*Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

#### **Table 1.**

*The photocatalytic degradation of organic pollutants in aqueous media using some d-block metal-based MOFs as photocatalysts.*

MOF-5 is a highly efficient photocatalyst that would most likely succeed due to the light source. Visible light irradiation (cut-off filter ʎ > 380 nm) would significantly degrade TiO2 and ZnO activity due to a lack of uptake at wavelengths ʎ > 350 nm (**Figure 17c**) [98].

MOF-5 exhibited opposite morphology forward into various compounds, including large phenolic molecules that can flexibly disperse into the micro pores of MOF-5 deteriorated significantly faster than small ones can gain access to the inner of MOF-5,

#### **Figure 17.**

*(a) The comparison of calculated bandgap for TiO2 and MOF-5; (b) plots for photocatalytic degradation of phenols using TiO2, ZnO, and MOF-5; (c) the plausible mechanism of photocatalytic degradation using MOF-5 as a photocatalyst (reproduced from Ref. [97]).*

as investigated by Garcia and his colleagues. Researchers have studied the photodegradation of DTBP and P, where DTBP is 2,6-di-tert-butylphenol and P is significantly larger. They found that DTBP deteriorated at a similar rate to P in terms of MOF-5 at first (**Figure 18a**) [100]. MOF-5 has demonstrated size-selective photocatalytic activity. When a mixture containing Pmix and DTBPmix was exposed to radiation, DTBP deteriorated 4,4'-fold greater in comparison to P after 180 minutes

#### **Figure 18.**

*(a) Photodegradation curves for phenol (P) and 2,6-di-tert-butylphenol (DTBP) of the pure species at 40 mg L<sup>1</sup> ; (b) irradiation of a mixture of 20 mg L<sup>1</sup> of both molecules using MOF-5 as a photocatalyst (reproduced from Ref. [99]).*

*Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

of irradiation, degrading nearly 50% of the phenol and 100% of the DTBP (**Figure 18b**) [101].

Porous MOFs with 2.85 eV bandgap energy have shown photocatalytic properties for the degradation of methyl orange (MO) in an aqueous solution. The concentration of MO in water must have gradually decreased over time in the presence of light, implying perceptible decay of MO. MO can be degraded completely into colorless molecules in 120 minutes, implying that UV light was far more effective than visible light for this type of photocatalytic activity [102]. Furthermore, the UTSA-38 catalyst was recovered from the reaction mixtures with simple filtration, with no discernible loss of catalytic performance. The main pathways proposed by UTSA-38 for MO photoreduction when exposed to UV or visible light are depicted in **Figure 19a**. Charged particles reduced oxygen (O2) to oxygen radicals, which then changed into hydroxyl radicals (OH° ), which were efficient at decaying MO [104].

The Langmuir–Hinshelwood kinetic has been successfully applied to heterogeneous photodegradation. The relationship between the initial degradation rate and the initial dye concentration of the organic substrate can be written as *r0 = k0C0/(1 + K0 C0)*. The photodegradation of the four dyes in [Co2(4,4'-bpy), Ni2, Zn2, and H2O has been studied. The majority of these reactions produced very low *K0* values, which were discovered. A low value of *K0* indicates poor adsorption, despite the fact that *K0* is the equilibrium adhesion coefficient. The photocatalysts [Co2(4,4'-obb)2, [Ni2('bpy')2H2O, and [Zn2 bpy) performed better than commercial TiO2 catalysts under laboratory conditions (**Figure 19b**) [101]. These MOF catalysts were previously reported, but their kinetic rates and degradation efficiencies have been summarized in **Table 2**.

The photocatalytic decomposition of organic dyes in [Co2(4,4'-bpy),] a simple mechanism has been proposed further. One electron moves from the HOMO to the LUMO when exposed to UV light and 2H2O. The excited M2+ center decomposes rapidly to its ground state. If any molecules are within an acceptable distance but have the proper orientation, transitional energetic compounds may form. This results in the

#### **Figure 19.**

*(a) Photodegradation mechanism for methyl orange by UTSA-38 in the presence of UV-visible or visible light; (b) absorbance plots for degradation of methyl orange solution degraded by UTSA-38 in the presence of different light sources, such as UV-visible, visible, and dark light (reproduced from Ref. [103]).*


#### **Table 2.**

*The kinetic parameters for dye degradation using [Co2(4,4'-bpy)](4,4' obb)2, [Ni2(4,4'-bpy)2](4,4'-obb) 2H2O, as well as [Zn2(4,4'-bpy)](4,4'-obb)2 [105].*

