An amino-decorated NbO-type metal–organic framework for high C₂H₂ storage and selective CO₂ capture

Abstract

A novel amino-decorated NbO-type metal–organic framework [Cu₂(C₂₂H₁₁NO₈)(H₂O)₂]·(DMF)₆·(H₂O), (ZJU-8, ZJU = Zhejiang University; DMF = N,N-dimethylformamide) has been synthesized and structurally characterized. The activated ZJU-8a exhibits both a high C₂H₂ uptake capacity (195 cm³ g⁻¹ at 1 bar) and a moderately high adsorption selectivity for CO₂ over N₂ (12.27) and CH₄ (7.38) at room temperature.

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1. Introduction

Emerging as a novel class of crystalline porous materials, microporous metal–organic frameworks (MOFs) have great potential applications in gas storage and separation due to their large surface areas, tunable pore sizes and shapes.^1–4 Meanwhile, the pore surfaces can be systematically modified through the immobilization of specific sites, such as open metal sites and functional groups including –NH₂, and –COOH, to induce their stronger interactions with gas molecules.^5,6

The NbO-type series of MOFs, self-assembled from a tetratopic bis(isophthalic acid) linker and a copper paddle-wheel Cu₂(COO)₄ cluster, have already attracted extensive attention since the first NbO-type MOF-505 was reported ten years ago.^7–9 Specifically, ligand extension and decoration are good strategies to tune their porosities and functionalize their pore surfaces for gas storage and separation. For example, the introduction of the Lewis basic sites into the framework leads ZJU-5 to exhibit stronger interaction with C₂H₂ molecules,^7b while the central “dynamic” pyrimidine groups within UTSA-76 significantly enhances the methane storage and working capacities.^7c Herein, we report a novel amino-decorated NbO-type metal–organic framework [Cu₂(L)(H₂O)₂]·(DMF)₆·(H₂O) (ZJU-8, ZJU = Zhejiang University; DMF = N,N-dimethylformamide) built from the tetracarboxylate link H₄L (Scheme 1) (H₄L = 2′-amino-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid). The activated ZJU-8a exhibits high porosity with a Brunauer–Emmett–Teller (BET) surface area of 2501 m² g⁻¹ and pore volume of 1.0224 cm³ g⁻¹. With multifunctional adsorption sites, ZJU-8a shows a high C₂H₂ uptake capacity of 195 cm³ g⁻¹ at 1 bar and 298 K; meanwhile, ZJU-8a exhibits a moderately high adsorption selectivity for CO₂/N₂ (15/85) of 12.27 and CO₂/CH₄ (50/50) of 7.38 at 298 K, respectively.

Scheme 1 The organic linker H₄L for the construction of ZJU-8.

2. Experimental

2.1. Gas sorption measurements

A Micromeritics ASAP 2020 surface area analyser was used to measure gas sorption isotherms. In order to remove guest solvent molecules in the framework, the as-synthesized sample of ZJU-8 was treated with anhydrous acetone for 3 days and then activated at 373 K under high vacuum until the outgas rate was <5 μm Hg min⁻¹ before measurements.

2.2 Fitting of pure component isotherms

Experimental adsorption isotherms for CO₂, N₂ and CH₄ in ZJU-8a were measured at 273 and 298 K and fitted on the basis of the dual site Langmuir–Freundlich equation:where p (kPa) is the pressure of the bulk gas at equilibrium with the adsorbed phase, N (mol kg⁻¹) is the adsorbed amount per mass of adsorbent, N^max₁ and N^max₂ (mol kg⁻¹) are the saturation capacities of sites 1 and 2, b₁ and b₂ (kPa⁻¹) are the affinity coefficients of sites 1 and 2, and n₁ and n₂ represent the deviations from an ideal homogeneous surface. The fitting parameters of DSLF equation are presented in Table S3.†

Adsorption isotherms and gas selectivities calculated by Ideal Adsorbed Solution Theory (IAST)¹⁰ for mixed CO₂/N₂ (15/85) and CO₂/CH₄ (50/50) in the ZJU-8a. The adsorption selectivities, S_ads, are defined by the following equation:

in which p_i the bulk gas pressure of species i, and q_i the component molar loading of species i.

2.3 Estimation of the isosteric heats of gas adsorption

A virial-type expression was employed to calculate the enthalpies of adsorption on ZJU-8a. The data were fitted using the equation:where the p is pressure, n is the amount adsorbed, and A₀, A₁, etc., are virial coefficients.

