Transition metal-based metal–organic frameworks for environmental applications: a review

Abstract

Rapid industrialization is deteriorating air and water quality by exposing life to a wide range of pollutants, thus calling for efficient and affordable remediation strategies. Metal–organic frameworks (MOFs) are emerging materials for environmental remediation applications due to their high surface area, ordered porous structure, and application-specific tailoring of properties. In particular, transition metal-based frameworks are advanced adsorbents and catalysts for the remediation of organic and gaseous pollutants. Physicochemical properties are mainly dependent on the choice of the metal center, the oxidation state, and organic linkers. Bimetallic-, polyoxometalate-, and metal oxide-incorporated frameworks find applications as photocatalysts for decontamination of dyes, phenolic compounds, pesticides and pharmaceutical drugs under ultraviolet (UV)/visible radiations. Large surface area coupled with high activity of transition metal frameworks allows the capture and removal of inorganic and volatile organic pollutants. Transition metal frameworks convert gaseous pollutants into value-added chemicals. Frameworks containing synthetic and natural fibers are currently studied to remove chemical warfare agents.

Introduction

According to the 2020 edition of the World Water Development Report, 1.6 billion people lack access to clean water (UNESCO 2019). Moreover, the quality of water sources is rapidly decreasing due to the incorporation of highly toxic chemicals. Anthropogenic activities, such as agricultural and industrial, are responsible for releasing hazardous compounds, including dyes, pesticides, phenolic, and chlorinated compounds. Furthermore, daily activities have been adding emerging contaminants such as pharmaceuticals, personal care products, and endocrine-disrupting compounds at trace concentrations. These toxic and carcinogenic micropollutants could be a severe threat to the environment (De Andrade et al. 2018; Rojas and Horcajada 2020). Another high-priority environmental concern is associated with the increasing demand for fossil fuels such as coal, petroleum, and natural gas. The uncontrolled burning of fossil fuels leads to the emission of obnoxious and toxic gases, generally termed as CO_x, NO_x, and SO_x, along with volatile organic compounds and volatile sulfur-organic compounds. These gases severely deteriorate air quality and are directly or indirectly linked to global warming and acid rains (Kim 2018; Han et al. 2019).

Numerous physicochemical and biological methods for water and air decontamination have been developed. Many of these strategies have limited applicability, difficulty in scaling up, and high operational cost. Such is the case of ultrafiltration, reverse osmosis, or biological treatment of water due to large energy consumption, high cost, or slow operational kinetics. In the case of air decontamination processes, the accepted method of chemical adsorption involves high energy consumption and experiences solvent loss. In that sense, the development of economical, robust, and scalable technologies remained a challenging task for environmental engineers and researchers.

The discovery of metal–organic frameworks (MOFs) opened the door for the development of porous materials for gas storage at ambient conditions. Among the attractive physicochemical properties of these inorganic–organic hybrids are a large surface area (~ 1000–10,000 m² g⁻¹), pore volume, excellent water and thermal stability, and an enormous structural variability (more than 60,000 MOFs have been reported in the Cambridge crystallographic database by changing metal centers and/or organic linkers). Such features allow the design of MOFs for a wide variety of applications in CO₂ capture, gas storage, gas separation, sensing, catalysis of organic reactions, adsorption, activation of small molecules, and biomedical imaging. The increasing publications per year index related to the use of MOFs for environmental applications corroborate the increasing interest in the field (Fig. 1a). Furthermore, the application of MOFs is not restricted to one specific type of pollutant. MOF-based materials have been used for the removal of heavy metals, recovery of precious metals (Geisse et al. 2019; Lou et al. 2019; Jiang et al. 2019), the capture of NO_x, CO₂, and other toxic gases (Kim 2018; McGrath et al. 2019) and photocatalytic degradation of different organic and inorganic pollutants (Fig. 1b) (Bedia et al. 2019).

Fig. 1

b Number of publications per year related to metal–organic frameworks (MOFs) for environmental applications (2015–2020), b publications percent of MOFs for heavy metal removal, recovery of precious metal and rare-earth elements, air treatment, and photocatalytic degradation of organic compounds (Statics extracted from Web of Science database:(Clarivate Accelerating innovation 2019) Keywords used: MOFs, environmental applications, water remediation, air decontamination)

Heterogeneous photocatalysis is an efficient strategy for eliminating organic pollutants, where semiconductors such as TiO₂, ZnO, and ZnS remain the most popular choices. However, the use of these photocatalysts presents some significant drawbacks such as poor material recovery, fast electron–hole pair recombination, and low solar activity due to a wide bandgap energy (E_g ~ 3.2–3.7 eV). The oxides and sulfides of some 3d-block transition metals are known to have a narrow bandgap energy (1.11 eV for CoFe₂O₄), making them suitable for exploiting the entire UV–visible region of the solar spectrum (Gupta et al. 2020c). MOFs with Zn(II), Cu(I)/Cu(II), Co(II)/Co(III), and Fe(II)/Fe(III) as metal centers are capable of acting as photocatalysts, allowing the degradation of organic pollutants under UV/visible light. The organic linkers in the MOF structure can act as light “receptionists,” activating the transition metal centers via ligand-to-metal cluster charge transfer. The excited MOF is capable of producing electron–hole pairs that can be transferred to the surface and participate in photo-redox reactions. The weak coordination of d-block transition metals with ligands within the MOF favors the creation of coordination vacancies where the coordinatively unsaturated metal centers behave as Lewis acid catalytic sites (Kökçam-Demir et al. 2020). Moreover, the light-harvesting properties of these coordination polymers could be tuned by using conjugated organic linkers. The MOFs could exploit the entire solar spectrum and are considered as next-generation photocatalysts. Since one of the first applications of MOFs was gas capture and storage, it is possible to use MOFs as adsorbents/catalysts for the removal of toxic gases, which may come out as an economical and direct approach to alleviate the growing level of air pollution (Fig. 2).

Fig. 2

Transition metal-based metal–organic framework (MOF) for environmental applications. The broad ultraviolet/visible absorption range of MOF-based materials makes them excellent candidates for photodegradation of organic compounds, while the large surface area and porosity boost their use as materials for toxic gas decontamination

This review provides an updated overview of transition metal-based MOFs materials for environmental applications, particularly organic pollutants degradation and air decontamination strategies. The first section is dedicated to the more recent synthetic approaches for upgrading the catalytic centers of MOFs with a focus on the strategies related to the metallic nodes. The relevant physical and chemical properties of the MOF, such as surface area, thermal stability, resistance to acidic or basic media, as well as the optical properties, are discussed in the subsequent section. Major sections of this review highlight the applicability of MOFs in the catalytic degradation of organic dyes, pharmaceutical drugs, pesticides, and phenolic compounds. The adsorptive and catalytic removal of inorganic and organic gaseous pollutants using MOFs has been discussed in great lengths in the interest of readers. MOF’s potential to remediate harmful toxicants like chemical warfare agents have been addressed to broaden the domain of the coordination polymers in environmental applications. Finally, we have addressed the challenges and prospects of MOFs in air and water decontamination.

Synthesis of metal–organic frameworks

The photocatalytic activity of metal–organic frameworks (MOFs) is due to its high porosity and excellent light absorption capability. Opposite to zeolites and aluminophosphates, which present a restricted number of building units, SiO₄/AlO₄ and AlO₄/PO₄, respectively, tuning of linkers or metal nodes provides a more extensive range of surface area, porosity, and chemical environment in MOFs. The coordination polymers are versatile materials that allow a rational design of the active catalytic site and its environment with a remarkable degree of exactitude (Gascon et al. 2014). In particular, MOFs containing transition metals as nodes have gained interest due to significant structural and morphological variability. In that sense, MOFs containing Zn, Cu, Cd, Co, and Fe centers have been reported as photocatalysts for the degradation of organic pollutants under UV/visible source (Vu et al. 2019; Cheng et al. 2020). The metal nodes usually have one or more weakly coordinated ligands (water molecules or solvents) that can be eliminated without altering the crystal structure. Once the coordination vacancy is formed, the metal center behaves as Lewis acid, accepting electrons from a photosensitive molecule present in the media (Fig. 3a). However, not only the metallic node plays a major role in the MOF photocatalytic performance, the organic linker and the pore space could participate as well in the photo-redox reaction. The encapsulation of a photocatalytic active species inside the MOF pores represents another strategy to ensure high photocatalytic performance (Fig. 3c) (Chandra et al. 2019; Malik and Nath 2019). Besides, the porous structure of the inorganic polymers favors the charge transfer by lowering the recombination rate for electron–hole pairs due to the narrow distance that charge carriers must travel to be transported to the pore surface (Li et al. 2018b).

Fig. 3

a Organic pollutants photodegradation using metal–organic frameworks (MOFs) showing the excitation of the organic linker after irradiation and the transference of the excited electron to the metal node through ligand-to-metal charge transfer, b toxic gases physical adsorption on MOFs surface, c encapsulation of photoactive species inside MOF pores, d modification of metal nodes (bimetallic MOFs, metal oxides or polyoxometalates), e commonly used organic ligands for enhancing the light-harvesting ability of MOF: i. 2-hydroxyterephthalic acid, ii. 1,2,4-triazole, iii. trimesic acid, iv. 2-aminoterephthalic acid

Finally, the organic linkers are responsible for absorbing photons and inducing a charge-separate state by single electron transfer from the ligand to the metal nodes (Dhakshinamoorthy et al. 2016). The extent of conjugation in the ligand (and the metal coordination) determines the energetic requirements for a favorable transition. In that sense, the preferred ligands are aromatic polycarboxylates, 2-aminoterephthalic acid, and porphyrins (Fig. 3e) (Dhakshinamoorthy et al. 2016). As mentioned earlier, MOF applications can be extended to the physicochemical adsorption (Fig. 3b) and catalytic degradation of toxic gases. The variation of metal nodes, i.e., the introduction of another metal ion (Fig. 3d), leads to variation in the porosity of the material and active sites for adsorption, influencing the gas capture capacity (He et al. 2020). In the following subsections, an overview of synthetic approaches for MOFs with engineered photocatalytic centers has been provided with a focus on the modification of metallic nodes of the MOFs and/or encapsulation of active species. More detailed information on the synthesis of MOFs can be collected from the previously published reviews (Stock and Biswas 2012; Lee et al. 2013).

Monometallic ion-based metal–organic frameworks

Materials scientists develop strategies that allow the designing of specific materials with desired characteristics (Howarth et al. 2017; Liu et al. 2017b; Diercks et al. 2018). Different aspects have to be considered for photocatalytic applications, such as absorption of photons in the entire solar spectrum, efficient charge separation, presence of catalytic centers, and fast charge transfer kinetics (Zeng et al. 2016; Gong et al. 2020; Pratim Bag and Sahoo 2020). Metal–organic frameworks (MOFs) represent one of the most diverse family of materials with characteristics that are required for photocatalytic applications: the organic linker may behave as a photon absorber and the isolated metallic ions as catalytic centers to promote the charge transfer or the formation of oxidant species (Cheng et al. 2018; Kang et al. 2019). Till now, thousands of MOFs have been reported with a well-controlled structure in one-, two-, or three-dimensions obtained by changing the metal centers and/or the organic linkers (Gangu et al. 2016; Safaei et al. 2019).

The literature has plenty of specialized synthetic routes for the fabrication of MOFs (Bayliss et al. 2014; Yap et al. 2017; Guo et al. 2019; Zhu and Liu 2019). This large variety offers the possibility of obtaining MOFs with diverse properties such as vacancies, crystallinity, and morphology that could contribute to their performance. Figure 4 summarizes some of the most common strategies adopted for the synthesis of MOFs. These techniques have been extensively reviewed and are commonly employed to obtain MOFs for different applications (Flügel et al. 2012; Lee et al. 2013; Rubio-Martinez et al. 2017; Zhang et al. 2017). Although solvothermal is the most popular strategy for synthesizing MOFs, other strategies have been reported to obtain the solids at mild conditions (Klimakow et al. 2010; Khan and Jhung 2015; Mehta et al. 2018; Vaitsis et al. 2019). Moreover, several green strategies have been developed to avoid high energy consumption or the generation of many residues (Sánchez-Sánchez et al. 2015; Reinsch 2016). Lee et al. (2015) compare seven different synthetic routes on the properties of ZIF-8 (Lee et al. 2015). Sonochemical and dry-gel routes offer lower-sized particles. Solvothermal and mechanochemical are attractive for incorporating magnetite nanoparticles into MOFs. The route followed for the synthesis of the material has an impact on the MOF yield.

Fig. 4

Synthetic approaches for preparing metal–organic frameworks: solvo/hydrothermal, ionothermal, electrochemistry, spry dryer, microwave, sol–gel, mechanochemical, dry-gel, sonochemistry, supercritical, flow chemistry

Template-directed synthesis of MOFs is a strategy to form composite@MOF photocatalysts (Yang et al. 2019b). It could enhance the charge separation and transfer process, improving the photocatalytic behavior. Synthetic routes like Langmuir–Blodgett (Benito et al. 2016) and electrodeposition (Li et al. 2016) could be used for the development of MOF films. Another strategy to improve photocatalytic properties could be the exfoliation of the MOF to obtain 2D structures. In this way, the metallic centers would be more accessible for catalytic reactions (Zhu and Liu 2019). The synthesis method should be selected based on the linker and metal salt required to prepare the MOF with controlled composition and morphology. The employment of additives to achieve the directional growth of porous solids is a recurrent strategy to control the shape and size and obtain exotic morphologies (Ranft et al. 2013; Liu et al. 2018a).

Bimetallic ions-based metal–organic frameworks

The presence of second metal in a bimetallic catalyst has a synergistic effect on its catalytic activity, which has been reviewed extensively in the literature (Notar Francesco et al. 2014). The introduction of a second metal ion as the node for preparing metal–organic frameworks (MOFs) enhances the photocatalytic performance of the material, compared to their single metal analogs. The improved catalytic response is most likely related to synergistic effects among the components of the MOF-based photo-responsive material. The possibility of multiple oxidation states in many of the transition metals enables easy electron transfer, making their redox chemistry attractive. Iron-based bimetallic MOFs generated hydroxyl (^·OH) radicals from heterogeneous Fenton reaction with hydrogen peroxide (H₂O₂) (Zhao et al. 2017; Tang and Wang 2020). Sun et al. (2017) fabricated a family of bimetallic Fe-M MOFs using manganese, cobalt, and nickel as the second metal (M). The bimetallic MOFs were synthesized via the solvothermal reaction of terephthalic acid with variable Fe/M ratios. The Fe–M MOFs were tested for heterogeneous Fenton-like degradation of phenolic compounds. The better catalytic performance of Fe–Mn MOF, compared to the other two bimetallic MOFs, has to do with the redox chemistry of Co and Ni and their role in the Fenton reaction. A priori, either the Co(III)/Co(II) or Mn(III)/Mn(II) could decompose H₂O₂ and generate ^·OH radicals. However, the lower incorporation of Co ions into the Fe–MOF skeleton hinders the ^·OH radical formation and, subsequently, Fe–Co–MOF photocatalytic efficiency. Unlike Mn(II), Ni(II) does not participate in the Fenton reaction, which leads to poor degradation performance of Fe–Ni-MOF (Sun et al. 2017). Thus, it is highly recommendable to choose suitable second metal ions as these occupy catalytic centers and lower the catalytic activity, if not chosen properly.

