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A multinuclear solid-state NMR study of templated and calcined chabazite-type GaPO-34

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<mark>Journal publication date</mark>19/07/2012
<mark>Journal</mark>The Journal of Physical Chemistry C
Issue number28
Volume116
Number of pages10
Pages (from-to)15048-15057
<mark>State</mark>Published
<mark>Original language</mark>English

Abstract

The open-framework gallophosphate GaPO-34 is prepared with either 1-methylimidazole or pyridine as the structure-directing agent. C-13 and H-1 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. P-31 MAS NMR confirms the presence of three crystallographic P sites, while Ga-71 MAS and MQMAS NMR spectra reveal three crystallographic Ga sites: two tetrahedral and one six-coordinate. Simulations of Ga-69 MAS NMR spectra from these results are in good agreement with spectra acquired at B-0 = 20.0 T, and assignments are supported by first-principles calculations. F-19 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 GaPO4, 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.