Hafnia-Silica Cryogels: Solvent-Assisted Textural and Catalytic Control in the Citronellal Cyclization.

A series of hafnia-silica mixed oxides of HfO₂(SiO₂)₈ stoichiometry were prepared by a sol-gel method followed by freeze-drying. Two hafnium precursors, Hf(OiPr)₄ and Hf(OtBu)₄ were examined under a variety of aqueous-alcoholic systems. A decisive dependence of the alcoholic solvent on the gel formation and the surface and porosity of the final materials was found, which in turn were key to the enhancement of catalysis. Hence, acid hydrolysis and gelification of Hf(OiPr)₄ in tert-butyl alcohol, followed by lyophilization, yielded a material with the highest mesoporosity and specific surface area. This catalyst behaved most efficiently in the acid-catalyzed cyclization of citronellal to isopulegol at room temperature. Furthermore, this cryogel also presented combined Lewis acidity and hydrogen bonding that resulted in higher diastereoselectivity for the above reaction. These results highlight the effect of how the textural and, therefore catalytic, properties of the cryogels depend decisively on the solvents chosen for their synthesis. [ABSTRACT FROM AUTHOR]