An anionic metal–organic framework based on angular tetracarboxylic acid and a mononuclear copper ion for selective gas adsorption

Abstract

An anionic metal–organic framework [CuL]·2(CH₃)₂NH₂·DMF (ZJNU-55, L = 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalate) was synthesized by a solvothermal reaction of a custom-designed angular diisophthalate and CuCl₂·2H₂O, and structurally characterized by single-crystal X-ray diffraction. Notably, although the ligand bears a close resemblance to the previously reported diisophthalate used to construct PCN-307, the resultant MOF's structure is totally different from that of PCN-307. The framework exhibits a mononuclear Cu(COO)₄ inorganic secondary building unit, instead of the typical dinuclear Cu₂(COO)₄ paddlewheel unit. The combination of Cu(COO)₄ with the deprotonated organic ligand results in a three-dimensional network structure featuring helical nanotube channels along the crystallographic c axis. TOPOS software analyses indicate that two alternative simplifications based on the organic linkers can produce a 4-connected binodal net with the Schläfli symbol (4·6²·8³)(4·6⁴·8) or a (3,4)-connected 3-nodal network with the Schläfli symbol (6·8·9)₂(6·8³·9²), which have been rarely observed in MOF chemistry. Furthermore, its gas adsorptive properties were systematically investigated. The activated ZJNU-55a exhibits highly selective adsorption of C₂H₂ and CO₂ over CH₄ with the adsorption selectivities of 39.2–64.9 and 13.1–29.2 at 298 K, respectively. As far as we are aware, this is a rare example of a MOF constructed from mononuclear Cu(COO)₄ for selective gas adsorption. This work also demonstrates that subtle ligand modification can drastically affect the structures and properties of the resultant MOF.

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1. Introduction

Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are emerging as a rapidly expanding class of porous materials, which are readily constructed from the self-assembly of suitable metal ions/clusters that serve as nodes and organic ligands that act as linkers. The key advantage that MOFs have over traditional porous materials such as activated carbons and zeolites is the easy tuning of the shape, size, and chemical nature of their pores, which offers them with great promise for a broad spectrum of potential applications including but not limited to gas storage and separation,¹ chemical sensing,² drug delivery³ and heterogeneous catalysis.⁴ Benefiting from their wide range of applications coupled with aesthetical structure characteristics, rapid progress related to MOF chemistry has been made in the last few decades.

Since an organic ligand is one main component of MOFs, the judicious selection of the organic ligand is very critical in the construction of MOFs. Among the various organic ligands, multicarboxylate ligands, especially those containing m-benzenedicarboxylate/isophthalate moieties, are very promising because isophthalate is a very powerful subunit to construct porous MOFs.⁵ Recently, we became interested in the utilization of linear diisophthalate ligands to target new porous MOFs for gas storage and separation because it has been well demonstrated that a combination of linear diisophthalate ligands and dicopper paddlewheel typically yields NbO-type MOFs,⁶ such as the earliest reported MOF-505,⁷ as well as NOTT-10x series,⁸ which provide an excellent platform to investigate the effect of functional groups on gas adsorption properties.^8a,9 Although significant progress has been made in the preparation of porous MOFs by using linear diisophthalates as organic ligands, the use of angular/nonlinear diisophthalates featuring a reduced symmetry compared to the linear diisophthalates is relatively less investigated, but appealing because novel structures might be formed from such types of ligands. For example, Zhou et al. recently used a series of angular diisophthalates bearing different functional groups to produce isostructural MOFs PCN-305–308 with a new network topology.¹⁰ To date, the number of porous MOFs based on angular diisophthalates is still vanishingly small.^10,11

Based on the above considerations, by only simply substituting the central benzene ring of the recently published angular diisophthalate (5-methyl-1,3-bis(3,5-dicarboxylphenyl)benzene, Scheme 1)¹⁰ used to construct PCN-307 with the pyrimidine group, we designed and synthesized a new angular diisophthalate ligand, 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalic acid (Scheme 1), which contains two separated isophthalate subunits connected to central pyrimidine moieties through carbon–carbon single bonds. The introduction of nitrogen atoms is expected to enhance selective gas adsorption. Against our expectation, although the new ligand bears a close resemblance to the one used to construct PCN-307,¹⁰ the resultant MOF's structure is totally different from that of PCN-307 upon solvothermally reacting with Cu²⁺ ions. As a consequence, their distinct gas adsorption properties were also observed. In this work, we wish to report the solvothermal synthesis, single-crystal structure as well as gas adsorption properties of this MOF.

