Synthesis And Characterization Of Ordered Porous Silica Templated By Gemini Quaternary Ammonium Surfactants

Mesoporous materials have developed well in many fields, such as catalysis, function materials, pharmaceutical chemistry, environmental protection, adsorption and separation because they possess regular pore structure, extremely high specific surface area and large pore volume. Zeolite molecular sieves have been used for several decades, but the pore diameters of zeolites are generally less than 1.2 nm, this make they be restricted in catalysis such as molecular whose size between 1-2 nm. Supermicroporous materials whose pore diameter between 1-2 nm make up the gap between zeolite and mesoporous materials.Gemini surfactants are a new class of amphiphilic molecules containing two head groups and two aliphatic-chains, linked by a spacer at or near the head groups. Because of the unique structure, gemini surfactants have some superior properties, such as higher surface activity, lower critical micelle concentration (cmc), better wetting properties and be easier to form various kinds of aggregates compared with the corresponding monovalent surfactants. Thus, gemini surfactants have potential applications in the synthesis of porous silicas as structure-directing agents.In this paper, a family of designed gemini surfactants C14H29(CH3)2N+-(CH2)s-N+(CH3)2C14H29·2Br-(designated as C14-S-14, S=4,6,8,10) was utilized as structure-directing agent to prepare ordered porous MCM-41 silica. The samples were characterized by small angle X-ray diffraction, high-resolution transmission electron microscopy and N2 adsorption/desorption analysis.Through analysing the characterization results, a list of conclusion were discovered.(1) The obtained materials all possessed 2D-hexagonal periodic structure (space group 2D-p6mm).(2) The structure of materials and pore diameter were influenced by the the length of the spacer group of the gemini surfactants. When S=2, the yeiled materials is cubic structure with pm 3 n symmetry (previous work of our group). When S=4-10, as the spacer length increased, the pore diameter decreased obviously from mesoporous range to supermicroporous range. When S=4, the pore diameter were about 2.5 nm, when S=6 and 8, the diameter were between 2.2-2.3 nm, which were both in the mesoporous range; when S=10, the pore diameter was in the range of 1.8-1.9nm, which was in the supermicroporous range.(3) Two types mesoporosity, that is, framework-confined mesoporosity (primary pores) and voids between particles (textural mesoporosity) existed in the samples through, and primary pores were dominant.(4) The self-assembly ability of C14-S-14 was stronger than that of tetradecyltrimethylammonium bromide (TTAB, which was regarded as the corresponding monomer of gemini surfactant C14-S-14) in controlling the orderly pore structure and pore size of the porous materials.