Moderately polarized carborane-MOF with inverse C₂ selectivity for one-step polymer-grade ethylene purification

Abstract

Ethylene purification from ternary C₂ mixtures (C₂H₂/C₂H₄/C₂H₆) typically requires multi-step processes due to competing adsorption mechanisms in conventional adsorbents. Herein, we report a carborane-based metal–organic framework (CB-Zn-DPG) featuring moderately polarized pore surfaces that inverts traditional selectivity trends. Unlike polar MOFs (C₂H₂ > C₂H₄ > C₂H₆) or nonpolar MOFs (C₂H₆ > C₂H₄ > C₂H₂), CB-Zn-DPG simultaneously captures both C₂H₂ (61.5 cm³ g⁻¹) and C₂H₆ (44.0 cm³ g⁻¹) preferentially over C₂H₄ (42.4 cm³ g⁻¹) at 298 K and 1.0 bar, with higher isosteric heats for impurities (C₂H₂ 31.5 kJ mol⁻¹; C₂H₆ 29.8 kJ mol⁻¹vs. C₂H₄ 25.9 kJ mol⁻¹). DFT calculations reveal multi-site van der Waals interactions at the channel centers for C₂H₂/C₂H₆, while C₂H₄ adsorbs weakly at the pore corners. Dynamic breakthroughs confirm direct production of >99.99% polymer-grade C₂H₄ from equimolar C₂H₄/C₂H₆ (50/50) and ternary C₂H₂/C₂H₄/C₂H₆ (1/90/9) mixtures in a single column operation. This work establishes pore-surface polarity engineering as a paradigm for challenging hydrocarbon separations.

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New concepts

Ethylene (C₂H₄) purification from acetylene (C₂H₂) and ethane (C₂H₆) impurities is a critical yet challenging industrial process, traditionally requiring energy-intensive multi-step operations. In this study, we report a novel carborane-based metal–organic framework, CB-Zn-DPG, constructed from Zn²⁺, p-carborane dicarboxylate, and meso-α,β-di(4-pyridyl) glycol (DPG), which achieves one-step production of polymer-grade C₂H₄ (>99.99% purity) from ternary C₂H₂/C₂H₄/C₂H₆ mixtures under ambient conditions. Key innovations include: (1) CB-Zn-DPG exhibits reversed adsorption affinity (C₂H₂ > C₂H₆ > C₂H₄) due to its moderately polarized pore surfaces, enabling direct C₂H₄ purification from complex mixtures; (2) the framework features a two-fold interpenetrated dia topology with 1D channels (5.4 × 3.7 Ų), combining high stability (thermal stability up to 240 °C), recyclability, and hydrophobicity; (3) DFT calculations reveal that C₂H₂ and C₂H₆ are anchored at channel centers via multiple van der Waals interactions, while C₂H₄ adsorbs weakly at the pore corners; (4) dynamic breakthrough experiments validate direct C₂H₄ purification from equimolar C₂H₄/C₂H₆ (50/50) and ternary C₂H₂/C₂H₄/C₂H₆ (1/90/9) mixtures, outperforming many benchmark materials. In general, our work not only provides a new material for one-step C₂H₄ purification from C₂H₂/C₂H₄/C₂H₆ mixtures, but also establishes boron-cluster engineering as a versatile strategy to overcome capacity-selectivity trade-offs in multifunctional adsorbent design.

