Recent Advances in C2 Gases Separation and Purification by Metal-Organic Frameworks
Recent Advances in C2 Gases Separation and Purification by Metal-Organic Frameworks
The energy consumption of chemical separation processes is a significant part of total global energy consumption each year, which accounts for about half of industrial energy consumption, with the separation of low-carbon hydrocarbon gases being the most important.[1-3] Traditional separation methods used in industry include low temperature distillation, catalytic hydrogenation and solvent extraction, which consume large amounts of energy and also suffer from environmental pollution.[4, 5] At the same time, these traditional separation methods are not in line with the principles of green chemistry.[6-9] It runs counter to the current international advocacy of carbon peaking and carbon neutrality. Therefore, new separation technologies need to be developed to achieve the goal of low carbon and environmentally friendly separation.[10-13] In contrast, adsorption-based separation is considered as new generation of separation technology that is more energy efficient and environment friendly due to the high recyclability of the adsorbent materials used and the fact that no additional solvents are required in the separation process.[14, 15] The development of materials with excellent adsorption and separation properties is key to the development of this technology and determines to a large extent the actual separation of the gas mixture. Therefore, it is essential to explore the synthesis of materials with excellent adsorption and separation capabilities to achieve the dual carbon goal.[16-18]
Up to now, main types of separation materials are metal-organic frameworks (MOFs), silica gels, activated alumina, activated carbons, molecular sieve carbons, zeolites, porous organic frameworks (POFs), hydrogen-boned organic frameworks (HOFs), porous organic cages (POCs), etc.[19-22] Among them, MOFs have shown potential applications in areas including gas storage and separation, [23, 24] catalysis, [25, 26] luminescence, [27, 28] proton conduction[29, 30] and biomedical applications[31, 32] due to their ultra-high specific surface area, structural versatility and designability as well as ease of modification and post-synthesis. Compared to other separation materials, [34, 35] the inherent characteristics of MOFs make them more promising for separation applications, [36-38] especially for low-carbon hydrocarbons. One of the most appealing features is that MOFs have well-defined structures, which feasibly clarifies the mechanism in the adsorption separation process and facilitates the targeted synthesis of desired structures according to the characteristics of the gas mixture to be separated. Another important feature lies in that certain MOFs structures have the ability of trapping specific gases in the form of a molecular sieving or gate-opening effect. In addition, the intrinsic pore structures of MOFs facilitate the modification of the pores according to the physicochemical properties of the target gas,  which cannot be found in molecular sieves and activated carbons.
Typically, MOFs materials achieve effective separation of gas mixtures by physical adsorption. The separation is generally driven by the van der Waals forces between the framework structure and the gas molecules, hydrogen bonding and the difference in the ability of the open metal sites to act on the gas molecules. Currently, the mechanisms of gas screening action generally include size sieving, thermodynamic effects, kinetic effects and conformational screening or a combination of such effects. Size sieving: it is based on different sizes of the molecules of the gas mixture to be separated, and is designed to synthesize the right sized orifice to achieve effective separation. This separation method is generally beneficial for the separation of gas mixtures with large differences in molecular size. Thermodynamic effects: the source of the distinction between the forces acting on different gas molecules by thermodynamic separation comes mainly from differences in the forces acting on the gas molecules by the skeletal structure, or by the forces acting at specific structural sites on the framework. Specific sites of action can be anchored to the framework by selecting ligands with different functional groups or by post-modification. Kinetic effects: kinetic separation is generally based on the different diffusion rates of gas molecules in the pore channel. Conformational screening: conformational sieving is achieved by controlling the different conformations of gas molecules in the MOFs pore channel. Different separation mechanisms are available for different types of gas molecules. Depending on the target molecule, a suitable separation method needs to achieve the goal of efficient separation.
Since the physical and chemical properties of C2 (C2H2, C2H4 and C2H6) molecules are very similar both in size and boiling point (Figure 1), the separation process is very difficult and has been listed as "Seven chemical separations to change the world". Among them, ethylene (C2H4) is an important chemical raw material, its production and purity are one of the important indexes to measure the development level of a country's chemical industry. As the variety of C2H4 downstream products (polyethylene, ethylene oxide, ethylene glycol, styrene, etc.) continues to increase, its consumption also climbs year by year. This paper summarizes the research progress of MOFs materials in the field of C2 gas separation in recent years (Figure 2), explains the separation mechanism, and proposes the future research directions of MOFs in the field of C2 gas separation, taking into account the problems existing in the current research.
SEPARATION OF C2H4/C2H6
The separation of ethane (C2H6) is an important step in the purification process of C2H4, which is required by the chemical industry for purity greater than 99.5%.[44, 45] Currently, the C2H6/C2H4 separation method used in industry is through low temperature distillation, which requires significant energy consumption. Adsorption-based separation is considered to be one of the most important methods to achieve low-energy separation.[46, 47] Depending on whether the gas molecule being preferentially adsorbed is C2H4 or C2H6, MOFs materials can be classified as C2H4 adsorbents and C2H6 adsorbents.
C2H4-Selective Adsorbent. Among the MOFs materials reported so far, there are more materials that preferentially adsorb C2H4 than C2H6. This is mainly due to the strong interaction between the C2H4 molecules and the framework structure, where the double bonds in C2H4 molecules generate strong interaction with the metal sites in MOFs framework.
