High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies
Résumé
The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al-5(OH)(PO4)(3)(PO3OH)(4)] [NH3(CH2)(2)NH3](2) [2H(2)O], compound 1, and [Al-5(PO4)(5)(PO3OH)(2)] [NH3(CH2)(3)NH3](2) [H2O], compound 2. We have previously reported the structure of the periodic part of 1 by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocksdetermined by analysis of the Al-27-P-31 correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of 2, using the powder synchrotron diffraction data as cost function. Both 1 and 2 are layered materials, in which the inorganic layers contain five P and seven Alinequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO4, AlO5, and AlO6, and the interlayer space contains the amines and water molecules. In 1, the inorganic layers are stacked on each other with a 42 element of symmetry along the c-axis, while they are stacked with a 180 degrees rotation angle in 2. By analysis of a set of high-resolution 1D and 2D NMR spectra (P-31, Al-27, H-1, N-15, C-13, Al-27-P-31, H-1-P-31, and H-1-N-14) the structure analysis of 1 and 2 is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data.