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J. Phys. Chem. B 2006, 110, 22618-22627
Interplay of Bonding and Geometry of the Adsorption Complexes of Light Alkanes within Cationic Faujasites. Combined Spectroscopic and Computational Study Evgeny A. Pidko,*,† Jiang Xu,‡ Barbara L. Mojet,‡ Leon Lefferts,‡ Irina R. Subbotina,§ Vladimir B. Kazansky,§ and Rutger A. van Santen† Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, NL-5600 MB EindhoVen, The Netherlands, Catalytic Processes and Materials, Faculty of Science and Technology, Institute of Mechanics Processes and Control Twente (IMPACT), UniVersity of Twente, Enschede, The Netherlands, and N. D. Zelinsky Institute of Organic Chemistry of Russian Academy of Sciences, Moscow, Russia ReceiVed: June 5, 2006; In Final Form: August 24, 2006
A FT-IR spectroscopic study of methane, ethane, and propane adsorption on magnesium and calcium forms of zeolite Y reveals different vibrational properties of the adsorbed molecules depending on the exchanged cation. This is attributed to different adsorption conformations of the hydrocarbons. Two-fold η2 coordination of light alkanes is realized for MgY, whereas in case of CaY zeolite quite different adsorption modes are found, involving more C-H bonds in the interaction with the cation. The topological analysis of the electron density distribution function of the adsorption complexes shows that when a hydrocarbon coordinates to the exchanged Mg2+ ions, van der Waals bonds between H atoms of the alkane and basic zeolitic oxygens significantly contribute to the overall adsorption energy, whereas in case of CaY zeolite such interactions play only an indirect role. It is found that, due to the much smaller ionic radius of the Mg2+ ion as compared to that of Ca2+, the former ions are significantly shielded with the surrounding oxygens of the zeolitic cation site. This results in a small electrostatic contribution to the stabilization of the adsorbed molecules. In contrast, for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation significantly contributes to the adsorption energy. This is in agreement with the experimentally observed lower overall absorption of C-H stretching vibrations of alkanes loaded to MgY as compared to those for CaY zeolite. The preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes that, in turn, strongly depends on the size and location of the cations in the zeolite cavity.
1. Introduction Cationic forms of faujasite are widely applied as selective adsorbents for gas separation and purification, as well as catalyst supports and catalyst components. Recently, it was found that the alkali- and alkaline-earth exchanged Y zeolites promote selective oxidation of hydrocarbons with O2.1-12 The reaction can take place via either photochemical or thermal activation. One notes that these cations do not show any redox properties and cannot dissociate either hydrocarbon or oxygen molecules. Thus, the activation of the reagents in these zeolites is different from the conventional processes.13-15 The initial coordination of the reagents to the exchanged cations is very important both for gas separation and for their chemical activation. It was shown by us16 that in case of the photochemical process the main role of the cations is to prearrange reactants in the zeolite cavity via adsorption so that they can be activated in the subsequent reaction steps. On the other hand, for the thermal oxidation of propane, the lower activity of the magnesium exchanged Y zeolite as compared to that of CaY was attributed by Xu et al.12 to different adsorption * Corresponding author. Tel.: +31 40 247 2189. Fax: +31 40 245 5054. E-mail:
[email protected]. † Eindhoven University of Technology. ‡ University of Twente. § Russian Academy of Sciences.
properties of the corresponding ions stabilized at the zeolitic cation sites. Obviously, depending on the adsorption fashion and the bonding realized within the resulting complex, the properties and the reactivity of the adsorbed molecules significantly vary. However, there is only limited information concerning activation of alkane molecules loaded into the zeolite. Most of the studies reported17-23 deal mainly with methane adsorption to cationic or hydrogen forms of zeolites, because of the ease of interpretation of the spectroscopic data due to the high symmetry and the size of the molecule. Upon adsorption, the C-H breathing (ν1) mode of CH4 is no longer symmetryforbidden and therefore becomes active in the IR spectrum. The value of the red-shift of the corresponding band has been correlated to the polarizing ability of the exchanged cation.19,23 The adsorption of heavier alkanes by zeolites has been studied to a much lesser extent because it is very difficult to separate the IR spectrum of the hydrocarbon-cation adsorption complexes from those formed due to the physical adsorption of the hydrocarbon on zeolite walls.19 Ethane adsorption on ZnZSM524 or CuZSM-525 is very strong and results in formation of two-fold coordinated (η2) C2H6 to the exchanged cations. Such a strong interaction results in a significant perturbation of the adsorbed molecules that obviously influences the corresponding IR spectrum. The vibration corresponding to the fully symmetrical C-H stretching (ν1) of the gaseous C2H6 was found
10.1021/jp0634757 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/20/2006
Adsorption of Light Alkanes to Cationic Faujasites to be the most sensitive to the nature of the adsorption center. Recently, Pidko and Kazanskii26 have shown by means of a combined DRIFTs and theoretical study that, depending on the nature of the cation site, different adsorption geometries of ethane can be realized, and hence different vibrational properties of the adsorbed alkane are observed. In addition, IR absorption studies showed that propane also adsorbs on alkaline-earth exchanged Y zeolite and polarizes at the supercage cation sites.10,11 However, for light alkanes in cationic zeolites, the exact adsorption geometries as well as their chemical bonding features are not well understood yet. Herein, we report a combined spectroscopic and computational study of methane, ethane, and propane adsorption on magnesium and calcium exchanged Y zeolites. Also, we provide a detailed analysis of the nature of chemical bonding of light alkanes to zeolitic cations. The Atoms in Molecules theory proposed by Bader27 is applied. Using this approach, one can reveal not only covalent interactions but also van der Waals and hydrogen bonds between the interacting species and determine their relative contribution to the total adsorption energy. 2. Experimental and Computational Details The magnesium and calcium exchanged Y zeolites were prepared by triple wet ion exchange of the sodium form of Y zeolite (Akzo Nobel, Si/Al ) 2.5) with a 0.1 M aqueous solution of, respectively, magnesium chloride or calcium nitrate (Merck) for 20 h at 363 K under stirring. After the third ion-exchange, the material was washed three times with distilled water, filtered, and dried at 363 K overnight. The chemical composition of the resulting zeolites was analyzed by X-ray fluorescence. The M2+/ Al ratio was 0.34 for MgY and 0.45 for CaY zeolite. Powder X-ray diffraction showed no collapse of the structures. FT-IR measurements were performed for the self-supported wafers pressed from the zeolite powder (30 mg) using a Bruker Vector 22 FT-IR spectrometer with a mercury cadmium telluride detector. A miniature cell, equipped with NaCl transparent windows, which can be evacuated to pressures below 10-7 mbar, was used for the activation of zeolites and for the spectroscopic studies. The samples were activated in a vacuum (