The first example of MOF materials used to separate C2H4/C2H6 mixtures was Cu-BTC (BTC = benzene-1, 3, 5-tricarboxylate) reported by Wang et al. The double bond of C2H4 can form a strong π-complex with the unsaturated Cu(Ⅱ) ions of the Cu-BTC lattice under low pressure. Considering the similar molecular sizes of C2H4 (3.3 Å) and C2H6 (3.9 Å), benzene rings may play a role in the preferential sorption of C2H4, resulting in strong selective adsorption. The adsorption capacity of C2H4 is thus larger than that of C2H6, achieving the separation of C2H4/C2H6 mixtures. Experiment for single-component adsorption of C2H4 showed a value in excess of 6 mmol/g at 295 K. Grand Canonical Monte Carlo (GCMC) calculations predict a separation selectivity for this material of more than 2 at 298 K and 100 kPa.
In 2011, Bao et al. reported a magnesium-based metal organic backbone (Mg-MOF-74). The two-site Sips model agrees well with the adsorption equilibrium data. The intracrystalline diffusion coefficients are obtained by fitting the absorption curves to a simplified microporous diffusion model (Figure 3). The heat of adsorption and Henry's constant for olefins suggest that it strongly interacts with the open sites of the MOFs backbone via π-complexation. GCMC simulations show that all adsorbates are preferentially adsorbed by the open metal sites, with one molecule bound to each metal site. To predict the adsorption performance of Mg-MOF-74 for C2H4/C2H6 separation, adsorption equilibrium selectivity, combined equilibrium and kinetic selectivity, and adsorbent selection parameters for the variable pressure adsorption process were estimated. The relatively high values of adsorption selectivity indicate that it is feasible to use Mg-MOF-74 as an adsorbent for C2H4/C2H6 separation in the adsorption process.
In order to obtain better performance, Long et al. improved the separation selectivity by enhancing the force between the framework structure and C2H4 molecules. The selectivity of C2H4/C2H6 separation is improved by taking advantage of the interaction of metal sites in the framework structure with the double bonds in C2H4 The adsorption experimental test results show that Fe2(dobdc) has strong adsorption affinity with unsaturated olefins. Neutron powder diffraction studies indicated that the adsorption site of C2H4 lay within the Fe2(dobdc) channel, with Fe-C distances falling in the 2.42-2.60 Å range. Correspondingly, the interactions of C2H6 with the metal cations were weaker, and the Fe-C distances are around 3 Å. The results of theoretical calculations by ideal adsorbed solution theory (IAST) show that the separation selectivity coefficient of C2H6/C2H4 is 13-18 at 318 K. The actual experimental test results of gas breakthrough separation show that the Fe material can be separated at 318 K and 100 kPa to obtain C2H4 with 99%-99.5% purity.
In addition to the separation of C2H4 by using the strong interaction between C2H4 and metal sites, effective separation of C2H6 can also be achieved by molecular sieving. To achieve an effective molecular sieving effect, [Ca(C4O4)(H2O)] synthesized from calcium nitrate and squaric acid was reported by Chen et al. in 2018. The structure has rigid 1D channels in which the pores are similar in size to C2H4 molecules. By taking full advantage of the size, shape and stiffness of the pores, transportation of C2H6 gas molecules is effectively prevented. It was confirmed by gas breakthrough separation experiments in ambient conditions with high C2H4 production efficiency. At the same time, the material is simple to synthesize and can be easily prepared in kilogram yields by environmentally friendly methods, while the structure has good water stability, which is important for potential industrial applications.
Researchers achieved effective separation of C2H4 gas by combining molecular sieving effects with open metal sites. The structural pore channels are precisely tuned by using reticular chemistry, while CuI is introduced to enhance the force of the framework structure on the C2H4 molecules (Figure 4). In 2019, Li's group found rapid adsorption of C2H4 driven by the strong affinity of π-complexation and notably lessening ethane uptake due to its size-sieving effect by constructing Cu-chelated UiO-66-type MOFs. This rare synergy of specific π-complex interactions and effective size sieving effects in CuI@UiO-66-(COOH)2 resulted in an ultra-high IAST selectivity of 80.8 for 50/50 C2H4/C2H6 mixtures under ambient conditions, outdistancing most previously reported benchmark porous materials. Detailed break-through experiments on 50/50 v/v C2H4/C2H6 mixtures further confirm the excellent separation performance. This work may provide some guidance for the development of new porous materials for improved olefin/paraffin separation performance.
C2H6-Selective Adsorbent. Typically, the C2H4 molecule is smaller than C2H6 and contains a double bond that interacts with the open metal sites in MOFs structure. As a result, most MOFs have a stronger force on C2H4 than on C2H6, and thus the adsorption capacity of C2H4 is greater than that of C2H6. This leads to the fact that in the separation of C2H4/C2H6 mixtures, the C2H4 gas to be purified is preferentially adsorbed and requires a further desorption step to obtain the C2H4 product, and that the purity of the desorbed C2H4 gas is not high due to the presence of co-adsorption and requires further purification.
Therefore, from the point for energy saving and emission reduction and the achievement of carbon peaking and carbon neutrality goals, it is necessary to obtain pure C2H4 gas by onestep separation. In order to achieve the purification of C2H4 gas, it is necessary to synthesize materials that obtain preferential adsorption of C2H6 molecules.
