- Email: [email protected]
Facile Green Synthesis of Zirconium Based Metal-Organic Framework having Carboxylic Anchors
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 9 (2019) 522–527
www.materialstoday.com/proceedings
GMSP&NS’18
Facile Green Synthesis of Zirconium Based Metal-Organic Framework having Carboxylic Anchors Madhu N Nimbalkar and Badekai Ramachandra Bhat* Catalysis and Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore - 575025, Karnataka, India
Abstract Highly stable nano porous metal-organic framework having free carboxyl functional group was synthesized by modulated hydrothermal method. The method enunciates simple and efficient greener routes for the synthesis of UiO-66 type metal organic frameworks. The obtained MOF was characterized by Powder XRD, BET surface area, TG and SEM analysis. The SEM analysis shows that the size of the as synthesized MOF was in between 150 nm to 300 nm. From the surface area analysis, the total pore volume and the BET surface area were found to be 0.2692 cm3/g and 505 m2/respectively. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India. Keywords:Green synthesis;Hydrothermal synthesis;metal-organic frameworks;Zirchonium chloride
1. Introduction Since its inception, MOFs enticed scientific community with huge interest. This is because of the possibility of combination of various metal ions along with number of organic ligands resulting in vast structural formation and their application in various fields. This can be evidenced by the increase in number of publications over the past decade [1]. Though MOFs have huge potential in diversified applications, the main drawback lies with their stability, especially in the presences of moisture. Only few of the synthesized MOFs are able to hold the framework integrity in aqueous condition. ___________ * Corresponding author. Tel.: +91-824-2474033; fax: +91-824-2474033 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
523
Nomenclature MOFs BDC-NH2 BETC UiO-66 ATR
metal-organic frameworks 2-aminoterepthalic acid 1,2,4,5-benzenetetracarboxylic acid Universitetet i Oslo Attenuated total reflectance
Synthesis of MOFs was progressed from kinetically controlled methods as compared to their predecessors Zeolites, such as slow evaporation methods at initial stages to present solvothermal synthesis. Solvothermal routes offer robust and rapid formation of framework due to entropy driven hydration process forming M-O-M linkages and clusters which are building blocks of MOFs [2]. It is also found that time along with temperature have leveraging effect on framework stability [3]. By optimizing experimental conditions like temperature and duration of synthesis it is possible to obtain more denser and stable structure. Zirconium based metal-organic frameworks attracted considerable importance due their exceptional chemical and mechanical stability. UiO-66 is the one of the most researched zirconium MOF built from Zr6(O)4(OH)4 nodes and benzene dicarboxylate linkers, which was first synthesized by Cavaka et al [4]. The superior stability of Zirconium MOFs is due the Zr (IV) ion’s high affinity for oxygen donor ligands and maximum coordination number makes it difficult for ligand displacement by keeping framework intact. Functionalization of this highly stable MOF opens wide opportunities in various applications. One such way is to use functionalized ligands during synthesis. Many attempts were made to use greener methods for sustainable synthesis of zirconium based MOFs and ended with poor crystallinity and repeatability [5]. In recent years modulator approach has been used, where ligands having one coordination sites were used during synthesis of MOF which competes with linkers to form complex with metal cations, there by altering the rate of crystal formation [6]. Acetic acid seems to be the best modulating agent as it offers no solubility issue for hydrothermal synthesis. The solubility of the ligands in aqueous condition is the main criteria for the green synthesis of zirconium based MOFs [6]. Even with water soluble ligands it was difficult to obtain crystal solids as the reaction mass turns into amorphous solids [7]. But the presence of water is essential for the formation of UiO-66 type MOFs, as Zr6(O)4(OH)4 structure consists – OH and –O bridges [8]. In fact synthesis with BDC-NH2 ligands requires water to obtain well-ordered single crystal [6]. Yang et al first reported the synthesis of UiO-66(COOH)2 with BETC ligands under aqueous condition without using any modulators[5]. But reproducibility often questioned by many researchers [9]. Ragon et al conducted a comprehensive study on aqueous synthesis of UiO-66 (COOH)2 using high throughput technique with HCl and water mixture at 100 ° C to 150 ° C[7] . It was concluded that equimolar ratio of metal ion and ligand with optimum dosage of HCl/water is necessary for crystal formation. Excess of acid prevents deprotonation of ligands reducing the reaction rates. The rate of crystal formation increases as the temperature increased from 100 ° C - 150 ° C. Hu et al used nitrate salt of Zirconium and acetic acid as modulator along with BETC ligands to obtain nano sized crystalline UiO-66(COOH)2 by hydrothermal method. Zirconium chloride was avoided which might be due to its hygroscopic nature and formation of gel like products [10]. Here, we report modified hydrothermal method for the synthesis of UiO-66(COOH)2 metal-organic framework with 1, 2, 4, 5-benzene tetra carboxylic acid and zirconium chloride as metal source. 2. Materials and Methods 2.1 Materials Zirconium chloride (ZrCl4) (99.5%, Reactor grade) was purchased from Alfa Aesar. 1,2,4,5benzentetracarboxylic acid was purchased from Sigma Aldrich, Acetic acid were supplied by Spectrochem Pvt Ltd, India. All chemicals were used as received without further treatment.
