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• Recently, nano-sized catalysts have been widely paid attention to in the organic synthesis, biotechnology, advanced materials and other high-technology fields, and shown brand new catalytic activity [1-3]. Bimetallic nanoparticles (NPs) have been reported to exhibit solid-state properties different from those of their monometallic counterparts [4-6]. They exhibit better catalytic activity than their corresponding monometallic catalysts in many reactions [7-9]. In the case of bimetallic catalysts, the two different components may complement one another with regard to characteristics such as surface stability, oxygen mobility and electronic properties. Among the various metallic catalysts, Au NPs are of increasing interest due to their versatility and exceptional catalytic properties in different reactions at low temperature [10, 11]. Cu-based catalysts are also particularly effective for the Ullmann coupling reaction [12-14]. Au-Cu systems have shown excellent catalytic performance in many reactions, including CO oxidation and propene epoxidation [15-17]. However, because of the small particles of nano-catalysts, it is difficult to separate the catalyst from the reaction system. This separated process may be an expensive and time-consuming process accompanied by large amounts of wastewater [18-20]. It is very important that catalysts are readily separated from the reaction mixture to allow recycling of the catalyst and purification of the products. Recently, magnetic Fe₃O₄ nanoparticles as novel supported materials have been widely concerned due to their capacity for magnetic separation, as well as their chemically modifiable surfaces and excellent stability [21, 22].

  Synthesis of organic dyes and pigments involves a large amount of chemical reaction and catalysts [23-25]. Ullmann coupling reaction is a very important C-C bond formation reaction in the organic dye and pigment industry [26]. Some important organic dyes and pigments, such as C.I. Reactive Dye Blue 19, C.I. Reactive Dye Blue 49, and C.I. Pigment Red 177 (PR 177) are synthesized by Ullmann coupling reaction. The coupling reactions are catalyzed by an excess of copper powder at very high temperature (above 200 ℃) over prolonged reaction time [12, 27]. In addition to the long reaction time and low catalytic efficiency, a large amount of copper powder is not easily separated from the reaction system. There is an urgent need to develop a new catalyst system with a combination of low temperature activity, high catalytic selectivity and excellent separability [28, 29]. It would be interesting to develop an Au-Cu bimetallic nano-catalyst and apply to the Ullmann coupling reaction in the dye industry.

  Considering that magnetic separation is more efficient and attractive than traditional centrifugation or filtration, we decided to immobilize the Au and Cu NPs on a magnetic support. So in the present study, a new nano-catalytic system based on Au and Cu NPs supported on magnetic Fe₃O₄ colloidal nanocrystal clusters with a silica coating, Fe₃O₄@SiO₂-Au/Cu, was designed and prepared. As shown in Fig. 1a, a Fe₃O₄@SiO₂ core-shell nanostructure was used as the support material. The SiO₂ surface was first functionalized with amino groups using 3-aminopropyltriethoxysilane (APTES), as confirmed by the Fourier transform infrared spectroscopy (FTIR) spectrum in Fig. S1 (Supporting information). Compared with the original Fe₃O₄@SiO₂, the sample after APTES modification generated an obvious peak at approximately 1630 cm^-1, which is characteristic of amino groups. The zeta potential value of the material was also increased from -60 mV to approximately -40 mV, indicating the successful functionalization of the Fe₃O₄@SiO₂ with -NH₂ groups. Then Au and Cu NPs with dimeter of 3 nm to 5 nm as shown in Fig. S2 (Supporting information) were synthesized by direct reduction method using HAuCl₄ and CuSO₄ aqueous solution as precusors, trisodiun citrate dehydrate as the capping agent and NaBH₄ as the reducing agent. The sequential adsorption of Au and Cu NPs gave the novel Fe₃O₄@SiO₂-Au/Cu nanoparticles.

