1. Introduction
Ethylene (C
2H
4) is a pivotal product in the global chemical industry [
1], [
2], [
3]. Catalytic cracking, steam cracking, and other C
2H
4 production processes are often conducted on mixtures containing C
2H
4 along with propane (C
3H
8), propylene (C
3H
6), ethane (C
2H
6), acetylene (C
2H
2), carbon dioxide (CO
2), and other compounds [
4], [
5], [
6], [
7], [
8]. In the traditional separation process, to obtain polymer-grade C
2H
4 from crude C
2H
4, an amine solution must be employed to absorb and thus remove CO
2, a selective catalytic hydrogenation must be implemented to purify alkynes, and multiple stages of cryogenic distillation must be carried out; however, this process is very energy intensive, and its input costs are high (
Fig. 1) [
9], [
10], [
11]. Because they can be conducted at ambient temperature using efficient and flexible devices, physisorption-centered separation processes hold the potential to significantly curtail energy consumption in gas separation [
12], [
13], [
14], [
15], [
16]; thus, these processes are expected to partially replace traditional energy-intensive C
2H
4 separation technologies. The appeal of adsorption separation technology lies in the breakthrough innovations that have been achieved in the field of adsorbents. The limits of commercial adsorbents (e.g., zeolite, activated carbon, silica gel, and alumina) have already been recognized: namely, poor adsorption capacity and poor selectivity for hydrocarbon mixtures due to the scarcity of adsorption sites and an insufficient recognition mechanism, especially for the isolation of hydrocarbons from multi-component mixtures containing mutually similar hydrocarbons (Fig. S1 in Appendix A) [
17], [
18], [
19], [
20], [
21], [
22]. As a means of addressing the challenges in these industrially important hydrocarbon-separation processes, metal-organic frameworks (MOFs) have provided good opportunities and platforms in the realm of gas separation owing to their porosity and diversity [
23], [
24]. Over the past ten years, a relatively complete structural design method based on MOF topology has been developed, along with a functional group surface cooperative regulation strategy [
25], [
26], [
27].
Research focusing on the MOF-based direct separation of C
2H
4 from binary mixtures (i.e., C
2H
4/C
2H
6, C
2H
4/C
2H
2, C
3H
8/C
3H
6, and C
2H
2/CO
2) over the past ten years [
1], [
4], [
8], [
12], [
13], [
14], [
26], ternary mixtures (i.e., C
2H
4/C
2H
6/C
2H
2 and C
2H
4/C
2H
2/CO
2) over the past five years [
5], [
6], [
7], [
15], [
19], [
28], [
29], [
30], [
31], and quaternary mixtures (i.e., C
2H
6/C
2H
4/C
2H
2/CO
2) over the past three years [
32], [
33], [
34], [
35] has produced continuous breakthroughs. Notably, as the studied mixtures get closer in composition to the actual industrial gas, the separation becomes more difficult, and the observed adsorption selectivity (especially between C
2H
6 and C
2H
4) and separation performance of the reported adsorbents decrease significantly. Indeed, the key stumbling block that limits the C
2H
4 separation efficiency of multi-component mixtures is generally the ineffective purification of C
2H
4 from C
2H
6. Li et al. [
14] and Lin et al. [
36] reported the achievement of ethylene purification from binary C
2H
6/C
2H
4 mixtures using the top-performing C
2H
6-selective adsorbents Fe
2(O
2)(dobdc) (dobdc
4− = 2,5-dioxido-1,4-benzenedicarboxylate) and Cu(Qc)
2 (HQc = quinoline-5-carboxylic acid), which demonstrated notable C
2H
6/C
2H
4 selectivities of 4.4 and 3.4, respectively. Subsequently, Gu et al. [
7] reported the introduction of Lewis basic sites into UiO-67 (UiO: Universitetet i Oslo) for the direct purification of ethylene from ternary mixtures; however, the materials were reported to afford C
2H
6/C
2H
4 selectivities of only 1.49 and 1.7, respectively. Chen et al. [
32] integrated three kinds of MOFs characterized by different selectivities into a fixed bed for the direct production of polymer-grade ethylene from ternary and quaternary mixtures; notably, the C
2H
6/C
2H
4 selectivity was decreased to 1.7. Recently, Wu et al. [
31] proposed a robust Al-MOF (Al-PyDC, H
2PyDC = 2,5-pyrroledicarboxylate) with multiple supramolecular binding sites for highly efficient one-step C
2H
4 purification from ternary mixtures, although its C
2H
6/C
2H
4 selectivity only reached 1.9. With the ultimate goal of simplifying the C
2H
4 separation process, reducing its cost and energy consumption, and creating an efficient adsorbent that affords the one-step purification of ethylene from multi-component refinery gas, the key target of the present study was to weaken the adsorption of C
2H
4 in comparison with other refinery gas molecules (i.e., C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2). The approach adopted herein is obviously different from the traditional strategy utilized to design efficient adsorbents [
37], [
38].
