Thermo-, Electro-, and Photocatalytic CO₂ Conversion to Value-Added Products over Porous Metal/Covalent Organic Frameworks

ConspectusThe continuing increase of the concentration of atmospheric CO₂ has caused many environmental issues including climate change. Catalytic conversion of CO₂ using thermochemical, electrochemical, and photochemical methods is a potential technique to decrease the CO₂ concentration and simultaneously obtain value-added chemicals. Due to the high energy barrier of CO₂ however, this method is still far from large-scale applications which requires high activity, selectivity, and stability. Therefore, development of efficient catalysts to convert CO₂ to different products is urgent. With their well-engineered pores and chemical compositions, high surface area, elevated CO₂ adsorption capability, and adjustable active sites, porous crystalline frameworks including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are potential materials for catalytic CO₂ conversion. Here, we summarize our recent work on MOFs and COFs for thermocatalytic, electrocatalytic, and photocatalytic CO₂ conversion and describe the structure-activity relationships that could guide the design of effective catalysts.The first section of this paper describes imidazolium-functionalized porous MOFs, including porous liquid and cationic MOFs with nucleophilic halogen ions, which can promote thermocatalytically CO₂ cycloaddition reaction with epoxides toward cyclic carbonates at one bar pressure. A porous liquid MOF takes on the role of a CO₂ reservoir to tackle the low local CO₂ concentrations in gas-liquid-solid heterogeneous reactions. Imidazolium-functionalized MOFs with halogen ions for CO₂ cycloaddition could avoid the use of cocatalysts, and this leads to milder and more facile experimental conditions and separation processes.In a section dealing with the electrocatalytic CO₂ reduction reaction (CO₂RR), we developed a series of conductive porous framework materials with fast electron transmission capabilities, which afford high current densities and outperform the traditional MOF and COF catalysts that have been reported. The intrinsically conductive two-dimensional 2D MOFs and COFs nanosheets based on the fully π-conjugated phthalocyanine motif with excellent electron transport capability were prepared, and strong electron transporters were also integrated into metalloporphyrin-based COFs for CO₂RR. Cu₂O quantum dots and Cu nanoparticles (NPs) can be uniformly dispersed on porous conductive MOFs/COFs to afford synergistic and/or tandem electrocatalysts, which can achieve highly selective production of CH₄ or C₂H₄ in CO₂RR.A third section describes our efforts to facilitate electron-hole separation in CO₂ photocatalysis. Our focus is on regulation of coordination spheres in MOFs, fabrication of the architecture of MOF heterojunctions, and engineering MOF films to facilitate photocatalytic CO₂ reduction.Finally, we discuss several problems associated with the studies of MOFs and COFs for CO₂ conversion and consider some prospects of the fabrication of effective porous frameworks for CO₂ adsorption and conversion.