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Historical Developments in Synthesis Approaches and Photocatalytic Perspectives of Metal-Organic Frameworks

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Mohd Muslim and Musheer Ahmad

Submitted: 25 July 2022 Reviewed: 16 August 2022 Published: 03 October 2022

DOI: 10.5772/intechopen.107119

Photocatalysts IntechOpen
Photocatalysts New Perspectives Edited by Nasser Sayed Awwad

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Photocatalysts - New Perspectives

Edited by Nasser S. Awwad, Saleh Saeed Alarfaji and Ahmed Alomary

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Abstract

Metal–organic frameworks (MOFs) have witnessed fast-growing development in inorganic chemistry as well as material chemistry due to their attractive tunable property, structural specificity, high surface area, and porosity of 3D structures. The conventional semiconductor nature of MOFs is dependent on the photoactive organic ligands and their optimization with incorporated active metal center ion, which have enjoyed these properties in the photocatalytic mechanism via efficient photogenerated charge carriers under the illumination of sunlight (UV-Visible) and other different types of lights. To improve photocatalytic efficiency, a wide range of MOFs could be easily designed to cover and harvest UV irradiation from the sunlight. A wide variety of MOFs have been designed and synthesized as photocatalysts for photocatalytic degradation of organic pollutants, photocatalytic specific redox in organic synthesis, and function in photoelectrodes. In addition, the mechanisms and current challenges for MOFs in photocatalytic degradation of organic pollutants will be thoroughly discussed. This chapter discusses recent research advances in the use of MOFs as emerging photocatalysts.

Keywords

  • metal–organic frameworks
  • photocatalytic degradation
  • photocatalysis mechanism
  • photoelectrodes
  • photocatalytic selective redox

1. Introduction

1.1 General introduction

Metal–organic frameworks (MOFs) are crystalline three-dimensional (3D) hybrid materials composed of metal ions and metal clusters linked by polydentate organic ligands [1]. MOF metal centers act as templates, connecting to organic linkers via coordinative metal–ligand interactions and electrostatic attraction. MOFs esthetical chemistry is determined by the interaction of a specific metal secondary building unit with organic ligands [2]. The organic linker may have the same topology but a different metric, creating an isoreticular set of structures that share the same basic net. The importance of coordination bonds and other weak interactions (pi-electron, H-bond, or Van der Waals interaction) in MOF synthesis could be taken for granted. MOFs have high crystallinity, large surface area, high pore volume, and low framework density [3]. They are promising materials for a variety of applications, including clean energy storage (methane and hydrogen), CO2 capture, absorption, and various separation processes [4, 5, 6]. In general, MOFs are made up of two parts: cluster or metal ion nodes and organic linkers that connect the SBUs, resulting in crystalline structures with significant porous texture development. MOFs can also be used as thin-film devices, for biomedical imaging, light harvesting, optical luminescence, catalysis, and other various applications [7]. MOF-5 and HKUST-1 are two well-known MOFs used as a photocatalyst in the synthesis of synthetic organic molecules (Figure 1) [9]. The interactions of a specific metal secondary building unit with organic SBUs determine the chemistry of MOFs. The combination of these structures results in an enormous number of possibilities for synthesizing various MOFs with tailored functional properties [10].

Figure 1.

Schematic representation of the synthesis of MOF-5 and HKUST-1 using different secondary building units (SBUs) and organic linkers. Free spaces in the framework are represented by yellow and blue spheres (reproduced from Ref. [8]).

The various types of MOFs are produced by using various SBU and organic linkers. As can be seen, different pore shapes of the MOFs framework can be achieved depending on the organic linker. Polytopic organic linkers include carboxylates, phosphonates, sulphides, azoles, and heterocyclic compounds [8]. Several SBUs and organic ligands used in the synthesis of MOFs are depicted in Figure 2. When Yaghi et al. synthesized MOF-5 in 1995, they made the first reference to the synthesis of metal–organic frameworks. Since then, a large number of these materials have been studied and classified into various categories in the literature [3, 11, 12]. UIO-66 from Universiteti I Oslo, MIL from Materials of Institute Lavoisier, and ZIF-based MOFs from Zeolite Imidazolate Framework, among many others MOFs have been used in the photocatalytic degradation [13, 14].

