A ligand conformation preorganization approach to construct a copper–hexacarboxylate framework with a novel topology for selective gas adsorption

Abstract

The development of porous MOFs with novel structures and functional properties is a key research topic in the MOF chemistry and materials field. In this article, by employing a ligand conformation preorganization strategy, we designed a nonplanar methyl-substituted triisophthalate ligand, which was used to successfully construct a copper–hexacarboxylate framework with a novel topological structure. More importantly, the obtained MOF not only exhibits good hydrolytic stability but also shows utility as an adsorbent for efficient separation and purification of C₂H₂ and natural gas under ambient conditions. At ambient temperature and atmospheric pressure, the C₂H₂ and CO₂ uptake capacities reach up to 149.1 and 83.1 cm³ (STP) g⁻¹, while the IAST-predicted C₂H₂/CH₄ (50/50, v/v), C₂H₂/CO₂ (50/50, v/v), and CO₂/CH₄ (50/50, v/v) adsorption selectivities are as high as 22.2, 3.81, and 5.13.

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1. Introduction

Porous metal–organic frameworks (MOFs) are emerging as a family of advanced organic−inorganic hybrid crystalline porous materials constructed by assembling inorganic metal ions or clusters and organic linkers via metal–organic coordination bonds. Such materials have found a large variety of potential applications including but not limited to selective gas/molecule adsorption and separation,¹ catalysis,² sensing,³ and drug delivery,⁴ most likely because they offer a high degree of diversity and tunability in terms of structure, chemical composition, porosity, and functionality compared to traditional porous materials. Up to now, design and construction of porous MOFs with intriguing structures and functional properties is a key research topic in the realm of MOF chemistry and materials.

It is well known that the final structures of MOFs commonly rely on a number of factors such as the organic ligands, the metal ions, the metal–ligand ratio, solvent, reaction temperature and time, and pH value, in which the judicious choice of organic ligands and metal ions is very critical. We recently became particularly interested in employing isophthalate-containing multicarboxylate, including diisophthalate and triisophthalate, as organic ligands to fabricate copper-based MOFs, primarily because of the strong coordination ability of the isophthalate moiety and its propensity to engender metal–organic polyhedral cages with predesigned open copper sites upon self-assembly with copper ions, which is beneficial for gas adsorption.⁵ During our studies, it has been found that the final structures of the resultant MOFs are largely dictated by the ligand's conformations. For instance, for copper-bent diisophthalate systems, they are prone to form MOFs with variable topologies which are heavily dependent on the ligand's conformations. By capitalizing on conformational flexibility, we very recently fabricated a pair of supramolecular isomers using a bent diisophthalate ligand.⁶ When the bent diisophthalate building block becomes more preorganized, the self-assembly pathway and product become significantly more predictable. By employing steric hindrance imposed by methyl groups to preorganize the conformation of a bent diisophthalate ligand, we more recently successfully targeted an ssa-type MOF in which the organic building blocks only adopt one conformation.⁷ When shifting our attention from copper–diisophthalate systems to copper–triisophthalate ones, we note that it is required that the terminal three isophthalate units of the C₃-symmetric triisophthalate ligand employed must be coplanar for the formation of rht-type MOFs, as reported in the literature.⁸ Thus, it is reasonable to speculate that the generation of interesting MOFs with novel architectures and topologies will occur if the peripheral isophthalate units of the triisophthalate ligands are not in a plane. For this purpose, we employed the ligand conformation preorganization strategy to design a nonplanar methyl-substituted triisophthalate ligand, namely, (1,3,5-trimethylbenzene-2,4,6-triyl) triisophthalate (H₆L, Scheme 1). The introduction of methyl groups in the central benzene unit of the ligand is expected to help fix the ligand's conformation in which the terminal isophthalate moieties are almost perfectly perpendicular to the central benzene unit. It should be pointed out that this ligand has been utilized to construct Zn²⁺/Co²⁺-based MOFs by other research groups,⁹ but copper-based MOFs based on this ligand have been not reported so far. In addition, a structurally similar BHB ligand but without methyl groups on the central phenyl ring has been used to construct a copper-based MOF UTSA-20 with a novel zyg topology.¹⁰ However, different with our ligand, the terminal isophthalate moieties of the BHB ligand are not perpendicular to the central phenyl ring with a torsion angle of ca. 34° due to the absence of the steric hindrance effect induced by methyl groups. With the ligand H₆L in hand, we successfully constructed a copper-based MOF under solvothermal conditions, which features a novel topological structure instead of rht-type and zyg-type ones.

