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

In this study, the nature of the hydroxyl groups present on γ-alumina, γ-Al₂O₃, pretreated at various temperatures has been reinvestigated by ¹H NMR spectroscopy. The peaks are assigned by comparison between experimental and simulated spectra, in agreement with previous IR studies. The lowest chemical shifts δ correspond to OH groups strongly bound to the most acidic Al atoms (Al_IV and Al_V). High chemical shifts δ 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(CH₂<i>t</i>Bu)₄, <b>1</b>, on γ-Al₂O₃ at various pretreatment temperatures <i>T </i>(350, 500, 700 °C), referred to as <b>1-</b>γ-Al₂O_3‑(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–CH₂<i>t</i>Bu and Al–CH₂<i>t</i>Bu for <b>1</b>-γ-Al₂O_3‑(500) and <b>1</b>-γ-Al₂O_3‑(700), hence the existence of surface cationic low coordinated hafnium complexes. In contrast, for <b>1</b>-γ-Al₂O_3‑(350), 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