Figure 2.

Synthetic scheme for different zirconium-based metal–organic frameworks (MOFs) were synthesized using the same secondary building units (SBUs) and different organic ligands (reproduced from Ref. [8]).

1.2 Historical developments

MOF-5 was first synthesized by Yaghi and his colleagues in 1999 as Zn4O(BDC)3·(DMF)8(C6H5Cl) using zinc nitrate and H2BDC (1,4-benzenedicarboxylate) as a precursor. Structural transformation of MOFs occurs when exposed to variable water concentration environments [15]. Despite the fact that such a structure can be reversed by thermal treatment of the frameworks. Hausdorf and his colleagues studied the photocatalytic activity of zinc carboxylate-based MOFs (MOF-5) in water. Furthermore, Laurier et al. reported in 2013 that when exposed to visible light, iron(III)-based MOFs can photodegrade Rhodamine 6G in an aqueous solution [16]. Meanwhile, Serre and Sanchez synthesized the Ti8O8(OH)4(O2C–C6H4–CO2)6 (MIL-125(Ti)) in 2009 [17]. When exposed to visible light, iron(III)-based MOFs can photodegrade Rhodamine 6G in an aqueous solution. The Fe-O cluster itself could indeed act as a semiconductor to absorb visible light and then induce electrons from organic ligands and entire photocatalysts’ surface [18].

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2. Advance features of MOFs

2.1 Ultrahigh porosity of MOFs

Metal–organic frameworks (MOFs) with ultrahigh porosity are useful in a variety of applications, such as gas storage, separation, and catalysis. It is usually vulnerable to conscience because of the large void space inside the crystal framework. Expanding the organic linker chains should lead to increased porosity of MOFs in general [19]. The porous nature of high porosity MOFs was first demonstrated in the 1990s. The reported metal–organic framework (MOF-2010) had a large Langmuir surface area (6240 m2 g−1) and pore volume (3.60 cm3 g−1 and 0.89 cm3 cm−3) (Figure 3) [21]. The porous nature of high porosity MOFs was first demonstrated in the 1990s, with no encapsulation of guest molecules in their pores. MOFs frameworks exhibit ultrahigh porous behavior with reversible gas storage properties [22]. They are excellent candidates for use in the creation of novel and valuable MOF materials.

Figure 3.

(a) The connectivity of pyr and qom nets along with [3, 6] coordinates; (b) pairs of pyr nets; (c–e) qom is not self-dual; (d) qom connectivity with dual tiling net; (e) different net from the original net of the (c); (f) porous net of MOF-177; (g) porous net of MOF-180; and (h) porous net of MOF-200. Where yellow ball indicates the porous cages, Zn is blue; O is red; C is black, and hydrogen atoms are omitted for clarity (reproduced from Ref. [20]).

2.2 Ultrahigh surface area of MOFs

The isoreticular expansion premise has made significant progress in the generation of ultrahigh surface area MOFs. This method has been used to summarize some of the appreciable surface areas of MOFs, such as MOF-2105 and NU-1004 [23, 24]. When the solvent is removed, an increase in linker causes MOFs to collapse. Supercritical carbon dioxide activation has proven useful in addressing this issue. Although a long chain of linkers can result in the formation of interpenetrated structures, synthesizing MOFs in topographic networks can mitigate this tendency. Ultrahigh surface area of MOFs has been developed to overcome the problem of water shortage (or high humidity) consistency [22, 25]. This has led to the development of NU-1106 and DUT-327, both of which are based on the rht and umt topologies. Large surface area MOFs (NU-1103) have been reported with a larger surface area of 5646 m2 g−1 (BET area of 6550 m2g−1) (Figure 4) [23, 26, 27].

Figure 4.

