Two different pore architectures of cyamelurate-based metal–organic frameworks for highly selective CO₂ capture under ambient conditions

Abstract

In the context of porous coordination materials toward CO₂ capture and separation, two new metal–organic frameworks termed IRH-6 and IRH-7 were synthesized with square and rhombic microchannel pores, respectively. These materials exhibit high CO₂ uptakes of 2.67 mol kg⁻¹ (IRH-6) and 2.78 mol kg⁻¹ (IRH-7) at 100 kPa and 298 K. Grand canonical Monte Carlo simulation demonstrated strong non-covalent interactions between quadripolar CO₂ molecules and these nitrogen-rich frameworks. CO₂/CH₄ (50 : 50), CO₂/N₂ (15 : 85), and CO₂/H₂ (15 : 85) gas mixtures were investigated by ideal adsorbed solution theory and showed excellent CO₂ selectivity under ambient conditions for both porous materials. In particular, a remarkable increase in the CO₂ selectivity to 102 for IRH-7 over 31 for IRH-6 was observed for the CO₂/CH₄ binary mixture, which highlights the effect of pore aperture modification on preferential CO₂ uptake over other conventional gases.

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Introduction

Anthropogenic carbon dioxide (CO₂) emissions have attracted much concern over recent decades in order to find practical solutions to critical climate change issues.¹ Worldwide programs and policies have been implemented to tackle climate challenges, which significantly reduce anthropogenic CO₂ emissions. In parallel, scientists have developed promising technologies for CO₂ capture and sequestration, and have developed alternative energy systems that work with renewable energy to reduce CO₂ emissions.^2–4 Impressive efforts have been invested in developing efficient CO₂ capture materials such as inorganic^5–7 and organometallic porous materials.^8–11

As an emerging subclass of porous coordination polymers, metal–organic frameworks (MOFs) have been widely studied for their remarkable features such as high specific surface areas, tunable porosity, thermal and chemical stability, and so forth.¹² The functionalization of these materials allows better interactions with guest molecules and improves their retention as well as their catalytic conversion, which makes MOFs promising materials for diverse applications.¹³ Over the last few decades, MOFs have been reported as successful platforms for several valuable applications such as gas capture and sequestration, catalysis and chemical conversion, electrochemical applications, chemical detection, water treatment, drug delivery, and white light emission.^14–21 In particular, a special category of microporous MOFs was proved to be efficient for selective CO₂ capture, featuring both a tailor-made framework and perfect surface potential constructed exclusively to host CO₂ molecules.^22–24 The design of such materials can be achieved under solvothermal synthesis conditions using polytopic organic linkers and metal ions. The structure of organic linkers and the coordination mode of metal ions facilitate the architecture prediction of MOFs.²⁵ To date, many strategies have been adopted to design MOFs with high CO₂ retention, such as incorporating active Lewis sites and polar functional groups in ligands and creating open metal sites within the frameworks, which enhanced MOFs performances for gas mixtures separation, mainly for CO₂ over N₂, H₂, C₂H₂, and CH₄.^26–31 However, the geometric prediction of MOFs remains random, leaving sometimes a large gap between the desired structure and the obtained one, which can affect the MOF properties. In this case, post-synthetic modification of MOFs is required to get the desired performances. This study reports the successful synthesis and characterization of two new microporous neodymium-based MOFs termed IRHs 6 and 7 (IRH = Institut de Recherche sur l'Hydrogène) with two different pore apertures supported by nitrogen-rich cyamelurate (Cy) linkers.³² Furthermore, a successful structural modification from the square pore aperture in IRH-6 to the rhombic one in IRH-7via the Cy–Nd intermediate complex has improved the selective CO₂ uptake over CH₄. The strategic design of these microporous MOFs is based on the small size of Cy linkers and the high coordination number of oxyphilic Nd³⁺ ions. In addition, abundant heteroatoms (O and N) incorporated in Cy linkers ensure a good amount of accessible Lewis-base sites which enhance selective CO₂ capture over other gases (CH₄, N₂, H₂).

