Synthesis And Adsorption Properties Of Hierarchical Porous Silica-carbon Composites

This paper is focused on the design, synthesis and application of hierarchical porous silica/carbon composites with micrometer and nanometer sized pores. The obtained products have complex pore structures, high surface areas, large pore volumes, good permeability and large amount of homogeneous adsorption sites. As a result, they are beneficial for the removal of organic pollutants from water.Expanded graphite(EG) which possesses lots of micrometer-ranged interconnected pores was used as carbon source in this project. Then various silica/carbon composites with different hierarchical pore structures, silica/C molar ratios and aluminum metal contents have been prepared in aqueous solutions that contain templates, siliceous sources, EG and metal species. Textural properties and morphologies of the synthesized hierarchical silica-carbon composites were characterized by the combination of X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM) and N2 adsorption-desorption measurements. Formation mechanisms of the hexagonal silica/EG and the cubic mesoporous silica/EG composites were studied and compared. Factors such as the preparation methods, ratios of the reagents, acidities of the solutions, organic additives and so on during the synthesis processes were thoroughly discussed.Those synthesized silica/carbon composites were used as adsorbents for the removal of organic pollutants such as methylene blue(MB), Rhodamine B(Rh B). The adsorption isotherms and kinetics of organic molecules over those composites were thoroughly evaluated and analyzed. Diverse hierarchical micro/nanoporous silica/carbon composites with different structures and morphologies can be synthesized through both traditional hydrothermal stirring method and a new shaking strategy. Stirring the reaction mixtures is beneficial for improving the hydrolysis and condensation rates of siliceous sources and then enhancing the interactions between silica and carbon. However, stirring the solution also has the disadvantages of partially destroying the macrostructures of EG. On the other hand, shaking the reaction solutions is a much more moderate way than stirring. So porous silica/EG composites with complete structures and excellent adsorption performances can be prepared by the novel shaking method in mild acidic condition with ethanol as additives. During the synthesis, aluminum chloride and its self-generated acidity is the most important factor for the formation of porous silica/carbon composites. This weak acidic condition also makes it possible for the metal species to deposit into the structure of those composites. Besides, butanol are also important for cubic mesoporous silica to be grown onto the pores of EG.The surface areas, pore size distributions and pore volumes of the produced composites were determined by the molar ratios of the siliceous sources to carbon in reagents. Surface areas and adsorption capacities of the silica/EG composites synthesized with relatively low TEOS/C molar ratios were much lower than those synthesized with higher TEOS/C molar ratios. So it is very important to control the TEOS/C molar ratio in reaction for synthesizing the hierarchical porous silica/EG composites. All porous silica/carbon composites show a greater adsorption capacity to methylene blue than EG and pure silica. Adsorption capacity of the composites will be further improved if aluminum metal species were introduced into the structures of the samples. In addition, methylene blue molecules probably adsorb on the homogeneous surface of the cubic silica/EG composites, while Rhodamine B molecules seem adsorb on the same cubic silica/EG composites as a heterogeneous way. Kinetic studies show that almost all of the hierarchical porous silica/carbon composites have rapid uptakes of MB.