A novel 3-D microporous magnesium-based metal–organic framework with open metal sites

Abstract

A novel 3-dimensional (3-D) Mg(II) metal–organic framework (MOF) [Mg₄(bdc)₄(DEF)₄]_n (1) was synthesized by the solvothermal reaction of 1,4-benzenedicarboxylic acid (H₂bdc) and magnesium nitrate hexahydrate in N,N′-diethylformamide (DEF). Single-crystal structural analyses reveal that the bdc dianion connects two dinuclear units to form a tetranuclear unit. The dinuclear units consist of a Mg(II) ion that is tetra-coordinated to four bridging oxygen atoms and a Mg(II) ion that is hexa-coordinated to four bridging oxygen atoms, and two pendant DEF molecules. These special arrangements result in novel zig-zag patterned 1-D rhombic channels containing coordinated DEF molecules. Heating 1 to 400 °C provides a porous DEF-free MOF (3), as confirmed by thermogravimetric analysis (TGA), elemental analysis, powder X-ray diffraction (PXRD) and BET surface area. Remarkably, removing the coordinated DEF molecules from 1 while retaining the porosity might lead to the formation of open metal sites, which are favorable for the adsorption of various gas molecules. Adsorption experiments and ideal adsorbed solution theory (IAST) calculations show that 3 has much larger H₂ and CO₂ uptakes and a higher CO₂/N₂ selectivity than a sample activated at 300 °C (2), which still contains coordinated DEF molecules.

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Introduction

Metal–organic frameworks (MOFs) are a new class of microporous and mesoporous materials that have received much attention because of their extremely large surface areas and high porosities.^1–8 These materials are synthesized by the self-assembly of metal corners and organic linkers. Compared to more traditional adsorbents, such as carbon-based materials or zeolites, MOFs can be more easily tailored for specific applications due to their structural versatility.^2,9 By combining appropriate organic and inorganic building blocks, optimal host/guest interactions can be created within MOF pores that have predefined sizes and functionalities.¹⁰ Due to these attractive properties, MOFs have been used for various applications, such as gas storage,^6,11–15 gas separations,^14,16 catalysis,^17–20 ion exchange^21–23 and drug delivery.^24–26 In particular, these materials have been actively studied for CO₂ separation and capture over the past few years.^1,27–31 Bae and Snurr¹⁰ recently evaluated the potential of over 40 MOFs and three commercial adsorbents for adsorptive CO₂ separation based on five adsorbent evaluation criteria and showed that several MOFs are promising for CO₂ separations.

Compared to MOFs containing transition metals, those containing alkaline earth metals, specifically Mg-based MOFs, are relatively small.^32–45 The use of lightweight Mg is of particular interest due to its less toxicity than the transition metals. Additionally, Mg closely resembles Zn but is less flexible than Zn and frequently forms octahedral complexes. 1,4-Benzenedicarboxylic acid (H₂bdc) is known to be a very good organic connector for forming predictable network structures. However, very few bdc-linked Mg(II) complexes exist, and most of them consist of [Mg₃(bdc)₃(solvent)₄] trinuclear units in which all three Mg(II) ions are octahedrally coordinated.^32,46 This type of complex has also been reported for Mg(II) complexes with naphthalene-dicarboxylic acid and 4,4′-biphenyldicarboxylic acid ligands.^37,42,43 To the best of our knowledge, a 3-D bdc-linked Mg(II) MOF formed from [Mg₄(bdc)₄(solvent)₄] units in which the Mg(II) ions are not octahedrally coordinated has not yet been reported.

In this paper, we report the synthesis and crystal structure of a novel 3-D Mg(II) MOF (1). By heating 1 at 400 °C, a DEF-free version (3) was obtained, as evidenced by TGA and elemental analysis. Powder X-ray diffraction (PXRD) patterns, BET surface areas and pore size distributions showed that heating 1 at 400 °C leads a slightly transformed but permanently porous structure 3. Moreover, the removal of coordinated DEF molecules from 1 while retaining the porosity indicates that 3 might have open metal sites, which are known as favorable adsorption sites for various gas molecules. Actually, 3 exhibits considerably larger H₂ and CO₂ uptakes as well as higher CO₂/N₂ selectivities, compared to a sample activated at a lower temperature 300 °C (2) that still has coordinated DEF molecules.

