Progress of MOFs and their derivatives for mitigating air pollution

Abstract

The post-industrial revolution era has witnessed unprecedented economic and technological growth, leading to a significant surge in population and industrial advancements. However, this rapid progress has been accompanied by a concerning increase in environmental degradation, resulting in mass extinctions and posing a serious threat to both ecosystems and human health. Addressing these pressing challenges requires innovative solutions. Metal-organic frameworks (MOFs), which are crystalline structures made of metal ions or clusters interwoven with organic ligands, are one intriguing technique. MOFs have gotten a lot of interest because of their amazing specific surface area, tunable pore size, and adaptability. Their development holds significant potential for mitigating industrial waste gas emissions and improving environmental quality across various applications. This comprehensive review delves into the pivotal role of MOFs in air purification. Beginning with an exploration of the hazards, origins, and complexities of haze, the review meticulously examines the applications of MOFs in addressing various pollutants, including SO₂, NO_x, PM_2.5, automobile exhaust, coal-fired flue gas, fuel emissions, and incineration byproducts. Each section provides insight into design principles, adsorption mechanisms, and transformation processes for effective pollutant mitigation. Overall, this review demonstrates an array of effective and environmentally sound technical methodologies, underscoring the pivotal role of MOFs in combating multifaceted air pollution. It serves as a valuable resource for researchers and practitioners seeking sustainable solutions to complex environmental challenges.

Graphical abstract

1 Introduction

1.1 The harm of haze

Although the overconsumption of natural resources has contributed to industrialization and economic development, resulting in significant material progress, the environmental costs have been high, especially in terms of atmospheric changes, which have caused serious problems [1]. The use of fossil fuels like coal and oil has led to the regular occurrence of smog events that draw attention from all around the world [2]. Haze pollution is characterized by fine particulates matter and reduced visibility to less than 10 km. The impact of smog pollution has far-reaching implications, including adverse effects on traffic [3, 4], human health [5] (Table 1), infant stunting [23], and environmental quality [24]. Research has indicated a clear correlation between air pollution and the quality of human life [25,26,27,28].

Table 1 Effects of air pollution on human health

The issue of smog can be traced back to the industrialization of developed nations like the United Kingdom and the USA [29]. In a study by Leleveld et al. [30], global satellite observations, ground-based observations, and atmospheric chemistry models were employed to analyze the impact of worldwide atmospheric pollution sources on premature mortality. It was discovered that the usage of solid fuels for cooking and heating results in indoor air pollution, which claims the lives of almost 3.54 million people per year. The primary cause of death in China is air pollution, which causes more deaths than traffic accidents and HIV combined. In Southeast Asia, India has the highest mortality rate, with 650,000 premature deaths yearly.

1.2 Causes of haze

Atmospheric visibility is intrinsically linked to PM_2.5 particle concentration in the air. Smog, a phenomenon that results in impaired visibility, is primarily caused by sulfates, ammonium, and nitrates carried by PM_2.5 particles. Both natural and man-made processes, such as the burning of fossil fuels or biomass, industrial processes, and automobile emissions, can produce these particles. With a cycle ranging from days to weeks, PM_2.5 particles can travel thousands of kilometers, making them a significant contributor to haze formation. The small size of PM_2.5 particles enables them to scatter light efficiently, which significantly impacts visibility. The photochemical oxidation of volatile organic compounds (VOCs) in metropolitan settings, where emissions of VOCs from biogenic and anthropogenic sources are large, produces various semi-volatile and non-volatile chemicals that are essential to the conversion process of gas into particles. The vapor phase oxidation of volatile organic molecules by free radicals such as OH, NO₃, or ozone further exacerbates the issue.

During the daytime, the oxidation of VOCs induced by hydroxyl radicals (OH) results in the formation of various organic products with oxygen-containing functional groups. These products include aldehydes, ketones, alcohols, carboxylic acids, hydrogen peroxide, percarboxylic acids, and peroxyacylnitrates [31]. The initial oxidation step generates some compounds that undergo subsequent oxidation processes. This leads to multigenerational products with lower volatility and higher solubility than the parent chemical. In urban environments, aromatics such as toluene, xylene, and trimethylbenzene account for a substantial portion (over 20%) of all volatile organic chemicals [32, 33]. Human activities are the primary cause of aromatic hydrocarbon emissions, particularly from automobiles, fuel-powered vehicles, and industries. Toluene concentration in urban areas may range from 1 to 200 pounds, depending on the source and location of emissions [32,33,34]. Aromatic hydrocarbons typically have a short lifespan of 0.5 day to 2.5 days and are not reactive with OH free radicals. However, when OH aromatic reaction occurs, it can produce two types of chemicals: ring-opening chemicals such as glyoxal and methyl glyoxal, and ring-preserving chemicals such as benzaldehyde and cresol [35]. Regardless of the initial toluene level, it has been noted that under low nitrogen oxides (NO_x), circumstances, the oxidation product of toluene-OH on pre-existing ammonium sulfate aerosol produces 30% of the secondary organic aerosol. Aromatic oxidation also affects black carbon (BC) particles, altering their composition and increasing their density, hygroscopicity, and optical qualities.

1.3 Application of metal-organic framework and their derivatives in air pollution control

Air pollution levels vary across regions and cities due to differences in natural and climatic conditions, topography, air pollution sources, and energy consumption. While national policies play a significant role, local measures need to be implemented to address specific air pollutants [36, 37]. Sustainable development in the environment includes the protection of biodiversity and the reduction of atmospheric C0₂ concentrations and pollutants [38]. Despite numerous studies on catalysts since the 1990s, removing haze under lean operating conditions remains a significant challenge. In China, studies have been conducted to enhance high velocity catalytic activity [39, 40]. In the realm of environmental catalysis, the removal of methane from compressed natural gas vehicle exhaust is a significant challenge due to its resistance to conventional three-way catalysts. Metal-organic framework (MOF) possess a highly porous structure and an enormous specific surface area, enabling them to effectively adsorb various air pollutants, including fine particulate matter (PM_2.5 and PM₁₀), VOCs, NO_x, and sulfur dioxide (SO₂). The pore size of MOFs can be controlled by selecting different organic ligands, allowing them to adsorb pollutants of different sizes. Due to their structural stability, MOFs can maintain excellent adsorption performance under harsh environmental conditions, making them suitable for use as catalysts for automotive exhaust purification, converting harmful gases into harmless substances. This property is particularly well-suited for industrial waste gas and exhaust purification applications. Furthermore, due to the diverse composition and tunable structure of MOFs, they are considered suitable precursors for the preparation of porous materials. They are typically prepared through thermal decomposition or solution infiltration under specific conditions. By precisely controlling the precursor and synthesis process of MOFs, it is possible to prepare derivative materials with more diverse pore sizes, appearances, compositions, and properties than the precursor material itself. The main methods for preparing these derivative materials include direct thermal decomposition of MOFs [41], co-thermal decomposition with other precursors [42], and thermal decomposition of composite materials [43], as well as solution infiltration followed by heat treatment [44]. Researchers interest in MOFs materials has grown due to their potential to help with air purification and pollution management in this setting (Table 2).

Table 2 Separation of atmospheric pollutants by MOF catalysts

2 Adsorption and conversion of sulfur dioxide using MOFs

Sulfur dioxide is a prevalent industrial exhaust gas that threatens both the environment and human health. Scientists have made significant progress in developing a range of MOFs adsorbents, including MOF-74, MFM-300, MFM-601, MOF-160, Ni8(L)6, and UiO-66, which can effectively adsorb and convert sulfur dioxide. These MOFs materials exhibit automatic air purification and separation, allowing them to treat various harmful gases and exhaust emissions. The continued development of MOF materials is essential to reducing industrial exhaust emissions and enhancing environmental quality. It is imperative to note that air pollution can have a profound impact on the Earth natural cycles, particularly the sulfur cycle [31]. The atmosphere is naturally exposed to sulfur dioxide through various sources such as volcanic eruptions, spring water, air currents, organic decomposition, and anaerobic bacteria. However, significant quantities of this gas are released into the atmosphere through human activities such as fossil fuel burning, food production, transportation, and activities in the oil industry. The increasing levels of sulfur dioxide in the atmosphere have resulted in severe damage to ecosystems and loss of biodiversity, and pose a significant threat to human health. Inhalation of sulfur dioxide through the lungs and skin can result in respiratory illnesses and fatalities, especially when concentrations reach up to 100 ppm [82]. Once released into the atmosphere, SO₂ readily dissolves in water to form sulfuric acid (H₂SO₄) in contact with oxygen. These acids cause soil and water pollution through wet sedimentation, affecting the growth of forests and crops. In most urban areas, the formation of PM_2.5 is associated with SO₂.

