Microporous Metal–Organic Framework Based on Ligand-Truncation

Aug 24, 2017 - By using the ligand-truncation strategy, a microporous metal–organic framework (1) with high surface area was designedly synthesized...
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Microporous Metal−Organic Framework Based on Ligand-Truncation Strategy with High Performance for Gas Adsorption and Separation Jianqiang Liu,†,∥ Wenjing Wang,‡,§,∥ Zhidong Luo,† Baohong Li,† and Daqiang Yuan*,‡ †

School of Pharmacy, Guangdong Medical University, Dongguan 523808, China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China § University of the Chinese Academy of Sciences, Beijing 100049, China ‡

S Supporting Information *

ABSTRACT: By using the ligand-truncation strategy, a microporous metal−organic framework (1) with high surface area was designedly synthesized. MOF 1 shows a new (4, 4)-connected net with a Schläfli symbol of (44.62)(43.62.8)2(42.82.102) and exhibits a high H2 capture capacity (193 cm3 g−1 at 1 atm and 77 K) and selectivities for CO2 over N2 and CH4 at low pressure.

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ylic acid (H6NBPD).6 Later, Liu and his co-workers designed a C2-symmetric bent ligand H4tpta ([1,1′:3′,1″-terphenyl]3,3″,5,5″-tetracarboxylic acid, derived from the common hexacarboxylate compound of 3,3″,5,5″-benzene-1,3,5-triylhexabenzoic acid, as the optimum linker to fabricate MOF JLULiu22 (JLU = Jilin University) (Scheme 1).7 These results revealed that the desymmetrization strategy of ligands in building new MOFs is challenging, and a slight perturbation on the feature of ligands may transform MOFs into completely different structures. Herein, we wish to report a case which confirms the attracting application of the “ligand-truncation” strategy. In this work, we explored the “ligand-truncation” strategy to desymmetrize our previously designed ligand of C3-symmetric H6PHB into a C2-symmetric 2,6-di(3′,5′-dicarboxylphenyl)pyridine (H4L) linker (Scheme 1), which has an appropriate length of 7.5 Å and so is the optimum candidate to fabricate the porous MOFs.1m The H4L ligand with a 120° space angle from the bilateral isophthalate moieties may induce the fabrication of a Cu(II)-paddlewheel threshold that has been found in the well-known MOP-1.1k−m Furthermore, it has been confirmed that the grafting of functionalized N-donor groups (such as amine and pyridine) into the pores of MOFs can obviously

INTRODUCTION As an emerging new porous material, metal−organic frameworks (MOFs) have great potential for gas capture applications due to their adjustable chemical postsynthetic modification, which can be carefully predesigned to modulate and boost gas molecular recognition.1 Several assembling methodologies have been explored for the building of porous MOFs with excellent properties so far.2,3 The most popular way to build new MOFs materials is to use highly symmetric multidentate carboxylate ligands (such as tetracarboxylate and hexacarboxylate ligand) that are parent matrixes, joined by rigid and/or flexible moieties.1 Recently, we have designed and built an uncommon 3,3,5-c PMOF of [Cu4(μ2-O)(PHB)1.5(H2O)2]· (DMF)5(H2O)4 (GDMU-2: GDMU = Guangdong Medical University) (H6PHB = 3,3′,3″,5,5′,5″-pyridyl-1,3,5 -triylhexabenzoic acid) using a less explored hexacarboxylate ligand. It represents the smallest ntt-MOF using an N-functionalized hexacarboxylate ligand and shows high H2 uptake of 240.7 cm3 g−1 at 1 atm and 77 K.4 However, Matzger et al. have proposed a new synthetic strategy named linker-directed matrix desymmetrization, which could be achieved by tailoring substituent entities from high symmetry linkers.5 Li and his co-workers first designed and synthesized a graceful MOF using a desymmetrized ligand 5,5′-(9-oxo-9,10-dihydroacridine-2,7diyl)-diisophthalic acid (H4OADDI) that was generated by the trimming of the ligand 4′,4″,4‴-nitrilotribiphenyl-3,5-dicarbox© XXXX American Chemical Society

