Structure refinement, electron microscopy, and solid-state magic angle spinning nuclear magnetic resonance characterization of AlPO 4-52: An aluminophosphate with a large cage

Abstract

Synchrotron powder X-ray diffraction (PXRD) data for calcined AlPO 4-52 were refined in space group P 3 1c using the Rietveld method. The previously proposed topology (AFT), an ABC six-ring structure type with an AABBCCAACCBB stacking sequence, was confirmed. The refined unit cell dimensions are a = b = 13.715 (1) Å and c = 29.676 (3) Å. Possible causes for peak broadening in the synchrotron PXRD profile were examined. A series of PXRD patterns obtained at various temperatures indicated that the calcined sample used for synchrotron PXRD was partially rehydrated. Electron diffraction patterns showed particle texture effects but no evidence for faulting or disorder. Individual crystallites of AlPO 4-52 are roughly equidimensional hexagonal prisms. Preferred orientation of the crystallites was not evident. Solid-state magic angle spinning nuclear magnetic resonance (MAS n.m.r.) spectra of calcined never-rehydrated AlPO 4-52 showed that all aluminum and phosphorus atoms were tetrahedrally coordinated. However, MAS n.m.r. spectra of a calcined rehydrated sample showed a 1:2 ratio of tetrahedral to octahedral aluminum atoms due to extraframework water coordination. The 31P MAS n.m.r. spectra are consistent with phosphorus atoms located exclusively in double six-rings.