Scheme 1 Chemical structure of the organic ligands employed to construct PCN-307 (left) and ZJNU-55 (right).

2. Experimental

2.1 Materials and methods

Dimethyl 5-(pinacolboryl)isophthalate was synthesized according to the method reported in the literature.¹² All other chemicals employed were purchased from commercial suppliers and are used without further purification. ¹H and ¹³C{¹H} NMR (Nuclear Magnetic Resonance) spectra were obtained with a Bruker AV400 or AV600 NMR spectrometer that operates at 400 or 600 MHz for ¹H NMR spectroscopy and 100 or 150 MHz for ¹³C NMR spectroscopy. The spectra were recorded in CDCl₃ or DMSO-d₆, and chemical shifts are reported as δ values in ppm relative to TMS (tetramethylsilane). The FTIR (Fourier transform infrared) spectra were recorded using a Nicolet 5DX FTIR spectrometer on KBr discs in the 4000–400 cm⁻¹ region and reported in cm⁻¹ unit. Elemental analyses for C, H and N were carried out with a Vario EL III CHNOS elemental analyser. TGA (thermogravimetric analysis) was performed with a Netzsch STA 449C thermal analyser with a heating rate of 5 K min⁻¹ under a nitrogen atmosphere. PXRD (Powder X-ray Diffraction) measurements were carried out with a Philips PW3040/60 automated powder diffractometer using Cu-K_α radiation (λ = 1.54178 Å). The simulated PXRD pattern was calculated using Mercury 1.4.1 from the single-crystal X-ray diffraction structure. Gas sorption isotherms were measured using a Micromeritics ASAP 2020 HD88 surface-area and pore-size analyser. The temperature of 77 K was maintained using liquid nitrogen, while other temperatures were maintained using a circulating water bath. The gases used had the following purities: N₂ (99.9999%), CO₂ (99.999%), C₂H₂ (99.9%), and CH₄ (99.99%).

2.2 Single-crystal X-ray crystallography

Crystallographic data were recorded on a Bruker APEX II CCD area-detector diffractometer using Mo-K_α radiation (λ = 0.71073 Å) at T = 296(2) K. Absorption corrections by multi-scan were applied. Structures were solved by direct methods and refined by full-matrix least squares against F² using the SHELXTL-2014 software package. All non-H atoms were refined anisotropically. Hydrogen atoms attached to carbon were placed in geometrically idealized positions and refined using a riding model. PLATON/SQUEEZE¹³ was employed to calculate the diffraction contribution of the disordered solvent molecules, which could not be modelled as discrete atomic sites. The structure was further refined using the data generated. Details of the crystal parameters, data collection and refinement of ZJNU-55 are summarized in Table S1.† Crystallographic data for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. CCDC 1492066.

2.3 Synthesis and characterization of the organic ligand

Tetramethyl 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalate. 4,6-Dichloro-2-methylpyrimidine (0.50 g, 3.07 mmol), dimethyl 5-(pinacolboryl)isophthalate (2.45 g, 7.67 mmol), anhydrous Cs₂CO₃ (3.00 g, 9.20 mmol), and Pd(Ph₃P)₄ (0.18 g, 0.15 mmol) were added to a 250 mL flask equipped with a stirring bar and a reflux condenser. The flask was evacuated and refilled with nitrogen. The process was repeated three times. After that, dry dioxane (80 mL) was added via syringe. The resulting mixture was magnetically stirred under reflux for 48 h and allowed to cool to ambient temperature. After the solvent had evaporated, the resulting residue was extracted with CH₂Cl₂ three times. The organic extracts were combined, washed with brine, dried with anhydrous magnesium sulfate, vacuum filtered and concentrated under reduced pressure. The crude reaction mixture was recrystallized from toluene to give tetramethyl immediately as a white solid in 35.5% yield (0.52 g, 1.09 mmol). ¹H NMR (CDCl₃, 400.1 MHz) δ (ppm): 9.017 (d, J = 1.2 Hz, 4H), 8.855 (t, J = 1.2 Hz, 2H), 8.081 (s, 1H), 4.046 (s, 12H), 2.946 (s, 3H); ¹³C NMR (CDCl₃, 150.9 MHz) δ (ppm): 169.259, 165.830, 163.281, 138.116, 132.625, 132.391, 131.540, 110.078, 52.631, 26.350.

