Microwave-Assisted Preparation And Applications Of Nanoporous Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a new generation of porous materials constructed from molecular building blocks bearing reactive organic functional groups. Differing from recently developed metal-organic frameworks (MOFs), COFs are formed using covalent bonds. COFs have low densities because they are entirely composed of light elements such as H, B, C, and O, thus increasing the mass percentage of guests that can be adsorbed into the solids. Furthermore, COFs exhibit remarkably high surface areas in comparison with other porous materials, and much higher thermostabilities (up to500℃), while MOFs that are maintained by coordination bonding are not robust enough to exhibit permanent porosity after the removal of guest molecules or at high temperatures. This makes COFs promising for a range of applications including gas store and catalysis.Although some traditional techniques, such as solvothermal and high-temperature ionothermal methods, are applied to prepare crystalline COFs, the reaction and crystallization processes using these traditional methods usually utilize high temperature and high pressure, as well as take long time. In this paper, we use a facile and high-efficient synthesis of COFs under microwave irradiation. The main contents of this thesis are as follows:1. A covalent triazine-based framework (CTF) was synthesized by using a novel MW-enhanced high-temperature ionothermal method and remarkably high capacity hydrogen adsorption in the solids at low pressure. The approach is a simple yet highly energy-and time-efficient way for the preparation of this kind of covalent triazine-based porous material through a high-temperature polymerization reaction catalyzed by Lewis acids. Clearly, Amorphous CTF samples with high surface areas (up to2390m2g-1) and high hydrogen adsorption capacities (up to1.78wt%at1bar and77K) could be easily obtained in tens of minutes by using such a MW-enhanced ionothermal method, while the polymerization reaction has to be carried out at high temperatures (400-700℃) for a long reaction time (20-116h) using conventional high-temperature ionothermal method. 2. A novel type of magnetic porous carbonaceous polymericmaterial, composite (CTF=covalent triazine-based framework), has been synthesized by a f acile microwave-enhanced high-temperature ionothermal method. By selecting ZnCl2as a reaction medium and the Lewis acid catalyst, and choosing FeCl3-6H2O as an iron oxide precursor, a series of CTF/Fe2O3composites with different y-Fe2O3contents has been prepared in60min. The resulting samples were characterized by the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), vibration sample magnetometer (VSM), and N2sorption-desorption isotherms. The obtained CTF/Fe2O3composites exhibit high surface areas (930-1149m2g-1), and their saturation magnetizations at300K vary from1.1to5.9emu g-1, depending respectively on different Fe2O3contents (6.43-12.43wt%) in the CTF/Fe2O3composites. The CTF/Fe2O3composites were applied to remove organic dye from aqueous solution by selecting methyl orange as a model molecule, and both high adsorption capacity (291mg g-1, corresponding to0.889mmol g-1) and fast adsorption kinetics (kads=4.31m2mg-1min-1) were observed.3. A new type of COF nanoparticle, i.e. melamine-based porous polymeric network SNW-1, was synthesized by a microwave-assisted synthesis route. Although the synthesis of SNW-1has to be carried out at180oC for3d under conventional reflux conditions, SNW-1nanoparticles could be obtained in6h by using such a microwave-assisted method. The results obtained have clearly demonstrated that microwave-assisted synthesis is a simple yet highly efficient approach to nanoscale COFs or other porous polymeric materials. Remarkably, the as-synthesized SNW-1nanoparticles exhibit extremely high sensitivity and selectivity, as well as fast response to nitroaromatic explosives such as2,4,6-trinitrotoluene,2,4,6-trinitrophenylmethylnitramine and picric acid without interference by common organic solvents, which is due to the nanoscaled size and unique hierarchical porosity of such fluorescence-based sensing material.