Cages to save the planet:Porous Organic Frameworks 拯救地球：笼状有机框架

Introduction

The concentration of CO2 has risen significantly since the 19th century, from less than 300 ppm to a record high of 421 ppm in 2022. This increase has resulted in a cascade of severe weather events, ocean acidification, and rising sea levels. To mitigate the effects of global warming, the concepts of 'carbon peaking' and 'carbon neutrality' have been introduced, focusing on reducing CO2 emissions through various means, including carbon capture, utilization, and storage (CCUS). Among the technologies for CO2 adsorption, porous materials stand out due to their low energy consumption, ease of regeneration, large adsorption capacity, and good selectivity.

Porous Organic Frameworks for CO2 Capture

Porous organic frameworks, including MOFs and COFs, have garnered substantial research interest due to their structural designability, controllable porous structures, and adjustable morphologies. These materials offer large surface areas with numerous active sites to adsorb CO2 molecules through physical and/or chemical effects. The porosity of these frameworks can be customized to optimize CO2 capture performance by controlling the shape and size of the pores. Additionally, the rational design and fabrication of these materials allow for modification with functional groups, providing an additional advantage for CO2 capture applications.

MOF and Related Materials

MOFs, with their high surface area, tunable pore size, and ability to tune chemical and physical properties, have garnered considerable attention for carbon capture. MOFs made from renewable resources, including metal ions and organic linkers, are considered a sustainable option for carbon capture. Pristine MOFs with open metal sites and controllable typologies enhance gas-adsorbent interactions, where open metal sites exhibit high affinity for gas molecules, and the appropriate topology with high specific surface area generates more adsorption sites for interaction with gas molecules. High selectivity makes MOFs and related materials preferred carbon capture materials, ensuring that carbon dioxide is captured while other gases, such as oxygen, are not absorbed.

Modified MOFs

Despite the potential of MOFs for gas adsorption, pristine MOFs have some drawbacks as capture materials. They can also adsorb other gases, such as water vapor, which can reduce their overall efficiency and increase the amount of energy for regeneration. Additionally, MOFs face stability issues under certain conditions, such as high humidity and high temperatures, which are common in industrial settings. Post-synthetic modification (PSM) provides an effective way to overcome these limitations and achieve enhanced stability and/or selectivity. Functionalization has been proven to be an effective strategy during the PSM process, where functional groups can be linked to the metal centers or organic ligands of MOFs to alter their chemical and physical properties.

A new framework, pip2−Mg2(dobpdc) (pip2 = 1-(2-aminoethyl)piperidine), that exhibits two-step CO2 uptake andachieves an unusually high CO2 capacity approaching 1.5 CO2 per diamine at  saturation.

COF and Related Materials

COFs, similar to MOFs, are a series of conjugated organic polymeric 2D or 3D materials assembled by covalent bonds connecting organic building blocks. COFs have a large internal surface area, making them ideal for adsorbing large amounts of CO2, while the pore size can be adjusted by tuning molecular design. COFs are constructed by covalent bonds, which endow them with high stability and resistance to degradation. This stability also makes COFs suitable for survival in harsh environments and allows COFs to be regenerated and reused without significant loss of performance.

COF-999 is an exceptional material for the capture of CO2 from open air as evidenced by its cycling stability, facile uptake of CO2 (reaches half capacity in 18.8 min) and low regeneration temperature (60 °C).

Conclusions and Outlooks

In conclusion, MOFs, COFs, and other porous framework materials have shown significant potential for CO2 capture and separation due to their high surface area, porosity, and designable structures. However, challenges remain in achieving high CO2 capture performances and selectivities under practical conditions. Many organic frameworks exhibit unsatisfactory structural stability for practical applications, especially in the presence of moisture or acidic gases. Designing framework materials with covalent bonded interactions can improve their thermal and chemical stability, while the combination with carbonaceous materials can enhance stability further. Reducing the cost of MOFs, COFs, and other organic framework materials is crucial for their transition from laboratory synthesis to industrial production. Theoretical calculations should be employed to elucidate the CO2 adsorption mechanism in these materials, guiding the optimal structural design for effective CO2 adsorption sites. Finally, exploring beyond MOFs and COFs, such as hydrogen-bonded organic frameworks (HOFs) and conjugated polymers (CPs), may offer more possibilities for CO2 adsorption with flexible porous structures, excellent cyclability, and enhanced adsorption or separation performances.

LinkChem Porous Organic Frameworks Building Block Series

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