An rht-type metal–organic framework constructed from an unsymmetrical ligand exhibiting high hydrogen uptake capability

Abstract

An unsymmetrical hexatopic carboxylate acid (H6-1) ligand containing one acrylamide unit and two triazole groups was designed and synthesized, and then used to construct a metal–organic framework (MOF Cu-ABTA) via a solvothermal reaction with Cu(NO₃)₂. A powder X-ray diffraction investigation indicated that the MOF Cu-ABTA possesses a (3,24)-connected rht-network, as does our previously reported MOF NTU-105 with three triazole groups. The gas sorption measurements revealed that Cu-ABTA shows high porosity and a very high H₂-uptake capability with a strong H₂ binding affinity as demonstrated by the enhanced isosteric heat of adsorption, which make it a highly promising candidate for clean energy storage.

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Introduction

The design and construction of chemical or physical adsorbents for clean energy storage (such as hydrogen) and carbon dioxide capture is vital to addressing the persisting challenges in global energy issues and environmental sustainability.^1–7 Various porous materials have been developed over the past decades. Among them, metal–organic frameworks (MOFs), an emerging unique class of crystalline solid-state materials, show great promise owing to their adjustable chemical functionality, structural diversity, and ease of functionalization via pre- and/or post-synthesis.^8–16

Typically, MOFs are constructed by the self-assembly of metal ions or clusters with multidentate organic linkers via metal coordination bonds under solvothermal conditions. Rational design and preparation of MOFs with targeted ordered structures and topologies can be achieved by the methodology of reticular chemistry due to the inherent modular nature of the materials.^17,18 It has been well documented that the solvothermal reaction of copper(II) ions with a dendritic hexacarboxylate acid ligand consisting of three coplanar isophthalate units having an overall C₃-symmetry could exclusively generate a (3,24)-connected framework with rht topology.^19,20 Such network should be explained by the supermolecular building block (SBB) approach in contrast to that of the molecular building block (MBB).^21–23 Cuboctahedral metal–organic polyhedron (MOP), with a chemical formula [Cu(II)₂(isophthalate)₂H₂O₂]₁₂ and acting as SSB, has been cross-linked by a rigid triangular central core through the 24 vertices from the 5-position of each isophthalate moiety. These highly-connected rht-MOFs have demonstrated a series of specific advantages, such as the absence of framework interpenetration and the high concentration of open-metal sites, which make them promising porous materials in gas storage and separation applications.^19–36

For example, the groups of Zhou,^24–26 Schröder^27–29 and Hupp^30,31 reported a variety of rht-MOFs constructed from phenyl and/or alkyl cross-linked hexacarboxylate ligands. Bai³² and Li³³ groups developed acrylamide and secondary amine-decorated rht-MOFs to enhance the uptake of CO₂. We³⁴ and other groups^35,36 independently reported the same three triazole functionalized rht-MOF NTU-105 exhibiting high H₂ and CO₂ uptake properties. It was found that all of the dendritic hexacarboxylate linkers in the cases of rht-MOFs are symmetric, meaning that the same connecting groups were employed between the core unit and the three isophthalates in the ligands (Scheme 1). However, MOFs constructed by unsymmetrical hexacarboxylate ligands are rarely reported.

Scheme 1 Synthetic strategy and an example in this work.

Herein, by utilizing the strategy for reducing symmetry as proposed in Scheme 1, we designed and synthesized an unsymmetrical hexacarboxylate ligand, i.e., 5,5′-(4,4′-(5-(3,5-dicarboxyphenyl-carbamoyl)-1,3-phenylene)bis(1H-1,2,3-tri-azole-4,1-diyl))diisophthalic acid, to construct a MOF Cu-ABTA under the solvothermal reaction with Cu(NO₃)₂. Powder X-ray diffraction investigation indicated that Cu-ABTA still possesses the (3,24)-connected rht-network. The gas sorption measurements revealed that Cu-ABTA shows a high porosity and a very high H₂ uptake capability with a strong binding affinity as demonstrated by its enhanced isosteric heat of adsorption.