  Figure 1

  

  Figure 1.  (a) The stepwise fabrication of Fe₃O₄@SiO₂-Au/Cu magnetic nanocomposites. TEM images of (b) Fe₃O₄@SiO₂ and (c) the Fe₃O₄@SiO₂-Au/Cu nanoparticles. The inset in (c) shows the size distribution of Au/Cu NPs on the silica surface, calculated by measuring the diameters of at least 200 particles in the TEM images. (d) High-angle annular dark-field STEM image of the Fe₃O₄@SiO₂-Au/Cu nanocomposite, with elemental mapping of Fe, Si, N, Au, Cu and (e) XPS survey of the Fe₃O₄@SiO₂-Au/Cu nanocomposite and high-resolution Au 4f and Cu 2p spectra.

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  The surface morphology of the magnetic core-shell nanocomposites was determined by transmission electron microscopy (TEM). Figs. 1b and c present TEM images of the Fe₃O₄@SiO₂ nanostructure before and after deposition of the Au and Cu NPs. The Au and Cu NPs were evidently deposited on the SiO₂ surface in a highly dense and well dispersed manner. The inset in Fig. 1c shows the size distribution of the Au/Cu NPs, which exhibits a Gaussian distribution with an average size of approximately 5 nm. Both the small size of the NPs and their high degree of dispersion likely contributed to the activity of the catalyst.

  Scan transmission electron microscopy (STEM) images of the Fe₃O₄@SiO₂-Au/Cu are presented in Fig. 1d, along with a high-angle annular dark-field image and elemental mapping of the nanostructure. Energy-dispersive X-ray (EDX) mapping of elemental Fe, Si, N, Au and Cu confirmed the presence of the expected Au and Cu in the structure of the nanocomposite. The mapping of N also supports our assumption that the SiO₂ surface was modified with a uniform layer of highly dense -NH₂ groups, which was a prerequisite for the uniform deposition of the Au and Cu NPs. The Au and Cu maps in Fig. 1d demonstrate that the Au and Cu NPs were highly dispersed on the nanocomposite surface, consistent with the TEM image in Fig. 1c.

  X-ray photoelectron spectroscopy (XPS) data obtained for the Fe₃O₄@SiO₂-Au/Cu are shown in Fig. 1e. The spectra show that there was no Fe on the surface of the nanocomposite, indicating that the Fe₃O₄ clusters were completely coated with silica. Both Au 4f and Cu 2p peaks were generated by the Fe₃O₄@SiO₂-Au/Cu. Their binding energies of 84.0 eV and 932.2 eV, respectively, suggest the presence of metallic Au and Cu in the NPs. The content ratio of Au and Cu showed by XPS was around 1.04:1 (Au:Cu), which was consistent with the molar ratio of the Au and Cu NPs added.

  This novel Fe₃O₄@SiO₂-Au/Cu catalyst was applied to the Ullmann coupling reaction of bromamine acid for the synthesis of 4, 40-diamino-1, 10-dianthraquinonyl-3, 30-disulfonic acid (DAS) as shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  The Ullmann coupling reaction of bromamine acid.

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  DAS is an important intermediate for the synthesis of a highgrade organic pigment PR 177. There was a lot of waste water produced in the traditional preparation process of DAS which used excessive copper powder as catalyst. For comparison purposes, the synthesis was carried out over several catalysts, including unsupported Au NPs, Cu NPs, Au/Cu NPs, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-Au/Cu. The TEM images in Fig. S2 demonstrate that both the unsupported Au and Cu NPs had diameters in the range of 3 nm to 5 nm. In a typical trial, 0.5 mmol of bromamine acid, 10 mL of H₂O₂ and 1.5 mmol of the catalyst were mixed in a 50 mL threeneck, round-bottom flask and heated to 90 ℃ under nitrogen. To monitor the extent of the reaction, 1 mL aliquots of the reaction solution were removed at regular time intervals and analyzed by HPLC after filtration. The percent conversion of the bromamine acid and the selectivity for the desired product DAS were calculated as

  Conversion (%) = (Rc-Rl)/Rc × 100%

  Selectivity (%) = Pg/(Rc-Rl) × 100%

  Where Rc is the original moles of the reactant, Rl is the moles of the reactant remaining after the reaction, and Pg is the moles of the product generated.

