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Article

Three-Dimensional Hydrogen-Bonded Porous Metal-Organic Framework for Natural Gas Separation with High Selectivity

by
Wenyan Dan
*,
Guangfeng Wei
and
Xiangdong Fang
*
College of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Yangpu, Shanghai 200092, China
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(2), 424; https://doi.org/10.3390/molecules29020424
Submission received: 8 December 2023 / Revised: 30 December 2023 / Accepted: 8 January 2024 / Published: 15 January 2024
(This article belongs to the Section Inorganic Chemistry)
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Abstract

:
A 3D hydrogen-bonded metal-organic framework, [Cu(apc)2]n (TJU-Dan-5, Hapc = 2-aminopyrimidine-5-carboxylic acid), was synthesized via a solvothermal reaction. The activated TJU-Dan-5 with permanent porosity exhibits a moderate uptake of 1.52 wt% of hydrogen gas at 77 K. The appropriate BET surface areas and decoration of the internal polar pore surfaces with groups that form extensive hydrogen bonds offer a more favorable environment for selective C2H6 adsorption, with a predicted selectivity for C2H6/CH4 of around 101 in C2H6/CH4 (5:95, v/v) mixtures at 273 K under 100 kPa. The molecular model calculation demonstrates a C–H···π interaction and a van der Waals host–guest interaction of C2H6 with the pore walls. This work provides a strategy for the construction of 3D hydrogen-bonded MOFs, which may have great potential in the purification of natural gas.

Graphical Abstract

1. Introduction

Metal-organic frameworks, as self-assembled porous materials, have been widely explored in the last two decades due to their excellent performance in the areas of gas storage and separation [1,2], heterogeneous catalysis [3], drug delivery [4], luminescence [5,6,7], electrochemistry [8], and magnetism [9]. The structure and properties of MOFs depend on the nature of their metal cations and bridging ligands. Designing MOFs with an eye for novel structures and utilities through the sagacious choice of various metal ions and ligands always constitutes one of the most intriguing research topics in chemistry and materials science [10,11,12]. Among the factors that may influence the structures of MOFs, hydrogen bonding is capable of providing overall structural rigidity and diversity [13], and the stability of the corresponding molecular networks can be enhanced by augmenting the number or strength of the hydrogen bonds in which each tectonic subunit participates [14]. However, hydrogen bonds are greatly affected by various factors, such as intermolecular distances, temperature, pressure, and solvents [15]. As a result, reports concerning the design and synthesis of functional MOFs involving both coordination bonds and hydrogen bonds are rather scarce in the chemical literature [16,17].
Three-dimensional hydrogen-bonded MOFs are usually assembled based on coordinated hydrogen bond interactions. According to the structural characteristics of reported 3D hydrogen-bonded MOFs, they can be constructed in three ways (Figure 1). The first is that the metal and ligand form a 0D cluster including an MOP (metalorganic polyhedron) or a macrocycle including MOCs (metal-organic cubes) and MOSs (metalorganic squares), which are further linked together along three directions to generate 3D frameworks [18,19,20]. Eddaoudi and Liu reported some hydrogen-bonded MOFs made with MOCs and MOSs [21,22,23,24] containing 3D hydrogen-bonding interactions. In this respect, MOC-2 and MOC-3, consisting of indium (III) and 4,5-dicyanoimidazole, were constructed from vertex-to-vertex hydrogen-bonded MOCs [21]. The second is that 1D chains of metal and ligands are hydrogen-bonded along two orthogonal dimensions to give a 3D framework. Román synthesized a copper(II)-isophthalato MOF containing a 9-methyladenine nucleobase, exhibiting features of the second 3D hydrogen-bonded MOFs [25]. The last choice is similar to pillar-layered structures, in which the 2D networks of a metal and ligand are further interconnected by hydrogen bonds to generate 3D frameworks, as shown by {[Zn(apc)2]·H2O}n and DAT-MOF-1 [26,27].
However, great synthetic challenges still exist in constructing 3D hydrogen-bonded MOFs due to the less tunable character of hydrogen bonds. Using preorganized 2D metal organic sheets assembled by hydrogen bonds may be an alternative strategy to solve the above morass, albeit encountering the quandary of selecting suitable metal centers as well as ligands.
With recent developments in MOFs and HOFs (hydrogen-bonded organic frameworks), the separation properties of MOF and HOF materials have attracted considerable research and commercial interest based on the conception of CO2 capture [28] and purification of natural gas [29], ethane [30], and butadiene [31]. In fact, the high porosities of materials are not prerequisites for their gas separation applications. As reported in the literature, MOFs and HOFs exhibiting excellent performance for gas separations are those of moderate porosities, with BET surface areas less than 1000 m2/g [32]. The design and synthesis of new porous MOF materials with hydrogen-bonding features may serve as a new concept to take advantage of the structural features of both materials with the aim of gas separation applications. Liu’s group reported some hydrogen-bonded MOFs like ZSA-7, ZSA-8, and ZSA-9. ZSA-7 and ZSA-8 exhibit high CO2 adsorption abilities (109.8 and 114.0 cm3·g−1 at 273 K, respectively, under 1 bar) and outstanding natural gas selectivity separation, especially for CO2 over CH4 (39.8 and 35.7 for CO2/CH4 = 0.5/0.5 and 76.0 and 51.6 for CO2/CH4 = 0.05/0.95, respectively, under 1 bar at 298 K) [33]. Natural gas is a hydrocarbon mixture primarily consisting of 85% CH4, 9% C2H6, 3% C3H8, 2% N2, and 1% C4H10. To reduce the percentage of ethane content, it is very important to purify natural gas. With CH4 as a premium choice for greenhouse effect reduction, using MOFs for the industrial separation of CH4 and C2H6 in natural gas becomes urgent and necessary. As a fruitful approach, selectivity enhancements in gas separation have been successfully carried out through the precise control of the pore size and environment of MOFs [34]. However, the selectivity of C2H6 and CH4 is low in the reported MOFs, and it may be due to the inappropriate BET surface areas and weak interactions of C2H6 with the cavity surface of the MOF frameworks [30,32]. In this respect, 3D hydrogen-bonded MOFs may be an excellent choice of porous materials for CH4/C2H6 purification of natural gas.
Based on our preliminary study [26], we herein used the aminopyrimidylcarboxylate ligand (2-aminopyrimidine-5-carboxylic acid) and Cu(NO3)2 to deliver a 3D hydrogen bonded framework (namely TJU-Dan-5) through a solvothermal reaction, which consists of 2D sql networks connected by hydrogen bonds. The 1D permanent porosity of TJU-Dan-5 exhibits higher C2H6/CH4 selectivity (101:1, at 273 K/100 kPa) due to C–H···π interactions between C2H6 and the pore wall. In addition, TJU-Dan-5 displays a high selectivity for equimolar C2H6/C2H4 mixture gas (selectivity: 2.46 at 298 K/100 kPa). This work demonstrates that internal hydrogen bonding implanted in the pore of MOFs provides both enhanced structural integrity and C2H6 friendly pore surfaces.

