Bifunctional RhIII-Complex-Catalyzed CO2 Reduction and NADH Regeneration for Direct Bioelectrochemical Synthesis of C3 and C4

Abstract

Bioelectrochemical synthesis is emerging as an ecofriendly method for CO₂ fixation, typically using electrochemically regenerated NAD(P)H to supply reducing equivalents for formate dehydrogenase (FDH) to convert CO₂ to formate. However, the efficiency of this process is hindered by unfavorable thermodynamic conditions. In this study, we developed a one-pot bioelectrochemical system featuring a rhodium-based catalyst [Cp*Rh(bpy)Cl]²⁺ (Rh^III-complex or [Rh^III-H₂O]²⁺), which works cooperatively with enzymatic cascades involving acetyl-CoA synthase (ACS), acetaldehyde dehydrogenase (ACDH), alcohol dehydrogenase (ADH), formolase (FLS), and d-fructose-6-phosphate aldolase mutant FSA^A129S to convert CO₂ into various C₂₊ chemicals. The bifunctional Rh^III-complex concurrently catalyzes gas-phase reduction of CO₂ to formate at 15.8 mM/h and NADH regeneration at 0.24 mM/min. Formate production is 83.2 times faster than using one of the best aerobic FDHs from Clostridium ljungdahlii (ClFDH), resulting in a 3.6-fold increase in methanol production rate (0.43 mM/h) compared to the tandem enzymatic system (0.12 mM/h). Additionally, the bifunctional Rh^III-complex cooperates with enzymatic cascades to produce dihydroxyacetone (C₃) and L-erythrulose (C₄) with yields of 2.63 and 1.93 mM, respectively. This study highlights the synergy between electrochemical and enzymatic catalysis, providing an alternative for electroenzymatic CO₂ reduction to produce value-added C₂₊ compounds with enhanced productivity.