Tetrazole–Viologen-based Flexible Microporous Metal–Organic

Article pubs.acs.org/IC

Tetrazole−Viologen-based Flexible Microporous Metal−Organic Framework with High CO2 Selective Uptake Ya-Ping Zhao,†,‡ Yan Li,§ Cai-Yan Cui,†,‡ Yu Xiao,†,‡ Rong Li,†,‡ Shuai-Hua Wang,*,† Fa-Kun Zheng,*,† and Guo-Cong Guo† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China § Department of Chemistry, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: A flexible metal−organic framework (FMOF) with functionalized pores was hydrothermally synthesized to improve CO2 affinity and selectivity. The obtained FMOF exhibits a reversible shrinking and swelling framework transformation, which is triggered by the adsorption of CO2 rather than by the adsorption of N2 and CH4. At ambient temperature and an atmospheric pressure, this FMOF shows not only a high CO2 uptake (98 cm3 g−1, 19.3 wt %) but also a good calculated adsorption selectivity for CO2 over both CH4 and N2 (CO2/CH4 50:50 v/v: 28.6:1, CO2/N2 15:85 v/v: 210.4:1 calculated by ideal adsorbed solution theory), indicating potential applications in the purification of natural gas and industrial flue gas.

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example, the zeolite-like IFMC-1,7a constructed by 4,5-di(1Htetrazol-5-yl)-2H-1,2,3-triazole, exhibits both high uptake and selectivity for CO2 over those of N2 at ambient temperature. The absorption property of IFMC-1 was ascribed to the quadrupole−dipole interactions between CO2 and uncoordinated N atoms of N-rich tetrazole rings. A zinc-tetrazolate framework with multipoint interactions around the aromatic tetrazole rings also presents similar CO2 adsorption behavior.7b Recently, more attention has been paid to flexible metal− organic frameworks (FMOFs) for the adsorption of CO2. Being a subclass of MOFs, FMOFs with reversible structural transformation often show the unusual stepwise or uncommon hysteretic gas adsorption behavior and are expected to be beneficial to gas adsorption and separation.8 Although some reported FMOFs have been supposed to be a superior platform for gas storage and purification, only a few display a high CO2 adsorption and selectivity ability over both N2 and CH4.1a,3e,9 Herein, an FMOF with functionalized channels using N-rich tetrazole rings was synthesized so as to achieve selective adsorption of CO2 versus N2 and CH4. A semirigid ligand 1,1′bis(tetrazolmethyl)-4,4′-bipyridinium (btzmb), prepared via in situ [2 + 3] cycloaddition reaction of azide and the presynthesized 1,1′-bis(cyanomethyl)-4,4′-bipyridinium dibromide ((bcmb)Br2), was used to construct the target FMOF (Scheme 1). By introducing an auxiliary ligand 1,2,4,5benzenetetracarboxylic acid (H4btec), a new three-dimensional

INTRODUCTION The capture of CO2, the main cause of global warming and climate change, has been a worldwide concern in both industrial and environmental aspects, such as the separation of CO2 from postcombustion processing or low-grade natural gas.1 One of the effective methods for the separation of CO2 from the gas mixtures above is the development of novel materials with the high adsorption capacity and selectivity for CO2.2 With the extraordinarily high specific surface area and chemically tunable structure at the molecular level, porous metal−organic frameworks (MOFs) constructed by linking inorganic units with organic linkers to make extended networks are the most promising adsorbents.3 Nowadays, increasing efforts have been dedicated to the adsorption of CO2 on MOFs at ambient temperature. Several approaches, such as tuning pore size distribution,3d introducing unsaturatedly coordinated metal sites,4 functionalizing the pore surfaces of the framework, or making the network inherently flexible,1a,5 have been proposed to improve the selective adsorption of CO2. Among these, the modification of organic linkers on the pore surface have been demonstrated to be effective to the enhancement of the selective storage of CO2 on MOFs, which can be attributed to the electrostatic interaction or hydrogen bonding between CO2 guest molecules and the introduced polarizing groups or electron-rich atoms such as carboxylate groups, hydroxide, N, and F atoms.6 Tetrazole groups contain electron-rich N atoms, and many tetrazole derivatives have been applied to the assembly of porous coordination polymers to enhance the CO2 adsorption.7 For © XXXX American Chemical Society

Received: February 6, 2016

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DOI: 10.1021/acs.inorgchem.6b00320 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

model, and those of lattice water molecules are not included. The final structures were refined using a full-matrix least-squares refinement on F2. All of the calculations were performed by the SHELXTL-2014 program package11 of crystallographic software. The guest water molecules in crystal 1 may lose or evaporate during data collection or handling samples. The accurate number of guest water molecules is further identified by elemental analysis and thermogravimetric analysis. Pertinent crystal data and structural refinement results are listed in Table 1. Selected bond distances and angles for 1•S are listed in Tables S1 in the Supporting Information.

