SBA-15-Supported Mixed-Metal Oxides: Partial Hydrolytic Sol−Gel Synthesis, Adsorption, and Structural Properties

Abstract

Several bimetallic mixed oxides−silica nanocomposites were prepared using SBA-15 as a support and suitable metal-containing reagents. The oxide precursors were obtained by reacting two metal alkoxides, namely, titanium(IV) isopropoxide [Ti(OiPr)4], zirconium propoxide [Zr(OPr)4], and aluminum isopropoxide [Al(OiPr)3], or one of these alkoxides with a solution of nickel nitrate hexahydrate [Ni(NO3)2·6H2O] in isopropyl alcohol (iPrOH). The final condensation of the bimetallic species was achieved by a partial hydrolytic sol−gel route in the pores of SBA-15 silica using small amounts of HCl with concomitant slow evaporation of the solvent. High loadings of these oxides were deposited after several deposition cycles. The as-prepared nanocomposites were characterized by thermogravimetric analysis, whereas nitrogen adsorption at −196 °C was measured for the materials calcined at 300 and 600 °C. Nitrogen adsorption isotherms and the calculated pore-size distributions showed that the deposition of large amounts of the aforementioned oxides in SBA-15 occurred according to an “island-type” mechanism, resulting in the formation of pore constrictions but without damaging the ordered structure of SBA-15. Small-angle X-ray diffraction (XRD) analysis provided additional evidence for the presence of these oxides in the ordered mesopores after thermal treatment below 300 °C and for the diffusion of these species to the external surface of SBA-15 after calcination above 600 °C. Changes in the Al coordination in these oxides with an increase in the temperature of calcination were monitored by Al27 MAS NMR, showing the existence of surface, framework, and oxygen-deficient Al3+ sites. After consecutive depositions, the resulting composites were calcined at higher temperatures (600−800 °C) and characterized by wide-angle powder XRD, which revealed the existence of nanoparticles of double and single oxide phases formed in the aforementioned temperature range. The crystallite size of these nanoparticles estimated using the Scherrer formula were, in general, larger than the silica pore widths, suggesting the surface diffusion of these oxides and sintering of small nanoparticles during thermal treatment.