Supercritical CO2 as a reaction and impregnation medium in the synthesis of Pd–SiO2 aerogel inverse opals

Pd–SiO2 aerogel inverse opals were prepared for the first time using supercritical CO2 (scCO2). Colloidal crystals formed by 3D-arrays of monodisperse spherical polymer particles (opals) were used as templates. In one approach, a pre-made large surface area SiO2 aerogel inverse opal prepared in scCO2 was impregnated with palladium hexafluoroacetylacetonate [Pd(hfac)2] in scCO2 and thermally treated to get Pd–SiO2. In another approach, tetraethylorthosilicate and Pd(hfac)2 dissolved in scCO2 reacted directly on the polymeric template in one step. Both, impregnation and reaction experiments were carried out at 40 °C and 85 bar. After the removal of template, large surface area porous materials replicating the structure of the original template were obtained (inverse opals). These materials presented hierarchical porosity with ordered macropores of mesoporous walls. Palladium (between 1 and 3 mol% by EDX) was incorporated uniformly throughout the SiO2 matrix forming small clusters (by TEM). The effect of Pd incorporation on the aerogel porosity was studied.