5,5′-(Pyrimidin-2-methyl-4,6-diyl)diisophthalic acid. Tetramethyl 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalate (0.52 g, 1.09 mmol) was dissolved in THF (40 mL) and MeOH (40 mL), and then 6 M NaOH aqueous solution (40 mL) was added to this solution. The resultant mixture was stirred at 323 K for 24 h, and cooled to room temperature. After the removal of the solvent under reduced pressure, the residue was dissolved in water and filtered. Upon cooling in an ice water bath, the filtrate was acidified with 6 M HCl to pH = 5. The resulting precipitation was collected by suction filtration, and dried under vacuum at 343 K to give the target compound in an almost quantitative yield. ¹H NMR (DMSO-d₆, 600.1 MHz) δ (ppm): 9.047 (d, J = 1.2 Hz, 4H), 8.664 (t, J = 1.2 Hz, 2H), 8.610 (s, 1H), 2.844 (s, 3H); ¹³C NMR (DMSO-d₆, 100.6 MHz) δ (ppm): 168.404, 167.403, 163.369, 137.475, 133.825, 132.569, 131.934, 110.966, 26.576; selected FTIR (KBr, cm⁻¹): 1714, 1606, 1581, 1539, 1444, 1228, 1111, 754, 677, 654.

2.4 Synthesis and characterization of ZJNU-55

The organic ligand (5.0 mg) and CuCl₂·2H₂O (10.0 mg) in a solvent mixture of N,N-dimethylformamide (DMF, 1.5 mL), MeOH (0.5 mL) and water (0.1 mL) were placed in a 10 mL vial. After the addition of 6 M HCl (50 μL), the vial was capped tightly and heated at 363 K for 60 h. After this time, blue crystals suitable for X-ray diffraction analysis were harvested in 22% yield based on the used organic ligand. The crystalline product is insoluble in the common organic solvents. Based on the single-crystal structure analysis, TGA and microanalysis, ZJNU-55 can be best formulated as [CuL]·2(CH₃)₂NH₂·DMF. Elemental analysis for C₂₈H₃₃CuN₅O₉, calcd (%): C, 51.97; H, 5.14; N, 10.82. Found (%): C, 51.89; H, 5.21; N, 10.77. Selected FTIR (KBr, cm⁻¹): 3421, 1622, 1574, 1419, 1363, 1246, 1107, 1022, 789, 768, 723, 660.

2.5 Fitting of pure-component isotherms

The pure component C₂H₂, CO₂ and CH₄ adsorption isotherm data measured at 278, 288 and 298 K were fitted with the singe-site Langmuir–Freundlich model
with the T-dependent parameter b
where q and q_sat are the amounts adsorbed at equilibrium pressure p, and the monolayer saturation capacity in mmol g⁻¹, b and v are Langmuir and Freundlich constants, respectively, and p is the pressure of the bulk gas in kPa. The Langmuir–Freundlich parameters are provided in Table S2. Fig. S8† provides a comparison of the experimental isotherm data for C₂H₂, CO₂ and CH₄ in ZJNU-55a with the isotherm fits.

2.6 Dynamic breakthrough experiment

The dynamic breakthrough experiments were performed at 298 K and 1 atm using a lab-scale fix-bed reactor. Detailed information of the setup can be found in our previous publication.^11d In a typical experiment, 260 mg of ZJNU-55 was packed into a stainless steel column (4.0 mm I.D./6.4 mm O.D. × 130 mm) with silica wool filling the void space. The sorbent was activated in situ in the column with a vacuum pump at 298 K for 48 h. A helium flow (5 mL min⁻¹) was introduced after the activation process to purge the adsorbent. The flow of He was then turned off while a gas mixture (50 : 50 C₂H₂–CH₄ and 50 : 50 CO₂–CH₄) at 5 mL min⁻¹ was allowed to flow into the column. The effluent from the column was monitored using a mass spectrometer (HPR 20).

