Journal of Colloid and Interface Science 416 (2014) 198–204

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Hollow metal–organic framework polyhedra synthesized by a CO2–ionic liquid interfacial templating route Li Peng, Jianling Zhang ⇑, Jianshen Li, Buxing Han, Zhimin Xue, Binbin Zhang, Jinghua Shi, Guanying Yang CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

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Article history: Received 24 September 2013 Accepted 25 October 2013 Available online 15 November 2013 Keywords: Gas–ionic liquid interface Metal–organic framework Hollow polyhedra Gas separation Catalyst

a b s t r a c t We proposed for the first time a CO2–ionic liquid (IL) interfacial templating route for the production of hollow MOF microparticles. By adding the gaseous CO2 into IL phase under stirring, a large number of CO2 bubbles are generated, which provide numerous CO2–IL interfaces, serving as the nucleation or agglomeration centers for the MOF nanocrystals. By this strategy, the hollow and mesoporous Zn–BTC (BTC: 1,3,5-benzenetricarboxylic) tetrahedroids were fabricated. The morphologies of the Zn–BTC polyhedra can be easily controlled by CO2 pressure. The as-synthesized Zn–BTC hollow microparticles have shown potential applications in gas separation and catalysis. Furthermore, the CO2–IL interface templating approach has been successfully applied to the fabrication of microsized Zn–BDC (BDC: 1,4-benzenedicarboxylic) hollow prisms. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Metal–organic frameworks (MOFs) [1] have recently emerged as a class of promising organic–inorganic hybrid materials with diverse structural topologies, tunable functionalities and a wide range of applications in catalysis [2], separation [3], solar energy conversion [4], biological and medical applications [5]. A burgeoning research interest in the field is to fabricate porous MOFs in higher order structures or assemblies, which will reinforce the usefulness of MOF materials and expand the scope of utilization [6]. For example, the hollow particles, an important class of materials with large internal cavities and thin shells, present a variety of applications in different fields [7]. However, the reports on MOFs in hollow form have been scarcely published. The currently available methods for hollow MOF synthesis are liquid–liquid interface templating route [8], solid–solid interface templating route [9], and spray-drying methodology [10]. For all the reported cases, the hollow MOF crystals are in the most common spherical form [8–10]. The interface offers an important path for the self-assembly and chemical manipulation of nanocrystals [11,12]. Conventionally, hollow particles are produced by liquid–liquid interface [8,13] and solid–solid interface [9,14] templating route, which use liquid or solid as the internal core. Interestingly, N2 bubbles [15,16] and CO2 droplets [17,18] have been utilized as the internal cores for ⇑ Corresponding author. Fax: +86 10 62559373. E-mail address: [email protected] (J. Zhang). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.041

the interfacial templating fabrication of hollow particles. The utilization of CO2 is very attractive because CO2 is nontoxic, nonflammable, abundant, tunable, and can be easily recaptured and recycled after use [19–22]. In general, water is used as the immiscible phase with N2 or CO2 for interfacial templating method to synthesize hollow particles. The N2–water interface and CO2– water interface templating routes allow the production of hollow particles from water-soluble reactants. In recent years, ionic liquids (ILs) have received tremendous attention owing to their negligible vapor pressures, high chemical and thermal stability, wide liquid temperature range and wide electrochemical windows [23–25]. Most importantly, ILs can solvate a wide range of organic and inorganic reagents, excelling the conventional solvents (water and organic solvent). These unique features have made ILs promising media for the design and preparation of novel MOFs [26–31]. Commonly, ILs behave as solvents, structure-directional template, or charge-compensating groups in the reaction systems for MOF formation. Here we propose for the first time a CO2–ionic liquid (IL) interface templating route for the production of hollow MOF microparticles. IL can simultaneously solubilize the inorganic and organic MOF precursors, which have markedly different solubility characteristics; the CO2 bubbles generated in IL can provide numerous CO2–IL interfaces, serving as the nucleation or agglomeration centers for the MOF nanocrystals. The monodisperse hollow Zn–BTC (BTC: 1,3,5-benzenetricarboxylic) tetrahedroids with a mesoporous structure were formed via this strategy. The morphologies of the MOF polyhedra can be easily controlled by CO2 pressure. The

