Carbazole-based porous organic polymers for carbon dioxide capture and catalytic conversion

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Carbon dioxide is a kind of notorious greenhouse gas, and its rapidly increased concentration has caused global ecological and environmental problems. The efficient capture and conversion of carbon dioxide not only possess significance for environment, but also contribute to the development of industry and economy. Porous organic polymers (POPs) are a new class of porous materials that possess high porosity, high thermal/chemical stability, low density, designable structures and compositions, and facile functionalization strategies. POPs have been widely researched and applied in gas storage/separation, biomedicine, contaminant treatment, energy, catalysis and other fields. Polycarbazoles and polymers containing carbazole units can be constructed through various reactions depending on the structure and property of monomers. The oxidative coupling reaction and other reactions, such as Sonogashira-Hagihara coupling reaction, Suzuki coupling reaction, and Yamamoto reaction, have been employed to the construction of conjugated porous polymers. Monomers with suitable functional groups are utilized to fabricate hypercrosslinked porous polycarbazoles. Triazine-based polymers can be prepared from nitriles by a cyclotrimerization reaction. Covalent organic frameworks are constructed through Schiff-base reaction between amine-modified monomers and aldehyde compounds. Moreover, electropolymerization is a facile method to deposit porous films with a high monomer density on the surface of electrodes. Carbazole-based porous organic polymers possess microporous structures and abundant nitrogen contents, which show high sorption capacity and selectivity for carbon dioxide. For example, CPOP-1 constructed from 1,3,5-tri(9-carbazolyl)-benzene possesses a high adsorbing capacity of 21.2wt% at 273 K and 1.0 bar. Furthermore, the introduction of functional moieties, such as pyridine and hydrazine, helps to improve the adsorption performance. The transformation of linear carbon dioxide requires a significant energy input because of its thermodynamic stability, the dissociation energy of C=O bond is ~750 kJ mol–1. The participation of catalysts is essential to promote the efficiency of conversion. Polycarbazoles with intrinsic catalytic activities can be facilely constructed through polymerizations of functionalized monomers and post-modification strategies. The combination of porosity, adsorption, and catalytic property endows the polymers with excellent performances in chemical conversions of carbon dioxide (such as formylation and cycloaddition reaction), electrocatalytic and photocatalytic reduction reactions. As compared with analogous molecular catalysts, the high porosity of polycarbazoles not only guarantees that the active sites are fully exposed to reactants, but also gathers the substrates and restricts the reaction region to achieve the enhanced efficiency and selectivity. As heterogeneous catalysts, it is facile to separate and recycle the polymers from the catalytic system, and the catalysts still maintain high catalytic performance in cycling experiments.