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Ion exchange; Zeolite A, X, Y; MAS NMR; Mössbauer spectroscopy; XPS; XRD
Abstract:
During ion exchange under atmospheric conditions, zeolites A, X, and Y facilitate the oxidation of Fe2+ to Fe3+ to very different extents, ranging from complete oxidation of the ferrous cations to ferric species to almost complete preservation of the ferrous state. Drying the exchanged zeolites at elevated temperature favors the oxidation reaction. The chemical environments of the low-silica zeolites A and X seem to promote the oxidation reaction more strongly than that of the more siliceous zeolite Y. Iron-rich surface layers are formed on the external surface of the zeolite particles, resulting in an apparent over-exchange especially for the low-silica zeolites. The structural integrity of the zeolite samples is differently affected by the exchange in the weakly acidic exchange solutions. The observed structural damage ranges from an almost unaffected framework, as observed for zeolite Y after one exchange, to structural collapse, as observed for zeolite X after repeated exchanges.
