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Research Article  |  Open Access  |  7 Apr 2024

Biological metal–organic frameworks for natural gas purification and MTO product separation

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Chem Synth 2024;4:22.
10.20517/cs.2023.69 |  © The Author(s) 2024.
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The use of porous solid adsorbents is an effective and excellent approach for the separation and purification of methanol-to-olefins product and methane (CH4). In this particular study, a series of adenine (AD)-based biological metal–organic frameworks (Bio-MOFs) {Their general formula is Cu2(AD)2(X)2 [X = formic acid, acetic acid (AA), and propionic acid]} were proposed, which exhibited remarkable efficiency in the purification of CH4 and the separation of C3H6 from methanol-to-olefins product, ultimately yielding purified C2H4. The experimental findings demonstrate that different terminal ligands induce alterations in the pore microenvironment, consequently leading to variations in adsorption capacities and stability. Specifically, Cu-AD-AA exhibits the highest adsorption capacity and selectivity among the three MOFs, as confirmed by static adsorption isotherm testing and theoretical evaluation using ideal adsorbed solution theory (IAST) simulation. At 298 K and 1 bar, Cu-AD-AA exhibits 786 and 10.9 selectivity for C3H8/CH4 and C3H6/C2H4, respectively, surpassing the majority of MOFs materials. Furthermore, breakthrough experiments conducted in ambient conditions reveal that Cu-AD-AA possesses commendable separation capabilities, enabling one-step purification of C2H4 at varying proportions (C2H4/C3H6 = 50:50, 50:20, and 90:10), along with satisfactory recycling performance. Importantly, the synthesis of Cu-AD-AA utilizes simple and easily obtainable raw materials, thereby offering advantages such as cost-effectiveness, low toxicity, and facile synthesis that enhance its potential for industrial applications.


Bio-MOFs, MTO product separation, natural gas purification, porous solid adsorbents


In recent years, light hydrocarbons as important chemical raw materials and fuels have gained significant application in various fields[1,2]. Natural gas is widely recognized as a clean energy that can be extensively applied in power generation, domestic fuel, and even in the automotive industry[3-5]. Pyrolysis gas, which is a major source of natural gas, primarily consists of methane (CH4) and light hydrocarbons (C2-C3) such as acetylene (C2H2), ethylene (C2H4), ethane (C2H6), and propane (C3H8)[6,7]. However, the presence of these light hydrocarbon impurities not only decreases the conversion rate of CH4 but also affects the stability of cyclic processes during storage and the safe transportation through pipelines. Therefore, it is necessary to remove these impurities to ensure the purity of the natural gas before it can be liquefied and transported[8]. Interestingly, these C2 and C3 light hydrocarbons also serve as essential raw materials in the petrochemical industry. Effective separation of these light hydrocarbons from pyrolysis gas not only enhances the purity of CH4 but also enables efficient utilization of C2-C3 molecules.

In addition to CH4 purification, the industrialization of the methanol-to-olefins (MTO) reaction has witnessed significant advancements in recent years[9,10]. MTO reaction is a significant and advanced method for producing C2H4 from coal and natural gas, with the product consisting of approximately 21 wt% C3H6 and 51 wt% C2H4[11,12]. Therefore, the separation of C3H6 and C2H4 in MTO product is crucial for downstream applications[13,14]. However, the separation of C3H6 and C2H4 poses a complex and challenging task due to their relatively low boiling points (225.4 K for C3H6 and 169.4 K for C2H4) and similar kinetic diameters (4.6 Å for C3H6 and 4.2 Å for C2H4)[15-17]. Due to the relatively low boiling point of these light hydrocarbons, conventional methods for separating light hydrocarbons often rely on high pressure and low temperature distillation processes. Nevertheless, this approach suffers from several drawbacks including high energy consumption, large footprint, and complex operation[18]. To address these issues, combining physical adsorbents with pressure swing adsorption (PSA) technology has emerged as a promising alternative[19]. This method, based on the principle of physical adsorption, can be conducted at ambient temperature and pressure, not only offering a simple operational procedure but also ensuring low energy consumption. Among various physical adsorbents, metal–organic frameworks (MOFs) materials have gained significant attention in materials science due to their excellent properties[20-22].

MOFs are crystalline structures formed through coordination bonds between metal ions or clusters and organic ligands[23-25], which exhibit higher specific surface area, porosity, and adjustable structure compared to traditional materials such as porous silicon, activated carbon, and zeolite[26-28]. As a result, it offers a broader range of potential applications; for example, it has fantastic uses in the fields of gas adsorption and separation, energy conversion, drug delivery, water vapor capture, catalysis, and sensing[29-33]. In industrial settings, MOFs materials can be designed to purify natural gas and separate MTO product based on slight physical differences between these light hydrocarbons[34]. For example, C3H8 and C2H6 molecules have slightly larger sizes and polarizabilities than CH4. By designing pore size close to C3H8 and C2H6 molecules and enhancing interaction between the framework and gas molecules, efficient natural gas purification can be achieved[35]. Similarly, for C3H6 and C2H4, pores can be designed based on their difference in polarizability, and the introduction of specific organic functional groups can increase the van der Waals interaction between the host material and guest molecules, thereby facilitating the efficient separation of C3H6/C2H4 mixtures[36]. In addition, MOFs materials, which operate on the principles of physical adsorption, possess low adsorption enthalpy, exceptional thermal stability, and regenerability, enabling efficient adsorption and desorption at moderate temperatures. This characteristic can significantly save energy consumption[37].