3. Results and discussion

3.1 Synthesis and characterization

The organic ligand was conveniently synthesized by a Pd-catalyzed Suzuki cross-coupling reaction between 4,6-dichloro-2-methylpyrimidine and dimethyl 5-(pinacolboryl)isophthalate followed by hydrolysis and acidification in a moderate yield (see the Experimental section for synthetic details). Solvothermal reaction of CuCl₂·2H₂O and the organic ligand in a mixed solvent of DMF, MeOH and H₂O at 363 K for 60 h in the presence of HCl afforded blue needle-shaped single crystals of ZJNU-55 with the molecular formula [CuL]·2(CH₃)₂NH₂·DMF (Fig. S1†). Its structure was determined by single-crystal X-ray diffraction and the phase purity of the bulk materials was verified by a fairly good match of the experimental powder X-ray diffraction pattern with the one simulated from the single-crystal structure data (Fig. S2†). The TGA performed under a nitrogen atmosphere from 298 to 1073 K showed a weight loss of 25.1%, which corresponds to the loss of counterions and solvent molecules (calcd 25.5%) (Fig. S3†). FTIR spectra showed that the characteristic bands corresponding to the free carboxylate groups were not observed in the region of v = 1690–1730 cm⁻¹ (Fig. S4†), indicating full deprotonation of the organic ligands.

3.2 Single-crystal structural analysis

The single-crystal X-ray diffraction reveals that ZJNU-55 crystallizes in the chiral hexagonal space group P6₂ with unit cell parameters of a = b = 22.7393(15) Å, c = 10.7316(10) Å. Apart from the guest molecules, the asymmetric unit consists of one crystallographic independent Cu²⁺ ion and one deprotonated organic ligand. Concerning the organic ligand, the nitrogen atoms in the central pyrimidine ring did not coordinate with the Cu²⁺ ions, whereas four carboxylate groups were fully deprotonated and participated in coordination with Cu²⁺ ions. Every ligand utilizes four carboxylate groups adopting the monodentate coordination mode to link four separated Cu²⁺ ions (Fig. 1a). Different from what was observed in MOFs constructed from the analogous diisophthalates where the organic ligand usually adopts two different conformations, the organic ligand is incorporated into the as-synthesized ZJNU-55 in only one conformation where the dihedral angles between terminal isophthalate moieties and the central pyrimidine ring are 36.43° and 42.37°, respectively. Regarding the inorganic secondary building unit (SBU), although it is well documented that the assembly of Cu²⁺ ions with the ligands containing carboxylate groups usually affords a binuclear dicopper paddlewheel cluster,^8b,11b,14 the inorganic SBU in this compound is unexpectedly a mononuclear Cu(COO)₄ in which the Cu(II) ion is coordinated by four carboxylate oxygen atoms (O1, O3#4, O5#1 and O8#5; symmetry codes: #1: 1 − x, 1 − y, z; #4: y, −x + y, −1/3 + z; #5: −x + y, 1 − x, −2/3 + z) from four different organic ligands in a square-planar coordination geometry, which is rarely observed in copper-multicarboxylate frameworks (Fig. 1b). The Cu–O_carboxylate distances span from 1.903 to 1.972 Å, which is comparable to those observed in other copper complexes based on the carboxylic acids.¹⁵ Notably, this type of SBU is negatively charged, which results in a negative framework that needs positive ions to balance the whole charge of the structure. The combination of Cu(COO)₄ with the deprotonated organic linker generates a non-interpenetrated three-dimensional network. Along the crystallographic c axis, two different types of channels (channel A and channel B) can be observed with the dimensions of ca. 2 and 8 Å, respectively, when the van der Waals radii of the atoms are taken into account (Fig. 1c). The nitrogen atoms as well as uncoordinated carboxylate oxygen atoms point towards the channels. It is notable that channel A can be viewed as an infinite nanotube which is comprised of four left-handed helical chains with a helical pitch of 42.926 Å and two right-handed helical chains with the helical pitch of 21.463 Å, which are weaved together by sharing the metal ions (Fig. 1d). These two types of helical chains are formed by alternatively linking isophthalate moieties and Cu²⁺ ions. The channels are occupied by the disordered solvent molecules and dimethylammonium counterions generated via decomposition of the DMF solvent, which could not be characterized owing to the very weak diffraction intensity. PLATON calculation indicates that the effective free volume of ZJNU-55 is 47.3% of the crystal volume (2270.9 Å of the 4805.62 ų unit cell volume) when the solvent molecules and counter ions are not considered.

