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Enhanced hydrothermal stability of Cu MOF by post synthetic modification with amino acids
Vacuum 164 (2019) 449–457
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Enhanced hydrothermal stability of Cu MOF by post synthetic modification with amino acids
T
Reetu Rania,c, Akash Deepa,c, Boris Mizaikoffb, Suman Singha,c,∗ a
CSIR- Central Scientific Instruments Organisation, Sector 30-C, Chandigarh, 160030, India Institute of Analytical and Bioanalytical Chemistry, Ulm University, Ulm, 89081, Germany c Academy of Scientific & Innovative Research (AcSIR), Ghaziabad, 201002, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Amino acids Hydrothermal stability Metal organic frameworks (MOFs) Post synthetic modification (PSM)
In this work, we report the solvothermal synthesis of copper metal organic framework (Cu MOF) also known as HKUST 1 and its functionalization with amino acids to increase hydrothermal stability. MOFs are porous materials made up of organic and inorganic components. Owing to properties like large surface area, tunable pore size, pore volume and desirable functionality via Post Synthetic Modification (PSM), MOFs are gaining attention in various applications such as catalysis, energy storage, pollutant removal and sensing. But most of the MOFs show poor hydrothermal stability that limits their applications in aqueous samples. Various characterization techniques such as Powder X-Ray Diffraction (PXRD), Fourier-Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Zeta potential study indicated successful modification of Cu MOF with different amino acids. Thermogravimetric analysis (TGA), contact angle and SEM results after hydrothermal conditioning showed enhanced water stability for glycine, lysine and tyrosine functionalized Cu MOF compared to pristine MOF. The pristine Cu MOF possessed contact angle of 39.6° which got increased for modified Cu MOFs showing transition behaviour of Cu MOF from hydrophilic to hydrophobic after functionalization.
1. Introduction Metal Organic Frameworks (MOFs) are a subclass of coordination compounds containing both inorganic and organic components with highly porous structures [1–3]. A variety of MOFs can be designed for various applications depending upon combination of organic linkers with different metals [4,5]. Owing to properties like large surface area, tunable pore size, pore volume and desirable functionality via post synthetic modification (PSM), MOFs are gaining attention in various applications such as catalysis, energy storage, pollutant removal and sensing [6–9]. But most of the MOFs show poor hydrothermal stability hindering their applications in aqueous samples [10]. Making these MOFs moisture resistant can help to increase their applicability for various fields. Taking example of HKUST 1, which is a copper MOF with a paddlewheel structure and open metal coordination sites, get attacked by water molecules and collapse of structure is observed in aqueous samples and high humidity conditions [11,12]. Basically, the hydrothermal stability of MOFs depends upon strength of bond between metal oxide cluster and linker [13]. Various methods have been reported to improve hydrothermal stability of different MOFs via ligand
∗
functionalization. Some authors tried to improve hydrothermal stability via incorporating hydrophobic ligands so as to prevent attack of water molecules on metal linker coordination bonds [14]. Ageing properties of HKUST 1 were improved by using a hydrophobic mixed matrix membrane with polyvinylidene difluoride (PVDF) [15]. Hybridization with different hydrophobic groups such as carbon nano tubes [16], silica [17], attapulgite clay [18], and graphite oxide [19] is also reported for improving hydrothermal stability of Cu MOF. All these methods improved the hydrothermal stability of Cu MOF, but their synthesis process is quite tedious. Moreover, the pre-functionalization of linkers often causes problem in getting desired MOF because most of the functional groups are unstable in harsh MOF synthesis conditions. Also, the steric hindrances caused by these groups interfere in the crystallization process, thus affecting the porosity of MOF [20]. Therefore, post synthetic modification of synthesized MOFs can be an alternative way to circumvent this limitation of poor hydrothermal stability [21,22]. Nadeen Al-Janabi et. al. did post synthetic modification of CuBTC using glycine amino acid to improve hydrothermal stability for selective adsorption of CO2 from aqueous samples [23]. In present work, an attempt has been made to explore the effect of different amino
Corresponding author. CSIR- Central Scientific Instruments Organisation, Sector 30-C, Chandigarh, 160030, India. E-mail addresses: [email protected], [email protected] (S. Singh).
https://doi.org/10.1016/j.vacuum.2019.01.011 Received 31 July 2018; Received in revised form 7 January 2019; Accepted 9 January 2019 Available online 15 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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pattern of the synthesized samples is shown in Fig. 2 which displays the main diffraction peaks of Cu MOF at 2θ = 5.778°, 6.725°, 9.538°, 11.694°, 13.137°, 14.716°, 16.375°, 17.258°, 19.036°, 20.262°, 25.951° and 29.406° corresponding to Miller indices (111), (200), (220), (222), (400), (331), (422), (511), (440), (600), (731) and (751) respectively [25,26]. In PXRD pattern of functionalized Cu MOF, besides the peaks of parent MOF, the other peaks, characteristics of amino acids are also observed. The PXRD pattern of Gly@Cu MOF, showed peaks at 14.992°, 22.327°, 28.227° are for glycine [27], PXRD pattern of Lys@Cu MOF, showed peaks at 18.382°, 20.680°, 24.802° for lysine [28]. The PXRD pattern of Tyr@Cu MOF, showed peaks at 15.797°, 18.034°, 20.938° and 21.966°, 27.364° for tyrosine [29] and the PXRD pattern of Cys@Cu MOF showed peaks at 20.768°, 28.340° for cysteine [30]. The presence of PXRD peaks corresponding to amino acids along with peaks of parent Cu MOF confirms the successful functionalization of synthesized metal organic framework. The intense peaks at small angles (2θ) are characteristics of microporous material which possesses numerous tiny pores or cavities. Comparing the diffraction peaks of functionalized samples with parent Cu MOF, almost similar diffraction pattern is observed which indicates that the crystal structure of MOF is well preserved after functionalization except cysteine where some degree of amorphousness is observed. However, the intensity of main diffraction peaks of Cu MOF decreased after functionalization and the degree of Bragg diffraction angle of some planes also got shifted. These changes can be attributed to pore filling effect of channels in porous materials [31].
