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 (FaldDH) 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.