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Water-Stable In(III)-Based Metal−Organic Frameworks with RodShaped Secondary Building Units: Single-Crystal to Single-Crystal Transformation and Selective Sorption of C2H2 over CO2 and CH4 Zhen-Ji Guo,†,‡ Jiamei Yu,*,† Yong-Zheng Zhang,‡ Jian Zhang,‡ Ya Chen,‡ Yufeng Wu,*,† Lin-Hua Xie,*,‡ and Jian-Rong Li‡ †

Institute of Circular Economy, Beijing University of Technology, Beijing 100124, P. R. China Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China

‡

S Supporting Information *

ABSTRACT: Three new water-stable In(III)-based metal− organic frameworks, namely, [In 3 (TTTA) 2 (OH) 3 (H 2 O)]· (DMA) 3 (BUT-70, DMA = N,N-dimethylacetamide), [In3(TTTA)2(CH3O)3] (BUT-70A), and [In3(TTTA)2(OH)3] (BUT-70B), with rod-shaped secondary building units (SBUs) and an new acrylate-based ligand, (2E,2′E,2″E)-3,3′,3″-(2,4,6trimethylbenzene-1,3,5-triyl)-triacrylate (TTTA3−) were obtained and structurally characterized. BUT-70A and -70B were generated in a single-crystal to single-crystal transformation fashion from BUT-70 through guest exchange followed by their removal. The solvents used for guest exchange were methanol and dichloromethane, respectively. Single-crystal structure analyses show that the guest exchange and removal process is accompanied by the substitution of coordinated water molecules of In(III) centers with uncoordinated carboxylate O atoms of TTTA3− ligands. Moreover, hydroxyl groups bridging two In(III) centers are also replaced by methoxyl groups in the transformation from BUT-70 to -70A. Overall, three metal−organic frameworks (MOFs) are constructed by infinite chains consisting of corner-sharing InO4(OR)2 (R = H or Me) octahedral entities, which are interconnected by TTTA3− ligands to form three-dimensional frameworks. Unlike most reported MOFs with infinite chains as SBUs, such as well-known MIL-53 and M-MOF-74, which have one-dimensional channels along the chain direction, the BUT-70 series contain two-dimensional intersecting channels. The Brunauer−Emmett−Teller surface area and pore volume of BUT-70A were estimated to be 460 m2 g−1 and 0.18 cm3 g−1, respectively, which are obviously lower than those of BUT-70B (695 m2 g−1 and 0.29 cm3 g−1). Gas adsorption experiments demonstrated that BUT-70A and -70B are able to selectively adsorb C2H2 over CO2 and CH4. At 1 atm and 298 K, BUT-70A uptakes 3.1 mmol g−1 C2H2, which is 3.6 times that of the CO2 uptake and 7.2 times that of the CH4 uptake. Compared with BUT-70A, BUT-70B presents an even higher C2H2 uptake of 3.9 mmol g−1 at the same conditions, but slightly lower Ideal Adsorbed Solution Theory C2H2/CO2 and C2H2/CH4 selectivities. Acetylene (C2H2), as a highly flammable and reactive gas, is widely used in lighting, welding, and cutting metals (acetyleneoxygen flame) and is also the basic raw material for the manufacture of acetaldehyde, acetic acid, benzene, synthetic rubber, and synthetic fiber. The high-purity C2H2 is generally required in those applications; nevertheless, impurities such as carbon dioxide (CO2) and methane (CH4) always exist in the acetylene production. The similar molecular size, sublimation point, and critical temperature of C2H2, CO2, and CH4 make their separations highly challenging.4 Traditional purification method by cryogenic distillation is generally applied for separating C2H2 from CO2 and CH4, which however suffers

1. INTRODUCTION As a new class of porous materials, metal−organic frameworks (MOFs) composed of metal ions and organic ligands have shown great potential in gas storage, chemical separation, catalysis, ion exchange, biomedicine, molecular recognition, sensing, etc.1 Through the assembly of well-defined secondary building units (SBUs) and given organic ligands, the design of numerous MOF structures with regular pore, large specific surface area, as well as relatively low framework density has been realized. In recent years, researchers have been devoted to fine-tune the pore structure, improve the stability, and thereby explore the practical use of MOFs.2 Gas separation is one of the most promising applications for MOFs. Some studies indeed have shown that MOFs are highly potential as a new generation of porous materials for gas separations.3 © 2017 American Chemical Society

Received: November 27, 2016 Published: February 6, 2017 2188

DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

Article

Inorganic Chemistry Scheme 1. Synthesisa of Ligand H3TTTA

(a) I2, HIO4·2H2O, H2SO4, AcOH, 90 °C, 10 h; (b) Pd(PPh3)4, DMF, AcOK, 120 °C, 8 h; (c) THF, EtOH, H2O, NaOH, 80 °C, 12 h; (d) dilute aqueous HCl solution.