2. Results and Discussion

2.1. Structure Description

The crystallographic data and structure refinement information are listed in Table S1. TJU-Dan-5, Cu(apc)2, is crystallized in the monoclinic space group of C2/c. The asymmetric unit consists of one crystallographically independent Cu2+ ion and two molecules of apc− ligand (Figure S1). As shown in Figure S2, the Cu2+ ion coordinates four carboxylate oxygen (Cu–O, 1.976(4) Å and 1.972(4), Table S2) and one pyrimidine nitrogen atom (Cu–N, 2.177(4) Å) has a distorted tetragonal pyramid conformation. The Hapc carboxylate groups coordinate the copper ion in a bidentate fashion (Figure S2), with the C–O bond distances of the carboxylate group ranging from 1.254(6) Å to 1.265(6) Å. Compared with the reported structure [26], Hapc adopted new coordination modes (III and IV, Figure S3). One apc ligand connects three copper ions through two Cu–O bonds and one Cu–N bond (mode IV, Figure S3). The other one apc ligand chelates two copper ions through two Cu–O bonds, and the aminopyrimidine group does not coordinate with copper ion (mode III, Figure S3). The free aminopyrimidine forms a hydrogen bond between amino and carboxylate instead (dN–H···N = 2.988(6) Å, Figure S3 and Table S3). It is noteworthy that internal hydrogen bonds exist within aminopyrimidine groups (dN–H···N = 2.984(6) Å, 3.051(7) Å, 3.054(6) Å, Figure S4 and Table S3).
The Cu paddle-wheel SBU(Cu2(CO2)2N2) produced by two CuO4N1 tetrahedrons are corner-shared to form a 2D network along the a axis (Figure 2a, green). The other 2D network (Figure 2b, red) is also composed of Cu paddle-wheel SBU and a 1D helix chain along the b axis. A similar pillared-layer 3D framework is generated by internal hydrogen bonds among apc ligands of the 2D network (Figure 1, resulting in a one-dimensional hexagonal channel with the pore size of 5.0 Å × 6.0 Å by the van der Waals radius along c axis (Figure 3). The void volume is 1272.9 Å3 (36.5%) as estimated by PLATON using a probe of 1.2 Å and the Connolly surface area is shown in Figure 4. Usually, 4, 4′-Bipyridine or analogue linker usually bridges the porous pillared-layer frameworks through coordination bonds [35]. However, the internal hydrogen bonds within aminopyrimidine groups link the 2D layers to construct the 3D porous pillar-layer network of TJU-Dan-5, the structure type rarely reported in MOFs. In contrast to most MOFs with metal ions or clusters coordinating with organic ligands, the framework of TJU-Dan-5 provides a new example of 3D hydrogen-bonded MOFs (Figure 1). It is thus interesting to further explore the topology analysis of such hydrogen-bonded frameworks for the rational design of reticular chemistry of MOFs.
The topology of TJU-Dan-5 was analyzed by using the TOPOSPro program [36]. Firstly, the two-dimensional network formed by the metal cluster and ligands was investigated. Considering Cu paddle-wheel SBU(Cu2(CO2)2N2) as a 4-connected node, each layer yields a 2D 4-connected uninodal net with point symbol of {44.62}, which is a sql net (Figure 2b). Furthermore, hydrogen bonds participating in the 3D framework of TJU-Dan-5 should be considered in the process of analyzing the topology. The two crystallographically independent apc− ligands with different hydrogen bonds are regarded as two 3-connected nodes (Figure 5a), Cu paddle-wheel SBU(Cu2(CO2)2N2) as a 6-connected node, and the total 3-D network displays a trinodal (3,3,6)-connected net with point (Schläfli) symbol {63}4{66.84.105}. According to the search results from the Reticular Chemistry Structure Resource (RCSR) database and the literature, only seven trinodal (3,3,6)-connected nets have been reported, including the symbol names of brk, muo, tsa, tsy, xbq, zxc [37], and another one with point symbols of {42.6}{43}{45.64.86} [38]. Therefore, TJU-Dan-5 provides a completely new topology in MOF crystal nets.

