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

摘要

Wall ion exchange (WIE) and subsequent calcination were applied to prepare porous TiO2-based composite oxides using a composite of TiOSO4 center dot xH(2)SO(4)center dot xH(2)O and cetyltrimethylammonium bromide (TS). Chromium oxyanion HCrO4- in an aqueous solution was ion-exchanged stoichiometrically with HCrO4- in TS at pH values in the range of 2.2-5.5. Calcination of the HCrai-introduced TS (Cr-TS) at 673 K for 2 h in air produced Cr-Ti oxide with surface areas of 296-430 m(2) 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 Cr2O3- during drying of the Cr-TS. The Cr2O7- 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 m(2) 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. (C) 2014 Elsevier Inc. All rights reserved.