Evolution of structure and of grafting properties of β-alumina with pretreatment temperature

In this study, the nature of the hydroxyl groups present on β-alumina, β-Al ₂O ₃, 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(CH ₂tBu) ₄, 1, on β-Al ₂O ₃ at various pretreatment temperatures T (350, 500, 700 °C), referred to as 1-β-Al ₂O _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-CH ₂tBu and Al-CH ₂tBu for 1-β-Al ₂O _3-(500) and 1-β-Al ₂O _3-(700), hence the existence of surface cationic low coordinated hafnium complexes. In contrast, for 1-β-Al ₂O _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.