Attempts to produce uniform Fe(III) siting in Fe content SOD and LTA zeolites -: An EPR and Mossbauer study

The authors' studies dealing with Fe(III) content sodalites (SOD) and NaA (LTA) zeolites [Appl. Catal. A 223 (2002) 147] revealed a lesser degree of framework (FW) aluminium substitution by Fe(III) than anticipated on the basis of previous publications [Solid State Ionics 53-56 (1992) 1282]. The fairly high stability of SOD structure "withstood" the attack by diluted mineral acids to some degree in that amounts in about 30-40% excess compared to the ion-exchange capacity of the zeolite did not result in complete dissolution, but in amorphisation only, while the solid habit was retained. Fe content LTA samples exhibited lower tolerance against mineral acids. The acid treatment of SOD's (or industrial by-products with high SOD content) combined with addition of reducing and/or complexing agents (like sulphur dioxide, hydroxylamine, EDTA, "phen", "dipy", etc.) permits extraction of extra-framework (EFW), or EFW and FW iron, if necessary. The recrystallisation of fully deferrated, amorphised material makes the synthesis of various zeolites possible with acceptable white colour. X-band EPR and Mossbauer spectra of SOD's and LTA samples and those enriched in Fe-57 tracer, revealed uniform Fe(III) siting in the (partially) deferrated and recrystallised samples, although at low Fe(III) levels. It is an interesting observation that this uniform siting is dominantly FW in SOD's and EFW in LTA's with unexpected low symmetry, unfilled co-ordinations. Besides that, these investigations may contribute to the understanding of Fe(III) ion substitution in three-dimensional tectosilicate frameworks, the LTA preparations can be attractive as selective oxidation catalysts for reactants not excluded from the structure by sieving effects. (C) 2002 Elsevier Science B.V All rights reserved.