Preparation of WO3 aerogel catalysts using supercritical CO2 drying

Single tungsten oxide aerogels (WO3), binary oxide aerogels (WO3–Al2O3) and ternary oxide aerogels (WO3–SiO2–Al2O3) were prepared using standard sol–gel route. Tungsten oxide tetraethoxide (WO(OCH2CH3)4) was used as the sol–gel precursor. The excellent properties of the gels obtained by the sol–gel synthesis were preserved upon supercritical drying with CO2. After supercritical drying at 40 °C and 100 bar, all aerogels were calcined to 800 °C. The influence of the synthesis parameters on the catalytic activity of WO3 as supported on silica and/or alumina aerogels was investigated through the transformation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine. Despite including WO3 into single and mixed silica and alumina aerogels, high specific surface areas (284–653 m2 g−1) were preserved. Higher conversion was obtained for catalysts with higher ratios of WO3 in the mixed silica–alumina aerogels that were calcined at 800 °C.