MULTIPLE-QUANTUM AL-27 MAS NMR-SPECTROSCOPY OF MICROPOROUS ALPO-40 AND SAPO-40

Two-dimensional triple-and quintuple-quantum Al-27 magic-angle spinnin g (MQ MAS) n.m.r. spectra of microporous AIPO-40 and SAPO-40 have been recorded at 9.4 and 14.1 T. The resolution of the 5Q spectra of AIPO- 40 is amazing, at least 11 resonances being resolved, The 3Q spectra f urther reveal the presence of 2 other much fainter peaks. Because the AIPO-40 aluminium sites have different quadrupole coupling constants t he 9.4 T spectra are slightly better resolved than the 14.1 T spectra, suggesting that in MQ n.m.r. it may sometimes be advantageous to work at lower magnetic fields. The 3Q spectra of SAPO-40 are poorly resolv ed displaying a main broad peak and two faint resonances. This is beca use the introduction of even a small amount of Si into the framework o f AIPO-40 generates a distribution of Al sites and a dispersion of che mical shifts. MO Al-27 MAS n.m.r. together with P-31 MAS n.m.r. eviden ce discard an orthorhombic, Peen, and monoclinic, P112/n, structure fo r as-prepared or calcined dehydrated AIPO-40, suggesting a space group with even lower symmetry that proposed to date. (C) Elsevier Science Inc. 1997.