OPTICAL AND X-RAY ABSORPTION-SPECTROSCOPY STUDIES OF IRON MFI-TYPE ZEOLITE PREPARED IN FLUORIDE MEDIUM

Two samples of MFI-type ferrisilicate molecular sieves that were synthesized in the presence of fluoride ions, and having a Si/Fe molar ratio of 31 and 56, were characterized by optical and X-ray (at the iron K-edge) absorption spectroscopies. In the as-synthesized samples, the Fe3+ ions are present in the zeolite framework in a four-fold coordination with an average Fe-O distance approximately 1.86-1.87 angstrom. After template removal by calcination and subsequent hydration, most of the iron atoms are in a distorted tetrahedral environment with a second-neighbor oxygen subshell at an Fe-O distance of 2.18 angstrom. This second subshell might be assigned to water sorbate molecules in interaction with Fe3+ Lewis acid sites in the framework.