Solvent- and Temperature-Induced Multiple Crystal Phases: Crystal

Oct 14, 2016 - Three S-containing highly porous metal−organic frameworks featuring with high adsorption capacities and high selectivity for methylen...
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Solvent- and Temperature-Induced Multiple Crystal Phases: Crystal Structure, Selective Adsorption, and Separation of Organic Dye in Three S‑Containing {[Cd(MIPA)]n}n− Homologues Bao-Xia Dong,* Meng Tang, Wen-Long Liu,* Yi-Chen Wu, Yong-Mei Pan, Fan-Yan Bu, and Yun-Lei Teng College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, P. R. China S Supporting Information *

ABSTRACT: Three S-containing highly porous metal−organic frameworks, namely, [((CH3)2NH2)Cd(MIPA)]n·xG (n = 4 (1), 2 (2), 1 (3) G = guest of DMF/DMA and H2O), featuring with high adsorption capacities and high selectivity for methylene blue (MB), were successfully assembled by subtly varying the synthesis conditions including solvents and temperatures. Compounds 1−3 crystallize in P21/c, I2/a, and R3̅m, respectively. The anionic frameworks of them are homologues and multiple crystal phases of the {[Cd(MIPA)]n}n−. Compounds 1 and 2 demonstrate the same 3,6-connected binodal net (42·6)(48·64·83), encapulating ca. 18 Å × 18 Å hexagonal channels and three other types of micropous channels. Compound 3 shows a different topology of a 4,4-connected binodal net (43·62·8), which has the highest crystalline density, highest solvent-accessible void, and accessible special surface area, as well as the highest symmetry among them. Circular channel, ca. 13 Å × 13 Å, and two other types of micropore channels are found in six directions. The dye adsorption process for these homologues is proven to be charge-selective and size-selective, in which only the cationic guest (MB+) with a suitable size could enter the frameworks of them and exchange with the dimethylammonium cations. The MB maximum adsorption capacities are 395.1 mg g−1 for 1, 286.6 mg g−1 for 2, and 373.2 mg g−1 for 3, respectively. Moreover, for three kinds of dye mixtures, 3 exhibits the best adsorption kinetics, which is probably due to the suitable pore size, highest solvent-accessible void, and the moderate maximum adsorption capacity of MB.

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INTRODUCTION Metal−organic frameworks (MOFs), known as organic− inorganic hybrid solids with infinite, uniform, and porous framework structures, are built from organic linkers (usually nitrogen or oxygen-containing ligands) and inorganic metal (or metal-containing cluster) nodes.1,2 As a nascent field, MOFs have the advantages over conventional inorganic porous materials in that their structures and functions are designable and readily modulated.3,4 Intrigued by their unique characteristics, novel structures are continually increasing, and potential applications involving gas storage, heterogeneous catalysis, adsorption of volatile organic compounds (VOCs), and drug delivery, are continuously being explored.5−14 The textile industry uses >10 000 synthetic dyes that have been specifically designed to be stable against oxygen, water, and sunlight, which in turn, are difficult to degrade.15,16 Adsorption treatment arises as one of the more feasible methods for rapidly and effectively removing dyes from the contaminations because the adsorbent can be regenerated and the whole progress is much milder. Recently, many MOFs have been applied to the adsorption and separation of dyes such as methylene blue, malachite green, rhodamine B, methyl orange, xylenol orange, etc. The ionic MOFs may have more unique advantages such as selective © XXXX American Chemical Society

adsorption of cationic or anionic dyes by host−guest electronic interactions and/or guest−guest exchange interactions.14−21 Sulfur-containing compounds, e.g., thiolates, are of increasing interest which has arisen from their fascinating electroactive properties.22−24 Compared to O, N donor atoms (as the hard acid) making strong electronic barriers between the metal centers and the rest of the organic molecule, S donor atoms (as the soft acid) can decrease the lowest unoccupied molecular orbital (LUMO) and increase the highest occupied molecular orbital (HOMO) energy of the ligand.25−27 Thiolate-based organic ligands have been extensively used in the synthesis of small molecule analogues of metal-containing active sites of metalloproteins through well-organized sulfur bridges.28,29 However, most of the sulfur-containing coordination compounds are nonporous, which induces lots of functions of such compounds to remain unexplored. Recently, a S-containing porous materials of IFMC-28 has come into sight, which exhibits selective adsorption ability for Cu2+ ions thanks to the existence of S-donors. It was constructed by a predesigned Received: July 4, 2016 Revised: September 8, 2016

