Distortable functionalized ligand implantation in ultra-microporous MOFs for efficient C₂H₂ purification

Abstract

Adsorption-based separation techniques are pivotal in the purification of C₂H₂, leveraging their eco-friendly and energy-efficient attributes. However, the reusability of adsorbents is highly restricted by the trade-off between adsorption capacity and adsorption enthalpy, which is required to be urgently addressed. Herein, we demonstrated that the implantation of distortable ligands with functional groups is efficient in solving the above issue. The functional groups provided binding sites for C₂H₂ adsorption, while the distortable ligands optimized the pore structures to reduce adsorption enthalpy, thus balancing the trade-off between adsorption capacity and enthalpy. A series of ultra-microporous MOFs was successfully constructed by employing different pyridine ligands. The adsorption capacity of SNNU-504 after optimization was effectively enhanced, while the adsorption enthalpy hardly increased due to the adjustable ligand torsion angle. Breakthrough experiments and GCMC simulations further verified the potential C₂H₂/CO₂ separation applications.

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Introduction

The separation of CO₂ from C₂H₂ is of paramount importance in various industrial processes, particularly in the purification of acetylene for applications in chemical synthesis and welding.^1,2 However, the similar physical properties of CO₂ and C₂H₂, such as their molecular size and boiling points, lead to a huge challenge in their separation.^3–8 Traditional methods, such as cryogenic distillation and solvent extraction, are energy-intensive and inefficient.^9–12 Therefore, it is imperative to explore novel approaches to address the current challenges associated with the separation of C₂H₂/CO₂.

Metal–organic frameworks (MOFs) with a high surface area and tunable pore structures have emerged as promising candidates for widespread applications (e.g., gas adsorption, separation, catalysis, and fluorescence sensing).^13–23 Due to their tunable pore structures, functionalized binding sites, and surface chemistries, MOFs are ideal adsorbents for the selective separation of gas mixtures.²⁴ To achieve selective gas uptake capacity and high separation efficiency, a variety of strategies have been explored in reported works.^25–31 Among these methods, pore engineering, including pore size regulation and pore environment modulation, which is effective for achieving molecular sieve separation and introducing preferential binding sites, stands out.^2,6,32–36 However, the high adsorption capacity via pore engineering is achieved by the improved adsorption enthalpy, which inevitably leads to the difficulty in desorption and poor reusability.^37–43 For example, Z. Niu et al. proposed a novel super-strong C₂H₂ nano-trap based on multi-site interactions,⁴⁴ which can efficiently capture C₂H₂ molecules due to the strong interaction between the adjacent open metal sites of the nano-trap and C₂H₂. However, the ultra-strong acetylene nano-trap produces a higher adsorption enthalpy of C₂H₂ (79.1 kJ mol⁻¹) than that of SiF₆²⁻ series compounds,⁴⁵ which also limits this material's practical industrial applications due to its weak recycling. F. Zheng et al. reported three isostructural frameworks with different pore sizes (namely CPL-n),⁴⁶ indicating that the adsorption heat of C₂H₂ in CPL-2 (30.8 kJ mol⁻¹) and CPL-5 (31.3 kJ mol⁻¹) with larger pore sizes is lower than that in CPL-1 (40.2 kJ mol⁻¹) with a smaller pore size because of the strongest confinement effect in the smallest channel; whereas the large pores inevitably reduced the adsorption and separation efficiency. Hence, it is of great significance to break the trade-off between adsorption capacity and adsorption enthalpy to achieve both high adsorption performance and excellent reusability.

In this study, we demonstrated that implanting a distortable functionalized ligand into ultra-microporous MOFs is an efficient strategy to balance the trade-off between adsorption capacity and adsorption enthalpy. The enlarged pore size reduces adsorption enthalpy, while the functionalized ligand offers binding sites to improve adsorption capacity (Scheme 1). Due to the steric hindrance, different torsion angles can adjust the pore sizes to reduce adsorption enthalpy, while the functionalized sites provide active sites for gas adsorption to enhance the adsorption ability. A series of trinuclear nickel metal–organic frameworks (Ni-MOFs) were constructed utilizing the ligand H₃PTO, nickel as the metal component, and various dipyridine ligands as auxiliary ligands. SNNU-504 after the introduction of binding sites shows the highest uptake for C₂H₂ among the four ultra-microporous MOFs, being 2.1 times that of SNNU-501 without an increase of adsorption enthalpy, because of the slight increase in the aperture caused by the ligand torsion angle. In addition, SNNU-504 after optimization achieves a new breakthrough in separation performance for the C₂H₂ mixture than the other three ultra-microporous MOFs, which can be verified by GCMC simulations.

