Straightforward Loading of Imidazole Molecules into Metal–Organic

Communication Cite This: J. Am. Chem. Soc. 2017, 139, 15604-15607

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Straightforward Loading of Imidazole Molecules into Metal−Organic Framework for High Proton Conduction Yingxiang Ye,† Weigang Guo,† Lihua Wang,† Ziyin Li,† Zhengju Song,† Jun Chen,† Zhangjing Zhang,*,† Shengchang Xiang,† and Banglin Chen*,†,‡ †

Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, 32 Shangsan Road, Fuzhou 350007, People’s Republic of China ‡ Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States S Supporting Information *

sites. It is still not clear whether the bound imidazole or free imidazole molecules are more favorable to the proton conduction, although Lan and Zhou et al.13 developed one MOF in which the bound imidazole molecules play more important roles than the free ones for the higher proton conductivity. That promoted us to carry out the in-depth studies on the imidazole included MOF materials for proton conduction. To our surprise, we realized a MOF material whose free imidazole included sample exhibits much higher proton conductivity than the one with the bound ones, reaching 1.82 × 10−2 S cm−1 at 90% RH and 70 °C as one of the highest proton conductive porous materials. Moreover, such a free imidazole included the MOF material Im@(NENU-3) ([Cu 12 (BTC) 8 (H 2 O) 12 ][HPW 12 O 40 ]·[(CH 3 ) 4 N] 2 ·9H 2 O· 14.5Im) can be straightforwardly prepared through one-step direct evaporaion of the imidazole molecules into the porous pristine MOF NENU-3 ([Cu12(BTC)8(H2O)12][HPW12O40]· [(CH3)4N]2·25H2O, H3BTC = 1,3,5-benzenetricarboxylic acid) (Scheme 1). The crystalline MOF NENU-314 was chosen as the host to encapsulate imidazole molecules. This is because the inclusion of the polyoxometalate anions into the HKUST-115 framework can significantly enhance both hydrothermal and chemical stability.12 The previous effort to incorporate the imidazole hydrochloride molecules into NENU-3 did confirm the high stability but only led to a material with the moderate proton conductivity of 5.94 × 10−4 S cm−1 at 90 °C and 70% RH.12 The well-developed two-step approach readily leads to imidazole-loaded proton conductor Im-Cu@(NENU-3a) (Scheme 1). Interestingly, Im-Cu@(NENU-3a) still keeps high crystallinity, which motivated us to determine its single crystal X-ray structure. As expected, the open Cu sites on the activated NENU-3a have been occupied by the imidazole m o l e c u l e s i n t h e r e s u l t in g I m - C u @ ( N E N U - 3 a ) ([Cu 1 2 (BTC) 8 (Im) 1 2 ][HPW 1 2 O 40 ]·[(CH 3 ) 4 N] 2 ·8H 2 O· 0.25Im). Im-Cu@(NENU-3a) exhibits higher proton conductivity of 3.16 × 10−4 S cm−1 at 70 °C and 90% RH than the parent material NENU-3, but much lower than the one developed recently,13 indicating that the bound imidazole molecules are not the prerequisite for high proton conductive materials. This observation motivated us to prepare free

ABSTRACT: A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal−organic framework (NENU-3, ([Cu12(BTC)8(H2O)12][HPW12O40])·Guest). The resulting material Im@(NENU-3) exhibits a very high proton conductivity of 1.82 × 10−2 S cm−1 at 90% RH and 70 °C, which is significantly higher than 3.16 × 10−4 S cm−1 for Im-Cu@(NENU-3a) synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in Im-Cu@(NENU-3a) isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within Im@(NENU-3) significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks.

P

roton exchange membrane fuel cell (PEMFC) has been recognized as the most promising candidate for vehicle applications owing to their high-power densities and ultralow emission features.1 The state-of-the-art PEMFCs with commercial Nafion as electrolytes can reach high conductivity of 10−1− 10−2 S cm−1 under 60−80 °C and 98% relative humidity (RH).2 The high costs and complex synthesis process of Nafion membranes have hindered their large-scale usage; it is thus desirable, though challenging, to develop new proton exchange materials with high conductivity. The emerging crystalline porous materials including metal−organic frameworks (MOFs)3 and covalent-organic frameworks (COFs)4 have been developed as the very promising materials as the proton conductors,5 attributed to their rich structural tunability,6 high surface areas7 and functional pore cages/channels to provide and accommodate various proton carriers.8 Since Kitagawa pioneered the research on the immobilization of imidazole (Im) molecules as the proton carriers into a porous MOF to facilitate the proton conduction,9 it has become a general strategy for developing high proton conductive materials.10,11 Generally, imidazole molecules were incorporated into the porous materials through a two-step approach, i.e., activation of porous materials followed by the inclusion of imidazole molecules either through vapor evaporation9,11 or immersion of imidazole molecules.12 Such a methodology apparently faciliates the binding of imidazole molecules to open metal © 2017 American Chemical Society