The isosteric heat of adsorption, Q_st, defined as

2.4 Synthesis and characterization of ZJU-8

A mixture of the organic link H₄L (15 mg, 0.0356 mmol) and Cu(NO₃)₂·2.5H₂O (30 mg, 0.0645 mmol) was dissolved into 8 mL mixed DMF/H₂O solvent (3/1, v/v) in a 20 mL screw-capped vial, then 100 μL HCl (37%, aq.) was added, the mixture was sonicated until homogenous. The vial was capped and placed in a precise oven at 80 °C for 1 day. The resulting blue rhombic shaped crystals were washed with DMF three times to afford ZJU-8 materials. Yield: 68%, calculated based on the ligand. According to the single-crystal structure determination, elemental analyses and thermogravimetric analysis (Fig. S1†), ZJU-8 can be formulated as [Cu₂(L)(H₂O)₂]·(DMF)₅·(H₂O). TGA under a nitrogen atmosphere reveals a weight loss of 43.77% up to 528 K, corresponding to the loss of free DMF and H₂O molecules and terminal water molecules (calcd 43.52%). Elemental analysis: calcd (C₃₇H₅₂Cu₂N₆O₁₆, %): C, 46.10; H, 5.03; N, 8.72; found: C, 46.01; H, 5.31; N, 9.13.

3. Result and discussion

ZJU-8 was synthesized via a solvothermal reaction of Cu(NO₃)₂·2.5H₂O and amino-decorated ligand H₄L at 80 °C. The structure was characterized by single-crystal X-ray diffraction analysis and is isoreticular to other NbO-type MOFs. The framework of ZJU-8 is built from paddle-wheel Cu₂(CO₂)₄ secondary building units (SBUs) which were connected by L⁴⁻. The copper atoms will become unsaturated upon axial aqua ligand removal. In the crystal structure, there exist two kinds of cages (Fig. 1) which are alternately stacked along the c axis. The first cage with a spherical-like pore of about 10.5 Å in diameter is formed from six ligands and six paddlewheel SBUs; while the second one with a shuttle-shaped cage of approximately 9.5 × 22.5 Å is constructed from twelve ligands and six paddlewheel SBUs, each cage has open triangular windows of 5.0 Å in dimension along c axis. PLATON program¹¹ showed the accessible pore volume of ZJU-8 is 63.3% (7355 ų out of 11 618 ų per unit cell volume), which is slightly smaller than the non-modified prototype NOTT-101 (ref. 8a) of 67.4% (7801 ų out of 11 569 ų per unit cell volume). After removal of the coordinated water, the framework density is 0.690 g cm⁻³.

Fig. 1 (a) a spherical-like cage of about 10.5 Å in diameter and (b) a shuttle-shaped cage of about 9.5 × 22.5 Å (Cu, blue; C: gray; O: red; H: black) in the crystal structure of ZJU-8.

To confirm the permanent porosity of ZJU-8a, N₂ sorption isotherm at 77 K was measured. Prior to measurements, the acetone-exchanged sample of ZJU-8 was vacuum-dried at 373 K to obtain the activated ZJU-8a. The phase purity of both the fresh and de-solvated samples were confirmed by powder X-ray diffraction (Fig. S2†), suggesting ZJU-8a maintains its crystalline nature. The reversible type-I N₂ sorption isotherm at 77 K (Fig. 2) clearly indicated the microporous nature of the framework. At P/P₀ = 0.99, ZJU-8a can adsorb 661 cm³ g⁻¹ N₂ with the corresponding pore volume of 1.0224 cm³ g⁻¹. The calculated Brunauer–Emmett–Teller (BET) surface area is 2501 m² g⁻¹ (Fig. S4†), which is slightly larger than the nitro-decorated NJU-Bai 14 (ref. 9a) of 2384 m² g⁻¹, benzothiadiazole-based ZJNU-40 (ref. 7g) of 2209 m² g⁻¹ and naphthyl-modified ZJU-7 (ref. 7e) of 2198 m² g⁻¹. Obviously, the pore volume and BET surface area of ZJU-8a are systematically smaller than those of the prototype NOTT-101 (ref. 7f) (1.080 cm³ g⁻¹ and 2805 m² g⁻¹) because of the occupancy of the amino groups inside the pore spaces.

Fig. 2 N₂ sorption isotherms of ZJU-8a at 77 K. Solid symbols: adsorption, open symbols: desorption.

The explosive nature of C₂H₂ needs us to store it under suitable low pressure. Among diverse porous materials, MOFs are promising ones for such an important application. The high surface area, open metal sites and functional amino groups within the framework of ZJU-8a encouraged us to study its C₂H₂ uptake capacity. C₂H₂ sorption isotherms (Fig. 3) were measured at both 273 K and 298 K. ZJU-8a exhibits high acetylene uptakes of 272 and 195 cm³ g⁻¹ at 1 bar at 273 K and 298 K, respectively. The gravimetric acetylene storage at 298 K of 195 cm³ g⁻¹ uptakes is among the highest for MOFs (e.g. HKUST-1 (ref. 12a) (201 cm³ g⁻¹), CoMOF-74 (ref. 12b) (197 cm³ g⁻¹), ZJU-5 (ref. 7b) (193 cm³ g⁻¹), ZJU-70 (ref. 12c) (191 cm³ g⁻¹), NOTT-101 (ref. 12d) (184 cm³ g⁻¹), PCN-16 (ref. 7i) (176 cm³ g⁻¹)) and FJI-H8 (ref. 12e) (224 cm³ g⁻¹). To our best knowledge, ZJU-8a shows the highest gravimetric acetylene uptake capacity among these series of NbO-type MOF materials at room temperature (Table S4†). Considering the framework density of 0.690 g cm⁻³, the volumetric capacities are 188 and 134 cm³ cm⁻³ at 273 K and 298 K, respectively. The initial Q_st of acetylene adsorption is 29.6 kJ mol⁻¹ (Fig. S5†), which is comparable to those in other MOFs with copper paddlewheels.

