A Flexible and Stable Interpenetrated Indium Pyridylcarboxylate

Mar 6, 2019 - An interpenetrated indium pyridylcarboxylate framework, [NH2(CH3)2][In(L)2]·2.5DMF·5H2O (1), has been synthesized by employing a ...
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A Flexible and Stable Interpenetrated Indium Pyridylcarboxylate Framework with Breathing Behaviors and Highly Selective Adsorption of Cationic Dyes Bin Zhang,†,‡ Qian-Qian Chu,‡ Ke-Fen Yue,*,† Shi-Hui Zhang,† Bo Liu,*,‡ and Yao-Yu Wang† †

Inorg. Chem. Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/06/19. For personal use only.

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, National Demonstration Center for Experimental Chemistry Education, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China ‡ College of Chemistry & Pharmacy, Northwest A&F University, Yangling 712100, P. R. China S Supporting Information *

ABSTRACT: An interpenetrated indium pyridylcarboxylate framework, [NH2(CH3)2][In(L)2]·2.5DMF·5H2O (1), has been synthesized by employing a pyridylcarboxylate ligand, 4(3-carboxylphenyl)picolinic acid (H2L), and an In3+ ion, with both chemical stability and framework flexibility. The desolvated 1 exhibits an uncommon breathing sorption behavior and shows highly selective adsorption for C2H2, C2H4, and CO2 over CH4. Furthermore, 1 shows rapid and higher adsorption efficiency for methylene blue and neutral red in aqueous solution.

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INTRODUCTION

Organic dyes, as a kind of common industrial material, are widely applied in many industrial applications by virtue of their varied colors and high stability, such as inks, medicines, textiles, and so on. However, it is unpredicted that the toxicity of organic dyes causes irreversible damage to the ecological environment and human health. Also, the nondegradability of organic dyes brings environmental problems. So, effectively and selectively removing dyes is necessary and meaningful. At present, activated carbon, zeolites, and MOFs have been utilized. Compared with MOFs, low selectivity and efficiency are drawbacks for activated carbon and zeolites. Adjustable pore sizes and charges of the frameworks for MOFs are strong factors in adsorbing dyes. In particular, high adsorption capacity, high selectivity, reusability, and their ability to degrade dyes give MOFs broad application prospects.18−21 Thus, MOFs have been considered to be promising dye adsorbents. In this work, a flexible and stable interpenetrated anionic MOF, [NH2(CH3)2][In(L)2]·2.5DMF·5H2O (1), was successfully constructed. Relying on a pyridylcarboxylate ligand, 4-(3carboxylphenyl)picolinic acid (H2L), with and In3+ ion, 1 can maintain the integrity of the main framework in mild or harsh solutions. Given its appropriate pore size, charge of the framework, and superior stability, 1 shows rapid and higher adsorption efficiency for methylene blue and neutral red in

With unceasingly thorough studies, metal−organic frameworks (MOFs), as functional porous materials, have gained a substantial amount of attention in the fields of storage and separation owing to their large surface area, tailorable pore diameters, and modifiable pore environment.1−5 According to the classification, flexible or dynamic porous structures, also considered to be the third generation of porous coordination polymers, have also attracted fevered research concerns because of their attractive characteristics.6−8 Generally, these kinds of MOFs with structural flexibility exhibit a reversible dynamic behavior upon different external stimuli, including the presence/removal of specific guest molecules or even varied temperature and mechanical pressure.9−11 Peculiarly, MOFs show guest-induced “breathing effect” or “gate-opening” behaviors, which can dynamically respond to certain guest molecules and, upon gas/vapor adsorption, undergo reversible structural transformations from small to large aperture.12−17 Thus, the flexible nature of MOF possesses a wide foreground in the selective adsorption and separation of gases. With these unique superiorities, however, the stability of the dynamic MOFs has been neglected, and few studies were carried out on those reported flexible MOFs. The need for stability has gradually been noted as a pivotal obstruction to be overcome especially for the future industrialization of MOFs, and the robust and flexible MOFs are more suitable for application as adsorption/separation materials. © XXXX American Chemical Society

Received: January 15, 2019

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DOI: 10.1021/acs.inorgchem.9b00113 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry water. In particular, after desolvation, 1 exhibits interesting breathing behaviors and highly selective separation of C2H2, C2H4 and CO2 from CH4.

