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Mesoporous Metal–Organic Frameworks with Size-, Shape-, and Space-Distribution-Controlled Pore Structure Weina Zhang, Yayuan Liu, Guang Lu, Yong Wang, Shaozhou Li, Chenlong Cui, Jin Wu, Zhiling Xu, Danbi Tian, Wei Huang, Joseph S. DuCheneu, W. David Wei, Hongyu Chen, Yanhui Yang, and Fengwei Huo* Porous materials, such as silica,[1] carbon,[2] and zeolites,[3] can interact with atoms, ions, molecules, or even larger guest species not only at external surfaces but also throughout the whole internal pore system, and therefore, are regarded as powerful tools for many important applications. Metal–organic frameworks (MOFs),[4] a new member of porous crystalline materials, have been attracting considerable attention as promising candidates for gas storage,[5] gas separation,[6] chemical sensing,[7] heterogeneous catalysis,[8] etc. Sizes, shapes, and chemical environments of the cages or channels within MOFs are of paramount importance for the above-mentioned applications, in which large surface areas, high free volumes, and structure functionalities are often highly desirable. However, most of the reported MOFs so far are restricted to microporous regime.[9] The small pore size inherently
W. Zhang, Z. Xu, Prof. W. Huang, Prof. F. Huo Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM) Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) Nanjing Tech University (NanjingTech) 30 South Puzhu Road Nanjing 211816, PR China W. Zhang, Y. Liu, Prof. G. Lu, Prof. S. Li, C. Cui, J. Wu, Prof. F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798, Singapore E-mail: [email protected] Prof. Y. Yang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive Singapore 637459, Singapore Dr. Y. Wang, Prof. H. Chen Division of Chemistry and Biological Chemistry Nanyang Technological University 21 Nanyang Link Singapore 637371, Singapore Z. Xu, Prof. D. Tian College of Science Nanjing Tech University (NanjingTech) Puzhu Road Nanjing 211816, PR China J. S. DuChene, Prof. W. D. Wei Department of Chemistry and Center for Nanostructured Electronic Materials University of Florida Gainesville, FL 32611, USA
DOI: 10.1002/adma.201405752
Adv. Mater. 2015, DOI: 10.1002/adma.201405752
limits the diffusion of chemical species and their interactions with active sites in MOFs.[10] One of successful strategy from zeolites, silica, and carbon is the fabrication of mesopore structure which has expanded a large variety of potential and existing commercial applications.[1–3] Hence, it is worthwhile to develop methods to fabricate MOFs with mesopores so as to enhance the molecular diffusion while preserving their molecular sieve properties. To date, two major synthetic strategies have been explored to synthesize mesoporous-MOFs (meso-MOFs).[11] One is through ligand extension, either to increase the length of organic ligands[12] or to use bulky organic scaffolds[13] to form mesopores inside MOFs. In this case, the largest pore size reported is 9.8 nm in MOF-74 by increasing the length of organic linker to 5 nm.[14] Besides the difficulties in complex ligands synthesis, interpenetration, disintegration, and instability of frameworks almost inevitably occur in MOFs with extended organic linkers, which prevent this functionalization method from being generally adopted in the formation of meso-MOFs. Another approach, the surfactanttemplate method,[4d,15] has been introduced to increase the pore size in MOFs. For example, the Zhou group has successfully used cetyltrimethylammonium bromide (CTAB) as soft template to build meso-MOFs.[16] In this system, surfactant molecules first self-assembled into micelles serving as a soft template for MOFs growth and were subsequently removed to generate mesopores. The pore diameter of the resulting MOFs could be tuned from 3.8 to 31.0 nm. Nevertheless, as is well known, small molecular micelles are usually unstable under the synthesis conditions of most MOFs, so that only a few series of MOFs (such as carboxylic acid ligands) can be obtained by the surfactant-template method. Recently, some new methods have been successively developed to prepare the meso-MOFs, such as the gelation process,[17] and switchable solvent.[18] Moreover, the above methods are suitable for preparation of intrinsic meso-MOFs, but lack of control over the shape, position, and space distribution of mesopores in MOFs makes it hard to meet the demand for the growing applications of MOFs. It is well known that the potential applications of MOFs can be further developed and extended by encapsulating various nanoparticles (NPs) within the frameworks matrix so that the functionalized MOFs can exhibit the novel chemical and physical properties endowed by NPs.[7b,19] Thus, to the best of our knowledge, general and versatile strategies of synthesizing functionalized MOFs with size-, shape-, and space-distribution-controlled mesopores have been rarely reported, in spite of the need and the significance in application of functionalized meso-MOFs. Herein, we report a facile strategy of crafting mesopores inside MOFs through encapsulation of NPs followed by etching. Especially, the mesopore morphology, hierarchical structure, and space
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Scheme 1. The preparation of functionalized meso-MOFs.
distribution can be tuned by control of the size and shape of NPs, and the encapsulation conditions (Scheme 1). Compared with the traditional fabrication method of meso-MOFs, this unique strategy endows the meso-MOFs with flexibility and designability, as well as with potential applications in fine chemistry and biomedicine. Interestingly, two kinds of NPs can be incorporated simultaneously in MOFs, where one kind of NPs is used as the sacrificial template for mesopores and another kind of NPs serves as the catalytic active sites inside MOFs. It is worth noticing that the obtained meso-MOFs–NPs composites not only maintain welldefined crystal structures but also preserve their microporosity throughout the crystal surrounding the mesopores. As a result, the functionalized meso-MOFs show good selectivity as well as enhanced conversion rate in the catalytic hydrogenation reaction.