Experimental

Materials and methods

Unless specifically mentioned, all chemicals are commercially available and were used as received. Tetrahydrofuran (THF) and triethylamine (TEA) were distilled over sodium/benzophenone ketyl and CaH₂ under nitrogen atmosphere prior to use, respectively. Di-tert-butyl 5-azidoisophthalate and MOFs NTU-105 (ref. 34) and Cu-TPBTM (ref. 32) were prepared as described in the literature for comparison. IR-spectra were recorded as KBr-pellet on a Perkin-Elmer 1760X FT-IR spectrometer. NMR spectra were taken on a Bruker AV400 at room temperature. EI mass spectra were obtained in positive ion mode on a Waters GCT Premier. MALDI-TOF-MS were taken on a Bruker Daltonics Microflex spectrometer. High-resolution mass spectral measurements were carried out on a Waters Q-tof Premier MS. Elemental analyses (C, H, and N) were obtained from a EuroVector Euro EA Elemental Analyzer. Thermogravimetric analysis (TGA) was carried out on a TGA-Q500 thermoanalyzer with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The powder X-ray diffraction (PXRD) measurements were taken on a Bruker D8 diffractometer using Cu-K_α radiation (λ = 1.5418 Å) at room temperature.

Synthesis of organic linker and MOF

Di-tert-butyl 5-(3,5-diiodobenzamido)isophthalate (4). 3,5-Diiodobenzoic acid (0.32 g, 0.86 mmol) and di-tert-butyl 5-aminoisophthalate (0.25 g, 0.86 mmol) were added to dry THF (20 mL). Then, EDC·HCl (0.18 g, 0.95 mmol) was added to the above mixture, which was heated to reflux overnight. The reaction mixture was cooled to room temperature and evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH₂Cl₂/EtOAc, 100 : 2) to give compound 4 as a white solid (0.31 g, 0.48 mmol, yield: 56%) with the following properties: ¹H NMR (400 MHz, CDCl₃) δ 8.37 (s, 3H), 8.24 (d, J = 1.2 Hz, 1H), 8.15 (d, J = 1.4 Hz, 2H), 7.85 (s, 1H), 1.61 (s, 18H). ¹³C NMR (101 MHz, CDCl₃) δ 164.73, 163.47, 148.42, 137.90, 137.84, 135.70, 133.17, 126.92, 125.26, 95.14, 82.08, 28.32. EI-MS: m/z calcd for C₂₃H₂₅I₂NO₅: 649.3, found: 649.0 [M]⁺.

Di-tert-butyl 5-(3,5-bis((trimethylsilyl)ethynyl)benzamido)is ophthalate (5). Compound 4 (0.53 g, 0.82 mmol) was dissolved into a mixture of newly-distilled THF (20 mL) and TEA (10 mL) under argon. Then, CuI (20 mg), Pd(PPh₃)₄ (60 mg) and trimethylsilylethyne (0.19 g, 1.93 mmol) were added into the reaction mixture successively, then stirred at 50 °C under Ar atmosphere overnight. Upon completion of the reaction, the solution was evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH₂Cl₂/EtOAc, 100 : 2) to give compound 5 as a light brown solid (0.46 g, 0.78 mmol, yield: 95%) with the following properties: ¹H NMR (400 MHz, CDCl₃) δ 8.40 (d, J = 1.4 Hz, 2H), 8.38 (d, J = 1.3 Hz, 1H), 7.89 (d, J = 1.5 Hz, 2H), 7.87 (s, 1H), 7.74 (t, J = 1.4 Hz, 1H), 1.62 (s, 18H), 0.26 (s, 18H). ¹³C NMR (101 MHz, CDCl₃) δ 164.76, 164.46, 138.34, 138.08, 134.99, 133.34, 130.38, 126.77, 125.00, 124.52, 102.96, 96.93, 82.01, 28.33, −0.02. MALDI-TOF MS: m/z calcd for C₃₃H₄₃NO₅Si₂: 589.9, found: 612.1 [M + Na]⁺.