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Table 1. Crystallographic Data and Structural Refinements for 1 complex molecular formula fw temperature (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc(g cm−3) F(000) Rint reflns collected GOF on F2 R1a [I > 2σ(I)] wR2b (all data)

EXPERIMENTAL SECTION

Materials and Instrumentation. All of the materials were purchased from commercial sources. The Fourier transform infrared (FTIR) spectra were obtained in the 4000−400 cm−1 range with KBr pellets on a Nicolet Avatar 360 FTIR spectrometer. The carbon, hydrogen, and nitrogen microanalyses were measured with a PerkinElmer 2400C elemental analyzer. Thermal gravimetric analyses (TGA) were performed by using a Netzsch TG209F3 instrument at a heating rate of 5 °C min−1 under a nitrogen stream. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker D8 ADVANCE diffractometer employing Cu Kα radiation. The absorptive experiments for dyes were recorded by a Shimadzu UV2450 spectrophotometer, which used deionized water as the blank. Synthesis of [NH2(CH3)2][In(L)2]·2.5DMF·5H2O (1). Single crystals of 1 were synthesized as follows: a mixture of In(NO3)3 (12 mg, 0.04 mmol), H2L (4 mg, 0.02 mmol), N,N-dimethylformamide (DMF; 3 mL), and HNO3 (0.4 mL; 2.2 mL of HNO3 in 10 mL of DMF) was sealed in a 10 mL screw-capped flask and then heated at 100 °C for 48 h. After cooling to room temperature, colorless block crystals were collected by filtration and dried in air (yield of 80% based on H2dcpy). Elem anal. Calcd for 1: C, 35.85; H, 5.54; N, 10.22. Found: C, 35.93; H, 5.52; N, 10.24. IR (KBr, cm−1): 3417 (w), 3059 (w), 2920 (w), 1668 (s), 1585 (s), 1489 (s), 1399 (s), 1372 (s), 1195 (w), 1094 (m), 926 (w), 874 (w), 831 (w), 766 (m), 700 (w), 657 (w), 562 (w), 498 (w). Sorption Measurements. All of the gas sorption isotherms were obtained on the ASAP 2020 M adsorption equipment. The fresh samples were steeped in acetone, which need to be constantly renewed for 5 days and then dried overnight at 120 °C under vacuum to remove guest molecules prior to measurements. Single-Crystal X-ray Crystallography. The structure of compound 1 was obtained through Mo Kα radiation (λ = 0.71073 Å) on a Bruker AXS SMART CCD area detector diffractometer under ambient conditions. The structure was determined by direct methods and refined using the SHELXTL program package. All non-hydrogen atoms were confirmed with anisotropic technology. The diffraction contribution of the highly disordered solvent molecules located in the structure was eliminated by applying the program SQUEEZE implemented in PLATON. The final formula of 1 was ascertained by combining the crystallographic data, elemental microanalyses, and TGA data. The selected structural refinement results for compound 1 were summarized in Table 1. CCDC 1888354 contains the crystallographic data of 1. Dye Adsorption and Separation. All of the experiments were carried out at room temperature. Negatively charged methyl orange (MO) and orange II (OrII) and positively charged neutral red (NR), methylene blue (MLB), methyl violet (MV), and rhodamine B (RhB) were employed to assess the ability of adsorption for 1. The freshly assynthesized samples (10 mg) were transferred into the six aqueous solutions of organic dye, respectively (4 mL, 10 ppm), and then the absorptive experiments are recorded by UV−vis spectroscopy at a predetermined time in dark conditions. The experiment about the selectivity of mixed organic dyes for 1 was divided into two groups [MLB (2 mL, 10 ppm) and MO (2 mL, 10 ppm) and also MLB (2 mL, 10 ppm) and OrII (2 mL, 10 ppm)]. The maximum adsorption capacity of NR and MLB was investigated in different initial concentrations of dye solutions (10 mg sample) for 48 h. After that, we measured the UV−vis absorption by a supernatant to obtain the value of the quantity Qe. Qe = (C0 − Ce)V/m (C0 and Ce represent the initial and equilibrium concentrations of the dyes, respectively; V represents the volume of the dyes; m represents the mass of the sample). To ensure the of accuracy of the maximum adsorption amounts, five sets of experimental data were selected to be calculated for MLB and NR.

