A tetranuclear-cluster-based MOF with a low-polarity pore environment for efficient C₂H₆/C₂H₄ separation

Abstract

Removing trace ethane (C₂H₆) from ethylene (C₂H₄) to obtain high-purity C₂H₄ is of great industrial significance. However, the construction of C₂H₆-selective metal–organic frameworks (MOFs) remains challenging, mainly because C₂H₄ has a small dynamic diameter and a large quadrupole moment. It has been reported that ultramicroporous MOFs with tailored pore sizes and a weakly polar pore environment can effectively purify C₂H₄ from C₂H₆–C₂H₄ mixtures. For this purpose, we successfully designed and synthesized one novel MOF material ZJNU-400 with a non-polar pore environment. The channel size of ZJNU-400 is equivalent to the size of the C₂H₆ molecule, which can provide a more accessible channel surface for C₂H₆. The pore environment of ZJNU-400 is rich in low-polarity aromatic groups, and uncoordinated N and O atoms can interact strongly with C₂H₆ molecules. Therefore, ZJNU-400 exhibits significant C₂H₆/C₂H₄ (50/50) selectivity (2.82) and C₂H₆ uptake (64.3 cm³ g⁻¹) at 298 K and 1 bar, which are superior to many C₂H₆-selective adsorbents. A breakthrough experiment shows that ZJNU-400 can purify C₂H₄ from a C₂H₆/C₂H₄ mixture in one step. GCMC and DFT calculations further revealed that the multiple synergistic interactions between the MOF and C₂H₆ enable efficient separation of the C₂H₆–C₂H₄ mixture. These results indicate that ZJNU-400 is a promising adsorbent for C₂H₆/C₂H₄ separation.

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10th anniversary statement

Ten years ago, when the Chinese Chemical Society and Royal Society of Chemistry jointly initiated this new journal, I was kind of concerned about the manuscript submission, both in terms of quality and quantity. Gladly, I have witnessed the great success of this Inorganic Chemistry Frontiers under the leadership of Editor-in-Chief Dr Song Gao and editorial board members. I have always considered this journal as one of the two most important publication platforms for the international Inorganic Chemistry community. As a new Associate Editor of ACS Inorganic Chemistry, I congratulate the great success of Inorganic Chemistry Frontiers and wish it continuous growth and success. Banglin Chen, University Distinguished Chair Professor at Fujian Normal University and Zhejiang Normal University.

1. Introduction

As the largest petrochemical raw material for the production of polypropylene, plastics, coatings and other chemical commodities, ethylene is the core of the petrochemical industry and an important cornerstone for measuring the level of the national petrochemical industry. More than 170 million tons of C₂H₄ are produced globally each year, mainly through light diesel cracking, naphtha cracking or C₂H₆ cracking.^1–4 Among them, naphtha cracking is currently the most industrialized, but the cracking products contain impurities such as H₂, CH₄, C₂H₂, C₂H₆, and C₃H₈.^5–8 Compared with the separation of light hydrocarbons with other carbon numbers, the separation of C₂ gases (C₂H₂, C₂H₄ and C₂H₆) is the most difficult because the molecular sizes and boiling points of these gases are very similar.^9–12 A typical industrial separation process is to first remove ethane through low-temperature and high-pressure technology, and then use catalytic hydrogenation or solvent extraction to remove acetylene and finally obtain polymerization-grade C₂H₄ through a distillation tower.^13–15 The corresponding energy consumption required for this separation is approximately 7.3 GJ per ton of ethylene, accounting for approximately 0.3% of global energy consumption.³ Therefore, there is an urgent need to explore alternative technologies and materials that can effectively separate and purify these light hydrocarbons under mild conditions to achieve the one-step removal of C₂H₂ and C₂H₆ from the C₂ hydrocarbon ternary gas mixture to obtain pure grade C₂H₄.