Results and discussion

In our previous work, we have provided an instance of selective CO₂ adsorption for three lanthanide-based MOFs supported by the Cy linker termed IRHs-1–3 where the preferential CO₂ uptake has been improved by the small pore size and the nitrogen-rich surface.³³ Herein, our study aims to evaluate the effect of aperture modification of novel neodymium-based MOFs to improve their preferential CO₂ adsorption. Under solvothermal conditions, we synthesized a new MOF isomorphous to the IRH (1–3) series denoted as IRH-6 with a square pore aperture. Furthermore, this framework was broken down under hydrothermal conditions to an intermediate molecular complex denoted Cy–Nd with seven coordinated water molecules, which was subsequently used to reconstruct again a new tridimensional framework termed IRH-7. This new MOF was obtained by the self-assembly of Cy–Nd molecules under solvothermal conditions. In a water-deficient medium, the Nd³⁺ metal ions of the Cy–Nd complex change their coordination environment by releasing five coordinated water molecules, and binding to the free O–N coordination sites of the Cy⁻¹ anion by auto-assembly. This mechanism leads to the reconstruction of a new extended IRH-7 with a rhombic pore aperture (Fig. 1). In our previous work, we have highlighted different possible coordination modes of cyamelurate–lanthanide systems, resulting in four different types of coordination systems; we observed the Ln contraction effect on the ionic radii, which affects the coordination spheres along the lanthanide series.³⁴ Herein, because of the position of its ionic radius in the series, the Nd³⁺ ion allows different coordination modes and jumps from a simple molecular complex configuration to an extended 3D coordination network.

Fig. 1 Synthesis routes of IRH-6, Cy–Nd and IRH-7.

Structural description

The structural elucidation by single-crystal X-ray diffraction (SCXRD) shows a porous structure for IRH-6 with the general formula [Nd(Cy)(H₂O)₂]_n crystallizing in the P4₁22 space group of the tetragonal system. A symmetrical view of IRH-6 structure (Fig. S1†) shows that Cy linkers bonded to four Nd³⁺ ions by coordination bonds to both oxygen and nitrogen sites, and thus Nd³⁺ ions exhibit a coordination number of 10 provided by four Cy and two H₂O molecules. Each Cy linker displays the μ₄-bridging coordination mode into 4-connected nodes while each metal ion acts as a 4-c node to form a rigid three-dimensional microporous framework of the square pore aperture and the pts-sqc183 topology (Fig. 2a–c).

Fig. 2 Views of pores structures, corresponding topology, and respective tilling for (a–c) IRH-6 and (d–f) IRH-7. Carbon atoms are shown in gray, oxygen atoms in red, hydrogen atoms in white, nitrogen atoms in blue, and neodymium atoms in green.

In water medium at 100 °C for 3 days, IRH-6 breaks down to the heptahydrate molecular complex of Cy–Nd with the general formula [Nd(Cy)(H₂O)₇]_n which crystallizes in the P2₁/n space group of the monoclinic system. In this structure, Nd³⁺ ions have an ennea-coordination environment of NdNO₈ (Fig. S2 and S3†) formed by seven oxygen atoms from seven water molecules, and one nitrogen and one oxygen atom from a Cy linker in a slightly distorted tricapped trigonal prism around the Nd³⁺ ion. In an acetonitrile/water (10 : 1) mixture at 80 °C, metal knots of Cy–Nd lose five coordinated water molecules and recombine to form the tridimensional framework of IRH-7 with a rhombic pore aperture (Fig. 2d), in which the Cy linkers have the same coordination mode as that in IRH-6 (Fig. S4†). However, different bond angles and space groups are obtained (Table S1†). IRH-7 has a general formula of [Nd(Cy)(H₂O)₂]_n and crystallizes in the C222₁ space group of the orthorhombic system with the simplified topology of pts-sqc183 (Fig. 2e and f). IRHs 6 and 7 exhibit effective accessible pore apertures of about 4.9 Å and 3.6 Å (regardless of the van der Waals radii), respectively. As illustrated in Fig. S5 and S6†, the guest accessible void volume was assessed to be 43% (IRH-6) and 42% (IRH-7) of the unit cell volume as determined using PLATON.³⁵ All crystallographic data are given in Table S1† and the selected coordination bonds and angles are given in Tables S2–S10.†

Characterizations

The phase purity of the as-synthesized IRH-6, Cy–Nd, and IRH-7 was investigated by comparing simulated and experimental powder X-Ray diffraction (PXRD) patterns (Fig. S7–S9†). This result was strengthened by the scanning electron microscopy (SEM) images, which substantiate the identical crystal morphologies of the as-synthesized samples with an average size of 50–200 μm without contamination by amorphous phases. The SEM (Fig. 3) and optical images (Fig. S10–S12†) show octahedral crystals for IRH-6 and elongated needles for IRH-7, while Cy–Nd exhibits parallelepiped crystals.