Scheme 1. Synthetic Route of [Zn2(btec) (btzmb)]n·8nH2O (1·S)

(3D) FMOF of [Zn2(btec) (btzmb)]n·8nH2O (1•S, S = solvent) was obtained via a hydrothermal reaction, wherein two mutually perpendicular channels are formed in the framework. Compound 1 behaves with an interesting “breathing effect” induced by CO2 rather than by CH4 and N2. The highly selective CO2 uptake over N2 and CH4 was observed, which can be attributed to the gated sorption behavior and strong interactions of CO2 with the pore surface.

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Table 1. Pertinent Crystal Data and Structure Refinement Results for 1•S 1•S formula Mr (g mol−1) size (mm) cryst shape cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρ (g cm‑3) T (K) F(000) Reflns Rint params S on F2 R1 (I > 2σ(I))a wR2 (I > 2σ(I))b CCDC

EXPERIMENTAL SECTION

Materials and Instruments. All reagents except 1,1′-bis(cyanomethyl)-4,4′-bipyridinium dibromide ((bcmb)Br2) were commercially obtained and used without any treatment. In situ temperature-dependent powdered X-ray diffraction (PXRD) patterns were recorded on a Ultima-IV diffractometer with Cu Kα radiation (λ = 1.540 56 Å) at a speed of 2°/min. Ex situ PXRD data were collected on a Rigaku Miniflex II diffractometer using Cu Kα radiation (λ = 1.540 598 Å) at 40 kV and 40 mA in the range of 5° ≤ 2θ ≤ 80°. The simulated PXRD patterns were derived from the Mercury Version 1.4 software (http://www.ccdc.cam.ac.uk/products/mercury/). Thermogravimetric (TG) measurements were performed using a METTLER TOLEDO apparatus, in which the samples were heated in an Al2O3 crucible at a heating rate of 10 K min−1 with a stream of nitrogen. The elemental analyses were performed on an Elementar Vario EL III microanalyzer. The FT-IR spectra were obtained on a PerkinElmer Spectrum using KBr disks in the range of 4000−400 cm−1. Gas sorption isotherms of dehydrated 1 were measured using an ASAP2020 gas adsorption instrument up to 1 atm of gas pressure. The highly pure N2 (99.999%), CO2 (99.999%), and H2 (99.999%) were used in the sorption experiments. The compound 1•S was activated at 120 °C for 8 h to obtain the dehydrated framework 1. Syntheses. ((bcmb)Br2. ((bcmb)Br2 was synthesized via nucleophilic substitution reaction. A mixture of bromoacetonitrile (2.40 g, 20.0 mmol) and 4,4′-bipyridine (1.56 g, 10.0 mmol) was dissolved into 30 mL of dimethylformamide in a 25 mL round-bottom flask. The above solution was heated at 80 °C for 15 h under reflux condition, and a yellow precipitate was finally obtained by filtration followed by washing three times using acetonitrile. The mass spectrum (MS) and 1 H and 13C nuclear magnetic resonance (NMR) analyses were used to characterize the prepared compound (bcmb)Br2 (Figures S1−S3 in the Supporting Information). [Zn2(btec) (btzmb)]n·8nH2O (1•S, S = solvent). A mixture of ZnBr2 (112.5 mg, 0.5 mmol), NaN3 (78 mg, 1.2 mmol), H4btec (50.8 mg, 0.2 mmol), and (bcmb)Br2 (59.2 mg, 0.2 mmol) in 8.0 mL of distilled water was sealed into a 25 mL Teflon-lined stainless steel vessel and then heated to 140 °C and kept at this temperature for 3 d. After it cooled to room temperature at a rate of 5 °C/h, yellow crystals of 1•S were obtained with 80% yield (based on Zn). Anal. Calcd for C24H14N10O8Zn2·8H2O (1•S): C, 34.10; H, 3.57; N, 16.57%. Found: C, 34.05; H, 3.52; N, 16.62%. IR spectrum and PXRD patterns are plotted in Figures S4 and S7 in the Supporting Information, respectively. Single-Crystal Structure Determination. Single-crystal X-ray diffraction measurements were performed on a Rigaku Pilatus CCD diffractometer at 293 K, which was equipped with Mo Kα radiation (λ = 0.710 73 Å), using the ω-scan technique for collections of the intensity data sets, and corrected for Lp effects. The primitive structures were solved by the direct methods and reduced by CrystalClear software.10 The subsequent successive difference Fourier syntheses yielded the other non-hydrogen atoms. The hydrogen atoms of ligands were added geometrically and refined using the riding

