N-heterocyclic carbene-functionalized metal–organic frameworks for the chemical fixation of CO2

Abstract

N-heterocyclic carbenes (NHCs) are a class of molecules with a lone pair of carbene electrons and thus, they have the ability to activate CO₂ to form imidazolium carboxylates. The incorporation of activated, metal-free NHC moieties into metal–organic frameworks (MOFs) without the decomposition of metal–carboxylate coordination motifs is highly desired owing to the high CO₂ affinity and versatile chemical functionalities in MOFs. Herein, we have summarized the recent in situ generation approaches to form metal-free NHC-functionalized MOFs, which are a unique class of CO₂-conversion catalysts with high catalytic activity, selectivity and stability, superior to those of homogenous and other heterogeneous NHC analogues. The NHC-functionalized MOFs for catalytic CO₂ reduction include reactions such as the hydroboration of CO₂, hydrosilylation of CO₂, N-methylation using CO₂ and hydrogenation of CO₂ to formic acid. Overall, the synthetic strategy of metal-free NHC-functionalized MOFs, the unique catalytic pathways of NHC-functionalized MOFs, and potentially new research directions of NHC-functionalized MOFs are discussed, which will guide researchers to attempt to design new NHC-MOFs and extend their catalytic applications in the chemical fixation of CO₂.