Fig. 1 The coordination environment of the organic ligand (a) and the Cu(COO)₄ unit (b) in ZJNU-55. Copper atoms are shown in the polyhedral model and the organic ligand was represented in the sphere and ball model. Symmetry codes: #1: 1 − x, 1 − y, z; #2: x − y, x, 1/3 + z; #3: 1 − y, 1 + x − y, 2/3 + z; #4: y, −x + y, −1/3 + z; #5: −x + y, 1 − x, −2/3 + z. (c) A three-dimensional network with two different types of channels A and B viewed along the crystallographic c axis. (d) A side view of channel A comprised of six helical chains sharing the metal ions. (e) Schematic representation of a 4-connected binodal net with the Schläfli symbol (4·6²·8³)(4·6⁴·8) and a (3,4)-connected network with the Schläfli symbol (6·8·9)₂(6·8³·9²). The hydrogen atoms are omitted for clarity.

From the perspective of a topological network, each Cu²⁺ ion links four organic ligands, and therefore can be regarded as a 4-connected node, while each organic ligand in turn connects with four Cu²⁺ ions and thus can also be taken as a 4-connected node. As a result, the whole framework can be rationalized as a novel 4-connected binodal net with the Schläfli symbol (4·6²·8³)(4·6⁴·8) (Fig. 1e). Alternatively, if the organic ligand is treated as having a pair of three-connected nodes, the overall structure possesses a novel (3,4)-connected trinodal net with the Schläfli symbol (6·8·9)₂(6·8³·9²), which is previously unreported.

3.3 Permanent porosity

Gas-adsorption measurements were performed on a Micromeritics 2020 HD88 surface-area-and-pore-size analyser. Before gas adsorption measurements, the as-synthesized sample was activated by multiple solvent exchanges with low-boiling-point dry acetone followed by evacuation under dynamic vacuum at 295 K for 24 h, generating the desolvated ZJNU-55a. No colour change after solvent removal also indicates the absence of the common dinuclear Cu(II) paddlewheel cluster in the structure, which is consistent with the above single-crystal structure analyses. PXRD analysis suggests that the activated sample is highly crystalline and retains its original structure (Fig. S2†). As shown in Fig. 2a, the activated ZJNU-55a exhibits a type-I N₂ adsorption isotherm at 77 K, indicating a typical microporous material. At the maximum relative pressure, the N₂ uptake capacity reaches 118 cm³ (STP) g⁻¹, which corresponds to a pore volume of 0.1825 cm³ g⁻¹. Deduced from nitrogen isotherm data, the Brunauer–Emmett–Teller (BET) and Langmuir surface areas are estimated to be 450.1 and 508.6 m² g⁻¹, respectively (Fig. S5†), which are relatively moderate compared to other reported copper-based MOFs constructed from angular diisophthalate linkers such as PCN-305–308 (1376–1927 and 2235–2929 m² g⁻¹ for BET and Langmuir surface areas),¹⁰PCN-124 (1372 and 2002 m² g⁻¹ for BET and Langmuir surface areas),¹¹ⁱNJU-Bai10 (2883 and 3107.9 m² g⁻¹ for BET and Langmuir surface areas),^11h[Cu₂L] (L = 1,3-bis(3,5-dicarboxylphenylethynyl)benzene, 1879 and 2489 m² g⁻¹ for BET and Langmuir surface areas),^11j and PMOF-3 (1840 and 2020 m² g⁻¹ for BET and Langmuir surface areas).^11k It should be mentioned that further increasing the activation temperature leads to inferior results probably due to the partial collapse of the framework (Fig. S6†), and therefore in this situation a room temperature of 295 K was employed as the optimum activation temperature.