In order to weaken C
2H
4 adsorption through pore geometry design, four kinds of pillar-layered MOFs were systematically constructed: Ni(BTC)(pyrazine) [
39], Ni(BTC)(pyridine)
0.67(H
2O)
1.33, Ni(BTC)(DMF)
2 [
40], and Co(BTC)(DMF)
2, which were dubbed TYUT-10, TYUT-11, TYUT-12, and TYUT-13, respectively (TYUT: Taiyuan University of Technology; BTC: trimesic acid; DMF:
N,
N′-dimethylformamide; see Figs. S2-S9 and Tables S1 and S2 in Appendix A). Through the precise regulation of interlayer pore segmentation, we obtained suitable segmented channels between layers in TYUT-12 that are decorated with functional groups. In fact, the abundance of methyl groups, carboxylate oxygens, and nitrogen atoms in the interlayer ring-mounted channels results in a strong recognition ability for C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2 molecules that relies on C-H∙∙∙O, C-H∙∙∙N, and C-H∙∙∙π interactions; however, the C
2H
4 molecule is mainly recognized in flat pores through relatively weak π∙∙∙π interactions. As a result of its unique pore system, TYUT-12 does not just demonstrate exceptional C
2H
6/C
2H
4 adsorption selectivity (4.56); its use also permits—for the first time—the direct production of high-purity C
2H
4 (> 99.95%) from six-component refinery gas mixtures (C
3H
8/C
3H
6/C
2H
6/C
2H
4/C
2H
2/CO
2) through a single breakthrough operation conducted under ambient conditions, thus demonstrating the great potential of this MOF for realizing this challenging C
2H
4 separation process.
2. Materials and methods
2.1. Preparation of samples
(1) Synthesis of Ni(BTC)(pyrazine) (TYUT-10): The creation of the single-crystal sample was conducted using a method previously described, incorporating slight adjustments [
39].
(2) Synthesis of Ni(BTC)(pyridine)0.67(H2O)1.33 (TYUT-11): A mixture containing Ni(NO3)2·6H2O (0.029 g, 0.1 mmol), H3BTC (0.021 g, 0.1 mmol), and pyridine (81 µL, 1.0 mmol) was dissolved in 10 mL of DMF, sealed within a 20 mL scintillation vial, and subjected to a 373 K reaction for 48 h. Subsequently, the mixture was slowly cooled to ambient temperature. Upon reaching room temperature, the supernatant was discarded, and the residue was washed with DMF three times. The resulting material was then air-dried at room temperature to obtain green crystal samples (yield: 0.0176 g, 51.2% based on the BTC ligand; elemental analysis calculated (%) for Ni1.5C18.5H9.5NO11: C 43.62, H 1.91, N 2.68; found (%): C 43.53, H 1.86, N 2.75).
(3) Synthesis of Ni(BTC)(DMF)
2 (TYUT-12): The conventional preparation method used for the single-crystal sample was in accordance with a previous reported method [
40]. A 10 g level synthesis method was developed as follows: Ni(NO
3)
2·6H
2O (8.70 g, 0.03 mol) and H
3BTC (6.30 g, 0.03 mol) were dissolved in 300 mL of DMF, transferred into a 500 mL round bottom flask, and refluxed at 393 K for 15 h. After the reaction concluded, the solvent was cooled to room temperature. The resultant green block crystals were gathered, subjected to three washes with DMF, and subsequently air dried (yield: 11.47 g, 92.5% based on the BTC ligand). The abovementioned reacted solvent can be recovered and reused more than five times.
