NATURE OF THE ADSORBATE-CATALYST INTERACTIONS IN THE ZEOLITE CHEMISTRY. A STUDY BASED ON THE ELECTRON CHARGE DENSITY

ZALAZAR, M. F.; PERUCHENA, N. M.

Santiago de Compostela

Congreso; Ninth Triennal Congress of the World Association of Theoretical and Computational Chemists, WATOC2011; 2011

Since the 70's, experimental and theoretical efforts have been carried out to gain an understanding of the mechanism for the conversion of methanol to hydrocarbons catalyzed by acidic zeolites. In the last years of active work, not only have the suggested mechanisms from the experimental field been theoretically examined, but also new ones have been proposed from theoretical calculations. The results which have been obtained so far still arouse controversies and the discussions are still strongly persistent. Therefore, a study from the analysis of the electron density, aimed to the analysis of the different species which have been proposed as transition states, TS, and reaction intermediates in these complex reactions over zeolite catalysts, can be very useful to understand the different proposed mechanisms. A key step in the conversion of methanol to hydrocarbons (MTH) over the H-ZSM-5 catalyst is the alkene methylation reaction. In this reaction a water molecule is formed, and the methyl cation is located between that water molecule and the alkene double bond. In the present work we carried out a topological analysis of the proposed transition state for this reaction in order to characterize and quantify the intermolecular interactions established between the reactive species. The main purpose of this work is to provide an answer to the following questions: (a) Which bonds are established? (b) What is the nature of bonding interactions? (c) How influence the lattice atoms on stabilizing the TS?. The study was performed in the framework of atoms in molecules theory (AIM), using a cluster model 38T with an overall composition C2H61O59Si37Al. Electron densities were obtained at MP2 and B3LYP levels using a 6-31++G(d,p) basis set, using the Gaussian 03 program. The bond and atomic properties were calculated using the Aim2000 package. Our results shows that the water molecule interacts with the oxygen atoms of the zeolite framework (by hydrogen bonds, HB) and with the methyl cation by a closed shell interaction. At the same time, the carbocation is stabilized by an interaction with a carbon atom of the alkene double bond and by four HB interactions with zeolite oxygen atoms. In summary, the methyl cation is multi-coordinated, H2O¡P¡P¡PCH3+¡P¡P¡Pƒà(C=C). The stability of the TS is achieved due to the stabilizing effect of the surrounding zeolite framework on the carbocation species.