The local structure of aluminium sites in zeolites

Abstract

The long range ordering and thus the average structure of crystalline zeolites can be determined by various diffraction and spectroscopic techniques. It has, however, proved difficult to establish the local structure surrounding these aluminium sites by diffraction methods. The most useful information has come from theoretical studies (M. Brandle et al., J. Chem. Phys., 1998, 109, 10379; U. Eichler et al., J. Phys. Chem. B, 1997, 101, 10035) which suggest that the Al–O distance associated with the proton is longer than other Al–O interatomic distances. Employing in situ X-ray absorption fine structure spectroscopy (EXAFS) of the aluminium edge at 1565.6 eV, we report individual bond lengths angles for the local aluminium environment of neutral and acidic zeolites. For two acidic zeolites we find that there is indeed one Al–O distance that is significantly longer than those in a neutral material. We also show that for the average T-atom positions our EXAFS results are consistent with X-ray diffraction measurements, to an accuracy of ca. 0.01 A. Changes in bond angles show how the zeolite structure distorts to accommodate Bronsted acidity.