acids namely glycine, cysteine, tyrosine and lysine on hydrothermal stability of Cu MOF (HKUST 1). Out of these, glycine is simplest with hydrogen in side chain and non-polar in nature and lysine, tyrosine, cysteine are polar amino acids. Amino acids hinder the attack of water molecules on Open Metals Sites (OMSs) of Cu MOF and prevent the weakening of metal and linker coordination bond which finally helps in enhancement of hydrothermal stability of Cu MOF [23]. 2. Experimental section 2.1. Materials and methods All the reagents used were of analytical grade and were used as received without further purification. Cupric nitrate tri hydrate (Sigma Aldrich), 1,3,5-benzenetricarboxylic acid (trimesic acid, H3BTC) (Sigma Aldrich), ethanol (Merck) were used to synthesize Cu MOF. Cysteine (Sigma Aldrich), tyrosine (Sigma Aldrich), glycine (TCI), and lysine monohydrochloride (CDH) were used for post synthetic modification of Cu MOF. Ultrapure water (18.2 MΩ cm), ethanol and methanol (Merck) were used for washing, purification and activation purpose. Ultrapure water (18.2 MΩ cm) was obtained from a Millipore water purification system. PXRD studies were performed with D8-Advance, Bruker using Cu Kα radiations with accelerating voltages 40 kV and current of 40 mA and 2θ ranging from 5° from 30°. FTIR spectra were obtained on Spectrum two, Perkin Elmer, in the range of 400 cm−1 to 4000 cm−1 with resolution of 4 cm−1. SEM studies were performed on Hitachi S4300 SE/N at operating voltage 7 kV. EDS was done on Hitachi SU8010 on clean silicon wafer. Thermodynamic stability of the prepared samples was tested on thermogravimetric analyzer (Model SDTQ600,TA Instruments) in temperature range of 50–600 °C. Contact angle studies were done on DSA-100E, Kruss. Zeta potential studies were performed on Malvern ZS90, Malvern.
3.1.2. Morphology study SEM and EDS was performed to investigate the surface morphology and elemental composition of Cu MOF before and after amino acid functionalization. SEM and EDS image of Cu MOF are shown in Fig. 3. The SEM (Fig. 3 (a)) shows presence of irregularly shaped plate like morphology of MOF and EDS (Fig. 3 (b)) confirmed the presence of copper (Cu), along with oxygen (O) and carbon (C). The mapping of individual elements viz copper, carbon and oxygen is also shown in Fig. 3(c–e). The parent Cu MOF possesses average particle size of 1.0 μm. Similarly, Fig. 4 −7 show SEM and EDS images of Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF, respectively. The SEM image of glycine functionalized Cu MOF (Fig. 4(a)) showed irregular needle shaped morphology of glycine and plate like morphology of Cu MOF. EDS (Fig. 4 (b)) showed presence of Cu, C, O, N. Presence and distribution of these elements was further confirmed with elemental maps of Cu, C, O, and N as shown in Fig. 4(c–f). Similarly, the SEM results of lysine and tyrosine functionalized Cu MOF showed morphology of both Cu MOF and representative amino acid. Lysine showed rectangular crystals (Fig. 5(a)), and tyrosine demonstrated rod to columnar shaped crystals (Fig. 6(a)). EDS results of these functionalized Cu MOFs further confirms the presence of all the elements viz. Cu, C, O, and N of parent MOF and respective amino acids lysine (Fig. 5(b–f)) and tyrosine (Fig. 6(b–f)). The SEM images of cysteine functionalized Cu MOF showed elongated spindle shape crystals of cysteine (Fig. 7(a)). But morphology of parent Cu MOF is not well preserved after cysteine functionalization which is clearly shown in Fig. 7 (a). EDS results showed presence of the elements Cu, C, O, N and S as shown in Fig. 7(b–g). All these results of SEM and EDS confirm successful functionalization of Cu MOF with amino acids glycine, lysine and tyrosine which is consistent with the results of PXRD.
2.2. Synthesis of Cu MOF The synthesis of Cu MOF is done by using previously reported hydrothermal method [24]. In a typical synthesis, 2.2174 g of Cu (NO3)2.3H2O was dissolved in 30 mL absolute ethanol and 1.05 g of trimesic acid was dissolved in another 30 mL absolute ethanol. Then, both the solutions were transferred to a 100 mL Teflon lined stainless steel autoclave and placed in an oven for 12 h at 120 °C for heat treatment. The solution turned into blue crystals which was further filtered and washed with ethanol and water and dried in oven at 145 °C for 12 h. The MOF was activated by immersing in methanol for 48 h and afterward dried at 145 °C. 2.3. Post synthetic modification of Cu MOF with different amino acids The as synthesized Cu MOF was functionalized with different amino acids using previously reported method [23]. Fixed amount of amino acids was dissolved in ethanol and heated on a hot plate at 50 °C for one hour with continuous stirring. After that, the synthesized Cu MOF was added in this solution of amino acids and stirred for 24 h on the hotplate at 50 °C. As prepared samples then cooled to room temperature and filtered through a Whatman filter paper and washed with ethanol and methanol several times to remove the unreacted amino acids. After washing, samples were dried in vacuum oven at 120 °C for 15 h. Fig. 1 shows the synthesized Cu MOF and amino acid functionalized Cu MOF.