Fig. 3 SEM images of the as-synthesized phases of IRH-6, Cy–Nd, and IRH-7.

IRHs 6 and 7 were activated before gas sorption measurements. A solvent exchange process for 48 hours was carried out in anhydrous methanol which was regenerated every 8 hours. Thereafter, a final activation was carried out overnight at 110 °C in a dynamic vacuum to provide activated IRHs 6 and 7. FTIR and PXRD analyses were performed to investigate the crystallinity and chemical stability of IRHs 6 and 7 during the activation process. The PXRD patterns of the solvent exchanged and activated samples of IRHs 6 and 7 were in good agreement with those of the simulated patterns (Fig. S7 and S8†). Furthermore, the FTIR spectra of the as-synthesized IRHs 6 and 7 were unveiled and were similar to those after solvent exchange and also after the complete activation (Fig. S13 and S14†). Besides, their thermal stability was tested using a TG analyzer under a nitrogen atmosphere in a temperature range of 30–800 °C (10 °C min⁻¹). The results are displayed in Fig. S16 and S17.† The TG curves of the as-synthesized IRHs 6 and 7 show a first weight loss of 5% and 7% respectively, starting at ∼110 and 115 °C, which can be assigned to the release of free H₂O molecules in the pores. The solvent exchange curves disclose a first weight loss of 5% (IRH-6) and 7.5% (IRH-7) starting at ∼35 °C, which is assigned to the release of the exchanged MeOH molecules. Common weight loss at ∼180 °C was observed for the as-synthesized, exchanged, and activated IRHs 6 and 7, which can be attributed to the release of coordinated H₂O molecules in the frameworks.³³ After that, no weight loss was observed up to ∼400 °C. TGA and FTIR analysis confirmed the total activation of IRHs 6 and 7, indicating the complete removal of solvent guest molecules from the frameworks. The good chemical and thermal stability in addition to the permanent porosity of IRHs 6 and 7, provides a good basis for the subsequent adsorption tests.

Gas adsorption studies

CO₂ adsorption isotherms collected at 273 K show a type-I adsorption isotherm (Fig. S19†) for the activated IRHs 6 and 7, thus highlighting their permanent porosity. The experimental Brunauer–Emmett–Teller surface areas for IRH-6 and IRH-7 were about 501 m² g⁻¹ and 462 m² g⁻¹, respectively; these values were slightly lower than the theoretical ones calculated for IRH-6 (652 m² g⁻¹) and IRH-7 (547 m² g⁻¹). Furthermore, IRHs 6 and 7 depict uniform microporous structures with an experimental mean pore size of 5.4 Å for IRH-6 and 3.6 Å for IRH-7, which are consistent with the theoretically calculated values of 4.9 Å and 3.6 Å for IRH-6 and IRH-7, respectively (Table S11†).

