Phase Transfer Directed Synthesis of Hollow Zeolitic Imidazolate

Communication pubs.acs.org/crystal

Phase Transfer Directed Synthesis of Hollow Zeolitic Imidazolate Frameworks-67 Nanocages Bo Yu,†,‡ Dejiang Zhang,†,‡ Shichao Du,†,‡ Yan Wang,†,‡ Mingyang Chen,†,‡ Jie Hou,†,‡ Shijie Xu,†,‡ Songgu Wu,†,‡ and Junbo Gong*,†,‡ †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R. China ‡ Collaborative Innovation Center of Chemistry Science and Engineering, Tianjin 300072, P.R. China S Supporting Information *

ABSTRACT: Cubic zeolitic imidazolate frameworks (ZIFs) nanocage was prepared by using a multitemplate and phase transfer directed method. Phase transfer of aqueous metal ions on the interface of cubic Cu2O template was achieved by coordinating etching and precipitating process, in which cubic morphology was preserved and hollow metal oxide nanocages were obtained due to the removal of templates. Therefore, cubic ZIFs nanocages could be formed by the transformation of metal oxide cages, which was a self-template transforming process. The former metal oxide nanocages will not only be the template, but also the metal source for the construction of ZIFs as well. This phase transfer directed method will offer a further development of porous metal−organic frameworks with functional hollow structure.

M

acid) were concentrated to modify the surface of templates.10,11,13,19−21 These groups and bridging agents will contribute to the self-assembly of MOFs by step-by-step trapping metal cations and organic ligands in turn from aqueous solution. Unfortunately, the thickness of the MOF shell will only be tuned by varying the assembly cycle number using such a step-by-step assembly method,11,13,19 which finally results in complex operations. Herein, we report a novel phase transfer directed method using Cu2O as the cubic template to prepare hollow porous zeolitic imidazolate frameworks (ZIFs) materials, which are a class of typically porous zeolitic-like MOFs with high chemical and thermal stabilities as well as tunable network topologies analogous to those in zeolites.22,23 This phase transfer directed method includes two stages. The former one is the phase transfer of metal cations from aqueous phase to hydroxide phase. The other following one is the transformation of metal hydroxides to porous ZIFs, which can be considered as the phase transfer of organic ligands from aqueous phase to ZIFs phase as well. Therefore, the generation of hollow ZIFs nanocages could be achieved by pH directed formation of hollow metal hydroxide nanocages followed by self-template synthesis of porous ZIFs (Figure 1a). In the case of this work, Cu2O nanoparticles were used as the sacrificial template due to the various crystal morphology, especially nonspherical shapes such as cubes, octahedra, and

etal−organic frameworks (MOFs) are a new class of porous crystalline materials with network topologies consisting of metal building blocks and organic ligands.1 This kind of porous framework attempts to attract considerable attention in gas storage,2 selective separation,3−5 catalysis,6,7 and even drug delivery.8,9 In order to expand and reinforce the utilization of MOFs, previous studies have concentrated on the fabrication of porous MOFs materials with various forms including hollow,10,11 core−shell,12,13 and yolk−shell14,15 forms. However, the formation of these special forms always requires a sacrificial template for expected cubic, spherical, rodshaped, or other morphological particles of porous materials. The considerable difficulty is how to promote the nucleation and growth of MOFs on the surface of templates. Generally, the methods that have been reported can be divided into two main categories. The first one is a self-template method, which is the on-site generation of MOFs shell on metal oxide templates by the organic acid ligand etching process. Cu or Zn metal oxides are always considered as templates attributed to their various particle shape and higher activity related to organic acid ligands. The selected metal oxides are not only as the template but also as the metal source of building blocks in MOFs. Therefore, MOFs obtained via self-template method are still limited by the metal source of the template (ZnO for ZnMOFs, Cu2O, or CuO for Cu-MOFs).12,16−18 The other one is the step-by-step method, which unlocks the limited metal source from templates and allows nonmetallic materials to be used as the sacrificial template. In order to improve the affinity between templates and MOFs, special chemical groups (−SO3H and −COOH) and bridging agents (mercaptoacetic © XXXX American Chemical Society

Received: September 21, 2016 Revised: November 15, 2016 Published: November 22, 2016 A

DOI: 10.1021/acs.cgd.6b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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was employed as the coordinating etchant and the chemical route could be described as follows Cu 2O + xS2 O32 − + H 2O → [Cu 2(S2O3)x ]2 − 2x + 2OH− (1)

S2 O3

2−

+ H 2O ⇌

HS2 O3−

+ OH

−

(2)