A

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data and Structure Refinement for Compounds 1−3

a

compounds

1

2

3

molecular formula empirical formula (the anion framework) formula weight temperature (K) crystal system space group a/Å b/Å c/Å α β γ V/Å3 Z Dc (g cm−3) μ/mm−1 F(000) Rint GOF R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b

[NC2H8]4[C32H12Cd4O16S4] C32H12Cd4O16S4 1230.26 150(2) monoclinic P21/c 14.0370(14) 33.781(4) 18.982(2) 90 103.684(3) 90 8745.3(16) 4 0.934 1.086 2352 0.0441 1.016 0.0354 0.0830

[NC2H8]2[C16H6Cd2O8S2] C16H6Cd2O8S2 615.13 293(2) monoclinic I2/a 14.425(5) 33.813(5) 19.194(4) 90 104.061(18) 90 9081(4) 8 0.900 1.046 2352 0.0413 1.035 0.0311 0.0726

[NC2H8][C8H3CdO4S] C8H3CdO4S 307.56 293(2) trigonal R3̅m 23.0075(18) 23.0075(18) 20.808(2) 90 90 120 9538.9(18) 18 0.964 1.120 2646 0.0374 1.013 0.0281 0.0787

R1 = Σ∥F0| − |Fc|/Σ|F0|. bwR2 = |Σw(|F0|2 − |Fc|2)|/Σ|w(F02)2|1/2. Å) radiation on a Bruker-AXS D8 Advance X-ray diffractometer in the angular range 2θ = 5°−50° at 296 K. Each pattern is recorded with a 2 s per step scan. The UV absorption characteristics were measured on a Shimadzu UV-2550 UV−vis spectrophotometer. Synthesis of (NC2H8)4[Cd(MIPA)]4·xG (G = Guest Molecule, DMA, and H2O) (1). A total of 4.0 mL of DMA and 1.0 mL of H2O were added to a beaker, which contains Cd(NO3)2·4H2O (0.046 g, 0.15 mmol) and H3MIPA (0.010 g, 0.05 mmol) with stirring for 10 min. Then the mixture was transferred and sealed in a 23 mL Teflonlined stainless steel container and heated at 150 °C for 3 days. After slow cooling to room temperature with the rate of 5 °C h−1, colorless crystals 1, with a 75% yield based on Cd, were collected. The resulting crystals were washed with methanol and dried in a vacuum oven at 40 °C. Prominent FT-IR peaks for 1 (KBr Pellet, cm−1): 3419(s), 2787(m), 2470(w), 1598(s), 1546(s), 1426(s), 1363(s), 1295(w), 1248(w), 1183(w), 1104(w), 1020(w), 898(vw), 760(s), 730(s). Synthesis of (NC2H8)2[Cd(MIPA)]2·xG (G = DMF and H2O) (2). The preparation of 2 was similar to that of 1 except that 3.0 mL of DMF and 2.0 mL of H2O were used instead of the mixed solvent of DMA/ H2O (4:1). Colorless crystals of 2 were obtained with 50% yield based on Cd. The resulting crystals were washed with methanol and dried in a vacuum oven at 40 °C. Prominent FT-IR peaks for 2 (KBr Pellet, cm−1): 3406(s), 2783(m), 2464(w), 1805(w), 1598(s), 1545(s), 1425(s), 1361(s), 1104(w), 1019(w), 856(w), 759(s), 729(s). Synthesis of (NC2H8)[Cd(MIPA)]·xG (G = DMA and H2O) (3). A total of 2.0 mL of DMA and 3.0 mL of H2O were added to a beaker, which contains Cd(NO3)2·4H2O (0.046 g, 0.15 mmol) and H3MIPA (0.010 g, 0.05 mmol) with stirring for 10 min. Then the mixture was transferred and sealed in a 23 mL Teflonlined stainless steel container and heated at 160 °C for 3 days. After slow cooling to room temperature with the rate of 5 °C h−1, colorless crystals 3, with 40% yield based on Cd, were collected and washed with methanol. Finally, the resulting crystals were dried in a vacuum oven at 40 °C for further treatment and characterization. Prominent FT-IR peaks for 3 (KBr pellet, cm−1): 3414(s), 2785(m), 2467(w), 1832(w), 1601(s), 1555(s), 1424(s), 1359(s), 1105(w), 1020(w), 925(w), 748(s). X-ray Crystallographic Study. Single-crystal X-ray diffraction analysis data were collected on a Bruker Smart Apex II CCD diffractometer with Mo Kα monochromated radiation (λ = 0.71073 Å). All absorption corrections were performed by using the SADABS program. The structures were solved by direct methods and refined on