Received: August 28, 2017 Published: October 26, 2017 15604

DOI: 10.1021/jacs.7b09163 J. Am. Chem. Soc. 2017, 139, 15604−15607

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Journal of the American Chemical Society Scheme 1. Comparison of the One- and Two-Step Synthesis of the Proton Conductors Im@(NENU-3) and ImCu@(NENU-3a), and Their Corresponding Proton Conductivities

Figure 1. Nyquist plot of (a) Im-Cu@(NENU-3a) and (b) Im@(NENU-3) under 90% RH with different temperatures. Inset: Nyquist plot at 90% RH and 70 °C. (c) Arrhenius plots of Im@(NENU-3) and Im-Cu@(NENU-3a) between 25 and 70 °C at 90% RH. Least-squares fitting are shown as a solid line. (d) Compare proton conductivity of representative imidazole-loaded proton conductors with that of our samples under high humidity conditions, Im@(NENU-3) (1), Im-Fe-MOF (2), Im@Fe-MOF (3), and ImCu@(NENU-3a) (4).

imidazole molecules included Im@(NENU-3) in which the terminal sites were still occupied by water molecules while the free imidazole molecules were filled inside the pore spaces of the framework. The PXRD patterns of Im@(NENU-3) and Im-Cu@(NENU-3a) are identical with that of their parent material, indicating that they still maintain the integrity of the host skeleton (Figures S7−10). From the results of thermogravimetric analysis (Figure S5) and elemental analysis, the weight percentage of imidazole loading in Im@(NENU-3) was estimated to be 14.5 wt %, which corresponded to 43.5 imidazole molecules per crystal cell of the framework, and higher than 36.75 imidazole molecules (corresponding to 12.9 wt %) in the unit cell of Im-Cu@(NENU-3a). The alternating current (AC) impedance spectroscopy of a pelletized sample with controlled humidity and temperature was used to analyze the proton conduction behaviors of ImCu@(NENU-3a) and Im@NENU-3. As shown in Figure 1a,b, all the Nyquist plots display a single semicircle at high frequency, indicating proton conductivity through the bulk material, followed by a positively sloping capacitive tail at low frequencies. At 25 °C and 90% RH, the proton conductivity (σ) of Im-Cu@(NENU-3a) was found to be 5.13 × 10−6 S cm−1. Upon heating, the conductivity was further increased to 3.16 × 10−4 S cm−1 at 70 °C. At 70 °C and 90% RH, Im@NENU-3 exhibits much higher proton conductivity up to 1.82 × 10−2 S cm−1, about 2 orders of magnitude greater than that of ImCu@(NENU-3a), ranking it as one of the best reported MOFbased proton conductors.16 As shown in Figure 1d, the proton conductivity of Im@NENU-3 is remarkably higher than that of Im@Fe-MOF (4.23 × 10−3, at 60 °C and 98% RH),13 and slightly higher than that of Im-Fe-MOF (1.21 × 10−2 S cm−1 at 60 °C and 98% RH).13 The proton conductivities of Im-Cu@(NENU-3a) and Im@(NENU-3) were also measured at 70 °C with varying humilities to elucidate the correlation between conductivity and relative humidity (Figures S15 and S25). As expected, their conductivities decrease with the lower RH.17 The proton conductivities in Im-Cu@(NENU-3a) and Im@(NENU-3) remain almost unchanged in the consecutive heating and

cooling cycles (Figure S18), and Im@(NENU-3) can maintain high proton conductivity up to 12 h (Figure S26). The PXRD analysis confirms the structural integrity of the sample after AC impedance measurements (Figures S7−10). In order to gain insight into the proton transport pathway, we analyzed the single crystal data of Im-Cu@(NENU-3a) after its exposure to water vapor atmosphere (at 70 °C and 90% RH). To differentiate its difference from the as-synthesized ImCu@(NENU-3a), this one was termed as Im-Cu@(NENU3b). The structure of Im-Cu@(NENU-3b) is basically identical to that of Im-Cu@(NENU-3a) and isostructural to HKUST1,15 exhibiting two cuboctahedral cages (Cage A and Cage B) and one octahedral cage (Cage C) (Figure S1) with the sizes of 13, 11, and 7 Å, respectively. Notably, the Cage B only encapsulates a lattice water molecule (O1w) within the center, and the distance between O1w and the noncoordinated N atoms of the imidazole is 3.891 Å (Figures 2c). In addition, there is another independent lattice water molecule (O2w) within the channel, and the distance between O1w and O2w is up to 5.801 Å (Figures 2e and S3). Because of the muchreduced cages, there is only tiny amount of (about 0.26 wt %) the free imidazole molecules inside the cages. The immobilized and regularly bound imidazole molecules on the framework isolate the lattice water molecules and then block proton transport pathway, so the proton conduction in ImCu@(NENU-3a) can only be carried out through self-diffusion of the hydronium ions, leading to its low conductivity. For Im@(NENU-3), there are large amount of (14.5 wt %) free imidazole molecules for their successive hydrogen-bonding networks through themselves and the water molecules, thus significantly facilitating the proton conduction and generating high proton conductivity. To figure out the possible proton conduction mechanism, we calculated the activation energy (Ea) values from temperature dependent conductivity at 90% RH. The least-squares fit of the Arrhenius plots for Im-Cu@(NENU-3a) and Im@NENU-3 gave Ea values of 0.79 and 0.57 eV, respectively (Figure 1c). 15605

