DOI: 10.1002/cphc.201500209
Communications
Assembly of Mesoporous Metal–Organic Framework Templated by an Ionic Liquid/Ethylene Glycol Interface Xinxin Sang, Jianling Zhang,* Li Peng, Chengcheng Liu, Xue Ma, Buxing Han, and Guanying Yang[a] We propose a facile room-temperature synthesis of a metal–organic framework (MOF) with a bimodal mesoporous structure (3.9 and 17-28 nm) in an ionic liquid (IL)/ethylene glycol (EG) mixture. The X-ray diffraction analysis reveals that MOF formation can be efficiently promoted by the presence of the EG/IL interface at room temperature. The MOFs with mesoporous networks are characterized by SEM and TEM. The formation mechanism of the mesoporous MOF in EG/IL mixture is investigated. It is proposed that the EG nanodroplets in the IL work as templates for the formation of the large mesopores. The assynthesized mesoporous metal–organic framework is an effective and reusable heterogeneous catalyst to catalyze the aerobic oxidation of benzylic alcohols.
Metal–organic frameworks (MOFs),[1] also known as porous coordination networks, have recently attracted much attention for their potential use in gas storage,[2] separation,[3] biosensing[4] and catalysis,[5] owing to their fascinating structures and unusual properties such as permanent nanoscale porosity, high surface area, good thermostability, and uniformly structured cavities. MOFs are largely restricted to the microporous regime, which is not favourable to diffusion and mass transfer during their applications. The formation of mesoporous MOFs is of great interest.[6–12] Up to now, most meso-MOFs have small mesopore sizes (< 5 nm), and the preparation of large-pore meso-MOFs has been reported only sporadically.[6, 10, 12] The augmentation of the mesopore sizes of MOFs is still challenging. Ionic liquids (ILs) have attracted considerable attention as greener and safer solvents due to their peculiar properties, including the lack of measurable vapour pressure, non-flammable and their ability to dissolve a wide range of organic and inorganic compounds, in contrast to conventional solvents.[13–16] ILs have shown to be excellent media for the synthesis of a wide variety of MOFs.[9, 17–26] The use of ILs as solvents in the MOF preparation is termed as ionothermal synthesis.[27] Generally, MOFs prepared by ionothermal synthesis are sealed in autoclaves at temperatures higher than 100 8C for several days, which may cause the decomposition of ILs.[17, 18, 20–26] To achieve [a] X. Sang, Prof. J. Zhang, L. Peng, C. Liu, X. Ma, Prof. B. Han, G. Yang Beijing National Laboratory for Molecular Science CAS Key Laboratory of Colloid and Interface and Chemical Thermodynamics Institute of Chemistry Chinese Academy of Sciences (China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500209.
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the mesoporous MOF in IL systems at room temperature is an interesting topic.[9] Here we proposed a facile room temperature synthesis of mesoporous MOFs in ethylene glycol (EG)/IL mixture. By the EG/IL interfacial templating route, MOF networks with a bimodal mesoporous structure (3.9 and 17–28 nm) were synthesized. The porosity properties of the MOFs can be modulated by the EG content. The existence of the large mesopores in the MOFs can enhance mass transfer in applications such as catalysis, adsorption, and separation. The as-synthesized mesoporous MOF was used as an effective and reusable heterogeneous catalyst to catalyze the aerobic oxidation of benzylic alcohols. Cu3(BTC)2(H2O)3·xH2O (BTC = 1,3,5-benzenetricarboxylate) is one of the most extensively explored MOFs for its diverse use.[6, 10, 12, 28–31] First of all, Cu3(BTC)2 MOF was synthesized in a pure hydrophobic IL 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) at room temperature by the reaction of copper(II) acetate monohydrate (Cu(OAc)2·H2O) and 1,3,5benzenetricarboxylic acid (H3BTC). The X-ray diffraction (XRD) shows poor crystallization of the product synthesized in a pure IL at room temperature (Figure 1 a). It is consistent with the re-
Figure 1. XRD patterns of the sample synthesized in pure [Bmim][PF6] (a) and the MOF synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20 (b). c) Simulated XRD pattern of HKUST-1.
sults in literature that high temperatures of up to 100 8C is necessary for the preparation of crystallized MOFs when using IL as reaction medium.[27] The as-synthesized sample presents a morphology of irregular agglomerates (see the SEM image shown in Figure S1 of the Supporting Information). 2317
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Communications Then Cu3(BTC)2 MOF was synthesized in EG/[Bmim][PF6] mixture at room temperature. TX-100 was used as emulsifier to stabilize the EG/[Bmim][PF6] mixture, and its concentration was fixed at 0.25 mol·dm¢3. The EG volume ratio was 0.20. The Xray diffraction peak positions and relative intensities of the assynthesized MOF (Figure 1 b) agree well with those of the simulated HKUST-1 (Figure 1 c).[32] The XRD result proves the formation of crystallized Cu3(BTC)2 at room temperature in EG/ [Bmim][PF6] mixture. By combination with the XRD result of the product synthesized in pure IL (Figure 1 a), it is evident that the formation of Cu3(BTC)2 in IL at room temperature can be promoted well by the presence of EG. It can be attributed to the improved diffusion of reactants at the EG/IL interface, which results in fast nucleation and nanoparticle growth.[33] Figure 2 a and Figure 2 b show the SEM images of Cu3(BTC)2. Clearly, a mesoporous network is formed, with a pore diameter
Figure 2. SEM images (a,b) and TEM images (c,d) of the MOF synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20.
