A novel hydrophobic carborane-hybrid microporous material for reversed C₂H₆ adsorption and efficient C₂H₄/C₂H₆ separation under humid conditions

Abstract

Since ethylene (C₂H₄) is important feedstock in the chemical industry, developing economical and energy-efficient adsorption separation techniques based on ethane (C₂H₆)-selective adsorbents to replace the energy-intensive cryogenic distillation is highly demanded, which however remains a daunting challenge. While previous anionic boron cluster hybrid microporous materials display C₂H₄-selective features, we herein reported that the incorporation of a neutral para-carborane backbone and aliphatic 1,4-diazabicyclo[2.2.2]octane (DABCO) enables the reversed adsorption of C₂H₆ over C₂H₄. The generated carborane-hybrid microporous material ZNU-10 (ZNU = Zhejiang Normal University) is highly stable in humid air and maintains good C₂H₆/C₂H₄ separation performance under high humidity. Gas loaded single crystal structure and density-functional theory (DFT) calculations revealed that the weakly polarized carborane and DABCO within ZNU-10 induce more specific C–H^δ+⋯H^δ−–B dihydrogen bonds and other van der Waals interactions with C₂H₆, while the suitable pore space allows the high C₂H₆ uptake. Approximately 14.5 L kg⁻¹ of polymer grade C₂H₄ can be produced from simulated C₂H₆/C₂H₄ (v/v 10/90) mixtures under ambient conditions in a single step, comparable to those of many popular materials.

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Introduction

Polymer-grade ethylene (C₂H₄ > 99.9%) is significant feedstock to produce resins and plastics.¹ During the production, ethane (C₂H₆) is the major impurity and needs to be removed to improve the C₂H₄ quality. The current state-of-the-art technique to separate C₂H₄/C₂H₆ depends on cryogenic distillation at high pressure and low temperature.² The separation process is highly energy-intensive due to the similarity of the boiling points of C₂H₄ (−88.6 °C) and C₂H₆ (−103.7 °C) and requires very large distillation towers with 120 to 180 trays and high reflux ratios.³ Therefore, there is a need to develop novel technologies for efficient C₂H₄/C₂H₆ separation.⁴

Recently, physisorptive separation using porous solid adsorbents has attracted increasing attention due to its environmental friendliness and reduced energy consumption.⁵ Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs), as a new class of crystalline porous materials with a high surface area, structural tailorability and controllable properties, have been developed for the separation of C₂H₄/C₂H₆ hydrocarbon mixtures.⁶ Among them, most MOF materials, especially those decorated with polar functional groups or unsaturated metal sites, exhibit the preferential adsorption order of C₂H₄ > C₂H₆ based on their difference in physicochemical properties including molecular size, polarity and quadrupole moment (Table S2†).⁷ However, C₂H₄-selective MOFs are not the optimal selection for the adsorptive separation of C₂H₄/C₂H₆ mixtures. The concentration of C₂H₄ within the cracked gas mixtures is often over 10 times higher than that of C₂H₆, thus using C₂H₄-selective adsorbents for C₂H₄/C₂H₆ separation entails more adsorbent consumption and larger space occupation than using C₂H₆-selective adsorbents.⁸ Besides, the utilization of C₂H₄-selective adsorbents usually needs over four adsorption–desorption cycles to attain polymer-grade C₂H₄ due to the retention of C₂H₆ amidst adsorbent particles.⁹ In sharp contrast, using C₂H₆-selective adsorbents facilitates the direct acquisition of high-purity C₂H₄ through a single adsorption step, streamlining the separation process and effecting approximately a 40% reduction in energy consumption.¹⁰ Hence, the development of C₂H₆-selective adsorbents is highly imperative for industrial refinement of C₂H₄. In previous reports, reducing the polarity of pore surfaces in porous materials is promising to afford reversed adsorption of C₂H₆ over C₂H₄.¹¹ This phenomenon can be regarded as “like adsorbs like” when compared to the “like dissolves like” principle. However, the construction of C₂H₆-selective MOFs is still a difficult problem at present.

With the above consideration in mind, we envision that three dimensional neutral para-carborane (p-C₂B₁₀H₁₂) as a derivative of the [B₁₂H₁₂]²⁻ anion (Scheme 1) can serve as a weakly polarized functional group in MOFs to selectively bind C₂H₆ over C₂H₄. Although a plenty of dodecaborate [B₁₂H₁₂]²⁻ anion pillared MOFs have been developed in our group for gas separation with efficient C₂H₂/CO₂ and C₂H₂/C₂H₄ splitting performance,¹² carborane-based MOFs have been rarely reported,¹³ and no records of light hydrocarbon separation with carborane hybrid MOFs can be traced. On the other hand, carborane hybrid MOFs seem to display higher water stability when compared to benzene-based MOF analogues, which is a benefit for real applications with moisture.^13a,d

Scheme 1 Comparison of the surface electrostatic potential of boron clusters (dodecaborate dianion and p-carborane) and C₂ hydrocarbon gases (C₂H₆/C₂H₄).

