Studies on the porosity of SiO 2-aerogel inverse opals synthesised in supercritical CO 2

Abstract

A new technique to produce highly porous SiO 2-aerogels inverse opals in supercritical carbon dioxide (scCO 2) is being developed at our laboratory. Polystyrene latex particles decorated with methacrylic acid or a mixture of methacrylic and itaconic acid groups are organized in three-dimensional 3D-latex arrays and used as templates. The polymeric template is reacted with tetraethylorthosilicate (TEOS) or tetramethylorthosilicate (TMOS) dissolved in scCO 2 at 40 °C and 85 bar. The reaction is catalysed by organic acids attached to the latex particle surface. SEM and TEM images show that, upon calcination of the template, highly porous materials replicating the structure of the original template are obtained. N 2-adsorption isotherms reveal the presence of a large porosity located in the macropore wall. In all the cases, the maxima in the pore size distribution appear in the mesopore range. In this paper we investigate ways of controlling the porosity of the material by changing the precursors, the catalyst and/or its concentration. SEM, TEM and N 2-adsorption data show that, at the reaction conditions, reactivity of TEOS and TMOS on the templates is similar, yielding materials of comparable surface area. In the same way, the porosity of the aerogels obtained with templates impregnated using benzosulfonic (BSA) acid and p-toluene sulfonic acid is the same. Lower yields were obtained using monochloroacetic acid. The effect of the catalyst concentration is more important; as the concentration of BSA in the template increases, lower values are obtained for the maximum in the pore size distribution.