A NEXAFS Study on the Coordination Geometry of Aluminum in Minerals and Y Zeolite. Cornparison of Experiment and Theory

Abstract
Cuved-wave multiple scattering cluster calculations using the FEFF6 code were successfully used to interpret experimental Al K edge NEXAFS spectra of various minerals and Y zeolites, starting about 5 eV above threshold. The Al octahedral, tetrahedral and square planar geometries can be easily distinguished from each other utilizing the NEXAFS data. Semi-empirical molecular orbital calculations were used to interpret the whitelines below 5 eV in the data. A new assignment is given to the NEXAFS peaks found for Al atoms having a distorted octahedral coordination. Use of NEXAFS for determining the geometrical conformations of a Y zeolite involving Al atoms with mixed conformational geometry is emphasized. In H⁺Y zeolite, separate contributions to the NEXAFS spectra from tetrahedrally and octahedrally coordinated Al atoms are identified. There are also strong indications for the presence of penta-coordinated Al atoms accompanying the non-framework octahedrally coordinated Al atoms in the H⁺Y zeolite. We have found that detailed interpretations of the NEXAFS data complement the EXAFS and NMR results by providing unique geometrical information on the Al coordination.