Hydration of AlPO4-11 studied with X-ray powder diffraction and 27Al and 31P NMR

The influence of H2O adsorption in AlPO4 11 was studied by using x-ray powder diffraction and 27Al and 31P NMR. X-ray powder diffraction revealed a change of the crystal symmetry from IMa2 to PNa21 upon H2O adsorption, resulting in a diminished unit-cell vol. and more elliptical pores. NMR was used to study the hydration on an at. scale. The quadrupole coupling consts. (CQ) and asymmetry parameters (h) of the different Al sites in dry and wet AlPO4 11 were detd. by 27Al double rotation (DOR) and magic angle spinning (MAS) NMR and correlated with the symmetry of the Al site obtained from literature data. In contrast with earlier 27Al DOR measurements, spectral simulations of 27Al MAS and DOR NMR spectra measured at 7.0 T prove the preferential hydration of 1 tetrahedral Al site in AlPO4 11 (5 crystallog. different sites), transforming this site reversibly into octahedrally coordinated Al. The Al2 site become preferentially octahedrally coordinated based on the obsd. correlation between the x-ray diffraction data and CQ. The transformation upon hydration is a local process (comparable to the symmetry change obsd. upon adsorption of sorbates in ZSM 5) on the basis of 27Al DOR and 31P MAS NMR measurements performed on the partially hydrated sample. [on SciFinder (R)]