Nanocasting Synthesis of Iron-Doped Mesoporous Al−Ti Mixed Oxides Using Ordered Mesoporous Carbon Templates

Abstract

Mesoporous Al−Ti mixed oxides have been prepared by employing pyrrole-based nitrogen-doped mesoporous carbon templates with p6mm and Im3m symmetries. These oxides were obtained using isopropanol solutions of aluminum and titanium isopropoxides, Al(PriO)3 and Ti(PriO)4, respectively, by a combined nanocasting and sol−gel synthesis, involving hydrolysis of the aforementioned precursors in the pores of the carbon templates, followed by controlled thermal treatment of the resulting composites in nitrogen and oxidative removal of the templates in flowing air at 700 °C. The resulting mixed oxides and corresponding oxide−carbon nanocomposites were characterized by thermogravimetry, CHNS elemental analysis, nitrogen adsorption at −196 °C, small and wide angle X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy (HR TEM), energy-dispersive X-ray spectrometry (EDX), and elemental mapping. Nitrogen adsorption revealed that the ordered mesopores of the carbon templates were almost completely filled in a single nanocasting step, and the oxide inverse replicas possessed a specific surface area of about 200 m2/g, large pore volume, and uniform mesopores. The corresponding pore size distributions are narrow and centered at ∼7 nm; the latter value showed a tendency to increase with increasing temperature of the thermal treatment preceding the oxidative removal of the carbon template. In addition, HR TEM analysis confirmed ordered porosity of some oxide samples. The mixed oxide samples were found to be amorphous, if the temperatures of the thermal treatments in nitrogen and air did not exceed 700 °C. At higher temperatures, crystalline phases were gradually formed. The wide angle XRD analysis of the mixed oxides treated at 1000 °C revealed the presence of α-Al2O3 and rutile TiO2 phases. For some nanocomposites, the Magnéli phase Ti6O11 was also detected. Because the carbon templates used possessed iron phases due to employing iron chloride to catalyze polymerization of pyrrole during carbon synthesis, this residual iron was oxidized during carbon removal and transferred to the resulting oxide replicas. Elemental mapping and EDX analysis revealed a homogeneous distribution of Al, Ti, and Fe for all amorphous samples of mixed oxides with the Al/Ti atomic ratios close to those used in the synthesis.