Pore-structure control in bimetallic coordination networks for natural gas purification with record C₂H₆/CH₄ selectivity

Abstract

Developing effective adsorbents with high adsorption capacity and selectivity for separating methane (CH₄) from natural gas mixtures containing ethane (C₂H₆) and propane (C₃H₈) remains a significant challenge. Previous studies on CH₄/C₂H₆/C₃H₈ separation have primarily focused on enhancing C₃H₈/CH₄ selectivity, often neglecting the crucial role of C₂H₆/CH₄ selectivity, thereby limiting CH₄ productivity. Here, we present a strategy to modulate pore size and chemistry in two bimetallic coordination networks, CuIn(ina)₄ and CuIn(3-ain)₄, to enhance the separation of CH₄/C₂H₆/C₃H₈ mixtures. Remarkably, CuIn(3-ain)₄ exhibits a record C₂H₆/CH₄ selectivity and a benchmark low-pressure C₂H₆ adsorption capacity, achieving a CH₄ productivity of 7.92 mmol g⁻¹ with a purity exceeding 99.9999%, surpassing most known porous materials. Theoretical simulations reveal how selective adsorption can be finely tuned by adjusting pore size and geometry. Moreover, breakthrough experiments with ternary mixtures, along with regeneration and cycling tests, underscore the exceptional potential of CuIn(3-ain)₄ as a highly efficient adsorbent for natural gas separation.

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Introduction

Natural gas, predominantly methane (CH₄, 75%–90%), stands as a promising clean energy source and vital chemical raw material. However, natural gas extraction often yields a mixture containing minor amounts of ethane (C₂H₆), propane (C₃H₈), traces of carbon dioxide and other impurities (∼20%). These impurities not only degrade the combustion efficiency and conversion rate of CH₄ but also compromise the stability of its safe pipeline storage and transport.^1–4 Moreover, C₂H₆ and C₃H₈ are important raw materials for the manufacture of olefins and polymers.^5–8 Thus, separating C₂H₆ and C₃H₈ from CH₄ is essential for maximizing resource utilization and enhancing natural gas quality.^9,10

To date, low-temperature distillation, leveraging the boiling point disparities among CH₄ (112 K), C₂H₆ (184 K), and C₃H₈ (231 K), has dominated separation techniques. However, this method is marked by harsh operating conditions, high operating costs, and significant energy consumption.¹¹ Adsorption separation technology has surfaced as a highly promising option compared to low-temperature distillation due to its advantages of low energy consumption, low investment, and straightforward operation.^12–14 The key to achieving efficient adsorption separation lies in the development of high-performance adsorbents.¹⁵ Recently, metal–organic frameworks (MOFs), which comprise organic ligands and metal nodes, have attracted considerable interest for gas storage and separation due to their permanent porosity and functionalized pore environment.^16–20

Although MOFs have shown impressive potential for purifying natural gas, challenges remain. Natural gas typically contains approximately 5% C₃H₈ and 10% C₂H₆, so adsorbents with high adsorption capacity for C₂H₆ and C₃H₈ in the low-pressure region (5–10 kPa) are key to achieving efficient CH₄/C₂H₆/C₃H₈ separation.^21,22 Wu et al. have synthesized materials with strong adsorption capacity up to 2.93 mmol g⁻¹ of C₂H₆ at 10 kPa and 3.37 mmol g⁻¹ of C₃H₈ at 5 kPa through a channel methylation strategy.²² However, most MOFs are still insufficient for C₂H₆ and C₃H₈ uptake at low pressures.^23–29 This is partly due to the larger pore sizes of these materials, which are insufficient for recognizing C₃H₈ and C₂H₆ molecules from natural gas. For instance, JLU-Liu40 shows notable adsorption properties for C₂H₆ and C₃H₈ at 100 kPa, but its performance degrades at lower pressures.³⁰ Therefore, designing and synthesizing MOF materials that exhibit elevated gas capacities in the low-pressure range (5–10 kPa) is essential. The pore structure can be adjusted through functional group modifications, and more suitable pore sizes can also be achieved, offering a promising solution to this challenge (Scheme 1a).

Scheme 1 Schematic illustration of enhanced (a) low-pressure adsorption capacity of the framework for C₂H₆ through pore structure control, which improves C₂H₆/CH₄ selectivity, and (b and c) the increased CH₄ productivity, leading to efficient natural gas separation.

