Electron-Nuclear Double Resonance of Long-lived Radicals in Aged X- and Y-Type Zeolite Catalysts

During the conversion of 2-methylfuran to 2-methylthiophen on MNaX and MNaY zeolite catalysts (M = Li, Na, K, Rb, Cs), stable radicals are formed. We investigated these by ENDOR spectroscopy. 1H and 23Na ENDOR were observed with any aged catalyst. 7Li and 133Cs ENDOR were also observed, respectively, in lithium- and caesium-substituted zeolites. The 1H ENDOR consists of the superposition of a broad doublet and a narrow single line. 7Li and 23Na ENDOR are single Lorentzian lines centred about the respective free nucleus frequency. In contrast, 133Cs ENDOR is a well resolved doublet. An accurate lineshape analysis was carried out based on models available in the literature. It was found that the observed proton splittings must be ascribed to an isotropic hyperfine interaction. A correlation was found between the experimental proton coupling constant and the calculated lattice oxygen charge. The paramagnetic centre can reasonably be pictured as an AlO4. site. The caesium cation showed markedly different behaviour with respect to the lighter alkali-metal cations.