2.1 MOFs design for SO₂ adsorption

MOFs are a family of crystal materials made up of organic ligands and metal ions or clusters. These structures have a large surface area, adjustable pore sizes, and remarkable flexibility in terms of absorbing, holding, and adsorbing materials. Construction of MOFs involves the use of organic linkages such as carboxylic acids or azo ligands [84], as well as metal ions or clusters [85]. With varying properties of ligands and metal centers, these materials form one-, two-, or three-dimensional arrays. The versatility of MOFs allows them to exhibit various properties such as capture, repair, catalysis, chemical sensing, gas separation, and storage. Furthermore, MOFs can be easily prepared with different chemical compositions and ligand functions, at a lower cost than traditional porous materials, stimulating a growing interest in its application [86].

MOFs possess the unique ability to accommodate inorganic clusters including Keggin anions and metal nanoparticles. Catalytic sites can be added to the organic support that spans the metal nodes, or catalytic metal elements can be used to modify the organic connections. It is possible to customize the metal center in the MOFs to evoke desired functions by adjusting the stereoelectronic features of the connection. Zeolites, with their exceptional cation exchangeability, facilitate the preparation of metal nanoclusters, resulting in the uniform dispersion of electrostatically driven cation precursors. Furthermore, the high thermal stability of zeolites promotes the proper conversion of precursors, such as decomposition or calcination of organometallics. The various coupling of internal or exterior catalytic metal clusters with Brønsted or Lewis acid sites in zeolite materials has enabled numerous applications of these materials as metals [87]. Conventional desulfurization techniques such as limestone cleaning are effective in removing a significant proportion of SO₂ from exhaust gases. However, residual SO₂ can still have adverse effects on both the environment and human health [88]. Capturing the tiny quantities of SO₂ found in exhaust gases so becomes crucial. The creation of a strong and potentially irreversible metal-sulfur connection has made it difficult to use metals to capture this undesirable SO₂. This bond has the potential to disrupt other coordination contacts between the metal center and the ligand, resulting in structural collapse.

2.2 Adsorption and transformation mechanism of SO₂

Using one-dimensional [AlO4(OH)2] chains and biphenyl 3,3′, 5,5′-tetracarboxylate, NOTT-300 [46], also known as MFM-300 (Al) (Fig. 1), was produced. It has been demonstrated that SO₂ molecules and hydroxyl groups on Al SBU form hydrogen bonds. There is also a mild synergistic effect between neighboring C-H groups on the ligand benzene ring and SO₂ molecules. Furthermore, Marilyn and his colleagues [48] modified UIO-66 with alkaline groups such as -NH₂ in order to enhance the ability of MOFs to respond to acidic gases such as SO₂, NO₂, and CO₂. The findings suggest that the electronic properties of the MOF are altered by the interaction of these gases with the -NH₂ groups on the linker [90].

Fig. 1

A M-MOF-74. a Structure of M-MOF-74. Reprinted with permission from [89], Copyright 2005, American Chemical Society. b M-MOF-74 in contact with SO₂ (M = Mg, Zn). Reprinted with permission from [45], Copyright 2017, American Chemical Society. c The charge density varies with the increase of adsorption area. Blue represents Mg or Zn; red represents oxygen; black represents carbon; white represents hydrogen; and yellow represents sulfur. Reprinted with permission from [45], Copyright 2017, American Chemical Society.; B MFM-300. a structure of 300 and SO₂ molecules adsorbed in MFM-300 (In). Reprinted with permission from [90], Copyright 2020, American Chemical Society. b The main internal interaction of MFM-300-SO₂ Reprinted with permission from [46], Copyright 2012. c Isotherms of MFM-300 (Al) for adsorption of different gases. Reprinted with permission from [46], Copyright 2012; C MFM-601. a structure of MFM-601. Reprinted with permission from [47], Copyright 2018, Joseph H. Carter. b SO₂ binding sites in MFM-601. Reprinted with permission from [47], Copyright 2018, Joseph H. Carter. c The occupancy rate of SO₂ in MFM-601. Reprinted with permission from [47], Copyright 2018, Joseph H. Carter. d SO₂ adsorption isotherms of MFM-601. Adapted with permission from [47], Copyright 2018, Joseph H. Carter

The interaction between SO₂ molecules and the host skeleton can significantly impact MOF changes [90]. Mechanism investigations have revealed that strong contact between SO₂ and hydroxyl groups increases molecular interactions during the adsorption process, leading to an increase in SO₂ dipole moment. It was reported that MFM-601 (Fig. 1), based on Zr, exhibits excellent SO₂ adsorption performance [47]. By leveraging the dipole moment of SO₂, MFM-601 provides stable binding in the pore in addition to promoting the interaction between SO₂ molecules. Strong adsorption sites are provided by the MOFs nodes, and to improve SO₂ molecule capture, metal cations can be post-modified and added to the porous core material. Furthermore, the addition of Ba²⁺ to the defect site of nickel pyrazolate-based MOFs can also enhance their interaction with SO₂.

2.3 MOFs preparation for adsorption and conversion of SO₂

MOFs contain highly active metals that interact with sulfur dioxide (SO₂) to form Lewis acid, which aids in adsorption. Sun et al. [91] examined the adsorption characteristics of 12 distinct porous materials, such as MOF, zeolite, and ZIF, using molecular simulations, or Grand Canonical Monte Carlo. They discovered that because of the high density of exposed metal components in the framework, Mg-MOF-74 can remove CO₂, SO₂, and NO_x from the environment all at once. Moreover, Chabal et al. [45] used DFT analysis and in situ infrared spectroscopy to examine the relationship between SO₂, NO₂, and M-MOF-74 series metal frameworks. A case in point is the MOF-74 structure with zinc open metal sites as depicted in Fig. 1. Glover [80] conducted a study on various metals (M=Co, Mg, Ni, Zn) within the MOF-74 series, demonstrating the dynamic breakthrough capacity for removing hazardous gases including SO₂, from the air under dry and wet conditions. The findings indicate that the simulated Mg-MOF-74 material has an excellent adsorption effect on SO₂. However, due to competition between water and SO₂ molecules at open metal sites, the gas adsorption capacity is reduced in high humidity conditions. Water molecule adsorption often reduces M-MOF-74 ability to effectively absorb visitor molecules. In a separate research, due to water being the most common impurity in organic solvents, it often affects the yield and activity of products in many organic synthesis reactions and industrial processes. Guo et al. [92] utilized a solvothermal method to synthesize a smart fluorescent pyrene-based MOF (HPU-26), which can self-decompose and release ligands under the driving force of water molecules. The MOF exhibits multi-stimulus responsive properties, changing its fluorescent color from yellow-green to blue, making it easily visualized, and enabling detection of water content in common organic solvents.

NOTT-300 is comprised of MOF materials functionalized with OH hydroxyls. The MOF has an angular-extended octahedral chain composed of [AlO₄(OH)₂] linked by an H₄L ligand (bipheny-3,3′, 5,5′-tetracarboxylic acid). This frame structure has a diameter of 6.5 and features a 6.5 Å square channel with a free (cis-OH) hydroxyl group. MFM-300 (Al) (Fig. 1) exhibits remarkable selectivity in adsorbing CO₂ and SO₂ between 273 K and 303 K. At 273 K and 1 bar, a fully reversible desorption isotherm was reported, with a maximum adsorption of 7.0 mmol g^–1 for CO₂ and 8.1 mmol g^–1 for SO₂. Furthermore, the SO₂ isotherm indicates that SO₂ adsorption is maximized at extremely low pressures due to the strong contact between MFM-300 (Al) and SO₂, which is caused by the large dipole moment of SO₂. The -OH hydroxyl group of the MFM-300 (Al) skeleton is the main binding site for SO₂, according to Rietveld refinement by X-ray powder diffraction. Together with four weaker C-H-O=S=O hydrogen bonds, this group also produces strong Al-OH-O=S=O hydrogen bonds with nearby hydrogen atoms. O=S · · O=S=O is used by the second SO₂ molecule to create an intermolecular chain with the first SO₂ molecule when it enters the solution. Due to its high capture capacity under low pressure, MFM-300 is a promising candidate for SO₂ capture. Eddaoudi and Salama [93] found that the high SO₂ adsorption capacity of MFM-300 makes it ideal for SO₂ sensors. The capacity of this material to remove sulfur dioxide from flue gas selectively is confirmed by the fact that these sensors can efficiently collect sulfur dioxide even under actual flue gas settings.