524
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
2.2 Methods The synthesis of UiO-66(COOH)2 was carried out based on literature with a slight modification [9]. In this process, organic ligand (∼5 mmol) and ZrCl4 (∼5 mmol) were suspended in 50 mL of water/acetic acid mixed solvent (30 ml:20 ml), and the reaction mixture was heated to 130 ° C under reflux Condenser for 10 h to yield a powder product. The product was cooled to room temperature and filtered, washed with water and acetone. Then the sample was dried under vacuum at 80 °C for 12 h to yield the final product with a yield of 90% based on the overall weight of ligand and metal salt. 3. Results and Discussion 3.1. PXRD From literature it was found that chloride salt of zirconium was not used along with acetic acid modulator for hydrothermal synthesis. When Zirconium chloride was used in aqueous condition gel like product obtained which needs further treatment to get powder [5]. This is due to the rapid reaction of metal salt with water molecule. In presence of modulator this reaction gets altered, provided the modulator is soluble in reaction condition. As the temperature of the reaction was increased these modulator were replaced by ligand molecule to from framework structure. By combining the outcome of the previous reports it was concluded that at higher concentration of metal ions, under aqueous conditions using acetic acid as modulator and at high temperature facilitates formation of UiO66(COOH)2 crystals. The crystallinity and phase purity were evaluated by powder x-ray diffraction (PXRD).The PXRD patterns( Fig.1.) of as synthesized MOF agree well with pristine UiO-66 with characteristic peaks at 7.4° and 8.5° representing the planes [111] and [220] respectively indicating an isostructural UiO-66 framework topology.
Fig. 1. PXRD spectra of synthesised MOF
3.2. FESEM The morphology of as synthesised UiO-66(COOH)2 has been studied by field-emission scanning electron microscope. Distinctive to the solvothermal synthesis, UiO-66 which has an octahedral crystal shape, the synthesized samples exhibit tiny quasi-spherical particles (Fig. 2.) with a size of 150−300 nm similar to modulated hydrothermal synthesis [9]. This might be attributed to the higher rate of nucleation under water-reflux conditions and the presence of added modulators.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
525
Fig. 2. SEM images of MOF particles
3.3. BET Surface area analysis To evaluate the surface area and porosity, N2 sorption isotherm at 77K were used. The isotherm data indicates a hybrid type I/IV isotherm (Fig.3.) with hysteresis between adsorption and desorption branches. This hybrid kind of isotherm is due to the presence of micro and meso pores in MOF crystal structure. The BET Surface area was found to be 505 m2 g-1, which is slightly higher than the earlier reports [9].
Fig. 3. N2 Adsorption isotherm
3.4. FTIR and TGA The TGA analysis of as synthesised MOF shows three different kind of weight loss (Fig.4.), the loss in the beginning (25 ° C to 100 ° C) was due to the escape of water molecules from the framework. With increase in temperature up to 400 ° C the weight loss is may be due to the egression of free acid present in the pores and finally above 525 ° C, the structure disintegration take place ,but there is no clear indication on gap between later two stages. IR spectra has taken by ATR technique (Fig. 5.), band at 1701 cm-1 represents the free COOH group on the benzene ring while the intense bands at 1593 and 1419 cm-1 are associated with the in- and out of-phase stretching modes of the carboxylate group linked to metal centre.
526
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
Fig. 4. TGA studies of MOF
Fig. 5. IR spectra of MOF.