Experimental

Materials

All the reagents and solvents employed in this study were commercially available and used as supplied without further purification. Mg(NO₃)₂·6H₂O and 1,4-benzenedicarboxylic acid were obtained from Sigma Aldrich Chemical Co (St. Louis, USA). Ultra-high-purity N₂ (99.999%), H₂ (99.999%) and CO₂ (99.999%) were purchased from Air Korea Co., Ltd., and used as received.

Synthesis of [Mg₄(bdc)₄(DEF)₄]_n (1)

A mixture of 1,4-benzenedicarboxylic acid (100 mg, 0.60 mmol) and Mg(NO₃)₂·6H₂O (154 mg, 0.60 mmol) in 3 ml diethylformamide (DEF) was heated to 120 °C in a sealed 5 ml screw-top glass bottle for 18 h. The mixture was allowed to cool to room temperature, and the resulting crystals were filtered from the reaction mixture and washed with DEF. The yield was 130 mg (75% based on 1,4-benzenedicarboxylic acid). Anal. calcd for C₅₂H₆₀Mg₄N₄O₂₀: C, 53.92; H, 5.22; N, 4.84%. Found: C, 53.27; H, 5.08; N, 4.48%.

Preparation of 2

Compound 2 was prepared by heating 1 at 300 °C for 30 minutes in a tube furnace under a nitrogen atmosphere. Anal. found: C, 50.31; H, 3.98; N, 2.2%.

Preparation of 3

Compound 3 was prepared by heating 1 at 400 °C for 30 minutes in a tube furnace under a nitrogen atmosphere. It is assumed that all four DEF molecules are removed at 400 °C. Anal. calcd for C₃₂H₁₆Mg₄O₁₆: C, 50.99; H, 2.14; N, 0.0%. Found: C, 49.51; H, 2.54; N, 0.0%.

Physical characterization

Elemental analysis (C, H, and N) was performed using a Perkin-Elmer 2400 series II elemental analyzer. Thermal analyses were performed on a Mettler Toledo TGA/SDTA 851 thermal analyzer in a dynamic dinitrogen atmosphere (flow rate = 30 cm³ min⁻¹). The sample was heated in an alumina crucible at a rate of 10 °C min⁻¹. The powder X-ray diffraction (PXRD) patterns were recorded using an Ultima IV diffractometer (Rigaku Co., Japan) with nickel-filtered Cu Kα radiation (k = 1.5418 Å). The sample was scanned over the 2θ range from 3° to 50° with a step size of 2° and step counting time of 1 minute. The N₂ adsorption and desorption isotherms at 77 K were obtained using an Autosorb-iQ system (Quantachrome Instruments, USA). The H₂ (77 K), CO₂ (293 K) and N₂ (293 K) adsorption isotherms were measured using a Tristar 3020 system (Micromeritics Instruments, USA). A specially designed air circulator (Protech Korea Instruments, Korea) was used to maintain a constant temperature. Before the gas adsorption measurements, the sample was heated in a tube furnace (Lenton Thermal Designs, UK) at two different temperatures (300 °C and 400 °C) for 30 minutes under a dynamic dinitrogen atmosphere (flow rate = 50 cm³ min⁻¹). The furnace temperature was slowly increased from room temperature to the final temperature at 5 °C min⁻¹. Then, the samples were further evacuated under vacuum at 100 °C for 2 h and at 200 °C for 30 minutes.

