Spatial disposition of square-planar mononuclear nodes in metal–organic frameworks for C₂H₂/CO₂ separation

Abstract

The efficient separation of acetylene (C₂H₂) from its mixture with carbon dioxide (CO₂) remains a challenging industrial process due to their close molecular sizes/shapes and similar physical properties. Herein, we report a microporous metal–organic framework (JNU-4) with square-planar mononuclear copper(II) centers as nodes and tetrahedral organic linkers as spacers, allowing for two accessible binding sites per metal center for C₂H₂ molecules. Consequently, JNU-4 exhibits excellent C₂H₂ adsorption capacity, particularly at 298 K and 0.5 bar (200 cm³ g⁻¹). Detailed computational studies confirm that C₂H₂ molecules are indeed predominantly located in close proximity to the square-planar copper centers on both sides. Breakthrough experiments demonstrate that JNU-4 is capable of efficiently separating C₂H₂ from a 50 : 50 C₂H₂/CO₂ mixture over a broad range of flow rates, affording by far the largest C₂H₂ capture capacity (160 cm³ g⁻¹) and fuel-grade C₂H₂ production (105 cm³ g⁻¹, ≥98% purity) upon desorption. Simply by maximizing accessible open metal sites on mononuclear metal centers, this work presents a promising strategy to improve the C₂H₂ adsorption capacity and address the challenging C₂H₂/CO₂ separation.

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1. Introduction

Acetylene (C₂H₂) is one of the most essential feedstocks in petrochemical industries with a global market of 5.6 billion US$ in 2018 and projected to reach 11.42 billion US$ in 2023.¹ It is manufactured by hydrocarbon cracking or partial nature gas combustion, in which carbon dioxide (CO₂) is an inevitable byproduct.² Thus, the removal of CO₂ is a must-do step before further production of value-added chemicals. Yet, the C₂H₂/CO₂ separation is a challenging task owing to their very close molecular sizes/shapes (C₂H₂, 3.34 Å × 3.32 Å × 5.70 Å; CO₂, 3.33 Å × 3.18 Å × 5.36 Å, Fig. S1†) and similar physical properties (boiling points: C₂H₂, 189.3 K; CO₂, 194.7 K).³ The industrial purification of C₂H₂ from CO₂ is carried out by cryogenic distillation and liquid absorption,^4–7 both of which are highly energy-intensive and often associated with potential safety hazards.

Adsorptive separation enables efficient gas purification and is deemed an energy-saving alternative technology. Porous materials that can selectively adsorb one type of molecule over others have recently aroused great interest and are deemed promising to replace traditional industrial practices. For instance, metal–organic frameworks (MOFs), a.k.a. porous coordination polymers (PCPs), are porous crystalline solids constructed from the self-assembly of organic linkers and metal ions/clusters.^8–20 Although recently emerged, MOFs stand out in gas separation studies and the reticular chemistry enables exquisite control over pore size and surface chemistry to realize preferential binding or even molecular sieving effects.^21–28 With regard to C₂H₂/CO₂ separation, however, the trade-off between adsorption capacity and selectivity still poses a daunting challenge for materials scientists.

Surface engineering with electronegative elements (e.g., N, O, and F) usually facilitates the binding of C₂H₂ over CO₂ due to their difference in the quadrupole moments (−13.4 × 10⁻⁴⁰ and +20.5 × 10⁻⁴⁰ C m² for CO₂ and C₂H₂, respectively) and electrostatic potentials (Fig. S1†).^29–48 For example, a highly fluorinated MOF material SIFSIX-Cu-TPA exhibits a very large C₂H₂ adsorption of 185 cm³ g⁻¹ at 298 K and 1 bar with a moderate C₂H₂/CO₂ selectivity of 5.4.³¹ This approach, however, may not necessarily favor C₂H₂ adsorption as the overall mismatch of electrostatic potentials between the framework surface and C₂H₂ molecule could disrupt and even reverse the selectivity.⁴⁹