In this study, NPs with controlled size, shape, and spatial distribution were encapsulated in MOFs via the ingenious encapsulation strategies and the particles were then subjected to etching, giving rise to mesoMOFs with well-defined crystal structures. Here, the zeolitic imidazolate framework material,[20] ZIF-8, was taken as an example to study the NPs encapsulation and mesopores creation process. It is well known that Pt NPs display outstanding catalytic activity and Au NPs have always been regarded as a wellestablished model in experiment due to easy analysis. Hence, Pt and Au NPs were selected as the catalysts and the sacrificial templates, respectively, in this study, but not limited in noble metal NPs (Figure S2, Supporting Information).[21] Pt and Au NPs with uniform shape and size were synthesized using established methods and their surfaces were functionalized with poly(vinyl pyrrolidone) (PVP) either during or after synthesis. In a typical experiment, methanolic solutions of zinc nitrate (25 × 10−3 M, 10 mL), 2-methylimidazole (25 × 10−3 M, 10 mL), and Au NPs with the absorbance value of 4 (0.5 mL) were mixed briefly and allowed to react for a certain time before the second-time introduction of Au NPs (1.0 mL). The mixture was then kept at room temperature for 24 h under static conditions. The as-prepared ZIF-8-Au crystals were added into the etching solution of KI and I2 with gentle shaking. As can be observed by naked eye, the red color of ZIF-8-Au was gradually changed to white during etching (Figure S5a, Supporting Information), indicating the removal of Au NPs. The meso-MOF obtained through the above method was denoted as meso-ZIF-8. The element analysis and UV–vis spectra also confirmed the thorough removal of Au NPs via etching (Table 1). In attempt to increase the quantity of mesopores, the volume of Au NPs in the initial mixture was systematically adjusted from 0.1 to 1.5 mL. However, we observed that the quantity of Au NPs encapsulated in
Table 1. Metal nanoparticles content and surface area data for ZIF-8-Au, ZIF-8-Pt-Au, meso-ZIF-8, and meso-ZIF-8-Pt. M contenta) [%]
Sample
BET SAb) [m2 g−1]
Langmuir SA [m2 g−1]
Pore volumec) [cm3 g−1]
Mesopore diameterd) Mesopore volumee) [nm] [cm3 g−1]
Au
Pt
3.9
0
1275.2
1702.7
0.604
–
0.003
Meso-ZIF-8
0
0
1359.2
1814.3
0.667
14.2
0.031
ZIF-8-Pt-Au
3.9
2.0
1214.9
1621.5
0.589
–
0.01
0
2.0
1323.6
1767.4
0.653
13.8
0.035
5.0
0
1263.0
1687.3
0.621
–
0.002
0
0
1335.2
1783.5
0.678
3.8/13.9
0.166/0.029
ZIF-8-Au
Meso-ZIF-8-Pt ZIF-8-Au 13 nm-Au 3 nm Hierarchical meso-ZIF-8
a)M is the metal nanoparticles; b)BET SA is the Brunauer–Emmett–Teller surface area; c)The pore volume is the total specific pore volume determined by using the adsorption branch of the N2 isotherm at P/P0 = 0.99; d)Mesopore diameters were determined from the local maximum of the Barrett–Joyner–Halenda (BJH) distribution of pore diameters obtained in the adsorption branch of the N2 isotherm; e)Mesopore volumes were obtained from the BJH cumulative specific adsorption volume of pores of 1.70–300.00 nm in diameter.
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ZIF-8 hardly increased. Concomitantly, the size of ZIF-8-Au crystal decreased from ca. 307 to ca. 184 nm (Figure S1a–e, Supporting Information). On the other hand, if the 0.5 mL Au NPs were introduced in the beginning of reaction and followed by an extra 1.0 mL Au NPs were introduced after a reaction time of 10 min, the size of ZIF-8-Au crystal can be kept the same as that of ZIF-8-Au which obtained by introduce of 0.5 mL Au NPs only in the beginning of reaction while the quantity of Au NPs in ZIF-8 was increased (Figure S1b,f, Supporting Information). These results suggested that the homogeneous nucleation of ZIF-8 could be affected by the quantity of Au NPs presented in the beginning of the reaction, while PVP-modified Au NPs introduced in later stages of the reaction could only successively adsorb on to the continuously forming fresh surfaces of the growing MOFs spheres without altering the crystal structure, which is consistent to the result in open literature.[19g] Therefore, by changing the strategy of adding Au NPs, we could easily gain control over the quantity of Au NPs within ZIF-8. Moreover, the spatial distribution of the Au NPs can also be easily tuned. For instance, when 1.0 mL Au NPs were added into the reaction mixture after a reaction time of 20 min instead of 10 min, these Au NPs would spread closer to the edges of ZIF-8 (Figure 1c) instead of gathering in middle (Figure 1a). Consequently, NPs can leave mesopores in different positions within ZIF-8 after etching, which was demonstrated by transmission electron microscopy (TEM) images (Figure 1b,d). More importantly, the etching process had no significant effect on the meso-ZIF-8 crystal structure, as the morphology of the ZIF-8 crystals remained intact after etching. We believe that this is straightforward strategy to successively prepare meso-ZIFs, which remained unreported using existing methods (ligand extension and surfactant-template methods) probably due to the weak interaction between N-containing ligands and metal ion. Thus, our strategy offers a promising way to expand the team of meso-MOFs. We find that the encapsulation strategy can be successfully extended to other MOFs, as well as other NPs possessing different sizes, shapes, and compositions.[19g] For example, Au NPs with different size (3, 13, and 50 nm), Au nanowires (Au NWs), Ag nanocubes (Ag NCs), Pd NPs, and Au@Cu2O NPs (Figure S2, Supporting Information) can also be used as sacrificial templates for diversified mesopore (Figure 1 and Figure S3 and S4, Supporting Information). Interestingly, two or three kinds of NPs with different size could be incorporated simultaneously in MOFs and the spatial distribution with MOFs could be controlled by the sequence of their addition during the MOFs formation reaction. Taking advantage of this flexibility, NPs can leave different sizes or shapes of mesopores in different positions within ZIF-8 after simple etching. For example, we could produce the hierarchical meso-MOFs containing cores rich in 13.9 nm mesopores and the transition layers rich in 3.8 nm mesopores by sequent encapsulation Au NPs with diameter of 13 and 3 nm as well as simple etching (Figure 1g,h). In addition, hierarchical meso-MOFs containing core rich in sphere mesopores and transition layers rich in linear mesopore also could be obtained by sequent encapsulation 13 nm Au NPs and 2 nm Au NWs as well as simple etching (Figure 1o,p). Meanwhile, meso-UiO-66 and meso-ZIF-67 can also be obtained via this strategy (Figure 1i,r). Hence, different sizes and shapes of