Di-tert-butyl 5-(3,5-diethynylbenzamido)isophthalate (6). Excess K₂CO₃ (1.38 g, 10 mmol) was added to a solution of compound 5 (0.68 g, 1.15 mmol) in CH₃OH (10 mL) and CH₂Cl₂ (10 mL) and then stirred at room temperature for 5 h. The mixture was poured into water (30 mL) and extracted by CH₂Cl₂ (2 × 50 mL). The combined organic layer was washed by diluted HCl (0.1 M, 80 mL × 1) and brine (70 mL × 2), dried over anhydrous Na₂SO₄ and evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH₂Cl₂/EtOAc, 100 : 3) to give compound 6 as a white solid (0.50 g, 1.12 mmol, yield: 97%) with the following properties: ¹H NMR (400 MHz, CDCl₃) δ 8.39 (s, 3H), 7.96 (d, J = 1.5 Hz, 2H), 7.91 (s, 1H), 7.77 (t, J = 1.4 Hz, 1H), 3.19 (s, 2H), 1.61 (s, 18H). ¹³C NMR (101 MHz, CDCl₃) δ 164.77, 164.55, 138.58, 138.02, 135.30, 133.27, 131.09, 126.86, 125.21, 123.55, 82.05, 81.66, 79.59, 28.31. EI-MS: m/z calcd for C₂₇H₂₇NO₅: 445.5, found: 445.2 [M]⁺.

Tetra-tert-butyl 5,5′-(4,4′-(5-(3,5-bis(tert-butoxycarbonyl)phenylcarbamoyl)-1,3-phenylene)bis(1H-1,2,3-triazole-4,1-diyl))diisophthalate (^tBu6-1). Compound 6 (0.42 g, 0.94 mmol) and di-tert-butyl 5-azidoisophthalate (0.75 g, 2.35 mmol) were added into a mixture of THF/H₂O (40 mL/10 mL), which was degassed by Ar for 1 h. Then, CuSO₄ (60 mg) and sodium ascorbate (120 mg) were added into the mixture, which was stirred at 60 °C under Ar for 3 days. Then, the mixture was cooled down to room temperature and extracted by CH₂Cl₂ (50 mL × 2). The combined organic layers were dried over Na₂SO₄, and evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH₂Cl₂/EtOAc, 100/1) to give compound ^tBu6-1 as a white solid (0.56 g, 0.52 mmol, yield: 55%) with the following properties: ¹H NMR (400 MHz, CDCl₃) δ 8.70 (s, 1H), 8.68 (d, J = 1.2 Hz, 2H), 8.57 (d, J = 1.1 Hz, 4H), 8.55 (s, 2H), 8.53 (d, J = 1.3 Hz, 4H), 8.51 (s, 1H), 8.41 (s, 1H), 1.66 (s, 36H), 1.63 (s, 18H). ¹³C NMR (101 MHz, CDCl₃) δ 165.46, 164.88, 163.83, 147.44, 138.61, 136.99, 136.10, 134.28, 133.05, 131.36, 130.50, 126.42, 125.99, 125.14, 124.70, 118.95, 117.88, 82.83, 81.85, 28.30. EI-MS: m/z calcd for C₅₉H₆₉N₇O₁₃: 1084.2, found: 1106.1 [M + Na]⁺.

