from first principles PP-I-1

PP-III-108Acidic Properties and Location of Zn Cation in Mordenite by IRSpectroscopy and DFT Simulations via CH 4 AdsorptionSubbotin A.N., Sokolova N.A., Kuz’min I.V., Subbotina I.R., Zhidomirov G.M.,Kazansky V.B.N.D. Zelinsky Institute of Organic Chemistry RAS, Moscow, Russiasubbot@ioc.ac.ruIn many cases chemical activation of different molecules adsorbed by cation exchangedzeolites results from their polarization by cations. In ZSM-5 where the negatively charged[AlO 4 ] tetrahedra are strongly distantly separated from each other only one half of them isneutralized by bivalent cations resulting in superacidic adsorption Lewis sites while thesecond part of negatively charged [AlO 4 ] tetrahedra is free. For Zn 2+ cations in ZnZSM-5zeolite the unusual type of cationic positions results to dissociative adsorption of CH 4molecule with the formation of bridged hydroxyl group and Zn-CH 3 structure [1]. In thiswork we presented evidence by IR spectroscopy that dissociative adsorption of CH 4 moleculeoccurs on Zn 2+ cations in mordenite with small Si/Al (Si/Al =6). To explain the obtainedresults acidic properties of Zn 2+ cations in dependence of location of these cations in structureof mordenite with different membered rings were studied by DFT simulation of adsorption ofCH 4 in mordenite. This provide the vibrational frequencies, intensities of IR bands andbinding energies of adsorbed CH 4 molecule with Zn 2+ cations located in different positions indifferent-membered rings of mordenite. The main attention was paid to theoretical studies ofadsorption of CH 4 on Zn 2+ in 12-membered rings in main channel with far distance Al atomsin this rings and preferred location of Zn 2+ near one of this.It was found that adsorption of adsorption of CH 4 on Zn 2+ location near one of Al atom of Alin 8-membered rings with far distance of Al atoms and in 12 -membered rings with neardistance of Al atoms leads to hydrogen-bonding of CH 4 with oxygen of lattice. This resultscould lead to increasing to shift of vibrational frequencies, intensities of IR bands that notsupported by IR spectroscopy of adsorbed CH 4 . Nevertheless the experimental data ofvibrational frequencies, intensities of IR bands of adsorbed CH 4 are in good agreement withtheoretical calculation of adsorbed CH 4 on 12 -membered rings with far distance of Al atoms.It was calculated that adsorption of CH 4 on mordenite in this case could occur to dissociativeadsorption of CH 4 molecule at moderately elevated temperatures.[1] V.B.Kazansky, A.I. Serykh, E.A.Pidko; J. Catal. 225 (2004) 369-373268