pores can be created in various MOFs, indicating the versatility of the method and can thus seek a broad range of applications in the field of sensing, gas storage, and catalysis. Based on the above hypothesis, functionalized meso-MOFs could be achieved by simultaneously encapsulating multiple kinds of NPs within MOFs, such as Pt and Au NPs, where Au NPs were used as the sacrificial templates for mesopores and Pt NPs were served as catalytic active sites. In principle, by employing this strategy, molecular diffusion could be enhanced during heterogeneous catalysis due to the presence of mesopores while the unique size selectivity of MOFs could be preserved thanks to the intact microporosity. The encapsulation process is similar to that of ZIF-8-Au. 0.5 mL Au NPs (Abs = 4) and 0.3 mL Pt NPs (1.3 × 10−3 M) were mixed with methanolic solutions of zinc nitrate, and 2-methylimidazole to form the initial mixture. Another 1.0 mL Au NPs were added after a reaction time of 15 min and the final mixture was kept at room temperature for 24 h under static conditions. The products were shorted as ZIF-8-Pt-Au, which was treated by the same etching procedure as ZIF-8-Au. As expected, meso-ZIF8-Pt crystals with well-dispersed mesopores and intact Pt NPs were successfully obtained, which was confirmed by the TEM images (Figure 1e,f). It is well known that the different kinds of NPs have different etching solutions. By a suitable match of NPs, the selective etching of NPs will produce more kinds of functional meso-MOFs to meet the demand for the growing applications of MOFs. The porosity, crystal structure, and thermo stability of meso-ZIF-8-Pt were studied by N2-sorption measurements, X-ray diffraction (XRD), and thermogravimetric analyses (TGA), respectively. The N2 adsorption–desorption isotherm of meso-ZIF-8 and meso-ZIF-8-Pt nanostructure display properties intermediate between type-I behavior, characteristic of microporous materials, and type IV behavior, characteristic of mesoporous materials (Figure 2a,c,e). These results are indicative of their porous structure, which contains both mesopores and micropores. The steep increase in N2 uptake at low relative pressure indicates the microporous structure of ZIF-8 crystal before and after etching. Notably, mesopores were created after etching, as evidenced by the presence of hysteresis loop in N2 adsorption–desorption isotherms and the pore-distribution study (Figure 2b,c,f). As can be seen from the pore-distribution study, the meso-ZIF-8 and meso-ZIF-8-Pt possess a mesopore system with a diameter of 14.2 and 13.8 nm, respectively, which are consistent with the results observed by TEM. Nevertheless, the wide peak in pore size distribution analysis indicated that our strategies prepared the meso-MOFs with less uniform mesopore, compared with the existed methods, ligand extension, and surfactant template.[14,16] In addition, compared with ZIF-8-Au (1275 m2 g−1), ZIF-8-Pt-Au (1214 m2 g−1), and ZIF8-Au 13 nm-Au 3 nm (1263 m2 g−1), meso-ZIF-8 (1359 m2 g−1), meso-ZIF-8-Pt (1323 m2 g−1), and hierarchical meso-ZIF-8 (1335 m2 g−1) show a slight increase in gravimetric Brunauer– Emmett–Teller (BET) surface areas (Table 1). Meso-ZIF-8, meso-ZIF-8-Pt, and hierarchical meso-ZIF-8 also have detectable mesopore volume, which further confirms the existence of mesopores inside ZIF-8 after etching Au NPs (Table 1). Interestingly, the hierarchical meso-MOFs containing micropore 0.34 as well as mesopore 3.8 and 13.9 nm were further confirmed
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Figure 1. TEM images of meso-MOFs and functionalized meso-MOFs. a,c) ZIF-8-Au obtained when Au NPs were introduced 10 or 20 min after the initiation of reaction; b,d) meso- ZIF-8 obtained by etching of Au NPs; e) ZIF-8-Pt-Au consisting of 2.9 nm Pt NPs-rich cores, 13 nm Au NPs-rich transition layers, NP-free shells prepared by sequentially adding 2.9 nm Pt NPs and a part of 13 nm Au NPs at the beginning of reaction and the rest 13 nm Au NPs at 15 min; f) meso-ZIF-8-Pt obtained by etching Au NPs; g) ZIF-8-Au 13 nm-Au 3 nm consisiting of 13 nm Au NPs-rich cores, 3 nm Au NPs-rich transition layers; h) meso-ZIF-8 with two kinds of mesopore obtained by etching Au NPs; i,k,m,q,s) are ZIF-8-Ag NC, UiO-66-Pd, ZIF-8-Au NW, ZIF-67-Au, and ZIF-8-Au(50 nm); j,l,n,r,t) are meso-ZIF-8, meso-UiO-66, and meso-ZIF-67 obtained by etching i,k,m,q,s); o) ZIF-8-Au 13 nm-AuNW consisting of 13 nm Au NPs-rich cores, Au NWs-rich transition layers; p) meso-ZIF-8 with sphere and linear mesopore obtained by etching Au NPs and Au NWs. Image a scale bar is 500 nm and other scale bars are 200 nm.
by BET analysis and pore-size distribution (Figure 2e,f). In addition, linear meso-ZIF-8 also contains both mesopores and micropores. These results demonstrate that, by using our novel encapsulation and etching strategy, functionalized meso-MOFs have been successfully synthesized, in which microporous frameworks grow around mesopores. The XRD spectra of ZIF8-Au, ZIF-8-Pt-Au, ZIF-8-Au 13 nm-Au 3 nm, more importantly, meso-ZIF-8, meso-ZIF-8-Pt, and hierarchical meso-ZIF-8 all show well-defined peaks (011, 002, 112, 022, 013, and 222)
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consistent with those of intrinsic ZIF-8, confirming the phase purity and the framework stability after removing Au NPs (Figure 3). Nevertheless, the peaks of Au and Pt NPs are too weak to be clearly observed in XRD spectra, probably due to their low concentration, small sizes as well as the encapsulation by ZIF-8 crystal. TGA spectra (Figure S5b–d, Supporting Information) indicate that meso-ZIF-8 and meso-ZIF-8-Pt composites are less thermally stable than ZIF-8-Au and ZIF-8-Pt-Au, which could largely be attributed to the existence of mesopores.
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COMMUNICATION Figure 2. Pore-structure analysis of meso-ZIF-8. a) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Au and meso-ZIF-8. Inset shows hysteresis generated from the mesoporosit of meso-ZIF-8; b) Pore-size distribution of ZIF-8-Au and meso-ZIF-8; c) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Pt-Au and meso-ZIF-8-Pt. Inset shows the hysteresis loop generated from the mesopores of meso-ZIF-8-Pt; d) Pore-size distributions of ZIF8-Pt-Au and meso-ZIF-8-Pt; e) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Au 13 nm-Au 3 nm and meso-ZIF-8; f) Pore-size distributions of ZIF-8-Au 13 nm-Au 3 nm and meso-ZIF-8. Inset is magnifying pore size distribtion of meso-ZIF-8. Pore-size distributions were calculated from adsorption braches by using BJH method.
At the same time, meso-ZIF-8-Pt, ZIF-8-Pt-Au composites are less thermally stable than meso-ZIF-8 and ZIF-8-Au, possibly due to the movement and decomposition of PVP chains (PVP glass transition temperature, 175 °C; decomposition temperature, 435 °C). With the increasing quantity of encapsulated
Adv. Mater. 2015, DOI: 10.1002/adma.201405752
PVP-modified NPs, ZIF-8 will contain an increasing amount of PVP, leading to inferior thermo stability. The diffusion rate of molecules within the as-obtained mesoZIF-8-Pt composites is enhanced with the aid of the mesoporous structure, and in the meanwhile, the molecular sieving
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Figure 3. Powder XRD patterns of ZIF-8-Au, meso-ZIF-8, ZIF-8-Pt-Au, meso-ZIF-8-Pt, ZIF-8-Au 13 nm-Au 3 nm, and hierarchical meso-ZIF-8.