ligand of H2DMTDC (3,4-dimethylthieno[2,3-b]thiophene2,5-dicarboxylic acid) and [Zn4O(CO2)6] secondary building block.30 Most recently, Yoon et al.31 obtained a new Zr (IV)based MOF of Zr-btdc (btdc = 2,2′-bithiophene-5,5′dicarboxylic acid), which is an isostructure of UiO-67. It shows an enhancement of gas sorption capacities and isosteric heat of adsorption (Qst) toward H2 and CO2, which indicates that the incorporation of electronegative atoms in a ligand can increase the interaction between gas molecules and a further increase of the gas sorption capacity of MOFs. 5-Mercaptoisophthalic acid (H 3 MIPA, see Figure S1, Supporting Information) is a multifunctional ligand with a combination of hard and soft donor atoms (two −COOH and one −SH), which can bind the metal ions to achieve a high connectivity of the framework. Herein, we report three highly porous frameworks, [((CH3)2NH2)Cd(MIPA)]n·xG (n = 4 (1), 2 (2), 1 (3); G = guest of DMA/DMF and H2O, DMA = N,Ndimethylacetamide, DMF = N,N-dimethylformamide), which crystallize in P21/c, I2/a, R3̅m space groups, respectively. The anionic frameworks of them are homologues and multiple crystal phases of the {[Cd(MIPA)]n}n−. It should be noted that these desolvated homologues all exhibit high adsorption capacities and high adsorption selectivity toward cationic dye of methylene blue (MB). Furthermore, the anionic framework of 3, possessing the highest symmetry and highest solventaccessible void, exhibits the fast MB adsorption rate in three kinds of dye mixtures.

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EXPERIMENTAL SECTION

Materials and General Methods. All chemicals purchased were of reagent grade and were used as received. 5-Mercapto-isophthalic acid (MIPA) was synthesized according to a documented procedure.32 FT-IR spectrum (KBr pellets) was recorded in the range of 4000−400 cm−1 on a BRUKER TENSOR 27 Fourier-transform infrared spectrometer. Thermal gravimetric analyses (TGA) measurements were performed on a TG model STA 449 F3 Netzsch instrument at a ramp rate of 5 °C min−1, by heating the sample under argon. Powder X-ray diffraction (PXRD) data were collected with Cu Kα (λ = 1.5406 B

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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F2 by full-matrix least-squares methods using the SHELXTL package.33 Anisotropic thermal parameters were used to refine all Cd, C, S, and O atoms. After locating and refining the framework and the counterions, the difference Fourier map showed many peaks of very low electronic density, suggesting an extensive disordered of the solvent molecules. Thus, the solvent molecules reside in those regions of diffuse electron density, and the counterions were treated by the PLATON/ SQUEEZE procedure.34 The hydrogen atoms attached to carbon positions were placed in geometrically calculated positions. The crystal data and structure refinement results of squeezed compounds 1−3 are summarized in Table 1. Selected bond lengths and angles are listed in Table S1. Atomic coordinates and equivalent isotropic displacement parameters for compounds 1−3 are listed in Table S2. Crystallographic data for the structure reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC numbers of 1482474−1482476 for 1−3. Dye Sorption Measurements. Adsorption experiments were carried out at constant temperature (298 K) in methanol solution. A stock solution of methylene (200 ppm) was prepared by dissolving MB (molecular formula: C16H18ClN3S·3H2O, molecular weight: 373.9 g mol−1) in methanol solution. The MB methanol solutions with different concentrations were prepared by successive dilution of the stock solution with methanol. The MB concentrations were determined using the absorbance (at λ = 653 nm) of the solutions after getting the UV spectra of the solution with a spectrophotometer (Shimadzu UV spectrophotomer, UV-25502). The calibration curve was obtained from the spectra of the standard solutions (1−16 ppm), which is fitted by the equation of y = 0.21467x − 0.01234 very well (R2 = 0.99991). Before adsorption, the adsorbents were activated by exchanging with methanol and then dried overnight under a vacuum. Ten milligrams of samples (1−3) were dispersed in a 100.0 mL methanol solution containing 40 ppm MB, respectively, at constant temperature of 298 K until the equilibrium was achieved. The kinetics of adsorption was determined by analyzing the adsorptive uptakes of the dye from methanol solution at different time intervals. After the adsorption process, the supernatants were analyzed to determine the remaining dye concentration in the solution via UV-25502. The amount of the dye adsorbed onto the smaples was determined according to the change of concentration before and after adsorption. The amount of dye absorbed at equilibrium, qe (mg g−1), was obtained by35

