Mild Conditions Of Synthesis Of Mesoporous Silica

Biomineralization is the study of the formation, structure and properties of inorganic solids deposited in biological systems. That is, under certain chemical conditions, the inorganic ions in solution are transformed to solid minerals under the specific control of the organic species. Biomimetic or bio-inspired materials chemistry is currently a promising field to synthesize functional inorganic materials. The interaction on the interface of inorganic-organic composites controls the growth of inorganic materials and thus tunes their structures and morphologis. Ordered materials with mesoscale structures, which are generally synthesized through the cooperative assembly between amphiphilic organic molecules (templates) and inorganic species, have attracted great attention due to their potential applications to catalysis, separations and nanodevices. However, comparing with the biosilification in nature, the synthesis of MCM-41 is always under either acidic or basic conditions and under hydrothermal treatment. It is a great challenge to synthesize mesoporous silicas under mild conditions of near-neutral pH and at ambient temperature. The near-neutral-pH synthesis can not only broaden some applications of mesoporous material such as in-situ encapsulation, but also inspire the studies of biomineralization.In this thesis, with salts as the mineralization agents, mesoporous silicas with specific morphologies under mild conditions of near-neutral pH and ambient temperature were synthesized and were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and N2 adsorption-desorption, etc. We have investigated: 1) the synthesis of tubular mesoporous silica with Na2EDTA as mineralization agent and cationic surfactant Cetyltrimethyl Ammonium Bromide (CTAB) as template, 2) high yield mesoporous silica fibers with NaNO3 as mineralization agent at low temperature and 3) plate-like SBA-15 with pluronic block copolymer P123 as template under neutral or near neutral pH. The main content involves:1) With the aid of the organic salt of Na2EDTA (disodium ethylene diamine tetraacetic acid), wormhole-like mesoporous silica with wall thickness of around 3 nm was synthesized at near neutral pH and room temperature. The characterization of N2 sorption measurement indicated the pore size and specific surface area of the calcined mesoporous silica is 2.8 nm and 706 m2/g, respectively. By deconvolution of the 29Si NMR spectra, the Q4/Q3 ratios are determined to be 2.5 (as-synthesized) and 2.7 (calcined),which are higher than those of mesoporous silica synthesized under either acidic or basic conditions. Temperature has a great influence on the mesostructure of the products. Mesoporous silica can always be obtained at the temperature of 2-15℃, while the regularity of mesostructure decreasd greatly once temperature exceeding 50℃. The incorporation amount of surfactant in as-obtained product is low and the surfactant can be easily removed by ethanol extraction. By characterizations of XRD, N2 adsorption-desorption, SEM, and TEM, the effects of reactant ratio and temperature on the structure and morphology of mesoporous silica were investigated in detail. Mesoporous silica can always be obtained with the reactant composition in molar ratio of 0.05~0.2CTAB/ 0.1~2 TEOS /0.1~0.5 Na2EDTA/100~500H2O. An increase of the amount of CTAB leads to a larger d spacing value of the mesostructure correspondingly. Temperature has a great influence on the morphology of mesoporous silica. With the increase of synthesis temperature, the tubular morphology transforms to particle aggregates. A formation mechanism of (S+E-)+-I- has been proposed.2) By the SEM and TEM characterizations, the morphology and pore channel of as-obtained mesoporous silica tubes were investigated in detail. The formation mechanism of tubular morphology was proposed. It is considered that Na2EDTA plays ternary roles in the synthesis of mesoporous silica, one as catalyst for the hydrolysis and condensation of TEOS, another is to co-assemble with micelles of CTAB to give rise to the disordered mesoporous structure of silica and the other is to act as crystal template to induce the formation of tubular morphology. The addition of ethanol easily induces the formation of the mesoporous silica tubes. At near-neutral pH, the cases of addition of various organic cosolvents are also investigated and it has been found that the dielectric constant of the added cosolvent exerted great influence on the morphology of the mesoporous silica.3) It has been found that the addition of ZnSO4 to CTAB-TEOS-EDTA (CTE) synthesis system improves the structure order greatly and makes a structural transformation from wormhole-like to ordered hexagonal texture. The amount of ZnSO4 has a great influence on the mesostructure. With EDTA:ZnSO4 increasing from 0.5:0.5 to 0.5:0.1, the d spacing value and wall thickness increase correspondingly. Mesoporous Fe/silica with Fe content as high as 6.9 wt% was synthesized by use of EDTANaFe instead of Na2EDTA. XRD and nitrogen sorption measurements indicate the obtained product has a mesostructure with d spacing value of 3.7 nm, the supramicroporous channels with the BET (Brunauer, Emmett & Teller) surface area of 758 m2/g. A proportion of Fe has been testified to be incorporated into the framework by EPR. It supplies an efficient way to introduce other heteroatom into the mesoporous silica framework.4) Without any addition of acid or base, at near-neutral pH, by use of the catalysis of NaNO3 for the hydrolysis and condensation of TEOS, mesoporous silica has been synthesized in the presence of anionic surfactant CTAB. NO3- plays binary roles in the synthesis, one as catalyst for TEOS, the other is to combine with CTA+ to form long cylindrical micelles to give rise to mesostructure. At low temperature of 8~15℃, mesoporous silica fibers with yield of almost 100% were synthesized. The fiber has a hexagonal cross section with a diameter of around 200 nm. It has been found that temperature has great effect on the structure and morphology of the product. On one hand, extremely low temperature results in no solid product, on the other hand, an increase of temperature leads to a decrease of structure regularity and meanwhile the morphology transforms to particle aggregates. By use of Al(NO3)3, the structure regularity and surface area of the obtained mesoporous material have been improved greatly.5) At near-neutral pH of 5.2, with triblock copolymer P123 as template, mesoporous silica with high yield of plate-like morphology and ordered hexagonal structure has been facilely synthesized in the presence of Na2EDTA. After calcinations and treatment with ZnSO4 solution, the sample exhibits a specific surface area of 323 m2/g and a pore diameter of 5.2 nm. The synthesis method with the addition of salts at near-neutral pH and room temperature widens the synthesis scope of SBA-15 type mesoporous materials.