Scheme 1 The schematic diagram of the pore structure optimization strategy.

Experimental section

Materials and methods

Nickel nitrate nonahydrate (Ni(NO₃)₂·9H₂O, Aladdin), nickel chloride hexahydrate (NiCl₂·6H₂O, Aladdin), trans(p-carbonyl)-triphenylphosphine (H₃PTO, ChouHe Pharm), 4,4-bipyridine (4,4-bpy, TCI), N,N-dimethylformamide (DMF; Sinopharm), N,N-dimethylacetamide (DMA; Sinopharm), ethanol (EtOH, Sinopharm), 1,4-dioxane (1,4-DO; Aladdin), tetra-fluoroboric acid (HBF₄, Aladdin), isopropyl alcohol (iPA; Sinopharm), and acetone (Sinopharm) are all commercially available; 3,5-bis(3-pyridyl)-4-amino-1,2,4-triazole (3-apt) and 3,5-bis (4-pyridyl)-4-amino-1,2,4-triazole (4-apt) were synthesized in the lab and the methods were as follows.

Synthesis of 3-apt. 2.6 g of 3-cyanopyridine (25 mmol), 3 mL of 85% hydrazine hydrate and 1 mL of anhydrous ethanol were added to a 25 mL polytetrafluoroethylene reactor liner, then covered with a stainless steel reactor shell, and placed in an oven at 120 °C for 24 hours. After the reaction, the oven was cooled to room temperature and washed with anhydrous ethanol, and 3-apt ligand was obtained by drying.

Synthesis of 4-apt. The methods were the same as in the synthesis of the 3-apt ligand except for 4-cyanopyridine being used as the raw material.

Synthesis of compounds

Synthesis of SNNU-501. H₃PTO (9 mg, 0.02 mmol) and Ni(NO₃)₂·9H₂O (29 mg, 0.1 mmol) were placed into a reactor of 20 mL with PTFE material, and 5 mL of DMA and 0.2 mL of H₂O were added to the mixture. Following a 10 minute ultrasound treatment, the reactants were fully dissolved, resulting in a slightly green solution. The reactor was then sealed and placed in an oven, where it was heated to 150 °C for three days. After cooling to room temperature, SNNU-501 was obtained by washing the crystal with fresh DMF.

Synthesis of SNNU-502. A mixture consisting of H₃PTO (9 mg, 0.02 mmol), NiCl₂·6H₂O (24 mg, 0.1 mmol) and 4,4-bpy (10 mg, 0.1 mmol) was placed in a 20 mL reactor. Subsequently, 3 mL of DMF, 2 mL of EtOH, 1 mL of 1,4-DO and 100 μL of HBF₄ were added to the reactor. Following a 10 minute ultrasound treatment, the reactants were fully dissolved, resulting in a slightly green solution. The reactor was then sealed and placed in the oven and heated to 150 °C for 4 days. After cooling to room temperature, SNNU-502 was obtained by washing with fresh DMF.

Synthesis of SNNU-503. H₃PTO (9 mg, 0.02 mmol), Ni(NO₃)₂·9H₂O (29 mg, 0.1 mmol) and 3-apt (10 mg, 0.04 mmol) were placed in a reactor of 20 mL PTFE material, and 5 mL of DMA and 0.2 mL of H₂O were added. Following a 10 minute ultrasound treatment, the reactants were fully dissolved, resulting in a slightly green solution. The reactor was then sealed and placed in the oven and heated to 150 °C for 3 days. After cooling to room temperature, SNNU-503 was obtained by washing with fresh DMF.

Synthesis of SNNU-504. H₃PTO (9 mg, 0.06 mmol), Ni(NO₃)₂·9H₂O (29 mg, 0.1 mmol) and 4-apt (10 mg, 0.04 mmol) were placed in a reactor of 20 mL PTFE material, and 5 mL of DMA and 0.3 mL of HBF₄ were added. Following a 10 minute ultrasound treatment, the reactants were fully dissolved, resulting in a slightly green solution. The reactor was then sealed and placed in the oven and heated to 150 °C for 3 days. After cooling to room temperature, SNNU-504 was obtained by washing the crystal with fresh DMF.

