DFT—Quantum chemical study of the HZSM-5-cyclohexene interaction pathways

• Published in: Journal of molecular catalysis. A, Chemical Chemical, 2005-07, Vol.236 (1), p.194-205
• Main Authors: Cuán, Angeles, Martínez-Magadán, José Manuel, García-Cruz, Isidoro, Galván, Marcelo
• Format: Article
• Language: English
• Subjects: Alkoxy species; Carbenium ion; Catalysis; Chemistry; Cyclic olefins; Density functional calculations; Exact sciences and technology; General and physical chemistry; Ion-exchange; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Zeolites; Zeolites: preparations and properties
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/j.molcata.2005.04.021

A theoretical analysis of the interaction between cyclohexene and a HZSM-5 zeolite model system is presented. Two different models were used to represent this catalyst: a ring structure model consisting of 10 TO 4 tetrahedral sites (where T = Si, Al) and a smaller model containing 3 TO 4 tetrahedral sites. Two different reaction pathways were studied: the hydrogen exchange between the HZSM-5 cluster model and the cyclohexene molecule, and the proton addition to double bond of the cyclohexene. The alkoxy species can be stabilized/destabilized depending of the arrangement restriction between the olefin and the local geometry of the active site. When cyclohexene molecule interacts with the 3T model the reaction involves an alkoxy species formation. Due to the allowed relaxation in the ring framework around the active site, the local arrangement restriction of the alkoxy species and therefore the covalent alkoxy bond is too weak giving rise to a more ionic intermediate species (carbenium ion-like), which is characterized as a minimum in the PES. By using the non-interacting system, as a reference, and based on the calculated changes in the total energy of the different complexes, one is able to determine that both carbenium ion-like and alkoxy species are destabilized with respect to physisorbed complexes. A theoretical analysis of the interaction between cyclohexene and HZSM-5 zeolite model systems is presented. Two different reaction pathways are fully characterized: hydrogen exchange and proton addition to the double bond of the olefin. The comparisons of two representations of the catalytic site indicate that a ring model gives rise to a more ionic intermediate species. These intermediates are characterized as minima in the PES, but they are destabilized with respect to the corresponding physisorbed complexes. In addition, the structures of the transition states for both trajectories are determined. Transition states geometries involved in, (a) the hydrogen exchange reaction and (b) proton addition to the cyclohexene double bond. ▪