STUDY OF THE ALUMINOPHOSPHATES ALPO4-21 AND ALPO4-25 BY AL-27 DOUBLE-ROTATION NMR

Aluminum-27 double-rotation NMR in a magnetic field of 11.7 T distinguishes the extremely distorted five-coordinated aluminum sites in the molecular sieve precursor ALPO4(-21). Upon calcination, ALPO4(-21) transforms to ALPO4(-25), which has two tetrahedral aluminum sites with similar isotropic chemical shifts that cannot be resolved in an 11.7 T field. The two tetrahedral environments, however, have a different quadrupole coupling constants and are distinguished by double rotation at 4.2 T field. The quadrupole coupling constants obtained for these sites indicates that the tetrahedral aluminum environments are less distorted in the hydrated material.