2.2. Characterization of TJU-Dan-5

The thermal stability of TJU-Dan-5 was further evaluated via a thermogravimetric analysis (TGA) and varied-temperature PXRD. A sharp weight loss above 270 °C in the TGA curve corresponded to the departure of the organic ligands and the collapse of the framework (Figure S5), while the varied-temperature PXRD patterns obtained at increasing temperatures show that the framework collapses above 270 °C (Figure S6). The framework of TJU-Dan-5, like most MOFs based Cu-paddle-wheel SBU, is stable up to 250 °C in air, both suggesting that hydrogen-bonded MOFs materials are structurally robust.

2.3. Gas Adsorption of H2 and N2

Adsorption experiments were thus carried out to evaluate the rigidity and permanent porosity of TJU-Dan-5. The N2 sorption at 77 K for activated TJU-Dan-5 shows type I adsorption isothermal behavior, characteristic of a microporous material with permanent porosity (Figures S7 and S8). The Brunauer–Emmett–Teller (BET) and Langmuir surface area of TJU-Dan-5 were determined to be 748.7 m2·g−1 and 914.2 m2·g−1, respectively. The experimental pore volume was calculated to be 0.323 cm3·g−1. The pore size distribution of TJU-Dan-5, analyzed by the NLDFT model, displays a peak of6.0 Å in the micropore region, which is close to the size of the hexagonal channel in the crystal structure of title compound (Figure S7, inset). Hydrogen adsorption reveals a moderate uptake of 1.52 wt% at 77 K (1 atm) (Figure S9). TJU-Dan-5 may be studied as a potential hydrogen storage material. The Qst (H2) is slightly lower than that of MOF-74(Zn) but considerably higher than those of other porous materials, such as MOF-177, JUC-48, and PCN-17 [39]. The isosteric heat of H2 sorption, calculated by fitting the adsorption data at 77 K and 87 K to the Virial equation, was found to be 7.98 kJ/mol at zero coverage (Figures S10 and S11).

