Determination of temperature-induced changes in the structure of the nearest environment of aluminum in mordenite zeolites from x-ray absorption near-edge structure spectra

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The structure of the nearest environment of aluminum in mordenite zeolites was investigated at temperatures up to 1000 K using the Al K x-ray absorption near-edge structure (XANES) spectra. The interatomic Al-O distances and the number of coordinating oxygen atoms were determined by Fourier transformation of Al K-XANES spectra and fitting of the contribution of the nearest oxygen atoms. The exactness and stability of the obtained values of structural parameters were verified using the Al K-XANES spectrum of the model Na-mordenite zeolite. To determine the coordination of aluminum in H-mordenites in the temperature range under study, fitting of the contribution of coordinating oxygen atoms was performed using both the single-sphere model of the nearest environment of aluminum and two-sphere fitting for the most probable structural models of the nearest environment of aluminum. Such models were chosen based on the analysis of the specific preedge features and fine structure of the XANES spectra, as well as NMR data. With this approach, the simultaneous presence of tetra-and hexacoordinated aluminum in H-mordenite at T = 300 K was established and the concentrations of such mixed states were determined. The presence of strong distortions of the oxygen tetrahedron coordinating the aluminum atom in dehydrated H-mordenite at 575 K was shown and the Al-O distances for such distortions were determined. The simultaneous presence of tri-and tetracoordinated aluminum in H-mordenite at 985 K was established and the fraction of tricoordinated aluminum was estimated quantitatively.