behavior of ZIF-8 matrix is preserved in catalysis. Liquid-phase hydrogenations of n-pentene, n-hexene, n-heptene, and ciscyclooctene were taken as the example to study the enhanced performance of meso-ZIF-8-Pt in catalysis. The reactions were carried out under static conditions without external stirring and thus, the rate of conversion relied solely on the molecular self-diffusion. As shown in Figure 4, hydrogenation of linear olefin catalyzed by ZIF-8-Pt exhibited low percentage of conversion (n-pentene: 30%, n-hexene: 9%, n-heptene: 7%) presumably owing to the slow molecular diffusion through small pore apertures (3.4 Å) of ZIF-8 under static state and the low reaction temperature. By contrast, the meso-ZIF-8-Pt (Figure 1f) catalyzed reactions showed evidently higher conversion rate (n-pentene: 44%, n-hexene: 16%, n-heptene: 11%). The results demonstrate the ability of mesopores within MOFs to enlarge the diffusion space for molecules, and therefore to facilitate catalytic reactions. The reusability of meso-ZIF-8-Pt as catalyst for hydrogenation of olefin was demonstrated by three consecutive runs. Notably, meso-ZIF-8-Pt not only maintained well-defined crystal structures but also preserved their mesopores after catalytic reaction, which was confirmed by the BET, TEM, and XRD measurements (Figure S6–S9, Supporting Information).
Furthermore, neither ZIF-8-Pt nor meso-ZIF-8-Pt exhibited the propensity to catalyze the hydrogenation reaction of the sterically more demanding cyclooctene, which is consistent with the small pore size of ZIF-8 (3.4 Å) and also suggests the absence of Pt NPs on the outer surface of the composite. These results demonstrate that the functionalized meso-MOFs enhanced the diffusion rate of molecules as well as preserved the molecular size selectivity, due to the protecting microporous frameworks growing around the mesopores. The functionalized meso-MOFs were achieved subtly depending on three key factors. One is synthesis of NPs. Different from the existed methods for the preparation of mesoMOFs, in our strategies, the shape and size of mesopore are all decided by the NPs templates. To the best of our knowledge, it is simple strategy to be able to design and synthesize mesoMOFs with the controlled size, shape, and functional group base on the needs of specific chemical, biological, and material applications. In addition, given the rapid development in nanomaterials synthesis, a large tool box of NPs is available to be taken advantage of and thus, it is highly possible to endow the strategy with more flexibility and designability. The second factor is the encapsulation strategy. Given that the methods of incorporating NPs of different kinds, shapes and sizes in MOFs have been well developed,[19c,g] our synthetic strategy is thus general and versatile and can seek a broad range of applications in the field of sensor, storage, medicine, and catalysis. The third factor is the selective etching. It is well known that different kinds of NPs have different etching agents. In this case, Au NPs as sacrificial templates are easy to be etched by KI/I2 solutions; oppositely, Pt NPs as functional group remain intact under the condition of KI/I2 solution. Hence, the selective etching of NPs will bring the more possibility for the functionalized meso-MOFs. In conclusion, the presented work demonstrates a general and facile strategy for the synthesis of meso-MOFs through an encapsulation and etching process. The meso-MOFs could not only maintain a well-defined crystal structure but also possessed mesopores with controlled size, shape, and spatial distribution by simply adjusting the kinds of NPs and the encapsulation conditions. Particularly, the obtained functionalized meso-MOFsPt hybrid materials displayed high catalytic activity originating from the mesopores and good selectivity due to the protection of the microporous frameworks in catalytic hydrogenation reactions. In addition, the hierarchical meso-MOFs were achieved by flexible design of NPs encapsulation. Compared with traditional pristine meso-MOFs, the strategy enables us to rationally design and synthesis meso-MOFs with tunable mesopore sizes, shapes, and different functional species, which can further expand the potential field of applications of meso-MOFs.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Figure 4. Catalysis behaviors of ZIF-8-Pt and meos-ZIF-8-Pt.
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This work was supported by the AcRF Tier 2 (RG 17/12) from Ministry of Education, Singapore, and the Singapore National Research Foundation
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Received: December 16, 2014 Revised: February 4, 2015 Published online:
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under the Campus for Research Excellence and Technological Enterprise Programmed Nanomaterials for Energy and Water Management, and the Foundation for Distinguished Young Scholars of Jiangsu Province (BK20140044).
[12] a) O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Nature 2003, 423, 705; b) S. Cantrill, Nat. Chem. 2012, 4, 516; c) O. K. Farha, J. T. Hupp, Acc. Chem. Res. 2010, 43, 1166; d) B. Wang, A. P. Cote, H. Furukawa, M. O’Keeffe, O. M. Yaghi, Nature 2008, 453, 207; e) Y. K. Park, S. B. Choi, H. Kim, K. Kim, B. H. Won, K. Choi, J. S. Choi, W. S. Ahn, N. Won, S. Kim, D. H. Jung, S. H. Choi, G. H. Kim, S. S. Cha, Y. H. Jhon, J. K. Yang, J. Kim, Angew. Chem. Int. Ed. 2007, 46, 8230; f) L. Q. Ma, J. M. Falkowski, C. Abney, W. B. Lin, Nat. Chem. 2010, 2, 838. [13] P. Thanasekaran, T. T. Luo, J. Y. Wu, K. L. Lu, Dalton Trans. 2012, 41, 5437. [14] H. X. Deng, S. Grunder, K. E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gandara, A. C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki, J. F. Stoddart, O. M. Yaghi, Science 2012, 336, 1018. [15] a) D. Bradshaw, S. El-Hankari, L. Lupica-Spagnolo, Chem. Soc. Rev. 2014, 43, 5431; b) S. Cao, G. Gody, W. Zhao, S. Perrier, X. Peng, C. Ducati, D. Zhao, A. K. Cheetham, Chem. Sci. 2013, 4, 3573. [16] a) L. G. Qiu, T. Xu, Z. Q. Li, W. Wang, Y. Wu, X. Jiang, X. Y. Tian, L. D. Zhang, Angew. Chem., Int. Ed. 2008, 47, 9487; b) L. B. Sun, J. R. Li, J. Park, H. C. Zhou, J. Am. Chem. Soc. 2012, 134, 126; c) Y. Zhao, J. Zhang, B. Han, J. Song, J. Li, Q. Wang, Angew. Chem., Int. Ed. 2011, 50, 636. [17] L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen, C.-Y. Su, Nat. Commun. 2013, 4, 1774. [18] L. Peng, J. Zhang, Z. Xue, B. Han, X. Sang, C. Liu, G. Yang, Nat. Commun. 2014, 5, 4465. [19] a) H.-L. Jiang, T. Akita, T. Ishida, M. Haruta, Q. Xu, J. Am. Chem. Soc. 2011, 133, 1304; b) R. Ameloot, M. B. J. Roeffaers, G. De Cremer, F. Vermoortele, J. Hofkens, B. F. Sels, D. E. De Vos, Adv. Mater. 