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Controlled Synthesis and Structural Analysis on the Ordered Porous Materials

Author: YuTing
Tutor: ZhaoDongYuan
School: Fudan University
Course: Inorganic Chemistry
Keywords: metal-organic framework polycarboxylic acid mesoporous materials electron crystallography small-angle X-ray scattering titanium carbide/carbon nanocomposite
CLC: O631
Type: PhD thesis
Year: 2007
Downloads: 1467
Quote: 2
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Porous materials, especially those with controllable size, shape, uniformity andframework component of pore space, have attracted lots of scientific andtechnological interests, due to their high specific surface areas and confined space forthe interaction with encountered matter. As encouraged by the mutual promotion ofscientific research and industry application in the field of zeolite, the later developingporous materials are also expected to disserve these concerns. In the late 1980s andearly 1990s, concepts from zeolite synthesis were branched and developed in theactive fields of metal-organic frameworks (MOFs) and mesoporous materials (2-50nm). Those years found the successful synthesis of MOF-n and M41S, more orderedporous materials appear with different pore structures and framework components,which promise distinct properties for applications. In some extent, controllablesynthesis of porous materials on micro or meso-scale is being approached.Nevertheless, it should be helpful to direct the synthesis, elucidate mechanism andutilize the porous materials with full knowledge of pore size, shape, topology and etc.By noticing the different "crystal" essence of MOFs and mesoporous materials,different insights into their structures would be preferred. As for MOFs, pore topologysimplified from the complicated structural details is required. As for mesoporousmaterials, how to clarify the pore arrangement and structural details is challengingand interesting. Furthermore, there are lots of opportunities to enrich the frameworkcomponents of mesoporous nanocomposites by taking the advantages of MOFs andmesoporous materials.This thesis is divided into three major parts. In the first part, the use of lowsymmetrical polycarboxylic acid in the controlled synthesis of MOFs is mainlydescribed. In the second part, 3-D reconstruction of mesostructures and developmentof this methodology is offering a tool to interact with the synthesis of mesoporousmaterials. In the third part, a metal-organic polymer precursor is utilized in thecontrolled multi-components organic-organic self-assembly of hybrid mesostructuredpolymers and their derivatives-mesoporous titanium carbide/carbon nanocomposites.At the far beginning of the focused researches on MOFs, high symmetrical andmultiple-linking organic ligands were preferred in the design synthesis. However, lowsymmetrical ligands are also gradually involved in preparation of MOFs because oftheir easy availability, simplicity or good performance of chelating metals. In this part, isophthalic acid has been firstly employed in the synthesis of MOFs by simplemethods and strategies. It is observed that the bending configuration of linker oftenresults in low-dimensional structures if there would be no other means to connect thethree-dimension. The observations are further supported by the two additionalstructures from 2, 5-thiophenedicarboxylic acid, which has similar bendingconfiguration. Furthermore, self-assembly of citric acid with Co, Ni, Cu or Zn hasbeen conducted with pyrazine acting as base and/or the second ligand. For those latetransition metals, the tendency to form low dimensional structures is still obviousconsidering structural fragments of these metal citrates. Due to the significant role ofacetate played in some above products, an in situ reaction for generation of acetatefrom acetylacetone is utilized to control coordination environment of rare earth metalsreducing cross-linking extent of MOFs. As comparison, trimesic acid has been alsochosen as a high symmetrical ligand in the controlling synthesis of MOFs. A galliumtrimesate structure and a manganese trimesate structure are given to demonstrate thatthe ligand symmetry is only one of the key factors that account for symmetry of thewhole structure, while metal oxygen cluster accounts more for the final structure.The second part is mainly focused on the controlled synthesis and structuralsolution of mesoporous materials templated by block copolymers. Diffraction patternsof mesoporous silica FDU12s varies with synthetic temperatures even if the materialswere derived from the same starting recipe. By applying TEM techniques, the spacegroups of all the FDU-12 products herein have been determined to be Fm(?)m,independent from synthetic temperatures. For such cage-like mesoporous materials,the enlargement of cage window without structural regularity loss is as important asmechanism survey of this process. 3-D reconstructions of mesostructures of FDU-12sare fulfilled by using electron crystallography, small-angle X-ray scattering (SAXS),PXRD, and nitrogen sorption methods. Structural evolutions of FDU-12s are alsotraced by the difference between electrostatic potential maps, which point out thatwindow enlargement could be recognized by finding diffraction intensity growth of220 reflections. After solving the "phase problem" by electron crystallography,intensity extracted from 2-D SAXS pattern has been attempted to achieve averageelectron density map for a bimodal mesoporous carbon, showing the general structuraland formation mechanism difference to CMK-5 materials.In the third part, ordered mesoporous titanium carbide/carbon nanocompositeswith different titanium contents are fabricated with the cooperative organic-organic self-assembly of titanium citrate, resol and block copolymer (F127) via EISA andcarbothermal reduction. The nanocomposite materials possess two-dimensional (2-D)hexagonal arrayed cylindrical channels (space group of p6mm) with titanium carbidenanocrystals embedded in the amorphous carbon matrix. The size of titanium carbidenanocrystals is controlled in the range of 4-6 nm. The ordered mesoporous titaniumcarbide/carbon composites have high surface areas (600-800 m2 g-1), large porevolumes (0.43-0.49 cm3 g-1) and narrow pore size (3.8-4.8 nm). PXRD patternssuggest that the formation temperature of nanocrystalline titanium carbide is no lessthan 950℃under Ar atmosphere, consistent with thermogravimetric analysis. Below950℃, crystalline titania (TiO2) phase is unjustifiable from the series of WXRDpatterns which is rare compared with the previous reports. The use of titanium citrateas a precursor is considered to be a key factor to make a high disperse titaniumspecies into the phenolic polymer frameworks and further prevent titanium oxidefrom crystallization at low temperature. It is featured that the incorporation oftitanium carbide nanocrystals can increase the oxidation-resistance performance ofmesoporous carbon frameworks in air. Furthermore such nanocomposites may enablea facile improvement of electrical properties of mesoporous carbons for the use ofelectrode materials and the general synthesis strategy can also be extended to generatein situ nanoreactors with other early transition metal carbide nanocrystals in thecarbon pore walls as substitutive catalyst of noble metals.

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CLC: > Mathematical sciences and chemical > Chemistry > Polymer chemistry ( polymer ) > Polymer physics and physical chemistry of polymers
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