High-Resolution
Structural Characterization of Two
Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR
Crystallographies
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Abstract
The
syntheses and structure resolution process of two highly complex
powdered aluminophosphates with an original 5:7 Al/P ratio are presented:
[Al<sub>5</sub>(OH)(PO<sub>4</sub>)<sub>3</sub>(PO<sub>3</sub>OH)<sub>4</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>]<sub>2</sub> [2H<sub>2</sub>O], compound <b>1</b>, and [Al<sub>5</sub>(PO<sub>4</sub>)<sub>5</sub>(PO<sub>3</sub>OH)<sub>2</sub>] [NH<sub>3</sub>(CH<sub>2</sub>)<sub>3</sub>NH<sub>3</sub>]<sub>2</sub> [H<sub>2</sub>O], compound <b>2</b>. We have previously
reported the structure of the periodic part of <b>1</b> 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 blocks determined by analysis of the <sup>27</sup>Al–<sup>31</sup>P correlation pattern of the two-dimensional
(2D) NMR spectrum in the structure search process, we first determine
the periodic structure of <b>2</b>, using the powder synchrotron
diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five
P and seven Al inequivalent atoms, with aluminum atoms that are found
in three different coordination states, AlO<sub>4</sub>, AlO<sub>5</sub>, and AlO<sub>6</sub>, and the interlayer space contains the amines
and water molecules. In <b>1</b>, the inorganic layers are stacked
on each other with a 4<sub>2</sub> element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation
angle in <b>2</b>. By analysis of a set of high-resolution 1D
and 2D NMR spectra (<sup>31</sup>P, <sup>27</sup>Al, <sup>1</sup>H, <sup>15</sup>N, <sup>13</sup>C, <sup>27</sup>Al–<sup>31</sup>P, <sup>1</sup>H–<sup>31</sup>P, and <sup>1</sup>H–<sup>14</sup>N), the structure analysis of <b>1</b> and <b>2</b> 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