X-ray single crystal structure determination

The single crystal structures of SNNU-501/502/503/504 were obtained using a Bruker D8 Venture (PHOTON II CPAD detector, Mo Kα as an X-ray source for SNNU-501; Cu Kα for SNNU-502/504; Ga Kα for SNNU-503) single crystal X-ray diffractometer. The structural determination and refinement were conducted via the direct method employing the SHELXS program in conjunction with OLEX software. All non-hydrogen atoms were refined using anisotropic parameters. Comprehensive crystallographic data and structure refinement parameters for the four compounds are presented in Table S1,† while selected bond lengths and angles are detailed in Tables S2–4.†

Powder X-ray diffraction (PXRD)

The crystallinity and phase purity of the sample were assessed utilizing powder X-ray diffraction (PXRD) with a Rigaku Mini Flex 600 instrument, employing Cu Kα radiation. The analysis was conducted at a working voltage of 40 kV and a current of 40 mA, within a scanning range of 5 to 50 degrees, at a scanning speed of 10 degrees per minute.

Thermogravimetric analysis (TGA)

The thermogravimetric analyses (TGA) were carried out on a NETSCHE STA-449C thermal analyzer under a nitrogen atmosphere (temperature range, 25–600 °C) at a heating rate of 2 °C min⁻¹.

Gas adsorption

Prior to the gas adsorption test, the synthesized sample was soaked in a low-boiling solvent for 4 days (SNNU-501: iPA; SNNU-502/503/504: acetone). In the meantime, the fresh solvent was changed twice a day, and then the activated sample was obtained by heating and pumping at 60 °C under vacuum conditions for 10 hours. To evaluate the permanent porosity of SNNU-501/502/503/504, CO₂ adsorption measurements were performed at 195 K. Furthermore, the adsorption capacities of the activated samples for single-component gases (CO₂, C₂H₂, C₂H₄, C₂H₆) were tested at different temperatures to evaluate the feasibility of the four compounds as adsorbent materials for separating CO₂ from the C₂H₂/CO₂ mixture, and the purity of the gases used for adsorption experiments is 99.999%.

Ideal adsorbed solution theory (IAST) calculations

The IAST calculations were used to predict the selectivity of binary mixtures from experimental pure-gas isotherms. To perform the integrations required by IAST, single component isotherms should be fitted with the Langmuir–Freundlich (LF) equation to give the best fit to the experimental data. Subsequently, the parameters derived from this equation were utilized to calculate IAST selectivity, thereby assessing the separation efficiency of the mixture.

Breakthrough experiments

To further verify the actual separation efficacy of the binary C₂H_x/CO₂, breakthrough experiments were conducted for activated SNNU-501/502/503/504 (with respective weights of SNNU-501: 0.8064 g; SNNU-502: 0.3944 g; SNNU-503: 0.4396 g and SNNU-504: 0.4356 g) at 298 K. The activated samples were placed in stainless steel columns measuring 12 cm in length, with an inner diameter of 0.44 cm and an outer diameter of 0.6 cm. Prior to each test conducted at ambient pressure, the columns were purged with helium gas for a duration of 12 hours at room temperature, utilizing a flow rate of 20 mL min⁻¹ to eliminate air and any residual gas molecules present in the breakthrough column. Subsequently, the C₂H_x/CO₂ (50/50) mixture with a total flow rate of 2 mL min⁻¹ was fed into the column for testing.

Grand canonical Monte Carlo (GCMC) simulations

The determination of guest molecular adsorption sites in the MOF skeleton will provide effective guidance for the design of gas storage and separation materials. Hence, GCMC theoretical simulations were adopted to calculate the interaction and adsorption behavior of these four compounds on C₂H₂ at the molecular level and obtain the possible adsorption sites and density distribution in the skeleton to explore the adsorption and separation mechanism of C₂H₂/CO₂. All simulations were performed in Materials Studio 8.0. Using a 2 × 2 × 2 supercell as the simulation box, the Forcite module is used to optimize the crystal cell and guest molecules. The combined average of the gas adsorption sites was calculated by using the equilibrium steps of 1 × 10⁷ and the generation steps of 1 × 10⁷. The Lennard–Jones interaction has a cutoff radius of 18.5 Å and uses a universal force field. Additionally, long-distance electrostatic interactions are treated by the Ewald summation technique.

Results and discussion

Design and structural analysis

As discussed above, MOFs with enlarged pore sizes and functionalized binding sites are promising to balance the trade-off between adsorption efficiency and adsorption enthalpy. Here, we demonstrate that distortable functionalized ligands are promising to achieve both pore size and binding site modulation. Three ligands (4,4-bpy, 3-apt, and 4-apt) with different distorted angles were used to construct the desirable MOF frameworks (Scheme 2). A series of [Ni₃O(COO)₆] trinuclear cluster-based metal–organic framework materials with the same inorganic secondary building units were synthesized by incorporating various pyridines as secondary second ligands into the parent framework composed of the tricarboxylic acid ligand H₃PTO and metal Ni. Single crystal-X-ray diffraction analysis revealed that the parent framework, designated as SNNU-501, crystallized in the orthorhombic system within the Pnma space group (see Table S1†).