(4) Synthesis of Ni(BTC)(DMF-D7)2 (TYUT-12-D7): For the neutron powder diffraction (NPD) experiments, deuterated DMF (DMF-D7) was used to prepare TYUT-12-D7 in order to reduce the influence of the hydrogen (H) element on the test. The preparation method was as follows: A combination of H3BTC (0.105 g, 0.5 mmol) and Ni(NO3)2·6H2O (0.145 g, 0.5 mmol) was dissolved in 5 mL of deuterated DMF-D7. This mixture was sealed within a 20 mL small vial and subjected to a 393 K reaction for 2 d, followed by a gradual cooldown to room temperature. Once the temperature reached ambient levels, the supernatant was discarded, and the obtained crystals were washed three times with DMF-D7. The crystals were then allowed to air dry at room temperature (yield: 0.172 g, 80.75%). The solution (DMF-D7) after the above reaction can be recycled and used three times, and the TYUT-12-D7 product can be obtained by adding 80% of the basic raw materials each time (Fig. S10 in Appendix A).
(5) Synthesis of Co(BTC)(DMF)2 (TYUT-13): A blend of Co(NO3)2·6H2O (0.146 g, 0.5 mmol) and H3BTC (0.105 g, 0.5 mmol) was dissolved in 10 mL of DMF and enclosed within a 23 mL Teflon-lined autoclave. The mixture was subjected to a reaction at 393 K for 48 h, then gradually cooled down to room temperature. The prepared crystals were washed three times with DMF. The crystals were then allowed to air dry at room temperature, resulting in the acquisition of purple crystal samples (yield: 0.168 g, 81.6% based on the BTC ligand; elemental analysis calculated (%) for Co3C45H51N6O24: C 43.75, H 4.23, N 6.77; found (%): C 43.69, H 4.13, N 6.80).
2.2. Single-crystal X-ray diffraction and NPD
Crystallographic data acquisition was conducted using a Bruker D8 VENTURE PHOTON II area-detector diffractometer (Bruker AXS GmbH, Germany), employing graphite-monochromated Ga Kα radiation with a wavelength (
λ) of 1.34139 Å (1 Å = 10
−10 m) for TYUT-11 and Mo Kα radiation (
λ = 0.71073 Å) for TYUT-13 using the ω-scan technique. The SAINT program [
41] was used for the integration of diffraction data and the intensity correction for the Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the SADABS program [
42]. The structures were solved using direct methods and refined with the full-matrix least-squares technique based on
F2 using the SHELXL-97 program [
43]. All non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were introduced at the calculated positions.
The NPD data were collected using the multi-physics instrument (MPI) at the China Spallation Neutron Source, with a
Q range of 1.1-30 Å
-1 [
44], [
45]. The detectors were calibrated using a National Institute of Standards and Technology (NIST) silicon powder standard prior to the measurements. The C
2D
4-loaded powder sample with a mass of about 3 g was loaded in a vanadium can at room temperature. NPD data were acquired at a temperature of 10 K under vacuum for 3 h. Deuterated gas, C
2D
4, was used to reduce the large incoherent neutron scattering produced by the H atoms. The empty vanadium can and background data were obtained for data reduction and correction using the program Mantid. A Rietveld analysis was performed using GSAS-II. Herein, a
P1 phase model was employed for the structural refinement. In order to obtain a reliable and stable refinement result, the bonds in the C
2D
4, deuterated DMF-D
7, and 1,3,5-benzenetricarboxylic acid were constrained by rigid-body models to reduce the number of parameters. An isotropic thermal motion model was used for rigid bodies with the same type.
The crystallographic data is synopsized in Tables S1 and S2. The X-ray crystallographic data and NPD data related to TYUT-11, TYUT-12·0.72C2D4, and TYUT-13 have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under the deposition numbers 2110177, 2246837, and 2115139, respectively.
