Photocatalysis of Covalent Organic Frameworks

*Hui Liu and Yingjie Zhao*

## **Abstract**

The development of clean and sustainable energy is gaining attention in light of the current energy crisis and global warming. An ideal way to utilize renewable solar energy is to convert clean energy through photocatalysis. This includes splitting water, reducing CO2, regenerating coenzymes, etc. Photocatalysis relies heavily on photocatalysts. It has recently become popular to use organic porous polymers in this process. Covalent organic frameworks (COFs), as one of the organic porous polymers, have the characteristics of high crystallinity, porosity, and structural designability that make them perfect platforms for photocatalysis. An overview of recent advances in COF photocatalysts is presented in this chapter. The photocatalytic applications of COFs with different ligation and different structures were first discussed, including photocatalytic hydrogen evolution, CO2 conversion, coenzyme regeneration, and conventional organic reactions. Finally, conclusions and prospects were provided in the last section.

**Keywords:** covalent organic frameworks, photocatalysis, photocatalytic hydrogen evolution, photocatalytic CO2 reduction

## **1. Introduction**

As fossil fuels (e.g. oil, coal, and natural gas) were excessively utilized, energy crisis and climate issues gradually became global concerns [1–3]. Developing "green energy" as a replacement for traditional fossil fuels will be the most efficient solution to these problems. Photocatalysis is a green technology with important application prospects in energy and environment. Photocatalytic reaction refers to the process of semiconductor photocatalysts promoting the conversion of compounds under light conditions, which can effectively convert light energy into chemical energy. On the one hand, photocatalytic degradation of organic pollutants and photocatalytic reduction of CO2 is helpful to solve environmental problems. On the other hand, photolysis of hydrogen and oxygen in aquatic products can develop new energy for human beings. It should be noted that photocatalyst materials play an important role in the whole photocatalytic process. During the photocatalytic process, electron–hole pairs in the photocatalysts are generated by absorption of photons with higher energy than the band gap. In redox reactions, electrons and holes migrate to the surface, catalyzing the reaction. The first photoelectrochemical water splitting on TiO2 was reported by Fujishima and Honda in 1972 [4]. Since then, a series of inorganic photocatalysts including TiO2 [5, 6], cadmium sulphide (CdS) [7, 8], zinc oxide (ZnO) [9] and silver

phosphate (Ag3PO4) [10] emerged and lead the research over the past several decades. Its practical application was limited, however, by the requirements for UV light, heavy metal toxicity, and photo corrosion. In recent years, using organic polymers like linear conjugated polymers (CPs) [11–14], graphitic carbon nitride (g-C3N4) [15–17], conjugated microporous polymers (CMPs) [18–23], covalent organic frameworks (COFs) [24–27] and covalent triazine-based frameworks (CTFs) [28–31] as photocatalysts became hot topics. Among them, COFs have drawn wide attention due to advantages of easy structure design, large surface area, tunable electronic properties and band gaps, and diverse synthesis methods.

COFs are a new type of crystalline organic porous polymer based on covalent bond connection [32]. COFs with two-dimensional (2D) or three-dimensional (3D) networks could be prepared according to the construction units of different building blocks. The unique structures of COFs also bring a couple of important advantages: (1) the structural designability endows them with enhanced visible-light absorption and tunable band structure; (2) the porous structure is conducive to the adsorption of substrates and the transport of products; (3) the strong covalent bonds endow COFs with good stability, which thereby prolongs the lifetime of the photoactive structure; (4) the ordered conjugated structure is beneficial not only to the absorption of light energy but also to the transport of excited electrons. As a result of these potential advantages, COFs have been extensively used in gas storage and separation [33–36], catalysis [37–39], optoelectronics [40], sensing [41–43], and energy storage [44, 45]. There have been many excellent reviews about the synthesis, structures, and applications of COFs materials [46–55]. It can be said that COFs material has many advantages as a heterogeneous photocatalyst.