Representation of large pores (indicated by blue spheres) and small pores (indicated by purple spheres) in the ftw topological networks of NU-1103 (reproduced from Ref. [23]).

2.3 MOFs with Lewis acid frameworks

Multicomponent reactions (MCRs) combine three or more reaction partners in one skillet to produce organic products. MCRs have played an important role in drug discovery and pharmaceutical applications. Brønsted and Lewis acids have been used to accelerate multiple MCR reactions at the same time [28]. Metal–organic frameworks (MOFs) have emerged as an important class of crystalline porous materials for the development of high-efficiency single-site solid photocatalysts. MOFs are composed of inorganic metal ions or clusters and organic linkers with organic atoms and molecules (Figure 5a) [20, 29]. A set of strict MOFs with acidic sites based on electron-deficient high-valent metallic sources (ZrIV, HfIV, etc.) have been developed and used to catalyze biologically important transformations. The acidity of Brønsted and Lewis acids was increased by converting immaculate doped or other Zr-capping substituents (Figure 5b) [28, 30]. A 2D MOF with self-supporting nanostructure morphological characteristics and freely available Lewis acidic Zr-OTf sites has outperformed two three-dimensional (3D) MOFs for the fabrication of a wide range of synthesized tetrahydroquinoline and aziridine carboxyl group derivative products (Figure 5c). Zr6OTf-BTB outperformed the relatively homogeneous standard Sc(OTf)3 in terms of significantly higher turnover numbers and 9–14 times longer catalyst lifetime [31]. It was eventually used to effectively create a few biologically active drug targets via MCRs.

Figure 5.

The generation of strongly Lewis acidic Zr-OTf sites in Zr6OTf-BPDC, Zr6OTf-BTC, and Zr6OTf-BTB is illustrated and compared using (a) MOF nodes and ligands, (b) structures and pore distributions, and (c) coordination defects or capping residues (yellow color) (purple: Zr, red: O, gray: C, yellow: Lewis acidic site). Where H atoms are omitted for clarity (reproduced from Ref. [28]).

2.4 Flexible and porous MOFs

Porous coordination polymers (CPs) or metal–organic frameworks (MOFs) have received a great deal of attention as smart materials. MOF-based materials, such as MOF composite materials, have piqued the interest of electrochemical energy storage and conversion researchers [32]. In addition to MOFs, there are also soft porous crystals (SPCs), which appear to be reversible or multistable crystalline solids with long-range structural ordering and repairable state transformation [33]. Flexibility frequently comes at the expense of decreased stability, and porosity loss is common. Polymeric guests prevent the framework from collapsing spontaneously, resulting in novel and stable porous phases [34]. This strategy was also used to stabilize highly porous MOFs after activation, preserving porosity. Polymerization of monomer units within MOF pore spaces is widely acknowledged as a simple and convenient method for polymer-porous material interbreeding. Kitagawa and colleagues demonstrated the incorporation of common vinyl polymers, polypyrrole (PPy), and polythiophene (PTh) into appropriate nanochannels of different MOFs [35]. Polypyrrole (PPy) is a polymer that has been shown to be a good electrical conductor for superconductors and has been used to improve MOF electrical properties and encourage their use in energy storage applications and supercapacitors [36] (Figure 6). Researchers believe that inserting a conducting polymer guest into such materials could change the porous behavior of the host frameworks.

Figure 6.

The reversible phase transformation for stabilization of soft nature MOFs (reproduced from Ref. [37]).

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3. Synthesis approaches of MOFs

The solvothermal synthesis method has been used for the synthesis of MOFs using organic solvents, such as N, N dimethylformamide (DMF), methanol (MeOH), ethanol (EtOH), and acetonitrile [38, 39]. To form and self-assemble MOF crystals, metal components and organic linkers are commonly dissolved in an organic solvent. The heating rate during synthesis is typically less than 220°C, and crystal growth times range from a few hours to several days. Significant advances in the synthesis of MOFs have been made after two decades. Several useful synthesis techniques for MOFs and their utility have been reported, including electrochemical, microwave-assisted, mechanochemical, and sonochemical methods [40] (Figure 7a, b).