2011, 23, 1788; c) W. Zhang, G. Lu, C. Cui, Y. Liu, S. Li, W. Yan, C. Xing, Y. R. Chi, Y. Yang, F. Huo, Adv. Mater. 2014, 26, 4056; d) W. Zhang, G. Lu, S. Li, Y. Liu, H. Xu, C. Cui, W. Yan, Y. Yang, F. Huo, Chem. Commun. 2014, 50, 4296; e) Y. Liu, W. Zhang, S. Li, C. Cui, J. Wu, H. Chen, F. Huo, Chem. Mater. 2013, 26, 1119; f) T. Tsuruoka, H. Kawasaki, H. Nawafune, K. Akamatsu, ACS Appl. Mater. Interfaces 2011, 3, 3788; g) G. Lu, S. Z. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Y. Qi, Y. Wang, X. Wang, S. Y. Han, X. G. Liu, J. S. DuChene, H. Zhang, Q. C. Zhang, X. D. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. H. Yang, J. T. Hupp, F. W. Huo, Nat. Chem. 2012, 4, 310. [20] a) A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe, O. M. Yaghi, Acc. Chem. Res. 2010, 43, 58; b) X. Huang, J. Zhang, X. Chen, Chin. Sci. Bull. 2003, 48, 1531. [21] a) G. Frens, Nat. Phys. Sci. 1973, 241, 20; b) Y. Wang, Q. Wang, H. Sun, W. Zhang, G. Chen, Y. Wang, X. Shen, Y. Han, X. Lu, H. Chen, J. Am. Chem. Soc. 2011, 133, 20060; c) J. Zeng, Y. Zheng, M. Rycenga, J. Tao, Z.-Y. Li, Q. Zhang, Y. Zhu, Y. Xia, J. Am. Chem. Soc. 2010, 132, 8552; d) Z. Sun, Z. Yang, J. Zhou, M. H. Yeung, W. Ni, H. Wu, J. Wang, Angew. Chem., Int. Ed. 2009, 48, 2881; e) Y. Li, E. Boone, M. A. El-Sayed, Langmuir 2002, 18, 4921; f) T. Teranishi, M. Hosoe, T. Tanaka, M. Miyake, J. Phys. Chem. B 1999, 103, 3818; g) C. C. Huang, Z. Yang, K. H. Lee, H. T. Chang, Angew. Chem., Int. Ed. 2007, 46, 6824.
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Mesoporous Metal–Organic Frameworks with Size-, Shape-, and Space-Distribution-Controlled Pore Structure Weina Zhang, Yayuan Liu, Guang Lu, Yong Wang, Shaozhou Li, Chenlong Cui, Jin Wu, Zhiling Xu, Danbi Tian, Wei Huang, Joseph S. DuCheneu, W. David Wei, Hongyu Chen, Yanhui Yang, and Fengwei Huo* Porous materials, such as silica,[1] carbon,[2] and zeolites,[3] can interact with atoms, ions, molecules, or even larger guest species not only at external surfaces but also throughout the whole internal pore system, and therefore, are regarded as powerful tools for many important applications. Metal–organic frameworks (MOFs),[4] a new member of porous crystalline materials, have been attracting considerable attention as promising candidates for gas storage,[5] gas separation,[6] chemical sensing,[7] heterogeneous catalysis,[8] etc. Sizes, shapes, and chemical environments of the cages or channels within MOFs are of paramount importance for the above-mentioned applications, in which large surface areas, high free volumes, and structure functionalities are often highly desirable. However, most of the reported MOFs so far are restricted to microporous regime.[9] The small pore size inherently
W. Zhang, Z. Xu, Prof. W. Huang, Prof. F. Huo Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM) Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) Nanjing Tech University (NanjingTech) 30 South Puzhu Road Nanjing 211816, PR China W. Zhang, Y. Liu, Prof. G. Lu, Prof. S. Li, C. Cui, J. Wu, Prof. F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798, Singapore E-mail: [email protected] Prof. Y. Yang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive Singapore 637459, Singapore Dr. Y. Wang, Prof. H. Chen Division of Chemistry and Biological Chemistry Nanyang Technological University 21 Nanyang Link Singapore 637371, Singapore Z. Xu, Prof. D. Tian College of Science Nanjing Tech University (NanjingTech) Puzhu Road Nanjing 211816, PR China J. S. DuChene, Prof. W. D. Wei Department of Chemistry and Center for Nanostructured Electronic Materials University of Florida Gainesville, FL 32611, USA
DOI: 10.1002/adma.201405752
Adv. Mater. 2015, DOI: 10.1002/adma.201405752
limits the diffusion of chemical species and their interactions with active sites in MOFs.[10] One of successful strategy from zeolites, silica, and carbon is the fabrication of mesopore structure which has expanded a large variety of potential and existing commercial applications.[1–3] Hence, it is worthwhile to develop methods to fabricate MOFs with mesopores so as to enhance the molecular diffusion while preserving their molecular sieve properties. To date, two major synthetic strategies have been explored to synthesize mesoporous-MOFs (meso-MOFs).[11] One is through ligand extension, either to increase the length of organic ligands[12] or to use bulky organic scaffolds[13] to form mesopores inside MOFs. In this case, the largest pore size reported is 9.8 nm in MOF-74 by increasing the length of organic linker to 5 nm.[14] Besides the difficulties in complex ligands synthesis, interpenetration, disintegration, and instability of frameworks almost inevitably occur in MOFs with extended organic linkers, which prevent this functionalization method from being generally adopted in the formation of meso-MOFs. Another approach, the surfactanttemplate method,[4d,15] has been introduced to increase the pore size in MOFs. For example, the Zhou group has successfully used cetyltrimethylammonium bromide (CTAB) as soft template to build meso-MOFs.[16] In this system, surfactant molecules first self-assembled into micelles serving as a soft template for MOFs growth and were subsequently removed to generate mesopores. The pore diameter of the resulting MOFs could be tuned from 3.8 to 31.0 nm. Nevertheless, as is well known, small molecular micelles are usually unstable under the synthesis conditions of most MOFs, so that only a few series of MOFs (such as carboxylic acid ligands) can be obtained by the surfactant-template method. Recently, some new methods have been successively developed to prepare the meso-MOFs, such as the gelation process,[17] and switchable solvent.[18] Moreover, the above methods are suitable for preparation of intrinsic meso-MOFs, but lack of control over the shape, position, and space distribution of mesopores in MOFs makes it hard to meet the demand for the growing applications of MOFs. It is well known that the potential applications of MOFs can be further developed and extended by encapsulating various nanoparticles (NPs) within the frameworks matrix so that the functionalized MOFs can exhibit the novel chemical and physical properties endowed by NPs.[7b,19] Thus, to the best of our knowledge, general and versatile strategies of synthesizing functionalized MOFs with size-, shape-, and space-distribution-controlled mesopores have been rarely reported, in spite of the need and the significance in application of functionalized meso-MOFs. Herein, we report a facile strategy of crafting mesopores inside MOFs through encapsulation of NPs followed by etching. Especially, the mesopore morphology, hierarchical structure, and space
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Scheme 1. The preparation of functionalized meso-MOFs.
