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Synthesis and properties of ferrocene confined within UiO-67 MOFs
Microporous and Mesoporous Materials 264 (2018) 133–138
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis and properties of ferrocene confined within UiO-67 MOFs Shucheng Liu, Jiao Xu, Engao Dai, Junjie Qiu, Yi Liu
T
∗
School of Physical Sciences, Guizhou University, Guiyang 550025, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Porous materials Metal-organic frameworks Ferrocene
Ferrocene was encapsulated into the cavities of the metal-organic framework UiO-67 using a solvent-free evaporation method. The obtained host-guest compounds were characterized by XRD, TEM, EDS, TG, XPS, N2 adsorption-desorption, magnetic hysteresis and cyclic voltammetry studies. The results show that the micropores of UiO-67 are successfully occupied by ferrocene molecules and the ferrocene@UiO-67 composite shows an interesting ferromagnetic behavior at room temperature. Both the oxidation and reduction peak are observed in cyclic voltammograms due to the reversible conversion between Fe2+ and Fe3+ of ferrocene confined within UiO-67 framework. The results reveal that the novel properties of ferrocene embedded inside MOFs carries a substantial potential with regard to the synthesis of ferro-magnet or catalytic materials.
1. Introduction Ferrocene is a sandwich compound composed of a pair of planar cyclopentadienyl ring of 6p-electrons and 6d-electrons on Fe(II) atom [1–4]. Ferrocene is capable of engaging in electron transfer processes. Therefore, ferrocene has frequently been used as an electron transfer mediator for various electro-catalytic reactions. Previous studies have demonstrated that hollow structures (cages, bowls, capsules etc.) with nanometer-sized cavities have ability to encapsulate large guest molecules and can regulate specific reactions inside the cavities. In particular, encapsulation of ferrocene in carbon nanotubes (NTs) can combine both advantages of NTs and novel properties of ferrocene. The electronic properties of ferrocene-filled NTs are greatly modified due to the charge transfer between ferrocene molecules and NTs [5]. Intercalation and exfoliation of graphene oxide using covalently attached ferrocene was achieved and the composites showed interesting magnetic behavior [1]. Recently, the organometallic host-guest chemistry of porous crystalline organic frameworks (COF) was studied by infiltration of ferrocene. The structure of the inclusion compound (FeCp2)4@COF102 has a unique arrangement of FeCp2 molecules which replicates the host structure [6]. These works extend the scope of these new classes of low dense materials for possible applications in the fields like catalysis and sensing. Metal-organic frameworks (MOFs) with high surface areas and welldefined pore structures have been demonstrated as novel functional and
∗
robust materials because of their designable framework structures modularly built from metal clusters as nodes and organic ligands as struts [7–10]. Recent work on host-guest chemistry of MOFs is mostly focused on desorption and storage features of small gas molecules, i.e. CO2, H2, CH4 and NH3 as guests inside MOFs. In view of developing molecular-based switching systems, control of large guest molecular motions inside the cavity by external stimuli is an attractive task. FeCp2(∼5 Å diameter) is an ideal probe molecule for adsorption studies due to its chemical and thermal robustness [11–15]. In this work, we have started a study to investigate the host-guest chemistry of MOFs by introducing volatile ferrocene using the solventfree gas phase infiltration method. The result reveals that the redox properties of ferrocene embedded inside MOFs are very interesting and carries a substantial potential with regard to the synthesis of ferromagnet or catalytic materials. Herein, we choose UiO-67 as the presentative example of host materials. UiO-type MOFs (UiO stands for University of Oslo), which is composed of {Zr6O4(OH)4} oxo cluster nodes and dicarboxylate linkers, are known for their excellent thermal and chemical stabilities [16–23]. Among these types of MOFs, UiO-67 crystal is based on the Zr6O4(OH)4 building unit, forming lattices by a 12-fold connection through the biphenyl dicarboxylic acid (BPDC) linkers, resulting in a face centered cubic (f c c) structure (a = 27.1 Å). The structure of UiO-67 contains two types of cages: an octahedral cage (Φ ∼18 Å) that is face sharing with 8 tetrahedral cages (Φ ∼ 11.5 Å) [17,19], as showing in Scheme 1.
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.micromeso.2018.01.018 Received 23 July 2017; Received in revised form 9 December 2017; Accepted 12 January 2018 Available online 17 January 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
Microporous and Mesoporous Materials 264 (2018) 133–138
S. Liu et al.
2.3. Physical measurements X-ray powder diffraction (XRPD) patterns were recorded using a PANalytical X'Pert powder diffractometer with CuKα radiation. Transmission electron microscope (TEM) measurement was performed by use of a FEI Tecnai G2 F20. Thermogravimetric analysis (TGA) were performed using a Simultaneous Thermal Analyzer (STA 449C, NETZSCH) in the temperature range between 30 °C and 800 °C in a N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was measured with Thermo Escalab 250Xi. Nitrogen adsorption/desorption isotherms were measured at 77 K using surface area and pore size analyzer (3H2000PS2). The magnetic properties of sample were determined using a vibrating sample magnetometer (VSM, VersaLab, Quantum Design) at room temperature.