Herein, we reported a novel microporous MOF termed ZNU-10 (ZNU = Zhejiang Normal University) with cluster para-carboranyl dicarboxylic acid (1,12-p-C₂B₁₀H₁₀-(COOH)₂) and 1,4-diazobicyclo[2.2.2]octane (DABCO) as the backbones. A pillar-layered framework with pcu topology and saturated metal sites is generated when assembled with classical paddle-wheel dicopper(II) SBUs. As a result, these pores with uniform electrostatic potential not only help to reverse the adsorption sequence of C₂H₆ over C₂H₄, but also contribute to improving the water stability of the framework due to the hydrophobic surface of the pore channel. Despite its higher affinity towards C₂H₆, the C₂H₆ adsorption heat (26.4 kJ mol⁻¹) in ZNU-10 is quite low, allowing its facile regeneration. In situ gas loaded single crystal structures and density-functional theory (DFT) calculations were applied to reveal the adsorption mechanism. In the poorly polarized channels, C₂H₆ molecules are stacked more closely and feature stronger van der Waals interactions and C–H^δ+⋯H^δ−–B dihydrogen bonding to the framework due to the larger molecular size and polarizability. Experimental breakthrough studies confirmed that one-step acquisition of high-purity C₂H₄ from C₂H₄/C₂H₆ mixtures can be realized with a high production of ca. 14.5 L kg⁻¹.

Results and discussion

Greenish-blue single crystals of ZNU-10 {Cu(1,12-p-C₂B₁₀H₁₀-(COO)₂)(DABCO)_0.5} were produced by a solvothermal reaction of Cu(NO₃)₂·3H₂O, p-C₂B₁₀H₁₀-(COOH)₂ and DABCO in a mixed DMF/MeOH/H₂O (v/v/v = 2/2/1) solution (Fig. S1†). Single crystal X-ray diffraction (SCXRD) analysis showed that Cu₂(COO)₄ paddle wheel secondary building units (SBUs) are formed between two Cu²⁺ and four carboxylate ligands (Fig. 1A). Each SBU paddle wheel is connected by p-carborane ligands to form a two-dimensional (2D) square grid layer (Fig. 1B). The layers are further pillared by the organic DABCO ligands to generate a 3D framework with pcu topology and saturated metal sites (Fig. 1C). The analysis of the surface electrostatic potential of ZNU-10 indicates that ZNU-10 features a uniform and poorly polarized pore surface (Fig. 1D). This is because ZNU-10 consists of a neutral carborane backbone and aliphatic organic building blocks. When compared, ZNU-1 {CuB₁₂H₁₂(dpe)₂}^12a constructed from −2 charged [B₁₂H₁₂]²⁻ backbones and aromatic ligands displays similar 1D channels but significantly different electrostatic potential on the pore surface. Notable charge separation can be observed on the isosurface of ZNU-1, namely the anionic boron cluster surface displays highly negative electrostatic potential while the organic linker surface displays very positive electrostatic potential (Fig. 1F). As we know, ZNU-1 is one of the best MOFs to selectively capture C₂H₂ from various mixtures.^12a,b Due to the completely different electrostatic potential distribution, ZNU-10 is yet supposed to be a C₂H₆ selective adsorbent for one-step C₂H₄ acquisition from C₂H₄/C₂H₆ mixtures.

Fig. 1 (A and B) Single layer of ZNU-10 constructed from carborane dicarboxylic acid and Cu(II) paddle wheels. (C) 3D structure of ZNU-10 viewed along the b axis. (D) Isosurface of ZNU-10 viewed along the c axis. (E) Structure of ZNU-1 viewed along the a axis. (F) Isosurface of ZNU-1 viewed along the a axis.