Introduction

Ethylene (C₂H₄), serving as an essential foundational material within the petrochemical industry, is widely employed in the manufacture of resins, plastics, and numerous other organic chemical products.^1,2 Industrial-scale ethylene production predominantly relies on steam cracking or thermal decomposition processes using feedstocks like naphtha or ethane. A significant challenge arising from these methods is the inevitable co-production of impurities, specifically acetylene (C₂H₂) and ethane (C₂H₆). The presence of these contaminants detrimentally affects downstream processes, particularly ethylene polymerization reactions, leading to a substantial degradation in the quality of the resultant polyolefin materials. Consequently, the effective elimination of both acetylene and ethane impurities is a critical processing requirement.^3–7 Separating ethylene from ethane and acetylene remains a significant challenge. This difficulty stems from the close resemblance in their molecular dimensions, boiling points and key physicochemical properties (Table S2). Current industrial protocols for segregating acetylene, ethane and ethylene (C₂H₂/C₂H₄/C₂H₆) gas mixtures typically involve sequential separation steps. Selective acetylene removal is generally achieved through catalytic hydrogenation, utilizing catalysts containing precious metals (e.g., Pd-based) under conditions of elevated temperature and pressure. Ethane separation, conversely, is conventionally accomplished via energy-intensive cryogenic distillation.⁸ This reliance on scarce noble metal catalysts contributes significantly to high operational costs and raises potential environmental sustainability concerns. Moreover, cryogenic distillation imposes a substantial energy burden. Therefore, there exists a clear and pressing demand for the development of novel, highly efficient and energy-conserving technologies capable of effectively separating C₂H₂/C₂H₄/C₂H₆ mixtures.

Gas separation employing porous solid materials represents a more energy-efficient, environmentally benign and economically viable strategy, particularly under mild operating conditions.^9–26 Metal–organic frameworks (MOFs), a class of novel adsorbents, are formed through the assembly of inorganic metal ions or clusters with organic bridging ligands. These materials exhibit exceptional promise for gas separation due to their exceptionally high specific surface areas, remarkable porosity, and highly tunable structural features.^27–39 Significant differences exist in the quadrupole moments of C₂ gas molecules: acetylene possesses a quadrupole moment of approximately 7.2 × 10⁻²⁶ esu cm², ethylene has a value of about 1.5 × 10⁻²⁶ esu cm², while ethane exhibits the lowest value at roughly 0.65 × 10⁻²⁶ esu cm². To exploit these differences, the strategic incorporation of open metal sites (or clusters) or polar functional groups onto the internal surfaces of the MOF pores enables the preferential adsorption of molecules characterized by higher dipole and/or quadrupole moments. Consequently, the majority of MOFs (eg, ZNU-1, Fig. S18)⁴⁰ demonstrate strong adsorption affinities for unsaturated C₂ hydrocarbons, typically following the established selectivity trend of C₂H₂ > C₂H₄ > C₂H₆ (Scheme 1a).^41–45 This inherent selectivity pattern underscores a significant challenge for the direct, single-step separation of ethylene using conventional MOFs. Notably, ethane possesses the highest polarizability among the C₂ hydrocarbons (Table S2). MOFs with non-polar surfaces (e.g., ZNU-10, Fig. S19) display the opposite selectivity trend of C₂H₆ > C₂H₄ > C₂H₂ (Scheme 1b), which also fails to purify C₂H₄ from ternary mixtures.⁴⁶ Therefore, an alternative approach involves designing MOFs featuring pore surfaces rich in moderately polarized environments (Scheme 1c). This can be achieved by integrating neutral aromatic carborane units and functional hydroxy groups during synthesis. Such frameworks have the potential to show higher adsorption affinity for C₂H₆ and C₂H₂ over C₂H₄, offering a viable pathway for achieving efficient and single-step C₂H₄ purification. However, the construction of carborane hybrid MOFs with additional decorated hydroxy groups is still not realized.

Scheme 1 Illustration of adjusting the C₂H₂/C₂H₄/C₂H₆ selectivity trend by changing the pore surface polarity. (a) MOFs with highly polarized pore surface exhibit selectivity of C₂H₂ > C₂H₄ > C₂H₆; (b) MOFs with non-polar pore surface exhibit selectivity of C₂H₆ > C₂H₄ > C₂H₂; (c) MOFs with moderately polarized pore surface exhibit selectivity of C₂H₂ ∼ C₂H₆ > C₂H₄.

