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. 2022 Jun 8;8(23):eabn9231. doi: 10.1126/sciadv.abn9231

Synergistic binding sites in a hybrid ultramicroporous material for one-step ethylene purification from ternary C2 hydrocarbon mixtures

Peixin Zhang 1,2,†, Yao Zhong 1,†, Yan Zhang 3, Zhenliang Zhu 1, Yuan Liu 1, Yun Su 1, Jingwen Chen 1, Shixia Chen 1, Zheling Zeng 1, Huabin Xing 2, Shuguang Deng 4, Jun Wang 1,*
PMCID: PMC9176739  PMID: 35675397

Abstract

One-step separation of C2H4 from ternary C2H2/C2H4/C2H6 hydrocarbon mixtures is of great significance in the industry but is challenging due to the similar sizes and physical properties of C2H2, C2H4, and C2H6. Here, we report an anion-pillared hybrid ultramicroporous material, CuTiF6-TPPY, that has the ability of selective recognition of C2H4 over C2H2 and C2H6. The 4,6-connected fsc framework of CuTiF6-TPPY exhibits semi–cage-like one-dimensional channels sustained by porphyrin rings and TiF62− pillars, which demonstrates the noticeably enhanced adsorption of C2H2 and C2H6 over C2H4. Dynamic breakthrough experiments confirm the direct and facile high-purity C2H4 (>99.9%) production from a ternary gas mixture of C2H2/C2H6/C2H4 (1/9/90, v/v/v) under ambient conditions. Computational studies and in situ infrared reveal that the porphyrin moieties with large π-surfaces form multiple van der Waals interactions with C2H6; meanwhile, the polar TiF62− pillars form C–H•••F hydrogen bonding with C2H2. In contrast, the recognition sites for C2H4 in the framework are less marked.

An anion-pillared HUM realizes one-step C2H4 purification from ternary C2 hydrocarbon mixture by synergistic binding sites.

INTRODUCTION

As the largest feedstock in the petrochemical industry, the purification of olefins, e.g., ethylene and propylene, collectively accounts for approximately 0.3% of global energy (1, 2). The annual production of ethylene (C2H4) exceeded 190 million metric tons in 2019 and will continuously expand in the foreseeable future (3). In industry, C2H4 is mainly produced by steam cracking of naphtha or thermal dehydrogenation of ethane, in which the impurities of acetylene (C2H2) and ethane (C2H6) are inevitably entrained (4). The trace amount of C2H2 (1000 to 5000 parts per million) will poison the catalysts of polyethylene production and even lead to an explosion; meanwhile, high levels of C2H6 will compromise the polymer production efficiency (5). Now, catalytic hydrogenation is used to remove C2H2 from C2H4 at high temperature and pressure (6), while the C2H6-C2H4 mixtures are separated by thermal-driven cryogenic distillation with large reflux ratio and >150 trays at −25°C and 23 bar (7). Therefore, physisorption-based separation with high-energy efficiency is considered as a promising alternative to separate C2H4 from C2H2-C2H4-C2H6 ternary mixtures in a single step under mild operation conditions (8, 9).

Anion-pillared hybrid ultramicroporous materials (HUMs) are an intriguing subclass of metal-organic frameworks (MOFs); the pore size and chemistry can be exquisitely modulated through the reticular principle and crystal engineering strategies (10–13). Another important dimension is that the altering of anion pillars (e.g., SiF62−, TiF62−, GeF62−, ZrF62−, and NbOF52−) can induce the distortion of pore shapes and the rotation of organic ligands (14, 15), thus anion-pillared HUMs have emerged as the promising C2 light hydrocarbon adsorbents for binary separations such as C2H2/C2H4 (16), C2H2/CO2 (17), and C2H4/C2H6 (18). However, the single-step C2H4 purification from ternary C2 mixtures has never been achieved in adsorptive separations using anion-pillared HUMs, because the quadrupole moment and kinetic diameter of C2H4 [1.5 × 10−26 electrostatic unit (esu) cm2 and 4.1 Å] locate between those of C2H2 (7.2 × 10−26 esu cm2 and 3.3 Å) and C2H6 (0.65 × 10−26 esu cm2 and 4.4 Å) (19). Thus far, only nine reports have potentially realized the single-step separation of C2H4 from C2H2/C2H4/C2H6 ternary mixtures, and only seven MOFs have demonstrated their potentials in practical applications with dynamic breakthrough experiments (5, 20–27).