qe =

(C0 − Ce)V m

To study the selective adsorption of dye mixture, 10 mg of samples (1−3) were added into 100.0 mL of dye mixture (MO/MB, PR/MB, or RhB/MB) at 298 K. The initial concentration ratio of each mixed dye in solution was set to be 1:1 (10 ppm: 10 ppm). The dye mixture solutions containing the adsorbent were mixed well with magnetic stirring.

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RESULTS AND DISCUSSION Synthesis and Structural Description. [((CH3)2NH2)Cd(MIPA)]n·xG (n = 4 (1), 2 (2), 1 (3); G = Guest of DMA/DMF and H2O). Compounds 1−3 were obtained by employing the predesigned H3MIPA ligand (Figure S1) to react with Cd(NO3)2·4H2O under solvothermal conditions at 150−160 °C for 3 days. Single-crystal X-ray diffraction analyses reveal that 1 and 2, crystallizing in the monoclinic P21/c (No. 14) and I2/a (No. 15) space groups, respectively, exhibit similar anionic structure. They were synthesized from DMA/H2O = 4:1 and DMF/H2O = 3:2, respectively. In contrast, compound 3 was obtained in the mixed solvent of DMA/H2O = 2:3 with higher water content. It crystallizes in the trigonal R3̅m (No. 166) space group with higher symmetry. The asymmetric unit of the solvent-free anionic structures of 1−3 contain four, three, and one crystallographically independent CdII centers, and four, two, one MIPA3− ligands, respectively, in which Cd(2), Cd(3) in compound 2, and Cd(1), MIPA3− in compound 3 are half-occupied, as shown in Figure 1. The whole structures for them are balanced by dimethylammonium cations, which originate from the decomposition of the DMA/DMF molecules. All CdII centers in 1−3 are six-coordinated by four oxygen atoms and two sulfur atoms in a distorted octahedral [CdO4S2] geometry. The Cd−O distances are in the range of 2.214(2)−2.604(2) Å for 1, 2.235(2)−2.557(2) Å for 2, and 2.362(2)−2.391(2) Å for 3.

(1)

where C0 and Ce are the concentrations of dye at initial and equilibrium, respectively (mg L−1). V is the volume of the solution (L), and m is the weight of adsorbent used (g). The removal percentage η of dye was calculated using the following equation: η=

C0 − Ce × 100% C0

(2)

The adsorption rate constant was calculated using pseudo-first- or pseudo-second-order reaction kinetics36,37 using eqs 3 and 4, respectively.

ln(qe − qt) = ln(qe) − k1t −1

(3) −1

where qt (mg g ) and qe (mg g ) represent the amount of adsorbed at t and equilibrium time, respectively. k1 (min−1) represents the adsorption rate constant for pseudo-first-order adsorption which is calculated from the plot of ln(qe − qt) against t.

t t 1 = + t 2 qt qe k 2qe

Figure 1. Illustration of the asymmetric units of 1 (a), 2 (b), and 3 (c) after PLATON/SQUEEZE treatment (all H atoms are omitted for clarity); symmetry code for 1, #1 (x − 1, −y + 1/2, z − 1/2); #2 (x, −y + 1/2, z − 1/2); #3 (−x, −y + 1, −z + 1); #4 (−x + 1, y + 1/2, −z + 3/2); #5 (x, −y + 1/2, z + 1/2); symmetry code for 2, #1 (−x + 3/2, y, −z); #2 (−x + 3/2, −y + 1/2, −z + 1/2); #3 (−x + 1/2, y, −z); #4 (x − 1, −y + 1/2, z − 1/2); #5 (−x + 2, −y, −z + 1); #6 (x + 1/2, −y, z); #7 (−x + 5/2, y, −z + 1); symmetry code for 3, #1 (−x + y − 1/3, −x + 1/3, z + 1/3); #2 (y − 1/3, x + 1/3, −z + 1/3); #3 (−x + 1/3, −y + 2/3, −z + 2/3); #4 (−x + y, y, z).

