Spectroscopic characterization of sol-gel-derived mixed oxides

Al2O3-, TiO2-, SiO2-, and ZrO2-based mixed-oxide supports were synthesized by sol-gel techniques using 2,4-pentanedione as the complexing/templating agent. These materials have been investigated by employing nitrogen adsorption, Al-27, Si-29, and H-1 solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and Fourier transform laser Raman spectroscopy (FT-Raman). Nitrogen adsorption isotherms of the mixed oxides Al2O3-SiO2 and TiO2-SiO2 were of type IV, with hysteresis loops of type II indicating well-defined pore structure, while other mixed oxides Al2O3-TiO2, Al2O3-ZrO2, TiO2-ZrO2, and ZrO2-SiO2 exhibited type II hysteresis loops, indicating pores of undefined nature. Al-27 MAS NMR Studies on the alumina-based mixed oxides showed the presence of four-, five-, and six-coordinated aluminum species, the number of four-coordinated framework aluminum sites being highest in the Al2O3-SiO2 mixed oxide. Si-29 CP MAS NMR spectra of the oxides Al2O3-SiO2, TiO2-SiO2, and ZrO2-SiO2 indicated the substitutional insertion of the cations (Al, Ti, and Zr) into the silica framework. DRIFTS spectra of these mixed oxides, where the vibrations corresponding to Si-O-M' bonds were observed, are in conformity with Si-29 CP MAS results. FT-Raman studies indicated the amorphous nature of the oxides, except in the case of the TiO2-SiO2 sample where the peaks corresponding to titania microcrystallites were noticed.