A high-resolution MAS NMR study of the structure of fluorinated NiW/γ-Al2O3 hydrotreating catalysts

The structures of fluorinated oxidic and sulfided W/gamma-Al2O3 and NiW/gamma-Al2O3 catalysts were studied by means of multinuclear MAS NMR and BET measurements. The quantitative H-1 and F-19 MAS NMR results show that fluorination of gamma-alumina favors the formation of polymeric tungstate in the catalysts and promotes the sulfidation of tungsten. Addition of tungsten to fluorinated gamma-Al2O3 leads to the transformation of part of the type-S1 fluorine, bonded to one aluminum atom, to the type-S3 fluorine, bonded to three aluminum atoms. Sulfidation results in a reorganization of the hydroxyl groups on the surface of fluorinated alumina and in a partial hydrolysis of the alumina network because of the H2O produced during sulfidation and the inductive effect of fluorine on the alumina lattice. Nickel reduces the NMR signal intensities in the oxidic samples because of its strong paramagnetic effect on the H-1, F-19, and Al-27 resonances. Sulfidation decreases the paramagnetic effect of nickel because of the combination of nickel with WS2 into the Ni-W-S structure.