Microporous Lanthanide Metal–Organic Framework Constructed from

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Microporous Lanthanide Metal−Organic Framework Constructed from Lanthanide Metalloligand for Selective Separation of C2H2/CO2 and C2H2/CH4 at Room Temperature Jing-xin Ma,†,‡ Jiarui Guo,† Hailong Wang,‡ Bin Li,‡ Tianlin Yang,† and Banglin Chen*,‡ †

State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering and College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China ‡ Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States S Supporting Information *

ABSTRACT: A novel pillar-layer porous lanthanide metal−organic framework [Tb3(ODA)3(BPDC)3Na2]n·Gx (UTSA-222, G = guest molecules) was constructed from an organic ligand [1,1′-biphenyl]4,4′-dicarboxylate (BPDC2−) and a lanthanide metalloligand [Tb(ODA)]+ (H2ODA = oxydiacetic acid). The UTSA-222 contains two-dimensional intersecting channels with the Brunauer−Emmett−Teller surface area and pore volume of 703 m2 g−1 and 0.344 cm3 g−1, respectively, for the activated sample. It shows moderately high adsorption selectivity for C2H2/CO2 and C2H2/ CH4 separations at 1 atm and room temperature.

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INTRODUCTION

Lanthanide metal−organic frameworks (Ln-MOFs), due to their specific properties arising from 4f electrons, such as luminescence and magnetism, have received progressively more attention.6 Compared with their d-block transition-metal counterparts, porous Ln-MOFs have been much less developed for their potential application in gas adsorption and separation, partly because of the large coordination spheres and flexible coordination geometries of lanthanide ions. Various strategies have been developed to construct porous Ln-MOFs.7 Among these methods, lanthanide metalloligand approach by making use of [Ln(ODA)3]3− (H2ODA = oxydiacetic acid), has been used to assemble Ln-MOFs or heterometallic MOFs (HMOFs), though without detailed gas sorption studies.8 Detailed studies on the crystal structures of the complexes containing [Ln(ODA)3]3− indicate that [Ln(ODA)]+ units can be easily self-assembled together to form a two-dimensional (2D) layer in which there exist accessible uncoordinated sites of lanthanide ion on the both sides of the plane. If such accessible sites can be occupied by dicarboxylate organic linker to connect the 2D layers together, a pillar-layer Ln-MOF can be formed (Scheme 1). Herein we report the synthesis, structure, and gas separation properties of the novel pillar-layer microporous LnMOF, [Tb3(ODA)3(BPDC)3Na2]n·Gx (UTSA-222), from the lanthanide metalloligand ([Tb(ODA)]+) and a dicarboxylate ligand (BPDC2− = [1,1′-biphenyl]-4,4′-dicarboxylate).

In the past decade, the gas separation materials based on metal−organic frameworks (MOFs) have been extensively investigated.1 Because MOFs can be readily self-assembled from metal ions/clusters with multidentate organic linkers, judicious selection of molecular building blocks can provide enormous opportunities to tune their pore sizes2 and functionalization at the molecular level of such microporous materials for their specific separation of small gas molecules. Recently, a large number of new MOFs as absorbents for gas separations, such as CO2/N2, CO2/CH4, and CO2/H2, and hydrocarbon separations have been developed.3,4 Among the diverse gas separations, C2H2/CO2 separation is one of the most challenging ones owing to their very similar boiling points (189.2 vs 194.7 K) and geometrical dimensions (3.32 × 3.34 × 5.70 Å vs 3.18 × 3.33 × 5.36 Å).5 Furthermore, C2H2 is a basic raw material for various industrial and consumer products such as rubber and plastics. The main source of acetylene is the oxidative coupling of methane in the process of converting natural gas (NG) now. So, separation of C2H2 from CH4 is an important industrial process to provide a cleaner source of C2H2 for further chemical processing and transformation. The traditional cryogenic distillation and solvent extraction for such gas separations are comparatively energyconsuming and pollutive. One of the energy-efficient and environmentally friendly strategies for these separations is the adsorptive separation technology under ambient conditions through the usage of porous adsorbents. © 2017 American Chemical Society

Received: March 28, 2017 Published: June 5, 2017 7145

DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150

Article

Inorganic Chemistry Scheme 1. Lanthanide Metalloligand [Tb(ODA)]+ and Assembling Strategies of UTSA-222