Scheme 2 The formation of the ligand torsion angle in the compounds after utilizing the different second ligands.

As illustrated in Fig. 1a, the inorganic secondary building unit [Ni₃(μ₃-O)(COO)₆] consists of six interconnected Ni²⁺ centres. Each Ni centre is coordinated to oxygen atoms from four distinct PTO³⁻ carboxylic groups, resulting in an average Ni–O coordination bond length of 2.082 Å. This arrangement leads to the formation of a planar quadrilateral configuration in the equatorial plane. In addition, the bridged μ₃-O atom (the average bond length of the Ni-μ₃-O coordination bond is 1.999 Å) and an oxygen atom from the water molecule (the average bond length of the Ni–O coordination bond is 2.170 Å) are linked to the centre of Ni. These connections occur in a manner that is perpendicular to the equatorial plane, thereby establishing a conventional Ni trinuclear cluster with an octahedral geometry, with an average interatomic distance of 3.462 Å between the nickel atoms. The asymmetric unit is shown in Fig. S1a.† Each H₃PTO ligand is interlinked with three [Ni₃(μ₃-O)(COO)₆] secondary building units (SBUs), and each SBU is further coordinated to six distinct PTO³⁻ ligands, resulting in the formation of a three-dimensional porous framework designated as SNNU-501 (Fig. S1b†). This framework exhibits two distinct sizes of ultra-microporous windows along the [001] direction. The smaller window is formed by the interaction of four separate [Ni₃(μ₃-O)(COO)₆] SBUs, with the P=O groups in the independent ligand on both sides pointing to the interior of the quadrilateral channel, resulting in a window size of 3.8 Å. The larger channel consists of four different H₃PTO ligands connecting two different [Ni₃(μ₃-O)(COO)₆] SBUs, forming a pore window with a diameter of 5.6 Å (Fig. 1a). In this structure, the directivity of the trinuclear clusters makes the channels contain abundant coordination water molecules, which exposes more metal unsaturated coordination sites in the pores after activation and provides more possibilities for the modification of the pores.

Fig. 1 (a) The structure and topology simplified diagram of SNNU-501; (b–d) the secondary ligand; and the analysis of the structure (e–g) and different pockets (h–j) for SNNU-502/503/504.

The trinuclear metal Ni cluster and the triangular carboxylic acid ligand H₃PTO are simplified with octahedral and tetrahedral geometries (one vertex of the green tetrahedron is an uncoordinated oxygen atom in the P=O group), respectively, forming the pore structure illustrated in Fig. 1. From a topological perspective, the [Ni₃(μ₃-O)(COO)₆] SBU and the PTO³⁻ ligand in the structure are considered to be three-connected and six-connected nodes, respectively. Consequently, SNNU-501 can be represented as a three-dimensional network characterized by 3,6-connectivity, with the corresponding topological symbol being {4^2.6}2{4^4.6^2.8^9}. It exhibits two distinct types of pores with diameters of 3.8 Å and 5.6 Å. Taking further SNNU-501 as the parent structure, a series of trinuclear cluster-based Ni-MOFs (SNNU-502/503/504, Fig. 1e–g) were formed by the introduction of different pyridine derivatives as second ligands (Fig. 1b–d, simplified as purple-red rods) after solvent modulation during the process of their synthesis, resulting in a more diverse pore environment across the various structures. It is noted that the shapes and sizes of the different kinds of pores and windows in this series of compounds will be determined by the size of the secondary ligand added and the coordination geometry. The construction of SNNU-502 is achieved through the introduction of the 4,4-bpy ligand, wherein the nitrogen atoms located at both ends of the 4,4-bpy ligand occupy two adjacent open metal sites in pocket B to connect two opposing trinuclear metal Ni clusters, resulting in pocket B being blocked and thus keeping pocket A about 2 Å in size (Fig. 1h).