a

isotherms were obtained with a Micromeritics ASAP2020 surface area and pore analyzer. All the gases used were of 99.999% purity. Additional crystallographic information is available in the Supporting Information. 2.2. Syntheses and Transformation. Synthesis of (2E,2′E,2″E)3,3′,3″-(2,4,6-Trimethylbenzene-1,3,5-triyl)-triacrylic acid (H3TTTA). A mixture of mesitylene (4.81 g, 40 mmol), I2 (12.70 g, 50 mmol), HIO4·2H2O (9.12 g, 40 mmol), and H2SO4 (1 mL) in acetic acid (40 mL) and water (8 mL) was stirred at 90 °C for 10 h. The reaction mixture was diluted with water (200 mL), and the resulting precipitate was filtered, washed with water, and acetone to give 1,3,5-triiodo-2,4,6trimethylbenzene (2) as white solid (16.8 g, 79%). To a 500 mL three-necked, round-bottomed flask, 2 (15 g, 30 mmol), methyl acrylate (13 g, 150 mmol), CH3COOK (28 g, 285 mmol), and Pd(PPh3)4 (0.88 g, 0.76 mmol) was added. The flask was connected to Schlenk line, evacuated, and refilled with nitrogen. 150 mL of N,N-dimethylformamide (DMF) was degassed (2 h) and added through a canula. The flask was equipped with a water condenser and reflux under the nitrogen for 24 h. The solvent was evaporated on rotary evaporator. 150 mL of H2O was added and then extracted with CH2Cl2. The organic phase was dried with MgSO4. After removal of the CH2Cl2 solvent, the crude product was column-chromatographed over silica gel using ethyl acetate/petroleum ether (1:5, v/v) to obtain pure product (3) (11 g, ∼77% yield) based on 1,3,5-triiodo-2,4,6trimethylbenzene. (2E,2′E,2″E)-Trimethyl 3,3′,3″-(2,4,6-trimethylbenzene-1,3,5-triyl) triacrylate (3) (11 g) was suspended in a mixed solvent of tetrahydrofuran (THF; 40 mL) and MeOH (40 mL), to which 20 mL of 10 M NaOH aqueous solution was added (Scheme 1). The mixture was stirred under reflux overnight, and the THF and MeOH were removed under a vacuum. Dilute HCl was added to the remaining aqueous solution until the solution was at pH = 6. The solid was collected by filtration, washed with water, and dried to give the final product of (2E,2′E,2″E)-3,3′,3″-(2,4,6-trimethylbenzene-1,3,5triyl)-triacrylic acid (4, H3TTTA) as white solid (10 g, 88.2% yield). 1H NMR (deuterated dimethyl sulfoxide, 400 MHz): δ = 12.45 (s, 3H), 7.66 (d, 3H), 5.93 (d, 3H), 2.18 (m, 9H) (Figure S1). Synthesis of [In3(TTTA)2(OH)3(H2O)](DMA)3 (BUT-70). A mixture of H3TTTA (20 mg, 0.06 mmol) and InCl3 (12 mg, 0.054 mmol) was dissolved in 2 mL of mixed solvent of DMA and H2O (1 mL/1 mL) with 0.25 mL of HBF4 (48%, aq.) in a screw-capped vial (4 mL). The reaction system was heated at 120 °C for 36 h and then cooled to room temperature. Colorless crystals of [In3(TTTA)2(OH)3(H2O)](DMA)3 (BUT-70) were obtained by filtration and washed with DMA. Anal. Calcd (%) for In3C48H62O19N3: C, 43.33; H, 4.66; N, 3.16. Found: C, 42.89; H, 4.10; N, 3.47. For PXRD pattern of assynthesized material, see Figure 3a. For FT-IR and TGA spectra, see Figures S2 and S3 in the Supporting Information, respectively. Single-Crystal to Single-Crystal Transformations from BUT-70 to [In3(TTTA)2(CH3O)3] (BUT-70A) and [In3(TTTA)2(OH)3] (BUT-70B). As-synthesized BUT-70 sample was soaked in fresh DMA for 24 h, and the extract was discarded. Fresh solvent (anhydrous dichloromethane or methanol) was subsequently added. The sample was immersed in the given solvent for 24 h, and then the extract was discarded and fresh solvent was added. This procedure was repeated twice. The guest-free phase [In3(TTTA)2(CH3O)3] (BUT-70A) or [In3(TTTA)2(OH)3]

