Interaction of C3-C5 Alkenes with Zeolitic Bronsted Sites: π-Complexes, Alkoxides, and Carbenium Ions in H-FER

We use a chemically accurate (4 kJ/mol) hybrid MP2:(PBE+D2) + Delta CCSD(T) method to determine relative stabilities of all possible pi-complexes, alkoxides, and carbenium ions formed from propene, butene, and pentene with the Al(2)O(7) Bronsted acid site in H-FER. The energetic order is carbenium ions > tert-alkoxides > pi-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 CS surface species are 2-propoxide, 2-butoxide, and the 2-methyl-2-butene pi-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 pi-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, pi-complexes), and the Gibbs free energy order becomes carbenium ions and tert-alkoxides > primary and secondary alkoxides > pi-complexes.