Highly Selective CO2‐Capturing Polymeric Organic Network Structures

The combustion of hydrocarbon fuels to satisfy the current energy requirements increases the emission of gases that contribute to global-warming[1] and thus to climate change, such as carbon dioxide (CO2). Porous structures including COFs (covalent organic frameworks),[2] PIMs (polymers of intrinsic microporosities),[3] HCPs (hyper-cross-linked polymers),[4] CMPs (conjugated microporous polymers),[1c,5] and CTFs (covalent triazine-based frameworks)[6] have been considered to be very successful in capturing CO2 with high capacities. Meanwhile, there is still the great challenge of realizing highly selective CO2-capturing materials for practical use in ambient conditions, which could allow many practical applications and help to significantly slow the greenhouse effect. Herein, we report new highly selective CO2-capturing polymeric organic networks (PONs) synthesized using (methanetetrayltetra-4,1-phenylene)tetrakisboronic acid and three kinds of halogen-substituted struts that include heterocyclic groups. The structures and morphology of the networks are studied by solid-state NMR, Fourier-transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM). PON-1 has only phenyl group, while PON-2 and -3 contain pyridyl and thiophenyl groups, respectively. While all the PONs have moderate surface areas, they are very attractive for use in capturing a large amount of CO2 under ambient conditions. Moreover, it was found that PONs offer high selective adsorption for CO2 over CH4 and H2 under ambient conditions. (Methanetetrayltetra-4,1-phenylene)tetrakisboronic acid[8] has been used as a tetrahedral building unit to assemble into highly porous three-dimensional network structures. It also has the potential to react with various dihalogenated struts, and there are many useful coupling reactions between boronic acids and halogen atoms on diverse aromatic and heterocyclic compounds.[7] For the efficient synthesis of porous structures, we selected representative conditions of the coupling reaction using (methanetetrayltetra-4,1-phenylene)tetrakisboronic acid and 1,4-dibromobenzene to give a tetrahedral structure (Supporting Information, Table S1). The reactions were carried out at 90 °C in the presence of water for 5 days to give sufficient