3 Adsorption and conversion of nitrogen oxides using MOFs

Since the onset of the industrial revolution, fossil fuel extraction, production, and consumption have steadily increased. Unfortunately, this has led to the release of an ever-increasing number of harmful chemicals such as nitrogen oxides (NO_x) into the atmosphere. The environment and public health are seriously threatened by the emission of these pollutants. Thus, it is essential to provide hazardous gas control and treatment a lot of attention [90]. Among the nitrogen oxide family, nitrogen dioxide is more toxic than nitric oxide. Nitrogen dioxide is highly corrosive and can damage materials irreparably. In the past, traditional “capture” techniques for nitrogen oxides, including selective catalytic reduction [94], activated carbon adsorption, and zeolite, have been employed. However, the reduction of NO₂ to NO during carbon surface oxidation is an inevitable outcome of all these techniques [95]. As a result, researchers have studied various strategies for reducing NO₂ to NO, hoping to overcome this significant challenge. Han et al. [96]. used a MOF with variable valence vanadium metal centers, MFM-300 (VIII, IV), to adsorb and reduce nitrogen dioxide (NO₂), and revealed the interaction between NO₂ molecules and the active MOF through in situ X-ray diffraction technology. The vanadium metal center in the MOF has REDOX activity. The nitrogen dioxide is absorbed in MFM-300 (VIII) pore and reduced to NO at the same time, and the vanadium metal center of MOF is oxidized from trivalent to quadrivalent. Molecularly porous materials, or MOFs, have emerged as a promising field of research in gas separation and have received considerable attention from researchers.

3.1 MOFs design principle for NOx adsorption

MOFs are renowned for their enormous specific surface area, porosity, and adaptable topological structure to various demands. Different metal and organic ligands are used in the MOFs synthesis process, which can produce alkaline or acidic crystalline porous MOFs networks. This chemical diversity makes it possible to customize MOFs composition according to their specific needs. MOFs central cavities with varying shapes can be obtained using the same ligand and resulting in a variety of porosities in the final product. This high porosity is comparable to activated carbon and is an advantage over traditional porous materials like zeolite. Because MOFs have a high-specific surface area and porosity, they are perfect for a variety of uses.

Because of their special qualities, MOFs have emerged as viable options for the adsorption of dangerous gases. The adsorption capacity of graphite oxide composite materials and HKUST-1 for ammonia was examined in a recent research [97]. HKUST-1 has shown to be an effective adsorbent for H₂S and NO₂ [98], but the structural integrity of the framework is compromised during the adsorption process, rendering it unusable for further use. MOFs with different ligands exhibit distinct structural properties and, therefore, varying adsorption abilities. Research has been conducted to explore the effect of different oxidized foreign metals introduced into MOFs on gas adsorption [99]. The doping of metal ions can provide additional adsorption sites and improve efficiency. A study conducted by Amani et al. [52] involved the addition of Ce³⁺ to MOFs and doping Ce³⁺ with Zr-based MOFs as a NO₂ adsorbent. The surface chemistry, structural characteristics, and adsorption performance of the material were analyzed. It was discovered that oxidation-reduction events occurred and the number of active sites increased, indicating that metal doping is advantageous for NO₂ adsorption.

3.2 Adsorption and transformation mechanism

Research shows that the porosity, stability, and corrosion resistance of NO₂ can be improved using metal-doped MOFs. When the number of carboxylic acid reaction ligands is restricted as in the case of benzenediazonium chloride, a mixed Ce-Zr-MOF phase is formed. The Ce³⁺ interacts with carboxyl groups, thereby limiting the amount of space available for water and adsorbent molecules to enter the pores. The increase in NO₂ adsorption is attributed to redox oxidation. The crystal surface structural properties regulate the development of new micropores in Ce doping which in turn increases NO₂ adsorption [52].

The capability of Cu-based skeleton HKUST-1 for NO₂ adsorption is demonstrated in Fig. 2. The graphite oxide composite material GO-HKUST-1 of HKUST-1 exhibits a range of 2.43–2.91 mmol/g for dry NO₂ adsorption and 0.83–1.28 mmol/g for 70% humidity-induced NO₂ adsorption [102]. It is noteworthy that the BET surface areas of both HKUST-1 and GO-HKUST-1 witnessed a significant reduction of 90% and 80%, respectively. Findings from differential thermogravimetric analysis and infrared spectroscopy analysis indicated that MOF degradation occurred due to the reaction between Cu nodes and NO₂, leading to the formation of bidentate nitrates [52].

Fig. 2

HKUST-1: a MOF catalyzed one-pot synthesis of β-aryl sulfonamides derivative via olefin aziridination. Reprinted with permission from [53], Copyright 2021, Sajan Pradhan. b Unit cell of bulk structure. Reprinted with permission from [100], Copyright 2020, Royal Society of Chemistry. c Adsorption of N₂ in composite materials HKUST-1 and HKUST-1/GO at standard temperature and pressure; the filled sign denotes adsorption and the open symbol desorption. Reprinted with permission from [101], Copyright 2022, Paulina Jagódka

The study of UiO-66-NH₂ has allowed for more research on the modification of UiO-66 by linkers. NO₂ was discovered to have an adsorption capacity of 20.3 mmol/g when it was dry, but 31.2 mmol/g when it was found to be at 80% humidity [103], showing a considerable increase in the adsorption rate. Nuclear magnetic resonance, X-ray photoelectron spectroscopy, infrared, and other characterization methods were used to investigate the mechanism behind the adsorption of NO₂ by UiO-66-NH₂. These studies suggested that NH₂ groups facilitate the production of diazonium ions, which in turn react with NO₂. McGrath et al. have reported that aryl diazonium salts generate phenol while releasing N₂ gas to remove NO₂ [104]. The metal framework does not break down when bonds cleave, even if chemical processes do occur within it.

To create UiO-66-Ox, free carboxylic acid was added to UiO-66. The NO₂ capture capacity of UiO-66-Ox was found to be 8.4 mmol/g, which was much greater than that of UiO-66 through a microbreakthrough experiment [104]. The interaction between free COOH groups and NO₂, which raises gas adsorption rates, is responsible for this enhancement. However, the structure of UiO-66-O_x decomposes automatically after exposure to NO₂. Metal ions doping was explored as an alternative to functionalize Zr-UiO-66 nodes and connections. UiO-66 and UiO-67 were doped with Ce³⁺ during the dry synthesis process. Greater NO₂ adsorption is achieved by the Ce-doped UiO-66 and UiO-67 structures because they have more adsorption sites. However, the structure stability of the Ce-doped framework deteriorated when exposed to NO₂ [52].

The Zr-based MOF [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃ has been identified as a highly effective yet reversible NO₂ sensor. This is achieved through the use of calixarene as a linker and a unique charge transfer process between calixarene and NO₂ at ambient temperature [56]. The charge transfer is attributed to the creation of N₂O₄, which undergoes disproportionation into NO⁺ and NO₃^–. The study identified that the development of purple charge transfer complexes occurs under the encapsulation of NO⁺ molecules, along with the coordination of NO₃^– and Zr⁴⁺ sites, which was verified by UV-visible and infrared spectra. It was further observed that the MOF crystallinity remained unaltered even after repeated exposure to NO₂. Figure 2 illustrates that low NO₂ concentrations below 1 ppm have a major effect on elimination in addition to having an adsorption capacity of 14.1 mmol/g. The creation of one-dimensional helical chains of (NO₂ ····· N₂O₄) ∞ monomer dimers in MFM 300 (Al) enhances the interaction between porous host molecules and guest molecules (Fig. 3). Furthermore, the stability of the MFM-300 (Al) structure is demonstrated by its ability to undergo five NO₂ adsorption and desorption cycles. Furthermore, studies show that copper-containing HKUST-1 can adsorb about 9 mmol g^–1 of NO at 296 K, making it one of the most promising porous materials with remarkable adsorption ability [105]. The robust interaction between NO and Cu ions within the framework can be ascribed to this. Furthermore, researchers have explored selective Ni doping in the Mg-MOF-74 framework to improve adsorption. Mg-MOF-74 doped with 40% Ni²⁺ was found to demonstrate excellent NO adsorption and transport capability [54].