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C24H30N10O16Zn2 845.32 0.39 × 0.12 × 0.09 prism monoclinic P21/c 19.377(5) 16.277(4) 10.720(3) 101.384(5) 3314.6(15) 4 1.694 293 1728 6127 0.043 463 1.09 0.045 0.156 1451897

R1 = ∑(F0 − Fc)/∑F0. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2.

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RESULTS AND DISCUSSION Crystal Structure. Single-crystal X-ray diffraction analysis reveals that compound 1•S belongs to the P21/c space group. In each asymmetric unit, there are two crystallographically independent Zn(II) atoms, one btzmb ligand, one btec4− ligand, and eight lattice water molecules. Both the Zn1 and Zn2 centers are four-coordinated by two carboxylate O atoms from two btec4− ligands and two tetrazole N atoms from two btzmb ligands to form a distorted tetrahedral geometry (Figure 1). The Zn(II) ions were connected by btzmb ligands to form

Figure 1. Asymmetric unit in compound 1. Symmetry codes: A: −x, 1 − y, −z; B: −1 − x, 1 − y, −z; C: −x, 0.5 + y, 0.5 − z; D: −x, 2 − y, −z; E: x, 1.5 − y, −0.5 + z; F: x, 1.5 − y, 0.5 + z. B

DOI: 10.1021/acs.inorgchem.6b00320 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry two-dimensional (2D) corrugated layers parallel to the bc plane, and the tetrazole rings linked by bipyridinium moieties are located at the top or bottom of the wave plane (Figure 2a). The

Figure 2. Structure description of 1: (a) The 2D waved layer formed by Zn(II) with the btzmb ligand in the bc plane. (b) The 3D framework viewed along the c axis. (c) The 3D framework viewed along the a axis.

adjacent layers are further cross-linked by the coordination of carboxylate groups of btec4− ligands with Zn(II) ions, generating a porous 3D framework. There are two types of open channels: type I (Figure 2b) and type II (Figure 2c) extending along the c-axis and a-axis, respectively, in the framework. Notably, tetrazole rings are parallel to type I channels and bipyridinium moiety to type II channels, respectively, decorating the pore surface. The uncoordinated carboxylate O atoms of the btec4− ligands point to both type I and II channels (Figure S5 in the Supporting Information). The type I and II channels have the rectangular cavities of effective size ∼6.2 × 6.8 Å2 and 6.0 × 4.0 Å2, respectively (Figure S6 in the Supporting Information). The solvent-accessible volume of 1 is 1085.6 Å3, with solvent-accessible voids of 32.8%, as calculated by PLATON.12 The purity of 1•S was independently confirmed by elemental analysis and by the similarity between simulated and experimental PXRD (Figure S7 in the Supporting Information). Thermogravimetric analysis (TGA) under N2 atmosphere (Figure S8 in the Supporting Information) shows that the weight decrease of 16.90% below 150 °C should correspond to the loss of eight guest H2O molecules in each asymmetric unit (calcd: 17.05%). The framework collapse, reflected by the drastic mass loss of the TGA curve, occurred above 320 °C. Dynamic Sorption Properties. The porosity of 1 was characterized by N2 adsorption at 77 K (Figure S9). The activated sample has a Langmuir surface area of 757 m2·g−1 and a Brunauer−Emmett−Teller surface area of 493 m2·g−1. Interestingly, being measured up to 1 atm, compound 1 shows two-step N2 adsorption behavior with little desorption hysteresis. The CO2 adsorption behavior measured at 273 and 298 K is illustrated in Figure 3. Similar to the N2 adsorption at 77 K, the isotherms of CO2 at both temperatures also display uncommon two steps. For the isotherm at 273 K, the first stage reaches the inflection point of 18.1 cm3 g−1 at 54 mmHg, and the uptake achieves a saturation of 122 cm3 g−1 (23.9 wt %) at 760 mmHg. At 298 K, the CO2 adsorption capacity of 1 is 16.7 cm3 g−1 at 130 mmHg in the first step, reaching a value up to

Figure 3. (a) CO2 adsorption isotherms of 1 at 273 and 298 K. (b) CO2, CH4, and N2 adsorption isotherms for 1 at 298 K.

