A new boron cluster anion pillared metal organic framework with ligand inclusion and its selective acetylene capture properties

Abstract

The separation of acetylene (C₂H₂) from carbon dioxide (CO₂) and ethylene (C₂H₄) is important in industry but challenging due to their similar physical properties. Herein, a novel microporous boron cluster pillared metal–organic framework BSF-10 was synthesized with ligand inclusion for efficient C₂H₂/CO₂ and C₂H₂/C₂H₄ adsorption separation. The free dipyridyl ligands in the large pore reduce the porosity of BSF-10 but stabilize the framework. The available narrow pores without inclusion of ligands are suited for the accommodation of C₂H₂ by cooperative dihydrogen bonding. High C₂H₂ capacity and high C₂H₂ selectivity over CO₂ and C₂H₄ are achieved. The practical separation ability was confirmed by the breakthroughs using C₂H₂/CO₂ and C₂H₂/C₂H₄ gas mixtures with good recyclability. The dynamic separation factor of 2.8 for the equimolar C₂H₂/CO₂ mixture is comparable to those of many benchmark materials.

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The separation of C₂H₂/CO₂ and C₂H₂/C₂H₄ to produce C₂H₂ and C₂H₄ in high purity is important in industry.^1–4 C₂H₂ is a vital fundamental material for the synthesis of various organic chemicals and polymers. Produced from the cracking of hydrocarbons or partial combustion of natural gas, CO₂ is a contaminant and needs to be removed.³ On the other hand, the deep removal of C₂H₂ is a must in the process of ethylene production to obtain polymer grade C₂H₄ (>99.996%) since trace C₂H₂ can poison the ethylene polymerization catalysts by forming metal acetylides.⁴ Current state-of-the-art technologies for the separation and purification of C₂H₂ from other gases largely rely on cryogenic distillation, partial hydrogenation, or solvent extraction, which are either energy-intensive or associated with pollution. When compared, physisorption separation using porous materials is more energy-efficient and eco-friendly.^1,2 However, due to their similar molecular diameter and polarity (Table S1†), efficient C₂H₂/CO₂ and C₂H₂/C₂H₄ separation is still difficult to achieve.

The design of advanced porous materials is vital for many applications such as gas separation⁵ and catalysis.⁶ Recent years have witnessed families of porous metal organic frameworks (MOFs) with excellent properties for physisorption separations.^7–16 In this context, inorganic anion pillared MOFs as a unique subclass with strong Lewis basic functional anion sites, developed firstly by the Zaworotko group¹⁷ and Kitagawa group,¹⁸ have been of particular interest for selective gas separation due to their high molecular recognition competence. These inorganic anion pillared MOFs can be classified to five series to date according to the anions used: (1) MFSIX networks pillared by hexafluorometallate anions (e.g. SiF₆²⁻, GeF₆²⁻, and TiF₆²⁻),^19,20 (2) MO_xF_y pillared networks with anions containing octahedral metal centers bonded through both O and F atoms (e.g. NbOF₅²⁻ and TaOF₅²⁻),^21–23 (3) networks cross-linked by tetrahedral MO₄²⁻ oxyanions (e.g. CrO₄²⁻, MoO₄²⁻, SO₄²⁻),²⁴ (4) DICRO coordination networks that are sustained with Cr₂O₇²⁻ dianions,²⁵ and (5) boron cluster anion pillared supramolecular metal organic frameworks (BSFs) featuring closo-[B₁₂H₁₂]²⁻ or closo-[B₁₂H₁₁I]²⁻ as the inorganic pillars.^26–32

