Construction and optimization of a photo−enzyme coupled system for sustainable CO₂ conversion to methanol

Using reduced nicotinamide adenine dinucleotide (NADH) as cofactor, CO₂ can be reduced into methanol catalyzed by formate dehydrogenase (FDH), formaldehyde dehydrogenase (F_aldDH) and alcohol dehydrogenase (ADH). However, poor stability of soluble enzymes and the stoichiometric consumption of NADH are major restrictions. Herein, the three enzymes were co-immobilized on a hollow fiber membrane (HFM) module, which was then integrated with a photocatalytic NADH regeneration system to constitute a photo−enzyme coupled system (PECS) for the synthesis of methanol. First, the multi-enzyme immobilization process was optimized and the enzyme-bearing membrane was characterized. Then, the influencing factors of PECS were investigated. The results show that using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as the activators, a total immobilization efficiency of 64.5% could be obtained, which was superior to sequential immobilization. Under the optimum immobilization conditions, the specific activity reached 0.397 mmol g⁻¹ h⁻¹. For the PECS, NADH concentration, pH value and manipulation parameters had great impacts on the synthesis of methanol. With 10 mmol L⁻¹ NAD⁺ and H₂O as electron donor, the methanol yield after 5 h could reach 38.6%, 3.81 times that of enzyme-catalyzed system, proving the PECS was feasible for a continuous synthesis of methanol.