Before gas adsorption experiments, we investigated the chemical and thermal stability of ZNU-10 to optimize the activation conditions. We find that ZNU-10 is highly stable in many organic solvents including DMA, EtOH, n-hexane, acetone, CH₂Cl₂, DMF, MeCN, MeOH and so on, identified by a microphotograph (Fig. 2A) and powder X-ray diffraction (PXRD) (Fig. 2B). Temperature varied PXRD and thermogravimetric analysis (TGA) indicated that ZNU-10 is stable below 320 °C (Fig. 2C and D). Such thermal stability is superior to that of all the boron cluster anion hybrid MOFs and most Cu(II) based MOFs (Fig. S10 and S11†).¹² According to the above results, the as-synthesized ZNU-10 was soaked in MeOH for solvent-exchange and activated under 120 °C to fully remove all the guest molecules. Besides, we find that ZNU-10 is highly stable in humid air. The as-synthesized single crystals retained the crystallinity and quality over 3 months under an ambient humid atmosphere or heating at 120 °C for 3 days under vacuum (Fig. 3D, inset), identified by single crystal X-ray analysis.

Fig. 2 (A) Microphotograph of single crystals and (B) PXRD of ZNU-10 after soaking in different solvents for 3 months. (C) Temperature varied PXRD of ZNU-10. (D) TGA curve of ZNU-10 (inset: microphotograph of single crystals of ZNU-10 after heating at 120 °C for 3 days or exposure to humid air for 3 months).

Fig. 3 (A) N₂ sorption isotherms for ZNU-1 and ZNU-10 at 77 K. (B) C₂H₄ and C₂H₆ adsorption isotherms for ZNU-10 at 298 K. (C) C₃H₆ and C₃H₈ adsorption isotherms for ZNU-10 at 298 K (inset: adsorption isotherms in the pressure region of 0–10 kPa). (D) The isosteric heat of adsorption (Q_st) of C₂H₄ and C₂H₆ for ZNU-10 and ZNU-1. (E) IAST selectivity of equimolar C₂H₆/C₂H₄ and C₃H₈/C₃H₆ at 298 K. (F) Water sorption isotherms for ZNU-10 and ZNU-1 at 298 K (inset: contact angle testing of ZNU-10).

The permanent porosity of ZNU-10 was established by N₂ gas adsorption experiments at 77 K. The adsorption isotherm corresponds to the representative type I adsorption isotherm, indicative of its inherent microporous characteristics (Fig. 3A). At P/P₀ = 0.95, the N₂ adsorption capacity of ZNU-10 was 293 STP cm³ g⁻¹ (13.1 mmol g⁻¹), and the Brunauer–Emmett–Teller (BET) surface area and micropore volume are calculated to be 1171 m² g⁻¹ and 0.45 cm³ g⁻¹, respectively. The calculated pore size is centered at 8.59 Å (Fig. S13†), which is very close to the pore size of 7.7–10 Å from the single crystal structure evaluation (Fig. S6 and S7†). When compared, ZNU-1 only displays a BET surface area of 532 m² g⁻¹ and a pore volume of 0.20 cm³ g⁻¹ (Fig. S14†). Therefore, the porosity of ZNU-10 (Fig. S4 and S5†) is significantly increased when compared to that of ZNU-1.

Single component C₂H₆ and C₂H₄ adsorption measurements at 298 K were conducted for ZNU-10 and ZNU-1. At 1.0 bar, the C₂H₆ and C₂H₄ uptakes on ZNU-10 were 47.6 and 35.1 STP cm³ g⁻¹ (2.13 and 1.57 mmol g⁻¹), respectively (Fig. 3B, right). This uptake trend is opposite to that of ZNU-1 (30.9 and 37.1 STP cm³ g⁻¹ for C₂H₆ and C₂H₄) (Fig. 3B, left). With the temperature decreasing to 278 K, the C₂H₆ and C₂H₄ uptakes on ZNU-10 were improved to 67.9 and 55.1 cm³ g⁻¹ (Fig. S17 and S18†). The reversed adsorption of C₃H₈ and C₃H₆ is also observed in the low pressure region for ZNU-10 (Fig. 3C), which is quite rare in MOFs.¹⁴

In order to quantitatively compare the adsorption affinity between the framework and gas molecules, we calculated the adsorption heats (Q_st) of ZNU-10 and ZNU-1 using the Clausius–Clapeyron equation after fitting the isotherms to Langmuir equations with excellent accuracy (Tables S3 and S4†). The Q_st values of C₂H₆ and C₂H₄ for ZNU-10 were 26.4 and 23.2 kJ mol⁻¹, respectively; while those for ZNU-1 were 34.1 and 34.9 kJ mol⁻¹, respectively (Fig. 3D). These Q_st values are consistent with the slope of the adsorption isotherms in the low pressure region. Notably, the C₂H₆ adsorption heat on ZNU-10 is much lower than those in most C₂H₆-selective materials such as Fe₂(O₂)(dobdc) (67 kJ mol⁻¹),¹⁵ MAF-49 (60 kJ mol⁻¹),¹⁶ UiO-NDC (58 kJ mol⁻¹),¹⁷ NKMOF-8-Br (40.8 kJ mol⁻¹)¹⁸ and FJI-H11-Me (38.9 kJ mol⁻¹),¹⁹ and even lower than that of C₂H₆-disfavored ZNU-1. This extremely low adsorption heat allows the facile regeneration of ZNU-10 for recyclable use.

