Adsorption of HF on gibbsite calcined at various temperatures: A solid-state NMR study of low-level fluorinated systems

The fluorination mechanism of low-level fluorinated materials obtained by calcination of Bayer gibbsite at different temperatures was studied by 27Al and 19F solid-state NMR spectroscopy and X-ray diffraction. Gibbsite was calcined at 400, 600, 800, and 1000 degrees C followed by treatment with anhydrous HF/N2 gas at 100 degrees C. Three types of Al-F species on the surface of fluorinated gibbsite calcined at 400, 600, and 800 degrees C are proposed on the basis of the 19F magic angle spinning (MAS) NMR spectra, which show peaks at about -132, -137, and -163 ppm, while the spectrum of fluorinated gibbsite calcined at 1000 degrees C showed five additional peaks, at about -139, -164, -176, -183, and -191 ppm. The fluorination process in aluminas is based on elementary exchange reactions in which F- replaces surface -OH groups according to their basicity. The 27Al MAS NMR and 19F-27Al cross-polarization-MAS experiments confirmed the presence of octahedral Al species.

Automated summary

The fluorination mechanism of low-level fluorinated materials obtained by calcination of Bayer gibbsite at different temperatures was studied using 27Al and 19F solid-state NMR spectroscopy and X-ray diffraction. Different types of Al-F species on the surface of fluorinated gibbsite were proposed based on 19F magic angle spinning (MAS) NMR spectra, showing distinct peaks at various ppm values. The fluorination process in aluminas involves elementary exchange reactions where F- replaces surface -OH groups, confirmed by 27Al MAS NMR and 19F-27Al cross-polarization-MAS experiments.