3.1.3. FTIR and Raman analysis In order to further confirm the formation of amino acid functionalized Cu MOF, FTIR and Raman analysis were performed. The FTIR spectra of synthesized Cu MOF along with amino acid modified Cu MOF are shown in Fig. 8. All the IR bands are in good agreement with the previous literature of Cu MOF [32,33]. The IR spectrum of Cu MOF is mostly comprised of vibrations due to BTC linker. The region below 1150 cm−1, is mainly attributed to vibrational modes of benzene ring of
3. Results & Discussion 3.1. Characterization of bare and amino acid functionalized Cu MOF 3.1.1. Crystallinity analysis PXRD analysis was done to examine the crystallinity and phase purity of bare and amino acid functionalized Cu MOF. The PXRD 450
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Fig. 1. Synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF
BTC. The single peak at 1111 cm−1 and two peaks at 731 cm−1 and 761 cm−1 correspond to in-plane and out-of-plane (δC–H) bending modes of benzene ring respectively. The region of spectrum from 1150 cm−1 to 1700 cm−1 represents characteristic peaks of carboxylate groups of the BTC. The weak peaks at 1185 cm−1 and 1272 cm−1 are assigned to υCOstretching vibrational modes. The strong band at 1371 cm−1 and 1645 cm−1 belong to asymmetric stretching (υasymC–O2) and symmetric stretching (υsymC–O2) modes of carboxylate group respectively. The two peaks at 1448 cm−1 and 1565 cm−1 represent stretching modes associated with the aromatic rings (υC=C). The broad band at 3419 cm−1 is associated to stretching vibrational modes of water (υOH). The presence of O–H stretching mode confirms that the pristine MOF is hydrated. The vibrations associated with metal linker bond are present below 600 cm−1. These vibrational modes are difficult to observe with IR method. The bonding between Cu-O appears at 491 cm−1. FTIR spectra of amino acid modified Cu MOF show all characteristics peaks of Cu MOF along with the additional peak at 3247 cm−1 due to stretching vibrational modes in N-H group [34] confirming that Cu MOF is successfully functionalized with amino acids. Results of FTIR were further confirmed with Raman spectroscopy
Fig. 2. PXRD pattern of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF (navy), and Cys@Cu MOF (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a) SEM image of Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen. 451
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Fig. 4. (a) SEM image of Gly@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen.
Fig. 5. (a) SEM image of Lys@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen. 452
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Fig. 6. (a) SEM image of Tyr@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen.
Fig. 7. (a) SEM image of Cys@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen, (f) Elemental mapping of Sulphur. 453
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Table 1 Zeta potential study of synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF, and Cys@Cu MOF. MOF
Zeta potential (mV)
Cu MOF Gly@Cu MOF Lys@Cu MOF Tyr@Cu MOF Cys@Cu MOF
−1.61 −17.7 −4.87 −19.9 0.42
3.1.4. Zeta potential study Zeta potential study has been done for the characterization of the surface charge of MOF before and after functionalization. Zeta potential of parent MOF was about −1.61 mV at pH 7.4 as shown in Table 1 and it was −17.7 mV, −4.87 mV, −19.9 mV and 0.42 mV for Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF, respectively. At pH 7.4, the surface of Cu MOF will be negatively charged because the pH of point of zero charge for Cu MOF is 4.0 and above this pH, the deprotonation of carboxylic group of BTC linker is observed. This results in negative charge at Cu MOF. The results obtained are as per previously reported literature [33]. After functionalization, there is variation in zeta potential of Cu MOF as each amino acid has different charge at pH 7.4 and their interaction with MOF results in an overall charge to the surface.
Fig. 8. FTIR spectra of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF(navy), and Cys@Cu MOF(green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.1.5. Thermal stability study Thermogravimetric analysis (TGA) was performed to check thermal stability of prepared samples. The TGA curve of parent and functionalized Cu MOF are presented in Fig. 10. Two major weight loss steps were observed for synthesized Cu MOF. Firstly, the moisture was removed from MOF till 130 °C. After removal of solvent, a stable plateau was observed up to 310 °C. After 310 °C, predominant weight loss step was observed due to degradation of organic linkers which results in complete collapse of MOF structure. The remaining residue was CuO powder [39]. Thermal studies showed that the synthesized Cu MOF was thermally stable up to 310 °C. Comparing the thermal stability of amino acid functionalized Cu MOF with pristine Cu MOF, in all four samples; the first weight loss due to water is much lower than the parent. This helps to conclude that amino acids replaced the water molecules (present at OMS of Cu MOF) which are basically responsible for catalytic degradation of Cu MOF. This resulted in more hydrophobic nature of functionalized samples [23]. Although amino acid functionalization makes Cu MOF hydrothermally stable but there is decrease in
Fig. 9. Raman spectra of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF (navy), and Cys@Cu MOF (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(Fig. 9). Peaks located at 1612 cm−1 to 1005 cm−1 correspond to the stretching vibrational modes of benzene ring (υ C=C(Ar)). The peaks located at 745 cm−1 and 829 cm−1 represent out-of-plane ring bending (δC=C(Ar)) and out-of-plane C–H bending of the benzene ring (δC- H(Ar)) [35]. Peaks located at 1461 cm−1 and 1545 cm−1 are characteristic peaks of carboxylate group of BTC associated with symmetric (υsymCOO−) and asymmetric stretching (υassymCOO−) modes of carboxylate group respectively. The main peaks of metal linker bonding are present in low frequency region from 100 to 600 cm−1 [36]. Peaks associated with Cu–O stretching mode are observed at 195 cm−1 and 501 cm−1. The peaks at 448 cm−1 and 274 cm−1 are ascribed to Cu-Cu stretching mode (υ Cu-Cu) [37]. Raman spectra of amino acid modified Cu MOF possess all the characteristics peaks of Cu MOF along with some additional peaks of every amino acid. The bands at 448 cm−1, 501 cm−1 and 274 cm−1 got shifted in case of all four amino acids showing interaction between amino acids with metal center of Cu MOF. This observation reveals that amino acids are replacing water molecules from OMS of Cu MOF [23]. The additional peak at 1158 cm−1 in amino acid modified Cu MOFs is associated with N-H stretching vibrational mode [38], confirming the successful modification of Cu MOF by amino acids.
