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Using reduced nicotinamide adenine dinucleotide (NADH) as cofactor, CO 2 can be reduced into methanol catalyzed by formate dehydrogenase (FDH), formaldehyde dehydrogenase (F ald DH) 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 H 2 O 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. [Display omitted] • Three dehydrogenases were co-immobilized on hollow fiber membranes (HFMs). • CO 2 was converted to methanol by the photo-enzyme coupled system (PECS). • H 2 O was used as the electron donor that renders the process green and sustainable. • Methanol yield after 5 h reached 38.6% in PECS, 3.81 folds that of enzyme system. [ABSTRACT FROM AUTHOR]

The overexploitation of fossil fuels has led to a significant increase in atmospheric carbon dioxide (CO 2) concentrations, thereby causing problems, such as the greenhouse effect. Rapid global climate change has caused researchers to focus on utilizing CO 2 in a green and efficient manner. One of the ways to achieve this is by converting CO 2 into valuable chemicals via chemical, photochemical, electrochemical, or enzymatic methods. Among these, the enzymatic method is advantageous because of its high specificity and selectivity as well as the mild reaction conditions required. The reduction of CO 2 to formate, formaldehyde, and methanol using formate dehydrogenase (FDH), formaldehyde dehydrogenase (F ald DH), and alcohol dehydrogenase (ADH) are attractive routes, respectively. In this review, strategies for overcoming the common limitations of enzymatic CO 2 reduction are discussed. First, we present a brief background on the importance of minimizing of CO 2 emissions and introduce the three bottlenecks limiting enzymatic CO 2 reduction. Thereafter, we explore the different strategies for enzyme immobilization on various support materials. To solve the problem of cofactor consumption, different state-of-the-art cofactor regeneration strategies as well as research on the development of cofactor substitutes and cofactor-free systems are extensively discussed. Moreover, aiming at improving CO 2 solubility, biological, physical, and engineering measures are reviewed. Finally, conclusions and future perspectives are presented. • Common limitations of enzymatic CO 2 reduction and solutions are presented. • Enzyme immobilization techniques on various supports are elaborated and compared. • Cofactor regeneration pathways, substitutes and free systems are discussed. • Biological, physical, engineering measures for improving CO 2 solubility are reviewed. • Perspectives on enzymatic CO 2 reduction to formate/methanol are put forward. [ABSTRACT FROM AUTHOR]

[Display omitted] • Amino-functionalized polyethylene-silica (APS) composite membranes were fabricated. • Silica loading rate was tunable and reusability of APS after 7 washes was over 99%. • Formate dehydrogenase (FDH) immobilized on APS showed greatly enhanced activity. • Reusability was much improved compared to particles or membrane attached FDH. Enzymatic CO 2 conversion represents a highly efficient, energy-conserving, and environmentally sustainable approach to mitigating the increasing pressure caused by huge carbon emissions and generating valuable products. However, several challenges associate with the process including the instability and recovery difficulty of free enzyme, as well as low solubility of CO 2 in mild conditions. Immobilizing enzyme on the amino-functionalized supports presents a viable solution to these issues. Considering the wide application of polymer hollow fiber membranes in the field of CO 2 gas–liquid membrane contactor and the characteristics of hydrophilic, biocompatible, and high specific surface area of silica nanoparticles, the amino-functionalized polyethylene (PE)-silica (APS) composite membranes were fabricated via different routes. The morphologies, the loading rates of modified SiO 2 and reusability of APS membranes were characterized for a crude screening. Following that, seven kinds of APS membranes were used as the carriers of formate dehydrogenase (FDH) that could catalyze the reduction of CO 2 to formate, and their immobilization efficiency and catalytic performance were compared. The results show that 1# and 2# membranes based on carboxyl-modified PE with a loading rate of 5.5 %∼5.6 % and the reusability of above 99 % were more ideal carriers than others. The enzyme activity recovery rate of APS-immobilized FDH (FDH-APS) was separately 367 % and 256 %, proving the intensification action of carriers on the enzymatic reaction. After 10 cycles of use, the relative activity of two kinds of FDH-APS was around 70 % and 78 %, much higher than corresponding particles or membrane attached FDH (25.3 % and 20.4 %). After storage at 4 °C for 20 days, the relative activity of FDH-APS was 82.7 % and 81.0 %, while it was only 61.4 % for free enzyme. [ABSTRACT FROM AUTHOR]

