Three metal-organic framework isomers of different pore sizes for selective CO₂adsorption and isomerization studies

Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) with tunable pore sizes, shapes and functionalities have excellent prospects in many applications, such as carbon capture. Molecular sieving can usually enable very high CO₂adsorption selectivity but has rarely been achieved, because it is difficult to precisely control the pore size in the range of 3-4 Å. We report here three MOF isomers built from Cd^II, terephthalic acid and 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine with the same stoichiometric ratio, among which1and2are framework-catenation isomers and2and3are framework-topological isomers.1contains 2-fold interpenetrated networks (topology ofpcu) and 1D ultra-micropores and shows highly selective adsorption of CO₂over N₂and CH₄, which is mainly ascribed to the molecular sieving effect of the framework.2contains apcunetwork with 3D interconnected micropores, and3contains akagnetwork with much larger pores of 15 Å. Framework isomerization, in this case, was shown to be a feasible way of tuning the pore size of a MOF for selective CO₂adsorption. The effects of hydrothermal reaction conditions and additives on the structures and the formation of the MOF isomers were also studied.