Received: April 4, 2017

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DOI: 10.1021/acs.inorgchem.7b00851 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Well-Designed Object of the “Ligand-Truncation” Strategy

could not be modeled properly, the scattering contribution from the disordered guest molecules was removed by the SQUEEZE subroutine of the PLATON software suite. The final formula of 1 was confirmed by crystallographic data combined with other chemical analyses. The refined data and selected parameters for the compound are enumerated in Tables S1 and S2, respectively. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) for 1 indicates a rapid weight loss of 41.3% from room temperature to 285 °C, which accords with the departure of disordered solvents and H2O molecules (calcd: 46.1%). MOF 1 then decomposes with the increase of temperature; a weight loss of 32.6% is ascribed to the loss of the organic L ligands (calcd: 39.1%) (Figure S2). The powder XRD pattern for 1 after adsorption− desorption tests is also in good agreement with the simulated one, indicating the stability of the material (Figure S1). Also, the powder XRD patterns of different solvent exchanged samples are similar to the simulated one, which proved the stability of the structure. The IR, PXRD, and TGA of desorbed samples can also confirm its stability (Figures S1−S3). Gas Sorption. Low-pressure gas sorption isotherms were carried out on a Micromeritics ASAP 2020 analyzer. The sample was first degassed at 80 °C for 10 h to remove the adsorbed solvents before measurement. The ultrahigh purity (UHP, 99.999% purity) grade of N2, H2, CH4, and CO2 gases were used throughout the adsorption experiments. The N2 isotherm was measured at 77 K (liquid nitrogen temperature). Pore size distribution (PSD) data were obtained from the N2 isotherms based on the no-local density functional theory (NLDFT) model. The isosteric heat of adsorption (Qst) for H2 was calculated using the data collected at 77 and 87 K. The data were fitted first using a virial-type expression composed of parameters ai and bi (eq 1). Then, the Qst was calculated from the fitting parameters using eq 2, where p is the pressure, T is the temperature, R is the universal gas constant, N is the amount adsorbed, and m and n determine the number of terms required to adequately describe the isotherm. In order to evaluate the efficacy of 1 for CO2/CH4 and CO2/N2 separation, the selectivity was calculated by ideal adsorbed solution theory (IAST) along with the pure component isotherm fits.

augment the CO2 uptake capacity and selectivity. We present the successful fabrication of a new MOF {[Cu2(L)(H2O)6]n· 4nDMF} (1), which contains both the unsaturated metal sites and accessible pyridine units. Interestingly, 1 does not have the MOP feature, but it preserves the paddlewheel cluster similar to those observed in GDMU-2. Fascinatingly, 1 preserves the porous characteristic with the alignment of open metal sites and functional sites, shows a high performance for H2 capture (193 cm3 g−1 at 77 K), and also shows selective separation of CO2 over N2 and CH4.

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EXPERIMENTAL SECTION

Materials and Instrumentation. The ligand was obtained from Jinan Henghua Sci. & Tec. Co. Ltd. without further purification. Powder X-ray diffraction (PXRD) patterns were collected at room temperature on a Rigaku D/max-2550 diffractometer using Cu Kα radiation (λ = 1.5418 Å). Elemental analyses for C, H, and N were measured by vario MICRO (Elementar, Germany). The thermal gravimetric analyses (TGA) were performed on TGA Q500 thermogravimetric analyzer under a N2 condition with a heating rate of 10 °C min−1. Gas adsorption isotherms were measured by a Micromeritics ASAP 2020 M surface area analyzer. Synthesis of {[Cu2(L)(H2O)6]n·4nDMF}. A single crystal of compound 1 was obtained by the mild reaction of Cu(NO3)2·3H2O (0.15 mmol) and 2,6-di(3′,5′-dicarboxylphenyl)pyridine (H4L) (0.05 mmol) in DMF (2 mL) with HNO3 (0.45 mL) at 105 °C for 72 h. After the mixture was cooled to room temperature, bright-blue block crystals were obtained (yield 55%, based on Cu). C33H49Cu2N5O14 (%) Anal. Calcd: C, 40.61; H, 4.35; N, 4.98. Found: C, 40.69; H, 4.60; N, 4.55. FTIR (KBr): 3321 (w), 1714 (m), 1603 (m), 1561 (s), 1436 (m), 1353 (s), 1248 (m), 1109 (w), 922 (w), 890 (w), 768 (s), 727 (m), 643 (m) cm−1. To confirm the purity and the stability of 1, the PXRD matching analyses were carried out by the LeBail route. By comparing the calculated and experimental patterns, the fitting results showed very good agreement with the initial structure, which indicated the as-synthesized product is pure and stable (Figure S1 and Table S3). Single-Crystal X-ray Structure Determination. Crystallographic data for 1 were collected on a Bruker Apex-II CCD diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 123 K. The structure was solved by direct methods and refined by full-matrix least-squares on F2 using SHELXL-2014/6. All non-hydrogen atoms were refined anisotropically. Since the highly disordered guest molecules were trapped in the channels of 1 and

ln p = ln N +

1 T

m

n

∑ aiNi + ∑ biNi i=0

i=0

(1)

m

Q st = − R ∑ aiNi i=0

(2)