from the high energy consumption and potential risk of C2H2 explosion when it is pressurized to absolute pressure over 2 bar. Therefore, highly efficient, safe, and energy-saving separation of C2H2 from CO2 and CH4 is quite significant. The emergence of microporous materials with selective adsorption properties (e.g., MOFs) is thus a viable option in this regard.5 However, in crystal engineering of MOFs, single-crystal to single-crystal (SCSC) transformation is a very intriguing phenomenon that has attracted widespread attention in the past few years, because such a process can facilitate the extraction of clear relation between the property change and the structure transformation by the analysis of single crystal structures.6 During SCSC transformation, the rearrangement of molecular components in the crystals takes place, sometimes resulting in slight changes of the coordination environment of metal ions and ligands but significant change of their properties, which are not easily interpreted and rationalized by other characterization techniques, such as powder X-ray diffraction (PXRD), infrared spectrum (IR), and elemental analysis (EA). Furthermore, SCSC transformation also provides an alternative approach for obtaining new MOF materials with unique property. In this work, a new acrylate-based ligand, ((2E,2′E,2″E)3,3′,3″-(2,4,6- trimethylbenzene-1,3,5-triyl)triacrylate (TTTA3−), as a variant of the well-known benzene-1,3,5tricarboxylate (BTC3−) ligand, was designed and synthesized. The reaction of InCl3 with H3TTTA resulted in a threedimensional (3D) structural MOF, [In3(TTTA)2(OH)3(H2O)]·(DMA)3 (BUT-70, BUT = Beijing University of Technology, DMA = N,N-dimethylacetamide). On the basis of interesting SCSC processes, the activation treatment of BUT-70 with different solvents followed by their removal produced two structurally different guest-free phases, [In3(TTTA)2(CH3O)3] (BUT-70A) and [In3(TTTA)2(OH)3] (BUT-70B). It was found that the MOFs are porous and waterstable, and the transformations among them are reversible. In addition, gas adsorption studies show that BUT-70A and -70B can selectively adsorb C2H2 over CO2 and CH4, being potentially useful in their separations.

2. EXPERIMENTAL SECTION 2.1. Materials and General Methods. All general reagents and solvents (AR grade) were commercially available and used as received. Fourier transform infrared (FT-IR) spectra were measured on a IRAffinity-1 instrument, and 1H NMR data of H3TTTA were collected on a Mercury 300 NMR spectrometer. BRUKER D8-Focus Bragg− Brentano X-ray Powder Diffractometer with a Cu sealed tube (λ = 1.541 78 Å) was used to record the PXRD data at room temperature (r.t.). Thermal gravimetric analysis (TGA) data were performed by using a TGA-50 (SHIMADZU) thermogravimetric analyzer under air atmosphere with a heating rate of 10 °C min−1. Gas adsorption 2189

DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

Article

Inorganic Chemistry

Figure 1. (a) Coordination environment of In3+ atoms in BUT-70; (b) infinite 1D chain consisting of corner-sharing InO4(OH)2 octahedral entities; (c) the 3D framework structure viewed along the crystallographic (101) direction (color code: In, dark cyan; C, dark gray; O, red; octahedral geometry constituted by In and O: green; hydrogen atoms and guest molecules are not shown for clarity in (b, c)). the bulk gas at equilibrium with the adsorbed phase (kPa), b1 and b2 are the affinity coefficients of sites 1 and 2 (kPa−1).

(BUT-70B) was obtained after the solvent-exchanged sample was evacuated under vacuum at 80 °C for 4 h. For BUT-70A, Anal. Calcd (%): C, 42.85; H, 3.57. Found: C, 42.98; H, 4.15. For PXRD pattern of as-synthesized material, see Figure 3b. For FT-IR and TGA spectra, see Figures S2 and S3 in the Supporting Information, respectively. For BUT-70B, Anal. Calcd (%): C, 41.14; H, 3.14. Found: C, 42.57; H, 3.99. For PXRD pattern of as-synthesized material, see Figure 3c. For FT-IR and TGA spectra, see Figures S2 and S3 in the Supporting Information, respectively. 2.3. X-ray Crystallography. The diffraction data of the BUT-70, BUT-70A, and -70B were collected with an Agilent Supernova CCD diffractometer equipped with a mirror monochromated enhanced Cu Kα radiation (λ = 1.541 84 Å). The data sets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm.7 The structures were solved by direct methods and refined by full-matrix least-squares on F2 with anisotropic displacement using the SHELXTL software package.8 Hydrogen atoms of ligands were calculated in ideal positions with isotropic displacement parameters. Crystal data and structure refinements and selected bond angles and distances are shown in Tables S1−S4 in the Supporting Information, and additional crystallographic information can be found there as well. 2.4. Analysis of Gas-Adsorption Data. Fitting of SingleComponent Gas-Adsorption Isotherm. The single-component C2H2, CO2, and CH4 adsorption isotherms were fitted with the following dual-site Langmuir equation, where N is the adsorbed amount per mass of adsorbent (mmol g−1), N1max and N2max are the saturation capacities of sites 1 and 2 (mmol g−1), p is the pressure of

