Controllable Preparation of Nanoscale MetalâOrganic Frameworks by

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Controllable Preparation of Nanoscale MetalOrganic Frameworks by Ionic Liquid Microemulsions Weizhong Zheng, Xiaolei Hao, Ling Zhao, and Weizhen Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Industrial & Engineering Chemistry Research

Controllable Preparation of Nanoscale Metal-Organic Frameworks by Ionic Liquid Microemulsions Weizhong Zhenga, Xiaolei Haoa, Ling Zhaoa,b , and Weizhen Suna,* a

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China a,b Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

ABSTRACT ： With the particles size down to nanoscale, nano metal-organic frameworks (NMOFs) with well-controllable dimension exhibit more potential applications in drug delivery, biosensing and biomedical imaging. Although the microemulsion method provides an efficient approach for preparing nano particles, the synthesis of NMOFs with narrow size distribution is a great challenge. In this work, the nanoscale zeolitic imidazolate frameworks (NZIFs), being considered as the subclass of MOFs, were synthesized by the ionic liquid-containing microemulsion system of H2O/BmimPF6/TX-100. The obtained NZIFs have extremely small size of no more than 2.3 nm, narrow distribution of less than 0.5 nm, and good thermal stabilities. By adding ethanol into H2O/BmimPF6/TX-100 system, the [Cu3(BTC)2(H2O)3]n (HKUST-1) has been successfully synthesized with similar nano dimensions to NZIFs. The molecule dynamic simulation reveals that one new microemulsion was formed, in which the ethanol and water molecules were capsuled by *

Additional Supporting Information may be found in the online version of this article. Correspondence concerning this article should be addressed to W. Z. Sun at [email protected]. 1

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the surfactant TX-100 and BmimPF6. This new microemulsion is beneficial to the dissolution of organic ligand 1,3,5-benzenetricarboxylic acid. Hopefully this work can provide new insights into the green production of nanoscale MOFs. Keywords: MOFs, IL microemulsion, Well-controllable dimension. 1. INTRODUCTION Metal-organic

frameworks

(MOFs),

constructed

with

the

self-assembly of metal ions or metal clusters and organic bridging ligands, are a novel class of crystalline porous materials1-4. To date, porous MOFs with high surface area and designable structures have attracted tremendous attention owing to their potential applications in gas sorption5, energy storage6, catalysis7 and selective separation8 etc. With the particles size down to nanoscale, MOFs with well-controllable dimension exhibit an additional potential application in catalysis9, drug delivery10, biosensing11 and biomedical imaging12. Till now, two major strategies regarding the synthesis of the nanoscale MOFs (NMOFs) have been reported according to the previous researches13-16. One strategy concerns the controlled precipitation of MOFs,

including

ultrasounds14,

thermal

conditions17

and

surfactant-directed12, and the other strategy is related to the self-assembly of MOFs, such as templates and microemulsions18. Due to the existence of many factors that affects the precipitation of precursors, it is

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challenging to synthesize different kinds of NMOFs in the same or similar reaction conditions. In contrast, microemulsions provide an efficient approach for preparing NMOFs. In microemulsions, the dispersed phase is considered as numerous “nano reactors” that can well control the growth of the particles. However, the dissolution of organic ligands in microemulsions is still an obstacle for synthesizing NMOFs with well-controlled sizes5, 19, 20. In our recent work, nanoscale zeolitic imidazolate frameworks (ZIFs) with well-controllable size distribution were prepared in conventional reverse microemulsion21. Although such a conventional reverse microemulsion is confirmed to be an efficient mean to synthesize NMOFs, the large amount of spent heptane causes serious environmental pollution and its recovery is cost-intensive, particularly for large-scale production of NMOFs. Herein, we employ a green method to synthesize NMOFs using ionic liquid microemulsions (ILMEs) which are considered as an environmentally friendly system to construct nanoscale materials, since ionic liquids (ILs) possess many excellent properties, including outstanding dissolution performance, designable combination of cations and anions, negligible vapor pressure, and easy recovery22-24. In the pioneering work, Han et al. synthesized La(BTC)(H2O)6 with different nanoscale shapes by using H2O/TX-100/BmimPF6 microemulsions, which provided a possible environmentally friendly system to achieve the