Scheme 1. Illustration of ferrocene confined in UiO-67 MOFs.
2.4. Electrochemical measurements 5 mg of ferrocene@UiO-67 powder was dispersed in a 1 mL mixture of distilled water and ethanol (3: 1 v/v). Then, 10 mL of 5 wt% Nafion was added to the above mentioned solution. The mixed solution was sonicated for at least 30 min to form a homogeneous ink. 5 mL of the mixed solution was drop-cast onto a glassy carbon electrode with a diameter of 5 mm for the electrochemical measurements. All the electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CHI Instruments Inc., Shanghai) using a typical three-electrode mode with an electrolyte solution of 6 M KOH, a Pt counter electrode, an Ag/AgCl (saturated KCl) reference electrode, and a modified glassy carbon working electrode. For comparison, the electrochemical measurements of UiO-67 and blank glassy carbon electrode as work electrode were performed under the same conditions. 3. Results and discussion Powder X-ray diffraction patterns (Fig. 1) of ferrocene@UiO-67 suggested that the crystal structure of UiO-67 is maintained without damage when ferrocene molecules are encapsulated into the cavity of UiO-67. Inspection of the PXRD-data reveals that all peaks are at the same position of the pristine powder, which indicates that no contraction or change in the symmetry of the crystals happens due to the rigidity of UiO-67. A change in Bragg peak intensity at low angle was noticed, which can be associated to the presence of ferrocene molecules within the pores of UiO-67. This is explained by taking account of scattering factors due to loading of ferrocene. Moreover, the powder pattern did not show any reflections corresponding to FeCp2, which rules out the presence of crystals of FeCp2 outside the framework. TEM (Fig. 2) show that the framework of UiO-67 are maintained by the loading of ferrocene, which exhibit a well-defined mono disperse octahedral microcrystals with particle size in the range of 150–300 nm. The element composition of the sample is qualitatively determined by EDS attached to TEM. Confine of ferrocene within UiO-67 was further confirmed by EDS analysis of ferrocene@UiO-67 which showed the presence of Fe peaks in the spectrum (Fig. 2c). The relative atomic percentage of 0.19%, 90.72%, 6.34% and 1.49% for Fe, C, O and Zr respectively were obtained by the EDS analysis (Table 1). Moreover, the EDS mapping of ferrocene@UiO-67 (Fig. 2f) also confirms that the ferrocene are uniformly distributed in the frameworks. Fig. 3 shows the thermal behavior of ferrocene in UiO-67 framework. The thermogravimetric analysis (TGA) profile of ferrocene@UiO67 exhibits no weight drop until 110 °C. Above 110 °C, there was a 25% weight loss until temperature reached 280 °C, which was attributed to the release of ferrocene from the UiO-67 channels. The curve shows a
Fig. 1. XRD patterns of UiO-67 and ferrocene@UiO-67.
2. Experimental 2.1. Synthesis of UiO-67 Synthesis of UiO-67 was performed by dissolving 1.2 mmol of zirconium (IV) chloride (ZrCl4), 1.2 mmol of 4,4′-Biphenyl dicarboxylic acid (H2BPDC) and 30 equivalents of acetic acid (CH3COOH) in 30 mL of N,N-dimethyformamide (DMF) at room temperature. The mixture was placed in a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 48 h. The product was cooled to room temperature, washed three times with DMF, and dried at room temperature. Excess H2BPDC and DMF in the pores were removed using a high temperature treatment at 300 °C for one day. A pale yellow powder was obtained.
2.2. Synthesis of ferrocene @UiO-67 150 mg of dry and activated UiO-67 and 300 mg of ferrocene were placed in two separate glass boats in a Schlenk bottle. The bottle was then evacuated to 10−3 mbar for 5 min and sealed. The reaction bottle was kept at 110 °C for 72 h.
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Fig. 2. TEM of UiO-67(a); ferrocene@UiO-67 (b,d,e); EDS and EDS mapping of ferrocene@UiO-67 (c,f).
Table 1 The element composition of ferrocene@UiO-67 determined by EDS. Element
Weight %
Atomic %
Uncertainty %
Correction
k-Factor
C(K) O(K) Fe(K) Cu(K) Zr(K)
76.869 7.161 0.737 5.648 9.583
90.719 6.344 0.187 1.259 1.489
1.073 0.188 0.052 0.143 0.281
0.173 0.514 0.994 0.997 0.999
6.279 1.980 1.480 1.757 3.927
stability plateau in the range of 280–450 °C before a second step of an additional 28% weight loss occurs (450–600 °C), which corresponds to the thermal decomposition of the UiO-67 framework. The chemical state of materials was investigated using X-ray photoelectron spectroscopy (XPS) to probe the properties of the inner-shell electrons (Fig. 4). High-resolution narrow-scan spectra were obtained for the C1s, O1s, Zr3d and Fe2p core levels. As observed from C1s spectra, the most prominent peak at 284.7 eV is assigned to sp2-hybridized carbon (C=C) on the benzene ring. The peak at 288.6 eV is attributed to HO−C=O groups of the BPDC linkers. The O1s spectrum reveals only one peak at 531.8 eV, which can be attributed to the oxygen components on the carboxylate groups of the BPDC linkers. The two peaks at 182.6 and 185.0 eV in the Zr3d spectrum can be ascribed to the Zr-O bonds. For the Fe2p spectrum, two peaks at 711.2 and 724.3 eV corresponded to Fe 2p3/2 and Fe 2p1/2, are typical of ferrocene. All of these results further confirm the coexisting chemical states of ferrocene and UiO-67 in the composites. The N2 adsorption-desorption isotherms and pore size distributions
Fig. 3. TGA of UiO-67 and ferrocene@UiO-67 under a N2 atmosphere.