Thanks to the progress of reticular chemistry, MOFs materials have enabled precise regulation of pore environments to selectively adsorb specific gas molecules[38]. Generally, two strategies are employed for regulating pores in MOFs. One involves substituting metal ions to achieve sub-angstrom precision regulation[39], while the other adjusts ligands to accurately regulate the structure by altering the length and introducing functional groups[40,41]. However, the first approach is limited by different coordination modes of metal ions and restricted choices of metals, making it difficult for widespread use. In contrast, the second technique, which utilizes organic ligands, is more commonly adopted due to the wide range of options available. In the pursuit of industrial-grade production, factors such as environmental friendliness and cost-effectiveness must be considered. Traditional MOFs heavily rely on ligands produced through organic synthesis[42]. Many of these ligands, particularly those with special functional groups (such as -NH2, -CH3, -CH2CH3 and -CF3), require customization, leading to significant costs. Moreover, most of these ligands are toxic and unsuitable for large-scale production. In contrast, Bio-MOFs composed of biomolecules and metal ions have gained attention among various MOFs materials due to their low cost, low toxicity, good reproducibility, and adherence to the principles of green chemistry[43,44]. Inexpensive biomolecules, such as amino acids, proteins, peptides, and porphyrins, can serve as ligands for synthesizing Bio-MOFs[45,46]. However, the study of Bio-MOFs for light hydrocarbon separation remains relatively limited compared to their use in carbon dioxide capture[47,48].

Based on the aforementioned considerations, we have developed a solvothermal synthesis method for three Bio-MOFs with the traditional lvt topology. The rigid ligand adenine (AD), abundant in nitrogen atoms and amino groups, is an excellent choice for constructing stable MOFs with active sites. While Cu-AD-AA and Cu-AD-PA structures were previously reported using diffusion methods, in this study, we successfully synthesized them through solvothermal techniques[49], which are simpler and more scalable for large-scale production, resulting in defect-free crystals suitable for gas separation applications. Cu-AD-FA was reported for the first time, and its structure was analyzed by single-crystal X-ray diffraction experiments. Structural analysis reveals that different terminal carboxylic acid ligands significantly influence the pore environment (channel size, pore volume, active site) of materials. It is noteworthy that previous researchers have conducted some gas adsorption and separation studies for Cu-AD-AA and Cu-AD-PA materials. For example, in 2012, Pérez-Yáñez et al. explored the CO2/H2 and CO/H2 separation properties of these two materials[50]. Similarly, in 2020, Li et al. investigated the separation performance of Cu-AD-PA specifically for C2H2/CO2[51]. However, a comprehensive investigation into the separation ability of light hydrocarbons, especially for C2H4/C3H6 separation, has not been done so far. Therefore, the light hydrocarbon adsorption separation of these materials was systematically studied in this work. Among them, Cu-AD-AA has exhibited excellent adsorption and separation capacity, demonstrating great potential in natural gas purification and MTO product separation.


Materials and methods

The chemicals used in this investigation were acquired from commercial sources and were not subjected to further purification. Scanning electron microscope (SEM) images were captured using a JEOL JSM-7800F microscope. Dynamic light scattering measurements were conducted at 25 oC on a Malvern Zetasizer Nano-ZS90 instrument (detection range: 3-3,000 nm). Powder X-ray diffraction (PXRD) data were collected using a Rigaku D/max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Thermogravimetric analysis (TGA) data were obtained using a TGA Q500 thermogravimetric analyzer under air conditions at a heating rate of 10 oC min-1. Surface area was measured using N2 adsorption isotherms at 77 K with a Micromeritics ASAP 2420 instrument. CH4, C2H2, C2H4, C2H6, C3H6 and C3H8 adsorption isotherms were determined using a Micromeritics ASAP 2020 instrument. The breakthrough experiments were conducted in a homemade HPMC41 gas separation test system (Nanjing Hope Analytical Equipment Co., Ltd).

Synthesis of Cu-AD-FA

A mixture of Cu(NO3)2·3H2O (12 mg, 0.05 mmol), AD (16 mg, 0.12 mmol), N,N-dimethylformamide (DMF, 2.0 mL) and FA (0.3 mL) was sealed in a 20 mL vial and heated at 65 oC for 24 h. The green octahedron crystals were gathered and washed with EtOH and then dried in air [20 mg, yield 64%, based on Cu(NO3)2·3H2].

Synthesis of Cu-AD-AA

A mixture of Cu(NO3)2·3H2O (2.4 g, 0.010 mol), AD (3.2 g, 0.024 mol), N,N-dimethylacetamide (DMA, 100 mL), H2O (30 mL) and AA (70 mL) was sealed in a 250 mL vial and heated at 65 oC for 24 h. The green powders were gathered and washed with EtOH and then dried in air [3.12 g, yield 61%, based on Cu(NO3)2·3H2O]. The microcrystals of Cu-AD-AA exhibit a regular octahedral shape which was examined by SEM tests [Supplementary Figure 1A]. These microcrystals reveal an average diameter of 615 nm, as determined by dynamic light scattering tests [Supplementary Figure 1B].

