| Abstract
| - The local aluminum structure in zeolite mordenite was studied at temperatures up to 1000 K in a vacuum byAl K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The interatomic aluminum−oxygendistances and the number of coordinating oxygen atoms were determined by Fourier transform analyses ofexperimental Al K-edge XANES spectra and the fits of the nearest oxygen atoms contributions, using alimited number of variables. The values of fixed parameters for Fourier transform and fit are establishedfrom the spectrum of Na−mordenite, considered the reference compound for the studied zeolites H−mordenites,which was also used to test the accuracy and the stability of the determined structural parameters. To revealthe aluminum coordination in H−mordenite at various temperatures, the Fourier transform peak of thecoordinating oxygen polyhedron was fitted first with a single-shell model, and the obtained structuralinformation was refined by the fits, on the basis of the most plausible models for the aluminum coordinationenvironment. The choice of such models for each temperature was performed according to the qualitativepredictions on the aluminum local atomic structure provided by the preedge data analysis and 27Al magicangle spinning (MAS) NMR experiments. By this method, the presence of sixfold aluminum atoms, asidefrom the fourfold ones, in H−mordenite at room temperature was revealed quantitatively, and the concentrationsof these mixed coordinations were determined; the structural distortion of the oxygen tetrahedron aroundaluminum in dehydrated H−mordenite at T = 575 K was found to be strong, and the corresponding Al−Odistances for this distortion were obtained; for H−mordenite at 985 K, the presence of threefold coordinatedaluminum atoms, aside from the fourfold ones, was revealed, and an estimate of the amount of threefoldaluminum was given.
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