A novel methoxy-decorated metal–organic framework exhibiting high acetylene and carbon dioxide storage capacities†

Xing Duan *^a, Yuanjing Cui ^b, Yu Yang ^b and Guodong Qian *^b
aCollege of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310027, PR China. E-mail: star1987@hdu.edu.cn
^bState Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China. E-mail: gdqian@zju.edu.cn

First published on 28th December 2016

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Introduction

Over the past few decades, metal organic frameworks (MOFs), as a new type of crystalline porous multifunctional materials, have attracted tremendous interest from scientists in the fields of chemistry and materials science due to their exceptionally high surface area and useful and tunable porosity.^1–3 Meanwhile, the pore surfaces and sizes of metal organic frameworks can be systematically functionalized and varied/tuned because of their diversity in architecture and composition, so MOF materials show rapid development in a variety of application areas, including catalysis,⁴ gas storage,^2a,f–h molecular separation,^2d drug delivery,⁵ and luminescence.^2c Although numerous MOFs used for storage of small molecule gases have been reported and studied, quite a lot of efforts have been pursued to further enhance their gas adsorption capacity.

To develop microporous metal organic framework materials with high gas adsorption performance, a series of strategies, such as improving the density of metal open sites, increasing the pore volume and surface areas, enhancing the quality of crystals and optimizing the activation conditions, taking advantage of framework catenation and interpenetration, adjusting the pore sizes and modifying the pore surfaces, has been extensively explored.⁶ Designing and synthesizing MOFs with open metal sites (OMSs) is one of the best strategies to enhance gas storage properties. The well-known MOF material HKUST-1 (ref. 7) with open copper coordination sites in a large cuboctahedral cage has a high acetylene adsorption capacity of 201 cm³ g⁻¹. Xiang et al.⁸ reported a porous material with high densities of open metal sites, CoMOF-74, which exhibits a high volumetric acetylene storage capacity of 230 cm³ cm⁻³. However, high densities of open metal sites result in high adsorption enthalpy, which goes against the regeneration of crystal materials. On the other hand, pore size also plays an important role in the aspect of gas adsorption. For promoting the capability of capturing gas molecules, maximizing the guest–framework interaction by matching the pore space with the size of the gas molecules can be identified as an effective approach. To adjust the pore size, several strategies have been reported, such as inserting a symmetry-matching regulated secondary linker or anchoring a metal ion/cluster/functional groups at the cage/channel centers, which make the pore space of the primary framework partition into multiple domains.^6d–j Zhao et al.^6d introduced a tripyridyl-type linker into a MIL-88-type structure to obtain a family of CPM-33 materials which exhibit superior CO₂ adsorption capacity. In particular, CPM-33b shows the highest CO₂ adsorption value of 126.4 cm³ g⁻¹ among MOFs without OMSs and is comparable to ZnMOF-74 and NiMOF-74.^2b,9 Herein, with the purpose of improving the gas uptake capacity, we substitute benzene with dimethoxynaphthalene into terphenyl-3,3′,5,5′-tetracarboxylic acid to obtain a new tetracarboxylic organic linker H₄DTPD (Scheme 1) and its corresponding first microporous MOF, [Cu₂(DTPD)(H₂O)₂]·(DMF)₅·(H₂O)₂ (ZJU-12; H₄DTPD = 5,5′-(2,6-dimethoxynaphthalene-1,5-diyl)diisophthalic acid, ZJU = Zhejiang University), with optimized pore space and open metal sites. The activated ZJU-12a exhibits a moderately high BET (Brunauer–Emmett–Teller) surface area of 2316 m² g⁻¹ and a high C₂H₂ storage capacity of 244 cm³ g⁻¹ and CO₂ capture capacity of 134 cm³ g⁻¹ at room temperature.

Experimental

Synthesis of the organic linker

H₄DTPD was synthesized as shown in Scheme 2.

Dimethyl 5-amino-isophthalate (50 g) was added to 15% hydrobromic acid (900 mL) and cooled to 0 °C. A sodium nitrite solution (2.5 M, 120 mL) was added slowly with stirring to obtain a solution of diazonium bromide. The solution of diazonium bromide was slowly added to a solution including CuBr (49 g) and 15% hydrobromic acid (450 mL) under stirring, while the temperature was always kept under 0 °C. The mixture was kept stirring under room temperature overnight after the addition was completed. The solution was filtered, and the filter cake was dissolved in CCl₂H₂, dried with MgSO₄, filtered and concentrated in a vacuum. The crude product was purified by column chromatography (silica gel, ethyl acetate/petroleum ether, 1:8 v/v) to obtain dimethyl 5-bromobenzene-1,3-dicarboxylate as a white powder. Yield: 85%. ¹H-NMR (500 MHz, CDCl₃): δ = 3.95 (s, 6H), 8.35 (d, 2H), 8.61 (s, 1H) ppm.