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production of NMOFs20. They emphasized that the shape of the dispersed phase has a crucial effect on determining the morphology of the MOF nuclei formed at the beginning, which further controls the shape of the final MOFs. Nevertheless, the mean particle size of MOFs obtained in their work seems to be larger than 200 nm with slightly poor particle size distribution, which was ascribed to the fact that the coordination reaction proceeds at the water/IL interface, because metal ions and organic ligands are dissoluble in water phase and IL phase, respectively. In the H2O/TX-100/BmimPF6 microemulsion, some nanoparticles with the mean size of about 3~5 nm such as Pd25 and Pd4Au26, were successfully synthesized. However, it is still a challenge to synthesize MOFs with extremely small size less than 10 nm in such microemulsion, especially when the organic ligands constructing the MOFs are not water-soluble. In

the

present

work,

nanoscale

ZIF-8

and

ZIF-67

with

well-controllable dimension and uniform particle size distribution were successfully synthesized on the basis of the H2O/TX-100/BmimPF6 microemulsion. By adding ethanol into H2O/TX-100/BmimPF6 system to improve the dissolution of organic ligands, the [Cu3(BTC)2(H2O)3]n (HKUST-1) was further synthesized with similar nano dimensions to NZIFs. The mechanism of controlling the crystal growth in the dispersed phase was proposed and further confirmed by molecular dynamics simulation. In addition, from the industry of interest, a green method of

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demulsification and the recycling of ILs were presented. Hopefully the process introduced here can well confine the growth of the nanoparticles and provide a new environment-friendly way to produce nanoscale MOFs in large-scale. 2. EXPERIMENTAL 2.1 Synthesis of MOFs in ILMEs Ionic liquid microemulsion containing water, TX-100 and BmimPF6 was constructed according to the previous report27, and denoted as ILME-1. The BmimPF6 was synthesized on the basis of the pioneering work28. The ZIF-8 and ZIF-67 nanocrystals (NZIF-8 and NZIF-67) were synthesized in ILME-1 using a direct mixing method21, 29. The quaternary component system composed of water, ethanol, TX-100 and BmimPF6 (denoted as ILME-2) was formed by adding ethanol in ILME-1. Similarly, the HKUST-1 nanocrystal (NHKUST-1 b) was synthesized in ILME-2 using a direct mixing method29, 30. For comparison, HKUST-1 nanocrystal (NHKUST-1 a) was synthesized in ILME-1. To remove the solvent, the samples were immersed in dichloromethane (CH2Cl2) and ethanol solution (V:V=1:1) with stirring at room temperature for 48 h. The detailed synthesis procedures of BmimPF6 and MOFs were described in the Supporting Information. The whole schematic illustration for the process of MOFs synthesis is shown in Figure 1.

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Figure 1. Schematic illustration for the process of MOFs synthesis.

2.2 Molecular dynamic simulation The quaternary component system composed of water, ethanol, TX-100 and BmimPF6 was investigated via molecular dynamic (MD) simulation in order to confirm the location of ethanol in ILME-2. The initial simulated box was built and maintained at the size of 6.0×6.0×6.0 nm3 using PACKMOL software31. To reduce the computational cost, water and ethanol molecules were initially wrapped by the TX-100, and BmimPF6 was placed in the most outside layer of the built box with the corresponding molecule number of 300, 100, 80, and 400, respectively. The obtained box was simulated using GROMACS 4.5 package. Firstly, the energy minimum was performed for 5000 steps to eliminate the overlap in the initial box. Then, 8 ns quenching simulation was carried out under the Canonical ensemble (NVT) with Hoover-Nose thermostat using the relaxation time of 0.5 ps from 300 K to 500 K, and then back to

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300

K.

