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Rapid hydrothermal synthesis of MIL-101(Cr) metal–organic framework nanocrystals using expanded graphite as a structure-directing template
Inorganic Chemistry Communications 35 (2013) 265–267
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Rapid hydrothermal synthesis of MIL-101(Cr) metal–organic framework nanocrystals using expanded graphite as a structure-directing template Le-Ting Yang, Ling-Guang Qiu ⁎, Sheng-Mei Hu, Xia Jiang, An-Jian Xie, Yu-Hua Shen Laboratory of Advanced Porous Materials, School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China
a r t i c l e
i n f o
Article history: Received 11 May 2013 Accepted 21 June 2013 Available online 29 June 2013 Keywords: MIL-101(Cr) Expanded graphite Hydrothermal synthesis Rapid synthesis Nanoscale
a b s t r a c t Nanoscale MIL-101(Cr) metal–organic frameworks with giant pores, very large Brunauer–Emmett–Teller surface areas, and small dimensions in the nanoscale regime, were synthesized in high yields by an expanded graphite (EG)-templated hydrothermal strategy. Compared with a conventional hydrothermal method, the size of the MIL-101(Cr) crystals can be well controlled at nanoscale and the reaction time can be largely shortened from 8 to 2 h using such an EG-templated hydrothermal method. The size of the MIL-101(Cr) crystals can be well controlled and the synthesis cycle can be largely shortened for many practical applications. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Remarkable progresses on metal–organic frameworks (MOFs) have been achieved in recent years [1], which are due to the huge porosity and easy tunability of their pore size and shape by changing the connectivity of the inorganic moiety and the nature of the organic linkers [2], and their potential applications in gas storage [3], gas adsorption and separation [4], catalysis [5], sensing [6], and drug delivery [7]. More recently, increased efforts have been devoted to the synthesis of nanoscale metal–organic frameworks (NMOFs) and their potential applications in various fields [8] because miniaturizing MOF crystals down to nanoscale could integrate the functions of nanomaterials into the MOFs without changing the characteristic features of the MOF crystals. Meanwhile, by controlling of the size and shape of NMOFs, the physical/chemical properties of the MOFs can be rationally tailored [19,20]. Although significant progresses of NMOFs have been achieved in recent years, a simple, rapid, environmentally friendly and easy scale-up synthetic method for the fabrication of NMOFs still remains a challenge. Among the numerous MOFs reported so far, MIL-101(Cr) is one of the most widely studied MOF materials [9,10]. MIL-101(Cr) is a very important material due to its mesoporous cage, huge surface area [11], and high hydrothermal/thermal stability, and therefore has been widely studied for adsorption [12], catalysis [13] and drug delivery [11]. To date, MIL-101(Cr) is mainly synthesized by a hydrothermal method, but the reaction requires relatively long times (8 h or more) (see Table S1). From the view of economics and industrial production perspective, rapid synthesis of nanoscale MIL-101(Cr) is ⁎ Corresponding author. Tel./fax: +86 551 5108212. E-mail address: [email protected] (L.-G. Qiu).