Emerging adsorbents like metal–organic framework materials have shown promising results in effectively separating binary or ternary gas mixtures.^16–18 The highly polarized or high electric field gradient surfaces, like those found in open metal sites (OMSs), of MOFs interact strongly with gases containing unsaturated bonds such as C₂H₂ and C₂H₄, making it relatively simple to design C₂H₄-selective MOFs.^19,20 However, to obtain high-quality C₂H₄, a further desorption process is required after selective adsorption of ethylene, which results in considerable energy loss.²¹ Therefore, it is more practical and economical to develop C₂H₆ selective adsorbents that can directly produce high-purity C₂H₄, thus reducing energy consumption. Unfortunately, C₂H₆ selective MOFs typically have poor selectivity due to the lack of suitable strong binding sites.^22,23 A feasible strategy to design adsorbents that preferentially bind C₂H₆ over C₂H₄ is to reduce OMSs and increase the effective contact area between C₂H₆ molecules and the host skeleton, forming multiple interactions such as introducing Lewis N/O negative potential sites. Precisely regulating the pore structure and enhancing the interaction between host MOFs and guests are more beneficial for customizing ultramicroporous C₂H₆-first selective MOF adsorbents.^24–26 Recently, significant progress has been made by researchers in developing C₂H₆ selective MOF adsorbents. For example, Gücüyener and colleagues introduced the first C₂H₆ selective adsorbent, ZIF-7.²⁷ Under lower pressure, the abundant electronegative N sites on the pore surface facilitate a strong interaction between C₂H₆ molecules and benzimidazole ligands, resulting in a substantial increase in C₂H₆ adsorption by ZIF-7. This MOF exhibits a strong adsorption capacity for C₂H₆/C₂H₄ mixtures and higher adsorption selectivity. Meanwhile, Xia and colleagues studied the PCN-250 material and discovered that C₂H₆ molecules are primarily distributed around the N atoms of the ligand.²⁸ The pore size effect and different van der Waals interactions give rise to the reverse adsorption behaviour of C₂H₆/C₂H₄. Furthermore, Lin et al. developed Cu(Qc)₂,⁵ which introduces aromatic rings to create an inert pore environment by reducing the number of polar open metal binding sites in MOFs. This pore environment maximizes C₂H₆ adsorption by the adaptive pore structure of the skeleton.²⁹

Herein, we carefully selected the N-rich tetracarboxylic acid ligand H₄DDBP [2,2′-(pyrazine-2,6-diyl)diterephthalic acid] and successfully synthesized ZJNU-400 by utilizing network chemistry theory. The characteristic of the structure is the formation of a saturated coordinated tetranuclear Mn cluster, resulting in a framework devoid of OMSs and exhibiting exceptional stability. ZJNU-400 possesses a high pore volume, an appropriate pore size, and an inert pore environment containing functionalized N-sites that enhance its capacity for selective ethane adsorption. To investigate the impact of the pore environment of MOFs on C₂H₆/C₂H₄ separation, we conducted a comprehensive analysis of the structure, adsorption isotherms, simulated separation ratios, and other properties of the MOF material. Notably, the optimized aperture (6.4 × 6.7 Ų) of the one-dimensional channel provides an increased effective contact area with the C₂H₆ molecule. Our findings suggest that the presence of N-rich sites within the framework promotes enhanced hydrogen bonding between the framework and C₂H₆; additionally, channels constructed by benzene rings create a low-polarity pore environment conducive to affinity with C₂H₆. Consequently, ZJNU-400 demonstrates highly C₂H₆-selective adsorption and excellent C₂H₄ separation performance.

2. Experimental section

2.1 Materials and measurements

The chemicals used were obtained from ordinary sources, which can be used directly without re-treatment processes. Powder X-ray diffraction (PXRD) measurements were recorded using a Rigaku D/max-2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Then, by employing a TGA Q500 thermogravimetric analyzer, thermal gravimetric analyses (TGAs) were performed. The crystallographic data of ZJNU-400 were collected on a Bruker Apex II CCD diffractometer by using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The comprehensive crystallization data of ZJNU-400 are given in Table S1.† Crystallographic data for ZJNU-400 (2161591†) have been delivered by the Cambridge Crystallographic Data Centre. Topology structure information on the compounds was calculated using TOPOS 4.0. The synthesized sample of ZJNU-400 was guest-exchanged frequently with MeOH for three days and then evacuated under dynamic vacuum at 373 K for 10 h to obtain the activated sample for the gas adsorption experiment by utilizing an Autosorb iQ.