The abundant polar anion pillars (MF62−) in HUMs are strong C2H2 recognition sites as hydrogen bonding acceptors, enabling pervasive high C2H2 uptake and selectivity over C2H4 (28–30). For instance, Cu(4,4′-dipyridylsulfone)2(TiF6) (ZUL-100) exhibited the highest C2H2 adsorption capacity of 2.96 mmol g−1 at an ultralow pressure of 0.01 bar and resulted in an outstanding C2H2/C2H4 selectivity of 175 at 298 K and 1 bar (31). Regarding the C2H4/C2H6 separation, unsaturated C2H4 is preferentially adsorbed via the electron π-complexion mechanism, leading to the preferential C2H4 adsorption over C2H6 (32). In contrast, C2H6-selective adsorption can substantial reduce energy consumption and afford high-purity C2H4 by avoiding the C2H4 recovery through heating or vacuuming (33). To date, only limited low-polarity or aromatic-rich MOFs have exhibited such unusual adsorption behavior (34, 35). Considering that C2H6 has larger polarizability (44.4 × 10−25 to 44.7 × 10−25 cm3 for C2H6 and 42.5 × 10−25 cm3 for C2H4) and van der Waals surface area (75 Å2 for C2H6 and 61 Å2 for C2H4), low-polarity MOFs featuring abundant aromatic or aliphatic ligands can yield selective C–H•••π and van der Waals interactions with C2H6 molecules (25). However, the organic ligands of existing anion-pillared HUMs fail to provide sufficient low-polarity π-surfaces to selectively bind C2H6 over C2H4. To the best of our knowledge, the selective adsorption of C2H2 and C2H6 over C2H4 from C2H2/C2H4/C2H6 ternary mixtures has never been documented in anion-pillared HUMs.

Here, we report an example of anion-pillared HUMs, CuTiF6-TPPY [5,10,15,20-tetra(4-pyridyl)-21h,23h-porphyrin], which can one-step separate C2H4 from C2H2/C2H4/C2H6 ternary mixtures. The rarely reported quadritopic pyridyl-based ligand creates semi–cage-like one-dimensional (1D) channels with a proper cavity size of 5.0 Å by 8.0 Å rather than the 1D penetrating frameworks using two connected linear ligands (Fig. 1A). The properly positioned anion pillars (TiF62−) provide strong recognition sites for C2H2 with C–H•••F hydrogen bonds. Meanwhile, the porphyrin moieties with large π-surfaces form multiple van der Waals interactions (C–H•••N bonds) with C2H6. Those specific binding sites are absent for C2H4, which results in efficient single-step purification of C2H4 (purity, >99.9%) from a ternary mixture of C2H2/C2H4/C2H6 in one adsorption column at room temperature.

Fig. 1. Schematic illustration of the structure of CuTiF6-TPPY.

Fig. 1.

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(A) Schematic illustration of the modularity with different 2D layer net and anion pillars that form 3D pcu or fsc topology anion-pillared HUMs. (B) Schematic illustration of the building blocks (CuII, TiF62−, and TPPY organic ligand) and the 3D fsc network topology of CuTiF6-TPPY. (C) Pore size and shape. (D) The normalized x-ray absorption near-edge structure (XANES) spectra at the Cu K-edge. a.u., arbitrary units. (E) Fourier transformation of the extended x-ray absorption fine structure (EXAFS) spectra in the R space. FT, Fourier transform. (F) The EXAFS fitting curve for CuTiF6-TPPY.

