HYDRATION OF ALPO4-11 STUDIED WITH X-RAY-POWDER DIFFRACTION AND AL-27 AND P-31 NMR

The influence of water adsorption in AlPO4-11 has been studied with X-ray powder diffraction and Al-27 and P-31 NMR. With X-ray powder diffraction, a change of the crystal symmetry from IMA2 to PNA21 is observed upon water adsorption, resulting in a diminished unit-cell volume and more elliptical pores. NMR was used to study the hydration on an atomic scale. With Al-27 double rotation (DOR) and magic angle spinning (MAS) NMR, the quadrupole coupling constants (C(Q)) and asymmetry parameters (eta) of the different Al sites in dry and wet AlPO4-11 were determined and correlated with the symmetry of the Al site obtained from literature data. In contrast with earlier Al-27 DOR measurements,1 spectral simulations of Al-27 MAS and DOR NMR spectra measured at 7.0 T prove the preferential hydration of one tetrahedral aluminum site in AlPO4-11 (five crystallographically different sites), transforming this site reversibly into octahedrally coordinated aluminum. 2 From the observed correlation between the XRD data and C(Q), it was concluded that the Al2 site becomes preferentially octahedrally coordinated. On the basis of Al-27 DOR and P-31 MAS NMR measurements performed on the partially hydrated samples, it is shown that the transformation upon hydration is a local process, comparable to the symmetry change observed upon adsorption of sorbates in ZSM-5.3,4