distribution can be tuned by control of the size and shape of NPs, and the encapsulation conditions (Scheme 1). Compared with the traditional fabrication method of meso-MOFs, this unique strategy endows the meso-MOFs with flexibility and designability, as well as with potential applications in fine chemistry and biomedicine. Interestingly, two kinds of NPs can be incorporated simultaneously in MOFs, where one kind of NPs is used as the sacrificial template for mesopores and another kind of NPs serves as the catalytic active sites inside MOFs. It is worth noticing that the obtained meso-MOFs–NPs composites not only maintain welldefined crystal structures but also preserve their microporosity throughout the crystal surrounding the mesopores. As a result, the functionalized meso-MOFs show good selectivity as well as enhanced conversion rate in the catalytic hydrogenation reaction.
In this study, NPs with controlled size, shape, and spatial distribution were encapsulated in MOFs via the ingenious encapsulation strategies and the particles were then subjected to etching, giving rise to mesoMOFs with well-defined crystal structures. Here, the zeolitic imidazolate framework material,[20] ZIF-8, was taken as an example to study the NPs encapsulation and mesopores creation process. It is well known that Pt NPs display outstanding catalytic activity and Au NPs have always been regarded as a wellestablished model in experiment due to easy analysis. Hence, Pt and Au NPs were selected as the catalysts and the sacrificial templates, respectively, in this study, but not limited in noble metal NPs (Figure S2, Supporting Information).[21] Pt and Au NPs with uniform shape and size were synthesized using established methods and their surfaces were functionalized with poly(vinyl pyrrolidone) (PVP) either during or after synthesis. In a typical experiment, methanolic solutions of zinc nitrate (25 × 10−3 M, 10 mL), 2-methylimidazole (25 × 10−3 M, 10 mL), and Au NPs with the absorbance value of 4 (0.5 mL) were mixed briefly and allowed to react for a certain time before the second-time introduction of Au NPs (1.0 mL). The mixture was then kept at room temperature for 24 h under static conditions. The as-prepared ZIF-8-Au crystals were added into the etching solution of KI and I2 with gentle shaking. As can be observed by naked eye, the red color of ZIF-8-Au was gradually changed to white during etching (Figure S5a, Supporting Information), indicating the removal of Au NPs. The meso-MOF obtained through the above method was denoted as meso-ZIF-8. The element analysis and UV–vis spectra also confirmed the thorough removal of Au NPs via etching (Table 1). In attempt to increase the quantity of mesopores, the volume of Au NPs in the initial mixture was systematically adjusted from 0.1 to 1.5 mL. However, we observed that the quantity of Au NPs encapsulated in
Table 1. Metal nanoparticles content and surface area data for ZIF-8-Au, ZIF-8-Pt-Au, meso-ZIF-8, and meso-ZIF-8-Pt. M contenta) [%]
Sample
BET SAb) [m2 g−1]
Langmuir SA [m2 g−1]
Pore volumec) [cm3 g−1]
Mesopore diameterd) Mesopore volumee) [nm] [cm3 g−1]
Au
Pt
3.9
0
1275.2
1702.7
0.604
–
0.003
Meso-ZIF-8
0
0
1359.2
1814.3
0.667
14.2
0.031
ZIF-8-Pt-Au
3.9
2.0
1214.9
1621.5
0.589
–
0.01
0
2.0
1323.6
1767.4
0.653
13.8
0.035
5.0
0
1263.0
1687.3
0.621
–
0.002
0
0
1335.2
1783.5
0.678
3.8/13.9
0.166/0.029
ZIF-8-Au
Meso-ZIF-8-Pt ZIF-8-Au 13 nm-Au 3 nm Hierarchical meso-ZIF-8
a)M is the metal nanoparticles; b)BET SA is the Brunauer–Emmett–Teller surface area; c)The pore volume is the total specific pore volume determined by using the adsorption branch of the N2 isotherm at P/P0 = 0.99; d)Mesopore diameters were determined from the local maximum of the Barrett–Joyner–Halenda (BJH) distribution of pore diameters obtained in the adsorption branch of the N2 isotherm; e)Mesopore volumes were obtained from the BJH cumulative specific adsorption volume of pores of 1.70–300.00 nm in diameter.
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ZIF-8 hardly increased. Concomitantly, the size of ZIF-8-Au crystal decreased from ca. 307 to ca. 184 nm (Figure S1a–e, Supporting Information). On the other hand, if the 0.5 mL Au NPs were introduced in the beginning of reaction and followed by an extra 1.0 mL Au NPs were introduced after a reaction time of 10 min, the size of ZIF-8-Au crystal can be kept the same as that of ZIF-8-Au which obtained by introduce of 0.5 mL Au NPs only in the beginning of reaction while the quantity of Au NPs in ZIF-8 was increased (Figure S1b,f, Supporting Information). These results suggested that the homogeneous nucleation of ZIF-8 could be affected by the quantity of Au NPs presented in the beginning of the reaction, while PVP-modified Au NPs introduced in later stages of the reaction could only successively adsorb on to the continuously forming fresh surfaces of the growing MOFs spheres without altering the crystal structure, which is consistent to the result in open literature.[19g] Therefore, by changing the strategy of adding Au NPs, we could easily gain control over the quantity of Au NPs within ZIF-8. Moreover, the spatial distribution of the Au NPs can also be easily tuned. For instance, when 1.0 mL Au NPs were added into the reaction mixture after a reaction time of 20 min instead of 10 min, these Au NPs would spread closer to the edges of ZIF-8 (Figure 1c) instead of gathering in middle (Figure 1a). Consequently, NPs can leave mesopores in different positions within ZIF-8 after etching, which was demonstrated by transmission electron microscopy (TEM) images (Figure 1b,d). More importantly, the etching process had no significant effect on the meso-ZIF-8 crystal structure, as the morphology of the ZIF-8 crystals remained intact after etching. We believe that this is straightforward strategy to successively prepare meso-ZIFs, which remained unreported using existing methods (ligand extension and surfactant-template methods) probably due to the weak interaction between N-containing ligands and metal ion. Thus, our strategy offers a promising way to expand the team of meso-MOFs. We find that the encapsulation strategy can be successfully extended to other MOFs, as well as other NPs possessing different sizes, shapes, and compositions.[19g] For example, Au NPs with different size (3, 13, and 50 nm), Au nanowires (Au NWs), Ag nanocubes (Ag NCs), Pd NPs, and Au@Cu2O NPs (Figure S2, Supporting Information) can also be used as sacrificial templates for diversified mesopore (Figure 1 and Figure S3 and S4, Supporting Information). Interestingly, two or three kinds of NPs with different size could be incorporated simultaneously in MOFs and the spatial distribution with MOFs could be controlled by the sequence of their addition during the MOFs formation reaction. Taking advantage of this flexibility, NPs can leave different sizes or shapes of mesopores in different positions within ZIF-8 after simple etching. For example, we could produce the hierarchical meso-MOFs containing cores rich in 13.9 nm mesopores and the transition layers rich in 3.8 nm mesopores by sequent encapsulation Au NPs with diameter of 13 and 3 nm as well as simple etching (Figure 1g,h). In addition, hierarchical meso-MOFs containing core rich in sphere mesopores and transition layers rich in linear mesopore also could be obtained by sequent encapsulation 13 nm Au NPs and 2 nm Au NWs as well as simple etching (Figure 1o,p). Meanwhile, meso-UiO-66 and meso-ZIF-67 can also be obtained via this strategy (Figure 1i,r). Hence, different sizes and shapes of