in the range of 2–30 nm. The TEM image shown in Figure 2 c gives a further evidence for the formation of a mesoporous network. From the magnified TEM image (Figure 2 d), nanoparticles of 5 nm and small mesopores of 3 nm can be clearly observed. Cu3(BTC)2 is further characterized by FT-IR spectrum and X-ray photoelectron spectroscopy (XPS). The FT-IR spectrum of the Cu3(BTC)2 synthesized in this work is nearly identical to that of the HKUST-1 synthesized in literature (Figure S2).[7, 32] The characteristic FT-IR vibrations of IL and EG cannot be observed in the FT-IR spectrum of the MOF, proving that EG and IL are removed completely. The XPS results prove the formation of the Cu3(BTC)2 (Figure S3). The thermogravimetric analysis shows that the MOF can keep stable up to 300 8C (Figure S4), similar to that reported in literature.[12] The mesoporosity properties of the MOF was investigated by the N2 adsorption–desorption method. As shown in FigChemPhysChem 2015, 16, 2317 – 2321
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Figure 3. N2 adsorption–desorption isotherms and mesopore size distribution curves (inset) of the MOF synthesized in EG/[Bmim][PF6] mixture with different EG volume ratios: a) 0.20, b) 0.35, c) 0.50, d) 0.60.
ure 3 a, the N2 adsorption–desorption isotherm of the MOF exhibits an intermediate mode between type I and type IV, which is related to micropores and mesopores of the materials investigated, respectively. Such an obvious hysteresis is associated with the porous material presenting a network structure.[34] The Brunauer–Emmett–Teller (BET) surface area and total specific pore volumes are 544.5 m2 g¢1 and 0.636 cm3 g¢1, respectively. The mesopore size distribution curve, calculated from Barrett–Joyner–Halenda analysis, shows a bimodal mesoporous structure with pore size distributions centered at 3.9 and 16.5 nm, respectively (inset of Figure 3 a). The MOFs were also synthesized in EG/IL mixtures with different EG volume ratios at room temperature. The products were characterized by FT-IR, XRD, SEM and TEM images (Figure S5–S8). The results show that all the MOFs synthesized present mesoporous structures. Figure 3 presents N2 adsorption-desorption results of the MOFs synthesized with different EG volume ratios. All the MOFs demonstrate similar obvious hysteresis and bimodal porous structures. Table 1 summarizes the porosity properties of the MOFs. The BET surface area values of the four MOFs are lower than those of the HKUST1 reported in literature (~ 1000 m2 g¢1).[9, 31] It can be ascribed to the pore impenetration of the interconnected MOF networks synthesized in this work.[7, 35] The BET surface area values of the
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Table 1. Porosity properties of the Cu3(BTC)2 synthesized in EG/[Bmim] [PF6] mixture with different EG volume ratios (REG).
REG
SBET[a] [m2 g¢1]
Smeso[b] [m2 g¢1]
Vt[c] [cm3 g¢1]
Vmeso[d] [cm3 g¢1]
Dmeso[e] [nm]
0.20 0.35 0.50 0.60
544.5 653.3 486.7 762.7
92.6 85.8 105.2 115.0
0.636 0.674 0.634 0.670
0.388 0.364 0.340 0.328
3.9 3.9 3.8 3.9
16.5 20.6 22.3 27.6
[a] SBET is the BET specific surface area. [b] Smeso is the specific mesopore surface area [c] Vt is the total specific pore volume. [d] Vmeso is the specific mesopore volume obtained from the BJH cumulative specific adsorption volume. [e] Dmeso is the mesopore diameter.