Synthesis of Cu-AD-PA

A mixture of Cu(NO3)2·3H2O (2.4 g, 0.010 mol), AD (3.2 g, 0.024 mol), N-Methyl-2-pyrrolidone (NMP, 100 mL), H2O (30 mL) and PA (60 mL) was sealed in a 250 mL vial and heated at 65 oC for 24 h. The green powders were gathered and washed with EtOH and then dried in air [3.07 g, yield 57%, based on Cu(NO3)2·3H2O]. The microcrystals of Cu-AD-PA exhibit a regular octahedral shape as examined using SEM tests [Supplementary Figure 1C]. These microcrystals reveal an average diameter of 615 nm as determined by dynamic light scattering tests [Supplementary Figure 1D]. The container photographs for Cu-AD-AA and Cu-AD-PA after the synthesis reaction are shown in Supplementary Figure 2.

X-ray crystallography

Crystallography data of Cu-AD-FA was collected at room temperature on a Bruker Apex III CCD single crystal diffractometer by graphite-monochromated Mo-Kα (λ = 0.71073 Å). The structure was solved by a direct method and then refined by a least square method on F2 in Olex2. The hydrogen atoms were added geometrically. The formula of Cu-AD-FA is Cu2(AD)2(FA)2(DMF)1.5. The CCDC number of Cu-AD-FA is 2312625. Crystal data and structural refinement results are summarized in Supplementary Table 1.


Crystal structure description

By adjusting the size of the terminal ligands in the channel in a unified structure, the channel environment can be effectively improved to achieve a small regulation and a large influence[52,53]. For example, the aqueous synthesis of MOF-808, previously reported by our group, improved the water adsorption performance by modifying the carboxylic acid-based modulator, enabling the straightforward tuning of the pore environment[54]. In this study, a classic Bio-MOF platform is utilized, which was composed of AD ligand, Cu2+ ions and terminal ligand (FA, AA, PA). It is worth mentioning that similar structures were observed in Bio-MOF-11-14 series, which consisted of cobalt ions with AD and terminal carboxylic acid ligands. Specifically, Bio-MOF-11 was isoreticular to Cu-AD-AA, and Bio-MOF-12 was isoreticular to Cu-AD-PA[48,49]. It provides significant evidence for the outstanding designability of this series of materials as a reticular chemistry platform. The unit cell parameters for these MOFs can be found in Supplementary Table 2, indicating that as the size of the terminal ligand increases, the unit cell parameters are slightly elongated along the a and b axes but compressed along the c axis. The theoretical pore volumes for the three MOFs are 0.35 (Cu-AD-FA), 0.28 (Cu-AD-AA), and 0.23 cm3/g (Cu-AD-PA), respectively, due to the increased density of the skeleton.

The structure of Cu-AD-FA was determined by single-crystal X-ray structure analysis, and the other two structures were proved by PXRD characterization [Supplementary Figure 3]. The structure of these three Bio-MOFs is illustrated in detail in Figure 1. In Figure 1A, Copper dimer clusters are connected to AD ligands, forming a three-dimensional structure. Each copper dimer is linked by four AD ligands and two monocarboxylic acids. Each AD ligand links to two copper dimers, resulting in one nitrogen atom and one amino group per AD within the structure, which can serve as polar adsorption sites. These MOFs exhibit the same lvt topology as the aforementioned MOFs [Supplementary Figure 4A]. However, different terminal ligands cause slight changes within the pore [Figure 1B-D]. The steric hindrance of the terminal ligands gradually transforms the pore size from narrow to more open. To observe this change more directly, Supplementary Figure 4B-D show that the distance between nitrogen atoms and amino groups at the same position increases as the size of the terminal ligands enlarges (3.2 Å for Cu-AD-FA, 4.4 Å for Cu-AD-AA and 5.4 Å for Cu-AD-PA). Instead of conforming to a traditional one-dimensional channel, the structure of this series exhibits three-dimensional intercrossed channels [Figure 1E-G]. Notable alterations occur within the interior environment of the cavity due to variations of the terminal ligands. The inner wall of the channel is non-uniformly smooth; thus, measurements primarily focus on the length of crossed positions and long cavity length. In the Cu-AD-FA structure depicted in Figure 1H, there are two dimensions of cavities within the Z-shaped channel: a longer cavity measuring approximately 11.2 Å in length and a narrower cavity with a width of 8.2 Å. Unlike Cu-AD-FA, Cu-AD-AA incorporates methyl group into its skeleton, which widens the pore size. Consequently, both the long cage and narrower cavity increase to approximately 12.7 Å and 8.8 Å, respectively [Figure 1I]. Notably, more pronounced changes occur with Cu-AD-PA, where ethyl group is introduced into the Cu-AD-FA structure. While the long cavity size remains around 12.7 Å, the narrow cavity reduces to about 6.8 Å [Figure 1J]. This slight change in the tunnel environment is highly intriguing, prompting us to investigate their adsorption properties concerning light hydrocarbons.