The separation selectivity of C₂H₆/C₂H₄ is further calculated based on ideal adsorption solution theory (IAST). The C₂H₆/C₂H₄ selectivity on ZNU-10 is 1.68∼1.64 in the range of 1–100 kPa (Fig. 3E), which is approximately 3.2 times that of ZNU-1 (S = 0.51 at 100 kPa). This C₂H₆/C₂H₄ selectivity is also higher than those of many C₂H₆ selective MOFs such as UiO-NDC (1.35),¹⁷ UPC-612 (1.4),²⁰ TJT-100 (1.2),²¹ LIFM-63 (1.56),²² JNU-2 (1.6)²³ and Azole-Th-1 (1.46).²⁴ Besides, the C₂H₆/C₂H₄ selectivity on ZNU-10 remains consistent regardless of the C₂H₆/C₂H₄ ratio (Fig. S22†). The equimolar C₃H₈/C₃H₆ selectivity for ZNU-10 and ZNU-1 from 1 to 100 kPa is calculated to be 1.91–1.00 and 0.21–0.78, respectively, further indicating the opposite affinity of ZNU-10 and ZNU-1 towards olefins and paraffins.

To further explore the pore difference of ZNU-10 and ZNU-1, we performed a water vapor adsorption test. The results showed that at a relative humidity of 0.9, the water vapor adsorption capacity of ZNU-10 was only 35.6 mL g⁻¹, much lower than that of ZNU-1 (ca. 162 mL g⁻¹) under the same conditions (Fig. 3F). These results showed that ZNU-10 is more hydrophobic and has high potential to remove the C₂H₆ impurity from C₂H₄/C₂H₆ mixtures under humid environmental conditions. The water stability is also confirmed by soaking the ZNU-10 crystals in water. Initially, ZNU-10 floated on the surface of water due to its low density (0.933 cm³ g⁻¹) and hydrophobicity. After some time, ZNU-10 crystals sunk to the bottom of water and became blue powders slowly. Nonetheless, the PXRD pattern of the water-soaked ZNU-10 is still consistent with that of the as-synthesized one (Fig. S25†). We also measured the water contact angle of ZNU-10, and the large water contact angle of >114° also indicated the hydrophobic nature of ZNU-10 (Fig. S26†).

Due to the high stability of ZNU-10 single crystals, the in situ single crystal structure of ZNU-10 with the loading of C₂H₆ or C₂H₄ was studied. We found that on average 0.7 C₂H₆ and 0.55 C₂H₄ molecules are adsorbed by a carborane unit (Fig. 4A and B), which corresponds to ca. 45 and 35 STP cm³ g⁻¹ of C₂H₆ and C₂H₄, respectively. These uptake values are close to the gas uptake from the adsorption isotherms at 1.0 bar. For both gases, two different binding sites are observed. The first site is located close to the window consisting of two carboranes and two DABCO (Fig. S9A and B†). There are four such sites in a single unit cell. The second site is located close to the window consisting of four carboranes (Fig. S9C and D†). There are two such sites in every unit cell. Therefore, six gas molecules can be adsorbed in a unit cell at maximum. However, the single crystal structure analysis indicated that the occupancy of gas molecules at each position is less than one. This could be explained by the low adsorption heats of C₂H₆ and C₂H₄ in ZNU-10 and the high symmetry of the framework. Due to the disorder, the contact distance between the gas molecules and the framework is not precise. Nonetheless, more close van der Waals interactions and C–H^δ+⋯H^δ−–B dihydrogen bonds are observed for C₂H₆ molecules. For example, the closest C–H^δ+⋯H^δ−–B distance between C₂H₆ and ZNU-10 is 2.10 Å, while that for C₂H₄ is 2.32 Å.

Fig. 4 (A and B) In situ single crystal structure of C₂H₆@ZNU-10 and C₂H₄@ZNU-10 viewed along the c axis. (C and D) DFT calculated C₂H₆ and C₂H₄ adsorption configuration in ZNU-10 viewed along the b axis and the corresponding binding energies.