In 2010, Jorge Gascon et al. reported the first example of a structure that selectively recognized C2H6 molecules. The zeolitic imidazole backbone ZIF-7 allows selective adsorption of C2H6 gas from C2H4/C2H6 mixtures. In dynamic gas breakthrough separation experiments, pure C2H4 gas can be obtained in a single step separation (Figure 5). It was found that the reason for this phenomenon is the selective recognition of specific gases by ZIF-7, which leads to a structurally flexible switch gate effect, where specific pressure thresholds control the uptake and release of individual gas molecules. For different gas molecules the threshold pressure will then be different, thus offering the possibility of selective separation of gases. This property makes ZIF-7 ideal for the separation of C2H4 from C2H6 compared to most microporous materials that selectively adsorb C2H4. This is the first reported material to achieve the inverse separation of C2H4/C2H6 gas mixture.
In 2014, Pires et al. reported the metal-organic backbone IRMOF-8 has a higher C2H6 adsorption capacity than C2H4 over a wide range of pressures in the temperature range near room temperature. In these temperature and pressure ranges, the separation selectivity factor is between 3.4 and 1.6. Finally, the ability to separate C2H6/C2H4 mixtures was confirmed by fixed bed separation experiments, where C2H6 gas was preferentially adsorbed in the separation column and C2H4 gas was obtained as a pure gas from the exit of the separation column. Based on the calculations, it can be found that the difference in the interaction of C2H6 and C2H4 with the two adjacent aromatic rings in IRMOF-8 plays an important role in the selective adsorption results.
Owing to the open metal sites in all MOF structures, the double bond in C2H4 shows strong interactions with the open metal sites. There is little difference in their adsorption capacity for C2H4/C2H6, which prevents efficient C2H4 purification. Experimental results by IRMOF-8 show that non-polar pore channels will be more favorable for the adsorption of C2H6 molecules. Lin et al. reported a method to facilitate the separation of C2H6 from C2H4 that involves controlling the pore structure in two iso-reticulated ultra-microporous MOFs with weakly polar pore surfaces to enhance the binding affinity for C2H6. Under ambient conditions, the conventional complex structure shows very little difference in absorption and selectivity for C2H6/C2H4, but the small pore structure in this structure significantly increases the selectivity for C2H4/C2H6 separation (Figure 6c). Neutron powder diffraction studies clearly show that the material exhibits adaptive adsorption behavior towards C2H6, which allows it to maintain a continuous tight van der Waals contact with C2H6 molecules in its optimized pore structure (Figure 6a, b), thus preferentially binding C2H6 over C2H4. Gas adsorption isotherms, crystallographic analysis, molecular modelling, selectivity calculations and breakthrough experiments provide comprehensive evidence that this unique MOF material is a highly efficient material for C2H4 purification (Figure 6d).
The analysis of gas structure shows that C2H6 molecules have more C-H bonds than C2H4. Therefore, an effective purification of C2H4 gas can be achieved by introducing into the structure groups that have a stronger effect on the C-H bond. Based on this idea, Liao et al. reported a complex MAF-49 with a C2H6 adsorption capacity exceeding that of C2H4 due to the fact that the pore surface of MAF-49 is rich in regularly arranged and negatively charged nitrogen atoms (Figure 7). Theoretical calculations show that C2H6 molecules form three strong C-H…N hydrogen bonds and three weak C-H…N electrostatic interactions with MAF-49. For C2H4 molecule, only two weaker C-H…N hydrogen bonds and two very weak C-H…N electrostatic interactions are formed. This is due to the spatial site resistance and electrostatic repulsion existing between the two C-H parts of the molecule. Gas adsorption tests showed that the C2H6 uptake rate of MAF49 is higher than that of C2H4 at a low pressure of 316 K. This result was also verified by gas breakthrough separation experiments, where an inflow gas mixture of 15:1 C2H4/C2H6 resulted in 99.995% pure C2H4.
The MOF structures reported so far that preferentially adsorb C2H6 have saturated metal site, and therefore the difference in their adsorption capacities for C2H6 and C2H4 is not significant, resulting in poor selectivity and purification efficiency for C2H6/C2H4 separation. Thus, Li et al. reported an example of a strategy to peroxide metal sites in the structure, yielding a MOF material Fe2(O2)(dobdc) with peroxide iron sites. High-resolution neutron diffraction experiments confirm that the preferential binding of C2H6 at the peroxide site is established by the deuterium bond C-D…O (D…O 2.17-2.22 Å), and that the D…O distance is smaller than that of C2H4. The IAST selectivity of Fe2(O2)(dobdc) for C2H6/C2H4 (50:50) is 4.4. Finally, it was confirmed by using fixed bed separation experiments that the C2H6/C2H4 (50:50) gas mixture passed through a column filled with Fe2(O2)(dobdc) material. C2H4 gas with a purity of 99.99% can be obtained.
SEPARATION OF C2H2/C2H4
As one of the most important feedstocks for petrochemical industry, C2H4 has an annual production capacity of over 150 million tons. During the production of C2H4 by steam cracking, trace amounts of acetylene (C2H2) are inevitably produced.[62, 63] The presence of C2H2 can lead to the formation of metal acetylide from the catalysts used in the polymerization of C2H4, resulting in catalyst poisoning. Therefore, the C2H2 content needs to be reduced to at least 40 ppm. It is worth noting that the separation of C2H2 and C2H4 is very challenging due to their similar physical properties.[51, 65, 66] Conventional C2H2/C2H4 separation methods, which mainly include low temperature distillation and catalytic hydrogenation, are energy intensive and not conducive to the achievement of the two-carbon goal. Therefore, adsorption-based separation is considered to be one of the best ways to achieve low-energy separations.
