Identification of fluorine sites at the surface of fluorinated gamma-alumina by two-dimensional MAS NMR

By means of Al-27 triple quantum Magic-Angle Spinning Nuclear Magnetic Reso nance (3QMAS NMR) and Al-27{F-19} WISE MAS NMR, we were able to detect thre e different Al-F sites on the surface of fluorinated gamma-alumina. Three F -19 resonances at 9, 20, and 33 ppm (from C6F6) correlated to Al-27 resonan ces in the octahedral range. While the positions of the maxima in the Al-27 dimension were ill-defined due to the inherently low efficiency of the Al- 27{F-19} CPMAS process, the center of gravity of the lines shifted signific antly upfield in that dimension with increasing wt.% F. Tentatively, these three resonances were assigned to Al-VI(O6-nFn)(n = 1, 2, 3) environments o n the F/gamma-Al2O3 surface. At F contents above levels corresponding to th e full fluorination of the gamma-Al2O3 surface, neoformation of an AlF3. 3H (2)O phase was also evidenced F-19 resonance at -8 ppm and with an Al-27 re sonance at -17 ppm. (C) 2000 Elsevier Science B.V. All rights reserved.