On the other hand, a more reliable approach is surface engineering with open metal sites (OMSs) to preferentially bind C₂H₂ molecules via π-complexation.^50–56 Most of the reported clusters in MOFs offer one accessible OMS per metal, such as mononuclear square-pyramidal,⁵⁷ dinuclear paddle-wheel,⁵⁸ and trinuclear μ₃-oxo clusters.⁵⁹ It is interesting to point out that a simple square-planar mononuclear center may be able to provide as many as two binding sites for C₂H₂ molecules. The material UTSA-74a is constructed with both square-planar and tetrahedral mononuclear zinc centers. Although the square-planar zinc center can provide two accessible binding sites, the tetrahedral one has no accessible binding sites due to the rigid coordination geometry.⁵¹ The material NKMOF-1-Ni is a rare MOF example constructed with square-planar mononuclear centers.⁶⁰ As strong as it may seem in the binding of C₂H₂ over CO₂ (C₂H₂/CO₂ selectivity = 249.3), its C₂H₂ adsorption capacity (85.7 cm³ g⁻¹) is relatively low, likely due to the short Ni⋯Ni distance (5.75 Å) allowing only one C₂H₂ molecule in between. Intuitively, by employing organic linkers of right size and geometry as spacers, it is plausible to realize the maximum utilization of binding sites of square-planar metal centers, and therefore, achieve not only high C₂H₂/CO₂ selectivity but also large C₂H₂ adsorption capacity (Scheme 1).

Scheme 1 A simple concept to enhance C₂H₂ adsorption via spatial disposition of square-planar mononuclear metal centers with tetrahedral spacers.

Herein we report a microporous MOF (termed JNU-4, JNU = Jinan University) featuring a 4,4-connected pts topology with square-planar mononuclear copper(II) centers spaced by tetrahedral organic linkers. Structural analysis shows that every copper center has both OMSs orientated toward the channels, and thus, spatially accessible for C₂H₂ molecules. As a result, the activated JNU-4 (JNU-4a) exhibits excellent C₂H₂ adsorption capacity, particularly at 298 K and 0.5 bar (200 cm³ g⁻¹). Computational studies confirm that C₂H₂ molecules indeed preferentially occupy both OMSs of the square-planar copper centers, and the simulated adsorption data are consistent with the experimental results. Breakthrough measurements demonstrate that JNU-4a is capable of efficiently separating C₂H₂ from C₂H₂/CO₂ mixtures over a broad range of flow rates, realizing by far the largest C₂H₂ capture capacity and fuel-grade C₂H₂ production even under humid conditions.

2. Results and discussion

2.1. Structure and porosity analysis

JNU-4 was synthesized according to a modified procedure previously reported by Krautscheid.⁶¹ The solvothermal reaction of 5-(3-methyl-5-(pyridin-4-yl)-4H-1,2,4-triazol-4-yl)-1,3-benzenedicarboxylic acid (H₂MPTBDC) and Cu(NO₃)₂·3H₂O in a mixture of acetonitrile, water, and nitric acid yielded dark blue crystals of JNU-4. Single crystal X-ray diffraction (SCXRD) data reveal that the as-synthesized JNU-4 crystallized in the monoclinic P2₁/c space group. There are two crystallographically independent Cu(II) atoms in the asymmetric unit, both of which are in four-coordination with the oxygen/nitrogen atoms of MPTBDC linkers, affording Cu(COO)₂(N-pyridine)₂ and Cu(COO)₂(N-triazole)₂ of similar square planar geometry. One Cu atom coordinates with two O atoms from carboxylate groups and two N atoms from pyridine groups, while the other coordinates with two O atoms from carboxylate groups and two N atoms from triazole groups. The tetrahedral MPTBDC linkers and the square planar copper centers are further linked to generating a three-dimensional (3D) MOF with a 4,4-connected pts topology network. The copper centers are sufficiently spaced by the organic linkers and the two binding sites are potentially accessible for C₂H₂ molecules (Fig. 1 and S2†). The phase purity of the bulk JNU-4 was verified by comparison of the simulated and experimental powder X-ray diffraction (PXRD) patterns (Fig. S3†). The N₂ adsorption/desorption measurements of JNU-4a (see the ESI† for the detailed activation method) at 77 K display a reversible type I adsorption isotherm with a saturated adsorption of 400 cm³ g⁻¹, characteristic of a microporous material (Fig. 2a). The pore size distribution (PSD) calculated using Non-Localized Density Functional Theory (NL-DFT) methods shows two main peaks centered at 4.8 and 9.3 Å (Fig. 2a inset), which are arguably in the right range for realizing both high C₂H₂ adsorption and C₂H₂/CO₂ selectivity.⁶² The Brunauer–Emmett–Teller surface area of JNU-4a was calculated to be 1533 m² g⁻¹ and the pore volume to be 0.62 cm³ g⁻¹ at P/P₀ = 0.9 (Fig. S4†), which matches well with the crystal structure (0.60 cm³ g⁻¹), indicating that the sample can be fully desolvated while maintaining the structural integrity. The high surface area, suitable pore size, and square planar mononuclear centers of JNU-4a encouraged us to further investigate its C₂H₂ adsorption and C₂H₂/CO₂ separation. The OMS density of JNU-4a is about 4.8 mmol cm⁻³ (Table S5†), which is mediocre compared to other MOFs, likely due to its high specific surface area (1533 m² g⁻¹) and large pore volume (0.62 cm³ g⁻¹).