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Communications four MOFs are roughly increased with the increasing EG content. However, the SBET of the MOF synthesized with REG = 0.50 is the lowest, which might result from the more serious pore impenetration. All four of the MOFs present bimodal mesoporous structures. The small mesopores (~ 3.9 nm) stay nearly unchanged with EG content. The large mesopores are increased with the increasing EG content, that is, 16.5, 20.6, 22.3, and 27.6 nm as the EG volume ratios are 0.20, 0.35, 0.50, and 0.60, respectively. The results in Table 1 indicate that the MOF porosities can be easily tuned by varying EG content. The above results prove that the presence of EG can induce and accelerate the MOF formation in hydrophobic IL at room temperature. To get information on the mechanism of the mesoporous MOF formation, the phase behaviour of the [Bmim][PF6]/EG/TX-100 systems with different EG content was studied. The EG/TX100/[Bmim][PF6] systems present a translucent appearance. The microstructures of EG/TX100/[Bmim][PF6] systems were characterized by small-angle X-ray scattering (Figure S9). The EG droplets are spherical, and the droplet diameters are 30, 36, 42, and 62 nm for the emulsions with EG content of 0.20, 0.35, 0.50, and 0.60, respectively. Clearly, the EG droplets dispersed in IL are nanosized and can swell up upon increasing the EG content. By combination with the large mesopore size change of the Cu3(BTC)2 with EG content (Table 1), it is evident that the emulsions with larger droplets favour the formation of MOFs with larger mesopore size. It indicates that the EG nanodroplets dispersed in IL may work as templates for the large mesopore formation. Based on the experimental results, a possible mechanism for the formation of mesocellular MOF in EG/IL mixtures is proposed (Scheme 1). In the emulsion, the EG droplets (in dozens of nanometers) stabilised by surfactant TX-100 coexist with TX-
Scheme 1. Formation of bimodal mesoporous MOF in EG/IL mixture.
100 micelles, which are usually in a few nanometers. For the MOF synthesis, Cu2 + ions are confined in the EG phase and BTC3¢ ions are dispersed in the IL phase. The nanosized framework building blocks form from the linkage of Cu2 + and BTC3¢ that preferentially assemble at the EG/IL interface, due to their hybrid composition and mid-range zeta potentials (Scheme 1 a, b I).[36–38] As time passes, more MOF nanoparticles form and flocculation occurs, leading to the formation of a three-dimensional network of interconnected aggregates[39] (Scheme 1 bII,c). During this process, the EG droplets and micelles act as templates for mesopore formation (Scheme 1 c). Therefore, after removing the surfactant and solvents, the MOF network with a bimodal mesoporous structure is obtained (Scheme 1 d). ChemPhysChem 2015, 16, 2317 – 2321
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Cu3(BTC)2 has been used as catalyst for alcohol oxidation with its advantages of large surface area, interesting topology and low cost, especially in comparison to noble-metal catalysts.[12, 28, 31] The catalytic performance of the as-synthesized Cu3(BTC)2 for the oxidation of three aromatic alcohols to the corresponding aldehydes was tested under experimental conditions identical to those catalyzed by commercial Cu3(BTC)2,[28] using 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a co-catalyst, molecular oxygen as an oxidant and acetonitrile as solvent. The selectivity of aromatic aldehydes was > 98 %. As can be seen from Figure 4, cinnamyl alcohol and benzyl alcohol are converted completely to the corresponding aldehydes in 2 and 6 h respectively. The conversion of 4-methylbenzyl alcohol is
Figure 4. Time conversion plot for the aerobic oxidation of a) cinnamic alcohol, b) benzyl alcohol, and c) 4-Methylbenzyl alcohol catalysed by the Cu3(BTC)2 synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20. Reaction conditions: benzyl alcohol 0.2 mmol, catalyst 30 mg, acetonitrile 1 mL, TEMPO (0.5 equiv), Na2CO3 (1 equiv), 75 8C, oxygen atmosphere.
lower than the above two alcohols, that is, conversion reaches 91 % as the reaction time is 18 h. It can be attributed to the presence of the electron-donating methyl group substituent, which may have a negative influence on the alcohol oxidation. The catalytic activities of the as-synthesized MOFs for the oxidation of the three alcohols are remarkably higher than those of the commercial microporous Cu3(BTC)2, that is, the aldehyde yields were 62 %, 89 %, and 65 % for the oxidation of cinnamyl alcohol, benzyl alcohol, and 4-methylbenzyl alcohol, respectively, even with a prolonged reaction time of 22 h.[28] Such high catalytic activities of the Cu3(BTC)2 synthesized in this work can be attributed to their mesoporous network structure, which is beneficial to the diffusion of substrates and products.[12] The reusability of the as-synthesized Cu3(BTC)2 for the alcohol oxidation was investigated. As an example, Figure 5 shows the reusability of the Cu3(BTC)2 synthesized in EG/[Bmim][PF6] mixture with EG volume ratio of 0.20 for the oxidation of cinnamic alcohol. The catalyst shows no evident drop of catalytic
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Communications solution was generated. The desired amount of EG was added into the solution, and the mixture was stirred for 0.5 hour. The concentration of TX-100 was fixed at 0.25 mol dm¢3. The small-angle X-ray scattering (SAXS) experiment was carried out at Beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF). The wavelength was 1.38 æ, and the distance of sample to detector was 2.10 m. In a typical experiment, the freshly prepared mixture was added into the sample cell, and the X-ray scattering data were recorded.