2.4. Separations of C2H6/CH4 and C2H6/C2H4

Separation of C2H6 from natural gas is crucial in sufficient utilization of natural gas and CO2 reduction in the earth atmosphere. The unique channel sizes of TJU-Dan-5 us to investigate its potential in the C2H6/CH4 separation from natural gas. In this respect, single-component sorption isotherms of CH4 and C2H6 were measured at 273 K and 298 K, respectively. For TJU-Dan-5, the absorbed amounts of CH4 at 273 and 298 K are only 23.6 and 15.2 cm3·g−1, respectively (Figure 6), whereas the corresponding C2H6 amounts of gas uptake at 273 and 298 K are 50.3 and 45.03 cm3·g−1(Figure 6). More importantly, under low pressure below 30 kPa, TJU-Dan-5 takes up much more C2H6 than CH4. TJU-Dan-5 takes up a much different amount of C2H6 and CH4, suggesting of separation selectivity of TJU-Dan-5 in C2H6 and CH4. In addition, C2H2, C2H4, and CO2 gas sorption isotherms of TJU-Dan-5 were measured at 273 K and 298 K, respectively (Figure 7). TJU-Dan-5 can take up the amounts of 57.79 cm3·g−1 and 50.06 cm3·g−1 for C2H2, 47.86, 41.64 cm3·g−1 and 36.25 cm3·g−1 for C2H4, and 59.27 cm3·g−1 and 36.25 cm3·g−1 for CO2 at different temperature levels (Figure 4).
To obtain binding energy at low coverage, isosteric adsorption heats of C2H6, CH4, C2H2, and C2H4 for TJU-Dan-5 were calculated through the Virial equation and using the Clausius–Clapeyron relation, respectively (Figures S12 and S13). As shown in Figure S12, the isosteric adsorption heat of C2H6 is higher than the others. At zero coverage of C2H6 interaction with the most energetically favored adsorption sites, the enthalpy of 35.5 kJ mol−1 can be attributed to stronger van der Waals host–guest interactions and C–H···π interaction between TJU-Dan-5 and C2H6. Pyridine rings presented in TJU-Dan-5 may bring C2H6 in close contact with the pore walls. To visualize the preferential binding sites of C2H6, DFT-based structural potential energy surface searching identified strong C–H···π interactions between C2H6 and the framework (the distances between H and pyrimidine centroids: 2.76 and 3.25 Å in Figure 7) and van der Waals host–guest interaction (the nearest distances between H and pyrimidine: 2.61 and 2.81 Å in Figure 7). In fact, the calculated adsorption energy of C2H6 on TJU-Dan-5 is 34.7 kJ/mol, in an excellent agreement with the experimental data.
The ideal adsorbed solution theory (IAST) is used to calculate C2H6/CH4, C2H6/C2H4, C2H2/CH4 [40] and gas mixture selectivity respectively (see the Supporting information, Equations (S1) and (S2)). TJU-Dan-5 shows the selectivity of C2H6/CH4 (68 at 50:50), C2H6/CH4 (67 at 5:95), 298 K (100 kPa); C2H6/CH4 (146 at 50:50), C2H6/CH4 (101 at 5:95), 273 K (100 kPa); C2H2/CH4 (15 at 50:50), 298 K (100 kPa); C2H2/CH4 (25at 50:50), 273 K (100 kPa) (Figure 8). In the calculation, TJU-Dan-5 exhibits greater separation ratios of C2H6/CH4 at both 273 K and 298 K. The high selectivities of TJU-Dan-5 in C2H6/CH4 separation are better than those of MOFs or HOFs (Table 1) reported at room temperature, such as FJI-C4 [41], UTSA-33 [42], SNNU-Bai67 [43], VNU-18 [44], SBMOF-2, and HOF-BTB [45,46]. These results may be attributed to the stronger affinity between the pore environment of TJU-Dan-5 and C2H6, making TJU-Dan-5 a good choice for natural gas purification. In addition, it is worth noting that the selectivity for C2H6/C2H4 equimolar mixtures is 2.46 at 298 K (100 kPa) (Figure 9), only lower than some leading MOFs in gas separation, such as MAF-49 (2.9) [30], Zn-FBA (2.9) [47], Cu(Qc)2 (3.4) [48], and Fe2(O2)(dobdc) (4.4) [49], supporting our prediction of TJU-Dan-5 utilization in gas separation.
Dynamic breakthrough experiments toward C2H6/CH4 (5:95, v/v) and C2H6/C2H4 (50:50, v/v) were carried out at 298 K (Figure 10). The activated TJU-Dan-5 sample was packed into a column, with the binary gas mixtures of CH4/C2H6 and C2H4/C2H6 flowing at 5.0 mL min−1 at room temperature. As shown in Figure 10a, CH4 was immediately monitored in the outlet of the fixed bed, while the C2H6 was retained in the column packed with TJU-Dan-5 for 70 min/g. The results demonstrate that TJU-Dan-5 is capable of trapping C2H6 molecules from CH4/C2H6 mixtures. High-purity CH4 products (≥99.99%) can be directly obtained, and during the time of 0 to 66 min/g (C2H6 does not reach its breakthrough point), the amounts of CH4 captured in TJU-Dan-5 were calculated to be 299.31 mL/g. In Figure 10b, C2H4 was eluted from a column before C2H6 with different retention times of 7.5 and 10.3 min·g−1, respectively, in an equimolar mixture. The purity of CH4 was determined to be ≥99.04%. The breakthrough curves are kept over three cycles, indicating that the sample is sufficiently stable at the given conditions. In addition, the PXRD experiments further confirm the working stability of TJU-Dan-5 following the sorption experiments and breakthrough tests (Figure S14). Separation of absolute C2H6 from CH4/C2H6 mixture can be achieved by using activated TJU-Dan-5 under ambient conditions.

