NMR characterization and Rietveld refinement of the structure of rehydrated AlPO4-34

The triclinic form of AlPO4-34, a microporous aluminophosphate with the cha bazite (CHA) topology, adopts a rhombohedral symmetry upon calcination. The framework structure of this phase remains intact under ambient conditions, but it distorts dramatically, though reversibly, in the presence of water. Following these structural changes in situ by X-ray diffraction revealed t hat there are actually two stable rehydrated phases, which differ from each other by one water molecule in the channel. Both of these phases have tric linic unit cells that are closely related to that of the calcined rhombohed ral phase. The structure of the low-temperature (10 degrees C), fully rehyd rated phase (phase B) was elucidated by combining high-resolution synchrotr on powder diffraction with solid-state NMR techniques. Coordination of thre e of the six Al atoms to water molecules causes the deformation of the fram ework and the reduction of the symmetry. Rietveld refinement of the structu re of phase B in the triclinic space group P1 (a = 9.026, b = 9.338, c = 9. 508 Angstrom, alpha = 95.1 degrees, beta = 104.1 degrees, and gamma = 96.6 degrees) converged with R-F = 0.079 and R-WP = 0.176 (R-exp = 0.087). Frame work connectivities derived from the structure were used to assign P-31 NMR lines as well as part of the Al-27 NMR signal.