4. Conclusions Based on previous literature, here we extended the greener method for the hydrothermal synthesis of UiO66 (COOH)2 MOF using easily available Zirconium chloride salt with BETC ligand and acetic acid as modulator. Presence of water soluble acid modulator is necessary for crystal phase formation as it competes with other ligands during framework formation and alters the rate of reaction facilitating crystalline phase formation. Temperature also affects the rate of product formation; optimum temperature for hydrothermal synthesis is 130 ° C. This method is environmentally benign and can be scaled up to multi gram level. Presence of free carboxylic groups in the framework amplifies the properties of MOF for wide range of applications.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
527
Acknowledgements MN is thankful to the National Institute of Technology Karnataka, Surathkal, for providing essential facilities to carry out this research work. References [1] [2] [3] [4]
Long, Jeffrey R., and Omar M. Yaghi, Chem Soc Rev. 38(2009): 1213-1214 Natarajan, Srinivasan, Partha Mahata, and Debajit Sarma, J. Chem. Sci. 124(2012): 339-353. Forster, Paul M., Norbert Stock, and Anthony K. Cheetham,Angew. Chem. Int. Ed. 44 (2005): 7608-7611. Cavka, Jasmina Hafizovic, Søren Jakobsen, Unni Olsbye, Nathalie Guillou, Carlo Lamberti, Silvia Bordiga, and Karl Petter Lillerud,J. Am. Chem. Soc. 13(2008): 13850-13851. [5] Yang, Qingyuan, Sébastien Vaesen, Florence Ragon, Andrew D. Wiersum, Dong Wu, Ana Lago, Thomas Devic, Angewandte Chemie. 125 (2013): 10506-10510. [6] Schaate, Andreas, Pascal Roy, Adelheid Godt, Jann Lippke, Florian Waltz, Michael Wiebcke, and Peter Behrens, Chem. Eur. J.17 (2011): 6643-6651. [7] Ragon, Florence, Betiana Campo, Qingyuan Yang, Charlotte Martineau, Andrew D. Wiersum, Ana Lago, Vincent Guillerm, J. Mater. Chem. A. 3(2015): 3294-3309. [8] Valenzano, Loredana, Bartolomeo Civalleri, Sachin Chavan, Silvia Bordiga, Merete H. Nilsen, Søren Jakobsen, Karl Petter Lillerud, and Carlo Lamberti,Chem. Mater. 23(2011): 1700-1718. [9] Hu, Zhigang, Yongwu Peng, Zixi Kang, Yuhong Qian, and Dan Zhao, Inorg. Chem. 54(2015): 4862-4868. [10] Bueken, Bart, Niels Van Velthoven, Tom Willhammar, Timothée Stassin, Ivo Stassen, David A. Keen, Gino V. Baron, Chem. Sci. 8(2017): 3939-3948.
ScienceDirect Materials Today: Proceedings 9 (2019) 522–527
www.materialstoday.com/proceedings
GMSP&NS’18
Facile Green Synthesis of Zirconium Based Metal-Organic Framework having Carboxylic Anchors Madhu N Nimbalkar and Badekai Ramachandra Bhat* Catalysis and Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore - 575025, Karnataka, India
Abstract Highly stable nano porous metal-organic framework having free carboxyl functional group was synthesized by modulated hydrothermal method. The method enunciates simple and efficient greener routes for the synthesis of UiO-66 type metal organic frameworks. The obtained MOF was characterized by Powder XRD, BET surface area, TG and SEM analysis. The SEM analysis shows that the size of the as synthesized MOF was in between 150 nm to 300 nm. From the surface area analysis, the total pore volume and the BET surface area were found to be 0.2692 cm3/g and 505 m2/respectively. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India. Keywords:Green synthesis;Hydrothermal synthesis;metal-organic frameworks;Zirchonium chloride
1. Introduction Since its inception, MOFs enticed scientific community with huge interest. This is because of the possibility of combination of various metal ions along with number of organic ligands resulting in vast structural formation and their application in various fields. This can be evidenced by the increase in number of publications over the past decade [1]. Though MOFs have huge potential in diversified applications, the main drawback lies with their stability, especially in the presences of moisture. Only few of the synthesized MOFs are able to hold the framework integrity in aqueous condition. ___________ * Corresponding author. Tel.: +91-824-2474033; fax: +91-824-2474033 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
523
Nomenclature MOFs BDC-NH2 BETC UiO-66 ATR
metal-organic frameworks 2-aminoterepthalic acid 1,2,4,5-benzenetetracarboxylic acid Universitetet i Oslo Attenuated total reflectance
