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, ABET, were increased by 10% with the in- crease 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, ABET 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.