Herein, we reported a novel carborane hybrid MOF, CB-Zn-DPG (also termed as ZNU-22), constructed from Zn²⁺, carborane dicarboxylate (p-C₂B₁₀H₁₀-(COOH)₂) and meso-α,β-di(4-pyridyl) glycol (DPG). CB-Zn-DPG adopts the dia topology and undergoes two-fold interpenetration, generating linear one-dimensional channels with dimensions of 5.4 × 3.7 Ų. The frameworks facilitate stronger interactions with acetylene (C₂H₂) and ethane (C₂H₆) molecules, enabling superior separation performance for C₂H₂/C₂H₄/C₂H₆ mixtures. The separation capabilities for C₂H₂, C₂H₄ and C₂H₆ were evaluated through single component adsorption isotherms, ideal adsorbed solution theory (IAST) selectivity calculations and dynamic breakthrough experiments. Furthermore, combined density functional theory (DFT) computational analysis elucidated the underlying separation mechanism. Both C₂H₂ and C₂H₆ molecules are fixed near the channel center via multiple short-range van der Waals interactions, whereas C₂H₄ molecules reside in the channel corners experiencing weaker interactions. Dynamic separation tests confirmed that CB-Zn-DPG achieves one-step purification to polymer-grade ethylene from equimolar C₂H₄/C₂H₆ mixtures and ternary C₂H₂/C₂H₄/C₂H₆ (1/90/9) mixtures. The incorporation of neutral aromatic carborane units—a classic type of boron cluster—combined with functional hydroxy groups, enables the creation of moderately polarized pore surfaces (Fig. S20). This strategy effectively inverts the conventional adsorption selectivity trends for C₂ hydrocarbons, providing a new design paradigm for tackling challenging gas separations.

Results and discussion

The crystalline material CB-Zn-DPG was prepared through solvothermal synthesis using precursors Zn(NO₃)₂·6H₂O, p-C₂B₁₀H₁₀-(COOH)₂ and meso-α,β-di(4-pyridyl)glycol (DPG). The reaction employed a three-solvent blend of N,N′-dimethylformamide (DMF), methanol (MeOH) and deionized water, combined in a 1 : 1 : 1 volume ratio. The crystallization was conducted at a controlled temperature of 80 °C for 24 hours, producing phase-pure colorless block-like crystals ideal for structural characterization (Fig. 1a and Fig. S1). Single-crystal X-ray diffraction (SCXRD) analysis showed that CB-Zn-DPG crystallizes in the orthorhombic Pnna space group. Unit cell parameters are provided in Table S1. In CB-Zn-DPG, every Zn(II) node is connected to two carboxylate ligands and two pyridyl ligands, while every organic ligand links two Zn(II) cations (Fig. 1b and Fig. S2). This connection belongs to the diamondoid (dia) topological type (Fig. 1c). As shown in Fig. 1d and e, CB-Zn-DPG displays a two-fold interpenetrated dia network and a 1D accessible channel along the b-axis, generating a micropore in the cross-section of 5.4 × 3.7 Ų. The accessible void space of CB-Zn-DPG is ca. 21.0% (Fig. S3).

Fig. 1 (a) The building units of CB-Zn-DPG, including Zn²⁺, carborane dicarboxylate and organic linker DPG. (b) Coordination mode of CB-Zn-DPG. (c) The dia topological network of CB-Zn-DPG. (d) Two-fold interpenetrated dia nets in CB-Zn-DPG and the 1D pore channels viewed along the b-axis. (e) The dimensions of the pore channel.

