HYDRATION OF ALPO4-11 STUDIED WITH X-RAY-POWDER DIFFRACTION AND AL-27AND 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 diffr action, a change of the crystal symmetry from IMA2 to PNA21 is observe d upon water adsorption, resulting in a diminished unit-cell volume an d more elliptical pores. NMR was used to study the hydration on an ato mic scale. With Al-27 double rotation (DOR) and magic angle spinning ( MAS) NMR, the quadrupole coupling constants (C(Q)) and asymmetry param eters (eta) of the different Al sites in dry and wet AlPO4-11 were det ermined and correlated with the symmetry of the Al site obtained from literature data. In contrast with earlier Al-27 DOR measurements,1 spe ctral simulations of Al-27 MAS and DOR NMR spectra measured at 7.0 T p rove the preferential hydration of one tetrahedral aluminum site in Al PO4-11 (five crystallographically different sites), transforming this site reversibly into octahedrally coordinated aluminum. 2 From the obs erved 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 part ially hydrated samples, it is shown that the transformation upon hydra tion is a local process, comparable to the symmetry change observed up on adsorption of sorbates in ZSM-5.3,4