2.3. Adsorption and breakthrough experiments
Adsorption and desorption isotherms were acquired using an Intelligent Gravimetric Analyzer (IGA-001, Hiden Isochema, UK; detection limit of 0.1 μg based on an ultra-high-precision microbalance). All samples were CH2Cl2-exchanged over three times in 2 d, and were then activated under high vacuum (0.1 Pa) at 373 K for 12 h. Adsorption equilibrium data was gathered after maintaining the weight for a minimum of 30 min to achieve adsorption equilibrium at each predetermined pressure point on the isotherm. In each adsorption test, the mass of the preactivated materials was approximately 80 mg. To obtain kinetic adsorption data at 298 K, the mass of the sample was collected in real time after increasing the pressure from 0.1 Pa to 100 kPa (20 kPa∙min−1) and was then maintained at 100 kPa for over 60 min.
Dynamic separation experiments for the C2H6/C2H4, C2H2/C2H4, CO2/C2H4, C2H6/C2H4/C2H2, C2H6/C2H4/C2H2/CO2, and C3H8/C3H6/C2H6/C2H4/C2H2/CO2 mixtures were conducted with a flow rate of 2 mL∙min−1 (298 K, 100 kPa). For this experimentation, 1.50 g of TYUT-12 with a particle size of approximately 50 μm was loaded into a stainless steel column with the dimensions ϕ4 mm × 120 mm. Subsequently, the sample was activated by vacuum at 393 K for a duration of 12 h. The reaction vessels were positioned within a temperature-controlled environment, maintaining a temperature of 298 K. Mass flow controllers were employed to regulate the flow rates, while the gas stream effluent from the adsorption bed was detected using an Agilent 490 gas chromatograph (Agilent Technologies, USA). Before this experiment, the materials were activated by purging the column with helium (He) gas for a duration of 1 h at 393 K. The desorption experiments were conducted under helium flow (10 mL∙min−1) at room temperature for C2H6/C2H4 and at 333 K for C2H2-containing mixtures. An ABR automated breakthrough analyzer (Hiden Isochema, UK) was used to conduct single-component breakthrough experiments for TYUT-12 at a temperature of 298 K and a pressure of 100 kPa. The ABR automated breakthrough analyzer was also employed to perform quaternary mixture breakthrough experiments for TYUT-12 at 298 K and a pressure of 300 kPa.
3. Results and discussion
The series of pillar-layered MOFs was constructed using metallic nickel (Ni) or cobalt (Co), BTC, and the “pillar” compounds (pyrazine, pyridine, or DMF;
Fig. 2). The obtained materials are composed of graphene-like layers, where the repeating unit, which extends in every direction, consists of three metal ions and a molecule of BTC, forming a hexagon-like hole. The role of the pillars is to connect or support the layers, so that porous channels are formed between them. In the case of TYUT-10, a three-dimensional (3D) structure, with 3D channels extending in all directions, is formed as a result of the pillar, the bidentate ligand pyrazine, connecting the graphene-like layers that compose the material. In contrast, by replacing pyrazine with the monodentate ligand pyridine, the pillar can only play a supporting role for the layers, so that TYUT-11, a two-dimensional (2D) MOF with staggered layers, is produced. From the perspective of the structural and channel differences illustrated in Figs. 2 and S2, the formation of the layers causes the 3D channels in TYUT-10 to be segmented into each layer, which can result in blocking the gas diffusion between different layers, thus enhancing the system’s ability to discriminate between different gases. Notably, the abundant porosity and high specific surface area of TYUT-10 result in a very high adsorption capacity (∼4 mmol∙g
−1) for C
2H
6 and C
2H
4 but a low C
2H
6/C
2H
4 selectivity (1.37). Given the separation of pores located in different layers, C
2H
6 and C
2H
4 can only diffuse within layers, while diffusion between different layers is prevented. The resulting confinement effect causes the C
2H
6/C
2H
4 selectivity in TYUT-11 to increase to 1.55 with respect to TYUT-10 (Figs. S11 and S12 and Tables S3-S5 in Appendix A).
