A Multinuclear Solid-State
NMR Study of Templated
and Calcined Chabazite-Type GaPO-34
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Abstract
The open-framework gallophosphate GaPO-34 is prepared
with either
1-methylimidazole or pyridine as the structure-directing agent. <sup>13</sup>C and <sup>1</sup>H NMR spectra for these two variants of
the as-made GaPO-34 are fully assigned, confirming the presence of
the protonated amine and water within the pores of both materials. <sup>31</sup>P MAS NMR confirms the presence of three crystallographic
P sites, while <sup>71</sup>Ga MAS and MQMAS NMR spectra reveal three
crystallographic Ga sites: two tetrahedral and one six-coordinate.
Simulations of <sup>69</sup>Ga MAS NMR spectra from these results
are in good agreement with spectra acquired at <i>B</i><sub>0</sub> = 20.0 T, and assignments are supported by first-principles
calculations. <sup>19</sup>F MAS NMR proves the presence of Ga-bridging
fluoride within the as-made materials, leading to the six-coordinate
gallium. Calcination removes the organic species and fluoride, yielding
a microporous chabazite-type GaPO<sub>4</sub>, containing one tetrahedral
Ga site. Exposure to moist air yields calcined, rehydrated GaPO-34
containing four-, five-, and six-coordinate gallium. Upon heating
this material, loss of crystallinity is observed by powder X-ray diffraction
and NMR, with the latter revealing a range of P and Ga environments.
The thermal instability of calcined, rehydrated GaPO-34 contrasts
with the isomorphous aluminophosphate, showing that apparently analogous
materials may have important differences in reactivity