N/O-functionalized MOFs with nonpolar channels for efficient natural gas purification and carbon capture

Abstract

A porous PyC–Zn-MOF, which possesses a rectangular pore structure and abundant nitrogen/oxygen bifunctional sites, is reported. It demonstrated effective separation of CH₄/C₂H₆/C₃H₈ and CO₂/N₂, achieving a CH₄ purity of >99.9% with a yield of 7.88 L kg⁻¹ in a single adsorption–desorption cycle, underscoring its outstanding potential for industrial application.

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Against the backdrop of today's global energy transition, natural gas is considered a relatively clean fossil fuel, but its composition is complex, containing methane (CH₄), higher hydrocarbons such as ethane (C₂H₆) and propane (C₃H₈), and impurities including carbon dioxide (CO₂) and nitrogen (N₂).^1,2 Methane itself is a potent greenhouse gas; leakage during extraction, transport, and storage exacerbates climate change, offsetting its combustion benefits.³ Ethane and propane are valuable chemical feedstocks, yet their coexistence with methane makes separation challenging, leading to resource waste and reduced combustion performance.^4,5 Meanwhile, CO₂ reduces calorific value and causes pipeline corrosion,⁶ while N₂ dilutes natural gas, lowering its energy density.⁷ Therefore, treating natural gas for methane purification and CO₂ capture not only improves combustion efficiency but also provides feedstock for carbon capture and storage (CCS),^8,9 thereby simultaneously advancing the decarbonization of natural gas utilization and the development of carbon management technologies.^10,11

Current natural gas separation technologies include cryogenic distillation, membrane separation, and physical adsorption. Cryogenic distillation is energy-intensive and inefficient for similar boiling points.¹² Membrane separation has low selectivity and poor stability under high pressure.¹³ Compared to the aforementioned methods, physical adsorption offers high efficiency and low energy consumption, but lacks high-performance adsorbent materials.¹⁴ Traditional adsorbents often fail to achieve both high capacity and selectivity, driving the development of novel materials.^15,16

Metal–organic frameworks (MOFs) are an ideal solution due to their unique structural properties, such as tuneable porosity, high surface area, and excellent stability, providing a versatile platform for advanced separation materials.^17,18 Based on their unique physicochemical properties, MOFs have shown significant application potential in fields such as gas adsorption and separation,¹⁹ energy,²⁰ chemical sensing,^21,22 microelectronic devices,²³ biomedicine,²⁴ dye adsorption and separation,²⁵ and advanced electrochemistry.²⁶ In particular, for gas separation, MOFs offer superior selectivity and efficiency over traditional materials, driving research into tailored MOFs for specific needs.^27,28

To address challenges in natural gas purification, we synthesized a novel metal–organic framework, PyC–Zn-MOF, based on a pyrazole–carboxylic acid ligand (PyC). The material features a rectangular pore framework with N/O sites constructed around Zn(II) metal nodes. These sites enable selective adsorption by forming electrostatic interactions with gas molecules such as CH₄, C₂H₆, and C₃H₈. Due to the differing hydrogen atom counts in these three gas molecules, their electrostatic interactions with the pore walls vary in strength, enabling efficient natural gas purification. Additionally, in CO₂/N₂ separation, the electrostatic potential within the rectangular channels favors interactions with CO₂ molecules, while inert N₂ molecules are poorly adsorbed, resulting in outstanding CO₂/N₂ separation performance. Thus, PyC–Zn-MOF, with its unique rectangular pore structure and N/O-modified pore walls, holds broad application prospects in natural gas purification and CO₂ capture.

As shown in Fig. 1c, PyC–Zn-MOF is constructed from the PyC ligand. The pyrazole moieties within the ligand coordinate with trinuclear zinc clusters to form secondary building units (SBUs) of Zn₃(µ₃-O)(COO⁻)₃. These SBUs are then linked by PyC ligands to form the three-dimensional framework of PyC–Zn-MOF (Fig. S1 and S2). This MOF features rectangular channels lined with accessible N/O adsorption sites (Fig. 1d).

Fig. 1 Molecular electrostatic potential maps of (a) N₂, CO₂, CH₄, C₂H₆, and C₃H₈ and (b) the PyC–Zn-MOF. (c) Synthesis route of PyC–Zn-MOF. (d) and (e) Crystal structure of the three-dimensional framework, depicted from different perspectives.