a

1 C26H14InN2O8 597.21 100(10) monoclinic I2/a 18.4359(7) 22.1499(6) 20.6432(8) 90 106.994(4) 90 8061.7(5) 8 0.984 2376.0 0.0309 14022 1.041 0.0457 0.1313

R1 = ∑|Fo| − |Fc|/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

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RESULTS AND DISCUSSION High-quality single crystals of 1 were obtained under the solvothermal reaction of H2L and In(NO3)3 in a DMF solvent at 100 °C for 3 days. Structural analysis shows that 1 crystallizes in a monoclinic space group I2/a. Its asymmetric unit is composed of an independent In3+ ion and two L2− ligands. Figure 1a shows that each In3+ is a seven-connected node linked by two nitrogen atoms and five oxygen atoms from four different L2− ligands, giving a mononuclear [InO5N2] unit. Meanwhile, every fully deprotonated L2− ligand connects with two In3+ ions, generating a 3D network with hexagonal channels (sizes of 19.0 Å × 28.9 Å, including the van der Waals radius of the atom) in the axial direction (Figure 1b). In order to improve the structure stability, two of the same subnets interpenetrate each other to form an interpenetrated framework, in which a new 1D open channel is formed with a size of 12.3 Å × 12.9 Å (Figure 1c), and the PLATON calculation manifests the overall free void of 50%.22 In addition, the cavity is occupied by dimethylamine cations to balance the charge of the framework. It is worth pointing out that massive uncoordinated carboxylate oxygen atoms decorate the inner wall of the channels, leading to the polarity of the pore surface.23 The measured PXRD data of 1 verified the purity of the synthesized samples (Figure S1). TGA reveals a weight loss of 16% below 190 °C, which matches that of the removal of free solvents and the collapse of the framework that occurs at 357 °C (Figure S2). Furthermore, the chemical stability was examined through immersion of 1 in water and even acidic and basic solutions for 12 h. Figure 2 proves that the crystalline integrity of the framework remains intact after treatment with solutions in pH = 2−13. Evidently, the good stability of 1 makes it possible for it to be applied in practical applications. To assess the porosity of 1, a N2 adsorption study was performed at 77 K. Interestingly, up to 1 atm, the phenomenon of breathing is found in the N2 sorption isotherm (Figure 3). During the adsorption, a low adsorption amount of 9.9 cm3 g−1 B

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Figure 1. (a) Coordination environment of an In3+ ion. (b) Independent network viewed along the a axis. (c) 2-fold interpenetrating framework of 1. (d) Topological scheme of 1.

Figure 2. PXRD profiles of 1 after being soaked in water and acidic and basic solutions for 12 h.

Figure 3. N2 sorption isotherms of 1 at 77 K.

examined at different temperatures under 1 atm. Strikingly, all adsorption isotherms in Figure 4 display hysteresis desorption, the hysteresis of which is related to the prevention of gas escaping from the flexible interpenetrated framework during the adsorption−desorption process. At 273 K, the sorption isotherms of C2H2, C2H4, CO2, and CH4 exhibit diverse breathing adsorption behaviors, and the inflection points are 469, 470, 150, and 460 Torr, respectively. This phenomenon is largely due to the effects of the different interactions of gas molecules with the framework. On the other hand, 1 shows obvious selective adsorption of C2H2; especially, CH4 shows a lower adsorption amount (4.1 cm3 g−1) in contrast to those for C 2 H 2 , C 2 H 4 , and CO 2 at the same conditions, the corresponding adsorption quantities of which are 32.1, 17.6,

