Modulating CO2 Adsorption in Metal–Organic Frameworks via Metal

Article Cite This: Inorg. Chem. 2018, 57, 6135−6141

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Modulating CO2 Adsorption in Metal−Organic Frameworks via Metal-Ion Doping Ping Cui,§ Ji-Jing Li,† Jie Dong,† and Bin Zhao*,† †

Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry, Tianjin, and Co-Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China § School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China S Supporting Information *

ABSTRACT: One novel metal−organic framework {[Ni6(OH)4(BTB)8/3(H2O)6]· 4H2O·9DMF}n (DMF = dimethylformamide) based on 1,3,5-benzenetribenzoic acid (H3BTB) was successfully synthesized under basic condition at room temperature, featuring a unique twofold interpenetrated pcu-type net with [Ni6(OH)4(COO)8(H2O)6] cluster as building block. Its gas-adsorption behaviors were investigated and modified by utilizing metalII-doped procedure, which was certified to be a highly effective pathway in enhancing CO2 uptake capacity.

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INTRODUCTION Carbon dioxide (CO2), as the primary greenhouse gas and a major source of ocean acidification, is causing climate change and global warming.1,2 Global CO2 emissions continue to rise rapidly over the century because of combustion of carbon-based fuels (coal, natural gas, and oil) and certain chemical reactions (e.g., manufacture of cement).3 Obviously, it is far from enough if humans only expect the ecological systems themselves to keep the balance of nature.4 From the aspects of energy use and environmental conservation, new energy and low carbon technology are one of the most demanding needs of global society. Thus, reducing CO2 emissions without large energy expenditures is of prime importance in various key industrial applications related to energy, environment, and health. As one of the most effective strategies to reduce CO2 emissions, CO2 capture and sequestration (CCS) technologies can greatly mitigate the greenhouse gas emissions from new and existing emission sources. In recent years, a large amount of porous materials has been designed as high-capacity adsorbents for CO2 sequestration.5−10 Among them, metal−organic frameworks (MOFs) have been demonstrated to be very effective for gas adsorption/separation and even for toxic gas removal.11 In contrast to the known porous materials (e.g., zeolites and carbon-based and polymer-based materials), MOFs, as a leading class of porous crystalline materials, are composed of metal or metal oxide clusters interconnected via organic linkers, which possess well-defined porous structures, high surface area, tunable porosity, and desired chemical functionalities.12−14 These advantages make them good candidates for CO2 capture. Until now, a variety of methods have been developed to improve the adsorption capacity of MOF materials.15 Among © 2018 American Chemical Society

them, the doping on MOFs has been widely utilized recently.16−18 Doping of different metals into the nodes in MOFs can generate the defects in MOFs, which would enhance their intrinsic properties and thus affect their gas uptake. However, incorporation of mixed metal nodes into MOFs normally results in the instability of frameworks and complicated topologies of MOFs. Successful construction of doped MOFs in one step resides in the selection of appropriate MOF frameworks and secondary metal atoms, which makes it a great challenge for the chemists. We were interested in this field inspired by the seminar works made recently and reported here a series of stable CoIIdoped MOFs in one step, which illustrated enhanced porosity and improved CO2 adsorption capacity compared with the pure species. A new self-interpenetrated MOF {[Ni6(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1) was constructed from 1,3,5-benzenetribenzoic acid (H3BTB) and nickel salt as reactants, with high porosity calculated from the singlecrystal structure, which will be a benefit for gas adsorption and storage. Next, its doping process by incorporation of Co atoms was comprehensively investigated, and the corresponding products {[Ni6−xCox(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1a−1c; DMF = dimethylformamide) were obtained. Given high porosity and rich adsorptive sites resulting from introducing bimetallic nodes into MOFs, we also studied their adsorption behaviors for CO2 and observed the effect of introducing the secondary metal on the gas-adsorption properties. Received: March 19, 2018 Published: May 10, 2018 6135

DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

Article

Inorganic Chemistry Scheme 1. Schematic Representation for the Synthesis of 1a

a

Two-fold interpenetrated nets are shown with blue and gold colors, respectively. Hydrogen atoms and solvent molecules are omitted for clarity.