Rigid Regular Porous Structures. The first MOFs for C2H2/C2H4 separation possessed chiral pores in M'MOFs by using chiral diamine (R, R)-1, 2-cyclohexanediamine. By introducing different carboxylic acid BDC and CDC into their isomorphic M'MOF-2 and M'MOF-3, adjustment of the chiral pore size can be achieved. The slightly smaller pores in M'MOF-3 are much more selective for C2H2/C2H4 separation than M'MOF-2a. Thus, the targeted modulation of pore channels in the microporous structure by this strategy achieves an effective separation of specific gases (Figure 8).
The adsorption-based separation of C2H2/C2H4 suffers from a trade-off effect between adsorption capacity and separation selectivity. This means the adsorption capacity and separation selectivity are often not combined at the same time. Therefore, in order to overcome the trade-off effect in the C2H2/C2H4 separation process, in 2016, Cui et al. reported a series of SIFSIX materials that overcome these effects to some extent. The SIFSIX series of materials are linked by the inorganic anion SiF62- and the pore size can be adjusted by varying the length of the chosen ligand. Due to the weak basic SiF62- nodes, they are able to interact strongly with the weakly acidic gas separation C2H2 and thus selectively adsorb C2H2 molecules. As a result, the SIFSIX structures reported in this article have significantly higher C2H2 adsorption capacities than C2H4. Among them, SIFSIX-2-Cu-i has an adsorption capacity of 2.1 mmol/g at 298 K and 0.025 bar, with a high selectivity of 39.7-44.8 for C2H2/C2H4 separation. Both DFT calculations and neutron diffraction experiments show that each C2H2 molecule binds simultaneously to two F atoms from different networks through C-H…F hydrogen bonding, resulting in a strong interaction with the framework. Thus, a high adsorption capacity and separation selectivity for C2H2 were achieved. The SIFSIX-2-Cu-i structure can effectively purify C2H4 to 99.998%. This method is also applicable to the separation of gas mixtures. Specific binding sites and suitable pore sizes are necessary for the separation of specific gases.
Wu et al. reported a fluorinated microporous material (FJI-W1), a structure with excellent water and thermal stability. Gas adsorption tests showed that FJI-W1 has a high adsorption capacity of 150 and 159 cm3/g for C2H2 and C3H4, respectively. Dynamic breakthrough separation experiments showed that the separation times for C2H2/C2H4 (1/99, v/v) and C3H4/C3H6 (1/99, v/v) are up to 230 and 600 min/g, respectively (Figure 9). The DFT calculations demonstrate that C2H2 can form a C-H…F bond with a SiF62- and C3H4 has two C-H…F hydrogen bonds formed with a SiF62-. In addition, the results of different gas flow rates and cycle breakthrough tests show the FJI-W1 has significant separation capability for C2-C3 alkynes/olefins.
Soft Porous Crystal. The suitable pore structure and adsorption sites of the robust MOFs help to achieve selective adsorption of C2H2 gas, but the forces are often too strong for the regenerative use of the material. Therefore, materials with moderate forces need to be synthesized to achieve effective separation of acetylene gas. In 2017, Li et al. reported an example of a flexible MOF to effectively remove acetylene from C2H2/C2H4 gas mixtures. Results from neutron diffraction experiments showed that specific binding sites in the structure and a unique pore structure work together to enable ELM-12 to selectively adsorb C2H2 rather than C2H4 under ambient conditions. Results from fixed bed breakthrough separation experiments also show that when a 1:99 of C2H2: C2H4 mixture is passed through a column filled with ELM-12, C2H4 purity in excess of 99.999% is obtained.
An example of a flexible MOF [Cu(dps)2(SiF6)] (known as NCU-100 or UTSA-300Cu) for the removal of trace C2H2 is reported by Wang et al. UTSA-300-Zn has relatively small 3.5 × 3.9 × 4.1 Å3 molecular cages. During the adsorption of 298 K C2H2 gas, the gate-opening effect occurs in the structure when the adsorption pressure reaches 0.2 bar and C2H2 molecules can only enter the molecular cage (Figure 10). Accordingly, the size of the molecular cage in the structure is elongated by Cu-F to 3.6 × 4.3 × 4.2 Å3. The adsorption of C2H2 gas can be achieved at relatively low-pressure conditions. The adsorption capacity of C2H2 at 298 K and 1.0 bar is 4.57 mmol g-1 without the adsorption of C2H4 gas. Dynamic breakthrough experiments demonstrate the potential of this MOF for the highly selective separation of C2H2/C2H4.
Whether for rigid or flexible MOF materials, pore size is crucial for separation selectivity. The adsorption capacity depends mainly on pore size, specific surface area and host-guest interaction. In addition to in situ pore size modulation, post-synthetic modification, intercalation and auxiliary synthetic modulation are also effective pore size modulation strategies. Finally, a single MOF material can no longer meet the requirements of fine separation, and a MOF material with a complex effect of strong hydrogen bond recognition by anions and a sieving mechanism can combine high adsorption capacity and separation selectivity. Therefore, the modulation and functionalization of the pore structure is an effective strategy to address the efficient separation of MOF materials.
SEPARATION OF C2H2/CO2
C2H2 has been widely used as an important industrial gas, either as a fuel in welding/cutting or in the production of commercial chemicals such as vinyl chloride, acrylic acid and 1, 4-butynediol.[72-75] The current industrial production of C2H2 is usually through the cracking of hydrocarbons and therefore inevitably produces carbon dioxide (CO2) impurities. The purification of C2H2 from CO2 is important to obtain high purity C2H2 for the manufacture of related chemicals.[76-78] In this context, the synthesis of selective adsorbents with a strong C2H2 capture capacity is of great importance for the purification of C2H2 gas.
