Evolution of Structure and of Grafting Properties of γ-Alumina with Pretreatment Temperature

In this study, the nature of the hydroxyl groups present on gamma-alumina, gamma-Al2O3, pretreated at various temperatures has been reinvestigated by H-1 NMR spectroscopy. The peaks are assigned by comparison between experimental and simulated spectra, in agreement with previous IR studies. The lowest chemical shifts delta correspond to OH groups strongly bound to the most acidic Al atoms (Al-IV, and Al-V). High chemical shifts delta are assigned to OH groups making hydrogen bonds. A large range of values is found depending on the strength of these bonds. The structure of the surface complexes obtained by grafting Hf-(CH(2)tBU)(4), 1, on gamma-Al2O3 at various pretreatment temperatures T (350, 500, 700 degrees C), referred to as 1-gamma-Al2O3-(T), and of their thermolysis products has been determined, by a combined experimental (mass balance, in situ IR,) and theoretical (DFT calculations) study. These results unambiguously prove the presence of two kinds of neopentyl-metal bonds, Hf-CH(2)tBu and Al-CH(2)tBu for 1-gamma-Al2O3-(500) and 1-gamma-Al2O3-(700), hence the existence of surface cationic low coordinated hafnium complexes. In contrast, for 1-gamma-Al2O3-(330), only neutral species exist Hence, temperature pretreatment has a key role for controlling the chemistry of the alumina surface (density of OH groups, presence of highly Lewis acidic Al), the grafting mode of the Hf precursor, and the formation of cationic low coordinated active centers.