The introduction of 3-apt into the structure generates SNNU-503, being isomorphic to SNNU-501, in which 3-apt connects two similar trinuclear Ni metal clusters. However, it is noteworthy that pocket B does not remain blocked, so SNNU-503 still produces two different pore sizes (Fig. 1i). The size of pocket A in SNNU-501 was reduced from 3.8 Å to 2.6 Å, and the size of aperture B was reduced from 5.6 Å to 3.4 Å. SNNU-502 and SNNU-503 do not have the same pore channels due to the insertion of the second ligand at a different connection location.

SNNU-504 was constructed by adding the ligand of 4-apt, in which a two-dimensional layer with the P=O group on the surface is formed by the triangular ligand PTO³⁻ and the trinuclear Ni cluster, and was connected to 4-apt to form a pillar-layer structure with pore sizes of 3.6 Å and 6 Å (Fig. 1g, j and Fig. S2†). The structural difference between the pillar-layer structures of SNNU-504 and SNNU-503 can be attributed to the convergence of the linear 4-apt ligand connected adjacent trinuclear cluster along the C-axis, maybe caused by differences in ligand torsion angles (Fig. 2a–c).

Fig. 2 (a–c) The structural analysis for SNNU-501/502/503/504 along the c axis and (d–f) the PXRD patterns of four compounds.

Among the four compounds, SNNU-503 and SNNU-504 have abundant N atoms by the introduction of –NH₂ functional groups and pyridine ring, which are expected to provide more binding sites for C₂H₂ molecules and achieve effective separation for the C₂H₂/CO₂ mixture.

The PXRD patterns of SNNU-501/502/503/504 are shown in Fig. 2d–f, indicating that the diffraction peak positions of the synthesized four compounds were basically consistent with the simulation of single crystal data, which proved that the test samples were of pure phase with high crystallization. Although SNNU-502 and SNNU-503 are isostructural, due to the slight difference in pore sizes, the XRD peaks of SNNU-502 are slightly shifted. According to the PXRD patterns after adsorption and breakthrough experiments, the main peak of the four compounds is still maintained, indicating that they can keep their frame structure after the corresponding experiments and have good stability.

Thermogravimetric analysis (TGA)

The TGA curves (Fig. S3†) show that the MOF skeletons of these four compounds have similar thermal stability, which can be stabilized to about 360 °C. The compound of SNNU-501 has obvious weight loss before 170 °C, while the behaviours of SNNU-502, SNNU-503 and SNNU-504 occur before 100 °C, and the weight loss is being reduced from 18% of SNNU-501 to about 13%, which may be due to the decrease of the number of solvent molecules contained in the structure after the addition of a secondary ligand for pore partitioning. The weight loss of them before 170 °C can be attributed to the loss of free solvent molecules and coordination water molecules in the pore, as the temperature continues to rise to nearly 360 °C, the ligands decompose, and the skeleton structure of the crystal gradually collapses. All these four compounds have a similar decomposing temperature at nearly 360 °C because of the same component of the frame structure composed of metal Ni and the carboxylic acid ligands.

Gas adsorption

Before the test of gas adsorption performance, the guest molecule occupied volumes in the structure of SNNU-501/502/503/504 were calculated using PLATON software to be 8302.2, 6028.7, 5635.5 and 6593.6 ų, respectively, accounting for 72.1%, 65.5%, 52.5% and 61.5% of the total crystal volume. The permanent porosity of the four compounds was characterized by CO₂ adsorption at 195 K, under ambient pressure.

As shown in Fig. S4,† the CO₂ uptakes of SNNU-501, 502, 503 and 504 are as follows: 63.0 cm³ g⁻¹, 58.5 cm³ g⁻¹, 80.2 cm³ g⁻¹ and 116.7 cm³ g⁻¹, and the corresponding BET specific surface areas were 278 m² g⁻¹, 260 m² g⁻¹, 357 m² g⁻¹ and 518 m² g⁻¹, respectively. Among the four compounds, SNNU-504 showed the highest gas adsorption capacity at 195 K and the highest BET specific surface area, indicating that it has the most excellent adsorption capacity and separation potential. Fig. 3a–c show that the adsorption capacities of CO₂, C₂H₂, C₂H₄ and C₂H₆ by SNNU-501, SNNU-502, SNNU-503 and SNNU-504 at 298 K were 18.5/20.7/12.7/12.3 cm³ g⁻¹, 22.5/21.9/16.6/12.1 cm³ g⁻¹, 19.1/23.8/16.6/13.4 cm³ g⁻¹ and 29/43.1/16.9/24.5 cm³ g⁻¹, respectively. It is not difficult to notice that the C₂H₂ adsorption capacity at 298 K of SNNU-504 is 2.1 times that of the parent framework SNNU-501 without modification. The corresponding adsorption isotherms at 273 K were 28/35/22/18.6 cm³ g⁻¹, 36.8/40.8/25.4/17.3 cm³ g⁻¹, 32/38/26.5/19.6 cm³ g⁻¹ and 48.9/59.7/39.6/34.5 cm³ g⁻¹, respectively (Fig. S5†). Obviously, the adsorption capacity of the four compounds for C₂H₂ and CO₂ molecules with a smaller kinetic diameter (∼3.3 A) is higher than that of C₂H₄ and C₂H₆ with a larger molecular size, and these four compounds show the highest uptake gap for C₂H₂/CO₂ than that for C₂H₄/CO₂ and C₂H₆/CO₂. This illustrates the greater C₂H₂/CO₂ adsorption and separation potential in skeletons than for the other two mixtures.

