Mesoporous silica synthesis in sub- and supercritical carbon dioxide

Abstract

Mesoporous silicas were synthesized from sodium silicate (Na₂Si₃O₇) and tetraethylorthosilicate (TEOS) with Pluronic F127 (polyethylene oxide-polypropylene oxide-polyethylene oxide, EO₁₀₆PO₇₀EO₁₀₆) triblock copolymer using sub- and supercritical carbon dioxide (SubCO₂ and SCO₂) respectively, as solvents. Templates were removed using liquid carbon dioxide (LCO₂) and SCO₂. The most efficient template removal was achieved by LCO₂ − 92.7% (w/w), followed by LCO₂ with ethanol entrainer − 85.6% (w/w), and by methanol − 78.8% (w/w). The best efficiency of template removal by SCO₂ was 50.7%. Values of specific surface areas, A_BET, were increased by 10% with the increase of an ageing time from 6 to 24 hours for Na₂Si₃O₇-based silicas at aqueous synthesis conditions, whereas the use of SCO₂ reduced this value by 19.4%. For TEOS-based silicas synthesized using SCO₂, A _BET values increased by 3.8 times. Application of SCO₂ for synthesis of TEOS-based silicas resulted in higher mesopore volumes of 0.719 and 1.241 mL/g with an average mesopore width varying from 3.4 to 3.9 nm. Although Na₂Si₃O₇-based silicas have almost similar mesopore width range, their mesopore volumes were 7 times less than those for TEOS-based silicas. Formation of mesopores in Na₂Si₃O₇- and TEOS-based silicas was at the expense of micropores when synthesized in SCO₂.