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Table 1. Crystallographic Data Collection and Refinement Result for UTSA-222 formula formula weight temperature/K crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ (mm−1) F(000) crystal size, mm3 GOF Rint R1, wR2 [I ≥ 2σ(I)] R1, wR2 [all data] largest diff. peak and hole (e Å−3)

EXPERIMENTAL SECTION

Materials and Instrumentation. All reagents and solvents were commercially available and used without further purification. Elemental analyses for C, H, and O were performed with a Vario EL cube CHNOS analyzer. The infrared (IR) spectrum was recorded on a Nicolet FT-170SX instrument using KBr discs in the 400−4000 cm−1 region. Thermogravimetric analysis (TGA) was performed from 23 to 800 °C under a nitrogen atmosphere using a NETZSCH STA 449 F3 analyzer with a heating rate of 5 °C min−1. Powder X-ray diffraction (PXRD) patterns were measured by a Rigaku SmartLab diffractometer operated at 40 kV and 44 mA with a scan rate of 1.0° min−1. Gas adsorption isotherms were measured using a Micromeritics ASAP 2020 surface area analyzer. Fluorescence measurements were made on a Hitachi F-7000 spectrofluorimeter. Synthesis of UTSA-222. A mixture of Tb(NO3)2·6H2O (10.0 mg, 0.022 mmol), H2ODA (7 mg, 0.052 mmol), and NaHBPDC (6.6 mg, 0.025 mmol) was dissolved into a mixed solvent of dimethylformamide (DMF) and H2O (5 mL; v/v = 8:1) in a screw-capped vial (20 mL), to which 5 μL of 2 M HNO3 was added. The vial was capped and heated in an oven at 80 °C for 4 d. Colorless block crystals were obtained by filtration and washed several times with DMF to afford UTSA-222 in 75% yield based on Tb(NO3)2·6H2O. IR (KBr, cm−1): 3430(s), 2922(m), 2853(m), 1604(s), 1519(s), 1414(s), 1117(m), 1043(m), 850(m), 776(m). Elemental analysis of UTSA-222a: Found (Calcd): C, 40.73% (40.70%); H, 2.30% (2.28%); O, 27.09% (27.11%). X-ray Collection and Structure Determination. The crystal data for UTSA-222 were collected on an Agilent Supernova CCD diffractometer equipped with a graphite-monochromatic enhanced Mo Kα radiation (λ = 0.710 73 Å) at 293 K. The data sets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structure was solved by direct methods and refined by full matrix leastsquares methods with the SHELX-2013 program package. The solvent molecules, DMF and H2O, in the complex are highly disordered. The SQUEEZE subroutine of the PLATON9 software suit was used to remove the scattering from the highly disordered guest molecules. Anisotropic thermal parameters were used to refine all non-hydrogen atoms. Hydrogen atoms were located geometrically and refined isotropically. The crystal data and structure refinement details are summarized in Table 1.

C54H36Na2O27Tb3 1639.57 293 trigonal R32 (No. 155) 12.6214(15) 12.6214(15) 45.452(9) 90 90 120 6270.4(2) 3 1.303 2.581 2379 0.11 × 0.15 × 0.23 1.025 0.077 0.0345, 0.0632 0.0405, 0.0648 −1.884, 1.103

Figure 1. X-ray crystal structure of UTSA-222 showing (a) 2D layer formed by lanthanide metalloligand, [Tb(ODA)]+; (b) the 3D framework; (c) the sqc4 framework topology; (d) the open “8” font-style channels along the a and b axes. Green: Tb; yellow: Na; red: O; gray: C; white: H.

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distance between the layers is 15.64 Å. Simplification of the network structure by TOPOS program reveals that UTSA-222 has the sqc4 topological type with Schläfli symbol {36.418.53.6} (Figure 1c). There exist 2D open “8” font-style intersecting channels of ∼9.3 × 5.1 Å along the c axis (Figure 1d). The solvent-accessible volume is calculated to be 44.8% by using PLATON. Luminescent Properties of UTSA-222. The solid-state photoluminescence of UTSA-222 was investigated at room temperature. The emission spectrum (excited at 398 nm) exhibits five main characteristic emissions of the Tb(III) ions at 489, 544, 583, 621, and 645 nm correspond to the 5D4→7FJ (J = 6−2) transition for Tb3+ ions (Figure S2).