MOF Synthesis and Characterization
Figure 5. Reusability of the Cu3(BTC)2. Reaction conditions: cinnamic alcohol 0.2 mmol, catalyst 30 mg, acetonitrile 1 mL, TEMPO (0.5 equiv), Na2CO3 (1 equiv), 75 8C, oxygen atmosphere, reaction time 2 h.
activity after four runs, indicating the high stability of the MOF. The resistance to deactivation in the aerobic oxidation may be attributed to the improved mass transport ability of guest species through the interconnected mesoporous network.[40, 41] The conversion decrease to 81 % as the catalyst is reused for the fifth time. The SEM image of the Cu3(BTC)2 after being used for five runs shows that some of mesopores of the MOF are partially blocked (Figure S10). The structural integrity of the MOF cannot be well preserved after being used for five runs, which can be proved by XRD pattern (Figure S11). Therefore, the reduced stability can be attributed to the damage of the pristine structure of MOF. In contrast, the commercial Cu3(BTC)2 lost 25 % yield for the second use in catalysing the oxidation of benzyl alcohol.[28] In summary, we have proposed a novel methodology for the formation of large-pore mesoporous MOF crystals by utilizing the EG nanodroplets in IL as mesopore templates. The porosity properties of the MOFs can readily be tuned by modulating the EG volume ratio. The as-synthesized large-pore mesoporous MOFs have shown highly catalytic activity for the oxidation of aromatic alcohols. The method can easily be applied to the synthesis of different kinds of MOFs.
Experimental Section Materials 1-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]; > 98 % purity) was provided by Lanzhou Greenchem ILS, LICP, CAS. Ethylene glycol (EG, A. R. Grade) was produced by Beijing Chemical Reagent Company. The surfactant Triton X-100 and copper (II) acetate monohydrate [Cu(OAc)2·H2O, A. R. Grade] were purchased from Alfa Aesar. 1,3,5-benzenetricarboxylic acid (H3BTC, purity 95 %) was provided by Aldrich. Benzyl alcohol (99 %), cinnamyl alcohol (98 %) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, purity 98 %) were supplied by Alfa Aesar. 4-methylbenzyl alcohol was purchased from J& K scientific Co., Ltd. Sodium carbonate and acetonitrile were provided by Beijing Chemical Reagent Company.
EG/[Bmim][PF6] Mixture Preparation and Characterization
The MOFs were synthesized using a similar synthetic method reported by Yaghi and coworkers.[42] Typically H3BTC (0.2 mmol) and Cu(OAc)2·H2O (0.2 mmol) were added into EG/[Bmim][PF6] mixtures (10 mL) with different EG volume fraction in a 25 mL flask. Then the mixture was stirred at room temperature for 24 h. Blue products were obtained after washing with ethanol for several times and drying at 60 8C under vacuum for 24 h finally. The morphology of MOFs was characterized by a HITACHI S-4800 SEM and TEM JeoL-1010 operated at 100 kV. The porosities were determined by N2 adsorption–desorption isotherms using a Quadrasorb SI-MP system. XRD analysis was performed on the X-ray diffractometer (Model D/MAX2500, Rigaka) with Cu Ka radiation. FT-IR spectra were obtained by a Bruker Tensor 27 spectrometer. The thermogravimetric analysis (TGA) measurement was carried out with a heating rate of 10 8C min¢1 using PerkinElmer Pyris 1 under N2 flow of 50 mL min¢1 to estimate thermal stability. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W AlKa radiation. The base pressure was about 3 Õ 10¢9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon.
Catalytic Reaction The conditions of catalytic reaction were similar to those reported by Garcia and coworkers.[28] Typically, the catalyst (30 mg), TEMPO (5 mg), sodium carbonate (17 mg) and aromatic alcohol (0.2 mmol) were loaded into a flask of 5 mL using 1 mL acetonitrile as solvent. The mixture was stirred at 75 8C under oxygen atmosphere for hours. After the desired time, the heterogeneous mixture was cooled and centrifuged. The liquid product was analysed by gas chromatography (Agilent 6820). For the reusability investigation, after reaction for 2 h, the catalyst was recovered by centrifugation, washed with acetonitrile and dried under vacuum. Then the solid was reused for a consecutive run.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (21173238, 21133009, U1232203, 21021003), the Ministry of Science and Technology of China (2009CB930802), and the Chinese Academy of Sciences (KJCX2.YW.H16). Keywords: heterogeneous catalysis · ionic liquids · mesoporous structures · metal–organic frameworks · template synthesis
The appropriate amounts of TX-100 and [Bmim][PF6] were first loaded into a capped glass vessel with a magnetic bar inside, and the mixture was vigorously stirred (1800 rpm) until a monophasic ChemPhysChem 2015, 16, 2317 – 2321
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Received: March 9, 2015 Revised: April 11, 2015 Published online on May 15, 2015
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Communications
Assembly of Mesoporous Metal–Organic Framework Templated by an Ionic Liquid/Ethylene Glycol Interface Xinxin Sang, Jianling Zhang,* Li Peng, Chengcheng Liu, Xue Ma, Buxing Han, and Guanying Yang[a] We propose a facile room-temperature synthesis of a metal–organic framework (MOF) with a bimodal mesoporous structure (3.9 and 17-28 nm) in an ionic liquid (IL)/ethylene glycol (EG) mixture. The X-ray diffraction analysis reveals that MOF formation can be efficiently promoted by the presence of the EG/IL interface at room temperature. The MOFs with mesoporous networks are characterized by SEM and TEM. The formation mechanism of the mesoporous MOF in EG/IL mixture is investigated. It is proposed that the EG nanodroplets in the IL work as templates for the formation of the large mesopores. The assynthesized mesoporous metal–organic framework is an effective and reusable heterogeneous catalyst to catalyze the aerobic oxidation of benzylic alcohols.
