1. Introduction

MOFs are special materials made by sticking metal bits and organic parts together. They've become popular because of the way metal and organic stuff can team up. These materials have lots of little holes and special spots that can do various things. Scientists use them to store gases and make reactions happen. The way MOFs are built is like putting together pieces of a puzzle. The metal parts and organic parts fit together to make different shapes and sizes. This makes MOFs flexible and useful for different tasks. For example, by changing the size of the pieces, scientists can adjust how much stuff the MOF can hold. Imagine MOFs as buildings with rooms that can do special things. The way the rooms are built, and the materials used can make them strong or weak [8]. Some MOFs are better at handling water and heat because of how they're built. Scientists can even change the metal parts to make the rooms interact differently with gases, which is handy for certain tasks. In simple words, MOFs are like versatile tools that scientists use to store things and make reactions go smoothly. They're like Lego sets that can be put together in different ways to do different jobs, like holding stuff or reacting with gases. The ability to adjust both structure and function lets MOFs work together by using their pores and special spots. This helps them increase the amount of gas they can hold while also separating different gases using various methods like thermodynamic sieving, kinetic sieving, gate-opening effects, and molecular sieving [9-11]. Additionally, to better understand the versatile nature of MOF materials, it is essential to explore their structural diversity. Figure 1 illustrates the crystal structures of IRMOF-n (n=1-8, 10, 12, 14, 6) with various ligands, showcasing the intricate arrangements of metal and organic components. This diversity in structure allows MOFs to exhibit unique properties tailored for specific applications. Moreover, Figure 2 provides a schematic representation of the formation of M-MOF-74 (M: Zn, Co, Ni, Mg), shedding light on the synthesis process and emphasizing the importance of metal selection in designing MOFs. There are five categories of MOF applications, including Carbon dioxide capture, Adsorption and Purification of CH₄, Separation of C₂H₂/C₂H₄, Separation of C2H4/C2H6, and Separation of C3H6/C3H8 by adsorption.

Figure 1. The crystal structures of IRMOF-n(n=1-8101214,6) with different ligands [4]

Figure 2. Schematic representation of the formation of M-MOF-74 (M: Zn, Co, Ni, Mg) [5]

2. Carbon Dioxide Capture

Carbon dioxide is a major greenhouse gas. In recent years, researchers have been using a new type of material called Metal-Organic Frameworks (MOFs) to capture CO₂ from the air. These MOFs have adjustable structures and functions, making them effective adsorbents for capturing CO₂ [6-8]. One type of MOF called SIFUSIX-MOFs contains silicon and other elements. They have a special structure that makes them excellent at capturing CO₂ compared to other gases. In the year 2013, Nugent and colleagues have been studying different SIFUSIX-MOFs to understand their ability to capture CO₂. They found that SiFUSIX-3-Zn has the best performance in terms of capturing CO₂, showing high CO₂ adsorption even at certain temperatures and pressures [9] (see Figure 3).

Figure 3. (a) The structures and ligands of SIFSIX-2-Cu, SIFSIX-2-Cu-i, and SIFSIX-3-Zn; (b) CO₂ adsorption isotherms of SIFSIX-2-Cu-i at different temperatures; (c) CO₂ adsorption isotherms of SIFSIX-3-Zn at different temperatures [10]

It's also good at selectively separating CO₂ from other gases, which is important for various applications. To make these materials more practical, researchers are trying to improve their sensitivity to water and maintain their adsorption capacity even in humid conditions [11]. In the year 2016, Chen and colleagues even modified the MOFs with specific chemical groups to enhance their ability to selectively capture CO₂. In one study, researchers attached certain groups to the surface of the MOFs to create a more efficient way of capturing CO₂ [12-14]. They observed interesting changes in CO₂ adsorption, suggesting that a special process happens when CO₂ is captured (see Figure 4).

Figure 4. At 100K, the solid filling models of mmen-Mn₂(dobpdc) (2) and CO₂-mmen-Mn₂(dobpdc) (b); before (c) and after (d) CO₂ adsorption, partial crystal structure of mmen-Mn₂(dobpdc) (e); CO₂ cooperative adsorption mechanism [15]

3. Adsorption and Purification of CH₄

Methane (CH₄) is an important component of natural gas, which is used for clean energy and in making various chemicals. In the past, when we tried to purify methane, we faced challenges because it had extra CO₂ and other gases mixed in. These impurities made it difficult to use methane effectively. To solve this, researchers are looking into new materials called MOFs (Metal-Organic Frameworks) that can help clean up methane and remove those unwanted gases. In the year 2013, the research group led by O. K. Farha studied different types of MOFs to see which ones can capture and hold methane the best [16]. They found that the size of the pores in the MOFs and their surface area play a big role in how well they can capture methane [17]. They also discovered new MOFs that have super high surface area and can capture a lot of methane (see Figure 5).