The dimethyl 5-bromobenzene-1,3-dicarboxylate (5.4 g), bis(pinacolato)diborane (6.0 g), Pd(dppf)₂Cl₂ (0.2 g) and potassium acetate (5.6 g) were added to 100 mL of dried 1,4-dioxane. The mixture was kept at 70 °C for 24 h under stirring. Afterwards, the resultant mixture was extracted with 50 mL of ethyl acetate. The organic layer was separated and dried with anhydrous MgSO₄. Then, the solvent was concentrated in a vacuum after filtration. Finally, the crude product was purified by column chromatography (silica gel, ethyl acetate/petroleum ether, 1:8 v/v) to obtain pure dimethyl (5-pinacolboryl)isophthalate. Yield: 66%. ¹H NMR (500 MHz, CDCl₃): δ = 1.37 (m, 12H), 3.95 (s, 6H), 8.64 (d, 2H), 8.76 (s, 1H) ppm.

Naphthalene-2,6-diol (10 g) and K₂CO₃ (20 g) were dissolved in DMF (50 mL), then iodomethane (10 mL) was added, and the mixture was kept under 85 °C for 3 hours and afterwards was filtered after adding water (100 mL). The crude product was purified via recrystallization of CCl₂H₂ to obtain 2,6-dimethoxynaphthalene. Yield: 85%. ¹H NMR (500 MHz, CDCl₃): δ = 3.90 (s, 6H), 7.66 (d, 2H), 7.12 (s, 2H), 7.15 (d, 2H) ppm.

A solution of 2,6-dimethoxynaphthalene (5 g) in a 100 mL mixed solvent of CHCl₃ and acetic acid (1:1) was added to NBS (9.6 g) under 0 °C. After the mixture reacted overnight at room temperature, the mixture was filtered. The solid was washed with water, saturated NaHCO₃ solution, ethanol and CHCl₃ to afford 1,5-dibromo-2,6-dimethoxynaphthalene as a white solid. Yield: 87%. ¹H NMR (500 MHz, CDCl₃): δ = 4.04 (s, 6H), 8.26 (d, 2H), 7.37 (d, 2H) ppm.

1,5-Dibromo-2,6-dimethoxynaphthalene (3 g), dimethyl (5-pinacolboryl)isophthalate (5.57 g) and K₂CO₃ (8 g) were added to 100 mL of anhydrous 1,4-dioxane, and the solution was deaerated under Ar for 15 min. Pd(PPh₃)₄ (0.47 g) was added to the reaction mixture with stirring, and the mixture was heated to 80 °C for 3 days under Ar. Afterwards, it was extracted with trichloromethane (150 mL). The organic layer was separated and was dried with anhydrous MgSO₄ and the solvent was removed in a vacuum. The crude product was purified by column chromatography to obtain tetramethyl 5,5′-(2,6-dimethoxynaphthalene-1,5-diyl)diisophthalate. Yield: 59.2%. ¹H-NMR (500 MHz, CDCl₃): δ = 3.77 (s, 6H), 3.96 (s, 12H), 8.27 (s, 4H), 8.77 (s, 2H), 7.24 (d, 2H), 7.43 (d, 2H) ppm.

Tetramethyl 5,5′-(2,6-dimethoxynaphthalene-1,5-diyl)diisophthalate (5 g) was then suspended in 150 mL of NaOH (13.9 g) aqueous solution, and 1,4-dioxane (40 mL) was added. The mixture was stirred under reflux until clarification. Dilute HCl was added to the aqueous solution until the pH of the solution was 2. The solid was collected by filtration, washed with a lot of water, and dried to give 5,5′-(2,6-dimethoxynaphthalene-1,5-diyl)diisophthalic acid (H₄DTPD, 96.7% yield). ¹H-NMR (500 MHz, DMSO): δ = 3.76 (s, 6H), 7.38 (d, 2H), 7.50 (d, 2H), 8.09 (s, 4H), 8.56 (s, 2H), 13.39 (s, 4H) ppm.

