Design Of Amphiphilic Template And Its Application In The Synthesis And Application Of New Large - Pore Ordered Mesoporous Materials

Owing to the high surface area, tunable pore size and structure, various framework composites and potential applications in diverse fields, including adsorption, separation, catalysis, sensors, energy conversion and storage, ordered mesoporous materials have attracted increasing attentions of scientists from chemistry, materials, physics, and biology. The synthesis of ordered mesoporous materials is based on the templating synthesis. The templates include soft template and hard template. The soft templating approach is facile and easy to be accomplished in large scale. The pore structure, size, and framework compositions of the ordered mesoporous materials can be well tunable by changing the molecular weight, hydrophobic/hydrophilic ratio, and the block compositions of the block copolymer. When the commercial Pluronic type (i.e., poly(ethylene oxide)-b-poly(propylene oxide)-b-poly-(ethylene oxide), PEO-b-PPO-b-PEO) triblock copolymers were used as the template, the pore size and pore wall thickness is small owing to the low molecular weight, which will limit the application involving large object. In addition, the thin pore wall of the mesoporous metal oxide is prone to collapse during the crystallization of the pore wall. Accordingly, design of new template for the synthesis of new large-pore ordered mesoporous materials is in much demand.In this dissertation, we apply the basic knowledge of the polymer synthesis to design a series of templates with different compositions and molecular weight, and then use the templates to synthesize new mesoporous silica, and carbon materials. By carefully control the properties of the template, the pore structure, size, and pore wall thickness of the ordered mesoporous materials can be well controllable. In addition, the applications of these mesoporous materials in catalysis, adsorption, electrochemical storage and drug delivery are also investigated.In Chapter2, a novel solvent evaporation induced aggregating assembly (EIAA) approach has been demonstrated for the synthesis of highly ordered mesoporous silica materials with large mesopores and window sizes by using water-insoluble block copolymer with large molecular weight as a template in a blend solvent containing a good solvent and a poor solvent (water). The mesoporous silica materials prepared by using large PEO-b-PMMA as a template, TEOS as a precursor in the acidic THF/H2O mixture medium have ordered fcc mesostructure, large tunable pore size of up to37.0nm, large window size (8.7nm), high surface area (508m2/g) and large pore volume (1.46cm3/g). Interestingly, the mesoporous silicas obtained from the EIAA approach show crystal-like morphology with large particle size (0.5-6μm). A mechanism which involves a good solvent evaporation induced continuous formation, aggregating and packing of the spherical silica/copolymer micelles at the interface of water-rich phase is proposed. The Au/OMS composites with well-dispersed Au nanoparticles (～4nm) showed an excellent catalysis for fast and efficient reduction of4-nitrophenol at room temperature. Because of highly confined small Au nanoparticles, the Au/OMS catalysts can be reusable without significant decrease of catalysis performance even after10cycles.In Chapter3, two different methods, the well-known solvent evaporation induced self-assembly (EISA) and newly-developed solvent evaporation induced aggregating assembly (EIAA) were both employed to synthesize ordered mesoporous silicas with a face centered cubic (fcc) mesostructure and large pore size (～18nm) by using an amphiphilic block copolymer poly(ethylene oxide)-b-poly(methyl methacrylate)(PEO-b-PMMA) as the template. Compared to the EISA process, the EIAA approach, featuring the presence of large amount water in the synthesis system and the self-assembly process at the liquid-liquid interface, gives rise to ordered mesoporous silica with thinner pore walls, higher surface area and larger pore volume. Through the EIAA process, discrete siliceous nanotubes and hollow nanospheres were also successfully synthesized by increasing the water content using block copolymer PEO-b-PMMA templates with different molecular-weights. Excess amount of water can significantly dilute the concentration of the pre-formed PEO-b-PMMA/silica composite micelles, thus inhibits the aggregating of the micelles to ordered mesostructure. Additionally, the introduction of ethanol in the initial water/tetrahydrofuran (THF) mixture solution can slow down the