Synthesis and characterization of a fluorinated anionic aluminophosphate framework UT-6, and its high-temperature dehydrofluorination to AlPO4-CHA

Abstract

The structure of an anionic aluminophosphate molecular sieve UT-6 has been determined by single-crystal X-ray diffraction and ¹⁹ F MAS NMR, ³¹ P CP MAS NMR and ²⁷ Al MQ MAS spectroscopy. Large crystals were grown in a non-aqueous synthesis system when hydrogen fluoride was used in trace amounts. The title compound, [Al ₃ P ₃ O ₁₂ F] ^- [C ₅ H ₅ NH] ⁺ ·0.15H ₂ O, crystallizes in the triclinic space group P (no. 2), with Z=2, a=9.118(1), b=9.161(1), c=9.335(1) Å, α=85.98(1), β=77.45(1), γ=89.01(1)°, V=759.25(14) Å ³ , R ₁ =0.0280 and wR ₂ =0.0830. UT-6 is a small pore material that has a three-dimensional network of channels running through the structure. The structure is closely related to that of chabazite. In addition to alternating tetrahedral phosphorus and aluminium atoms connected by bridging oxygens, there are also isolated pairs of octahedral aluminiums in the four-membered rings of the UT-6 framework that share two bridging fluorine atoms. The resulting negative charge on the framework is balanced by pyridinium cations that reside in the chabazite cages. Upon thermal treatment of UT-6, pyridine molecules and HF are removed from the structure and the material transforms into rhombohedral AlPO ₄ -CHA, as evidenced by in situ high-temperature powder X-ray diffraction, thermogravimetry and mass spectrometry. This represents the first solid-state transformation of an anionic aluminophosphate molecular sieve framework to an entirely neutral one.