pores can be created in various MOFs, indicating the versatility of the method and can thus seek a broad range of applications in the field of sensing, gas storage, and catalysis. Based on the above hypothesis, functionalized meso-MOFs could be achieved by simultaneously encapsulating multiple kinds of NPs within MOFs, such as Pt and Au NPs, where Au NPs were used as the sacrificial templates for mesopores and Pt NPs were served as catalytic active sites. In principle, by employing this strategy, molecular diffusion could be enhanced during heterogeneous catalysis due to the presence of mesopores while the unique size selectivity of MOFs could be preserved thanks to the intact microporosity. The encapsulation process is similar to that of ZIF-8-Au. 0.5 mL Au NPs (Abs = 4) and 0.3 mL Pt NPs (1.3 × 10−3 M) were mixed with methanolic solutions of zinc nitrate, and 2-methylimidazole to form the initial mixture. Another 1.0 mL Au NPs were added after a reaction time of 15 min and the final mixture was kept at room temperature for 24 h under static conditions. The products were shorted as ZIF-8-Pt-Au, which was treated by the same etching procedure as ZIF-8-Au. As expected, meso-ZIF8-Pt crystals with well-dispersed mesopores and intact Pt NPs were successfully obtained, which was confirmed by the TEM images (Figure 1e,f). It is well known that the different kinds of NPs have different etching solutions. By a suitable match of NPs, the selective etching of NPs will produce more kinds of functional meso-MOFs to meet the demand for the growing applications of MOFs. The porosity, crystal structure, and thermo stability of meso-ZIF-8-Pt were studied by N2-sorption measurements, X-ray diffraction (XRD), and thermogravimetric analyses (TGA), respectively. The N2 adsorption–desorption isotherm of meso-ZIF-8 and meso-ZIF-8-Pt nanostructure display properties intermediate between type-I behavior, characteristic of microporous materials, and type IV behavior, characteristic of mesoporous materials (Figure 2a,c,e). These results are indicative of their porous structure, which contains both mesopores and micropores. The steep increase in N2 uptake at low relative pressure indicates the microporous structure of ZIF-8 crystal before and after etching. Notably, mesopores were created after etching, as evidenced by the presence of hysteresis loop in N2 adsorption–desorption isotherms and the pore-distribution study (Figure 2b,c,f). As can be seen from the pore-distribution study, the meso-ZIF-8 and meso-ZIF-8-Pt possess a mesopore system with a diameter of 14.2 and 13.8 nm, respectively, which are consistent with the results observed by TEM. Nevertheless, the wide peak in pore size distribution analysis indicated that our strategies prepared the meso-MOFs with less uniform mesopore, compared with the existed methods, ligand extension, and surfactant template.[14,16] In addition, compared with ZIF-8-Au (1275 m2 g−1), ZIF-8-Pt-Au (1214 m2 g−1), and ZIF8-Au 13 nm-Au 3 nm (1263 m2 g−1), meso-ZIF-8 (1359 m2 g−1), meso-ZIF-8-Pt (1323 m2 g−1), and hierarchical meso-ZIF-8 (1335 m2 g−1) show a slight increase in gravimetric Brunauer– Emmett–Teller (BET) surface areas (Table 1). Meso-ZIF-8, meso-ZIF-8-Pt, and hierarchical meso-ZIF-8 also have detectable mesopore volume, which further confirms the existence of mesopores inside ZIF-8 after etching Au NPs (Table 1). Interestingly, the hierarchical meso-MOFs containing micropore 0.34 as well as mesopore 3.8 and 13.9 nm were further confirmed
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Figure 1. TEM images of meso-MOFs and functionalized meso-MOFs. a,c) ZIF-8-Au obtained when Au NPs were introduced 10 or 20 min after the initiation of reaction; b,d) meso- ZIF-8 obtained by etching of Au NPs; e) ZIF-8-Pt-Au consisting of 2.9 nm Pt NPs-rich cores, 13 nm Au NPs-rich transition layers, NP-free shells prepared by sequentially adding 2.9 nm Pt NPs and a part of 13 nm Au NPs at the beginning of reaction and the rest 13 nm Au NPs at 15 min; f) meso-ZIF-8-Pt obtained by etching Au NPs; g) ZIF-8-Au 13 nm-Au 3 nm consisiting of 13 nm Au NPs-rich cores, 3 nm Au NPs-rich transition layers; h) meso-ZIF-8 with two kinds of mesopore obtained by etching Au NPs; i,k,m,q,s) are ZIF-8-Ag NC, UiO-66-Pd, ZIF-8-Au NW, ZIF-67-Au, and ZIF-8-Au(50 nm); j,l,n,r,t) are meso-ZIF-8, meso-UiO-66, and meso-ZIF-67 obtained by etching i,k,m,q,s); o) ZIF-8-Au 13 nm-AuNW consisting of 13 nm Au NPs-rich cores, Au NWs-rich transition layers; p) meso-ZIF-8 with sphere and linear mesopore obtained by etching Au NPs and Au NWs. Image a scale bar is 500 nm and other scale bars are 200 nm.
by BET analysis and pore-size distribution (Figure 2e,f). In addition, linear meso-ZIF-8 also contains both mesopores and micropores. These results demonstrate that, by using our novel encapsulation and etching strategy, functionalized meso-MOFs have been successfully synthesized, in which microporous frameworks grow around mesopores. The XRD spectra of ZIF8-Au, ZIF-8-Pt-Au, ZIF-8-Au 13 nm-Au 3 nm, more importantly, meso-ZIF-8, meso-ZIF-8-Pt, and hierarchical meso-ZIF-8 all show well-defined peaks (011, 002, 112, 022, 013, and 222)
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consistent with those of intrinsic ZIF-8, confirming the phase purity and the framework stability after removing Au NPs (Figure 3). Nevertheless, the peaks of Au and Pt NPs are too weak to be clearly observed in XRD spectra, probably due to their low concentration, small sizes as well as the encapsulation by ZIF-8 crystal. TGA spectra (Figure S5b–d, Supporting Information) indicate that meso-ZIF-8 and meso-ZIF-8-Pt composites are less thermally stable than ZIF-8-Au and ZIF-8-Pt-Au, which could largely be attributed to the existence of mesopores.