Fig. 10. Thermal stability study of synthesized MOFs, Cu MOF (purple), Gly@Cu MOF (green), Lys@Cu MOF (red),Tyr@Cu MOF (black), and Cys@Cu MOF (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 454
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Fig. 11. Contact angle of synthesized MOFs, (a) Cu MOF, (b) Gly@Cu MOF, (c) Lys@Cu MOF, (d) Tyr@Cu MOF, and (e) Cys@Cu MOF.
in Table 2.
decomposition temperature, compared to the parent Cu MOF. In case of cysteine the decomposition temperature is even lower than 200 °C.
3.1.7. Hydrothermal conditioning Hydrothermal stability of parent Cu MOF and functionalized Cu MOFs was evaluated performing hydrothermal conditioning at high temperature and humid conditions [9]. For this, the samples were placed on filter paper and this filter paper was placed on a beaker filled with boiling water at 85 °C and kept in saturated water vapor for 5 h. After hydrothermal conditioning, the samples were dried at room temperature and SEM was recorded to observe the change in morphology of samples before and after hydrothermal conditioning. The result is shown in Fig. 12. As can be seen, the morphology of glycine, lysine and tyrosine functionalized Cu MOFs remained well preserved after hydrothermal conditioning. But the morphology of pristine Cu MOF and cysteine functionalized Cu MOF got highly damaged. This study concludes that glycine, lysine and tyrosine functionalized Cu MOFs are quite stable than parent Cu MOF and cysteine functionalized Cu MOF is very unstable.
3.1.6. Contact angle study Contact angle measurements are usually done to examine the wettability of various materials and are mainly useful in studying the effects of chemical functionalization on the material wettability. Hydrophilic materials possess contact angle below 90° and hydrophobic materials possess contact angle in between 90° and 150° [40]. Herein, the parent Cu MOF possess contact angle of 39.6° showing hydrophilic behaviour but after functionalization with amino acids, increase in contact angle was observed as shown in Fig. 11. All the four modified MOFs possess contact angle more than parent Cu MOF showing transition behaviour of Cu MOF from hydrophilic to hydrophobic after functionalization. Value of average contact angle of all samples is given Table 2 Average contact angle of synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF, and Cys@Cu MOF. MOF
Contact angle(º)
Cu MOF Gly@Cu MOF Lys@Cu MOF Tyr@Cu MOF Cys@Cu MOF
39.6 54.0 52.2 46.5 47.0
4. Conclusion In this work, the synthesis and functionalization of Cu MOF with amino acids namely glycine, lysine, tyrosine and cysteine is reported. The synthesis has been performed using post synthetic modification because pre-functionalization of linkers often causes problem to get desired MOF due to instability of most of the functional groups in harsh MOF synthesis conditions, steric hindrances caused by these groups and 455
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Fig. 12. SEM images of synthesized MOFs, (a) Cu MOF, (b) Gly@Cu MOF, (c) Lys@Cu MOF, (d) Tyr@Cu MOF, and (e) Cys@Cu MOF after hydrothermal conditioning.
Acknowledgements
interference in the crystallization process and thus affecting the porosity of MOF. FTIR, Raman and EDS results confirmed that Cu MOF was successfully functionalized with amino acids. PXRD and SEM results demonstrated that crystallinity and morphology of Cu MOF is well preserved after amino acids functionalization in case of glycine, lysine and tyrosine. But in case of cysteine functionalization, the crystalline nature of Cu MOF changed into amorphous. In TGA study, the first weight loss step due to evaporation of water is much lower in modified samples than parent MOF confirming idea of enhanced hydrothermal stability of modified one. The results were further proved with the contact angle measurements and hydrothermal conditioning methods. Out of all the amino acids lysine showed better enhancement in hydrothermal stability as proved from the results of SEM before and after hydrothermal conditioning. The cysteine functionalized Cu MOF showed least hydrothermal stability as shown in PXRD, SEM and TGA results. It strongly affected the morphology of parent Cu MOF also. In view of the above results, the improved performance of the amino acid functionalized Cu MOF offers a great potential for various applications such as sensing, adsorption, storage etc. The modified Cu MOFs can selectively adsorb analyte because of –NH2 and –COOH functionalization resulted from amino acid groups that help in enhanced selectivity, sensitivity in sensing application as well as better stability in aqueous environment.
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Conflicts of interest There are no conflicts of interest to declare.
