Water Formation in Non-Hydrolytic Sol-Gel Routes: Selective Synthesis of Tetragonal and Monoclinic Mesoporous Zirconia as a Case Study.

Several non-hydrolytic sol-gel syntheses involving different precursors, oxygen donors, and conditions have been screened aiming to selectively produce mesoporous t-ZrO ₂ or m-ZrO ₂ with significant specific surface areas. The in situ water formation was systematically investigated by Karl Fisher titration of the syneresis liquids. XRD and nitrogen physisorption were employed to characterize the structure and texture of the ZrO ₂ samples. Significant amounts of water were found in several cases, notably in the reactions of Zr(OnPr) ₄ with ketones (acetone, 2-pentanone, acetophenone), and of ZrCl ₄ with alcohols (benzyl alcohol, ethanol) or acetone. Conversely, the reactions of Zr(OnPr) ₄ with acetic anhydride or benzyl alcohol at moderate temperature (200 °C) and of ZrCl ₄ with diisopropyl ether appear strictly non-hydrolytic. Although reaction time and reaction temperature were also important parameters, the presence of water played a crucial role on the structure of the final zirconia: t-ZrO ₂ is favored in strictly non-hydrolytic routes, while m-ZrO ₂ is favored in the presence of significant amounts of water. ¹ H and ¹³ C NMR analysis of the syneresis liquids allowed us to identify the main reactions responsible for the formation of water and of the oxide network. The morphology of the most interesting ZrO ₂ samples was further investigated by electron microscopy (SEM, TEM).
(© 2020 The Authors. Published by Wiley-VCH GmbH.)