Figure 7.

(a) The popular synthesis methods for synthesis of MOFs; (b) the utility percentages of synthesis methods (reproduced from Ref. [41]).

The ability to manipulate and customize the morphology of mesoporous crystals, as well as their synthetic functionalization, is critical in delivering the desired characteristics and outcomes for MOF materials [42, 43, 44]. MOF crystal growth has led to the development of more advanced and tunable methods of synthesis for controlling MOF crystal morphology and size, as well as heavily doped to begin creating hybrid MOF crystals [45]. This section of the time frame that follows provides an overview of some of the key advances in this area of research (Figure 8).

Figure 8.

The time framework for the development of the MOF synthesis methods (reproduced from Ref. [46]).

3.1 Solvothermal method

Metal–organic frameworks can be successfully synthesized using solvothermal (Figure 9) and hydrothermal strategies, which appear to be very simple and well-known methods modified from zeolite synthesis [48]. In most cases, the metal component and the carboxylic acid linker are bonded in a suitable solvent. The ability to produce large crystals, less expensive, and use a high bandwidth of heat are just a few advantages of using this method over simpler approaches [49]. The DMF solvent is widely recognized as the best solvent for this purpose. Microwave-assisted applications can help to improve reaction efficiency but are limited by low product yield and long reaction times, high heat, and the use of toxic organic solvents, such as cadmium iodobenzene [50].

Figure 9.

Schematic representation of the solvothermal synthesis method for synthesis of metalorganic frameworks (reproduced from Ref. [47]).

3.2 Microwave-assisted method

Microwave-assisted synthesis methods make use of the interaction of electromagnetic waves with polar solvent ionic species (Figure 10). MIL-101(Fe) was synthesized at 150°C using dimethyl formamide, with a yield of 20% and particle sizes similar to 200 nm [51]. Increasing the concentration of water or the pH reduced crystal size. Microwave heating at 210°C was also used to synthesize Cr-Mil-101 MOF. Zr-based MOFs exhibit excellent chemical and thermal stability, resulting in strong coordination interactions of zirconium Zr(IV) ions with organic ligands [52]. The microwave was also used to help in the synthesis of Cr-MIL-101 at 95 °C and for a shorter duration of 9 minutes to generate MOF-5. A solvothermal method for preparing pure phase MIL-140 is likely to result in superior chemical stability with less preparation time than conventional electric heating strategies. In 2013, Ren et al. used this process to produce highly crystalline UiO-66 MOF octahedral-shaped crystals for H2 storage capacity [53].

Figure 10.

Schematic representation of the microwave-assisted method for synthesis of metalorganic frameworks (reproduced from Ref. [40]).

3.3 Vapor diffusion method

The vapor diffusion method was the first synthetic route used to create MOFs structure. This method produces high-quality crystals, but it requires high ligand solubility. For the first time, Smaldone and his colleagues characterized a series of cyclodextrin-based metal–organic frameworks (CD-MOFs) [54]. In addition, Forgan’s group created cyclodextrin-based metal–organic frameworks (-CD-MOFs) by combining -CD with K+, Rb+, and Cs+ in an aqueous medium and then vapor diffusion with MeOH [54]. They obtained single crystals of -CD-MOFs with crystal sizes ranging from 200 to 400 nm in 2–7 days (Figure 11). Wu et al. used this method to make [Pb(1,4-NDC)(DMF)] by dissolving Pb(NO3)2 and H2-1,4-NDC (naphthalene dicarboxylate) in DMF inside one vessel and triethylamine in the other. The continuous growth of MOF crystals was caused by the sustained diffusion of triethylamine from the outer to the inner container [56].

Figure 11.

Schematic representation of the vapor diffusion method for synthesis of metal–organic frameworks (reproduced from Ref. [55]).