Biological metal–organic frameworks for natural gas purification and MTO product separation

Figure 1. Structure of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA. (A) The composition structure of metal clusters; Ball and stick diagram of (B) Cu-AD-FA; (C) Cu-AD-AA; and (D) Cu-AD-PA. The three-dimensional intercrossed channels of (E) Cu-AD-FA; (F) Cu-AD-AA; and (G) Cu-AD-PA. Pore structure displayed by Connolly surface of (H) Cu-AD-FA; (I) Cu-AD-AA; and (J) Cu-AD-PA. Color scheme. White: Hydrogen; gray: carbon; blue: nitrogen; red: oxygen; green: copper.

Thermal and chemical stability

The thermal stability of these materials was investigated through TGA analysis and variable temperature PXRD tests [Supplementary Figures 5 and 6]. Variable temperature PXRD tests revealed that Cu-AD-FA could withstand temperatures up to 100 oC before its structure collapsed at 150 oC [Supplementary Figure 6A]. Conversely, Cu-AD-AA demonstrated good crystallization even at 200 oC, with slight broadening observed at 250 oC and a significant decrease in crystallinity at 300 oC, accompanied by weakened peak intensity [Supplementary Figure 6B]. Similarly, Cu-AD-PA showed comparable thermal stability to Cu-AD-AA, maintaining strong peak intensity without broadening up to 250 oC but experiencing decreased crystallinity beyond that temperature [Supplementary Figure 6C]. After soaking in different solvents for three days, PXRD tests proved that the three compounds have excellent chemical stability [Supplementary Figure 7]. The water stability test was conducted. After one day of soaking the three MOFs materials in water, the samples were reactivated for nitrogen adsorption measurements. The results show that the water stability of the three materials is not good enough, but their resistance to water is slightly different, among which Cu-AD-PA has the strongest resistance to water, followed by Cu-AD-AA and Cu-AD-FA [Supplementary Figure 8]. These differences can be attributed to the hydrophobicity changes of the terminal ligands.

Porosity of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA

Prior to the 77 K nitrogen adsorption test, these three MOFs underwent pre-activation. Approximately 300 mg of the synthesized sample was placed into a 40 mL centrifuge tube and immersed in EtOH. After a soaking period of four hours, centrifugation was conducted, and the supernatant was discarded. Subsequently, fresh EtOH was added, and the process was repeated. Following an additional two-day exchange period, the solid sample underwent centrifugation, followed by placement in a vacuum oven for degassing at room temperature for 20 min. The dried sample was then transferred to an adsorption tube and subjected to heating and degassing on a Micromeritics ASAP 2420 instrument for ten hours. It is worth noting that there was a slight difference in the pre-treatment temperatures. Cu-AD-FA required removal of guest molecules at a temperature of 40 oC, while Cu-AD-AA and Cu-AD-PA necessitated their removal at a temperature of 90 oC. This discrepancy arises from the poor thermal stability exhibited by Cu-AD-FA, as its skeleton tends to collapse with increasing temperature [Supplementary Figure 9].

The resulting 77 K nitrogen adsorption isotherm is depicted in Figure 2A. Among them, Cu-AD-AA exhibits the highest adsorption capacity, with its actual pore volume closely aligning with theoretical values. Conversely, Cu-AD-FA falls short of reaching its theoretical pore volume due to partial skeleton collapse, whereas Cu-AD-PA successfully achieves its theoretical pore volume. Compared to previously reported works[49], the three materials synthesized via solvothermal synthesis in this study show significantly fewer defects and notable improvements in pore volume and Brunauer-Emmett-Teller (BET) specific surface area [Table 1], indicating that these MOFs materials in this work are more suitable for gas adsorption and separation investigation. The pore size distributions of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA calculated by the nonlocal density functional theory (NLDFT) model, which are basically consistent with the dimensions measured by the crystal structure [Figure 2B]. The pore volume and thermal stability of these three materials exhibit a remarkable trade-off effect. Specifically, Cu-AD-AA demonstrates optimal properties, displaying enhanced stability and a high BET specific surface area.

Biological metal–organic frameworks for natural gas purification and MTO product separation

Figure 2. (A) N2 sorption isotherms for Cu-AD-FA, Cu-AD-AA and Cu-AD-PA at 77 K; (B) The pore size distributions of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA calculated by NLDFT model. NLDFT: Non-local density functional theory.

Table 1

BET surface area, Langmuir surface area, pore volume measured by experiment, theoretical pore volume and comparison of pore volume reported in the literature[49]