High Density of Open Metal Sites (OMSs). The C≡C triple bond in the C2H2 molecular structure forms strong interaction forces with the open metal sites in the metal-organic framework, which helps to increase the adsorption capacity of C2H2 gas. Therefore, increasing the number of open metal sites in the structure will increase the adsorption capacity of C2H2 gas and thus improve the separation efficiency.
The first example using MOF for selective adsorption of C2H2 over CO2 was realized by Kitagawa et al. They synthesized an ultra-microporous material with 1D channels (4 Å × 6 Å) that can accommodate C2H2 well via strong hydrogen-bonding interactions, showing considerable C2H2/CO2 separation. In 2016, Luo et al. reported an example of a new ZnII-MOF (UTSA-74). It has an fgl topology in which the 1D pore size is approximately 8.0 Å. The building block of UTSA-74 has two different Zn2+ sites, with Zn1 in a tetrahedral and Zn2 in an octahedral coordination configuration. After activation, the two water molecules on the Zn2 site can be removed form UTSA-74a with two binding sites for each Zn2 ion that can adsorb gas molecules (Figure 11). As a result, UTSA-74a has an adsorption capacity of 145 cm-3 g-1 for C2H2, an amount comparable to that of Zn-MOF-74. Interestingly, the accessible Zn2+ sites in UTSA-74a are bridged by CO2 molecules instead of being terminally bound in Zn-MOF-74. Therefore, at room temperature and 1 bar, UTSA-74a adsorbs much less CO2 (90 cm3 cm-3) than Zn-MOF-74 (146 cm3 cm-3), and UTSA-74a can be used for highly selective separation of C2H2/CO2. This result was verified by X-ray single crystal diffraction, gas adsorption isotherms, molecular simulations and gas breakthrough separation experiments.
MOFs with Anionic Groups. Although open metal sites are good for the selective separation of C2H2/CO2, no porous material can completely exclude CO2 from C2H2 based on physical adsorption. However, the open metal sites are not able to achieve complete non-adsorption of CO2 due to their strong effect on both C2H2 and CO2 molecules. The ideal porous material for C2H2/CO2 separation should adsorb one of the gas components and completely exclude the other. Therefore, in order to achieve the desired separation, it is necessary to not only design a framework structure with a pore size of 3.3 Å, but also introduce specific sites that can be used to specifically bind C2H2 molecules.
In 2017, Lin et al. reported an example of a novel microporous material [Zn(dps)2(SiF6)] (UTSA-300) that achieved complete non-adsorption of CO2 gas. The structure not only has a pore size of 3.3 Å but also contains several specific binding sites (Figure 12a). The MOF consists of cage-like structural units with hexafluorosilicate F sites and a 2D network of undulating 2D pore channels in the structure. The structure exhibits pore opening and closing transitions during activation, forming a closed pore framework with 0D cavities after a conformational change UTSA-300a (Figure 12b). Gas adsorption tests show that UTSA-300a exhibits excellent selectivity for C2H2/CO2 separation under ambient conditions and can reach an adsorption capacity of 76.4 cm3 g-1 for C2H2, but does not exhibit any adsorption effect (Figure 12c). Neutron powder diffraction and molecular modelling studies show that the C2H2 molecule binds to the two hexafluorosilicate F atoms in UTSA-300a, mainly through strong C-H, and that the heat of adsorption for C2H2 is 57.6 kJ mol-1. IAST calculations for the C2H2/CO2 separation selectivity of the binary mixture reach 743 at 298 K and 1 bar. Results of dynamic gas breakthrough experiments further demonstrate that the material can efficiently separate C2H2/CO2 to give pure C2H2 gas (Figure 12d), and C2H2 can interact stably with the electrostatic potential around SiF62-, leading to the opening of the pores. On the other hand, the pore structure cannot be opened for CO2 under ambient conditions.
Wu et al. reported an example of a new 3D fluorinated porous framework based on a pillar-cage type structure for the efficient adsorption and separation of C2H2. An example of a pillar-cage type fluorinated porous framework material with an ith-d topology has been constructed using an inorganic SiF62- anion supported pto topology, which has a huge cage cavity and a small window that allows for both high-capacity C2H2 storage and efficient C2H2/CO2 separation (Figure 13a). Unlike the classical pillar-layer type fluorinated porous framework, this material can effectively increase the adsorption capacity of C2H2 due to its huge cage-like cavities. At the same time, the synergistic effect of the ultra-microporous window and the SiF62- anion as a support column enhances the selectivity of the separation of C2H2 gas. In addition, the supporting effect of the SiF62- anion can also greatly enhance the thermal and chemical stability of the structure. Ultimately, the material exhibits an C2H2 adsorption capacity of up to 185 cm3 g-1 at 298 K, 1 atm (Figure 13b). It also shows excellent separation performance in C2H2/CO2 (1:1, v/v) gas mixture breakthrough experiments with a breakthrough time of up to 68 min g-1 (Figure 13c, d). This work provides a new strategy for the synthesis of hybrid porous materials, especially fluorinated porous materials, and offers new ideas for solving the adsorption and separation of low carbon gases.
