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Abstract

The stability of Co²⁺ - and Ni²⁺-exchanged zeolites strongly depends on the exchanged transition metal and the experimental conditions of the ion-exchange. An overexchange of both Co²⁺ and Ni²⁺ is observed with the zeolites A and X, enhanced at elevated temperatures. The interaction of the zeolite famework with acidic species formed during the exchange leads to structural damage. After the collapse of the zeolite frameworks during the calcination of the exchanged zeolites, the crystallization of CoAl₂O₄ and NiAl₂O₄ starts. During the calcination of Ni²⁺-exchanged zeolite A and X, NiO crystallizes prior to the formation of spinel and dissolves as the spinel crystallization starts. Depending on the zeolite precursor and the thermal conditions, SiO₂ phases such as quartz or cristobalite are formed. The amounts of crystalline and amorphous phases vary with calcination temperature and time. The phase formation processes are dependent on the ion-exchange procedures and thermal treatments. By controlling these parameters, materials with well-defined properties are accessible.