3. Materials and Methods

3.1. General Naterials and Methods

All reagents for syntheses were purchased from commercial sources. Thermogravimetric (TGA) analyses were investigated with a Mettler Toledo TGA/SDTA851 analyzer (Mettler Toledo, Zurich, Switzerland) in N2 atmosphere with a heating rate of 5 K min−1, from 30 °C to 800 °C. Elemental analyses (C, N, H) were measured on an Elementar Vario EL III microanalyzer (Elementar, Frankfurt, Germany). IR spectra were measured from a KBr pellets on a Thermo Scientific Nicolet IS10 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA) in the range of 4000–400 cm−1. Powder X-ray diffraction (PXRD) patterns were carried out using a Bruker D8 powder diffractometer (Bruker, Karlsruhe, Germany) at 40 kV, 40 mA for Cu Kα radiation (λ = 1.5406 Å), with a scan speed of 0.2 s/step and a step size of 0.05° (2θ).

3.2. Synthesis of TJU-Dan-5

[Cu(apc)2]n (TJU-Dan-5, Hapc = 2-aminopyrimidine-5-carboxylic acid) was obtained via solvothermal synthesis. A mixture of Hapc (0.014 g, 0.1 mmol), Cu(NO3)2·3H2O (0.013 g, 0.06 mmol), DMF (2.0 mL), and anhydrous methyl alcohol (0.5 mL) was placed in a 10 mL glass bottle and stirred for 1 h at room temperature. After the mixture was sealed in a Pyrex tube and heated at 60 °C for 2 days, the whole apparatus was cooled to room temperature. Blue plate crystals of TJU-Dan-5 were collected via filtration (yield: 55% based on Cu(NO3)2·3H2O). Elemental analysis (%): calcd. for [Cu(apc)2]n (339.75): C 35.35, H 2.37, N 24.74; found: C 35.41, H 2.50, N 24.60. IR (KBr, cm−1): 3401 cm−1 (m), 3303 cm−1 (w), 1686 cm−1 (w), 1646 cm−1 (w), 1601 cm−1 (m), 1508 cm−1 (w), 1414 cm−1 (m), 1357 cm−1 (m), 1246 cm−1 (w), 1181 cm−1 (w), 1084 cm−1 (m), 1003 cm−1 (w), 844 cm−1 (m), 810 cm−1 (w), 683 cm−1 (w), 602 cm−1 (w), 477 cm−1 (m).

3.3. X-ray Crystallography

The data for TJU-Dan-5 were collected from a single crystal at 296(2) K on a Bruker D8 VENTURE dual-wavelength Mo/Cu three-circle diffractometer with a microfocus sealed X-ray tube using mirror optics as monochromator and a Bruker PHOTON II detector (Bruker, Karlsruhe, Germany). MoKα radiation (λ = 0.71073 Å) was MoKα used in the diffractometer. All data were integrated with SAINT and a multi-scan absorption correction using SADABS was applied [57,58]. The structure was solved by direct methods using SHELXT and refined by full-matrix least-squares methods against F2 by SHELXL-2019/1 [59,60]. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically on calculated positions using a riding model with their Uiso values constrained to 1.5 times the Ueq of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for nitrogen and all other carbon atoms. The crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre [61]. CCDC 1830585 for TJU-Dan-5 includes the supplementary crystallographic data for this paper. These data can be acquired free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 21 April 2020), or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. This report and the CIF file were generated using FinalCif [62].

3.4. Gas Adsorption Measurements

The as-synthesized materials of TJU-Dan-5 were washed with DMF and CH3OH for three times, respectively. Solvent was exchanged with CH2Cl2 for nine times over three days. CH2Cl2 was further removed with supercritical liquid CO2 in a Tousimis Samdri PVT-30 critical point dryer (Tousimis, Rockville, MD, USA). N2 adsorption and desorption measurements were measured on a Quantachrome Autosorb-iQ gas adsorption analyzer (Quantachrome, Tallahassee, FL, USA) and pore size analyzer at 77 K. The pore size distribution of TJU-Dan-5 was analyzed by the NLDFT model utilizing N2 adsorption data at 77 K (calculation model: N2 at 77 K on carbon (slit/cylinder. pore, NLDFT equilibrium model); Eff. mol. Diameter (D): 3.54 Å). Before the gas adsorption tests, the experimental sample was immersed in CH3OH for 36 h, and then the exchanged sample was activated under vacuum at 100 °C for 10 h. Single H2 sorption experiments were measured on a Micromeritics ASAP 2020 adsorption analyzer (Micromeritics, Atlanta, GA, USA) at 77 K and 87 K via liquid N2 bath and liquid Ar bath, respectively. C2H6, C2H2, C2H4, CH4 and CO2 absorption experiments were performed on the Micromeritics ASAP 2020 adsorption analyzer at 273 K and 298 K via an ice-water bath and heating jacket, respectively. Two different temperatures isotherms were fitted to the Virial model and the isosteric heats of C2H6, C2H4, C2H2, and CH4 adsorption were calculated, respectively. The selectivity of C2H6 and C2H2 over CH4 was calculated with the ideal adsorbed solution theory (IAST) model (Equations (S1) and (S2)) at 273 K and 298 K.