2.2 Synthesis of ZJNU-400

A mixture of Mn(NO₃)₂·4H₂O (8 mg, 0.032 mmol), H₄DDBP (3 mg, 0.0073 mmol), and deionized water (H₂O, 0.15 mL) was added to 2 mL of N,N-dimethylacetamide (DMA), which was placed in a well-airtight 20 mL glass vial covered with tin foil and a bottle cap, and then the reactants were left to stand in an oven at 105 °C for 24 hours. After the reaction, octahedral crystals were obtained and washed several times with fresh DMA. The molecular formula is [Mn₂(DDBP)·2H₂O]·2(DMA). Found (wt%): C, 39.64; H, 2.45; N, 5.11; calcd (wt%): C, 40.01; H, 2.92; N, 5.80.

3. Results and discussion

3.1 Crystal structure

X-ray single crystal structure analysis shows that the structure of ZJNU-400 belongs to the monoclinic crystal system and P2₁/n space group (Table S1†). As shown in Fig. 1 and Fig. S1,† each tetranuclear manganese cluster of ZJNU-400 is connected through bridging O from the solvent, and then two pairs of centrally symmetric dinuclear manganese are connected through the four carboxyl groups of the two ligands to form a tetranuclear manganese cluster. The construction of multi-nuclear SBUs can enhance the stability of MOF materials, as demonstrated by the reported rigid Zr-MOF PCN-207³⁰ and MFM-133(Zr).³¹ Therefore, each tetranuclear cluster is surrounded by eight 4-connected organic ligands, forming a saturated coordination pattern. ZJNU-400 is periodically connected according to the above to form a three-dimensional network structure belonging to the flu topology, outlining the open quadrilateral windows with dimensions of 6.4 × 6.7 Ų along the y axis. Remarkably, the pore size fits neatly into a row of C₂H₆ or C₂H₄ molecules and is more in line with the size and shape of the C₂H₆ molecules.

Fig. 1 Single-crystal structure of ZJNU-400: (a) polyhedral simplification of organic and inorganic SBUs; (b) polyhedron view of the skeleton with the flu network; (c) 3D framework along the y axis; (d and e) 1D channel packed with C₂H₆ and C₂H₄, respectively.

3.2 Thermal and chemical stability

The consistency between the experimental results of ZJNU-400 and the simulated PXRD spectra shows that the crystal products were obtained, which have the corresponding phase purity (Fig. S2†). The chemical stability test of ZJNU-400 shows that the crystallinity of the skeleton remains unchanged after being soaked in HCl or NaOH aqueous solutions with a pH value of 5 to 12. TGA shows that ZJNU-400 has three weight loss platforms. The first weight loss platform is at 5–350 °C, which is about 10%, corresponding to the loss of one DMA solvent molecule and two water molecules. The second weight loss is 25.7% and the third weight loss is 14.7%, that is when the temperature is higher than 600 °C, the ligand decomposition structure begins to collapse (Fig. S3†).

3.3 Gas adsorption of ZJNU-400

The permanent porosity of ZJNU-400 was tested using its N₂ gas adsorption curve at 77 K. ZJNU-400 samples were exchanged three times each with chromatography-grade acetone solvent for 12 hours to remove solvent molecules in the crystal skeleton. The sample was then heated to 105 °C in an adsorption instrument and vacuum degassed to obtain the activated sample ZJNU-400. As shown in Fig. 2, the test result shows that the ZJNU-400 crystal shows a typical type-I N₂ adsorption isotherm, with a maximum saturated absorption capacity of 214 cm³ g⁻¹. The Brunauer–Emmett–Teller (BET) and Langmuir surface areas are 794 and 889 m² g⁻¹, respectively. Its total pore volume was found to be 0.33 cm³ g⁻¹, which is slightly lower than the theoretical pore volumes of its corresponding crystal structures, presumably due to insufficient nitrogen filling inside the pore surfaces.³² The density functional theory (DFT) method was used to calculate and determine its pore size distribution, which concentrated around 6–8 Å. This is consistent with the theoretical analysis performed using Materials Studio software.

Fig. 2 Sorption isotherms and pore size distribution of ZJNU-400 at 77 K.

