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Published in final edited form as: Chin J Chem Eng. 2020 Feb;28(2):10.1016/j.cjche.2019.09.005. doi: 10.1016/j.cjche.2019.09.005

Reversed ethane/ethylene adsorption in a metal–organic framework via introduction of oxygen

Ling Yang 1, Wei Zhou 2, Hao Li 3, Ali Alsalme 4, Litao Jia 5, Jiangfeng Yang 1, Jinping Li 1, Libo Li 1,5,6,7,*, Banglin Chen 3,*
PMCID: PMC8201596  NIHMSID: NIHMS1670569  PMID: 34131370

Abstract

Separation of ethane from ethylene is a very important but challenging process in the petrochemical industry. Finding an alternative method would reduce the energy needed to make 170 million tons of ethylene manufactured worldwide each year. Adsorptive separation using C2H6-selective porous materials to directly produce high-purity C2H4 is more energy-efficient. We herein report the “reversed C2H6/C2H4 adsorption” in a metal–organic framework Cr-BTC via the introduction of oxygen on its open metal sites. The oxidized Cr-BTC(O2) can bind C2H6 over C2H4 through the active Cr-superoxo sites, which was elucidated by the gas sorption isotherms and density functional theory calculations. This material thus exhibits a good performance for the separation of 50/50 C2H6/C2H4 mixtures to produce 99.99% pure C2H4 in a single separation operation.

Keywords: Metal–organic frameworks, Adsorptive separation, Open metal sites, Reversed ethane/ethylene adsorption, Density-functional theory calculation, Breakthrough curves

1. Introduction

Ethylene (C2H4) is a crucial starting chemical for the manufacture of many high-value organic products. In production plants, C2H4 is usually produced by thermal decomposition of ethane (C2H6) and steam cracking of naphtha. C2H6 will inevitably arise in these processes. Before plastic manufacturing process, separating C2H6 to produce polymer-grade (≥99.95% pure) C2H4 is very challenging, due to their similar molecular sizes and volatilities [1,2]. The current well-established method of producing this chemical is using a cryogenic high-pressure distillation process, which takes lots of energy in chemical industry. So developing an energy-efficient C2H6/C2H4 separation process is highly desired and is among the most important industrial separation processes in future [3].

Nonthermal adsorbent-based separation seems to be a promising alternative method in recent years, with low energy requirements and operating costs [4-6]. However, conventional porous materials such as zeolites and carbon-based materials are not satisfactory in this separation process due to poor adsorption selectivity and low capacity. As novel porous materials, metal–organic frameworks (MOFs) have attracted immense attention from both academia and industry in recent decades due to their excellent gas separation performance [7-19]. Unlike traditional porous materials, the pore surface and gas diffusion channels of MOFs could be easily functionalized and designed [20-30], which enables them to show promise for a wide variety of gas separation areas, including the important C2H6/C2H4 separation.

According to different binding affinities for C2H6 or C2H4, MOFs can be cataloged into C2H4-selective MOFs and C2H6-selective one in C2H6/C2H4 separation. Generally, C2H4-selective MOFs have polar binding centers like open metal sites (OMSs) [31] or narrow pore channels (0.3–0.4 nm) [32] that can selectively capture C2H4 from C2H6/C2H4 mixtures. But these adsorbents usually require three or four additional separation cycles to yield high-purity C2H4 product [1], which significantly increases the energy consumption of this method. The latter C2H6-selective MOFs is proposed to be much more efficient. C2H4 is afforded directly through a single adsorption step, which simplifies the process and results in high separation productivity [33,34]. However, construction of C2H6-selective MOFs is more challenging in view of the non-polarity and larger molecular size of C2H6 (C2H4: 0.48 × 0.42 × 0.33 nm3, C2H6: 0.48 × 0.41 × 0.38 nm3) [32]. Only a few examples have been identified so far, and they suffer from poor selectivities owing to lack of suitable strong binding sites for C2H6 [35-37]. In 2018, we reported a functionalized MOF [Fe2(O2)(dobdc)] with Fe–peroxo sites, which can induce stronger interactions with C2H6 than with C2H4 [38]. This strategy might provide us a tunable platform that can modify OMSs to realize ideal porous materials for C2H6/C2H4 separation. We thus synthesized Cr-BTC(O2) [Cr3(1,3,5-benzenetricarboxylate)2(O2)3], developed by Murray et al. [39], studied its adsorption mechanisms for binding C2H6, and investigated the C2H6/C2H4 separation performance. In Cr-BTC(O2), the O2 was believed to be a superoxo (O2)−, not a peroxo (O2)2−. We suspected that it may exhibit similar effect toward gas binding. Indeed, with the modified superoxo group, this MOF can preferentially bind C2H6 over C2H4, leading to the unusual reversed C2H6/C2H4 adsorption. This material thus can capture C2H6 from C2H6/C2H4 mixture to yield polymer-grade C2H4 under ambient conditions, as demonstrated by experimental breakthrough results.

