First-Principles Study on the Monomolecular Isomerization of Ethylbenzene within H-BEA Zeolite

To decipher the reaction network involving ethylbenzene in the petrochemical industry, the monomolecular isomerization process of ethylbenzene to xylene has been studied using density functional theory. With a combination of ab initio molecular dynamics simulations and metadynamics, free energy landscapes characterizing eight elementary steps, viz., ethylidene/methylene transfer, Buchner and retro-Buchner ring expansion, intramolecular hydride transfer, deprotonation of cationic bicyclo[4.1.0] hydrocarbon, and protonation of norcaradiene, have been determined. The Buchner and retro-Buchner ring expansions are found to be the rate-limiting step with the highest free energy barrier of 205 kJ/mol. While the destruction of the cationic diaryl intermediate via the reverse Friedel–Crafts alkylation step is kinetically comparable. Further charge analysis reveals that the highest free energy barrier for the Buchner and retro-Buchner ring expansion step can be attributed to the electron transfer between two strongly positively charged atom groups. This comprehensive computational study offers valuable insights into the intricate details of the ethylbenzene isomerization process, shedding light on the underlying mechanisms of this important reaction in the petrochemical industry.