Interaction of C₃–C₅ Alkenes with Zeolitic Brønsted Sites: π-Complexes, Alkoxides, and Carbenium Ions in H-FER
2020
Ren, Qinghua | Rybicki, Marcin | Sauer, Joachim
We use a chemically accurate (4 kJ/mol) hybrid MP2:(PBE+D2) + ΔCCSD(T) method to determine relative stabilities of all possible π-complexes, alkoxides, and carbenium ions formed from propene, butene, and pentene with the Al(2)O(7) Brønsted acid site in H-FER. The energetic order is carbenium ions > tert-alkoxides > π-complexes as well as primary and secondary alkoxide species. Primary carbenium ions are not stationary points on the potential energy surface. The energetically most stable C3, C4, and C5 surface species are 2-propoxide, 2-butoxide, and the 2-methyl-2-butene π-complex with energies of −78, −81, and −85 kJ/mol, respectively, for formation from the corresponding alkenes. Compared to the present results, the widely applied PBE+D2 approach overbinds all species, and the energy differences are 18–24, 25–45, and 48–71 kJ/mol for π-complexes, alkoxides, and carbenium ions. Enthalpies and Gibbs free energies are calculated for 323 and 623 K within the harmonic approximation. The calculated adsorption enthalpy of trans-2-pentene, −93 kJ/mol, is in agreement with the experimental value, −92 kJ/mol [Schallmoser et al. J. Am. Chem. Soc. 2017, 139, 8646]. Entropy favors the more mobile species (carbenium ions, π-complexes), and the Gibbs free energy order becomes carbenium ions and tert-alkoxides > primary and secondary alkoxides > π-complexes.
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