Fig. 1 (a) MPTBDC organic linker as a tetrahedral building unit, Cu(COO)₂(N-pyridine)₂ and Cu(COO)₂(N-triazole)₂ complexes as square-planar 4-connected building units, respectively. (b) Simplified pts topology of JNU-4, highlighting that every copper center has both OMSs orientated toward the channels accessible for C₂H₂ interaction. Color code: green, Cu; red, O; blue, N; gray, C. C₂H₂ molecules are shown in a ball-and-stick model depicted in gray.

Fig. 2 (a) N₂ adsorption/desorption isotherms of JNU-4a at 77 K. Filled/empty squares represent adsorption/desorption. (Inset) Calculated pore-size distribution using the Non-Localized Density Functional Theory (NL-DFT) method. (b) C₂H₂ and CO₂ single-component adsorption/desorption isotherms of JNU-4a at 298 K up to 1 bar. (c) Comparison of C₂H₂ gravimetric uptake in JNU-4a and some representative MOF materials at room temperature (0.5 bar and 1 bar). (d) Differential scanning calorimetry of JNU-4a upon introducing C₂H₂ or CO₂ at a flow rate of 20 mL min⁻¹ under ambient conditions (298 K and 1 bar).

2.2. Gas adsorption

Hence, we collected single-component C₂H₂ and CO₂ adsorption isotherms at different temperatures. As expected, JNU-4a manifests a steep adsorption curve with an adsorption capacity of 222 cm³ g⁻¹ at 298 K and 1 bar (Fig. 2b and S5†), which is superior to most of the top-performing materials, such as SIFSIX-Cu-TPA (185 cm³ g⁻¹),³¹ FJU-90a (180 cm³ g⁻¹),³² SIFSIX-3-Ni (73.9 cm³ g⁻¹),³³ ZJU-74a (85.7 cm³ g⁻¹),⁵⁰ UTSA-74a (108 cm³ g⁻¹),⁵¹ UPC-200(Al)-F-BIM (144.5 cm³ g⁻¹),³⁴ FeNi-M′MOF (96 cm³ g⁻¹),⁵² SNNU-27-Fe (182.4 cm³ g⁻¹),⁶² JCM-1 (75 cm³ g⁻¹),³⁵ and JNU-1 (64 cm³ g⁻¹),⁶³ and slightly lower than that for FJI-H8-Me (229 cm³ g⁻¹).⁶⁴ For an equimolar C₂H₂/CO₂ separation, the C₂H₂ uptake value at a partial pressure of 0.5 bar is considered an essential indicator of its separation potential. JNU-4a exhibits a gravimetric C₂H₂ adsorption capacity of 200 cm³ g⁻¹. To the best of our knowledge, this value is higher than those of reported top-performance porous materials for C₂H₂/CO₂ separation (Fig. 2c), including MIL-160 (172 cm³ g⁻¹),⁶⁵ Co-MOF-74 (174 cm³ g⁻¹),⁵⁵ SIFSIX-Cu-TPA (168.8 cm³ g⁻¹),³¹ and FJI-H8-Me (180 cm³ g⁻¹).⁶⁴ By contrast, the CO₂ adsorption capacity of JNU-4a at 0.5 bar is 109 cm³ g⁻¹, which is almost half of that for C₂H₂ under similar conditions. We further employed ideal adsorption solution theory (IAST)⁶⁶ to simulate the adsorption behavior for a 50 : 50 C₂H₂/CO₂ gas mixture. As shown in Fig. S6–S8,† JNU-4a shows a moderate IAST selectivity of C₂H₂ over CO₂, reaching up to 8.2 at 298 K and 1 bar, which is higher than that of many benchmark materials studied for C₂H₂/CO₂ separation, such as FJU-90a (4.3),³² MUF-17 (6.0),³⁶ SIFSIX-21-Ni (7.8),³⁸ UPC-200(Al)-F-BIM (3.15),³⁴ and SNNU-27-Fe (3.6).⁶²

