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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