In this chapter, we will focus on the photocatalytic applications of COFs including water splitting, CO2 reduction, coenzyme regeneration, and photocatalytic organic reactions. Finally, we will discuss the challenges and opportunities of COFs as photocatalysts.

## **2. Photocatalytic applications of COFs**

#### **2.1 Photocatalytic hydrogen evolution**

Hydrogen energy is regarded as the most promising clean energy in the 21st century because the only product of hydrogen combustion is water [56]. Hydrogen production from powder photocatalyst is expected to break the cost barrier and become the cheapest technology to decompose water to produce hydrogen, which is expected to surpass fossil fuels. In recent years, organic semiconductor materials have been extensively explored in photocatalytic hydrogen evolution. Among them, COFs with well-ordered conjugate structures showed great advantages not only in photocatalytic performance but also in the deep understanding of the structure–activity relationship [55–58]. A great deal of research work has focused on improving the performance of the photocatalyst by adjusting the building units and linkages of the COFs. In this section, representative building blocks and linkages of COFs for photocatalytic hydrogen evolution were summarized and discussed.

### *2.1.1 Hydrazone-linked COFs*

Hydrazone-linked COFs with hydrolytic and oxidative stability were prepared by condensation between hydrazide and aldehyde derivatives. Therefore,

### *Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

hydrazone-linked COFs provide a valuable design platform for water-splitting photocatalysts. In 2014, Lotsch group constructed a 2D COF (TFPT-COF) with hydrazone-linked through the condensation reaction of 2,5-diethoxyterephthalohydrazide and triazine-based aldehyde (**Figure 1**) [25]. This is the first report of COFs materials applied to photocatalytic hydrogen production. TFPT-COF is a mesoporous material with a pore size of 3.8 nm, a specific surface area of 1603 m<sup>2</sup> /g, and a pore volume of 1.03 cm3 /g. Compared with monomer, TFPT-COF has better visible light absorption performance, and the absorption boundary can reach more than 600 nm. TFPT-COF was selected as a photosensitizer, while Pt was used as the proton reduction catalyst. When illuminated with visible light, the hydrogen evolution rate in the first five hours reached 1970 μmol h−1 g−1 using 10 vol% aqueous triethanolamine (TEOA) solution as the sacrificial electron donor. The quantum efficiency was determined to be 2.2%. Under the same conditions, the hydrogen production efficiency of TFPT-COF was much superior to Pt-modified amorphous melon, g-C3N4, and crystalline poly (triazine imide) [59].

As a type of conjugated macrocycle, porphyrin displays unique photophysical and redox characteristics [60]. Wang and co-workers designed and synthesized

#### **Figure 1.**

*(a) Synthetic route and top view of the TFPT-COF. (b) Photocatalytic performance of Pt-modified TFPT-COF with different SED (black: sodium ascorbate solution; red: triethanolamine solution).*

**Figure 2.** *Schematic representation of the synthesis of MPor-DETH-COFs.*

four isostructural porphyrin-based 2D COFs (**Figure 2**) [61]. By incorporating different transition metals into the porphyrin rings, the physical and electronic properties of COFs were rationally tuned. There was a high level of crystallinity and surface area in all of the COFs. When illuminated with visible light while containing Pt as a co-catalyst and TEOA as a sacrificial electron donor, these four COFs exhibited tunable hydrogen evolution efficiency with the order of CoPor-DETH-COF (25 μmol g−1 h−1) < H2Por-DETH-COF (80 μmol g−1 h−1) < NiPor-DETH-COF (211 μmol g−1 h−1) < ZnPor-DETH-COF (413 μmol g−1 h−1). A molecular engineering approach was mainly responsible for the tunable photocatalytic performance. Moreover, hydrazone-linked COFs were also can be converted into oxadiazole-linked COFs via oxidization reaction, which endow COFs with narrow bandgaps and continuous π-electron delocalization for high photocatalytic activity (2615 μmol h−1 g−1) [62].