Fig. 2 (a) N₂ adsorption–desorption isotherm of ZJNU-55a at 77 K. Adsorption isotherms for (b) C₂H₂, (c) CO₂ and (d) CH₄ on ZJNU-55a at three different temperatures of 278 K, 288 K and 298 K. Solid and open symbols represent adsorption and desorption, respectively.

3.4 Selective gas adsorption

Acetylene produced by the thermal cracking of methane often contains unreacted methane, and efficient removal of CH₄ from C₂H₂ has thus become an important issue. In addition, the separation of CO₂ from CH₄ is also an industrially important process for natural gas purification because the coexistence of CO₂ with CH₄ not only leads to pipeline corrosion but also reduces the energy content of natural gas. The conventional cryogenic distillation is very energy-intensive for the above two separation processes. A number of groups have made significant contributions towards this goal, demonstrating the advantages of MOFs for these separation processes.^1i,6d,14d,16

The establishment of permanent porosity encourages us to explore its potential for C₂H₂/CH₄ and CO₂/CH₄ separation. Accordingly, the pure-component C₂H₂, CO₂ and CH₄ sorption isotherms for ZJNU-55a were measured up to 1 atm at three different temperatures of 278 K, 288 K and 298 K. As shown in Fig. 2b–d, all isotherms, except the CH₄ isotherm, show slightly hysteretic sorption behaviour. The isotherms are reproducible after five cycles (Fig. S7†). The most remarkable is that ZJNU-55a systematically adsorbs substantially more C₂H₂ and CO₂ than CH₄. At 298 K, for example, ZJNU-55a takes up a moderate amount of C₂H₂ (56.3 cm³ (STP) g⁻¹, 66.3 mg g⁻¹), and CO₂ (40.6 cm³ (STP) g⁻¹, 79.7 mg g⁻¹), but basically a negligible amount of CH₄ (8.4 cm³ (STP) g⁻¹, 6.0 mg g⁻¹) at 1 atm, indicating that ZJNU-55a is a promising material for C₂H₂/CH₄ and CO₂/CH₄ separation at room temperature. The acetylene adsorption amount is higher than that for Cu₂(pzdc)₂(pyz), the first MOF material reported by Kitagawa and co-workers for acetylene adsorption (43 cm³ (STP) g⁻¹ at RT and 1 atm),¹⁷ and comparable to that reported for Mn formate (51.2 cm³ (STP) g⁻¹ at 298 K and 1 atm).^16m Compared to PCN-307 (73.3 cm³ (STP) g⁻¹ at 297 K and 1 bar),¹⁰ZJNU-55a shows a lower CO₂ uptake capacity under similar conditions.

To evaluate the gas separation ability of ZJNU-55a, the ideal adsorbed solution theory (IAST) of Myers and Prausnitz¹⁸ was employed to predict the adsorption selectivities from the experimental pure-gas adsorption isotherms. Fig. 3a and b present the predicted adsorption selectivities for equimolar C₂H₂/CH₄ and CO₂/CH₄ mixtures in ZJNU-55a as a function of bulk pressures. It can be seen that the C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities decrease rapidly and then remain almost constant as the pressure increases, while the C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities increase with decreasing temperatures. For an equimolar C₂H₂/CH₄ mixture, the predicted selectivities are in the range of 39.2–64.9 for a range of pressures up to 110 kPa at 298 K (Fig. 3a), which is among the highest reported for MOFs (Table S3†). For an equimolar CO₂/CH₄ mixture, the predicted selectivities are in the range of 13.1–29.2 for a range of pressures up to 110 kPa at 298 K (Fig. 3b), which is significantly higher than that of PCN-307 (6.3–9.0 at 273 K).¹⁰ Such high selectivities further confirm that ZJNU-55a has a considerable potential in the adsorptive separation of C₂H₂/CH₄ and CO₂/CH₄ gas mixtures.

Fig. 3 IAST selectivities of (a) C₂H₂/CH₄ and (b) CO₂/CH₄ for equimolar binary gas mixtures in ZJNU-55a. The breakthrough curves for the equimolar C₂H₂/CH₄ (c) and CO₂/CH₄ (d) gas mixtures at 298 K and 1 atm.