Crystal data collection and refinement

Single-crystal X-ray diffraction data were collected at 305 K using a Bruker D8 Venture diffractometer and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data collection and structural refinement parameters and the crystallographic data for the complex are provided in Table S1.† The structure was solved by direct methods and refined by full-matrix least-squares on F² using SHELXL-97.⁴⁷ The non-hydrogen atoms were refined with anisotropic thermal parameters. The geometric positions of the hydrogen atoms bonded to carbon were included and given thermal parameters that were 1.2 times those of the atom to which they were attached. Absorption corrections were performed using the SADABS program.⁴⁸ Disordered atom positions were refined with isotropic displacement parameters. One of the disordered DEF molecules was modeled with an occupancy of 50% : 50% in two positions. For the other DEF molecules, all atoms except the oxygen atoms were modeled with an occupancy of 0.33% : 0.33% : 0.33% in three positions, and the oxygen atoms in these DEF molecules were modeled with an occupancy of 50% : 50% in two positions. The solvent hydrogen atoms were omitted from the model. All calculations were performed using SHELXS-97, SHELXL-97, PLATON v1.15,⁴⁹ ORTEP-3v2,⁵⁰ and the WinGX system Ver-1.64.⁵¹

Results and discussion

Structural description

Single-crystal X-ray diffraction measurements show that the [Mg₄(bdc)₄(DEF)₄]_n complex (1) crystallizes in the P2₁/n space group. As shown in Fig. S1,† the asymmetric unit of 1 contains two Mg(II) ions, one entire and two half bdc anions, and two coordinated diethylformamide (DEF) solvent molecules. The geometries of the Mg(1) and Mg(2) coordination spheres are given in Table S2.†

The coordination environment of the two Mg(II) ions are different; one Mg ion is tetra-coordinate, whereas the other Mg ion is six-coordinate, as shown in Fig. S2.† Mg(1) is tetra-coordinated to four bridging oxygen atoms and form tetrahedral geometry with the corresponding bond angles in the range of 99.97(12)–116.99(14)°, deviating from 109.5° – the angle for perfect tetrahedra. The bond lengths in the Mg(1) coordination sphere are Mg(1)–O(1) = 1.949(3) Å, Mg(1)–O(3) = 1.944(3) Å, Mg(1)–O(5) = 1.942(2) Å, and Mg(1)–O(7)^a (a = 1/2 − x, −1/2 + y, 1/2 − z) = 1.992(3) Å. Mg(2) is hexa-coordinated to four bridging oxygen atoms and two pendant DEF molecules. The bond lengths in the Mg(2) coordination sphere are Mg(2)–O(2) = 2.007(3) Å, Mg(2)–O(4) = 2.008(3) Å, Mg(2)–O(6)^b (b = 1 + x, y, z) = 2.038(2) Å, Mg(2)–O(8)^a = 2.071(3) Å, Mg(2)–O(10) = 1.974(13) Å, and Mg(2)–O(12) = 2.173(14) Å. The Mg–O distances are comparable to the reported Mg–O bond distances in the ranges from 1.991(4) to 2.293(4) Å.³⁴ The Mg(2) ion has a slightly distorted octahedral geometry with O(4) and O(12) occupying the axial positions (the trans O(4)–Mg(2)–O(12) angle is 174.1(4)°). The tetranuclear [Mg₄(bdc)₄(DEF)₄] units connect to each other to form novel zig-zag patterned 1-D rhombic channel, which contain coordinated DEF molecules, along different directions (Fig. 1 and S3†). After removing the DEF molecules from the cages, the framework becomes porous along the a direction as shown in Fig. 2a (the two diagonal lengths of the porous space are 6.3 and 14.0 Å). However, no porosity is observed along the b direction as shown in Fig. 2b, indicating that gas molecules cannot penetrate into the channels along the b direction. Therefore, gas adsorption only occurs in the channels along the a direction.

Fig. 1 View along the a direction showing coordinated DEF molecules in 1.

Fig. 2 Space-filling models of 1 viewed along the (a) a and (b) b axes after the DEF molecules are removed.

Most of the previously reported bdc-linked Mg(II) MOFs consist of [Mg₃(bdc)₃(solvent)₄] trinuclear units in which all three Mg(II) ions have octahedral geometries and each terminal magnesium ion coordinates to two solvent molecules.³² Therefore, these complexes are nonporous due to the presence of a diagonal dicarboxylate linker in the cubic unit. In this study, the cubic units are not diagonally linked, and thus, the structure is porous.

