Study On Synthesis Of Mesoporous Silica With Special Morphology

Mesoporous materials have attracted much more attention due to their advantages, such as high surface area, ordered arrangement, uniform pore structure, as well as their pore size could be adjusted in the range of 2 to 50 nm, and their potential application in the field of sorption, separation, catalysis, drug control-release, light, electricity and magnetism, etc. Due to the framework of the most current mesoporous materials is composed of SiO2, which only possess a little of weak acid centers, they are almost catalytic inactive. To make the silica based mesoporous materials possess catalytic active, embedding hetero atoms into the silica framework or modifying the framework of silicas with organic functional groups have been widely applied, and it is becoming one of the hottest topics in the field of mesoporous materials. Furthermore, synthesis of the mesoporous materials with special morphology by using multiple surfactants as co-templates is a novel attempt, it is a valuable attempt not only for improving the knowledge of materials science, but also for extending the application areas of mesoporous materials.The main topics of this dissertation are the preparation and characterization of the hetero atoms embedded mesoporous materials and the novel mesoporous silicas with special morphologies. A series of the metal embedded materials, and the mesoporous silicas with special morphologies, including solid sphere, hollow sphere, and leaf like shape, have been synthesized, their performances in catalysis, sorption, hydrogen storage, and medicine controlled release have been investigated, and some of the materials are modified by supporting precious metal or introducing functional groups. All the synthesized materials were characterized with SEM/TEM, XRD, XPS, et al, and some important information about the structure of materials has been obtained. Furthermore, a possible mechanism for the formation of hollow sphere structure also has been proposed.A new type of neodymium embedded mesoporous silica material has been synthesized successfully by a facile one-pot approach, it is important that the synthesized materials not only had the Nd incorporated into the framework of synthesized materials, but also exhibited a hollow sphere morphology. The effects of molar ratio of Si/Nd, synthesizing time and the removal method of templates have been investigated. Both XRD and XPS results confirmed the framework incorporation of Nd atoms into the framework of mesoporous silica when the molar ratio of Si / Nd is more than 10. It was found that the material showed high surface area, uniform pore size distribution, good thermal and hydrothermal stability. Meanwhile, it was also found that the Nd embedded hollow mesoporous silica material showed good catalytic activity and selectivity towards the oxidation of styrene. When the molar ratio of Si/Nd is 20, the conversion of styrene reached 16.21 %, and the selectivity of hyacinthin was high up to 95.07 %.By using 1, 12-diaminododecane (DADD) as main template, poly(vinylpyrrolidone) (PVP) and tri-block copolymer (F127) as co-templates, mesoporous silica with beautiful leaf-shape and solid sphere structures were successfully synthesized respectively. The mesoporous silicas showed high surface area, uniform pore wormlike pore structure and high thermal stability. By supporting 5 wt% Pd on the solid sphere mesoporous spheres with an impregnation-hydrogen reduction process, a high volume hydrogen storage material was prepared. It was found that the sample exhibited good hydrogen storage properties, the storage capacity was up to 2.59 wt% at room temperature and hydrogen pressure of 12atm, which might make it a promising hydrogen storage candidate material.Furthermore, a hollow sphere and cotton candy like mesoporous silica were synthesized by using DDA as main template, PVP and F127 as co-templates respectively. Based on the experimental results, a possible formation mechanism for hollow-sphere mesoporous morphology has been proposed. When the different molecular weight PVP with different chain length as co-template was applied, the different mesoporous materials were obtain. When long chain PVP-30 was used as co-template, the hollow sphere mesoporous silica with diameter of ca. 500 nm was synthesized; However, when short chain PVP-10 was used, the diameter was decreased to ca. 170 nm; indicating that the size of the hollow sphere was controlled by the chain length of the PVP polymers. In addition, the effect of PVP concentration was also investigated; it showed that the hollow sphere morphology will be disappeared when the concentration was lower than a critical value. It verified our proposed mechanism that the mesoporous structure might be dominated by main template DDA, and the morphology might be dominated by co-template PVP. Both hollow sphere and cotton candy like mesoporous silicas showed high surface area, uniform pore size, and large pore volume, e.g. the pore volume of the hollow sphere could be high up to 1.77 cm3g-1, and the surface area of the cotton candy like mesoporous silica could be 1255 m2g-1. The storage properties of the hollow sphere and the cotton candy like mesoporous silica were measured by loading Coumarin 480 dye molecules, the storage amount of hollow sphere is almost three times of that of the cotton candy like mesoporous silica, suggesting that the hollow sphere mesoporous silica with a large hollow space is a good guest molecule storage material.Finally, we attempted to prepare a novel type of intelligent nano storage materials by introducing a nanovalve into the hollow sphere materials, firstly the hollow sphere and solid sphere materials was synthesized by using DDA and PVP-10 as co-templates, then a pH value sensitive nanovalve was introduced on the mesoporous silica by using theα-cyclodextrin (α-CD) rings as capping agents and three different of anilino group linker molecules as stalks. The nanovalve's work relied on the hydrogen bonding interaction betweenα-CD and the stalk. When theα-CD ring is complexed with the stalk at neutral pH, the bulky cyclic component is located near the pore openings, thereby blocking departure of cargo molecules that were loaded in the nanopores and hollow interior of the particle. Protonation of the nitrogen atoms at lower pH (≤5 or 6) causes the binding affinity to decrease, releasing theα-CD and allowing the cargo molecules to escape. The effect of different stalk lengths, pH conditions and different mesoporous silica on the release of fluorescent dye cargo molecules is measured, and the results showed that nanovalves functionalized with short linkers molecules can release more molecules than that with long linkers molecules, the release rate of cargo molecules would be faster at lower pH condition, hollow mesoporous silica can release as much as two times more cargo molecules than solid mesoporous silica. Therefore, the hollow sphere mesoporous silica with nanovalves will potentially play an important role in the application of drug delivery.