Covalent Organic Frameworks

Covalent organic frameworks (COF) are a new class of 2D or 3D crystalline polymers formed by connecting organic building units into extended structures. High crystallinity is conducive to charge separation and transport, which makes COF have good catalytic activity. In addition, the well-defined pores, excellent stability and fine-tuned physical and chemical properties of COFs make them attractive photocatalysts.

Alfa Chemistry Catalysts provides COF materials and the organic linking agents required for their synthesis, including aldehydes, alkynes, boric acid and pyrocatechols, etc., which are used to construct functional porous materials.

Synthesis Method of COFs

Different synthesis methods, including solvothermal, ionic heating, and room temperature or microwave synthesis, are all suitable for the manufacture of COFs. [1]

• Solvothermal synthesis

Most COFs are synthesized using solvothermal methods, and the reaction process is usually heated (80-120 degrees Celsius) in a sealed container for 3 to 7 days. The temperature and pressure of the reaction system significantly affect the reaction yield, crystallinity and porosity.

• Ionization Thermal Synthesis

Molten metal salts and ionic liquids have been used to create ionic thermal conditions during COF synthesis. Compared with other organic solvents, ionic liquids have advantages in synthesizing COF with high periodicity and stability because of their reusability, the characteristics that can be changed by counter anions, and their environmental friendliness.

• Microwave assisted synthesis

Compared with traditional heating methods, microwave heating has several advantages, including faster reaction rate, higher product yield, lower energy use, milder reaction conditions, and increased selectivity during chemical reactions.

• Room temperature synthesis

Room temperature synthesis mainly includes two techniques: mechanochemical (MC) grinding and room temperature solvent method. Among them, MC synthesis is a simple, economical and environmentally friendly method, in which morphological changes and chemical bond breakage and reconstruction occur during the grinding process.[4]

Applications

In the field of photocatalysis, COFs materials have a wide range of applications, including heterogeneous photocatalysis, covering photocatalytic degradation and photocatalytic organic conversion, reduction and conversion of carbon dioxide into valuable materials, and desorption of hydrogen through moisture.

Research Cases

• Photocatalyst for hydrogen production

In an early study, Stegbauer and his colleagues synthesized a new type of crystalline hydrazone group 1,3,5-tris-(4-formyl-phenyl) through the condensation of TFPT and 2,5-diethoxy group. Triazine (TFPT)-COF-terephthalic hydrazide, which is the first COF used for photocatalytic hydrogen production using visible light.

Figure 1. Optical properties and photocatalytic hydrogen evolution of TFPT-COF. [2]

• CO₂ photocatalytic conversion

Yang et al. reported a 2D COF, which incorporates Re complex, with inherent light absorption and charge separation (CS) characteristics. The results show that this hybrid catalyst can effectively reduce CO₂ to form CO under visible light irradiation, with high selectivity (98%) and better activity than its homogeneous Re counterpart.

Figure 2. Proposed catalytic mechanism for CO₂ reduction. [3]

References

1. Qing Yang. (2020). "Covalent organic frameworks for photocatalytic applications," Applied Catalysis B: Environmental 276: 119174.
2. Linus Stegbauer. (2014). "A hydrazone-based covalent organic framework for photocatalytic hydrogen production," Chemical Science 5: 2789-2793.
3. Sizhuo Yang. (2018). "2D Covalent Organic Frameworks as Intrinsic Photocatalysts for Visible Light-Driven CO₂ Reduction," Journal of the American Chemical Society 140(44): 14614-14618.