Article pubs.acs.org/IC
Cite This: Inorg. Chem. 2018, 57, 1474−1482
A Comparative Investigation of Proton Conductivities for Two Metal−Organic Frameworks under Water and Aqua-Ammonia Vapors Ruilan Liu, Lili Zhao, Wei Dai, Chenglin Yang, Xi Liang, and Gang Li* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China S Supporting Information *
ABSTRACT: Our investigation on the proton conductivities of two water-stable isostructural 3D Co(II) MOFs, {[Co3(DMPhIDC)2(H2O)6]·2H2O}n (1) [DMPhH3IDC = 2-(3,4-dimethylphenyl)-imidazole-4,5-dicarboxylic acid] and {[Co3(mBrPhIDC)2(H2O)6]·2H2O} (2) [m-BrPhH3IDC = 2-(m-bromophenyl)-imidazole4,5-dicarboxylic acid], under water or aqua-ammonia vapor shows that the optimized proton conductivities of both 1 and 2 under aqua-ammonia vapor are 4.41 × 10−3 S· cm−1 and 5.07 × 10−4 S·cm−1 (at aqua-ammonia vapor from 1.5 M NH3·H2O solution and 100 °C), respectively, which are approximately 1 order of magnitude greater than those maximum values (8.91 × 10−4 S·cm−1 and 7.64 × 10−5 S·cm−1) under water vapor (at 98% RH and 100 °C). The plausible proton pathways and mechanisms of the MOFs have been proposed in terms of the structural analyses, activation energy calculations, water and NH3 vapor absorptions, and PXRD determinations.
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different RHs,11−35 only one literature concerns the MeOH vapor.36 More recently, our laboratory has preliminarily reported the proton conductivities of two CoII MOFs under aqua-ammonia vapor,37 which displays that the proton conductivities of the MOFs under aqua-ammonia vapor are significantly enhanced than those under water vapor. Notably, the substituted imidazole dicarboxylate-based MOFs containing both imidazole and carboxylate groups may benefit the proton transportation process. This exciting result inspires us to study more related MOFs in similar situations to confirm our conclusions. At the same time, the literatures on proton conductivity of imidazole dicarboxylate-based MOFs are very scarce.37 As an extension of our present work, herein we report in detail our further investigation of the proton conductive properties of two imidazole dicarboxylate-based CoII MOFs, {[Co3(DMPhIDC)2(H2O)6]·2H2O}n (1) [DMPhH3IDC = 2(3,4-dimethylphenyl)-imidazole-4,5-dicarboxylic acid] and {[Co3(m-BrPhIDC)2(H2O)6]·2H2O} (2) [m-BrPhH3IDC = 2-(m-bromophenyl)-imidazole-4,5-dicarboxylic acid], under water and aqua-ammonia vapors. Our investigation shows that both 1 and 2 have high proton conductivity, especially when exposed to aqua-ammonia vapor.
INTRODUCTION Due to their intriguing structural features (i.e., crystallinity, chemically functionalizable pores, and options for systematic structural variation), metal−organic frameworks (MOFs) have been extensively explored for their potential applications in gas storage and selective separation,1 catalysis,2−4 electrical conductivity,5−10 and proton conductivity.11−20 Recently, study on proton-conductive MOFs has been becoming an emerging area in the field of fuel cells, which is largely due to the fact that this type of MOF possesses good proton carriers, water-filled channels, acidic pores, or hydrogen-bonds supported nets. Indeed, very recently, several MOFs with excellent proton conductivities have been reported.20−24 Two aspects on promoting proton conductivities of MOFs have been well documented, including (1) by introducing proton conducting chemicals22,25−32 or proton carriers33−35 into the pores of MOFs and (2) by varying the ambient environmental factors [temperature, relative humidity (RH), and the vapor environment]11−36 and adopting post-treatment methods (by soaking in sulfuric acid solutions to produce derivative MOFs).23,24 For example, Cabezait and coauthors have obtained a MOF containing ammonia components, CaPiPhtA-NH3 [5-(dihydroxyphosphoryl)isophthalic acid = PiPhtA], which exhibits high proton conductivity of 6.6 × 10−3 S· cm−1 at 98% RH and 24 °C.23 Hong et al. have described a MOF, H+@Ni2(dobdc) (dobdc4+ = 2,5-dioxido-1,4-benzenedicarboxylate) with an excellent proton conductivity of 2.2 × 10−2 S·cm−1 at 95% RH and 80 °C, which was afforded by soaking [Ni2(dobdc)(H2O)2]·6H2O in sulfuric acid solutions.24 In addition, up to changing the vapor environment, while the proton conduction of MOFs is mainly investigated under © 2018 American Chemical Society
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EXPERIMENTAL SECTION
Reagents and Apparatus. All the chemicals used here were of analytical grade. The organic ligand m-H3BrPhIDC was prepared according to the literature procedure.38 Deionized water (distilled) was used throughout the experiments. Powder X-ray diffraction (PXRD) Received: November 8, 2017 Published: January 19, 2018 1474