Li et al. (2019a) reported a FeCo bimetallic MOF synthesized via a solvothermal method. The metallic precursors, FeCl₂.4H₂O and Co(NO₃)₂.6H₂O were dissolved in N´N-dimethylformamide/HF mixture with 2-hydroxyterephthalic acid as the organic linker. After mixing, the suspension was transferred into a Teflon sealed reactor and subjected to different time–temperature conditions. Fe(II) is considered as the most effective transition metal to activate H₂O₂ and peroxydisulfate, while Co(II) quickly generates sulfate radical (SO ^·−₄ ) and ^·OH radicals from peroxymonosulfate. Both processes are possible due to the oxidation of the metal ions in the presence of oxidants. The formation of highly reactive species, SO ^·−₄ and ^·OH radicals, allowed the prepared material to degrade methylene blue without any light source. Thus, the combination of Fe(II) and Co(II) in the MOF skeleton enhanced the catalytic activity compared to single Fe- or Co-based MOF (Li et al. 2019a). Fe-Zn bimetallic MOF was reported for photocatalytic degradation of methylene blue under visible light. The incorporation of a second metal node to the MOF structure lowered the bandgap to the visible light range, making the photodegradation process more efficient. The bimetallic MOF was prepared through a one-pot solvothermal method. The transition metal salts were dispersed in N´N-dimethylformamide/ethanol mixture with terephthalic acid as the linker. The mixture was thoroughly stirred and then heated at 150 °C for 20 h in a Teflon-lined stainless-steel Parr autoclave, yielding pale-yellow crystals. The prepared Fe-Zn bimetallic MOF exhibited good magnetic behavior, which further improved the catalyst recovery. Moreover, the aromatic linker is an accepted ligand to improve light response and transfer of the excited electrons, in this case, most likely to Fe(III) ions, considering that Zn(II) reduction to Zn is highly unlikely within the MOF structure. The Fe(III) ion behaves as a Lewis acid, which can easily reduce to Fe(II) by accepting an electron (Tiwari et al. 2019).

Gómez-Aviles et al. (2019) proposed a one-pot partial substitution of Ti ions with Zr in the NH₂-MIL-125(Ti). For the in situ substitution of Ti ions, different proportions of Zr(IV) butoxide solution and titanium isopropoxide were added in a 2-aminoterephthalic acid/N′N-dimethylformamide mixture under stirring. The reaction mixture was stirred, transferred to a Teflon-lined autoclave, and heated at 150 °C for 16 h, leading to the formation of yellow-colored crystals. The presence of amino groups (offering one pair of free electrons in the aromatic ligand structure) enables efficient metal–ligand charge transfer and extends the lifetime of the photo-excited states, which lowers the electron–hole recombination rate. In the mixed Ti-Zr metallic centers, there were new energy levels (due to the incorporation of Zr ions) below the conduction band or above the valence band, acting as new states for electron excitation, thus suppressing the charge recombination process. MOFs with low Zr(IV) content exhibited elevated porosity, high crystallinity, and low bandgap, which favored the acetaminophen degradation (Gómez-Avilés et al. 2019). He et al. (2020) reported the partial substitution of Cu with Fe and Co in HKUST-1 for CO₂ capture. The spray method is based on atomizing the precursor solutions (Cu, Co, and Fe salts; trimesic acid) into microdroplets. The microdroplets are carried through the tube furnace by airflow (5 L min⁻¹) at 200 °C. In the tube, the solvent evaporates with the initiation of the nucleation and crystallization processes. The incorporation of the second metal ion led to the expansion of the unit cell, resulting in a slightly higher surface area and pore volume (He et al. 2020).

Metal oxide-based metal–organic frameworks

The electron–hole recombination rates in monometallic and bimetallic metal–organic frameworks (MOFs) are still higher, which limits their photocatalytic activity. Aiming to reduce the charge recombination rate, loading MOFs with metal oxides is considered a suitable solution. Metal oxide semiconductors such as Fe₃O₄, α-Fe₂O₃, Cu₂O, TiO₂, and ZnO are popular choices due to their excellent photocatalytic activity. In that sense, a novel proposal is the modification (in situ/post-synthesis) of MOFs with other photocatalytic metal oxide semiconductors. Ahmad et al. (2019) synthesized M.MIL-100(Fe)@ZnO nanostructure for effective photo-Fenton-induced degradation of toxic organic compounds. Metal–organic assemblies were used as a template for preparing the stable mesoporous M.MIL-100(Fe) by the in situ self-assembly method. The metal–organic assemble precursor was prepared by dissolving metal precursors and organic linkers in dimethylformamide and transferring the mixture into a Teflon liner autoclave. The suspension was sonicated, sealed, and heated for 12 h at 150 °C. The powder collected by centrifugation was dispersed in 0.1 M HCl under stirring to destroy the template, leading to the formation of M.MIL-100(Fe). The ZnO nanospheres were loaded onto the M.MIL-100(Fe). The lower electron–hole recombination rate of ZnO and the MOF-ZnO heterojunction enhanced the degradation performance (Ahmad et al. 2019).

TiO₂ has been considered as one of the most employed semiconductor oxides in the preparation of photocatalysts, including the modification of MOFs for the degradation of volatile organic compounds and dyes. Various approaches have been adopted to introduce TiO₂ in the MOF structure, either within the pores or onto the surface. Yao et al. (2018) proposed the encapsulation of TiO₂ inside the pores of NH₂-UiO-66 for visible-light-responsive-photodegradation of volatile organic compounds. The TiO₂ nanoparticles were prepared separately by a microwave-assisted method. The TiO₂@MOF was obtained by self-assembly of the MOF around Zr(IV) ions pre-coordinated to the TiO₂. ZrCl₄ and TiO₂ nanoparticles were dispersed in N´N-dimethylformamide and 2-aminoterephthalic/acetic acid mixture was added. The reaction mixture was heated for 24 h at 120 °C under stirring, forming a yellow solid as the final product. The highly porous structure of the TiO₂@MOF led to enhanced volatile organic compounds capture, where 2-aminoterephthalic acid acted as a light-harvesting center and photocatalytic active site in the same space (Yao et al. 2018). Li et al. (2018d) reported a different synthetic strategy for preparing TiO₂@NH₂-MIL-88B(Fe). Ferric chloride and tetrabutyl titanate were used as metallic precursors with 2-aminoterephthalic acid as the organic ligand in the solvothermal synthesis. Unlike the material reported by Yao, TiO₂ nanoparticles were equally distributed on the material´s surface (Li et al. 2018d). Zhu and Yan (2018) presented another encapsulation strategy, where TiO₂ particles and the MOF were synthesized separately by the solvothermal method. TiO₂ was magnetically dispersed in N´N-dimethylformamide for 3 h, followed by the addition of MIL-125(Ti) to achieve co-precipitation of TiO₂ and MOF without stirring. MIL-125(Ti)@TiO₂ was separated and heated at 150 °C for 6 h to get the final product. The Ti-based MOF behaved as a light sensitizer, transferring photons to TiO₂. The synergistic effect of encapsulated TiO₂ and Ti in the MOF enhanced the degradation of rhodamine B than the non-encapsulated TiO₂ particles (Zhu and Yan 2018).

Li et al. (2018a) developed the first report on the incorporation of unstable Cu₂O nanocages inside ZIF-8 through a template protection-sacrifice method. The reported strategy is based on the preparation of the Cu₂O nanocages separately and capping with a silica shell to cover the surface. The Cu₂O@SiO₂ was loaded into the ZIF-8, followed by etching of the silica layer with the NaOH solution to yield hybrid Cu₂O@ZIF-8. The porous framework of the ZIF-8, along with the hollow morphology, achieved after etching SiO₂, expedite the diffusion of pollutants and offer a suitable space and surrounding for catalytic reactions. This approach protected Cu₂O inside the hybrid Cu₂O@ZIF-8 from oxidation after the catalytic hydrogenation of p-nitrophenol (Li et al. 2018a). Zhang et al. (2019b) reported the encapsulation of α-Fe₂O₃ nanoclusters in the pores of UiO-66 for visible-light-responsive photocatalysis. UiO-66 was synthesized via a solvothermal route using ZrCl₄ and terephthalic acid in N´N-dimethylformamide/acetic acid. The thermally activated MOF was suspended in an iron acetylacetonate solution and was stirred for 6 h. The prepared material was calcined for 6 h at 300 °C, which led to the formation of α-Fe₂O₃ nanoclusters within the pores of UiO-66 (Zhang et al. 2019b).

Polyoxometalate-based metal–organic frameworks

Polyoxometalates are poly-structures of cations linked through polyanion clusters with MO_x (x = 5,6) as the building units. Most often, transition metals such as W, Mo, V, and Nb in their highest oxidation state make polyoxometalates (Wang and Yang 2015; Jiang et al. 2018). The use of polyoxometalate units leads to open network metal–organic frameworks (MOFs). Polyoxometalate can either be a part of the MOF skeleton or encapsulated within the pores. Since revising all different types of polyoxometalate-based MOFs is beyond this review’s scope, limited information on the synthesis of polyoxometalate-based MOFs has been presented. A typical feature of polyoxometalate is the abundance of oxygen in its structure, acting as electron donors (Lewis base). At the same time, the transition metal ions provide empty orbitals to accept electrons, behaving as Lewis acids. Consequently, these structural assemblies can participate in redox reactions (Wang and Yang 2015). polyoxometalates have been used for a wide variety of applications due to their attractive redox behavior, tunable acid–base properties, and resistance to oxidative decomposition, sensitivity toward light and electricity, and high thermal stability. However, their high water solubility hinders their use as heterogeneous photocatalysts. The development of polyoxometalate-MOF is an attractive proposal to enhance its water stability (Chen and Zhang 2019). Zhao et al. (2018) reported two stable 3D Keggin-type polyoxometalate-based [Cu(I)₁₂(trz)₈] MOFs for photocatalysis. Two different polyoxometalates, [α-SiW₁₂O₄₀]⁴⁻ and [α-PW₁₂O₄₀]³⁻ used in the fabrication of MOFs resulted in different physical properties of the MOF. The [Cu(I)₁₂(trz)₈(H₂O)₂][α-SiW₁₂O₄₀]·2H₂O was prepared by a hydrothermal approach using Cu(NO₃)₂·3H₂O, 1-H-1,2,4-triazole (trz), K₄[α-SiW₁₂O₄₀]·17H₂O, and isonicotinic acid as a mixture in distilled water. The mixture was transferred into a Teflon-lined stainless-steel autoclave and heated for 72 h at 170 °C, yielding a yellow solid as the final product. The other polyoxometalate-MOF, [Cu(I)₁₂(trz)₈Cl][α-PW₁₂O₄₀] was developed following the same experimental procedure by replacing the Si-W–O Keggin polyoxometalate with K₃[α-PW₁₂O₄₀], resulting in dark brown crystals (Fig. 5). Despite structural differences between the MOFs, both presented relative narrow bandgap, resulting in efficient photocatalytic degradation of rhodamine B under visible light. Cu(I)-trz units connected with polyoxometalates promoted electron transfer between polyoxometalates by reducing the deactivation of excited states. Thus, introducing polyoxometalates into MOFs promotes visible-light response and separation of photogenerated electron–hole pairs.

Fig. 5

Formation processes of 3D Keggin-type polyoxometalate-based metal–organic frameworks using copper(I), 1-H-1,2,4-triazole with different polyoxometalate structures [α-SiW₁₂O₄₀]⁴⁻ and [α-PW₁₂O₄₀]³⁻ (Zhao et al. 2018)

Polyoxometalate may be used for improving the water stability of some MOFs, besides boosting photocatalytic activity. Chen et al. (2018) develop a cage-like Cu-MOF with H₄SiW₁₂O₄₀ as the polyoxometalate accommodated in cages of the Cu-MOF. The polyoxometalate-MOF ([(Cu₄Cl)(4-(4-carboxyphenyl)-1,2,4-triazole))₄].(HSiW₁₂O₄₀).31H₂O was prepared by a solvothermal reaction. The 4-(4-carboxyphenyl)-1,2,4-triazole), CuCl₂.3H₂O, and H₄SiW₁₂O₄₀ were dispersed in a water/CH₃CN mixture and heated at 120 °C for 48 h in a Teflon reactor, yielding green strip crystals as the final product. The Keggin-type polyoxometalate was introduced in the octahedral cages, resulting in a 3D arrangement of the SiW₁₂ in the cages sharing [Cu₄Cl]⁷⁺ vertices. The polyoxometalate-modified MOF was water-stable with an excellent photocatalytic response (Chen et al. 2018a). Zhong et al. (2018) proposed a similar approach to obtain robust and improved catalytic material by one-pot encapsulation of the H₃PW₁₂O₄₀·nH₂O Keggin-type polyoxometalate in the HKUST-1, [Cu₂(BTC)_4/3(H₂O)₂]₆[HPW₁₂O₄₀], named as PW@HKUST-1 by employing a novel liquid-assisted grinding method. The synthetic procedure consisted of dissolving the Cu(NO₃)₂·3H₂O and H₃PW₁₂O₄₀ at pH 4. The solvent was evaporated, and the powder was mixed with 1,3,5-benzene tricarboxylic acid and ethanol for 5 min in an agate mortar, resulting in a blue solid. The catalytic activity of the PW@HKUST-1 was found to be considerably higher than of the polyoxometalate and HKUST-1 (Zhong et al. 2018).

Physicochemical properties of metal–organic frameworks

Metal–organic frameworks (MOFs) are characterized by ultra-high surface area, tunable pore size and shape, and chemical functionality. Physicochemical properties can be tuned by changing the metallic nodes and the organic linkers constituting the MOF. The great variety of possible nodes-organic linkers combinations make these coordination polymers very suitable platforms for multiple applications such as drug delivery/release, gas storage, separation, and heterogeneous catalysis (Wang et al. 2015; Chen et al. 2020a). The MOF surface area, pore size, water stability, and bandgap are vital parameters for improving the material’s photocatalytic response. On the same note, for the removal of gaseous pollutants, the MOF should have a large surface area, suitable pore size, thermal and catalytic stability, and ability to withstand exposure to the harsh environment.

