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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]

[Display omitted] • Mesoporous silica (mSiO 2) with controllable structure were prepared and modified. • Structure and surface characters of mSiO 2 have great influence on its application properties. • PDA/PEI-mSiO 2 of 0.3 g enabled CO 2 reduction to be accelerated 77.4 times. • FDH and CA co-immobilized on PDA/PEI-mSiO 2 exhibit good reusability and stability. Enzymatic conversion of CO 2 to high-value chemicals is a significant route for the utilization of greenhouse gases in mild, high-selectivity and environment-friendly way, however, conversion efficiency is not yet satisfying. Here, mesoporous silica (mSiO 2) nanoparticles with controllable structure were prepared and modified by polydopamine (PDA) and polyethyleneimine (PEI), which were then used in an integrated process for CO 2 capture and conversion to formate for the first time. The effects of structure parameters of mSiO 2 and modification conditions on its application properties were investigated. The results show that in a range of 230 ∼ 500 nm, SiO 2 particles with smaller size exhibited better intensifying effect, while the optimal size was 410 nm for mSiO 2 with the same etching extent, attributed to a higher specific surface area. mSiO 2 itself was a robust CO 2 adsorbent and the addition of 0.01 g mSiO 2 (410) enabled CO 2 conversion to be accelerated 11.94 times compared to free enzyme. After modification, with 0.05 g PDA/PEI-mSiO 2(340) and PDA/PEI-mSiO 2(410) , the enzyme reaction was expedited up to 24 and 30.8 times of the system without particles due to an enhanced CO 2 uptake, as a collaborative result of mesoporous structure and amino-functionalization. Following that, PDA/PEI-mSiO 2(410) was used as the carrier for the co-immobilization of formate dehydrogenase and carbonic anhydrase, which could retain 86.7% activity after use for 10 times and 55.2% activity after 21 days at 4 °C, while 29.6% for free enzymes. [ABSTRACT FROM AUTHOR]

Polyethyleneimine (PEI) is a common modifier in the field of CO 2 adsorption but rarely used in the immobilization of enzyme catalyzing CO 2 conversion. Here, formate dehydrogenase (FDH) was immobilized on the surface of PEI modified polyethylene membrane or silica microspheres and then applied in the reaction of CO 2 to formate. After optimizing the immobilization conditions, the catalytic performance of immobilized FDH was respectively 4.3 and 7.4 times of free enzyme attributed to the promotion effect of the carrier. Polydopamine/PEI-SiO 2 attached FDH exhibited a higher activity and stability than biocatalytic membrane. Graphical abstract Unlabelled Image • FDH was immobilized on the surface of PEI modified carriers. • Enzyme activity increased instead of decreased for conventional carriers. • FDH-PDA/PEI-SiO 2 exhibited better catalytic properties than FDH-PEI-PAA-PE. • CO 2 conversion was accelerated up to 7.4 times after enzyme immobilization. [ABSTRACT FROM AUTHOR]