DOUBLE-ROTATION AND VARIABLE FIELD AL-27 NMR-STUDY OF DEALUMINATED Y-ZEOLITES

Al-27 MAS n.m.r. spectroscopy has been applied to dealuminated Y zeoli tes at two separate fields. The spectra clearly show that the nonframe work aluminum species associated with a peak observed at 30 ppm are di fferent in samples that had different hydrothermal treatments. In samp les that were treated with a single thermal dealumination, the peak at 30 ppm is caused by a tetrahedrally coordinated aluminum species with a sufficiently large quadrupole coupling to cause significant second- order shifts. The resonance at 30 ppm in a sample that was dealuminate d using two thermal treatments did not exhibit large second-order shif ts. These results were confirmed by double-rotation (DOR) n.m.r., whic h averages the anisotropy of the second-order quadrupolar interactions . The DOR spectra clearly show the existence of two tetrahedral specie s at 59 (framework) and 47 (nonframework) ppm in the sample treated wi th a single dealumination.