Suitable Pore Space. In 2020, Ma et al. reported a small molecule ultra-strong nano-trap material Cu-ATC, which acted as a nano-trap for C2H2. Unlike conventional hydrogen bonding or single metal open-site adsorption principles, Cu-ATC has a much higher adsorption capacity for C2H2 molecules due to the docking of two open Cu metal sites. The synergistic effect of this adsorption trap produces a superb enthalpy of adsorption of C2H2 up to 79.1 kJ mol-1 and a static adsorption capacity of C2H2 more than 100 cm3 g-1 at 298 K, thus demonstrating the feasibility of constructing highly efficient functional adsorbents based on the coordination effect (Figure 14).
Wu et al. further developed a series of cage-like porous materials with both ultra-high acetylene storage and excellent C2H2/CO2 separation capabilities.[84, 85] Due to the suitable cage-like cavity structure of all these complexes, they exhibit high C2H2 adsorption capacity at both 1 bar and 298 K. They also show good separation capacity for C2H2/CO2 mixtures due to the significant difference in the adsorption capacities of these complexes for C2H2 and CO2. Results of dynamic breakthrough experiments suggest that all these complexes exhibited good C2H2/CO2 separation capacity, with the longest separation time reaching 90 min g-1 (Figure 15a, b). The results of GCMC simulations indicate that the suitable pore structure and cage space are the key to the excellent separation performance of the FJI-H8-R series of complexes (Figure 15c, d). Therefore, such complexes can solve the trade-off phenomenon between adsorption capacity and separation performance to a certain extent, thus achieving effective separation of C2H2 and CO2 gas mixtures. We believe that this cage-cavity strategy provides a novel way to develop highperformance MOF materials with excellent C2H2/CO2 separation capability.
He et al. constructed a novel MOF (ZJNU-13) based on N-oxide ligands by using a solvothermal approach. The structure shows high adsorption capacity and excellent potential for C2H2 separation and purification due to the exposure of abundant open O donors on the pore surface and suitable pore size. The DFT calculations show that the C2H2 molecule has two binding sites which are located near the N-oxide functionalized dicarboxylate ligand cage B and the triangle windows connecting Cage A and Cage B (Figure 16), respectively. This work demonstrates that N-oxide is an effective strategy for the isolation and purification of C2H2.
Inverse Selective CO2/C2H2 Separation. Currently, most physical adsorbents interact more strongly with C2H2 than with CO2, making selective adsorption of CO2 over C2H2 a rarely reported phenomenon. The development of CO2-selective adsorbent materials (reverse CO2/C2H2 selectivity) remains extremely challenging. In 2016, chen et al. investigated two closely related structures SIFSIX-3-Ni and TIFSIX-2-Cu-i, and they are yin and yang with respect to their ability to separate C2H2 from CO2 and vice versa. Furthermore, it has also been demonstrated by breakthrough experiments that the selectivity between C2H2 and CO2 can be reversed in two closely related porous materials. The author attributed these phenomena to their detailed understanding of the distinct geometry of the primary and secondary binding sites, as well as the resulting differentiation between CO2 and C2H2.
Chen et al. synthesized an example of an ultra-microporous metal-organic framework using copper(Ⅱ) and a 5-fluoropyrimidin-2-olide (known as Cu-F-pymo) and activated the structure under different conditions, and found it could reverse the separation of CO2 from C2H2. Single-component gas adsorption experiments and modelling studies indicated that this molecular sieve effect was attributed to residual water molecules blocking. The breakthrough separation experiments also confirmed that the structure could directly harvest high purity acetylene (> 99.9%) from the gas mixture. Kitagawa et al. performed a precise modulation of the pore space by using amino groups as additional interaction sites, enabling them to increase the adsorption capacity of CO2 while inhibiting the adsorption of C2H2 gas (Figure 17). Based on this strategy, the authors synthesized two isomeric ultramicroporous PCP physisorption agents. They exhibit the highest CO2 uptake and CO2/C2H2 volume uptake ratios at 298 K. Results from DFT calculations show that for CO2 adsorption, the introduction of -NH2 group does not significantly change the orientation of the adsorbed CO2 molecules, as the position of the -NH2 group is parallel to that of the CO2 molecules. Furthermore, it enhances the interaction of the CO2 framework, resulting in a stronger affinity for CO2 adsorption than for C2H2. The results of gas breakthrough separation experiments provide strong evidence for the feasibility of this strategy. Zhao et al. report an example of an ultra-microporous metal-organic framework CeIV-MIL-140-4F based on charge transfer effects for efficiently reverse the CO2/C2H2 separation. The structure exhibits an excellent CO2 uptake capacity (151.7 cm3 cm-3) and an excellent selectivity for reverse CO2/C2H2 separation (above 40), which was verified by simulations and breakthrough separation experiments.
ONE-STEP SEPARATION OF THREE/FOURCOMPONENT MIXTURE
The high-purity preparation of C2H4 plays a pivotal role in the production of primary chemical raw materials. Traditional methods of C2H4 separation are cumbersome and energy intensive. The physical and chemical properties of C2H4 are very similar to those of C2H2, C2H6 and CO2, and the purification process requires a three-step process - an alkaline cell is used to separate CO2; a precious metal catalyst is used to convert C2H2 into C2H4 or C2H6 at high temperatures and pressures; and the purification of both C2H4 and C2H6 still relies on a stepwise distillation process.[91, 92] The different boiling points of the different components are used to control their stepwise flow out of the separation column and to collect them. Such a separation process is energy intensive.[93, 94]
Chen et al. achieved the first one-step efficient separation and preparation of C2H4 in a four-component system. It exploits the synergy between three high-performance ultra-microporous MOFs to achieve the one-step separation and preparation of high-purity C2H4 in a four-component mixed system. It was found that, by effectively tandem coupling the three MOF materials within a single adsorption column, C2H2, C2H6 and CO2 could be efficiently removed in sequence respectively, resulting in the onestep separation and collection of high purity ethylene (> 99.9%) at the end of the column. This physical adsorption process proceeded at ambient conditions and can significantly reduce the energy required for the C2H4 separation process.
