A COMBINED NMR AND XRD STUDY OF AFI AND AEL TYPE MOLECULAR-SIEVES

Calcined dehydrated AlPO4-5 has been studied by X-ray powder diffraction, P-31 MAS, and Al-27 MAS and double-rotation (DOR) NMR. The results indicate that three crystallographically different sites can be distinguished in the structure of dehydrated AlPO4-5 in the ratio 1: 1: 1. The observed splitting of the NMR spectra is correlated to the line width of the XRD peaks between 2theta = 19-degrees and 2theta = 23-degrees. Simulations of Al-27 DOR (4.7,9.4, and 11.7 T) and MAS spectra (7.0 and 9.4 T) yield the isotropic chemical shifts (delta(iso)), quadrupolar coupling constants (C(Q)), and the asymmetry parameters (eta) of the different sites in dry AFI. The interaction of water, methanol, ammonia, and acetonitrile with AlPO4-5 and AlPO4-11 is studied by XRD and Al-27 DOR and MAS NMR. Both water and ammonia are able to coordinate to part of the framework aluminum, leading to five- and/or six-coordinated aluminum. The relative amounts of five- and six-coordinated aluminum depend on the crystal structure and the interacting sorbate. Whereas water in AlPO4-11 is interacting with one specific aluminum site (Al2), the interaction of water with AlPO4-5 occurs randomly. This leads to a broadening of the tetrahedral signals at higher water loading (above 10-12 wt %). 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. Methanol and acetonitrile do not interact with framework atoms. These molecules are too big to coordinate to framework aluminum in AFI and AEL type molecular sieves. The broadening of the spectral features observed after methanol adsorption is apparently due to an increase in chemical shift dispersion brought about by nonspecific interactions of methanol with the framework, thereby causing a larger spread in Al-O-P angles.