Afterwards,

the

quenched

box

was

simulated

under

isothermal-isobaric ensemble (NPT) with Parrinello-Rahman barostat for 60 ns to equilibrate. Periodic boundary conditions (PBC) were employed in three directions. The particle mesh Ewald method was used to deal with long-range electrostatic interactions with a cutoff of 1.2 nm. The CGenFF force field was used to describe the interaction of water, ethanol, TX-10032 and BmimPF633. Water molecules were described by the extended simple point charge (SPC/E) model. 2.3 Characterization of MOFs In this work, power X-ray diffraction (PXRD) patterns of MOFs were characterized through the Bruker D8 Advance X-ray powder diffractometer using CuKα radiation (λ=1.54059 Å) at 40 kV and 40 mA at room temperature. In addition, transmission electron microscopy (TEM) images of MOFs were characterized by the FEI Tecnai G2 spirit BioTwin at 300 kV. The SDT Q600 thermal analyzer with a ramp rate of 10 oC /min from room temperature up to 800 oC in the nitrogen atmosphere was used to characterize thermal gravimetric analysis (TGA) of MOFs. Meanwhile, the nitrogen adsorption-desorption was characterized by the Micromeritics ASAP 2020M analyzer at 77 K. 3. RESULTS AND DISCUSSION 3.1 XRD of MOFs synthesized in ILMEs The powder XRD patterns of MOFs synthesized in ionic liquid

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microemulsions is presented in Figure 2. A good agreement can be seen between the XRD patterns of NZIFs (NZIF-8 and NZIF-67) and NHKUSTs (NHKUST-1a and NHKUST-1b) synthesized in the ILMEs and those of the corresponding reported and simulated patterns34-36. For example, the major peaks of the XRD patterns synthesized in the ILMEs, corresponding to the planes (011), (022), (112), (022), (013), and (222) for NZIFs and the planes (200), (220), (222), (400), (331), and (333) for NHKUST-1, agree well with those of the samples’ XRD patterns. Furthermore, the peak broadening can be observed compared to the samples’ XRD pattern. Therefore, it can be concluded that both of NZIFs and NHKUSTs are formed in ILMEs.

Figure 2. PXRD patterns of MOFs synthesized in ILMEs.

3.2 TEM image of the nanoparticles The TEM images of the nanoparticles synthesized in ILMEs are shown in Figure 3. It can be noticed that the NZIF particles (Figure 3a and Figure 3b) with remarkably small mean particle size and extremely narrow particle size distribution synthesized in the ILME-1 (including

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water, TX-100 and BmimPF6) are well dispersed and regularly spherical. The mean particle size of NZIF-8 and NZIF-67 are 2.2 nm (σ = 0.5) and 2.3 nm (σ = 0.4), respectively. The NZIF particles obtained in this work possess remarkably smaller mean particle size than the corresponding samples obtained in water and methanol, but are comparative to those obtained from the conventional reverse microemulsion21. In addition, compared with the La-MOFs prepared by Han20, the NZIF-8 and NZIF-67 synthesized in the ILME-1 have a smaller size (only 1/100 of La-MOFs particles). This should be ascribed to the different mechanism of controlling the crystal growth in the coordination reaction of metal ions and organic ligands. Using ILME-1 as reaction media the NHKUST-1a was synthesized. From the TEM image of NHKUST-1a (Figure 3d), the particles size is large (almost about 100 nm or even bigger). However, for the NHKUST-1b (Figure 3c) synthesized in the ILME-2 (including water, ethanol, TX-100 and BmimPF6), the nanoparticles are well dispersed and regularly spherical. The mean particle size of NHUKST-1b is 1.6 nm (σ = 0.4). Compared with the NHKUST-1a, the nanoparticles prepared in the ILME-2 prove that the ethanol as an additive in ILME-2 plays a decisive role in the particle size control.

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Figure 3. TEM image (a) of NZIF-8 synthesized in ILME-1, TEM image (b) of NZIF-67 synthesized in ILME-1, TEM image (c) of NHKUST-1b synthesized in ILME-2 and TEM image (d) of NHKUST-1a synthesized in ILME-1.

3.3 Thermal stability Figure 4 presents the thermal gravimetric analysis (TGA) traces of NZIF-8, NZIF-67 and NHKUST-1b synthesized in ILMEs. The MOFs obtained in this work have the similar thermal stability to those prepared in previous researches34-36. The decomposition temperature of NZIF-8 and NZIF-67 reaches up to 520 oC and 410 oC, respectively, which is in good agreement with the previous work that ZIF-8 exhibits a higher thermal stability compared to ZIF-6737. Additionally, NHKUST-1b is thermally stable up to 290 oC, which is also well consistent with the previous report36.