Photodegradation is a surface phenomenon where the reactive species generated by photocatalytic reactions attack the pollutants adsorbed on the surface. Hence, the greater the surface area, the higher the adsorption capacity, favoring the photodegradation of the pollutant. Pore size and volume are also of keen importance for adsorption and photocatalytic degradation. A wider pore size expedites the entrance of the targeted pollutant near to the photoactive sites, increasing the material performance. Various reports suggest that using mixed metal ions as the inorganic building units of MOFs provides a way to tune their acid–base and redox properties. Specifically, the secondary metal sites’ presence may improve the surface area of the resulting bimetallic MOFs due to the expansion of unit cells (Fig. 6b). Moreover, the bimetallic MOF’s pore volume increases with the atomic radii of the secondary metal ion (He et al. 2020).

Fig. 6

a Ultraviolet–visible diffuse reflectance spectroscopy and optical band gap energy estimation in the inset (Zhong et al. 2020), b nitrogen sorption isotherms at 77 K (He et al. 2020), c X-ray diffraction (XRD) patterns of MIL-101 exposed to the ambient atmosphere for several months (Du et al. 2019), d X-ray diffraction patterns for [(Cu₄Cl)(CPT)₄]·(HSiW₁₂O₄₀)·31H₂O soaked in water with pH 0–13 for one day (Chen et al. 2018a) (Reprinted with permission from Elsevier)

Light-harvesting ability in the solar spectrum is another fundamental property of a photocatalyst. From a practical point of view, organic linkers should absorb light in the visible region. MOF-based materials aim to facilitate easy electron transportation and increase the lifetime of photogenerated electron–hole pairs (Abazari and Mahjoub 2018). The light absorption in the visible region is marked by a low bandgap value (E_g ~ 1.6–3.2 eV) of the photocatalyst (Fig. 6a). Bimetallic and semiconductor metal oxides-based MOF, as well as their composites, can be used in the photodegradation of organic pollutants under the solar spectrum (Chen et al. 2020b). Another commonly used strategy for improving visible-light response is the use of conjugated organic or amine-functionalized linkers, which improve the electronic transference and the formation of charge carriers (Gómez-Avilés et al. 2019).

The structural stability of MOFs under strong acidic/alkaline conditions is of paramount importance to achieve the commercialization of MOFs for different environmental applications. The best approach to determine the structural stability is to expose the material to harsh conditions such as solutions of varying pH and then analyze either the solution phase for possible leaching of organic linkers/metal nodes or MOF structure by X-ray diffraction. An insignificant variation in the X-ray diffraction pattern of the MOF before and after being exposed to basic or acid media ensures high structural stability of the material (Fig. 6d) (Chen et al. 2018a). The removal of gaseous pollutants also requires MOF to be highly stable in the atmospheric conditions as structural instability leads to pore blockage and decrease in surface area, severely affecting the diffusion of gaseous pollutants inside the pores. Prolonged exposure of MOFs to ambient atmosphere could lower the stability by oxidizing the metal nodes (Fig. 6c) (Du et al. 2019).

Metal–organic frameworks for wastewater remediation

Without any pretreatment, industrial and municipal wastewaters are being dumped into clean water sources, adding all sorts of toxic organic chemicals. Metal–organic frameworks (MOFs) are in the limelight as an effective alternative to reduce the impact of these pollutants on human health by capturing and degrading them into non-toxic products. These coordination polymers have been employed in the design and development of novel adsorbents and photocatalysts to remove organic pollutants. In the following subsections, a general overview of the most updated reports on MOFs for wastewater remediation has been presented, which are primarily focused on the elimination of organic pollutants. Table 1 highlights the degradation performance of MOF-based photocatalysts for organic dyes, pharmaceutical drugs, pesticides, and phenolic compounds.

Table 1 Transition metal-based metal–organic frameworks as catalysts for the degradation of organic pollutants

Degradation of organic dyes

The majority of industrial wastewater is composed of dyes, toxic organic compounds, and colored substances. It is estimated that around 15% of the global dye production (nearly 800,000 tons per year) is directly discharged into water sources, without any pretreatment (Sarkar et al. 2017; Tarkwa et al. 2019). Consumer goods industries like textile, leather, paper and pulp, plastic, electroplating, food, medicine, and cosmetics are majorly responsible for dye pollution (Gupta et al. 2005). There are several dye classifications, including azo dyes, anthraquinone, indigoide, triphenylmethyl, and sulfur and phthalocyanine derivatives (Martínez-Huitle and Brillas 2009). As much as 70% of the synthetic dyes used in the industrial activities are azo dyes (those with azo-functional group “–N=N–”) (Gupta et al. 2020c). Organic dyes present complex and diverse chemical structures that put them in the category of persistent pollutants (the hydrolysis half-life of reactive blue 19 is 46 years) under ambient environmental conditions (Hao et al. 2000). Also, it has been reported that the textile wastewaters exhibit low dissolved oxygen, indicating that the organic dyes are non-biodegradable compounds (Hao et al. 2000).

Organic dyes in water can have toxic effects on human health, such as allergic reactions, dermatitis, nausea, and mental disorders. Many of the azo dyes have genotoxic, mutagenic, and carcinogenic effects on living beings. Moreover, they can react with disinfectants present in the water yielding hazardous by-products (Chung 2016; Gupta et al. 2020c). The presence of colored substances in water bodies leads to lower penetration of sunlight through the water. Consequently, a decreased photosynthesis rate and low dissolved oxygen levels are typical of dye polluted waters, interfering with the life cycle of aquatic life forms (Hassan and Carr 2018). A representative number of the commonly used dyes are highly soluble in water, hindering the removal of these pollutants by conventional methods. Consequently, the removal of dyes from water has become a significant challenge for stakeholders. The most employed strategies for dealing with colored waters are adsorption and degradation. Various porous materials have been investigated to remove dyes from aqueous media such as metal oxides (Gupta et al. 2020b), clays (Gil et al. 2011), mesoporous silica (Jadhav et al. 2019), activated carbon (Demirbas 2009), and graphene oxides (Gupta et al. 2020a). The popularity of metal–organic frameworks (MOFs) for the removal of organic dyes is based on their large surface area, high porosity, ability to interact with organic dyes via electrostatic interactions, and metal sites for the catalytic degradation of adsorbed dye molecules.

As discussed in previous sections, most MOFs have limitations in harvesting the entire solar spectrum, which could be resolved by coupling MOF with other semiconductors and sensitizers. Wang et al. (2019b) developed CdS@NH₂-MIL-125(Ti) heterostructure by depositing Ti-MOF over CdS nanorods. The CdS@MOF showed faster rhodamine B degradation under visible light as compared to CdS and MOF. The composite had a strong covalent conjunction from the Cd–Se–Ti bonding interactions, which resulted in a lower recombination rate of photogenerated electron–hole pairs and accelerated charge carrier transport (Wang et al. 2019b). Zhua et al. (2016) fabricated BiOBr/NH₂-MIL-125(Ti) composite for the degradation of rhodamine B under visible-light illumination. A low recombination rate of charge carriers and Ti³⁺–Ti⁴⁺ intervalence electron transfer led to the formation of O ^·−₂ over reduced Ti³⁺ (formed by the reduction of Ti⁴⁺ in the Ti–O cluster of MOF) (Zhu et al. 2016). Han et al. (2018) fabricated AgI/MIL‐53(Fe) composite for complete visible-light-assisted degradation of rhodamine B within 180 min. The composite showed a higher rate constant (0.030 min⁻¹) as compared to AgI (0.012 min⁻¹) and MIL‐53(Fe) (0.010 min⁻¹). The introduction of AgI in MOF significantly inhibited the recombination rate of the photogenerated electron–hole pairs (Han et al. 2018). α-Fe₂O₃ nanoparticles were strategically confined in the pores of UiO-66 to develop α-Fe₂O₃@UiO-66 heterostructure. Under visible light, the photocatalyst was able to degrade ~ 100% of dye within 50 min. The growth of α-Fe₂O₃ was confined to the pores UiO-66 and formed nanoclusters with a diameter of < 2 nm. The electron–hole transport distances shortened in the nanoclusters, which led to the transportation of electrons from bulk to the surface. A higher photocurrent intensity of α-Fe₂O₃@UiO-66 heterostructure under visible-light irradiation confirmed the phenomenon. The increased surface reaction sites and shortened carrier transport distance improved the photocatalytic activity of the heterostructure (Zhang et al. 2019b). Malik and Nath (2019) reported photocatalytic degradation of methylene blue dye over CdS@ZIF-8 in near-neutral conditions. The by-products generated during the photodegradation of methylene blue dye were analyzed by gas chromatography (GC)-mass spectrometry (MS). The analysis showed a small presence of intermediates after 2 h of irradiation, probably due to faster mineralization. Four major peaks at m/z ~ 283, 256, 167, and 125 were due to methylene blue dye, 3-amino-7-(dimethylamino)phenothiazin-5-ium, N-(3,4-dihydroxyphenyl)-N-methylformamide, and 4-aminobenzene-1,2-diol, respectively. Thus, it was conclusive that reactive oxygen species (^·OH, ^·O₂⁻) attacked N–CH₃ bonds leading to the oxidation of methyl groups to HCHO or HCOOH. The ^·OH radicals cleaved the central heterocyclic ring by attacking C–S and C–N bonds for complete mineralization (Malik and Nath 2019).

Abdpour et al. (2018) introduced a hybrid photocatalyst with MIL-100(Fe) and MIL-53(Fe) as p-type and n-type semiconductor materials, respectively. Decorating MIL-100(Fe) on the surface of MIL-53(Fe) resulted in electrostatic self-assembly of positive charged MIL-53 (Fe) and the negative charged MIL-100(Fe). This electrostatic interaction led to the development of a large heterojunction space, which facilitated an easy electron–hole transfer between MIL-53(Fe) and MIL-100(Fe) and increased the lifetime of the electron–hole pairs. This p–n heterojunction led to complete degradation of methyl orange within 80 min of visible-light illumination at neutral pH (Abdpour et al. 2018). A BiOI/ZnFe₂O₄/MIL-88B(Fe) ternary nanocomposite with n-p-n heterojunction was fabricated by the deposition of BiOI (p-type) and ZnFe₂O₄ (n-type) over MIL-88B(Fe) (n-type). The nanocomposite had a better dye degradation efficiency under visible light than the constituent photocatalysts. The electron-capturing through Fe-oxo clusters of MOF by converting Fe³⁺ to Fe²⁺ and transporting properties of MOF coupled with improved visible-light response and charge carrier transfer rate of BiOI/ZnFe₂O₄ led to an excellent photo-induced electron–hole pairs separation in the ternary composite (Khasevani and Gholami 2019b). Khasevani et al. (2017) successfully fabricated a novel quaternary Ag₃PO₄/BiPO₄@MIL-88B(Fe)@g-C₃N₄ nanocomposite. The nanocomposite showed low charge transfer resistance and superior charge separation efficiency, which was reflected in the acid blue 92 degradation under visible-light irradiation. Moreover, an excellent visible-light-harvesting ability, enhanced transportation, and mobility of photogenerated electrons in the specific π-conjugated networks of MIL-88B(Fe)@g-C₃N₄ had a unique role to play in the generation of reactive oxidation species (Khasevani et al. 2017).

Efforts in developing multi-step approaches have not been reflected in their photocatalytic efficiency. Most of these nanocomposites are far from harvesting the entire solar spectrum. For harnessing the visible part of the solar light, Liu et al. (2018b) developed a novel CuTz-1 for rapid photocatalytic degradation of multiple cationic, anionic, azo, and triarylmethane dyes in the presence of H₂O₂. CuTz-1 needed only 8, 15, and 22 min to completely degrade methyl orange, rhodamine B, and methylene blue dye. The faster degradation kinetics of the MOF could be linked to the predominance of Fenton-like reaction between Cu⁺ and H₂O₂, which generated ^·OH radicals for the degradation process. The effectiveness of the MOF was demonstrated by degrading a mixed solution of all four dyes under Xenon lamp irradiation and natural sunlight, where complete degradation was achieved within 21 and 80 min, respectively (Liu et al. 2018b).

Degradation of pharmaceutical drugs

In recent decades, thousands of tons of pharmaceutical drugs have been consumed, which led to the presence of more than 150 active pharmaceutical ingredients in sewage effluents and surface waters (Jones et al. 2005; Rayaroth et al. 2016). A large amount of these chemicals is excreted un-metabolized or as active metabolites, veto the removal processes in wastewater treatment plants, and enter the environment. Also, pharmaceutical products have a long half-life, leading to their accumulation beyond detectable limits and ultimately reaching life cycles. Some of them can have high environmental persistence longer than a year (Patel et al. 2019). The toxicity level of these compounds is estimated at trace level (ppb) for aquatic flora. These chemicals can also produce acute or chronic effects in humans. More accurately, estrogens hormones can induce endocrine disorders, which may restrict reproductive and sexual development (Prosser and Sibley 2015; Patel et al. 2019). Thus, there is a necessity to find effective methods for their complete removal from water even at trace levels. Catalytic degradation is one of the promising methodologies with improved treatment of persistent pollutants. The search for a suitable catalytic system is a debatable issue as each system is known for its pros and cons on the terms of affordability, ease of operation, and the formation of by-products. The ease of operation and possible mineralization make photocatalysis a viable option.

Gao et al. (2017) reported MIL-53(Fe) for the removal of clofibric acid and carbamazepine from deionized water, municipal wastewater, and river water. These compounds are drugs used to treat high blood lipids and epilepsy, respectively, but inhibit metabolism, reproduction, and embryonic development. The metal–organic framework (MOF)/H₂O₂/visible system degraded 98.2 and 90.1% (deionized water), 94 and 88% (river water), and 71 and 86% (municipal wastewater) of clofibric acid and carbamazepine, respectively, in 270 min. Apart from slow kinetics and requirement of a very low operational pH of 3, the process led to the formation of humic acid-like and fulvic acid-like organic matter as by-products, which require additional treatment process for complete removal from water (Gao et al. 2017). Fe-MOF with Fe(II)/Fe(III) coordinatively unsaturated centers, CUS-MIL-100(Fe), was developed for sulfamethazine degradation. The MOF could degrade 100% of sulfamethazine with 52% mineralization in 180 min using the heterogeneous Fenton reaction (Tang and Wang 2018). Fenton-like process and photocatalysis are better systems for the rapid mineralization of pollutants. MIL-53(Fe) in a modified visible/MIL-53(Fe)/Fe(III)/percarbonate system degraded 90% of sulfamethazine with 55% mineralization within 60 min. It was due to the significant contribution of Fenton reactions where adsorbed Fe(III) on MIL-53(Fe) reduced to Fe(II) by capturing photogenerated electron (Li et al. 2018c).