(4)

where k2 is the rate constant for pseudo-second-order adsorption (g mg−1 min−1), which is calculated from the plot of t/qt against t. The values of qe and k2 were determined from the slops and intercepts of the plot. C

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(ca. 7 Å × 7 Å), Cd5(MIPA)3 (ca. 7 Å × 7 Å), and Cd4(MIPA)4 (ca. 12 Å × 12 Å), are interconnected with the hexagonal channels in 1−2 (see Figure 3d,e and Tables S4−S5, Supporting Information). It is clear that the extending orientation of the [Cd2S2] group determines the different channel characters between 1−2 and 3. As for 3, the circular inner window is consist of Cd6(MIPA)3 (ca. 13 Å × 13 Å), in which three [Cd2S2] dimers locate in the vertexes of a triangle (Figure 4a,d). Besides, two kinds of Cd4(MIPA)4 microporous channels, with dimensions of ca. 9 Å × 9 Å and ca. 9 Å × 5 Å, are present in other five directions in 3 (as shown in Figure 4c,d and Table S6). Multiple Crystal Phases Analysis. Compounds 1−3 were assembled by subtly varying the synthesis conditions including solvents and temperatures. It is worth noting that the anionic frameworks of 1−3 are homologues and multiple crystal phases of the {[Cd(MIPA)]n}n−. Compounds 1 (P21/c) and 2 (I2/a) have similar anionic structures, which are different from 3. The anionic framework of 2 could be considered as a highly ordered arrangement of that of 1, since the solvent-free 1 has a twice asymmetric unit of 2. The L1, L3 and L2, L4, respectively, in 1 are equivalent to the L2 and L1 in 2, respectively, if taking the dihedral angles into account (Table 2). However, compound 2 shows inferior stability than 1 and tends to lose crystalline when exposing it to air. It is interesting that we observed a reversible temperature-induced monoclinic to triclinic phase transformation of 2 upon cooling. The in situ single-crystal Xray diffraction cell parameters data for 2 were collected between 90 and 250 K (Figure S3, Supporting Information). At 250 K, the unit cell parameters are a = 14.175 Å, b = 33.454 Å, c = 19.060 Å, and β = 103.31°, with a unit cell volume of 8795.3 Å3 (Table S7, Supporting Information). As the temperature decreasing to 240 K, the unit cell parameters start to change from monoclinic phase to triclinic phase (P1̅, a = 14.10 Å, b = 18.93 Å, c = 19.57 Å, α = 113.50°, β = 103.91°, γ = 103.31°, V = 4330 Å3). The unit cell parameters maintain the triclinic phase from 240 to 90 K. As the temperature increases from 90 to 250 K, the unit cell parameters change back. This phenomenon indicates that the encapsulated solvent molecules have an important effect on the lattice phase of the framework, and the change of temperature induces the change of guest-framework interaction. Furthermore, among these homologues, compound 3 has the highest symmetry of R3̅m which could be considered as a highly ordered rearrangement of 2. It is imaged that the hexagonal window shrinks to circular one when converting 2 to 3. Despite much effort, we did not observe direct single-crystal to single-crystal structure transformation among 1, 2, and 3. The solvent-accessible void fractions calculated using PLATON34 for 1−3 are ∼65.9% (5761.3 Å3), ∼67.3% (6113.7 Å3), and ∼65.3% (6227.4 Å3), respectively, of the total crystal volume of 8745.3 (at 150 K), 9081.0, and 9539.0 Å3, respectively. The bigger difference for the voids between 1 and 2 is ascribed to the data collecting temperature difference between them. The accessible special surface area, which is calculated by Material Studio 5.5 software, can reach 2169.7, 2236.4, and 2295.5 m2 g−1 for 1−3, respectively. Therefore, compound 3 exhibits a more compact porous structure (crystalline density of 0.964 g cm−3 higher than that of 1 and 2) with the highest solvent-accessible void and accessible special surface area, as well as the highest symmetry among these homologues.