Metal–organic frameworks (MOFs),[1] also known as porous coordination networks, have recently attracted much attention for their potential use in gas storage,[2] separation,[3] biosensing[4] and catalysis,[5] owing to their fascinating structures and unusual properties such as permanent nanoscale porosity, high surface area, good thermostability, and uniformly structured cavities. MOFs are largely restricted to the microporous regime, which is not favourable to diffusion and mass transfer during their applications. The formation of mesoporous MOFs is of great interest.[6–12] Up to now, most meso-MOFs have small mesopore sizes (< 5 nm), and the preparation of large-pore meso-MOFs has been reported only sporadically.[6, 10, 12] The augmentation of the mesopore sizes of MOFs is still challenging. Ionic liquids (ILs) have attracted considerable attention as greener and safer solvents due to their peculiar properties, including the lack of measurable vapour pressure, non-flammable and their ability to dissolve a wide range of organic and inorganic compounds, in contrast to conventional solvents.[13–16] ILs have shown to be excellent media for the synthesis of a wide variety of MOFs.[9, 17–26] The use of ILs as solvents in the MOF preparation is termed as ionothermal synthesis.[27] Generally, MOFs prepared by ionothermal synthesis are sealed in autoclaves at temperatures higher than 100 8C for several days, which may cause the decomposition of ILs.[17, 18, 20–26] To achieve [a] X. Sang, Prof. J. Zhang, L. Peng, C. Liu, X. Ma, Prof. B. Han, G. Yang Beijing National Laboratory for Molecular Science CAS Key Laboratory of Colloid and Interface and Chemical Thermodynamics Institute of Chemistry Chinese Academy of Sciences (China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201500209.
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the mesoporous MOF in IL systems at room temperature is an interesting topic.[9] Here we proposed a facile room temperature synthesis of mesoporous MOFs in ethylene glycol (EG)/IL mixture. By the EG/IL interfacial templating route, MOF networks with a bimodal mesoporous structure (3.9 and 17–28 nm) were synthesized. The porosity properties of the MOFs can be modulated by the EG content. The existence of the large mesopores in the MOFs can enhance mass transfer in applications such as catalysis, adsorption, and separation. The as-synthesized mesoporous MOF was used as an effective and reusable heterogeneous catalyst to catalyze the aerobic oxidation of benzylic alcohols. Cu3(BTC)2(H2O)3·xH2O (BTC = 1,3,5-benzenetricarboxylate) is one of the most extensively explored MOFs for its diverse use.[6, 10, 12, 28–31] First of all, Cu3(BTC)2 MOF was synthesized in a pure hydrophobic IL 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) at room temperature by the reaction of copper(II) acetate monohydrate (Cu(OAc)2·H2O) and 1,3,5benzenetricarboxylic acid (H3BTC). The X-ray diffraction (XRD) shows poor crystallization of the product synthesized in a pure IL at room temperature (Figure 1 a). It is consistent with the re-
Figure 1. XRD patterns of the sample synthesized in pure [Bmim][PF6] (a) and the MOF synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20 (b). c) Simulated XRD pattern of HKUST-1.
sults in literature that high temperatures of up to 100 8C is necessary for the preparation of crystallized MOFs when using IL as reaction medium.[27] The as-synthesized sample presents a morphology of irregular agglomerates (see the SEM image shown in Figure S1 of the Supporting Information). 2317
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Communications Then Cu3(BTC)2 MOF was synthesized in EG/[Bmim][PF6] mixture at room temperature. TX-100 was used as emulsifier to stabilize the EG/[Bmim][PF6] mixture, and its concentration was fixed at 0.25 mol·dm¢3. The EG volume ratio was 0.20. The Xray diffraction peak positions and relative intensities of the assynthesized MOF (Figure 1 b) agree well with those of the simulated HKUST-1 (Figure 1 c).[32] The XRD result proves the formation of crystallized Cu3(BTC)2 at room temperature in EG/ [Bmim][PF6] mixture. By combination with the XRD result of the product synthesized in pure IL (Figure 1 a), it is evident that the formation of Cu3(BTC)2 in IL at room temperature can be promoted well by the presence of EG. It can be attributed to the improved diffusion of reactants at the EG/IL interface, which results in fast nucleation and nanoparticle growth.[33] Figure 2 a and Figure 2 b show the SEM images of Cu3(BTC)2. Clearly, a mesoporous network is formed, with a pore diameter
Figure 2. SEM images (a,b) and TEM images (c,d) of the MOF synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20.
