ELECTRON NUCLEAR DOUBLE-RESONANCE OF LONG-LIVED RADICALS IN AGED X-TYPE 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. H-1 and Na-23 ENDOR were observed with any aged catalyst. Li-7 and Cs-133 ENDOR were also observed, respectively, in lithium- and caesium-substituted zeolites. The H-1 ENDOR consists of the superposition of a broad doublet and a narrow single line. Li-7 and Na-23 ENDOR are single Lorentzian lines centred about the respective free nucleus frequency. In contrast, Cs-133 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.