RESULTS

Pore structure and C2 adsorption property

The reaction of TPPY with Cu(BF4)2·4H2O and (NH4)2TiF6 in methanol solution at 60°C afforded a dark red powder of CuTiF6-TPPY (Fig. 1B; see the Supplementary Materials for synthetic and crystallographic details). Notably, CuTiF6-TPPY can also be successfully prepared by using other Cu salts such as Cu(NO3)2·3H2O and CuCl2·2H2O (fig. S5). Despite extensive attempts, high-quality single crystals of CuTiF6-TPPY cannot be obtained for single-crystal x-ray diffraction (XRD) studies. Therefore, multiple technologies were applied to determine the accurate crystal structure of CuTiF6-TPPY. First, the coordination environment of Cu ions on CuTiF6-TPPY was probed by Cu K-edge x-ray absorption fine structure (XAFS) analysis (36, 37). The normalized x-ray absorption near-edge structure (XANES) curves of Cu K-edge of CuTiF6-TPPY located between Cu phthalocyanine (CuPc; Cu─N4) and Cu fluoride (CuF2; Cu─F2), indicating the hybrid coordination of Cu─N and Cu─F bonds (Fig. 1D). To confirm this result, the Fourier transform (FT) k3-weighted extended XAFS (EXAFS) spectrum of CuTiF6-TPPY showed a main peak at 1.52 Å (Fig. 1E), which has a small offset compared to that of CuPc (Cu─N bond; 1.53 Å) and CuF2 (Cu─F bond; 1.53 Å), also confirming the hybrid coordination environment. Notably, no Cu─Cu coordination peak was observed at 2.2 Å, which unambiguously implied the complete absence of Cu nanoparticles or clusters. The fitting of the Cu K-edge EXAFS spectrum indicated a total coordination number of 5.8 ± 0.8, constituting by an average number of 4 and 2 for Cu─N and Cu─F path, respectively (Fig. 1F and table S1). In addition, the wavelet transform (WT) plot of CuTiF6-TPPY with the maximum WT centered at 4.5 Å−1 (fig.S1), which is close to that of CuPc and CuF2 but far from that of the Cu─Cu bond (6 to 8 Å−1), also confirming the existence of the Cu─N and Cu─F bonds in CuTiF6-TPPY. Inductively coupled plasma optical emission spectroscopy (ICP-OES) reveals that the Cu and Ti contents on CuTiF6-TPPY are 9.07 weight % (wt %) and 7.06 wt %, respectively, which are close to the theoretical contents of Cu (7.64 wt %) and Ti (5.73 wt %). The calculated molar ratio of Cu and Ti is close to 1:1, which confirms the absence of Cu chelation on porphyrin rings. Combined with the C and N contents by ultimate elemental analysis (table S3), the unit formula of CuTiF6-TPPY was determined to be Cu(TPPY)(TiF6). The x-ray photoelectron spectroscopy measurement also confirmed the element compositions in CuTiF6-TPPY (fig. S6 and table S3).

The Rietveld refinement of powder XRD (PXRD) data revealed that the as-synthesized CuTiF6-TPPY crystallizes in the orthorhombic crystal system with cell parameters of a = 13.858, b = 13.788, and c = 8.232 (fig. S2 and table S2) (38). The simulated PXRD pattern matches well with the experimental one, confirming the high phase purity of as-synthesized CuTiF6-TPPY (fig. S3). Individually, each Cu(II) atom was connected by four pyridyl groups from independent TPPY ligands to form a 2D layer network, which was further pillared by TiF62− anions and thus affords a 3D framework without interpenetration (Fig. 1B and fig. S7). The TiF62− pillars and pyridine rings are interconnected through hydrogen bonds with C–H•••F distances of 2.27 and 2.29 Å, leading to the titling of pyridine rings by 69.4° with respect to the crystal axials (fig. S8). Because of the high symmetries of the four connected TPPY linker and coordination model, CuTiF6-TPPY exhibits one type of semi–cage-like 1D channels with a cavity size of 5.0 Å by 8.0 Å (Fig. 1C and fig. S9), which is smaller than the 1D penetrating frameworks using two connecting linear ligand of SiFSiX-1-Cu (~8.0 Å; 1 = 4, 4′-bipyridine; fig. S10) (15, 28). Thermogravimetric analysis showed that the guest molecules can be completely removed at 80°C, and CuTiF6-TPPY showed a good thermostability until 320°C (fig. S11).

