Facile Fabricating Hierarchically Porous MetalâOrganic Frameworks

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Facile Fabricating Hierarchically Porous MetalOrganic Frameworks via a Template-Free Strategy Yu Cao, Yali Ma, Tao Wang, Xue Wang, Qisheng Huo, and Yunling Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01559 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 4, 2015

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Crystal Growth & Design

Facile

Fabricating

Hierarchically

Porous

Metal-Organic

Frameworks via a Template-Free Strategy Yu Cao, Yali Ma, Tao Wang, Xue Wang, Qisheng Huo and Yunling Liu* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China

*To whom correspondence should be addressed. Professor Yunling Liu College of Chemistry Jilin University Changchun 130012, P. R. China Fax: +86-431-85168624 E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Tunable

Cu-BTC

mesoporous

metal-organic

frameworks

(mesoMOFs)

are

prepared

conveniently through a template-free strategy under solvothermal conditions. Nanosized microporous Cu-BTC particles pack to form mesopores with sizes that can be controlled (26 to 72 nm) by simply varying the synthesis temperature. This template-free and controllable strategy can be applied using other organic solvents to obtain mesoMOF materials that possess similar tunable mesopores. Furthermore, the as-synthesized hierarchically porous MOFs materials are demonstrated as the electrocatalysts for oxygen reduction reaction (ORR) under mild conditions.


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INTRODUCTION Over the past few years, mesoporous metal-organic frameworks (mesoMOFs) have attracted intense attention.1-15 In comparison to traditional porous materials, such as microporous zeolites and mesoporous materials, mesoMOFs may offer the advantages of both mesoporous materials and microporous MOFs simultaneously, yielding enormous scientific and technological interest.16-20 Nevertheless, the facile synthesis of stable mesoMOFs remains a great challenge. MOF materials composed of organic ligands and metal ions or clusters have shown great potential for gas adsorption and separation, catalysis, and advanced applications.21-40 The ligandextension method was first proposed for the construction of mesoMOF crystals.41-43 However, the synthesis of extended organic ligands can be very complicated, and some mesoMOF crystals that have been obtained are not stable during the removal of the guest molecules. Inspired by the surfactant-directed synthesis strategy of mesoporous SiO2 materials, a self-assembly process was introduced to the synthesis of mesoMOFs. Accordingly, Han et al. reported well-ordered mesoporous MOF nanospheres from an ionic liquids/supercritical CO2/surfactant system.9 Yuan et al. prepared ordered mesoMOFs consisting of metal disulfonates with Pluronic-F127 and crown ether.10 Zhou et al. presented a cooperative template strategy to construct mesoMOFs, in which the chelating agent can harmonize the interaction between framework building blocks and surfactants well.11 To minimize the complexity of the synthesis system and reduce the consequential environmental pollution, a template-free strategy has been highlighted recently. Dai et al. reported a template-free synthesis of the hierarchical porous Zn-MOF-74 material obtained by solvent etching.12 Han et al. reported the template-free assembly of Cu3(BTC)2 mesoMOFs by using a CO2-expanded liquids system.13 Tian et al. used a random loose packing strategy to


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prepare interparticle porosity dominated mesoMOFs, and the mesopore apertures can be tuned by adding organic molecule fillers.14 In our previous work, highly ordered hexagonal mesostructured MOFs were obtained through direct cooperative self-organization of metal ions, ligands and cationic surfactants. Unfortunately, the mesostructure collapsed after the removal of surfactant.15 Inspired by the recent progress in this field, the development of a simple “green” low cost and template-free route to mesoMOFs is highly desirable. Herein, we propose a temperature-controlled and template-free strategy, and report the facile synthesis of Cu-BTC mesoMOFs. Compared to the most reported mesoMOFs, our template-free synthetic strategy is simple, convenient, and does not require the use of surfactant and additives or the employment of other special synthetic conditions, which requires additional steps to remove the template, thus adding difficulties in retaining the mesopores. In a typical synthesis, copper nitrate, acetic acid and triethylamine (TEA) were dissolved in ethanol, followed by the addition of benzene-1, 3, 5 tricarboxylic acid (H3BTC) under stirring. Then, the mixture was transferred into a Teflon-lined autoclave and reacted at different temperatures. The obtained mesoMOFs were named as Cu-BTC-n, in which n represented the synthesis temperature. Compared with traditional surfactants, acetic acid and trimethylamine used in this study are common, cheap, less dosage and have little pollution to the environment. For comparison, typical microporous HKUST-1 crystals were prepared according to the literature.44 This synthetic strategy is simple, convenient, and does not require surfactants and additives or other special synthetic conditions. The obtained mesoMOF materials have distinct characteristics of mesopores and narrow pore size distributions. The size of the mesopores, which are constructed by the stacking of microporous Cu-BTC nanoparticles, can be simply