DOI: 10.1021/acs.inorgchem.7b02851 Inorg. Chem. 2018, 57, 1474−1482
Article
Inorganic Chemistry
Figure 1. 3D solid state packing of MOF 2 containing 1D channels (a). The coordination (red color) and crystallization (green color) water molecules inside the channels (b). Water chain built by H-bonds (c). patterns were collected on a Rigaku D/MAX-3 with Cu Kα (λ = 1.5418 Å) irradiation. Water and ammonia vapors adsorption− desorption isotherms were applied by using a 3H-2000P Multistation Weight method analyzer at 298 K (BeiShiDe Instrument Technology (Beijing) Co. Ltd.). Syntheses of MOFs 1 and 2. MOF 1 was prepared by a reference method.39 The synthesis of MOF 2: A mixture of CoCl2·6H2O (0.0238g, 0.1 mmol), m-H3BrPhIDC (0.0312 g, 0.1 mmol), NaOH (0.05 mmol), and CH3CN/H2O (3/4, 7 mL) was sealed in a 25 mL Teflon-lined bomb and heated at 150 °C for 96 h. The reaction mixture was then allowed to cool to room temperature. Red cubeshaped crystals of 2 were collected, which were washed with distilled water and dried in air (45% yield based on Co). Calcd for C22H25Br2Co3N4O16.08: C, 28.17; H, 2.69; N, 5.97%. Found: C, 28.54; H, 2.35; N, 5.77%. IR (cm−1, KBr): 3334 (m), 2901 (m), 1957 (w), 1605 (s), 1488 (w), 1413 (s), 1271 (s), 1123 (s), 1069 (m), 996 (s), 885 (w), 861 (m), 787 (s), 738 (w), 694 (w), 547 (s), 441 (w). Crystal Structure Determinations. Crystal data and experimental details for MOF 2 are contained in Table S1. Measurement of 2 was made on a Bruker smart APEXII CCD diffractometer with a graphite-monochromated Mo-Kα radiation (λ = 0.710 73 Å). Single crystals of 2 were selected and mounted on a glass fiber. All data were collected at room temperature using the ω-2θ scan technique and corrected for Lorenz-polarization effects. A correction for secondary extinction was applied. The structure of 2 was solved by direct methods and expanded using the Fourier technique. The hydrogen atoms on C were positioned geometrically and refined using a riding model. The hydrogen atoms on O were found at reasonable positions in the differential Fourier map and located there. All the hydrogen atoms were included in the final refinement. All calculations were performed using the SHELXL-97 crystallographic software package.40 Selected bond lengths and bond angles and the parameters of hydrogen bonds are listed in Tables S2 and S3, respectively. CCDC (for 2) no. 1015604. Water Treatment and Activation. The crystals of 1 and 2 were immersed in deionized water for 1 month as well as in boiling water continuously heated to reflux for 24 h, then filtered and dried at room temperature to get the water-treated samples.
The activated samples of 1 and 2 are obtained via alcohol evacuation water in structure, then under vacuum at 70 °C for 12 h. Water and NH3 vapors adsorption measurements were conducted using the activated samples. Proton Conductivity Measurements. Electrochemical impedance spectroscopy (EIS) of 1 and 2 was carried out with a Princeton Applied Research PARSTAT 2273 impedance analyzer, in the frequency range of 0.1 Hz to 1 MHz by a conventional quasi-fourprobe method and AC voltage of 10 mV with Pt electrodes. Proton conductivity of the crystalline samples (ca. 26 mg) was tested on pellets prepared under a pressure at 3.0 MPa for 10 min at 5.0 mm in diameter. The thickness was measured by a Vernier caliper. The waterassisted conductivities were measured under different relative humidity conditions. The NH3·H2O-assisted conductivities were measured under different concentrations of aqua-ammonia vapor, which were obtained from various concentrations of NH3·H2O solution. The concentrated solution of NH3·H2O (14.80 M) was diluted by deionized water to concentrations of 1.5, 1.0, 0.5, 0.1, 0.05, 0.01, 0.005, and 0.001 M, respectively) for proton conduction determinations. The concentration of the above solutions was determined by a titration method. Samples were equilibrated for at least 8 h after each step in aqua-ammonia vapor and 16 h after each step in humidity. EIS spectra data was recorded by computer-controlled with the Power-Suite software. The total resistance (R) of the polycrystalline sample was obtained from the arc extrapolation to the low frequency Z′ side axis on the Nyquist plot. The corresponding proton conductivities were calculated by the equation σ = L/(RS), where σ = proton conductivity (S·cm−1), L = thickness of the pellet (cm), S = flat surface area of the pellet = πr2 (cm2) (r = radius of the pellet (cm)), R = resistance of the pellet measured (Ω). The activation energy values were calculated from the Arrhenius equation Tσ = σ0 exp(−Ea/kT) by the slope of the plots of log(σT) versus 1000/T.