Figure 5. (a) CH4 adsorption capacities for a series of classic MOFs (PCN-14, UTSA-20, HKUST-1, Ni-MOF-74, Ni-CPO27, NU-111, and NU-125) under 65 bar conditions, along with their working capacities (difference in uptake between two pressures) and comparison to the U.S. Department of Energy standards [31]; (b) Structural diagram of NU-1501; (c) Mass adsorption isotherms of CH₄ on NU-1501-Al at different temperatures [17].

One group of researchers created a MOF that can separate different gases from methane. For example, they made a MOF that can separate propane (C₃H₈) and ethane (C₂H₆) from methane [18]. This is important because sometimes, we want to use only the purest form of methane for different processes. They found that this new material can separate these gases well, which is helpful for industries (see Figure 6).

Figure 6. Schematic diagram of gas separation with C1/C2/C3 ternary components using MIL-142A [18]

Another group of researchers made a MOF with a special structure that has tiny pockets where methane can stick [19]. This MOF is good at separating methane from other gases like nitrogen (N2) or ethane. The special design of this material makes it a great candidate for cleaning up methane for various uses. In summary, people are working on finding new materials (MOFs) that can help us clean up methane and make it pure for different purposes. These materials have tiny spaces that can trap methane and separate it from other gases, making methane more valuable for clean energy and chemical production.

4. Separation of C₂ Hydrocarbon

4.1. Separation of C₂H₂/C₂H₄

Ethylene (C₂H₄) is one of the most widely used alkenes, crucial in the petrochemical industry. However, during production, it's common to have ethyne (C₂H₂) present in the ethylene mixture, which is harmful to the catalyst used in ethylene polymerization. Therefore, it's necessary to separate C₂H₂ from C₂H₄. Yet, their similar physical properties make this separation challenging [20-22]. The current methods, like low-temperature distillation and solvent extraction, are expensive and energy-consuming. People are exploring a more energy-efficient approach using porous materials for adsorption separation. In 2016, a research group led by Cui investigated several MOFs (Metal-Organic Frameworks) for C₂H₂/C₂H₄ separation [23, 24] (see Figure 7). These MOFs showed better adsorption performance for C₂H₂ compared to C₂H₄. Among them, Cu-I-based MOF (SIFSIX-2-Cu-i) exhibited a promising adsorption capacity of 2.1 mmol/g for C₂H₂ under 298 K and 0.025 bar, along with a high selectivity of 39.7-44.8 for C₂H₂/C₂H₄ separation. The unique interaction between each C₂H₂ molecule and the framework, facilitated by hydrogen bonding with different F atoms in the network, contributes to the exceptional adsorption performance and selectivity.

Figure 7. (a) Isotherms of C₂H₂ (circles) and C₂H₄ (triangles) in SIFSIX-1-Cu (red), SIFSIX-2-Cu (green), SIFSIX-2-Cu-I (blue), SIFSIX-3-Zn (light blue), and SIFSIX-3-Ni (orange) at 298K; (b) IAST (Ideal Adsorbed Solution Theory) selectivity comparison of 1% C₂H₂ mixture in SIFSIX materials with other C₂H₂/C₂H₄ separation MOFs; (c) Permeation simulation; (d) Comparative energy analysis with other MOFs [23]

In 2018, Li and their team focused on various MOFs for C₂H₂/C₂H₄ separation. A MOF with a pore size of 3.4 Å called SIFSIX-14-Cu-i (UTSA-200a) showed the best performance [25] (see Figure 8).

Figure 8. (a) The crystal structure of UTSA-200a; Theoretical models for C₂H₂ (b) and C₂H₄ (c) adsorption in UTSA-200a; (d) Crystal structure of C₂H₂ in UTSA-200a [25]

Its compact pore size perfectly matched C₂H₂ molecules while blocking larger C₂H₄. This structural advantage allowed UTSA-200a to achieve a high adsorption capacity of 116 cm³/cm³ under 298 K and 1 bar for C₂H₂. Its remarkable performance also extended to a C₂H₂/C₂H₄ mixture, where it displayed an impressive Ideal Adsorption Solution Theory (IAST) selectivity of over 6000 at the same conditions. In summary, researchers have been working on efficient methods to separate C₂H₂ from C₂H₄ mixtures. Using advanced materials like MOFs, they have achieved impressive results in terms of adsorption capacity and selectivity. This progress is significant for industrial processes that require pure ethylene while minimizing energy consumption.

4.2. Separation of C₂H₄/C₂H₆

The separation of ethene (C₂H₄) from ethane (C₂H₆) is an important industrial process used in the petrochemical industry. In the year 2018, Lin and colleagues found a way to precisely adjust the pore size of MOFs (Metal-Organic Frameworks) to separate C₂H₄ from C₂H₆, targeting this specific separation challenge. In 2018, researchers led by Lin developed a type of super microporous MOF called Ca(C₄O₄) (H2O) (UTSA-280M), with a one-dimensional rigid pore structure measuring about 3.8 Å x 3.8 Å. This pore size aligns perfectly with the smallest cross-sections of C₂H₄ (13.7 Å) and C₂H₆ (15.5 Å) (see Figure 9), allowing C₂H₄ to pass through while preventing C₂H₆ transport, achieving an ideal separation. UTSA-280M was proven through experiments to efficiently separate C₂H₄ from C₂H₆ under conditions of 298 K and 1 bar, with a high C₂H₄ production rate. Moreover, UTSA-280M is water-stable and can be easily prepared on a kilogram scale using environmentally friendly methods, making it a valuable candidate for industrial applications.

