STRUCTURAL INVESTIGATION OF ZINC-OXIDE CLUSTERING IN ZEOLITE-A AND SODALITE

The formation of small zinc oxide clusters in fully Zn-exchanged LTA z eolite and partially Zn-exchanged sodalite submitted to appropriate th ermal treatments was investigated by X-ray absorption spectroscopy at the Zn K-edge. The absence of extraneous ZnO or ZnAl2O4 particles as w ell as the changes in the zeolite framework produced by thermal treatm ents was checked by X-ray diffraction as well as Si-29 and Al-27 MAS N MR. In all the samples, the Zn cations are tetracoordinated to oxygen. In fully Zn-exchanged zeolite A, the EXAFS spectra result from the co ntribution of inequivalent Zn2+ cations belonging to clusters of diffe rent size located in either the sodalite cages or the cc-cages. In par tially Zn-exchanged sodalite, after calcination, the presence of alumi num belonging to the framework as second neighbors of Zn at 2.86 Angst rom is demonstrated, whereas Zn neighbors of Zn are moved away to 3.39 Angstrom instead of 3.24 Angstrom in crystalline ZnO. The results are consistent with the adsorption on the sodalite framework of clusters involving three Zn cations probably hydrolyzed and belonging either to the same cage or to three adjacent cages. (C) 1998 Elsevier Science B .V.