Fig. 3 The gas adsorption curves at 298 K: (a) SNNU-501; (b) SNNU-502/503; and (c) SNNU-504; (d) the comparison of uptakes under 0.15 bar at 298 K; (e) the adsorption enthalpy of C₂H₂ and CO₂; and (d) the selectivity ratio of SNNU-504 for C₂H₂/CO₂.

Moreover, SNNU-502 and SNNU-503 after the introduction of secondary ligands showed slightly higher gas adsorption capacity than the parent skeleton of SNNU-501, but this change was limited due to the limitation of the pore sizes. However, SNNU-504 exhibited the highest gas adsorption capacity for both CO₂ molecules and low carbon hydrocarbon molecules (C₂H₂, C₂H₄, C₂H₆) mainly due to the synergistic effect of a larger pore size, a Lewis N site and an aromatic ring caused by the secondary ligand.

In addition, the adsorption curve of C₂H₂ is steeper in the low-pressure range in comparison with the CO₂ molecule, and Fig. 3d shows the uptakes of C₂H₂ and CO₂ under lower pressure (0.15 bar) at 298 K, showing the stronger interaction with the skeleton for C₂H₂ than the CO₂ molecules. The adsorption gap of both for SNNU-501 is 6.9 cm³ g⁻¹ and for SNNU-502/503/504 is 9.8/9/13 cm³ g⁻¹, respectively, which can conclude that compared with SNNU-501, the modification of the pore environment and the introduction of high-density N atoms in the structure increased the adsorption capacity gap to C₂H₂ and CO₂, especially SNNU-504 shows the biggest uptake gap and excellent separation potential for C₂H₂/CO₂. The diagram of uptakes of C₂H₂ and CO₂ at 273 K is shown in Fig. S6.†

To evaluate the affinity of the SNNU-501/502/503/504 skeleton structures for different gases, the adsorption enthalpies (–Q_st) were calculated using the virial equation ((e1) and (e2) in the ESI†). From Fig. 3e, the adsorption enthalpy of CO₂ for SNNU-501, SNNU-502, SNNU-503 and SNNU-504 under zero loading was 16.5, 27.3, 42.5 and 38.8 kJ mol⁻¹, respectively, and the corresponding C₂H₂ adsorption was 47.2, 59.0, 57.8 and 47.6 kJ mol⁻¹, indicating that the force between C₂H₂ molecules and the skeleton is significantly stronger than CO₂ and showing the great C₂H₂/CO₂ mixture separation potential. It also can be found that the adsorption heat for C₂H₂ of SNNU-502/503 with the modification of pore size and the introduction of N atom increases significantly compared with SNNU-501, while SNNU-504 was almost unchanged, indicating that the value of adsorption heat would decrease with the increase of pore size after the introduction of binding sites in the ultra-microporous structure. The adsorption enthalpies (47.6 kJ mol⁻¹) of SNNU-504 for C₂H₂ are higher those of MOF-NH₂ (16.7 kJ mol⁻¹),⁴⁷ MIL-160 (31.8 kJ mol⁻¹),⁴⁸ and BUT-155 (30.7 kJ mol⁻¹),⁴⁹ but still lower than those of many materials reported for selective adsorption of C₂H₂/CO₂, including the ATC-Cu (79.1 kJ mol⁻¹),⁵⁰ CuI@UiO-66(COOH)₂ (74.5 kJ mol⁻¹),⁵¹ Ni(4-DPDS)₂WO₄ (77.2 kJ mol⁻¹),¹ SU-102-Li⁺ (72.0 kJ mol⁻¹),⁵² NKMOF-1-Ni (60.3 kJ mol⁻¹),⁵³ ZNU-1 (54.0 kJ mol⁻¹),⁵⁴ and MUF-17 (49.5 kJ mol⁻¹).⁵⁵ The corresponding adsorption enthalpies of C₂H₄ and C₂H₆ under the same conditions are also shown in Fig. 4, and the values are 42.4/32.2, 33.6/22.9, 41.1/33.5 and 39.9/37.7 kJ mol⁻¹ for SNNUN-501, SNNU-502, SNNU-503, and SNNU-504, respectively. Fig. S7–S10† show the fitting parameters of four compounds. The adsorption enthalpies for C₂H₂ first decreased and then increased during gas adsorption, which was caused by the steric resistance effects under low pressure and the binding site effects under high pressure.