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Enhanced hydrothermal stability of Cu MOF by post synthetic modification with amino acids
T
Reetu Rania,c, Akash Deepa,c, Boris Mizaikoffb, Suman Singha,c,∗ a
CSIR- Central Scientific Instruments Organisation, Sector 30-C, Chandigarh, 160030, India Institute of Analytical and Bioanalytical Chemistry, Ulm University, Ulm, 89081, Germany c Academy of Scientific & Innovative Research (AcSIR), Ghaziabad, 201002, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Amino acids Hydrothermal stability Metal organic frameworks (MOFs) Post synthetic modification (PSM)
In this work, we report the solvothermal synthesis of copper metal organic framework (Cu MOF) also known as HKUST 1 and its functionalization with amino acids to increase hydrothermal stability. MOFs are porous materials made up of organic and inorganic components. Owing to properties like large surface area, tunable pore size, pore volume and desirable functionality via Post Synthetic Modification (PSM), MOFs are gaining attention in various applications such as catalysis, energy storage, pollutant removal and sensing. But most of the MOFs show poor hydrothermal stability that limits their applications in aqueous samples. Various characterization techniques such as Powder X-Ray Diffraction (PXRD), Fourier-Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Zeta potential study indicated successful modification of Cu MOF with different amino acids. Thermogravimetric analysis (TGA), contact angle and SEM results after hydrothermal conditioning showed enhanced water stability for glycine, lysine and tyrosine functionalized Cu MOF compared to pristine MOF. The pristine Cu MOF possessed contact angle of 39.6° which got increased for modified Cu MOFs showing transition behaviour of Cu MOF from hydrophilic to hydrophobic after functionalization.
1. Introduction Metal Organic Frameworks (MOFs) are a subclass of coordination compounds containing both inorganic and organic components with highly porous structures [1–3]. A variety of MOFs can be designed for various applications depending upon combination of organic linkers with different metals [4,5]. Owing to properties like large surface area, tunable pore size, pore volume and desirable functionality via post synthetic modification (PSM), MOFs are gaining attention in various applications such as catalysis, energy storage, pollutant removal and sensing [6–9]. But most of the MOFs show poor hydrothermal stability hindering their applications in aqueous samples [10]. Making these MOFs moisture resistant can help to increase their applicability for various fields. Taking example of HKUST 1, which is a copper MOF with a paddlewheel structure and open metal coordination sites, get attacked by water molecules and collapse of structure is observed in aqueous samples and high humidity conditions [11,12]. Basically, the hydrothermal stability of MOFs depends upon strength of bond between metal oxide cluster and linker [13]. Various methods have been reported to improve hydrothermal stability of different MOFs via ligand
∗
functionalization. Some authors tried to improve hydrothermal stability via incorporating hydrophobic ligands so as to prevent attack of water molecules on metal linker coordination bonds [14]. Ageing properties of HKUST 1 were improved by using a hydrophobic mixed matrix membrane with polyvinylidene difluoride (PVDF) [15]. Hybridization with different hydrophobic groups such as carbon nano tubes [16], silica [17], attapulgite clay [18], and graphite oxide [19] is also reported for improving hydrothermal stability of Cu MOF. All these methods improved the hydrothermal stability of Cu MOF, but their synthesis process is quite tedious. Moreover, the pre-functionalization of linkers often causes problem in getting desired MOF because most of the functional groups are unstable in harsh MOF synthesis conditions. Also, the steric hindrances caused by these groups interfere in the crystallization process, thus affecting the porosity of MOF [20]. Therefore, post synthetic modification of synthesized MOFs can be an alternative way to circumvent this limitation of poor hydrothermal stability [21,22]. Nadeen Al-Janabi et. al. did post synthetic modification of CuBTC using glycine amino acid to improve hydrothermal stability for selective adsorption of CO2 from aqueous samples [23]. In present work, an attempt has been made to explore the effect of different amino
Corresponding author. CSIR- Central Scientific Instruments Organisation, Sector 30-C, Chandigarh, 160030, India. E-mail addresses: [email protected], [email protected] (S. Singh).
https://doi.org/10.1016/j.vacuum.2019.01.011 Received 31 July 2018; Received in revised form 7 January 2019; Accepted 9 January 2019 Available online 15 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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pattern of the synthesized samples is shown in Fig. 2 which displays the main diffraction peaks of Cu MOF at 2θ = 5.778°, 6.725°, 9.538°, 11.694°, 13.137°, 14.716°, 16.375°, 17.258°, 19.036°, 20.262°, 25.951° and 29.406° corresponding to Miller indices (111), (200), (220), (222), (400), (331), (422), (511), (440), (600), (731) and (751) respectively [25,26]. In PXRD pattern of functionalized Cu MOF, besides the peaks of parent MOF, the other peaks, characteristics of amino acids are also observed. The PXRD pattern of Gly@Cu MOF, showed peaks at 14.992°, 22.327°, 28.227° are for glycine [27], PXRD pattern of Lys@Cu MOF, showed peaks at 18.382°, 20.680°, 24.802° for lysine [28]. The PXRD pattern of Tyr@Cu MOF, showed peaks at 15.797°, 18.034°, 20.938° and 21.966°, 27.364° for tyrosine [29] and the PXRD pattern of Cys@Cu MOF showed peaks at 20.768°, 28.340° for cysteine [30]. The presence of PXRD peaks corresponding to amino acids along with peaks of parent Cu MOF confirms the successful functionalization of synthesized metal organic framework. The intense peaks at small angles (2θ) are characteristics of microporous material which possesses numerous tiny pores or cavities. Comparing the diffraction peaks of functionalized samples with parent Cu MOF, almost similar diffraction pattern is observed which indicates that the crystal structure of MOF is well preserved after functionalization except cysteine where some degree of amorphousness is observed. However, the intensity of main diffraction peaks of Cu MOF decreased after functionalization and the degree of Bragg diffraction angle of some planes also got shifted. These changes can be attributed to pore filling effect of channels in porous materials [31].