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adjusted by varying the synthesis temperature. This template-free strategy could be applied to other mesoMOFs using different organic solvents. Additionally, the obtained mesoMOF materials can be used as the electrocatalysts for oxygen reduction reaction (ORR) at room temperature and atmospheric pressure. EXPERIMENTAL SECTION Materials The ligand of benzene-1, 3, 5 tricarboxylic acid (H3BTC) was obtained from Sigma-Aldrich. Methanol (MeOH), ethanol (EtOH), acetonitrile (CH3CN), acetone, N, N-dimethyl formamide (DMF), acetic acid, trimethylamine (TEA), copper acetate, copper nitrate, copper chloride, and copper sulfate were purchased from Beijing Chemical Co. Inc. All materials were used as received without further purification. Synthesis of mesoporous MOFs. In a typical synthesis procedure of Cu-BTC-n, 0.435 g of Cu(NO3)2•3H2O (1.8 mmol), 0.62 mL of acetic acid and 0.50 mL of TEA were added into 12 mL of ethanol under stirring. After 1 hour, 0.210 g of H3BTC (1.0 mmol) was added into the solution. The mixture was stirred for an additional 2 hours to form a homogenous solution and then transferred into a Teflon-lined autoclave. The mixture was heated at different temperatures for 24 hours. After cooled naturally to room temperature, the solid was recovered by centrifugation, washed with ethanol for several times. Finally, to verify the stability of the materials, the product was treated with ethanol at 65 oC for 12 hours. The products were denoted as Cu-BTC-n, in which n represents the synthesis temperature.


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The same procedure were also carried out for the synthesis of Cu-BTC mesoMOFs using MeOH, CH3CN, acetone and DMF as the solvents instead of ethanol. Besides, Cu-BTC mesoMOFs samples were prepared by using copper chloride, copper sulfate and copper acetate as the metal source respectively. Characterization. X-ray powder diffraction (XRPD) patterns of the materials were recorded using a Rigaku D/max-2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 50 KV and 200 mA. The morphologies of the obtained Cu-BTC-n products were characterized by a HITACHI SU8020 scanning electron microscopy (SEM). SEM images in Supporting Information (SI) were recorded on a Field-emission scanning electron microscopy (FESEM) by using a Magellan 400, FEI microscope. Transmission electron microscopy (TEM) were obtained on an FEI Tecnai G2 F20 s-twin D573 field emission transmission electron microscope with an accelerating voltage of 200 kV. Thermalgravimetric analyses (TGA) were performed on a TA Q500 thermogravimetric analyzer with a heating rate of 10 oC min-1 under an air flow. N2 adsorption-desorption isotherms were measured at 77 K by using a Micromeritics ASAP 2420. The samples were degassed at 120 oC for 12 h prior to analysis. The Brunauer-Emmett-Teller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.05 to 0.3. The pore size distributions were estimated by using Barrett-Joyner-Halenda (BJH) method from adsorption branch of N2 isotherm. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The mesopore pore volume was obtained from BJH cumulative specific adsorption volume. Oxygen reduction reaction (ORR) measurements. The electrochemical tests were carried out with a typical three-electrode cell using CHI 660B (shanghai, China). A platinum wire was used as the counterelectrode and a saturated calomel electrode (SCE) as the reference electrode. The


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working electrode was prepared by applying the catalyst ink onto a pre-polished glassy carbon electrode (GCE). At firstly, the sample was dispersed in mixed solvent of DMF and ultrapure water (v/v=1:1) and ultrasonication for 10 min to form a homogenous sample ink (2 mg·mL−1). Then, a total of 10 µL well-dispersed sample ink was dropped onto the clean surface of the electrode (3 mm in diameter). The loading for each catalyst sample was about 283 µg·cm-2. After drying at room temperature, 5 µL of Nafion solution in ethanol (0.5 wt%) was applied onto the surface of the catalyst layer to produce a uniform protective film. The addition of the small amount of Nafion could effectively improve the dispersion of the catalyst suspension and enhance its binding onto the GCE surface. The as-prepared electrodes were dried at room temperature overnight. The electrolyte solutions were purged with O2 for 30 min before the experiments. During the electrochemical measurements, the headspace of the electrochemical cell was continuously purged with O2. All the experiments were conducted at room temperature. Finally, the reported potentials were converted to a reversible hydrogen electrode (RHE) by adding a value of (0.2438+0.059 pH) V. RESULTS AND DISCUSSION All of the Cu-BTC-n materials exhibited broad X-ray powder diffraction (XRPD) peaks, and all were in good agreement with the simulated XRPD pattern of HKUST-1 (Figure 1). Thermogravimetric analysis results indicated that all of the Cu-BTC-n mesoMOFs display good thermal stabilities, up to 280 oC, similar to HKUST-1 (Figure S1). The hierarchical pores of Cu-BTC-n mesoMOFs were revealed by N2 adsorption-desorption and transmission electron microscopy (TEM). As shown in Figure 2, N2 adsorption-desorption