5,5′-(4,4′-(5-(3,5-Dicarboxyphenylcarbamoyl)-1,3-phenylene)bis(1H-1,2,3-triazole-4,1-diyl))diisophthalic acid (H6-1). Excess trifluoroacetic acid (TFA, 10 mL) was added to a solution of the ester ^tBu6-1 (0.56 g, 0.52 mmol) in CH₂Cl₂ (10 mL). The mixture was stirred at room temperature over 6 h, obtaining a white precipitate. The suspension was filtered, washed with CH₂Cl₂ to remove the excess TFA, and then dried in air, giving H6-1 as a white solid (0.33 g, 0.45 mmol, yield: 87%) with the following properties: ¹H NMR (400 MHz, DMSO) δ 10.87 (s, 1H), 9.82 (s, 2H), 8.82 (s, 1H), 8.76 (d, J = 1.3 Hz, 4H), 8.74 (d, J = 1.4 Hz, 2H), 8.61 (d, J = 1.3 Hz, 2H), 8.54 (s, 2H), 8.24 (t, J = 1.4 Hz, 1H). ¹³C NMR (101 MHz, DMSO) δ 166.57, 165.69, 164.80, 146.81, 139.68, 136.84, 135.38, 133.05, 131.50, 130.90, 129.38, 125.16, 125.05, 124.81, 124.29, 123.43, 120.07. ESI-TOF-HRMS: m/z calcd for C₃₅H₂₂N₇O₁₃ 748.1276, found: 748.1273 [M + H]⁺.

MOF Cu-ABTA (Cu₃(1)(H₂O)₃·10DMF·12H₂O). Compound H6-1 (23 mg, 0.031 mmol) and Cu(NO₃)₂·3H₂O (23 mg, 0.095 mmol) were dissolved in N,N-dimethylformamide (DMF, 10 mL). Then, 10 drops of concentrated HNO₃ were added into the mixture. The solution was placed in a tightly capped 20 mL vial and heated in an oven at 75 °C for 3 days. The blue-green block crystals were collected after cooling to room temperature. Then, these crystals were washed several times with fresh DMF and dried in air (25 mg, yield: 55%). Elemental analysis calcd (%) for Cu₃(1)(H₂O)₃·10DMF·12H₂O: C 40.38, H 6.00, N 12.32; found: C 40.16, H 6.58, N 13.15. Selected IR (KBr) (cm⁻¹): 1658 (s), 1563 (m), 1423 (m), 1385 (s), 1102 (m), 1057 (m), 776 (m), 728 (m).

Gas sorption

Low-pressure gas sorption measurements were performed by using Quantachrome Instruments Autosorb-iQ (Boynton Beach, Florida, USA) with extra-high purity gases. The as-synthesized MOF NTU-105, Cu-TPBTM and Cu-ABTA crystals were immersed in CH₂Cl₂ for 12 h, during which time CH₂Cl₂ was replaced three times. The samples were then moved into a sample cell and dried under vacuum at 60 °C and 120 °C by using the “outgasser” function for 12 h and 5 h before the measurements, respectively. The Brunauer–Emmett–Teller (BET) surface area and the pore size distribution using the Non-Local Density Functional Theory (NL-DFT) model were calculated based on the N₂ sorption isotherm at 77 K in the Quantachrome ASiQwin 2.01 software package. The isosteric heat of adsorption (Q_st) for H₂ and CO₂, defined as
was determined by using the H₂ and CO₂ absorption isotherms at 77 and 87 K as well as 273 and 298 K, respectively (ASiQwin 2.01).

Results and discussion

Based on our reported MOF NTU-105,³⁴ herein, we changed one of the triazole units into an acrylamide group in order to design and synthesize an unsymmetric, hexatopic carboxylate ligand, i.e., 5,5′-(4,4′-(5-(3,5-dicarboxyphenylcarbamoyl)-1,3-phenylene)bis(1H-1,2,3-triazole-4,1-diyl))diisophthalic acid (H6-1). The synthetic procedure is shown in Scheme 2. Briefly, 3,5-diiodobenzoic acid and di-tert-butyl 5-aminoisophthalate were reacted in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) to give compound 4. The Sonogashira reaction of 4 with excess (trimethylsilyl)acetylene (TMSA) yielded compound 5, which was then deprotected by excess K₂CO₃ in the mixture of CH₃OH and CH₂Cl₂ to yield compound 6. Diethynyl 6 was reacted with di-tert-butyl 5-azidoisophthalate 7 in the presence of copper sulfate and sodium ascorbate under typical “click reaction” conditions to produce the tert-butyl ester ^tBu6-1, which was hydrolyzed in the mixture of trifluoroacetic acid (TFA) and CH₂Cl₂ to give the target ligand H6-1 with a high yield. All the compounds were characterized by ¹H NMR, ¹³C NMR and mass spectrum to confirm their identities.

