Adsorption and Degradation of Volatile Organic Compounds by Metal-Organic Frameworks (MOFs): A Review

Volatile organic compounds (VOCs) are a major threat to human life and health. The technologies currently used to remove VOCs mainly include adsorption and photocatalysis. Adsorption is the most straightforward strategy, but it cannot ultimately eliminate VOCs. Due to the limited binding surface, the formaldehyde adsorption on conventional photocatalysts is limited, and the photocatalytic degradation efficiency is not high enough. By developing novel metal-organic framework (MOF) materials that can catalytically degrade VOCs at room temperature, the organic combination of new MOF materials and traditional purification equipment can be achieved to optimize adsorption and degradation performance. In the present review, based on the research on the adsorption and removal of VOCs by MOF materials in the past 10 years, starting from the structure and characteristics of MOFs, the classification of which was described in detail, the influencing factors and mechanisms in the process of adsorption and removal of VOCs were summarized. In addition, the research progress of MOF materials was summarized, and its future development in this field was prospected.

A Microporous MOF Constructed by Cross-Linking Helical Chains for Efficient Purification of Natural Gas and Ethylene

Natural gas (NG) and ethylene (C₂H₄) are two raw materials of significant value for manufacturing versatile fine chemicals and/or polymers, and thus the development of solid adsorbing agents such as metal-organic frameworks (MOFs) applied to their depuration is very crucial but remains highly challenging. In this research, we designed and synthesized a ligand containing mixed N and O coordination donors, which was solvothermally assembled with Cu(II) ions to generate a microporous MOF. X-ray crystallography revealed that the title MOF incorporates one-dimensional (1D) homochiral helical chains that are datively cross-linked to form open channels in the three-periodic coordination framework. Furthermore, the behaviors of C₁-C₂ hydrocarbons and carbon dioxide (CO₂) adsorbed in the title MOF were systematically investigated, revealing its promising potential for the purification of both NG and C₂H₄. At 109 kPa and 298 K, the C₂/methane (CH₄), CO₂/CH₄, and acetylene (C₂H₂)/C₂H₄ adsorption selectivities are impressive, reaching as high as 62.9, 28.6, and 3.5, respectively. This work represents a unique MOF based on cross-linked homochiral helical chains exhibiting dual-function separation potentials for NG and C₂H₄ purifications.

Indium metal-organic frameworks based on pyridylcarboxylate ligands and their potential applications

Indium metal-organic frameworks (In-MOFs) based on pyridylcarboxylate ligands represent a subclass of MOFs featuring diverse structures, a high stability, and various properties. This review discusses the different aspects of In-MOFs including their design, synthesis and structures as well as their typical potential applications in adsorption and separation, catalysis, and chemical sensors. Importantly, the effect of pyridine on the properties and stability of frameworks has been carefully studied. The introduction of a pyridine group not only significantly enriches clusters of In3+ ions, but also enables flexible, controllably synthesized ionic or neutral frameworks to be fabricated. Based on this, we suggest that this type of In-metal organic framework (MOF) should receive more attention in the field of MOF design.

Coordination sphere hydrogen bonding as a structural element in metal-organic Frameworks

In the design of new metal-organic frameworks, the constant challenges of framework stability and structural predictability continue to influence ligand choice in favour of well-studied dicarboxylates and similar ligands. However, a small subset of known MOF ligands contains suitable functionality for coordination sphere hydrogen bonding which can provide new opportunities in ligand design. Such interactions may serve to support and rigidity the coordination geometry of mononuclear coordination spheres, as well as providing extra thermodynamic and kinetic stabilisation to meet the challenge of hydrolytic stability in these materials. In this perspective, a collection of pyrazole, amine, amide and carboxylic acid containing species are examined through the lens of (primarily) inner-sphere hydrogen bonding. The influence of these interactions is then related to the overall structure, stability and function of these materials, to provide starting points for harnessing these interactions in future materials design.

A Dy6-cluster-based fcu-MOF with efficient separation of C2H2/C2H4 and selective adsorption of benzene

A stable Dy-MOF was constructed based on hexanuclear clusters, and contains F-decorated pores and reveals separation performance for C₂H₂/CH₄, C₂H₄/CH₄ and C₂H₂/C₂H₄ and selective adsorption of benzene/cyclohexane and benzene/toluene.

Ultrasensitive and highly selective detection of formaldehyde via an adenine-based biological metal–organic framework

We demonstrate a successful design of an adenine-based BioMOF for highly sensitive formaldehyde recognition without the interference of other VOCs by utilizing its reactivity on Watson–Crick sites and MOF compartmentalization.

Doubly Interpenetrated Metal-Organic Framework of pcu Topology for Selective Separation of Propylene from Propane

Adsorptive separation is an appealing alternative technology to reduce the high energy and capital cost of the distillation separation of propylene/propane; however, it is very challenging to realize. A new flexible metal-organic framework (MOF) material [Zn₂(BDC-Cl)₂(Py₂TTz)] with a doubly interpenetrated pillared paddle wheel structure of pcu (primitive cubic) topology has been realized for this difficult separation for the first time. Through a judicious choice of linkers, the framework has small pore apertures that lead to much more propylene adsorption than propane. The selective adsorption relies on the sieving effect of the flexible framework. The column breakthrough experiment further demonstrated that efficient separation can be achieved under dynamic conditions.

