Formation of cyanide-functionalized SBA-15 and its transformation to carboxylate-functionalized SBA-15

Abstract

Cyanide-functionalized mesoporous silica SBA-15 (CN/SBA-15) with pore diameters larger than 6 nm has been prepared by a one-pot synthesis. The structure of CN/SBA-15 is affected by the amount of the functional silane 2-cyanoethyltriethoxysilane (CTES) and the conditions of co-condensation in the acidic aqueous solution of the triblock copolymer Pluronic P123: whereas direct mixing of the silica sources CTES and tetraethoxysilane (TEOS) leads to a disordered hybrid material, a highly ordered CN/SBA-15 is obtained if the synthesis solution containing CTES is prepared first and TEOS added afterwards. Treatment of the as-synthesized materials with concentrated sulfuric acid removes the copolymer template through ether cleavage and hydrolyses the –CN groups in one step, resulting in carboxylate-functionalized SBA-15 (CA/SBA-15). 13C CP/MAS NMR spectra show that for some CN/SBA-15 materials the copolymer template can be totally removed in this way, suggesting that all the hydrophilic ethylene oxide (EO) chains of the template are accessible to the acid. Nitrogen sorption measurements also indicate that these materials contain nearly no micropores in contrast to the pure-silica SBA-15 synthesized under the same conditions, which has a considerable proportion of micropores. The results suggest that the structural order and microstructures of the resulting CN/SBA-15 materials are strongly affected by assembly kinetics, which involve the hydrolysis and condensation of silica species and their interaction with the template.