3.4 Gel crystallization

Gel crystal growth is a useful strategy for MOF synthesis that involves adding an emulsifier to the reaction medium. Das et al. synthesized MOF [Ba2(O3P(CH2)3PO3)]3H2O by first dissolving the metal component, barium(II) chloride in water [57]. The mixture was thoroughly mixed prior to forming a gel, and sheets of ethanediphosphonic aqueous acidic remedy were equipped and nurtured for three weeks. Despite the fact that this procedure is time-consuming, additional separation phases are required to detoxify the product using the gel crystallization method (Figure 12).

Figure 12.

Synthesis of Fe-MOFs via PdCl2-mediated gel crystallization methods (reproduced from Ref. [58]).

3.5 Solventless method

Solventless synthesis methods are more advantageous because they allow the fabrication of MOFs without the use of toxic solvents and create a new identity through studies of a few solventless synthesis of MOFs [59]. Mechanosynthesis is a fast, scalable, and nontoxic method for producing MOFs. It involves ball bearings in a stainless-steel vessel with reagents and a stoichiometric amount of solvent [60]. Solventless synthesis reduces solvent toxicity by expelling it from the reaction. The nano-crystalline nature of the material is provided by the finishing, which can be biosynthesized quickly at room temperature. After the vessel has been completely closed and the reagent kits have been pulverized by proper mixing, the metal–organic framework is addressed (Figure 13).

Figure 13.

Schematic representation of solventless synthesis method of metal–organic frameworks (reproduced from Ref. [61]).

3.6 Sonochemical

The sonochemical synthesis of MOFs is a simple and effective method that involves exposing the mixture to sonogram waves with frequencies ranging from 20 kHz to 10 MHz. The advantages of this approach include the lack of additional heat required, the quick reaction time, and the creation of a narrow size distribution crystallization product (Figure 14). The sonochemical synthesis method is suitable for the synthesis of nanoscale crystals because particles are generated instantly inside local solvent cavity regions with a short total lifetime (ms) and dimensions in the 10 nm range. MOFs produced at room temperature using a sonochemical synthesis method have dimensions in the 10 nm range [63]. Fard et al. demonstrated the creation of a 2D MOF [Pb2(N3)(NO3)L2], (L = 8-hydroxy quinolate) in an aqueous medium [64].

Figure 14.

Sonochemical synthesis of the metal–organic framework (reproduced from Ref. [62]).

3.7 Electrochemical method

The electrochemical synthesis method provides an alternative approach to the sonochemical procedure that does not require an external source of heat [65] (Figure 15). Joaristi et al [67]. reported the synthesis of HKUST-1 on an anode side with only an isopropanol solution and a copper mesh. MIL-100(Fe) has been synthesized for the first time using a simple nucleation method [68]. The method involves dissolving 1,3,5-benzenetricarboxylic acid in ethyl alcohol and Milli-Q moisture in an electrolytic system under high pressure and temperature. MIL-100(Fe) crystals were grown on pure iron substrates using a Fe electrode at temperatures ranging from 110 to 190°C. Heating a solution of BDC: HNO3: H2O: AA: DMF contributed to UiO-66 anodic/cathodic film deposition on zirconium foil. The anodic deposition has been shown to improve MOF adhesion on zinc metal ions, in a study published in the Journal of Organic Chemistry and Biomaterials (JICB) by researchers at the University of California, Los Angeles [69].

Figure 15.

Schematic representation of electrochemical synthesis method of metal–organic frameworks (reproduced from Ref. [66]).

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4. Photocatalytic applications of MOFs

MOFs have recently received a lot of attention in the field of photocatalytic degradation. Photocatalysis is a green technology that converts sunlight into chemical energy [70]. MOFs have a large surface area and high porosity and are ideal candidates for photovoltaics. MOF breakthroughs in organic contaminant degradation, water splitting, and CO2 photoreduction [71]. TiO2 has a large bandgap (3.2 eV), which limits its photocatalytic properties in the UV light region. This region accounts for only 5% of the spectral region, resulting in low photocatalytic activity. In the last decade, far more effort has been expended to improve light utilization by changing TiO2 via anion/cation doping or incorporating it with other metals/semiconductors [72]. These evaluations are limited to delineating the engineering premise and the photocatalysts working principle.

