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

Calcined dehydrated AlPO4-5 has been studied by X-ray powder diffracti on, P-31 MAS, and Al-27 MAS and double-rotation (DOR) NMR. The results indicate that three crystallographically different sites can be disti nguished 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 wi dth of the XRD peaks between 2theta = 19-degrees and 2theta = 23-degre es. 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)), quadrup olar coupling constants (C(Q)), and the asymmetry parameters (eta) of the different sites in dry AFI. The interaction of water, methanol, am monia, and acetonitrile with AlPO4-5 and AlPO4-11 is studied by XRD an d 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-coordi nated aluminum. The relative amounts of five- and six-coordinated alum inum depend on the crystal structure and the interacting sorbate. Wher eas water in AlPO4-11 is interacting with one specific aluminum site ( Al2), the interaction of water with AlPO4-5 occurs randomly. This lead s to a broadening of the tetrahedral signals at higher water loading ( above 10-12 wt %). At low MAS frequencies some dipolar broadening occu rs, whereas at high spinning frequencies or 850-1000 Hz DOR the broade ning is caused by site distribution. Methanol and acetonitrile do not interact with framework atoms. These molecules are too big to coordina te to framework aluminum in AFI and AEL type molecular sieves. The bro adening of the spectral features observed after methanol adsorption is apparently due to an increase in chemical shift dispersion brought ab out by nonspecific interactions of methanol with the framework, thereb y causing a larger spread in Al-O-P angles.