Before adsorption measurements, CuTiF6-TPPY was exchanged with methanol and acetone and activated at 333 K under a high vacuum (<5 μm of Hg) for 12 hours. The activated CuTiF6-TPPY exhibited the intact PXRD pattern, implying the indistinctive structure distortion and rigid frameworks (fig. S3). The permanent porosity of CuTiF6-TPPY was investigated by N2 adsorption-desorption isotherm at 77 K (Fig. 2A), and the typical type I adsorption isotherm indicates the microporous nature of CuTiF6-TPPY (39). The Brunauer-Emmett-Teller (BET)–specific surface area and the pore volume were determined to be 685 m2 g−1 and 0.32 cm3 g−1, respectively (Fig. 2A and fig. S12). The pore size distribution (PSD) that was determined by the nonlocal density functional theory (NLDFT) method centered at 6.0 Å agreed well with the pore size (5.0 Å by 8.0 Å) derived from the crystal analysis. Notably, the pore structures can be remained after repeated breakthrough cycles and 4 months of storage, suggesting excellent structural robustness (fig. S13). Meanwhile, CuTiF6-TPPY can maintain its crystalline after soaking in various organic solvents for 1 week as verified by PXRD patterns (fig. S4).

Fig. 2. Single-component gas adsorption properties.

Fig. 2.

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(A) N2 adsorption-desorption isotherm at 77 K and NLDFT PSD curve. (B) C2H2, C2H4, and C2H6 adsorption isotherms at 298 K. (C) C2H2/C2H4 and C2H6/C2H4 ideal adsorbed solution theory (IAST) selectivity. Comparison plot of (D) C2H2/C2H4 IAST selectivity, (E) C2H6/C2H4 IAST selectivity, and (F) comprehensive C2 separation performances with representative porous materials.

To explore the adsorption and separation performances of CuTiF6-TPPY, single-component adsorption isotherms of C2 hydrocarbons were measured at 273, 288, and 298 K (Fig. 2B and fig. S14). Specifically, CuTiF6-TPPY exhibited an unusual adsorption behavior in the order of C2H2 > C2H6 > C2H4, indicating the potential of one-step separation C2H4 from C2H2/C2H4/C2H6 ternary mixtures. At 298 K and 1 bar, the uptake amount of C2H2, C2H6, and C2H4 on CuTiF6-TPPY reaches 3.62, 2.82, and 2.42 mmol g−1, giving a C2H2/C2H4 and C2H6/C2H4 uptake ratio of 1.50 and 1.17, respectively, outperforming most reported MOFs (table S7). In addition, the adsorption isotherms of C2 hydrocarbons are reversible, implying that CuTiF6-TPPY can be easily regenerated.

Furthermore, the ideal adsorbed solution theory (IAST) was applied to evaluate the C2H2/C2H4 and C2H6/C2H4 selectivity. Figure 2C showed that the IAST selectivity of C2H2/C2H4 (1/99) and C2H6/C2H4 (10/90) reaches 5.03 and 2.12 on CuTiF6-TPPY at 298 K and 1.0 bar. Note that the C2H2/C2H4 (1/99) selectivity (5.03) is the highest among all seven MOFs that achieved one-step C2H4 separation from C2H2/C2H4/C2H6 ternary mixtures (Fig. 2D). Meanwhile, the IAST selectivity of C2H6/C2H4 (10/90) is also superior to most top-ranking C2H6-selective MOFs (Fig. 2E). Therefore, CuTiF6-TPPY can be considered as the benchmark for one-step C2H4 separation from C2H2/C2H4/C2H6 ternary mixtures concerning the simultaneously outstanding C2H2/C2H4 and C2H6/C2H4 selectivities (Fig. 2F). Similarly, CuTiF6-TPPY also exhibited high IAST selectivities of 5.47 and 2.12 on C2H2/C2H4 (50/50) and C2H6/C2H4 (50/50) binary gas mixtures (fig. S18). Compared to the analogous SIFSIX-1-Cu, the C2H6/C2H4 separation performance was significantly improved attributing to the porphyrin moieties with large π-surfaces of CuTiF6-TPPY (fig. S15).

To evaluate the affinity of CuTiF6-TPPY to guest adsorbates, the isosteric heat of adsorption (Qst), a quantitative assessment of binding affinity, was calculated on the basis of fitted isotherms at three different temperatures by the Clausius-Clapeyron equation (40). The isotherm fitting details are provided in the Supplementary Materials (table S5 and fig. S17). The Qst of C2H2 and C2H6 is calculated to be 36.5 and 34.2 kJ mol−1 at zero coverage on CuTiF6-TPPY (fig. S19), both higher than that of C2H4 (29.6 kJ mol−1). This result indicates that CuTiF6-TPPY has higher affinities toward C2H2 and C2H6 than C2H4, which is consistent with the order of adsorption capacities. All the Qst of C2 adsorbates are located in the physisorption range and lower than many ultramicroporous MOFs (table S6), implying the low energy consumption for regeneration (41).