4.1 Fundamental processes

MOF-based photocatalysis, like the vintage photodegradation phenomenon formed for conventional semiconductor photocatalysts, followed four basic steps of processes (Figure 16), which operate from their crystalline structure. The four fundamental operating processes in MOF-based heterogeneous catalysis are as follows [74]:

Figure 16.

Fundamental steps in a traditional photocatalysis system (reproduced from Ref. [73]).

4.1.1 Photoexcitation step

MOF-based photocatalysts could occur via inorganic SBU or organic ligands. MOFs generally have low absorptions in the UV-Vis region, depending on their fluorophore centers [75]. The spectra shown above are frequently associated with various π–π* transitions of aromatic units. MOF photocatalysts exhibit a few distinct absorption coefficient bands as a result of inorganic SBUs and organic ligands having distinct molecular orbitals, as well as their lowest energy bands. Semiconductor band theory fails to describe light absorption and subsequent transitions observed in MOFs. LMCT transitions are important for understanding the mechanisms and reaction products in photocatalysis involving MOFs. Even though photo-generated holes and electrons have the same energy stages for MOF molecular orbitals (HOMO and LUMO), this is critical for understanding MOF reactions [76].

4.1.2 Charge-transfer step

This procedure begins with the absorption of massive amounts of light energy, which results in the excitation of positive and negative charges from the bulk to the photocatalyst surface. The efficiency of these procedures is determined by the material type, crystallinity, and particle size. High charge mobility, for example, is frequently associated with higher crystalline semiconductor materials that exhibit focused charge mobility. The adsorption and photodegradation activities of various dyes on three Cd-containing MOFs have been investigated. Anionic dyes (e.g., sulphonated) were significantly adsorbed by Cd2(4,4′-bpy)3(S2O3)2, Cd1(4,.4′)·2H2O in the dark, but not by non-cationic dyes. The presence of chromophore centers, which may also serve as recombination centers where charge carriers are trapped and quenched, causes structural deficiency in inorganic photocatalysts, resulting in significant energy loss through realized heat [77]. The charge-transfer performance of semiconductor photocatalysts with small granules is satisfactory. If the particles are too small in size, the replication process may occur due to improved surface defects.

4.1.3 Surface activation step

Charge carriers travel to the photocatalyst surface, where they can be stimulated to perform specific chemical reactions. Photogenerated charge carriers are gathered by photoelectrodes and allowed to open circuit [78]. The first step in a fairly standard photocatalytic reaction is the physical adsorption of adsorbent materials. Anionic and cationic dyes showed different surface adsorption and catalytic properties. Furthermore, dye macromolecular adsorption resulted in non-covalent weak interactions rather than radical reforms in the cadmium thiosulphate-based MOF structures. The hydroxyl radical pathway is critical in the breakdown of anthraquinonic anionic dyes, whereas a surface-controlled N-de-ethylation reaction mechanism was proposed to explain the systematic degradation of cationic dyes via sequential intermediates, where MLCT inferred from Cd metal to HOMO as well as ligand-associated LUMO play dominant roles [79].

4.1.4 Charge-carrier recombination step

Photoluminescence (PL) spectrophotometry could be used to capture and interpret the energy released during recombination. Charge-carrier recombination accounts for the majority of energy loss in photocatalysts and PEC processes, and it remains one of the most difficult challenges to overcome [80]. Charge-transport recombination occurs in both the bulk and the surface of the photocatalyst. Reducing charge-carrier recombination from the surface and bulk phases is critical. Surface metallization with noble metals has been shown to be beneficial in charge-carrier combinations [81].