Powder X-ray diffraction (PXRD) was used to evaluate the phase purity of PyC–Zn-MOF. The distinctive diffraction peaks in the experimental pattern (Fig. S3) closely matched the simulated ones, indicating high phase purity. Furthermore, PXRD patterns of PyC–Zn-MOF following adsorption tests and after various treatments (Fig. S4) confirmed that the framework remained intact. The thermal stability of PyC–Zn-MOF was evaluated via thermogravimetric analysis (TGA) (Fig. S5). The sample exhibited a ∼20% mass loss below approximately 380 °C. The initial loss (0–200 °C) is attributed to the desorption of pore-confined water and partial solvent. The subsequent gradual loss (200–380 °C) likely corresponds to the partial decomposition of the organic components. Overall, the material demonstrates good thermal stability.

Nitrogen adsorption–desorption isotherms were measured at 77 K to probe the microporous characteristics of PyC–Zn-MOF (Fig. S6). The isotherm exhibits a sharp uptake at low relative pressures (P/P₀) and a plateau at higher pressures, characteristics of a Type I isotherm, confirming its microporous nature. Pore size distribution analysis via non-local density functional theory (NLDFT) indicated a predominant pore size of ∼0.47 nm (Fig. S7), in excellent agreement with the theoretical value, while the Brunauer–Emmett–Teller (BET) surface area was calculated to be 201.45 m² g⁻¹ (Fig. S8).

Molecular electrostatic potential (MEP) analysis of PyC–Zn-MOF (Fig. 1a and Table S1) reveals that the N/O sites within the pores carry a partial negative charge, enabling them to interact with specific gas molecules. For alkane separation, the partially positively charged hydrogen atoms in CH₄, C₂H₆, and C₃H₈ are potential interaction sites. Analysis of the surface electrostatic potential of Pyc–Zn-MOF (Fig. 1b and Table S1) reveals that N/O sites within the pores carry a negative charge, enabling them to interact with specific gas molecules for adsorption separation. Since hydrogen in CH₄/C₂H₆/C₃H₈ carries a positive charge, it is selected as the adsorption separation target. For CO₂ capture, the partially positively charged hydrogen atoms on the pyrazole rings of the framework can interact with the quadrupole moment of CO₂. Based on this, single-component adsorption isotherms for CO₂, N₂, CH₄, C₂H₆, and C₃H₈ were measured (Fig. 2). At 1 bar, the adsorption capacities (cm³ g⁻¹) at 273 K/298 K were 33.78/21.40 for CO₂, 2.54/1.10 for N₂, 10.00/5.87 for CH₄, 30.78/23.18 for C₂H₆, and 29.15/23.83 for C₃H₈, respectively. Notably, at 298 K and 0.1 bar, the CO₂ adsorption capacity of PyC–Zn-MOF surpasses that of most reported adsorbents (Fig. 3c and Table S2), being comparable to high-performance materials like MOF@CP-0²⁹ and CoIPA³⁰ and exceeding others such as SU-102,³¹ MIP-202,³² Ca-MOF-1,³³ and Mg-MOF-74.³⁴

Fig. 2 Single-component gas adsorption isotherms for (a) and (b) CO₂ and N₂ and (c) and (d) CH₄, C₂H₆, and C₃H₈ measured at 273 K and 298 K.

Fig. 3 (a) IAST-predicted adsorption selectivities for CO₂/N₂ (50/50), C₂H₆/CH₄ (10/85), and C₃H₈/CH₄ (5/85) mixtures. (b) Separation potential (Δq) for the corresponding mixtures. (c) Comparison of the CO₂ adsorption capacity at 298 K and 10 kPa of PyC–Zn-MOF with various porous materials. (d) Comparison of the C₂H₆/CH₄ and C₃H₈/CH₄ adsorption selectivity at 298 K and 100 kPa of PyC–Zn-MOF with various porous materials. (e) and (f) Dynamic breakthrough curves for (e) CO₂/N₂ (50/50, v/v) and (f) CH₄/C₂H₆/C₃H₈ (85/10/5, v/v/v) at 298 K.