Dynamic breakthrough experiments were carried out to check the viability of using ZJNU-55a to separate equimolar C₂H₂/CH₄ and CO₂/CH₄ gas mixtures. As shown in Fig. 3c and d, the separation of these two gas mixtures can be efficiently achieved. The dynamic C₂H₂ and CO₂ uptake capacities calculated as a result of the breakthrough time were found to be 1.92 and 1.10 mmol g⁻¹. Based on the adsorbed amount, the separation factors were calculated to be 26.4 and 7.1 for the equimolar C₂H₂/CH₄ and CO₂/CH₄ gas mixtures, respectively.

To understand the affinities of these materials towards gas molecules, the coverage-dependent adsorption enthalpies were calculated using the Clausius–Clapeyron equation based on the pure-component adsorption isotherms measured at three different temperatures of 278 K, 288 K and 298 K. Fig. 4 presents the data on the isosteric heats of adsorption for C₂H₂, CO₂ and CH₄ in ZJNU-55a as a function of component loadings. At low coverage, the isosteric heats of adsorption are 42.40 ± 0.95, 35.44 ± 0.14 and 25.21 ± 0.99 kJ mol⁻¹ for C₂H₂, CO₂ and CH₄, respectively. The Q_st of CO₂ is even higher than 22.836 kJ mol⁻¹ observed for PCN-307.¹⁰ Moreover, the isosteric adsorption heat follows the trend: C₂H₂ > CO₂ > CH₄. As demonstrated in the MOF literature,^16l the higher adsorption heats for CO₂ and C₂H₂ might to be due to the ionic nature of the framework and the basic sites such as pyrimidine nitrogen and uncoordinated carboxylate oxygen atoms in the pore surface enforcing their stronger interactions with C₂H₂ and CO₂ molecules, thus being responsible for the highly selective separation of C₂H₂ and CO₂ over CH₄.

Fig. 4 Dependence of the enthalpies of adsorption for C₂H₂, CO₂ and CH₄ with coverage.

4. Conclusions

In conclusion, a chiral anionic MOF with a novel topology was solvothermally synthesized from an angular diisophthalate ligand and CuCl₂·2H₂O. Although the ligand bears a close resemblance to the previously reported diisophthalate used to construct PCN-307, the resultant MOF's structure is totally different. The MOF is composed of a mononuclear Cu(COO)₄ inorganic SBU, instead of a typical dinuclear Cu₂(COO)₄ paddlewheel unit, which is connected with the deprotonated organic ligand to form a novel network structure. More importantly, although the inorganic SBU is mononuclear Cu(COO)₄, the framework after desolvation exhibits permanent porosity which has been established by various gas adsorption isotherms, showing the highly selectively adsorptive separation of C₂H₂ and CO₂ over CH₄. This is a rare example of a MOF constructed from mononuclear Cu(COO)₄ for selective gas adsorption. This work also demonstrates that subtle ligand modification, in our case only simply substituting the central benzene ring with the pyrimidine group, can drastically impact the structures and gas adsorption properties of the resultant MOFs.

Acknowledgements

We gratefully acknowledge the financial support of the Natural Science Foundation of China (no. 21301156), the Qianjiang talents project in Zhejiang province, China (ZC304015017), and the Natural Science Foundation of Zhejiang province, China (LR16B010001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Photograph of the crystals of ZJNU-55 (Fig. S1), PXRD (Fig. S2), TGA (Fig. S3), FTIR (Fig. S4), BET and Langmuir plots (Fig. S5), N₂ adsorption isotherms at 77 K of ZJNU-55 activated at different temperatures (Fig. S6), five cycles of C₂H₂ adsorption at 298 K (Fig. S7), comparison of the pure-component isotherm data with the fitted isotherms (Fig. S8), isosteric plots for C₂H₂, CO₂ and CH₄ adsorption (Fig. S9), ¹H and ¹³C NMR spectra (Fig. S10), crystal data and structure refinement for ZJNU-55 (Table S1), Langmuir–Freundlich parameters for adsorption of C₂H₂, CO₂, and CH₄ in ZJNU-55a (Table S2). CCDC 1492066. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00245e

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