Synthesis of ZJU-12

In a 20 ml glass vial, the organic linker H₄DTPD (10 mg, 0.0194 mmol) was dissolved in 12.85 mL of N,N-dimethyl formamide (DMF). Then, Cu(NO₃)₂·2.5H₂O (20 mg, 0.086 mmol) and 2.15 mL of H₂O were added into the vial, followed by 75 μL of HCl (37%, aq.). The glass vial was capped and placed in a precision oven at 80 °C for 3 days. Green rhombic shaped crystals were obtained after cooling to room temperature and washed with DMF three times to afford ZJU-12. Elemental analysis: calcd for [Cu₂(C₂₈H₁₆O₁₀)(H₂O)₂]·(DMF)₅·(H₂O)₂ (C₄₃H₅₉Cu₂N₅O₁₉, %): C, 48.04; H, 5.36; N, 6.52; found: C, 48.075; H, 5.564; N, 6.707.

Gas sorption measurements

A Micromeritics ASAP 2020 surface area analyzer was used to measure gas sorption isotherms. To obtain a guest-free framework, the fresh as-synthesized sample of ZJU-12 was washed with DMF, guest-exchanged with dry acetone at least 10 times, filtered and vacuumed at room temperature for 24 h and then at 383 K until the outgas rate was 5 μmHg min⁻¹ prior to measurements. An activated sample of ZJU-12 (100–120 mg) was used for the sorption measurements. The N₂ sorption measurement was maintained at 77 K with liquid nitrogen. The C₂H₂ and CO₂ sorption measurements were performed at 273 K with an ice-water bath and at 298 K with a water bath.

Results and discussion

The organic ligand H₄DTPD was successfully synthesized by addition reaction and Suzuki cross-coupling followed by hydrolysis of the ester and acidification. ZJU-12 was obtained by the solvothermal reaction of H₄DTPD and Cu(NO₃)₂·2.5H₂O in DMF/H₂O with addition of a small quantity of HCl at 80 °C for 3 days as blue rhombic shaped crystals. The formulae of ZJU-12 were confirmed by single-crystal X-ray diffraction studies, elemental analyses and thermogravimetric analysis (TGA, Fig. S1†). The consistency of the powder X-ray diffraction (PXRD) pattern of the as-synthesized samples with the simulated pattern from single-crystal data demonstrates the phase purity of the bulk crystal materials (Fig. S2†).

The single crystal X-ray crystallography (SXRD) analysis proved that ZJU-12 crystallizes in the trigonal space group Rm. As expected, ZJU-12 is built from paddle-wheel Cu₂(COO)₄ secondary building units (SBUs) connected to H₄DTPD linkers via carboxylate groups to have the well-known NbO topology (Fig. 1). The 3D framework of ZJU-12 has two types of cages along the c axis. The diameter of the spherical-like cage is about 6.4 Å, taking into account the van der Waals radii (Fig. 1a). Furthermore, the size of the irregular shuttle-shaped cage is approximately 3.2 × 20.6 Ų (Fig. 1b). The kinetic diameter and molecular dimensional size of C₂H₂ (3.3 Å, 3.32 × 3.34 × 5.7 ų) and CO₂ (3.3 Å, 3.18 × 3.33 × 5.36 ų) are a little lower than those of the cages, therefore these two gases can easily enter the cages of the crystal. Furthermore, due to the introduction of dimethoxynaphthalene, there are smaller sizes of windows along the a, b and c axes in ZJU-12 materials compared with the prototype framework NOTT-101. One, which has a value of about 1 Å, can be observed along the c axis (Fig. 1c) and the other two along the a axis are about 2 Å and 2.8 × 5.6 Ų (Fig. 1d). The smaller window sizes can effectively prevent gas molecules from escaping from the pore space of framework materials. The calculated accessible free pore volume of ZJU-12a is 62% (7414 ų out of 11960.5 ų), evaluated by the PLATON program.¹⁰