Scheme 1 The chemical structure of the organic ligand H₆L used to construct ZJNU-100.

The acetylene/methane (C₂H₂/CH₄), acetylene/carbon dioxide (C₂H₂/CO₂), and carbon dioxide/methane (CO₂/CH₄) separations are of high importance in the petrochemical industry. On one hand, as an important fuel and chemical raw material, C₂H₂ is typically produced by partial oxidation of natural gas. In this process, CO₂ is unavoidably generated. Therefore, removal of CO₂ and unconverted CH₄ from C₂H₂ is required for its practical applications. On the other hand, owing to its natural abundance and less adverse effects on the environment, natural gas is regarded as one of the most attractive energy carriers for resolving the current energy crisis and environmental issues caused by the massive consumption of fossil fuels. In addition to CH₄ as the major composition, natural gas also contains variable amounts of other gases such as CO₂, nitrogen (N₂), and light hydrocarbons. The acidic CO₂ not only reduces the energy content of natural gas, but also corrodes the equipment and pipeline especially in the presence of humidity. Additionally, landfill gas is also a very important source of CH₄. However, landfill gas often contains an unacceptable level of CO₂. Consequently, the capture and removal of CO₂ from CH₄ is very essential and particularly important in natural gas upgrading and landfill gas utilization. Among the strategies being developed to address the above-mentioned issues, adsorption-based separation using porous solids can greatly reduce the energy consumption associated with the classical separation techniques of cryogenic distillation and chemical scrubbing. In this context, development of porous solids capable of highly effective separation and purification of C₂H₂ and natural gas under ambient conditions is in urgent demand. To date, some experimental and computational studies have showed that many MOFs exhibit adsorption selectivity in C₂H₂/CH₄,¹¹ C₂H₂/CO₂ ¹² and CO₂/CH₄ ¹³ separations, surpassing most traditional adsorbents. Due to the same kinetic diameter (3.3 Å) of C₂H₂ and CO₂, the C₂H₂–CO₂ mixture is the most difficult to separate among the three pairs of gas mixtures. It should be mentioned that Lin et al. recently reported a flexible MOF UTSA-300 being capable of sieving CO₂ from C₂H₂.¹⁴ Herein, we presented the synthesis, characterization, and selective C₂H₂/CH₄, C₂H₂/CO₂, and CO₂/CH₄ adsorption properties of a new MOF based on copper ions and a hexacarboxylate ligand featuring a novel topological structure, which was rationally designed by the above-mentioned ligand conformation preorganization strategy.

2. Experimental

2.1 Materials and instruments

Unless otherwise noted, all commercially available chemical reagents and solvents were used as received without further purification. Dimethyl-5-(pinacolboryl) isophthalate was prepared according to the literature protocol.¹⁵ Thin layer chromatography (TLC) was carried out on aluminium sheets pre-coated with silica gel 60 F254 purchased from Merck, using ultra-violet (UV) light as the visualizing agent. Column chromatography was performed using silica gel (100–200 mesh, Qingdao Haiyang Chemical Co., Ltd). Proton and carbon nuclear magnetic resonance (¹H and ¹³C NMR) spectra were recorded on a Bruker AV 400 or AV600 spectrometer at room temperature. Residual protonated species in the deuterated solvents (CDCl₃ and DMSO-d₆) were used as internal references. Chemical shifts were reported in ppm (parts per million) and coupling constants were reported in hertz (Hz). Data were represented as follows: chemical shift, multiplicity (br = broad, s = singlet, d = doublet, and t = triplet), coupling constants, and integration. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 5DX FTIR spectrometer in the 400–4000 cm⁻¹ region using KBr pellets. Elemental analyses of C, H, and N were carried out with a PerkinElmer 240 CHN elemental analyser. Thermogravimetric analysis experiments were performed on a Netzsch STA 449C thermal analyser from room temperature to 1073 K under a N₂ atmosphere at a heating rate of 5 K min⁻¹. Powder X-ray diffraction (PXRD) patterns were collected using Cu-K_α radiation (λ = 1.5418 Å) on a Bruker D8 Advance powder diffractometer at ambient temperature in the angular range of 2θ = 5–45°. The simulated PXRD patterns were calculated using the single-crystal X-ray diffraction data and processed using the free Mercury program (version 1.4.1) provided by the Cambridge Crystallographic Data Center. Topological analyses were performed using the Topos 4.0 program. Gas adsorption isotherms were obtained with a Micromeritics ASAP 2020 HD88 surface area and pore analyser. Extra-high purity gases (N₂, 99.9999%; C₂H₂, 99.9%; CO₂, 99.999%; CH₄, 99.99%) were used for the adsorption–desorption measurements. A temperature of 77 K was maintained with liquid N₂, and other temperatures were controlled by a circulating water bath. The pore size distribution (PSD) was obtained from the N₂ adsorption data using the density functional theory (DFT) method and assuming a slit pore model.

