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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.