To estimate the permanent porosity, the dry acetone-exchanged ZJU-12 was activated under high vacuum to obtain the desolvated ZJU-12a. The N₂ adsorption isotherm of ZJU-12a, which exhibits reversible type-I adsorption behaviour, was acquired at 77 K. The highest adsorbed amount of N₂ for ZJU-12a is 606 cm³ g⁻¹ and the corresponding pore volume is 0.938 cm³ g⁻¹ (Fig. 2). The surface areas of ZJU-12a are evaluated to be 2316 m² g⁻¹ from Brunauer–Emmett–Teller (BET) (Fig. S3†) and 2567 m² g⁻¹ from Langmuir surface areas. The BET value is slightly lower than that for NOTT-101 (ref. 11a) (2805 m² g⁻¹) because of substitution of benzene with dimethoxynaphthalene, but is comparable to those for ZJU-25a^11b (2124 m² g⁻¹), NOTT-109 (ref. 11a) (2110 m² g⁻¹), NJU-Bai-14 (ref. 11c) (2384 m² g⁻¹), ZJNU-54a^11d (2134 m² g⁻¹) and ZJNU-44 (ref. 11e) (2314 m² g⁻¹). The pore size distribution calculated by the Horvath–Kawazoe model is in the range from 4 Å to 6 Å, which is consistent with the pore sizes from SXRD (Fig. S4†).

Acetylene is one of the important raw materials in industrial production, but the storage of acetylene is required to be under suitable low pressure, otherwise it has the possibility of explosion. Among all kinds of porous materials, MOF materials are considered to be the most promising ones for such an application. The optimized pore space and open metal sites within the framework of ZJU-12a encourage us to study its adsorption performance for acetylene and methane. We examine the acetylene adsorption of ZJU-12a. As shown in Fig. 3a, ZJU-12a can take up a large amount of acetylene. At 273 K and 298 K, the acetylene saturated adsorption capacity of ZJU-12a can reach 301 cm³ g⁻¹ and 244 cm³ g⁻¹, respectively. The gravimetric acetylene storage capacity of ZJU-12a at room temperature is significantly higher than those of examined porous MOF materials.^2f,7,12 In fact, the gravimetric C₂H₂ adsorption capacity of 244 cm³ g⁻¹ is the highest one ever reported (Table 1), indicating the promise of this novel microporous MOF material for practical acetylene storage applications. Taking the framework density of 0.799 g cm⁻³ without the coordinated waters into account, the volumetric capacities are 241 cm³ cm⁻³ and 195 cm³ cm⁻³ at 273 K and 298 K, respectively. The initial Q_st of C₂H₂ adsorption in ZJU-12a is calculated to be 29 kJ mol⁻¹ (Fig. 4), which is significantly lower than that for MOF-74 with high densities of open metal sites.

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The excessive emission of carbon dioxide (CO₂) is the major factor for the greenhouse effect. MOF materials are very promising solid porous adsorbents for CO₂ capture on account of their intrinsic advantages. We investigate the CO₂ adsorption performance, indicating that ZJU-12a exhibits a CO₂ uptake of 243 cm³ g⁻¹ at 273 K and 134 cm³ g⁻¹ at 298 K at 1 bar (Fig. 3b). Remarkably, the gravimetric CO₂ sorption capacity at room temperature is higher than those of MOF materials ZJNU-54a¹²ⁱ (120 cm³ g⁻¹), PCN-124 (ref. 13a) (114 cm³ g⁻¹), ZJNU-44 (ref. 13b) (116 cm³ g⁻¹), NPC-6 (ref. 13c) (108 cm³ g⁻¹), JLU-Liu21 (ref. 13d) (118 cm³ g⁻¹), NJU-Bai21 (ref. 13e) (115 cm³ g⁻¹) and NJU-Bai-14 (ref. 11c) (100 cm³ g⁻¹). The high C₂H₂ and CO₂ adsorption values demonstrate that open metal sites and optimized pore size can really enhance the interaction between the gas molecules and the framework. The initial adsorption enthalpy Q_st for CO₂ was calculated by the virial method and the value is 26.9 kJ mol⁻¹ (Fig. 4). Such low C₂H₂ and CO₂ adsorption enthalpy values meet the requirement of low energy consumption in the adsorbent recycling process.

Conclusions

In summary, by substituting benzene with dimethoxynaphthalene within the framework, we have developed a three-dimensional microporous metal–organic framework, ZJU-12. The activated ZJU-12a exhibits a moderately high BET surface area of 2316 m² g⁻¹. The optimized pore space and open metal sites within ZJU-12a have enabled this novel framework material to adsorb a large amount of C₂H₂ (244 cm³ g⁻¹) and CO₂ (134 cm³ g⁻¹) at room temperature. Thus, this new MOF can be a promising adsorbent for gas storage applications.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51472217, 51432001 and 51602087) and the Zhejiang Provincial Natural Science Foundation of China (No. LZ15E020001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1510153. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ce02291j

This journal is © The Royal Society of Chemistry 2017