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RESULTS AND DISCUSSION Crystal Structures. To address the potential of MOFs 1 and 2 as good candidates for proton conductors, we first focus our attention on the crystal structures of 1 and 2, considering
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DOI: 10.1021/acs.inorgchem.7b02851 Inorg. Chem. 2018, 57, 1474−1482
Article
Inorganic Chemistry
Figure 2. PXRD patterns of 1 (a) and 2 (b) for the simulated, as-synthesized, and after water treated samples.
pressure region (Figures S3 and S4). As indicated in Figures S3a and S4a, the rapid adsorption of dehydrated 1 and 2 begins from a relative pressure (P/P0) of 0.05, where the adsorbed water amount reached ca. 160 mg/g for 1 and 147 mg/g for 2. Then, the adsorbed water amount increases slowly with the increase of P/P0. The maximum absorption values are ca. 230 and 194 mg g−1 for 1 and 2, respectively, with P/P0 being 0.95, suggesting that the inside-channel spaces of 1 are more hydrophilic than are those of 2. As shown in Figures S3b and S4b, the ammonia vapor absorption and desorption isotherms exhibit that the ammonia vapor absorption values are ca. 137 mg g−1 at P/P0 of 0.05 for 1 and ca. 126 mg g−1 at P/P0 of 0.05 for 2. When P/P0 is equal to 0.95, the maximum ammonia vapor uptakes are ca.179 and 166.9 mg g−1 for 1 and 2, respectively. Proton Conduction of 1 and 2 under Water Vapor. As described above, the presence of water-filled channels, good thermal and water stabilities, and adsorption for water and NH3 vapors make MOFs 1 and 2 potential candidates as proton conductors. Alternating current (AC) impedance measurements were first carried out on a pressed sample under controlled RHs and different temperatures. Notably, MOFs 1 and 2 do not indicate any proton conductivity under anhydrous conditions. However, after the pellets of 1 and 2 were equilibrated at various RHs for 16 h, they show stable conductivity values. The proton conductivities of the MOFs were calculated from the fitting of the Nyquist plots.19,41 The proton conductivity of compound 1 was tested in the temperature range of 30−100 °C at 68%, 75%, 85%, 93%, and 98% RHs, respectively (Figures 3, 4, and S5−S9, and Table S5). It is to be pointed out that these plots display one semicircle with a characteristic spur at low frequencies, confirming the conducting species being H+ ions at the Pt electrodes. From these plots, we can get two points: (1) Under the fixed temperature, the humidity-dependent conductivity can be observed. As indicated in Figures S5 and 3, the calculated conductivities vary from 3.92 × 10−6 (68% RH) to 9.00 × 10−5 S·cm−1 (98% RH) at 30 °C (Figure S5). At 100 °C, the conductivities increase from 1.27 × 10−4 S·cm−1 (68% RH) to 8.91 × 10−4 S·cm−1 (98% RH) (Figure 3). It is obvious that the conductivity value increases with increasing humidity, which may be due to the slow fluidity of the water molecules in the channels under low humidity; as the humidity increases, the amount of water molecules that can enter the channels increases, which can form new H-bonds with a carboxyl oxygen atom and an imidazole N atom, and promote the transport of protons, increasing the conductivity. (2) The
the structure−property correlation. In the solid state, both 1 and 2 are isostructural and crystallized in the trigonal R3̅ space group. Since the crystal structure of 1 has been documented,39 we will focus on the structural discussion of 2. As 1, MOF 2 denotes a 3D framework containing infinite 1D-hexagonal or tetragonum channels, as shown in Figure 1a. Each m-BrPhIDC3− ligand adopts the same coordination fashion, μ3-kN,O:kN′,O′:kO″,O‴, bridging neighboring CoII ions. The central CoII ions exhibit two different coordination environments: [CoN2O4] and [CoO6] (Figure S1), in which the coordination N and O atoms are from m-BrPhIDC3− ligands and water molecules. It is to be pointed out that six coordination and two crystallization water molecules are in one asymmetric unit of the 3D structure (Figure 1b). These water molecules and carboxylate groups take part in the hydrogenbonding system. Therefore, the water chains supported by Hbonds are formed (Figure 1c). The similar case can be found in MOF 1 (Table S4), which may facilitate proton transfer along the chains. Water Stability of MOFs 1 and 2. As stated in our previous study,39 1 starts to lose two crystallization water molecules at