hydrolysis/condensation of silica precursor, which yields mesoporous silica spheres with1-5μm in diameter, large pore size (～16.8nm), windows size (～8.9nm), and high surface area (～482m2/g).In Chapter4, a solvent evaporation induced step-by-step aggregating assembly process has been developed for the synthesis of ordered dual mesoporous silica materials in an acidic THF/H2O solution by using nonionic block copolymer PEO-b-PMMA and cationic surfactant CnTAB as the co-templates. The obtained ordered dual mesoporous materials show high surface area (510～670m2/g) and uniform two sets of mesopores concentrated at about2.5and20nm, respectively, with small mesopores being distributed around the large mesopore. The small mesopore can be readily adjusted independently by using the CnTAB molecules with different length of hydrophobic chains. In addition, small Au nanoparticles (about4nm in diameter) can be encapsulated in the ordered dual mesoporous silica. The obtained Au/ODMS materials exhibited an excellent performance in catalyzing the epoxidation of styrene with a high conversion (95.4%) and selectivity (82.6%) toward styrene oxide.In Chapter5, ordered mesoporous carbons (OMCs) with face-centered cubic (fcc) and2-D hexagonal (p6m) symmetries have been successfully synthesized by using lab-made poly(ethylene oxide)-block-poly(methyl methacrylate) diblock polymers (PEO-b-PMMA) with different PEO/PMMA ratios as a template. The synthesis process undergoes an evaporation induced self-assembly (EISA) by using low-molecular weight phenolic resol as a carbon source and tetrahydrofuran (THF) as a solvent, and followed with thermal polymerization and removal of template during the carbonization process. The atom transfer radical polymerization (ATRP) method was used to prepare the diblock copolymers with different molecular weight and compositions of PEO and PMMA segments by simply changing the PEO initiator and polymerization time. For the first time, we have obtained OMCs with2D hexagonal mesostructure (p6m symmetry) and large pore size (8.6-12.1nm) by using PEO-b-PMMA with long PMMA segment as the templates. Notably, the pore size of the ordered mesoporous carbons can be tuned in the range of8.6-22.0nm by slightly adjusting the hydrophobic PMMA length of the template or adding desired amount of PMMA homopolymer as a pore expander. Additionally, it is found that the pore wall thickness of OMCs with face-centered cubic symmetry can be adjusted from8.1to10.4nm by simply increasing the weight ratio of resol to the template.In Chapter6, we report a one-pot controllable method to synthesize N-doped ordered mesoporous carbons (NMC) with a high N content by using dicyandiamide as a nitrogen source via evaporation induced self-assembly process. In this synthesis, resol molecules can bridge Plunoric F127template and dicyandiamide via hydrogen bonding and electrostatic interactions. During thermosetting at100℃for formation of rigid phenolic resin and subsequent pyrolysis at600℃for carbonization, dicyandiamide provides N species while resol can form a stable framework, thus ensuring the successful synthesis of ordered N-doped mesoporous carbon. The obtained N-doped ordered mesoporous carbons possess tunable mesostructures (p6m and Im3m symmetry) and pore size (3.1-17.6nm), high surface area (494-586 m2/g), and high N content (up to13.1wt%). Ascribed to the unique feature of large surface area and high N contents, NMC materials show high CO2capture of2.8-3.2mmol/g at298K and1.0bar, and exhibit a good performance as the supercapacitor electrode with the specific capacitances of262F/g (in1M H2SO4) and227F/g (in6M KOH) at a current density of0.2A/g.In Chapter7, a smart azobenzene-derivated cationic surfactant with photoresponse was designed for use as the structure-directing agent to mesoporous silica nanospheres. Interestingly, the surfactant confined in the silica matrix retains the photoresponse property, which can tune the molecular size and polarity via the cis-trans isomerism. After UV irradiation for1h,80%of surfactants in the silica framework can be released in the H2O/EtOH mixed solvent and recycle from the solution. The recycled surfactant can be used as the structure-directing agent to mesoporous silica nanospheres, which exhibits a facile, green, environmentally friendly method to mesoporous silica nanospheres. The mesoporous silica nanospheres after the removal of AZTMA templates show open framework with accessible pores, radial mesopores, high surface area (～381m2/g), uniform pore size (2.7nm) and spherical diameters (～300nm).In Chapter8, the whole thesis is summarized.