In order to enhance the ability to selectively adsorb C2H6, DMF molecules were utilized as pillars in the construction of pillar-layered MOFs with increased C2H6 binding affinity resulting from multiple van der Waals interactions; the MOFs obtained in this way were dubbed TYUT-12 (metal: Ni) and TYUT-13 (metal: Co). In comparison with the porous TYUT-10, the 2D TYUT-12, in which the DMF pillars act as layer supports, comprises interlayer channels that are completely segmented by the staggered layers. Connected hexagonal channels are formed between layers, at the center of which are independent oblate spherical cavities. Through the interlayer restriction and pore adjustment in the structure of TYUT-12, as well as the introduction of methyl groups, carbonyl oxygens, and nitrogen (N) atoms (resulting from the addition of DMF), the adsorption selectivity of TYUT-12 toward gaseous C3H8, C3H6, C2H6, C2H2, and CO2 over C2H4 is enhanced. As shown by the adsorption curves of TYUT-12, this MOF’s adsorption capacity and affinity for C3H8, C3H6, C2H6, C2H2, and CO2 are significantly stronger than those for C2H4. In particular, the difference in adsorption between C2H6 and C2H4 is significantly enhanced with respect to TYUT-10 and TYUT-11. In contrast, due to the difference in the coordinating abilities of Co and Ni, the coordinated DMF in TYUT-13 has a weaker interaction with Co, while its hydrogen bond with the carboxylate oxygens from the above layers is stronger, leading to structural distortion and interlayer distance reduction (Figs. 3 and S9). As a result, this MOF’s capacity for the adsorption of the mentioned gaseous molecules is drastically reduced. Through the tailor-made interlayer pore segmentation control in TYUT-12, in the optimized interlayer confinement channels, the adjustment of the specific adsorption sites of the benzene ring sandwich with weak π···π interactions achieves the goal of weakening ethylene adsorption. This material was thus ultimately selected as the best candidate for the direct purification of C2H4 from multi-component refinery gas mixtures (C3H8/C3H6/C2H6/C2H4/C2H2/CO2).
The adsorption of gaseous light hydrocarbons on TYUT-12 at different temperatures (273, 288, and 298 K) was investigated in detail; according to the relevant results, over a wide temperature range, the adsorption capacities for C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2 were significantly higher than that for C
2H
4 (
Figs. 3(a) and (b) and Figs. S13-S15 in Appendix A). Especially under low pressure (< 10 kPa), the uptake of C
2H
6 by TYUT-12 was higher than the uptakes of C
2H
4, C
2H
2, and CO
2, indicating that TYUT-12’s interlayer channels and functional groups effectively provide rich and strong binding sites for C
2H
6 molecules. In the case of large gas adsorption differences, as shown in
Figs. 3(c) and (d), TYUT-12 exhibited outstanding equimolar C
2H
6/C
2H
4 adsorption selectivities at 100 kPa (4.56 at 298 K and 7.20 at 273 K) and a high C
2H
6/C
2H
4 adsorption ratio at 10 kPa (2.21 at 298 K and 1.80 at 273 K)—values that are generally higher than those of the top-performing MOFs reported in the literature (
Fig. 3(e) and Fig. S16 in Appendix A) [
46]. Moreover, the equimolar C
3H
8/C
2H
4, C
3H
6/C
2H
4, C
2H
2/C
2H
4, and CO
2/C
2H
4 adsorption selectivities afforded by TYUT-12 were determined to be in the 3.2-41 range at 298 and 273 K. Even for the gas adsorption selectivity of a 1:99 gas composition (all the compositions of gas mixtures are volume ratio unless otherwise specified), TYUT-12 exhibited good performance (Fig. S17 in Appendix A). Its excellent adsorption selectivities for C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2 with respect to C
2H
4 point to the great potential of TYUT-12 for the efficient purification of C
2H
4 from multi-component mixtures.