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isotherms of Cu-BTC-n mesoMOFs are a combination of type I and type IV, which is characteristic of microporous and mesoporous materials. As apparent in Figure 2, the hysteresis loop gradually moves to higher relative pressure in products that were synthesized using elevated synthesis temperature, indicating that higher synthetic temperatures yield larger mesopore sizes. The isotherms of all Cu-BTC mesoMOFs exhibit a type H1 hysteresis loop with sharp adsorption and desorption branches, which correspond to narrow mesopore size distributions (inset in Figure 2). It is worth noting that all the materials were treated with ethanol at 65 oC for 12 h to demonstrate that the pores among the particles are stable and can be retained as they dispersed in solutions. All of the Cu-BTC-n mesoMOFs have high mesoporous BET surface areas, large pore volumes and large pore sizes. The detailed porosity properties are listed in Table 1. The CuBTC-25 mesoMOF (the synthesis temperature is 25 oC) has the biggest BET surface area of 1048 m2 ·g-1, with a total pore volume of 0.68 cm3 ·g−1. The average pore diameter of the mesopore is about 26 nm. The BET surface area of Cu-BTC-n (n = 65, 85, 110, 120) declines from 954 to 651 m2·g-1 as the synthesis temperature increases, while the pore volume raises to 1.0 cm3·g-1 and the pore size increases to 61 nm. Evidently, higher temperature results in larger mesopores, which is also supported by the TEM observations. As shown in Figure 3, all of the obtained Cu-BTC mesoMOF samples consist of agglomerates (assembly of rigidly joint nanoparticles). When the synthesis temperature is lower, such as 25, 65, 85 oC, the obtained CuBTC mesoMOF samples are made up of smaller nanoparticles as well as smaller mesopores (Figure 3a-3c). While increasing the synthesis temperature, the nanoparticles and mesopores obviously increase (Figure 3d-f). In addition, when the synthesis temperature is to 140 oC, the


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pores become larger and some macropores appear; the largest pore size reaches 72 nm, which is in the range of macropores. Nevertheless, all of the Cu-BTC mesoMOFs contain micropores with a diameter of 0.86 nm calculated by Horvath-Kawazoe analysis (Figure S2), in good agreement with the micropore diameter (0.86 nm) estimated from crystallographic data of original HKUST-1. Furthermore, when the synthesis temperature is increased up to 160 oC, an unknown crystalline phase appears and N2 sorption indicates that this phase does not contain mesopores (Figure S3 and Figure S4). Above all, the optimal synthesis temperature is 110 oC, which results in good-quality Cu-BTC mesoMOFs materials with larger mesopores, surface area and pore volume. The above results demonstrate that by using the template-free strategy, stable hierarchically micro- and mesoporous Cu-BTC mesoMOFs have been successfully synthesized. It is worth noting that compared with reported Cu-BTC mesoMOF materials, our materials have relatively higher BET surface area, larger mesopore and larger pore volume (Table S1).5, 8, 11, 13-14, 45-46 Most importantly, the mesopore size of Cu-BTC mesoMOFs can be easily controlled in a wide range by a judicious choice of the synthesis temperature. Cu-BTC mesoMOFs with large mesopores, constructed by the aggregates of microporous nanoparticles, can also be confirmed using scanning electron microscopy (SEM) (Figure 4). SEM micrographs show that the packing of Cu-BTC nanoparticles built up the mesopores. The small size of nanoparticles for Cu-BTC mesoMOFs can be proven by the broadening of the XRPD peaks (Figure 1).The size of the Cu-BTC-n mesoMOFs nanoparticles range from 15 to 26 nm, as calculated using the Scherrer formula with XRPD patterns (Table S2).47-48 With the