Gas diffusion within the framework is directly related to the kinetic diameter of the gaseous molecules (Table S12†);³⁶ thus the design of tailored pores regarding the mean path of guest gas molecules and their collisions with the pore aperture remains the most promising gas separation technique. IRH 6 and 7 pore sizes and shapes, their permanent porosity, and nitrogen-rich surfaces support their preferential CO₂ capture. The experimental adsorption isotherms are illustrated in Fig. 4 and adsorption capacities are given in Table S13.† The CO₂ adsorption isotherms of IRHs 6 and 7 exhibit exponential adsorption isotherms saturated under high pressures. CO₂ uptakes quickly reach high adsorption values of 2.67 mol kg⁻¹ for IRH-6 and 2.78 mol kg⁻¹ for IRH-7 under 100 kPa at 298 K. This behavior can be attributed to favorable electrostatic interactions of CO₂ with the nitrogen sites and the coordinated water molecules incorporated in the frameworks, and to the kinetic diameter of CO₂ molecules suitable with the pore aperture of these frameworks. N₂ and H₂ adsorptions were much lower and those under 100 kPa at 298 K were only 0.07 mol kg⁻¹ (N₂) and 0.05 mol kg⁻¹ (H₂) for IRH-6 compared to those of IRH-7, which do not exceed 0.05 mol kg⁻¹ (N₂) and 0.06 mol kg⁻¹ (H₂) under the same conditions. These low adsorption values can be explained by the weak electrostatic interactions of H₂ and N₂ nonpolar gases with these nitrogen-rich frameworks. A remarkable decrease in CH₄ uptake was observed for IRH-7 (0.08 mol kg⁻¹) over IRH-6 (1.03 mol kg⁻¹) under 100 kPa at 298 K. This can be justified by the narrowing of the pore aperture of IRH-7, which significantly reduces the adsorption of CH₄ of kinetic diameter greater than the pore diameter of this porous material.³⁷ IRHs 6 and 7 maintain the same exponential CO₂ adsorption behavior after several adsorption cycles without any loss of capacity (Fig. S20 and S21†). The PXRD patterns after adsorption, heated to 130 °C and 160 °C and exposed to humidity, match well those of the as-synthesized ones, which confirms the stability of IRHs 6 and 7 under chemical and thermal stress conditions (Fig. S22 and S23†).

Fig. 4 Experimental adsorption isotherms of CO₂, CH₄, N₂, and H₂ at 298 K for IRH-6 and IRH-7.

Molecular simulation

Grand Canonical Monte Carlo (GCMC) simulations were performed to further investigate the preferential CO₂ uptake of IRHs 6 and 7. Basically, GCMC is a computational method based on random trial movement steps (insertion, translation, rotation) applied to sorbate molecules inside a simulation box containing a rigid frame. In this system, the chemical potential, temperature, and volume are maintained fixed, while sorbate–sorbate and sorbate–framework electrostatic and van der Waals interactions are continuously evaluated using universal force field (UFF) parameters and Lennard-Jones (LJ) potential; detailed descriptions are given in the Experimental section.^38–40 We simulate the CO₂ density distributions under 100 kPa at 298 K within IRH 6 and 7 microchannel pores as shown in Fig. 5; the CO₂ molecules are distributed homogeneously in the pores and mainly occupy the center of the unidirectional channels. The intense blue regions over the probability density map indicate strong adsorption sites within the surfaces, which are located in between coordinated water molecules and in front of Cy nitrogen sites incorporated in both frameworks. The simulated annealing calculations disclose strong CO₂ binding sites near the coordinated H₂O molecules and the Cy nitrogen sites. No binding sites were observed around Nd³⁺ ions because their highly occupied coordination environment which blocks any contact with the CO₂ molecules. As illustrated in Fig. S24,† CO₂ molecules were electrostatically trapped in the framework through non-covalent interactions with the hydrogen atoms of coordinated H₂O molecules and nitrogen atoms of Cy linkers. A total of eight interactions with an average distance of 3.36 Å (O5–H5A⋯O1(CO₂)¹, O5–H5B⋯O2(CO₂)¹, C3–N1⋯O1(CO₂)², O4–H4A⋯O2(CO₂)², O4–H4B⋯O1(CO₂)³, C3–N4⋯O2(CO₂)³, O4–H4B⋯O1(CO₂)⁴, C3–N4⋯O2(CO₂)⁴) are responsible for holding CO₂ molecules in the framework and play the pivotal role for the high CO₂ selectivity.

Fig. 5 (a) IRH-6 and (b) IRH-7 Monte-Carlo simulated CO₂ adsorption under 100 kPa at 298 K. Carbon atoms are shown in gray, oxygen atoms in red, hydrogen atoms in white, nitrogen atoms in blue, and neodymium atoms in green.

Ideal adsorbed solution theory (IAST)