4.2 MOF-based photocatalytic degradation of organic pollutants

MOFs are composed of metal-containing nodes linked by organic ligands via strong covalent bonds. When exposed to light, a few MOFs begin to behave like semiconductor materials, implying they could be useful as photocatalysts [82]. New research has demonstrated porous MOF materials to be a new type of photocatalyst. MOFs have a promising future, despite the fact that they have not been widely investigated to date. It is simple to synthesize MOFs with tunability in light absorption capacity, thereby activating appealing photocatalytic properties. Research into the application of MOFs in this area has so far been largely unexplored. In this chapter, we highlight the importance of photocatalysis in the degradation of organic pollutants into MOFs. The reaction pathway as well as the impact of external variables on electrocatalytic activity are discussed. The main issues in photocatalytic degradation and potential opportunities have been thoroughly discussed [83].

4.3 Photocatalytic properties of d-block metal-based MOFs

MOFs offer a unique opportunity for the discovery of new catalysts capable of degrading organic pollutants. More effort has been devoted to developing innovative photocatalyst materials based on MOFs. MOFs could also have potential applications in the environmentally friendly removal of organic compounds. Some d-block metal-based MOFs have good photocatalytic efficiency for organic pollutants. The d-block transition metal MOFs are important for their contributions to a variety of fields such as magnetism, catalysis, gas separation, drug delivery, and so on. The transition metal (Zn(II), Cu(II), and Cd(II)) based MOFs that have been studied as photocatalysts for photocatalytic degradation of an organic pollutant under UV, visible, or UV-vis light illumination are summarized in Table 1 [95].

MOFsLight irradiationDegradation (%)Time (min)Organic pollutantsRef.
Cd-based MOFs
Cd(4-bpah)(1,3-bdc)(H2O)UV40240MB[84]
Cd (btec)0.5(bimb)0.5Vis90540X3B[12]
Cd(npdyda)(H2O)2UV13.7150MO[85]
Ni-based MOFs
[Ni2(4,4’-bpy)2](4,4’-obb)2·H2OUV5090RhB[86]
[Ni2(4,4’-bpy)2](4,4’ obb)2.·H2OUV8090MB[87]
Co-based MOFs
[Co2(4,4’-bpy)](4,4’-obb)2UV6090RhB[85]
Co(btec)0.5(4,4’-bimb)Vis80540X3B[88]
[Co2(4,4’-bpy)](4,4’-obb)2UV9090MB[86]
Zn-based MOFs
[Zn2(4,4’-bpy)](4,4’-obb)2UV4090RhB[86]
MOF-5UV50180Phenol[89]
MOF-5UV100180DTBP[90]
(UTSA-38)UV–vis100120MO[91]
[Zn(1,4-biyb)(adtz)]·H2OUV62180MB[88]
Fe-based MOFs
Fe2(bhbdh)Vis9015RhB(H2O2)[92]
MIL-53(Fe)Vis2020MB(H2O2)[93]
Fe2(bhbdh)Vis9015MO(H2O2)[92]
MIL-53(Fe)UV–vis1140MB[93]
Cu-based MOFs
Cu(ptz)(I)Vis9824MB(H2O2)[93]
[Cu3(3-dpsea)(1,3,5-btc)2(H2O)5]UV5645MB[94]
Cu(ptz)(I)Vis10035RhB(H2O2)[88]
Cu(dm-bim)Vis9534MO[94]
Cu(dm-bim)Vis10040RhB[94]

Table 1.

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

The MOF-5 is made up of Zn4O clusters that are orthogonally linked by 1,4-bdc linkers at the corners of a cubic framework structure. This MOF was discovered to have a broad absorption band in the wavelength range of 500–840 nm. MOF-5 is a highly efficient photocatalyst that would most likely succeed due to the light source [96]. MOF-5 may improve overall photocatalytic activity efficiency and photodegradation of phenol, like TiO2, could occur via a network of reactions, such as the formation of a radical cation by electron transfer from phenol to MOF (Figure 17a). It degraded phenol in aqueous solutions in a manner similar to commercial TiO2 and could improve overall photocatalytic activity efficiency (Figure 17b). 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].

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]).

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

Figure 18.

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

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].

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]).

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.

