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-ZrO2 or m-ZrO2 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 ZrO2 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 ZrCl4 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 ZrCl4 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-ZrO2 is favored in strictly non-hydrolytic routes, while m-ZrO2 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 ZrO2 samples was further investigated by electron microscopy (SEM, TEM).

Tuning Texture and Morphology of Mesoporous TiO2 by Non-Hydrolytic Sol-Gel Syntheses.

The development of powerful synthetic methodologies is paramount in the design of advanced nanostructured materials. Owing to its remarkable properties and low cost, nanostructured TiO2 is widely investigated for applications such as photocatalysis, energy conversion or energy storage. In this article we report the synthesis of mesoporous TiO2 by three different non-hydrolytic sol-gel routes, and we investigate the influence of the synthetic route and of the presence and nature of the solvent on the structure, texture and morphology of the materials. The first route is the well-known ether route, based on the reaction of TiCl4 with iPr2O. The second and third routes, which have not been previously described for the synthesis of mesoporous TiO2, involve the reaction of Ti(OiPr)4 with stoichiometric amounts of acetophenone and benzoic anhydride, respectively. All materials are characterized by XRD, N2 physisorption and SEM. By playing with the non-hydrolytic route used and the reaction conditions (presence of a solvent, nature of the solvent, calcination), it is possible to tune the morphology and texture of the TiO2. Depending on the reaction conditions, a large variety of mesoporous TiO2 nanostructures could be obtained, resulting from the spontaneous aggregation of TiO2 nanoparticles, either rounded nanoparticles, platelets or nanorods. These nanoparticle networks exhibited a specific surface area up to 250 m² g-1 before calcination, or up to 110 m² g-1 after calcination.