Dual Microreactor Concept for Efficient Enzymatic Direct Air Capture and Formate Generation through CO2 Reduction Combining Golden Hydrogen’s Potential

Enzyme-mediated direct CO2 hydrogenation with upstream enzyme-mediated direct air capture (DAC) in interconnected multiphase fixed-bed microreactors (FBMRs) has been envisioned for the first time for the bioconversion of atmospheric CO2. Both processes are catalyzed by the thermostable hydrogen-dependent CO2 reductase (HDCR) enzyme from the thermophilic acetogenic bacterium Thermoanaerobacter kivui and the human carbonic anhydrase II (hCA II) enzyme immobilized on the surface of solid particles, respectively. The performance of the integrated enzymatic processes was evaluated using 3D models linking Euler–Euler equations of multiphase flow and mass transport equations in liquid and gas phases and diffusion/enzymatic reaction models within hCA II and HDCR enzyme layers. When coupled to an enzymatic DAC FBMR that extracts on-road CO2 under traffic congestion or CO2 from the atmosphere, the direct CO2 hydrogenation FBMR charged with the same enzyme loading and operating at 70 °C can reduce more CO2 than the DAC FBMR can remove due to the enhanced interphase mass transfer. The coupled multiphase FBMRs with immobilized hCA II/HDCR enzymes can also operate with higher CO2 concentrations (CO2 emissions from residential, commercial, and public services buildings), but the enzyme-mediated hydrogenation process (with large HDCR enzyme loadings) is controlled by mass transfer and reverse formate oxidation. FBMRs, which can potentially be installed on heavy-duty and marine vehicles, must be designed and operated under conditions that ensure that the benefits of high CO2 capture and reduction rates outweigh the cost of energy requirements. Formate must be removed from the reaction system to ensure liquid recirculation and no formate inhibition of the CO2 reduction.