Use of a single physical adsorbent for the separation of complex systems offers significant process advantages to fill multiple separation materials. Based on this, Cao et al. used the precise structural design of MOFs to embed specific adsorption sites for multiple impurity gases into a single porous adsorbent at the same time. For the first time, selective adsorption of C2H6, C2H4 and CO2 molecules in a single MOF has been achieved, thus enabling the high-purity preparation of polymer-grade C2H4 in quaternary mixed systems in a single step. Xu et al. designed a Th-MOF for the efficient purification of C2H4 from binary C2H6/C2H4 and ternary C2H4/C2H2/C2H6 mixtures. The structure was able to separate C2H4 (purity > 99.9%) from not only binary C2H6/C2H4 (1:9, v/v) mixtures at 100 kPa and 298 K but also from a C2H6/C2H2/C2H4 ternary mixture (9:1:90, v/v/v) with excellent separation (Figure 18). Density functional theory calculation indicates that the separation mechanism is due to stronger van der Waals interactions between the ethane and MOF skeletons.
Wang et al. reported an example of a non-polar pore with a methyl modification having the ability to purify C2H4 from binary C2H6/C2H4 mixtures and ternary C2H6/C2H4/C2H2 mixtures in one step (Figure 19). Theoretical calculations show multiple interactions between the C2H6 or C2H2 molecule and the pore wall, whereas the C2H4 molecule interacts weakly with the pore wall and is not favorable for C2H4 adsorption due to the repulsive effect of methyl groups.
CONCLUSIONS AND PERSPECTIVE
Efficient separation of hydrocarbon gases is one of the key technologies for modern industrial production. Advances in separation technology not only contribute to energy saving and emission reduction of energy consumption, but also to the achievement of dual carbon goal. Among these, the separation of low carbon hydrocarbons is considered as one of the seven separation challenges and is a difficult and new challenge for high quality production processes. If physical separation technology replaces the traditional thermally driven separation technology, energy consumption can be reduced to half or even one third of the original amount. One of the key technologies to achieve is considered to be the adsorption-based separation. Therefore, the synthesis of materials with high adsorption capacity and high selectivity has become the focus and difficulty of research.
With the increasing variety of MOF materials and MOF composites in recent decades, their application in the field of gas adsorption and separation is promising. Actual separation of gases is not only related to the pore size and specific surface area of the MOF material, but also to the specific recognition sites present in the structure. Therefore, it is important to design and synthesize materials with right pore structure to achieve the target function. Currently, the pore character of MOF materials is generally regulated by in situ synthesis (replacement of metal ions, organic ligands) and post-synthetic modifications.
At present, there are several problems in C2 gas adsorption and separation: (1) Materials that can combine both high adsorption capacity and high separation selectivity in C2H2/C2H4 separation still need to be investigated in depth, in order to achieve complete sieving of C2H2/C2H4 mixtures. (2) In C2H6/C2H4 separation, C2H6 has a larger molecular size compared to the C2H4 molecule, but its quadrupole moment is smaller. Current research has focused on using its rich hydrogen atoms and intermolecular hydrogen bonding to achieve inverse separation of C2H6, but the materials reported so far suffer from low separation selectivity and poor separation results. (3) In C2H2/CO2 separation, the materials reported so far preferentially adsorb C2H2 gas, resulting in poor purity of target gas and requiring multiple steps to achieve the desired purity. Therefore, how to achieve preferential adsorption of CO2 is a difficult area of current research. Despite the effective separation of C2 gas achieved by MOFs materials, there are still some critical issues that need to be addressed.
Moreover, regarding to the industrial application of MOFs materials, the following two major challenges are faced. i) Low-cost mass preparation of MOFs materials: how to solve the problem of green, low-cost and mass preparation of MOFs materials is a major problem. ii) Design of the adsorption separation process: how to scale up the experiments in a millimeters-scale laboratory separation device is another major challenge. The amount of sample used in the laboratory process is at the gram level, and how to scale up the production is a problem that cannot be avoided. The solution of the above two problems will provide the scientific basis for the application in the adsorption separation industry.
ACKNOWLEDGEMENTS: The Postgraduate Innovative Research Projects of Tianjin. COMPETING INTERESTS
The authors declare no competing interests.
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Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0132
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Figure 3 Adsorption equilibrium isotherms of C2H6 (a) and C2H4 (b) on Mg-MOF-74 at 278, 298, and 318 K. Solid lines represent the dual-site Sips model fits. Reprinted with permission from ref. . Copyright 2011 American Chemical Society.
Figure 4 X-ray single crystal structure of UiO-66-type MOFs, indicating that 2-connected linkers bridge 12-connected [Zr6(μ3-OH)8(O2C-R)12] molecular building blocks (MBBs) to form the 3D fcu-topology frameworks (a); The pore window size can be systemically modulated via the judicious choice of organic linkers (b); Contracted after the configuration of copper(I) ions (c). Reprinted with permission from ref. . Copyright 2019 Wiley-VCH.