Fig. 3

A Fe²⁺-MOF-5: a Illustration of the secondary MOF-5 building unit in which Fe²⁺ replaces one of the Zn²⁺ ions to produce Fe²⁺-MOF-5. Reprinted with permission from the source [55], Copyright 2015, American Chemical Society. b NO disproportionation at Fe²⁺ nodes. Reprinted with permission from the source [55], Copyright 2015, American Chemical Society. c The green, blue, and gold X-ray adsorption spectra of Fe-MOF-5 are shown. Reprinted with permission from the source [55], Copyright 2015, American Chemical Society; B [[Zr₆O₄(OH)₄(AA)₆]₂(cal)₃: a Structure of the novel MOF [Zr₆O₄(OH)₄(AA)₆]₂(cal)₃ in partial crystal form. Reprinted with permission from reference [56], Copyright 2018, Angewandte Chemie International Edition. b Structure representation and basic grid topology of 2-methylpropyl hydrogen carbonate and connectors. Reprinted with permission from reference [56], Copyright 2018, Angewandte Chemie International Edition. c N₂ adsorption isotherm of sample [Zr₆O₄(OH)₄(AA)₆]₂(cal)₃ extracted by asoxet at 77 K. Reprinted with permission from reference [56], Copyright 2018, Angewandte Chemie International Edition; C MFM-300(Al): a, b The interaction between MOF and guest molecules at binding sites I and II, and c Formation of a 1D spiral chain in the MFM-300 (Al) channel. Reprinted with permission from reference [57], Copyright 2018, Nature Materials. d The MFM-300 (Al) adsorption isotherm for NO₂ and other gases at 298 K. Reprinted with permission from reference [57], Copyright 2018, Nature Materials. e Equal volume adsorption heat (Qst) and entropy of NO₂ adsorbed by MFM-300 (Al) (ΔS). Grey is C, Blue is N, White is H, Red is O, and Green is Al. Reprinted with permission from reference [57], Copyright 2018, Nature Materials

In a recent study, it was discovered that NO binds strongly to unsaturated redox-active Fe²⁺ in Fe-MOF-74, resulting in the formation of a Fe-NO adduct. The oxidation of the Fe²⁺ site was correlated with a distinct color change when exposed to NO. In addition, the study was extended to a different iron-ordered mesoporous siliceous framework where Fe²⁺ swaps MOF-5 (Fig. 3) and disproportionates NO to produce nitrite iron complexes and N₂O [55].

4 Adsorption and conversion of VOCs and PM_2.5 using MOFs

Urbanization and industrialization have contributed to the rise of air pollution, particularly in the form of PM_2.5 (pneumatic diameter 2.5 μm), which is dangerous for human health and daily activities [106]. Exposure to PM_2.5 can lead to respiratory and central nervous system disorders, blood disorders, and cancer [107]. Therefore, it is imperative to tackle PM_2.5 to improve air quality. However, conventional filters [108, 109], made from petroleum-based materials, pose a risk of secondary pollution due to their non-degradable properties. Cellulose (a natural polymer) offers superior adsorption capacity, chemical resistance, and flexibility, making it a promising alternative to petroleum-based polymers in the production of biodegradable air filters [110]. However, traditional cellulose-based filters have limitations such as poor gas adsorption capability and susceptibility to bacterial infections [111]. Therefore, reducing air pollution requires the development of an effective and sustainable air filter membrane that can capture and degrade PM_2.5.

Particle filtering membranes should have a high affinity, stability, robust porosity connectivity, and a sizable specific surface area [112]. Metal-organic frameworks (MOFs) coated electroret filters (E-MOFilters) are a new and promising technology that can effectively remove both particulate matter (PM) and volatile organic compounds (VOCs) [113]. E-MOFilters have demonstrated an 85% efficacy in PM filtering and a 90% efficiency in toluene adsorption [61]. It is worth noting that E-MOFilters are chiefly responsible for reducing VOCs levels through adsorption with the ability to solely absorb VOCs contaminants. Although the non-destructive adsorption technique used in E-MOFilters is simple, the adsorption quantity may decrease over time due to saturation and concentration gradient desorption [114]. Another possible solution to disrupt VOCs is the development of photocatalytic oxidation method proposed by Fukushima and Honda in 1972 [115]. Photocatalysts are a promising solution for decomposing VOCs into harmless products, such as water and carbon dioxide. Among the various photocatalysts, because of its great stability, robust resistance to corrosion, and non-toxicity, titanium dioxide (TiO₂) stands out among the other photocatalysts [116]. High excitation energy, low electron-to-oxygen transfer rate, and high electron pair recombination rate are some of the variables that restrict its photocatalytic efficacy. In order to get around these restrictions, scientists have been investigating the possibility of using metal-organic framework (MOF)/semiconductor composite materials, such as MIL-100(Fe)/TiO₂, ZIF-8/ZnO, ZIF-8/TiO₂, UiO-66-NH₂/TiO₂, UiO-66-NH₂/g-C₃N₄, MIL-125(Ti)/GO, MIL-125-NH₂/BiOCl, and MIL-125-NH₂/Bi₂WO₆, as possible photocatalysts to break down volatile organic compounds (VOCs) [117].

4.1 MOFs design principle for adsorption and conversion of VOCs and PM_2.5

Through the process of intercepting and filtering, MOF can absorb particles and other hazardous gaseous pollutants. This results in enhanced filtration effectiveness, which can be further improved depending on pore size [118]. Composite materials comprising semiconductor particles and MOFs have been developed to act as photocatalysts that photodegrade VOCs molecules. MOFs have an abundance of adsorption sites to retain a significant quantity of VOCs, while semiconductor particles act as catalysts to photodegrade them. By combining graphene oxide (GO) with NH₂-MIL-125 (Ti) using a microwave heating technique, a heterojunction photocatalyst was created. The produced 10-GO/NH₂-MIL-125 (Ti) showed significant improvement in photocatalytic efficiency for gaseous acetaldehyde. According to the results, the composite materials enhanced photocatalytic activity may be attributed to increases in visible light absorbance, carrier density, charge transfer efficiency, and a decrease in electron-hole recombination rate.

Huang and colleagues created TiO₂@NH₂-MIL-125 as a photocatalyst to remove formaldehyde (Table 3). Comparing the photocatalytic removal rate of HCHO to that of pure TiO₂ and NH₂-MIL-125, the findings showed a considerable improvement after treatment (Fig. 4c), with a 90% removal rate [70]. Zhang used solvent evaporation to create TiO₂-UiO-66-NH₂ nanocomposites in a different work (Fig. 4d) (Table 3). Because of the strong adsorption capacity of NH₂-MIL-125, the high dispersibility of TiO2, and the effective interfacial charge transfer synergy between TiO₂ and NH₂-MIL-125, the nanocomposite photocatalysts demonstrated a conversion efficiency of 72.7% for toluene, which is 9.7 times greater than UiO-66-NH₂ [71]. Moreover, acetaldehyde conversion efficiency was greater than UiO-66-NH₂. These findings show that heterojunction formation can greatly enhance the performance of MOF/semiconductor photocatalysts in VOCs removal.

Table 3 Performance summary of MOFs in PM_2.5 capture and VOCs degradation

Fig. 4

a PM_2.5 SEM pictures before and after PTZ filtering (a-1, a-2). Reprinted with permission from [67], Copyright 2023, Journal of Colloid and Interface Science.; b (b-1) Ag-MOFs@CNF @SEM images of CF crystals, (b-2) ZIF-8@CNF and SEM images of CF crystals. Reprinted with permission from [69], Copyright 2019, Carbohydrate Polymers; c Schematic diagram of TiO₂@NH₂-MIL-125 composite. Reprinted with permission from [70], Copyright 2019, Applied Catalysis B: Environmental; d Composite schematic diagram of TiO₂-UiO-66-NH₂ Reprinted with permission from [71], Copyright2020, Chemical Engineering Journal; e QF for air filters such as PTZ-1. Reprinted with permission from [67], Copyright 2023, Journal of Colloid and Interface Science

A polyacrylonitrile@TiO₂/zeolite imidazolic acid framework-8 (PTZ) hybrid membrane was used by Yang et al. to study the performance of PM_2.5 filtration and photodegradation [67]. The results showed that the enhanced PTZ membrane greatly increased the capture efficiency of PM_2.5 and that the mixed structure produced by PTZ showed greater PM_2.5 degradation effectiveness compared to PT, with an increase from 66% to 85%. In the meantime, Ma studied Ag-MOFs@CNF, where ZIF-8 and Ag-MOF crystals were successfully deposited on the cellulose fiber surface, as demonstrated by the ZIF-8 filter, SEM, and EDS data. The intermediate layer of ZIF-8 filter material was observed to have a denser and smaller pore structure than the higher and lower layers (Fig. 4b). This is because ZIF-8 crystals and Ag-MOFs can mix to form a complex, dense composite filter porosity structure (Table 3).