MIL-125-NH₂@Zr was fabricated for the photocatalytic degradation of acetaminophen under solar irradiation. Acetaminophen is a popular antipyretic and analgesic drug and has been detected in urban wastewaters. The doping of Ti-MOF with Zr provided energy levels below the conduction band or above the valence band, which acted as new states for electron transfer and decreased the possibility of charge recombination. These improved optical properties led to the complete degradation of acetaminophen with 65% mineralization in 180 min. Even after 180 min of irradiation, no aromatic intermediate was detected, and only organic acids, such as maleic, malonic, and oxalic acids were observed, which showed the extraordinary mineralization capability of the bimetallic MOF (Gómez-Avilés et al. 2019). Ag/AgCl is a composite photocatalyst known for a high surface plasmon resonance, enhancing the visible-light-harvesting ability. Huang et al. (2018) integrated Ag/AgCl with MIL-88A(Fe) to form Ag/AgCl@MIL-88A(Fe) nanocomposite for ibuprofen (popular nonsteroidal anti-inflammatory drug) degradation. The nanocomposite was faster than its constituents, illustrating that Ag/AgCl incorporation to MOFs is an excellent approach in harnessing visible light (Fig. 7a). The intermediates and end-products of ibuprofen degradation were probed by ion chromatography (IC) and liquid chromatography (LC)-MS–MS. The ion chromatography analysis confirmed the formation of formic and acetic acid as the end-products where the concentration of both acids increased with the irradiation time. The ibuprofen degradation was initiated by the cleaving of the alkyl side chain (forming 2 and 3 as intermediates), decarboxylation (forming 1 as intermediate), or both at the same time (forming 4 and 5 as intermediates). Since C–C bonds in aliphatic side chains are less stable than C=C bonds in aromatic rings, these bonds are cleaved first by the reactive species, which is followed by ring-opening and complete mineralization of ibuprofen (Fig. 7b) (Huang et al. 2018).

Fig. 7

a Photodegradation of ibuprofen over Ag/AgCl@MIL-88A(Fe) nanocomposite under visible-light irradiation, b proposed photocatalytic degradation pathway of ibuprofen with the presence of ACMA-2 nanocomposite (Huang et al. 2018)

Degradation of pesticides

The use of pesticides represents a widespread strategy to control the growth of unwanted plants and animals to safeguard agricultural production. More than 5.5 million tons of pesticides are released every year into the environment (Yang et al. 2019a; Oladipo et al. 2020; Pirsaheb and Moradi 2020). Herbicides are considered as the principal pesticide (~ 45%), followed by insecticides, fungicides, and bactericides (Mahour et al. 2014; Rawtani et al. 2018). Human exposure to pesticides causes carcinogenicity, neurotoxicity, reproductive toxicity, and metabolic toxicity (Oladipo et al. 2018; Khodkar et al. 2019). Also, prolonged exposure to these chemicals can significantly enhance the frequency of chromosomal aberrations, which could result in a higher risk of blood cell mutations (Oladipo 2018; Khodkar et al. 2019). Considering their high toxicity, persistence nature, and bio-accumulation potentials, the United Nations Environment Programme Governing Council and the United States Environmental Protection Agency (USEPA) classified some pesticides as persistent organic pollutants (Mahour et al. 2014; Matthies et al. 2016). Therefore, complete mineralization (conversion into less harmful compounds) for these pollutants is mandatory. As alternatives to the commonly used conventional approaches for pesticide treatment, advanced oxidation processes are particularly attractive because they can mineralize highly toxic and hazardous organic pollutants. Transition metal-based metal–organic frameworks (MOFs) have shown efficient catalytic properties and good stability for advanced oxidation processes. This subsection presents a comprehensive overview of the recent progress in utilizing MOFs for pesticide degradation from wastewater.

Zhong et al. (2020) designed Cu₂O/MIL(Fe/Cu) composite by growing Cu₂O on MIL-100(Fe) as a photo-Fenton catalyst for the degradation of thiacloprid (pesticide banned for its carcinogenicity and reproductive toxicity). Cu₂O onto the MOF surface was introduced to improve the visible-light absorption, leading to an increased catalytic rate for pesticide degradation as compared to constituent photocatalysts. After MOF modification, the charge transfers between copper oxide and MIL(Fe/Cu) improved via Cu bridges. A reduction in bandgap from 2.5 to 1.3 eV was observed, which increased the Fe²⁺/Fe³⁺ redox reaction. Cu₂O/MIL(Fe/Cu) achieved 100% of thiacloprid degradation in 18 min, with 82.3% of mineralization after 80 min. Density functional theory (DFT) study corroborated that the amount of Cu on the modified MOF could enhance electron transition and photosensitivity. Under UV/visible-light irradiation, the photogenerated e⁻ drove Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ redox reactions. The holes and ^·OH/O ^·−₂ radicals (from H₂O₂) were responsible for the photocatalytic degradation of pesticides. During the process, reactive species (^·OH, O ^·−₂ ) attacked the cyanoimino group (–CN), followed by the cleaving of the heterocyclic ring (Fig. 8a) (Zhong et al. 2020).

Fig. 8

a Photo-Fenton catalytic thiacloprid degradation pathway from the resulting intermediates. The possible reaction mechanism for photo-Fenton degradation of thiacloprid employing Cu₂O/MIL(Fe/Cu) (Reprinted with permission from (Zhong et al. 2020), b Interaction mechanism for photo-Fenton degradation of atrazine employing BiOBr/UiO-66. Degradation steps from the resulting intermediates in atrazine degradation (Reprinted with permission from (Xue et al. 2018))

The pore size in MOF is an essential factor in controlling the catalytic process as pores of appropriate sizes allow the incoming and outgoing molecules in the cavities, which could help maintain the active site density. The microporous MOFs can adsorb small molecules; meanwhile, mesoporous MOF composites with open cavities provide sites for high catalytic activity. M.MIL-100(Fe)@ZnO nanospheres were used for the photocatalytic degradation of atrazine. M.MIL-100 (Fe)@ZnO nanospheres showed a 92% degradation and 49% mineralization of atrazine within 120 min. Improved molecular diffusion through mesopores in the modified MOF favored the process, which was not observed in the microporous MIL-100(Fe). Also, ZnO nanospheres incorporated in the MOF pores improved the charged separation efficiency by lowering the charge recombination rate of MIL-100(Fe) (Ahmad et al. 2019). Xue et al. (2018) evaluated the photocatalytic response of BiOBr/UiO-66 MOF for atrazine degradation. The modified MOF degraded atrazine with an insignificant loss in the activity after four cycles. The presence of competing impurities and anions in mineral water, tap water, and river water lowered the atrazine degradation performance. The degradation process is inhibited when anions are presented in the wastewater, as anions like nitrate and bicarbonate are powerful radical scavengers. The ^·OH, O ^·−₂ , and h⁺ are the reactive species involved in the atrazine degradation process over BiOBr/UiO-66. The generated radicals attacked atrazine molecules leading to dechlorination, dealkylation, and C–N bond dissociation (Fig. 8b) (Xue et al. 2018). A novel UiO-66/CdIn₂S₄ heterojunction nanocomposite was developed to mineralize triclosan from wastewater. Modified MOF with CdIn₂S₄ had a lower charge recombination rate than parent MOF, which enhanced the mineralization efficiency of the modified material. The nanocomposite showed six times higher efficiency than CdIn₂S₄ with 90% mineralization after 180 min. The main reactive species present in the degradation process were e⁻, O ^·−₂ , h⁺, and ^·OH, which were important for pesticide mineralization. The nanocomposite maintained high photocatalytic activity even after four cycles (Bariki et al. 2020). Thus, MOF-based composites are potential photocatalysts for the remediation of pesticides from wastewater.

Degradation of phenolic compounds

Phenols are organic compounds commonly present in wastewaters from oil refineries, petrochemical, pharmaceuticals, paper, textile, and food industries (Farzaneh et al. 2010; Martínková et al. 2016; Villegas et al. 2016). The United States Environmental Protection Agency has designated phenolic compounds as priority pollutants. Though lethal effects in humans are not documented, humans exposed to phenol (> 1 mg L⁻¹) in drinking water could cause skin rashes, irregular breathing, muscle weakness, weight loss, or gastrointestinal symptoms such as diarrhea and nausea (Mohammadi et al. 2015; Villegas et al. 2016). Heterogeneous photocatalysts could harness solar energy for the in situ generation of reactive species (^·OH, O ^·−₂ ), which are capable of mineralizing phenolic contaminants even at low concentrations (Chun et al. 2000; Ahmed et al. 2010).

Garcia’s research group was one of the pioneers in proposing the use of metal–organic frameworks (MOFs) as photocatalysts for the degradation of pollutants (Alvaro et al. 2007; Llabrés i Xamena et al. 2007). The organic linkers are thought to behave as light-absorbing chromophores able to harvest photons from a wide region of the solar spectrum, while the metal ions in the scaffold are responsible for the photocatalytic activity. MOF-5 exhibited similar photocatalytic activity as commercial TiO₂ P25 and ZnO when comparing the moles of phenol degraded per metal ion in the material. This comparison originated from the fact that terephthalic units in MOF-5 are thought to be inactive (Alvaro et al. 2007). Additionally, a reverse shape selectivity (i.e., large molecules are degraded faster than small molecules) during the photocatalytic degradation of phenol and 2,6-di-tert-butylphenol using MOF-5 was reported (Llabrés i Xamena et al. 2007). This characteristic behavior was reciprocated by Wang et al. (2017) during photocatalytic degradation of phenol using MOFs obtained with transition metals (M=Co, Zn, Ni, Cd) as nodes and 5-aminoisophthalic acid as the organic linker. The large pore size of the framework (Zn > Cd > Co > Ni) disfavored the photocatalytic degradation of phenol, indicating that small pore size is needed to grasp and further degrade the phenol molecules. Also, solubility was proposed to play an important role during the photocatalytic degradation of phenol, while [ZnL(H₂O)] and [CdL(H₂O)H₂O were insoluble, [CoL(H₂O)] was partially soluble, and [Ni-(HL)₂(H₂O)₂] was completely soluble in water, which may have led to the homogeneous Fenton reaction (Wang et al. 2017).

The photocatalytic activity of MOF can be enhanced by controlling the specific surface area. Masoomi et al. (2016) improve the photocatalytic degradation of phenol under UV/visible light by enhancing the specific surface area of TMU-5 by incorporating a varying proportion of Cd²⁺ ions (Masoomi et al. 2016). A close relationship between specific surface area and photocatalytic degradation of 2-chlorophenol has been reported over three MOFs composed of dicarboxyl‐functionalized Fe(III)‐salen complexes and Zn and Cd nodes. The highest photocatalytic degradation rate was recorded over Zn(Fe–L)Cl. It is assumed that the higher the specific surface area, the more exposed are the metal centers of the framework for interaction and photocatalytic activity (Li et al. 2015a). The photocatalytic degradation of other phenolic compounds has also been performed using MOF. [Zn(BDC)(DMF)] photocatalyst has been reported to degrade 97% of p-nitrophenol under 15 min of solar illumination. Apart from MOF being highly reusable for photocatalytic degradation of p-nitrophenol, the possible reduction of p-nitrophenol into p-aminophenol over [Zn(BDC)(DMF)] was an important and promising process for the transformation of a pollutant into a valuable chemical for the preparation of drugs (Samuel et al. 2018). There is a common agreement regarding the mechanism followed during the degradation of phenolic compounds. The highly reactive ^·OH radicals are considered as the main species involved in the photocatalytic reaction. These are generated in situ by direct oxidation of hydroxyl ions or water molecules (Li et al. 2015a; Wu et al. 2015; Masoomi et al. 2016; Gong et al. 2019a) or by H₂O₂ reduction (Liang et al. 2015; Zhang et al. 2015; Khandan et al. 2018; Samuel et al. 2018; Ahmad et al. 2019; Li et al. 2019c). In some research works, O ^·−₂ radicals are another reactive species involved in the photodegradation mechanism formed by photogenerated electron transfer to adsorbed oxygen molecules (Alvaro et al. 2007; Masoomi et al. 2016; Gong et al. 2019a; Hu et al. 2019).

The formation of composites is a widely employed strategy to improve the performance of the MOF. The mechanism responsible for performance improvement depends on the type of material coupled with the photocatalyst. Liang et al. (2015) incorporated palladium particles in MIL-100(Fe) to form Pd@MIL-100(Fe). Deposition of Pd improved the visible-light-harvesting ability of MOF and increased the separation of charge carriers, avoiding recombination of electron–hole pairs in the photocatalyst. In this way, a 65% degradation and 20% mineralization of bisphenol A were possible in 240 min under visible light (Liang et al. 2015). Likewise, platinum particles were employed to improve the transfer of photogenerated electrons to H₂O₂ in Pt-UiO-66-NH₂ MOF and boost phenol degradation (Chen et al. 2020c). The coupling of MOF with magnetic ferrite nanoparticles is another viable strategy to improve the photocatalytic activity, coupled with the easy recovery of the MOF from reaction media. Magnetic Fe₃O₄/MIL-53(Fe) nanocomposite was fabricated by the solvothermal method (Zhang et al. 2015). Improved visible-light-induced degradation of p-nitrophenol was observed as compared to conventional metal oxides. Iron oxides are also known for improving the performance by a photo-Fenton coupled process. Gong et al. (2019a) reported the development of Fe₃O₄@GO@MIL-100(Fe) composites with a large surface area of 1048.1 m² g⁻¹ and a pore volume of 0.4 cm³ g⁻¹ as compared to Fe₃O₄@GO (surface area ~ 79.4 m² g⁻¹ and pore volume ~ 0.1 cm³ g⁻¹). While the Fe₃O₄@GO degraded 40% of 2,4-dichlorophenol, Fe₃O₄@GO@MIL-100(Fe) nanocomposite degraded 100% of the pollutant in visible light/H₂O₂ system (Gong et al. 2019a).