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Figure. 4 TGA traces of MOFs synthesized in ILMEs.

3.4 Adsorption-desorption isotherms and porosity properties The porous structure properties of MOFs synthesized in ILMEs were evaluated through the N2 adsorption-desorption isotherms method, as shown in Figure 5. For NZIF-8, NZIF-67 and NHKUST-1b, the adsorption isotherms exhibit a similar trend to the type IV, indicating that they own obvious microporous structure.

Figure. 5 The N2 adsorption-desorption isotherms of MOFs synthesized in ILMEs.

The BET surface area, microprobe volume and pore diameter of MOFs synthesized in ILMEs are shown in Table 1. The MOFs (NZIF-8, NZIF-67 and NHKUST-1b) synthesized in ILMEs exhibit a BET surface

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area in the range of 675~800m2/g. Actually, the MOFs synthesized at room temperatures usually own smaller surface areas compared with those synthesized at high temperatures38-40. Indeed, the surface areas of MOFs in this work are comparative to those synthesized MOFs at aqueous room temperatures conditions40, 41. On the other hand, for the synthesized MOFs in this work, some chemical substances e.g. 2-MeIM, H3BTC, TX-100, and BmimPF6 may be adsorbed inside the pores of MOFs and cannot be easily removed through subsequent disposals such as washing and drying. Table 1. The porosity properties of MOFs synthesized in ILMEs

a

Sample

Solvent

SBET (m2/g)

Vmicro (cm3/g)

Da (nm)

NZIF-8

ILME-1

799.7

0.222

0.6

NZIF-67

ILME-1

675.2

0.225

0.6

NHKUST-1b ILME-2

700.9

0.260

0.8

The pore diameter is referred to the pore size corresponding to the peak position

on the pore size distribution (PSD) curve.

3.5 Growth mechanism of the nanoparticle synthesized in ILMEs In microemulsions, the dissolution of organic ligands into the dispersed phase plays an important role in well controlling the particle size. For ZIFs, the building rocks, i.e. 2-methylimidazole (2-MeIM), can dissolve into water, and thus the coordination reaction proceeds within nano water droplets and the nanoscale ZIF-8 and ZIF-67 are synthesized in the ILME-1. However, most organic ligands such as trimesic acid (H3BTC) that is used to synthesize HKUST-1 are insoluble or slightly

12


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soluble in water. Therefore, a new method needs to be designed to dissolve organic ligands in the droplets of ILMEs. As excellent solvents, ethanol exhibits a good dissolving capacity for organic compounds. To reveal the important role of ethanol in the controlling mechanism of particles size, the molecular dynamic (MD) simulations were performed. By MD simulations, the microemulsion system can be found to be well established and the ethanol is observed to be well dispersed in the water droplet, as shown in Figure 6. Therefore, as an additive, ethanol is added into the ILME-1 to form a novel ionic liquid microemulsion (ILME-2) with quaternary component for improving the dissolution of organic ligands. IL removed

Tail (TX-100)

TX-100 removed

Head (TX-100)

Ethanol

Water

IL

Figure 6. The equilibrated snapshot of ILME-2 by MD simulations.

The growth mechanism of HKUST-1 nanoparticle in ILME-2 is similar to that of ZIF nanoparticles in the ILME-1 since for both ILME-1 and ILME-2, the metal ions and organic ligands are dissolved into the water droplets. Therefore, the growth mechanism of HKUST-1 nanoparticle in ILME-2 is discussed as follows, as shown in Figure 7 and that of ZIF nanoparticle is presented in Figure S1 in the Supporting

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Information. The Cu(NO3)2·3H2O and trimesic acid are dissolved into ethanol

aqueous

solution

(50

wt%),

respectively.