### *2.1.2 Azine-linked COFs*

The azine-linked COFs are synthesized by the condensation of aldehyde derivatives with hydrazine. The electronic band structure and steric hindrance can be regulated by introducing different heteroatoms into COFs. Lostch and co-workers synthesized a series of azine-linked COFs and investigated the connection between the number of nitrogen atoms in their structures and the photocatalytic hydrogen production efficiency (**Figure 3**) [63]. Pt and 10 vol% TEOA were selected as the cocatalyst and sacrificial electron donor, respectively. The photocatalytic performance was gradually enhanced with the increase of nitrogen content in the central benzene ring. The hydrogen production efficiencies of N0-COF, N1-COF, N2-COF, and N3-COF were 23, 90, 438, and 1703 μmol h−1 g−1, respectively. Through theoretical calculation, it is found that the planarity of COFs increased with the increase of nitrogen element content. Therefore, N3-COF has the best planarity, which was conducive to the photogenerated electron migration and thus improves the corresponding photocatalytic activity. In addition to the above work, the authors also reported a series of planar pyrene-azine-COFs (A-TEXPY-COFs) with varied nitrogen atoms in the peripheral aromatic units [64]. The photocatalytic efficiency of A-TEXPY-COFs was regulated by nitrogen element content in the peripheral aromatic units. The

#### **Figure 3.**

*(a) The formation of Nx-COF. (b) Photocatalytic H2 evolution of Nx-COFs as photocatalyst over 8 h under visible light irradiation.*

*Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

photocatalytic efficiency exhibited a decreasing tendency with an increasing nitrogen element content. Quantum-chemical calculations suggested that the thermodynamic driving force increased with the decrease of nitrogen content. In addition, Lostch and co-workers also developed cobaloxime as a co-catalyst instead of platinum, a precious metal [65]. The HER rate of N2-COF could reach 782 μmol h−1 g−1 with TEOA as SED.

#### *2.1.3 β-ketoenamine-linked COFs*

The *β*-ketoenamine-linked COFs could be synthesized through Schiff base reaction and this kind of COFs shows great potential in photocatalytic hydrogen production due to its excellent solvent stabilities of keto−enamine. In 2018, Cooper et al. synthesized two COFs (S-COF and FS-COF) with ultra-high photocatalytic hydrogen evolution activity using benzo-bis(benzothiophene sulfone) moieties (**Figure 4**) [27]. Pt acted as the co-catalyst, while the sacrificial electron donor was changed to 0.1 M ascorbic acid. The average photocatalytic HER rate of S-COF and FS-COF were measured to be 4.44 mmol h−1 g−1 and 10.1 mmol h−1 g−1, respectively. Both S-COF and FS-COF have sufficient stability under visible light radiation, and FS-COF could work stably for 50 hours under ascorbic acid conditions. It was noted that HER rate of 1.32 mmol h−1 g−1 for FS-COF was obtained even in the absence of Pt. Moreover, higher HER of 16.1 and 16.3 mmol h−1 g−1 were achieved when FS-COF were filled with Eosin Y and WS5F, respectively.

With the introduction of functional organic units in COFs, tunable platforms have become possible. Acetylene as functional group has received significant attention in the field of photocatalysis. For instance, the *β-*ketoenamine-linked COFs (TP-EDDA and TP-BDDA) were modified with acetylene and diacetylene groups by Thomas et al. (**Figure 5**) [66]. TP-BDDA has a narrower band gap than TP-EDDA due to its enhanced conjugated structure. Therefore, TP-BDDA showed better activity of photocatalytic hydrogen evolution. With Pt as co-catalyst and 10% TEOA as sacrificial electron donor, TP-BDDA exhibited an HER of 324 μmol h−1 g−1, substantially higher than that of TP-EDDA (30 μmol h−1 g−1). In addition, the optical and electronic properties of *β*-ketoenamine-linked COFs could be adjusted by slightly changing the structure of their building blocks. Seki and co-workers synthesized a series of *β*-ketoenamine-linked isoreticular COFs based on 1,3,5-triformylphloroglucinol and 4,4″-diamino-substituted p-terphenyl derivatives [67]. Among these COFs,

**Figure 4.** *Synthetic routes for FS-COF and S-COF.*

**Figure 5.** *Scheme of the synthesis of TP-EDDA and TP-BDDA COFs.*

electron-deficient benzothiadiazole substituted COF (BtCOF150) exhibited better hydrogen evolution efficiency of 750 μmol h−1 g−1.

