Preparation of porous MTiOx (M = Cr, Mo, or W) composite oxides from a mesostructured material of titanium oxysulfate and surfactant micelles based on a wall ion exchange–calcination method

• Porous TiO2-based composite oxides were prepared through wall ion exchange (WIE).
• Surface areas and pore sizes of Cr–Ti oxides were 296–430 m2 g−1 and 1.7–2.5 nm.
• The Cr/Ti ratio of 0.30 or above was needed to form pore structure.
• Only Cr ions at pore surface were oxidized to Cr(VI) upon calcination at 673 K.
• Porous Mo- and W-Ti oxides with surface areas of ca. 200 m2 g−1 were also obtained.

Wall ion exchange (WIE) and subsequent calcination were applied to prepare porous TiO2-based composite oxides using a composite of TiOSO4·xH2SO4·xH2O and cetyltrimethylammonium bromide (TS). Chromium oxyanion HCrO4− in an aqueous solution was ion-exchanged stoichiometrically with HSO4− in TS at pH values in the range of 2.2–5.5. Calcination of the HCrO4−-introduced TS (Cr–TS) at 673 K for 2 h in air produced Cr–Ti oxide with surface areas of 296–430 m2 g−1, pore diameters of 1.7–2.5 nm, and a wormhole-like pore structure. These high surface areas could be achieved at ratios of Cr and Ti in the Cr–Ti oxide of 0.30–0.39 or above. XAFS, FT-IR, and UV–vis diffuse reflectance spectra indicated the occurrence of dimerization of HCrO4− to Cr2O72− during drying of the Cr–TS. The Cr2O72− species was then reduced to Cr(III) species upon calcination at 473–573 K. The Cr(III) species located on the surfaces of the pore walls were reoxidized to Cr(VI) upon calcination at 673 K, whereas those inside the pore walls remained unchanged. The present WIE–calcination method was applied to oxyanions of Mo, W, and V but did not produce the stoichiometric exchange that occurred in the Cr–TS system. Porous Mo- and W-Ti oxides with surface areas of 197–258 and 218–273 m2 g−1, respectively, were obtained upon treatment at approximately pH 10 and subsequent calcination at 673 K, although the reaction mechanisms were unknown. No porous V-Ti oxides with a high surface area could be obtained.

Figure optionsDownload as PowerPoint slide