Pore volume
Theoretical pore volume
Reference pore volume

Gas adsorption behavior of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA

To comprehensively elucidate the impact of pore microenvironment changes on gas adsorption properties, adsorption isotherms of CO2 and C1-C3 light hydrocarbon gas molecules were tested at 273 and 298 K, respectively [Supplementary Figures 10-13]. In order to more clearly describe the adsorption properties of these MOFs materials, the adsorption isotherms at 298 K and their adsorption enthalpies (Qst) were shown in Figure 3. The adsorption capacity of C3 and C2 molecules for the three MOFs is significantly higher than that of CH4 [Figure 3A-C]. This discrepancy stems from the greater polarizability and larger molecular size of C2 and C3, facilitating stronger interactions with the framework [Supplementary Table 3]. Notably, the variance in adsorption capacity is most pronounced at low pressures. Detailed specific adsorption values at 10 kPa and 101 kPa at 298 K can be found in Supplementary Table 4. Regarding C2 molecules, the presence of polar sites within the pores plays a crucial role in their binding ability. In terms of a horizontal comparison, C2H2 possesses the smallest pKa value and can form hydrogen bonds with numerous amino and nitrogen atoms within the pores, leading to higher adsorption capacity than C2H4 and C2H6. C2H4 and C2H6 exhibit similar sizes and polarities, consequently resulting in comparable adsorption capacities. From a longitudinal perspective, the channel environment undergoes minimal changes from Cu-AD-FA to Cu-AD-AA and then to Cu-AD-PA, yet these modifications significantly influence the adsorption capacity of C2 molecules. Specifically, the adsorption capacity of Cu-AD-FA for C2H2 at 10 and 101 kPa is merely 18 and 42 cm3/g, while under the same conditions, the adsorption capacity of Cu-AD-AA for C2H2 reaches as high as 32 cm3/g and 67 cm3/g. Clearly, besides the role of the amino group, the dispersion force between methyl and C2H2 also contributes positively. The adsorption capacity of Cu-AD-PA for C2H2 is 29 and 60 cm3/g, which may explain the lower pore volume compared to Cu-AD-AA, although it still surpasses the adsorption capacity of Cu-AD-FA. Similarly, under analogous conditions, the adsorption capacities of C2H4 and C2H6 follow a similar gradient trend, with Cu-AD-AA exceeding both Cu-AD-PA and Cu-AD-FA. Similar to the C2 molecule, the adsorption capacity of Cu-AD-FA for the C3 molecule remains the lowest among the three. The adsorption capacity of Cu-AD-PA for C3 molecules is inferior to that of Cu-AD-AA, possibly due to a reduction in pore volume and the free rotation of the ethyl group. At 298 K, 1 bar, the C3H8 adsorption capacities of the three MOFs were 21 (Cu-AD-FA), 74 (Cu-AD-AA) and 33 cm3/g (Cu-AD-PA), respectively. Under the same conditions, the adsorption capacities for C3H6 were 21, 75 and 37 cm3/g, respectively. Notably, Cu-AD-AA demonstrates a remarkable adsorption capacity for C3H6, with an adsorption amount of 59 cm3/g at 10 kPa and 298 K, surpassing several high-performance C3H6 adsorbents such as nickel nitroprusside (Ni-NP)[55], NUM-7a (an ultramicroporous Mn-based MOF)[56], NKMOF-11 (a new ultramicroporous MOF)[57], and hydrogen-bonded organic framework (HOF)-16a[58]. Additionally, Cu-AD-AA also exhibits significant C3H8 uptake (56 cm3/g) at 10 kPa. Clearly, the internal environment of the Cu-AD-AA channel facilitates the adsorption of C3 molecules. In terms of CO2 adsorption, Cu-AD-AA demonstrated the highest adsorption capacity among the three materials, reaching 65.45 cm3/g at 298 K and 1 bar. Cu-AD-FA exhibited a slightly lower adsorption capacity of 58.44 cm3/g, while Cu-AD-PA displayed the lowest adsorption capacity at 28.72 cm3/g.

Biological metal–organic frameworks for natural gas purification and MTO product separation

Figure 3. CO2, CH4, C2H2, C2H4, C2H6, C3H6 and C3H8 adsorption isotherms of (A) Cu-AD-FA; (B) Cu-AD-AA; (C) Cu-AD-PA at 298 K and 1 bar. Qst curves of (D) Cu-AD-FA; (E) Cu-AD-AA; (F) Cu-AD-PA.

To assess the interaction strength between the host framework and these light hydrocarbons and CO2, the adsorption enthalpies for the three MOFs were calculated. The Virial equation was employed to fit the adsorption curves at 273 K and 298 K, from which the Qst values were derived [Supplementary Figures 14-17]. As depicted in Figure 3D, the Qst values of Cu-AD-FA for various gases are as follows: CO2 (28.3 kJ/mol), CH4 (15.7 kJ/mol), C2H2 (24.8 kJ/mol), C2H4 (21.7 kJ/mol), C2H6 (21.3 kJ/mol), C3H6 (25.1 kJ/mol), and C3H8 (24.2 kJ/mol). In contrast, Cu-AD-AA exhibits significantly higher Qst for C3H6 (48.7 kJ/mol) and C3H8 (43.1 kJ/mol) compared to CH4 (16.4 kJ/mol). Additionally, the Qst of Cu-AD-AA for CO2, C2H2, C2H4, and C2H6 are 25.87, 34.2, 32.0, and 29.9 kJ/mol, respectively [Figure 3E]. On the other hand, Cu-AD-PA shows the following Qst for various gases: CO2 (16.83 kJ/mol), CH4 (15.1 kJ/mol), C2H2 (32.7 kJ/mol), C2H4 (28.2 kJ/mol), C2H6 (26.9 kJ/mol), C3H6 (27.8 kJ/mol), and C3H8 (26.3 kJ/mol) [Figure 3F]. A clear comparison reveals that Cu-AD-AA demonstrates a stronger affinity towards C3 and C2 molecules, while Cu-AD-FA exhibits the weakest affinity for these specific gas molecules. Based on these findings, it is evident that Cu-AD-AA holds the highest potential for effectively separating natural gas and MTO product.