Before gas adsorption measurement, the as-synthesized CB-Zn-DPG was activated under vacuum at 75 °C for 10 h. The permanent porosity of CB-Zn-DPG was established through N₂ (kinetic diameter: 3.64 Å) sorption at 77 K. As shown in Fig. 2a, this material achieves notable adsorption capacities of 152.5 cm³ g⁻¹ for N₂ under the specified cryogenic conditions. The N₂ adsorption isotherm displays a classic type I behavior, confirming the material's microporous nature. The DFT calculated pore size distribution of CB-Zn-DPG is centered at 5.5 Å. The H–K method yields a distribution centered at 4.8 Å (Fig. S6). Additionally, a 273 K CO₂ adsorption test was carried out (Fig. S7). The results show that CB-Zn-DPG exhibits a CO₂ uptake of 51.2 cm³ g⁻¹ at 1.0 bar. The pore size distributions of CB-Zn-DPG, calculated using the DFT and H–K method based on the CO₂ adsorption isotherms, locate in the range of 3.4–4.7 Å and 4.5–6.3 Å, respectively (Fig. S8, S9 and Tables S3, S4). The above results are in very close agreement with the single crystal analysis.

Fig. 2 (a) N₂ sorption isotherms at 77 K and (inset) corresponding DFT pore size analysis of CB-Zn-DPG. (b)–(d) C₂H₂, C₂H₄ and C₂H₆ adsorption isotherms of CB-Zn-DPG at (b) 298 K, (c) 278 K and (d) 308 K. (e) PXRD patterns of CB-Zn-DPG after different treatments. (f) Four cycles of C₂H₂ adsorption at 298 K on CB-Zn-DPG.

Given its moderately polarized pore surface and microporous characteristics, we further investigated its potential application for single-step C₂H₄ purification from a ternary mixture including C₂H₂, C₂H₄ and C₂H₆. Subsequently, the C₂H₂, C₂H₄ and C₂H₆ adsorption isotherms on CB-Zn-DPG were collected at 298, 278 and 308 K (Fig. 2b–d and Fig. S10–S12). The C₂H₂ and C₂H₆ uptakes of 61.5 and 44.0 cm³ g⁻¹ are superior to that of C₂H₄ (42.4 cm³ g⁻¹) at 298 K and 1.0 bar. At low pressures, the C₂H₆ adsorption uptake exceeds those of C₂H₂ and C₂H₄. Moreover, at 298 K, the C₂H₆ isotherm exhibits steeper slopes than the C₂H₂ and C₂H₄ isotherms in the low-pressure region, demonstrating the stronger binding between C₂H₆ and the framework. When the test temperature was varied, the adsorption trend of each gas was similar to that at 298 K. CB-Zn-DPG still exhibited the highest C₂H₂ adsorption capacity at 1.0 bar, and those for C₂H₆ and C₂H₄ remained comparable. Specifically, the measured C₂H₂ uptakes are 70.5 and 57.0 cm³ g⁻¹ at 278 and 308 K, respectively. These values exceed those for C₂H₆ (49.2, 38.8 cm³ g⁻¹) and C₂H₄ (49.9, 37.0 cm³ g⁻¹) under the same conditions. Additionally, the C₂H₆ and C₂H₂ adsorption isotherms also rise more steeply than that of C₂H₄ in the low-pressure region at 278 and 308 K. This observation suggests that CB-Zn-DPG can directly purify C₂H₄ from C₂ mixtures.

Since the stability of materials is important in industrial applications, the chemical stability of CB-Zn-DPG was evaluated by powder X-ray diffraction (PXRD). The diffraction peak positions remained nearly identical for the freshly synthesized sample under various conditions: after being activated, after being soaked in different solvents, after one week of air exposure, and after subsequent C₂H₂ adsorption–desorption cycles. This consistency confirms the material's excellent chemical stability and high crystallinity (Fig. 2e). Fig. S4 also shows that CB-Zn-DPG exhibits high stability in organic solvents such as acetone, DCM, DMA, DMF, EtOH, MeCN and n-hexane. In addition, this MOF remains chemically stable at different pH conditions (pH range from 6 to 12, Fig. S5). The thermogravimetric analysis (TGA) curve of CB-Zn-DPG indicates its thermal stability to be about 255 °C (Fig. S6), and the variable-temperature PXRD patterns show that it maintains a relatively high crystallinity at temperatures ⩽240 °C (Fig. S7). The combined TGA and variable-temperature PXRD results establish the optimal thermal stability of CB-Zn-DPG near 240 °C. In addition, Fig. 2f displays four continuous cycles of C₂H₂ adsorption at 298 K. The C₂H₂ adsorption capacity remains virtually constant, demonstrating the excellent recyclability of CB-Zn-DPG.

