Synthesis Of Ordered Mesoporous Silica Templated By N-alkyldimethylamino Acetate

According to the classification made by IUPAC, porous materials can be arranged in three main categories, depending on their pore size (diameter, d), micro-(d < 2 nm), meso- (2 nm < d < 50 nm), and macroporous materials (d > 50 nm).Mesoporous materials are usually synthesized by self-assembly of surfactants through sol-gel chemistry process. Compared with the classical porous materials, mesoporous materials have the characters as follows: (1)regular and well-defined channel systems; (2)narrow pore size distribution with tunable pore size in the range of 1.3～30.0nm; (3) high BET specific surface areas; (4) excellent thermal and hydrothermal stability; (5) particles with different shapes. Mesoporous materials are promising catalysts, catalyst supports, chromatographic packing materials, adsorbent, and the host or template for the synthesis of advanced nanostructures.In this thesis, silica based mesoporous materrials were synthesized using a family of betaine zwitterionic surfactant (N-alkyldimethylamino acetate, abreviated as BS-n) as template. The resulting samples were characterized by small angle X-ray diffraction(SXRD), nitrogen adsorption -desorption analysis, and high recognization transmission electron microscopy(HRTEM). We have studied the effect of the pH value, the concentration of sodium nitrate and sodium chloride on the structure and properties of the resulting materials.Using BS-12 as template, the effect of pH value is essential on the structure and properties of the product. Sample obtained at pH3.0 had relativly high specific surface area, but its pore size distribution is a little wide. When pH>4.6, only low-ordering samples could be got. In the range of pH3.5～4.6, mesoporous materials with uniform pore size and high specific surface area could be obtained. Especially, the total specific surface area of sample obtained at pH3.5 was 1337m²/g, and the BJH pore size of it was 1.49nm. The pore size which observed by HRTEM was consistent with the BJH analysis. Clearly, this sample belonged to super-microporous material.Similar to BS-12 system, pH value affected the result products significantly in the BS-14 system. The structures of samples synthesied at pH2.5～4.6 were close, the specific areas were in the range of 944～1211m²/g, and the pore sizes were about 2.22nm. The sample got at pH4.1 had the highest ordering among them. The structure of it was close to hexagonal phase characterized by SXRD, and the pore channels were wormlike taken by HRTEM. When the pH value was as high as 6, the sample showed no diffraction peak. The effect of the sodium nitrate were studied at pH4.1 and pH5.1 respectivly. At the two different pH value, the pore size of the result sample enlarged, but the ordering, pore volume and specific surface area decreased. However, at the condition of pH4.1 and NaNO₃ 5.0mol/L, unique hollow silica spheres with mesostructured shell was discovered.In the BS-18 system, the effects of the pH value also followed the rule that low pH value resulted in low ordering samples, increasing pH value properly could enhance the ordering of samples, however continuing increasing pH value reduced the ordering of the samples. At pH5.1, the samples had the highest ordering. SXRD and HRTEM showed that the structure of this sample belonged to hexaganol phase. Based on sample got at pH5.1, the influence of NaNO₃ and NaCl on the samples were investigated. The SXRD pattern showed that increasing the concentration of NaNO₃ made the 26 of the result sample decrease. The nitrogen adsorption-desorption analysis exhibited that the pore size enhanced, but the specific surface area fell down. Compared to NaNO₃, the effect of NaCl was milder.