The isosteric heats of adsorption (Q_st) at zero coverage were calculated³⁵ from the adsorption isotherms at 273 K and 298 K (Fig. S9 and S10): CO₂ (27.00 kJ mol⁻¹), N₂ (23.58 kJ mol⁻¹), CH₄ (13.77 kJ mol⁻¹), C₂H₆ (29.43 kJ mol⁻¹), and C₃H₈ (35.28 kJ mol⁻¹). The synergistic effect between the active N/O sites and the pore confinement,³⁶ coupled with the optimal fit of the C₃H₈ molecular dimensions within the pore,³⁷ results in the highest Q_st for C₃H₈ (Fig. S10). As shown in Table S1, the Q_st trends for CO₂/N₂ correlate with MEP negativity; CO₂ exhibits the most negative MEP (−3.57 × 10⁻²) and highest Q_st (27.00 kJ mol⁻¹), indicating preferential electrostatic recognition of its quadrupole moment. Conversely, alkane adsorption is dominated by dispersion forces rather than MEP. Despite CH₄ having a more negative MEP than C₂H₆, its Q_st is significantly lower. As the carbon chain lengthens, increased polarizability and van der Waals contact area enhance Q_st, with C₃H₈ (35.28 kJ mol⁻¹) benefiting most from pore confinement and dispersion effects.

The ideal adsorption solution theory (IAST) was applied to predict the selectivity for binary gas mixtures.³⁸ The IAST selectivities for equimolar CO₂/N₂ (50/50) and for C₂H₆/CH₄ (10/85) and C₃H₈/CH₄ (5/85) mixtures on PyC–Zn-MOF were 2.35, 21.19, and 101.17, respectively (Fig. 3a). The trend S (C₃H₈/CH₄) > S (C₂H₆/CH₄) can be attributed to the greater number of partially positive hydrogen atoms in C₃H₈, which engage in stronger and more numerous electrostatic interactions with the negatively charged N/O atoms in the framework.^39,40 Consequently, PyC–Zn-MOF exhibits higher adsorption selectivity toward the C₃H₈/CH₄ system than toward the C₂H₆/CH₄ system. Notably, the selectivities of PyC–Zn-MOF for C₂H₆/CH₄ (10/85) and C₃H₈/CH₄ (5/85) surpass those of many reported adsorbents (Fig. 3d and Table S3), such as TPOA-F (17.7 and 99),⁴¹ InOF-1 (17 and 90),⁴² JUC-106 (13 and 75),⁴³ and ZUL-C1 (22 and 73).⁴⁴ This further demonstrates PyC–Zn-MOF's outstanding separation capability for both C₂H₆/CH₄ and C₃H₈/CH₄ mixtures, enabling the capture of trace amounts of C₃H₈ and C₂H₆ from CH₄/C₂H₆/C₃H₈ mixtures.

Furthermore, the separation potential (Δq), a key metric combining selectivity and capacity, further confirms the outstanding performance of PyC–Zn-MOF (Fig. 3b and Fig. S11). For gas mixtures with varying compositions, this material exhibits significantly different Δq values: for the CO₂/N₂ system with an equal volume ratio (50/50), Δq is 0.53 mol kg⁻¹; under non-equimolar mixtures (C₂H₆/CH₄ = 10/85 and C₃H₈/CH₄ = 5/85), Δq values increased to 2.23 and 8.11 mol kg⁻¹, respectively. These data conclusively demonstrate the excellent purification capability of PyC–Zn-MOF for C₂H₆ and C₃H₈ in binary mixtures with CH₄.

Dynamic breakthrough experiments were conducted with CO₂/N₂ (50/50) and CH₄/C₂H₆/C₃H₈ (85/10/5) mixtures to evaluate its practical separation performance. The separation capability was characterized by analyzing the breakthrough curves (Fig. 3e and f). As shown in Fig. 3e, the experiment using an equimolar CO₂/N₂ mixture at 298 K and a flow rate of 2 mL min⁻¹ showed that nitrogen (N₂) encountered minimal resistance while passing through the adsorbent bed and broke through the column first, whereas CO₂ exhibited significant interaction with the adsorbent, resulting in its selective capture and prolonged retention. Specifically, PyC–Zn-MOF exhibited outstanding dynamic adsorption performance for CO₂, with N₂ and CO₂ breakthrough times of 30.0 and 45.0 min g⁻¹, respectively. These breakthrough results strongly corroborate the CO₂ preferential adsorption observed in the static adsorption isotherm studies, further validating the material's excellent CO₂/N₂ separation selectivity. Notably, this outstanding separation performance is consistent with the strong CO₂ adsorbent binding energy obtained from theoretical calculations, indicating that the specific pore structure and active sites of PyC–Zn-MOF can effectively distinguish between CO₂ and N₂ molecules, highlighting its significant application potential in fields such as carbon capture.

