Conversion of Carbon Dioxide into Molecular-based Porous Frameworks.

ConspectusThe conversion of carbon dioxide (CO) to value-added functional materials is a major challenge in realizing a carbon-neutral society. Although CO is an attractive renewable carbon resource with high natural abundance, its chemical inertness has made the conversion of CO into materials with the desired structures and functionality difficult. Molecular-based porous materials, such as metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs), are designable porous solids constructed from molecular-based building units. While MOF/COFs attract wide attention as functional porous materials, the synthetic methods to convert CO into MOF/COFs have been unexplored due to the lack of synthetic guidelines for converting CO into molecular-based building units.In this Account, we describe state-of-the-art studies on the conversion of CO into MOF/COFs. First, we outline the key design principles of CO-derived molecular building units for the construction of porous structures. The appropriate design of reactivity and the positioning of bridging sites in CO-derived molecular building units is essential for constructing CO-derived MOF/COFs with desired structures and properties. The synthesis of CO-derived MOF/COFs involves both the transformation of CO into building units and the formation of extended structures of the MOF/COFs. We categorized the synthetic methods into three types as follows: a one-step synthesis (); a one-pot synthesis without workup (); and a multistep synthesis which needs workup ().We demonstrate that borohydride can convert CO into formate and formylhydroborate that serve as a bridging linker for MOFs in the Type-I and Type-II synthesis, representing the first examples of CO-derived MOFs. The electronegativity of coexisting metal ions determines the selective conversion of CO into formate and formylhydroborate. Formylhydroborate-based MOFs exhibit flexible pore sizes controlled by the pressure of CO during synthesis. In pursuit of highly porous structures, we present the Type-I synthesis of MOFs from CO via the in situ transformation of CO into carbamate linkers by amines. The direct conversion of diluted CO (400 ppm) in air into carbamate-based MOFs is also feasible. Coordination interactions stabilize the intrinsically labile carbamate in the MOF lattice. A recent study demonstrates that the Type-III synthesis using alkynylsilane precursors enables the synthesis of highly porous and stable carboxylate-based MOFs from CO, which exhibit catalytic activity in CO conversion. We also extended the synthesis of MOFs from CO to COFs. The Type-III synthesis using a formamide monomer affords stable CO-derived COFs showing proton conduction properties. The precise design of CO-derived building units enables expansion of the structures and functionalities of CO-derived MOF/COFs. Finally, we propose future challenges in this field: (i) expanding structural diversity through synthesis using external fields and (ii) exploring unique functionalities of CO-derived MOF/COFs, such as carriers for CO capture and precursors for CO transformation. We anticipate that this Account will lay the foundation for exploring new chemistry of the conversion of CO into porous materials.