  The catalytic results are summarized in Table 1. When we optimized the reaction temperature, we found that if the temperature was lower than 90 ℃, it was difficult for bromamine acid to be dissolved under aqueous condition. When the temperature was higher than 100 ℃, there would be by-products caused by the hydrolysis and debromination of bromamine acid. So we chose 90 ℃ as the catalytic reaction temperature. As can be seen, the reaction did not proceed within 120 min in the absence of a catalyst or over Fe₃O₄@SiO₂, indicating that Fe₃O₄@SiO₂ was only a supporting material which had no catalytic activity. Among the catalysts studied, Au and Cu were found to show high selectivity and good conversion, respectively. The conversion of bromamine acid reached 95.48% over the Cu NPs while the selectivity for DAS was as high as 87.26% over the Au NPs catalyst. These data confirm that a combination of Au and Cu should improve both the conversion and selectivity and, in fact, the Au/Cu catalyst showed both high conversion and selectivity under the same reaction conditions. The conversion and selectivity were 96.25% and 86.34% at 120 min when using this material. The effect of reaction time was also examined and it was found that the conversion fell dramatically, from 96.25% to 53.17%, when the reaction time was decreased from 120 min to 60 min. In contrast, both the conversion and selectivity remained unchanged when the reaction time was prolonged from 120 min to 180 min. Thus, 120 min was regarded as the appropriate reaction time for this catalytic system. Interestingly, using the Fe₃O₄@SiO₂-Au/Cu as the catalyst, the conversion increased markedly to 90.22% after 60 min, whereas only 53.17% conversion was obtained over the unsupported Au/Cu NPs and there was no conversion in the absence of any catalyst or over Fe₃O₄@SiO₂. Thus, it appears that the use of Au/Cu NPs supported on Fe₃O₄@SiO₂ not only solves the problem of catalyst separation from the reaction mixture but also improves the dispersion of the active Au/Cu NPs on the SiO₂ surface, thus raising the catalytic activity of the Fe₃O₄@SiO₂-Au/Cu nanoparticles.

  Table 1

  

  Table 1.  Effect of different catalytic systems on the synthesis of DAS.^a

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  The recyclability of the Fe₃O₄@SiO₂-Au/Cu was also investigated. The catalyst was easily recovered by separation with an external magnet after the reaction and was subsequently recycled under the same reaction conditions. The results in Fig. 2 show that the material could be recycled five times with no evident loss of its catalytic activity, indicating good stability. For the 1st, 2nd, 3rd, 4th and 5th cycle, the conversion was 97.35%, 97.2%, 96.68%, 96.34% and 95.89%; the selectivity was 88.67%, 88.89%, 88.56%, 88.32% and 88.17%, respectively. In addition, inductively coupled plasma (ICP) analysis showed that the amounts of Au and Cu in the supernatant after the reaction were negligible, demonstrating that there was no loss of the Au and Cu active species during the repeated trials, which is consistent with the stability results.

  Figure 2

  

  Figure 2.  (a) Reusability studies of Fe₃O₄@SiO₂-Au/Cu during the synthesis of DAS. Reaction conditions: Bromamine acid (0.5 mmol), H₂O as solvent (10 mL), catalyst (1.5 mmol), 90 ℃, 120 min, under a N₂ atmosphere. Yields were determined by HPLC. (b) Photographic images of the Fe₃O₄@SiO₂-Au/Cu before (left) and after (right) magnetic separation by an external magnet.