N = N1max ×

b1p b2 p + N2max × 1 + b1p 1 + b2 p

Calculation of Isosteric Heat of Adsorption. Isosteric heats of adsorption (Qst) for CO2, C2H2, and CH4 as a function of the quantity adsorbed were estimated based on the gas adsorption data recorded at 273 and 298 K, respectively. Qst for CO2, C2H2, and CH4 adsorption were calculated with the following Clausius−Clapeyron equation, where T is the temperature, R is the universal gas constant, and C is a constant. The Qst values at different gas loading N were obtained from the slopes of the plots

(ln P)N = −

Q st 1 +C R T

Calculations of Ideal Adsorbed Solution Theory Adsorption Selectivity. The selectivity of preferential adsorption of component 1 over 2 in a mixture containing them can be formally defined as the following equation, where q1 and q2 are the absolute loadings. The calculations of Sads are based on the Ideal Adsorbed Solution Theory (IAST) of Myers and Prausnitz.9 These calculations are performed according to the previously reported works.10 Sads = 2190

q1/q2 p1 /p2 DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

Article

Inorganic Chemistry

3. RESULTS AND DISCUSSION Single-crystal structural analysis shows that BUT-70 crystallizes in the monoclinic system, space group P21/c. The asymmetric unit of its structure contains two TTTA3− ligands, four In3+ atoms, three μ2−OH− groups, one coordinated water molecule, and three DMA solvent molecules. As shown in Figure 1a, the four independent In3+ atoms (In1, In2, In3, and In4) are all coordinated with six O atoms in an octahedral geometry, among which In1 and In4 are on the 21 axis with half occupancy. In1, In3, and In4 are coordinated by four carboxylate O atoms from four TTTA3− ligands at the equatorial positions with the In−O bond lengths in the range from 2.1171(6) to ∼2.2172(4) Å, and two μ2−OH− groups at the apical positions with the In−O bond lengths in the range from 2.078(3) to ∼2.102(3) Å, respectively, while In2 is coordinated with three carboxylate O atoms provided by three TTTA3− ligands (O5, O7, and O16) and one hydroxyl O atom (O2) at the equatorial positions (In−O: 2.094(4) to ∼2.179(4) Å), and the two apical positions are occupied by another hydroxyl O atom (O1) (In−O: 2.102(3) Å) and a water molecule (O4) (In−O: 2.172(5) Å). Each of the three unique hydroxyl groups (O1, O2, and O3) bridges two neighboring In3+ atoms (In1/In2, In2/In3, and In3/In4). There are two types of unique TTTA3− ligands in BUT-70, and their carboxylate groups are all coordinated with neighboring In3+ atoms in a bi-monodentate fashion, except one that is coordinating with In3 through one carboxylate O atom (O14) and hydrogen bonding to a hydroxyl group (O3) through the other uncoordinated carboxylate O atom (O13) (d(O3···O13) = 2.724(8) Å). Note that all the TTTA3− ligands are in a distorted conformation, where the peripheral vinyl groups are tilted by 53.5° to ∼68.6° dihedral angles with respect to the central phenyl core due to the spatial hindrance between vinyl groups and neighboring methyl substituents. As a result, the coordination geometry of the TTTA3− ligands is much different from that of the well-studied tricarboxylate ligand BTC3− with a coplanar geometry. The connection of the In3+ atoms with the carboxylate and μ2−OH− groups results in an infinite one-dimensional (1D) chain (rod-shaped SBU) consisting of corner-sharing InO4(OH)2 octahedra arranging along the crystallographic (101) direction (Figure 1b). Each 1D chain is connected with six neighboring symmetrically equivalent ones by the TTTA3− ligands, leading to the 3D framework of BUT-70 (Figure 1c). The total potential solventaccessible void volume in the framework is 51.2% of the whole structure as estimated by PLATON,11 which is occupied by three DMA molecules per formula according to single-crystal structure analysis, TGA, and elemental analyses (see Figure S3 in the Supporting Information and Experimental Section). To check the porosity of BUT-70, we performed N2 adsorption experiment at 77 K after the sample was solventexchanged with methanol and subsequent evacuation under high vacuum at 80 °C. Unexpectedly, the saturated N2 uptake of the activated BUT-70 at P/P0 = 1 is only 128 cm3 g−1. The Brunauer−Emmett−Teller (BET) and Langmuir surface areas are estimated to be 460 and 545 m2 g−1, respectively, and the pore volume is calculated to be 0.18 cm3 g−1. This is much lower than the calculated value (0.47 cm3 g−1) based on the single-crystal data provided that all the guests are removed and the framework retains intact. We repeated the adsorption experiment with prolonged time for the solvent exchange and vacuum evacuation processes but obtained the same results. It was speculated that the structure of BUT-70 might partially

collapse during the sample activation (transformed to BUT70A actually as discussed below). Then, the fresh BUT-70 sample was solvent-exchanged with a more inert and volatile solvent, dichloromethane (DCM), instead of methanol. N2 adsorption isotherm was recorded at 77 K for the DCMexchanged BUT-70 sample after it was further evacuated under high vacuum at 80 °C (transformed to BUT-70B actually as discussed below). It shows an elevated saturated N2 uptake of 188 cm3 g−1 at P/P0 = 1 (Figure 2). The BET and Langmuir

