A Multinuclear Solid-State NMR Study of Templated and Calcined Chabazite-Type GaPO-34

Abstract

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