Characterization of methyl viologen in the channels of zeolite L

The degree of exchange of methyl viologen (MV2+) within the channels of zeo lite I, microcrystals was determined as a function of the amount of MV2+ ad ded to an aqueous zeolite L suspension. Care was taken to remove molecules that have been adsorbed on the outer zeolite surface. Thus, the obtained va lues for the maximum occupation probability per unit cell are 0.78 for the commercial and 0.85 MV2+ for the self-synthesized potassium zeolite L for w hich the equilibrium constant was found to be in the order of 10(4) at room temperature. Adsorption isotherms and BET results show that the self-synth esized 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 presen ted. They are compared with a MVCl2-KBr pellet and MV2+-Y zeolite spectra. The MV2+-L zeolite spectra indicate weak interactions 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 t he zeolite can be used as an internal standard for fast and nondestructive determinations of the MV2+ loading. Raman spectra an better suited for this purpose than IR. The reason for this is that the TR intensities of the zeo lite framework vibrations at about 1050 cm(-1) are much higher than those f or all MV2+ modes, while the only strong zeolite framework Raman band at ab out 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 c hannels 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 d egrees.