Fig. 4 The calculated adsorption enthalpy of four compounds for different gases: (a) SNNU-501; (b) SNNU-502; (c) SNNU-503; and (d) SNNU-504.

The separation selectivity of the equimolar quantity of C₂H_x/CO₂ was calculated according to the ideal adsorption solution theory (IAST) ((e3)–(4) in ESI†), and the fitting parameters are shown in Fig. S11–14.† From Fig. 3f, the selectivity ratio of the equimolar quantity of C₂H₂/CO₂ for SNNU-501, SNNU-502, SNNU-503 and SNNU-504 at 298 K was 3.34, 4.22, 4.46 and 4.55, respectively, and SNNU-504 shows the highest selectivity ratio of the C₂H₂/CO₂ mixture among these four compounds. In addition, the IAST selectivity of the four compounds for C₂H₄/CO₂ and C₂H₆/CO₂ (1/1, v/v) shows that SNNU-501 and SNNU-502 had no obvious separation selectivity for either mixture (Table S5†). SNNU-503 and SNNU-504 have low IAST separation selectivity for the C₂H₄/CO₂ mixture, and SNNU-504 showed a selectivity ratio of 1.28 for C₂H₆/CO₂.

The further separation selectivity for the C₂H₂/CO₂ mixture with different components at 298 K was calculated, and the results are shown in Fig. S15.† In addition, SNNU-504 still showed good separation selectivity even for 1% CO₂ in a binary C₂H_x/CO₂ mixture (Fig. 3f) and can be used for removing trace amounts of CO₂ from C₂H₂. In general, all the results above show that SNNU-504 has excellent gas uptakes and C₂H₂/CO₂ selectivity compared with SNNU-501 without functional group modification and SNNU-502/503 with smaller size, maybe due to the synergistic effect of the suitable pore size and –NH₂ functional group.

Breakthrough experiments

Fig. 5 shows that C₂H₂ and CO₂ molecules pass through the penetration column at the same time in both of the SNNU-501 and SNNU-502, and had no practical separation effect. In contrast, SNNU-503/504 can effectively separate the C₂H₂/CO₂ mixture, and SNNU-504 demonstrates superior performance than that of SNNU-503 with a lower purity.

Fig. 5 Breakthrough curves of C₂H₂/CO₂ for four compounds at 298 K, under 1 bar with a gas flow rate of 2 mL min⁻¹: (a) SNNU-501; (b) SNNU-502; (c) SNNU-503; and (d) SNNU-504.

According to Fig. 5c, CO₂ is preferentially detected from the outflow gas at 15.4 min g⁻¹ after adsorption saturation, while C₂H₂ reaches the breakthrough point and passes through the breakthrough column at 24.5 min g⁻¹. Hence, the breakthrough separation time of SNNU-504 for C₂H₂/CO₂ is about 9.1 min g⁻¹, indicating that it can effectively separate CO₂ from the mixture of C₂H₂/CO₂. However, SNNU-501 with limited active sites and SNNU-502/503 with the small pore size limit the adsorption capacity of guest molecules, so that the three compounds reach adsorption saturation in a short time to pass through the breakthrough column at the same time. In addition, the flow rate of C₂H₂ gas flowing out of the breakthrough column due to the strong force of the skeleton on C₂H₂ is smaller than that of CO₂, showing greater mass transfer resistance, which is consistent with the results of adsorption heat.

Furthermore, the breakthrough experiments of four compounds for the C₂H₄/CO₂ and the C₂H₆/CO₂ mixture were conducted, and the results show that they have no obvious separation performance (Fig. S16†). In summary, the great C₂H₂/CO₂ separation ability of SNNU-504 among these four compounds was further verified by the breakthrough experiments, which is in keeping basically with the results of selectivity calculations.