2.2 Single-crystal X-ray diffraction

The single-crystal structure data for ZJNU-100 were collected on a Bruker SMART APEX II diffractometer using graphite-monochromated Cu-K_α (λ = 1.54178 Å) radiation. The structure was solved by direct methods using the SHELXS-2017 program and refined by full-matrix least squares on F² using the SHELXL-2017 program. The non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters. In the refinement, hydrogen atoms were treated as riding atoms using the SHELXL default parameters. The guest molecules were highly disordered and randomly dispersed, and thus their locations cannot be determined. To resolve this issue, the electron density peaks of guest molecules were removed by the SQUEEZE procedure of the PLATON program suite.¹⁶ The detailed structure refinement parameters are summarized in Table S1 in the ESI.†

2.3 Synthesis and characterization of the organic ligand

A mixture containing 1,3,5-tribromo-2,4,6-trimethyl-benzene (1.00 g, 2.80 mmol), dimethyl 5-(pinacolboryl) isophthalate (2.96 g, 9.25 mmol), Cs₂CO₃ (4.11 g, 12.61 mmol), and Pd(PPh₃)₄ (0.24 g, 0.21 mmol) was introduced into 100 mL of anhydrous 1,4-dioxane in a 250 mL single-necked round-bottom flask equipped with a magnetic stirring bar and a refluxing condenser. The flask was evacuated and then back-filled with N₂ three times in order to ensure that the reaction is carried out under a N₂ atmosphere. The resulting mixture was allowed to reflux for 3 days with continuous stirring. After that, the reaction mixture was filtered while hot through a plug of Celite and washed with chloroform, tetrahydrofuran (THF), and 1,4-dioxane. After the organic solvent was removed under reduced pressure, the residue was purified with column chromatography on silica gel using petroleum ether and ethyl acetate (1/1, v/v) as an eluent to afford the hexamethyl ester as a white solid (0.88 g, 1.26 mmol, 45%). ¹H NMR (CDCl₃, 400.1 MHz) δ (ppm): 8.711 (t, J = 1.6 Hz, 3H), 8.121 (d, J = 1.6 Hz, 6H), 3.984 (s, 18H), 1.675 (s, 9H).

The hexamethyl ester intermediate (0.88 g, 1.26 mmol) was suspended in a mixed solvent composed of THF and methanol (80 mL, v/v = 1/1), and then an aqueous solution of 3 mol L⁻¹ NaOH (20 mL) was added. The resultant mixture was stirred and refluxed overnight. After the reaction mixture was allowed to cool to room temperature, the organic solvents were removed, and the remaining solution was acidified with concentrated hydrochloric acid in an ice-water bath to give a white precipitate, which was collected by filtration, washed with water, and dried in a vacuum oven at 343 K to produce 0.75 g of the target compound H₆L (1.22 mmol, 97%). ¹H NMR (DMSO-d₆, 400.1 MHz) δ (ppm): 13.300 (br, 6H), 8.486 (t, J = 1.2 Hz, 3H), 8.020 (d, J = 1.2 Hz, 6H), 1.621 (s, 9H); ¹³C NMR (DMSO-d₆, 150.9 MHz) δ (ppm): 167.001, 142.288, 138.408, 134.530, 133.155, 132.404, 129.106, 19.879; selected FTIR (KBr, cm⁻¹): 1701, 1601, 1448, 1390, 1269, 1207, 1134, 1111, 922, 760, 690, 669, 606.