3.5. Breakthrough Measurements

The breakthrough experiment was carried out using a multi-constituent adsorption breakthrough curve analyzer (BSD-MAB, Beishide Instrument Technology (Beijing) Co., Ltd., No. 607, Building 1, Brilliant International, Shangdi 10th Street, Haidian District, Beijing, China). The gas separation properties of TJU-Dan-5 (1.26 g) were examined by breakthrough experiments using 0.05 (C2H6): 0.95 (CH4) and 0.5 (C2H6): 0.5 (C2H4) gas mixtures flowing through the activated samples packed into the same glass column (6.0 mm inner diameter, 65 mm in length), respectively. The gas mixture passed through the column at a rate of 5 mL/min. The composition of the effluent gas was detected by a Mass spectrometry.

3.6. Density Functional Theory Calculations

Spin polarized density functional theory (DFT) calculations were performed using VASP packages with projected augmented wave (PAW) pseudo-potentials [63,64,65]. The exchange–correlation energy was treated based on the generalized gradient approximation (GGA) by using Perdew–Burke–Ernzerhof (PBE) functional [66]. The plane-wave cutoff energy was set to 450 eV. Brillouin zone sampling was restricted to the Gamma point [67]. The DFT-D3(BJ) method of Grimme were employed to describe long-range vdW interactions [68,69]. In this work, the crystal structure of the TJU-Dan-5 framework was obtained according to the experimental characterization results. The optimized lattice parameters of TJU-Dan-5 are 11.13 Å × 13.34 Å × 13.16 Å, with α = 106.74°, β = 90.01°, γ = 65.39°, which are close to the experimental characterization results. The preferential binding sites of small molecules were searched via the stochastic surface walking (SSW) method [70,71], which has been implemented in a computing program (LASP www.lasphub.com (accessed on 21 July 2022)). LASP is now available on market, and it can interface with VASP for all the functionalities. All atoms were fully relaxed during the lattice optimization. The Quasi-Newton l-BFGS method is used for geometry relaxation until the maximal force on each degree of freedom is less than 0.05 eV/Å.

4. Conclusions

A new 3D hydrogen bonding MOF (TJU-Dan-5) was successfully synthesized and characterized. As observed in the solid-state structure, two 2D metal coordination planes of TJU-Dan-5 were ligated by hydrogen bonds, resulting in a 1D porous channel. Albeit no open metal site in TJU-Dan-5, the permanent porosity of TJU-Dan-5 results in moderate performance for adsorption of H2 gas as well as high C2H6/CH4 and C2H6/C2H4 selectivity. The molecular model calculation result reveals strong C–H···π interactions between C2H6 and the pore wall, suggesting intermolecular hydrogen bonding in favor of pore integrity and C2H6 selection. We are exploring the possibility of diversifying the 3D hydrogen-bonded MOFs family for gas separation and further developing this synthetic concept.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29020424/s1, Figure S1: The asymmetric unit of compound TJU-Dan-5; Figure S2: The coordinated environment around the metal center of TJU-Dan-5; Figure S3: The coordination mode of Hapc; Figure S4: Hydrogen bonds in TJU-Dan-5; Figure S5: TGA curve of active TJU-Dan-5; Figure S6: XRD patterns of TJU-Dan-5 in air from room temperature to 270 °C; Figure S7: Nitrogen sorption isotherm of TJU-Dan-5 at 77 K; Figure S8: BET surface area plot for TJU-Dan-5; Figure S9: Hydrogen sorption isotherm of TJU-Dan-5; Figure S10: Isosteric heat of adsorption Qst of H2 for TJU-Dan-5; Figure S11: The H2 isotherms at 77 K and 87 K and the Virial equation fits for TJU-Dan-5; Figure S12: Isosteric heat of adsorption Qst of C2H6, C2H4, C2H2, CH4 for TJU-Dan-5; Figure S13: The gas isotherms at 273 K and 298 K and the Virial equation fits for TJU-Dan-5; Figure S14: PXRD patterns of TJU-Dan-5 after sorption experiments and breakthrough tests; Table S1: Crystal data and structure refinement of TJU-Dan-5; Table S2: Selected bond lengths (Å) and angles (º) for TJU-Dan-5; Table S3: Hydrogen bonds for TJU-Dan-5; Table S4: Parameters for DSLF isotherm fits at 298 K.

