Solid-State NMR Studies of the As-Synthesized AlPO₄-5/TPAF Microporous Aluminophosphate

Abstract

As-synthesized AlPO4-5 aluminophosphate molecular sieve prepared in a fluoride medium, using the quaternary tetrapropylammonium (TPA+) cation template, is characterized by a variety of solid-state NMR techniques. 19F MAS spectra show two distinct populations, with the same isotropic chemical shift. While 31P MAS experiments distinguish four different crystalline environments, both 19F−31P CP-MAS and HETCOR experiments reveal that only two signals belong to the AlPO4-5 framework, whereas the remaining two are attributed to crystalline byproducts. 27Al MAS and 3Q-MAS spectra indicate, in addition to the tetrahedral environment, the presence of pentacoordinated Al species in close proximity to fluorine atoms, as is additionally shown by the 19F−27Al HETCOR experiment. An 27Al−{19F} REDOR experiment strongly supports the existence of an Al−F bond (rAl-F < 2 Å which forms the fifth coordination position of one of the twelve aluminum sites of the unit cell.