Catalytic properties of nanoscale iron-doped zirconia solid-solution aerogels

Nanoscale iron-doped zirconia solid-solution aerogels are prepared via a simple ethanol thermal route using zirconyl nitrate and iron nitrate as starting materials, followed by a supercritical fluid drying process. Structural characteristics are investigated by means of powder X-ray diffraction (XRD), thermal analyses (TG/DTA), N(2) adsorption measurements and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results show that the resulting iron-doped solid solutions are metastable tetragonal zirconia which exhibit excellent dispersibility and high solubility of iron oxide. Further, when the Fe:(Fe+Zr) ratio x is lower than 0.10, all of the Fe(3+) ions can be incorporated into ZrO(2) by substituting Zr(4+) to form Zr(1-) (x)Fe(x)O(y) solid solutions. Moreover, for the first time, an additional hydroxyl group band that is not present in pure ZrO(2) is observed by DRIFTS for the Zr(Fe)O(2) solid solution. This is direct evidence of Fe(3+) ions incorporated into ZrO(2). These Zr(1-) (x)Fe(x)O(y) solid solutions are excellent catalysts for the solvent-free aerobic oxidation of n-hexadecane using air as the oxidant under ambient conditions. The Zr(0.8)Fe(0.2)O(y) solid-solution catalyst demonstrates the best catalytic properties, with the conversion of n-hexadecane reaching 36.2 % with 48 % selectivity for ketones and 24 % selectivity for alcohols and it can be recycled five times without significant loss of activity.