are shown in Fig. 5 and the textural parameters are also listed in Table 2. For UiO-67, the adsorption-desorption isotherms exhibit type-I isotherm with typical microporous characteristics. The specific BET surface area of UiO-67 was measured as 1662 m2 g-1. The pore size distribution curve of UiO-67 using the BJH model exhibits narrow pores distribution centered at 1.99 nm with the total pore volume 0.94 cm3 g1 . N2 adsorption of ferrocene @UiO-67 was also measured at 77 K and
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Fig. 4. XPS of ferrocene@UiO-67.
Table 2 Porosity characteristics of samples. Sample
SBET (m2g−1)
d pore (nm)
Vpore (cm3g−1)
Smicropore (m2g−1)
Vmicropore (cm3g−1)
UiO-67 ferrocene @UiO-67
1662 26
2.26 20.47
0.94 0.13
1566 0
0.61 0
that the 86.2% of pores in UiO-67 are occupied by ferrocene. We used vibrating sample magnetometer (VSM) to measure the magnetic hysteresis curve of the samples. Fig. 6 shows the hysteresis loop of ferrocene and ferrocene filled UiO-67 obtained at the room temperature, respectively. The shape of the hysteresis of ferrocene@ UiO-67 indicates that the composites are ferromagnetic at room temperature. The hysteresis exhibits an average coercivity of HC = 0.9499 T and the saturation magnetization is measured to be 1.0 emu/g. The observed value of HC and saturation magnetization for the present composites is compared to that of the bulk ferrocene [5]. In consideration of micropore volume (0.61 cm3 g-1 by the N2 adsorption measurement) of UiO-67, we can estimate that the magnetization per pore volume is about 1.49 emu/cm3 in ferrocene@UiO-67. The electrochemical behaviors of ferrocene@UiO-67 composites were tested by a three-electrode system using 6 mol/L KOH as the electrolyte at different scan rates in a potential range of 0–600 mV (vs. Ag/AgCl) (Figs. 7 and 8). Notably, a pair of symmetric redox peaks (240 mV and 375 mV at scan rate of 10 mV/s, respectively) appeared in
Fig. 5. N2 adsorption isotherms of UiO-67and ferrocene@UiO-67 at 77 K. The inset shows the BJH pore size distribution curve.
the N2 adsorbed amount decreased drastically as compared to UiO-67. Especially, it was observed that, when ferrocene was introduced, the micropore surface area and the micropore volume of UiO-67 decreased to 0. The results suggested that ferrocene molecules occupied the micropore spaces of UiO-67. From value of pore volume, it is estimated
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CV curves and were not observed in UiO-67 and blank glassy carbon electrode, which was attributed to the reversible conversion between Fe2+ and Fe3+ in ferrocene [24]. When the scan rate is fast enough, the redox peak position shifts to high potential, and the peak currents are greatly enhanced. The linear relationship between the peak current and the scan rate indicates that the electrode is reversible (Fig. 8b). The behavior of cyclic voltammograms of the ferrocene-loaded MOF can be explained by a charge-hopping mechanism [24], where the ferrocene molecules remain localized in the pores of the MOF and mediate electron transport across the layer. In this case, the Fe2+/Fe3+ redox reaction takes place at the MOF-electrolyte interface. The redox processes corresponding to the conversion between different irons oxidation states can be described by following reactions:
Fe 2 + + OH− ↔ Fe3 + (OH ) + 2e− This process is illustrated in Scheme 2. The small size of an anchored ferrocene molecule (∼5 Å) relative to the channel diameter of UiO-67(∼18 Å and ∼11.5 Å) permits ferrocene to install in channels and the electrochemical reaction occurs in the channels. Under the oxidation process, OH− is adsorbed on ferrocene, as a result, some free electrons are released to contribute the oxide current peak. Under the reduction process, OH− is released to contribute the redox current peak. The results show that the installed ferrocene molecules are electrochemically reversible, implying effective site-to-site redox hopping and potential usefulness as a redox shuttle in electro catalytic systems.
Fig. 6. Magnetic hysteresis curves of ferrocene and ferrocene@UiO-67.
