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[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]

Enzymatic conversion of CO 2 to formate is a highly significant pathway for CO 2 utilization, but current level of conversion efficiency remains unsatisfactory due to low solubility of CO 2 under mild conditions. CO 2 -philic materials can significantly augment the reaction by raising local concentration of substrate. Herein, the intercalated Ti 3 C 2 T x , nano-silica (SiO 2) and carbon nitride (CN) with abundant CO 2 adsorption sites were introduced to fabricate Ti 3 C 2 T x based nanocomposites, Ti 3 C 2 T x /SiO 2 /CN (TSCN), for the first time. As the controls, binary composites including Ti 3 C 2 T x /SiO 2 (TS), Ti 3 C 2 T x /CN (TCN) and SiO 2 /CN (SCN) were also studied. For each kind of material, the composition, structure, morphology, and application performance in the enzymatic CO 2 conversion were characterized and influencing factors were investigated. Among these composites, TSCN exhibited the best application performance and the enzyme reaction was accelerated up to 23.9 times that of the blank system. The value could reach as high as 35.3 after polyethyleneimine (PEI) modification. Following that, PEI modified composites were used as carrier of formate dehydrogenase (FDH) and apparent specific activity was up to 2.8 times that of free enzyme, which could retain 88.2% of the initial value after 10 cycles associated with the structural integrity. This work provides new impetus to CO 2 capture and conversion in a clean and mild way that shows bright application prospects. [ABSTRACT FROM AUTHOR]

[Display omitted] • Intercalation and modification of Ti 3 C 2 T x were studied. • The relation between the structure and the application performance of Ti 3 C 2 T x was illustrated. • NH 4 OH/KCl intercalation brought a higher specific surface area and better intensifying effect. • With the addition of 20 mg PEI-Ti 3 C 2 T x , the enzymatic CO 2 conversion was expedited up to 9.2 times. • PEI-Ti 3 C 2 T x exhibited a good reusability. Ti 3 C 2 T x as a typical MXene material has a multilayer structure similar to accordion, whose interlayer spacing and specific surface area can be regulated by intercalation and modification. In this work, the effects of several kinds of cation-containing intercalators including NH 4 OH, KOH and NH 4 OH/KCl on the structure of Ti 3 C 2 T x and its application performance in enzymatic CO 2 conversion were investigated for the first time. The results show that Ti 3 C 2 T x intercalated with NH 4 OH/KCl had larger layer spacing (25.25 Å) and specific surface area (20.85 m2/g), and better enhancing effect. The initial reaction rate was 0.379 mM/h, which was separately 1.04, 1.42, 1.76 and 5.29 times that of Ti 3 C 2 T x /KOH, Ti 3 C 2 T x /NH 4 OH, Ti 3 C 2 T x and the blank system. Following that, Ti 3 C 2 T x /NH 4 OH/KCl was modified by polyethyleneimine (PEI) and dopamine (DA) and the concentration of modifier was optimized. PEI modified Ti 3 C 2 T x /NH 4 OH/KCl (PEI-Ti 3 C 2 T x) was superior to PEI/DA modified that in the N content, CO 2 adsorption capacity and application performance. With the addition of 20 mg PEI-Ti 3 C 2 T x , the initial reaction rate was 0.658 mM/h, which was 1.74 and 9.2 times that of before modification and the blank system, respectively. PEI-Ti 3 C 2 T x also exhibited a good reusability and the initial reaction rate was 89.4% of the initial value after 10 cycles. [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]