The icosahedral closo-dodecaborate [B₁₂H₁₂]²⁻ is a stable dianionic boron cluster consisting of 12 identical B–H vertices (Fig. 1A).³³ It is considered as a three-dimensional aromatic analogue to two-dimensional aromatic benzene due to the electron delocalization.³⁴ While other anions use electronegative F and O to coordinate transition metals, [B₁₂H₁₂]²⁻ utilizes hydride, which is not common in coordination polymers. In 2019, we reported the first BSF material termed BSF-1, which is constructed from closo-[B₁₂H₁₂]²⁻ dianions, Cu²⁺ ions and 1,2-di(pyridin-4-yl)ethyne (L4, Fig. 1B) with N⋯Cu and B–H⋯Cu coordination as well as B–H⋯H–C dihydrogen bonds (Fig. 1C).²⁶ Every hexacoordinate Cu center serves as a six-connected node. Four pyridyl units from different ligands comprise the equatorial plane and the axial positions of the Cu nodes coordinate to hydrogen atoms from [B₁₂H₁₂]²⁻, which bridge two Cu nodes to generate an infinite Cu-dodecaborate chain (Fig. 1D). BSF-1 displays high separation selectivity for C₃H₈/CH₄ and C₂H₆/CH₄. The extension of the organic linker to L5 (Fig. 1B) produces a isostructural MOF BSF-3 with improved porosity, which shows superior C₂H₂ capacity as well as enhanced C₂H₂/C₂H₄ and C₂H₂/CO₂ separation selectivity compared to BSF-1.²⁸ BSF-4 with L2 and BSF-9 with L3 were expected to show similar gas separation performance due to the similar length of organic linkers.^29,30 However, the interpenetration symmetry difference endows BSF-9 with symmetrical interpenetration mode and a record high C₂H₂/CO₂ separation selectivity in robust MOFs without open metal sites, which is over 5 fold that in BSF-4 with asymmetrical interpenetration mode.³⁰ Another approach to tune the porosity of BSFs is to alter the interpenetration degree. Eliminating the interpenetration of BSFs by controlling the binary solvent system can afford enhanced theoretical porosity. However, these non-interpenetrated MOFs (eg, BSF-5, 6, 7, 8) are actually very unstable and lose most of the porosity after guest removal due to the framework collapse.³⁰ Thus, it is still very challenging to design BSFs with a desirable stable porous structure and the interpenetration mode in BSFs is still difficult to predict and control.

Fig. 1 (A and B) Structure of closo-dodecaborate [B₁₂H₁₂]²⁻ and organic ligands. (C) General coordination mode in BSFs. (D) General 1D pore channels of interpenetrated BSFs.

Herein, we would like to report another approach to tune the porosity and stability of BSFs. 1,4-di(pyridin-4-yl)buta-1,3-diyne (L6, Fig. 1B) with a N⋯N distance of 12.1 Å and negligible steric hindrance was utilized as the organic linker. A new boron cluster anion pillared metal organic framework BSF-10 with ligand inclusion was prepared. BSF-10 is asymmetrically interpenetrated. There is one large pore with a square shaped window and two small pores with narrow rectangular shaped windows. In the large pore, L6 is included and unable to be removed by soaking into polar solvents or heating under vacuum. This ligand inclusion reduces the porosity of BSF-10 but enhances the stability for practical applications, which has never been reported in BSFs.⁶ The available narrow pores without inclusion of ligands are suited for the accommodation of C₂H₂ by cooperative dihydrogen bonding. Thus, the high capacity of C₂H₂ (65.0 cm³ g⁻¹) as well as good separation selectivity for C₂H₂/CO₂ (13.52–5.86) and C₂H₂/C₂H₄ (4.09–2.93) under ambient conditions are achieved. The practical separation ability was further confirmed by the dynamic breakthroughs using C₂H₂/CO₂ and C₂H₂/C₂H₄ gas mixtures with good recyclability. The dynamic separation factor of 2.8 for equimolar C₂H₂/CO₂ mixtures is comparable to those of many benchmark materials.