To gain more insights into the gas adsorption behavior, modeling studies using DFT calculations were further performed. The calculated average bonding energy between ZNU-10 and two gases located at different sites based on the in situ single crystal structures is −32.1 and −22.8 kJ mol⁻¹ for C₂H₆ and C₂H₄, respectively (Fig. 4C and D), consistent with the experimental Q_st trend. In site I, C₂H₆ interacts with two carborane backbones by multiple C–H⋯H–B interactions. The contact between C₂H₆ and DABCO is also observed, with H⋯H distances of 2.08–2.67 Å and H⋯C distances of 2.90–3.03 Å. In site II, there only exist weak C–H⋯H–B interactions with a distance of >2.4 Å. When compared, C₂H₄ displays slightly shorter contact distances (2.18 and 2.21 Å in site I and 2.29 in site II) but decreased binding energy due to the fact that the overall interactions are fewer. This is consistent with the fact that C₂H₄ has small molecular size and thus the confinement in the nanospace of ZNU-10 will be weaker.

To evaluate the feasibility of ZNU-10 for practical C₂H₄ purification from binary C₂H₄/C₂H₆ mixtures, lab-scale breakthrough experiments were conducted for the C₂H₄/C₂H₆ (90/10) mixture to simulate the real separation conditions (Fig. S22†). Fig. 5A shows that high-purity C₂H₄ (>99.9%) was obtained between 44 and 58 min. The polymer grade C₂H₄ (>99.9%) productivity is estimated to be as high as 14.5 L kg⁻¹ (647 mmol kg⁻¹), superior to those of NKU-301 (9.5 L kg⁻¹),¹⁴ Zr-TCA (6.51 L kg⁻¹),²⁵ MOF-801 (5.73 L kg⁻¹),²⁶ UiO-66 (0.46 L kg⁻¹)²⁵ and many other popular materials under the same conditions.¹¹ Besides, ZNU-10 displays high recyclability and resistance to humidity. The breakthrough time is almost the same for multiple cycles or under highly humid conditions (Fig. 5B). Besides, ZNU-10 can be readily generated under Ar purge at room temperature within 1 h (Fig. S24†), which is consistent with the low adsorption heat of C₂H₆ and C₂H₄ in ZNU-10. The regeneration can also be achieved by desorption under vacuum for 1 h. In brief, the combination of high productivity of pure C₂H₄, good recycling performance and facile regeneration conditions of the material makes ZNU-10 as a potential adsorbent for practical C₂H₄ purification.

Fig. 5 (A) The column breakthrough curves of ZNU-10 for C₂H₄/C₂H₆ (90/10) mixtures at 298 K. (B) Repeated column breakthrough curves of ZNU-10 for C₂H₄/C₂H₆ (90/10) mixtures under dry and humid conditions.

Conclusions

In conclusion, we have reported a novel hydrophobic carborane hybrid metal–organic framework ZNU-10 with reversed C₂H₆ adsorption properties for efficient capture of C₂H₆ from C₂H₄/C₂H₆ mixtures to produce high-purity C₂H₄ in a single step. ZNU-10 displays distinct electrostatic potential distribution in the pore surface compared to the analogue cluster boron anion pillared ZNU-1. The difference leads to reversed C₂H₄/C₂H₆ hydrocarbon adsorption orders in ZNU-10 and ZNU-1. Notably, ZNU-10 exhibits good chemical/thermal stability with resistance towards water adsorption due to its hydrophobic pore surface. The excellent C₂H₄/C₂H₆ separation performance is fully illustrated by breakthrough experiments with good recyclability and capacity retention under high humidity. In general, our work demonstrates the versatility and importance of boron cluster functionalities, which can be tuned to achieve desirable gas adsorption properties. Considering the large library of boron cluster compounds, this work will open a new avenue in boron cluster chemistry for light hydrocarbon separation applications.

Data availability

All the data supporting this article have been included in the main text and the ESI.†

Author contributions

L. W.: investigation, structural determination and analysis, funding, and supervision; S. W.: synthesis, characterization, and adsorption experiments; J. H.: DFT calculation; Y. J.: investigation and discussion; J. Li: adsorption experiments; Y. H.: breakthrough experiments; Y. H.: breakthrough experiments; T. B.: supervision; B. C.: supervision and funding; Y. Z.: concept, supervision, draft and funding.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 22205207, 22378369 and 22008209), Major project of Natural Science Foundation of Zhejiang Province (LD24B060001) and Jinhua Industrial Key Project (No. 2021-1-088).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2287884, 2287885, 2287887 and 2287888. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc00424h

‡ These authors contributed equally to this work.

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