Using reduced nicotinamide adenine dinucleotide (NADH) as cofactor, formate dehydrogenase (FDH) can reversely catalyze CO 2 reduction to formic acid. However, the industrialized application of FDH is fairly limited due to its high cost and stoichiometric cofactor consumption. Enzyme immobilization and cofactor regeneration are vital solutions to these problems. Ascribed to the benefits of utilizing a cheap, environmentally friendly, and renewable energy source, photocatalytic NADH regeneration is naturally sustainable and promising. Nonetheless, the rate-matching and compatibility between photocatalysis and enzyme catalysis have not been well revealed. In this study, polyethyleneimine (PEI) modified hollow fiber membrane (HFM) as the enzyme support was in-situ integrated with a visible-light-driven NADH regeneration system using thiophene-modified macroporous graphitic carbon nitride (ATCN-CN) as the catalyst. First, NADH regeneration conditions were regulated to accommodate enzyme catalysis. Then, the photo-enzyme coupled system (PECS) for the sustainable synthesis of formic acid with triethanolamine (TEOA) or H 2 O as electron donor was constructed, and the influencing factors were investigated. At last, the PECS was compared with other methods as well as a system that used FDH immobilized on silica nanoparticles. The results show that the reaction pH, electron donor, and catalyst concentration have a significant impact on NADH regeneration. For the PECS with TEOA as electron donor, the optimal pH value and photocatalyst concentration for formic acid synthesis are 7.0 and 1.2 mg mL−1, respectively. With the initial addition of 1 mmol L−1 NAD+, a formic acid yield of 35.3% was obtained after 5 h, standing out among the reported values. H 2 O as a green and low-cost electron donor meets the desire for low-cost photocatalysis. Although the production of formic acid after 5 h was lower in the PECS with H 2 O as electron donor than in the TEOA system, it was still 14.6 times that of the immobilized enzyme system without cofactor regeneration, and the yield of formic acid could reach 14.8% and 98.3% with 1 or 0.1 mmol L−1 NAD+, validating that current PECS is feasible to a sustainable and green synthesis of solar fuel from CO 2. [Display omitted] • CO 2 was converted to formic acid by the photo-enzyme coupled system (PECS). • The rate-matching and compatibility of photo and enzyme catalysis was studied. • Yield reached 35.3% after 5 h with 1 mmol L−1 NAD+ and triethanolamine (TEOA). • Yield reached 14.8% and 98.3% after 5 h with 1 or 0.1 mmol L−1 NAD+ without TEOA. [ABSTRACT FROM AUTHOR]

Enzymatic conversion of CO 2 into high-added chemicals is advantageous in mild reaction conditions and high specificity, for which enzyme immobilization and cofactor regeneration are often necessary. Although many efforts have been made, unfortunately, the turnover number (TN) of cofactor does not exceed 100 (~1 usually). Here, an integrated process of formate synthesis from CO 2 catalyzed by hollow fiber membrane immobilized enzyme succeeded with a simple UV/TiO 2 photocatalytic coenzyme regeneration was explored. By optimizing the operating conditions and NADH concentration, TN reached 125 after 4.5 h. This process can be extended to the production of methanol and other similar systems. Unlabelled Image • A high-efficiency and sustainable production of formate from CO 2 is achieved. • A HFM bioreactor is coupled with a photocatalytic cofactor regeneration unit. • Special ventilation and liquid circulation greatly accelerate the reaction. • TN and TOF are obviously higher than most of the reported values. • When NADH concentration is 0.01 mM, TN can reach 125. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 was converted into formaldehyde by the coupled photo-enzyme catalysis. • The enzyme reduction of CO 2 is predominant compared to the photo-reduction. • EDTA is more efficient than H 2 O while the decomposition cannot be excluded. • For the sequence enzymatic reaction, the second step is rate-limiting. • The yield of formaldehyde can reach 3.83% with the addition of 1 mM NAD+. Using the reduced nicotinamide adenine dinucleotide (NADH) as coenzyme, formate dehydrogenase (FDH) and formaldehyde dehydrogenase (FADH) can catalyze the reduction of CO 2 to formaldehyde. In this study, a hollow fiber membrane module was used as the enzyme-bearing reactor and gas distributor, and then coupled with a UV/TiO 2 photocatalytic NADH regeneration system. First, photocatalytic CO 2 reduction with varied pH value and electron donor was studied as a control. Then, CO 2 reduction catalyzed by the photo-enzyme coupled system was investigated and the limiting step of the reactions was analyzed. At last, the ratio of FDH and FADH, and cofactor concentration were optimized, and the operation stability was examined as well. The results show that the optimal pH value for the production of formaldehyde by photocatalysis and photo-enzyme coupling catalysis is respectively 6.5 and 7.0 with H 2 O as electron donor. At pH 7.0, the initial reaction rate is 0.035 and 0.104 mmol/(L·h), and the formaldehyde production after 5 h is 0.117 and 0.393 mmol/L, proving the enzymatic reduction of CO 2 is predominant in the coupled system. With EDTA as electron donor, the optimal pH value is 5.5 for the coupled system, and the formaldehyde production is much higher than that with H 2 O at all pH values, however, part of them might come from EDTA. For the sequence reactions, the second step is a rate-limiting step, and cofactor regeneration is vital for enzyme catalysis. The optimal loading ratio of FDH and FADH is 1:0.3, and with the addition of 1 mmol/L NAD+ or NADH, the yield of formaldehyde can reach 3.83% and 6.47% after 4 h. The photo-enzyme coupled system shows a good manipulation stability during 48 h. [ABSTRACT FROM AUTHOR]