Overview of Porous Materials: Activated Carbon, Zeolites, MOFs and COFs

Porous materials are materials that have small pores or cavities throughout their structure, allowing for the passage of fluids or gases. These materials, with their myriad of microscopic voids, have gained substantial attention in various fields such as catalysis, adsorption, and separation technologies due to their remarkable properties including large surface area, tuneable pore size, and distinctive porosity. In addition, they facilitate applications starting from energy storage to drug delivery systems. Porous materials could be organic or inorganic, with structures ranging from amorphous to crystalline, and sizes from nanometers to millimeters. The blog gives an overview of four primary types of porous materials: activated carbon, zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs).

Activated Carbon

Activated carbon, a highly porous form of carbon, is renowned for its exceptional adsorption capacity. It is manufactured from carbon-rich materials such as wood, coal, and coconut shell through a series of processes: carbonization, activation, and washing. The activation process, by steam or chemical, aids in developing a complex internal pore structure which enhances the surface area and adsorption capacity. These characteristics make it ideal for applications such as water purification, gas purification, and metal extraction.

Zeolites

Zeolites, another class of porous materials, are crystalline aluminosilicates with a distinctive feature of regular and tunable pore structure. The highly regular structure is made up of tetrahedra that is connected to form a three-dimensional network of pores and channels. Zeolites have excellent ion-exchange, selective adsorption, and catalytic properties due to their internal framework made of TO4 tetrahedra (T=Si, Al), and find extensive applications in oil refining, petrochemical industries, and environmental remediation.

Metal-Organic Frameworks (MOFs)

MOFs, relatively new additions to the domain of porous materials, are made of metal ions or clusters coordinated to organic ligands. They are synonymous with high porosity, large surface area and tunable pore size. They offer remarkable flexibility in structural design, allowing for precise tuning of their functional properties. MOFs have potential applications in gas storage, catalysis, drug delivery and sensing applications.

Covalent Organic Frameworks (COFs)

Like MOFs, COFs are also a new class of porous materials, albeit based on the covalent bonding of light atomic components. These materials are remarkably resistant to solvents and have high thermal stability. Notably, they are characterized by the capability of harmonizing their structure with specific applications due to the flexibility and designability of the covalent bond. COFs are extensively explored for energy storage, catalysis, adsorption, and sensing applications.

Advantages of MOF and COFs

While activated carbon and zeolites mentioned have various advantages, the drawbacks of activated carbon, such as limited control over the pore sizes and shapes, and high production energy costs, limit their applications; zeolites' high thermal instability and difficulty in structural modification also pose significant challenges. Oppositely, MOFs and COFs have unique advantages that are superior to activated carbon and zeolites. The tailorability of their pore size, shape, and functionality indicates their potential in selective sensing and separation tasks. Their structured design allows for the precise incorporation of active sites, indicating their efficacy in catalysis. Moreover, their exceptional porosity and high surface area enable them to store copious quantities of gases, making them ideal for energy storage applications. Despite being relatively new, MOFs and COFs are populated for their flexibility, tunability, and multifunctionality, making them a promising class of porous materials with a plethora of applications.