Direct-air capture conversion of CO2 in fixed-bed microreactors with immobilized formate dehydrogenase and glucose dehydrogenase: Concept feasibility

Conversion of CO2 from the atmosphere (via direct air capture) into formate via an enzyme-mediated CO2 reduction process, coupled with in situ regeneration of NADH, in fixed-bed microreactors (FBMRs) with co-immobilized oxygen-tolerant formate dehydrogenase (FDH) from Cupriavidus necator and glucose dehydrogenase (GDH) was proposed for the first time. Intensified CO2 conversion process was explored at different operating conditions, FDH and GDH loadings, NADH and glucose concentrations via a 3D model describing the two-phase flow, mass transport in two fluid phases, and diffusion-reactions in enzymatic layer. Enzymatic reduction of CO2 in FBMRs, with an appropriate FDH loading and an efficient glucose/GDH NADH regeneration system (with appropriate glucose and GDH loadings) that suppresses formate oxidation, allows higher CO2 conversions due to enhanced interphase mass transfer and gas-liquid interfacial area. FBMRs need to be designed and operated under appropriate conditions to ensure that the benefit generated by the high CO2 reduction rate outweighs the cost of pressure drop and the formate needs be removed from reaction system to maintain a concentration that ensures no formate inhibition for CO2 reduction reaction. FBMRs perform much better in summer, with lower pressure drop. Overall, FBMRs with immobilized FDH/GDH enzymes, integrated in modular units, are attractive for green CO2 conversion.