On the other hand, for multicomponent separation systems, the “cask effect” is a significant factor, meaning that the productivity of the target gas depends on the adsorption of fewer impurities.³¹ In natural gas purification, the adsorption capacity for C₂H₆ and selectivity towards CH₄ are generally lower compared to C₃H₈, necessitating greater focus on C₂H₆/CH₄ separation.³² However, the molecular size and chemical properties of C₂H₆/CH₄ are more similar than those of C₃H₈/CH₄ (Table S1†), thus greatly hindering the separation of C₂H₆/CH₄.³³ Previous research has mostly favoured C₃H₈/CH₄ selectivity, often compromising CH₄ productivity. The poor C₂H₆/CH₄ separation performance and low CH₄ productivity of most MOFs with good C₃H₈/CH₄ separation are bottlenecks in the purification of natural gas using MOFs. Therefore, there is an urgent need to develop MOF materials with high C₂H₆/CH₄ selectivity. As shown in Scheme 1b, the effective period of CH₄ purification, highlighted in orange, is limited, while the interval between the breakthroughs of C₂H₆ and C₃H₈ is prolonged, leading to inefficiency. If the breakthrough time of C₂H₆ is shifted backward, thereby enhancing the efficiency of the adsorption cycle, this issue can be effectively resolved.³¹ MOFs with high C₂H₆ adsorption capacity and high C₂H₆/CH₄ selectivity are preferred. When C₂H₆ and C₃H₈ break through similarly or even simultaneously, MOFs can exhibit higher separation performance and efficiency (Scheme 1c). However, few materials following this strategy have been reported to date.

In this study, two bimetallic coordination networks, CuIn(ina)₄ and CuIn(3-ain)₄, were prepared and their adsorption and separation properties were systematically investigated.^34,35 The introduction of amino groups not only regulates the pore size but also improves the chemical environment of the pores. As a result, CuIn(3-ain)₄ exhibits remarkable adsorption capacity and the highest C₂H₆/CH₄ selectivity for C₂H₆ and C₃H₈ in the low-pressure region. In breakthrough experiments, the breakthrough times of C₂H₆ and C₃H₈ are close to each other, which greatly improves the productivity and efficiencies of CH₄ purification. Additionally, theoretical calculations confirm the high performance of CuIn(3-ain)₄ in CH₄/C₂H₆/C₃H₈ separation.

Results and discussion

In selecting an appropriate framework, it is crucial to seek a highly stable and easily functionalized porous material. We chose the highly stable and tunable CuIn(ina)₄ and functionalized it with amino groups to obtain the new material CuIn(3-ain)₄. Both materials exhibit doubly interpenetrating diamond (dia) network structures, forming one-dimensional channels along the a- and c-axes, respectively. Each Cu(I) ion is coordinated by four pyridine nitrogen atoms from separate ligands, forming a typical tetrahedral structure. Additionally, each In(III) ion is coordinated by eight carboxylate oxygen atoms from four ligands, forming slightly distorted tetrahedral units (Fig. 1). The powder X-ray diffraction (PXRD) pattern of synthesized CuIn(3-ain)₄ corresponded well with the simulated structure (Fig. S1†), verifying the phase purity of the sample. Thermogravimetric analysis (TGA) showed that CuIn(ina)₄ and CuIn(3-ain)₄ maintained thermal stability up to 573 K and 523 K, respectively (Fig. S2†). The permanent porosity of these two materials was assessed through CO₂ adsorption experiments at 195 K (Fig. 2a and S3, S4†), which generated a Brunauer–Emmett–Teller (BET) surface area of CuIn(ina)₄ and CuIn(3-ain)₄ of 472 and 429 m² g⁻¹, and the total pore volumes were 0.22 and 0.20 cm³ g⁻¹, respectively. The adsorption isotherms for both materials exhibited type-I adsorption–desorption behavior, which suggested that micropores were dominant. Using the Horvath–Kawazoe (H–K) method, pore-size distributions were determined to be 6.7 Å for CuIn(ina)₄ and 5.4 Å for CuIn(3-ain)₄.

Fig. 1 Polyhedral representation of (a) CuIn(ina)₄ and (b) CuIn(3-ain)₄. (c and d) Schematic view of the accessible pore channel (yellow cylinder).