Author Contributions

Writing—original draft, W.D.; investigation, W.D.; data curation, W.D.; software, G.W.; methodology, W.D.; resources. W.D.; writing—review and editing, W.D. and X.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by National Natural Science Foundation of China (Grant Number: 21971195), the Research Project of Shanghai Science and Technology Commission (Grant number: 21ZR1467800), and the Fundamental Research Funds for the Central Universities (Grant Number: C1-22120180252).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Material.

Acknowledgments

We are extremely grateful to Guiyang Zhang and Binbin Tu for help with gas adsorption determination; to Yanfeng Bi and Jia Li for help with selective calculations; and to Xin Xu for valuable discussion.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Molecules 29 00424 g001
Figure 1. Schematic diagram showing the construction of 3D hydrogen-bonded MOFs in three ways (blue balls represent a metal cluster or macrocycle; the orange rods represent hydrogen bonds among ligands; red balls represent a metal or metal cluster; and the grey rods represent organic ligand parts).
Figure 1. Schematic diagram showing the construction of 3D hydrogen-bonded MOFs in three ways (blue balls represent a metal cluster or macrocycle; the orange rods represent hydrogen bonds among ligands; red balls represent a metal or metal cluster; and the grey rods represent organic ligand parts).
Molecules 29 00424 g001
Molecules 29 00424 g002
Figure 2. Structure of TJU-Dan-5. (a) Packing diagram of TJU-Dan-5 along c axis, (red: one two-dimensional coordination layer; green: the other two-dimensional coordination layers). (b) The two-dimensional layer can be simplified as sql topology (green grid) along a axis.
Figure 2. Structure of TJU-Dan-5. (a) Packing diagram of TJU-Dan-5 along c axis, (red: one two-dimensional coordination layer; green: the other two-dimensional coordination layers). (b) The two-dimensional layer can be simplified as sql topology (green grid) along a axis.
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Molecules 29 00424 g003
Figure 3. Packing view of TJU-Dan-5. (a) Yellow balls are added to highlight the porosity along c axis, (b) yellow columns are added to highlight the 1D channels, along b axis (C: dark gray; N: dark blue; O: red; H: light gray, Cu2(CO2)2N2: two light blue tetrahedrons. Hydrogen bonds within aminopyrimidine groups are represented in orange dotted lines.
Figure 3. Packing view of TJU-Dan-5. (a) Yellow balls are added to highlight the porosity along c axis, (b) yellow columns are added to highlight the 1D channels, along b axis (C: dark gray; N: dark blue; O: red; H: light gray, Cu2(CO2)2N2: two light blue tetrahedrons. Hydrogen bonds within aminopyrimidine groups are represented in orange dotted lines.
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Molecules 29 00424 g004
Figure 4. Connolly surface area (orange/grey curved surface) calculated with probe atomic radii of 1.4 Å in TJU-Dan-5. (a) Along Z axis, (b) along X axis.
Figure 4. Connolly surface area (orange/grey curved surface) calculated with probe atomic radii of 1.4 Å in TJU-Dan-5. (a) Along Z axis, (b) along X axis.
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Molecules 29 00424 g005
Figure 5. The simplified node and topology of TJU-Dan-5: (a) the simplified apc− ligands in two 3-connected nodes (orange lines represent hydrogen bonds) and the simplified Cu paddle-wheel SBU Cu2(CO2)2N2 in 6-connected node; (b) the network of TJU-Dan-5 showing in (3,3,6)-connected new topology.
Figure 5. The simplified node and topology of TJU-Dan-5: (a) the simplified apc− ligands in two 3-connected nodes (orange lines represent hydrogen bonds) and the simplified Cu paddle-wheel SBU Cu2(CO2)2N2 in 6-connected node; (b) the network of TJU-Dan-5 showing in (3,3,6)-connected new topology.
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Molecules 29 00424 g006
