Synthesis of large-pore silica with cage-like structure using sodium silicate and triblock copolymer template

Silicas with large (about 10 nm in diameter), uniform, cage-like pores were synthesized from sodium silicate and poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) B50-6600 triblock copolymer template (EO39BO47EO39) under acidic conditions. The synthesis procedure was analogous to that reported by others for FDU-1 silica with highly ordered cubic Im3m structure, but in our study, an expensive tetraethyl orthosilicate (TEOS) was successfully replaced by cost-efficient sodium silicate. Adsorption and thermogravimetric properties of the silicas synthesized from sodium silicate were found to closely resemble those of FDU-1 silicas synthesized from TEOS, but the pore volumes and specific surface areas of the former were found to be lower in most cases. The uniform cage-like structures assembled from sodium silicate formed at room temperature and 318 K, but no ordered structure was obtained at 333 K, whereas in the case of TEOS, an ordered structure formed also at 333 K. The sample synthesized at room temperature from sodium silicate had a smaller pore size when compared to the sample obtained at 318 K, whose pore size could be further enlarged by heating at 373 K for 6 h, but longer heating was detrimental for the uniformity of the cage-like structure, as seen from nitrogen desorption data. Similar undesirable structural changes were observed for FDU-1 synthesized from TEOS and heated at 373 K for 24 or more hours. It is concluded that the synthesis of uniform large-pore cage-like structure using cost-efficient silica source and triblock-copolymer template can be successfully achieved and some control over the pore diameter can be exercised, but ordered porous structure forms less readily and is more difficult to tailor when compared to that obtainable using the expensive TEOS.