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Efficient regeneration of nicotinamide adenine dinucleotide (NADH) is of great significance for the application of oxidoreductase-catalyzed reactions. Photocatalytic regeneration is advantageous for the utilization of cheap, abundant, and clean solar energy. Graphitic carbon nitride (g-C 3 N 4) has attracted wide attention due to its non-toxicity, wide range of precursors, suitable band gap, and superior chemical and thermal stability. In this work, mesoporous g-C 3 N 4 (MPCN) was prepared using silica with different size as the template. The preparation process of MPCN was optimized and the effect of the template size on the structure and properties of MPCN were investigated. The results demonstrate that the smaller the template size, the larger the specific surface area of MPCN, and thus the better the photocatalytic performance. After optimization, the catalytic activity of MPCN with 7 nm silica as the template (MPCN-7) was 17.4 mM/(g min) and the NADH yield after 40 min was 70.6 %. MPCN-7 was also modified by 3-aminothiophene-2-carbonitrile (ATCN) and polyvinylpyrrolidone (PVP). Under same conditions, the catalytic activity of ATCN-MPCN, PVP-MPCN, and ATCN/PVP-MPCN was upgraded from 10.8 to 49.5, 16.4, 29.2 mM/(g min), respectively. The final yield of NADH rose from 66.9 % (40 min) to 76.3 % (12 min), 72.8 % (25 min) and 82.0 % (15 min). [Display omitted] • Mesoporous carbon nitride with a series of silica as the template was prepared. • The preparation process including mixing method and mass ratio was optimized. • The template size had great impact on structure and catalytic property of catalyst. • Thiophene and polyvinylpyrrolidone modification was studied. • A high coenzyme regeneration yield of 82 % was achieved after 15 min. [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]