Scheme 2 Synthetic route for ligand H6-1.

A solvothermal reaction of H6-1 and Cu(NO₃)₂ in N,N-dimethylformamide (DMF) in the presence of a small amount of HNO₃ at 75 °C for 3 days yielded block green-blue crystals of MOF Cu-ABTA (Cu₃(1)(H₂O)₃·10DMF·12H₂O). The phase purity of a bulk sample was confirmed from powder X-ray diffraction (PXRD) measurements. From the comparison of Fourier transform infrared (FT-IR) spectra of MOF Cu-ABTA and ligand H6-1 as shown in Fig. 1, the band at ∼3070 and 2600 cm⁻¹ corresponding to the O–H stretching vibration in the ligand disappeared completely in that of the MOF. In addition, the band at 1712 cm⁻¹ attributed to C=O vibration of the linker shifted to 1658 cm⁻¹ in that of the MOF. These results clearly confirm the coordination of copper ion with carboxylate units of the ligand.

Fig. 1 FT-IR spectra of ligand H6-1 and as-synthesized MOF Cu-ABTA.

The crystals of as-synthesized MOF Cu-ABTA show a high quality under microscope (Fig. S1 in ESI†). Its PXRD pattern is almost identical to the simulated one based on the single crystal data of MOF NTU-105 (ref. 34) as shown in Fig. 2, along with the very good matches between these peaks (Fig. S2 in ESI†). Furthermore, the simulation for PXRD data of Cu-ABTA shows a tetragonal space group I4/m with cell parameters a = 30.03 Å, b = 30.03 Å, c = 43.11 Å, α = β = γ = 90°, which are very close to those of NTU-105 (Fig. S3 in ESI†). The results of PXRD comparison and simulation indicate that Cu-ABTA still possesses the same (3,24)-connected rht-framework as our reported MOF NTU-105 (Fig. 3).

Fig. 2 Powder XRD patterns of as-synthesized MOF Cu-ABTA (upper) and simulation from MOF NTU-105 (lower).

Fig. 3 Schematic representation of (3,24)-connected rht-type network in MOF Cu-ABTA.

To remove the solvent guest molecules inside the framework, fresh MOF Cu-ABTA crystals were immersed several times in CH₂Cl₂ and degassed stepwise under a dynamic vacuum at 60 and 120 °C. A color change from green-blue to deep purple-blue was observed, which is a typical phenomenon for dicopper paddlewheel frameworks, indicating the generation of open Cu^II sites. The desolvated MOF Cu-ABTA sample was subject to nitrogen sorption measurement at 77 K, as shown in Fig. 4. The isotherm exhibits a reversible pseudo-type I sorption behavior with a small step prior to the plateau, indicating the presence of hybrid micro- and mesopores within the framework. The overall uptake of nitrogen is 766 cm³ g⁻¹ at 1 atm. The Brunauer–Emmett–Teller (BET) surface area and the total pore volume calculated from the nitrogen sorption isotherm are 2840 m² g⁻¹ and 1.19 cm³ g⁻¹, respectively, which are comparable with some rht-type MOFs, although a little bit lower than our reported NTU-105 (Table S1 in ESI†). The pore size distribution analysis based on the nonlocal density functional theory (NLDFT) indicates that the materials exhibit three dominant pore diameters centered at 1.11, 1.27 and 2.03 nm, respectively (Fig. S6†).