To explore the host–guest interactions between the gases and the framework, the isosteric heats of adsorption (Q_st) for CB-Zn-DPG were calculated using the Clausius–Clapeyron equation. At near-zero loading, the Q_st values are 31.5 kJ mol⁻¹ for C₂H₂, 29.8 kJ mol⁻¹ for C₂H₆ and 25.9 kJ mol⁻¹ for C₂H₄ (Fig. 3a). This trend mirrors the observed adsorption capacities. To quantitatively assess the C₂H₄ purification potential of CB-Zn-DPG for C₂ mixtures, we evaluated the selectivity using the ideal adsorbed solution theory. The separation selectivities were determined by fitting the adsorption isotherms to the single-site Langmuir equation for C₂H₄ and the dual-site Langmuir equation for C₂H₂ and C₂H₆ (Table S5–S7). As shown in Fig. 3b, the IAST selectivities for equimolar C₂H₂/C₂H₄ and C₂H₆/C₂H₄ at 278, 298 and 308 K under 1.0 bar are 1.94/1.49, 1.73/1.50 and 1.81/1.39, respectively (Fig. S16 and S17). At 298 K, the C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivity observed for CB-Zn-DPG surpasses that of many reported porous materials, including azole-Th-1 (1.06 and 1.46),⁴⁷ MOF-525 (1.44 and 1.25),⁴⁸ NPU-1 (1.40 and 1.32),⁴⁹ UPC-612 (1.07 and 1.41),⁴⁸ and UPC-613 (1.40 and 1.47),⁴⁸ but is inferior to those ZJNU-115 (2.05 and 1.56),⁵⁰ ZSTU-2 (2.36 and 1.62),⁵¹ Zn-ATA (1.79 and 1.81),⁵² UiO-67-(NH₂)₂ (2.1 and 1.7)⁴² and MOF-303 (2.4 and 1.7)⁵³ (Fig. 3c).

Fig. 3 (a) The isosteric heat of adsorption for C₂H₂, C₂H₄ and C₂H₆ on CB-Zn-DPG. (b) The C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivity of CB-Zn-DPG at 298 K. (c) Plot of the C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivity at 1.0 bar for CB-Zn-DPG and other MOFs.

To investigate the distinct adsorption behaviors of C₂H₂, C₂H₄ and C₂H₆, DFT calculations were carried out. For the C₂H₂ molecule, the optimal binding site is positioned at the center of the channels (Fig. 4a). The H atom of acetylene interacts with the framework with distances of 2.40 and 2.92 Å. One oxygen atom from the carboxyl group forms one C–H⋯O–C interaction with acetylene at a distance of 2.72 Å. Additionally, the H atom of the pyridine ring interacts with acetylene to form a C≡C⋯H–C interaction (2.92 Å), indicating stronger framework interaction (Fig. 4b). Regarding C₂H₆, its preferred binding site similarly resides in the channel center (Fig. 4c). The negatively charged pore surface of CB-Zn-DPG engages with the positively charged C–H bonds. C₂H₆ is tightly bound through eight C–H⋯H–B interactions (2.47–2.95 Å) and one C–H⋯O–C (2.80 Å) interaction (Fig. 4d). For the C₂H₄ molecule, the primary adsorption site is located at the side of the pore channels, as shown in Fig. 4e. C₂H₄ exhibits four C–H⋯H–B (2.49–2.99 Å), one C–H⋯O–C (2.99 Å) and three C–H⋯π (3.17–3.20 Å) interactions, leading to the weakest adsorption affinity due to the long distance (Fig. 4f). The computed binding energy values of CB-Zn-DPG with individual C₂H₂, C₂H₄ and C₂H₆ molecules were quantified as −61.2, −58.2 and −62.9 kJ mol⁻¹, respectively (Table S8). Collectively, the CB-Zn-DPG framework demonstrates notably stronger adsorption toward C₂H₂ and C₂H₆ than C₂H₄. This adsorption trend aligns well with the single-component adsorption experiments.