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increase of the synthesis temperature, the nanoparticles became larger (15.2, 16.3, 17.6, 18.4, 22.5, 26.3 nm for 25, 65, 85, 110, 120, 140 oC, respectively), as well as the mesopores. The SEM results were consistent with those of the N2 sorption study and TEM analysis. Compared with the traditional synthesis of HKUST-1, the presence of acetic acid and TEA plays an important role in the synthesis of Cu-BTC mesoMOFs. The introduction of acetic acid and TEA can adjust the crystallization rate of the crystal and prevent the formation of big crystals. As shown in Figure S5, when only acetic acid is introduced, the peaks in the XRPD pattern of Cu-BTC MOF are narrow and sharp, indicating that the crystals are larger (calculated nanoparticle size is around 66 nm). This material exhibits a N2 adsorption-desorption isotherm that is classified as type I, which further proves that no mesoporous MOFs are present in this sample (Figure S6). When only TEA is introduced, the peaks in the XRPD pattern become broader, but the N2 sorption isotherm is not type IV, indicating that no mesoporous MOFs are gained. When adding acetic acid and TEA simultaneously, the broad peaks of the XRPD pattern and the N2 sorption isotherm are type I and type IV, indicating that the Cu-BTC mesoMOF material with small particles and hierarchical pores is successfully obtained (Figure S5 and Figure S6). To estimate the amount of nitrogen in the obtained materials, the energy dispersive spectroscopy (EDS) characterization was carry out and Cu-BTC-110 was taken as an example. EDS result shows that there exist very small amounts of nitrogen, which indicated that the obtained materials maybe include tiny triethylammonium acetate of trimesate salt (Figure S7). Moreover, water as the co-solvent in the synthesis of HKUST-1 should be avoided in the synthesis of Cu-BTC mesoMOFs. The existence of water inhibits the formation of the mesopores (Figure 5).


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On the basis of the above experimental results, a possible procedure for the temperaturecontrolled synthesis of tunable Cu-BTC mesoMOFs is shown in Scheme 1. At first, copper ions and acetates form the known [Cu2(CH3COO)4] building units. Then, these building units react with the deprotonated BTC3- organic ligands to generate the HKUST-1 nanoparticles.49 After that, the nanosized MOF particles co-assemble into mesostructures by aggregation. The crystallization temperature is the key to the size of the MOF particles. Here, low temperature leads to the formation of smaller microporous nanoparticles; tight packing of these nanoparticles thus results in small pores (left in Scheme 1). At high temperature, the Cu-BTC nanoparticles become bigger and the loose packing cause larger pores. Therefore, the pore size of Cu-BTC-n mesoMOFs can be finely controlled by tuning the synthesis temperature. Furthermore, the approach used for the synthesis of Cu-BTC mesoMOFs can also be extended to other solvents, such as methanol, n-propanol, DMF, acetone and acetonitrile, and the copper nitrate can be replaced by copper chloride, copper sulfate and copper acetate. The obtained mesoMOFs materials possess similar tunable mesopores (see SEM images, XRPD patterns and N2 adsorption-desorption isotherms in Figure S9-S19 and Table S3). It indicates that the Cu-BTC mesoMOFs can be synthesized in various systems, providing more choices for the future large scale preparation. To explore the applications of MOF materials without dopants as the electrocatalysts for ORR, the Cu-BTC MOF material, HKUST-1, was prepared according to the literature. The ORR activity of the Cu-BTC modified electrode was measured by cyclic voltammetry. In the N2 saturated solution of 0.1 M KOH, featureless voltammetric current was observed within the potential range involved, whereas in 0.1 M KOH solution saturated with O2, a substantial reduction occurred with an obvious ORR catalytic current. The onset and peak potentials for


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ORR were found at +0.82 V and +0.52 V (vs. RHE), respectively (as shown in Figure 6a). For comparison, Cu-BTC-110 was fabricated onto the GCE, demonstrating an onset potential at +0.85 V and a peak at +0.54 V (vs. RHE), respectively (Figure 6b). The similar onset and peak potentials might be assigned to the redox process of Cu2+/Cu+ in the MOF.50 It is worth noting that the ORR current density is 1.3 times increased, from 0.428 mA·cm-2 of the HKUST-1 modified electrode to 0.572 mA·cm-2 of the Cu-BTC-110 modified electrode. Compared with the HKUST-1 modified electrode, the increase of the catalytic current was possibly due to the more exposed catalytic potential in the porous Cu-BTC-110 material. The hierarchical porous CuBTC-110 also accelerate the diffusion of O2 and thus improve the catalytic current. In addition, the stability of the electrocatalysts was confirmed by XRPD and the result shows that the structure of Cu-BTC-110 material remains stable after the ORR electrochemical measurements (Figure S8). The above results demonstrate that MOFs materials have potential applications toward ORR. CONCLUSION In summary, we describe a simple, convenient temperature-controlled assembly of a series of Cu-BTC mesoMOFs by using a template-free strategy. The Cu-BTC mesoMOF materials have distinct characteristics of mesoporous materials and narrow pore size distributions. The mesopores, which are constructed by the stacking of microporous Cu-BTC nanoparticles, can be simply adjusted by varying the synthesis temperature. This template-free strategy could be applied to other organic solvents. Moreover, Cu-BTC mesoMOFs with large mesopores provide the possibility for potential application in ORR, and further research is currently ongoing in our group.