CO2/N2 Breakthrough Curve Measurements. The column contained 0.270 g of crystal 1 for the experiment using a 15:85 CO2/ B

DOI: 10.1021/acs.inorgchem.7b00851 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry N2 mixture. The flow rates (mL min−1 at 293 K and 101.325 kPa) of gases were regulated by mass flow controllers. Before breakthrough experiments, the samples were activated at 373 K for 12 h under vacuum condition. After that, the columns were filled with a He flow. In the meantime, as a carrier gas, the He was also used to clean the system. Then, the CO2/N2 (15:85, 4 mL min−1) mixture gas was stabilized by flowing through the alternative vent line for 20 min before being introduced to the column.

channel in the whole structure (Figure 1c), and the channel has a maximum pore diameter of 7.65 Å and pore limiting diameter of 6.05 Å calculated by Poreblazer.9 This phenomenon is also found for PCN-305.7 The topology of the structure can be simplified to a trinodal 4-c net, where the nodes represent the 4-connecting ligand and the two 4-connecting SBUs (Figure 1d). This net has (44.62)(43.62.8)2(42.82.102) topology. Compared to PCN-305, it adopts the 4-nodal net with a topological symbol of (4.64.8)2(42.64)(64.82)2(66). The different topology can be ascribed to the distinct SBU. The calculation by PLATON with the probe radius of 1.8 Å illustrates a total solvent accessible volume amounts to 8592.7 Å3 per unit cell, which accounts for 63.9% of the cell volume, offering possibilities for gas adsorption. Gas Adsorption. The permanent porosity of 1 was confirmed by the nitrogen adsorption studies at 77 K (Figure 2a). The nitrogen adsorption of 1 exhibits a reversible type I isotherm and reaches saturation at 526 cm3 g−1 at 1 bar, indicating the microporous nature of the sample. The Brunauer−Emmett−Teller (BET) and Langmuir specific surface areas of 1 are 1860 and 2256 m2 g−1, respectively. The experimental total pore volume of 1 is 0.80 cm3 g−1. The PSD based on NLDFT adsorption models using the nitrogen adsorption data at 77 K exhibits a narrow distribution of micropores at 5.9−22 Å, which is consistent with the internal structural features of cages (Figure S7). Meanwhile, H2, CO2, and CH4 uptake capacities were also measured. At the temperatures of 77 and 87 K, we investigated the gravimetric H2 uptake in 1 (Figure 2b). Both the H2 isotherms show good reversibility without hysteresis, showing that the interaction between H2 and the framework of 1 is physisorption. The H2 uptake capacity is up to 193 cm3 g−1 (1.78 wt %, 8.9 mmol g−1) at 1 atm and 77 K, and 133 cm3 g−1 (1.18 wt %, 5.9 mmol g−1) at 1 atm and 87 K, respectively. Although the value of H2 uptake capacity is less than that of the record MOFs, such as [Cu(Me-4py-trz-ia)] (3.07 wt %)10 and PCN-12 (3.05 wt %),11 it is still comparable with many famous porous MOF materials reported in the literature at the same conditions.1h,12 The high H2 uptake capacity could be due to the alignment of open Cu(II) active sites in 1. Additionally, the Qst for H2 is about 7.5 kJ mol−1 at zero coverage, and then slightly decreases to 5.4 kJ mol−1 at high coverage (Figure S8). The value of the Qst is higher than those of reported MOFs, such as UMCM-150 (7.3 kJ mol−1),13 PCN-46 (7.2 kJ mol−1),14 MOF-5 (4.7−5.2 kJ mol−1),15 and MOF-177 (4.4 kJ mol−1).16