Fig. 2 (a) Pore size distribution for CuIn(ina)₄ and CuIn(3-ain)₄ analyzed by the H–K method. CH₄, C₂H₆, and C₃H₈ adsorption isotherms at 298 K for (b) CuIn(ina)₄ and (c) CuIn(3-ain)₄. (d) Comparison of C₂H₆ (10 kPa, 298 K) and C₃H₈ (5 kPa, 298 K) uptake among representative MOFs. (e) IAST selectivity of CuIn(ina)₄ and CuIn(3-ain)₄ for C₂H₆/CH₄ (10/85, v/v) and C₃H₈/CH₄ (5/85, v/v) at 298 K. (f) Comparison of the C₂H₆/CH₄ (10/85, v/v) and C₃H₈/CH₄ (5/85, v/v) selectivities with reported MOFs.

Single-component adsorption isotherms for CH₄, C₂H₆, and C₃H₈ on CuIn(ina)₄ and CuIn(3-ain)₄ were determined at 273, 298, and 313 K (Fig. 2b, c and Fig. S5–S10†). The results showed that C₃H₈ isotherms for both materials exhibited a steeper increase than those for C₂H₆, with both increasing more significantly than those for CH₄, suggesting that the skeleton has a strong affinity for C₃H₈ and C₂H₆. The higher affinity for C₃H₈ compared to C₂H₆ contributes to a higher C₃H₈ uptake (3.25 mmol g⁻¹ in CuIn(ina)₄ and 3.01 mmol g⁻¹ in CuIn(3-ain)₄) compared to C₂H₆ (3.14 mmol g⁻¹ in CuIn(ina)₄ and 2.96 mmol g⁻¹ in CuIn(3-ain)₄) at 298 K and 100 kPa. The larger pore volume of CuIn(ina)₄ results in higher accommodation of both C₃H₈ and C₂H₆ compared to CuIn(3-ain)₄. The corresponding gas occupancy values of C₃H₈ in CuIn(ina)₄ and CuIn(3-ain)₄ are 4.34 and 5.42 molecules per unit cell, while for C₂H₆ are 4.19 and 5.34 molecules per unit cell, respectively (Table S2†); the gas occupancy values for both gases are higher in CuIn(3-ain)₄, demonstrating that C₂H₆ and C₃H₈ are more efficiently packed within CuIn(3-ain)₄. The densities of C₂H₆ and C₃H₈ in CuIn(ina)₄ are 0.43 and 0.65 g mL⁻¹, and in CuIn(3-ain)₄, they are 0.44 and 0.66 g mL⁻¹, respectively (Table S3†), further demonstrating the dense packing of C₂H₆ and C₃H₈ in CuIn(3-ain)₄. It is worth noting that the adsorption capacity for C₂H₆ and C₃H₈ of CuIn(3-ain)₄ in the low-pressure region is significantly higher than those of most of the other reported materials (Fig. 2d).^36,37 The adsorption capacity for C₃H₈ can reach up to 2.92 mmol g⁻¹ at 5 kPa, and the adsorption capacity for C₂H₆ can reach up to 2.71 mmol g⁻¹ at 10 kPa. These results suggest that CuIn(3-ain)₄ holds considerable promise for the separation of natural gas. Moreover, the isosteric heat of adsorption (Q_st) was obtained by fitting the isotherms of CH₄, C₂H₆, and C₃H₈ at 273 K and 298 K. At zero loading, the Q_st for C₃H₈ and C₂H₆ reached 51 and 39 kJ mol⁻¹, respectively, notably exceeding that of CH₄ (35 kJ mol⁻¹) (Fig. S12†). This suggests a strong affinity of C₃H₈ and C₂H₆ towards CuIn(3-ain)₄, further highlighting its potential as an effective adsorbent for natural gas separation.

We calculated the ideal adsorbed solution theory (IAST) selectivity of C₃H₈/CH₄ (5/85, v/v) and C₂H₆/CH₄ (10/85, v/v) at 298 K to further assess the separation ability of these two MOFs for CH₄/C₂H₆/C₃H₈ mixtures. As shown in Fig. 2e, for CuIn(3-ain)₄, the C₂H₆/CH₄ selectivity reaches 99 at 100 kPa, which is the highest IAST selectivity of the C₂H₆/CH₄ mixture reported to date. In addition, the selectivity of C₃H₈/CH₄ is as high as 945. To the best of our knowledge, this C₃H₈/CH₄ selectivity surpasses all reported values in other porous materials, except for MOF-303 (5114),³⁸ such as ZUL-C2 (741),³⁹ BSF-2 (681)²⁷ and LIFM-ZZ-1 (485)⁴⁰ (Fig. 2f).^41,42 In comparison, CuIn(ina)₄ exhibits a similar C₃H₈/CH₄ (5/85, v/v) selectivity (1164) but a lower C₂H₆/CH₄ (10/85, v/v) selectivity (32), possibly due to the lower C₂H₆ adsorption capacity of CuIn(ina)₄ at low pressure. These results indicate the superior separation performance of CuIn(3-ain)₄ for CH₄/C₂H₆/C₃H₈ mixtures compared to CuIn(ina)₄.