During the separation process, gas diffusion is an important limiting factor; therefore, the adsorption kinetics of the gas molecules in TYUT-12 were investigated in detail (Figs. S18, S19, and Table S6 in Appendix A) [
47]. The time needed to reach the adsorption equilibrium was similar for C
2H
6, C
2H
4, C
2H
2, and CO
2, while the calculated diffusion coefficients of C
3H
8 and C
3H
6 were significantly smaller than those of C
2H
6 and C
2H
4, indicating the obvious diffusion restriction of larger molecules in the pore channels. The characteristics of the desorption curves also indicate that the time required for C
3H
8 desorption is the longest and the time for C
2H
6 and C
2H
4 desorption is relatively shorter—a trend that is related to molecular size and the interaction with the structures. The values for the isosteric heat of adsorption (
Qst) for the processes of C
3H
8, C
3H
6, C
2H
6, C
2H
4, C
2H
2, and CO
2 adsorption on TYUT-12 were calculated using a virial equation based on the adsorption curves obtained at different temperatures (
Fig. 3(f) and Figs. S20 and S21 in Appendix A). Near zero coverage, the
Qst values for the adsorptions of C
3H
8, C
3H
6, C
2H
6, C
2H
4, C
2H
2, and CO
2 on the said MOF were calculated to be 39.2, 35.8, 36.6, 31.4, 32.4 and 34.8 kJ∙mol
−1, respectively. Notably, the order of the
Qst values of the mentioned gases is consistent with their adsorption behavior, as inferred from the results of the single-component gas adsorption experiments (
Figs. 3(a) and (b)); the
Qst values also point to the unique C
2H
4 adsorption environment of TYUT-12, which results in C
2H
4 exhibiting the weakest binding affinity for the said MOF.
In order to demonstrate the C
2H
4 separation potential of TYUT-12 from gas mixtures, key separation experiments were conducted on the binary mixtures C
2H
6/C
2H
4 (50:50, 10:90, and 5:95), C
2H
2/C
2H
4 (50:50), and CO
2/C
2H
4 (50:50); the ternary mixture C
2H
6/C
2H
4/C
2H
2 (9:90:1); the quaternary mixture C
2H
6/C
2H
4/C
2H
2/CO
2 (9:89:1:1 and 9:85:1:5); and the six-component mixture C
3H
8/C
3H
6/C
2H
6/C
2H
4/C
2H
2/CO
2 (9:9:10:70:1:1) (
Fig. 4). As shown in
Fig. 4(a), C
2H
4 was efficiently separated from an equimolar C
2H
6/C
2H
4 mixture by passing the said mixture over a fixed bed of TYUT-12 under ambient conditions; using this approach, high-purity C
2H
4 (99.95%) was directly obtained from the outlet over a period of time. The dynamic uptakes of C
2H
6 and C
2H
4 during the separation process were 1.16 and 0.83 mmol∙g
−1, respectively, which were slightly lower than the static adsorption capacities due to the effect of micropore diffusion. Moreover, in the separation of C
2H
6/C
2H
4 (10:90 and 5:95) (Fig. S22 in Appendix A), higher purity C
2H
4 products with a longer separation time were obtained. Separation experiments were also conducted on the equimolar C
2H
4-containing binary mixtures C
3H
8/C
2H
4, C
3H
6/C
2H
4, C
2H
2/C
2H
4, and CO
2/C
2H
4. As shown by the data reported in Fig. S23 in Appendix A, all the gas mixtures afforded the isolation of high-purity C
2H
4, indicating the potential of TYUT-12 to be used in the separation of C
2H
4 from mixtures containing C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2. In the cases of the ternary mixture C
2H
6/C
2H
4/C
2H
2 (9:90:1) and the quaternary mixture C
2H
6/C
2H
4/C
2H
2/CO
2 (9:89:1:1 and 9:85:1:5) (
Figs. 4(b) and (c) and Figs. S24 and S25 in Appendix A), TYUT-12 also demonstrated a good C
2H
4 separation performance, with high-purity C
2H
4 (99.98%) being obtained. As for the challenging task of carrying out the direct purification of C
2H
4 from six-component refinery gas mixtures (C
3H
8/C
3H
6/C
2H
6/C
2H
4/C
2H
2/CO
2, 9:9:10:70:1:1), which has never before been realized, TYUT-12 again afforded a clear and efficient C
2H
4 separation. Based on the unique C
2H
4 binding affinity and locations in TYUT-12, five of the component gases of the mixture (C
3H
8, C
3H
6, C
2H
6, C
2H
2, and CO
2) were adsorbed onto the TYUT-12, while C
2H
4 was eluted from it at first, then quickly reached equilibrium. Correspondingly, C
2H
4 at over 99.96% purity was isolated in one step from the six-component mixture C
3H
8/C
3H
6/C
2H
6/C
2H
4/C
2H
2/CO
2 (9:9:10:70:1:1;
Fig. 4(d)). The amount of C
2H
4 captured under dynamic conditions in the TYUT-12 was calculated to be 0.90 and 0.86 mmol∙g
−1 in the separation processes conducted on quaternary or six-component mixtures, respectively; thus, about 0.78 and 0.60 mmol∙g
−1 C
2H
4 productivity (> 99.95% purity) can be obtained by one separation process (
Fig. 4(e) and Fig. S26 in Appendix A). As shown in Fig. S27 in Appendix A, it is worth mentioning that the material still exhibits good quaternary mixture (C
2H
6/C
2H
4/C
2H
2/CO
2, 9:89:1:1) separation under high pressure (300 kPa), and the corresponding C
2H
4 productivity can be increased to 1.27 mmol∙g
−1 (standard temperature and pressure).