#### *2.1.4 Imine-linked COFs*

In COF synthesis, amine linkages are commonly formed by condensation between aldehydes and amines, which are abundant and easily accessible. Various attempts have been made to construct imine-linked COFs for high photocatalytic activity since the 2010 report on photocatalytic H2 production from conjugated poly(azomethine) networks [68].

The introduction of D-A strategy into COFs was beneficial to enhance charge separation and transport ability and thus enhaning photocatalytic activity. In 2020, Wen and co-workers constructed PyTz-COF with a band gap of 2.20 eV using the electron-rich pyrene (Py) and electron-deficient thiazolo[5,4-d]thiazole (Tz) [69]. Overlapping orbitals between Py and Tz units enabled electron transfer and charge separation. With ascorbic acid as SED and Pt as co-catalyst, the hydrogen production rate of PyTz-COF was up to 2072.4 μmol h−1 g−1. Besides, Dong et al. prepared a benzothiadiazole (BT)-based COF (BT-TAPT-COF) using the same strategy [70]. For at least 64 hours, BT-TAPT-COF demonstrated efficient and steady photocatalytic H2 evolution. The maximum HER rate was 949 μmol h−1 g−1 with SED (ascorbic acid) and co-catalyst (Pt). In addition, Chen and co-workers synthesized BT-based COFs that are efficient photocatalysts by introducing halogen moieties into D-A arrangements (**Figure 6**) [71]. It was found that Py-ClTP-BT-COF with co-catalyst (Pt) and SED (ascorbic acid) has superior HER rate of 8875 μmol h−1 g−1. At 420 nm, Py-ClTP-BT-COF achieved an apparent quantum efficiency (AQE) of 8.45%, which was higher than most reported COF-based photocatalysts at that time. Besides, the HER rate of Py-ClTP-BT-COF could also reach 2200 μmol h−1 g−1 without co-catalyst.

Although imine-linked COFs show the potential of photocatalytic hydrogen production, but their stability and conjugation degree are still their shortcomings, which also restricts the further improvement of their photocatalytic efficiency.

### *2.1.5 Vinylene-linked COFs*

It has been recently discovered that COFs with vinylene-linked have been fabricated via knoevenagel condensation or aldol condensation. As compared to imine, hydrazone, and azine-linked COFs, vinylene-linked COFs show greater conjugation *Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

**Figure 6.**

*(a) The formation of Py-XTP-BT-COFs and stacking mode of Py-ClTP-BT-COF. (b) Photocatalytic hydrogen evolution of Py-XTP-BT-COFs under visible light irradiation.*

degree. For vinylene-linked COFs, extended conjugation through C=C linkages not only enhances photothermal stability but also enhances absorbance and exciton migration. It was anticipated that vinylene-linked COFs, which combine chemical stability with crystallinity, porosity, and the ability to conjugate, would be highly photoactive. Although the vinylene-linked has many advantages, the poor reversibility of this linkage makes the construction of vinylene-linked very challenging because the bad self-adjusting process is unfavorable for the formation of highly ordered structures.

