EXAFS as a probe of local coordination environments of transition metal ions in polycrystalline zeolite catalysts. Co(i i) in hydrated zeolite –A and –Y

Abstract

Synthetic zeolites exchanged with transition metal or rare earth ions are of immense importance in catalysis, both commercially and in basic research; however, difficulty in growing single crystals and problems of long range disorder make crystallographic studies formidable tasks. In this study, extended x-ray absorption fine structure (EXAFS) spectroscopy has been successfully applied to characterize the local coordination of Co(ii) ions in polycrystalline zeolite –A and –Y, thereby establishing the utility of this technique for structural study of intrazeolitic complexes. The Co(ii) ions in Co18.2Na20.7–Y⋅xH2O are found to be ’’solutionlike’’, six-coordinate with Co–O bond distances of 2.06(1) Å, but in Co4.3Na3.4–A⋅xH2O and Co5.5Na1.0–A⋅xH2O, they are five coordinate with Co–O distances of 2.07(1) Å. The latter result is compatible with several published spectroscopic measurements, but is in conflict with a previous single crystal x-ray diffraction study; this discrepancy has been critically analyzed. The experimental data were collected for ∼17 h per sample on an ’’in house’’ EXAFS facility employing a standard 1 kW Mo x-ray tube and focussing optics. Hence, EXAFS appears to be a powerful tool for investigating short range order even in materials as complex as zeolites.