Understanding the Nanoconfinement Effect on the Ethanol-to-Propene Mechanism Catalyzed by Acidic ZSM-5 and FAU Zeolites

JB Yin and XX Guo and YX Sun and S Han and QG Li, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 310-334 (2021).

DOI: 10.1021/acs.jpcc.0c07614

We have carried out a two-layer our own n-layered integrated molecular orbital and molecular mechanics (MM) study on the mechanism of ethanol to propene on H-ZSM-5 and H-FAU zeolites. The entire mechanism is divided into four reaction pathways (I, II, III, and IV). In reaction pathways I and II, ethanol is converted into propene in acidic zeolites. In reaction pathways III and IV, the coreaction of ethanol with propene was investigated. The rate-determining steps are the dehydration of ethanol (pathways I and II) and the ethylation of propene, 1-pentene, and 2-pentene by ethanol (pathways III and IV) over two zeolites. Four pathways have almost the same reactivity on two zeolites, and H-ZSM-5 and H-FAU are both favorable for the formation of propene. Six types of reaction steps are involved in all four pathways, and the calculated results demonstrate the following order of reactivity on two zeolites: proton transfer > deprotonation > dimerization > beta-scission > ethylation dehydration of ethanol. The medium-pore ZSM-5 leads to entropy-increased transition-state (TS) structures more easily than the large-pore FAU zeolite in pathway I. The ZSM-5 framework has a stronger stabilizing effect on the formation of TS structures than the FAU framework in pathways I, II, and IV on the basis of the analysis of MM energy values; no clear variation trend of MM energies is found in pathway III. The difference charge density, reduced density gradient, and localized orbital locator plots reveal the nature of TS structures and explain the complicated van der Waals attractive interaction and spatial repulsive interaction between different molecular fragments in the TSs from the point of view of electron transfer. The diffusion behaviors of ethanol and olefin molecules are investigated by molecular dynamics simulations. The ZSM-5 and FAU zeolites describe different diffusion pictures for different sizes of molecules at low and high temperatures because of their different zeolitic topological channels.

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