#### **Figure 20.**

*(a) X3B photodegradation experiments: (i)X3B/[Mn3(btc)2(bimb)2] ii) X3B/UV light (without catalyst); iii) X3B/[Mn3(btc)2(bimb)2] (iv) X3B/[Mn3(btc)2(bimb)2]; 4H2O/visible light; X3B/[Mn3(btc)2(bimb)2]; and (v) 4H2O/tert-butyl alcohol/UV light. 4H2O/UV light. (b) X3B photodegradation experiments: (i) X3B/ [Co3(btc)2(bimb)2] (ii) X3B/UV light (without catalyst); (iii) X3B/[Co3(btc)2(bimb)2] (iv) X3B/ [Co3(btc)2(bimb)2]; 4H2O/tert-butyl alcohol/UV light; 4H2O/visible light, as well as (v) X3B/ [Co3(btc)2(bimb)2]. UV light/4H2O [Mn3(btc)2(bimb)2] UV/vis diffuse-reflectance spectra [Co3(btc)2(bimb)2] and 4H2O (black line) 4H2O (red line) with a background of BaSO4. (d) A simplified model of X3B's photocatalytic reaction mechanism with [Mn3(btc)2(bimb)2]. [Co3(btc)2(bimb)2] and 4H2O (reproduced from Ref. [106]).*

cleavage of the C–N bond and the gradual N-deethylation of RhB. The HOMO and LUMO MOFs have different bandgap sizes (4.04 and 3.72 eV, respectively), resulting in photocatalytic degradation differences (**Figure 20a** and **b**) [107]. Despite the fact that the two MOFs share the same hierarchical architecture, different focal metal ions *Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

result in different radioactivity levels. Mn3(btc)2(bimb)2]4H2O could be assigned to ligand-to-metal charge transfer (LMCT), as shown in **Figure 20c**. In the latter MOF, two additional peaks at 547 and 721 nm are detected, which are most likely the result of the spin-allowed transition of d<sup>7</sup> Co2+ ion. The photocatalytic properties of [Mn3(btc)2(bimb)2] have been improved. Under UV light, the aforementioned was able to degrade X3B almost completely in 10 hours [87]. The energy bandgap between 4H2O and Co3 has also been found to be larger under UV light. Mn3(btc)2(bimb)2]. 4H2O could be attributed to their distinct UV/vis absorption properties. The HOMO is primarily attributed by the oxygen and (or) nitrogen 2p bonding orbitals. The LUMO is caused by empty Mn(Co) orbitals (conduction band). Electrons were transferred from oxygen and (or) nitrogen to Mn throughout the photoinduced process. In this case, one electron was extracted from the water molecule and aerated to produce the OH° hydroxyl radicals [101]. Meanwhile, electrons in the LUMO combined with oxygen adsorbed on the MOF surfaces to form O2, which was then converted to hydroxide (OH) (**Figure 20d**).

#### **4.4 Photocatalytic selective redox in organic synthesis**

MOFs can be used to promote photocatalytic oxidations in the absence or presence of another semiconductor. This is important because the oxidation of alcohols to aldehydes and ketones is an important reaction in organic synthesis [71]. **Table 3** summarises the studies that describe the use of MOFs as photocatalysis catalysts. Amine-functionalized UiO-66 has been reported as a high-efficiency and highselectivity visible-light photocatalyst for the selective aerobic oxygenation of various organic compounds such as alcohols, olefins, and cycloalkanes.

The -NH2 group inside the bdc linker introduces a new absorption edge in the diffuse reflectance UV/Vis spectrum of NH2-UiO-66 at λ max (450 nm). Exposure to visible light increased the conversion of the studied alkenes steadily over time. MOFs can be used as photocatalysts for H2 generation, CO2 reduction, photooxygenation, and nitro reduction [115]. The experimental results show that the solvent used and the reacting precursors now influence the final product selectivity. From b-methylstyrene, styrene, and 1,2-diphenylethylene, epoxides with selectivity values


**Table 3.**

*Summary of photooxidation reactions catalyzed using MOFs-based photocatalysts.*

**Figure 21.** *NH2-UiO-66 catalyzes photooxidation of various substrates (reproduced from Ref. [91]).*

```
Figure 22.
Nitrobenzene photoreduction catalyzed by Pt(1.5)/NH2-Ti-MOF (reproduced from Ref. [117]).
```
ranging from 15 to 65% were obtained. Cyclooctene resulted in low conversion due to its larger kinetic diameter, particularly when compared to the pore diameter of NH2- UiO-66. An 18O-isotope labeling experiment for such photocatalytic epoxidation of cyclooctene has shown that the product contains oxygen (**Figure 21**). This is consistent with the fact that oxygen can be inferred from molecular sufficient oxygen in the gaseous state [116].