MOFs coupled with conventional photocatalysts form Z-scheme photocatalysts with better charge separation. Cobalt and nickel-metal nodes and organic linkers were combined with CuWO₄ to form [CoNi(μ₃-tp)₂(μ₂-pyz)₂]/CuWO₄ composite. Apart from the lower recombination of charge carriers, the MOF/CuWO₄ composite had a large surface area, which led to a fourfold increase in the photocatalytic degradation performance for p-nitrophenol (Ramezanalizadeh and Manteghi 2018). Hu et al. (2019) reported the coupling of NH₂-MIL-125(Ti) with BiOCl for the photocatalytic degradation of bisphenol A under visible light. Band edge positions were sketched from X-ray photoelectron spectroscopy and UV–visible diffuse reflectance spectroscopy measurements. The study confirmed the formation of the heterojunction, which lowered the charge recombination rate and improved the degradation of bisphenol A. It is important to mention that NH₂-MIL-125(Ti) alone did not exhibit good photocatalytic activity (Hu et al. 2019). Ahmad et al. (2019) formed a ZnO decorated MIL-100(Fe) composite for the degradation of phenol and bisphenol A. The composite exhibited improved performance in comparison to the constituent photocatalysts. This behavior was attributed to the larger surface area that improved the contact with the pollutant and the photo-Fenton coupled process promoted by iron in the MOF (Ahmad et al. 2019). A similar effect was observed in g-C₃N₄/PDI@NH₂-MIL-53(Fe) composite synthesized by the solvothermal method. The matching of the conduction band of the photocatalysts facilitated a faster charge transfer and separation of photo-induced electron–hole pairs. Moreover, the formation of ^·OH through H₂O₂ reduction was promoted by iron species in the framework. Bisphenol A and p-nitrophenol were degraded fully after 10 and 30 min, respectively (Li et al. 2019c). Though most of the studies lack the degradation pathway of pollutants, which makes it difficult to understand the underlying degradation mechanism and impact of the study, Meng et al. (2017) reported visible-assisted degradation of bisphenol A over PCN-222. The GC–MS analysis confirmed a new peak at m/z ~ 108 along with the characteristic peak of bisphenol A at m/z ~ 213. This peak was assigned to p-benzoquinone, which was an intermediate product. Since p-benzoquinone is a resonating structure of p-hydroquinone, it could be assumed that singlet oxygen produced in the process cleaved and oxidized the single bonds to yield phenol like species in the initial stage. The high concentration of p-benzoquinone in the solution after 2 h suggested that the radicals were partly successful in ring-opening, which led to incomplete mineralization (Meng et al. 2017). Such drawbacks could be eliminated by adding oxidants, which could enhance the degradation rate and at the same time could increase the mineralization efficiency.

Metal–organic frameworks for air decontamination

Large emissions from industrial units, thermal power plants, incinerators, and automobiles are rich in toxic gases like SO_x, NO_x, volatile organic compounds, volatile sulfur-organic compounds, NH₃, and H₂S. The complex atmospheric chemistry of these gases aided by solar radiation and water vapors leads to photochemical smog, acid rain, and the greenhouse effect, which pose a severe threat to human health and the environment. Long-term exposure to air pollutants causes asthma, lung cancer, psychological complications, autism, and low birth weight. Sporadic events, like the historic 1952 Great Smog of London and 1966 New York City smog, or the more recent, 2015 Southeast Asian haze have shown the extent of deterioration done by the anthropogenic activities the air quality. The need of the hour is to develop effective and affordable technologies to trap and eliminate these gaseous pollutants. Many physicochemical processes including absorption, membrane separation, and cryogenic distillation have been developed for the removal of toxic components of gas streams. The absorption of toxic gas in a solvent is highly dependent on the strength of the interaction of gas molecules with the solvent. For sour gas sweetening, alkanolamines are a widely used class of chemical solvents, which selectively absorb acidic gases (CO₂ and H₂S) by dissociation. Though absorption is an efficient process, continuous loss of solvent, high energy consumption for the regeneration of spent solvent, and relatively low recovery of absorbed gas limit the process (Vega et al. 2020; Shah et al. 2017). Membrane treatment of toxic gas is attractive when the pressure of feed gas is high and is more suitable for the treatment of natural gas near the source. Moreover, the process does not require large space for operation, unlike the solvent absorption process. The process is limited by slow mass transfer at ambient conditions and periodic membrane replacement. For lowering the operational cost, membrane-solvent hybrid processes have been proposed. These gas–liquid membrane contactors offer high surface area per unit contactor volume with promisingly large H₂S/CO₂ selectivity (Shah et al. 2017). Researchers are focused on the adsorptive and catalytic removal of gaseous pollutants from the contaminated air. So far, numerous solid adsorbents like zeolites, lime, active carbon, metal oxides, and silica, in the pristine or modified form, have been extensively reported for the removal of SO_x/NO_x/CO_x pollutants (Rezaei et al. 2015). The adsorptive and catalytic removal of toxic gases is in limelight for decades and appreciated for their technical simplicity and operation at ambient conditions. One of the limiting factors is the low surface area for sorbent-sorbate interactions. Large surface area coupled with transition metals (for adsorption/catalytic degradation) in metal–organic frameworks (MOFs) has attracted the environmental researchers to explore these materials for the detoxification of contaminated air. Moreover, tunable MOFs provide possible modifications in the structure and functionality for selective removal of toxic gases. In this section, we have provided discussions on the role of MOFs in the removal of major inorganic and organic gaseous pollutants. In the literature, it has been well established that MOFs have significantly high NO_x/SO_x/CO_x adsorption capacities as compared to other porous solid adsorbents (Rezaei et al. 2015). For this reason, the review has reserved discussions and comparisons among different MOFs.

Inorganic sulfur compounds

Hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) are the common atmospheric sulfur pollutants, which are majorly contributed by biogenic activities, farm activities, and thermal plants. These sulfur compounds in the oxidized form as SO₃ are responsible for the formation of acid rains that severely deteriorates the water quality. These gases are known to be toxic to humans, even at a low concentration. Though the exposure of more than 100 ppm of H₂S or SO₂ is found deadly for humans, the lower concentration (~ below 1 ppm) of these gaseous pollutants could cause discomfort. The USEPA has fixed 15 ppb and 30 ppb, as the yearly average exposure limit for H₂S and SO₂, respectively. To reach such a low threshold requires special adsorbents with large surface area and high density of active sites. The open metal sites (electron-deficient) of a highly porous metal–organic framework (MOF) could strongly interact with H₂S (Lewis base or electron-pair donor) and SO₂ (σ-donor/π-acceptor), responsible for the extremely high adsorption capacity of MOFs. Many MOFs with 3d transition metals as the nodes have been studied for the adsorptive removal of SO₂ from flue gases (Fig. 9a). Hungerford et al. (2018) developed pillared MOFs, M-DMOF-TM with Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ as the metal node for SO₂ removal. Though the MOFs showed good physisorption of dry SO₂, the SO₂ adsorption in 85% humidity led to the structural collapse due to the formation of sulfate/sulfite acid. The stability trend: Co < Ni < Cu > Zn in the humid condition was explained by the metal–ligand bond strength (in the same order as predicted by the Irving-Williams series) (Hungerford et al. 2018).

Fig. 9

a SO₂ adsorption efficiency of metal–organic frameworks (MOFs) (Tan et al. 2013; Glomb et al. 2017; Carter et al. 2018; Hungerford et al. 2018; Brandt et al. 2019; Smith et al. 2019), b H₂S adsorption capacity of MOFs (Liu et al. 2017a; Huang and Wang 2019), c NH₃ adsorption capacity of metal–organic frameworks (MOFs, Morris et al. 2011; Rieth et al. 2016; Glomb et al. 2017; Chen et al. 2018b; Rieth and Dinca 2018; Zhang et al. 2020b), d Toluene adsorption capacity of MOFs (Yang et al. 2011; Bahri et al. 2016; Vellingiri et al. 2016a; Zhang et al. 2019c, d)

Glomb et al. (2017) reported significantly higher SO₂ adsorption capacity for Zn-based MOFs with urea-functionalized dicarboxylate linkers in the dry conditions, probably due to the high polarity of the gas molecules and the involvement of hydrogen bonding with the N–H and C=O bonds of the linker. Even for the dry SO₂, the diffraction pattern of MOF-1 showed broadening of reflections and a higher amorphous background, which hinted toward the partial decomposition of the MOF network (Glomb et al. 2017). The highest SO₂ adsorption capacity of 1645 mg g⁻¹ was reported for MOF-177, which was linked to its exceptionally high surface area of 4100 m² g⁻¹ and a total pore volume of 1.51 cm³ g⁻¹. The MOF-177 showed poor adsorption behavior at low partial pressures, which limited its application for selective SO₂ adsorption from lowly concentrated flue gas (< 500 ppm). Moreover, the MOF-177 showed a 55% decrease in the surface area after dry SO₂ adsorption, making it unsuitable for humid SO₂ (Brandt et al. 2019).

The higher adsorption of SO₂ onto Ni(BDC)(TED)_0.5 than its Zn analog was reported by Tan et al. (2013), where the Ni-MOF adsorbed 291 and 638 mg g⁻¹ of SO₂ at 0.11 and 1.13 bar, respectively (Tan et al. 2013). Since Zn-based MOFs have been predicted to be less stable for SO₂ adsorption, the obvious choices for practical applications are Ni- and Cu-based MOFs. The more pragmatic approach toward using the MOFs for selective uptake of SO₂ from flue gas was recorded for the fluorinated MOF. The breakthrough experiments with the KAUST-7 recorded high SO₂ uptake of 89.6 mg g⁻¹ (500 ppm, SO₂/N₂: 0.05/99.95 mixture) and 0.64 mg g⁻¹ (500 ppm, SO₂/CO₂/N₂: 0.05/10/89.95). The analog MOF, KAUST-8, showed even higher uptake of 102.4 mg g⁻¹ (500 ppm, SO₂/N₂: 0.05/99.95 mixture). The fluorinated MOFs were stable for the adsorption of dry and 40% humid SO₂, which was reflected in their respective X-ray diffraction pattern for used MOFs (Tchalala et al. 2019).

Smith et al. (2019) developed a thermally and chemically stable MOF, MFM-170, for the selective and reversible uptake of SO₂ from flue gases. MFM-170 retained its structural integrity for ten weeks of exposure to moist SO₂. The SO₂ adsorption capacity of MFM-170 (1240 mg g⁻¹) decreased to 1037 mg g⁻¹ for MFM-170·H₂O, where the water molecules at the Cu(II) site of MFM-170 reduced the adsorption capacity. The in situ single-crystal X-ray diffraction pattern of MFM-170∙5.46SO₂ confirmed six distinct binding sites with the open Cu(II) sites as the thermodynamically favored binding site for SO₂ molecules. Exceptional SO₂/CO₂ selectivity was noticed in the breakthrough studies for fully activated and water-saturated MFM-170 (Smith et al. 2019).

Wang et al. (2020) reported theoretical considerations on the SO₂/CO₂ selectivity of Cu(II) sites of CuBTC MOF. The shorter bond distance and high binding energy for [Cu²⁺···O=S=O] interactions than that for [Cu²⁺···O=C=O], as calculated from the grand canonical Monte Carlo and density functional theory analysis suggested strong electrostatic interaction of SO₂ with the Cu²⁺ sites (Wang et al. 2020). The SO₂ adsorption onto Cu₂(2,3-pyrazinedicarboxylate)₂(pyrazine), named as CPL-1 MOF, with SO₂/CO₂ (9), SO₂/N₂ (368), and SO₂/CH₄ (74) selectivity was possible via molecular sieving effect. The density functional theory -based static binding energy of SO₂ molecules (36.3 kJ mol⁻¹) on CPL-1 MOF was found significantly higher than of CO₂ molecules (13.8 kJ mol⁻¹), which accounted for the SO₂/CO₂ selectivity. The density functional theory calculations predicted O–(hydroxyl and carbonyl) and H–(pzdc ligands)-based binding sites in CPL-1, where the SO₂ molecules were trapped by S^δ+···O^δ− electrostatic interactions at a distance of 2.70 Å and O^δ−···H^δ+ hydrogen bonds at a distance of 2.19–2.85 Å. In this work, the authors were able to prepare MOF in kilogram scale with similar crystallinity and performance at a fabrication cost of $230 kg⁻¹ (Zhang et al. 2020a). The exceptional selectivity of MFM-170, SO₂/CO₂ (35), SO₂/N₂ (944), SO₂/CO (203), and SO₂/CH₄ (260), coupled with zero loss in the SO₂ adsorption performance for 50 adsorption/desorption cycles, made MFM-170 as one of the suitable MOF for the capture of SO₂ from flue gases (Smith et al. 2019).

For the selective adsorption of H₂S from syngas, Liu et al. (2017a) reported numerous MOFs with different organic linkers and Cr, Fe, Cu, Zn, and Zr as transition metal nodes (Fig. 9b). Among all the MOFs studies, Cu- and Zn-containing MOFs showed better adsorption performances due to the strong covalent interaction of the H₂S molecules with free Zn and Cu sites. The chemisorption process was driven by the formation of Cu–S and Zn–S bonds at the expense of weak Cu–O and Zn–O bonds, respectively. For other MOFs, including MIL-101(Cr), UiO-66, and ZIF-8, reversible adsorption behavior with poor selectivity was recorded due to the involvement of physical forces. Considering the trade-off between H₂S/CO₂ selectivity and uptake, UiO-66 and MIL-101(Cr) were chosen as promising MOFs for H₂S capture (Liu et al. 2017a). Alivand et al. (2019) reported selective H₂S adsorption using conventional MIL-101(Cr) and improved MIL-101(Cr) with excess ligand-clusters. Apart from the improvement in the surface and porous properties, the modified MOF had ~ 60% more H₂S adsorption capacity than the conventional one. The increased adsorption behavior was attributed to the creation of unsaturated Cr³⁺ sites for stronger interactions of polar gases at low pressures (Alivand et al. 2019).

