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Inorganic Chemistry Communications
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Copper nanoparticles embedded in metal–organic frameworkMIL-101(Cr) as a high performance catalyst for reduction ofaromatic nitro compounds
Fang Wu, Ling-Guang Qiu ⁎, Fei Ke, Xia JiangLaboratory of Advanced Porous Materials, School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China
In this paper, we describe the preparation of Cu nanoparticles (NPs) loaded on a MIL-101 (Cr) metal–organicframework and its highly enhanced heterogeneous catalysis for reduction of aromatic nitro compounds. Theobtained Cu/MIL-101(Cr) nanocomposites were characterized by powder X-ray diffraction (PXRD), elementalanalysis, scanning electronmicroscopy (SEM), transmission electronmicroscopy (TEM), inductively coupled plas-ma atomic emission spectroscopy (ICP-AES), and nitrogen adsorption–desorption isotherms at 77 K. The result re-veals that both small CuNPswith diameter of 2–3 nmandCuNPswith average diameter of 100 nmare formed, andthe small Cu NPs are embedded in MIL-101(Cr). The obtained Cu/MIL-101(Cr) nanocomposites showed highlyenhanced catalytic activity for the reduction of aromatic nitro compounds.
Transition metal nanoparticles have attracted much attention dueto their unique physics and chemistry properties, and various applica-tions such as magnetic [1], catalytic [2], and optic properties [3]. How-ever, small size and high surface energy of metal nanoparticles oftenlead to agglomeration. Therefore, polymers, surfactants and colloidsare often used as protecting agents to increase the stability of the metalnanoparticles [4]. Meanwhile, metal nanoparticles immobilized in po-rous materials exhibited specific physical and chemical properties [5].Metal–organic frameworks (MOFs), a class of hybrid porous materialsassembled by polydentate bridging legends with metal ions or metalion clusters, have gained considerable attention in recent years owingto their potential applications in many areas such as catalysis, delivery,magnetic resonance imaging, gas adsorption, and sensing [6–8]. It hasbeen clearly demonstrated that the encapsulation of metal nanoparticlesin metal–organic frameworks (MOFs) is an effective method [9–12] forimproving heterogeneous catalysis in comparison with other poroussolids, such as mesoporous carbons [13] and silicas [14] since their highsurface area, well defined cavities and easier functionalization. Varioussyntheticmethods, including solvent-free gas-phase loading [15], solu-tion impregnation methods [16], incipient wetness impregnation [17],and solid grinding [18] have been developed to synthesize metalnanoparticles within MOFs.
Herein, we report a solution impregnation method for the fabricationof metal/MOF nanocomposites by hydrazine reduction under microwaveirradiation in a very short time. In the present work, we illustrate the Cu/MIL-101(Cr) nanocomposites by such microwave-assisted hydrazine
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reduction method. It has been demonstrated that Cu/MIL-101(Cr)nanocomposites could be synthesized under microwave irradiationin 2 min. MIL-101(Cr) was chosen as a carrier because MIL-101(Cr)has large pores (2.9 nm and 3.4 nm), high surface area, and excellentchemical and thermal stability [19], and copper ions can diffuse intothe pores of MIL-101(Cr) and nucleated to form Cu NPs withinthe pores by hydrazine reduction. The results reveal that microwavesynthesis is a simple and highly efficient approach for the fabricationof metal/MOF nanocomposites. Significantly, the as-synthesizedmetal/MOF nanocomposites exhibited a highly enhanced catalytic activ-ity in hydrogenation of aromatic nitro compounds.
The pure sample MIL-101(Cr) was characterized by powder X-raydiffraction (PXRD), as shown in Fig. 1. All the diffraction peaks can beassigned to the crystalline MIL-101 (Cr), and no obvious peaks fromimpurities can be observed, indicating the formation of MIL-101(Cr)in this work. The PXRD data (Fig. 1c and d) of the Cu/MIL-101(Cr)composites prepared by hydrazine reduction under microwave irra-diation suggest that MIL-101(Cr) MOF still maintained the structureafter the formation of Cu NPs. Furthermore, the PXRD patterns ofCu/MIL-101(Cr) nanocomposites exhibit no peaks attributable to crys-talline Cu, though Cu content in MIL-101(Cr) was determined by ICP tobe 2.4 wt.%. The results reveal that amorphous Cu was formed, due tostrong thermodynamic driving force toward crystallization under sucha microwave irradiation condition [20].
Fig. 2 shows the nitrogen adsorption–desorption isotherms(Fig. 2a) of MIL-101 (Cr) and Cu/MIL-101(Cr) composites measured at77 K and the corresponding pore size distribution analysis (Fig. 2b).The Brunauer–Emmett–Teller (BET) specific surface area (SBET) ofthe pure MIL-101(Cr) was calculated to be 3023 m2 g−1, and its
Fig. 1. Powder X-ray diffraction patterns (a) simulated from the crystallographic dataof MIL-101(Cr), and PXRD patterns of (b) MIL-101 (Cr) nanocrystals synthesized bythe hydrothermal methods, (c) and (d) Cu/MIL-101(Cr) nanocomposites before andafter catalysis, respectively.
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pore volume was determined to be 1.46 cm3 g−1. The pore sizes ofMIL-101(Cr) analyzed by using the Barrett–Joyner–Halenda (BJH)method were calculated to be 2.36 nm. However, after the loading ofCu NPs, the BET surface area of MIL-101(Cr) decreased significantlyfrom 3023 m2 g−1 to 1482 m2 g−1. Accordingly, the pore volumeof MIL-101(Cr) also decreased remarkably from 1.46 cm3 g−1 to0.72 cm3 g−1, clearly indicating that Cu ions were embedded innanopores created in MIL-101(Cr).