Thermal behavior

To investigate the thermal stability of 1, thermogravimetric analyses (TGA) and powder X-ray diffraction (PXRD) experiments were performed. As shown in Fig. 3, an initial weight loss due to the loss of uncoordinated solvent molecules is observed in the thermogravimetric curve. A second weight loss (34.31%) occurs in the temperature range of 146–388 °C and is attributed to the loss of the four coordinated DEF molecules (calc. weight loss = 34.91%). These molecules are removed in two successive steps; the two DEF molecules that are weakly coordinated to the magnesium ions (Mg–O_DEF distance of 2.17 Å) are removed in the temperature range of 146–275 °C, and the other two DEF molecules, which are strongly coordinated to the magnesium ions (Mg–O_DEF distance of 1.97 Å), are removed in the temperature range of 275–388 °C. Remarkably, the following thermogravimetric curve shows a steady plateau up to 550 °C and then the framework of 1 starts to collapse from 550 to 640 °C.

Fig. 3 TGA curve of 1.

The PXRD pattern of the as-synthesized sample of 1 matches well with the simulated pattern from the single-crystal structure. The as-synthesized samples were heated at 300 °C and 400 °C, respectively. After heating 1 at 300 °C under a nitrogen atmosphere for 30 minutes, some of the coordinated DEF molecules remain in the pores, and elemental analysis results show that a partially evacuated MOF (2) is obtained. When 1 is heated at 400 °C under a nitrogen atmosphere for 30 minutes, all of the free and coordinated DEF molecules are removed to give a DEF-free MOF (3) as confirmed by elemental analysis (see the Experimental section), which verifies the absence of nitrogen atoms in 3. In contrast, elemental analyses of 1 and 2 show that considerable amounts of nitrogen atoms, possibly from DEF molecules, are included in these structures. It should be noted that DEF molecules are the only source for nitrogen atoms in these structures. The TGA curves of 2 and 3 (Fig. S4†) further verifies that 3 is a DEF-free MOF and 2 still has coordinated DEF molecules. The PXRD patterns of 2 and 3 show that although the structure is slightly transformed, the overall crystallinity is retained (Fig. 4). Broadened diffraction peaks might be due to formation of mesopores from thermally derived defect,⁵² which is also coincided with appearance of a new peak at a low angle in the PXRD pattern of 3. However, the single-crystal structures of 2 and 3 have not yet been obtained.

Fig. 4 PXRD patterns of (a) 1 (simulated), (b) 1 (as-synthesized), (c) 2 (partially evacuated) and (d) 3 (DEF-free).

To verify that 2 and 3 are permanently porous, the N₂ adsorption and desorption isotherms were measured at 77 K (Fig. S5†). The N₂ adsorption isotherms of 2 and 3 exhibit type I profiles, which are characteristic of microporous materials. The obtained BET surface areas of 2 and 3 are 193 and 378 m² g⁻¹, respectively. Interestingly, a hysteresis in the adsorption and desorption isotherms is observed for 3. This may be attributed to the formation of mesopores, possibly from partial breakage of some metal–ligand bonds during heating at 400 °C. This is supported by the pore size distributions obtained by the DFT method (Fig. S6a†). The pore size distributions obtained from the density functional theory (DFT) and Horváth–Kawazoe (H–K) methods show that 3 has significantly larger amounts of micropores and mesopores than 2 (Fig. S6†), which explains the higher surface area of 3. A similar phenomenon has been reported for post-synthetic annealing of MOF-552, which led to formation of vacancy sites due to partial decomposition of the bridging carboxylates of the framework linker. This resulted in enhanced gas adsorption behavior as well as improved structural stability. In this study, the PXRD patterns, BET surface areas and pore size distributions suggest that heating 1 at 400 °C leads a slightly transformed but permanently porous structure 3. Moreover, removing the coordinated DEF molecules from 1 while retaining the crystallinity might lead to the formation of open metal sites, which are favorable for the adsorption of various gas molecules.