One of the many observations reported by Liu et al. (2017a) was the significantly higher adsorption of H₂S onto the amine analog of UiO-66 as compared to the parent MOF. Apart from reversible adsorption of H₂S molecules in the tetrahedron aromatic cage of the UiO-66, the reactive interaction of −NH₂ groups with H₂S (forming NH₃HS) led to irreversible adsorption of H₂S over UiO-66-NH₂ (Liu et al. 2017a). Joshi et al. (2018) made detailed observations on the enhanced H₂S uptake from natural gas simulant mixtures by amine analogs of UiO-66(Zr), MIL-125(Ti), and MIL-101(Cr). In the fixed-bed experiments with 1% H₂S/99% CH₄ and 1% H₂S/10% CO₂/89% CH₄ test mixtures, the trend: MIL-125 < UiO-66 < MIL-101 observed for the adsorption capacity was following the pore diameter. The MOFs showed decreased H₂S uptake in the presence of CO₂ in the test mixtures except for MIL-101. The presence of hydroxyl groups in MIL-125 and UiO-66 served as the CO₂ physisorption sites, which were absent in MIL-101. The study suggested adsorptive uptake of H₂S over amine analogs with a higher H₂S/CO₂ selectivity from natural gas (Joshi et al. 2018). Monte Carlo molecular simulation was performed by Sokhanvaran et al. (2019) to understand the adsorption of H₂S, CO₂, and CH₄ onto parent and hydroxyl-functionalized MIL-47(V). The simulated adsorption selectivity of the MOF was in the order H₂S/CH₄ > H₂S/CO₂ > CO₂/CH₄. Polar H₂S molecules interacted more strongly with the polar functionality of the MOF than the CO₂ molecules, which is nonpolar with a weak electrical quadrupole moment (Sokhanvaran et al. 2019). Thus, theoretical and experimental considerations showed that MOFs own intrinsic selectivity for H₂S molecules in multi-component systems with a higher proportion of CO₂ and CH₄ molecules, which is a good indication for their utilization in the desulfurization of natural gas. In most of these studies, the regeneration of MOFs was poor, which led to the non-reusability of MOFs for a large number of cycles.

The thermal catalytic conversion of H₂S to sulfur using MOF as a catalyst is a more feasible approach toward the desulfurization process. Liu et al. (2017a) hinted toward converting H₂S to S₈ at 200 °C onto MIL-100(Fe) gel. The temperature was sufficient to break the energy barrier, which led to the transfer of an electron from S to Fe, resulting in the reduction of Fe(III) and the production of S₈ in the pores of MOF (Liu et al. 2017a). Zheng et al. (2020) presented a detailed study on the catalytic conversion of H₂S over three Fe-based MOFs, MIL-100, MIL-101, and MIL-53. The Fe-MOF showed superior H₂S conversion performance than commercial Fe₂O₃, where the trend: MIL-100 (100%) > MIL-101 (79%) > MIL-53 (77%) at 100 °C followed matched with the trend observed for surface area and the concentration of Lewis acid sites. The density functional theory calculations predicted larger H₂S adsorption energy (E_ads ~ 0.96 eV) with shorter Fe–S bond length (2.788 Å) for MIL-100 as compared to those observed for MIL-101 (E_ads ~ 0.31 eV, Fe–S ~ 2.806 Å), which further confirmed the stronger binding of H₂S onto MIL-100 (Zheng et al. 2020). In the context of adsorption-degradation duality, TiO₂@MIL-101(Cr) was developed for the UV-assisted photo-oxidation of H₂S. The TiO₂-coated MOF showed improved H₂S conversion efficiency with a higher degree of reusability without any regeneration (Sheng et al. 2017). Thus, catalytic or photocatalytic conversion of H₂S using MOFs are preferred methods for the desulfurization process.

Inorganic nitrogen compounds

Nitrogen oxides (NO and NO₂) are the primary pollutants responsible for the formation of photochemical smog, acid rain, and the destruction of the ozone layer. These gaseous pollutants are being added to the air from automobile combustion engines and thermal power plants. Adsorptive removal of NO_x is an unconventional approach, which is in the early stages of development. In the literature, reports on adsorptive removal of NO_x using metal–organic frameworks (MOFs) are limited. But discussing these published works seems reasonable to compare the NO_x removal efficacy of MOFs via adsorptive and catalytic processes. Peterson et al. (2016) reported significant improvement in the NO₂ adsorption capacity of UiO-66-NH₂ (930 mg g⁻¹) over UiO-66 (400 mg g⁻¹) in the dry conditions. In 80% relative humidity, the adsorption capacity reached 1400 and 610 mg g⁻¹ for UiO-66-NH₂ and UiO-66. Another feature of this study was the lower formation of NO during the adsorption process (4.5%) as compared to conventional active carbon (23%) (Peterson et al. 2016).

Li et al. (2019b) reported preferential and reversible adsorption of NO₂ over a Zn-based MOF, MFM-520, with a high degree of reusability. The in situ single-crystal X-ray diffraction study of NO₂@MFM-520 confirmed the dimerization of NO₂ to form N₂O₄, where each pore can accommodate one N₂O₄ molecule. The retention of N₂O₄ molecules in the pore was supported by sixteen hydrogen bonding (O^δ−···H^δ+ ~ 2.74 and 3.74 Å) interactions of aromatic –CH groups of linker and eight dipole interactions (N^δ+···O^δ− ~ 3.68 Å) of carboxylate O atoms with the N₂O₄ molecule. The NO₂-loaded MOF soaked in water for 10 min yielded a 0.09 M HNO₃ solution. Moreover, the MOF was fully regenerated and showed no performance loss for 125 cycles. Thus, this MOF-based process could be used as a powerful method for air clean-up and cheap production of HNO₃ (as opposed to conventional energy-intensive oxidation of NH₃) (Fig. 10a) (Li et al. 2019b).

Fig. 10

a Adsorption and conversion of NO₂ into HNO₃ over a transition metal-based metal–organic framework (MOF), b photocatalytic conversion of NO_x to HNO₃/HNO₂ over a MOF as a powerful method for air clean-up and cheap production of HNO₃

The selective catalytic reduction (SCR) of NO with NH₃ is an efficient post-treatment catalytic process to convert NO to N₂ for NO_x emission control. Though transition metal oxides are commonly used as catalysts for commercial applications, transition metal-based MOFs are expected to show even better results owing to a large surface area and porosity. Sun et al. (2018) studied the deNO_x efficacy of Ni-BDC in the selective catalytic reduction of the NH₃ process in the 100–500 °C range. More than 90% NO conversion was possible at a temperature of 275 °C, which decreased sharply after 440 °C due to the destabilization of the Ni-BDC framework (Sun et al. 2018). Better deNO_x performance was recorded for MnO_x@MIL-125(Ti), where the catalyst showed quantitative NO conversion in the range of 175–425 °C. The Mn oxide-MOF catalyst was a significant improvement over commercially used MnO_x/TiO₂ catalysts. The in situ diffuse reflectance infrared Fourier transform spectroscopy confirmed two kinds of NH₃ species on the surface of the MnO_x@MIL-125(Ti) catalyst, i.e., coordinated NH₃ on Lewis sites and NH₄⁺ on Brønsted acid sites. The adsorption of NO and O₂ led to the formation of NO⁻, NO₂⁻, and NO₃⁻ on the surface of MnO_x@MIL-125(Ti). The two species, NO₂⁻ and NO₃⁻, are active in the selective catalytic reduction reaction. Furthermore, the rapid disappearance of NO⁻ species in the presence of NH₃ predicted the formation of [NO⁻(NH₃)], as possible intermediates, which rapidly decomposed into N₂ and H₂O (Sun et al. 2019a).

The catalytic removal of NO was studied as a synergy between CuBTC and the non-thermal plasma (NTP) process. The non-thermal plasma modified CuBTC (CuBTC-P) showed ~ 98% NO removal as compared to 21 and 29%, respectively, for CuBTC and non-thermal plasma de-nitration process. The CuBTC-NTP synergetic method generated unsaturated Cu(II) sites, which were reduced to Cu(I) after electron capture during the non-thermal plasma discharge process. The primary process was reducing NO to N₂ by the high-energy electrons of non-thermal plasma and the Cu(I) active sites of CuBTC. The CuBTC-non-thermal plasma de-nitration synergetic method showed the merits of being effective at ambient temperature without reducing agents (as opposed to the conventional selective catalytic reduction method) (Gong et al. 2019b). Three iso-compositional and structurally different Fe-based MOFs, MIL-53, MIL-88B, and MIL-101, have been explored as potential photocatalysts for converting NO to HNO₃. Among the three MOFs, MIL-101 showed better NO conversion efficiency of 76, 61, and 61% under solar, UV, and visible radiation, respectively (Fig. 10b). Also, the MOF was found highly stable, where conversion efficiency was more than 70% for eight consecutive cycles (Nguyen et al. 2018). To some extent, the involvement of transition-based MOFs in the selective catalytic reduction processes for reductive removal of NO_x seems a reasonable choice in the mitigation of NO_x from the polluted air.

Ammonia (NH₃) emitted in large quantities from livestock breeding, fertilizer application, and industrial production is another form of inorganic nitrogen that is considered toxic to living beings. Ammonia reacts with SO₂, NO_x, etc., to form PM_2.5 (fine particulate matter with an aerodynamic diameter of < 2.5 μm), which has been linked to respiratory disorders in densely polluted countries. NH₃ molecules are basic and have a high affinity for Lewis or Brønsted acidic active sites such as unsaturated Lewis acidic open metal sites of transition metal-based MOFs. In the literature, most of the studies dealing with the adsorptive removal of NH₃ molecules are focused on using MOFs with 3d transition metals as nodes (Fig. 9c).

Reith et al. (2016) reported three isostructural azolate MOFs with Mn, Co, and Ni as metal nodes for the adsorptive removal of NH₃. The three MOFs showed significantly higher NH₃ uptake than UiO-66-NH₂ at 1 bar and 298 K. The comparatively higher NH₃ uptake by Mn₂Cl₂(BTDD)(H₂O)₂ suggested MOF structural rearrangement, which resulted in the opening of a second coordination site on the Mn ions. These MOFs maintained adsorption capacity for multiple cycles, even with the loss in surface area, suggesting good accessibility to NH₃-binding open metal sites (Rieth et al. 2016).

The study by Rieth and Dinca (2018) developed another series of isoreticular azolate MOFs containing Co, Ni, and Cu as the nodes and 1H,5H-benzo(1,2- d),(4,5-d′)bistriazole) (BBTA) as the linker. The BBTA series was a significant improvement over the larger-pore bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) (BTDD) series in the NH₃ uptake capacity. The smaller pore in BBTA series allowed increased hydrogen-bonding interactions of the metal-bound NH₃ molecules with the additional NH₃ molecules adsorbed in the pore, which is less likely to occur in the larger-pore BTDD series. The breakthrough performance of Co₂Cl₂BBTA in 80% relative humidity was nearly 45% lower than in dry conditions, which was still superior to conventional adsorbents like zeolites and AC (Rieth and Dinca 2018). A series of MOFs with terephthalic acid (BDC) as the linker, Cu(BDC), Zn(BDC), and Cd(BDC) was synthesized for the removal of low concentrates of NH₃ from polluted air. The adsorption of NH₃ onto MOF led to the structural changes, where 3D (Cu(BDC) and Cd(BDC)) and 2D (Zn(BDC)) MOFs transformed into 1D MOFs. The high polarity and coordination ability of NH₃ molecules with the metal nodes resulted in the formation of metal-NH₃ bonds at the expense of framework metal-linker bonds. The regeneration and reversibility of the framework were possible after 80 min of heating at 220 °C under vacuum. Though the overall regeneration process was energy-intensive, Cu(BDC) showed reusability for ten adsorption-regeneration cycles (Chen et al. 2020d).

Volatile aromatic compounds

Volatile aromatic compounds, including benzene and benzene homologs (toluene, xylene, ethylbenzene, etc.), are ubiquitous in the polluted urban atmosphere primarily released by chemical processing industries. These pollutants are difficult to degrade and linked to respiratory disorders and cancers in human beings. Even at low concentrations, these gaseous pollutants could cause human discomforts in the indoor environment. Thus, the olfactory threshold for benzene, toluene, and p-xylene has been set to 2.70, 0.33, and 0.06 ppm, respectively (Vellingiri et al. 2016b). Adsorption technology, with the aid of active carbon, amorphous silica, or zeolites, has been frequently adapted for the mitigation of benzene and its analogs (Kim et al. 2020). Though these adsorbents are cost-effective, limited surface area and binding sites have disfavored their adsorption efficacies. A major segment of metal–organic frameworks (MOFs) has one or more phenyl rings in their structural linkers, the phenyl ring of volatile aromatic compounds could interact with the MOF surface via π–π interactions similar to those observed in active carbon. Another strong interactive force favorable to the adsorption process is the cation-π interaction between the metal node of a MOF and the phenyl ring of volatile aromatic compounds. Though there are several volatile aromatic compounds of environmental interests, this subsection is limited to a discussion on benzene and toluene removal with some perspectives on other volatile aromatic compounds.

In the literature, reports are available on the adsorptive removal of several volatile aromatic compounds onto transition metal-based MOFs, MIL-101(Cr) (Yang et al. 2011), MOF-199(Cu) (Vellingiri et al. 2016b), and Cu-MOF (Yang et al. 2015). Yang et al. (2015) demonstrated exceptional volatile aromatic compounds adsorption capacity over a porous Cr-based MOF, MIL-101 (Yang et al. 2011). The maximum adsorption capacity was recorded for benzene (1291 mg g⁻¹), whereas the minimum was for m-xylene (727 mg g⁻¹). The odd adsorptive behavior was not supported by the polarity of the volatile molecules (Yang et al. 2015), which suggested that the cation-π interactions were not the driving forces in the adsorption of the compounds. The possible reason could be the matching of the molecular volume of volatile aromatic compounds with that of the cylindrical pore size and volume of MOF. Volatile aromatic compounds molecules are considered as rectangular solids shape, where benzene, toluene, ethylbenzene, and p-xylene have a plane diameter smaller than those of o-xylene and m-xylene. The volatile aromatic compounds molecule with smaller plane diameter could easily enter the pores of comparable size as compared to larger plane diameter molecules, which was the main reason behind significant adsorption of benzene due to the least diameter (Yang et al. 2011).

For Cu-MOF with azolate linkers, the larger-sized xylene or ethylbenzene was preferred more than the smaller-sized benzene and toluene, suggesting the absence of host–guest selectivity. One of the possible explanations for this behavior was the hydrophobic interactions between the alkyl-branched aromatics with the n-C₄H₉-filled pores of MOF (Yang et al. 2015). The adsorptive removal of benzene has been performed over CuBTC and MIL-101, where the adsorption capacity was 450 and 900 mg g⁻¹, respectively. Based on the enthalpy and performance considerations, it was established that CuBTC adsorbed benzene via strong interactions as compared to the MIL-101, but the later had higher adsorption capacity due to a larger cavity volume (Belarbi et al. 2019). MIL-(Cr)-Z1 with an adsorption capacity of 766 mg g⁻¹ in 80% relative humidity was a good adsorbent for the removal of benzene in humid conditions. The adsorption performance decreased as a function of increasing temperature, suggesting weak physical interaction of benzene onto the MOF (Zhu et al. 2017).

