-34 shows a C 2 selectivity of 16.5%, contrasting with ZFO + SSZ-13, which shows a 4.1% reduction in C 2 selectivity and a 4.6% increase in propane production, with propylene selectivity maintaining at 19.5% for both. This result strongly suggests that ethylene-to-propane conversion is dominant over ethylene-to-propylene conversion on SSZ-13.…”

A computational investigation of the decomposition of acetic acid in H-SSZ-13 and its role in the initiation of the MTO process

“…Larger changes in barriers of around ΔΔ G ‡ = 50 kJ mol −1 were observed in a recent investigation of the aromatic cycle, where the second acid site actively participated in the reaction mechanism. 144 We considered the involvement of a second acid site also for the reactions studied in this work, but did not find a mechanism where that seemed beneficial and we consequently performed no computational investigation of zeolite model with more than one acid site. In an earlier study, selected barriers for the MTO initiation mechanism were compared for H-SSZ-13, H-SZM-5, H-BEA and H-SAPO-34.…”