Time-dependent adsorption rate curves were obtained to evaluate the kinetic adsorption performance of the adsorbent at 298 K and 100 kPa. As shown in Fig. 3, the initial adsorption rates of CuIn(3-ain)₄ for all three gases (CH₄: 0.32 mmol g⁻¹ s⁻¹, C₂H₆: 0.54 mmol g⁻¹ s⁻¹, and C₃H₈: 0.46 mmol g⁻¹ s⁻¹) were higher than those of CuIn(ina)₄ (CH₄: 0.12 mmol g⁻¹ s⁻¹, C₂H₆: 0.27 mmol g⁻¹ s⁻¹, and C₃H₈: 0.23 mmol g⁻¹ s⁻¹). This discrepancy can be explained by differences in the pore size of the MOF materials and the internal amino functional groups, impacting how gas molecules diffuse and get trapped. The initial adsorption rates of the three gases followed the order of C₂H₆ > C₃H₈ > CH₄. Despite having a higher adsorption capacity, the larger size of the C₃H₈ molecules resulted in a slower adsorption rate. The slowest adsorption rate was observed for CH₄, possibly caused by the weak binding affinity of the framework for CH₄. The adsorption rate of CuIn(3-ain)₄ for C₂H₆ was faster than that of C₃H₈, in contrast to the thermodynamic effect. The synergistic interplay between the thermodynamic and kinetic effects optimizes the practical separation performance of CuIn(3-ain)₄.

Fig. 3 Kinetic sorption measurements and instant adsorption rates. (a) CH₄, (b) C₂H₆, and (c) C₃H₈ for CuIn(ina)₄; and (d) CH₄, (e) C₂H₆, and (f) C₃H₈ for CuIn(3-ain)₄.

Given the excellent CH₄/C₂H₆/C₃H₈ separation properties, density functional theory (DFT) calculations were conducted to investigate the interactions of CH₄, C₂H₆, and C₃H₈ with the framework based on the Grand Canonical Monte Carlo (GCMC) results. As shown in Fig. 4, the CH₄ adsorption site in both materials is located near the center of the channel, which is insufficient for strong interactions between CH₄ and the framework due to its small molecular size (CuIn(ina)₄: 3.00–3.41 Å, CuIn(3-ain)₄: 3.06–3.10 Å). Both C₂H₆ and C₃H₈ tend to bind near the ligands in CuIn(ina)₄. C₂H₆ interacts with the framework through C–H⋯O interactions (2.96 to 3.01 Å) with the carboxylic acid group and C–H⋯π interactions (3.24 Å) with the neighboring pyridine ring. In CuIn(3-ain)₄, the introduction of the amino group narrows the pore size and enhances the C–H⋯π interaction between C₂H₆ and the pyridine ring (3.10–3.15 Å). Additionally, C₂H₆ forms a C–H⋯N interaction (3.17 Å) with the amino group, leading to a stronger binding affinity of CuIn(3-ain)₄ for C₂H₆. The larger size of the C₃H₈ molecule results in shorter interaction distances with the framework. C₃H₈ forms stronger interactions with CuIn(3-ain)₄ (2.80–3.08 Å) compared to CuIn(ina)₄ (2.87–3.29 Å). This results in a higher affinity for C₃H₈ adsorption by CuIn(3-ain)₄ at lower pressures (1 kPa). Interestingly, GCMC simulations further revealed spatially distinct adsorption sites of C₂H₆ and C₃H₈ in CuIn(3-ain)₄, minimizing competitive adsorption which is expected to optimize actual separation performance. In addition, the host–guest interactions were further investigated using Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. S15,† compared with the CH₄-loaded material, C–H stretching vibration peaks at 2950 cm⁻¹ were observed after the adsorption of C₂H₆ and C₃H₈, with a more pronounced peak intensity following C₃H₈ adsorption. This observation further confirms the existence of stronger interactions between C₃H₈ and the MOF pore walls. Furthermore, gas adsorption density distributions within the adsorbents were examined using GCMC simulations. Specifically, the adsorption densities of C₂H₆ and C₃H₈ at 1 kPa were only slightly lower than those at 100 kPa, indicating a strong adsorption affinity at low pressure, which is consistent with the experimental results (Fig. S16–S21†).

