Characterization of iron in Fe-VPI-5 and its thermally transformed product, FAPO-8

Iron incorporation in VPI-5 occurs within a narrow range of gel composition and hydrothermal treatment conditions. Physico-chemical characterization was carried out by X-ray diffraction, SEM, thermogravimetry and solidstate NMR. The ESR studies showed that Fe³⁺ incorporation occurs into lattice sites only; no occluded Fe species are observed. A significant amount of framework iron was removed during the thermal transformation of Fe-VPI-5 to FAPO-8, forming aggregates of occluded iron species in the pores. During the process, part of the Fe³⁺ reduced to Fe²⁺ with concurrent observations of a signal near zero field.