4. Conclusions In summary, we have shown that UiO-67 can be filled with ferrocene as identified by XRD, TEM, EDS, TG and XPS. The magnetic measurements reveal that the magnetic structure of UiO-67 is significantly modified, and ferromagnetic behavior are formed due to ferrocene encapsulation. The oxidation and reduction peak are observed in cyclic voltammograms due to the conversion between Fe2+ and Fe3+ in ferrocene. The combined findings have important implications for the use of this MOF composite as electro-catalysts and electro-analytical devices in electrochemical cells. Acknowledgements Fig. 7. Cyclic voltammograms of UiO-67, ferrocene@UiO-67 and glassy carbon electrode.
This work was supported by the National Natural Science Foundation of China (21261006).
Fig. 8. Cyclic voltammograms of ferrocene@UiO-67(a); The linear relationship between the peak current and the scan rate (b).
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Scheme 2. Illustration of conversion between different irons oxidation state in ferrocene.
References
[14] A.C. Gomes, C.I.R. Magalhães, T.S.M. Oliveira, A.D. Lopes, I.S. Gonçalves, M. Pillinger, Dalton Trans. 45 (2016) 17042–17052. [15] J. Guo, J.X. Liu, M. Lan, Y.Z. Hu, S.L. Wang, J.Q. Wen, Y. He, F.H. Gao, X. Zhang, S.J. Zhang, G. Xiang, M.A.C. Willis, F.S. Boi, RSC Adv. 7 (2017) 4753–4758. [16] S. Chavan, J.G. Vitillo, D. Gianolio, O. Zavorotynska, B. Civalleri, S. Jakobsen, M.H. Nilsen, L. Valenzano, C. Lamberti, K.P. Lillerud, S. Bordiga, Phys. Chem. Chem. Phys. 14 (2012) 1614–1626. [17] M.J. Katz, Z.J. Brown, Y.J. Colón, P.W. Siu, K.A. Scheidt, R.Q. Snurr, J.T. Hupp, O.K. Farha, Chem. Commun. 49 (2013) 9449–9451. [18] P.W. Siu, Z.J. Brown, O.K. Farha, J.T. Hupp, K.A. Scheidt, Chem. Commun. 49 (2013) 10920–10922. [19] J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, J. Am. Chem. Soc. 130 (2008) 13850–13851. [20] H. Wu, Y.S. Chua, V. Krungleviciute, M. Tyagi, P. Chen, T. Yildirim, W. Zhou, J. Am. Chem. Soc. 135 (2013) 10525–10532. [21] M. Kandiah, M.H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E.A. Quadrelli, F. Bonino, K.P. Lillerud, Chem. Mater. 22 (2010) 6632–6640. [22] C.H. Lau, R. Babarao, M.R. Hill, Chem. Commun. 49 (2013) 3634–3636. [23] N. Ko, J. Hong, S. Sung, K.E. Cordova, H.J. Park, J.K. Yang, J. Kim, Dalton Trans. 44 (2015) 2047–2051. [24] J.X. Liu, M. Paradinas, L. Heinke, M. Buck, C. Ocal, V. Mugnaini, C. Wöll, ChemElectroChem 3 (2016) 713–718.
[1] M.B. Avinash, K.S. Subrahmanyam, Y. Sundarayya, T. Govindaraju, Nanoscale 2 (2010) 1762–1766. [2] Y.K. Park, S.B. Choi, H. Kim, K. Kim, B.H. Won, K. Choi, J.S. Choi, W.S. Ahn, N. Won, S. Kim, D.H. Jung, S.H. Choi, G.H. Kim, S.S. Cha, Y.H. Jhon, J.K. Yang, J. Kim, Angew. Chem. Int. Ed. 46 (2007) 8230–8233. [3] W.Y. Sun, T. Kusukawa, M. Fujita, J. Am. Chem. Soc. 124 (2002) 11570–11571. [4] I. Hod, W. Bury, D.M. Gardner, P. Deria, V. Roznyatovskiy, M.R. Wasielewski, O.K. Farha, J.T. Hupp, J. Phys. Chem. Lett. 6 (2015) 586–591. [5] J. Cheng, X.P. Zou, G. Zhu, M.F. Wang, Y. Su, G.Q. Yang, X.M. Lü, Solid State Commun. 149 (2009) 1619–1622. [6] S. B.Kalidindi, K. Yusenko, R.A. Fischer, Chem. Commun. 47 (2011) 8506–8508. [7] H. Li, M. Eddaoudi, M. O'Keeff, Nature 402 (1999) 276–279. [8] K.L. Mulfort, J.T. Hupp, J. Am. Chem. Soc. 129 (2007) 9604–9605. [9] B. Sareeya, H. Satoshi, H. Masakazu, M. Mizuno, T. Kawamura, D. Tanaka, N. Yanai, S. Kitagawa, Nat. Mater. 8 (2009) 831–836. [10] B. Liu, H. Shioyama, T. Akita, J. Am. Chem. Soc. 130 (2008) 5390–5391. [11] S. Hermes, F. Schröder, S. Amirjalayer, R. Schmid, R.A. Fischer, J. Mater. Chem. 16 (2006) 2464–2472. [12] M. Müller, O.I. Lebedev, R.A. Fischer, J. Mater. Chem. 18 (2008) 5274–5281. [13] F. Cao, Y. Ding, L. Chen, C.R. Zhang, Nanoscale 5 (2013) 10000–10006.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis and properties of ferrocene confined within UiO-67 MOFs Shucheng Liu, Jiao Xu, Engao Dai, Junjie Qiu, Yi Liu
T
∗
School of Physical Sciences, Guizhou University, Guiyang 550025, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Porous materials Metal-organic frameworks Ferrocene
Ferrocene was encapsulated into the cavities of the metal-organic framework UiO-67 using a solvent-free evaporation method. The obtained host-guest compounds were characterized by XRD, TEM, EDS, TG, XPS, N2 adsorption-desorption, magnetic hysteresis and cyclic voltammetry studies. The results show that the micropores of UiO-67 are successfully occupied by ferrocene molecules and the ferrocene@UiO-67 composite shows an interesting ferromagnetic behavior at room temperature. Both the oxidation and reduction peak are observed in cyclic voltammograms due to the reversible conversion between Fe2+ and Fe3+ of ferrocene confined within UiO-67 framework. The results reveal that the novel properties of ferrocene embedded inside MOFs carries a substantial potential with regard to the synthesis of ferro-magnet or catalytic materials.
