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The e.s.r. spectra of Y zeolites containing different exchangeable cations have been recorded following y-irradiation at 77 K. The samples varied in their cation, proton and A1 contents. The spectra were assigned to Ycentre type defects and were separated into two types: the electron being trapped either by H+ ions, if they are present, or in the insulator material. The stability of the Ycentre is strongly dependent on the presence of electron scavengers such as H+.Discussion of the e.s.r. spectra leads to the conclusion that there is a random distribution of g3 and to a lesser extent of g2 components of the g tensor showing that some fluctuations occur in T-0-Si (T = Si or Al) bond angles and T-0 bond lengths. This heterogeneity of the material closely corresponds to its heterogeneity in acid strength and catalytic activity.It is postulated and evidence is given that the paramagnetic defects correspond to T U S i groups which constitute the zeolitic framework itself, i.e., that the method proposed allows the solid itself to be studied. This conclusion has been applied to the study of aluminium deficient zeolites, It is concluded that silica migrates towards aluminium vacancies during aluminium removal, rebuilding the framework, and that heterogeneity of the material decreases during removal of aluminium.

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