Metal-Organic Frameworks and Porous Coordination Polymers: Properties and Applications

Metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) generally have defined structures, permanent porosity, and high specific surface areas. They are synthesized from metal and organic building blocks by solvothermal reactions or self-assembly. MOFs and PCPs are considered to be a class of the most promising materials for hydrogen storage and for gas separation such as carbon dioxide capture from the flue gas or natural gas. However, gas adsorption on the pore surface of MOFs and PCPs is physisorption, and the interaction energy between the adsorbents and gas is too weak. Therefore, even though large amount of gases can be stored in the materials at a low temperature, the storage capacity falls down to very low values at ambient temperature. To enhance hydrogen gas storage in MOFs and PCPs, we have modified their pore spaces by various methods such as generating accessible metal sites, fabricating metal nanoparticles (NPs), including proper organic guests, and incorporating specific metal ion binding sites in the ligand. The generation of accessible metal sites is based on “Kubas” interactions of hydrogen molecules with the metal ions. The production of metal nanoparticles in PCPs, without using extra reducing agent and NP-stabilizing agents just at room temperature, is based on the redox chemistry between the redox-active components of the MOFs or PCPs and the metal ions. To capture CO2 selectively from the industry flue gas that contains not only CO2 but also other gases, we have developed smart 3-dimensional (3D) PCPs with very small pores, which are highly flexible. Since CO2 molecule has much higher polarizability and quadrupole moment than other gases, it would interact much more strongly with the flexible PCPs with very small pores and open up the windows while other gases cannot. We have also created various strategies such as post-synthetic modification of pore space with highly flexible carboxyl pendants, impregnation of metal ions in the pores of a MOF, and inclusion of branched polyethylenimine units in the pores of porous organic polymer. These induce stronger interactions with the CO2 molecules, and enhance the gas uptake capacities and the selectivity of CO2 adsorption In this review, properties and applications of metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) are described. Many MOFs and PCPs are highly flexible and responsive to external stimuli. Sometimes they transform their structures to others by maintaining the single crystallinity. For decades, MOFs and PCPs have been regarded as a class of the promising materials for hydrogen storage and carbon dioxide capture applications since they adsorb large amounts of gases at low temperatures. However, their gas uptake capacities decrease dramatically at ambient temperature compared to those at low temperatures because they physic-sorb gases by weak interaction energies. Therefore, to enhance gas storage and separation abilities of MOFs and PCPs at ambient temperature, we have modified their pore spaces. In this review, some characteristic properties of MOFs and PCPs will be introduced, and various strategies for modifying the pore spaces of PCPs and MOFs for hydrogen storage and carbon dioxide capture will be presented.