Adjusting the porous and structural properties of mesoporous silica by the addition of organic modifiers

Abstract The effects of the organic modifiers 2-cyanoethyltriethoxysilane and 3-aminopropyltriethoxysilane on the porous properties of silicates synthesised by co-condensation with tetraethyl orthosilicate were investigated. Materials were synthesised in aqueous solutions of cetyltrimethylammonium bromide or n -dodecylamine surfactants. Preparations using cetyltrimethylammonium bromide and sodium hydroxide gave MCM-48, with an average pore diameter of 25 A, while addition of 10 mol% (of total silica) 2-cyanoethyltriethoxysilane improved the mesoporosity characteristics; the modified preparation gave a more intense X-ray diffraction pattern confirming a more ordered structure with a greater volume of regular mesopores. Disordered materials with pores 145 A in diameter were formed using 2-cyanoethyltriethoxysilane modifier and ammonia as base. All preparations incorporating 3-aminopropyltriethoxysilane modifier were disordered and microporous. For silicates synthesised using n -dodecylamine surfactant, mesopores with diameter 118 A were formed using 2-cyanoethyltriethoxysilane while preparations with 3-aminopropyltriethoxysilane and in the absence of a modifier were microporous. The particle sizes of the silicates displayed a clear correlation with the pore diameters: microporous silicates were 0.5–1 μm in particle diameter, MCM-48 phase (25/27 A) were 1–2 μm, while relatively large mesoporous materials (>100 A) had particle dimensions 10–100 μm.