1. Introduction Ferrocene is a sandwich compound composed of a pair of planar cyclopentadienyl ring of 6p-electrons and 6d-electrons on Fe(II) atom [1–4]. Ferrocene is capable of engaging in electron transfer processes. Therefore, ferrocene has frequently been used as an electron transfer mediator for various electro-catalytic reactions. Previous studies have demonstrated that hollow structures (cages, bowls, capsules etc.) with nanometer-sized cavities have ability to encapsulate large guest molecules and can regulate specific reactions inside the cavities. In particular, encapsulation of ferrocene in carbon nanotubes (NTs) can combine both advantages of NTs and novel properties of ferrocene. The electronic properties of ferrocene-filled NTs are greatly modified due to the charge transfer between ferrocene molecules and NTs [5]. Intercalation and exfoliation of graphene oxide using covalently attached ferrocene was achieved and the composites showed interesting magnetic behavior [1]. Recently, the organometallic host-guest chemistry of porous crystalline organic frameworks (COF) was studied by infiltration of ferrocene. The structure of the inclusion compound (FeCp2)4@COF102 has a unique arrangement of FeCp2 molecules which replicates the host structure [6]. These works extend the scope of these new classes of low dense materials for possible applications in the fields like catalysis and sensing. Metal-organic frameworks (MOFs) with high surface areas and welldefined pore structures have been demonstrated as novel functional and
∗
robust materials because of their designable framework structures modularly built from metal clusters as nodes and organic ligands as struts [7–10]. Recent work on host-guest chemistry of MOFs is mostly focused on desorption and storage features of small gas molecules, i.e. CO2, H2, CH4 and NH3 as guests inside MOFs. In view of developing molecular-based switching systems, control of large guest molecular motions inside the cavity by external stimuli is an attractive task. FeCp2(∼5 Å diameter) is an ideal probe molecule for adsorption studies due to its chemical and thermal robustness [11–15]. In this work, we have started a study to investigate the host-guest chemistry of MOFs by introducing volatile ferrocene using the solventfree gas phase infiltration method. The result reveals that the redox properties of ferrocene embedded inside MOFs are very interesting and carries a substantial potential with regard to the synthesis of ferromagnet or catalytic materials. Herein, we choose UiO-67 as the presentative example of host materials. UiO-type MOFs (UiO stands for University of Oslo), which is composed of {Zr6O4(OH)4} oxo cluster nodes and dicarboxylate linkers, are known for their excellent thermal and chemical stabilities [16–23]. Among these types of MOFs, UiO-67 crystal is based on the Zr6O4(OH)4 building unit, forming lattices by a 12-fold connection through the biphenyl dicarboxylic acid (BPDC) linkers, resulting in a face centered cubic (f c c) structure (a = 27.1 Å). The structure of UiO-67 contains two types of cages: an octahedral cage (Φ ∼18 Å) that is face sharing with 8 tetrahedral cages (Φ ∼ 11.5 Å) [17,19], as showing in Scheme 1.
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.micromeso.2018.01.018 Received 23 July 2017; Received in revised form 9 December 2017; Accepted 12 January 2018 Available online 17 January 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
Microporous and Mesoporous Materials 264 (2018) 133–138
S. Liu et al.
2.3. Physical measurements X-ray powder diffraction (XRPD) patterns were recorded using a PANalytical X'Pert powder diffractometer with CuKα radiation. Transmission electron microscope (TEM) measurement was performed by use of a FEI Tecnai G2 F20. Thermogravimetric analysis (TGA) were performed using a Simultaneous Thermal Analyzer (STA 449C, NETZSCH) in the temperature range between 30 °C and 800 °C in a N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was measured with Thermo Escalab 250Xi. Nitrogen adsorption/desorption isotherms were measured at 77 K using surface area and pore size analyzer (3H2000PS2). The magnetic properties of sample were determined using a vibrating sample magnetometer (VSM, VersaLab, Quantum Design) at room temperature.