Figure 2. N2 adsorption isotherms of BUT-70A and -70B recorded at 77 K.

surface areas are estimated to be 695 and 780 m2 g−1, respectively, and the pore volume is calculated to be 0.29 cm3 g−1. Although the pore volume of the DCM-treated BUT-70 sample is higher than that of methanol-treated one by 58.2%, it is still much lower than the expected value on the single-crystal data of BUT-70. There are two possible reasons for these findings: (1) the sample was probably partially degraded (leading to framework collapse) after the activation, and (2) severe structural transformations might happen during the activation process by the two different solvent-exchange methods. We then further examined the framework stability of BUT70. First, it was found that PXRD pattern of as-synthesized BUT-70 matches well the simulated pattern derived from its single-crystal structure data, suggesting the phase purity of the bulk sample (Figure 3a). Then, activated samples as abovementioned were checked by PXRD. As shown in Figure 3b,c, the PXRD peaks of both BUT-70A and -70B are intense and sharp, indicating high crystalline nature of them. Through comparing the PXRD patterns of BUT-70, -70A, and -70B, obviously the structure of BUT-70 transformed after the activation, as evidenced by the disappearance of some PXRD peaks and the appearance of new peaks in the PXRD patterns for both BUT-70A and -70B. Different PXRD patterns of BUT-70A and -70B show that they have different framework structures, although they were initially expected to be an identical guest-free phase of BUT-70. These results demonstrate that the framework of BUT-70 is not collapsed after the solvent exchange and removal but transforms to new phases with robust framework structures. The observed low N2 uptakes for the guest-free BUT-70 phases can thus be attributed to the structural transformations instead of structural degradation during the activation. Fortunately, the single crystals remained transparent, and no visible cracks were observed after BUT-70 samples were solvent-exchanged with DCM or methanol. The single-crystal structures of BUT-70A and -70B were successfully determined 2191

DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

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Inorganic Chemistry

70A and -70B are only slightly changed in comparison with those of BUT-70 (Table S1 in the Supporting Information). The structural analysis of BUT-70B reveals that the activation process removed not only the guest molecules but also the coordinated water molecules. The high flexibility of the MOF should be related with the flexible trans-ethylene arms of TTTA3− ligand. By comparing the structure of BUT-70 and -70B, we propose that the single-crystal to single-crystal (SCSC) transformation process is as follows. As shown in Figure 4, when the coordinated water molecule released, In2 atom became five-coordinated, leading to an unstable coordination configuration. The TTTA3− ligands coordinated with In2, In1, and In3 started to change their conformations, so that the only uncoordinated carboxylate O atom (O13, hydrogen-bonding with a hydroxyl group in BUT-70) could approach and finally coordinate with In2 atoms to stabilize the whole framework structure. The atom positions for In1, In2, O1, and O2 also obviously shifted to adapt the new coordination geometry of In2 (Figure 4b). As a result, in BUT-70B all the four unique In3+ atoms are coordinating with four carboxylate O atoms at the equatorial positions (In−O bond: 2.112(4) to ∼2.210(4) Å) and two μ2−OH− groups at the apical positions (In−O bond: 2.084(3) to ∼2.104(4) Å), and all the carboxylate groups of TTTA3− ligands are coordinated with two neighboring In3+ atoms in a bimonodentate fashion (Figure S5a in the Supporting Information). As in BUT-70, the framework of BUT-70B is constructed by the corner-sharing InO4(OH)2 octahedral entities-based 1D chains, which are interconnected by TTTA3− ligands. It contains 49.5% total potential solvent-accessible void volume as estimated by PLATON. The structure of BUT-70A is almost identical to that of BUT-70B, except that the hydroxyl groups are replaced by methoxyl groups, which should be derived from methanol molecules used in solvent exchange (Figure 4b). As the methoxyl groups are bulkier than the hydroxyl groups, the total potential solvent-accessible void volume of BUT-70A (45.3%)

Figure 3. (a, b) PXRD patterns of BUT-70 and -70A: simulated (black), as-made (red), and treated in water for 24 h at r.t. (blue); (c) PXRD patterns of BUT-70B: simulated (black), as-made (red), treated in boiling water at r.t. for 24 h (blue), for 5 d (dark yellow), and treated by HCl aqueous solutions (pH = 4) at r.t. for 24 h (purple); observed diffraction: green tick marks, systematic absences: pink tick marks.