Dynamic breakthrough experiments

Dynamic transient breakthrough experiments were further carried out to evaluate the actual performances of CuTiF6-TPPY for separating C2H2/C2H6/C2H4 (1/9/90, v/v/v) ternary mixture at a flow rate of 2.5 ml min−1 under ambient conditions. As shown in Fig. 3A, C2H4 first eluted through the column at 49.5 min, followed by C2H6 at 60.5 min; whereas, the breakthrough of C2H2 occurred at an elongated time of 220 min. Note that the ability to remove trace C2H2 is the best among all the one-step C2H4 purification adsorbents. The concentration of C2H4 at the outlet was monitored by online gas chromatography, and high-purity C2H4 (>99.9%) can be obtained in one step with a collection window of 11 min. In addition, the clean separation of C2H4 can also be obtained at a higher flow rate of 5.0 ml min−1 (fig. S21). In contrast, the analogous SIFSIX-1-Cu can barely separate binary C2H6/C2H4 (10/90, v/v) and ternary C2H2/C2H6/C2H4 (1/9/90, v/v/v) gas mixtures (fig. S16).

Fig. 3. Dynamic breakthrough curves and cycling tests.

Fig. 3.

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The dynamic breakthrough curve of (A) C2H2/C2H6/C2H4 (1/9/90, v/v/v) mixture at 2.5 ml/min. Dynamic breakthrough curves of C2H6/C2H4 (10/90, v/v) binary mixture (B) at the flow rate of 5.0 ml/min and (C) 2.5 and 6.0 ml/min. (D) Five continuous breakthrough cycles at the flow rate of 5.0 ml/min on CuTiF6-TPPY at 298 K.

Considering the close breakthrough time of C2H6 and C2H4 compared to that of C2H2/C2H4, detailed breakthrough experiments with various C2H6/C2H4 compositions and flow rates were conducted. As shown in Fig. 3B, for C2H6/C2H4 (10/90, v/v) binary gas mixture, C2H4 and C2H6 eluted through the column at 19.6 and 28.4 min, respectively, affording a time interval of 8.8 min to produce high-purity C2H4 (>99.9%). The productivity of C2H4 (>99.9%) was calculated to be 1.09 mmol g−1, which is superior or comparable to top-performing materials, such as Azote-Th-1 (1.13 mmol g−1; 10/90, v/v) (21), Zn(batz) (MAF-49) (0.28 mmol g−1; 50/50, v/v) (42), Fe2(O2)(dobdc) (0.79 mmol g−1; 50/50, v/v) (33), and Mn5IIMnIII(u3-O)2-(CH3COO)3(Tripp)2(BDC)3 (NPU-1) (0.28 mmol g−1; 1/2, v/v) (20). Moreover, clean separations can also be obtained at total flow rates of 2.5 and 6.0 ml min−1 (Fig. 2C). Similarly, the breakthrough curve of C2H6/C2H4 (50/50, v/v) revealed that the high-purity C2H4 (99.9%) can be collected from a relatively low concentration of C2H4 at a large flow rate of 8.0 ml min−1 (fig. S21). For practical applications, the material stability is critical; five continuous cycles for the separation of C2H6/C2H4 mixture after facile regeneration with a He flow of 20 ml min−1 at 298 K displayed no noticeable deterioration in retention time during the stability test (Fig. 3D and fig. S23). In regeneration processes, high-purity C2H6 cannot be obtained even at smaller He flow rates of 5 and 10 ml min−1 (fig. S24). Time-dependent gas uptake profiles were further gravimetrically recorded (fig. S25A), almost identical slopes of C2H4 and C2H6 for each pressure stage confirmed the close adsorption and desorption rates. Furthermore, in the desorption curves (fig. S25B), rapid gas desorptions were both completed within 3 min due to their close interaction strengths with CuTiF6-TPPY. Note that this phenomenon does not compromise the one-step separation performances because C2H4 is directly collected at the exit of adsorption column rather than in the desorption process. Moreover, the reproducibility for the synthesis process and separation performances of CuTiF6-TPPY were demonstrated by the intact XRD patterns, C2 adsorption capacities, and dynamic breakthrough performances on 10 parallelly synthesized batches (figs. S30 to S32).