GCMC simulations

GCMC theoretical simulations were adopted to calculate the adsorption behaviour of these four compounds on C₂H₂ and CO₂ at the molecular level. Fig. S17† shows that more C₂H₂ molecules were adsorbed in these four frameworks than CO₂ molecules at 298 K, and the phenomenon of higher packing density for C₂H₂ molecules is in accordance with the results of the highest adsorption capacity for C₂H₂ than CO₂ measured by the experiments among the four compounds.

The further simulations between SNNU-504 and C₂H₂ molecules under different pressures at 298 K (Fig. S18†) show that C₂H₂ molecules are preferentially adsorbed around the [Ni₃O(COO)₆] trinuclear cluster with the open metal sites through H–C_C2H2⋯Ni under low pressure; with the increase of pressure, the C–H_C2H2⋯π interaction between the π bond on the benzene ring from the H₃PTO ligand and the H atom from the C₂H₂ molecule becomes the second adsorption site (site II) in the quadrilateral channel, it makes the density distribution of acetylene molecules mainly leaning towards the four benzene rings inside the pore channel. When the pressure reaches 0.8 bar, a third kind of interaction site would be generated, and it could be attributed to the hydrogen bond of C–H_C2H2⋯N between the C₂H₂ molecule and the N atom from the pyridine ligand. While irregular quadrilateral pore channel composed from the P=O functional group of carboxylate ligand PTO and pyridine ligand (4-apt) generate the C–H_C2H2⋯O and C–H_C2H2⋯π interaction, named as for sites IV. Fig. 6(a and b) shows the different sites for C₂H₂ and CO₂ in SNNU-504.

Fig. 6 Density distributions of SNNU-504 for CO₂ (a) and C₂H₂ (b). GCMC simulations of adsorption binding sites for C₂H₂: (c) SNNU-501; (d) SNNU-502; (e) SNNU-503; and (f) SNNU-504 (OMS: open metal sites; LBS: Lewis basic sites).

The interaction sites in detail for C₂H₂ molecules in the structure of four compounds are shown in Fig. 6(c–f), and SNNU-501/502/503 show the weaker interaction in comparison with SNNU-504 under 1 bar at 298 K. It is worth noting that in addition to H–C_C2H2⋯Ni (3.4712 Å), C–H_C2H2⋯π (3.0958–3.8455 Å), C–H_C2H2⋯N (3.47 Å) and C–H_C2H2⋯O (3.2124–3.5880 Å) that exist in SNNU-504 (Fig. 6f), more weaker interaction sites between the ligand and C₂H₂ molecules linked by the bond of C–H_PTO⋯C_C2H2 appear in SNNU-501/502/503 (Fig. 6c–e), which verified the stronger interaction between SNNU-504 and guest C₂H₂ molecules than the other three compounds and further explained the excellent adsorption and separation performance of SNNU-504 for C₂H₂/CO₂.

Conclusions

In summary, we have successfully synthesized a series of ultra-microporous MOF materials (SNNU-501/502/503/504) by adding different pyridine ligands, realizing the regulation of pore size and the introduction of interaction sites. To solve the trade-off effect between adsorption capacity and adsorption enthalpy in ultra-microporous MOF structures, the adsorption capacity of C₂H₂ was increased by introducing binding sites into the parent frame of SNNU-501 with the ultra-microporous structure on the one hand. On the other hand, we optimized the pore size by the difference of the ligand torsion angle, thereby reducing the positive correlation between adsorption capacity and adsorption enthalpy in the ultra-microporous structure. The results show that the adsorption capacity of SNNU-504 after optimization was 2.1 times that of unoptimized SNNU-501 at 298 K with hardly an increase in adsorption enthalpy. In addition, SNNU-504 exhibited the best acetylene mixture separation performance and also achieved a new breakthrough compared with the other three compounds. This work also provides a reference for solving the trade-off effect in the process of C₂H₂ adsorption and adsorption enthalpy in ultra-microporous MOF materials.

Author contributions

Hong-Juan Lv: methodology, data collection and analysis, and writing the original draft. Yunhui Zhai: investigation and formal analysis. Ying-Ying Xue: data analysis. Jiao Lei: investigation and data analysis. Wenyu Yuan: checking, editing and funding acquisition. Quan-Guo Zhai: supervision, checking, and funding acquisition.

Data availability

All data are available in the main text or the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (22071140, 52302103 and 22172118), the China Postdoctoral Science Foundation (2022TQ0200), the Natural Science Foundation of Shaanxi Province (2021JLM-20 and 2023-JC-QN-052), and the Fundamental Research Funds for the Central Universities (GK202101002).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2414499, 2414500, 2414501 and 2416694. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00475f

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