Gas separation behavior of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA

As illustrated in Supplementary Figures 18-20, the adsorption data (CO2, CH4, C2H2, C2H4, C2H6, C3H6, and C3H8) obtained from experiments for Cu-AD-FA, Cu-AD-AA, and Cu-AD-PA were fitted using the dual-site Langmuir-Freundlich equation. The fitting parameters are within the error range and are listed in Supplementary Tables 5-7. The theoretical selectivity of Cu-AD-FA, Cu-AD-AA, and Cu-AD-PA for C2H2/CH4, C2H4/CH4, C2H6/CH4, and C3H8/CH4 was predicted by ideal adsorbed solution theory (IAST) calculations. The selectivity for C3H8/CH4 (0.5:0.5) was found to be 36, 746, and 31 for Cu-AD-FA, Cu-AD-AA, and Cu-AD-PA, respectively [Figure 4A]. For C2H6/CH4 (0.5:0.5), the selectivity was 29, 50, and 28, respectively [Figure 4B]. Similarly, for C2H2/CH4 (0.5:0.5), the selectivity was 53, 81, and 68 for Cu-AD-FA, Cu-AD-AA, and Cu-AD-PA, respectively, and for C2H4/CH4 (0.5:0.5), the selectivity was 34, 54, and 34 [Supplementary Figure 21]. Notably, Cu-AD-AA exhibits higher selectivity for C3H8/CH4 and C2H6/CH4 than many previously reported famous MOFs. Cu-AD-AA demonstrated higher selectivity for C3H8/CH4 compared to ZUL-C2 (632)[59], PCN-224 (609)[60], FJI-C1 (471)[61] and BSF-1 (353)[62]. The selectivity of Cu-AD-AA for C2H6/CH4 is lower than that of top-performing MOFs such as ZUL-C2 (91) and BSF-2 (53)[63]. However, it remains greater than the selectivity of most reported MOFs materials. In summary, Cu-AD-AA demonstrates excellent potential as a physical adsorbent for CH4 purification, given its high selectivity towards C3H8/CH4 and C2H6/CH4. A more detailed comparison of C3H8/CH4 and C2H6/CH4 is found in Supplementary Table 8. Furthermore, the separation selectivity for gas molecules with similar sizes, such as C2H2/CO2, C2H4/C2H6, and C3H6/C3H8, was investigated. For C2H2/CO2, it is interesting that Cu-AD-FA is preferred for CO2 adsorption; Cu-AD-AA and Cu-AD-PA are preferred for C2H2 adsorption. The selectivity of CO2/C2H2 (0.5:0.5) for Cu-AD-FA is 1.47 and selectivity of C2H2/CO2 (0.5:0.5) for Cu-AD-AA and Cu-AD-PA are 2.10 and 8.86, respectively [Supplementary Figure 22]. Both C2H2/CO2 selectivity and C2H2 adsorption capacity of Cu-AD-PA are higher than those synthesized by the diffusion method in the literature, demonstrating the significance of this study once again. The selectivity of Cu-AD-FA, Cu-AD-AA and Cu-AD-PA for C2H4/C2H6 (0.5:0.5) is 1.16, 1.04 and 1.27, respectively. Their C3H6/C3H8 (0.5:0.5) selectivity is 1.53, 1.64 and 3.16, respectively [Supplementary Figure 23].

Biological metal–organic frameworks for natural gas purification and MTO product separation

Figure 4. (A) Selectivity of equimolar mixtures of C3H8 over CH4 for Cu-AD-FA, Cu-AD-AA and Cu-AD-PA; (B) Selectivity of equimolar mixtures of C2H6 over CH4 for Cu-AD-FA, Cu-AD-AA and Cu-AD-PA; (C) Selectivity of equimolar mixtures of C3H6 over C2H4 for Cu-AD-FA, Cu-AD-AA and Cu-AD-PA; (D) Comparison of IAST selectivity of C3H6/C2H4 (0.5/0.5) with other reported MOFs. IAST: Ideal adsorbed solution theory; MOFs: metal–organic frameworks.