crucial not only for fundamental understanding but also for viable applications in industry. Very recently, microwave (MW)-assisted hydrothermal technique has been tried in the synthesis of MIL-101(Cr) to reduce the reaction time and get smaller nanoscale crystals. For example, Zhao et al. reported the MW-assisted synthesis of MIL-101(Cr). MIL-101(Cr) crystals with a Brunauer–Emmett–Teller (BET) specific surface area (SBET) of 3054 m2 g−1 could be obtained in 1 h [14]. Khan et al. have suggested the synthesis of nanosized MIL-101(Cr) with a SBET of 3223 m2 g−1 and yield of 38% by optimizing the reaction conditions like pH levels and water concentrations through a MW-assisted hydrothermal technique [15]. It has clearly been demonstrated that MIL-101(Cr) synthesized by the MW-assisted method shows fast crystallization [16,17] and decreased sizes [18], but low yields [15]. Therefore, more work should be conducted for the contribution of rapid hydrothermal synthesis of MOF nanocrystals. Herein, we describe the rapid hydrothermal synthesis of MIL-101(Cr) MOF nanocrystals by choosing expanded graphite (EG) as a structuredirecting template. EG is a kind of modified graphite which possesses a layered structure with expanded interlayer spacings. It is well known that functional groups such as \OH, and \COOH can be grafted into the graphite sheets after acid and high temperature treatments [19]. This may have a significant effect on the template-directed nucleation and crystallization of MOF crystals [20]. In this work, we illustrate a highly efficient synthesis of nanoscale MIL-101(Cr) crystals using an EG-templated hydrothermal method. The nanosized MIL-101(Cr) crystals with sizes of around 200 nm and relatively higher BET surface areas (up to 3751 m2 g−1) can be produced in fairly good yields (43%) in 2 h, while no product could be obtained in the absence of EG under the
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.06.034
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L.-T. Yang et al. / Inorganic Chemistry Communications 35 (2013) 265–267
Fig. 1. PXRD patterns of typical MIL-101(Cr): simulated MIL-101(Cr); (a) sample A; (b) sample B; (c) sample C and (d) sample D.
same hydrothermal conditions. The results suggest that such a novel kind of EG-templated hydrothermal synthesis route is a highly time- and energy-efficient method for synthesizing nanoscaled MIL-101(Cr) crystals with controllable sizes. The rapid synthesis of nanoscaled MIL-101(Cr) crystals consists in the hydrothermal reaction of H2BDC (166 mg at 1 mmol) with chromium (III) nitrate Cr(NO3)3 · 9H2O (400 mg at 1 mmol), fluorhydric acid (HF at 40%) (0.2 mL at 1 mmol), and H2O (4.8 mL at 265 mmol), in the presence of various amounts of EG (2 mg for sample A, 5 mg for B, 10 mg for C, and 50 mg for D, respectively), and the temperature was set at 493 K for 2 h. After cooling to room temperature, the solid products were recovered by centrifugation. The as-synthesized MIL-101(Cr)
Fig. 2. SEM images of samples A (a), B (b), C (c), and D (d, f) prepared by using the EG-templated hydrothermal synthesis method at 493 K with various EG contents of (a) 0.002 g, (b) 0.005 g, (c) 0.01 g, and (d, f) 0.05 g, respectively, and (e) SEM images of MIL-101(Cr) crystals synthesized by conventional hydrothermal synthesis at 493 K for 8 h.
crystals were further purified by the following two-step processes using hot ethanol and aqueous NH4F solutions, because a significant amount of non-reacted terephthalic acid is present both outside and within the pores of MIL-101(Cr) as described previously [9]. The solvothermal treatment was performed using ethanol at 333 K for 2 h each time, up to four times. The resulting solid was soaked in 1 M of NH4F solution at 343 K for 10 h and immediately centrifuged, and washed with hot water. The solid was finally dried overnight at 423 K under an air atmosphere. Fig. 1 shows the powder X-ray diffraction (PXRD) patterns of MIL-101(Cr) obtained by the EG-templated hydrothermal method in the presence of various EG contents in the reaction mixture. As can be seen from Fig. 1, compared with the simulated MIL-101 XRD patterns from its crystal structural data, most peaks from the resulting MIL-101(Cr) crystals are preserved except the peak at 2θ = 26.6° from EG, suggesting that this pattern is in agreement with those previously reported for MIL-101(Cr). The peak at 26.6° can be attributed to the EG used as the template during the synthesis, and the sharpness and intensity of the peak is increased with an increase of the content of EG but not obvious. Furthermore, the diffraction peak width of the as-synthesized MIL-101(Cr) has a broad trend due to the smaller size of the MIL-101(Cr) crystals in nanoscale obtained in the presence of EG, as further confirmed by scanning electronic microscopy (SEM) results as shown below. To gain a better understanding of the morphology of the assynthesized MOF crystals synthesized by the EG-templated method, the morphologies of the as-synthesized MIL-101(Cr) were studied by SEM, and the results are shown in Fig. 2. It can be seen from SEM images of pure MIL-101(Cr) crystals (see Fig. 2e) that MIL-101(Cr) crystals with an octahedral shape and a size range of about 800 nm were produced by a conventional hydrothermal method. The crystals grow on the surface of EG (see Fig. 2f), which means that EG acts as a structure-directing template and the mechanisms of EG layers as template of crystals growth is proposed in Fig. S1. Although the shape of the MIL-101(Cr) crystals synthesized by the EG-templated hydrothermal method (see Fig. 2a–d) are the same as the samples from the the conventional hydrothermal method, it is obvious that the addition of a small amount of EG resulted in pronounced changes in the size of the MIL-101(Cr) crystals. And the MIL-101(Cr) crystals obtained here have very small dimensions with nanometer sizes as compared with those synthesized by the conventional hydrothermal method. The dimensions of the as-prepared MIL-101(Cr) decrease and become more homogeneous in size with increasing of the EG:Cr(NO3)3 · 9H2O molar ratio from 0.024:1 to 0.6:1, the size range of samples A–D are approximately 500 ± 250, 400 ±
Fig. 3. (a) N2 adsorption–desorption isotherms (P/P0 = 1), at 77 K for the MIL-101(Cr) MOFs A (■), B (●), C (Y) and D (▲), the isotherms for B (●), C (Y) and D (▲) are vertically offset by 300, 1000, and 1350 cm3 g−1, respectively; (b) Distributions of pore diameters obtained using the BJH method; the distributions for B, C, and D are vertically offset by 10, 20, and 30 cm3 g−1, respectively.
L.-T. Yang et al. / Inorganic Chemistry Communications 35 (2013) 265–267
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Table 1 Synthesis conditions and porosity of as-prepared MIL-101 (Cr). Sample
A B C D E a b
Reaction conditiona EG:Cr(NO3)3 · 9H2O:H2BDC:HF:H2O (mol ratio)
Reaction result Heating time
Particle size (nm)b
Yield (%)
SBET (m2 g−1)
Vt (cm3 g−1)
0.024:1:1:1:278 0.06:1:1:1:278 0.12:1:1:1:278 0.6:1:1:1:278 0:1:1:1:278
2 2 2 2 8
500 400 300 200 800
38 43 37 33 35
3182 3751 2196 2247 2962
1.9 1.8 1.2 1.2 1.6
h h h h h
Reaction temperature: 493 K; hydrothermal heating. Particle size: average particle size observed by SEM (error: ~50%).
200, 300 ± 150, and 200 ± 100 nm, respectively. It may be because it was hard to form a larger size since the crystals grow too fast in the presence of a large amount of EG. The results clearly indicate that hydrothermal synthesis by choosing EG as a template agent not only provides an easy way for the fabrication of MOF nanocrystals with controllable crystal sizes, but also may offer a good opportunity for rapid synthesis of MOF nanoparticles with various structures. The porosity properties of the as-prepared MIL-101(Cr) were investigated using N2 adsorption–desorption isotherms (see Fig. 3), and the results are