Vellingiri et al. (2016a) reported toluene adsorption onto six MOFs with higher adsorption capacity recorded for UiO-66(NH₂) and ZIF-67 (Fig. 9d). The hydrogen bonding between the methyl of toluene and –N–H bonds of UiO-66(NH₂) and ZIF-67 dominated over the π-π interactions. The involvement of physical forces in the adsorption process led to a lowering in the toluene adsorption capability of MOFs with the increasing temperature. MOFs other than UiO-66(NH₂) performed poorly in 25% relative humidity due to the competitive nature of water molecules with toluene for the binding sites. Based on the adsorption capacity and reusability, UiO-66(NH₂) was screened out as the best performing MOF in the study (Vellingiri et al. 2016a). Bahri et al. (2016) reported one of the highest adsorption capacities for toluene over MIL-101(Cr), where the larger surface area and mesoporous cavities favored the adsorption of toluene molecules. The 30% relative humidity seriously affected the performance of MIL-101 with a ~ 80% loss in the toluene uptake. MIL-53 had a good resistance against the humidity with adsorption capacity dipping to 692 mg g⁻¹ at 30% relative humidity from 730 mg g⁻¹ in dry conditions. In the case of MIL-101, Cr³⁺ metal sites preferably interacted with polar H₂O molecules, forming water clusters on the MOF surface and preventing the volatile aromatic compound molecules from reaching the binding sites. On the contrary, the interactions between H₂O and carboxylate and hydroxy groups via hydrogen bonding led to the MOF structure’s deformation. The flexible framework allowed the expansion of pores and compensated for the loss due to the pore filling of MOF with water molecules (Bahri et al. 2016). Besides adsorption, photocatalytic degradation is another powerful method for the remediation of volatile aromatic compounds. Zhang et al. (2016) reported visible-light-induced degradation of toluene over NH₂-MIL-101(Fe). The MOF photocatalyst adsorbs in the entire UV/visible range with a bandgap of 1.32 eV. The photocatalyst was able to degrade ~ 80% of toluene in 600 min, which was considerably faster than P25 Degussa, which was due to the organic-linker-to-metal-cluster charge transfer in MOF, increasing the photo-charge separation efficiency (Zhang et al. 2016).

Li et al. (2020) proposed adsorption–oxidation removal of toluene using MIL-88B. In the first step, MOF was used as an adsorbent and adsorb 100% of 460 ppm toluene gas. In the second stage, the MOF acted as a photocatalyst and converted 85% of the adsorbed toluene into CO₂ under a 100 mW cm⁻² solar irradiation within 60 min. The MOF showed similar adsorption–oxidation performances for benzene and p-xylene. Besides good conversion efficiency, the MOF showed remarkable reusability with only a 6% loss in the performance after 100 adsorption–oxidation cycles (Li et al. 2020). In an unconventional approach, TiO₂ nanoparticles-incorporated NH₂-UiO-66 was studied for the photocatalytic degradation of styrene under visible light. After 600 min of illumination, the MOF degraded ~ 99% of styrene with a mineralization efficiency of 35% (Yao et al. 2018). The mineralization efficiency in the foremost factor in judging the applicability of the photocatalytic process. Though visible illumination is partially effective in mineralization, the evolution of the metal oxide-integrated MOF hybrids could resolve the issue.

Volatile sulfur-organic compounds

Representative volatile sulfur-organic compounds, including methyl mercaptan, dimethyl sulfide, and dimethyl disulfide, are colorless and odorous gases. These can cause skin and eye irritation in humans at a very low concentration and could be lethal if exposed to a high concentration. The emission of volatile sulfur-organic compounds is primarily linked to industrial sources like oil refineries, pharmaceuticals, tanneries, and paper mills. According to Pearson’s hard-soft acid–base theory, volatile sulfur-organic compounds (soft bases) preferentially interact with soft Lewis acid sites (Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺) via acid–base interactions. In the literature, multiple reports have been dedicated to the use of Cu-based metal–organic frameworks (MOFs), especially MOF-199 for the adsorptive removal of volatile sulfur-organic compounds (Li et al. 2015b; Ma et al. 2016; Deng et al. 2018). Deng et al. (2018) utilized chemically and thermally activated MOF-199 for the adsorption of volatile sulfur-organic compounds in single and multi-component studies. The best performance was shown by chemically and thermally treated MOF-199-T2, where the adsorption capacity for methyl mercaptan, dimethyl sulfide, and dimethyl disulfide was 14.6, 10.6, and 21.2 mg g⁻¹, respectively. The low energy requirement for the breaking of C–S (259 kJ mol⁻¹) and S–S (266 kJ mol⁻¹) bonds as compared to the C–H bonds (347 kJ mol⁻¹) and the high affinity of Cu sites to form CuS favored the adsorption of dimethyl disulfide over other volatile sulfur-organic compounds in a multi-component study. Moreover, the presence of two methyl groups and larger size promoted the van der Waals interactions between dimethyl disulfide molecules and the aromatic groups of MOF. These interactive forces and the preferential adsorption of dimethyl disulfide were further validated by the density functional theory calculations (Deng et al. 2018). Li et al. (2015b) reported ethyl mercaptan and dimethyl sulfide adsorption over MOF-199. The deconvoluted X-ray photoelectron spectra of Cu 2p and S 2p confirmed the presence of Cu–S–C₂H₅ species and the formation of CuS, which was visible as the MOF turned black (Li et al. 2015b). Similar results were observed for methyl mercaptan adsorption onto MOF-199. The MOF showed significantly better methyl mercaptan adsorption performance than the commercial AC, where the maximum adsorption capacity of 75 mg g⁻¹ was recorded for 5 ppm methyl mercaptan concentration (Ma et al. 2016).

Wang et al. (2019a) developed a reduced CuBTC (Cu(I)/Cu(II)-BTC) to remove dimethyl disulfide with the adsorption capacity of 147 mg(S) g⁻¹. The density functional theory calculation predicted higher interaction energy of dimethyl disulfide with the Cu(I)/Cu(II)-BTC structure (70.65 kJ mol⁻¹) than the parent Cu(II)-BTC (20.28 kJ mol⁻¹), confirming stronger interaction of dimethyl disulfide with Cu(I) sites (Wang et al. 2019a). Other than Cu-based MOF, Zn-based MOF, IRMOF-3 (Wang et al. 2014), and HKUST-1 (Ma et al. 2018) have been explored for the adsorption of volatile sulfur-organic compounds. Wang et al. (2014) studied ethyl mercaptan and dimethyl sulfide adsorption onto IRMOF-3 in a fixed-bed reactor. The adsorption capacities decreased as a function of increasing temperature, suggesting the involvement of physical forces in the volatile sulfur-organic compounds adsorption process. The physisorption of volatile sulfur-organic compounds was further confirmed by X-ray photoelectron spectroscopy, where the peaks associated with ZnS (161.7 eV) and elemental S (164.4 eV) were absent, and the “S” peaks corresponding to volatile sulfur-organic compounds were observed (Wang et al. 2014).

Chemical warfare agents

Though a sizable population in the world may never experience chemical warfare agents in their lifetime, these toxicants have been given the utmost priorities in war-torn nations. The Geneva protocol of 1925 prohibited the use of these compounds, but could not prevent the development and stockpiling of these weapons of mass destruction. As preventive measures against the use of chemical warfare agents in wars, accidental leakage of chemical warfare agents in production units, and for the destruction of existing stockpiles, several effective detoxification strategies have been proposed. The adsorptive or catalytic removal of these hazardous compounds is explored as these methods are significantly fast and effective. In the literature, metal–organic frameworks (MOFs) have been studied both as adsorbents and catalysts for chemical warfare agents´ removal on many occasions, which deserves a brief discussion in this review. Soman, sarin and o-ethyl S-[2-(diisopropylamino)ethyl]-methylphosphonothioate are organophosphate compounds, which are considered as nerve agents. The systematic molecular simulation study by Agrawal et al. (2018) on the adsorptive interactions of simulants (dimethyl methylphosphonate DMMP and dimethyl p-nitrophenyl phosphate) and real (soman and sarin) chemical warfare agents with ~ 3000 different MOFs. The study confirmed that simulants are as good as chemical warfare agents for adsorption study (Agrawal et al. 2018).

In the literature, most of the papers published on the removal of chemical warfare agents simulants are centered on the use of Zr-based MOFs, especially parent and analogs of UiO-66 and UiO-67 (Ploskonka and DeCoste 2019; Ruffley et al. 2019; Wang et al. 2019c). Ploskonka and DeCoste (2019) studied multiple G-nerve agents and stimulants hydrolysis onto four Zr-MOFs, UiO-66, UiO-67, NU-1000, and PCN-222 in the N-ethylmorpholine buffer solution (pH 10). All the four MOFs showed faster hydrolysis of sarin with a half-life (t_1/2) in the range of 2–7 min. The dimethyl p-nitrophenyl phosphate hydrolysis was comparatively slow (t_1/2 ~ 17–19 min) and reached 100% within 120 min. The adsorption of G agents or simulants onto UiO-66 and UiO-67 occurred at uncoordinated Zr sites via organophosphate group of the adsorbate (Ploskonka and DeCoste 2019).

In another recent study, Harvey et al. (2019) employed density functional theory-calculated infrared spectroscopy to resolve the binding sites of sarin molecules onto UiO-66. Though the study predicted three additional binding sites, ZrOH, fully coordinated Zr site, and μ₃-OH for sarin molecules, the under-coordinated Zr was the preferred sites for sarin adsorption (Harvey et al. 2019). The hydrolysis was facilitated through hydrogen bonding between the bridged hydroxyl group and the leaving group of the adsorbate molecule. A water molecule hydrogen-bonded to a neighboring Zr–OH initiated a nucleophilic attack on the P, forming P–OH and the leaving P–X. The P–X bond’s scission was done by the Brønsted acidic nature of the neighboring Zr–OH₂ site. A similar mechanism was proposed for NU-1000 and PCN-222, but with the addition of another pathway. The terminal –OH on the same Zr atoms on which the organophosphate interacted was utilized in the hydrolysis process (Ploskonka and DeCoste 2019). The theoretically calculated energy of terminal displacement of Zr–OH₂ groups by the sarin molecules by Momeni and Cramer highlighted the importance of the displacement process in the overall reaction kinetics. The study predicted faster kinetics for UiO-66 than NU-1000, which was experimentally observed in the study (Momeni and Cramer 2018).

The role of moisture on the hydrolysis of dimethyl p-nitrophenyl phosphate was probed by Wang et al. (2019c), where UiO-66, UiO-66-NH₂, and NU-1000 wetted with different amounts of water were tested for dimethyl p-nitrophenyl phosphate hydrolysis. Overall, the water loading favored the hydrolysis process, where higher activity and faster kinetics were recorded for MOFs soaked with 400 wt% water (Wang et al. 2019c). Ruffley et al. (2019) reported the adsorption of dimethyl methylphosphonate onto differently functionalized UiO-67-X, with X as −H, −CH₃, −SH, −NH₂, −N₃,–NO₂, −Br, and −Cl. Based on the density functional theory calculations, UiO-67-NH₂, UiO-67-CH₃, and UiO-67 showed strong, intermediate, and weak binding with dimethyl methylphosphonate, respectively (Ruffley et al. 2019).

Bis(2-chloroethyl) sulfide, commonly known as mustard gas, was used during World War I. It is a notorious chemical warfare agent with cytotoxic, vesicant, and blistering effects on exposed skin. The extent of damage reported in World War I, continuous use of this chemical warfare agents in the post-world war era, and a large stockpile of bis(2-chloroethyl) sulfide in all major countries have forced researchers to develop remediation strategies to counter the damaging effects of bis(2-chloroethyl) sulfide in the future. The safety guidelines prohibit the use of bis(2-chloroethyl) sulfide in the laboratory scale, and for this reason, studies using 2-chloroethyl ethyl sulfide as a bis(2-chloroethyl) sulfide simulant have been reported in the literature. Buru et al. (2020) developed H₅PV₂Mo₁₀O₄₀-incorporated NU-1000 activated by heat (PV₂Mo₁₀@NU-1000–80 °C) and supercritical CO₂ (PV₂Mo₁₀@NU-1000-scCO₂). In the study, though 100% 2-chloroethyl ethyl sulfide conversion was possible with PV₂Mo₁₀@NU-1000–80 °C and PV₂Mo₁₀@NU-1000-scCO₂, the reaction rate was slow, and instead of forming non-toxic singly oxidized 2-chloroethyl ethyl sulfoxide, doubly oxidized 1-chloro-2-(ethylsulfonyl)ethane was majorly formed as the oxidation product in all experimental cases (Buru et al. 2020). The same group reported another polyoxometalate (H₃PW₁₂O₄₀)-incorporated NU-1000 (PW₁₂@NU-1000) for the oxidative conversion of 2-chloroethyl ethyl sulfide. The MOF (t_1/2 ~ 13 min) and H₃PW₁₂O₄₀ (t_1/2 ~ 5 min) showed selective toward 1-chloro-2-(ethylsulfonyl)ethane and 2-chloroethyl ethyl sulfoxide products, respectively. The heated MOF-polyoxometalate hybrid (PW₁₂@NU-1000-120 °C) decreased the t_1/2 value to 3 min with intermediate 2-chloroethyl ethyl sulfoxide selectivity of ~ 59%. Supercritical CO₂-activated MOF-polyoxometalate hybrid (PW₁₂@NU-1000-scCO₂) had t_1/2 of 1 min and 90% preference to 2-chloroethyl ethyl sulfoxide. This variation in the selectivity could be due to the involvement of different mechanisms occurring on the MOF node or the polyoxometalate. An easy diffusion of the substrate to polyoxometalate in mesopores and rapid oxidation of sulfide led to the selective formation of singly oxidized products (Buru et al. 2018).

Buru et al. (2018) studied photocatalytic oxidation of 2-chloroethyl ethyl sulfide using porphyrin- (PCN-222), pyrene- (NU-1000), and perylene- (UMCM-313) based MOFs under light-emitting diode. PCN222, NU-1000, and UMCM-313 had t_1/2 value of 11, 6, and 4 min, respectively, in the bulk state, which was following the measured singlet oxygen quantum yield, Φ(¹O₂)^* of the corresponding linkers (porphyrin ~ 0.38, pyrene ~ 0.40, and perylene ~ 0.45). The higher Φ value for a photosensitizer decreased the half-life of the 2-chloroethyl ethyl sulfide oxidation reaction. All three MOFs were found selective and generated the singly oxidized product, which is considered crucial in the implementation of MOFs in mustard gas oxidation. The thin films of MOF showed delayed photo-oxidation of 2-chloroethyl ethyl sulfide, but the turnover frequencies were ten times higher than those observed in the bulk MOF. The increased activity was linked to the lower MOF loading and greater MOF exposure to the incident radiation (Buru et al. 2018). The photo-oxidation of 2-chloroethyl ethyl sulfide is fast in pure O₂ and gets significantly delayed in the air (Liu et al. 2015). The faster oxidative 2-chloroethyl ethyl sulfide conversion over MOFs in the presence of air makes the process convenient from a practical standpoint. Zhang et al. (2018) reported photo-oxidation of 2-chloroethyl ethyl sulfide onto photoactive UiO-68-TBTD. The MOF was faster in selective oxidation of 2-chloroethyl ethyl sulfide to 2-chloroethyl ethyl sulfoxide (t_1/2 ~ 3 min) in the air than H₂-TBTD (t_1/2 ~ 6.5 min) (Zhang et al. 2018). This study was a significant improvement over the previously published one on the grounds of practicality and reusability.