After three consecutive breakthrough cycles (Fig. S12), the material demonstrated excellent cycling stability with no significant degradation in separation performance. This excellent cycling stability, combined with the material's inherently high CO₂ selectivity, suggests its broad application prospects in industrial gas separation. In particular, PyC–Zn-MOF can efficiently and stably separate CO₂ in processes such as flue gas carbon capture, contributing to carbon neutrality goals. Furthermore, this material holds significant value in natural gas purification, where it can effectively remove trace CO₂ impurities to enhance gas quality and meet standards for pipeline transmission and industrial applications.⁴⁵

PyC–Zn-MOF exhibits significant differences in dynamic separation performance for ternary mixtures of CH₄/C₂H₆/C₃H₈. Breakthrough experiments (Fig. 3f) revealed a stepwise elution profile: CH₄ broke through first at 19.8 min g⁻¹, followed by C₂H₆ at 39.7 min g⁻¹, and finally C₃H₈ at 119.0 min g⁻¹. This sequence directly reflects the differing interaction strengths between the gas molecules and the active sites within the material's pores, which arises from the weaker interactions between CH₄ and the MOF.⁴⁶

In contrast, C₂H₆ and C₃H₈ are selectively retained due to stronger adsorption interactions. Notably, C₃H₈ exhibits a significantly higher affinity for the MOF than C₂H₆, allowing PyC–Zn-MOF to achieve efficient separation of these components. Practically, this material demonstrates outstanding methane purification capability, producing CH₄ with a purity exceeding 99.9% in a single step. Its methane productivity reaches 7.88 liters per kilogram of adsorbent (L kg⁻¹). This performance indicates that PyC–Zn-MOF holds significant application value in natural gas upgrading (removing heavier hydrocarbons such as ethane and propane) and liquefied petroleum gas (LPG) recovery. Its high separation efficiency stems from its precisely tuned pore size and surface chemistry, offering a novel solution for the industrial separation of multicomponent hydrocarbon mixtures.⁴⁷

To further elucidate the adsorption mechanism for natural gas and flue gas purification, grand canonical Monte Carlo (GCMC) simulations were employed to map the adsorption densities of N₂, CO₂, CH₄, C₂H₆, and C₃H₈ (Fig. 4). The results reveal that CH₄, C₂H₆, and C₃H₈ exhibit strong adsorption densities in the N/O-rich regions of PyC–Zn-MOF, while CO₂ shows strong adsorption densities around the pyrazole hydrogen atoms. This phenomenon is consistent with the predictions from the molecular electrostatic potential analysis.

Fig. 4 Spatial distribution of adsorption densities from GCMC simulations for (a) N₂, (b) CO₂, (c) CH₄, (d) C₂H₆, and (e) C₃H₈ within the PyC–Zn-MOF.

In summary, we synthesized a novel PyC–Zn-MOF with rectangular pores and N/O bifunctional sites for natural gas upgrading and carbon capture. At 298 K and 100 kPa, the material shows excellent selective adsorption for C₂H₆ and C₃H₈via differentiated electrostatic interactions. Breakthrough experiments confirm successful CO₂/N₂ and CH₄/C₂H₆/C₃H₈ separation, yielding >99.9% pure CH₄ (7.88 L kg⁻¹). Simulations attribute CO₂ capture to pore-wall hydrogen interactions, while alkane affinity (C₃H₈ > C₂H₆ > CH₄) scales with carbon number. These findings highlight the potential of electrostatic-governed separation for industrial purification.

W. Xu and X-Z. Guo: investigation, methodology, data curation, and writing – original draft. N. Ma: data curation. Z. Xing and J. Li: funding acquisition and investigation. J. Chang: conceptualization, funding acquisition, project administration, supervision, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the results of this study are included in the supplementary information (SI). Detailed synthesis procedures; adsorption; breakthrough, IAST and DFT simulations were listed in SI. Additional raw data (e.g., original breakthrough experiment logs and DFT calculation output) are available from the corresponding author upon request. See DOI: https://doi.org/10.1039/d6cc00792a.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22472023 and 22572025), the Jilin Province Science and Technology Development Program (20250102077JC), and the Fundamental Research Funds for the Central Universities (2412024QD014 and 2412023QD019).

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Footnote

† These authors contributed equally to this work.

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