Jiang and co-workers first synthesized sp2 carbon frameworks by knoevenagel condensation. Then, a strong electron-withdrawing group (3-ethylrho-danine, ERDN) was introduced at the edge of sp2 c-COF to obtain D-A heterojunction (**Figure 7**) [72]. In consequence, visible light was clearly absorbed more widely. As a result of the pushand-pull effect, exciton migration and charge transport were also facilitated. When illuminated with visible light in the presence of Pt and TEOA, sp2 c-COFERDN exhibited an HER of 2.12 mmol g−1 h−1, higher than sp2 c-COF (1.36 mmol g−1 h−1). According to Zhang and co-workers, vinylene-linked COFs have been investigated as effective photocatalytic water splitter [73, 74]. As an example, they designed and synthesized two triazine-cored COFs (g-C18N3-COF and g-C33N3-COF) with unsubstituted olefin linkages [74]. The higher HER rate was observed in g-C18N3-COF (14.6 μmol h−1) and

**Figure 7.** *Schematic representation of the synthesis and photocatalytic hydrogen evolution of sp2 c-COFERDN.*

g-C33N3-COF (3.7 μmol) under the same conditions than in imine-linked COF with a similar topology and unit cell parameters. Furthermore, the ordered conjugated

structure of g-C18N3-COF yielded a high photocurrent of 45 μA cm−2 at 0.2 V vs. RHE. In order to achieve high photocatalytic performance, effective charge separation and transport must be achieved. Structures built between donors and acceptors are proving to be highly effective. According to Zhao et al., sp2 -carbon-linked COFs with periodic D-A structures were shown in a recent study (**Figure 8**) [75]. Cyano-vinylene linkages are incorporated into an electron-deficient benzobisthiazole structure to form a conjugated structure. It was found that the two-dimensional networks exhibited a strong D-A effect when electron-rich benzotrithiophene was introduced. Taking advantage of the highly conjugated and ordered structure of D-A, BTH-3 displayed an attractive photocatalytic HER of 15.1 mmol h−1 g−1 in the presence of co-catalyst (Pt) and sacrificial electron donor (0.1 M ascorbic acid).

## **2.2 Photocatalytic CO2 reduction**

Energy crisis and environmental concerns like greenhouse gas emissions can be addressed by converting CO2 into chemical fuels using solar energy [76]. In comparison to photocatalytic hydrogen evolution, CO2 reduction involves more complicated mechanisms. A variety of products could be generated, including carbon monoxide, formic acid, hydrocarbons, and alcohols. As part of the photocatalytic CO2 reduction system, COFs often served as photosensitive supports [77–82].

A post-synthetic strategy was used by Huang and his colleagues to prepare a re-doped COF for CO2 reduction (**Figure 9a**) [83]. A triazine-based COF was firstly prepared via reaction between 2,2-bipyridyl-5,5-dialdehyde (BPDA) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl) trianiline (TTA). After bipyridine ligand was reacted with Re(CO)5Cl, the Re moiety was integrated into COF as a homogeneous photocatalyst for CO2 reduction. Re-COF was found to be an effective photocatalyst for reducing CO2 to CO with high selectivity (98%) (**Figure 9b**). Because the COFs

**Figure 8.** *Synthetic routes and chemical structures of COFs BTH-1, 2, 3.*

*Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

#### **Figure 9.**

*(a) Synthesis of COF and Re-COF. (b) Proposed Catalytic Mechanism for CO2 reduction.*

are crystalline and porous, they demonstrated better activity than homogeneous Re catalysts. Cooper and co-workers combined the rhenium complex with vinyl-linked conjugated Bpy-sp2 c-COF affording a Re-Bpy-sp2 c-COF heterogeneous photocatalyst [84]. In this case, bipyridine unit offered the ligation to Re complex, was integrated into the vinyl-linked Bpy-sp2 c-COF via Knoevenagel condensation. To produce the desired photocatalyst, [Re(CO)5Cl] was loaded on the Bpy-sp2 c-COF. In the presence of TEOA, the yield of CO was 1040 μmol g−1 h−1 and the selectivity for H2 was 81% under visible light irradiation. The dye-sensitization process improved selectivity to 86% and CO production rate to1400 μmol g−1 h−1. Furthermore, platinum will generate syngas when added. Moreover, platinum could be tuned to adjust the chemical composition of the gas obtained.