Figure 5 Breakthrough profile obtained for an equimolar mixture of C2H4 and C2H6 on a column packed with ZIF-7 pellets at 25 ℃ and 1 bar. Reprinted with permission from ref. . Copyright 2010 American Chemical Society.
Figure 6 Preferential binding sites for C2D6 and C2D4 molecules and the close van der Waals contacts within the rhombic cavity of aromatic rings (a, b); C2H6 and C2H4 sorption isotherms for Cu(ina)2 and Cu(Qc)2 at 298 K (c); Experimental column breakthrough curves for equimolar C2H6/C2H4 mixture in an absorber bed packed with Cu(Qc)2 (d). Reprinted with permission from ref. . Copyright 2018 American Chemical Society.
Figure 7 Preferential adsorption sites for (a) C2H6 and (b) C2H4. Reprinted with permission from ref. . Copyright 2015 Springer Nature.
Figure 8 C2H2 (blue square), CO2 (red dot) and C2H4 (green triangle) on M'MOF-3a (a) and M'MOF-2a (b) at 295 K. Reprinted with permission from ref. . Copyright 2011 Springer Nature.
Figure 9 Cyclic column-breakthrough curves of FJI-W1 with 2 mL/min flow rate at 298 K for (a) 1/99 C2H2/C2H4 and (b) 50/50 C2H2/C2H4. Density functional theory (DFT) calculated the interaction sites of FJI-W1 with the gas molecules (c) C2H2 and (d) C3H4. Reprinted with permission from ref. . Copyright 2022 American Chemical Society.
Figure 10 Crystal structure and simplified diagram of (a) NCU-100m⊃C2H2 (in parallel projection) and (b) NCU-100a⊃C2H2. C2H2 adsorption from Rietveld refinements in (c) Site I and (d) in both sites of NCU-100a. Reprinted with permission from ref. . Copyright 2020 American Chemical Society.
Figure 11 DFT-D optimized structure of (a) UTSA-74⊃C2H2 and (b) X-ray single crystal structure of UTSA-74⊃CO2 in which the local coordination environments are shown on the right. Reprinted with permission from ref. . Copyright 2016 American Chemical Society.
Figure 12 (a) Preferential binding sites for C2D2 molecules (sites I and II). (b) Hirshfeld surface (di) displaying C-D…F interactions (red area). (c) C2H2, CO2 and C2H4 sorption isotherms for UTSA-300a at 273 K. (d) Experimental column breakthrough curves for equimolar C2H2/CO2. Reprinted with permission from ref. . Copyright 2017 American Chemical Society.
Figure 13 (a) The distribution of two different types of cages in the framework; (b) C2H2 and CO2 uptakes at 298 K and 1 bar; (c) Three-cycle tests with a 2 mL min-1 flow rate at 298 K. (d) Comparison of 0.1 bar C2H2 uptake and breakthrough time among representative MOFs. Reprinted with permission from ref. . Copyright 2021 Wiley-VCH.
Figure 14 (a) The distribution of two different types of cages in the framework; (b) C2H2 and CO2 uptakes at 298 K and 1 bar; (c) Three-cycle tests with a 2 mL min-1 flow rate at 298 K. (d) Comparison of 0.1 bar C2H2 uptake and breakthrough time among representative MOFs. Reprinted with permission from ref. . Copyright 2021 Wiley-VCH.
Figure 15 (a) Breakthrough curves of FJI-H8-Me for separating CO2/C2H2 (50:50) at 298 K and 1 bar; (b) Five cycles of tests for the separating CO2/C2H2 are shown in different color lines; Possible binding sites calculated by GCMC simulation for C2H2 (c) and CO2 (d). Reprinted with permission from ref. . Copyright 2021 Wiley-VCH.
Figure 16 The optimized adsorption sites for isolated gas molecule (a-b) C2H2 and (d-e) CO2, as well as (c) C2H2-C2H2. The (f) shows the ESP fitted atomic charges for some atoms in ZJNU-13. The units for bond distance, binding energy, and atomic charge are Å, kJ mol-1, and |e|, respectively. The atoms are represented by balls with different colors (C, grey; H, white; N, blue; O, red; Cu, brown). Reprinted with permission from ref. . Copyright 2020 American Chemical Society.
Figure 17 Schematic illustration of the "opposite action" strategy for boosting CO2/C2H2 selectivity. (a) Different binding modes of CO2 and C2H2 in microporous channels. (b) Additional interacting site is precisely introduced to not only provide enhanced CO2-framework interaction but also suppress C2H2 adsorption by driving out some of the initially adsorbed C2H2 molecules; the expulsion of the adsorbed C2H2 molecules is driven by space limitations originating from the interference in the pore-guest interaction, resulting in high inverse selectivity. Reprinted with permission from ref. . Copyright 2021 Wiley-VCH.
Figure 18 C2H6/C2H4/C2H2 (90/9/1, v/v/v) ternary mixture separation of Azole-Th-1. Reprinted with permission from ref. . Copyright 2020 Springer Nature.
Figure 19 Breakthrough curves for C2H6/C2H2/C2H4 at 298 K (a) (5/5/5, v/v/v); (b) (6/6/3, v/v/v); (c) (2/10/10, v/v/v); (d) (3/3/10, v/v/v). Reprinted with permission from ref. . Copyright 2022 Wiley-VCH.
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