2. Experimental

2.1. Synthesis of materials

2.1.1. Cr-BTC [Cr3(BTC)2]

The synthetic method described by Murray et al. [39] was improved as follows: anhydrous Cr(CO)6 (1.09 g, 5 mmol), trimesic acid (H3BTC, 0.623 g, 3 mmol), and anhydrous DMF (50 ml) were added to a 100ml three-neck flask in glove box filled with 99.999% N2. The reaction mixture was heated to 393 K and stirred for three days to form purple precipitate. Methanol exchange was repeated six times during 2 days, and the solid was collected by filtration and dry in vacuum to yield Cr3(BTC)2 solvent as an orange powder. Cr3(BTC)2 solvent sample was fully activated by heating under dynamic vacuum (<10−2 Pa) at 433 K for 24 h and then cooled down to room temperature to yield Cr3(BTC)2 as light green powder. Cr3(BTC)2 is air-sensitive, and needs to be handled and stored in a dry box under N2 atmosphere.

2.1.2. Cr-BTC(O2) [Cr3(BTC)2(O2)3l

Cr3(BTC)2 was oxidized under carefully controlled conditions: about 1 g Cr3(BTC)2 sample was transferred into a 500 ml flask in a dry glove box, then sealed and evacuated to 10−2 Pa. Pure O2 (>99.999%) was slowly dosed to the bare Cr3(BTC)2 sample to 103 Pa at a rate of 50 Pa·min−1 under 298 K and kept it for 30 min. The O2 pressure was brought up to 105 Pa and was maintained constant over 1 h to reach equilibrium. At last, the sample was evacuated to remove the free O2 gas molecules and yield the deep green Cr3(BTC)2(O2)3. Cr2(BTC)3(O2)3 is air-sensitive, the water molecule in the air will cause its gradual destruction, so the sample needs to be handled in a dry box under N2 atmosphere.

All chemical reagents and solvents were commercially available, and used without further purification.

2.2. Characterizations

The gas sorption isotherms were collected on an Intelligent Gravimetric Analyzer (IGA 001, Hiden, UK). The purities of the N2, C2H4 and C2H6 are higher than 99.999%. To maintain the experimental temperatures, ice-water bath (273 K) and water bath (298 K) were used. The breakthrough curves were measured on a homemade apparatus; the details were provided in supporting information.

2.3. Density-functional theory calculation

Density-functional theory (DFT) calculations were performed to gain better understanding of the superoxo groups in the adsorption of C2H6 and C2H4. Due to the orientational disorder of the superoxo groups on the Cr metal sites, direct calculation based on the MOF crystal structure is not straightforward. In this work, we adopted a simplified cluster model to evaluate the gas binding. Each cluster consists of two Cr2+ ions and four H2BTC− groups, forming a charge-neutral Cr2 paddlewheel. The cluster was put in a 2.5 nm × 2.5 nm × 25 nm supercell. For this system, a cutoff energy of 544 eV and a 2 × 2 × 2 k-point mesh (generated using the Monkhosrt-Pack scheme) were found to be enough for total energy to converge within 0.01 meV per atom. The cluster models (without and with superoxo sites) were first fully optimized with respect to atomic coordinates. The ground state of the Cr2 dimer was found to be antiferromagnetic. For C2H6 or C2H4 adsorption, various possible binding configurations were considered and fully relaxed. The lowest-energy structures were identified as the optimal binding structures.