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  In summary, we have developed an efficient nano-catalyst, Fe₃O₄@SiO₂-Au/Cu NPs, to catalyze Ullmann coupling reaction of bromamine acid in water. The active Au and Cu NPs were highly dispersed on the Fe₃O₄@SiO₂ surface. This material exhibits high catalytic activity. Using water instead of organic solvents as the reaction medium, a bromamine acid conversion of 97.35% and selectivity for DAS of 88.67% were obtained at 90 ℃ over 120 min. Both of these values are much higher than those observed when using monometallic catalysts. This new nano-catalyst can also be easily recycled and reused several times without any obvious loss of its initial catalytic activity. The advantages of Fe₃O₄@SiO₂-Au/Cu NPs include high efficiency, easy and rapid separation of the used material, and green and mild reaction conditions, making it a potentially useful and attractive approach to industrial production.

  Acknowledgments

  This work was financially supported by the Shanghai Natural Science Foundation (No. 13ZR1400300) and National Key R & D Program of China (No. 2017YFB030900).

  Appendix A. Supplementary data

  Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.11.040.

• 1. [1]

     R.N. Dhital, C. Kamonsatikul, E. Somsook, et al., J. Am. Chem. Soc. 134(2012) 20250-20253. doi: 10.1021/ja309606k

  2. [2]

     M. Banach, J. Pulit-Prociak, J. Clean. Prod. 141(2017) 1030-1039. doi: 10.1016/j.jclepro.2016.09.180

  3. [3]

     K. Tian, W.J. Liu, S. Zhang, H. Jiang, Green Chem. 18(2016) 5643-5650. doi: 10.1039/C6GC01231K

  4. [4]

     T. Mitsudome, K. Miyagawa, Z. Maeno, et al., Angew. Chem. Int. Ed. 129(2017) 9509-9513. doi: 10.1002/ange.v129.32

  5. [5]

     A. Lehoux, L. Ramos, P. Beaunier, et al., Adv. Funct. Mater. 22(2012) 4900-4908. doi: 10.1002/adfm.v22.23

  6. [6]

     X. Liu, A. Wang, L. Li, et al., J. Catal. 278(2011) 288-296. doi: 10.1016/j.jcat.2010.12.016

  7. [7]

     X.M. Fu, Z.J. Liu, S.X. Cai, et al., Chin. Chem. Lett. 27(2016) 920-926. doi: 10.1016/j.cclet.2016.04.014

  8. [8]

     S. Bhattacharjee, C.S. Tan, J. Clean. Prod. 156(2017) 203-213. doi: 10.1016/j.jclepro.2017.03.148

  9. [9]

     X. Weng, M. Guo, F. Luo, Z. Chen, J. Chem. Eng. 308(2017) 904-911. doi: 10.1016/j.cej.2016.09.134

  10. [10]

      M. Comotti, W.C. Li, B. Spliethoff, F. Schüth, J. Am. Chem. Soc. 128(2006) 917-924. doi: 10.1021/ja0561441

  11. [11]

      T. Tabakova, L. Ilieva, P. Petrova, et al., Chem. Eng. J. 260(2015) 133-141. doi: 10.1016/j.cej.2014.08.099

  12. [12]

      F. Benaskar, N. Patil, V. Engels, et al., Chem. Eng. J. 207(2012) 426-439. http://www.sciencedirect.com/science/article/pii/S1385894712008844

  13. [13]

      V. Engels, F. Benaskar, N. Patil, et al., Org. Process Res. Dev. 14(2010) 644-649. doi: 10.1021/op9003423

  14. [14]

      K.J. Shi, X. Zhang, C.H. Shu, et al., Chem. Commun. 52(2016) 8726-8729. doi: 10.1039/C6CC03137D

  15. [15]

      T.C. Ou, F.W. Chang, L.S. Roselin, J. Mol. Catal. A:Chem. 293(2008) 8-16. doi: 10.1016/j.molcata.2008.06.017

  16. [16]

      X.M. Liao, V. Pitchon, P.H. Cuong, W. Chu, V. Caps, Chin. Chem. Lett. 28(2017) 293-296. doi: 10.1016/j.cclet.2016.06.043

  17. [17]

      M.Y. Liu, W. Zhou, T. Wang, et al., Chem. Commun. 53(2016) 4694-4697.