RESULTS AND DISCUSSION Single Crystal X-ray Structure. Single-crystal X-ray diffraction studies show that UTSA-222 crystallizes in the trigonal system with a space group of R32. As expected, the Tb(III) ions and ODA2− ligands form [Tb(ODA)]+ metalloligands, which were connected with each other by Tb−O bonds to obtain 2D layers (Figure 1a) with a Tb···Tb distance of 6.315(6) Å, and all uncoordinated sites of Tb (III) ions located two sides of the layer (Figure S1). These layers were bridged by the pillar BPDC2− ligands in which the torsion angle between BPDC2− was 24.4° to produce a three-dimensional (3D) framework (Figures 1b and 2). The shortest Tb···Tb 7146

DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150

Article

Inorganic Chemistry

cm3 g−1) and only tiny amount of CH4 (13.4 cm3 g−1) at 296 K, indicating UTSA-222a has the potential for C2H2/CH4 and C2H2/CO2 separation at ambient conditions. The C2H2/CO2 uptake ratio of UTSA-222a is close to 2.0 at 296 K, which is lower than that of MAF-2 (3.7) but higher than that of other adsorbents (1.0−1.7).11b As established before, the relatively high C2H2 uptake is mainly attributed to moderate pore sizes and the π···π stacking between C2H2 molecules and phenyls of BPDC2− ligands.1e,13a Meanwhile, because of the strong Lewis acidity of C2H2, the HCC−H···phenyl interactions could also contribute to the C2H2 adsorption.13b To investigate UTSA-222a’s feasibility to selectively separate C2H2 from binary C2H2/CO2 and C2H2/CH4 mixtures, we utilized the Ideal Adsorbed Solution Theory (IAST; Supporting Information) to calculate the adsorption selectivity. Figure 4 presents the values of the adsorption selectivity for the C2H2/CO2 and C2H2/CH4 mixtures as a function of total

Figure 2. Three BPDC2− ligands and two Na+ ions formed a pillar that bridged six Tb (III) ions in adjacent layers, and the torsion angle between BPDC2− in a pillar was 24.4°. Green: Tb; yellow: Na; red: O; gray: C. All H atoms were omitted for clarity.

Gas Adsorption Properties of UTSA-222a. The permanent porosity was established by the N2 sorption isotherm at 77 K. Prior to gas adsorption measurement, the as-synthesized UTSA-222 was solvent-exchanged with dry acetone, followed by vacuum activation at 353 K to generate activated UTSA-222a. PXRD of UTSA-222a matches with the as-synthesized one, indicating that the 3D framework is intact after activation (Figure S3). As shown in Figure S5, the N2 sorption isotherm shows a type-I adsorption behavior with an uptake of 173.3 cm3 g−1, characteristic of a microporous material. The Langmuir surface area, the Brunauer−Emmett− Teller (BET) area, and pore volume were 790 m2 g−1, 703 m2 g−1, and 0.344 cm3 g−1, respectively, based on the N2 sorption isotherm at 77 K. The unique structure and permanent microporosity of UTSA-222a prompt us to examine its potential applications as an adsorbent for industrially important gases, such as C2H2, C2H4, CH4, and CO2. The sorption isotherms of C2H2, C2H4, CH4, and CO2 were collected up to 1 atm at 273 and 296 K. As shown in Figure 3, the isotherms are completely reversible and show no hysteresis. At 1 atm, UTSA-222a exhibited a C2H2 uptake of 103.4 and 85.3 cm3 g−1 at 273 and 296 K, respectively. The C2H2 adsorption capacity at room temperature is much higher than that of UTSA-68 (70.1 cm3 g−1),3c ZJU-30 (51.8 cm3 g−1),10 MIL-53 (72 cm3 g−1),11a MAF-2 (70 cm3 g−1),11b and UTSA-35 (65.0 cm3 g−1)12 but slightly lower than MOF-508 (90 cm3 g−1).11a In stark contrast to C2H2, UTSA-222a can adsorb much smaller amount of CO2 (42.7

Figure 4. IAST calculations of C2H2/CH4 and C2H2/CO2 adsorption selectivities for UTSA-222a at 296 K.