2.4 Synthesis and characterization of ZJNU-100

H₆L (5.0 mg, 8.2 μmol), CuCl₂·2H₂O (10.0 mg, 58.6 μmol), deionized water (0.3 mL), N,N-dimethyl formamide (DMF, 1.5 mL), and 6 mol L⁻¹ HCl (70 μL) were charged in a 20 mL glass vial. The mixture was heated at 363 K for 48 h. After being cooled down to room temperature, the blue block-shaped crystals of ZJNU-100 suitable for single-crystal X-ray diffraction (SCXRD) analysis were harvested by suction filtration, and washed with fresh DMF (yield: 65% based on H₆L). ZJNU-100 is insoluble in common solvents such as DMF, THF, 1,4-dioxane, acetone, methanol, dichloromethane, chloroform, and water. Based on single-crystal X-ray determination, TGA and microanalyses, ZJNU-100 can be best formulated to be [Cu₂₄(HL)₆(L)₃(H₂O)₂₄]·52DMF. Selected FTIR (KBr, cm⁻¹): 3423, 1655, 1630, 1578, 1437, 1369, 1255, 1101, 779, 727, 700, 663, 611. Elemental analysis calcd (%) for C₄₅₃H₅₈₀N₅₂O₁₈₄Cu₂₄: C, 48.48; H, 5.21; N, 6.49; Found: C, 48.38; H, 5.20; N, 6.54.

3. Results and discussion

The synthesis of the organic ligand, (1,3,5-trimethylbenzene-2,4,6-triyl) triisophthalate, was facilely accomplished using Suzuki cross-coupling reaction between 1,3,5-trimethyl-2,4,6-tribromobenzene and dimethyl 5-(pinacolboryl) isophthalate followed by hydrolysis and acidification. ¹H and ¹³C NMR confirmed the structure and purity of the ligand. Subsequently, the ligand was subjected to a solvothermal reaction with copper chloride dihydrate in acidified mixed solvents of DMF and water, affording blue block-shaped crystals which we termed ZJNU-100 (Fig. S1†). The PXRD pattern of the bulk phase of ZJNU-100 coincides with the simulated one generated from the single-crystal X-ray diffraction data, verifying that the bulk product is phase-pure (Fig. S2†). TGA analysis showed a weight loss of 37.9% (calcd. 37.7%) from room temperature to 573 K, which is attributed to the departure of solvent molecules and terminal water molecules (Fig. S3†). More importantly, as evidenced by PXRD (Fig. S2†) and 77 K N₂ adsorption measurements (Fig. S6†), ZJNU-100 is stable in aqueous HCl/NaOH solutions in the pH range of 3–11 for 24 h at room temperature, which is favourable for practical industrial applications.