in the range of 2–30 nm. The TEM image shown in Figure 2 c gives a further evidence for the formation of a mesoporous network. From the magnified TEM image (Figure 2 d), nanoparticles of 5 nm and small mesopores of 3 nm can be clearly observed. Cu3(BTC)2 is further characterized by FT-IR spectrum and X-ray photoelectron spectroscopy (XPS). The FT-IR spectrum of the Cu3(BTC)2 synthesized in this work is nearly identical to that of the HKUST-1 synthesized in literature (Figure S2).[7, 32] The characteristic FT-IR vibrations of IL and EG cannot be observed in the FT-IR spectrum of the MOF, proving that EG and IL are removed completely. The XPS results prove the formation of the Cu3(BTC)2 (Figure S3). The thermogravimetric analysis shows that the MOF can keep stable up to 300 8C (Figure S4), similar to that reported in literature.[12] The mesoporosity properties of the MOF was investigated by the N2 adsorption–desorption method. As shown in FigChemPhysChem 2015, 16, 2317 – 2321
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Figure 3. N2 adsorption–desorption isotherms and mesopore size distribution curves (inset) of the MOF synthesized in EG/[Bmim][PF6] mixture with different EG volume ratios: a) 0.20, b) 0.35, c) 0.50, d) 0.60.
ure 3 a, the N2 adsorption–desorption isotherm of the MOF exhibits an intermediate mode between type I and type IV, which is related to micropores and mesopores of the materials investigated, respectively. Such an obvious hysteresis is associated with the porous material presenting a network structure.[34] The Brunauer–Emmett–Teller (BET) surface area and total specific pore volumes are 544.5 m2 g¢1 and 0.636 cm3 g¢1, respectively. The mesopore size distribution curve, calculated from Barrett–Joyner–Halenda analysis, shows a bimodal mesoporous structure with pore size distributions centered at 3.9 and 16.5 nm, respectively (inset of Figure 3 a). The MOFs were also synthesized in EG/IL mixtures with different EG volume ratios at room temperature. The products were characterized by FT-IR, XRD, SEM and TEM images (Figure S5–S8). The results show that all the MOFs synthesized present mesoporous structures. Figure 3 presents N2 adsorption-desorption results of the MOFs synthesized with different EG volume ratios. All the MOFs demonstrate similar obvious hysteresis and bimodal porous structures. Table 1 summarizes the porosity properties of the MOFs. The BET surface area values of the four MOFs are lower than those of the HKUST1 reported in literature (~ 1000 m2 g¢1).[9, 31] It can be ascribed to the pore impenetration of the interconnected MOF networks synthesized in this work.[7, 35] The BET surface area values of the
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Table 1. Porosity properties of the Cu3(BTC)2 synthesized in EG/[Bmim] [PF6] mixture with different EG volume ratios (REG).
REG
SBET[a] [m2 g¢1]
Smeso[b] [m2 g¢1]
Vt[c] [cm3 g¢1]
Vmeso[d] [cm3 g¢1]
Dmeso[e] [nm]
0.20 0.35 0.50 0.60
544.5 653.3 486.7 762.7
92.6 85.8 105.2 115.0
0.636 0.674 0.634 0.670
0.388 0.364 0.340 0.328
3.9 3.9 3.8 3.9
16.5 20.6 22.3 27.6
[a] SBET is the BET specific surface area. [b] Smeso is the specific mesopore surface area [c] Vt is the total specific pore volume. [d] Vmeso is the specific mesopore volume obtained from the BJH cumulative specific adsorption volume. [e] Dmeso is the mesopore diameter.
