The incorporation of iron ions in AlPO4-5 molecular sieves after microwave synthesis studied by EPR and Mossbauer spectroscopy

FeAPO-5 samples with up to 3 mol% Fe were synthesized by microwave crystallization from Fe(II)- and Fe(III)containing gels. The samples were studied as-synthesized and during thermal treatment in air, N-2 and NH3 using in situ EPR and Mossbauer spectroscopy. The iron ions are mainly incorporated in lattice positions substituting for aluminium. During calcination, Fe(II) is easily oxidized to Fe(III). The Fe(III) ions in both the as-synthesized and rehydrated calcined samples are located on the inner pore surface. They have octahedral symmetry coordinating to framework oxygen atoms and ligand molecules in the pores as well. The non-framework ligands are released on calcination leaving behind tetrahedral Fe(III). Structural changes of the iron coordination sphere caused by adsorption/desorption processes are reversible. (C) 1998 Elsevier Science B.V.