During this coordinating etching process, Cu2O templates will be coordinating dissolved by the involved S2O32− forming soluble [Cu2(S2O3)x]2−2x complexes, and the OH− released by the etching process can induce the precipitation of metal cations from the aqueous solution. Considering that ZIF-67 consists of tetrahedral Co(II) metal centers and imidazolate bridging ligands, Co(II) was involved in the coordinating etching process. After red colored Cu2O templates were completely etched and transformed to soluble complexes, green colored Co(OH)2 could be easily distinguished (Figure 1b). It is found that Co(OH)2 was precipitated and aggregated on the surface of templates, and the cubic morphology of nanoparticles was well preserved (Figure 2c). When the coordinating etching process starts from the surface of Cu2O nanocrystals after the addition of Na2S2O3, OH− will be accumulated and released from the solid−liquid interface. Therefore, the interface provides a microenvironment where the local concentration of OH− or pH is the highest. This special distribution of pH or OH− concentration improves the affinity between the Cu2O template and free aqueous Co(II) cations. The pH gradient is significant as a virtual force field trapping and precipitating aqueous Co(II) to Co(OH)2 on the template. Then, core−shell structural Cu2O@Co(OH)2 nanocrystals will be obtained, while S2O32− still diffuses deep inside to etch the Cu2O core. Xray diffusion (XRD) result suggests that all characteristic peaks of Cu2O nanocrystals at 29.5, 36.4, 42.3, 61.3, 73.5, and 77.3 degrees (JCPDS No. 05−0667, the all-range XRD spectrum of Cu 2 O could be obtained in Supporting Information) disappeared in the pattern of synthesized Co(OH)2 (Figure S1) and ZIF-67 (Figure 3) nanoparticles. In order to further understand the phase transfer process, the ICP test is conducted to trace the aqueous Co, Cu, and S during the addition of Na2S2O3 into the solution (Figure 3b). The decreasing concentration of Co suggests precipitation of Co(OH)2 with the addition of Na2S2O3, while the increasing concentration of Cu is attributed to the coordinating etching process. XRD and ICP results indicate a dissolution process of Cu2O templates during the precipitation of Co(OH)2. Therefore, the remaining cubic nanoparticles should be the Co(OH)2 shell formed by the phase transfer, and show an obviously hollow structure (Figure 2d). The transformation of hollow Co(OH)2 cubic nanocages to porous ZIF-67 was achieved by a self-templating method12,17 (Figure 1a). Cubic Co(OH)2 nanocages are considered not only as the template but the Co(II) source as well for the generation of porous ZIF-67. XRD result shows that all characteristic peaks of hollow ZIF-67 were in good agreement with those of ZIF-67 nanocrystals formed by a typical hydrothermal method (Figure 3), and both of them suggest a SOD zeolitic topological net framework.27 Prospectively, the porous ZIF-67 transformed from cubic Co(OH)2 still maintained the cubic morphology (Figure 4a and b), and the hollow shell structure could be obvious under the highmagnification transmission electron microscopy (TEM) analysis (Figure 4c and d). However, it is found that the thickness of the ZIFs shell significantly increased compared with the

Figure 1. (a) Synthetic scheme of hollow ZIF-67 nanocages from cubic Cu2O and hollow Co(OH)2 templates. (b) Color changes form red Cu2O to green Co(OH)2 and then to purple ZIF-67.

other highly symmetrical structures. Cubic Cu2O nanocrystals were obtained by the typical reduction method, and they showed red color (Figure 1b) and 500−800 nm cubic morphology (Figure 2a and b). Generally, the fabrication

Figure 2. SEM images of cubic Cu2O nanocrystals (a,b) and hollow Co(OH)2 nanocages (c,d).

process of hollow materials always consists of on-site generation of shell materials and removal of templates followed behind.10,11,14 However, in this phase transfer process, the synthetic strategy is designed based on the interaction and association of accurately controlled hydrolysis of aqueous metal cations and etching of Cu2O templates. According to Pearson’s hard and soft acid−base principle,24 the insoluble Cu2O nanocrystals will coordinate with some soft base ligands (CN−, SCN−, S2O32−, etc.) forming soluble complexes in aqueous solution.25,26 Therefore, sodium thiosulfate (Na2S2O3) B

DOI: 10.1021/acs.cgd.6b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. (a) XRD results of Cu2O template and ZIF-67. (b) Tracing of aqueous Co and Cu during the addition of Na2S2O3.

