Study of Confinement Effects in H-Beta and H-ZSM-5 Zeolites by Topological Analysis of Electron Density

ZALAZAR, M. FERNANDA; PAREDES, ESTEBAN NADAL; DUARTE, DARIO; PERUCHENA, NÉLIDA MARIA

Santiago de Chile

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists WATOC 2014; 2014

Pontificia Universidad Católica de Chile

Zeolites are microporous/nanoporous solids widely used in fine chemicals and petrochemicals as heterogeneous catalysts. They possess pores, cavities and channels with well-defined molecular dimensions. These three-dimensional cavities provide a selective environment, in which chemical reaction occurs. The confinement effect was proposed to explain the interactions between the zeolite framework and the adsorbed molecule. The confinement effects play an important role on adsorption and catalytic properties of zeolites by stabilizing adsorbed molecules, intermediates, and reaction transition states. In the present work we study the host-guest interactions between catalyst and confined molecules and their relationship with energies involved in the reaction. We selected the reaction of methylation of benzene by methanol in H-ZSM-5 and H-beta zeolites as a case of study. The study was performed in the framework of density functional theory (DFT) and atoms in molecules theory (QTAIM). The zeolite catalyst has been modeled by an extended cluster model 46T for H-ZSM-5 and 52T for H-Beta, with an overall composition H49O68Si45Al and H63O73Si51Al, respectively. Electron densities were obtained at M06-2X and B3LYP levels using a 6-31++G(d,p) basis set and the Gaussian 09 program. The bond and atomic properties were calculated using the AIMAll software. We have employed similar QTAIM analysis in our previous works on the reaction of alkenes over acidic zeolite.[1-4] The topological analysis shows in both catalysts the presence of two principal interactions between the confined molecule with the active site (acid and basic site) and other interactions among the two confined species (methanol and benzene). In addition, several weak interactions with the zeolite framework [denoted as CM-HM...Oz; CB-HB...OZ; OZ...pi(CC)] are observed. The contribution of these weak interactions to the confinement effect is higher in HZSM-5 than in H-Beta, this is reflected in a greater stabilization of confined species within the cavity. Our results show that electron density analysis of host-guest interactions between organic species and two zeolite catalysts with different topologies provide valuable information about the confinement effect and their relationship with energetic parameters of the reaction. Acknowledgments: The authors acknowledge to UNNE, CONICET and FONCYT-ANPCYT for financial support. References: [1] M. F. Zalazar, N. M. Peruchena, J. Phys. Chem. A, 111 (2007) 7848. [2] M. F. Zalazar, D. J. Duarte, N. M. Peruchena, J. Phys. Chem. A, 113 (2009) 13797. [3] M. F. Zalazar, N. M. Peruchena J. Mol. Model., 17(2011) 2501. [4] M. F. Zalazar, N. M. Peruchena, J. Phys. Org. Chem., 27 (2014) 327.