2.3. Adsorption enthalpy

To quantitatively depict the large adsorption difference between C₂H₂ and CO₂ on JNU-4a, their isosteric heat of adsorption (Q_st) values were calculated from the fitting of single-component gas adsorption isotherms collected at different temperatures. As shown in Fig. S9–S11,† the Q_st for C₂H₂ at zero coverage was calculated to be 26.8 kJ mol⁻¹, significantly higher than that for CO₂ (19.7 kJ mol⁻¹). To further experimentally quantify the Q_st for C₂H₂ and CO₂ on JNU-4a, we performed differential scanning calorimetry measurements of heat flow upon introducing C₂H₂ or CO₂ at a flow rate of 20 mL min⁻¹, respectively, at 298 K and 1 bar (Fig. 2d). The obtained experimental Q_st for C₂H₂ and CO₂ is 34.4 kJ mol⁻¹ and 20.6 kJ mol⁻¹, respectively, indicative of a significantly stronger binding affinity for C₂H₂ than CO₂. It is worth mentioning that the Q_st for C₂H₂ on JNU-4a is lower than those on UTSA-300 (57.6 kJ mol⁻¹),⁶⁷ SIFSIX-3-Ni (36.7 kJ mol⁻¹),³³ and most of the benchmark MOFs with high-density OMSs, such as ATC-Cu (79.1 kJ mol⁻¹),⁵⁶ NKMOF-1-Ni (60.3 kJ mol⁻¹),⁶⁰ ZJU-74a (45 kJ mol⁻¹),⁵⁰ and CuI@UiO-66-(COOH)₂ (74.5 kJ mol⁻¹)⁵⁴ (Fig. S12†). Such a low Q_st value implies that JNU-4a could be regenerated under mild conditions, which would be highly beneficial for energy-efficient C₂H₂/CO₂ separation. To prove the easy regeneration and recyclability of JNU-4a, we carried out twenty continuous gas adsorption/desorption measurements for C₂H₂ and CO₂ without applying heat at the degassing stage. As expected, no obvious loss of C₂H₂ and CO₂ adsorption capacity was observed (Fig. S13 and S14†).