Photocatalysis that results in charge separation may promote both oxidation and reduction (via the reaction with photogenerated holes). All of these methods must occur at the same rate, but depending on the material, either of the two half-reactions could occur. Pt/NH2-Ti-MOF was discovered to act as a photocatalyst for such nitrobenzene reduction under visible light illumination (500W Xe lamp), resulting in aniline as the final product. Other photocatalytic reductions of aromatic nitro groups are discussed further below. Pt(1.5)/NH2-Ti-MOF (3.3 mmol<sup>1</sup> ) demonstrated superior catalytic activity to NH2-Ti-MoF (2.3 mol<sup>1</sup> ), suggesting that hoarded Pt also acts as a co-catalyst in this framework (**Figure 22**). The reaction selectivities appear to be nearly identical regardless of the presence of Pt species.

#### **4.5 Functions of MOFs in photoelectrodes**

MOFs play critical roles in increasing photoelectrode efficiency and achievability during the fabrication process of photomicrography devices. They improve lightharvesting capability, carrier separation efficiency, carrier potential efficiency, and electrode potential efficiency. MOF photovoltaics can be used to improve

## *Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

light-harvesting capability and accelerate carrier separation efficiency. Light usage efficiency is the most important factor influencing solar energy conversion efficiency in PEC system applications. Improving light resource efficiency as much as possible is critical to improving photoelectrochemical performance [118]. TiO2 has a low light optimum utilization and a bandgap (3.0–3.2 eV), but it plays an important role in photoelectrode mechanism studies. Because of their adjustable bandgap and absorptivity, MOFs are thought to be effective photosensitizers [119]. They first grew ZIFs in situ on semiconducting ZnO. They sulfurized ZnO@Zn-ZIF, ZnO@Co-ZIF, and ZnO@ZnCo-ZIF to obtain high surface area shells with abundant porosity. They eventually succeeded in fabricating honeycomb ZnO@ZnS, ZnO@CoS, and ZnO@ZnS/CoS heterojunction photoelectrodes (**Figure 23a**). The structure properties after vulcanization provided long photoelectric effect transmitting pathways and an abundance of exposure catalyst surface to achieve effective optical absorption [119]. The sulphide MOFs elevated the photoelectrochemical spectral range to red-shift to varying degrees in the ultraviolet-visible spectral range (UV-vis). MOFs are frequently used as photosensitizers to extend the absorption of visible light in order to improve light usage. Liu et al. used a hydrothermal process to create an ultra-thin MIL-101 (Fe) layer on the surface of Mo: BiVO4 (**Figure 23b**) [121].

MOF photogeneration is a critical step in improving photoelectrode PEC efficiency. The energy levels of MOFs highest occupied molecular orbital and lowest unoccupied molecular orbital can be changed to better match semiconducting levels of energy and charge-transport carriers [122]. A team of researchers in China's Zhejiang Province of Hebei has developed a novel way to improve charge separation at the electrolyte/semiconductor interface. They used a hydrothermal deposition technique to form a binary photoanode from their 3D bimetallic MOFs and BiVO4 (**Figure 24a**). When exposed to visible light, the holes produced by BiVO4 after absorbing photons migrated to CoNi-MOFs, and Co2+ and Ni2+ were able to capture and oxidize the holes.

#### **Figure 23.**

*(a) The fabrication and formation of cellular ZnO@ZnS/CoS are depicted schematically. (b) UV-vis diffuse reflectance of ZnO, ZnO@ZnS, ZnO@CoS, and ZnO@ZnS/CoS. (a-b) are adapted from Ref. Elsevier. All rights reserved. (c) UV-vis spectra of photoanodes BiVO4, MIL-101(Fe)/BiVO4, Mo: BiVO4, and MIL-101(Fe)/Mo: BiVO4 (reproduced from Ref. [120]).*

#### **Figure 24.**

*(a) CoNi-MOFs/BiVO4 schematic illustration; (b) BiVO4 and CoNi-MOFs/BiVO4 charge separation efficiency; (c) The evolution of H2 and O2 gases in comparison to the evolution predicted by the current generation and faradaic efficiency. (a–c) (reproduced from Ref. [123]).*

The heavy metal ions served as active sites for the interfacial H2O to O2 reaction. The photogeneration of CoNi-MOFs/BiVO4 has produced O2 and H2 at energies close to theoretical, and the Faraday effectiveness was approximately 90%, demonstrating that the majority of the charges were isolated in time to produce O2 or H2 (**Figure 24b** and **c**) [124].