Fig. 3 C₂H₂ sorption isotherms at 273 K and 298 K. Black: 273 K; red: 298 K; solid symbols: adsorption, open symbols: desorption.

Carbon dioxide (CO₂), which is a major greenhouse gas, has attracted more and more attention owing to the greenhouse effect. MOF materials have shown great potential as adsorbents for CO₂ capture and separation to reduce anthropogenic CO₂ emission and lower the concentration of CO₂ in the atmosphere. It has been reported that the incorporating of nitrogen-containing organic groups is capable of increasing CO₂ binding affinity.^13,14 Herein, we inspected the role of –NH₂ group in the process of selective adsorption of CO₂/N₂ and CO₂/CH₄. Accordingly, CO₂, CH₄ and N₂ sorption isotherms were measured at 273 K and 298 K at low pressure. As show in Fig. 4, ZJU-8a can systematically take up much more CO₂ than CH₄ and N₂, indicating that ZJU-8a has potential application in the separation of CO₂/N₂ and CO₂/CH₄ mixtures. ZJU-8a exhibited a CO₂ uptake capacity of 165 cm³ g⁻¹ at 273 K and 95 cm³ g⁻¹ at 298 K at 1 bar, respectively. Remarkably, the gravimetric CO₂ adsorption capacity at room temperature is slightly higher than those of prototype MOF NOTT-101 (ref. 12d) (83 cm³ g⁻¹), NOTT-125 (ref. 13e) (93 cm³ g⁻¹), HNUST-1 (ref. 9b) (93 cm³ g⁻¹), UTSA-40 (73 cm³ g⁻¹)^7j and NU-135 (ref. 9f) (79 cm³ g⁻¹).

Fig. 4 CO₂ (blue), CH₄ (red) and N₂ (green) sorption isotherm at 273 K (a) and 298 K (b). Solid symbols: adsorption, open symbols: desorption.

The well-known Ideal Adsorbed Solution Theory (IAST) was used to calculate the adsorption selectivities and CO₂ uptake of ZJU-8a for the following binary gas mixtures: CO₂/N₂ (15/85) and CO₂/CH₄ (50/50), which mimic post-combustion capture applications and biogas treatment. As shown in Fig. 5, ZJU-8a exhibits high selectivity for these two mixture compositions. Specifically, the IAST selectivity in a 15 : 85 molar ratio of CO₂ and N₂ mixtures has high value of 43.6 at 273 K and 12.3 at 298 K at a very low pressure. The CO₂/CH₄ selectivities were calculated to be 8.2 at 273 K and 7.4 at 298 K at low pressure for an equimolar gas mixture, which are systematically higher than those found in prototype MOF NOTT-101,^7g suggesting that incorporation of –NH₂ group into the framework is a promising method to enhance CO₂ adsorptive separation capacity. Fig. S6 and 7† present IAST calculations for gas uptake from 15/85 CO₂/N₂ and 50/50 CO₂/CH₄ gas mixtures maintained under isothermal conditions at 273 K and 298 K, respectively. Apparently, ZJU-8a can take up much more CO₂ in all the cases. To understand the interaction between the framework and gas molecules, the isosteric enthalpy of adsorption, Q_st, was calculated by using the pure component isotherm fitted by the virial method. As show in Fig. S8–10,† the Q_st of CO₂, CH₄ and N₂ are 21.9, 13.1 and 9.7 kJ mol⁻¹ at zero loading, respectively, indicating that ZJU-8a exhibits a higher binding affinity for CO₂ than those for CH₄ and N₂ during the adsorption process.

Fig. 5 The adsorption selectivity predicted by IAST of ZJU-8a for CO₂/N₂ (15/85) (a) and CO₂/CH₄ (50/50) (b) at 273 and 298 K, respectively.

4. Conclusions

In summary, we have successfully constructed a novel amino-decorated NbO-type metal–organic framework ZJU-8. With high BET surface area of 2501 m² g⁻¹ and pore volume of 1.0224 cm³ g⁻¹, the activated ZJU-8a exhibits high gravimetric acetylene storage capacity of 195 cm³ g⁻¹ at 1 bar at room temperature. Meanwhile, the strategy of incorporating –NH₂ group into framework through the enhanced interactions with acidic CO₂ and narrowing down the pore sizes has been proved a promising method to enhance CO₂ adsorption and separation capacities.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 51272231 and 51229201), Program for Innovative Research Team in University of Ministry of Education of China (IRT13R54), The Project was supported by the Natural Science Foundation of Zhejiang province, China (LZ15E020001), Grant AX-1730 from the Welch Foundation (B. C.).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, Thermogravimetric analysis, synthesis route of the ligand, calculation of BET surface area of ZJU-8, comparison of C₂H₂ adsorption on various NbO-type MOFs and the isosteric enthalpy of adsorption. CCDC 1406816. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra12700a

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