Properties of hybrid silicas synthesized using different cationic surfactants with cyclic head

Full text: The M41S materials family consists of mesoporous molecular sieves (MCM-41, MCM-48 and MCM-50) developed by Mobil, that have several applications due to their exceptional characteristics. Recently, the hybrid hexagonal mesoporous silica, MCM-41, was applied as catalyst in base catalyzed reactions. Its activity can be attributed to the presence of siloxi anions inside the pores and on the surface of the silica. Generally, the synthesis of this catalyst uses cetyltrimethylammonium bromide (CTABr) as directing agent of mesopores. However, other works show that CTABr can be substituted by other cationic surfactants, what can improve the structure organization or the catalytic activity of MCM-41. In this sense, this work aims to study the influence of different cationic surfactants with cyclic hydrophilic heads (cetylmethylimidazolium (CMImBr), cetylvinylimidazolium (CVImBr), cetylallylimidazolium (CAImBr) and cetylpyridinium (CPyBr) bromides on the formation of aqueous micelles and mesoporous hybrid silicas synthesized from them. The small angle X-ray scattering (SAXS) curves of aqueous surfactant dispersions showed high scattering intensities to smaller head sizes, indicating that the increasing of the sized of the hydrophilic head hinders the micelle formation. The SAXS curves also show that the increasing of the surfactant concentration reduces de intermicellar distance for all surfactants. The hybrid silicas were synthesized using tetraethyl orthosilicate (TEOS) as silica source and the above surfactant dispersions and were characterized by X-ray diffraction. The results showed the formation of hexagonal MCM-41 structure with CPyBr, CMImBr and CVImBr. The silicas prepared with CAImBr just have a mesoporous structure with low organization. The structural organization depends on the surfactant concentration, which increases by the increasing of available cations to micelle formation. (author)