Synthesis, Characterization And Application Of Mesoporous Materials Templated By Special Anionic Surfactant

In the two decades of evolution and development for ordered mesoporous materials, the template was gradually extended from initial cationic surfactant to neutral surfactant, and the structure of the template was gradually extended from the initial single alkyl chain with single charge, to multi-alkyl chains with multi-charge. Among all the routes for synthesis of mesoporous silica, the application of anionic surfactant is limited, which is mainly due to the isoelectric point of silicate species in aqueous solution is pH 2.0. That means the silica species are positively charged when pH value is smaller than 2.0, while silicate species are negatively charged when pH is larger than this value. Therefore, an acidic system can guarantee a strong interaction between organic and inorganic phase, but not conducive to the dissolution and micellar-based self-assembly of anionic surfactants, while a larger pH value is conducive to self-assembly of the anionic surfactant, but will greatly reduce the interaction between the organic template and inorganic species. This problem has not been solved until the concept of structure-directing agent was introduced. The classic method developed in recent years for synthesizing mesoporous silica by using amino acid type anionic surfactant as template is to use silane coupling agent with a positively charged end group and alkoxy silicone groups as structure-directing agent. Compared to the previous method using cation and neutral template, it is a big advantage for the use of amino acid type anionic surfactant as template to synthesize mesoporous silica with diverse mesoscopic structure through a simple experimental method to control. However, the mechanism of amino acid-based anionic surfactant templated synthesis method has not been explored, and all of the products are pure silicon-based mesoporous materials, while their doped and related applied research are very few.Under these considerations, the author carried out related research projects. The main contents of this paper are as follows:1. N-lauroyl-L/D-alanine sodium salts were synthesized through acylation. A series of mesoporous silica materials were prepared at room temperature by using these two compounds with different proportion as structure-directing agent, and 3-aminopropyltrimethoxysilane as co-structure directing agent. By means of microscopic analysis and nitrogen adsorption-desorption tests, it is found that mesoporous silica particles with two-dimensional hexagonal distorted rod morphology could be obtained by using structure directing agent with single chirality, while mesoporous silica particles templated by mixed system show irregular morphology, and their surface areas are also reduced accordingly. The circular dichroism spectra studies for the template solutions show that the helical distorted aggregation already exist in chiral template aqueous solution, and the racemic mixture will make this helical structure disappearance, which confirms that the synthesis mechanism is in line with liquid crystal templating mechanism. 2. N-hexadecyl ethylenediaminetriacetic acid was synthesized using multi-step reaction, and the relevant characterizations were performed to verify the structure and composition of the product are in line with the target product. The critical micelle concentration of the surfactant was tested through fluorescence probe method, and the coordination ability and coordination composition of the product and metal ions were also tested by means of spectral analysis.A series of NiO/mesoporous silica composites with different doping amounts were obtained by using the above surfactant as structure-directing agent, and 3-aminopropyl trimethoxysilane as co-structure directing agent through adjusting the Ni²⁺ concentration in template solution. The products were characterized by microscopic analysis, nitrogen adsorption-desorption tests, and the structure and composition analysis, indicating that the formed cubic NiO nanoparticles were located in silica mesopores, and gathered into clusters with the size about 15nm. With microscopy analysis of products calcinated at different temperatures, it is found that the NiO nanoparticles formed at lower temperatures were firstly gathered into nanobelts, and then shrinked into spherical clusters with the temperature increases. This particular distribution is mainly due to the large surface energy of nanoparticles and the magnetic properties of NiO. The aggregation of NiO nanoparticles also retained rich mesoporous structure of the composites, accompanied with a large surface area and narrow pore size distribution. The density of NiO clusters in the silica matrix increases with the increase of Ni²⁺ in template solution, and the Ni/Si atomic ratio in the obtained products shows a linear relationship with the amount of added Ni²⁺, which means that the component of the composites can be adjusted by changing the amount of added metal ions in template solution.A series of ZnO/mesoporous silica composites were obtained under mild conditions by using N-hexadecyl ethylenediaminetriacetic acid as structure-directing agent, and N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride as co-structure directing agent through adjusting the Zn²⁺ concentration in template solution. The composition of the composites can also be adjusted by changing the adding amount of Zn²⁺ in template solution. Compared to NiO/mesoporous silica composites, ultrafine ZnO nanoparticles are highly dispersed in the silica pores, and there are relatively strong host-guest interaction existing between the nanoparticles and the ordered silica pore wall. By adjusting the Zn²⁺ concentration in template solution, the surfactant packing parameter and the curvature of mixed micelles were effectively changed, which achieves an mesophase evolution of composites from three-dimensional cubic Fd3m to the two-dimensional hexagonal p6mm to bicontinuous cubic Pn3m and to the lamellar phase.A series of Co₃O₄/mesoporous silica composites with different doping amounts were obtained by using N-hexadecyl ethylenediaminetriacetic acid as structure-directing agent, and 3-aminopropyl trimethoxysilane as co-structure directing agent through adjusting the Co2+ concentration in template solution. The generated Co₃O₄ nanoparticles have uniform distribution in the ordered silica pores. By gradually increasing the Co2+ concentration in template solution, the mesophase of composites evolves from three-dimensional cubic Pm3n to the two-dimensional hexagonal p6mm to bicontinuous cubic Ia3d and to the lamellar phase.Our experiments also tested the photoluminescence properties of ZnO/mesoporous silica composites, which demonstrate the quantum size effect of metal oxide in these nanocomposites. We also use the obtained NiO/mesoporous silica composites as catalyst to decompose methyl orange solution under UV light, whose results show that the composites has good catalytic activity and good renewability.