Computational simulation studies

To reveal the host-guest interactions between the framework and adsorbates, we performed the grand canonical Monte Carlo (GCMC) calculations, first-principles dispersion-corrected DFT calculations, and in situ infrared (IR) experiments. The C2 gases were adsorbed in two major areas, i.e., the organic ligand of the TPPY region (region I) and the TiF62− anion-pillared region (region II; Fig. 4, A and B). The contribution density for C2 gases is in the order of C2H2 > C2H6 > C2H4 that is consistent with the adsorption capacities (fig. S27). As for region II, the contribution densities for C2H2 and C2H6 were higher than that of C2H4, confirming that the TiF62− anions could provide stronger interactions with C2H2 and C2H6 over C2H4 (Fig. 4, A and B, and fig. S26A). As for region I, the C2H2 molecules were uniformly distributed along with the whole porphyrin ring; C2H6 molecules were mainly adsorbed at the center of porphyrin rings and the junction of pyridine and porphyrin rings. In contrast, C2H4 molecules were loosely distributed near the porphyrin rings (fig. S27B). Meanwhile, DFT calculations were conducted to identify the adsorption sites for C2 gases. The C2H2 molecule is adsorbed around TiF62− anions via a strong H bonding (C–H•••F 1.90 Å), yielding C2H2 binding energy of −44.1 kJ mol−1 (Fig. 4C). Meanwhile, the C2H6 molecules can be captured by three adsorption sites that are provided by both TiF62− anion and TPPY ligand: (i) the synergy of C–H•••π (3.01 Å) and C–H•••F (2.46 Å and 2.90 Å) interactions (Fig. 4D), (ii) the C–H•••N interactions with porphyrin rings (Fig. 4E), and (iii) multiple van der Waals forces between C2H6 and pyridine rings (Fig. 4f). The C2H6 binding energy at three sites is calculated to be −40.75, −36.2, and −40.2 kJ mol−1, respectively. In sharp contrast, the C2H4 molecules are weakly adsorbed by the C–H•••F bonding with TiF62− anion and multiple C–H•••N van der Waals interactions between two interlayers of TPPY ligands with much longer corresponding bonding distances (fig. S26). The C2H4 binding energy is calculated to be −28.4 and −25.0 kJ mol−1 at two binding sites, respectively. The trend and value of calculated binding energy between CuTiF6-TPPY and C2 adsorbates are similar to the experimental adsorption heat at zero coverage, confirming that anion pillars (TiF62−) with strong polarization and porphyrin moieties with large π-surfaces played a synergistic role in the confined micropore channels for the selective recognition of C2H2 and C2H6 over C2H4. Furthermore, the in situ IR experiments with corresponding C2 gas loadings confirmed the relatively strong interactions between C2H2/C2H6 and CuTiF6-TPPY (fig. S28 and detailed discussion in the Supplementary Materials). The time-dependent in situ IR spectra with C2 gas loadings indicated that C2H2 and C2H6 can be adsorbed in a stronger and faster manner than C2H4 by CuTiF6-TPPY (fig. S29).

Fig. 4. Theoretical studies for C2 distribution density and binding sites in CuTiF6-TPPY.

Fig. 4.

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Computational simulations for the density distribution of (A) C2H2 and (B) C2H6 on CuTiF6-TPPY at 100 kPa and 298 K. The DFT calculated binding sites of (C) C2H2 and (D to F) C2H6 in CuTiF6-TPPY. The closest contacts between framework atoms and the gas molecules are defined by the distances (in angstrom), and the distances include the van der Waals radius of atoms. [Framework: C, gray (80%); H, white; N, blue; F, cyan; Cu, pink; Ti, silvery; gas: C, orange; H, white].