Research indicates that ultramicroporous MOFs with tailored pore sizes (≈7 Å) and a low-polarity pore environment show promise as candidates for ethylene purification.²⁵ A systematic C₂ gas adsorption test was performed on ZJNU-400. As depicted in Fig. 3a, b and Fig. S4,†ZJNU-400 exhibited significantly high gas uptake for C₂H₂, C₂H₆, and C₂H₄ at 273/298 K, with values reaching 125.1/82.8, 88.5/64.3, and 89.8/59.2 cm³ g⁻¹, respectively. These results highlight the superior storage capacities of ZJNU-400 for C₂H_n hydrocarbons, particularly for C₂H₆ surpassing those of other materials, such as MAF-123-Mn (35 cm³ g⁻¹),³³ UiO-66-2CF₃ (41 cm³ g⁻¹),³⁴ and ZIF-8 (56 cm³ g⁻¹)³⁵ under the same conditions at 298 K. Moreover, it is noteworthy that ZJNU-400 demonstrates a higher storage capacity for C₂H₆ compared to C₂H₄. This preference is evident from the steep slope observed for C₂H₆ adsorption by ZJNU-400 at low pressure compared to that of C₂H₄. The findings suggest that ZJNU-400 holds potential for efficient separation of C₂H₆/C₂H₄ mixtures due to its higher affinity for C₂H₆. This deduction is supported by the calculated Q_st values of ZJNU-400 for C₂H₄ and C₂H₆, which are determined to be 26–32 and 27–36 kJ mol⁻¹, respectively (Fig. 3c and Fig. S5†). Therefore, ZJNU-400 has the potential for efficient separation of C₂H₆/C₂H₄ mixtures and high-capacity storage of C₂H₄ to solve the trade-off problem.

Fig. 3 (a–b) C₂H₂, C₂H₄, and C₂H₆ adsorption and desorption isotherms of ZJNU-400; (c) the Qst of ZJNU-400 for the above gases; (d) the selectivity of ZJNU-400 for C₂H₆/C₂H₄ equimolar mixtures at 298 K; (e) comparison of the adsorption selectivities for C₂H₆-selective MOF materials.

Based on the ideal adsorption solution theory (IAST), the C₂H₂/C₂H₄ and C₂H₆/C₂H₄ adsorption selectivity values of ZJNU-400 are as high as 1.7 and 2.8 in an equimolar gas mixture at 298 K and 1 bar (Fig. 3d and Table S2†). As far as we know, the C₂H₆ adsorption selectivity of the ZJNU-400 adsorbent material for C₂H₄ is comparable to those of the ultrahigh benchmark selectivity materials currently studied, Fe₂(O₂)(dobdc) (4.4),⁴ Cu(Qc)₂ (3.5)⁵ and Co(AIN)₂ (2.9),³⁶ and significantly better than those of other general materials, such as Ni(bdc)(ted)_0.5 (2.1),³⁷ IRMOF-8 (1.9),³⁸ Azole-Th-1 (1.6),³⁹ MIL-53(Al) (1.3),⁴⁰etc. (Fig. 3e and Table S3†).

3.4 Dynamic column breakthrough studies

Combined with moderate C₂H₆/C₂H₄ reverse selectivity and high C₂H₆ adsorption capacity, ZJNU-400 shows the potential for C₂H₆/C₂H₄ separation. Subsequently, in order to verify the separation ability of ZJNU-400, a dynamic breakthrough experiment of the mixture was conducted under Ar as the carrier gas, in which the C₂H₆–C₂H₄ (50 : 50, V : V) mixture was introduced as the feed, under the conditions of 298 K and 100 kPa. The gas mixture was flowed through the packed ZJNU-400 column at a total flow rate of 2 mL min⁻¹. As shown in Fig. 4 and Fig S6,†ZJNU-400 preferentially adsorbs C₂H₆ over C₂H₄. According to the breakthrough results, the breakthrough time interval of the 50/50 mixture is 16.4 min g⁻¹ for C₂H₆ and 47.3 min g⁻¹ for C₂H₄, which is comparable to those of some MOFs with good C₂H₆/C₂H₄ separation performance, such as Ni(TMBDC) (DABCO)_0.5,⁴¹ M-PNMI⁴² and MIL-53-BDC.⁴³ During these intervals, high-grade C₂H₄ (>99.9%) products can be collected directly at the outlet. The extended breakthrough time interval observed in the experiment indicates the efficient separation of C₂H₆–C₂H₄ mixtures at near room temperature, suggesting the practical feasibility of C₂H₆/C₂H₄ separation without the need for precise temperature control. Furthermore, cycling experiments demonstrated the stability of ZJNU-400 in terms of its capture capacity and separation performance for C₂H₆, reinforcing its efficacy as an efficient and practical adsorbent capable of capturing C₂H₆ and facilitating the separation of C₂H₄–C₂H₆. These findings highlight the potential of ZJNU-400 as a viable solution for C₂H₆/C₂H₄ separation in industrial applications.