can be observed at a low relative pressure (P/P0 = 0.6). Above P/P0 > 0.6, the isotherm displays a sharp increase and then ultimately reaches saturation, the N2 uptake of which is 106 cm3 g−1 at 1 atm. It is noteworthy that the trace of the desorption isotherm is not following the adsorption branch. The appearance of a distinct hysteresis loop in the stepwise adsorption isotherm should be attributed to the dynamic structural transformation between narrow- and large-pore phases. Similar to the present case, most breathing adsorption behaviors were related to the bending of metal−ligand bonds and the sliding of interpenetrated frameworks.24 To further investigate the potential performance of the pore, the adsorption capacities of C2H2, C2H4, CO2, and CH4 were C

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Figure 4. Gas sorption isotherms at 273 and 298 K: C2H2 (a); C2H4 (b); CO2 (c); CH4 (d).

and 16.3 cm3 g−1, respectively. To estimate the practical separation ability for C2H2, theoretical gas mixtures of C2H2/ CH4, C2H2/C2H4, and C2H2/CO2 are conducted by the ideal absorbed solution theory (IAST) model. As Figure 5 shows, at

and the C2H4/CH4 and CO2/CH4 selectivities for an equimolar mixture are 11 and 5, respectively. The above high selectivity is probably caused by the cationic framework, free carboxylate oxygen atoms, and [H2N(CH3)2]+ cations, which induce the open channels to be highly polar by generating an extra electric field, finally strengthening the affinity between the quadrupolar gas molecules and framework.29−31 The permanent porosity, superior chemical stability, and unique anionic nature of the framework inspire us to exploit its potential application toward dye sorption and separation in aqueous solution. Hence, differently shaped and charged organic dyes were employed to check the adsorption ability of 1. All dye-uptake analyses were conducted under ambient temperature. The sample of 1 was immersed in aqueous solutions including organic dyes, respectively, and then the absorptive experiments are recorded by UV−vis spectroscopy. For MLB and NR, Figure 6 indicates that efficient adsorption phenomena were observed over 45 min by the distinct color change and evident decrease of the absorbance peak, indicating that the dye molecules could rapidly enter into the channels of 1 because of suitable dimensions of the dye molecules and ionexchange process.32 The adsorption efficiencies are reflected in Figure 6b,d, and MLB has been completely adsorbed in 25 min, while NR takes about 30 min to be fully adsorbed. For MV and RhB, some variations of the color and concentration in aqueous solution were found, showing that the adsorption capacity is inextricably linked with the size of the dye molecule between ion exchange and comparisons between dye molecule sizes of RhB (15.6 × 13.5 × 4.2 Å3) as well as MV (3.5 × 13.0 × 13.7 Å3) and channel size (12.3 × 12.9 Å2), and it can be inferred that the narrow channel prevents the entry of RhB and MV; on the contrary, there was little change in the colors and concentrations in aqueous solution for MO and OrII, suggesting that charge repulsion is a barrier, while the dye

Figure 5. IAST adsorption selectivities and isotherms of 1 for C2H2/ CH4, C2H2/C2H4, C2H2/CO2, CO2/CH4, and C2H4/CH4 at 298 K.

298 K and 1 atm, the simulated adsorption selectivities for the equimolar mixture of binary C2H2/CH4, C2H2/C2H4, and C2H2/CO2 are 35, 5, and 5, respectively. The selectivity of C2H2/CH4 can match numerous prominent MOFs that own open metal sites or active functional groups, such as SNNU65-Cu-Fe (37.3),25 ZJU-199 (27.3−33.5),26 M′MOF-20 (34.9),27 and BUT-70 (23.3−47.9),28 indicating the potential application of 1 in a real process of C2H2/CH4 separation. In addition, the selectivities of 1 for C2H4 and CO2 over CH4 in binary C2H4/CH4 and CO2/CH4 mixtures were also evaluated, D

DOI: 10.1021/acs.inorgchem.9b00113 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. UV−vis spectra of MLB (a) and NR (c) in aqueous solutions at different times and variable adsorption amounts (Qt) with time for MLB (b) and NR (d).