Figure 9. (a) Crystal structure of UTSA-280. Green, light coral and gray nodes represent Ca, O, and C atoms, respectively; (b) Coordination environment of formate ligand and calcium ion; (c) Coordination constraints of C₄O₄^-2 ligand; (d) Single-component adsorption isotherms of CO₂ (black) at 195K, N₂ (blue) at 77K, ethene (black) and ethane (indigo) at 298K; (e) IAST (Ideal Adsorbed Solution Theory) adsorption selectivity comparison for equimolar ethene/ethane mixture among different MOFs at 298K [26]

In industrial production, obtaining high-purity C₂H₄ requires the removal of small amounts of C₂H₆. MOFs like UTSA-280M, with their strong C₂H₆ adsorption, can potentially lower production costs and energy consumption. In 2018, Li and the team employed a reverse approach by using Fe₂(0.2) (dobdc) to create a MOF with strong C₂H₆ adsorption for efficient C₂H₆/C₂H₄ separation [27] (see Figure 10).

Figure 10. (a) Crystal structure of Fe₂(dobdc); (b) Crystal structure of Fe₂(O₂) (dobdc); (c) Crystal structure of Fe₂(O₂)(dobdc)C₂D₆. Iron atoms are represented in green; Carbon in deep gray; Oxygen in pink; O₂^2- in red; Hydrogen or Deuterium in white; Carbon in C₂D₆ in blue [27]

Overall, researchers are exploring innovative methods to achieve efficient separation of C₂H₄ from C₂H₆ using MOFs with tailored pore sizes. These advancements hold the potential to revolutionize industrial processes while reducing environmental impact.

5. Separation of C₃H₆/C₃H₈ by Adsorption

The separation of C₃H₆/C₃H₈ is a crucial and challenging industrial process [28-30]. MOF-74 is a well-known type of MOF with open metal sites (OMS) that can interact effectively with various gases [31]. In 2012, Long and his team investigated the use of Fe-MOF-74 for low-carbon hydrocarbon separation, including C₃H₆/C₃H₈ separation [32, 33] (see Figure 11).

Figure 11. (a) Crystal structure of Fe-MOF-74; (b) Interaction modes of OMS, C₃H₆, and C₃H₈ in Fe-MOF-74; (c) Adsorption isotherms of C₃H₆ and C₃H₈ on Fe-MOF-74 at 318 K [32]

The Fe-MOF-74 could adsorb C₃H₆ more than C₃H₈ due to ion-induced dipole interactions. The separation is achieved through coordination with the unsaturated metal cation in the MOF structure. In 2018, Wang and the team used Y-based MOF (Y-abtc) with a pore window size suitable for the kinetic diameter difference between C₃H₆ (4.68 Å) and C₃H₈ (5.1 Å) to achieve C₃H₆ molecular sieving [34] (see Figure 12).

Figure 12. (a) Building units and structure of Y-abtc; (b) Adsorption isotherms of C₃H₈ and C₃H₆ on Y-abtc at 25°C; (c) Multicomponent penetration results of equimolar C₃H₈ and C₃H₆ mixture on Y-abtc at 25°C [34]

The Y-abtc quickly adsorbs C₃H₆ while excluding C3H8. Multi-component breakthrough experiments showed that highly pure C₃H₆ (99.5%) can be obtained from the obtained C₃H₆/C₃H₈ mixture. Considering its thermal and hydrothermal stability, as well as its effective C₃H₆/C₃H₈ separation, this work demonstrated the potential of Y-abtc as an alternative adsorbent for separating C₃H₆/C₃H₈ mixtures. MOFs are promising candidates for gas separation due to their evenly distributed pore sizes, various functional sites, and adjustable pore sizes. Gas separation techniques based on different adsorption mechanisms are being explored and applied to gases like C₃H₄/C₃H₆ and others. These techniques have shown promising results in achieving successful gas separation.

6. Conclusion

In conclusion, researchers are harnessing the unique properties of Metal-Organic Frameworks (MOFs) to address pressing challenges in gas separation and purification. Whether capturing carbon dioxide to combat climate change, purifying methane for clean energy applications, or achieving precise separations of gases like ethylene and ethane, MOFs offer a versatile and innovative solution. Their tunable structures, adjustable pore sizes, and specialized interactions enable them to capture specific gases with impressive efficiency and selectivity. As industries evolve and environmental concerns grow, these advancements in gas separation technology hold significant promise for transforming industrial processes while reducing energy consumption and minimizing environmental impact.