SCXRD studies show that ZJNU-100 is a porous three-dimensional network crystallizing in the hexagonal space group of P6/mmm, with the unit cell parameters a = b = 63.7383(15) Å, and c = 25.3645(7) Å. Each copper ion is five-coordinated by four oxygen atoms belonging to four distinct carboxylate groups and one oxygen atom coming from the terminal water molecule, thus representing a square-pyramidal geometry. The Cu–O_carboxylate bond lengths are in the range of 1.854–2.238 Å. As expected, two neighbouring copper ions were linked to each other by four bidentate bridging carboxylate groups to generate a dicopper paddlewheel secondary building unit (SBU) with the Cu–Cu distances lying in the range of 2.593–2.708 Å. These inorganic dicopper paddlewheel units were further interconnected by the organic ligands, eventually resulting in the formation of a complicated three-dimensional network. Noteworthily, some of the organic ligands were fully deprotonated, while the others were partially deprotonated, with a free carboxylate group being immobilized in the pore surface and pointing into the centre of the pore. As such, the uncoordinated carboxylate oxygen atoms are capable of serving as potential binding sites for incoming guest molecules. Besides, the water molecules occupied at the apical position of the bimetallic Cu₂(COO)₄ node can be readily removed during the activation process to generate Lewis acidic open metal sites. Therefore, ZJNU-100 features two different types of functional active sites for gas adsorption. Interestingly, a close inspection of the crystal structure revealed that there exist seven types of metal–organic polyhedral cages with different shapes and sizes in the overall network. As illustrated in Fig. 1, cage A consists of eight dicopper paddlewheel units and four organic ligands; cage B is composed of six dicopper paddlewheel units and three organic ligands; cage C encompasses nine dicopper paddlewheel units, twelve isophthalate units and four organic ligands; cage D is constructed by linking twelve isophthalate units and twelve organic ligands with eighteen dicopper paddlewheel units; cage E is comprised of six dicopper paddlewheel units and eight organic ligands; cage F is built up from eight dicopper paddlewheel units and four organic ligands; cage G is assembled from six dicopper paddlewheel units, six isophthalate units and four organic ligands. The diameters of the spheres representing the void of the cages A–G were estimated to be about 6.5, 3.0, 12.0, 12.0, 12.0, 6.2 and 8.2 Å based on van der Waals radii, respectively. To the best of our knowledge, ZJNU-100 represents the first example of MOFs incorporating seven different types of cages in the overall structure. After the removal of the solvent molecules and the terminal water molecules, the accessible void fraction was estimated to be 64.4% of the unit cell volume, and the framework density is 0.7802 g cm⁻³. Topologically, if the dicopper paddlewheel units are regarded as 4-connected nodes and the organic ligands are considered to be 5-connected and 6-connected nodes, respectively, the overall network of ZJNU-100 can be simplified to be a (4,5,6)-connected network with the point (Schläfli) symbol of {4²·6²·8²}₃{4³·6³}₄{4³·6⁷}₂{4⁴·6²}₅{4⁴·6⁴·8²}₂{4⁴·6⁶}₂{4⁵·6¹⁰}₂{4⁶·6⁹}. Furthermore, if the 5-connected and 6-connected ligands are considered to consist of three and four 3-connected nodes, respectively, the overall network of ZJNU-100 can be simplified to be a (3,4)-connected network with the point symbol of {6·7·8}₈{6·7²}₈{6²·8²·10²}₅{6²·8}₄{6³·8²·9}₄{7²·8}₁₀{7⁴·10·12}₂{7⁴·10²}. The topological structure has not been observed for copper–multicarboxylate frameworks so far.

Fig. 1 Single-crystal X-ray diffraction structure of ZJNU-100 showing (a–g) seven different types of metal–organic polyhedral cages, and (h) 3D network architecture. The hydrogen atoms and the terminal water molecules are omitted for clarity.

To characterize the permanent porosity and pore textural properties of ZJNU-100, we performed the N₂ adsorption and desorption experiments at 77 K. Prior to gas adsorption, the as-synthesized ZJNU-100 was subjected to guest exchange with dry acetone, and the acetone-exchanged ZJNU-100 was evacuated under dynamic vacuum at room temperature for 24 h and then at 353 K until the degas rate of 0.27 Pa min⁻¹ (2 μmHg min⁻¹) was reached. As depicted in Fig. 2, the N₂ adsorption isotherm is fully reversible and features a type-I profile, reflecting that ZJNU-100 is a porous material with permanent microporosity. Based on the N₂ adsorption isotherm and the saturated N₂ uptake of 487.2 cm³ (STP) g⁻¹, the Brunauer–Emmett–Teller (Langmuir) specific surface areas and pore volume are estimated to be 1933 (2113) m² g⁻¹ and 0.754 cm³ g⁻¹, respectively (Fig. S5†). The BET surface area of ZJNU-100 is significantly higher than that of UTSA-20 (1156 m² g⁻¹)¹⁰ based on the BHB ligand, NPC-5 (1140 m² g⁻¹)^9a and SDU-1 (779 m² g⁻¹)^9a constructed from the H₆L ligand, but lower than that of some other copper-triisophthalate frameworks such as Cu-TDPAT (1938 m² g⁻¹),¹⁷ SDU-6 (2826 m² g⁻¹),¹⁸ PCN-61 (3000 m² g⁻¹),¹⁹ NU-125 (3120 m² g⁻¹),²⁰ Cu-TPBTM (3160 m² g⁻¹),²¹ and NOTT-112 (3800 m² g⁻¹)²² (Table S2†). The inset in Fig. 2 shows the DFT PSD of ZJNU-100, falling in the range of 5.36–15.91 Å with the maximum value of 5.90, 8.04 and 11.79 Å.