A synergistic catalyst (Ni-TpBpy) was developed by Zou and co-workers by combining COF (TpBpy) with a single Ni site. [85]. They first constructed a 2,2′-bipyridine-based COF using 5′-diamino-2,2′-bipyridine. Then, the single Ni sites were loaded via treatment with Ni(ClO4)2. In the presence of [Ru(BPY)3]Cl2, the CO yield reached 4057 μmol g−1 within 5 hours, and the selectivity was as high as 96%. A unique microenvironment around single Ni sites in TpBpy was found to be responsible for the excellent photoactivity.

Metalloporphyrin (TAPP) complexes have shown good absorption of visible light and potential to reduce CO2 emissions [86]. Taking that into account, Lan and co-workers constructed a variety of porphyrin-tetrathiafulvalene covalent organic frameworks for CO2 reduction (**Figure 10**) [87]. By introducing electron-rich tetrathiafulvalene (TTF) molecules into COFs, a donor-acceptor effect was created. Consequently, the generated charge carriers could be efficiently separated and transferred from TTF to TAPP moiety. With TTCOF-Zn as photocatalyst, the yield of CO is 12.33 μmol and the selectivity is close to 100% after 60 hours of visible light irradiation without adding any sacrificant and noble metal cocatalyst. In order to further enhance reduction efficiency, the same group prepared a series of Z-scheme photocatalysts based on COF semiconductors, which combined inorganic semiconductors (TiO2, Bi2WO6, and α-Fe2O3) with COFs (COF-316/318) [88]. It was shown that photocatalytic CO production could reach 69.67 μmolg−1h−1 without additional photosensitizers and sacrificial agents.

#### **Figure 10.**

*(a) Schematic of the synthesis of TTCOF-M. (b)Schematic of the mechanism of TTCOF-M CO2RR. (c)Theoretical simulation UV/Vis DRS of TTCOF-Zn and scheme of PET route under light excitation (inset).*

Recent studies have shown that COFs without metal active centers can also be used for photocatalytic reduction of carbon dioxide. It is obvious that the reduction efficiency is much less than that of metallic-supplemented COFs, but metal-free COFs are also of great significance for the development of future carbon dioxide reduction. With these concerns, Liu and co-workers prepared a β-ketoenamine-based 2D COF, termed TpBb-COF (**Figure 11**), by condensation of 2,6-diaminobenzo[1,2-d:4,5-d′] bisthiazole and 1,3,5-triformylphloroglucinol, exhibited an excellent CO production in gas–solid system without using any photosensitizer and sacrificial agent [89]. Interestingly, the reduction of CO2 concentration was more beneficial to CO production, and when CO2 concentration was reduced to 30.0%, the CO generation rate increased to 89.9 μmol g−1 h−1. The mechanism of photocatalytic reduction of carbon dioxide and the rate equation between the production rate of CO and the concentrations of CO2 are given.

In recent years, 3D COFs have also made progress in the field of photocatalytic CO2 reduction. Zhang and co-workers designed eight-linked structural units (TTEP) based on functional porphyrin rings and obtained a 3D COF with pcb topology through Scheff base reaction under solvothermal synthesis conditions (NUST-5 and NUST-6) (**Figure 12**) [90]. Based on the excellent photoelectric characteristics of porphyrin units, these two COFs have shown great application value in the field of CO2 adsorption and photocatalytic CO2 reduction. After 10 hours of visible light irradiation, the photocatalytic CO2 reduction performance test showed that the CO yield of NUST-5 and NUST-6 was 54.7 μmol g−1 and

*Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

**Figure 11.** *Schematic of the synthesis of TpBb-COF.*

**Figure 12.**

*Illustration for the reticular design and synthesis for NUST-5 (R = H) and NUST-6 (R = Me) 3D COFs with pcb topologies.*

76.2 μmol g−1, respectively. In addition, the CH4 yields of NUST-5 and NUST-6 were 17.2 and 12.8 μmol g−1, respectively. The calculated CO/CH4 ratios are 76% and 86%.

