Bifunctional RhIII-complex catalyzed CO2 reduction and NADH regeneration for direct bioelectrochemical synthesis of C3 and C4

Abstract

Bioelectrochemical synthesis is emerging as an eco-friendly method for CO₂ fixation. These systems typically rely on electrochemically regenerated NAD(P)H to provide the necessary reducing equivalents for formate dehydrogenase (FDH) to convert CO₂ into formate. However, the efficiency of these systems is currently unsatisfactory due to the unfavorable dynamics of the CO₂-to-formate conversion by FDH. In this study, we developed a one-pot cooperative bioelectrochemical system featuring a rhodium-based catalyst [Cp*Rh(bpy)Cl]²⁺ (Rh^III-complex or [Rh^III-H₂O]²⁺) working cooperatively with enzymatic cascades of 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 several C₂₊ chemicals. The bifunctional Rh^III-complex concurrently catalyzes the reduction of CO₂ to formate at a rate of 15.8 mM/h and NADH regeneration at a rate of 0.24 mM/min. The formation of formate is 83.2 times faster than using one of the best aerobic FDH from Clostridium ljungdahlii (ClFDH), resulting in a 3.6 times enhanced methanol production rate of 0.43 mM/h in the bioelectroenzymatic system (Rh^III-complex-ACS-ACDH-ADH) compared to that of 0.12 mM/h in tandem enzymatic system (ClFDH-ACS-ACDH-ADH). Bifunctional Rh^III-complex also works cooperatively with tandem enzymatic cascades to produce dihydroxyacetone (C₃) and L-erythrulose (C₄) at the yield of 2.63 mM, and 1.93 mM, respectively. This study leveraged the synthetic capabilities of both electrochemical catalysis and enzymatic catalysis, offering an alternative for electroenzymatic CO₂ reduction to yield value-added compounds with enhanced productivity.