3. Results and Discussion

3.1. Structure analysis

Cr-BTC has a three-dimensional framework structure depicted in Fig. 1a, where Cr2 paddlewheel complexes are linked via triangular BTC3− to form an isostructural structure of Cu-BTC. The as-synthesized Cr-BTC sample was exchanged with methanol for six times, and heated at 433 K under vacuum over 48 h to fully expose the Cr(II) OMSs. Next, O2 molecule was induced to bind Cr(II) OMSs under carefully controlled conditions to form the Cr-superoxo sites in Cr-BTC(O2) (Fig. 1b and c). Cr-BTC(O2) maintains the framework structure of Cr-BTC, with a Brunauer–Emmett–Teller surface area of 1135 m2·g−1 (Fig. S1a). Bulk purity of Cr-BTC(O2) sample was confirmed by powder X-ray diffraction (PXRD) patterns, infrared spectroscopy analysis, and the measured surface area (Fig. S1).

Fig. 1.

Fig. 1.

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(a) Illustration of the structure of Cr-BTC, and the cluster models of Cr2BTC4 (b) and Cr2BTC4(O2)2 (c) used in the calculations (Cr, navy; C, gray; O, red; O2−, orange, H, white) [39].

3.2. Gas adsorption and separation

The C2H6 and C2H4 sorption isotherms of Cr-BTC and Cr-BTC(O2) were collected at 273 and 298 K (Figs. 2 and S2). As shown in Fig. 2a and b, the C2H6 and C2H4 adsorption isotherms of Cr-BTC and Cr-BTC (O2) show type I character with a smooth increase with pressure. At 105 Pa and 298 K, C2H4 and C2H6 adsorption capacities of Cr-BTC are 6.03 mmol·g−1 (135 cm3·g−1) and 5.27 mmol·g−1 (118 cm3·g−1), respectively. While, after O2 molecule bonded, the selectivity of two gases is totally reversed. The gas uptakes of C2H6 and C2H4 on Cr-BTC(O2) reach 3.30 mmol·g−1 (74 cm3·g−1) and 2.89 mmol·g−1 (65 cm3·g−1), respectively, at the same conditions. The corresponding isosteric heat (Qst) of C2H6 adsorption on Cr-BTC(O2) was calculated to be 37.2 kJ·mol−1 at zero coverage (Figs. 2d and S3), which is apparently higher than that of C2H4 (23.5 kJ·mol−1). Furthermore, C2H6/C2H4 (50/50) selectivity of Cr-BTC(O2) calculated by ideal adsorbed solution theory (IAST) is over 3 at the low pressure region, which decreases gradually as the pressure increases (Figs. 2e and S4). This is mainly due to the strong interactions between C2H6 molecules and Cr-superoxo sites at low pressure. According to these results, we further demonstrated the separation performance of Cr-BTC(O2) for C2H6/C2H4 (50/50) mixtures under dynamic conditions. Breakthrough experiments were performed in a packed column containing ~ 1 g of Cr-BTC(O2) samples at room temperature (298 K). As shown in Fig. 2c, the breakthrough data clearly demonstrate that Cr-BTC(O2) can effectively separate C2H6/C2H4 mixtures: the C2H4 gas passed through the adsorption bed first, while C2H6 was retained in the packed column. The concentration of C2H6 in the outlet effluent was below 0.1%, thus producing C2H4 with a purity of >99.99%. To ensure the regenerability of Cr-BTC(O2), C2H6/C2H4 (50/50) separation cycling experiments were performed at the same conditions (Fig. 2f). The experimental cycling results indicate that there was no noticeable loss of C2H6/C2H4 separation capacity in continuous 4 cycles.

Fig. 2.

Fig. 2.

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(a, b) Experimental C2H6 and C2H4 sorption isotherms of Cr-BTC and Cr-BTC(O2) at 298 K. (c) Breakthrough curves of 50/50 (v/v) C2H6/C2H4 mixture on Cr-BTC(O2) at 298 K. (d) Isosteric heat of C2H6 and C2H4 adsorption on Cr-BTC(O2). (e) Predicted mixture adsorption selectivity of Cr-BTC(O2) predicted by IAST method at 298 K. (f) 50/50 C2H6/C2H4 separation cycles lasting for 40 min. Each separation process was carried out at 298 K and 1.01 bar, while regeneration was performed using vacuum at 298 K for 2 min.