Scheme 1. Illustration of ferrocene confined in UiO-67 MOFs.
2.4. Electrochemical measurements 5 mg of ferrocene@UiO-67 powder was dispersed in a 1 mL mixture of distilled water and ethanol (3: 1 v/v). Then, 10 mL of 5 wt% Nafion was added to the above mentioned solution. The mixed solution was sonicated for at least 30 min to form a homogeneous ink. 5 mL of the mixed solution was drop-cast onto a glassy carbon electrode with a diameter of 5 mm for the electrochemical measurements. All the electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CHI Instruments Inc., Shanghai) using a typical three-electrode mode with an electrolyte solution of 6 M KOH, a Pt counter electrode, an Ag/AgCl (saturated KCl) reference electrode, and a modified glassy carbon working electrode. For comparison, the electrochemical measurements of UiO-67 and blank glassy carbon electrode as work electrode were performed under the same conditions. 3. Results and discussion Powder X-ray diffraction patterns (Fig. 1) of ferrocene@UiO-67 suggested that the crystal structure of UiO-67 is maintained without damage when ferrocene molecules are encapsulated into the cavity of UiO-67. Inspection of the PXRD-data reveals that all peaks are at the same position of the pristine powder, which indicates that no contraction or change in the symmetry of the crystals happens due to the rigidity of UiO-67. A change in Bragg peak intensity at low angle was noticed, which can be associated to the presence of ferrocene molecules within the pores of UiO-67. This is explained by taking account of scattering factors due to loading of ferrocene. Moreover, the powder pattern did not show any reflections corresponding to FeCp2, which rules out the presence of crystals of FeCp2 outside the framework. TEM (Fig. 2) show that the framework of UiO-67 are maintained by the loading of ferrocene, which exhibit a well-defined mono disperse octahedral microcrystals with particle size in the range of 150–300 nm. The element composition of the sample is qualitatively determined by EDS attached to TEM. Confine of ferrocene within UiO-67 was further confirmed by EDS analysis of ferrocene@UiO-67 which showed the presence of Fe peaks in the spectrum (Fig. 2c). The relative atomic percentage of 0.19%, 90.72%, 6.34% and 1.49% for Fe, C, O and Zr respectively were obtained by the EDS analysis (Table 1). Moreover, the EDS mapping of ferrocene@UiO-67 (Fig. 2f) also confirms that the ferrocene are uniformly distributed in the frameworks. Fig. 3 shows the thermal behavior of ferrocene in UiO-67 framework. The thermogravimetric analysis (TGA) profile of ferrocene@UiO67 exhibits no weight drop until 110 °C. Above 110 °C, there was a 25% weight loss until temperature reached 280 °C, which was attributed to the release of ferrocene from the UiO-67 channels. The curve shows a
Fig. 1. XRD patterns of UiO-67 and ferrocene@UiO-67.
2. Experimental 2.1. Synthesis of UiO-67 Synthesis of UiO-67 was performed by dissolving 1.2 mmol of zirconium (IV) chloride (ZrCl4), 1.2 mmol of 4,4′-Biphenyl dicarboxylic acid (H2BPDC) and 30 equivalents of acetic acid (CH3COOH) in 30 mL of N,N-dimethyformamide (DMF) at room temperature. The mixture was placed in a 100 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 48 h. The product was cooled to room temperature, washed three times with DMF, and dried at room temperature. Excess H2BPDC and DMF in the pores were removed using a high temperature treatment at 300 °C for one day. A pale yellow powder was obtained.
2.2. Synthesis of ferrocene @UiO-67 150 mg of dry and activated UiO-67 and 300 mg of ferrocene were placed in two separate glass boats in a Schlenk bottle. The bottle was then evacuated to 10−3 mbar for 5 min and sealed. The reaction bottle was kept at 110 °C for 72 h.
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Fig. 2. TEM of UiO-67(a); ferrocene@UiO-67 (b,d,e); EDS and EDS mapping of ferrocene@UiO-67 (c,f).
Table 1 The element composition of ferrocene@UiO-67 determined by EDS. Element
Weight %
Atomic %
Uncertainty %
Correction
k-Factor
C(K) O(K) Fe(K) Cu(K) Zr(K)
76.869 7.161 0.737 5.648 9.583
90.719 6.344 0.187 1.259 1.489
1.073 0.188 0.052 0.143 0.281
0.173 0.514 0.994 0.997 0.999
6.279 1.980 1.480 1.757 3.927
stability plateau in the range of 280–450 °C before a second step of an additional 28% weight loss occurs (450–600 °C), which corresponds to the thermal decomposition of the UiO-67 framework. The chemical state of materials was investigated using X-ray photoelectron spectroscopy (XPS) to probe the properties of the inner-shell electrons (Fig. 4). High-resolution narrow-scan spectra were obtained for the C1s, O1s, Zr3d and Fe2p core levels. As observed from C1s spectra, the most prominent peak at 284.7 eV is assigned to sp2-hybridized carbon (C=C) on the benzene ring. The peak at 288.6 eV is attributed to HO−C=O groups of the BPDC linkers. The O1s spectrum reveals only one peak at 531.8 eV, which can be attributed to the oxygen components on the carboxylate groups of the BPDC linkers. The two peaks at 182.6 and 185.0 eV in the Zr3d spectrum can be ascribed to the Zr-O bonds. For the Fe2p spectrum, two peaks at 711.2 and 724.3 eV corresponded to Fe 2p3/2 and Fe 2p1/2, are typical of ferrocene. All of these results further confirm the coexisting chemical states of ferrocene and UiO-67 in the composites. The N2 adsorption-desorption isotherms and pore size distributions
Fig. 3. TGA of UiO-67 and ferrocene@UiO-67 under a N2 atmosphere.