after the guest-exchanged BUT-70 crystals were in situ heated at 80 °C under N2 flow for ∼4 h to remove guest molecules. BUT-70A and -70B remain in the monoclinic system, space group P21/c. The unit cell axes and unit cell volumes of BUT-

Figure 4. (a) Schematic representation of the transformation between BUT-70, -70A, and -70B; (b) the partial of 1D chains in BUT-70A, -70, -70B, and -70A, respectively, to show the transformation process (color code: C, dark gray; H, white; O, red; In, darkcyan; hydrogen atoms in the MOFs are omitted for clearly). 2192

DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

Article

Inorganic Chemistry Table 1. Gas Adsorption Data in BUT-70A and -70B BUT-70A BUT-70B

273 298 273 298

K, K, K, K,

1 1 1 1

atm atm atm atm

CO2 [cm3 g−1]

CH4 [cm3 g−1]

C2H2 [cm3 g−1]

C2H4 [cm3 g−1]

C2H6 [cm3 g−1]

C3H6 [cm3 g−1]

C3H8 [cm3 g−1]

37.9 19.3 57.1 31.3

16.1 9.7 14.3 9.6

93.8 69.5 120.5 87.1

53.0 36.9 94.2 49.0

62.7 42.7 68.9 53.3

72.1 57.4 76.7 69.8

69.4 53.0 69.3 64.5

structure of BUT-70 could be recovered after BUT-70A or -70B were immersed in mother liquid (1:1 DMA and H2O) at room temperature, which was confirmed by single-crystal X-ray diffraction experiments and structure analyses. Moreover, BUT70B could be transformed into BUT-70A by simply immersing the crystals of BUT-70B in methanol and subsequent evacuation. However, BUT-70A was not able to transform into BUT-70B when its crystals were immersed in DCM, suggesting that the replacement of methoxyl groups on the 1D InO4(OCH3)2 chains of BUT-70A by the hydroxyl groups requires the presence of water in solvent. Thus, the transformation from BUT-70A to -70B requires two steps, immersing in the mother liquid and then in DCM (Figure 4a). To test the chemical stabilities of the three MOFs, their samples were treated in water. After being soaked in water at r.t. for 24 h, the measured PXRD patterns of BUT-70 and -70A show retained crystallinity with minor changes of the PXRD peaks, implying water stability of their robust frameworks (Figure 3a,b). BUT-70B was further demonstrated to be stable in harsher conditions. Its samples were treated in boiling water for 24 h and 5 d, as well as in the dilute aqueous HCl solution (pH = 4) at r.t. for 24 h, respectively. The PXRD patterns of all these treated samples show again retained crystallinity and unchanged structure (Figure 3c). Considering their unique pore structures and water stability, BUT-70A and -70B are potentially useful for the gas separation. We performed the adsorption studies of CO2, CH4, C2H2, C2H4, C2H6, C3H6, and C3H8 on the two MOFs, and the results are summarized in Table 1, Figures S8 and S9 in the Supporting Information. Clearly, the MOFs adsorb more C2H4, C2H6, C3H6, and C3H8 than CH4, which should be related to their higher molecule weights and thus stronger guest−host interactions with the MOF frameworks. Notably, the results show that BUT-70A uptakes quite different amounts of C2H2, CO2, and CH4 at 273 or 298 K in all measured pressures (Figure 5a, Table 1), although the kinetic diameters of the three gases are very close (C2H2: 3.3 Å, CO2: 3.3 Å, CH4: 3.8 Å), especially for C2H2 and CO2. At 298 K, the uptake ratios of C2H2 to CO2 and of C2H2 to CH4 are 3.61 and 7.16, respectively. For BUT-70B, the adsorption capacity of C2H2, CO2, and CH4 are higher with respect to those of BUT-70A (Figure 5b, Table 1) at the same conditions, but the uptake ratios of C2H2 to CO2 and of C2H2 to CH4 are lower, being 2.79 and 9.07, respectively. The uptake ratios of C2H2/CO2 and C2H2/CH4 at room temperature and 1 atm were used to tentatively evaluate the performance of the two MOFs in the separation of these gases. It was found that the uptake ratios of C2H2/CO2 for BUT-70A (3.61) and -70B (2.79) at room temperature are higher than those of most reported MOFs, except MOF-2 (3.875),18 which, however, can only uptake a much lower amount of C2H2 (31 cm3 g−1) at room temperature and 1 atm (Table 2). The uptake ratios of C2H2/CH4 for BUT-70A (7.16) and -70B (9.07) are also moderately high compared with those of the reported MOFs. Furthermore, IAST adsorptive selectivity for the binary

