CATION ENVIRONMENT AND MIGRATION IN IRON-EXCHANGED ZEOLITE NA-Y STUDIED BY ESR

The location, immediate environment and mobility of various Fe(III) ce ntres in Fe, Na-Y zeolite are discussed on the basis of their ESR char acteristics during and after dehydration/rehydration cycles, adsorptio n/desorption of pyridine and low temperature physisorption of oxygen. Apart from the g approximate to 2.0 signal associated with hexaaqua-Fe (III) complexes (species A), two types of ESR signals, characteristic of centres with different degrees of hydration, and contributing to ab sorption at g approximate to 4.3, have been identified. On the basis o f their ESR characteristics they have been described as [Fe(H2O)(6-n)O -n]((3 - 2n)+) (species B) and [Fe(OH)(n)O-4 - n]((n - 5)+) (species C ), appearing under mild and severe dehydration conditions respectively . In the temperature range 293-373 K dehydration of Fe,Na-Y zeolite is reversible. Rehydration restores the original spectrum which indicate s both mobility and coordination flexibility of Fe(III) centres in Fe- exchanged Y zeolite in this temperature range. On rehydration of a sam ple dehydrated at 473 K that parr of the ESR spectrum associated with Fe(III) aqua complexes cannot be restored. It is postulated that this effect might be due to partial clustering of hydrated iron species. Ex periments with adsorption of pyridine and low temperature physisorptio n of oxygen have shown that species B and C are sensitive to both reag ents, which implies their location in supercages of the zeolite struct ure.