Since protective gears like masks and suits are deployed to save humans from the toxic effects of chemical warfare agents, MOFs like UiO-66-NH₂ integrated into fibers (Dwyer et al. 2018; Kim et al. 2018; Chen et al. 2019b; Shen et al. 2019) or self-supported filters (Liang et al. 2018a) have been explored for the purpose. Liang et al. (2018a) developed Zr-MOF filters by growing UiO-66, UiO-67, and UiO-66-NH₂ onto the ZrO₂ nanofiber mat as the source of Zr ions. The orchestrated Zr-MOF filters were evaluated for the catalytic hydrolysis of chemical warfare agent surrogate dimethyl p-nitrophenyl phosphate in an N-ethylmorpholine buffer solution (pH 10). In 60 min, UiO-66-NH₂ and UiO-67 showed 100% dimethyl p-nitrophenyl phosphate conversion to p-nitrophenoxide, whereas it was 90% for UiO-66. The t_1/2 values were in the order of UiO-66-NH₂ (1.9 min) > UiO-67 (6.6 min) > UiO-66 (21.3 min). The faster degradation kinetics of UiO-66-NH₂ was due to the involvement of amine-group (Brønsted base) in the nucleophilic attack on the soft carbon center of dimethyl p-nitrophenyl phosphate molecules (Wang et al. 2019c). Moreover, the larger pore size of UiO-67 favored the diffusion process and allowed easy access of active sites to dimethyl p-nitrophenyl phosphate molecules (Liang et al. 2018a).

Shen et al. (2019) integrated UiO-66-NH₂ in cellulose sponge for the catalytic hydrolysis of dimethyl p-nitrophenyl phosphate in an N-ethylmorpholine buffer solution. The MOF-cellulose sponge and MOF powder showed similar dimethyl p-nitrophenyl phosphate conversion performance after 100 min, where the t_1/2 value was 8 and 9 min, respectively. Apart from being good catalysts, the sponge showed high mechanical strength until 32 kPa (Shen et al. 2019). UiO-66-NH₂ was successfully grown on the surface of cotton fabric, which was used for the adsorption and hydrolysis of soman. In a more direct approach, the ability of MOF-cotton fabric to withstand chemical warfare agents was tested using an aerosol vapor–liquid assessment group (device to analyze military protective fabrics). The study reported a 20-fold and 10-fold dip in the soman and bis(2-chloroethyl)sulfide permeability of MOF-cotton fabric compared to the cotton fabric. Moreover, the conversion of toxic soman to non-toxic methylphosphonic acid was feasible (Kim et al. 2018). Thus, it is possible to integrate such MOF-fiber hybrids in chemical warfare agent resistant protective suits. The more realistic situation in battlefields requires the destruction of chemical warfare agents without the need for water and toxic organic bases. The possible replacement of N-ethylmorpholine is a solid-phase base like polyethyleneimine. The Zr-MOF/polyethyleneimine composites were developed by a simple vortex mixing of MOF powder and polyethyleneimine particles.

The MOF-808/polyethyleneimine showed a quantitative conversion of dimethyl p-nitrophenyl phosphate under 50% relative humidity in 960 min (t_1/2 ~ 96 min). The possible real-world application of MOF/polyethyleneimine composites was further explored by coating MOF-808/polyethyleneimine composite on a flexible fiber. The kinetics of dimethyl p-nitrophenyl phosphate conversion was even faster on the MOF-808/polyethyleneimine/fiber, where in the same experimental condition; the complete conversion was possible within 240 min (t_1/2 ~ 24 min). The MOF-808/polyethyleneimine/fiber completely hydrolyzed soman and o-ethyl S-[2-(diisopropylamino)ethyl]-methylphosphonothioate within 60 min. The MOF/polyethyleneimine/fiber showed remarkable tolerance in different conditions experienced in a warzone like wet and dry perspiration, CO₂-rich air, gasoline-rich atmosphere, and after exposure to air for 100 days. The overall impression of this study was that the MOF/polyethyleneimine/fiber could be successfully integrated into protective suits (Fig. 11) (Chen et al. 2019b).

Fig. 11

Hydrolysis of dimethyl p-nitrophenyl phosphate in N-ethylmorpholine buffer over metal–organic framework (MOF)-fiber composite and destruction of 2-chloroethyl ethyl sulfide over MOF-fiber composite in O₂/blue light-emitting diode system. Atom color legend: C, gray; N, blue; O, red; S, orange; P; green, Cl, light green; H, white

Challenges and perspectives

The challenges being faced by old and new technologies are the driving force for the development of new functional materials that can eliminate toxicants present in water and air. Metal–organic frameworks (MOFs) are a sub-class of inorganic–organic hybrids with notable and advantageous physicochemical properties suitable for capturing pollutants and degrading them into non-toxic by-products by different chemical reactions. Though the commercialization of MOFs for remediation of contaminated water and air is in the infancy period and need proper exploration, the literature has been enriched with numerous related publications, which have been reviewed here.

Transition metal-based MOFs in the form of monometallic MOFs, bimetallic MOFs, and composites have the potential to eliminate toxic organic pollutants from wastewater of varying origins. These are novel catalysts for degrading and mineralizing organic pollutants under low power UV/visible radiations, and ultrasonication, which make these materials energy-efficient. However, the possible use of MOFs in wastewater remediation requires high structural stability in the operating conditions. Though it has been claimed that MOFs are stable in a wide pH range, such reports are scarce and cannot be generalized and applied to all MOFs. Likewise, their stability against oxidants and radiation sources needs to be addressed along with the reusability. The best possible way to evaluate MOF’s structural integrity is to subject them in acidic/basic solutions, oxidants, and radiation sources and then compare the X-ray diffraction patterns before and after exposure (Mateo et al. 2019). The use of toxic heavy metals (Cd, Cr, and Co), organic linkers (4,4-bipyridine and trimesic acid), solvents (N´N-dimethylformamide, chloroform, and acetone) and probable toxicity of synthesized nanoscale MOF crystals could be a roadblock in their commercialization for wastewater treatment (Kumar et al. 2019). Another issue that is commonly ignored in these research works is the cost of fabricating MOFs. Since there is no good estimation of MOF’s manufacturing cost, it is not easy to estimate and compare the cost-to-performance ratio with other conventional materials. Though limiting the number of organic linkers for MOF synthesis is not feasible, the least toxic transition metals like Fe and Zn can be prioritized. Moreover, toxic solvents can be fully removed by adopting solvent-free synthesis. Since the synthesis of MOF via solvent-free routes like liquid-assisted grinding and aqueous synthesis are comparatively cheaper ($13–36 kg⁻¹) than organic solvent (solvothermal) synthesis ($35–71 kg⁻¹), following alternative solvent-free routes could reduce the cost of production (DeSantis et al. 2017). Many of these studies also lack screening of degradation by-products and their toxicity and column studies with real wastewater from industrial and domestic sources. Another pitfall is that some of the pollutants like pesticides are toxic even at μg L⁻¹ concentrations, and the studies should focus on achieving the minimum acceptable limit of such pollutants after the treatment process. The practical exploitation of MOFs needs a complete understanding of the reaction pathways and their applicability in real wastewater and needs to be explored in great length from commenting on its applicability in wastewater treatment facilities. Nevertheless, the field is in the flowering stage, and future works could help in deciding MOFs applicability in wastewater treatment.

MOFs were first studied for the capture and storage of nonpolar gases. So, application of MOFs in adsorptive or catalytic removal of inorganic and organic gaseous pollutants is evident. The transition metal-based MOFs, especially Cr- and Cu-based MOFs, have shown high adsorption capacity for toxic gases with high selectivity against CO₂ and CH₄. In the literature, more studies are focused on the adsorptive removal and less on the catalytic removal of gases. The high adsorption of gaseous pollutants at ambient conditions is favorable only if the process is reversible and repetitive for hundreds of cycles for economical use of the porous adsorbents. Though a handful of works are focused on discussing the reusability issues, many researchers chose not to report, which makes it difficult to compare the advantages of adsorption over catalysis. The catalytic removal of gaseous pollutants via thermal or photolytic reactions over MOFs is a more suitable approach. Some of the works have confirmed the significant conversion of toxic pollutants like NO_x into nitric and nitrous acid under the effect of solar or UV-irradiation. Also, the selective catalytic reduction of NO with NH₃ or thermal degradation of volatile organic sulfur compounds to yield S₈ makes MOFs, versatile catalysts for air detoxification. Though the MOFs have many advantages to highlight for gaseous pollutant removal, the materials are thermally unstable and degrade even at a low temperature of 300–400 °C. The thermal stability of MOFs can be improved by increasing the coordination forces with a proper choice of organic linkers and metal nodes. Moreover, metal nodes of MOFs, especially those containing transition metals, are hydrophilic and coordinated with water molecules, which lowers the active site density and compromises with the structural integrity by hydrolysis reactions. Developing hydrophobic MOFs by choosing hydrophobic linkers and grafting oleophilic functionalities on the open metal sites can prevent the inflow of water molecules, which will enhance the performance. Moreover, the MOF stability should be evaluated by keeping MOFs in different atmospheric conditions to see the deterioration of MOFs under atmospheric oxygen (Du et al. 2019). So far, the studies on the air detoxification are limited to answering whether a MOF can remove a particular gaseous pollutant, and if yes, then to what extent? The future research works should address mineralization efficiency, the toxicity of by-products, reusability, and MOF stability. Other than that, the practical use of MOFs could be indoor air decontamination, which requires gas-phase studies at ultra-low concentrations (ppm) for many pollutants as the United States Environmental Protection Agency limit for such pollutants is in the ppb range.

Good progress has been made in evaluating MOFs for protection against chemical warfare agents as adsorbents or catalysts. Chemical warfare agents are a special class of toxic pollutants and need unique and expensive methods for removal. MOFs blended with natural or synthetic fibers, and their use in protective gear and suits will be a humanitarian effort in saving countless lives in war-torn nations. The studies dealing with MOFs for chemical warfare agents´ removal have addressed many drawbacks of efficacy in different environmental conditions, regenerations, the toxicity of by-products, reusability, and possible testing in real conditions (using Aerosol Vapor–Liquid Assessment Group assessment). The overall impression is that there are higher chances of commercialization of MOFs in the military than in civilian applications. Every possible assumption of the commercialization of MOFs is based on the cost of fabrication, low eco-toxicity, and high efficacy without relying on secondary chemicals or processes. Therefore, looking for economic precursors is essential for an affordable method for MOF fabrication. The least-step approach toward finding an alternative for a multi-step process for the development of a MOF could be a cost-cutting alternative. A more systematic evaluation of MOF toxicity to humans and other living beings may be another convincing approach for betting over MOFs in the wastewater treatment process. Though the present development of MOFs for environmental applications is limited due to shallow understanding, MOF’s role in the future is highly probable. We believe that in the coming years, transition metal-based MOFs could be an alternative to pre-existing adsorbents and catalysts and integrated into commercial air and gas decontamination processes.

The future of MOFs is dependent on the ease with which the synthesis technology could be transferred from academic/laboratory‐scale to a large industrial scale. Methods for industrial production of MOFs should be scrutinized on various parameters, including cost, availability of raw materials, technical simplicity, yields, purity and quality of synthesized MOFs, and environmental impact of the process. The quality of the adopted methodology is commonly evaluated by the space‐time yield of the synthesis reaction. Space‐time yield is defined as the mass of a MOF produced in kilogram per cubic meter of the reaction mixture in one day of synthesis. Methods with high space‐time yield values are considered best for large-scale production of MOFs. The space‐time yield values of popular MOFs synthesized using different protocols have been discussed in the literature (Rubio-Martinez et al. 2017). There is no point in developing materials with a poor environment footprint for environmental applications. For this reason, along with parameters like space‐time yield, hidden and largely ignored parameter, i.e., environmental footprint should be considered before commercializing the process. Many popular MOFs like HKUST-1, MOF-5, UiO-66 series, ZIF-67, and ZIF-8 manufactured by Badische Anilin und Soda Fabrik, MOF Apps, and MOF Technologies are commercially available on Sigma-Aldrich (Rubio-Martinez et al. 2017). To date, high cost is the most important factor that is preventing the MOFs from replacing conventional adsorbents for environmental applications. But, with the advancement of the field, new players are entering the market for the commercialization of lesser-known MOFs and will certainly introduce MOFs up to their use for real-world environmental applications.

Conclusion

This review deals with the environmental applications of metal–organic frameworks (MOFs) for air and water detoxification. The large surface area, tunable porosity, and ease of functionalization make the chemistry and applications of MOF-based materials attractive to materials scientists. Thus, the development of new photocatalysts and gas adsorbents are being boosted by the tailored synthesis of novel coordination polymers where the metallic node and the organic linker are chosen to improve the final application of the MOF. The most updated synthetic routes for transition metal-based MOFs are discussed. The electron-transfer properties of the metallic node and the organic linker define the photocatalytic activity of a MOF. The most commonly used transition metals are Fe, Cu, Cd, and Fe, whereas aromatic polycarboxylates and porphyrins are the preferred ligands. Combinations of the mentioned metals/ligands are proven to be the most effective for photodegradation of organic pollutants, which is driven by an adequate light-harvesting capacity, low recombination rate, and large surface area. At the same time, the use of two metals instead of one as metallic nodes increases the pore size and thus favors the gas adsorption process. Also, bimetallic MOFs are relatively stable and could act as reusable photocatalysts/adsorbents. The MOF-based materials exhibit high adsorption capacity for a large variety of inorganic and organic gaseous pollutants. MOF’s commercialization is challenging due to its high production cost, but the growing number of business enterprises could pave the path for large-scale synthesis of MOFs-based materials. The positive approach toward finding technologies to integrate MOFs in the consumer lives will be the driving force toward the commercialization of MOFs for environmental applications.