listed in Table 1. SBET of samples A, C and D were determined to be 3182, 2196, and 2247 m2 g−1, respectively, and total pore volumes (Vt) of all the samples varied from 1.2 to 1.9 cm3 g−1; significantly, SBET of sample B was measured to be 3751 m2 g−1. This value surpasses the SBET of most MIL-101(Cr) obtained by the hydrothermal method (see Table S1), as well as those of MIL-101(Cr) crystals prepared by the microwave-assisted method (see Table S1). Although the mechanism is not very clear until now, EG used here may act as a nucleation template, which induces fast kinetics of crystal nucleation and controls the crystal growth of nanosized MIL-101(Cr) crystals because of large amounts of functional groups, such as \OH, and \COOH, grafted into the graphite sheets. In conclusion, we have clearly demonstrated rapid synthesis of nanoscale MIL-101(Cr) MOF crystals in a highly time- and energy-efficient manner. Compared with the conventional hydrothermal method, which commonly requires 8 h at 493 K, MIL-101(Cr) nanocrystals can be facilely prepared in a much shorter reaction time (2 h) by such an EG-templated hydrothermal method. EG, a high thermal conductivity material with functional groups such as \OH, and \COOH that can be grafted into the graphite sheets and porous structure, plays a crucial role in determining the reaction rate and crystal morphology. This may have a significant effect on the template-directed nucleation and crystallization of MOF crystals. The observations made in this investigation obviously provide a new way for rapid preparation of MOF nanocrystals under mild synthetic conditions. Such a templated hydrothermal synthesis method may also be applied to synthesize other MOF-type materials, especially in the case where the hydrothermal method is not very efficient for rapid synthesis of the MOF crystals. It is not very clear yet but it seems that EG acts as a nucleation template, and provides a strong interaction between the chromium octahedral motif and the effective nuclei formation of MIL-101(Cr) during the hydrothermal reaction. Rapid syntheses of other MOF-type materials and investigations on the precise role of EG in the synthesis of MOF nanocrystals are in progress. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC, 20971001), the NSFC-CAS Joint Fund for Research Based on Large-Scale Scientific Facilities (10979014), the Program for New Century Excellent Talents in University, Ministry of Education, China (NCET-08-0617), and the National “211 Project” of Anhui University.
Appendix A. Supplementary material Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.inoche.2013.06.034. References [1] G. Férey, Hybrid porous solids: past, present, future, Chem. Soc. Rev. 37 (2008) 191–214. [2] O.M. Yaghi, M. O'Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular synthesis and the design of new materials, Nature 423 (2003) 705–714. [3] Z.J. Zhang, Y.G. Zhao, Q.H. Gong, Z. Li, J. Li, MOFs for CO2 capture and separation from flue gas mixtures: the effect of multifunctional sites on their adsorption capacity and selectivity, Chem. Commun. 49 (2013) 653–661. [4] C.M. Brown, Y. Liu, T. Yildirim, V.K. Peterson, C.J. Kepert, Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study, Nanotechnology 20 (2009) 20. [5] H.H. Zhao, H.L. Song, L.J. Chou, Nickel nanoparticles supported on MOF-5: synthesis and catalytic hydrogenation properties, Inorg. Chem. Commun. 15 (2012) 261–265. [6] (a) Z. Jin, H.Y. Zhao, D.J. Yang, X.D. Yao, G.S. Zhu, A novel 3D porous cadmium(II) MOF based on conjugated ligand with potential application for sensing small linear conjugated molecule, Chem. Commun. 