Fig. 2 77 K N₂ adsorption and desorption isotherm of ZJNU-100. The solid and open symbols refer to adsorption and desorption, respectively. The inset shows the DFT PSD of ZJNU-100.

Next, we investigated the potential of ZJNU-100 for C₂H₂/CH₄, C₂H₂/CO₂, and CO₂/CH₄ separations. Accordingly, simple-component sorption isotherms for C₂H₂, CO₂, and CH₄ were collected at three different temperatures of 278 K, 288 K, and 298 K and pressures up to 106.6 kPa (800 mmHg). As shown in Fig. 3, no obvious hysteresis was observed for all isotherms at all the temperatures and pressures investigated, indicating that the adsorption and desorption processes are completely reversible. Most remarkably, the gas adsorption capacity of ZJNU-100 follows the hierarchy: C₂H₂ > CO₂ > CH₄. Concretely, ZJNU-100 is capable of adsorbing up to 149.1 cm³ (STP) g⁻¹ of C₂H₂ at 298 K and 106.6 kPa. As expected, the C₂H₂ uptake increases with the decreasing temperature due to exothermic physical adsorption, reaching 171.5 cm³ (STP) g⁻¹ at 288 K, and 192.4 cm³ (STP) g⁻¹ at 278 K. Comparatively, the amount of C₂H₂ adsorbed in ZJNU-100 exceeds those of the vast majority of MOF materials reported thus far under the same measurement conditions such as PCM-48 (25.54 cm³ (STP) g⁻¹),²³ ZJNU-62 (33.9 cm³ (STP) g⁻¹),²⁴ APPT-Cd-ClO₄⁻ (39.3 cm³ (STP) g⁻¹),²⁵ ZJNU-63 (39.4 cm³ (STP) g⁻¹),²⁶ ZJNU-61a (48.0 cm³ (STP) g⁻¹),²⁷ FJI-C4 (72.5 cm³ (STP) g⁻¹),^11f MFM-130a (85.9 cm³ (STP) g⁻¹),²⁸ FJI-Y9-ht (119.2 cm³ (STP) g⁻¹),²⁹ UTSA-34b (120.8 cm³ (STP) g⁻¹), ZJU-199 (128 cm³ (STP) g⁻¹),³⁰ and UPC-21 (139.5 cm³ (STP) g⁻¹).³¹ As for CO₂ adsorption, the uptake amount of CO₂ at 106.6 kPa is 83.1 cm³ (STP) g⁻¹ at 298 K, 105.4 cm³ (STP) g⁻¹ at 288 K, and 131.7 cm³ (STP) g⁻¹ at 278 K. The CO₂ uptake of ZJNU-100 is also comparable to and better than those of some reported copper–hexacarboxylate frameworks such as HNUST-5 (56 cm³ (STP)),³² Cu-NTTA (65.5 cm³ (STP) g⁻¹),³³ GDMU-2 (74 cm³ (STP) at 273 K),³⁴ and UTSA-20 (82.4 cm³ (STP) g⁻¹)¹⁰ (Table S2†). In contrast, ZJNU-100 only adsorbs a very limited amount of CH₄, with the CH₄ uptake of 20.8 cm³ (STP) g⁻¹ at 298 K, 25.7 cm³ (STP) g⁻¹ at 288 K, and 31.8 cm³ (STP) g⁻¹ at 278 K at 106.6 kPa, respectively. To evaluate the binding energy between gas molecules and the framework, we calculated the coverage-dependent isosteric heat (Q_st) of gas adsorption from multiple temperature-dependent isotherms according to the Clausius–Clapeyron equation. During the entire adsorption process, the order of Q_st values is consistent with the uptake amounts (Fig. 4a). Specially, at near zero coverage, the Q_st values for C₂H₂, CO₂, and CH₄ adsorption are 32.8 ± 2.2, 24.6 ± 0.4 and 15.7 ± 1.2 kJ mol⁻¹ respectively.

