Research On The Synthesis And Characteristic Of Novel Nanoporous Carbon/Silica Composites

Nanoporous materials are widely applied for adsorption, separation, purification, chromatograph and catalytic fields, etc. The fundamental understanding and application for high-performance adsorption and hemogenerous catalysts required the nanoporous materials with activated species co-existing with hydrophobic surface, also having higher surface area and particular pore size distribution. These materials can be obtained from layered materials by using soft chemistry methods by pillaring and intercalation techniques. Utilization of layered materials with a thinner layer is crucial and challenging in this route because these layered materials permit the final nanoporous materials to have a more exposed surface and higher specific surface area due to the thin pore wall. Among all the layered materials, graphite is a layered material having the thinnest atomic layer. A nanoporous material with thin graphitic layers has been sought with expectation for high-efficiency adsorption and gas-storage capability. Because of its neutral wall nature, some species can be intercalated into graphite layers under controlled conditions to form the graphite-intercalated compounds typical of a nonporous sandwiched structure. The weaker forces between layers give an unstable structure. However, oxidation of graphite in a liquid phase containing strong oxidants donates oxygen to the base planes of the graphite layers,leading to the production of graphite oxide (GO) with thin walls and a great minus charge density balanced by counteractions such as protons in the interlayer galleys. The graphite oxide exhibits rich intercalation chemistry owing to its excellent swelling and intercalation properties, similar to clay minerals. Different species such as transition metal, organic molecules and some different polymers such as ethylene oxide(PEO), polyfurfuryl alcohol (PFA) and polyaniline,etc. can be intercalated into the GO layers to form intercalated composites. It is possible to synthesis a nanoporous material having thin carbon walls co-exiting with species by adding a robust bridging/pillaring species into the interlayer of GO. Just based on the novel soft chemistry route, Dr. Z.-M Wang firstly synthesized the nanoporous carbon/silica composites with higher surface area from introducing tetraethoxylsilane (TEOS) into surfactant-preexpanded graphite oxide. The obtained novel porous nanocomposites with carbon and silica have a high surface area greater than 1000m2/g and present medium hydrophilic property falling between typical activated carbon and hydrophilic silica.However, this method (called as traditional method) exists some disadvantages. First, this method requires a great amount of organic silicon sources, much of which are thrown away and wasted after the synthesis is finished. Second, when organic silicon source (TEOS) is introduced, the process consumes a long hydrolyzing time. Third, after TEOS is introduced into the surfactant-pre-expanded precursors, the obtained sample becomes irregular and disorder structure. Therefore, from the viewpoint of high-efficiency and economic synthesis methods, this synthetic method needs to be developed and improved. For this purpose, under the direction of Dr. Wang Z.-M, this thesis gives a very successful development on the synthetic approach. In this thesis, mechanochemcial method is introduced to synthesize the novel porousnanocomposites. The result shows that the synthetic time decreases to 15 mins from initial one week, and the consuming amount of organic silicon source decrease 10 times.A mechanochemical method, which applies mechanical energy in the form of grinding, milling, and so forth to bring about chemical reaction, can be a simple and effective way to improve the TEOS intercalation reaction into GO layers. In this thesis, the mechanochemical method is applied to introduce a controlled amount of TEOS into the interlayer spaces of GO preexpanded by a long-chain surfactant.In this thesis, some interesting and new results are shown. When small amounts of TEOS are introduced, the samples with expanded and ordered layered structure can be obtained, and with the increase of TEOS amount, the interlayer distances of samples increase. When the amount of introduced TEOS increases to a larger amount, the regularly layered structure of GO is destroyed and disordered. The silicon content in samples increases with the increase of TEOS amount at initial time, and then become stable when introduced TEOS gets to a certain value. In this synthetic process using mechanochemical method, not all the TEOS molecular become stable silica, and just part of TEOS molecular are hydrolyzed into stable silica species, and other part of TEOS molecular escaped away into air. X-ray Diffraction (XRD) results show that the position of diffraction peak of samples at small angle degree moved to the larger angle degree and become constant after about 9 hours when samples are laid into air. The diffuse reflectance infrared Fourier transform (DRIFT) results show that with the increase of introduced TEOS amount, the structure of silica in samples changes from a disordered structure with enhanced band into a more condensed silica-network structure. The analysis of N2 adsorption isotherm shows that the surface area of the obtained nanoporous carbon/silica compositesincreases with the increase of silica content at the beginning, when getting to its maximum, and become a little decreased. All the N2 adsorption isotherms of samples exhibit clear characteristic of mesopores. At small amount of introduced TEOS, samples nearly only have mesopores, and at larger TEOS amount, some micropores also exist in samples.In this thesis, the reproductive experiments are also carried out. In the second part of this thesis, the synthetic experiments of samples with a heavier weight are carried out, and a new pore-size-analysis method is tried to analyses the porosity of the obtained nanocomposites with a heavier weight. The αs-plot method is a very effective method to analyses porosity of a solid. Before using αs-plot analysis, a standard reference need be chosen. However, at present, there is not suitable standard reference αs data to analysis the porosity of the nanocomposites obtained in this thesis because the porous nanocomposites is composed of carbon and silica. IUPAC just gives the standard αs data of carbon material and silicon material, respectively. In this thesis, the simulated standard αs data of the nanocomposites with carbon/silica is tried and used first time in this field to analyses the porosity of the obtained composites.The further study in this thesis is carried out on the adsorption property of the obtained novel nanoporous composites to water and hexane. H2O adsorption results show that the surface hydrophilicity of samples is improved after samples experience the first H2O adsorption, and some pores in samples are disappeared inversely. The porosities of samples become decreased and pore structure is changed after the first-run H2O adsorption, among them, the mesoporsity enhanced and microporosity decreased inversely. The hexane adsorption results show that the adsorption amount of hexane in the obtained samples increases with the surface area of samples, and hexane adsorption nearly has noinfluence to the surface of samples. The adsorption amounts of hexane aremore than the adsorption amount of H2O in all the same samples,indicating the hydropobicity of carbon layer in samples is kept.