Adsorption, separation, and catalytic properties of densified metal-organic frameworks

• Recent developments in the densification of MOFs for industrial applications.
• Densified MOFs exhibit improved chemical stability, and better mechanical strength.
• Summary of densified MOF-5 with enhanced volumetric hydrogen storage capacity.
• Improved liquid chromatography and acidic gas separation properties of densified MOFs.
• Progress and challenges in the densification of MOFs.

Metal-organic frameworks (MOFs) are one of the widely investigated materials of 21st century due to their unique properties such as structural tailorability, controlled porosity, and crystallinity. These exceptional properties make them promising candidates for various applications including gas adsorption and storage, separation, and catalysis. However, commercial applications of MOFs produced by conventional methods including solvothermal or hydrothermal synthesis are rather limited or restricted because they often produce fine powders. The use of MOF powders for industrial applications often results in pressure drop problems similar to the case with zeolites and limited robustness against water. To realize these materials for practical applications, densification of MOFs (by increasing pellet density) is routinely employed to form pellets, extrudates or beads to improve the overall density, volumetric adsorption, mechanical and thermal properties. However, the improvements come with some drawbacks such as reduction in overall porosity, surface area, and gravimetric adsorption capacity. Thus, optimizing the properties of densified MOF's by tuning the pellet density is very crucial for realizing these materials for industrial applications. Methods that increase the packing density in MOFs (for example by intentional interpenetration, etc.), which is different from pellet density, is not the scope of this review. In this review, the properties and applications of densified MOFs with different metal clusters and organic linkers are discussed.