Fig. 4 The DFT optimized adsorption configuration and binding energies of (a) and (b) C₂H₂, (c) and (d) C₂H₆, and (e) and (f) C₂H₄ in CB-Zn-DPG.

As presented in Fig. 5, fixed-bed breakthrough experiments evaluated the dynamic separation of C₂H₂, C₂H₄ and C₂H₆ for one-step purification of C₂H₄. Specifically, introducing an equal-molar C₂H₄/C₂H₆ mixture (50/50) at a constant flow rate of 1 mL min⁻¹ and 298 K resulted in C₂H₄ emerging at the column outlet after 437.6 s g⁻¹, while C₂H₆ broke through much later at 2392.9 s g⁻¹ (Fig. 5a). For the C₂H₂/C₂H₄/C₂H₆ mixture (1/90/9) at 1 mL min⁻¹, C₂H₄ appeared at 458.1 s g⁻¹, whereas both C₂H₂ and C₂H₆ were retained significantly longer, breaking through at 2180.9 s g⁻¹ and 2317.7 s g⁻¹, respectively (Fig. 5b). These findings demonstrate that CB-Zn-DPG successfully achieved polymer-grade C₂H₄ (>99.99%) directly from both the C₂H₄/C₂H₆ (50/50) and C₂H₂/C₂H₄/C₂H₆ (1/90/9) gas mixtures.

Fig. 5 CB-Zn-DPG breakthrough curves for (a) a C₂H₄/C₂H₆ (50/50) mixture and (b) a C₂H₂/C₂H₄/C₂H₆ (1/90/9) mixture at 1 mL min⁻¹.

Conclusions

In conclusion, we have successfully synthesized a novel carborane hybrid MOF, CB-Zn-DPG, featuring a two-fold interpenetrated dia topology and moderately polarized pore surfaces. This material demonstrates excellent potential for the separation of C₂H₄ from C₂ hydrocarbon mixtures. Gas adsorption isotherms and DFT calculations revealed significantly higher uptake capacities and binding affinities for C₂H₂ and C₂H₆ compared to C₂H₄. Crucially, dynamic breakthrough experiments validated its ability to directly produce polymer-grade C₂H₄ (>99.99%) from C₂ mixtures. This work not only advances carborane-MOF chemistry, but also provides a design blueprint for next-generation adsorbents targeting challenging hydrocarbon separations.

Author contributions

CL: synthesis, characterization, adsorption experiments, draft; GX: DFT calculation; YW, WY, YY, HW: breakthrough experiments; LW: investigation, structural determination and analysis, funding, supervision; YZ: concept, supervision, draft, funding.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supporting information (SI). Supporting information: experimental and characterization details. Some more additional data has also been mentioned in the corresponded place in the main text to further clarify the results in this research. See DOI: https://doi.org/10.1039/d5mh01641j.

CCDC 2479060 contains the supplementary crystallographic data for this paper.⁵⁴

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 22205207 and 22378369), the Major Project of Natural Science Foundation of Zhejiang Province (LD24B060001), and the Research Fund of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Key Laboratory of Advanced Catalysis and Adsorption Materials. We thank the staff at the BL17B1 beamline of the National Facility for Protein Science in Shanghai (NFPS), Shanghai Advanced Research Institute, CAS for providing technical support in X-ray diffraction data collection and analysis.

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