CHARACTERIZATION OF ZEOLITES AND AMORPHOUS SILICA-ALUMINAS BY MEANS OF AL-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 Tesla 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 (T2H), as well as the effects of hydration on T2H. 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 Si-29 n.m.r.