4.2 Adsorption and transformation mechanism

Yang et al. study [67] contrasted the QF of PTZ-1 with that of other air filters (Fig. 4a). PTZ-1 had exceptional PM_2.5 filtering ability, according to the study (Fig. 4e). Ma et al. [69] employed PM_2.5 and PM₁₀ as benchmarks to evaluate filter effectiveness. It was shown that pure cellulose filters were less effective in removing PM_2.5 and PM₁₀ than composite filters like MOFs@CNF and CF, particularly Ag-MOFs@CNF and the ZIF-8 filter (Table 3). This is attributed to bi-component MOFs, which improve the filter-PM interaction, thus increasing the filtering effectiveness of the composite filter [119]. The gradient filtering method in the multilayer structure can intercept more particles through the complex zigzag channels for particle passage, thereby increasing the frequency of effective collisions between the filter and particles [120]. ZIF-8 nanocrystals increase the filter’s specific surface area and improve its filtration capabilities [83]. Therefore, the multilayer Ag-MOFs@CNF filter offers a great deal of promise for PM removal.

ZIF-8 nanocrystals are known for their rich adsorption sites and specific surface areas that are crucial for gas adsorption and capture. These crystals possess organic and open metal adsorption sites that enhance the ability of the composite filter to adsorb gases significantly. ZIF-8 crystals really have a specific surface area of 1545.2 m²g^–1, which is 126 times more than that of a filter element made entirely of cellulose. Ag-MOFs@CNF has been significantly enhanced by the addition of ZIF-8 nanocrystals, making it a more effective gas adsorption material. Among the three adsorption sites, it is also notable that the nitrogen adsorption process begins at the metal centers (zinc). The gas fills all the holes in the ZIF-8 crystal as the pressure increases by diffusing onto the organic ligand. This enhances the overall performance of the composite filter, making it an excellent choice for gas adsorption and capture [83].

The mechanical characteristics of filter materials are primarily determined and crucially assessed by their compression performance, particularly Ag-MOFs@CNF and bicomponent MOFs. ZIF-8 filter exhibits better compressive strength. The inclusion of MOF crystals in composite filters leads to richer active sites and structural optimization, which enhance cellulose molecular chain interactions [121]. ZIF-8 crystals demonstrated excellent gas adsorption capacity for CO₂, CH₄, and N₂ in earlier studies by Russell et al. [122]. Moreover, MOF filters can selectively capture and adsorb SO₂/N₂ mixtures [123]. The findings suggest that the gas adsorption capability of Ag-MOFs@CNF and ZIF-8 air filters is highly effective. Based on the above, this filter has the potential to become a multifunctional air filter that can absorb toxic gases and purify air.

5 Other air pollutants separation using MOFs

5.1 Vehicle exhaust separation using MOFs

The extensive utilization of automobiles has resulted in elevated levels of exhaust emissions, which pose a severe threat to human health. The volatile organic compounds and nitrogen oxides present in the exhaust gas can quickly contaminate the atmosphere and contribute to air pollution. Moreover, they can cause respiratory problems among people [124]. In recent years, the vigorous development of new energy electric vehicles will reduce some carbon emissions, but the taxi in electric vehicles will even lead to the increase of NO₂ concentration and the rise of CO, SO₂, O₃, and particulate matter levels, resulting in a decline in air quality [125]. The handling of automobile emissions has gained significant attention in recent years as it affects public health [126]. Studies have demonstrated that exposure to benzene toxicity can result in symptoms like headaches and neurasthenia, as well as genetic damage. Long-term exposure to benzene can lead to human anemia [127], while many benzene can cause photochemical smog and secondary pollution in cities [128].

During vehicle operation, the combustible mixture undergoes a series of chemical reactions influenced by various factors, such as high temperature, high pressure, and electric sparks. These reactions produce nitrogen oxides (NO_x) [129], which comprise a family of toxic gases with varying toxicity levels [130]. NO_x present in the atmosphere can also contribute to acid rain [131]. Petroleum commonly contains elemental sulfur, which when burned, produces sulfur dioxide that dissolves in water to form sulfite. This can accelerate acid rain formation, which can harm animals and plants [132] as well as corrode and damage buildings [133]. Additionally, in the presence of sunshine, nitrogen oxides in exhaust gas can combine with hydrocarbons to create photochemical smog [134], which in turn generates secondary pollutants. This can lead to the development of respiratory system diseases and eye irritation [135], as nitrogen oxides and sulfur oxides can enter the body through the respiratory tract during human respiration, causing lung and respiratory ailments. Furthermore, exposure to nitrogen oxides can cause developmental delays in children [136]. Since their inception in 1974, ternary catalysts have undergone continuous improvement [137,138,139] and have become an essential component of automobile exhaust control [140]. Table 4 shows the effect of some catalysts in the treatment of automobile exhaust gas.

Table 4 Part of the catalyst used to treat automobile exhaust

5.1.1 MOFs design principles for vehicle exhaust separation

Photocatalysis is an emerging technology that offers several economic benefits. Among the many photocatalysts, titanium dioxide (TiO₂) is a popular choice for water splitting to produce hydrogen under ultraviolet light. Although TiO₂ demonstrates excellent photocatalytic performance and has been widely researched [115], its wide bandgap restricts its photo response to only ultraviolet light. To overcome this limitation, researchers have explored semiconductor photocatalysts with appropriate bandgaps in recent years [145]. Because of its special qualities—such as their porous structure, large-specific surface area, high thermal stability, and exceptional catalytic activity—porous coordination polymers, or MOFs, have become intriguing materials for a range of applications [146]. However, certain practical issues such as low sunlight utilization and limited photocatalytic activity [147], have hindered the full-scale utilization of MOFs. As a result, researchers have explored various methods to enhance the photocatalytic activity of MOFs through modifications. Many studies have been conducted to investigate the effectiveness of modified MOFs in addressing these issues. In a recent study, to facilitate photocatalytic oxidation, Liu et al. [148] used the UiO-66-NH₂@MIL-101 (Fe) heterostructure as a catalyst. Under visible light irradiation, Zhang et al. [149] used solvothermal synthesis to create NH₂-MIL-101 (Fe) hexagonal microspheres, which they then used to remove gaseous toluene. It was found that the addition of NH₂ to MIL-101 (Fe) enhanced the visible light absorption. MOF photocatalysts provide a variety of benefits over other catalysts, including improved structural stability from the organic and inorganic components of the framework, increased light adsorption range from its high porosity, and effective encouragement of rates for light-generated e^– and h⁺ separation and transfer due to its adjustability.

5.1.2 Adsorption and transformation mechanism of vehicle exhaust separation

Mahy et al. [150] conducted a study on a photocatalytic coating that degrades NO_x and organic compounds in exhaust gas. They tested the performance of the sample for photocatalytic degradation of NO_x and found that most samples had degradation rates between 10% and 45%. Ma et al. [151] mixed TiO₂ nanoparticles into asphalt and heated the mixture to 170 °C in order to get samples that had degrading activity for NO_x and CH. Nano TiO₂ coating materials have been produced by Yu et al. [152] to enhance the activity and stability of TiO₂ on road surfaces for relevant applications. Anatase-type nano-TiO₂, activated carbon powder, silane coupling agent, and deionized water were used to make the materials. Sodium dodecylbenzene sulfonate was added as a surfactant at different mass ratios to the road coverings. The study discovered that a mass ratio of around 1:2 between nano-TiO₂ and surfactant maximized catalytic degradation impact. Similarly, Singh et al. [153] optimized the parameters for loading TiO₂ on asphalt pavement to degrade SO₂ and NO₂, resulting in significant practical effectiveness. Wang et al. [154] employed an innovative approach of laying finely ground TiO₂ cement mortar on the road surface, then drying it and applying epoxy resin to fix the catalyst on the road surface to improve the wear resistance of the photocatalyst on the road surface. The NO_x removal rate, road wear resistance, and slip resistance were assessed, and the results showed that effective NO_x degradation could be achieved.

5.2 Coal-fired flue gas separation using MOFs

Coal combustion leads to the release of CO₂ and SO₂ as primary air pollutants. Global temperature increase is attributed to CO₂ emissions, with an addition of 0.2 °C every decade. The emissions of coal-fired exhaust gas, represented by the energy intensive steel industry, account for 7% of the global anthropogenic carbon dioxide emissions due to the combustion of a large amount of coal in the production process [155]. SO₂ emissions are also a severe concern, which can cause acid rain and environmental damage, leading to lung diseases in humans. Carbon capture and storage technology is used to lessen the effects of CO₂ emissions, which involves the extraction of CO₂ from the production point source and its injection underground for long-term storage. Conventional carbon capture technology involves the selective absorption of CO₂ by aqueous amine solutions, which form ammonium carbamate and ammonium bicarbonate. However, this process has several limitations such as amine volatilization, thermal degradation, and oxidative degradation, with low adsorption capacity and high cost. Therefore, the development of porous materials with high selectivity and adsorption capacity for gases is crucial for reducing atmospheric pollution. MOFs are a promising solution that can cater to these specific requirements [51], as shown in Fig. 5a.