are shown in Fig. 5 and the textural parameters are also listed in Table 2. For UiO-67, the adsorption-desorption isotherms exhibit type-I isotherm with typical microporous characteristics. The specific BET surface area of UiO-67 was measured as 1662 m2 g-1. The pore size distribution curve of UiO-67 using the BJH model exhibits narrow pores distribution centered at 1.99 nm with the total pore volume 0.94 cm3 g1 . N2 adsorption of ferrocene @UiO-67 was also measured at 77 K and
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Fig. 4. XPS of ferrocene@UiO-67.
Table 2 Porosity characteristics of samples. Sample
SBET (m2g−1)
d pore (nm)
Vpore (cm3g−1)
Smicropore (m2g−1)
Vmicropore (cm3g−1)
UiO-67 ferrocene @UiO-67
1662 26
2.26 20.47
0.94 0.13
1566 0
0.61 0
that the 86.2% of pores in UiO-67 are occupied by ferrocene. We used vibrating sample magnetometer (VSM) to measure the magnetic hysteresis curve of the samples. Fig. 6 shows the hysteresis loop of ferrocene and ferrocene filled UiO-67 obtained at the room temperature, respectively. The shape of the hysteresis of ferrocene@ UiO-67 indicates that the composites are ferromagnetic at room temperature. The hysteresis exhibits an average coercivity of HC = 0.9499 T and the saturation magnetization is measured to be 1.0 emu/g. The observed value of HC and saturation magnetization for the present composites is compared to that of the bulk ferrocene [5]. In consideration of micropore volume (0.61 cm3 g-1 by the N2 adsorption measurement) of UiO-67, we can estimate that the magnetization per pore volume is about 1.49 emu/cm3 in ferrocene@UiO-67. The electrochemical behaviors of ferrocene@UiO-67 composites were tested by a three-electrode system using 6 mol/L KOH as the electrolyte at different scan rates in a potential range of 0–600 mV (vs. Ag/AgCl) (Figs. 7 and 8). Notably, a pair of symmetric redox peaks (240 mV and 375 mV at scan rate of 10 mV/s, respectively) appeared in
Fig. 5. N2 adsorption isotherms of UiO-67and ferrocene@UiO-67 at 77 K. The inset shows the BJH pore size distribution curve.
the N2 adsorbed amount decreased drastically as compared to UiO-67. Especially, it was observed that, when ferrocene was introduced, the micropore surface area and the micropore volume of UiO-67 decreased to 0. The results suggested that ferrocene molecules occupied the micropore spaces of UiO-67. From value of pore volume, it is estimated
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CV curves and were not observed in UiO-67 and blank glassy carbon electrode, which was attributed to the reversible conversion between Fe2+ and Fe3+ in ferrocene [24]. When the scan rate is fast enough, the redox peak position shifts to high potential, and the peak currents are greatly enhanced. The linear relationship between the peak current and the scan rate indicates that the electrode is reversible (Fig. 8b). The behavior of cyclic voltammograms of the ferrocene-loaded MOF can be explained by a charge-hopping mechanism [24], where the ferrocene molecules remain localized in the pores of the MOF and mediate electron transport across the layer. In this case, the Fe2+/Fe3+ redox reaction takes place at the MOF-electrolyte interface. The redox processes corresponding to the conversion between different irons oxidation states can be described by following reactions:
Fe 2 + + OH− ↔ Fe3 + (OH ) + 2e− This process is illustrated in Scheme 2. The small size of an anchored ferrocene molecule (∼5 Å) relative to the channel diameter of UiO-67(∼18 Å and ∼11.5 Å) permits ferrocene to install in channels and the electrochemical reaction occurs in the channels. Under the oxidation process, OH− is adsorbed on ferrocene, as a result, some free electrons are released to contribute the oxide current peak. Under the reduction process, OH− is released to contribute the redox current peak. The results show that the installed ferrocene molecules are electrochemically reversible, implying effective site-to-site redox hopping and potential usefulness as a redox shuttle in electro catalytic systems.
Fig. 6. Magnetic hysteresis curves of ferrocene and ferrocene@UiO-67.
4. Conclusions In summary, we have shown that UiO-67 can be filled with ferrocene as identified by XRD, TEM, EDS, TG and XPS. The magnetic measurements reveal that the magnetic structure of UiO-67 is significantly modified, and ferromagnetic behavior are formed due to ferrocene encapsulation. The oxidation and reduction peak are observed in cyclic voltammograms due to the conversion between Fe2+ and Fe3+ in ferrocene. The combined findings have important implications for the use of this MOF composite as electro-catalysts and electro-analytical devices in electrochemical cells. Acknowledgements Fig. 7. Cyclic voltammograms of UiO-67, ferrocene@UiO-67 and glassy carbon electrode.
