Covalent Organic Frameworks (COFs) materials, or COFs materials for short, also known as organic zeolites, are porous crystalline framework materials formed by covalent bonding connections, which are mainly composed of C, H, O, N, B and other light elements. In terms of material properties, COFs have the advantages of low density, good chemical stability, high specific surface area, regular pore structure, simple functional design, and also luminescent properties in some cases. Researchers have further applied them to gas storage and separation, catalysis, and photovoltaics, and the results show great potential. We will focus on the applications of COFs in the above fields.
COFs have a gas storage performance that exceeds all reported adsorbent materials, and their high adsorption efficiency depends on their low crystalline density, high specific surface area, large pore volume, electron-rich network structure, and good chemical stability.Yaghi's research team systematically studied the adsorption of hydrogen, methane, and carbon dioxide by COFs as early as in 2009. It was found that the specific surface area and porosity of COFs profoundly affected their adsorption capacity for these gases, and the adsorption effect was enhanced with increasing specific surface area from one-dimensional to three-dimensional structures. This work not only shifts the COFs from synthesis to application, but also illustrates the adsorption ability of COFs for these gases, which lays the foundation for the subsequent development of COFs with relevant and better performance.
On the other hand, William A. Goddard, III et al. at the University of California introduced metals into COFs to increase the strength of the interaction between the adsorbent and the hydrogen molecules as a means to improve the storage capacity of COFs for hydrogen at room temperature. The researchers used PdCl2 to metallize COF-301 and synthesized COF-301-PdCl2, a material that is unique because of the hydroxyl groups adjacent to the imine bonds, which generate bonding sites to the metal. Also, they used hybridized DFT functional groups to estimate the interactions between hydrogen molecules and the backbone and to calculate simulated hydrogen adsorption isotherms. The results show that COF-301-PdCl2 is capable of storing up to 60 g L-1 of hydrogen at room temperature and 100 bar, exceeding the target value of the U.S. Department of Energy.
In addition COFs have significant photocatalytic properties. For the first time, the research team of Arne Thomas at the Technical University of Berlin has successfully prepared porous β-ketoenamine-conjugated two-dimensional COFs including either acetylene (C≡C) or diacetylene (C≡C-C≡C) fractions. The researchers compared the photocatalytic hydrogen production capabilities of the two COFs. The results show that the photocatalytic hydrogen production rate of COFs containing bi-alkynyl groups can be enhanced by more than 10-fold compared to COFs containing mono-alkynyl groups, which demonstrates the importance of the alkynyl groups in photocatalysis in the corresponding materials.