Gas adsorption studies

To confirm whether 3 has strong adsorption sites possibly from open metal sites, we measured the hydrogen adsorption isotherm of 3 at pressures up to 1 bar at 77 K (Fig. 5). For comparison, the hydrogen adsorption isotherm of 2 was also measured under the same conditions. The isotherms of both materials exhibit type I behavior, which is typical for microporous materials. Remarkably, 3 exhibits considerable H₂ uptake (0.78 wt%) at 77 K and 1 bar, especially compared to 2, which has an H₂ uptake of only 0.39 wt%. This result may be explained by strong interactions between the H₂ molecules and open metal sites in 3.

Fig. 5 H₂ adsorption isotherms of 2 and 3 at 77 K.

We also measured CO₂ and N₂ adsorption isotherms of 2 and 3 at 293 K as shown in Fig. 6a. For both samples, the adsorbed amount of CO₂ is much higher than the adsorbed amount of N₂. This can be explained by a larger quadrupole moment and a higher polarizability of CO₂ compared to N₂. Remarkably, 3 has considerably higher CO₂ and N₂ uptakes than 2, and its CO₂ adsorption is especially higher than that of 2. This is attributed to strong interaction of CO₂ with open metal sites in hierarchical porous structure of 3. CO₂ uptake of 3 at 1 bar (22.6 cm³ g⁻¹) is comparable to those of previously reported MOFs with open metal sites.^53,54

Fig. 6 (a) CO₂ and N₂ adsorption isotherms of 2 and 3 at 293 K and (b) IAST-predicted adsorption selectivities of 2 and 3 for a 10% : 90% = CO₂ : N₂ gas mixture.

To analyze the binary CO₂–N₂ mixture isotherms and CO₂/N₂ adsorption selectivities of 2 and 3, ideal adsorbed solution theory (IAST) was applied to the pure isotherms.⁵⁵ Previous studies have shown that IAST can accurately predict gas mixture adsorption in many zeolites^56–58 and MOFs.^31,56,59 To achieve good IAST predictions, the single-component isotherms should be fitted exactly. Hence, the dual-site Langmuir–Freundlich equation⁶⁰ was used to fit the experimental data accurately with R² values of greater than 0.9995. As shown in Fig. 6b, 3 has a higher CO₂/N₂ selectivity than 2 over the entire pressure range studied. This result can be explained by the presence of open metal sites in 3.

Conclusions

A new three-dimensional Mg(II)-based metal–organic framework (1) was synthesized. Single-crystal structural analysis reveals that the complex contains tetranuclear units connected by 1,4-benzenedicarboxylate dianions that form novel zig-zag patterned 1-D rhombic channel, which contain coordinated DEF molecules, in different directions. The Mg(II) ion coordination numbers in the complex are very interesting; one Mg(II) ion is tetra-coordinated, which is very rare for magnesium complexes, whereas the other Mg(II) ion is hexa-coordinated, as commonly observed for Mg(II) complexes.

Heating 1 at 400 °C led to a DEF-free (3) MOF, which was confirmed by TGA and elemental analyses. The PXRD pattern of 3 showed that although the structure is slightly transformed, the overall crystallinity is retained. The BET measurement confirmed that 3 retains permanent porosity. As expected, the BET surface area of 3 is considerably larger than that of 2, which still has coordinated DEF in the pores. Heating 1 at 400 °C results in the formation of open metal sites and some mesopores possibly from partial breakage of metal–ligand bonds. These hierarchical porous structure and open metal sites are favorable for the adsorption of various gas molecules. Gas adsorption experiments and IAST predictions show that 3 has considerably higher H₂ and CO₂ uptakes and CO₂/N₂ selectivity than 2. The superior performance of 3 can be explained by strong interactions between the gas molecules and open metal sites.

Acknowledgements

This work was supported by In-house Research and Development Program of the Korea Institute of Energy Research (KIER) (B6-2441), and the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea (CRC-14-1-KRICT). This work was also supported (in part) by the Yonsei University Future-leading Research Initiative of 2015 (RMS2 2015-22-0169).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, N₂ adsorption data, pore size distribution, and CIF file. CCDC 1424071. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra12946c

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