Fig. 4 Nitrogen sorption isotherm for MOF Cu-ABTA at 77 K (STP = standard temperature and pressure; filled symbols, adsorption; open symbols, desorption. Inset shows the BET specific surface area plot).

Due to its high porosity, we investigated the hydrogen uptake capability of MOF Cu-ABTA. As shown in Fig. 5, the desolvated framework demonstrates a very high uptake capacity up of to 307 cm³ g⁻¹ (2.75 wt%) at 77 K and 1 atm, which is among the highest values at low pressure in comparison with other isoreticular rht-MOFs under the same conditions (Table S1 in ESI†). To further investigate the binding affinity between hydrogen and the framework of Cu-ABTA, the isosteric heat of adsorption (Q_st) for H₂ was calculated from the isotherms at 77 and 87 K by using the Clausius–Clapeyron equation. In the uptake range of 12–166 cm³ g⁻¹, the Q_st value for Cu-ABTA is decreased from 7.36 to 6.20 kJ mol⁻¹, which is even higher than that of our reported MOF NTU-105 under the same uptake amount (5.96–5.17 kJ mol⁻¹, Fig. 5 and Fig. S8–S12 in ESI†). The high Q_st for Cu-ABTA indicates a stronger binding affinity between the hydrogen and framework, further showing that Cu-ABTA is a highly promising porous material for clean energy H₂ uptake and storage.

Fig. 5 Hydrogen adsorption isotherms for MOF Cu-ABTA at 77 and 87 K, respectively (upper); the comparison of isosteric heat for hydrogen adsorption between MOFs Cu-ABTA and NTU-105 under the same uptake amount (lower).

Furthermore, selective uptake of CO₂ for MOF Cu-ABTA was conducted to evaluate its potential application in gas separation. As shown in Fig. 6, CO₂ sorption isotherms reveal an uptake of 153 cm³ g⁻¹ (30.1 wt%) at 273 K and 1 atm with a Q_st of 21.1–20.7 kJ mol⁻¹ in the range of 0.96–80.1 cm³ g⁻¹ (Fig. S14†). For comparison, the N₂ and CH₄ uptake of Cu-ABTA under the same conditions were 7 and 29 cm³ g⁻¹, respectively, which shows that Cu-ABTA possesses a high selectivity toward CO₂ over N₂ and CH₄, indicating it should be a highly promising candidate for CO₂-selective adsorption.

Fig. 6 Various gas adsorption isotherms for MOF Cu-ABTA.

Conclusions

In summary, an unsymmetrical hexacarboxylate ligand possessing one acylamide and two triazole groups has been designed, synthesized and subsequently used to construct copper(II) rht-MOF Cu-ABTA via solvothermal reaction. Powder X-ray diffraction investigation indicated that the MOF still possesses the (3,24)-connected rht-network. The gas sorption measurements revealed that it shows a high porosity and very high capability for the selective capture of carbon dioxide and for hydrogen uptake with a strong binding affinity. The strategy of reducing ligand symmetry presented in this work may provide a new way to design and construct novel types of MOFs for exploring their potential applications in clean energy storage and environmental sustainability.

Acknowledgements

We are grateful for financial support from National Natural Science Foundation of China (21302072), Natural Science Foundation of Jiangsu Province (BK20130226), Natural Science Foundation of Jiangsu Higher Education Committee (13KJB150014), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China, Jiangsu Normal University (12XLR015 and 12XLR018), Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials (K201302), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The work was also supported by the National Research Foundation (NRF), Prime Minister's Office, Singapore under its NRF Fellowship (NRF2009NRF-RF001-015) and Campus for Research Excellence and Technological Enterprise (CREATE) Programme–Singapore Peking University Research Centre for a Sustainable Low-Carbon Future, as well as the NTU-A*Star Centre of Excellence for Silicon Technologies (A*Star SERC no.: 112 351 0003).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07101h

‡ These authors contributed equally to this work.

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