The Cd−S distances are in the range of 2.560(1)−2.623(1) Å for 1, 2.589(1)−2.624(1) Å for 2, and 2.601(1) Å for 3. Each deprotonated [MIPA]3− ligand in 1−3 serves as a μ4bridge connecting with four CdII centers, in which two carboxylate groups exhibit the same μ1-η1:η1 chelate coordination mode, and the mercapto group exhibits a μ2 coordination mode (Figure 2a). The dihedral angles between the central

Figure 2. Schematic presentations of the connection of [MIPA]3− ligand directly (a), and indirectly (b) through the mercapto group.

phenyl ring (O) and the [Cd2S2] (A), or [CdO2C] (B and C) groups for each ligand in 1−3 are shown in Table 2 (Table S3, Table 2. Dihedral Angles between the Central Phenyl Ring (O) and [Cd2S2] Group (A), [CdO2C] Groups (B and C), quasi-[Cd2S2] Groups (D and E), Respectively, in Compounds 1−3 compound 1

A

B

C

D

E

L1 L2 L3 L4 compound 2

70.074 61.288 89.356 62.093 A

15.282 28.15 10.23 18.542 B

17.884 25.749 11.189 24.377 C

70.074 44.601 89.356 60.071 D

73.931 60.442 87.586 53.998 E

L1 L2 compound 3

55.509 74.906 A

24.332 11.533 B

19.589 4.524 C

51.714 74.906 D

60.554 84.837 E

L1

59.784

24.557

24.556

51.446

51.459

the detailed presentation). Thanks to the existence of the mercapto group, all the CdII centers in 1−3 are dimeric (Figure 2b). As a result, the whole [MIPA]3− ligand is bonding to three [Cd2S2] groups (with two quasi-Cd2S2 groups, D and E). It should be noted that the [Cd2S2] groups have a great effect on the extending direction of the structure. The dihedral angles between the phenyl ring and the quasi-[Cd2S2] groups (D and E) have changed a lot compared with that between the phenyl ring and [CdO2C] groups (B and C), as shown in Table 2. Through the interconnection of [Cd2S2] and [MIPA]3−, three 3D open frameworks imparting nanoscale hexagonal (for 1−2) or circular channel (for 3) are formed, as shown in Figures 3 and 4. Compounds 1 and 2 demonstrate the same 3,6-connected binodal net (Figure S2a, Supporting Information), which has the Schläfli symbol of (42·6)(48·64·83). Compound 3 exhibits a different topology with a 4,4-connected binodal net of (43·62·8), as shown in Figure S2b (Supporting Information). As evidenced from the single-crystal X-ray diffraction analyses, the hexagonal window in both 1 and 2 (Figure 3a,b) is formed by the connection of six [MIPA]3− ligands and six dimeric [Cd2S2]. It shows a pore size of ca. 18 Å × 18 Å for the inner Cd6(MIPA)6 unit. Moreover, we also found that other three types of microporous channels, consisting of Cd4(MIPA)4 D

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. Representations of the hexagonal windows in 1 (a) and 2 (b); (c) view of the 3D open anionic framework of 1; view of the interconnected four types channels in 1 (d) and the corresponding profiling figures of them (e), colors for channels: 18 Å × 18 Å, green; 7 Å × 7 Å, pink; 12 Å × 12 Å, blue.

Figure 4. (a) Representation of the circular channel window in 3; (b) view of the 3D open anionic framework of 3; view of the interconnected three types channels (c) and the corresponding profiling figures of them (d), colors for channels: 13 Å × 13 Å, green; 9 Å × 9 Å, blue; 9 Å × 5 Å, pink.

than that for CH4 (4.6 cm3 g−1, 0.21 mmol g−1 and 0.32 wt %) and N2 (1.27 cm3 g−1, 0.05 mmol g−1 and 0.16 wt %) at the same temperature, as shown in Figure 5 and Table S8. The CO2/CH4 and CO2/N2 selectivities at 273 K for 1a are 15.9 and 53.9, respectively, which were evaluated by using the initial slope ratios estimated from Henry’s law38−40 constants for single-component adsorption isotherms, as summarized in Figure S5 and Table S9 (Supporting Information). It exhibits a higher isosteric heat of adsorption (Qst) of 37.0 kJ mol−1 for

Gas Sorption Properties. PXRD patterns for the assynthesized 1−3, and samples of 1 after treatment at different conditions are shown in Figure S4 (Supporting Information). The diffraction peaks of the as-synthesized MOFs match well with the simulated pattern on the basis of the single-crystal structure. Activated sample of 1a was obtained by exchange of the solvent of the as-synthesized samples with methanol. As expected, 1a exhibits higher affinity and capacity for CO2 (29.6 cm3 g−1, 1.32 mmol g−1 and 5.48 wt % at 273 K and 800 Torr) E

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 5. Gas sorption isotherms of CO2, CH4, and N2 for 1a at 273 K (a) and 298 K (b).