Fig. 5

a Schematic diagram of selective gas adsorption of MOF based on molecular sieve effect. Adapted with permission [156], Copyright 2020, Science of The Total Environment.; b Schematic diagram of recoverable stimulus response MOFs regulated by external stimuli for gas adsorption and release. Reprinted with permission from [157], Copyright 2017, American Chemical Society; c Molecular structure diagram of MOF-177. Reprinted with permission from [49]. Copyright 2005, Angewandte Chemie International Edition.; d The crystal structure of MIL-160. Reprinted with permission from [51], Copyright 2019, American Chemical Society.; e The crystal structure of NH₂-MIL-125 (Ti) [50]. Copyright 2017, American Chemical Society

5.2.1 MOFs design principles for coal-fired flue gas separation

In addition, Cui et al. [155] proposed a passive dual heat management strategy based on high latent heat and improved thermal conductivity. The mil-101 (Cr) ministry structure prepared has stable structure, high water absorption and desorption, and can spontaneously store and evaporate, taking away excess heat. In order to prove the high cyclic water absorption performance of mil-101 (Cr) under different conditions, they also explored the nitrogen adsorption isotherm of mil-101 (Cr), and compared with traditional adsorbents, the mil-101 (Cr) ministry obtained has better adsorption-desorption kinetics. MOF is a material produced from the self-assembly of organic ligand units and transition metals through coordination bonds. Compared to molecular sieves, MOF materials have a substantially wider specific surface area while maintaining the same morphology, structure, and controlled grain size [51, 158]. The main techniques for MOF regeneration include temperature swing adsorption (TSA), pressure swing adsorption (PSA), vacuum pressure swing adsorption (VSA), and their mixtures [159]. But to transmit pressure changes that cause adsorbed gas to escape from the wall, VSA and PSA need a lot of electrical energy [160]. Despite variable temperatures operations can utilize the waste heat generated by coal-fired sources, however, the use of microporous MOF adsorbents on a large scale is impractical due to their superior insulation properties. The poor heat conductivity of MOFs increases the cycle time significantly. Moreover, they may interact with electromagnetic radiation present in the environment, leading to structural changes at both small and large scales. Preloading guest molecules may also result in their expulsion from the pore, as shown in Fig. 5b. Britt et al. [79] developed a one-pot approach to manufacture MOF with different organic ligands. However, some of these frameworks exhibit low hydrothermal stability and compete with water during the adsorption process, leading to the collapse of the adsorbent structure and a decrease in saturation adsorption capacity.

5.2.2 Adsorption and transformation mechanism for coal-fired flue gas separation

Gas adsorption and separation using porous materials is a widely utilized method that involves the mechanisms of molecular screening, thermodynamic equilibrium, molecular dynamics, and quantum sieving [161, 162]. Molecular screening selectively adsorbs different gases based on their shape and kinetic diameter, allowing molecules with a kinetic diameter equal to or less than the pore diameter to pass through. The effectiveness of molecular screening can be influenced by temperature, as certain adsorbents have pore sizes that are sensitive to temperature. For optimal results, it is recommended to utilize equilibrium separation when the gas mixture components can easily pass through the interior of the adsorbent and its pore size is appropriately large. The interactions between molecules and the adsorbent surface have a major impact on the quality of selective adsorption. Additionally, the characteristics of the adsorbent molecules impact their contact strength. For instance, magnetic susceptibility, quadrupole moment, permanent dipole moment, and polarizability [163]. Dynamic separation is employed when equilibrium separation is not feasible. Precise tuning of the adsorbent pore size is crucial in dynamic separation, particularly when separating two molecules with varying dynamic diameters. In isotope separation, quantum sieving can be used to differentiate between guest molecules diffusion rates based on the compatibility of their pore widths with de Broglie waves.

5.3 Fuel exhaust separation using MOFs

Around 44% of global CO₂ emissions are a result of fuel combustion from sources such as coal, natural gas, and oil [164]. Over the past 100 years, the global average temperature has increased by 0.5 °C, and according to most global climate models, it will continue to rise by 1.5–3.0 °C over the next 100 years [165, 166]. So, it an urgent priority to capture CO₂ after combustion (Fig. 6a). For the past few decades, extensive studies have been conducted on CO₂ capture. Numerous carbon capture systems involving different techniques such as solid adsorption, electrochemical separation, low-temperature separation, and chemical adsorption of various solvents and membrane separation [171], nanofluids [172], ionic liquids [173], amino acid salt solutions [174], low-temperature separation, electrochemical separation, and solid adsorption have been widely researched. Danaci studied the effect of adsorption temperature on purity and recovery, as shown in Fig. 6e–f. This technology low energy usage and remarkable efficiency have drawn a lot of interest [175]. MOF has recently shown promising potential for CO₂ removal and can provide superior CO₂ adsorption capacity under low CO₂ partial pressure [176].

5.3.1 MOFs design principles for fuel exhaust separation

Because of their vast surface area, varied architectures, and high porosity, MOFs are very excellent CO₂ adsorbents. These three-dimensional microporous crystal structures are composed of a network of metal ligands and coordination bonds that connect the ligands to the central metal atom [177]. Due to the existence of these coordination bonds, MOFs are also known as coordination networks or coordination polymers at times [178, 179]. In MOF synthesis, the angstrom-level pore size and channel enable CO₂ storage [180]. Under high pressure, MOFs can store up to 12 times more CO₂ than empty containers, making them a significant asset to carbon capture and storage. Following the efficient capture of CO₂ by MOF, it is noteworthy that additional molecules will absorb and store it in the available pores and channels [177, 181]. This method is practical when dealing with confined spaces [182]. To optimize capture efficiency, MOFs with high CO₂ adsorption at low pressure are recommended. Darute et al. [76] have validated the carbon dioxide capture efficiency of polyethylene imine (PEI) injection into MIL-101. The adsorption capacity of carbon dioxide at 400 ppm is 1.0 mmol g⁻¹. Applications of MOF are numerous and include, but are not limited to, gas storage, CO₂ catalysis, CO₂ capture, VOCs detection, and gas purification and separation [183,184,185]. Currently, the physical adsorbents represented by MOF can adsorb gases of different concentrations. Figure 6d illustrates the adsorption capacity of several MOFs.

5.3.2 Adsorption and transformation mechanism for fuel exhaust separation

Research on the extraction of CO₂ gas from a CO₂ and N₂ mixture in Mg-MOF-74 was carried out by Mansour and Qasem. In their work, the TSA process was numerically simulated utilizing a four-step cycle involving feeding, rinsing, heating, and cooling [77]. Similarly, PSA operates by adsorbing the target gas from the gas mixture stream, followed by a vacuum pump that blows down the gas CO₂ flow. The light product is next pressured using the adsorption step’s output flow after the gas is released countercurrently at the free end. The process involves high-pressure operation during the adsorption step. In CO₂ mixture adsorption by the MOF, the mixture enters from the bottom and is adsorbed at the top. The exhaust process operates at medium pressure, utilizing a vacuum pump to exhaust N2 from the top, resulting in a higher CO₂ concentration. These four methods were also used by Rajagopalan and Rajendran [190] to gather CO₂ and create 94.8% pure CO₂ from CO₂ and N₂ combined with zeolite 13X as an adsorbent [191]. Adhikari and Lin [169] looked at how activated carbon (AC) affected MOF-74-Ni and MOF-74-Co ability to adsorb CO₂. The CO₂ adsorption rates of both modified MOFs were greater than those of the original structure. This is attributed to partially negatively charged oxygen atoms interacting with partially positively charged Pd atoms in polar CO₂. Figure 6c depicts the CO₂ of the original and upgraded structures at low pressure. Hu et al. [168] looked into absorption properties of pn-Mg2(dobpdc), mpn-Mg2(dobpdc), and dmpn-Mg2(dobpdc) showing CO₂ absorption between 2.7 and 4.1 mmol/g at 150 mbar and at 40 °C. Among these pn-Mg2(dobpdc) containing 2 amine groups showing highest absorption rate around 4.1 mmol/g and dmpn-Mg2(dobpdc) that has 2 methyl group and no free amine shows lowest absorption rate. Secondary amines have ability to interact with CO₂ through hydrogen bonding and enhance CO₂ absorption. This interaction was particularly evident in the high absorption rate demonstrated by pn-Mg2(dobpdc) compared to mpn-Mg2(dobpdc) that has one amine and one methyl, and dmpn-Mg2(dobpdc) no amine and two metyl groups (Fig. 6b).

