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 zeolites at two separate fields. The spectra clearly show that the nonframework aluminum species associated with a peak observed at 30 ppm are different in samples that had different hydrothermal treatments. In samples 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 dealuminated using two thermal treatments did not exhibit large second-order shifts. These results were confirmed by double-rotation (DOR) n.m.r., which averages the anisotropy of the second-order quadrupolar interactions. The DOR spectra clearly show the existence of two tetrahedral species at 59 (framework) and 47 (nonframework) ppm in the sample treated with a single dealumination.