Efficient C2Hn Hydrocarbons and VOC Adsorption and Separation in an MOF with Lewis Basic and Acidic Decorated Active Sites

To help address efficient separation of C₂H_n light hydrocarbons and C₂H₂/CO₂ in the chemical industry, the self-assembly via an azolate-carboxylate ligand and Co(II) ion gave rise to a new porous MOF material, [Co(btzip)(H₂btzip)]·2DMF·2H₂O (1) (H₂btzip = 4,6-bis(triazol-1-yl)isophthalic acid). In the MOF, the pores are modified by rich uncoordinated triazolyl Lewis basic N atoms and acidic -COOH groups, which strengthen interactions with C₂H_n hydrocarbons and CO₂ molecules, leading to high adsorption amounts for C₂H_2, C₂H₄, C₂H₆, and CO₂ and remarkable separation efficiency for C₂H_n-CH₄, CO₂-CH₄, and C₂H₂-CO₂ mixtures, as confirmed by breakthrough experiments on the realistic gas mixtures. The MOF also reveals outstanding selective adsorption ability for benzene/toluene, methanol/1-propanol, methanol/2-propanol, and 2-propanol/1-propanol isomers. Molecular simulations disclose the different adsorption sites in the MOF for various adsorbates.

A Highly Connected Trinuclear Cluster Based Metal-Organic Framework for Efficient Separation of C2H2/C2H4 and C2H2/CO2

One of the barriers for efficient gas separation is the trade-off between the selectivity and adsorption capacity. To address this issue, we synthesized an anionic trinuclear Co^II based 3D MOF (NbU-8), which is characterized by an ultramicroporous building unit (UBU) and Lewis basic binding sites on the pore surfaces. Remarkably, the combination of the two strategies can synergistically enhance the C₂H₂ adsorption capacity (182.9 cm³/g at 298 K) and simultaneously achieve a high separation performance toward C₂H₂/C₂H₄ and C₂H₂/CO₂ mixtures. Besides theoretical calculations, the separation efficiencies of C₂H₂/C₂H₄ and C₂H₂/CO₂ are also demonstrated using breakthrough experiments. Density functional theory calculations have further confirmed the -OH groups and ultramicroporous building units play an important synergistic effect in efficiently capturing acetylene molecules.

Insights into the Adsorption of VOCs on a Cobalt-Adeninate Metal-Organic Framework (Bio-MOF-11)

With increasingly severe air pollution brought by volatile organic compounds (VOCs), the search for efficient adsorbents toward VOC removal is of great significance. Herein, an adenine-based metal-organic framework, namely, bio-MOF-11 [Co₂(ad)₂(CH₃CO₂)₂·0.3EtOH·0.6H₂O, ad = adeninate], was synthesized via a facile method, and its VOC adsorption was reported for the first time. This novel bio-MOF-11 was investigated by employing four common VOCs (i.e., methanol, acetone, benzene, and toluene) as adsorbates. The saturated adsorption capacity of these targeted VOCs on bio-MOF-11 was estimated to be 0.73-3.57 mmol/g, following the order: toluene < benzene < acetone < methanol. Furthermore, with the adsorption temperature increasing from 288 to 308 K, the saturated adsorption capacity was reduced by 7.3-35.6%. It is worth noting that acetone adsorption is most sensitive to temperature ascribed to its low boiling point and strong polar nature. Meanwhile, owing to the molecular sieve effect, the adsorption capacity appears negatively correlated to the size of VOC molecules. Besides, the abundant exposed nitrogen atoms and amino groups in bio-MOF-11 cavities facilitate the adsorption of polar VOC molecules. This work promotes the fundamental understanding and practical application of bio-MOF for adsorptive removal of VOCs.

A robust Th-azole framework for highly efficient purification of C2H4 from a C2H4/C2H2/C2H6 mixture

Separation of C2H4 from C2H4/C2H2/C2H6 mixture with high working capacity is still a challenging task. Herein, we deliberately design a Th-metal-organic framework (MOF) for highly efficient separation of C2H4 from a binary C2H6/C2H4 and ternary C2H4/C2H2/C2H6 mixture. The synthesized MOF Azole-Th-1 shows a UiO-66-type structure with fcu topology built on a Th6 secondary building unit and a tetrazole-based linker. Such noticeable structure, is connected by a N,O-donor ligand with high chemical stability. At 100 kPa and 298 K Azole-Th-1 performs excellent separation of C2H4 (purity > 99.9%) from not only a binary C2H6/C2H4 (1:9, v/v) mixture but also a ternary mixture of C2H6/C2H2/C2H4 (9:1:90, v/v/v), and the corresponding working capacity can reach up to 1.13 and 1.34 mmol g-1, respectively. The separation mechanism, as unveiled by the density functional theory calculation, is due to a stronger van der Waals interaction between ethane and the MOF skeleton.

Optimizing supramolecular interactions in metal–organic frameworks for C2 separation

C₂ separation is of great importance in the petrochemical industry. This perspective presents current status and future challenges in the design of MOF materials for C₂ separation.