Site-directed cation ordering in chabazite-type AlxGa1- xPO4-34 frameworks revealed by NMR crystallography

Abstract

We report the first synthesis of the mixed-metal chabazite-type Al_xGa_1-xPO₄-34(mim) solid solution, containing 1-methylimidazolium, mim, as structure directing agent (SDA), from the parent mixed-metal oxide solid solution, γ-(Al_xGa_1-x)₂O₃. This hitherto unreported family of materials exhibits complex disorder, arising from the possible distributions of cations over available sites, the orientation of the SDA and the presence of variable amounts of water, which provides a prototype for understanding structural subtleties in nanoporous materials. In the as-made forms of the phosphate frameworks, there are three crystallographically distinct metal sites: two tetrahedral MO₄ and one octahedral MO₄F₂ (M = Al, Ga). A combination of solid-state NMR spectroscopy and periodic DFT calculations reveals that the octahedral site is preferentially occupied by Al and the tetrahedral sites by Ga, leading to a non-random distribution of cations within the framework. Upon calcination to the Al_xGa_1-xPO₄-34 framework, all metal sites are tetrahedral and crystallographically equivalent in the average R3̄ symmetry. The cation distribution was explored by ³¹P solid-state NMR spectroscopy, and it is shown that the non-random distribution demonstrated to exist in the as-made materials would be expected to give remarkably similar patterns of peak intensities to a random distribution owing to the change in average symmetry in the calcined materials.