A combined NMR and XRD study of AFI and AEL type molecular sieves

Calcined dehydrated AlPO4-5 was studied by x-ray powder diffraction, 31P MAS, and 27Al double-resonance (DOR) NMR. Three crystallog. different sites can be distinguished in the structure of dehydrated AlPO4-5 in the ratio 1:1:1. The obsd. splitting of the NMR spectra is correlated to the line width of the XRD peaks between 2q = 19 Deg and 2q = 23 Deg. Simulations of 27Al DOR (4.7, 9.4, and 11.7 T) and MAS spectra (7.0 and 9.4 T) yield the isotropic chem. shifts (diso), quadrupolar coupling consts. (CQ), and the asymmetry parameters (h) of the different sites. The interaction of H2O, MeOH, NH3, and MeCN with AlPO4-5 and AlPO4-11 is studied by XRD and 27Al DOR and MASNMR. Both H2O and NH3 are able to coordinate to part of the framework Al, leading to 5- and/or 6-coordinated Al. The relative amts. of 5- and 6-coordinated Al depend on the crystal structure and the interacting sorbate. Whereas H2O in AlPO4-11 is interacting with 1 specific Al site (Al2), the interaction of H2O with AlPO4-5 occurs randomly. This leads to a broadening of the tetrahedral signal at higher H2O loading (above 10-12%). At low MAS frequencies some dipolar broadening occurs, whereas at high spinning frequencies or 850-1000 Hz DOR the broadening is caused by site distribution. MeOH and MeCN do not interact with framework atoms. These mols. are too big to coordinate to framework Al in AFI and AEL type mol. sieves. The broadening of the spectra features obsd. after MeOH adsorption is apparently due an increase in chem. shift dispersion brought about by nonspecific interactions of MeOH with the framework, thereby causing a larger spread in Al-O-P angles. [on SciFinder (R)