Limitations of MOFs and new attempts to industrialize COFs
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. With these advantages, the application scope of COFs extends from gas storage to filter membranes, chemical sensors, and pollutant degradation catalysts, while their potential for environmental applications is gradually being emphasized.
Gas storage area
In 2009, Yaghi's group reported that COFs materials showed excellent performance in gas storage, with very high storage capacity for H, CH, and CO, etc. In 2020, Abuzeid et al. reported that the synthesized Cz-DHBDCOFs showed excellent CO absorption capacity, reaching 57.28 mg/g at 298K and 110.59 mg/g at 273K. The absorption capacity reached 110.59 mg/g at 273K.
Environmental applications
In 2020, Yaghi's group successfully combined different cofunctional groups into hydroxyl-containing multistage porous COFs using amidation, esterification, and thioesterification reactions, resulting in a series of adsorbents that can effectively remove a variety of pollutants (heavy metal ions and oxidizers) from water, with different chelating functional groups possessing different adsorption capacities and selectivity for heavy metal ions.
Industrialization field
In 2005, Prof. Yaghi's group synthesized the structure of COFs, which initiated the research boom in the field of COFs. Since the first COF was reported, the methodology of COFs synthesis has been an interesting research direction. So far, the traditional solvothermal method is still the most common way to prepare COFs, but there are still some unique methods (e.g., mechanochemical synthesis, ionothermal synthesis, hydrothermal synthesis, solvent-free synthesis, acoustic synthesis, and microwave-assisted synthesis) that can be used to prepare COFs according to the different organic building blocks and reaction mechanisms.
"Green chemistry is defined by IUPAC as "the design, invention, and application of chemical products and processes to reduce or eliminate the use and generation of hazardous substances". Solvothermal synthesis usually requires the use of hermetically sealed Pyrex tubes, long reaction times (3-7 days), high reaction temperatures, high boiling points, and hazardous organic solvent mixtures. Therefore, conventional solvothermal methods are usually not compatible with the principles of green chemistry.
However, all of the currently reported synthetic methods require the screening of suitable organic solvents as reaction media, which makes it difficult to scale up the synthetic system and does not have the advantage of industrialized production, restricting the promotion and application of COFs.
In view of this, Prof. Zhenjie Zhang of the School of Chemistry, Nankai University, developed a melt green synthesis strategy, which realized the gram-scale preparation of vinyl COFs with high crystallinity and stability without adding any solvent. As a green technology, melt green synthesis shows great potential in industrial practical synthesis process due to its unique advantages such as low cost, reduced use of hazardous chemicals, and simple process and handling. In addition to that, processing COFs into different shapes and being able to rationally handle the shapes and sizes so that they can be easily transported, stored, and used can greatly advance the industrial application of COFs. Most of the currently reported COFs exist in powder form after synthesis and activation, which hinders their further application in green chemistry. The new method of melt green synthesis makes it possible to manufacture cost-effective crystalline COFs in kilogram scale on a large scale, which successfully opens the door for industrialization of research results.
The corresponding Metal Organic Frameworks (MOFs), or MOFs for short, are a class of crystalline porous materials with a periodic network structure formed by interconnecting inorganic metal centers (metal ions or metal clusters) with bridging organic ligands through self-assembly. More than 80,000 MOFs have been reported so far, and they are widely used in various fields such as catalysis, energy storage, biomedicine, gas adsorption and separation, and sensors.
Although the research of MOFs is very hot, the MOFs materials applied in commerce are very limited, mainly due to the following reasons: 1) The cost of MOF material precursors is relatively expensive, the industrial production does not have a cost advantage, and the preparation method is more energy-consuming, and energy-saving and efficient processes are yet to be developed. 2) Many MOF materials have low mechanical strength and poor thermal stability, and they do not have the conditions for quantitative synthesis. 3) Most MOF materials have good single performance, comprehensive performance is difficult to meet the technical requirements, the use of the effect is not up to expectations.
One of the earliest achievements in the scale-up synthesis of MOF is the TruPick adsorbent, which stores and releases 1-methylcyclopropene to ripen fruits and vegetables. Shortly thereafter, another commercial MOFshi ION-X gas storage and delivery system, which is used to store hazardous gases such as arsine, phosphine, and boron trifluoride produced in the electronics industry.