CO2 at zero coverage, which is calculated on the basis of the adsorption isotherms at different temperatures through virial-eq (Figure S6). The CO2 sorption isotherms are reversible. Two consecutive cycles of CO2 sorption isotherms at 273 K exhibit good reversibility even without further evacuation treatment after every cycle test. As is well-known, CO2 has the ability to act as a Lewis acid and a Lewis base simultaneously. When acting as an acid, weak interactions take place between the electron-deficient C atom of CO2 and electron-rich sites of O or N from the functionalized benzenes, thus forming electron donor−acceptor complexes. The complexes are further stabilized through the formation of weak H bonds between the O atoms of the CO2 and the neighboring H atoms, where CO2 acts as a Lewis base. Such cooperative interactions are found in the majority of the functionalized benzenes which have been strategically selected for accurate quantum chemistry calculations.41,42 Moreover, as evidenced by Yoon’s work,31 the incorporation of an electronegative atom of S in the ligand will decrease electrostatic potential and induce a stronger interaction with adsorbent molecules. Therefore, we propose that the cooperative interactions of electrostatics and hydrogen bonding mainly contribute to the higher isosteric heat of adsorption of 1a for CO2. However, the N2 sorption experiment for 1a gives a limited result of 178.5 m2 g−1 for the Brunauer−Emmett−Teller (BET) surface area and 261.6 m2 g−1 for the Langmuir surface area, which is probably due to the part loss of the long-range orderly structure under activation, and the weak affinity of the framework toward N2. Besides, the activation for compounds 2 and 3 failed, and therefore, no more gas sorption tests for them were carried out. Dye Adsorption Properties. Although 1a exhibits limited uptake of CO2, it does not impede dye uptake from solution. The removal of dyes from effluents before discharge into natural bodies is extremely important from an environmental point of view. The adsorption ability of 1−3 was first tested by exposing 10 mg of the solid to 100.0 mL of a 40 ppm MB methanol solution for 18 h at 298 K, after which the supernatant was close to colorless (Figure S7). The maximum adsorption capacities were found to be 395.1 mg g−1 for 1, 286.6 mg g−1 for 2, and 373.2 mg g−1 for 3, respectively, as shown in Figure 6. All these values are larger than those reported over adsorbents like cotton waste (278 mg g−1),43 MOF-235 (187 mg g−1),44 graphene (154 mg g−1),45 and AlMCM-41 (78 mg g−1).46 In order to evaluate the rate of MB adsorption on these compounds, two common kinetic models of pseudo-first- or second-order are adopted. The rates of adsorption are clearly better described by pseudo-second-order

Figure 6. Effect of constant time on the adsorption of MB on 1−3.

than pseudo-first-order kinetics (Figure S8). The calculated values of k2, along with the relevant correlation coefficients (R2), are shown in Table 3. As is well-known, in the case of dye removal, the efficiency is determined mainly by the adsorption capacity of the adsorbents, selectivity for specific compounds, durability, and regenerability of the adsorbents.16 Among these factors, the selective adsorption of dyes is more attractive and challenging. To validate the selectivity of these homologues, three kinds of dye molecules with different sizes and charges (Scheme 1), that is, positively charged MB+ (molecule size 1.38 nm × 0.61 nm × 0.22 nm) and rhodamine B (RhB+, molecule size 1.54 nm × 1.15 nm × 0.48 nm), negatively charged methyl orange (MO−, molecule size 1.54 nm × 0.46 nm × 0.47 nm), and neutral phenol red (PR, molecule size 1.20 nm × 0.81 nm × 0.51 nm), have been chosen as models. The selectivity for MB by 1−3 was tested by soaking 10 mg of the fresh crystalline samples in 100.0 mL of dye mixtures of MO−/MB+, PR/MB+, and RhB+/MB+, respectively. Spectroscopic investigations of the supernatant are shown in Figure 7 and Figures S9−S10. After 2−4 h, the concentrations of MO−, PR, and RhB+ in methanol solution are nearly constant with time, but the concentration of MB+ in the methanol solution drastically decreased. Besides, the colors for three types of dye mixtures changed remarkably, from green to yellow for MO−/ MB+ mixture, from dark green to pale green for PR/MB+ mixture, and from dark purple to pink for the RhB+/MB+ mixture, respectively. Thus, it can be concluded that the ionexchange process for 1−3 is charge-selective and size-selective, in which only the cationic guest (MB+) with a suitable size F

DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 3. Parameters of Kinetic Models of MB with Different Crystals pseudo-first-order sample 1 2 3

k1 (min−1) 0.00629 0.00552 0.00797

qe (mg/g) 250.53 174.95 171.39

pseudo-second-order R2 0.97355 0.99879 0.98007

k2 (g/mg min−1) −5

5.50709 × 10 9.62474 × 10−5 1.08343 × 10−5

qe (mg/g)

R2

experimental value of qe(mg/g)

406.50 290.69 362.32

0.99927 0.99865 0.99953

395.17 286.60 373.21

Scheme 1. Dye Molecules Employed in This Work

Figure 7. UV−vis spectral changes of 100.0 mL dye mixture (10 ppm: 10 ppm) in the presence of 10 mg of 1: (a) MO and MB; (b) PR and MB; (c) RhB and MB. The inset pictures are photographs of the removal of MB in corresponding dye mixtures over time, as well as the comparison photograph of the single dye with 10 ppm.

Figure 8. Effect of constant time on the MB removal efficiency in different dye mixtures (a) MO−/MB+; (b) PR/MB+; and (c) RhB+/ MB+.

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CONCLUSION In conclusion, through the incorporation of a multifunctional ligand of H3MIPA with mixed hard and soft donor atoms, we successfully isolated three highly porous anionic frameworks with big hexagonal or circular channels. Compounds 1−3 crystallize in P21/c, I2/a, and R3̅m, respectively, which were synthesized by subtly varying the synthesis conditions including solvents and temperatures. Interestingly, these anionic frameworks are homologues and multiple phases of the {[Cd(MIPA)]n}n−. The gas sorption studies toward CO2 indicate that the activated 1a has a higher isosteric heat of adsorption of 37.0 kJ mol−1 for CO2 at zero coverage. Significantly, these homologues exhibit superior adsorption capacities (286.6− 395.1 mg g−1) and high selectivity toward cationic dye of MB adsorption. Moreover, the framework of 3, with the highest symmetry, highest solvent-accessible void, and accessible special surface area, exhibits the fastest adsorption kinetics in all three kinds of dye mixtures of MO−/MB+, PR/MB+, and RhB+/MB+, clearly indicating that 3 is a promising adsorbent for highly effective pollutant removing.

could enter the frameworks of them and exchange with the dimethylammonium cations. It is worthy to note that three homologues exhibit a different adsorption rate toward MB+ in each dye mixture. It takes 130, 190, and 75 min for compound 1 to reach a 96% decolorization ratio in MO−/MB+, PR/MB+, and RhB+/MB+ systems, respectively. For compound 2, it takes longer times of 170, 230, and 120 min, respectively. The most fast selectively adsorption of MB+ in each dye mixture system is embodied in compound 3, which need 100, 130, and 60 min, respectively, although 3 demonstrates lower maximum adsorption capacity comparing to 1. The superior adsorption kinetic of 3 in the mixed dye system is probably due to the suitable pore size, highest solvent-accessible void, and the moderate maximum adsorption capacity of MB+. Therefore, in combination with the adsorption capacity and adsorption rate, compound 3 is an ideal adsorbent that could serve as a highly effective material for selective dye separation. G

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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.cgd.6b00991. Tables with bond lengths and angles, dihedral angles, gas sorption capacities, gas adsorption selectivity, additional structures, figures, IR spectra, TGA curve, PXRD patterns, UV−visible spectra, and gas sorption isotherms (PDF) Accession Codes

CCDC 1482474−1482476 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 [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 Authors

*(B.-X.D.) E-mail: [email protected]. *(W.-L.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (Nos. 21371150, 21301152, 21573192, 21671169), Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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DOI: 10.1021/acs.cgd.6b00991 Cryst. Growth Des. XXXX, XXX, XXX−XXX