Three-dimensional Ordered Macroporous SiO ₂ Preparation And Characterization Of Microspheres

Reprocessing of nuclear fuel is a vital part of the sustainable development of nuclear energy. Extraction chromatography is a novel separation method with rapid development, which combines the high-selectivity of solvent extraction and high efficiency of chromatography techniques. This method possesses distinct advantages, such as simplicity, ease of operation, saving in reagent costs and minimum generation of secondary waste, showing great potential application in reprocessing of nuclear fuel. Therefore, the developing of varieties of novel extraction chromatography with good separation performance and high resistance to radiation, temperature and acid is of great significance.The research of this thesis is to fabricate macroporous silica spheres through colloidal crystal template technique in three steps, in order to offering desirable supports for novel silica-based extraction chromatography. Step1, emulsifier-free emulsion polymerization method was applied to synthesize monodisperse polystyrene (PS) particles and the effect of experimental parameters on diameter and monodispersity of PS latex particles were to be investigated. Step2, evaporation-induced assembling process in a suspension system was utilized to assemble PS latex particles into three-dimensionally ordered colloidal crystal ball templates and the conditions of assembling process were to be optimized. Step3, macroporous silica spheres were prepared after filling and consolidation of silica precursor, and final calcinations for PS removal. The inherent relationships between the process of filling and calcinations and the structure of pores and walls were explored.The research shows that (1) polystyrene particles with diameters in the range of325-420nm and monodispersity below0.05were prepared under different experimental conditions. The concentrations of buffer, initiator and comonomer were confirmed to be the main factors to influence the diameters and monodispersity of PS particles. The optimum conditions were obtained:the concentration of buffer (NaHC03) is between9.52and12.27mmol/L; the concentration of initiator (KPS) is between4.45and7.56mmol/L; the concentration of comonomer (AA) is between0.20and0.36mol/L.(2) The as-obtained colloidal crystal templates were in regularly spherical and PS particles were close-packed with each other, forming highly-ordered face-centered cubic (fcc) structure with a density of74%. The best fabricate conditions of colloidal crystal ball templates whose diameter was between100-150μm were obtained:the volume ratio of aqueous phase and oil phase is1:10; the stirring rate is fixed at450rpm; the adding method of PS latex is a slow dripping by a constant-voltage funnel. Consequently, the productivity can be reached to61.4%.(3) The immersing method and vacuum soaking method were both utilized to infuse precursor into template balls, then PS particles were removed by calcinations and the temperature programming was optimized:the sample is heated to300℃and kept for3hours (the heating rate is fixed at2℃/min). Then it is heated to600℃at the same heating rate for another3hours. Characterization of SEM and N2adsorption indicated that the SiO2spheres prepared by immersing method possess an interconnected macroporous structure, which transverse the whole spheres with a pore size of342nm. However, the skeleton is loose where exists numerous of mesopores with mean diameter of6.43nm, providing a high surface area of466.6m2/g and pore volume of0.750cm3/g. While precursor solution of high concentration was soaked into templates, the pore shrinkage can be reached to50%with a pore size of225nm Most of channels connecting the adjacent pores are blocked, even part of the pores are covered. The structures of SiO2spheres obtained by vacuum soaking method with precursor of low concentration are similar to those prepared by immersing method. The pore size is330nm, whereas the skeleton is denser with surface area of53.446m2/g and pore volume of0.115cm3/g, the average mesopore diameter is8.58nm.