Breakthrough repeatability tests demonstrated that TYUT-12 exhibits a stable separation performance and that high-purity C
2H
4 can be continuously obtained from quaternary gas mixtures (
Fig. 4(f) and Fig. S28 in Appendix A). Furthermore, the breakthrough and regeneration curves of single-component C
3H
8, C
3H
6, C
2H
6, C
2H
4, C
2H
2, and CO
2 gases on TYUT-12 confirmed that, under dynamic conditions, these gases can be easily desorbed from TYUT-12 by vacuuming (Figs. S29-S34 in Appendix A). The results of the dynamic separation experiments conducted on binary to six-component mixtures thus verified the ability of TYUT-12 to afford the isolation of high-purity C
2H
4 (> 99.95%) from different mixtures in one step, indicating the great potential of TYUT-12 to achieve the challenging industrial separation of C
2H
4 from complex light hydrocarbon mixtures.
To gain insights into and comprehensively understand the interactions between the host and guest, we conducted density functional theory (DFT) simulations, aiming to pinpoint the specific adsorption sites within TYUT-12 and to quantify the said MOF’s binding affinity toward C
3H
8, C
3H
6, C
2H
6, C
2H
4, C
2H
2, and CO
2 molecules. As shown in
Fig. 5, the simulation results indicated that the alkane molecules (C
3H
8 and C
2H
6) were preferentially located in the annular interlayer channel (site I), where they engaged in multiple interactions with N and O atoms from TYUT-12 functional groups via strong C-H∙∙∙O/N hydrogen bonds (2.51-3.01 Å). The preferential adsorption sites for the linear molecules (C
2H
2 and CO
2) were determined to be in site II, the narrow neck position in the annular interlayer channel, where several C-H∙∙∙O/N hydrogen bonds (2.85-3.48 Å) were calculated to form between the TYUT-12 framework and the guest molecule. The preferential adsorption sites for the olefin molecules (C
3H
6 and C
2H
4) were calculated to be in site III, where the guest molecules engage in relatively weak C-H∙∙∙π (2.85-3.46 Å) and π∙∙∙π interactions (3.64-3.69 Å) with two benzene rings from the host. Importantly, given its larger molecular size than C
2H
4, C
3H
6 can interact more strongly than C
2H
4 with the host through extra C-H∙∙∙O/N hydrogen bonds (2.87-3.19 Å). The data for the adsorption energy (
Eads) also shows that C
2H
4 has the minimum value among these gases (Table S7 in Appendix A). In a comparison of the DFT results for the series of TYUT-10/11/12/13 materials (Fig. S35 in Appendix A), the adsorption sites of C
2H
4 tended to be in the middle of the upper and lower layers and between the benzene rings, with TYUT-12 having the weakest interaction with C
2H
4. Significantly, the results from the theoretical calculations exhibited good consistency with the outcomes discussed in relation to the adsorption and breakthrough experiments.