To further explore the CO₂ adsorption selectivity of IRHs 6 and 7, ideal adsorbed solution theory (IAST) has been employed to predict the selectivity for CO₂/CH₄ (50 : 50), CO₂/N₂ (15 : 85), and CO₂/H₂ (15 : 85) gas mixtures, mimicking post/pre-combustion capture and natural gas sweetening situations.^41,42 The CO₂, CH₄, N₂, and H₂ single-component adsorption isotherms were fitted with the dual-site Langmuir–Freundlich (DSLF) model (eqn (S1)†).⁴³ Thereafter, the multi-component adsorption isotherms were predicted based on Gibbs adsorption equilibrium (eqn (S2)†) using the calculated fitting parameters (Table S14†). Simulated adsorption isotherms (Fig. 6) and IAST selectivities (S_IAST) were calculated following eqn (S3)† and further compared to those of selected materials from the literature (Table S15†). As expected, the CO₂ adsorption isotherms in all simulated mixtures for IRH-6 and IRH-7 have the same shape as that in the experimental isotherms with differences in absorption capacities. IRHs 6–7 exhibit a very fast adsorption tendency under low pressure to reach certain saturation under high pressure due to the favorable interactions of CO₂ within the frameworks. Moreover, N₂ and H₂ showed very low slope and adsorption capacity for both IRH structures, even at a low CO₂ concentration (15%) of simulated mixtures. As a result, IRH-6 and IRH-7 showed the same CO₂ selectivity factor of about 30 under 100 kPa at 298 K for the CO₂/N₂ (15 : 85) mixture, which exceeded the selectivity factor of 26.6 reported for MOF-505-K⁴⁴ and reached the selectivity factor of 31 for COF F-DCBP-CTF-1 observed for the CO₂/N₂ (15 : 85) mixture under ambient conditions.⁴⁵ Similarly, IRH-6 and IRH-7 exhibit high selectivity factors of about 29 and 30, respectively, for the CO₂/H₂ (15 : 85) mixture under 100 kPa at 298 K, and are nearly equivalent to the industrial NaX and NaY zeolites with a high CO₂ selectivity factor of 29 under the same conditions.⁴⁶ Under high pressure, IRH-6 and IRH-7 show the corresponding selectivity factors of 35 and 39 for the CO₂/H₂ (15 : 85) mixture under 4000 kPa at 298 K, respectively. In addition, IRH-7 shows a higher selectivity factor of 137 for the CO₂/N₂ (15 : 85) mixture compared to 55 exhibited by IRH-6 under 4000 kPa at 298 K. These results demonstrate the promising potential of IRHs 6 and 7 for CO₂/N₂ and CO₂/H₂ separation due to their high affinity for the polar CO₂ molecules. On the other hand, the remarkable effect of the pore aperture modification on increasing the CO₂ selectivity of IRH-7 over IRH-6 was demonstrated by the results of the CO₂/CH₄ (50 : 50) mixture. IRH-6 shows a selectivity factor of 31 for the CO₂/CH₄ (50 : 50) mixture under 100 kPa at 298 K, which is comparable to the selectivity values of 27 for IRH-3 under the same conditions.³³ This result is comparable to that of many previously reported MOFs, carbonaceous materials, and zeolites with good separation performance for the CO₂/CH₄ (50 : 50) mixture under ambient conditions such as UTSA-16 with a CO₂ selectivity factor of 29 ⁴⁷ and nitrogen-doped porous carbon CPC-600 with a CO₂ selectivity factor of 26.⁴⁸ After successful modification of the pore aperture, IRH-7 reached a selectivity factor of 102 for the CO₂/CH₄ (50 : 50) mixture under 100 kPa at 298 K which is comparable to exceptional UTSA-120 and zeolite-13X of high separation potential with selectivity factors of 96 and 103, respectively.^49,50 Since IRH-6 and IRH-7 have almost similar CO₂ adsorption isotherms, the difference in selectivity factors for the CO₂/CH₄ mixture (50 : 50) can be explained by the difference between the CH₄ adsorption capacity of IRH-7 with a pore aperture smaller than the kinetic diameter of CH₄ and the adsorption capacity of IRH-6 with a pore aperture higher than the kinetic diameter of CH₄ molecules. In the high-pressure range, the CH₄ adsorption capacity continued to increase moderately while the CO₂ adsorption capacity reached saturation for both materials in the simulated CO₂/CH₄ mixture, which explains the decrease in CO₂ selectivity of IRH-6 and IRH-7 to reach 31 and 102, respectively, under 4000 kPa at 298 K for the CO₂/CH₄ binary mixture.

Fig. 6 IAST adsorption isotherms and CO₂ selectivity of IRHs 6 and 7 for the CO₂/CH₄ (50 : 50), CO₂/N₂ (15 : 85), and CO₂/H₂ (15 : 85) mixtures.