MOFsk0 (min−1)DyesK0 (mg L−1)
[Ni2(4,4’-bpy)2](4,4’-obb)2·H2O0.029RBBR0.0015
[Co2(4,4’-bpy)](4,4’-obb)20.013RHB0.0035
[Zn2(4,4’-bpy)](4,4’-obb)20.020OG0.0029
[Co2(4,4’-bpy)](4,4’-obb)20.032MB0.0064
[Ni2(4,4’-bpy)2](4,4’-obb)2·H2O0.029OG0.0049
[Zn2(4,4’-bpy)](4,4’-obb)20.007RhB0.0020
[Ni2(4,4’-bpy)2](4,4’-obb)2·H2O0.008RhB0.0023
[Co2(4,4’-bpy)](4,4’-obb)20.031OG0.0022
[Zn2(4,4’-bpy)](4,4’-obb)20.023MB0.0029

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].

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 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 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 d7 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).

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]).

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 high-selectivity visible-light photocatalyst for the selective aerobic oxygenation of various organic compounds such as alcohols, olefins, and cycloalkanes.

CatalystStability evidencePhotoactivityPhotolysis sourceRef.
TiO2@HKUST-1[e]IR, XRD, reuse,89% conversion[b]sunlight[108]
CdS-NH2-UiO-66XRD, XPS, reuse31% conversion[b]300W Xe arc lamp[109]
NH2-MIL-125(Ti)XRD, reuse73% conversion[c]300W Xe lamp[110]
Au/MIL-125(Ti)–36% conversion[b]300W Xe arc lamp (λ = 320–780 nm)[111]
multicore Au@ZIF-8–51.6% conversion[b]500W Xe lamp[112]
MR-MIL-125(Ti)[d]reuse86.7 nmolg−1min−1 [b]150W Xe lamp[113]
NH2-UiO-66-F–53.9% conversion[b]26 W helical light bulb[114]
NH2-UiO-66–1.234 h−1 (TOF)[a]300W Xe lamp[91]

Table 3.

Summary of photooxidation reactions catalyzed using MOFs-based photocatalysts.

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 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].

Figure 21.

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

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−1) demonstrated superior catalytic activity to NH2-Ti-MoF (2.3 mol−1), 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.

Figure 22.

Nitrobenzene photoreduction catalyzed by Pt(1.5)/NH2-Ti-MOF (reproduced from Ref. [117]).

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 light-harvesting capability, carrier separation efficiency, carrier potential efficiency, and electrode potential efficiency. MOF photovoltaics can be used to improve 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].

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]).

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. 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].

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]).

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5. Conclusions and perspectives

Emerging metal–organic frameworks (MOFs) have been regarded as the most promising artificial photocatalysts for addressing numerous challenges in the disciplines of energy and environmental remediation due to their exemplary structure and diversity. The enslavement of novel photocatalysts has piqued the interest of numerous research groups. Semiconducting MOFs have a promising future as nascent photocatalysts, but they face significant barriers to widespread adoption. MOFs can be used for a wide range of applications, including healing metals and heavy metal cations, high antimicrobial applications, and photocatalytic indoor environmental remediation. Artificial photosynthesis, such as water splitting and CO2 photoreduction, is a novel and rapidly expanding application of MOFs. MOF-based photocatalysis equipment could be used for photovoltaic solar cells and detectors. On the surface of MOFs, direct CO2 capture and photoreduction from the atmosphere may be possible. Adsorption and separate redox active sites allow the constructed MOFs to mimic native plants. MOFs have a promising future as emerging photocatalysts. With the assistance of industrial partners and diverse stakeholders, the commercial exploitation of MOFs in the use of planet-saving solar energy photocatalysis innovations could be more influential. Numerous laboratory studies have shown their potential in the application of MOFs as photovoltaics.

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Acknowledgments

The authors are especially grateful to the Department of Applied Chemistry, ZHCET, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh U.P.-202002, India for providing extensive assistance during the completion of this project.

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Conflict of interest

The authors declare no conflict of interest.

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Declarations

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this chapter.

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Written By

Mohd Muslim and Musheer Ahmad

Submitted: 25 July 2022 Reviewed: 16 August 2022 Published: 03 October 2022

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

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