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RESULTS AND DISCUSSION The structure of 1 contains one crystallographically distinct L ligand bridging two distinct types of carboxylate dimer SBUs (Figures S4 and S5). One of the SBUs contains the Cu1 and Cu2 ions, while the other contains two symmetry related Cu3 atoms. Each ligand coordinates to four of these SBUs, two of each type, while each SBU connects four ligands. While all of the carboxylate oxygen atoms of each ligand are coordinated, the pyridyl nitrogen is uncoordinated. Unlike most MOFs bearing isophthalate moieties, 1 does not have cuboctahedral SBUs.8 Instead, it only shows 1D channels along the a and b axes and the free pyridyl groups oriented to the pores (Figure 1a,b and Figure S6). Hence, there is a three-dimensional (3D)

Figure 1. (a, b) View of the two different pores along the crystallographic b and c direction. (c) The three-dimensional nanoporous channel along the crystallographic c direction. (d) Topological representation of the trinodal 4-c net in the structure of 1. Blue spheres represent the ligands, brown spheres represent the Cu1/Cu2 SBUs, and orange spheres represent the Cu3 SBUs.

Figure 2. (a) N2 adsorption isotherm of 1 at 77 K and 1 atm. (b) H2 adsorption isotherm of 1 at 77 and 87 K. C

DOI: 10.1021/acs.inorgchem.7b00851 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Adsorption isotherms for CO2, CH4, and N2 at 273 K. (b) The adsorption selectivity for CO2/CH4 and CO2/N2 at 273 K.

Accession Codes

In addition, single-component low pressure gas adsorptions of CO2, CH4, and N2 were also measured. As shown in Figure 3a, the maximum adsorbed amounts of CO2, CH4, and N2 on 1 are up to 102, 28, and 9 cm3 g−1 at 273 K and 1 bar, respectively. In the meantime, CO2, CH4, and N2 uptake capacity at 293 K and 1 bar were also measured (Figure S9). The reason for preferential adsorption CO2 can be attributed to its much higher quadrupole moment.17 The significant difference of gas uptakes caused us to explore the CO2/CH4 and CO2/N2 separation performance. Here, the IAST method was used to predict binary mixture selectivity. To mimic the composition of the flue gas, IAST calculations were carried out assuming a CO2/N2 binary mixture with a molar ratio of 15:85. As shown in Figure 3b and Figure S10, the values of IAST CO2/N2 selectivity reach 55 and 25.8 at 273 and 293 K, respectively, which indicated that 1 could serve as a potential candidate for postcombustion CO2 capture. The column breakthrough curves of 1 were also measured using a binary 15:85 CO2/N2 mixture at 293 K and 1 bar to simulate more practical CO2 capture applications (Figure S11). The CO2 uptake of 1 in the columns was calculated as 1.1 mmol g−1 for the 15:85 CO2/N2 mixture at 293 K and 1 bar, which is lower than the values obtained from the single-component adsorption isotherms (2.9 mmol g−1 at 293 K and 1 bar). The reason could be attributed to the weak CO2/N2 selectivity and low adsorption rate under gas mixture conditions. On the other hand, the calculated CO2/CH4 selectivity ratio is up to 13.3 and 6.8 under 273 and 293 K from equimolar mixtures (Figure 3b and Figure S10), which is higher than those from the reported comparable structures (PCN-305, PCN-306, PCN-307, PCN308).7 The selectivity of CO2/CH4 makes 1 possibly useful for future natural gas upgrading.

CCDC 1486510 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected]. ORCID

Daqiang Yuan: 0000-0003-4627-072X Author Contributions ∥

J.L. and W.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the grants from the Science and Technology Plan Projects of Dongguan (2016108101005), and the Science Foundation funded project of Guangdong Medical University (Z2016001). We thank S. Ng and S. R. Batten for discussion and help.

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REFERENCES

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CONCLUSIONS In summary, on the basis of the ligand-truncation strategy, we have successfully constructed an uncuboctahedral microporous MOF (1) fabricated using the Cu-paddlewheel and functional tetracarboxylate ligand. MOF 1 exhibits high H2 capture as well as selective separation of CO2 over CH4 and N2.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00851. Additional figures and table providing PXRD, TGA, IR, adsorption information, and additional characterization details (PDF) D

DOI: 10.1021/acs.inorgchem.7b00851 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b00851 Inorg. Chem. XXXX, XXX, XXX−XXX