Conclusion

In summary, we reported the successful synthesis and pore architecture modification of new microporous MOFs termed IRHs 6 and 7 supported by nitrogen-rich Cy linkers and Nd³⁺ ions. The structural study and porosity analysis unveil two distinct square and rhombic pore apertures for IRH-6 and IRH-7 with median pore sizes of about 4.9 and 3.6 Å, respectively. Gas sorption studies disclosed high CO₂ uptakes over CH₄, N₂ and H₂ for IRHs 6 and 7 under ambient conditions. Moreover, modification of the pore aperture for IRH-7 significantly decreased its CH₄ uptake over IRH-6. The GCMC simulation showed strong binding sites for CO₂ molecules around the coordinated H₂O molecules and nitrogen sites incorporated into the IRH 6 and 7 microporous channels. In addition, IAST predicted excellent CO₂ selectivity of IRHs 6 and 7 for the CO₂/CH₄ (50 : 50), CO₂/N₂ (15 : 85), and CO₂/H₂ (15 : 85) mixtures under 100 kPa at 298 K. In particular, a remarkable selectivity factor of 102 of IRH-7 for the CO₂/CH₄ (50 : 50) mixture and high CO₂ selectivities of about 30 for the CO₂/N₂, 29 and 30 for CO₂/H₂ (15 : 85) mixtures were observed for IRH 6 and 7, respectively. These results demonstrated the efficiency of the designing microporous MOFs with polarized surfaces to selectively adsorb CO₂ molecules and highlighted the remarkable effect of pore aperture modification on increasing CO₂ selectivity.

Experimental

Materials and methods

All reagents and solvents employed were commercially available and used without further purification. The FTIR spectra were recorded with a Nicolet-iS-10 smart FTIR spectrometer using the transmittance mode within the 550–4000 cm⁻¹ range to collect data. The thermogravimetry analysis was performed using Diamond Pyris TGA/DTA apparatus from PerkinElmer and Mettler Toledo TGA/DSC1 and all data were collected within 25–800 °C at a rate of 5 °C min⁻¹. The scanning electron microscopy images were recorded using an EFSEM JEOL model 5500. Crystallographic data were collected using a Bruker APEX2 diffractometer with Cu Kα radiation. The structures were solved by direct methods using SHELXT-2018,⁵¹ and non-hydrogen atoms were refined anisotropically with least-squares minimization using SHELXL-2018.⁵² Hydrogen atoms were treated by first locating them from difference Fourier maps, recalculating their positions using standard values for distances and angles, and then refining them as riding atoms. Powder X-ray diffraction data were collected on an Xcalibur & Gemini Rigaku diffractometer with a dual fine-focus X-ray source (Cu Kα radiation) at room temperature. The samples were gently ground and mounted on a nylon loop. The data collection involved acquisition in transmission geometry of four different sections with increasing angular position, giving four different 2D frames that were integrated and combined to produce the final 1D X-ray diffraction patterns. Simulated powder diffraction patterns were generated from the corresponding CIFs resulting from the single-crystal analysis using the Mercury software of the Cambridge Crystallographic Data Centre.⁵³ A unique value of the FWHM for the diffraction peaks was adjusted to obtain a better match between the resolution of the experimental and the calculated patterns.

Synthesis procedures

Potassium cyamelurate. Potassium cyamelurate (K₃·Cy) was synthesized according to the method described in the literature reported by E. Horvath-Bordon et al.⁵⁴

IRH-6. Synthesis of IRH-6 was carried out according to the solvothermal method using a mixture of equimolar solvents of DMF/H₂O. Typically, 33.5 mg (0.1 mmol) of potassium cyamelurate was solubilized in 8 mL of H₂O, and 353.1 mg (0.6 mmol) of neodymium salt (Nd(NO₃)₃·6H₂O) was solubilized in 8 mL of N,N-dimethylformamide (DMF). The solutions were mixed at room temperature, stirred, and sonicated for 15 min. Thereafter, the mixture was sealed and heated at 90 °C for 24 hours. After cooling down to room temperature, as-synthesized IRH-6 (purple octahedral crystals) was recovered by filtration and washed twice with methanol (90% yield).