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Communications four MOFs are roughly increased with the increasing EG content. However, the SBET of the MOF synthesized with REG = 0.50 is the lowest, which might result from the more serious pore impenetration. All four of the MOFs present bimodal mesoporous structures. The small mesopores (~ 3.9 nm) stay nearly unchanged with EG content. The large mesopores are increased with the increasing EG content, that is, 16.5, 20.6, 22.3, and 27.6 nm as the EG volume ratios are 0.20, 0.35, 0.50, and 0.60, respectively. The results in Table 1 indicate that the MOF porosities can be easily tuned by varying EG content. The above results prove that the presence of EG can induce and accelerate the MOF formation in hydrophobic IL at room temperature. To get information on the mechanism of the mesoporous MOF formation, the phase behaviour of the [Bmim][PF6]/EG/TX-100 systems with different EG content was studied. The EG/TX100/[Bmim][PF6] systems present a translucent appearance. The microstructures of EG/TX100/[Bmim][PF6] systems were characterized by small-angle X-ray scattering (Figure S9). The EG droplets are spherical, and the droplet diameters are 30, 36, 42, and 62 nm for the emulsions with EG content of 0.20, 0.35, 0.50, and 0.60, respectively. Clearly, the EG droplets dispersed in IL are nanosized and can swell up upon increasing the EG content. By combination with the large mesopore size change of the Cu3(BTC)2 with EG content (Table 1), it is evident that the emulsions with larger droplets favour the formation of MOFs with larger mesopore size. It indicates that the EG nanodroplets dispersed in IL may work as templates for the large mesopore formation. Based on the experimental results, a possible mechanism for the formation of mesocellular MOF in EG/IL mixtures is proposed (Scheme 1). In the emulsion, the EG droplets (in dozens of nanometers) stabilised by surfactant TX-100 coexist with TX-
Scheme 1. Formation of bimodal mesoporous MOF in EG/IL mixture.
100 micelles, which are usually in a few nanometers. For the MOF synthesis, Cu2 + ions are confined in the EG phase and BTC3¢ ions are dispersed in the IL phase. The nanosized framework building blocks form from the linkage of Cu2 + and BTC3¢ that preferentially assemble at the EG/IL interface, due to their hybrid composition and mid-range zeta potentials (Scheme 1 a, b I).[36–38] As time passes, more MOF nanoparticles form and flocculation occurs, leading to the formation of a three-dimensional network of interconnected aggregates[39] (Scheme 1 bII,c). During this process, the EG droplets and micelles act as templates for mesopore formation (Scheme 1 c). Therefore, after removing the surfactant and solvents, the MOF network with a bimodal mesoporous structure is obtained (Scheme 1 d). ChemPhysChem 2015, 16, 2317 – 2321
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Cu3(BTC)2 has been used as catalyst for alcohol oxidation with its advantages of large surface area, interesting topology and low cost, especially in comparison to noble-metal catalysts.[12, 28, 31] The catalytic performance of the as-synthesized Cu3(BTC)2 for the oxidation of three aromatic alcohols to the corresponding aldehydes was tested under experimental conditions identical to those catalyzed by commercial Cu3(BTC)2,[28] using 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a co-catalyst, molecular oxygen as an oxidant and acetonitrile as solvent. The selectivity of aromatic aldehydes was > 98 %. As can be seen from Figure 4, cinnamyl alcohol and benzyl alcohol are converted completely to the corresponding aldehydes in 2 and 6 h respectively. The conversion of 4-methylbenzyl alcohol is
Figure 4. Time conversion plot for the aerobic oxidation of a) cinnamic alcohol, b) benzyl alcohol, and c) 4-Methylbenzyl alcohol catalysed by the Cu3(BTC)2 synthesized in EG/[Bmim][PF6] mixture with an EG volume ratio of 0.20. Reaction conditions: benzyl alcohol 0.2 mmol, catalyst 30 mg, acetonitrile 1 mL, TEMPO (0.5 equiv), Na2CO3 (1 equiv), 75 8C, oxygen atmosphere.
lower than the above two alcohols, that is, conversion reaches 91 % as the reaction time is 18 h. It can be attributed to the presence of the electron-donating methyl group substituent, which may have a negative influence on the alcohol oxidation. The catalytic activities of the as-synthesized MOFs for the oxidation of the three alcohols are remarkably higher than those of the commercial microporous Cu3(BTC)2, that is, the aldehyde yields were 62 %, 89 %, and 65 % for the oxidation of cinnamyl alcohol, benzyl alcohol, and 4-methylbenzyl alcohol, respectively, even with a prolonged reaction time of 22 h.[28] Such high catalytic activities of the Cu3(BTC)2 synthesized in this work can be attributed to their mesoporous network structure, which is beneficial to the diffusion of substrates and products.[12] The reusability of the as-synthesized Cu3(BTC)2 for the alcohol oxidation was investigated. As an example, Figure 5 shows the reusability of the Cu3(BTC)2 synthesized in EG/[Bmim][PF6] mixture with EG volume ratio of 0.20 for the oxidation of cinnamic alcohol. The catalyst shows no evident drop of catalytic
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Communications solution was generated. The desired amount of EG was added into the solution, and the mixture was stirred for 0.5 hour. The concentration of TX-100 was fixed at 0.25 mol dm¢3. The small-angle X-ray scattering (SAXS) experiment was carried out at Beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF). The wavelength was 1.38 æ, and the distance of sample to detector was 2.10 m. In a typical experiment, the freshly prepared mixture was added into the sample cell, and the X-ray scattering data were recorded.