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frameworks prior to the measurement. The isostatic heat (Qst) for the CO2 gas was analyzed with virial method previously reported in literatures.22 Syntheses. {[Ni6(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1). Ni(OOCCH3)2·4H2O (0.1 mmol, 0.0249g), H3BTB (0.1 mmol, 0.0438 g), and LiOH·H2O (0.1 mmol, 0.0042g) were dissolved in a mixture solvent of H2O (1.0 mL) and DMF (3.5 mL). The mixture was sealed in a 15 mL screw-top vial and allowed to stand at room temperature for 2 d to yield pale-green cube-shaped crystals suitable for X-ray diffraction. The obtained material was washed with DMF solvent and then collected by filtration in ca. 82% yield. Elemental analysis for 1: calcd: C, 49.15; H, 5.29; N, 5.21. Found: C, 49.22; H, 5.16; N, 5.09%. Formula is determined by considering the results of the TGA, EA, and magnetochemical measurement. Because of volatility of crystallization solvent, there is a discrepancy in elemental analytical data for samples. {[Ni6−xCox(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1a−1c). The synthesis of Co-doped 1a−1c was conducted the same way as that of 1 as described above, using different molar ratio of Co(OOCCH3)2· 4H2 O and Ni(OOCCH 3) 2 ·4H2 O metal salts instead of Ni(OOCCH3)2·4H2O as starting materials (Table S1). Purple cubeshaped crystals were obtained, with the shade depending on the Co content. The Co-containing materials obtained were washed with DMF solvent and then collected by filtration in ca. 75% (1a), 79% (1b), and 86% (1c) yields. ICP analyses and EA are presented in Table S2.

EXPERIMENTAL SECTION

Instrumental Procedures. Structural measurements were performed on an Oxford SuperNova TM Diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) by ωscan mode. An Oxford Cryosystems HeliX cryostat was used to maintain the measuring temperature to be essentially constant at 120(2) K. Data reduction and absorption corrections were made using the CrysAlispro software package. The structures were solved using the direct method and refined anisotropically by full-matrix leastsquares procedure on F2 with the SHELXL program in OLEX2 software package.19,20 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were fixed at their ideal positions and refined using the riding model with the isotropic displacement parameters to being 1.2 times Ueq of the attached carbon atoms. There is a large pore volume in the crystal structure, which is occupied by heavily disordered solvent molecules. Attempts to locate and refine were unsuccessful. To resolve this issue, the SQUEEZE program21 implemented in PLATON was employed to model this electron density and calculate a solvent-accessible volume. And then the scatters from highly disordered solvent molecules were removed from subsequent structure factor calculations. The resulting new HKL4 file with solvent-free diffraction intensities was generated and used to further refine the structure. In some cases, only O atoms of the coordinated solvents were left and refined. The contents of the solvent region are not represented in the unit cell contents in crystal data. Elemental analysis (EA) based on C, H, and N was measured on a PerkinElmer elemental analyzer. The powder X-ray diffraction (PXRD) data were performed on a Rigaku D/Max-2500 diffractometer, using Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Inductively coupled plasma (ICP) analysis was done on a Thermo Jarrell-Ash ICP-9000 (N+M) spectrometer. Thermal gravimetric analysis (TGA) was performed on the thermogravimetric analyzer of TGA Q500 at 5 °C/min from 30 to 800 °C under nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra DLD spectrometer with monochromatized Al Kα X-ray radiation as the X-ray source for excitation (hν = 1486.6 eV). Magnetic measurement was performed using a Quantum Design SQUID magnetometer. The temperature dependence of magnetic susceptibility was performed in the temperature range from 2.0 to 300 K with an applied field of 1000 Oe. The data were not corrected for diamagnetism. Adsorption Measurements and Isosteric Heat Calculation. Gas-adsorption measurement was performed on the Quantachrome Autosorb-1 automatic volumetric instrument. Before gas adsorption, the as-synthesized crystals were immersed in CH2Cl2 for 2 d to afford the CH2Cl2 solvent-exchanged samples, followed by drying overnight under dynamic vacuum at room temperature. Finally, these freshly prepared samples were degassed by using the “outgas” function of the surface area analyzer at 373 K for 12 h to yield the fully evacuated

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RESULTS AND DISCUSSION Crystal Structure and Characterization. Pale green crystals of 1 were afforded by treatment of H3BTB with Ni(OOCCH3)2·4H2O in a mixed DMF−H2O solvent system containing LiOH·H2O at room temperature (Scheme 1). A series of Co-doped MOFs (1a−1c) were obtained in appropriate metal-to-metal ratios (Table S1). The differences among them are the compositions of metal ions in frameworks. The PXRD analyses confirmed the isostructural relationship of 1a−1c with 1 (Figure 1). More importantly, the PXRD patterns of the Co-doped bimetallic MOFs suggested that the doping Co2+ was successful without disturbing the framework. The chemical composition of bimetallic MOF materials was determined by considering the ICP and EA (Table S2), resulting in the following materials: {[Ni4.1Co1.9(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1a, 32% Co and 68% Ni), {[Ni3.1Co2.9(OH)4(BTB)8/3(H2O)6]·4H2O·9DMF}n (1b, 48% Co and 52% Ni), {[Ni2.8Co3.2(OH)4(BTB)8/3(H2O)6]·4H2O· 9DMF}n (1c, 53% Co and 47% Ni). Substitution of Ni2+ center by Co2+ has a percentage up to 53%. Of particular note, the 6136

DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

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Inorganic Chemistry

Figure 1. Simulated and experimental PXRD patterns for isostructural MOFs 1 and 1a−1c.

mole fractions of Co2+ ion in the bimetallic MOFs are more than the mole fractions in the corresponding reactants, with indication of favoring for Co2+ ion incorporated into the MOF framework from solution than Ni2+ ion. This preferential behavior may be attributed to differences in the formation energy of the metal oxide clusters. Herein, 1 is chosen as a representative and described in detail. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in cubic space group Im3̅ with large unit cell a = b = c = 27.0485(7) Å. In the framework of 1, the [Ni6(OH)4(COO)8(H2O)6] secondary building units (SBUs) are defined by two μ2-OH and four H2O molecules bridging two [Ni3OH(COO)4H2O] entities (Figure 2a). The axial positions of the SBUs are terminally coordinated by H2O molecules. All six Ni atoms in the SBU are coplanar and are located in the octahedral environment. Considering the compounds reported previously23 and result of magnetochemical measurement of 1 (Figure S1), it is suggested that the oxidation state of metal ion is +2. Six such SBUs are knitted together by eight BTB3− anions to fabricate one pseudooctahedral cage with the inner cross-section distance as large as ∼2.4 nm taking van der Waals (vdw) radii into consideration (Figure 2b). The available pore space that can accommodate a sphere has an ∼1.4 nm diameter without touching the vdw atoms of the inner wall. 1 forms as two interpenetrated frameworks, because the planarity of the H3BTB ligand results in strong π−π interaction between ligands of the two independent nets (Figure 2c). Because of the twofold interpenetration, the window sizes of the cage (∼0.9 × 1.2 nm) will be reduced. Highly disordered DMF molecules reside in the cages. In this cage, the trigonal BTB3− anion serves as the faces and the rectangular SBUs as the vertices. In addition, each BTB3− anion bridges three [Ni6(OH)4(COO)8(H2O)6] SBUs, and each such SBU is surrounded by eight trigonal BTB3− anions. Therefore, they act as 3- and 8-connected nodes, respectively. This type of connectivity results in formation of the (3,8)-connected net or pcu-type MOF (Figure 2d). Although the framework is interpenetrated (Figure 2e), the total free volume of 1 after removal of the guest solvent molecules is estimated to be ∼59% as determined by PLATON routine.21 The guest entities within the pores could not be clearly determined by single-crystal X-ray diffraction but are

Figure 2. (a) Structure of the cage, in which the BTB3− ligand and [Ni6(OH)4(COO)8(H2O)6] SBU could be represented as 3- and 8connected nodes, respectively. (b) Space-filling representation of the cage, showing the void space (sky blue sphere). (c) Structure of the two-interpenetrated cage in space-filling mode, highlighting the π−π stacking (green color). (d) Schematic representation of the pcutopology. (e) Topological representation of the twofold interpenetration. Some hydrogen atoms and guest molecules were omitted for clarity. The polyhedron shows the cage within the framework.

characterized by EA and TGA (Figure S2). XPS analysis further confirms their non-hydrogen atoms (Figure S3). Gas Adsorptions. To confirm the porosity, the Ar and CO2 gas-adsorption experiments were performed on 1. PXRD patterns confirmed the phase purity of the as-synthesized sample (Figure 1). TGA showed its stability up to ∼400 °C under the N2 atmosphere (Figure S2). The Ar gas-adsorption isotherm for 1 at 87 K clearly exhibits a reversible type-I isotherm (Figure 3), indicative of permanent porosity. The apparent Brunauer−Emmett−Teller surface area (SBET) and Langmuir surface area (SLangmuir) are estimated to be 468.2 and 495.7 m2g−1, respectively. The pore size distribution is determined by the nonlocal density functional theory (NLDFT) model24 (Figure S4). These results above assess 6137

DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

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Figure 5. Qst of CO2 adsorption for 1 and 1a−1c calculated by the virial equation from the CO2 adsorption data at 273 and 298 K.