The mechanism for capturing CO₂ depends on the interaction between CO₂ and the adsorbent [187, 192]. Usually, dynamic sieving is used when the gas size is comparable to or less than the CO₂ molecules molecular size [159, 187]. MOF is a selective adsorbent for CO₂ in the flue gas stream, allowing CO₂ capture and separation. Danaci et al. [170] suggest higher CO₂ capture capacity and ease of regeneration of Mg-MOF-74 over UTSA-16 (Fig. 6e and Fig. 6f). The two adsorption sites of the MOF enhance its affinity for CO₂, resulting in better adsorption efficiency. Belmukhout et al. [187] have studied various MOF types including Mg-MOF-74, SIF6-3-Zn, and SIF-6-3-Cu, with CO₂ adsorption rates of 0.08 mmol/g, 0.13 mmol/g, and 1.24 mmol/g, respectively. The incorporation of hydroxyl groups on the Mg site of Mg-MOF-74 (Fig. 6e) significantly enhances CO₂ adsorption in the channel. MOFs with narrow pore sizes, which are selectively used for gas separation based on gas dynamic diameter, exhibit rapid adsorption kinetics. The separation of gases is primarily driven by gas dynamics. Compared to gases like CH₄, H₂, and N₂, CO₂ molecules with hole sizes less than 40 nm have a larger potential for separation [187]. Devic and Serre [186] conducted a study on various MOF structures based on M³⁺ cations. They identified one-dimensional pores of MIL-53 which resulted in the rhombic shape of MIL-53, while MIL-68 exhibited a triangular and hexagonal shape as depicted in Fig. 7a–d.

Fig. 7

a–d Structural framework of M³⁺ dicarboxylate and tricarboxylate systems in MIL-53, MIL-68, MIL-88, and MIL-100. Reprinted with permission from [186], Copyright 2014, Chemical Society Reviews. e Mg-MOF-74 structural framework. Reprinted with permission from [187]. Copyright 2016, Chemical Engineering Journal. f Molecular structure and framework diagram of UiO-66-NH₂. Reprinted with permission from [188]. Copyright 2016, Journal of Colloid and Interface Science. g Molecular structure diagram of ZIF-8-90. Reprinted with permission from [189]. Copyright 2021, American Chemical Society

Fig. 6

a Changes in atmospheric carbon dioxide concentration caused by global energy consumption. Reprinted with permission from [167], Copyright 2018, Advanced Sustainable Systems.; b In pn^–, mpn^–, and dmpn–Mg2(dobpdc), the isotherms for CO₂ adsorption at 40 °C are displayed (desorption data presented with open circles). The estimated partial pressure of CO₂ (150 mbar) in coal flue gas is shown by the dotted line. Reprinted with permission from [168], Copyright 2017, American Chemical Society.; c At 298 K, the modified MOF-74 (Co) structure and the original MOF-74 (Ni) structure CO₂ adsorption isotherms (open loop denotes desorption). Reprinted with permission from [169], Copyright 2016, Chemical Engineering Journal.; d The capacity of different MOFs to absorb CO₂ after burning. Reprinted with permission from [167], Copyright 2018, Advanced Sustainable Systems.; The impact of various parameters on the cost and separation efficiency of UTSA-16 (green) and Mg-MOF-74 (red). Adsorption temperature impact on CO₂ purity e and recovery rate f. Reprinted with permission from [170], Copyright 2020, Molecular Systems Design & Engineering

Fig. 8

a The composition of the metal organic framework MOF-5. Reprinted with permission from [159], Copyright 2012, American Chemical Society.; b The structure of the metal organic skeleton Cu BTC. Reprinted with permission from [159], Copyright 2012, American Chemical Society.; c The CPO-27 metal organic framework construction. Reprinted with permission from [159], Copyright 2012, American Chemical Society.; d Schematic diagram of the crystal structure of MIL-101. Each intersection point (right) of the illustrated structure corresponds to the superhedron of the chromium octahedral trimer and terephthalate moiety (left). Adapted with permission from [203], Copyright 2007, Advanced Materials. e Solid structures of MIL-47 (V) and MIL-53 (Al, Cr, Fe) (upper left). The structural schematics for MIL-101 (Cr) and MIL-100 (Cr) are shown in the upper right corner. The structures corresponding to the different inorganic components are shown in the diagram. At the end, water molecules and fluorine are shown in gray close together, while the metal, oxygen, and carbon atoms are shown in green, red, and black, respectively. Reprinted with permission from [80]. Copyright 2009, American Chemical Society

Fig. 9

a MIL-53 (Fe), MIL-53 (Cr), and MIL-53 (Al) adsorbed amounts of hydrogen sulfide at 303.1 K at pressures up to 2 MPa. Yellow: MIL-53 (Fe), Pink: MIL-53 (Cr), and Blue: MIL-53 (Al). Reprinted with permission from [80]. Copyright 2009, American Chemical Society.; b MIL-101, SBA-15, activated carbon, and HZSM-5 benzene adsorption isotherms. Reprinted with permission from [203], Copyright 2006, Advanced Materials

It has been discovered that MOFs are very efficient catalysts for the catalytic conversion processes of CO₂. MOFs can be utilized as a catalyst to reduce CO₂ to methanol, according to recent studies [78]. Development of functionalized MOFs like UiO-68-NHC has been created by combining imidazole bromide ligands with MOFs that include metal-free n-heterocyclic carbon (NHC). High yields of methanol have been produced using UiO-68-NHC capacity to transport hydrogen from silane to CO₂. After eight cycles, the yield of the MOF immobilized NHC catalyst is 90%, and it has a good selectivity for the necessary product methoxysilane. Yang et al. [193] have also developed MOF based on Ni30 clusters, which has been found to significantly impact the catalytic conversion of CO₂ and epoxides to cyclic carbonates. Kurisingal et al. work [194] focused on the synthesis of Cu BTC/Zr UiO-66 binary MOFs for CO₂ cycloaddition with epoxides in a solvent-free setting. There is a noticeable conversion impact, according to the data. The binding of Zr and Cu in MOF shows 99% selectivity for ECH carbonates, while the binding of Br in tetrabutylammonium bromide exhibits 99% selectivity for ECH carbonates. This study binary MOFs has outstanding recyclability; even after six cycles, there is no loss of catalytic activity. Additionally, Xiang et al. studied the double ligand ZIF-8-90 (Fig. 7g) [195], it produced an effective CO₂ and ECH cycloaddition. Compared to ZIF-8, the double ligand ZIF showed better stability and selectivity, and even after three cycles, there was no appreciable decrease in catalytic activity.

5.4 Waste incineration exhaust gas separation using MOFs

With the rapid growth of the global economy and the increase in urbanization, there has been a significant surge in urban household waste [196]. As per recent statistics, urban household waste generation across the world is estimated to be around 1.6 to 2 billion tons [196, 197]. Common methods for treating such waste include sanitary landfills, composting, and incineration. However, incineration of urban household waste emits SO_x and NO_x, with NO_x being mainly composed of NO and NO₂. The emission of NO_x has significant implications for both human health and the environment [198]. Researchers have focused a lot of attention on porous MOFs materials because of their varied pore architectures, open metal sites, pore function, and changeable porosity. In recent times, these substances have demonstrated outstanding abilities in the adsorption and segregation of diverse liquids and gases.

5.4.1 MOFs design principle for waste incineration exhaust gas separation

Numerous adsorption pathways have been proposed with regard to chemical adsorption between MOF and SO₂/NO_x active sites [199, 200]. The adsorption process is significantly influenced by acid-base interaction, complexation, and hydrogen bonding [201]. Metal ions, or metal ion clusters, and ligands make up MOFs. Typically, organic units have two, three, or four terminal ligands [202]. Figure 8 illustrates some typical MOFs with solvents that serve as the primary template molecules for MOF [204]. Most metal cations can be involved in the formation of MOFs. During the formation process, the length of ligands of the same metal type only needs to be adjusted, and the framework pore size relies on the ligand length [205].