acids namely glycine, cysteine, tyrosine and lysine on hydrothermal stability of Cu MOF (HKUST 1). Out of these, glycine is simplest with hydrogen in side chain and non-polar in nature and lysine, tyrosine, cysteine are polar amino acids. Amino acids hinder the attack of water molecules on Open Metals Sites (OMSs) of Cu MOF and prevent the weakening of metal and linker coordination bond which finally helps in enhancement of hydrothermal stability of Cu MOF [23]. 2. Experimental section 2.1. Materials and methods All the reagents used were of analytical grade and were used as received without further purification. Cupric nitrate tri hydrate (Sigma Aldrich), 1,3,5-benzenetricarboxylic acid (trimesic acid, H3BTC) (Sigma Aldrich), ethanol (Merck) were used to synthesize Cu MOF. Cysteine (Sigma Aldrich), tyrosine (Sigma Aldrich), glycine (TCI), and lysine monohydrochloride (CDH) were used for post synthetic modification of Cu MOF. Ultrapure water (18.2 MΩ cm), ethanol and methanol (Merck) were used for washing, purification and activation purpose. Ultrapure water (18.2 MΩ cm) was obtained from a Millipore water purification system. PXRD studies were performed with D8-Advance, Bruker using Cu Kα radiations with accelerating voltages 40 kV and current of 40 mA and 2θ ranging from 5° from 30°. FTIR spectra were obtained on Spectrum two, Perkin Elmer, in the range of 400 cm−1 to 4000 cm−1 with resolution of 4 cm−1. SEM studies were performed on Hitachi S4300 SE/N at operating voltage 7 kV. EDS was done on Hitachi SU8010 on clean silicon wafer. Thermodynamic stability of the prepared samples was tested on thermogravimetric analyzer (Model SDTQ600,TA Instruments) in temperature range of 50–600 °C. Contact angle studies were done on DSA-100E, Kruss. Zeta potential studies were performed on Malvern ZS90, Malvern.
3.1.2. Morphology study SEM and EDS was performed to investigate the surface morphology and elemental composition of Cu MOF before and after amino acid functionalization. SEM and EDS image of Cu MOF are shown in Fig. 3. The SEM (Fig. 3 (a)) shows presence of irregularly shaped plate like morphology of MOF and EDS (Fig. 3 (b)) confirmed the presence of copper (Cu), along with oxygen (O) and carbon (C). The mapping of individual elements viz copper, carbon and oxygen is also shown in Fig. 3(c–e). The parent Cu MOF possesses average particle size of 1.0 μm. Similarly, Fig. 4 −7 show SEM and EDS images of Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF, respectively. The SEM image of glycine functionalized Cu MOF (Fig. 4(a)) showed irregular needle shaped morphology of glycine and plate like morphology of Cu MOF. EDS (Fig. 4 (b)) showed presence of Cu, C, O, N. Presence and distribution of these elements was further confirmed with elemental maps of Cu, C, O, and N as shown in Fig. 4(c–f). Similarly, the SEM results of lysine and tyrosine functionalized Cu MOF showed morphology of both Cu MOF and representative amino acid. Lysine showed rectangular crystals (Fig. 5(a)), and tyrosine demonstrated rod to columnar shaped crystals (Fig. 6(a)). EDS results of these functionalized Cu MOFs further confirms the presence of all the elements viz. Cu, C, O, and N of parent MOF and respective amino acids lysine (Fig. 5(b–f)) and tyrosine (Fig. 6(b–f)). The SEM images of cysteine functionalized Cu MOF showed elongated spindle shape crystals of cysteine (Fig. 7(a)). But morphology of parent Cu MOF is not well preserved after cysteine functionalization which is clearly shown in Fig. 7 (a). EDS results showed presence of the elements Cu, C, O, N and S as shown in Fig. 7(b–g). All these results of SEM and EDS confirm successful functionalization of Cu MOF with amino acids glycine, lysine and tyrosine which is consistent with the results of PXRD.
2.2. Synthesis of Cu MOF The synthesis of Cu MOF is done by using previously reported hydrothermal method [24]. In a typical synthesis, 2.2174 g of Cu (NO3)2.3H2O was dissolved in 30 mL absolute ethanol and 1.05 g of trimesic acid was dissolved in another 30 mL absolute ethanol. Then, both the solutions were transferred to a 100 mL Teflon lined stainless steel autoclave and placed in an oven for 12 h at 120 °C for heat treatment. The solution turned into blue crystals which was further filtered and washed with ethanol and water and dried in oven at 145 °C for 12 h. The MOF was activated by immersing in methanol for 48 h and afterward dried at 145 °C. 2.3. Post synthetic modification of Cu MOF with different amino acids The as synthesized Cu MOF was functionalized with different amino acids using previously reported method [23]. Fixed amount of amino acids was dissolved in ethanol and heated on a hot plate at 50 °C for one hour with continuous stirring. After that, the synthesized Cu MOF was added in this solution of amino acids and stirred for 24 h on the hotplate at 50 °C. As prepared samples then cooled to room temperature and filtered through a Whatman filter paper and washed with ethanol and methanol several times to remove the unreacted amino acids. After washing, samples were dried in vacuum oven at 120 °C for 15 h. Fig. 1 shows the synthesized Cu MOF and amino acid functionalized Cu MOF.
