Covalent-organic porous framework (COF) integrated hybrid membranes for energy and environmental applications: Current and future perspectives

The discharging of toxic pollutants into the ecosystem is a growing concern worldwide. Every year a huge amount of various hazardous materials is released into aquatic ecosystems and air, leading to significant harm to humans and the environment. So, it is necessary to develop advanced engineered nanomaterials such as inorganic, metal organic framework (MOFs) and covalent organic framework (COFs) to protect the environment and people exposed to those harmful pollutants. Covalent organic frameworks (COFs) are porous crystalline materials known for excellent separation performance. COFs offer a large surface area, high porosity, low density, flexible molecular sieving, tunable pore size, and high stability, and are compatible with polymers to form MMMs. Furthermore, the modification of COFs by varying functional groups materials will develop high-performance membranes for gas and water treatment and fuel cell applications. This article has discussed the crucial properties of COF materials, such as molecular sieving, hydrophilicity, hydrophobicity, framework stability, and surface charge required for membrane performance enhancement. The different strategies for COF membrane fabrication, like in-situ growth, layer-by-layer stacking, blending, etc., were also highlighted. The COF membranes for water and wastewater treatment applications, gas separation, and fuel cells were also focused. The performance of literature reported COF membranes were discussed based on their water flux, rejection, gas permeance, selectivity factors, and proton conductivity. Modifying and designing membranes for various separation processes is done by altering the COF's structure and properties to utilize the membrane for practical applications. The fabrication challenges and utilization of COF membranes for different applications and the ways to move forward were addressed.