Post-synthesis modification

Cy–Nd. The molecular complex of Cy–Nd was obtained by the destruction of the IRH-6 framework under hydrothermal conditions. Typically, 100 mg of IRH-6 previously washed a couple of times with water at room temperature and well crushed using a mortar was immersed in 12 mL of water, stirred, and sonicated for 30 min. Thereafter, the mixture was sealed and heated at 100 °C for 3 days. After cooling down to room temperature, the as-synthesized Cy–Nd molecular complex (purple parallelepiped crystals) was recovered by filtration, washed twice with acetone, and dried in a dynamic vacuum (70% yield).

IRH-7. Synthesis of IRH-7 consisted of reconstructing the 3D framework using the Cy–Nd complex as a precursor in a solution of neodymium ions in excess. Typically, 100 mg of Cy–Nd previously dried and well crushed using a mortar was immersed with 100 mg of neodymium salt (Nd(NO₃)₃·6H₂O) in 16 mL of a mixture of MeCN/H₂O (10 : 1), and the above mixture was stirred, and sonicated for 30 min. Thereafter, the mixture was sealed and heated at 80 °C for 24 hours. After cooling down to room temperature, as-synthesized IRH-7 (purple needle crystals) was recovered by filtration and washed twice with methanol (90% yield).

Gas adsorption measurement

Pure-gas adsorption isotherms were collected using a homemade pure gas volumetric device (VP) at 298 K.^55,56 Before the measurements, 200 mg of the activated sample was heated (degassed) in a vacuum at 298 K for 24 h. The single-gas adsorption isotherms were measured via the standard volumetric approach consisting of a known amount of gas initially stored in the reference volume before being expanded into the measurement cell volume. The balance between the initial and final conditions was used to calculate the adsorbed amount. The process was repeated continuously until the final equilibrium pressure was reached. Afterward, the reverse procedure was used to determine the desorption isotherm.

Ideal adsorbed solution theory (IAST) calculations

IAST calculations were performed using the pyIAST Python package to predict mixed-gas adsorption isotherms from pure gas adsorption isotherms.⁵⁷ It was assumed that the adsorbates in the mixture were ideal and the adsorption equilibrium between adsorbed and gaseous phases follows the standard state specified by surface potential Φ_i given by the Gibbs adsorption equilibrium (eqn (S2)†).

Molecular simulation

Grand canonical Monte Carlo (GCMC) simulations were performed using the MS©2017 Dassault System 5.0 package [Accelrys Software Inc., San Diego, CA]. IRHs 6 and 7 were considered to be rigid frameworks. The QEq method and QEq_neutral1.0 parameters were used to determine partial charges for atoms in IRHs 6 and 7. The CO₂ partial charges for atoms were q(C) = +0.70e and q(O) −0.35e (e = 1.6022 × 10⁻¹⁹ C). The interaction energy between the framework and CO₂ was computed using the Coulomb and Lennard-Jones 6-12 (LJ) potentials. A cut-off radius of 15.2 Å was chosen for the LJ potential and Ewald & Group summation method was used to calculate electrostatic interactions between the framework and CO₂ molecules. A value of 1 × 10⁶ was used for all loading, equilibration and production steps. All simulations were carried out at 298 K.

Author contributions

ME and AD conceived the work. ME and MM carried out all syntheses. ND repeated to verify the synthesis procedure. ME carried out the solvent exchange and the activation of all materials, and performed all characterization techniques (IR, TGA, SEM). TM performed single-crystal X-ray diffraction for all samples. ME performed powder X-ray diffraction for all samples. ME and NF collected gas sorption data. ME carried out all adsorption studies, IAST and GCMC simulation studies. ME and AA carried out the comparison of the adsorption results with those of the reported materials. ME wrote the manuscript. All authors reviewed and edited the manuscript. AD supervised the entire project. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We are grateful to the Natural Sciences and Engineering Research Council of Canada (DDG-2022-00002), the Fonds De Recherche Du Québec – Nature et technologies, the Canadian Queen Elizabeth II Diamond Jubilee Scholarships, the Canadian Foundation for Innovation (37843), and Mitacs and the Université du Québec à Trois-Rivières. We dedicate this work to our families and friends.

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic details (including thermal atomic displacement ellipsoid plots), mass spectrum, ORTEP diagrams, and other crystallographic data, powder X-ray diffraction, infrared, thermogravimetry analysis, and photographs of the crystalline samples (PDF). CCDC 2104590–2104592. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02208g

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