Figure 3. Ar sorption isotherms for 1 measured at 87 K. Adsorption and desorption branches are shown with filled and open symbols, respectively.

its capacity to adsorb gas like CO2. The adsorption behaviors toward CO2 were measured at 273 and 298 K. The maximum uptake capacities of CO2 at 0.99 P/Po are 31.7 m3g−1 (1.4 mmol g−1) and 19.2 m3 g−1 (0.9 mmol g−1) at 273 and 298 K (Figure 4), respectively, which revealed that it is a promising

Figure 6. Comparison of Ar adsorption isotherms among 1 and 1a−1c at 87 K.

richest sample 1c are estimated to be 819.3 and 848.4 m2 g−1, respectively. It is worthy to note that the capacities change with the order of 1c > 1b > 1a > 1. Obviously, metal ion doping modified the surface area of MOFs. Similar to the Ar isotherms, all doped samples 1a−1c exhibited higher CO2 uptake capacity than that of 1 (Figure 7). Thus, the CO2 storage capacity decreases in the order of 1c > 1b > 1a > 1, following a similar trend as observed in the Ar adsorption isotherms. At 0.99 P/P0, the Co-richest sample 1c as expected possesses the maximum amount and has a storage capacity of 50.7 m3 g−1 (2.3 mmol g−1) at 273 K and 28.4 m3 g−1 (1.3 mmol g−1) at 298 K, which strongly improved the CO2 adsorption properties. Furthermore, the Qst of CO2 adsorption for 1a−1c was calculated (Figure 5 and Table S4). The Qst of doped 1a−1c is higher than that of 1 (21.1 kJ mol−1). Among them, the sample 1a shows the highest value of 39.0 kJ mol−1. Additionally, the near linearity in the CO2 isotherms until 1 atm indicates that the adsorption amounts are far from reaching the limiting capacities. It should be noted that there are up to six coordinated water molecules in one [Ni 6 (OH) 4 (COO)8(H2O)6] SBU for 1 and its derivatives (1a−1c). Since removal of the coordinated water molecules from the

Figure 4. CO2 sorption isotherms of 1 at 273 and 298 K. Filled and open symbols represent adsorption and desorption branches, respectively.

CO 2 storage material. The Q st of adsorption for CO 2 determined by both recording isotherms is estimated to be 21.1 kJ/mol at zero surface coverage (Figure 5). To investigate the effect of metalII doping on the gasadsorption properties, Ar and CO2 adsorption experiments for Co-doped 1a−1c were also performed. Ar adsorption was further employed to evaluate the porosity of the doped samples. As shown in Figure 6, all the uptake capacities of 1a−1c are higher than that of 1, thus suggesting the presence of porosity after the doping of different contents of CoII ions into 1. The Ar uptakes on 1a−1c are increased to 202.8 (1a), 207.1 (1b), and 263.0 (1c) m3 g−1, as presented in Table S3. Obviously, all doped samples show higher SBET and SLangmuir than that of 1 with increasing Co content, which is in good agreement with previous reports.17 The maximum SBET and SLangmuir for the Co6138

DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

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toward CO2. Influence on more different metal ions doping on MOFs performance is currently under investigation in our lab.

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ASSOCIATED CONTENT

S Supporting Information *

This part includes supporting figures, tables, XPS, pore size distribution (PDF), and these materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00730. (PDF) Accession Codes

CCDC 1828667 contain 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bin Zhao: 0000-0001-9003-9731 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from NSFC (21625103, 21571107, 21421001, and 21701188), 111 project (B12015) and NSF of Shandong Province (ZR2017MB005).

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REFERENCES

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Figure 7. CO2 sorption isotherms of 1 and 1a−1c at 273 (a) and 298 (b) K. Filled and open symbols represent adsorption and desorption branches, respectively.

SBUs may cause the exposed metal ions to be very active and unstable in the pretreatment session,25 the stability of MOFs before and after doping is not fully satisfactory (Figure S5). However, the doping of secondary metal ions into 1 would generate defects in MOF and further lead to the enhancement of the porosity, which can be reflected by the bigger BET surface area value of hybrid compounds, and thus, should be responsible for the improved gas-sorption behavior. In comparison, the hybrid bimetallic MOFs, NixCo1−x-ITHDs with excellent framework stability and ultralarge CO2 uptake capacity were reported by Lah group.26

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CONCLUSIONS In summary, the CoII-doping strategy has been successfully adopted to modify structural compositions of MOFs, with the aim of not only resulting in a higher surface area but also creating rich adsorptive sites for CO2 adsorption at room temperature. Obviously, the enhanced CO2 adsorption capability and enthalpy as compared to the mother MOF at the same conditions have confirmed that cobalt substitution has a great influence on controlling the affinity of bimetallic MOFs 6139

DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

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DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141

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DOI: 10.1021/acs.inorgchem.8b00730 Inorg. Chem. 2018, 57, 6135−6141