The selectivity for the separation of MTO product was also determined using IAST calculations [Figure 4C and Supplementary Figure 24]. Supplementary Figures 18 and 20 illustrate the adsorption capacities of C3H6 for Cu-AD-FA and Cu-AD-PA, which are initially higher than those of C2H4 at low pressures. However, the adsorption capacity of C2H4 surpasses that of C3H6 shortly thereafter. Consequently, these MOFs are unsuitable for effectively separating the C3H6/C2H4 mixture. In contrast, Cu-AD-AA exhibits a higher adsorption capacity for C3H6 compared to C2H4 in both low and high pressure regions [Supplementary Figure 19]. The IAST results presented in Figure 4C and Supplementary Figure 24 indicate that Cu-AD-AA demonstrates selectivity values of 10.9 (v/v = 0.1/0.9), 10.8 (v/v = 0.2/0.5), and 10.9 (v/v = 0.5/0.5) for different proportions of C3H6/C2H4. Conversely, the equimolar selectivity of Cu-AD-FA and Cu-AD-PA decreases to 1.1 and 1.9 at 101 kPa, indicating their inability to effectively separate C3H6 and C2H4. Notably, the C3H6/C2H4 selectivity of Cu-AD-AA exceeds that of numerous reported materials. As shown in Figure 4D, the selectivity of Cu-AD-AA ranks second, only surpassed by Zn2(oba)2(dmimpym) (15.6)[64], and is higher than those reported same type MOFs materials, such as CoV-(CF3)3bdc-tpt (10.1)[65], Y-pek-MOF-1 (9.0)[15], NEM-7-Cu (8.61)[66], Mn-dtzip (8.6)[67], spe-MOF (7.7)[16] and Zn-BPZ-SA (4.8)[36]. A more detailed comparison is listed in Supplementary Table 9.

Among the three MOFs, Cu-AD-AA has the best adsorption capacity and the highest selectivity, achieving a balance between adsorption capacity and selectivity. To assess Cu-AD-AA practical separation capability, dynamic breakthrough experiments were conducted at 298 K and 1 bar. Figure 5A illustrates the breakthrough of a gas mixture with a C3H8/CH4 ratio of 50/50 (flow rate: 2 mL/min). Over time, CH4 rapidly elutes from the column while C3H8 remains retained. Calculations indicate that a single breakthrough experiment yields high-purity CH4 at 1.25 mmol/g. Similarly, Figure 5B demonstrates that after the passage of a C2H6/CH4 gas mixture (50/50, 2 mL/min), CH4 breaks through first, followed by C2H6 after a certain period. The calculated CH4 yield is 0.22 mmol/g. These results indicate that Cu-AD-AA possesses exceptional natural gas purification capabilities.

Biological metal–organic frameworks for natural gas purification and MTO product separation

Figure 5. The breakthrough curves of the (A) C3H8/CH4 mixtures (v/v = 50/50 under a flow of 2 mL/min); (B) C2H6/CH4 mixtures (v/v = 50/50 under a flow of 2 mL/min); (C) C2H4/C3H6 mixtures (v/v = 50/50 under a flow of 2 mL/min); (D) C2H4/C3H6 mixtures (v/v = 50/20 under a flow of 4 mL/min) in an absorber bed packed with Cu-AD-AA at 298 K and 1 bar.

For MTO product separation, a mixture of C2H4:C3H6 was introduced into activated Cu-AD-AA samples packed in a column at flow rates of 2.0, 4.0, and 5.0 mL/min with ratios of 50:50 [Figure 5C], 50:20 [Figure 5D], and 90:10 [Supplementary Figure 25], respectively. The observations demonstrated that Cu-AD-AA effectively separated C2H4 and C3H6 mixtures, with preferential breakthrough of C2H4, while C3H6 eluted at 25, 14, and 13 min/g, respectively. Additionally, based on the breakthrough curves, the adsorption amounts of C3H6 by Cu-AD-AA from the mentioned C2H4 and C3H6 (50:50, 50:20, and 90:10) mixtures during a single breakthrough period were calculated to be 29.5, 12.1, and 8.2 cm3·g-1, correspondingly. Moreover, the breakthrough experiments were reproducible in at least three consecutive cycles with regeneration conditions involving vacuum or room temperature helium sweep. This repeatability aligns with its low Qst value, indicating high stability and durability. The stability of Cu-AD-AA was maintained after cycle tests, as evidenced by the PXRD analysis [Supplementary Figure 26]. Cu-AD-AA demonstrates desirable traits such as high thermal and chemical stability, elevated C3H6 absorption rate, and C2H4 selectivity, reducing energy consumption during the regeneration process. Consequently, it presents as a promising material for industrial MTO product separation.


In this study, three AD-based Bio-MOFs were successfully synthesized using a solvothermal method, employing monocarboxylic acids with varying sizes as coordinated terminal ligands. These MOFs exhibited distinct pore microenvironments arising from differences in terminal ligand sizes. Systematic investigation revealed a significant enhancement of thermal stability with larger terminal ligands. Furthermore, evaluation of their adsorption properties for light hydrocarbons revealed diverse capacities for adsorption and selective separation, attributable to their three-dimensional intercrossed channels and abundant active sites within. As a result, it demonstrated their aptitude for adsorbing C3 > C2 > C1 molecules, making them suitable for natural gas purification and MTO product separation. Qualitative analysis through IAST further confirmed the potential of these Bio-MOFs in CH4 purification and MTO product separation, with Cu-AD-AA exhibiting the most favorable separation properties. Quantification of separation ability was corroborated by dynamic breakthrough experiments. The inferior adsorption and separation capacity of Cu-AD-FA can be attributed to its poor stability and limited active sites compared to Cu-AD-AA. Although Cu-AD-PA demonstrated greater stability than Cu-AD-AA, its reduced pore volume and unrestricted rotation of ethyl groups within the pores contributed to significantly poorer adsorption and separation capabilities. The subtle variations in small channel environments among the three materials presented intriguing trade-offs, including the balance between high adsorption capacity and selectivity and the relationship between pore volume and stability. Based on the current findings, Cu-AD-AA appears to strike a favorable balance within these trade-offs, considering stability, adsorption capacity, and selectivity. It presents an encouraging prospect for designing separation materials with superior performance. Furthermore, the bio-based nature of Cu-AD-AA, coupled with its low toxicity and cost-effectiveness, further highlights the significance of this study. Combined with its straightforward synthesis method and robust separation capabilities, Cu-AD-AA exhibits substantial potential for various industrial applications.