2.4. Computational studies

To corroborate the adsorption capacity and the preferred C₂H₂ binding sites in JNU-4a, grand canonical Monte Carlo (GCMC) simulations were first carried out at 298 K.^68,69 The experimental and simulated isotherms are in reasonably good agreement, particularly for C₂H₂ at low pressures and for CO₂ over the entire pressure range (Fig. 3a). The density distribution and radial distribution functions of C₂H₂ and CO₂ at 298 K and 1 bar further reveal that C₂H₂ molecules are indeed predominantly located on both sides of Cu(COO)₂(N-triazole)₂ (site I) and Cu(COO)₂(N-pyridine)₂ (site II), whereas CO₂ molecules were observed only at site I (Fig. 3b, c and S15†). To describe the detailed interactions of gas molecules at both sites, dispersion-corrected density functional theory (DFT-D) calculations were then carried out. As shown in Fig. 3d–f, two C₂H₂ molecules at site I interact with Cu²⁺ through π-complexation with distances of 3.02 and 3.05 Å, respectively, which are comparable to the sum of the van der Waals radii of carbon (1.70 Å) and copper (1.40 Å) atoms. Additionally, one of the H atoms of C₂H₂ molecules is bound by uncoordinated carboxylate O atoms through C–H⋯O interaction with a distance of 1.95 Å (Fig. 3d). At site II, two C₂H₂ molecules interact with Cu²⁺ with distances of 3.31 and 3.96 Å, respectively, and both C₂H₂ molecules have an H atom bound by uncoordinated carboxylate O atoms through C–H⋯O interaction with distances of 1.72 and 1.95 Å (Fig. 3e). By contrast, both CO₂ molecules at site I interact side-on with Cu²⁺ through Cu⋯O=C=O interaction with distances of 3.80 and 3.81 Å, respectively, and the same interacting O atoms are also bound by H atoms of methyl groups through C=O⋯H interaction with distances of 2.70 and 3.14 Å, respectively (Fig. 3f). The static binding energies (ΔE) for C₂H₂ were calculated to be 41.7 and 33.2 kJ mol⁻¹ at site I and site II, respectively, which is much higher than that for CO₂ at site I (29.87 kJ mol⁻¹). The overall large difference between C₂H₂ and CO₂ in their static binding energies on JNU-4a is in good agreement with their respective experimental Q_st values. Thus, not only was the square planar copper center successfully integrated into the framework but also the two OMSs of copper clusters were proved accessible for C₂H₂ binding due to the right-sized organic linker as a spacing unit. The unprecedented maximum utilization of OMSs of the square planar metal center in JNU-4a enables both strong binding of C₂H₂ molecules and large C₂H₂ adsorption capacity, which prompted us to further investigate its C₂H₂/CO₂ separation performance.

Fig. 3 (a) Experimental and simulated adsorption isotherms of C₂H₂ (red) and CO₂ (blue) at 298 K and up to 1 bar. (b) GCMC simulated adsorption density distributions of C₂H₂ in JNU-4a at 298 K and 1 bar. (c) GCMC simulated adsorption density distributions of CO₂ in JNU-4a at 298 K and 1 bar. (d) DFT-D-calculated binding configurations of C₂H₂ at site I. (e) DFT-D-calculated binding configurations of C₂H₂ at site II. (f) DFT-D-calculated binding configurations of CO₂ at site I (green, Cu; dark gray, C; blue, N; red, O; white, H. C₂H₂ and CO₂ are represented in ball-and-stick models in (d)–(f), and the distance unit is Å).

2.5. Breakthrough experiments

Column breakthrough experiments were first performed on JNU-4a for a 50 : 50 C₂H₂/CO₂ mixture at a flow rate of 4 mL min⁻¹ at 298 K. As shown in Fig. 4a, a clean separation of C₂H₂/CO₂ was observed with a large separation window; CO₂ quickly broke through the column after 42 min g⁻¹, whereas the retention time of C₂H₂ on the column was 79 min g⁻¹, almost doubled that of CO₂. Moreover, the breakthrough curve for CO₂ exhibited a typical but very strong rolling-up at 42 min g⁻¹, with a C/C₀ value rising to 2.0, indicating that a great deal of the adsorbed CO₂ was displaced by C₂H₂ until breakthrough equilibrium, which can be rationalized by their large binding energy difference. Based on the breakthrough curves, the amount of C₂H₂ and CO₂ captured on the breakthrough column was estimated to be 160 cm³ g⁻¹ and 12.5 cm³ g⁻¹, respectively. Notably, such a captured amount of C₂H₂ is by far the highest among all reported porous materials (Fig. 4b and S17†), including MIL-160 (152 cm³ g⁻¹ or 142 cm³ g⁻¹ in a different report),^65,70 FeNi-M′MOF (66.7 cm³ g⁻¹),⁵² CuI@UiO-66-(COOH)₂ (64.7 cm³ g⁻¹),⁵⁴ and ZJU-74 (81.5 cm³ g⁻¹).⁵⁰ With the captured amount of C₂H₂ and CO₂, the separation factor was estimated to be 12.8, which is consistent with the IAST selectivity, and higher than that of most of the top-performing porous materials, such as CuI@UiO-66-(COOH)₂ (3.4),⁵⁴ FeNi-M′MOF (1.7),⁵² FJU-90a (2.1),³² NKMOF-1-Ni (2.6),⁶⁰ and SNNU-45 (2.9)³⁷ (Fig. S17 and S18†).