is lower than that of BUT-70B, which explains the lower saturated N2 uptake of BUT-70A relative to BUT-70B. However, in terms of the crystal structures of BUT-70A and -70B, their pore volumes are calculated to be 0.41 and 0.46 cm3 g−1, respectively. These calculated values are still much higher than their pore volumes experimentally evaluated based on the N2 adsorption isotherms (0.18 and 0.29 cm3 g−1, respectively). Analyzing the channels in BUT-70A or -70B, we found that they are irregular compared with common cylinder shaped ones in many MOFs. As shown in Figure S7 in the Supporting Information, the channels in BUT-70A and -70B are twodimensional (2D) intersecting along the crystallographic (100) planes. The cross-section diameters of the channels are calculated to be in the range of ∼5.5−6.5 Å and 5.8−6.9 Å by using a probe with varying radius via Materials Studio.12 Outer surfaces of the channels are roughly concaved with numerous sharp angles, much different from that of common cylinder-shaped channels. We believe that the special shape of the channels might lead to the preferred orientation of the adsorbed N2 molecules, and/or poor space commensurateness between the adsorbed N2 molecules and the pore packages. Thus, the channels of BUT-70A and -70B are not fully occupied by N2 molecules during the adsorption at 77 K even at the saturation pressures. As a result, the experimentally observed pore volumes are lower than the calculated ones from the single-crystal data as discussed above. Also note that the channels in BUT-70A and -70B are different from those in other MOFs constructed from 1D chain SBUs, such as MIL-53(Cr, Al),13 M2(dobdc) (also referred to as M-MOF-74 or CPO-27-M, M = Zn, Ni, Co, and Mg),14 MAF-X25, and MAF-X27.15 The channels in these reported MOFs are all 1D and are extending along the direction of the 1D chain SBUs. However, in BUT-70A and -70B, the channels are intersecting two-dimensionally along the crystallographic (100) planes (Figure S7b,d in the Supporting Information). None of the two directions of the channels (the crystallographic (011) and (01−1) directions) are parallel to the direction of the 1D chain SBUs (the crystallographic (101) direction). In addition, it is also noteworthy that the release of coordinated water molecules and the formation of new bonds between the In(III) atoms and the carboxylate O atoms during the SCSC transformation has not been observed for In(III)based MOFs, although some examples are known for the MOFs based on Cu(II), Zn(II), Cd(II), and Ni(II) ions.16 Fox example, Qin et al. reported the SCSC transformations of a 3D Cu-MOF where a μ2-H2O molecule bridging two Cu(II) centers was substituted by an uncoordinated carboxylate O atom of ligand after the as-synthesized MOF was guestexchanged with some solvents (acetone, 2-propanol, and 2butanol) at room temperature.17 This Cu-MOF transformed into derived structures without changing the space group; however, the SCSC transformations were irreversible, which means that the derived single crystals could not transform to the parent single crystal. In contrast, we found that the 2193

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Figure 6. IAST C2H2/CO2 and C2H2/CH4 adsorption selectivity for BUT-70A (a) and BUT-70B (b) at 298 K. Figure 5. CO2, CH4, and C2H2 adsorption isotherms of (a) BUT-70A and (b) -70B recorded at 298 K.

the range of 14.4−14.8 and 8.5−11.2, respectively. Those selectivities are higher than those of most reported MOFs (Table 2), such as ZJU-30a, UTSA-30a, UTSA-68a, ZJU-199, UTSA-50a, NOTT-101a, PCP-33, and HKUST-1. The C2H2/ CH4 IAST adsorption selectivity is high up to 66.6 for BUT-

C2H2/CO2 (50:50, v/v) and C2H2/CH4 (50:50, v/v) was more commonly used to evaluate the performance of MOFs or other adsorbents for the gas separations. As shown in Figure 6, C2H2/ CO2 IAST selectivities in BUT-70A and -70B at 298 K are in

Table 2. Comparison of Adsorption Data, Uptake Ratios, and IAST Selectivities for Selected MOFs uptake(cm3 g−1 RT, 1 atm) ZJU-26 HOF-3a Cu-TDPAT ZJU-40a NOTT-101a MOF-1 MOF-2 PCP-33 UTSA-74 UTSA-68a UTSA-50a HKUST-1 ZJNU-47a ZJNU-61 ZJU-30a ZJU-199 ZJU-72a UTSA-5a M′MOF-20a FJI-C4 UTSA-72a ZJU-10a FJI-C1 BUT-70A BUT-70B

C2H2

CO2

CH4

84 47 155.7 216 184 100 31 121.8 145.0 70.1 90.6 201 214 48.0 52.6 128.0 167.7 59.8 21 72.5 27.8 174 93.8 69.5 87.1