bulk gas pressure in UTSA-222a at 296 K. It can be seen that the selectivity of UTSA-222a for an equimolar C2H2/CO2 mixture lies in the range of 4.6−4.0 during the entire pressure range at room temperature, which is comparable to UTSA-50a (5.0)14 and UTSA-68a (5−3.4),3c higher than that of ZJU-30a (2.4−1.7) and UTSA-30a (4.0−3.4),3c highlighting UTSA222a as promising MOF for the C2H2/CO2 separation. We further evaluated the adsorption selectivity of C2H2 from C2H2/ CH4 mixture. At normal state, the methane content is above 90% in natural gas from high-grade gas field. Thus, three representative molar concentrations of C2H2 in C2H2/CH4 binary gas mixture, namely, 0.1, 0.05, and 0.01, were used to

Figure 3. Single-component adsorption isotherms for C2H2 (black), C2H4 (blue), CH4 (red), and CO2 (violet) of UTSA-222a at 273 K (a) and 296 K (b). Solid symbols: adsorption; open symbols: desorption. 7147

DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150

Inorganic Chemistry

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calculate the adsorption selectivities for mimicking natural gas upgrading. As depicted in Figure 4, the selectivities of C2H2 with respect to CH4 are in excess of 16 for a range of 100 kPa, implying the separation of C2H2 from natural gas will be quite straightforward. Conversion of natural gas is a highly efficient method to produce acetylene in modern chemical industry, and in general case the content ratio of C2H2 to CH4 approaches 1:1 in the cracking gas,16 so adsorption selectivity was also calculated for an equimolar C2H2/CH4 mixture. UTSA-222a has selectivities in the range of 16−19 (Figure 4) that are comparable to those found in UTSA-33a,3f UTSA-34a, and UTSA-34b.15b Given the fact that UTSA-222a has a relatively high uptake capacity as well, UTSA-222a is a potentially valuable material for the separation of C2H2 over CH4. During the industrialization process, the regeneration energy cost of a fixed bed, as one of the most important factors for assessing the performance of an adsorbent, must be considered. On the basis of the virial method, the isosteric heats of adsorption (Qst) of C2H2, CH4, and CO2 in UTSA-222a were calculated (Figure S8). The Qst for C2H2 at zero coverage is 26.00 kJ mol−1, which is lower than ZJU-30a (31.3 kJ mol−1),3c UTSA-33a (33 kJ mol−1),3f UTSA-67a (32 kJ mol−1)3e and HKUST-1 (30.4 kJ mol−1)11 and comparable to UTSA-68a (25.87 kJ mol−1), M′MOF-3a (25 kJ mol−1), and UTSA-100a (22 kJ mol−1).17 It is understandable that Qst for C2H2 at zero coverage in UTSA-222a is much lower than those in MOFs with high densities of open metal sites such as MOF-74 series (41−46 kJ mol−1) and UTSA-60a (36 kJ mol−1).18 These results suggest that energy required to regenerate UTSA-222a will be lower than those of MOF-74 series and UTSA-60a.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1-210-458-7428. ORCID

Bin Li: 0000-0002-7774-5452 Banglin Chen: 0000-0001-8707-8115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by an award AX-1730 from the Welch Foundation (B.C.), the Natural Science Foundation of Ningxia Province (Grant Nos. NZ13042 and NZ16031), and the NFSC (Grant No. 21261018)

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REFERENCES

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CONCLUSIONS In conclusion, a novel pillar-layer porous Ln-MOF (UTSA222) was designed and constructed from a lanthanide metalloligand strategy. The activated sample shows moderately high selective separation of C2H2 over CO2 and CH4. Compared with traditional pure organic linker approach to construct Ln-MOFs, the assembled lanthanide metalloligands have much more predictable structures, leading to more predictable Ln-MOF structures as well. In fact, by making use of different co-bicarboxylate organic ligands, it is quite possible that a variety of Ln-MOFs from this lanthanide metalloligand with different porosities might be synthesized for their gas separations. Furthermore, this work will also motivate us to explore more lanthanide metalloligands for the construction of Ln-MOFs for their diverse applications.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00762. Crystallographic data, TGA, PXRD, and additional figures (PDF) Accession Codes

CCDC 1487483 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. 7148

DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150

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DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150

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DOI: 10.1021/acs.inorgchem.7b00762 Inorg. Chem. 2017, 56, 7145−7150