Synthesis of MOFs was progressed from kinetically controlled methods as compared to their predecessors Zeolites, such as slow evaporation methods at initial stages to present solvothermal synthesis. Solvothermal routes offer robust and rapid formation of framework due to entropy driven hydration process forming M-O-M linkages and clusters which are building blocks of MOFs [2]. It is also found that time along with temperature have leveraging effect on framework stability [3]. By optimizing experimental conditions like temperature and duration of synthesis it is possible to obtain more denser and stable structure. Zirconium based metal-organic frameworks attracted considerable importance due their exceptional chemical and mechanical stability. UiO-66 is the one of the most researched zirconium MOF built from Zr6(O)4(OH)4 nodes and benzene dicarboxylate linkers, which was first synthesized by Cavaka et al [4]. The superior stability of Zirconium MOFs is due the Zr (IV) ion’s high affinity for oxygen donor ligands and maximum coordination number makes it difficult for ligand displacement by keeping framework intact. Functionalization of this highly stable MOF opens wide opportunities in various applications. One such way is to use functionalized ligands during synthesis. Many attempts were made to use greener methods for sustainable synthesis of zirconium based MOFs and ended with poor crystallinity and repeatability [5]. In recent years modulator approach has been used, where ligands having one coordination sites were used during synthesis of MOF which competes with linkers to form complex with metal cations, there by altering the rate of crystal formation [6]. Acetic acid seems to be the best modulating agent as it offers no solubility issue for hydrothermal synthesis. The solubility of the ligands in aqueous condition is the main criteria for the green synthesis of zirconium based MOFs [6]. Even with water soluble ligands it was difficult to obtain crystal solids as the reaction mass turns into amorphous solids [7]. But the presence of water is essential for the formation of UiO-66 type MOFs, as Zr6(O)4(OH)4 structure consists – OH and –O bridges [8]. In fact synthesis with BDC-NH2 ligands requires water to obtain well-ordered single crystal [6]. Yang et al first reported the synthesis of UiO-66(COOH)2 with BETC ligands under aqueous condition without using any modulators[5]. But reproducibility often questioned by many researchers [9]. Ragon et al conducted a comprehensive study on aqueous synthesis of UiO-66 (COOH)2 using high throughput technique with HCl and water mixture at 100 ° C to 150 ° C[7] . It was concluded that equimolar ratio of metal ion and ligand with optimum dosage of HCl/water is necessary for crystal formation. Excess of acid prevents deprotonation of ligands reducing the reaction rates. The rate of crystal formation increases as the temperature increased from 100 ° C - 150 ° C. Hu et al used nitrate salt of Zirconium and acetic acid as modulator along with BETC ligands to obtain nano sized crystalline UiO-66(COOH)2 by hydrothermal method. Zirconium chloride was avoided which might be due to its hygroscopic nature and formation of gel like products [10]. Here, we report modified hydrothermal method for the synthesis of UiO-66(COOH)2 metal-organic framework with 1, 2, 4, 5-benzene tetra carboxylic acid and zirconium chloride as metal source. 2. Materials and Methods 2.1 Materials Zirconium chloride (ZrCl4) (99.5%, Reactor grade) was purchased from Alfa Aesar. 1,2,4,5benzentetracarboxylic acid was purchased from Sigma Aldrich, Acetic acid were supplied by Spectrochem Pvt Ltd, India. All chemicals were used as received without further treatment.
524
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
2.2 Methods The synthesis of UiO-66(COOH)2 was carried out based on literature with a slight modification [9]. In this process, organic ligand (∼5 mmol) and ZrCl4 (∼5 mmol) were suspended in 50 mL of water/acetic acid mixed solvent (30 ml:20 ml), and the reaction mixture was heated to 130 ° C under reflux Condenser for 10 h to yield a powder product. The product was cooled to room temperature and filtered, washed with water and acetone. Then the sample was dried under vacuum at 80 °C for 12 h to yield the final product with a yield of 90% based on the overall weight of ligand and metal salt. 3. Results and Discussion 3.1. PXRD From literature it was found that chloride salt of zirconium was not used along with acetic acid modulator for hydrothermal synthesis. When Zirconium chloride was used in aqueous condition gel like product obtained which needs further treatment to get powder [5]. This is due to the rapid reaction of metal salt with water molecule. In presence of modulator this reaction gets altered, provided the modulator is soluble in reaction condition. As the temperature of the reaction was increased these modulator were replaced by ligand molecule to from framework structure. By combining the outcome of the previous reports it was concluded that at higher concentration of metal ions, under aqueous conditions using acetic acid as modulator and at high temperature facilitates formation of UiO66(COOH)2 crystals. The crystallinity and phase purity were evaluated by powder x-ray diffraction (PXRD).The PXRD patterns( Fig.1.) of as synthesized MOF agree well with pristine UiO-66 with characteristic peaks at 7.4° and 8.5° representing the planes [111] and [220] respectively indicating an isostructural UiO-66 framework topology.