Fig. 4 (a) The experimental breakthrough curves for C₂H₆–C₂H₄ (50 : 50) separation in a fixed bed packed with ZJNU-400 at 298 K and (b) three recyclability tests, respectively. All the experimental breakthrough curves were recorded at 1 bar with a flow rate of 2 mL min⁻¹.

3.5 Theoretical simulations

To further investigate the guest–host interactions and gain mechanistic insights at the molecular level, we conducted Grand Canonical Monte Carlo (GCMC) simulations. These simulations allowed us to obtain the gas density distribution, as shown in Fig. 5a and b. At a pressure of 100 kPa, the simulated adsorption amounts of C₂H₄ and C₂H₆ were found to be 2.6 and 3.2 mmol g⁻¹, respectively, which closely matched the experimentally measured loadings of 2.6 and 2.9 mmol g⁻¹. Interestingly, the simulated density distribution of C₂H₄ and C₂H₆ within the ZJNU-400 skeleton revealed two favorable sorption regions: clusters and benzene rings in the structure. Gas molecules were found to accumulate even more within these regions.

Fig. 5 Adsorption density distribution and possible binding sites calculated by GCMC (a and b) and DFT-D (c and d) simulations at 298 K and 100 kPa for C₂H₄ (left) and C₂H₆ (right) in ZJNU-400.

To gain further insight into the host–guest interactions between the framework and the adsorbate, we also performed first-principles dispersion-corrected density functional theory (DFT-D) calculations. These calculations allowed us to identify the adsorption sites in ZJNU-400 and calculate the corresponding binding energies. Consistent with the experimental adsorption data, we observed a slight shrinkage of the pores, which resulted in a shorter distance between the guest molecules and the backbone. Additionally, an increase in low-polar C–H binding sites and Lewis basic sites promoted higher binding energy for C₂H₆ compared to C₂H₄ in ZJNU-400. Fig. 5c and d show that the optimal adsorption sites are situated close to the tetranuclear manganese cluster of the rhombic pore window. In ZJNU-400, we observed multiple C–H⋯H–O dipolar interactions (2.5, 2.3, and 2.7 Å) between C₂H₆ and carboxylate groups. The proximity of the methyl group of C₂H₆ to the phenyl ring moiety of the ligand contributed to a larger number of stronger C–H⋯C van der Waals interactions (3.1, 3.4, and 2.9 Å). Furthermore, we noticed a shorter C–H⋯H–N bond (3.2 Å) between C₂H₆ and the negatively charged pyrazine nitrogen atom in the ligand, resulting in a relatively high binding force of −32.62 kJ mol⁻¹. In contrast, C₂H₄ exhibited fewer C–H bonds, leading to fewer CH=H⋯H–O and CH=H⋯C interactions. As a result, its binding energy was lower than that of C₂H₆, measuring −30.98 kJ mol⁻¹.

4. Conclusions

In summary, based on the principle of network chemistry, one new MOF adsorbent material was constructed using the N-containing ligand H₄DDBP. The formation of saturated coordination tetranuclear manganese clusters results in the absence of OMSs in the framework of ZJNU-400 non-existent OMSs. Through pore environment engineering, a precisely controlled pore structure with low-polar ultramicropores (6–8 Å) is constructed. The integrated pore surface environment is conducive to increasing C₂H₆ intake while reducing the adsorption of C₂H₄. Moreover, the uncoordinated N sites embedded in the framework tend to form C–H⋯C interactions with C₂H₆ over C₂H₄. These interactions improve the absorption rate of C₂H₆ and enable highly C₂H₆-selective one-step purification of C₂H₄. Consequently, this approach addresses the trade-off problem between adsorption capacity and separation selectivity, as well as energy saving.

Author contributions

Meng Feng: writing – original draft, preparation and synthesis of materials. Jiantang Li: resources, supervision, and data curation. Xirong Wang: experimental data analysis. Jingyu Wang: validation, formal analysis, and visualization. Dongmei Wang: writing – review & editing, software, supervision, and data curation. Banglin Chen: writing – review & editing and supervision.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†].

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52102189, 22301154) and the Jinhua City Project (2023-4-019).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2161591. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01685h

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