25 (2012) 74–78; (b) B.L. Chen, L.B. Wang, Y.Q. Xiao, F.R. Fronczek, M. Xue, Y.J. Cui, G.D. Qian, A luminescent metal-organic framework with lewis basic pyridyl sites for the sensing of metal ions, Angew. Chem. Int. Ed. 48 (2009) 500–503. [7] K.M.L. Taylor-Pashow, J. Della Rocca, Z.G. Xie, S. Tran, W.B. Lin, Postsynthetic modifications of iron-carboxylate nanoscale metal–organic frameworks for imaging and drug delivery, J. Am. Chem. Soc. 131 (2009) 14261–14263. [8] Z.Q. Li, L.G. Qiu, T. Xu, Y. Wu, W. Wang, Z.Y. Wu, X. Jiang, Ultrasonic synthesis of the microporous metal–organic framework Cu3(BTC)2 at ambient temperature and pressure: an efficient and environmentally friendly method, Mater. Lett. 63 (2009) 78–80. [9] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Mirgiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040–2042. [10] D.Y. Hong, Y.K. Hwang, C. Serre, G. Férey, J.S. Chang, Porous chromium terephthalate MIL-101 with coordinatively unsaturated sites: surface functionalization, encapsulation, sorption and catalysis, Adv. Funct. Mater. 19 (2009) 1537–1552. [11] P. Horcajada, C. Serre, M. Vallet-Regi, M. Sebban, F. Taulelle, G. Férey, Metal–organic frameworks as efficient materials for drug delivery, Angew. Chem. Int. Ed. 45 (2006) 5974–5978. [12] M. Latroche, S. Surblé, C. Serre, C. Mellot-Draznieks, P.L. Llewellyn, J.H. Lee, J.S. Chang, S.H. Jhung, G. Férey, Hydrogen storage in the giant-pore metal–organic frameworks MIL-100 and MIL-101, Angew. Chem. Int. Ed. 45 (2006) 8227–8231. [13] Y.K. Hwang, D.Y. Hong, J.S. Chang, S.H. Jhung, Y.K. Seo, J. Kim, A. Vimony, M. Daturi, C. Serre, G. Férey, Amine grafting on coordinatively unsaturated metal centers of MOFs: consequences for catalysis and metal encapsulation, Angew. Chem. Int. Ed. 47 (2008) 4144–4148. [14] Z.X. Zhao, X.M. Li, S.S. Huang, Q.B. Xia, Z. Li, Adsorption and diffusion of benzene on chromium-based metal organic framework MIL-101 synthesized by microwave irradiation, Ind. Eng. Chem. Res. 50 (2011) 22542261. [15] N.A. Khan, I.J. Kang, H.Y. Seok, S.H. Jhung, Facile synthesis of nano-sized metalorganic frameworks, chromium-benzenedicarboxylate, MIL-101, Chem. Eng. J. 166 (2011) 1152–1157. [16] S.H. Jhung, J.H. Lee, J.S. Chang, Microwave synthesis of a nanoporous hybrid material, chromium trimesate, Bull. Kor. Chem. Soc. 26 (2005) 880–881. [17] Z. Ni, R.I. Masel, Rapid production of metal–organic frameworks via microwaveassisted solvothermal synthesis, J. Am. Chem. Soc. 128 (2006) 12394–12395. [18] W. Lin, W.J. Rieter, K.M.L. Taylor, Modular synthesis of functional nanoscale coordination polymers, Angew. Chem. Int. Ed. 48 (2009) 650–658. [19] M. Jahan, Q.L. Bao, J.X. Yang, K.P. Loh, Structure-directing role of graphene in the synthesis of metal–organic framework nanowire, J. Am. Chem. Soc. 132 (2010) 14487–14495. [20] B. Liu, H. Shioyama, T. Akita, Q. Xu, Metal–organic framework as a template for porous carbon synthesis, J. Am. Chem. Soc. 130 (2008) 5390–5391.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Rapid hydrothermal synthesis of MIL-101(Cr) metal–organic framework nanocrystals using expanded graphite as a structure-directing template Le-Ting Yang, Ling-Guang Qiu ⁎, Sheng-Mei Hu, Xia Jiang, An-Jian Xie, Yu-Hua Shen Laboratory of Advanced Porous Materials, School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China
a r t i c l e
i n f o
Article history: Received 11 May 2013 Accepted 21 June 2013 Available online 29 June 2013 Keywords: MIL-101(Cr) Expanded graphite Hydrothermal synthesis Rapid synthesis Nanoscale
a b s t r a c t Nanoscale MIL-101(Cr) metal–organic frameworks with giant pores, very large Brunauer–Emmett–Teller surface areas, and small dimensions in the nanoscale regime, were synthesized in high yields by an expanded graphite (EG)-templated hydrothermal strategy. Compared with a conventional hydrothermal method, the size of the MIL-101(Cr) crystals can be well controlled at nanoscale and the reaction time can be largely shortened from 8 to 2 h using such an EG-templated hydrothermal method. The size of the MIL-101(Cr) crystals can be well controlled and the synthesis cycle can be largely shortened for many practical applications. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Remarkable progresses on metal–organic frameworks (MOFs) have been achieved in recent years [1], which are due to the huge porosity and easy tunability of their pore size and shape by changing the connectivity of the inorganic moiety and the nature of the organic linkers [2], and their potential applications in gas storage [3], gas adsorption and separation [4], catalysis [5], sensing [6], and drug delivery [7]. More recently, increased efforts have been devoted to the synthesis of nanoscale metal–organic frameworks (NMOFs) and their potential applications in various fields [8] because miniaturizing MOF crystals down to nanoscale could integrate the functions of nanomaterials into the MOFs without changing the characteristic features of the MOF crystals. Meanwhile, by controlling of the size and shape of NMOFs, the physical/chemical properties of the MOFs can be rationally tailored [19,20]. Although significant progresses of NMOFs have been achieved in recent years, a simple, rapid, environmentally friendly and easy scale-up synthetic method for the fabrication of NMOFs still remains a challenge. Among the numerous MOFs reported so far, MIL-101(Cr) is one of the most widely studied MOF materials [9,10]. MIL-101(Cr) is a very important material due to its mesoporous cage, huge surface area [11], and high hydrothermal/thermal stability, and therefore has been widely studied for adsorption [12], catalysis [13] and drug delivery [11]. To date, MIL-101(Cr) is mainly synthesized by a hydrothermal method, but the reaction requires relatively long times (8 h or more) (see Table S1). From the view of economics and industrial production perspective, rapid synthesis of nanoscale MIL-101(Cr) is ⁎ Corresponding author. Tel./fax: +86 551 5108212. E-mail address: [email protected] (L.-G. Qiu).
crucial not only for fundamental understanding but also for viable applications in industry. Very recently, microwave (MW)-assisted hydrothermal technique has been tried in the synthesis of MIL-101(Cr) to reduce the reaction time and get smaller nanoscale crystals. For example, Zhao et al. reported the MW-assisted synthesis of MIL-101(Cr). MIL-101(Cr) crystals with a Brunauer–Emmett–Teller (BET) specific surface area (SBET) of 3054 m2 g−1 could be obtained in 1 h [14]. Khan et al. have suggested the synthesis of nanosized MIL-101(Cr) with a SBET of 3223 m2 g−1 and yield of 38% by optimizing the reaction conditions like pH levels and water concentrations through a MW-assisted hydrothermal technique [15]. It has clearly been demonstrated that MIL-101(Cr) synthesized by the MW-assisted method shows fast crystallization [16,17] and decreased sizes [18], but low yields [15]. Therefore, more work should be conducted for the contribution of rapid hydrothermal synthesis of MOF nanocrystals. Herein, we describe the rapid hydrothermal synthesis of MIL-101(Cr) MOF nanocrystals by choosing expanded graphite (EG) as a structuredirecting template. EG is a kind of modified graphite which possesses a layered structure with expanded interlayer spacings. It is well known that functional groups such as \OH, and \COOH can be grafted into the graphite sheets after acid and high temperature treatments [19]. This may have a significant effect on the template-directed nucleation and crystallization of MOF crystals [20]. In this work, we illustrate a highly efficient synthesis of nanoscale MIL-101(Cr) crystals using an EG-templated hydrothermal method. The nanosized MIL-101(Cr) crystals with sizes of around 200 nm and relatively higher BET surface areas (up to 3751 m2 g−1) can be produced in fairly good yields (43%) in 2 h, while no product could be obtained in the absence of EG under the
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.06.034
266
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Fig. 1. PXRD patterns of typical MIL-101(Cr): simulated MIL-101(Cr); (a) sample A; (b) sample B; (c) sample C and (d) sample D.