BSF-10 was readily prepared by stirring a mixture of Na₂[B₁₂H₁₂]·2H₂O, Cu(NO₃)₂·3H₂O and L6 in MeOH at 35 °C for 48 h. Single crystals of BSF-10 were produced by layering a MeOH solution of L6 onto an aqueous solution of Na₂[B₁₂H₁₂]·2H₂O and Cu(NO₃)₂·3H₂O. X-ray structural analysis of BSF-10 revealed that it crystallizes in a three-dimensional (3D) framework in the monoclinic space group P21/c (Table S2†). The asymmetric interpenetration leads to the generation of two kinds of pore channels: large Pore I and small Pores II/III (Fig. 2A). The large Pore I features a square shaped window and the small Pores II/III display narrow rectangular shaped windows. Notably, L6 is included in the large pore, and unable to be removed by soaking BSF-10 into polar solvents or heating under vacuum (Fig. 2B). Analysis of the configuration of L6 indicates the existence of two forms of L6 in Pore I with a packing mode of AABB (Fig. 2C). Strong π⋯π interactions among guests as well as multiple weak B–H⋯H–C interaction exist, which stabilize the structure of L6@BSF-10 and prevent the release of L6. Notably, the Cu[B₁₂H₁₂] : L6 ratio has no influence on the structure of BSF-10. Even when excess Cu[B₁₂H₁₂] was used, the single crystals still show the composition of Cu[B₁₂H₁₂](L6)₃ with free L6 inside Pore I (Table S3†).

Fig. 2 Porous structure of BSF-10. (A) Generation of two kinds of pores by asymmetrical interpenetration. (B) L6 included in the large Pore I. (C) The packing mode of L6 in Pore I. (D and E) Structure of Pore II in two directions highlighting the closest B–H⋯H–B distance after subtracting the two van der Waals radii of hydrogen.

Pore II and Pore III are nearly identical. The closest distance between two opposite B–H units in BSF-10 is 5.034 Å (Fig. 2D and E). This distance is suited to trap an acetylene molecule by cooperative dihydrogen bonding (B–H^δ−⋯H^δ+-CC-H^δ+⋯H^δ−–B)²⁸ and thus be potential for C₂H₂/CO₂ and C₂H₂/C₂H₄ separation. Besides, the pore window size of ca. 4 Å (Fig. 3) defined by the opposite H(Py)⋯H(Py) is also close to the diameter of C₂H₂ (kinetic diameter 3.3 Å, 3D diameter: 3.32 × 3.34 × 5.70 ų), allowing tight C₂H₂ accommodation.

Fig. 3 Voids and pore windows of BSF-10 generated by a probe with the radius of 1.2 Å seen along (A) b-axis and (B) a-axis. The pore window dimensions are determined after subtracting the two van der Waals radii of hydrogen.

Inspired by the structural analysis, we were interested to investigate the potential of BSF-10 for the selective adsorption separation of C₂H₂ from CO₂ and C₂H₄. The first step is to confirm the permanent porosity of BSF-10 after solvent removal as BSFs with larger pores are not stable upon activation.³¹ N₂ gas adsorption experiments at 77 K were conducted after activating BSF-10 under vacuum at 75 °C for 12 h. A typical Type I isotherm was observed, indicating the microporous character of BSF-10. The BET surface area was calculated to be 426.3 m² g⁻¹ with a total pore volume of 0.216 cm³ g⁻¹ at P/P₀ = 0.95, (Fig. 4A), which is quite close to the calculated pore volume of 0.222 cm³ g⁻¹ based on the single crystal structure of BSF-10 using a probe with the radius of 1.2 Å (Fig. 3).

Fig. 4 (A) N₂ adsorption and desorption isotherms of BSF-10. (B and C) Single component adsorption isotherms of C₂H₂, CO₂ and C₂H₄ on BSF-10 at 298 and 278 K. (D) Adsorption heats of C₂H₂, CO₂ and C₂H₄ on BSF-10.

The establishment of permanent microporosity in BSF-10 motivated us to study the gas separation performance for C₂H₂/CO₂ and C₂H₂/C₂H₄. First of all, single component adsorption isotherms of C₂H₂, CO₂ and C₂H₄ were collected under 298 and 278 K. At 1.0 bar, the C₂H₂/CO₂/C₂H₄ uptakes were 65.0/25.8/39.5 cm³ g⁻¹ at 298 K and 78.1/53.0/40.2 cm³ g⁻¹ at 278 K, respectively (Fig. 4B and C). The isosteric heat of adsorption (Q_st) for BSF-10 was then calculated using the Clausius–Clapeyron equation after fitting the isotherms to the Langmuir–Freundlich equation with excellent accuracy (R² > 0.9999, Tables S4 and 5†). Q_st values at near-zero loading for C₂H₂, CO₂, and C₂H₄ were 34.8, 27.4, and 22.9 kJ mol⁻¹ (Fig. 4D). This trend of Q_st values is consistent with the slope of the adsorption isotherms, indicating the preferential adsorption of C₂H₂ over CO₂ and C₂H₄.