Fig. 4 Adsorption sites of (a) C₃H₈, (b) C₂H₆, and (c) CH₄ in CuIn(ina)₄ calculated by DFT simulations. Adsorption sites of (d) C₃H₈, (e) C₂H₆, and (f) CH₄ in CuIn(3-ain)₄ calculated by DFT simulations.

Dynamic breakthrough experiments were performed using ternary gas mixtures (CH₄/C₂H₆/C₃H₈, 85/10/5, v/v/v) at 298 K to verify the actual separation properties of CuIn(ina)₄ and CuIn(3-ain)₄. The results showed that CH₄ eluted immediately from both MOFs, while C₂H₆ was retained for 1 min g⁻¹ and 27 min g⁻¹ on CuIn(ina)₄ and CuIn(3-ain)₄, respectively (Fig. 5a). Moreover, C₂H₆ and C₃H₈ penetrated from the column almost simultaneously. As a result, the CH₄ productivity (purity: 99.9999%) increased from 0.08 to 7.92 mmol g⁻¹. This productivity surpasses those of most reported materials, such as NKMOF-15 (6.91 mmol g⁻¹),⁴³ SNNU-185 (6.85 mmol g⁻¹),⁴⁴ TIFSIX-Cu-TPA (5 mmol g⁻¹),³¹ BSF-3 (4.07 mmol g⁻¹),⁴⁵ and Ni(HBTC)(bipy) (3.82 mmol g⁻¹) (Fig. 5b).⁴⁶ Both adsorbed C₂H₆ and C₃H₈ can be fully desorbed under He flow (30 mL min⁻¹) at 373 K for 2 h (Fig. S22†). The reusability of CuIn(3-ain)₄ was evaluated through cyclic breakthrough experiments using the ternary gas mixtures (CH₄/C₂H₆/C₃H₈, 85/10/5, v/v/v) at 298 K (Fig. 5c) and cyclic adsorption of C₃H₈, C₂H₆ and CH₄ (Fig. 5d and Fig. S23–S25†). In addition, the diffraction peaks in the PXRD spectra without noticeable changes after cycling experiments (Fig. S26†) confirm the cycling stability of CuIn(3-ain)₄.

Fig. 5 (a) Dynamic breakthrough curves for CH₄/C₂H₆/C₃H₈ (85/10/5, v/v/v) with CuIn(ina)₄ and CuIn(3-ain)₄. (b) Comparison of CH₄ productivity in CH₄/C₂H₆/C₃H₈ (85/10/5, v/v/v) separation at 298 K among reported MOFs. (c) Cyclic breakthrough experiments for CH₄/C₂H₆/C₃H₈ with CuIn(3-ain)₄ at 298 K. (d) Cyclic adsorption of C₂H₆ and C₃H₈ at 298 K by CuIn(3-ain)₄.

Conclusions

In summary, the incorporation of functional sites (NH₂) into the porous structure enables precise tuning of pore chemistry and size, leading to exceptional separation performance and significantly enhanced adsorption of target molecules under low-pressure conditions. This advancement allows for the efficient separation of natural gas, even at low concentrations of C₂H₆ and C₃H₈. Remarkably, CuIn(3-ain)₄ exhibits record-high selectivity (99) for C₂H₆/CH₄. By combining thermodynamic and kinetic effects, CuIn(3-ain)₄ efficiently purifies CH₄ from natural gas, attaining a remarkable productivity of 7.92 mmol g⁻¹. DFT calculations confirm the strong affinity of C₃H₈ and C₂H₆ molecules towards the framework. This research provides a foundation for the future advancement of efficient adsorbents with strong affinity and sufficient selectivity for natural gas purification.

Author contributions

Q.-Y. Y. and L.-P. Z. conceived the idea of this research. L.-P. Z. carried out the experiments, analyzed the results and wrote the manuscript. Q.-Y. Y. led the project and edited the manuscript. All authors participated in and contributed to the preparation of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 22371221) and the National Key R&D Program of China (2024YFE0101800). Qing-Yuan Yang acknowledges the Programme of Introducing Talents of Discipline to Universities (B23025), State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (KF2023009), and the Shaanxi Fundamental Science Research Project for Chemistry and Biology (23JHQ007). We thank Mr. Chang Huang at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with single-crystal X-ray diffraction analyses.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00316d

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