98 cm3 g−1 (19.3 wt %) at 760 mmHg in the second step. Notably, such values have rarely been reported among FMOFs and comparable to those of the most famous rigid MOFs reported to date, although the specific surface area of 1 is relatively lower (Tables S2 and S3 in the Supporting Information).1a,3e,9a,d The desorption isotherms no longer trace along the adsorption isotherms, forming a large hysteresis loop. Previously reported FMOFs have shown this phenomenon. The pores appear to be in a shrinking form after the removal of guest molecules. Below the gate opening pressures, 54 mmHg at 273 K and 130 mmHg at 298 K, compound 1 with a shrinking structure adsorbs CO2 in the first step. Once the initial porous volume was filled with CO2 molecules, the further adsorption of CO2 can be promoted by the expansion of pore structure in the second step. When external pressures are reduced, the adsorbed CO2 trapped in the framework is not immediately released. The large hysteresis observed can be attributed to the strong interaction between polar functional groups and CO2. Since the crystal structure of guest-free framework 1 cannot be determined possibly due to the appearance of defects resulting from the removal of guest molecules in the single crystal, variable-temperature PXRD patterns of 1•S from 30 to 300 °C were performed under air atmosphere to further reveal the framework flexibility of 1 (Figures 4a and S10 in the Supporting Information). With the increase in temperature from 30 to 150 °C, the diffractions at the 2θ values of 9.07, 9.78, 21.09, and 27.42° shift to 9.63, 10.20, 22.34, and 28.56°, respectively. Upon the removal of guest H2O molecules, they however remain nearly unchanged at the temperature above 150 °C, which is consistent with the TGA results. The diffraction peaks of 1•S shift gradually to higher 2θ values, suggesting the shrinkage of unit cells with decreasing pore size.13 The PXRD patterns of 1•S were also collected at C

DOI: 10.1021/acs.inorgchem.6b00320 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) The variable-temperature PXRD patterns of 1•S from 30 to 300 °C. (b) The PXRD patterns of 1•S collected at 30, 150, and 30 °C through heating and cooling, respectively.

increasing temperatures from 30 to 150 °C and then at decreasing temperatures to 30 °C (Figure 4b). When cooled from 150 to 30 °C, the shifted PXRD pattern returns to the initial one measured first at 30 °C, proving the reversible shrinking-swelling of the frameworks by the removal and readsorption of guest water molecules. The bistable states of the framework 1, related to the stepped adsorption isotherms and large hysteresis, thus can be confirmed. Selective Sorption Studies. To explore the selectivity of CO2 capture, CH4 and N2 adsorption isotherms at ambient temperature were performed (Figure 3b). Different from CO2, no breathing phenomena can be observed in both CH4 and N2 adsorption. Compared with the high CO2 uptake, little of CH4 is captured (14.0 cm3 g−1), and N2 is hardly adsorbed at all (2.1 cm3 g−1). The selectivity for CO2 capture was calculated according to ideal adsorbed solution theory (IAST) based on a dual-site Langmuir−Freundlich (DSLF) simulation (Figures 5, S11, and S12 in the Supporting Information).14 The CO2/CH4 selectivity calculated for a 50/50 CO2/CH4 mixture is 28.6 for 1 at 1 atm and 298 K, and the CO2/N2 selectivity calculated for a 15/85 CO2/N2 mixture is 210.4 at the same condition. On the basis of adsorption isotherms measured and predicted, it can be seen that the selectivity of CO2/N2 is high, since the adsorption capacity of CO2 is evidently higher than that of N2 even at pressure lower than 0.15 atm. In addition, the Henry’s law selectivities for CO2/CH4 and CO2/N2 were calculated to be 14.5 and 45 at 298 K, respectively (Figures S13−S15 and Table S5 in the Supporting Information).16a To the best of our knowledge, the selectivity of 1 for CO2 is higher than that of the well-known MOFs reported (Tables S2 and S3 in the Supporting Information).3e,15 Meanwhile, the uptake capacity of CO2 and the selectivity values of CO2/CH4 or CO2/N2 are comparable to the values of typical tetrazolate-based MOFs MAF-66 and ZTF-1 (Table S4 in the Supporting Information).7a,16 The high CO2 uptake and selectivity of 1 can be attributed to the gated sorption behavior and host−guest interactions. The reversible structural transformation of 1, which is triggered by the absorption/desorption of CO2, is beneficial to the selective adsorption of CO2.17 In the meantime, the interactions between CO2 and the decorated pore surface of the framework are stronger than those between N2/CH4 and the surface. On the basis of the structure of 1, there are uncoordinated N atoms of tetrazole rings on the pore surfaces and uncoordinated carboxyl groups from btec4− ligands pointing to the channels. With a larger quadrupole moment of 1.4 × 10−39 C m2,18 CO2 has a