same hydrothermal conditions. The results suggest that such a novel kind of EG-templated hydrothermal synthesis route is a highly time- and energy-efficient method for synthesizing nanoscaled MIL-101(Cr) crystals with controllable sizes. The rapid synthesis of nanoscaled MIL-101(Cr) crystals consists in the hydrothermal reaction of H2BDC (166 mg at 1 mmol) with chromium (III) nitrate Cr(NO3)3 · 9H2O (400 mg at 1 mmol), fluorhydric acid (HF at 40%) (0.2 mL at 1 mmol), and H2O (4.8 mL at 265 mmol), in the presence of various amounts of EG (2 mg for sample A, 5 mg for B, 10 mg for C, and 50 mg for D, respectively), and the temperature was set at 493 K for 2 h. After cooling to room temperature, the solid products were recovered by centrifugation. The as-synthesized MIL-101(Cr)
Fig. 2. SEM images of samples A (a), B (b), C (c), and D (d, f) prepared by using the EG-templated hydrothermal synthesis method at 493 K with various EG contents of (a) 0.002 g, (b) 0.005 g, (c) 0.01 g, and (d, f) 0.05 g, respectively, and (e) SEM images of MIL-101(Cr) crystals synthesized by conventional hydrothermal synthesis at 493 K for 8 h.
crystals were further purified by the following two-step processes using hot ethanol and aqueous NH4F solutions, because a significant amount of non-reacted terephthalic acid is present both outside and within the pores of MIL-101(Cr) as described previously [9]. The solvothermal treatment was performed using ethanol at 333 K for 2 h each time, up to four times. The resulting solid was soaked in 1 M of NH4F solution at 343 K for 10 h and immediately centrifuged, and washed with hot water. The solid was finally dried overnight at 423 K under an air atmosphere. Fig. 1 shows the powder X-ray diffraction (PXRD) patterns of MIL-101(Cr) obtained by the EG-templated hydrothermal method in the presence of various EG contents in the reaction mixture. As can be seen from Fig. 1, compared with the simulated MIL-101 XRD patterns from its crystal structural data, most peaks from the resulting MIL-101(Cr) crystals are preserved except the peak at 2θ = 26.6° from EG, suggesting that this pattern is in agreement with those previously reported for MIL-101(Cr). The peak at 26.6° can be attributed to the EG used as the template during the synthesis, and the sharpness and intensity of the peak is increased with an increase of the content of EG but not obvious. Furthermore, the diffraction peak width of the as-synthesized MIL-101(Cr) has a broad trend due to the smaller size of the MIL-101(Cr) crystals in nanoscale obtained in the presence of EG, as further confirmed by scanning electronic microscopy (SEM) results as shown below. To gain a better understanding of the morphology of the assynthesized MOF crystals synthesized by the EG-templated method, the morphologies of the as-synthesized MIL-101(Cr) were studied by SEM, and the results are shown in Fig. 2. It can be seen from SEM images of pure MIL-101(Cr) crystals (see Fig. 2e) that MIL-101(Cr) crystals with an octahedral shape and a size range of about 800 nm were produced by a conventional hydrothermal method. The crystals grow on the surface of EG (see Fig. 2f), which means that EG acts as a structure-directing template and the mechanisms of EG layers as template of crystals growth is proposed in Fig. S1. Although the shape of the MIL-101(Cr) crystals synthesized by the EG-templated hydrothermal method (see Fig. 2a–d) are the same as the samples from the the conventional hydrothermal method, it is obvious that the addition of a small amount of EG resulted in pronounced changes in the size of the MIL-101(Cr) crystals. And the MIL-101(Cr) crystals obtained here have very small dimensions with nanometer sizes as compared with those synthesized by the conventional hydrothermal method. The dimensions of the as-prepared MIL-101(Cr) decrease and become more homogeneous in size with increasing of the EG:Cr(NO3)3 · 9H2O molar ratio from 0.024:1 to 0.6:1, the size range of samples A–D are approximately 500 ± 250, 400 ±
Fig. 3. (a) N2 adsorption–desorption isotherms (P/P0 = 1), at 77 K for the MIL-101(Cr) MOFs A (■), B (●), C (Y) and D (▲), the isotherms for B (●), C (Y) and D (▲) are vertically offset by 300, 1000, and 1350 cm3 g−1, respectively; (b) Distributions of pore diameters obtained using the BJH method; the distributions for B, C, and D are vertically offset by 10, 20, and 30 cm3 g−1, respectively.