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ASSOCIATED CONTENT Supporting Information. Characterization of Cu-BTC mesoMOFs; TGA curves of Cu-BTC-n mesoMOFs; The detailed porosity properties of Cu-BTC-n mesoMOFs; XRPD, SEM and N2 adsorption-desorption isotherms of Cu-BTC-n mesoMOFs, as well as XRPD, SEM and N2 adsorption-desorption isotherms of Cu-BTC products synthesized with other solvents and metal salts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(Y.L.) Fax: +86-431-85168624. E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 21171064, 21371067 and 21373095). The authors would like to thank Jacilynn Brant for helpful discussions. REFERENCES (1)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. Collected XRPD patterns of Cu-BTC-n mesoMOFs and simulated XRPD pattern of HKUST-1. Figure 2. N2 adsorption-desorption isotherms measured at 77 K of Cu-BTC-25, Cu-BTC-65, CuBTC-110 and Cu-BTC-140. The insets show the corresponding pore size distribution curves calculated by BJH method. For clarity, the sorption isotherms of Cu-BTC-85, Cu-BTC-120 and Cu-BTC-160 are shown in Figure S4. Figure 3. TEM images of the mesoporous Cu-BTC-n MOFs prepared at (a) 25 oC, (b) 65 oC, (c) 85 oC, (d) 110 oC, (e) 120 oC and (f) 140 oC, respectively; Scale bars = 500 nm. Table 1. Porosity properties of the Cu-BTC-n mesoMOFs obtained at different synthesis temperatures. Figure 4. SEM images of the mesoporous Cu-BTC-n MOFs prepared at (a) 25 oC, (b) 65 oC, (c) 85 oC, (d) 110 oC, (e) 120 oC and (f) 140 oC, respectively; Scale bars = 200 nm. Figure 5. N2 adsorption-desorption isotherms of Cu-BTC products synthesized with different volume ratio of H2O and EtOH. Scheme 1. Schematic illustration of a possible procedure for the formation of the hierarchical Cu-BTC-n mesoMOFs Figure 6. Cyclic voltammetry responses of (a) Cu-BTC-modified electrode and (b) Cu-BTC-110 modified electrode in N2 saturated (blue curves) and O2 saturated (red curves) 0.1 M KOH.


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Figure 1

Figure 2


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Figure 3


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Page 20 of 24

Table 1. Porosity properties of the Cu-BTC-n mesoMOFs obtained at different synthesis temperatures. a

sample Cu-BTC-25 Cu-BTC-65 Cu-BTC-85 Cu-BTC-110 Cu-BTC-120 Cu-BTC-140

b

c

d

e

SBET (m2g-1)

Smicro (m2g-1)

Smeso (m2g-1)

Vt (cm3g-1)

Vmeso (cm3g-1)

1048 954 905 795 651 612

729 608 566 558 489 474

319 346 339 237 162 138

0.82 1.056 1.144 1.129 1.001 0.734

0.63 0.83 0.89 0.96 0.81 0.516

f

Dmeso (nm) 26 31 34 47 61 72

a

SBET is the BET surface area.

b

Smicro stands for the t-plot micropore surface area calculated from the N2 sorption isotherm.

c

Smeso is the mesopore surface area estimated by subtracting Smicro from SBET.

d

Vt stands for the single point total pore volume determined by using the adsorption branch of the N2 isotherm at P/P0 = 0.99.

e

Vmeso is the mesopore volume obtained from the BJH adsorption cumulative volume.

f

Dmeso stands for mesopore diameter calculated from adsorption branch of N2 isotherm by using BJH method.


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Figure 4


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Figure 5

Scheme 1


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Figure 6


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Table of Contents Facile Fabricating Hierarchically Porous Metal-Organic Frameworks via a Template-Free Strategy Yu Cao, Yali Ma, Tao Wang, Xue Wang, Qisheng Huo and Yunling Liu*

A series of tunable Cu-BTC mesoporous Metal-Organic Frameworks (mesoMOFs) are prepared conveniently through a template-free strategy. The packed mesopores are constructed of microporous Cu-BTC nanoparticles and can be simply adjusted by controlling the synthesis temperature.


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Facile Fabricating Hierarchically Porous MetalâOrganic Frameworks

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