Compared with other functional materials, ZIFs have attracted considerable attention due to the high thermal stabilities and surface area by tunable network topologies. Thermogravimetric (TG) analysis shows a weight loss of 7% from 20 to 200 °C, corresponding to the release of solvent guest molecules in the porous structure (Figure S3). However, there was no significant weight loss observed between 200 to 450 °C, indicating a high chemical and thermal stability under the given condition. N2 adsorption capacity of hollow ZIF-67 and hydrothermal ZIF-67 were 17.8 mmol g−1 (399 cm3 g−1) and 15.8 mmol g−1 (354 cm3 g−1) at 77 K, respectively (Figure 5), which was similar to Co-ZIFs (SOD) reported in previous

Figure 4. SEM (a,b) and high-magnification TEM (c,d) images of cubic ZIF-67 nanocages.

hollow Co(OH)2 shell (Figure S2). This indicates that The involved organic ligands, 2-methylimidazole, will associate with Co(OH)2 and coordinate with the dissolved Co(II) released from hydroxides to form ZIF-67 shell. Porous ZIFs form a coating on the aggregated Co(OH)2 nanoparticles, and the Co(OH)2@ZIF-67 composites will be considered as the intermediate.12,17 During encapsulation of Co(OH)2 by porous ZIF-67, a typical self-template process will dominate the development of ZIFs. Organic ligands are able to diffuse through the pores of ZIF shells and reach the surface of Co(OH)2 cores where they continued to coordinate with the dissolved Co(II) to form ZIF-67. Meanwhile, the dissolved Co(II) will migrate from the inner core and get recrystallized or self-assembled outward at the ZIF-67 shell, which is similar to the Ostwald ripening process forming hollow spherical metal− or covalent−organic frameworks.28,29 Therefore, the morphology of synthesized ZIF-67 is preserved on the basis of Co(OH)2 nanocages, and the shell thickness of hollow cubic ZIF-67 nanocages could only be controlled by the thickness of the hollow Co(OH)2 shell before the complete transformation. Given that various metal hydroxides could be obtained by coordinating etching and precipitating processes,25,26 this method will be further developed for fabrication of other porous MOFs as well.

Figure 5. N2 adsorption capacity of hollow ZIF-67 nanocages and hydrothermal ZIF-67 nanocrystals at 77 K.

study.27 Therefore, the BET method (Brunauer−Emmett− Teller) surface area and pore volume of hollow ZIF-67 were calculated as 1151 m2 g−1 and 0.615 cm3 g−1, respectively, which were higher than those of hydrothermal ZIF-67 (1029 m2 g−1 and 0. 548 cm3 g−1). In summary, we report a novel phase transfer directed method to prepare hollow porous ZIF-67 nanocages. The synthesized hollow ZIF-67 nanocages show cubic morphology, high thermal stability, and high BET surface area. We believe that this method is not only designed for hollow porous ZIFs, but suitable for yolk−shell structural functional materials (such as Au or Ag or Pd metal@MOFs) as well, opening a new functional encapsulation route by porous MOFs materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01392. C

DOI: 10.1021/acs.cgd.6b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(23) Bennett, T. D.; Keen, D. A.; Tan, J. C.; Barney, E. R.; Goodwin, A. L.; Cheetham, A. K. Angew. Chem., Int. Ed. 2011, 50, 3067−3071. (24) Pearson, R. G. Hard and soft acids and bases; Dowden, Hutchinson & Ross, 1973; pp 1−52. (25) Wang, Z.; Luan, D.; Boey, F. Y.; Lou, X. W. J. Am. Chem. Soc. 2011, 133, 4738−4741. (26) Nai, J.; Tian, Y.; Guan, X.; Guo, L. J. Am. Chem. Soc. 2013, 135, 16082−16091. (27) Biswal, B. P.; Panda, T.; Banerjee, R. Chem. Commun. 2012, 48, 11868−70. (28) Kandambeth, S.; Venkatesh, V.; Shinde, D. B.; Kumari, S.; Halder, A.; Verma, S.; Banerjee, R. Nat. Commun. 2015, 6, 6786. (29) Lee, I.; Choi, S.; Lee, H. J.; Oh, M. Cryst. Growth Des. 2015, 15, 5169−5173.

Experimental section, XRD results, TEM and SEM images, and thermogravimetric analysis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junbo Gong: 0000-0002-3376-3296 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly acknowledge the financial support of National Natural Science Foundation of China (No. 81361140344 and No. 21376164), National 863 Program (No. 2015AA021002), Major National Scientific Instrument Development Project (No. 21527812), and Natural Science Foundation of Tianjin (No. 15JCZDJC33200).

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DOI: 10.1021/acs.cgd.6b01392 Cryst. Growth Des. XXXX, XXX, XXX−XXX