The morphologies and size or nanostructure of the as-preparedsamples were characterized by SEM and TEM, and the results areshown in Fig. 3. As can be seen from SEM images of the composites asshown in Fig. 3(a) and (b), the size and morphology of MIL-101(Cr)still kept average particle size of 200 nm with octahedral structureafter the encapsulation of Cu NPs in cages of MIL-101(Cr). Small CuNPs with a size range of 2–3 nm embedded in MIL-101(Cr) wereobserved from TEM images as shown in Fig. 3c. Meanwhile, largeCu NPs with an average size of 100 nm were also nucleated on thesurfaces of MIL-101(Cr) without growth restrain, and thus resulted in
Fig. 2. (a) Nitrogen adsorption–desorption isotherms and (b) pore size
Cu/MIL-101(Cr) nanocomposites, in which Cu NPs were formed bothin cages of MIL-101(Cr) and on surfaces of MIL-101(Cr) crystals (seeFig. 3d).
The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)in the presence of NaBH4 was used as a model reaction to investigatethe catalytic performance of the Cu/MIL-101(Cr) nanocomposites,and the reduction kinetics was investigated by UV–vis absorptionspectroscopy. When the Cu/MIL-101(Cr) nanocomposites were in-troduced into the mixture solution, the characteristic absorptionpeak of the 4-NP at 400 nm decreased in comparison with a con-comitant increase of the 4-AP at 300 nm. Since the concentrationof NaBH4 is much higher than that of 4-NP, it is rationally supposedthat the concentration of BH4
− remain constant during the reactionprocess. Hence the rate of the reaction could be reasonably as-sumed as a pseudo-first-order kinetics with regard to 4-NP only.The rate constant (k) value was calculated to be 0.97 min−1
under the experimental condition. This value is much higher thanthose of many other catalysts under ambient conditions [21–23]. Forcomparison, Cu NPs were also synthesized under such a microwaveirradiation condition in the absence of MIL-101(Cr), and catalytic per-formances of pure MIL-101(Cr) and Cu NPs were also investigatedunder the same catalytic reaction conditions. As can be seen fromFig. 4, the pure MIL-101(Cr) has no catalytic activity under the sameconditions. The rate constant value for the pure Cu NPs catalyst underthe same conditionwas estimated to be 0.81 min−1. The result revealshigh catalytic activity of the Cu/MIL-101(Cr) nanocomposites in com-parison pure Cu NPs and MIL-101(Cr) under the same conditions.Such an enhancement of catalytic activity may be attributed to thesmall Cu NPs with a size of 2–3 nm embedded in MIL-101(Cr), ratherthan Cu NPs formed around the MIL-101(Cr) crystals, because catalyticactivity of the Cu/MIL-101(Cr) nanocomposites is higher than that ofpure Cu NPs under the same reaction conditions as mentioned above.The mechanism of the catalytic reduction of 4-NP by Cu/MIL-101(Cr)nanocomposites may be explained as follows: MIL-101(Cr) adsorbed4-NP via π–π stacking interactions between the aromatic rings of 4-NPand the organic part of the framework. Such adsorption providesa high concentration of 4-NP near to the interface of the Cu NPsand MIL-101(Cr), leading to highly efficient contact between them.
Reduction of other aromatic nitro compounds was also studiedusing Cu/MIL-101(Cr) nanocomposites in the presence of NaBH4
under the same condition (see Table. S1). These results indicate that
distribution of MIL-101 (Cr) and Cu/MIL-101(Cr) nanocomposites.
Cu/MIL-101(Cr) nanocomposites have a high activity in hydrogenationof 2-nitrophenol (2-NP) and 4-nitroaniline (4-NA) to their corre-sponding amine forms, 2-aminophenol and 4-phenylenediamine,respectively. The rate constants of the catalytic reduction of 2-NPand 4-NA using Cu/MIL-101(Cr) nanocomposites were determinedto be 1.73 and 1.75 min−1, respectively (Fig. 5), suggesting againthat the catalyst shows a high activity in hydrogenation of aromaticnitro compounds.
In conclusion, we have successfully synthesized Cu/MIL-101(Cr)nanocomposites based under microwave irradiation. The nanocom-posites showed a high activity in hydrogenation of aromatic nitro com-pounds. MIL-101(Cr) supports not only enhance the catalytic activityof Cu NPs via a synergistic effect, but improve the Cu/MIL-101(Cr)nanocomposites easiness of separation for practical catalytic applications.
Fig. 4. Catalytic conversion of 4-NP to 4-AP over the Cu/MIL-101(Cr) nanocomposites,pure MIL-101(Cr) and Cu NPs.
Catalytic performance of such metal NPs encapsulated in MOF couldbe further improved by choosing noble metal such as Au [18],Ag [22], Pt [12], and Pd [24]. The preparation of Au, Ag and Pt NPsencapsulated in MOFs and investigations on their catalysis proper-ties are in progress.
Acknowledgments
Thisworkwas supported by theNationalNatural Science Foundationof China (NSFC, 20971001), the NSFC-CAS Joint Fund for Research Basedon Large-Scale Scientific Facilities (10979014), the Program for NewCentury Excellent Talents in University, Ministry of Education, China(NCET-08-0617), and the “211 Project” of Anhui University.
Fig. 5. Relationship of ln(Ct/C0) and reaction time t for the reduction of 2-NP, 4-NP, and4-NA over Cu/MIL-101(Cr) nanocomposites, respectively.
8 F. Wu et al. / Inorganic Chemistry Communications 32 (2013) 5–8
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.03.003.
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