DISCUSSION

In summary, we first synthesized and reported an anion-pillared HUM, CuTiF6-TPPY, that can one-step separate C2H4 from ternary C2H2/C2H4/C2H6 gas mixtures. The low-polarity π-surface areas provided by the porphyrin rings of TPPY can selectively adsorb C2H6 molecules via multiple van der Waals interactions. Meanwhile, the abundant TiF62− anions can strongly capture C2H2. As a result, CuTiF6-TPPY exhibited a higher adsorption capacity of C2H2 (3.62 mmol g−1) and C2H6 (2.82 mmol g−1) than that of C2H4 (2.42 mmol g−1) at 298 K and 1.0 bar. Notably, the highest C2H2/C2H4 IAST selectivity of 5.03 was obtained among the seven MOFs with the same adsorption behaviors. The superb IAST selectivity of C2H6/C2H4 (2.12) also outperformed most top-ranking MOFs. Dynamic breakthrough experiments with ternary and binary C2 gas mixtures have confirmed the direct and facile production of high-purity C2H4 (99.9%). Moreover, DFT calculations demonstrated the synergistic recognition sites by both polar anion pillars and porphyrin rings with large π-surface areas.

MATERIALS AND METHODS

Chemicals

All reagents were analytical grade and used as received without further purification. Cu(BF4)2·4H2O, Cu(NO3)2·3H2O, CuCl2·2H2O, and (NH4)2TiF6 were purchased from Aladdin Reagent Co. Ltd. TPPY was purchased from Jilin Province Extension Technology Co. Ltd. Ultrahigh purity–grade He (99.999%), N2 (99.999%), C2H2 (99.9%), C2H4 (99.99%), C2H6 (99.99%), and mixed gas (C2H6/C2H4 = 10/90, v/v; C2H6/C2H4/He = 25/25/50, v/v/v; C2H2/C2H6/C2H4 = 1/90/9, v/v/v) were purchased from Nanchang Jiangzu gas Co. Ltd. (China) and used for all measurements.

Preparation of powder CuTiF6-TPPY

A preheated water solution (2 ml) of Cu(BF4)2·4H2O (0.2 mmol) and (NH4)2TiF6 (0.2 mmol) were dropped into a preheated methanol solution (30 ml) of TPPY (0.1 mmol). Then, the mixture was heated at 333 K for 24 hours. The obtained dark red powder was exchanged with methanol and acetone for a day, respectively. CuTiF6-TPPY can also be prepared by using other Cu salts such as Cu(NO3)2·3H2O and CuCl2·2H2O at the same condition.

Sample characterization

PXRD patterns were collected using a PANalytical Empyrean Series 2 diffractometer with Cu─Ka radiation, at room temperature, with a step size of 0.0167°, a scan time of 15 s per step, and 2θ ranging from 5° to 50°. The contents of Cu and Ti were measured by ICP-OES (Agilent 5110, USA). The ultimate element analysis was conducted using a CHNS elemental analyzer (Vario MICRO). The N2 adsorption-desorption isotherms at 77 K were measured on a Micromeritics ASAP 2460 volumetric adsorption apparatus. The apparent BET-specific surface area was calculated using the adsorption branch with the relative pressure P/P0 in the range of 0.005 to 0.3. The total pore volume (Vtot) was calculated on the basis of the adsorbed amount of nitrogen at the P/P0 of 0.99. The PSD was calculated using the NLDFT methodology with nitrogen adsorption isotherm data and assuming a slit pore model. The time-dependent gas uptake profiles were recorded on an Intelligent Gravimetric Analyzer (IGA-100, HIDEN). The pressure was raised at a rate of 100 mbar min−1 and kept for 60 min to reach full adsorption equilibriums.

Gas adsorption measurements

The C2H2, C2H4, and C2H6 adsorption-desorption isotherms at 273, 288, and 298 K were measured volumetrically by the Micromeritics ASAP 2460 adsorption apparatus for pressures up to 1.0 bar. Before adsorption measurements, the samples were degassed using a high vacuum pump (<5 μm of Hg) at 333 K for over 12 hours.

Breakthrough experiments

The dynamic breakthrough experiments were carried out in a homemade apparatus under ambient conditions. The samples were activated under vacuum at 100°C for 12 hours and loaded (1.8 or 1.3 g) to the adsorption bed (Φ 6 mm by 150 mm). A carrier gas (He of ≥99.999%) was used to purge the adsorption bed for 1 hour, and then the gas flow was switched to the desired gas mixture at a certain flow rate. The recovery gas was connected to an analyzer port coupled with gas chromatography (7890B, Agilent) with a flame ionization detector.