Authors’ contributions

Provided financial support: Liu Y

Revised and finalized the manuscript: Liu Y, Zhang L

Designed the samples, performed experiments, and wrote the paper: Liu Y, Li W, Zhang L

Offered the experimental guidance and advice: Xue D

Assisted with data supplement: Wang Y, Wang D, Shi Z, Liu J

All authors contributed to the general discussion.

Availability of data and materials

Not applicable.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (Nos. 22171100 and U23A20360) and the 111 Project (B17020).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.


© The Author(s) 2024.

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Li W, Wang D, Wang Y, Shi Z, Liu J, Zhang L, Xue D, Liu Y. Biological metal–organic frameworks for natural gas purification and MTO product separation. Chem Synth 2024;4:22.

AMA Style

Li W, Wang D, Wang Y, Shi Z, Liu J, Zhang L, Xue D, Liu Y. Biological metal–organic frameworks for natural gas purification and MTO product separation. Chemical Synthesis. 2024; 4(3): 22.

Chicago/Turabian Style

Wen Li, Dan Wang, Yi Wang, Zhaohui Shi, Junxue Liu, Lirong Zhang, Dongxu Xue, Yunling Liu. 2024. "Biological metal–organic frameworks for natural gas purification and MTO product separation" Chemical Synthesis. 4, no.3: 22.

ACS Style

Li, W.; Wang D.; Wang Y.; Shi Z.; Liu J.; Zhang L.; Xue D.; Liu Y. Biological metal–organic frameworks for natural gas purification and MTO product separation. Chem. Synth. 2024, 4, 22.

About This Article

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This article belongs to the Special Issue Innovations in Functional Porous Frameworks
© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (, which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Author Biographies

Wen Li
Wen Li obtained his B.Eng. degree in Applied Chemistry from Beihua University in 2018, then he started his PhD study at Jilin University under the supervision of Prof. Yunling Liu. His current research interests primarily focus on designing MOFs materials with enhanced capabilities for capturing and separating small molecule gases.
Dan Wang
Dan Wang obtained her B.Eng. degree in Qingdao University of Science and Technology in 2021 and now she is studying for her M.Sc. degree in inorganic chemistry from Jilin University under the supervision of Prof. Yunling Liu. Her current scientific interest focuses on the design and synthesis of metal-organic frameworks for pollutants removal and detection.
Yi Wang
Yi Wang obtained her B.Sc in chemistry from China West Normal University in 2021 and currently is a master candidate at the College of Chemistry and Chemical Engineering, Shaanxi Normal University, under the supervision of Prof. Dong-Xu Xue. Her current research interest is in the design and synthesis of functional porous materials and their applications in gas storage and separation.
Zhaohui Shi
Zhaohui Shi received his B.Sc degree in chemistry from Jilin University in 2022. He is currently a M.S. candidate under the supervision of Prof. Yunling Liu at Jilin University. His research focuses on the design and synthesis of crystalline porous materials.
Junxue Liu
Junxue Liu obtained her B.Sc. degree in chemistry from Jilin University in 2020 and is currently carrying out her Ph.D. degree under the supervision of Prof. Jiyang Li in Jilin University (China). Her research is mainly focused on the synthesis of CDs and CDs@MOF nanocomposites and their application in the field of nanozymes.
Prof. Lirong Zhang
Prof. Lirong Zhang received her Ph.D. degree from Jilin University (China) in 2001, and then worked as a postdoc fellow at Max-Planck-Institute for Chemical Physics of Solids (2001-2002). She started teaching in the Department of Chemistry, Jilin University (2005), and was promoted to full professor (2015). Her research interests include the design and synthesis of metal-organic frameworks, gas adsorption and separation, carbon capture and conversion, pollutants removal and explosive detection et al.
Prof. Dong-Xu
Prof. Dong-Xu Xue obtained his Ph.D. degree from Sun Yat-Sen University (China) in 2008. He then worked as a postdoc fellow at University of South Florida (2008-2010) and King Abdullah University of Science and Technology (2010-2015), respectively. Since 2015, he joined School of Chemistry and Chemical Engineering of Shaanxi Normal University. His research interests include design and synthesis of metal-organic frameworks, gas adsorption and separation, energy gas storage, carbon capture and conversion et al.
Prof. Yunling Liu
Prof. Yunling Liu received his Ph.D. degree from Jilin University (China) in 2000, and then he joined the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry at Jilin University as assistant professor. He was promoted to associate professor in 2004, and full professor in 2008. He worked as a postdoc fellow at University of South Florida (2003-2005). His research focuses on the design and synthesis of functional directed porous materials applied in the energy field, with emphasis on gas storage and separation.

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