Figure 7. (a) Adsorption isotherm for MLB and NR with 1 at room temperature. (b) Maximum adsorption quantities for MLB and NR.

molecules enter the channels, despite suitable dimensions (Figure S3). The framework integrity of 1 performed by a dye adsorption experiment was investigated by PXRD and remains unchanged (Figure S4). It can be inferred from the above results that the efficient adsorption may be bound up with the charge and dimensions of the dye molecules. The most important factor for adsorption materials is their storage capacity. As illustrated in Figure 7 and Table S1, when the synthesized samples were submerged in MLB and NR aqueous solutions for 48 h, the maximum adsorption quantity of 1 for MLB and NR attained 410 and 202 mg g−1, respectively. The different adsorption amounts encourage us to explore the reasons behind this observation. By a comparison of molecular sizes and charges of the dyes, it is the most compelling answer that the higher adsorption amounts may be due to the exposed oxygen atoms in decorated channels. The energies of the framework with MLB and NR molecules were further confirmed by density functional theory calculation. The calculated adsorption energies of MLB and NR in 1 are −4.01984 and −3.63292 eV (Table 2 and Figure 8), respectively, showing that 1 can absorb MLB molecules more efficiently.33 As far as we know, the adsorption capacity

Table 2. Ground-State Energies of 1 and the Substrate Binding with Molecules dye

EMOF+dye (eV)

EMOF (eV)

Edye (eV)

ΔEadsorption (eV)

MLB NR

−1679.571 −1671.756

−1437.6367 −1437.6367

−237.91426 −230.48674

−4.01984 −3.63292

Figure 8. Structures of the MLB (a) and NR (b) molecules inside the pores of 1.

of 1 for MLB and NR is moderate compared to those of other MOFs (Table S2), but the advantage of the water stability E

DOI: 10.1021/acs.inorgchem.9b00113 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. UV−vis spectra of the mixture dyes of MLB and MO (a) and MLB and OrII (b) showing the selective adsorption of 1 at different times.

*E-mail: [email protected].

makes it a more promising candidate for absorbing dyes from water systems. Furthermore, the selectivity is a crucial factor for material application. Thus, the adsorption selectivity of 1 was tested. 1 was immersed in mixed solutions of different oppositely charged dyes (MLB and MO and also MLB and OrII). The selective adsorption behaviors in mixed solution were investigated by UV−vis spectrophotometry and photographs. Distinctly, as can be seen from Figure 9a, 1 shows high selectivity, which it can merely adsorb MLB from the aqueous solution of MLB and MO. The color changes of 1 from white to blue occurred, indicating MLB has been absorbed into pores. Similar phenomena also occur in the aqueous solution of MLB and OrII (Figure 9b). These results further indicate that 1 can serve as a promising superabsorbent material for selective MLB absorption from polluted water.

ORCID

Yao-Yu Wang: 0000-0002-0800-7093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for support from the NSFC (Grant 21601145), the NSF of Shaanxi Province (Grant 2017JQ2026), and the Education Committee of Shaanxi Province (Grant 17JS131).

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CONCLUSION In conclusion, a flexible interpenetrated anionic MOF has been constructed by adopting a pyridylcarboxylate ligand and an In3+ ion, which possesses an exceptional stability under water and harsh circumstances. Moreover, 1 shows high adsorption selectivity and capacity of MLB in aqueous solution. On the basis of the structural flexibility, 1 shows breathing sorption behaviors as well as highly selective separation of C2H2, C2H4, and CO2 from CH4. Those results indicate that 1 is a rare example, which possesses both high framework stability and flexibility.

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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.9b00113. Supplementary Figures S1−S5 and Tables S1−S3 and computational methods used for the adsorption energy of a molecule on the MOF surface (PDF) Accession Codes

CCDC 1888354 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [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

*E-mail: ykfl[email protected]. F

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DOI: 10.1021/acs.inorgchem.9b00113 Inorg. Chem. XXXX, XXX, XXX−XXX