### **2.3 Photocatalytic NADH regeneration**

As an environmentally friendly and sustainable method of converting solar energy, artificial photosynthesis is modeled after natural photosynthesis. An important component of artificial photosynthesis is coenzyme regeneration (NADH/NADPH) [91]. Many enzyme-mediated synthetic processes require NADH regeneration. Organic polymers with excellent visible light absorption have exhibited excellent photocatalytic performance under the irradiation of visible light [92–94].

A fully sp2-carbon conjugated COF (TP-COF) was synthesized and characterized for use in photocatalytic NADH regeneration in a previous study by Zhao et al. [95]. TP-COF (**Figure 13**) was used as a photosensitizer, while [Cp\*Rh(bpy)(H)]+ was selected as the electron mediator. Within 10 minutes of irradiation with 420 nm light, the NADH regeneration yield reaches 90.4%. The NADH regeneration system was then coupled with L-glutamate dehydrogenase (GDH) (a redox enzyme) to test the activity of the regenerated NADH. Benefiting from the high efficiency of NADH regeneration, α-ketoglutarate was efficiently converted to L-glutamate with a yield of 97% within 12 min.

Also, the linkage effect was investigated in the photocatalytic NADH regeneration [96]. Post-synthesis conversion of two imine-linked COFs (B-COF-1 and T-COF-1)

#### **Figure 13.**

*(a) Synthesis and structures of the TP-COF. (b) Illustration of the artificial PSI-induced coenzyme regeneration and photoenzymatic synthesis of l-glutamate by l-glutamate dehydrogenase (GDH).*

### *Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

into conjugated COFs (B-COF-2 and T-COF-2) was conducted (**Figure 14**). As a result of the subtle structure changes, the photoactivity was entirely different. NADH regeneration yielded 74.0% in 10 min with triazine-containing T-COF-2, much higher than with imine-linked precursors. Subsequently, Chen and co-workers introduced bipyridinium units into the framework structure and constructed a mesoporous alkene-linked COF as a porous solid carrier for co-immobilized formate dehydrogenase (FDH) and Rh-group electron medium [97]. By adjusting the incorporation amount of Rh electron medium, it was beneficial to regenerate NADH from NAD+ , with an apparent quantum yield (AQY) of 9.17 ± 0.44%. Finally, the assembled photocatalyst-enzyme coupling system can selectively convert CO2 to formic acid, with high efficiency and good reuse.

## **2.4 Photocatalysis for other organic reactions**

The photocatalytic organic reaction has been recognized as a green method for the synthesis of small molecules. In recent years, COFs have been frequently used for photocatalytic synthesis [48, 55, 98]. For instance, Wang and his group reported three COFs (LZU-190, LZU-191, LZU-192) with benzoxazole as the bonding type via a "killing two birds with one stone" strategy (**Figure 15**) [99]. The COFs structure was kept stable in strong acid and base (9 M NaOH, 9 M HCl), trifluoroacetic acid, boiling water, and light (all for 3 days). Subsequently, it was applied in the experiment of photocatalytic oxidation of arylboronic acids to phenols. Under visible light, all three COFs could oxidize arylboronic acids to phenols using air as oxygen source. Subsequently, LZU-190 was taken as an example to study substrate expansion and

#### **Figure 14.**

*Synthesis and Structures of B-COF-1,2; T-COF-1,2.*

**Figure 15.**

*One-Pot Construction of Benzoxazole-Linked COFs (LZU-190, LZU-191, and LZU-192) via the Cascade Reactions.*

#### **Figure 16.**

*Schematic representation of the TpTt synthesis and mechanistic representation of trans to cis photoisomerization of stilbene using the TpTt COF catalyst.*

#### *Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

mechanism, and electron spin-resonance spectroscopy (ESR) and isotope labeling experiments were used to confirm the mechanism of single electron transfer process.

