Conversion of Carbon Dioxide into Molecular-based Porous Frameworks

Abstract

The conversion of carbon dioxide (CO₂) to value-added functional materials is a major challenge in realizing a carbon-neutral society. Although CO₂ is an attractive renewable carbon resource with high natural abundance, its chemical inertness has made the conversion of CO₂ 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 CO₂ into MOF/COFs have been unexplored due to the lack of synthetic guidelines for converting CO₂ into molecular-based building units. In this Account, we describe state-of-the-art studies on the conversion of CO₂ into MOF/COFs. First, we outline the key design principles of CO₂-derived molecular building units for the construction of porous structures. The appropriate design of reactivity and the positioning of bridging sites in CO₂-derived molecular building units is essential for constructing CO₂-derived MOF/COFs with desired structures and properties. The synthesis of CO₂-derived MOF/COFs involves both the transformation of CO₂ 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 (Type-I); a one-pot synthesis without workup (Type-II); and a multistep synthesis which needs workup (Type-III). We demonstrate that borohydride can convert CO₂ 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 CO₂-derived MOFs. The electronegativity of coexisting metal ions determines the selective conversion of CO₂ into formate and formylhydroborate. Formylhydroborate-based MOFs exhibit flexible pore sizes controlled by the pressure of CO₂ during synthesis. In pursuit of highly porous structures, we present the Type-I synthesis of MOFs from CO₂ via the in situ transformation of CO₂ into carbamate linkers by amines. The direct conversion of diluted CO₂ (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 CO₂, which exhibit catalytic activity in CO₂ conversion. We also extended the synthesis of MOFs from CO₂ to COFs. The Type-III synthesis using a formamide monomer affords stable CO₂-derived COFs showing proton conduction properties. The precise design of CO₂-derived building units enables expansion of the structures and functionalities of CO₂-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 CO₂-derived MOF/COFs, such as carriers for CO₂ capture and precursors for CO₂ transformation. We anticipate that this Account will lay the foundation for exploring new chemistry of the conversion of CO₂ into porous materials.