Then,

the

above-obtained aqueous solutions are added into the mixture solution of TX-100 and BmimPF6, respectively, to construct ILME-2 (ILME-2A: dissolved metal ions; ILME-2B: dissolved organic ligands). Finally, the ILME-2A and ILME-2B are blended with vigorous stirring. In microemulsion, water droplets undergo a dynamic process of coalescence and separation. When the water droplet containing metal ions collides with the one composed of organic ligands, the compounds between the two droplets exchange and the coordination reaction further occurs. At the beginning, the MOF nuclei are formed by self-coordination of metal ions and organic ligands in the water droplets. The water droplets constrained the growth of MOFs and can well control the particle size.

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Figure 7. Schematic representation of the growth mechanism of HKUST-1 nanoparticle synthesized in ILME-2.

3.6 Demulsification and centrifugation of ILMEs at the end of reaction Ionic liquid microemulsions where at least one constituent is IL are thermodynamically stable system composed of two immiscible solvents stabilized by surfactants42. At the end of the self-coordination reaction, the ILMEs need to be broken to obtain the MOFs. Due to the high stability of the ILMEs and extremely small mean particle size of the MOFs, it is difficult to collect the nanoscale MOFs from this system by the direct centrifugation. In addition, for the conventional chemical demulsification,

new

organic

solvents

are

usually

added

into

microemulsions, which will contaminate the ILMEs and limits the reuse of microemulsion. According to the water/TX-100/BmimPF6 ternary

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phase diagram27, point A, the composition of ILMEs used in this work, is close to the two-phase region, as shown in Figure 8a. Thus, with the addition of a certain amount of water into the system, the composition of ILMEs can be adjusted to point B in the two-phase region. Then, by further centrifugation, the system will be divided into three phases (IL phase, water phase and products phase). The MOF crystals are located in the intermediate layer, with the water phase at the upper layer and the BmimPF6 phase in the bottom layer (see Figure 8b). The MOF crystals are constructed with metal clusters and organic bridging ligands. Thus, they are essentially composed of the hydrophilic domains and hydrophobic ones, meaning that the MOF crystals tend to locate at the interface between the water phase and BmimPF6 phase. In particular, with the particle size of MOFs down to nanoscale, the interface tension between MOF crystals, water, and the BmimPF6 phase plays the major role in determining the location of MOFs after centrifugation in the ILMEs, compared to the gravity of MOF crystals. The water phase was removed by syringe. The ionic liquid phase was collected by syringe and then evaporated under reduced pressure to remove water. The recovered IL (RIL) can be recycled for the next synthesis of MOFs. The products of MOFs are stayed in the centrifuge tube and washed for future use (Please see Supporting Information for the detailed procedure).

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Figure 8. Phase diagram (a) of the ILME-1 ternary system at 25 oC27; (b) the ternary system after demulsification and centrifugation. Point A is the composition of the ILME-1 where the ZIFs are synthesized; point B is the composition of the ternary system after demulsification by adding water; points C and D are the IL phase and water phase, respectively.

4. CONCLUSIONS In summary, nanoscale MOFs, including ZIF-8, ZIF-67 and HKUST-1 were prepared in ionic liquid microemulsions. By using ionic liquid microemulsions, small particle size, uniform size distribution and thermal stability of MOFs crystals were prepared. With addition of ethanol into ILME-1 to dissolve organic ligands into microemulsion, the quaternary component system of water/ethanol/TX-100/BmimPF6 has been demonstrated to be a novel microemulsion. This kind of ILME provides a potential method to enrich the synthesis strategies of MOFs and ethanol as an additive in ILME-2 play a very important role in the particle

size

control.

Furthermore,

the

synthesis

17


process

is


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environmentally friendly and requires less energy due to the use of green solvent

(ionic

liquid)

and

the

simple

demulsification

method.

Additionally, ionic liquid is easily recycled to synthesize MOFs. This work may provide a promising green way to synthesize nanoscale MOFs on a large-scale. AUTHOR INFORMATION Corresponding Author *(W.S.) E-mail: [email protected]. Telephone: +86 21 64253027 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The financial support by the National Natural Science Foundation of China (91434108) is gratefully acknowledged. REFERENCE 1.

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self-assembly in ionic liquids. Chem. Soc. Rev. 2013, 42, (3), 1096-1120.

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