L.-T. Yang et al. / Inorganic Chemistry Communications 35 (2013) 265–267
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Table 1 Synthesis conditions and porosity of as-prepared MIL-101 (Cr). Sample
A B C D E a b
Reaction conditiona EG:Cr(NO3)3 · 9H2O:H2BDC:HF:H2O (mol ratio)
Reaction result Heating time
Particle size (nm)b
Yield (%)
SBET (m2 g−1)
Vt (cm3 g−1)
0.024:1:1:1:278 0.06:1:1:1:278 0.12:1:1:1:278 0.6:1:1:1:278 0:1:1:1:278
2 2 2 2 8
500 400 300 200 800
38 43 37 33 35
3182 3751 2196 2247 2962
1.9 1.8 1.2 1.2 1.6
h h h h h
Reaction temperature: 493 K; hydrothermal heating. Particle size: average particle size observed by SEM (error: ~50%).
200, 300 ± 150, and 200 ± 100 nm, respectively. It may be because it was hard to form a larger size since the crystals grow too fast in the presence of a large amount of EG. The results clearly indicate that hydrothermal synthesis by choosing EG as a template agent not only provides an easy way for the fabrication of MOF nanocrystals with controllable crystal sizes, but also may offer a good opportunity for rapid synthesis of MOF nanoparticles with various structures. The porosity properties of the as-prepared MIL-101(Cr) were investigated using N2 adsorption–desorption isotherms (see Fig. 3), and the results are listed in Table 1. SBET of samples A, C and D were determined to be 3182, 2196, and 2247 m2 g−1, respectively, and total pore volumes (Vt) of all the samples varied from 1.2 to 1.9 cm3 g−1; significantly, SBET of sample B was measured to be 3751 m2 g−1. This value surpasses the SBET of most MIL-101(Cr) obtained by the hydrothermal method (see Table S1), as well as those of MIL-101(Cr) crystals prepared by the microwave-assisted method (see Table S1). Although the mechanism is not very clear until now, EG used here may act as a nucleation template, which induces fast kinetics of crystal nucleation and controls the crystal growth of nanosized MIL-101(Cr) crystals because of large amounts of functional groups, such as \OH, and \COOH, grafted into the graphite sheets. In conclusion, we have clearly demonstrated rapid synthesis of nanoscale MIL-101(Cr) MOF crystals in a highly time- and energy-efficient manner. Compared with the conventional hydrothermal method, which commonly requires 8 h at 493 K, MIL-101(Cr) nanocrystals can be facilely prepared in a much shorter reaction time (2 h) by such an EG-templated hydrothermal method. EG, a high thermal conductivity material with functional groups such as \OH, and \COOH that can be grafted into the graphite sheets and porous structure, plays a crucial role in determining the reaction rate and crystal morphology. This may have a significant effect on the template-directed nucleation and crystallization of MOF crystals. The observations made in this investigation obviously provide a new way for rapid preparation of MOF nanocrystals under mild synthetic conditions. Such a templated hydrothermal synthesis method may also be applied to synthesize other MOF-type materials, especially in the case where the hydrothermal method is not very efficient for rapid synthesis of the MOF crystals. It is not very clear yet but it seems that EG acts as a nucleation template, and provides a strong interaction between the chromium octahedral motif and the effective nuclei formation of MIL-101(Cr) during the hydrothermal reaction. Rapid syntheses of other MOF-type materials and investigations on the precise role of EG in the synthesis of MOF nanocrystals are in progress. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC, 20971001), the NSFC-CAS Joint Fund for Research Based on Large-Scale Scientific Facilities (10979014), the Program for New Century Excellent Talents in University, Ministry of Education, China (NCET-08-0617), and the National “211 Project” of Anhui University.
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