Additionally, the Q_st value for C₂H₂ in BSF-10 is higher than that of BSF-1 (30.7 kJ mol⁻¹), but lower than that of BSF-3 (42.7 kJ mol⁻¹), consistent with the C₂H₂ adsorption uptake trend of BSF-1 (2.35 mmol g⁻¹) < BSF-10 (2.90 mmol g⁻¹) < BSF-3 (3.59 mmol g⁻¹). Notably, this modest Q_st renders BSF-10 a suitable porous material for practical application with a low energy footprint for regeneration.

Since selectivity is as important as capacity to evaluate the separation performance, the C₂H₂/CO₂ and C₂H₂/C₂H₄ selectivity on BSF-10 at 298 K and 278 K was calculated using ideal adsorbed solution theory (IAST), which revealed that the selectivity for equimolar C₂H₂/CO₂ and C₂H₂/C₂H₄ at 298 K and 1.0 bar was 13.52–5.86 and 4.09–2.93, respectively (Fig. 5A). The selectivity is slightly increased under low pressure when the temperature reduced to 278 K, and is 17.4–5.2 for C₂H₂/CO₂ and 4.8–2.8 for C₂H₂/C₂H₄ (Fig. 5B). The C₂H₂/CO₂ selectivity with different C₂H₂ molar ratios was further calculated for BSF-10, which indicates that the decrease of the C₂H₂ molar ratio leads to the increased C₂H₂/CO₂ selectivity (Fig. 5C). The C₂H₂ capacity and the C₂H₂/CO₂ selectivity on BSF-10 are superior to those of many well-performing materials such as BSF-1 (52.5 cm³ g⁻¹, S = 3.4),²⁶ BSF-2 (41.5 cm³ g⁻¹, S = 5.1),²⁷ TCuI (49.3 cm³ g⁻¹, S = 5.3),³⁵ JNU-1 (63 cm³ g⁻¹, S = 5),³⁶ iMOF-7C (15.7 cm³ g⁻¹, S = 4),³⁷ FJU-36a (52.2 cm³ g⁻¹, S = 2.8),³⁸ SNNU-150-In (34.9 cm³ g⁻¹, S = 5.6),³⁹ PCM-48 (25.5 cm³ g⁻¹, S = 4.3),⁴⁰ and JXNU-5a (55.9 cm³ g⁻¹, S = 5),⁴¹ (Fig. 5D), but inferior to those of BSF-3 (80.4 cm³ g⁻¹, S = 16.3).²⁸ The C₂H₂/CO₂ uptake ratio of 2.52 at 298 K and 1 bar is also relatively high. This value is superior to those of most MOFs that separate C₂H₂/CO₂ by a thermodynamic mechanism,^42–52 but slightly lower than those of Cu^I@UiO66-(COOH)₂ (2.58),⁴³ MAF-2 (3.68),⁴⁷ and SIFSIX-21-Ni (3.1)^17c as shown in Fig. 6 and Table S8.†

Fig. 5 (A and B) IAST selectivity of C₂H₂/CO₂ and C₂H₂/C₂H₄ at 298 and 278 K. (C) IAST selectivity of C₂H₂/CO₂ with different C₂H₂ molar ratios. (D) Comparison of the C₂H₂ uptakes and C₂H₂/CO₂ selectivity among BSF-10 and others well-performing MOFs.

Fig. 6 Comparison of the C₂H₂/CO₂ uptake ratios at 298 K and 1 bar among BSF-10 and other well-performing materials in the context of C₂H₂/CO₂ separation by a thermodynamic mechanism.