DOI: 10.1021/jacs.7b09163 J. Am. Chem. Soc. 2017, 139, 15604−15607

Communication

Journal of the American Chemical Society

Im-Cu@(NENU-3a) with the much smaller interconnecting cages has not only isolated the terminal bound imidazole molecules from the lattice water molecules but also significantly limited its encapsulation of free imidazole molecules into the pore spaces to promote the proton transport pathway. In summary, we realized a very straightforward one-step strategy to incorporate a large amount of free imidazole molecules into a stable porous MOF for high proton conduction. This work will significantly facilitate the search for high proton conductive porous materials through the incorporation of a variety of proton carriers. Because a number of MOFs can readily encapsulate high density of free proton carriers such as imidazole molecules into their framework channels/cages, whereas the proton transportation pathways can be systematically adjusted and optimized through the tuning of the pore channels and/or pore metrics to promote the proton conduction, it is expected that some porous MOF materials with even higher proton conductivities will be developed in the near future through such a simple approach for their potential practical applications.

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Figure 2. BTC ligand and the paddle-wheel Cu2 units in ImCu@(NENU-3b) (a) and NENU-3 (b). The cuboctahedral Cage B in Im-Cu@(NENU-3b) (c) and NENU-3 (d) are fabricated by the BTC ligands and the Cu2 units, and with the different pore spaces and window sizes with much more free imidazole molecules inside the pore spaces of NENU-3. (e) Speculative pathway of the proton conduction for Im-Cu@(NENU-3a) based on single crystal X-ray data showing the actual positions of the absorbed water molecules within the structure. (f) Schematic view of the possible proton-conductive pathways in Im@(NENU-3). Water molecules are shown in violet or red. Red arc arrows show the protons hop along hydrogen-bonding networks. Red dashed arrows represent transport of protons through self-diffusion of protonated water. (Guest molecules and H atoms are omitted for clarity.)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09163. Experimental section, additional structural figures, characterization and electrochemical data (PDF) Data for 2(O20P0.5W6), 0.25(C432H336Cu48N96O192), 4(C2N0.5) (CIF) Data for 2(O20P0.5W6), 0.25(C432H384Cu48N96O192), 31(O), 4(C0.61N0.15) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected]

The difference in Ea values of these two materials are accounted to the difference in the concentration and mobility of the proton carriers in these two compounds. The protonconduction mechanism in Im-Cu@(NENU-3a) can be classified as the conventional vehicular mechanisms (>0.5 eV), because of lower concentration and mobility of the proton carriers.18 For Im@(NENU-3), the much higher proton conductivity and relatively low Ea value may be attributed to the incorporation of high concentration of free imidazole molecules, and the fast and efficient proton transfer pathway through the formation of hydrogen bonded networks among free imidazole and water molecules, resulting in the Grotthuss mechanism. Furthermore, the free water molecules also have possibility to accept protons and transport of protons through self-diffusion, resulting in partly vehicular mechanism. The proton conduction in Im@(NENU-3) can be classified as the combination of Grotthuss (hopping) and vehicle mechanisms (Figure 2f).18 It is interesting to compare with Im-Fe-MOF developed by Lan and Zhou et al. for high proton conduction as well.13 In Im-Fe-MOF, the size of the circular 1D channel is about 5.6 Å, and the distances between neighboring terminal bound imidazole molecules are about 7.735 and 14.496 Å, respectively, viewed along the c axis direction. In such a framework structure with 1D larger channel, the successive proton-hopping pathways can be realized through the hydrogen bonded networks among the terminal imidazole and absorbed water molecules. However, the significantly different framework of

ORCID

Yingxiang Ye: 0000-0003-3962-8463 Shengchang Xiang: 0000-0001-6016-2587 Banglin Chen: 0000-0001-8707-8115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21273033, 21673039 and 21573042), Fujian Science and Technology Department (2014J06003 and 2014H6007) and Welch Foundation (AX1730). S. X. gratefully acknowledges the support of the Recruitment Program of Global Young Experts.

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DOI: 10.1021/jacs.7b09163 J. Am. Chem. Soc. 2017, 139, 15604−15607