Influence of thermal treatments on phase composition and acidity of mesoporous tungsten oxide

• Thermal treatments significantly affect the structure and acidity of tungsten oxides.
• Calcination of composites at lower temperatures led to higher CO uptakes and acidity.
• Higher temperatures induce structural transformation and decrease in acidity.

We report on the synthesis of mesoporous tungsten oxides via hard templating method using silicotungstic acid as W precursor and having surface areas and pore diameters up to 68 m2/g and 12.8 nm, respectively. The relation between thermal treatments (calcination temperature of W-KIT-6 composites and reductive treatment of tungsten oxides), crystalline structure and acidity has been systematically investigated. The acidity of mesoporous tungsten oxides was investigated using common pyridine adsorption methods and chemisorption of probe molecules (CO, H2). Combination of XRD and FTIR techniques showed that monoclinic and orthorhombic phases of crystalline wall structure coexist in the calcination temperatures 550–800 °C. Increasing the calcination temperature and decreasing the W/Si mass ratio of composites favors the transformation of orthorhombic phase to more stable monoclinic. Moreover, the acidity as well as ability to chemisorbs CO decreases when the calcination temperature increases. The concentration of acid sites also depends on the temperature of reductive treatment of tungsten oxides: CO uptakes increase twice at the 250 °C and decrease by four times when the temperature increases to 450 °C.

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