Figure 6. CO2, CH4, C2H2, C2H4, and C2H6 adsorption isotherms for TJU-Dan-5 measured at 298 K (a) and 273 K (b).
Figure 6. CO2, CH4, C2H2, C2H4, and C2H6 adsorption isotherms for TJU-Dan-5 measured at 298 K (a) and 273 K (b).
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Figure 7. DFT-calculated optimized C2H6 adsorption site, van de Waals host-guest interactions (red dotted lines), and C–H···π interaction (black dotted lines) C2H6 and TJU-Dan-5. Color codes: Cu, pink; C, gray; H, white; O, red; and N, blue.
Figure 7. DFT-calculated optimized C2H6 adsorption site, van de Waals host-guest interactions (red dotted lines), and C–H···π interaction (black dotted lines) C2H6 and TJU-Dan-5. Color codes: Cu, pink; C, gray; H, white; O, red; and N, blue.
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Figure 8. Selectivity predicted via DSLF-IAST method for adsorption of equimolar binary mixture (C2H2/CH4, CH4/C2H6) in TJU-Dan-5 at 298 K (a) and 273 K (b).
Figure 8. Selectivity predicted via DSLF-IAST method for adsorption of equimolar binary mixture (C2H2/CH4, CH4/C2H6) in TJU-Dan-5 at 298 K (a) and 273 K (b).
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Molecules 29 00424 g009
Figure 9. Selectivity predicted via DSLF-IAST method for adsorption of equimolar binary mixture C2H6/C2H4 in TJU-Dan-5 at 298 K and 273 K.
Figure 9. Selectivity predicted via DSLF-IAST method for adsorption of equimolar binary mixture C2H6/C2H4 in TJU-Dan-5 at 298 K and 273 K.
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Figure 10. Experimental column breakthrough curves of TJU-Dan-5. (a) CH4/C2H6 (5/95) mixture under a flow of 5 mL·min−1, (b) C2H4/C2H6 (50/50) mixture under a flow of 5 mL·min−1 in an absorber bed packed with TJU-Dan-5 at 298 K and 1.0 bar. (The results were individually tested three times).
Figure 10. Experimental column breakthrough curves of TJU-Dan-5. (a) CH4/C2H6 (5/95) mixture under a flow of 5 mL·min−1, (b) C2H4/C2H6 (50/50) mixture under a flow of 5 mL·min−1 in an absorber bed packed with TJU-Dan-5 at 298 K and 1.0 bar. (The results were individually tested three times).
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Table 1. Comparison of some typical MOFs and HOF for the separation of C2H6/CH4 and C2H2/CH4 as predicted by IAST.
Table 1. Comparison of some typical MOFs and HOF for the separation of C2H6/CH4 and C2H2/CH4 as predicted by IAST.
MOFBETC2H6/CH4 (50:50)C2H6/CH4 (5:95)C2H2/CH4 (50:50)T (K)Gas Uptake C2H6 & CH4Refs.
TJU-Dan-5 *748146.5101.225.027350.3/23.6This work
686715.429845.03/15.2This work
UTSA-35 *742.715/1929651/9[50]
[Co2(5,4-PMIA)2(TPOM)0.5]92034.7/32.030081.9/16.9[51]
FJI-C4 *69039.7/5129866.3/18.4[41]
UTSA-33 *66020/17.129659/12[42]
Cu-TDPAT193816.4/154.3273217.7/51.6[52]
12.1/127.1298154.4/28.3[52]
JLU-MOF112 *15531224/298107.7/10.2[29]
SNNU-Bai67989.538.3/50.529891.2/15.8[43]
VNU-1890027.3/41.529872.8/18.5[44]
SBMOF-219526/1829859.8/16.2[45]
ZJNU-119 *95020.9/62.929889.2/37.5[53]
HOF-BTBa95517.7/12.527395.4/13.44[46]
13.79.329569.2/10.08[46]
PFC-5 *25619/1529825.9/8.0[54]
HOF-1425736.3 3.729844.2/7.8[55]
MAF-49 170//31636/22[30]
[Zn2(bdc)2(bpndi)]565175/49629844/8[56]
* The selectivity value at 1 bar.
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Dan, W.; Wei, G.; Fang, X. Three-Dimensional Hydrogen-Bonded Porous Metal-Organic Framework for Natural Gas Separation with High Selectivity. Molecules 2024, 29, 424. https://doi.org/10.3390/molecules29020424

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Dan W, Wei G, Fang X. Three-Dimensional Hydrogen-Bonded Porous Metal-Organic Framework for Natural Gas Separation with High Selectivity. Molecules. 2024; 29(2):424. https://doi.org/10.3390/molecules29020424

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Dan, Wenyan, Guangfeng Wei, and Xiangdong Fang. 2024. "Three-Dimensional Hydrogen-Bonded Porous Metal-Organic Framework for Natural Gas Separation with High Selectivity" Molecules 29, no. 2: 424. https://doi.org/10.3390/molecules29020424

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