On the nature of iron species in iron substituted aluminophosphates

The local structure of the iron centres in the as-synthesised and calcined FAPO-5 and FAPO-18 catalysts has been investigated using a combination of techniques, including X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Mossbauer spectroscopy and computational modelling. The calcination of the as-synthesised materials has been followed by combined in situ XRD/QuEXAFS measurements, which reveal the different nature of the iron species in the two catalysts. In the as-synthesised FAPO-5 catalyst all the iron centres are present as octahedrally coordinated Fe(III) ions; upon calcination, two water molecules are desorbed when the temperature is raised above 120degreesC to yield tetrahedrally coordinated Fe(III) ions in the active catalyst. In contrast, as-synthesised FAPO-18 contains a mixture of tetrahedrally coordinated Fe(II) ions and octahedral Fe(III) centres. Two independent processes occur upon calcination of FAPO-18: the desorption of water from the octahedral Fe(III) ions at ca. 120degreesC, and the oxidation of the tetrahedral Fe(II) ions at temperatures above 300degreesC; both materials possess tetrahedrally coordinated Fe(III) ions in the calcined state. The three dimensional structure and electronic state of the tetrahedral Fe(III) ions in the activated catalysts has been determined via ab initio quantum mechanical calculations and X-ray absorption spectroscopy.