To further verify the conformation of the C
2H
4 molecules adsorbed onto TYUT-12, high-resolution NPD experiments were conducted using a TYUT-12-D
7 sample synthesized using DMF-D
7 as the solvent. High-quality NPD data were collected at 10 K on C
2D
4-loaded TYUT-12 samples (
Fig. 6 and Fig. S36 in Appendix A), allowing the conformation of the C
2H
4 molecules in the structure to be determined. As expected based on the results of the DFT calculations, the C
2H
4 molecules were found to be located in the flat round pores (site III), surrounded by six DMF molecules and sandwiched between two benzene rings (
Figs. 6(e) and (f)). In this adsorption site, the C
2H
4 molecules engaged only in weak π∙∙∙π interactions (3.47-3.76 Å) with the benzene rings—an outcome that shows good concurrence with the findings derived from the DFT calculations. Hence, through the collected NPD data, we confirmed the uniqueness of the adsorption environment of the C
2H
4 molecules in TYUT-12 and were able to explain the efficacy of the one-step C
2H
4 separation from binary, ternary, quaternary, and six-component C
2H
4-containing hydrocarbon mixtures.
In addition, the synthesis of TYUT-12 was scaled up, and the stability of this MOF was investigated, as shown in
Fig. 7 and Figs. S37-S39 in Appendix A. Only easy-to-obtain chemical raw materials (i.e., nickel nitrate, H
3BTC, and DMF) are necessary for the preparation of TYUT-12, and the solvent after the reaction can be recycled; therefore, the synthesis of this MOF is relatively easy to scale up to the gram-level in the laboratory, and the samples prepared by this method maintain the original Brunauer-Emmett-Teller (BET) and adsorption properties. Notably, the powder X-ray diffraction (XRD) patterns of TYUT-12 were recorded after the material had undergone treatment under different conditions; these data indicated that TYUT-12 is characterized by a relatively stable structure. In detail, after being exposed to air for 18 months, no reduction in the C
2H
6 adsorption performance of TYUT-12 was observed. The crystal morphology after the stability test also remained (Fig. S38). Moreover, the stability of the material’s dynamic adsorption of C
2H
6 was verified by conducting cyclic breakthrough experiments. TYUT-12 was able to maintain its C
2H
6 capture capacity over 20 breakthrough cycles, and its regeneration could be realized via a simple outgas process. Its dynamic C
2H
6 adsorption performance was also maintained for five cycles under humid conditions (75%) (Fig. S39). In summary, for the separation of C
2H
4 from multi-component light hydrocarbon mixtures, TYUT-12 can be prepared via a simple, low-cost method, and the obtained material exhibits good structural and performance stability. These excellent characteristics demonstrate the great potential of this material for industrial applications.
4. Conclusions
This work demonstrates the successful construction of weak C2H4 adsorption locations through pore geometry design in a series of pillar-layered MOFs that can be used for the efficient one-step purification of C2H4. Based on its tailor-made pore environment, TYUT-12 exhibited outstanding adsorption selectivity for C3H8, C3H6, C2H6, C2H2, and CO2 over C2H4; more specifically, its C2H6/C2H4 selectivity reached a value of 4.56, marking a new record for C2H4 purification from multi-component mixtures. Theoretical calculations and NPD analysis indicated that the targeted weak pore confinement of C2H4 in TYUT-12 was achieved via relatively weak π∙∙∙π interactions (∼3.6 Å) with two adjacent benzene rings. The results of the breakthrough experiments indicated that high-purity C2H4 (> 99.96%) was directly produced from C2H6/C2H4/C2H2/CO2 quaternary mixtures and C3H8/C3H6/C2H6/C2H4/C2H2/CO2 six-component mixtures through a one-step separation process. The pore segmentation strategy applied herein based on pore engineering and functional group regulation is an effective method to achieve the precise molecular recognition of target products; it also offers new solutions and routes to increase the efficiency and environmental friendliness of olefin separation strategies in the petrochemical industry.
Acknowledgments
The research work was supported by National Key Research and Development Program of China (2022YFB3806800) and National Natural Science Foundation of China and (22278288 and 22090062). We gratefully acknowledge Juping Xu from the Spallation Neutron Source Science Center for the neutron powder diffraction.
Compliance with ethics guidelines
Yang Chen, Zhenduo Wu, Longlong Fan, Rajamani Krishna, Hongliang Huang, Yi Wang, Qizhao Xiong, Jinping Li, and Libo Li declare that they have no conflict of interest or financial conflicts to disclose.
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