MOF Synthesis and Characterization
Figure 5. Reusability of the Cu3(BTC)2. Reaction conditions: cinnamic alcohol 0.2 mmol, catalyst 30 mg, acetonitrile 1 mL, TEMPO (0.5 equiv), Na2CO3 (1 equiv), 75 8C, oxygen atmosphere, reaction time 2 h.
activity after four runs, indicating the high stability of the MOF. The resistance to deactivation in the aerobic oxidation may be attributed to the improved mass transport ability of guest species through the interconnected mesoporous network.[40, 41] The conversion decrease to 81 % as the catalyst is reused for the fifth time. The SEM image of the Cu3(BTC)2 after being used for five runs shows that some of mesopores of the MOF are partially blocked (Figure S10). The structural integrity of the MOF cannot be well preserved after being used for five runs, which can be proved by XRD pattern (Figure S11). Therefore, the reduced stability can be attributed to the damage of the pristine structure of MOF. In contrast, the commercial Cu3(BTC)2 lost 25 % yield for the second use in catalysing the oxidation of benzyl alcohol.[28] In summary, we have proposed a novel methodology for the formation of large-pore mesoporous MOF crystals by utilizing the EG nanodroplets in IL as mesopore templates. The porosity properties of the MOFs can readily be tuned by modulating the EG volume ratio. The as-synthesized large-pore mesoporous MOFs have shown highly catalytic activity for the oxidation of aromatic alcohols. The method can easily be applied to the synthesis of different kinds of MOFs.
Experimental Section Materials 1-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]; > 98 % purity) was provided by Lanzhou Greenchem ILS, LICP, CAS. Ethylene glycol (EG, A. R. Grade) was produced by Beijing Chemical Reagent Company. The surfactant Triton X-100 and copper (II) acetate monohydrate [Cu(OAc)2·H2O, A. R. Grade] were purchased from Alfa Aesar. 1,3,5-benzenetricarboxylic acid (H3BTC, purity 95 %) was provided by Aldrich. Benzyl alcohol (99 %), cinnamyl alcohol (98 %) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, purity 98 %) were supplied by Alfa Aesar. 4-methylbenzyl alcohol was purchased from J& K scientific Co., Ltd. Sodium carbonate and acetonitrile were provided by Beijing Chemical Reagent Company.
EG/[Bmim][PF6] Mixture Preparation and Characterization
The MOFs were synthesized using a similar synthetic method reported by Yaghi and coworkers.[42] Typically H3BTC (0.2 mmol) and Cu(OAc)2·H2O (0.2 mmol) were added into EG/[Bmim][PF6] mixtures (10 mL) with different EG volume fraction in a 25 mL flask. Then the mixture was stirred at room temperature for 24 h. Blue products were obtained after washing with ethanol for several times and drying at 60 8C under vacuum for 24 h finally. The morphology of MOFs was characterized by a HITACHI S-4800 SEM and TEM JeoL-1010 operated at 100 kV. The porosities were determined by N2 adsorption–desorption isotherms using a Quadrasorb SI-MP system. XRD analysis was performed on the X-ray diffractometer (Model D/MAX2500, Rigaka) with Cu Ka radiation. FT-IR spectra were obtained by a Bruker Tensor 27 spectrometer. The thermogravimetric analysis (TGA) measurement was carried out with a heating rate of 10 8C min¢1 using PerkinElmer Pyris 1 under N2 flow of 50 mL min¢1 to estimate thermal stability. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W AlKa radiation. The base pressure was about 3 Õ 10¢9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon.
Catalytic Reaction The conditions of catalytic reaction were similar to those reported by Garcia and coworkers.[28] Typically, the catalyst (30 mg), TEMPO (5 mg), sodium carbonate (17 mg) and aromatic alcohol (0.2 mmol) were loaded into a flask of 5 mL using 1 mL acetonitrile as solvent. The mixture was stirred at 75 8C under oxygen atmosphere for hours. After the desired time, the heterogeneous mixture was cooled and centrifuged. The liquid product was analysed by gas chromatography (Agilent 6820). For the reusability investigation, after reaction for 2 h, the catalyst was recovered by centrifugation, washed with acetonitrile and dried under vacuum. Then the solid was reused for a consecutive run.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (21173238, 21133009, U1232203, 21021003), the Ministry of Science and Technology of China (2009CB930802), and the Chinese Academy of Sciences (KJCX2.YW.H16). Keywords: heterogeneous catalysis · ionic liquids · mesoporous structures · metal–organic frameworks · template synthesis
The appropriate amounts of TX-100 and [Bmim][PF6] were first loaded into a capped glass vessel with a magnetic bar inside, and the mixture was vigorously stirred (1800 rpm) until a monophasic ChemPhysChem 2015, 16, 2317 – 2321
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Received: March 9, 2015 Revised: April 11, 2015 Published online on May 15, 2015
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