Banerjee and co-workers designed and synthesized a stable COF(TpTt) based on triazine units for photocatalytic E-Z isomerization of olefins (**Figure 16**) [100]. TpTt has obvious absorption of visible light and can catalyze conversion of trans-stilbene to cis-stilbene under the irradiation of blue light diode. As a result of the good stability of β-ketoenamine linkage, the photoactivity was retained after four cycles. This work also investigated the importance of visible light for this conversion, which begins when light is illuminated and stops when light is closed, suggesting that the reaction takes place through a photocatalytic pathway. In order to further explore the mechanism of photocatalysis, the author uses free radical quenching agent TEMPO for the controllable photocatalytic reaction. When using 4 equiv. TEMPO, cis product yield was significantly reduced to 3%, which confirmed the existence of free radicals and the importance of light catalytic reaction. For different trans olefin substrates, TpTt showed good isomerization of light yield. Besides, Wang et al. developed a red light-driven catalysis system combining the Por-sp2 c-COF with TEMPO [101]. When irradiated with 623 nm red light-emitting diode, amines were converted into imines in minutes using this catalytic system.

## **3. Conclusions**

In summary, COFs have been viewed as promising materials for photocatalysis applications. As a result of the large specific surface area and the porous structure, light and reactants could be absorbed more efficiently, further optimizing photocatalysis. Conjugated structures with a high degree of order could promote direct charge transport and efficient charge transfer. As an added benefit, COF materials are easily synthesized using the versatile toolbox of organic synthesis. The photoactive building blocks can be altered according to practical applications to synthesize COFs with different structures. A variety of influencing factors, such as pore size, band gap, and other factors, can be adjusted based on the structures. The aforementioned characteristics make COFs a suitable photocatalytic platform. Benefiting from these advantages, COFs have shown excellent capacity in harvesting visible light efficiently and have been frequently used for photocatalytic water splitting, CO2 reduction, coenzyme regeneration, dye degradation, some organic reaction, etc. There are, however, still some problems that need to be solved. It is still challenging to synthesize COFs materials due to the dichotomy of crystallinity and stability. Additionally, the exfoliation of layered 2D COFs during long-term photocatalysis would influence the performance. The exploration of new linkages and synthetic strategies to increase the stability and maintain the crystallinity of the COFs is necessary both for the development of COFs materials and the application of the photocatalyst. Both of these two factors are important to photocatalytic performance. Notably, the application of 3D COFs for photocatalysis is still rare. 3D COFs could avoid the exfoliation problem usually associated with 2D COFs. In addition, the large-scale synthesis of COFs materials is crucial to this field's future. From the point of view of photocatalysis, the efficiency is still relatively low in such as water splitting and CO2 reduction compared to the traditional inorganic materials. These organic materials should be explored in more detail for some specific applications. Besides, noble metals as co-catalyst and sacrificial agents are normally necessary at

the current study lever. It is desirable to avoid the use of noble metals as co-catalyst and sacrificial agents. This could be realized through the structure regulation of the COFs. For example, we could introduce different heteroatom doping (F, Cl, N) and D-A heterojunction structure into structure, which can make the COFs have more reactive active sites and stronger exciton separation and charge transport ability of the COFs material. Furthermore, COFs materials possess highly tunable structures, providing a great advantage for studying photocatalysis mechanisms. By experimenting with different COF catalysts, it is possible to conclude that the structure–activity relationship. We are likely to gain a better understanding of some key points concerning photo-induced charge carriers, such as the separation efficiency and the carrier transport, as time goes on. While traditional photocatalysis has been studied for half a century, the role of COFs materials in photocatalysis is still relatively new. Despite many challenges, we believe COFs materials will offer new development potential in the field of photocatalysis.

## **Acknowledgements**

This work was supported by the National Natural Science Foundation of China (31202117), Natural Science Foundation of Shandong Province (ZR2020ZD38).

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Hui Liu and Yingjie Zhao\* College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, China

\*Address all correspondence to: yz@qust.edu.cn

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Photocatalysis of Covalent Organic Frameworks DOI: http://dx.doi.org/10.5772/intechopen.107672*

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## **Chapter 5**
