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Molecules 2020, 25, 3653 12 of 23 However, this system complete the photocatalytic reaction and enzymatic reaction in the same environment, which will have a certain impact on the stability and activity of enzyme. For example, FDH has attracted much attention in recent years because it can directly reduce CO2 to formate without any other byproducts. FDH is divided into two types according to cofactor requirements: NADH-dependent FDH and metal-dependent FDH. Although metal-containing FDHs have a higher catalytic activity for CO2 reduction, these NADH-independent FDHs contain extremely unstable oxygen components, such as metal ions (tungsten or molybdenum), iron–sulfur clusters, and selenocysteine. The oxidation reaction of water may occur during photocatalysis to generate oxygen, which will affect the enzyme catalytic activity in the system, affect the stability of FDH, and then affect the final product of formic acid. The low compatibility of the photocatalysis and the biocatalysis in the system hindered its development. As is known, thylakoids in chloroplast were employed to couple the photoreaction and the biological reaction system by which the enzymatic reaction was separated from the water oxidation reaction to protect enzymes from inactivation. In order to achieve cooperation between photocatalysis and biocatalysis and improve compatibility, Zhan et al. [118] developed an artificial thylakoid by decorating the inner wall of protamine–titania (PTi) microcapsules with cadmium sulfide quantum dots (CdS QDs), and coupled with biocatalysis to form an artificial photosynthesis system. Cds QDs absorb visible light and generate electrons and holes. The electrons are transferred to the outer surface of the capsule through the heterostructure of Cds QDs and amorphous TiO2. Through the intermediate transfer of the rhodium complex, formate dehydrogenase converts CO2 to formic acid in the presence of NADH. The size-selective capsule wall separates photocatalytic oxidation and enzymatic reduction of CO2, thereby protecting the enzyme from inactivation that usually caused by photogenerated holes and active oxygen. Most enzymes are powdered reagents, which makes them difficult to separate from the substrate and cannot be recycled. It can effectively reduce the costs and simplify the product purification process through enzyme immobilization. Zeolite imidazolate framework (ZIF) is a type of MOF material that possesses well-defined pore structure, excellent chemical–thermal stability, extremely high surface area, and other excellent properties [119]. Moreover, ZIF is easy to prepare and has little effect on enzyme activity because the preparation is usually conducted at mild conditions. It has become one of the common methods for enzyme immobilization [120]. Zhou et al. [121] combined ZIF and TCPP to construct a photocatalytic multi-enzyme cascade biomimetic carbon sequestration system (TCPP&FF@ZIF-8 (FF = FateDH and FaldDH)). TCPP was used as the photocatalyst and ZIF-8 as the multi-enzyme immobilized carrier for FateDH and FaldDH. The catalytic system was then used to absorb CO2 and transform it to chemicals. Interestingly, the repeated stability investigation of the composite system showed that the residual activity of 3% TCPP and FF@ZIF-8 remained at 52.93 % after 10 batches of repeated use, suggesting that the system had excellent structural stability, light stability, and cycling stability. Using different photocatalysts, enzymes, and cofactors, various products and yields were obtained. Table 2 provides a simple comparison of the performance of different coupled photocatalytic/enzymatic CO2 reduction systems in recent years. Table 2. Performance comparison of different coupled photocatalytic/enzymatic CO2 reduction systems. Photocatalyst CCG-IP CNA H2TPPS C60 polymer film TiO2 C3N4(TPE-C3N4) CCGCMAQSP CdS QDs+PTi Enzyme FateDH, FaldDH, ADH FateDH, FaldDH, YADH FDH, AldDH, ADH FDH FDH MAF-7@FDH FateDH, FaldDH, ADH ClFDH Cofactors NADH + [Cp*Rh(bpy)H2O]2+Rh + TEOA NADH + [Cp*Rh(bpy)H2O]2+Rh + TEOA MV2+ NADH + TEOA NADH NADH + Rh + TEOA NADH + [Cp*Rh(bpy)H2O]2+ Rh + TEOA NADH + Rh Efficiency a CH3OH, 11.21 μM after 1 h CH3OH, 0.21 mM min−1 CH3OH, 6.8 μM after 100 min HCOOH, 239.46 μM after 2 h HCOOH, 1.634 mM after 4.5 h HCOOH, 16.75 mM after 9 h CH3OH, 110.55 μM after 2 h HCOOH, 1500 μM h−1 Ref. [122] [123] [124] [125] [126] [127] [116] [118] a The efficiency includes the products and rates.PDF Image | Research Progress in Conversion of CO2 to Valuable Fuels
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