In situ IR spectroscopic measurements

All the IR spectroscopic data are recorded in a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific Inc., USA) equipped with a liquid N2-cooled mercury cadmium telluride MCT-A detector. A vacuum cell, purchased from Specac Ltd., UK (product number P/N 5850c), is placed in the sample compartment of the IR spectrometer with the sample at the focal point of the beam. The cell is connected to different gas lines (C2H2, C2H4, and C2H6) and a vacuum line for evacuation. The MOF sample (powder, ∼30 mg) was placed in the cell and first annealed at 80°C under vacuum for activation and then cooled to room temperature for recording the reference spectrum and subsequent loading gas. C2H2 was introduced into the cell, and the spectra were recorded during the gas exposure until 50 min. After fully evacuating the same sample by pumping the cell, the reference spectrum was taken again, loading of C2H4 and C2H6 was performed separately, and the IR data were recorded in the same manner.

DFT calculations

First-principles DFT calculations were performed using the Materials Studio’s CASTEP code. All calculations were conducted under the generalized gradient approximation with Perdew-Burke-Ernzerhof. A semiempirical addition of dispersive forces to conventional DFT was included in the calculation to account for van der Waals interactions. A cutoff energy of 544 eV and a 2 × 2 × 2 k-point mesh were found to be enough for the total energy coverage within 0.01 meV atom−1. The structures of the synthesized materials were first optimized from the reported crystal structures. To obtain the binding energy, the pristine structure and an isolated gas molecule placed in a supercell (with the same cell dimensions as the pristine crystal structure) were optimized and relaxed as references. C2H2, C2H4, and C2H6 gas molecules were then introduced to different locations of the channel pore, followed by a full structural relaxation. The static binding energy was calculated by the equation EB = E(gas) + E(adsorbent) − E(adsorbent + gas).

GCMC calculations

All the GCMC simulations were performed in MS 2017R2 package. The crystal structure of the CuTiF6-TPPY was chosen after the DFT geometry optimization. The framework and the individual C2H2, C2H4, and C2H6 were considered to be rigid during the simulation. The charges for atoms of the CuTiF6-TPPY and C2 gases were derived from the Mulliken method. The simulations adopted the fixed pressure task, the Metropolis method in sorption module, and the universal force field. The interaction energies between the adsorbed molecules and the framework were computed through the Coulomb and Lennard-Jones 6-12 (LJ) potentials. The cutoff radius chosen was 18.5 Å for the LJ potential, and the electrostatic interactions were handled using the Ewald summation method. The loading steps and the equilibration steps were 1 × 107, and the production steps were 1 × 107.

X-ray absorption spectroscopy

The x-ray absorption spectroscopy measurements at Cu K (E0 = 8983 eV) edge were collected on the beamline BL01C1 in the National Synchrotron Radiation Research Center. The energy was calibrated accordingly to the absorption edge of pure Cu foil. The radiation was monochromatized by a Si (111) double-crystal monochromator. XANES and EXAFS data reduction and analysis were processed by Athena software (version 0.9.26) for background, pre-edge line, and post-edge line calibrations. The chemical valence of Cu in the samples was determined by the comparison with the reference Cu foil, CuF2, and CuPc. For the EXAFS part, the FT data in R space were analyzed by applying the first-shell approximate model for Cu─N contribution. The passive electron factor S0 was determined by fitting the experimental data on Cu foil and then fixed for future analysis of the measured samples.

Acknowledgments

Funding: The research work was supported by the National Natural Science Foundation of China (nos. 21908090, 22008099, 22108243, and 22168023) and the Natural Science Foundation of Jiangxi Province (no. 20192ACB21015).

Author contributions: P.Z. and Y. Zhong carried out the experimental work on synthesis, adsorption isotherm measurements, and breakthrough tests and performed the computational simulations. Y. Zhang, Y.S., and Z. Zhu took part in synthesis and recycling tests. J.C., Z.Z., and S.C. took part in modeling studies. J.W., H.X., and S.D. conceived the idea and analyzed the results. All authors contributed to the final version of the manuscript.

Competing interests: The authors declare that they have no competing interests.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

Supplementary Materials

This PDF file includes:

Figs. S1 to S32

Tables S1 to S7

References

Click here for additional data file. (2.6MB, pdf)

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Associated Data

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Supplementary Materials

Figs. S1 to S32

Tables S1 to S7

References

Click here for additional data file. (2.6MB, pdf)

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