Figure 5. Pressure-dependent selectivity profiles on 1 calculated by the IAST method (298 K): (a) the selectivity of CO2/CH4 (50/50); (b) the selectivity of CO2/N2 (15/85).

much stronger quadrupole−quadrupole interaction with the framework than CH4 and N2 with lower quadrupole moments of 0 and 4.7 × 10−40 C m2, respectively.19 Meanwhile, the bipyridinium moieties parallel to the II channels may serve as Lewis acidic sites, and two α-hydrogen atoms of the viologen moiety can form a C−H···O hydrogen bonding with CO2 as previously reported.20 To further explore the affinity of 1 to CO2 molecules, the isosteric heat of CO2 adsorption (Qst) was calculated by both the virial fitting method and Clausius− Claperyron equation (Figure S16 in the Supporting Information). The virial method was performed from the sorption isotherms performed at 273 and 298 K (Figure S13 in the Supporting Information). In the low CO2 uptake range (0−0.8 D

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Inorganic Chemistry mmol g−1), the Qst of 1 calculated by two methods agree well with each other, while in the range of 0.8−3.5 mmol g−1, the values calculated by Clausius−Claperyron equation are slightly higher than those obtained through virial method. This may be ascribed to the stepwise sorption isotherms. The Qst value at zero uptake for CO2 reaches 26.2 kJ mol−1, which is comparable to those of tetrazolate-based ZTF-1, UTSA-49, and MAF-66, proving the uncoordinated N atoms exposed to the pores (Figure S16 and Table S4 in the Supporting Information).16 With the increase in adsorption amount, the Qst of CO2 adsorption curve rises and reaches 38.1 kJ mol−1 at 4.4 mmol g−1 (98 cm−3 g−1), which is corresponding to those for MOFs functionalized with amine groups or open unsaturated metal sites such as UMCs (32−47 kJ mol−1),4b,8c,21 illustrating strong interactions between CO2 and functionalized pores. To further explore the above-mentioned strong interactions, the standard ab initio molecular dynamics calculation was performed using VASP package,22 employing the projector augmented wave method. Four CO2 molecules per unit cell are filled in the type-I void, and six CO2 molecules per unit cell are filled in the type-II void before simulation, and the 10 CO2 molecules are located at the high symmetry positions to reduce the effects of initial states on the results. As shown in Figure 6,

decorated pore surface of the framework are responsible for the enhanced selective adsorption of CO2 in compound 1. The simultaneous introduction of multiple functional groups and flexibility into MOFs is confirmed to be an effective approach for the enhancement of CO2 adsorption and separation performance, indicating potential applications in the purification and separation of nature gas and fuel gas.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00320. Experimental section, supplementary tables, structural figures, and additional characterizations (PDF) Crystallographic details (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-591-6317-3068. E-mail: [email protected]. (F.-K. Zheng) *E-mail: [email protected]. (S.-H. Wang) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Nature Science Foundation of China (21371170 and 21301059) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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REFERENCES

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Figure 6. Standard ab initio molecular dynamics calculation results with the btec4− ligand omitted for clarity. Green dashed lines: the distances between C atoms of CO2 and uncoordinated N atoms of tetrazole; yellow dashed lines: distances between O atoms of CO2 and α-H atoms; cyan dashed lines: distances between O atoms of CO2 and β-H atoms.

CO2 molecules locate inside the channels and form short contacts with the uncoordinated N atoms of tetrazolate. The C···N distances (2.82 and 2.83 Å), being smaller than the sum of van der Waals radii of C (1.70 Å) and N (1.55 Å), indicate strong interactions between CO2 molecules and tetrazolate moieties. Additionally, the distances between carbonyl O atom of the CO2 and the α-H and β-H atoms of bipyridinium are in the range of 2.03−2.23 Å, which can form weak C−H···O hydrogen bonds.23

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CONCLUSIONS In summary, an FMOF 1 was successfully constructed based on tetrazole−viologen ligands and can switch between shrinking and swelling forms, resulting in a two-step adsorption behavior for CO2. Compound 1 has a higher adsorption capacity and selectivity for CO2 over CH4 and N2 than most rigid MOFs and FMOFs reported. The strong interactions of CO2 with the E

DOI: 10.1021/acs.inorgchem.6b00320 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00320 Inorg. Chem. XXXX, XXX, XXX−XXX