Fig. 4 (a) Breakthrough curves (left y-axis) of C₂H₂ and CO₂ on JNU-4a for an equimolar mixture of C₂H₂/CO₂ (4.0 mL min⁻¹) at 298 K. Empty squares depict the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column (right y-axis). (b) Comparison of the estimated amount of C₂H₂ captured on the breakthrough column for an equimolar mixture of C₂H₂/CO₂ (JNU-4a and other top-performing materials). (c) Desorption curves of C₂H₂ and CO₂ after breakthrough equilibrium with helium gas (10.0 mL min⁻¹) sweeping at 298 K. Solid green squares represent the ratios of C₂H₂/CO₂ in the desorbed gas mixture. The yellow area highlights C₂H₂ with over 98% purity (105 cm³ g⁻¹) collectible during the desorption. (d) Continuous breakthrough experiments on JNU-4a for an equimolar mixture of C₂H₂/CO₂ at 298 K and the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column of nine cycles. In situ regeneration was carried out with helium gas (10.0 mL min⁻¹) sweeping at 298 K.

Although many MOF materials have been studied for C₂H₂/CO₂ separation, only a few of them reported C₂H₂ purity and productivity. The high separation factor of JNU-4a prompted us to conduct desorption experiments. Indeed, fuel-grade C₂H₂ (98% purity) can be achieved upon desorption and the productivity was estimated to be 105 cm³ g⁻¹ (Fig. 4c and S19†), which is sustainably higher than those of reported porous materials.^{33,44,49,71,72} To further evaluate the recyclability of JNU-4a, continuous breakthrough experiments were carried with helium sweeping at room temperature as the regeneration method (Fig. 4d and S20–S22†). Benefitting from its low Q_st, JNU-4a maintained the high C₂H₂ capture capacity after nine cycles, highlighting the easy regeneration and good recyclability of JNU-4a for C₂H₂/CO₂ separation. A high diffusion rate is crucial for industrial applications, which will drastically reduce the operation time.⁷³ Volumetric adsorption kinetics was first studied on a BEL MAX II sorption analyzer; both C₂H₂ and CO₂ exhibit a relatively high diffusion rate with diffusion constants (D/r²) calculated to be 3.00 × 10⁻³ and 2.59 × 10⁻³, respectively (Fig. S29a and b†). The data were consistent with the time-dependent gravimetric adsorption performed on a thermogravimetric analyzer; both C₂H₂ and CO₂ can reach adsorption saturation in a short time period (Fig. S29c†). To investigate the influence of the flow rate on the separation performance, column breakthrough experiments were carried out for a 50 : 50 C₂H₂/CO₂ mixture at different flow rates (up to 10.0 mL min⁻¹) (Fig. S23–S25†). The breakthrough times of C₂H₂ and CO₂ were reduced with the increase of flow rates, while the amount of the captured C₂H₂ was estimated to be 158.0 cm³ g⁻¹, 162.4 cm³ g⁻¹, and 168.3 cm³ g⁻¹ at flow rates of 6.0 mL min⁻¹, 8.0 mL min⁻¹, and 10.0 mL min⁻¹, respectively. The results suggest that the higher flow rate may increase the amount of C₂H₂ captured on the breakthrough column owing to the buildup of pressures.

