Solid-state NMR spectroscopy of anionic framework aluminophosphates: A new method to determine the Al/P ratio

A series of anionic framework aluminophosphates, with different Al/P ratios, have been investigated by various solid-state NMR techniques, including ²⁷Al, ³¹P magic angle spinning (MAS), ²⁷Al → ³¹P cross polarization (CP), ²⁷A1{³¹P} rotational echo double resonance (REDOR), and ³¹P{²⁷Al} transfer of population double resonance (TRAPDOR). Different Al coordinations (AlO_4b, AlO_5b, and AlO_6b) and P coordinations (PO_4b, PO_3bO_t, PO_2bO_2t, and PO_bO_3t), where b represents bridging oxygens and t represents terminal oxygens, can be unambiguously determined based on the solid-state NMR spectroscopy. Furthermore, a new method to determine the Al/P ratio of open-framework aluminophosphates has been established, which is useful for the understanding of unknown aluminophosphate structures.