L. Peng et al. / Journal of Colloid and Interface Science 416 (2014) 198–204

as-synthesized Zn–BTC hollow microparticles have shown potential applications in gas separation and catalysis. Furthermore, the CO2–IL interface templating approach has been successfully applied to the fabrication of microsized Zn–BDC (BDC: 1,4-benzenedicarboxylic) hollow prisms. The CO2–IL interface templating route has many advantages for the MOF synthesis. For example, the IL can simultaneously solubilize the inorganic and organic MOF precursors, which have markedly different solubility characteristics; the inner core (CO2) can be easily removed by depressurization; the process involves no volatile solvent, and the route can be applied to the synthesis of diverse kinds of MOFs owing to the well solvency of IL for a wide range of compounds. 2. Experimental section 2.1. Materials CO2 was provided by Beijing Analysis Instrument Factory with >99.95% purity. The surfactant N-ethyl perfluorooctylsulfonamide (N-EtFOSA, >95% purity) was purchased from Guangzhou Leelchem Corporation without any further purification. The IL 1,1,3,3-tetramethylguanidine trifluoroacetate (TMGT) was synthesized by direct neutralization of 1,1,3,3-tetramethylguanidine (TMG) with trifluoroacetate. TMG was produced by Alfa Aesar. Trifluoroacetate and triethylamine were provided by Beijing Chemical Reagent Company. Zinc acetate dihydrate (Zn(CH3COO)22H2O) (A.R. Grade) was purchased from Alfa Aesar. 1,3,5-Benzenetricarboxylic acid (H3BTC) and 1,4-benzenedicarboxylic acid (H2BDC) (95% purity) were purchased from Sigma–Aldrich. Propylene oxide (PO), tetrabutylammonium bromide (n-Bu4NBr) and ethyl acetate were A.R. Grade and purchased from Beijing Chemical Reagents Company. n-Dodecane (99% purity) was provided by Alfa Aesar. 2.2. MOF synthesis For the synthesis of Zn–BTC, Zn(CH3COO)22H2O (1 mmol) and H3BTC (0.1 mmol) were loaded into an autoclave containing N-EtFOSA/TMGT solution (5 g). Then triethylamine (0.3 mL) was added into the autoclave to deprotonate the linker. The temperature of the autoclave was controlled at 30 °C. CO2 was charged into the autoclave until the suitable pressure was reached. To ensure the gaseous state of CO2, the pressure was controlled to be lower than 7.2 MPa (the saturated vapor pressure of CO2 at 30 °C). After reaction for 5 h, CO2 was released by depressurization. The precipitate was collected and washed with ethanol. For the synthesis of Zn– BDC, the ligand was replaced by H2BDC, while other experimental conditions and procedure are the same with those above for the Zn–BTC synthesis. 2.3. MOF characterization The morphologies of the MOFs were characterized by a HITACHI S-4800 scanning electron microscope and TEM JEOL-1010 operated at 100 kV. To obtain cross-sectional view of the microparticle shell, the dry sample was embedded in epoxy resin (SPI-CHEM), then ultramicrotomed (Ultracut, Leica, Germany) to a thickness of approximately 60 nm, and collected on mesh grid for TEM imaging. The small-angle X-ray scattering (SAXS) experiment was carried out at Beamline 1W2A at the Beijing Synchrotron Radiation Facility (BSRF). The wavelength was 1.54 Å, and the distance of sample to detector was 2.78 m. The data were collected using a CCD detector (MAR) with a maximum resolution of 3450  3450 pixels. The mesopore size distribution was obtained from SAXS data by using an Irena tool suite within the Igor pro application software. The wide-angle powder X-ray diffraction (XRD) analysis of the MOF

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was performed on the X-ray diffractometer (Model D/MAX2500, Rigaku) with Cu Ka radiation. Thermogravimetric (TG) analysis was carried out on a PerkinElmer TGA instrument. The temperature range was from 50 to 750 °C with a heating rate of 10 °C/ min under nitrogen atmosphere. FT-IR spectra were obtained using a Bruker Tensor 27 spectrometer. The elemental analysis of C, H, and N was performed on FLASH EA1112 elemental analysis instrument. The Zn content of sample was determined by ICP-AES (VISTA-MPX). 2.4. Gas adsorption The desorption isotherms of CO2, N2 and CH4 were recorded at 273.2 K for degassed Zn–BTC in pressure range of 0.0004–1 atm on a TriStar II 3020 device. For each measurement, about 100 mg of the sample was degassed using the same procedure. 2.5. Catalytic test The formation of propylene carbonate (PC) via the reaction of CO2 and PO catalyzed by Zn–BTC is shown as follows. n-Bu4NBr was used as a promoter for the reaction.

The catalytic reactions were conducted in a 22 mL stainless steel reactor equipped with a magnetic stirrer. In a typical experiment, Zn–BTC (0.05 g), PO (20 mmol) and 0.12 g n-Bu4NBr were added into the reactor. The sealed reactor was put into a constant-temperature air bath at a desired temperature. CO2 was then charged into the reactor under stirring until the desired pressure was reached. After reaction for a certain time, the reactor was cooled down to room temperature and CO2 was released slowly to a cold trap containing ethyl acetate. Then the ethyl acetate in the cold trap was added into the reactor. The reaction mixture with internal standard n-dodecane was analyzed by GC instrument (Agilent 6820) equipped with a flame-ionized detector. 3. Results and discussion 3.1. Hollow Zn–BTC polyhedra Zn–BTC is one of the most extensively explored MOFs [32,33]. As examples, Fig. 1a and b shows the SEM images of the Zn–BTC synthesized at 6.3 MPa, the surfactant N-EtFOSA concentration being 2.0 wt% based on the IL TMGT. Clearly, the uniform and monodisperse microparticles were formed, with the average particle size of 3 lm. From a typical microparticle shown in Fig. 1c, the slightly truncated tetrahedroid was observed. The tetrahedroids are hollow inside, as demonstrated by the SEM image of the broken particles shown in Fig. 1d and the TEM image shown in Fig. 1e. The cross-sectional TEM image reveals that the shell of the hollow microparticle has a mesoporous structure and the mesopore size is about 6 nm (Fig. 1f). The mesoporous structure of the MOF was further proved by small-angle X-ray scattering (SAXS) technique [34,35]. As shown in Fig. 2A, the strong intensity increase in the low-q region (