3.1.3. FTIR and Raman analysis In order to further confirm the formation of amino acid functionalized Cu MOF, FTIR and Raman analysis were performed. The FTIR spectra of synthesized Cu MOF along with amino acid modified Cu MOF are shown in Fig. 8. All the IR bands are in good agreement with the previous literature of Cu MOF [32,33]. The IR spectrum of Cu MOF is mostly comprised of vibrations due to BTC linker. The region below 1150 cm−1, is mainly attributed to vibrational modes of benzene ring of
3. Results & Discussion 3.1. Characterization of bare and amino acid functionalized Cu MOF 3.1.1. Crystallinity analysis PXRD analysis was done to examine the crystallinity and phase purity of bare and amino acid functionalized Cu MOF. The PXRD 450
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Fig. 1. Synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF
BTC. The single peak at 1111 cm−1 and two peaks at 731 cm−1 and 761 cm−1 correspond to in-plane and out-of-plane (δC–H) bending modes of benzene ring respectively. The region of spectrum from 1150 cm−1 to 1700 cm−1 represents characteristic peaks of carboxylate groups of the BTC. The weak peaks at 1185 cm−1 and 1272 cm−1 are assigned to υCOstretching vibrational modes. The strong band at 1371 cm−1 and 1645 cm−1 belong to asymmetric stretching (υasymC–O2) and symmetric stretching (υsymC–O2) modes of carboxylate group respectively. The two peaks at 1448 cm−1 and 1565 cm−1 represent stretching modes associated with the aromatic rings (υC=C). The broad band at 3419 cm−1 is associated to stretching vibrational modes of water (υOH). The presence of O–H stretching mode confirms that the pristine MOF is hydrated. The vibrations associated with metal linker bond are present below 600 cm−1. These vibrational modes are difficult to observe with IR method. The bonding between Cu-O appears at 491 cm−1. FTIR spectra of amino acid modified Cu MOF show all characteristics peaks of Cu MOF along with the additional peak at 3247 cm−1 due to stretching vibrational modes in N-H group [34] confirming that Cu MOF is successfully functionalized with amino acids. Results of FTIR were further confirmed with Raman spectroscopy
Fig. 2. PXRD pattern of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF (navy), and Cys@Cu MOF (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a) SEM image of Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen. 451
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Fig. 4. (a) SEM image of Gly@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen.
Fig. 5. (a) SEM image of Lys@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen. 452
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Fig. 6. (a) SEM image of Tyr@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen.
Fig. 7. (a) SEM image of Cys@Cu MOF, (b) EDS analysis, (c) Elemental mapping of Copper, (d) Elemental mapping of Carbon, (e) Elemental mapping of Oxygen, (f) Elemental mapping of Nitrogen, (f) Elemental mapping of Sulphur. 453
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Table 1 Zeta potential study of synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF, and Cys@Cu MOF. MOF
Zeta potential (mV)
Cu MOF Gly@Cu MOF Lys@Cu MOF Tyr@Cu MOF Cys@Cu MOF
−1.61 −17.7 −4.87 −19.9 0.42
3.1.4. Zeta potential study Zeta potential study has been done for the characterization of the surface charge of MOF before and after functionalization. Zeta potential of parent MOF was about −1.61 mV at pH 7.4 as shown in Table 1 and it was −17.7 mV, −4.87 mV, −19.9 mV and 0.42 mV for Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF and Cys@Cu MOF, respectively. At pH 7.4, the surface of Cu MOF will be negatively charged because the pH of point of zero charge for Cu MOF is 4.0 and above this pH, the deprotonation of carboxylic group of BTC linker is observed. This results in negative charge at Cu MOF. The results obtained are as per previously reported literature [33]. After functionalization, there is variation in zeta potential of Cu MOF as each amino acid has different charge at pH 7.4 and their interaction with MOF results in an overall charge to the surface.
Fig. 8. FTIR spectra of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF(navy), and Cys@Cu MOF(green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.1.5. Thermal stability study Thermogravimetric analysis (TGA) was performed to check thermal stability of prepared samples. The TGA curve of parent and functionalized Cu MOF are presented in Fig. 10. Two major weight loss steps were observed for synthesized Cu MOF. Firstly, the moisture was removed from MOF till 130 °C. After removal of solvent, a stable plateau was observed up to 310 °C. After 310 °C, predominant weight loss step was observed due to degradation of organic linkers which results in complete collapse of MOF structure. The remaining residue was CuO powder [39]. Thermal studies showed that the synthesized Cu MOF was thermally stable up to 310 °C. Comparing the thermal stability of amino acid functionalized Cu MOF with pristine Cu MOF, in all four samples; the first weight loss due to water is much lower than the parent. This helps to conclude that amino acids replaced the water molecules (present at OMS of Cu MOF) which are basically responsible for catalytic degradation of Cu MOF. This resulted in more hydrophobic nature of functionalized samples [23]. Although amino acid functionalization makes Cu MOF hydrothermally stable but there is decrease in
Fig. 9. Raman spectra of synthesized MOFs, Cu MOF (black), Gly@Cu MOF (blue), Lys@Cu MOF (red),Tyr@Cu MOF (navy), and Cys@Cu MOF (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(Fig. 9). Peaks located at 1612 cm−1 to 1005 cm−1 correspond to the stretching vibrational modes of benzene ring (υ C=C(Ar)). The peaks located at 745 cm−1 and 829 cm−1 represent out-of-plane ring bending (δC=C(Ar)) and out-of-plane C–H bending of the benzene ring (δC- H(Ar)) [35]. Peaks located at 1461 cm−1 and 1545 cm−1 are characteristic peaks of carboxylate group of BTC associated with symmetric (υsymCOO−) and asymmetric stretching (υassymCOO−) modes of carboxylate group respectively. The main peaks of metal linker bonding are present in low frequency region from 100 to 600 cm−1 [36]. Peaks associated with Cu–O stretching mode are observed at 195 cm−1 and 501 cm−1. The peaks at 448 cm−1 and 274 cm−1 are ascribed to Cu-Cu stretching mode (υ Cu-Cu) [37]. Raman spectra of amino acid modified Cu MOF possess all the characteristics peaks of Cu MOF along with some additional peaks of every amino acid. The bands at 448 cm−1, 501 cm−1 and 274 cm−1 got shifted in case of all four amino acids showing interaction between amino acids with metal center of Cu MOF. This observation reveals that amino acids are replacing water molecules from OMS of Cu MOF [23]. The additional peak at 1158 cm−1 in amino acid modified Cu MOFs is associated with N-H stretching vibrational mode [38], confirming the successful modification of Cu MOF by amino acids.