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COMMUNICATION Figure 2. Pore-structure analysis of meso-ZIF-8. a) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Au and meso-ZIF-8. Inset shows hysteresis generated from the mesoporosit of meso-ZIF-8; b) Pore-size distribution of ZIF-8-Au and meso-ZIF-8; c) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Pt-Au and meso-ZIF-8-Pt. Inset shows the hysteresis loop generated from the mesopores of meso-ZIF-8-Pt; d) Pore-size distributions of ZIF8-Pt-Au and meso-ZIF-8-Pt; e) N2-adsorption–desorption isotherm at 77 K of ZIF-8-Au 13 nm-Au 3 nm and meso-ZIF-8; f) Pore-size distributions of ZIF-8-Au 13 nm-Au 3 nm and meso-ZIF-8. Inset is magnifying pore size distribtion of meso-ZIF-8. Pore-size distributions were calculated from adsorption braches by using BJH method.
At the same time, meso-ZIF-8-Pt, ZIF-8-Pt-Au composites are less thermally stable than meso-ZIF-8 and ZIF-8-Au, possibly due to the movement and decomposition of PVP chains (PVP glass transition temperature, 175 °C; decomposition temperature, 435 °C). With the increasing quantity of encapsulated
Adv. Mater. 2015, DOI: 10.1002/adma.201405752
PVP-modified NPs, ZIF-8 will contain an increasing amount of PVP, leading to inferior thermo stability. The diffusion rate of molecules within the as-obtained mesoZIF-8-Pt composites is enhanced with the aid of the mesoporous structure, and in the meanwhile, the molecular sieving
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Figure 3. Powder XRD patterns of ZIF-8-Au, meso-ZIF-8, ZIF-8-Pt-Au, meso-ZIF-8-Pt, ZIF-8-Au 13 nm-Au 3 nm, and hierarchical meso-ZIF-8.
behavior of ZIF-8 matrix is preserved in catalysis. Liquid-phase hydrogenations of n-pentene, n-hexene, n-heptene, and ciscyclooctene were taken as the example to study the enhanced performance of meso-ZIF-8-Pt in catalysis. The reactions were carried out under static conditions without external stirring and thus, the rate of conversion relied solely on the molecular self-diffusion. As shown in Figure 4, hydrogenation of linear olefin catalyzed by ZIF-8-Pt exhibited low percentage of conversion (n-pentene: 30%, n-hexene: 9%, n-heptene: 7%) presumably owing to the slow molecular diffusion through small pore apertures (3.4 Å) of ZIF-8 under static state and the low reaction temperature. By contrast, the meso-ZIF-8-Pt (Figure 1f) catalyzed reactions showed evidently higher conversion rate (n-pentene: 44%, n-hexene: 16%, n-heptene: 11%). The results demonstrate the ability of mesopores within MOFs to enlarge the diffusion space for molecules, and therefore to facilitate catalytic reactions. The reusability of meso-ZIF-8-Pt as catalyst for hydrogenation of olefin was demonstrated by three consecutive runs. Notably, meso-ZIF-8-Pt not only maintained well-defined crystal structures but also preserved their mesopores after catalytic reaction, which was confirmed by the BET, TEM, and XRD measurements (Figure S6–S9, Supporting Information).
Furthermore, neither ZIF-8-Pt nor meso-ZIF-8-Pt exhibited the propensity to catalyze the hydrogenation reaction of the sterically more demanding cyclooctene, which is consistent with the small pore size of ZIF-8 (3.4 Å) and also suggests the absence of Pt NPs on the outer surface of the composite. These results demonstrate that the functionalized meso-MOFs enhanced the diffusion rate of molecules as well as preserved the molecular size selectivity, due to the protecting microporous frameworks growing around the mesopores. The functionalized meso-MOFs were achieved subtly depending on three key factors. One is synthesis of NPs. Different from the existed methods for the preparation of mesoMOFs, in our strategies, the shape and size of mesopore are all decided by the NPs templates. To the best of our knowledge, it is simple strategy to be able to design and synthesize mesoMOFs with the controlled size, shape, and functional group base on the needs of specific chemical, biological, and material applications. In addition, given the rapid development in nanomaterials synthesis, a large tool box of NPs is available to be taken advantage of and thus, it is highly possible to endow the strategy with more flexibility and designability. The second factor is the encapsulation strategy. Given that the methods of incorporating NPs of different kinds, shapes and sizes in MOFs have been well developed,[19c,g] our synthetic strategy is thus general and versatile and can seek a broad range of applications in the field of sensor, storage, medicine, and catalysis. The third factor is the selective etching. It is well known that different kinds of NPs have different etching agents. In this case, Au NPs as sacrificial templates are easy to be etched by KI/I2 solutions; oppositely, Pt NPs as functional group remain intact under the condition of KI/I2 solution. Hence, the selective etching of NPs will bring the more possibility for the functionalized meso-MOFs. In conclusion, the presented work demonstrates a general and facile strategy for the synthesis of meso-MOFs through an encapsulation and etching process. The meso-MOFs could not only maintain a well-defined crystal structure but also possessed mesopores with controlled size, shape, and spatial distribution by simply adjusting the kinds of NPs and the encapsulation conditions. Particularly, the obtained functionalized meso-MOFsPt hybrid materials displayed high catalytic activity originating from the mesopores and good selectivity due to the protection of the microporous frameworks in catalytic hydrogenation reactions. In addition, the hierarchical meso-MOFs were achieved by flexible design of NPs encapsulation. Compared with traditional pristine meso-MOFs, the strategy enables us to rationally design and synthesis meso-MOFs with tunable mesopore sizes, shapes, and different functional species, which can further expand the potential field of applications of meso-MOFs.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Figure 4. Catalysis behaviors of ZIF-8-Pt and meos-ZIF-8-Pt.
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This work was supported by the AcRF Tier 2 (RG 17/12) from Ministry of Education, Singapore, and the Singapore National Research Foundation
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Adv. Mater. 2015, DOI: 10.1002/adma.201405752
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Received: December 16, 2014 Revised: February 4, 2015 Published online:
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under the Campus for Research Excellence and Technological Enterprise Programmed Nanomaterials for Energy and Water Management, and the Foundation for Distinguished Young Scholars of Jiangsu Province (BK20140044).
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