Preparation of WO3 aerogel catalysts using supercritical CO2 drying

Single tungsten oxide aerogels (WO 3 ), binary oxide aerogels (WO 3 -Al 2 O 3 ) and ternary oxide aerogels (WO 3 -SiO 2 -Al 2 O 3 ) were prepared using standard sol-gel route. Tungsten oxide tetraethoxide (WO(OCH 2 CH 3 ) 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 CO 2 . 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 WO 3 as supported on silica and/or alumina aerogels was investigated through the transformation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine. Despite including WO 3 into single and mixed silica and alumina aerogels, high specific surface areas (284-653 m 2 g -1 ) were preserved. Higher conversion was obtained for catalysts with higher ratios of WO 3 in the mixed silica-alumina aerogels that were calcined at 800°C.