38 21

12

ratio of uptakes(r.t., 1 atm) C2H2/CO2

C2H2/CH4

2.21 2.24

14

62.4 103.9 38.4 10 60.3 21.7 81 19.3 31.3

18.8 21.8 6.2 13.9 14.4 25.1 5.2 2.9 18.4 4.4 19 9.7 7.3 11.5

6.95 2.48 2.19 2 3.875 2.08 1.53 1.77 1.41 1.78 1.98

82 8−9

6−10

4.82

2.05 1.61 1.56 2.1 1.20 1.28

9.81 7.74 3.78 8.89 6.68 11.5 7.24 3.94 6.32

3.61 2.79

9.76 7.16 9.07

3.5−5 15.0 5.8−11

1.7−2.4 4.0−5.8

9.7

2194

14.4−14.8 8.5−11.2

ref

C2H2/CH4

7

22.4 87 84 50 8 58.6 95.0 39.6 64.4 113 108

IAST selectivity (r.t.) C2H2/CO2

27.3−33.5 39.7 34.9 51.0 26.5 61 39.3 66.6 23.3−47.9

5e 4c 19 20 20 18 18 21 22 22 23 24 25 26 27 28 29 5d 30 31 32 33 34 this work this work

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70A, which is higher than that of many reported MOFs (Table 2). In addition, we also calculated the isosteric heats of adsorption (Qst) of C2H2, CO2, and CH4 based on the isotherms recorded at 273 and 298 K for both BUT-70A and -70B. As shown in Figure S10 in the Supporting Information, with increasing the adsorption amount, the Qst for CO2 decreases from 24.6 to 23.2 kJ mol−1 for BUT-70A, and from 23.2 to 19.6 kJ mol−1 for BUT-70B, but the Qst for C2H2 gradually increases as C2H2 uptakes increases from 23.9 to 25.6 kJ mol−1 for BUT-70A, and increases from 23.0 to 27.0 kJ mol−1 then slightly discreases to 26.1 kJ mol−1 for BUT-70B. These results indicate that the frameworks of BUT-70A and BUT-70B show higher infinity to CO2 than to C2H2 at low loading, but as the adsorbed gases increase, the interactions between the frameworks and C2H2 become stronger, which is probably related to the special pore shape and internal surface functionality (methyl and vinyl groups) of pores in these MOFs.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.Y.) *E-mail: [email protected]. (Y.W.) *E-mail: [email protected]. (L.H.X.) ORCID

Ya Chen: 0000-0001-8477-1328 Lin-Hua Xie: 0000-0003-0017-3887 Jian-Rong Li: 0000-0002-8101-8493 Notes

The authors declare no competing financial interest. Simulation of the PXRD patterns was performed by the singlecrystal structure data and diffraction-crystal module of the Mercury program available free of charge via Internet at https://www.ccdc.cam.ac.uk/. Crystallographic data in this work has been deposited on the Cambridge Crystallographic Data Center (CCDC No. 1518180, 1515094 and 1515095). The data can be obtained free of charge from the Director, CCDC, 12 union Road, Cambridge CB2 1EZ, U.K. (Fax: +44− 1223−336033; e-mail: [email protected] or http://www. ccdc.cam.ac.uk).

4. CONCLUSIONS In summary, we have designed and synthesized a new acrylatebased ligand and isolated its In(III)-based MOF (BUT-70). This MOF undergoes an interesting reversible solvent-induced SCSC transformation accompanied by the substitution of coordination water molecules with uncoordinated carboxylate O atoms of the ligand, after it was solvent-exchanged with dichloromethane or methanol, followed by their removal. Additionally, hydroxyl groups bridging two In(III) centers were replaced by methoxyl groups when methanol was used for the guest exchange. The structural transformations led to two different guest-free phases of BUT-70 (BUT-70A and BUT70B). The MOFs are porous with 3D framework structures constructed by infinite 1D chains consisting of corner-sharing InO4(OR)2 (R = H or Me) octahedra. Unlike most reported MOFs constructed by 1D chains containing 1D channels along direction of the infinite chains, BUT-70, -70A, and -70B contain 2D intersecting channels along the crystallographic (100) planes. In addition, BUT-70A and -70B can also be mutually interconverted in an SCSC fashion by the solvent exchange at room temperature and subsequent removal. It was also demonstrated that these MOFs are highly stable in water. And, gas sorption studies reveal that both BUT-70A and -70B have good separation performances toward C2H2 over CO2 and C2H2 over CH4. This work demonstrates that the acrylatebased ligands are unique for construction of flexible and stable MOFs with selective gas adsorption property. Further studies to fine-tune porous structure with such kind of ligands and to explore the relationship between the MOF framework and adsorption property are underway.

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ACKNOWLEDGMENTS This work was financially supported from the Natural Science Foundation of China (21506003 and 21601008), the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20150309), and Project funded by China Postdoctoral Science Foundation (2015M580027).

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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.6b02840. 1

H NMR spectrum, FT-IR, TGA, details of structure refinement, additional structural figures, gas adsorption, and fitting adsorption data (PDF) X-ray crystallographic data in CIF format (CIF) 2195

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DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197

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DOI: 10.1021/acs.inorgchem.6b02840 Inorg. Chem. 2017, 56, 2188−2197