Characterization of zeolites and amorphous silica-aluminas by means of aluminum-27 nuclear magnetic resonance spectroscopy: A multifield, multiparameter investigation

We have obtained the aluminum-27 nuclear magnetic resonance (n.m.r.) spectra of a large number of ZSM-5 (MFI) zeolites and amorphous silica aluminas by using static, spin-echo, and "magic-angle" sample-spinning (MAS) techniques, at various magnetic field strengths in the 2.35-11.7 Tesia range. Our results give the mean nuclear quadrupole coupling constants (QCC), the static and MAS line widths, the chemical shifts and shift distributions, the spin-spin relaxation times (T_2H), as well as the effects of hydration on T_2H. We show that such a multiparameter analysis permits an unambiguous differentiation between framework and nonframework (ZSM-5 and amorphous silica-alumina) aluminum sites, which is of use in characterizing the structures of complex commercial zeolite catalysts. We present a theoretical approach to the determination of the QCC values in zeolites and silica-aluminas based upon the field dependence of the MAS or static line widths, which also yields an approximate bond-angle distribution that is found to be in accord with that deduced from X-ray diffraction and ²⁹Si n.m.r.