Fig. 1. PXRD spectra of synthesised MOF
3.2. FESEM The morphology of as synthesised UiO-66(COOH)2 has been studied by field-emission scanning electron microscope. Distinctive to the solvothermal synthesis, UiO-66 which has an octahedral crystal shape, the synthesized samples exhibit tiny quasi-spherical particles (Fig. 2.) with a size of 150−300 nm similar to modulated hydrothermal synthesis [9]. This might be attributed to the higher rate of nucleation under water-reflux conditions and the presence of added modulators.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
525
Fig. 2. SEM images of MOF particles
3.3. BET Surface area analysis To evaluate the surface area and porosity, N2 sorption isotherm at 77K were used. The isotherm data indicates a hybrid type I/IV isotherm (Fig.3.) with hysteresis between adsorption and desorption branches. This hybrid kind of isotherm is due to the presence of micro and meso pores in MOF crystal structure. The BET Surface area was found to be 505 m2 g-1, which is slightly higher than the earlier reports [9].
Fig. 3. N2 Adsorption isotherm
3.4. FTIR and TGA The TGA analysis of as synthesised MOF shows three different kind of weight loss (Fig.4.), the loss in the beginning (25 ° C to 100 ° C) was due to the escape of water molecules from the framework. With increase in temperature up to 400 ° C the weight loss is may be due to the egression of free acid present in the pores and finally above 525 ° C, the structure disintegration take place ,but there is no clear indication on gap between later two stages. IR spectra has taken by ATR technique (Fig. 5.), band at 1701 cm-1 represents the free COOH group on the benzene ring while the intense bands at 1593 and 1419 cm-1 are associated with the in- and out of-phase stretching modes of the carboxylate group linked to metal centre.
526
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
Fig. 4. TGA studies of MOF
Fig. 5. IR spectra of MOF.
4. Conclusions Based on previous literature, here we extended the greener method for the hydrothermal synthesis of UiO66 (COOH)2 MOF using easily available Zirconium chloride salt with BETC ligand and acetic acid as modulator. Presence of water soluble acid modulator is necessary for crystal phase formation as it competes with other ligands during framework formation and alters the rate of reaction facilitating crystalline phase formation. Temperature also affects the rate of product formation; optimum temperature for hydrothermal synthesis is 130 ° C. This method is environmentally benign and can be scaled up to multi gram level. Presence of free carboxylic groups in the framework amplifies the properties of MOF for wide range of applications.
Madhu N Nimbalkar et al. / Materials Today: Proceedings 9 (2019) 522–527
527
Acknowledgements MN is thankful to the National Institute of Technology Karnataka, Surathkal, for providing essential facilities to carry out this research work. References [1] [2] [3] [4]
Long, Jeffrey R., and Omar M. Yaghi, Chem Soc Rev. 38(2009): 1213-1214 Natarajan, Srinivasan, Partha Mahata, and Debajit Sarma, J. Chem. Sci. 124(2012): 339-353. Forster, Paul M., Norbert Stock, and Anthony K. Cheetham,Angew. Chem. Int. Ed. 44 (2005): 7608-7611. Cavka, Jasmina Hafizovic, Søren Jakobsen, Unni Olsbye, Nathalie Guillou, Carlo Lamberti, Silvia Bordiga, and Karl Petter Lillerud,J. Am. Chem. Soc. 13(2008): 13850-13851. [5] Yang, Qingyuan, Sébastien Vaesen, Florence Ragon, Andrew D. Wiersum, Dong Wu, Ana Lago, Thomas Devic, Angewandte Chemie. 125 (2013): 10506-10510. [6] Schaate, Andreas, Pascal Roy, Adelheid Godt, Jann Lippke, Florian Waltz, Michael Wiebcke, and Peter Behrens, Chem. Eur. J.17 (2011): 6643-6651. [7] Ragon, Florence, Betiana Campo, Qingyuan Yang, Charlotte Martineau, Andrew D. Wiersum, Ana Lago, Vincent Guillerm, J. Mater. Chem. A. 3(2015): 3294-3309. [8] Valenzano, Loredana, Bartolomeo Civalleri, Sachin Chavan, Silvia Bordiga, Merete H. Nilsen, Søren Jakobsen, Karl Petter Lillerud, and Carlo Lamberti,Chem. Mater. 23(2011): 1700-1718. [9] Hu, Zhigang, Yongwu Peng, Zixi Kang, Yuhong Qian, and Dan Zhao, Inorg. Chem. 54(2015): 4862-4868. [10] Bueken, Bart, Niels Van Velthoven, Tom Willhammar, Timothée Stassin, Ivo Stassen, David A. Keen, Gino V. Baron, Chem. Sci. 8(2017): 3939-3948.