The use of the same ligand and different metal components allows for the preparation of multiple MOFs with similar properties [206]. MOFs have several uses, such as the adsorption and storage of CO₂ and the storage of hydrogen [207], steam adsorption [208], chemical separation [209], biomedical [210], and catalysis [211]. The pore surfaces of MOFs are easily modified and selectively adsorb guest molecules with specific functional groups. As an adsorption material, MOFs hold great promise and have been extensively studied for their ability to adsorb and remove various harmful substances from the environment [212]. Furthermore, interactions between the adsorbent and MOFs can be enhanced through central metal coordination with unsaturated sites [213], functionalized ligands [214], and loaded active substances [215], resulting in effective adsorption and removal of harmful compounds. Enhancing adsorption in various host-guest interactions is facilitated by acid-base, hydrogen bonding, complexation, and coordination [215,216,217] with open metal sites.

5.4.2 Adsorption and transformation mechanism for waste incineration exhaust gas separation

Removing dangerous gasses is essential to preserving clean air quality. MOFs have favorable functional groups, open metal sites, and pore sizes and porosities. These features greatly facilitate particular interactions with host adsorbents. Using pressure swing adsorption methods, Hamon and collaborators [80] carried out comparative tests on stable and readily regenerated MOFs. Polar H₂S molecules and inorganic chains of MIL-53s (Al, Cr) demonstrate strong interaction, causing the pores to seal at low pressure. As pressure increases, the pores reopen and disrupt the strong interaction between H₂S and OH (at the MOF metal position). As a result, weak host-guest interactions cause all pores to fill. The results from the adsorption isotherm study of MOF on H₂S are presented in Fig. 9a. The study carried out at high pressure (1.6 MPa) revealed that MIL-53 (Al) and MIL-53 (Cr) had maximal H₂S adsorption capabilities of 13.12 and 11.77 mmol g^-1, respectively. MIL-47 and MIL-53s have identical adsorption capacities for Al and Cr at high pressure with pores reopening, according to a different study on MIL-47, which is similar to MIL-53s but does not include OH groups. The study also revealed a Type-I isotherm for H₂S adsorption.

Petit and Bandosz [218] conducted a study on the adsorption of NO₂ NH₃, and H₂S from the environment using composite materials made of MOF and graphite compounds. According to the study, new holes may form at the interface between the carbon layer and the MOFs as a result of the interaction between open metal sites and oxygen groups, producing composite materials with different properties. The experiment revealed that the GO/Cu-BTC composite material demonstrated a 12% improvement in NH₃ adsorption, 50% improvement in H₂S adsorption, and 4% improvement in NO₂ adsorption when compared to pure Cu-BTC.

Dathe et al. [219] have demonstrated that the adsorption of SO₂ by Cu-BTC impregnated with Ba(CH₃COO)₂ leads to the formation of small barium salt microcrystals inside Cu-BTC pores, thereby causing partial damage to the structure. The adsorption of SO₂ at high temperatures exceeds the stoichiometric capacity due to chemical bonding between metal cations and SO₂, leading to the formation of copper sulfate. As a result, Cu-BTC exhibits superior adsorption capacity at low temperatures. It has been demonstrated by Yazaydın et al. [220] that the inclusion of open metal sites in the Cu-BTC framework containing 4 wt% water molecules may considerably increase the capture ability of CO₂ and selectivity to N₂ and CH₄. Using a series of identically structured MOF of M-CPO-27s, Dietzel et al. [221] investigated the impact of metal centers on adsorption capacity and CO₂ adsorption selectivity. The results of the investigation show that at 298 K and high pressure (50 bar), the CO₂ adsorption rate of CPO-27 (Ni) is 51 wt%, and at 298 K and high pressure (50 bar), the CO₂ adsorption rate of CPO-27 (Mg) is 63 wt%.

According to Caskey et al. [222], at 1 atm, the CO₂ adsorption rates of CPO-27 (Co) and CPO-27 (Mg) were 30.6% and 35.2%, respectively. The use of small and uniform particles obtained by ultrasonic or microwave heating improves CO₂ adsorption [223, 224]. A total of 350 mg g^–1 of CO₂ was adsorbed by ultrasonically produced CPO-27 (Mg) at 298 K, demonstrating a high adsorption isotherm [225]. MOFs functionalized with amine enhance CO₂ adsorption through acid-base or electrostatic interactions. Jhung et al. [203] evaluated the adsorption of benzene by MIL-101 (Cr), which has a gas phase adsorption capability of 16.7 mmol g^–1. Their findings reveal that MIL-101 (Cr) has an adsorption capacity 5.5 times higher than SBA-15, 8.7 times higher than H-ZSM-5, and 2 times higher than activated carbon (Fig. 9b), owing to its large porosity. The microwave irradiation method is used to produce MIL-101 (Cr) in liquid-phase adsorption [226]. In addition, An et al. [227] based on high latent heat properties along with improved thermal conductivity have proposed a passive dual thermal management strategy. The prepared MIL-101 (Cr) MOF exhibits structural stability along with high water absorption/desorption capacities enabling spontaneous storage/release cycles which effectively dissipate excess heat. To demonstrate its superior cyclic water absorption performance under varying conditions; they also investigated nitrogen adsorption isotherms which revealed superior kinetics compared to traditional absorbents.

6 Conclusion

Undeniably, significant advancements have been made in global technology. Despite substantial progress in controlling air pollution, it remains a complex and multifaceted challenge, posing serious threats to public health and potentially impacting life expectancy. Of particular concern is the widespread issue of haze formation, which has multiple complex causes and diverse components. To address this issue effectively, comprehensive solutions have been developed for different gaseous pollutants. These encompass a range of treatment technologies aimed at adsorption, purification, and separation of various pollutants to enhance air quality and improve the human living environment while reducing incidence rates and extending lifespan.

SO₂ has extremely complex chemical properties and is often present in the atmosphere. Therefore, we have summarized a series of treatment plans here. Metal-organic frameworks (MOFs) including ZIF-8, HKUST-1, MIL-101, and UiO-66 have been the subject of extensive study, and the results indicate that these MOFs may effectively adsorb and convert SO₂. As crucial, other hazardous gases like nitrogen dioxide (NO₂) may also be addressed using MOFs because of their large surface area and versatile features. However, it is crucial to overcome challenges related to their structural integrity and reusability to fully harness their potential. Additionally, MOFs have demonstrated potential for mitigating PM_2.5 pollution by enhancing filtration efficiency and facilitating photocatalytic degradation. Innovative strategies such as Ag-MOFs@CNF @ZIF-8 and PTZ-1 nanofiber membranes hold great promise in reducing PM_2.5 levels and toxic gas concentrations while also extending their operational lifespan.

Advanced air purification technologies are essential to combat air pollution caused by the automotive industry. Using MOF-based technologies to lower emissions of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) is one potential approach that has surfaced. MOFs such as NH2-MIL-125 (Ti) and MIL-160 demonstrate impressive adsorption capacity and stability, making them a potential solution to this type of pollution. Additionally, given the growing concerns about climate change, it is crucial to capture and reduce carbon dioxide (CO₂) emissions. MOFs have the potential to optimize CO₂ capture by adjusting pore characteristics and incorporating structural functionalization, hence contributing to global efforts to mitigate CO₂ emissions.

To summarize, MOFs have proven to be adaptable and efficient materials that may be used to treat a wide variety of air contaminants. From capturing harmful gases to reducing fine PM_2.5 and mitigating CO₂ emissions, the adaptability and multifaceted applications of MOFs underscore their pivotal role in advancing air quality and promoting environmental sustainability.

6.1 Future perspectives

Currently, despite the significant potential of MOFs in adsorbing pollutants, there are still some limitations. For instance, the development of MOFs adsorbents tailored to specific pollutants is at an early stage. The existing methods for preparing and assessing the effectiveness of MOFs are largely confined to laboratory settings, and their performance in real-world applications remains to be thoroughly validated. Furthermore, the adsorption capacity of current MOFs is limited and necessitates replacement upon saturation. Additionally, the production cost of MOFs is relatively high. In the future, with further research and technological advancements, it is anticipated that both the application scope and adsorption capabilities of MOFs will be enhanced. It is recommended that researchers focus on optimizing the pore structure, surface chemistry, and functionalization of MOFs to enhance their pollutant adsorption capacity and selectivity while expanding their applicability. Emphasis should also be placed on developing renewable and biodegradable MOF materials to enable their reuse and minimize environmental impact. Policy makers are encouraged to provide financial support as well as technical assistance to improve essential production infrastructure for these materials. Enterprises should seek closer collaboration with research institutions to facilitate technology transfer for scaled-up production processes and optimization techniques aimed at reducing manufacturing costs. In conclusion, despite facing challenges related to air pollution control efforts; there exists substantial promise for MOFs in safeguarding human health and preserving environmental integrity.