Porous Aromatic Frameworks: Design, Synthesis And Characterization

Porous materials have been of continuously intense scientific and technological interest due to their vital importance to many applications including catalysis, gas separation and gas storage. Conventional porous materials consist of amorphous networks (e.g., activated carbon) and crystalline inorganic frameworks (e.g., zeolites). However, porous materials constructed by organic components have been expeditious advances in the past decade, particularly in the area of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). Very recently, a promising family of porous materials, porous organic frameworks (POFs) composed solely of light elements (C, H, O, N, B, etc,) have been the subject of intense recent interest owing to their high surface areas and good mechanical properties. Several pioneering and effective strategies have been employed to construct POFs, including condensation reaction of boronic acids, the dibenzodioxane-forming reaction, palladium-catalyzed Sonogashira-Hagihara cross-coupling reaction, palladium-catalyzed Suzuki cross-coupling reaction, nickel(0)-catalyzed Yamamoto-type Ullmann cross-coupling and trimerization reaction of aromatic nitrile compounds.One of the promising goals in the field of porous materials is the targeting synthesis of compounds with predicted topologies and demanded properties. Through assembling trimeric inorganic unit and organic linker, a chromium terephthalate-based solid, MIL-101, has been created with MTN zeotype architecture and large surface area, which can be potentially used as a nanomold. A very common octahedral inorganic building unit, zinc carboxylate cluster, can be connected with organic triangular unit via coordination bond to lead to a metal-organic framework, MOF-177, with expected high surface area and porosity. Via self-condensation and co-condensation reactions, a new class of porous material, covalent boron oxide-based frameworks (COFs), have been constructed by targeting known topology based on linking tetrahedral and triangular rigid organic unit.Based on our prevenient studies of porous materials, we have foused our research interests on the design, synthesis and characterization of porous aromatic frameworks (PAFs). In this thesis, we have synthesized three PAFs materials by skilful design and effective strategies. These materials were characterized by Fourier transform infrared spectroscopy (FT-IR), 13C solid-state NMR, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and N2 gas sorption. Futhermore, these series of PAFs materials have been explored their abilities in gas storge and liquid absorption.The detailed results are as follows:(1) We present a strategy that has enabled us to achieve, with the aid of computational design, a structure that possesses by far the highest surface area reported to date, as well as exceptional thermal and hydrothermal stabilities. Conceptually, replacement of the C-C covalent bonds of diamond with rigid phenyl rings should not only retain a diamond-like structural stability but also allow sufficient exposure of the faces and edges of phenyl rings with the expectation of increasing the internal surface areas. To synthesize PAF^-1, we selected tetrakis(4-bromophenyl) methane as the tetrahedral building unit, and the phenyl rings were coupled using the nickel(0)-catalyzed Yamamoto-type Ullmann cross-coupling reaction. PAF^-1 has a Langmuir surface area of 7100 m² g^-1, besides its exceptional surface area, PAF^-1 outperforms highly porous MOFs in thermal and hydrothermal stabilities, and demonstrates high uptake capacities for hydrogen and carbon dioxide. Moreover, the super hydrophobicity and high surface area of PAF^-1 result in unprecedented uptake capacities of benzene and toluene vapors at room temperature.(2) POFs possessing high thermal stabilities and high surface areas have been achieved for storage media. There is much opportunity to develop new combined issues in POFs with functional properties such as electronic and electroluminescent properties attributing to the conjugated texture. As a high stable conjugated material, poly(p-phenylene) (ppp) has attracted great interest in electronic device applications. To synthesize a new porous aromatic framework being composed of only phenyl rings, a monomer 1,3,5-tris(4-bromophenyl)benzene was employed. PAF-5 has been synthesized successfully using the Yamamoto-type Ullmann reaction. This material was characterized by Fourier transform infrared spectroscopy (FT-IR), ¹³C solid-state NMR, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and N₂ gas sorption. PAF-5 displaying high stability and high surface area exhibits excellent abilities to adsorb organic chemical pollutants at saturated vapour pressure and room temperature.(3) PAF-2 based on a tetraphenylmethane block and a triangular triazine ring been designed and synthesized in ionothermal condition. Characterization shows that such PAF-2 possesses high thermal and chemical stability, high surface area (Langmuir surface area 1109 m² g^-1), with narrow pore size distribution. Given the 3D aromatic framework of PAF-2 together with its large permanent surface area, we also explored its capability for adsorption of organic vapors such as benzene and cyclohexane. PAF-2 can adsorb large amount of benzene vapor but little cyclohexane vapor at room temperature with values of 138 mg g^-1 and 7 mg g^-1, respectively, at their saturated vapor pressures. PAF-2 shows the excellent selective sorption of benzene.