To confirm the practical separation ability of BSF-10 for C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures, dynamic breakthrough experiments were conducted. 0.233 g of BSF-10 powder was packed into a stainless column with a size of Φ 4.6 mm × 5 cm. After activating the sample by Ar purge at 75 °C for 12 h, an equimolar C₂H₂/CO₂ mixture was introduced. CO₂ appeared at the outlet within 12 min while C₂H₂ was retained in the column until 20 min (Fig. 7A). After complete breakthrough, Ar gas with a flow rate of 5 mL min⁻¹ was used to desorb the adsorbed C₂H₂ and CO₂ gases as well as to regenerate the material. Notably, nearly all the C₂H₂ and CO₂ gases are desorbed within 20 min at 75 °C and the material is regenerated for further use (Fig. 7D). Calculation of the areas of the breakthrough curves and desorption curves indicated that BSF-10 has a dynamic C₂H₂ capacity of 45 cm³ g⁻¹ with a C₂H₂/CO₂ separation factor of 2.8. This separation factor is slightly lower than that of benchmark materials such as CAU-10-H (3.4),⁴² Cu^I@UiO-66-(COOH)₂ (3.4)⁴³ and JCM-1 (4.4),⁴⁴ but higher than that of NKMOF-1-Ni (2.6),⁴⁵ FJU-90 (2.1)⁴⁶ and HOF-3a (2).⁵³ Besides, the dynamic C₂H₂ capacity of BSF-10 is also comparable to that of JCM-1 (49.3 cm³ g⁻¹). The regenerated BSF-10 was further used for repeated tests. Nearly no performance reduction was observed over 3 cycles (Fig. 7B). The same sample was further used for C₂H₂/C₂H₄ (1/99) mixture breakthrough experiments. C₂H₄ was detected at 15 min and C₂H₂ at 28 min, suggesting a good capture ability of trace C₂H₂ from bulky C₂H₄ (Fig. 7C). Desorption curves further indicated that the captured C₂H₂ can be desorbed within 20 min by Ar purge at 75 °C (Fig. 7D). Combining the high dynamic C₂H₂ capacity, high separation factor as well as facile regeneration conditions, BSF-10 is a good candidate for practical C₂H₂/CO₂ and C₂H₂/C₂H₄ separation.

Fig. 7 (A) Experimental breakthrough curves of C₂H₂/CO₂ (50/50) on BSF-10 at 298 K. (B) Comparison of the dynamic C₂H₂ uptake from the C₂H₂/CO₂ (50/50) mixture and breakthrough time among three cycles of breakthrough experiments. (C) Experimental breakthrough curves of C₂H₂/C₂H₄ (1/99) on BSF-10 at 298 K. (D) The desorption curves of BSF-10 after breakthrough experiments of C₂H₂/CO₂ (50/50) mixtures and C₂H₂/C₂H₄ (1/99) mixtures.

In conclusion, we prepared a novel microporous boron cage pillared metal–organic framework BSF-10 with unprecedented ligand inclusion. Free dipyridyl ligands occupy the large pores and interact with the framework by multiple interactions, which reduces the porosity of BSF-10 but enhances the stability of the framework. The narrow pores without inclusion of ligands are suited for the selective accommodation of C₂H₂ by cooperative dihydrogen bonding. Thus, high C₂H₂ capacity and high C₂H₂ selectivity over CO₂ and C₂H₄ are achieved. The practical separation ability was completely confirmed by the breakthroughs using C₂H₂/CO₂ and C₂H₂/C₂H₄ gas mixtures with good recyclability and facile regeneration conditions. The dynamic separation factor of 2.8 for the equimolar C₂H₂/CO₂ mixture is comparable to those of many benchmark materials. Boron cage pillared metal–organic frameworks constructed by non-linear organic linkers are under exploration. Besides, MOFs with dual C₂H₂/C₂H₄ and C₂H₂/CO₂ separation ability will be increasingly important for separating multicomponent gas mixtures using a single adsorbent, which currently largely relies on synergistic/tandem packed adsorbents in fixed-beds.⁵⁴

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Y. Zhang thanks the support of the Natural Science Foundation of China (No. 21908193) and Jinhua Industrial Key Project (2021A22648). L. Wang thanks the support of Open Research Fund of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University (KLMEACM202111). S. Duttwyler thanks the support of the National Natural Science Foundation of China (No. 21871231) and the Special Funds for Basic Scientific Research of Zhejiang University (No. 2019QNA3010 and K20210335).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2168393 and 2168394. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi00890d

‡ These authors contributed equally to this work.

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