  18. [18]

      S. Hu, Y. Guan, Y. Wang, H. Han, Appl. Energ. 88(2011) 2685-2690. doi: 10.1016/j.apenergy.2011.02.012

  19. [19]

      J.Y. Zhang, X. Huang, Q.Y. Shen, J.Y. Wang, G.H. Song, Chin. Chem. Lett. 29(2018) 197-200. doi: 10.1016/j.cclet.2017.05.012

  20. [20]

      A.C. Alba-Rubio, J. Santamaría-González, J.M. Mérida-Robles, et al., Catal. Today 149(2010) 281-287. doi: 10.1016/j.cattod.2009.06.024

  21. [21]

      Z. Abbasi, S. Rezayati, M. Bagheri, R. Hajinasiri, Chin. Chem. Lett. 28(2017) 75-82. doi: 10.1016/j.cclet.2016.06.022

  22. [22]

      L.L. Kong, L.Y. Fan, Chin. Chem. Lett. 27(2016) 827-831. doi: 10.1016/j.cclet.2016.01.055

  23. [23]

      F. Karcı, N. Şener, M. Yamaç, İ.Şener, A. Demirçalı, Dyes Pigments 80(2009) 47-52. doi: 10.1016/j.dyepig.2008.05.001

  24. [24]

      M.M.M. Raposo, M.C.R. Castro, M. Belsley, A.M.C. Fonseca, Dyes Pigments 91(2011) 454-465. doi: 10.1016/j.dyepig.2011.05.007

  25. [25]

      A. Asghar, A.A.A. Raman, W.M.A.W. Daud, J. Clean. Prod. 87(2015) 826-838. doi: 10.1016/j.jclepro.2014.09.010

  26. [26]

      C.W. Qian, W.L. Lv, Q.S. Zong, M.Y. Wang, D. Fang, Chin. Chem. Lett. 25(2014) 337-340. doi: 10.1016/j.cclet.2013.11.036

  27. [27]

      F. Benaskar, V. Engels, N.G. Patil, et al., Ind. Eng. Chem. Res. 52(2013) 18206-18214. doi: 10.1021/ie402956n

  28. [28]

      R.N. Dhital, C. Kamonsatikul, E. Somsook, et al., J. Am. Chem. Soc. 134(2012) 20250-20253. doi: 10.1021/ja309606k

  29. [29]

      A. Kamal, V. Srinivasulu, B. Seshadri, et al., Green Chem. 14(2012) 2513-2522. doi: 10.1039/c2gc16430b

• 

  Figure 1  (a) The stepwise fabrication of Fe₃O₄@SiO₂-Au/Cu magnetic nanocomposites. TEM images of (b) Fe₃O₄@SiO₂ and (c) the Fe₃O₄@SiO₂-Au/Cu nanoparticles. The inset in (c) shows the size distribution of Au/Cu NPs on the silica surface, calculated by measuring the diameters of at least 200 particles in the TEM images. (d) High-angle annular dark-field STEM image of the Fe₃O₄@SiO₂-Au/Cu nanocomposite, with elemental mapping of Fe, Si, N, Au, Cu and (e) XPS survey of the Fe₃O₄@SiO₂-Au/Cu nanocomposite and high-resolution Au 4f and Cu 2p spectra.

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  Scheme 1  The Ullmann coupling reaction of bromamine acid.

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  Figure 2  (a) Reusability studies of Fe₃O₄@SiO₂-Au/Cu during the synthesis of DAS. Reaction conditions: Bromamine acid (0.5 mmol), H₂O as solvent (10 mL), catalyst (1.5 mmol), 90 ℃, 120 min, under a N₂ atmosphere. Yields were determined by HPLC. (b) Photographic images of the Fe₃O₄@SiO₂-Au/Cu before (left) and after (right) magnetic separation by an external magnet.

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  Table 1.  Effect of different catalytic systems on the synthesis of DAS.^a

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•