Metalated carbon nitride with facilitated electron transfer pathway for selective NADH regeneration and photoenzyme-coupled CO2 reduction

Photoenzyme-coupled catalysis, featuring the integration of photocatalysis with enzymes, is very promising for next-generation green biomanufacturing. The presence of an Rh complex is a prerequisite for the efficient photocatalytic regeneration of the reduced form of nicotinamide adenine dinucleotide (NADH), which poses the issue of immobilizing homogeneous complexes. In this study, a novel immobilization method based on the thermal polymerization of 2,2'-bipyridine-5,5'-diamine (DABP) onto a polymeric carbon nitride (PCN) framework is proposed. PCNbpy4 is metalated by immobilizing Rh on the terminal bipyridine structure. Notably, partial DABP has the ability to undergo high-temperature thermal polymerization, resulting in the formation of N-doped graphene. This N-doped graphene can be grafted onto the terminal amino group, forming a potential electron transfer pathway. Additionally, N-doped graphene, because of its good electrical conductivity, guides the photogenerated electrons toward the anchored Rh sites. The catalyst achieves exclusive regeneration of 1,4-NADH with only a 0.12% Rh atomic ratio and realizes 80% NADH regeneration in 20 min. The competitive relationship between hydrogen production and NADH regeneration is also elucidated. Combined with formate dehydrogenase immobilized on a hydrophobic membrane, CO2 reduction to formate is accomplished efficiently, and the formate concentration can accumulate to 7 mmol L−1 within 48 h.