In industrial C₂H₂/CO₂ separation, the feed gases may contain water vapor, which will likely affect the separation performance of MOF materials with OMSs. Continuous single-component adsorption/desorption experiments were carried out on JNU-4a with moisturized C₂H₂ (10% relative humidity, RH). As shown in Fig. S26 and S27,† no obvious loss of adsorption capacity was observed after ten cycles of adsorption/desorption. Furthermore, we conducted eight cycles of continuous breakthrough experiments for an equimolar C₂H₂/CO₂ mixture with a flow rate of 6.0 mL min⁻¹ under humid conditions (10% RH) (Fig. 5a), and also no obvious loss of capture capacity was observed. The amount of C₂H₂ captured on the breakthrough column was estimated to be 145 cm³ g⁻¹ (Fig. 5b). Based on desorption curves, about 85 cm³ of fuel-grade C₂H₂ can be retrieved for 1 g of JNU-4a (Fig. 5c). Although lower than the productivity under dry conditions, it is still much higher than those of reported porous materials, including SIFSIX-3-Ni (56 cm³ g⁻¹, 99.9%),³³ SIFSIX-dps-Cu (55.6 cm³ g⁻¹, 99.9%),⁷¹ ZNU-1 (53.8 cm³ g⁻¹, 99.5%),⁴⁴ JXNU-11(Fe₂Ni) (44.3 cm³ g⁻¹, 95%),⁷² and Cd-NP (27.8 cm³ g⁻¹, 99.99%)⁴⁹ (Fig. 5d). Many MOFs with OMSs have been reported to be poisoned by water vapor, and it is worth pointing out that JNU-4a may be capable of efficiently separating C₂H₂/CO₂ even under humid conditions.

Fig. 5 (a) Continuous breakthrough curves of C₂H₂ and CO₂ on JNU-4a for an equimolar C₂H₂/CO₂ mixture (6.0 mL min⁻¹) under dry (0% RH) and humid conditions (10% RH). In situ regeneration was carried out with helium gas (10.0 mL min⁻¹) sweeping at 298 K. (b) One set of breakthrough curves (left y-axis) of C₂H₂ and CO₂ on JNU-4a for an equimolar C₂H₂/CO₂ mixture (6.0 mL min⁻¹) under humid conditions (10% RH). Empty squares depict the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column (right y-axis). (c) Desorption curves of C₂H₂ and CO₂ after breakthrough equilibrium with helium gas (10.0 mL min⁻¹) sweeping at 298 K. Solid green squares represent the ratios of C₂H₂/CO₂ in the desorbed gas mixture. The yellow area represents C₂H₂ with over 98% purity (85 cm³ g⁻¹) collectible during the desorption. (d) Comparison of the C₂H₂ productivity estimated from desorption curves for an equimolar C₂H₂/CO₂ mixture.

3. Conclusion

We have implemented a simple concept of two accessible OMSs per metal in MOFs by employing square-planar mononuclear centers as nodes and tetrahedral organic linkers as spacers. The 4,4-connected pts topology network (JNU-4) exhibits excellent C₂H₂ adsorption capacity, particularly at 298 K and 0.5 bar (200 cm³ g⁻¹). Computational modeling studies reveal that C₂H₂ molecules indeed preferentially occupy both OMSs of the square-planar centers. Breakthrough experiments demonstrate by far the largest C₂H₂ capture capacity (160 cm³ g⁻¹) from a 50 : 50 C₂H₂/CO₂ mixture and fuel-grade C₂H₂ production (105 cm³ g⁻¹, ≥98%) upon desorption. This work illustrates a straightforward design strategy to maximize the accessible OMSs in MOFs for C₂H₂ adsorption. Further optimization of the spacing linkers with the assistance of computational simulations would be key to advancing in this direction to address the challenging C₂H₂/CO₂ separation.

Data availability

The data that support the plots within this paper and other finding of this study are available from the corresponding authors upon reasonable request.

Author contributions

W. L. and D. L. conceived and designed the research. H. Z., W. L., and D. L. co-wrote the manuscript. H. Z. and X.-J. X. planned and executed the synthesis, characterization, and gas separation studies. Y. W. performed the theoretical simulations. D. L. and R.-J. W. carried out the structural analyses. All authors participated in and contributed to the preparation of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21731002, 21975104, 21901085, 22271120, 22201101, and 22150004), Guangdong Major Project of Basic and Applied Research (No. 2019B030302009), Guangdong Basic and Applied Basic Research Foundation (No. 2020A1515011005), Innovation Team Project in Guangdong Colleges and Universities (No. 2021KCXTD009), National Postdoctoral Program for Innovative Talent (No. BX20220132 and 2022M711327), and Outstanding Innovative Talents Cultivation Funded Programs for Doctoral Students of Jinan University (No. 2021CXB003).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2160960. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc04324f

‡ These authors contributed equally.

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