Characterization of Methylviologen in the Channels of Zeolite L

Brian Hennessya, Silke Megelskia, Claudia Marcollia, Valery Shkloverb, Christian Bärlocherb and Gion Calzaferria
aDepartment of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland
bLaboratory of Crystallography, ETH-Zentrum, CH-8092 Zürich, Switzerland

Abstract

The degree of exchange of methylviologen (MV2+) within the channels of zeolite L microcrystals was determined as a function of the amount of MV2+ added to an aqueous zeolite L suspension. Care was taken to remove molecules that have been adsorbed at the outer zeolite surface. The thus obtained values for maximum occupation probability per unit cell are 0.78 for the commercial and 0.85 MV2+ for the self synthesized potassium zeolite L for which the equilibrium constant was found to be in the order of 104 at room temperature. Adsorption isotherms and BET results show that the self synthesized zeolite has a larger specific surface area as well as a larger and thus more accessible pore volume for the MV2+ than the commercial sample. IR spectra of very thin layers on ZnSe in high vacuum and Raman spectra at ambient conditions, of MV2+-L zeolite at different loading levels, are presented. They are compared with MVCl2-KBr pellet and MV2+-Y zeolite spectra. The MV2+-L zeolite spectra indicate weak interaction between the MV2+ and the zeolite framework. They also indicate that the two pyridyl rings of the intercalated MV2+ are twisted. It was found that framework vibrations of the zeolite can be used as an internal standard for fast and non destructive determination of the MV2+ loading. Raman spectra are better suited for this purpose than IR. The reason for this is that the IR intensities of the zeolite framework vibrations at about 1050 cm-1 are much higher than all MV2+ modes, while the only strong zeolite framework Raman band at about 500 cm-1 is narrow and well isolated and of similar intensity to the relevant MV2+ signals at high loading. On the basis of Rietveld refinement of x-ray data and molecular modeling, a model of the MV2+ location in the channels of zeolite L is proposed. The MV2+ lies along the channel wall and the angle between the main MV2+ axis and the c-axis of the zeolite is 27.

published in: J. Phys. Chem. B 1999, 103, 3340.


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