3.3. Density-functional theory calculation

To understand the special role of the superoxo groups in the adsorption of C2H6 and C2H4, we performed DFT calculations. Due to the orientational disorder of the superoxo groups on the Cr metal sites, direct calculation based on the MOF crystal structure is not straightforward. Therefore, we adopted a simplified cluster model to evaluate the gas binding. Our cluster model consists of two Cr2+ ions and four H2BTC− groups, forming a charge-neutral Cr2 paddlewheel. We first calculated the C2H4 and C2H6 binding on the open Cr sites, and the optimized gas binding configurations are shown in Fig. 3(a, c).

Fig. 3.

Fig. 3.

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The preferential C2H6 and C2H4 adsorption sites in Cr-BTC (a, c) and Cr-BTC(O2) (b, d) obtained from DFT calculations (Cr, navy; C, gray; O, red; O2−, orange; H, white).

As expected, C2H4 binds strongly to the open Cr site through the C═C bond in a side-on mode, while C2H6 binds to the open Cr site relatively weakly through one—CH3 in an end-on configuration. The corresponding calculated static binding energies of the two molecules are 34.2 and 22.4 kJ·mol−1, respectively. Next, O2 molecules were introduced to the Cr sites to form a cluster model for Cr-BTC(O2). Two different initial O2 binding configurations (end-on and side-on) were considered. In both cases, the O2 got relaxed to the same end-on binding configuration, suggesting that the end-on orientation of O2 corresponds to the lowest energy configuration. The O—O bond length in the fully optimized structure is 0.128 nm, a typical value for superoxo species and slightly larger than that of molecular O2 (0.121 nm). Inspection of the charge density distribution also shows that there is notable amount of charge transfer from Cr2+ to O2, turning Cr2+ to nearly Cr3+, and O2 to (O2)−. These findings are consistent with the previously reported experimental results [39]. Subsequently, we calculated the gas binding on the structure where both Cr sites are pre-occupied by superoxo groups. Optimal gas binding configurations were identified and are shown in Fig. 3(b, d). C2H6 exhibits a larger affinity than C2H4 toward the superoxo group, through stronger O…H interaction (the hydrogen bond distance: 0.30 nm vs 0.33 nm). The stronger C2H6 binding is also partly due to the molecular geometry of C2H6 (trigonal pyramidal shape, in contrast to the trigonal planar shape of C2H4), which enables a better van der Waals interaction with the surrounding ligand surfaces in this particular MOF structure. As a net result, the calculated static binding energies of C2H4 and C2H6 on the superoxo site are 21.0 and 28.7 kJ·mol−1, respectively. Apparently, the effect of the superoxo functionalization in the structure is two-fold: a notable C2H6 binding enhancement, and a significant C2H4 binding strength reduction. Consequently, the binding preference between the two gases are reversed. These results are fully consistent with our experimental observations, and well explain the adsorption mechanisms qualitatively.

4. Conclusions

In summary, we have realized a functionalized MOF for the challenging task of C2H6/C2H4 separation. The basic mechanism of this MOF for the specific recognition of C2H6 has been clearly demonstrated through theoretical calculations. With the introduction of superoxo groups, Cr-BTC(O2) exhibits a higher binding affinity for C2H6 molecule, which realizes the efficient C2H6/C2H4 mixture separations under ambient conditions to directly produce high-purity C2H4. This work will provide a feasible strategy on the modification of OMSs within MOFs for stronger interactions with C2H6 than with C2H4, and prepare some useful porous materials for realization of the challenging C2H6/C2H4 separation.

Supplementary Material

Supp1

Acknowledgments

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21606163 and 21878205), Coal Bed Methane Joint Foundation of Shanxi (2016012006), Foundation of State Key Laboratory of Coal Conversion (J18-19-610), Welch Foundation (grant AX-1730), and the Distinguished Scientist Fellowship Program (DSFP) at KSU.

Footnotes

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2019.09.005.

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