This work was supported by the National Natural Science Foundation of China (21261006).
Fig. 8. Cyclic voltammograms of ferrocene@UiO-67(a); The linear relationship between the peak current and the scan rate (b).
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Scheme 2. Illustration of conversion between different irons oxidation state in ferrocene.
References
[14] A.C. Gomes, C.I.R. Magalhães, T.S.M. Oliveira, A.D. Lopes, I.S. Gonçalves, M. Pillinger, Dalton Trans. 45 (2016) 17042–17052. [15] J. Guo, J.X. Liu, M. Lan, Y.Z. Hu, S.L. Wang, J.Q. Wen, Y. He, F.H. Gao, X. Zhang, S.J. Zhang, G. Xiang, M.A.C. Willis, F.S. Boi, RSC Adv. 7 (2017) 4753–4758. [16] S. Chavan, J.G. Vitillo, D. Gianolio, O. Zavorotynska, B. Civalleri, S. Jakobsen, M.H. Nilsen, L. Valenzano, C. Lamberti, K.P. Lillerud, S. Bordiga, Phys. Chem. Chem. Phys. 14 (2012) 1614–1626. [17] M.J. Katz, Z.J. Brown, Y.J. Colón, P.W. Siu, K.A. Scheidt, R.Q. Snurr, J.T. Hupp, O.K. Farha, Chem. Commun. 49 (2013) 9449–9451. [18] P.W. Siu, Z.J. Brown, O.K. Farha, J.T. Hupp, K.A. Scheidt, Chem. Commun. 49 (2013) 10920–10922. [19] J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, J. Am. Chem. Soc. 130 (2008) 13850–13851. [20] H. Wu, Y.S. Chua, V. Krungleviciute, M. Tyagi, P. Chen, T. Yildirim, W. Zhou, J. Am. Chem. Soc. 135 (2013) 10525–10532. [21] M. Kandiah, M.H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E.A. Quadrelli, F. Bonino, K.P. Lillerud, Chem. Mater. 22 (2010) 6632–6640. [22] C.H. Lau, R. Babarao, M.R. Hill, Chem. Commun. 49 (2013) 3634–3636. [23] N. Ko, J. Hong, S. Sung, K.E. Cordova, H.J. Park, J.K. Yang, J. Kim, Dalton Trans. 44 (2015) 2047–2051. [24] J.X. Liu, M. Paradinas, L. Heinke, M. Buck, C. Ocal, V. Mugnaini, C. Wöll, ChemElectroChem 3 (2016) 713–718.
[1] M.B. Avinash, K.S. Subrahmanyam, Y. Sundarayya, T. Govindaraju, Nanoscale 2 (2010) 1762–1766. [2] Y.K. Park, S.B. Choi, H. Kim, K. Kim, B.H. Won, K. Choi, J.S. Choi, W.S. Ahn, N. Won, S. Kim, D.H. Jung, S.H. Choi, G.H. Kim, S.S. Cha, Y.H. Jhon, J.K. Yang, J. Kim, Angew. Chem. Int. Ed. 46 (2007) 8230–8233. [3] W.Y. Sun, T. Kusukawa, M. Fujita, J. Am. Chem. Soc. 124 (2002) 11570–11571. [4] I. Hod, W. Bury, D.M. Gardner, P. Deria, V. Roznyatovskiy, M.R. Wasielewski, O.K. Farha, J.T. Hupp, J. Phys. Chem. Lett. 6 (2015) 586–591. [5] J. Cheng, X.P. Zou, G. Zhu, M.F. Wang, Y. Su, G.Q. Yang, X.M. Lü, Solid State Commun. 149 (2009) 1619–1622. [6] S. B.Kalidindi, K. Yusenko, R.A. Fischer, Chem. Commun. 47 (2011) 8506–8508. [7] H. Li, M. Eddaoudi, M. O'Keeff, Nature 402 (1999) 276–279. [8] K.L. Mulfort, J.T. Hupp, J. Am. Chem. Soc. 129 (2007) 9604–9605. [9] B. Sareeya, H. Satoshi, H. Masakazu, M. Mizuno, T. Kawamura, D. Tanaka, N. Yanai, S. Kitagawa, Nat. Mater. 8 (2009) 831–836. [10] B. Liu, H. Shioyama, T. Akita, J. Am. Chem. Soc. 130 (2008) 5390–5391. [11] S. Hermes, F. Schröder, S. Amirjalayer, R. Schmid, R.A. Fischer, J. Mater. Chem. 16 (2006) 2464–2472. [12] M. Müller, O.I. Lebedev, R.A. Fischer, J. Mater. Chem. 18 (2008) 5274–5281. [13] F. Cao, Y. Ding, L. Chen, C.R. Zhang, Nanoscale 5 (2013) 10000–10006.
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