Fig. 10. Thermal stability study of synthesized MOFs, Cu MOF (purple), Gly@Cu MOF (green), Lys@Cu MOF (red),Tyr@Cu MOF (black), and Cys@Cu MOF (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 454
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Fig. 11. Contact angle of synthesized MOFs, (a) Cu MOF, (b) Gly@Cu MOF, (c) Lys@Cu MOF, (d) Tyr@Cu MOF, and (e) Cys@Cu MOF.
in Table 2.
decomposition temperature, compared to the parent Cu MOF. In case of cysteine the decomposition temperature is even lower than 200 °C.
3.1.7. Hydrothermal conditioning Hydrothermal stability of parent Cu MOF and functionalized Cu MOFs was evaluated performing hydrothermal conditioning at high temperature and humid conditions [9]. For this, the samples were placed on filter paper and this filter paper was placed on a beaker filled with boiling water at 85 °C and kept in saturated water vapor for 5 h. After hydrothermal conditioning, the samples were dried at room temperature and SEM was recorded to observe the change in morphology of samples before and after hydrothermal conditioning. The result is shown in Fig. 12. As can be seen, the morphology of glycine, lysine and tyrosine functionalized Cu MOFs remained well preserved after hydrothermal conditioning. But the morphology of pristine Cu MOF and cysteine functionalized Cu MOF got highly damaged. This study concludes that glycine, lysine and tyrosine functionalized Cu MOFs are quite stable than parent Cu MOF and cysteine functionalized Cu MOF is very unstable.
3.1.6. Contact angle study Contact angle measurements are usually done to examine the wettability of various materials and are mainly useful in studying the effects of chemical functionalization on the material wettability. Hydrophilic materials possess contact angle below 90° and hydrophobic materials possess contact angle in between 90° and 150° [40]. Herein, the parent Cu MOF possess contact angle of 39.6° showing hydrophilic behaviour but after functionalization with amino acids, increase in contact angle was observed as shown in Fig. 11. All the four modified MOFs possess contact angle more than parent Cu MOF showing transition behaviour of Cu MOF from hydrophilic to hydrophobic after functionalization. Value of average contact angle of all samples is given Table 2 Average contact angle of synthesized MOFs, Cu MOF, Gly@Cu MOF, Lys@Cu MOF, Tyr@Cu MOF, and Cys@Cu MOF. MOF
Contact angle(º)
Cu MOF Gly@Cu MOF Lys@Cu MOF Tyr@Cu MOF Cys@Cu MOF
39.6 54.0 52.2 46.5 47.0
4. Conclusion In this work, the synthesis and functionalization of Cu MOF with amino acids namely glycine, lysine, tyrosine and cysteine is reported. The synthesis has been performed using post synthetic modification because pre-functionalization of linkers often causes problem to get desired MOF due to instability of most of the functional groups in harsh MOF synthesis conditions, steric hindrances caused by these groups and 455
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Fig. 12. SEM images of synthesized MOFs, (a) Cu MOF, (b) Gly@Cu MOF, (c) Lys@Cu MOF, (d) Tyr@Cu MOF, and (e) Cys@Cu MOF after hydrothermal conditioning.
Acknowledgements
interference in the crystallization process and thus affecting the porosity of MOF. FTIR, Raman and EDS results confirmed that Cu MOF was successfully functionalized with amino acids. PXRD and SEM results demonstrated that crystallinity and morphology of Cu MOF is well preserved after amino acids functionalization in case of glycine, lysine and tyrosine. But in case of cysteine functionalization, the crystalline nature of Cu MOF changed into amorphous. In TGA study, the first weight loss step due to evaporation of water is much lower in modified samples than parent MOF confirming idea of enhanced hydrothermal stability of modified one. The results were further proved with the contact angle measurements and hydrothermal conditioning methods. Out of all the amino acids lysine showed better enhancement in hydrothermal stability as proved from the results of SEM before and after hydrothermal conditioning. The cysteine functionalized Cu MOF showed least hydrothermal stability as shown in PXRD, SEM and TGA results. It strongly affected the morphology of parent Cu MOF also. In view of the above results, the improved performance of the amino acid functionalized Cu MOF offers a great potential for various applications such as sensing, adsorption, storage etc. The modified Cu MOFs can selectively adsorb analyte because of –NH2 and –COOH functionalization resulted from amino acid groups that help in enhanced selectivity, sensitivity in sensing application as well as better stability in aqueous environment.
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Conflicts of interest There are no conflicts of interest to declare.
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