Fig. 3 C₂H₂, CO₂, and CH₄ adsorption and desorption isotherms of ZJNU-100 at three different temperatures of (a) 298 K, (b) 288 K, and (c) 278 K. The solid and open symbols represent adsorption and desorption, respectively.

Fig. 4 (a) The isosteric heat of C₂H₂, CO₂, and CH₄ adsorption in ZJNU-100 as a function of gas loadings. The IAST-predicted adsorption selectivities for the equimolar (b) C₂H₂–CH₄, (c) C₂H₂–CO₂, and (d) CO₂–CH₄ binary gas mixtures at three different temperatures of 298 K, 288 K, and 278 K.

The hierarchy of uptake capacity and isosteric adsorption heat for C₂H₂, CO₂, and CH₄ in ZJNU-100 suggests that it has promising potential for the separation of C₂H₂–CH₄, C₂H₂–CO₂, and CO₂–CH₄ binary gas mixtures. In order to assess the gas-mixture separation performance, we utilized the ideal adsorbed solution theory (IAST) model to predict the adsorption selectivity.³⁵ Prior to calculations, the pure-component experimental adsorption isotherms were fitted with the Langmuir–Freundlich equation (Fig. S7†). The fitting parameters obtained are provided in Table S3 in the ESI.†Fig. 4b–d show the IAST adsorption selectivities for the equimolar C₂H₂–CH₄, C₂H₂–CO₂, and CO₂–CH₄ gas mixtures at three different temperatures of 298 K, 288 K, and 278 K. It can be observed that the adsorption selectivity is closely correlated with the temperature and pressure. When the total gas pressure increases, the C₂H₂/CH₄ and C₂H₂/CO₂ adsorption selectivities decrease, while the CO₂/CH₄ adsorption selectivity slightly increases. With the decreasing temperature, the C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities increase, while the C₂H₂/CO₂ adsorption selectivity almost remains unchanged. In particular, the C₂H₂/CH₄, C₂H₂/CO₂, and CO₂/CH₄ adsorption selectivities at atmospheric pressure reach 22.2, 3.81, and 5.13 at 298 K, 25.8, 3.85, and 5.73 at 288 K, and 30.5, 3.88, and 6.46 at 278 K. Given the impressive adsorption selectivity, high uptake capacity, and good hydrostability, ZJNU-100 could be a promising solid adsorbent applied in the separation and purification of C₂H₂ and natural gas.

4. Conclusions

In summary, we employed the ligand conformation preorganization design strategy to construct a copper–hexacarboxylate framework. The obtained MOF not only shows a novel topological structure, high porosity and good hydrostability, but also displays selective C₂H₂/CH₄, C₂H₂/CO₂, and CO₂/CH₄ adsorption capabilities. The presented results will pave the way to the strategic design and synthesis of MOF materials with novel topologies and functional properties by sterically controlling the conformation of the organic building blocks.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 21771162, 21761003), the Natural Science Foundation of Zhejiang Province, China (LR16B010001), and the Qianjiang Talents Project in Zhejiang Province (ZC304015017).

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Footnote

† Electronic supplementary information (ESI) available: Electronic graphs (Fig. S1); PXRD patterns (Fig. S2); TGA curves (Fig. S3); FTIR spectra (Fig. S4); BET and Langmuir surface area plots (Fig. S5); comparison of 77 K N₂ isotherms and pore size distributions of ZJNU-100 before and after treatment with water (Fig. S6); gas adsorption isotherm fitting (Fig. S7); NMR spectra (Fig. S8); structure refinement parameters (Table S1); summaries of gas adsorption properties and pore textural parameters of some reported MOFs based on triisophthalate ligands (Table S2); fitting parameters (Table S3). CCDC 1877489. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qi01225c

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