Preparation And Properties Of Mesoporous SiO₂ And Mesoporous Magnetic Composites

In the last decade, the studies on the synthesis and applications of mesoporous materials were continuously an active filed in solid chemistry. This is due to that the mesoporous materials have unique and interesting structural characters including high surface-area, ordered, uniform and changeable pores, abundant surface silanol and biocompatibility. These characters provided good chances for the applications of porous materials in sorption and separation etc. In order to carry out different separation aims, it is important to prepare porous materials with different pore structure, adjustable morphologies at micro-, meso-, and macro-scale, and hydrothermal stability. At the same time, combining the mesoporous materials and magnetic materials has become another study hotspot,thus the composite materials would have high specific surface area and corresponding magnetic character, and then the separation process would became convenient if the composites was used in sepatations. So, the author of this thesis synthesized some mesoporous materials with different structure and morphology (block, sphere, and hollow sphere). Furthermore, mesoporous silica materials were combined with magnetic nanoparticles to synthesis superparamagntic mesoporous silica composites with high specific surface areas, and the composites have been used to recover and extract DNA. These investigations may pave the way for the application of mesoporous materials in the biomaterial separation fields. Such materials obtained may have great significance from theoretical and practical aspects. This thesis is arranged as the following:1. Spherical and hydrothermal stability of MSU-type mosoporous silicas were synthesized with Tween 80 surfactant by adjusting the concentration of 1-propanol. The obtained materials have been characterized by different techniques such as X-ray diffractions, scaning and transmission electron microscropy, and nitrogen adsorption-desorption analysis. The effect of the concentration of 1-propanol on the properties, for example, BET surface area, hydrothermal stability, and morphology of the materials has been investigated. In our system, high concentration of 1-propanol (40-90 v%)favors the formation of spherical morphology and the pore size of the materials can be tailored over several angstroms. Increase in the concentration of 1-propanol from 40-90 v % led to a continuously decrease in the BET surface area from 755-148 m2/g. The long-rang order of the pores decreased with the increasing of 1-propanol concentrations. The hydrothermal stability depends strongly on the concentration of 1-propanol. The mesoporous materials synthesized with 40-60 v% 1-propanol are more hydrothermally stable than other materials,for example, the material prepared with 60 v % 1-propanol maintain 80% of its original BET surface area in boiling water for 5 days. We provide a convenient new method to improve the hydrothermal stability of mesoporous materials through adjusting the concentration of 1-propanol. The using of 1-propanol as cosolvent introduces more flexibility and diversity into the designed synthesis of mesoporous materials. The spherical and hydrothermally stable mesoporous molecular sieves could be used for biological separations (for example: HPLC).2. Hollow mesoporous silica spheres with uniform diameter and shell thickness were synthesized by a sol-gel/emulsion (oil-in-water/ethanol) approach, in which cetyltrimethylammonium bromide (CTAB) surfactant was employed to stabilize and direct the hydrolysis of oil droplets of tetraethoxysilane (TEOS). Appropriate volume ratio of ethanol-to-water and CTAB concentration in the sol-gel/emulsion system of tetraethoxysilane (TEOS) can improve the stability of the oil droplets of TEOS, thus allowing preparation of the hollow mesoporous silica spheres. The hollow spheres remain the spherical nature even after calcination at 600℃for 6h indicating the hollow spheres have certain thermal stability. The diameters of the hollow spheres can be tuned in the range from 210 nm to 720 nm by varying the ratio of ethanol-to-water and their shell thickness can be mediated by changing the concentration of CTAB used in the system. BET surface areas of the hollow silica spheres are determined to be in the range of 924-1766 m2 g-1 and their pore sizes are around 3.10 nm as determined by BJH method. This work presents a facial method to synthesis hollow mesoporous silica spheres with controllable particles sizes and shell thickness. The hollow mesoporous silica spheres are expected to have a high potential for use in practical applications. 3. Mesoporous silica spheres embedded with the superparamagnetic iron oxide nanoparticles were prepared by a new sol-gel/emulsion approach. TEM imaging indicated a core-shell structure of the spheres, in which a substantial amount of the iron oxide nanoparticles was embedded in the cores. The resulting superparamagnetic silica spheres with a saturation magnetization value of 30 emu/g, attributed to the high mass fraction of iron oxide (48%) in the silica spheres,which could be enriched completely within 10 s under the application of a 0.4 T magnet. The composite spheres had a BET surface area of 726 m2 g-1 and a pore volume of 0.32 cm3 g-1. The pore size distribution curve calculated from the adsorption branch with the BJH method showed a sharp peak at 3.1 nm. TG/DTA curves, Raman and XRD spectra indicated that the mesoporous silica shelles protected the iron oxide phase from changing toα-Fe2O3 under calcination and the iron oxide phase in the calcinated composite spheres mainly isγ-Fe2O3. Adsorption and elution experiments of salmon sperm deoxyribonucleic acid (DNA) showed an excellent ability of the superparamagnetic mesoporous silica spheres to adsorb DNA due to their large specific surface area and pore volume. Electrophoresis experiments show that the superparamagnetic mesoporous silica spheres are qualified for efficient recovery of DNA fragments from agarose gel. The superparamagnetic mesoporous silica spheres are expected to have a high potential use in bio-molecule separation and purification.4. Highly magnetizable superparamagnetic mesoporous particles with different pore-sizes were prepared by a sol-gel method. The composite spheres had a BET surface area of 150-170 m2 g-1, the pore size could be adjusted from 2.7-5.8 nm using different synthesis parameters. The resulting superparamagnetic silica particles have a saturation magnetization value of 56 emu/g, which could be enriched completely within 3 s under the application of a 0.4 T magnet. Adsorption and elution experiments of salmon sperm deoxyribonucleic acid (DNA) showed an excellent ability of the superparamagnetic mesoporous silicapartilcles to adsorb DNA. The DNA adsorption efficient of the magnetic mesoporous particles are related to the pore diameters, which increased with the increasing of the pore sizes of the magnetic composite particles. Electrophoresis experiments show that the composites are qualified for efficient recovery of short DNA fragments from agarose gel and the efficient was equivalent to the commercial kit. The magnetic mesoporous materials are satisfied for purification DNA from thighbone of 4000 years old.