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Facile synthesis of hydrochar supported copper nanocatalyst for Ullmann CN coupling reaction in water
Molecular Catalysis xxx (xxxx) xxxx
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Facile synthesis of hydrochar supported copper nanocatalyst for Ullmann CeN coupling reaction in water Xin Gea, Meng Gea, Xinzhi Chenb, Chao Qianb, Xuemin Liua,*, Shaodong Zhoub,* a
School of Chemical and Material Engineering, Jiangnan University, Wuxi, PR China Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrochar Nanocatalyst Chitosan Copper eCN coupling reaction In water
The exploration of inexpensive and stable heterogeneous catalysts and application of green solvents for Ullmann CeN coupling reaction remain challenging. We present a facile fabrication of copper nanoparticles on hydrochar as prepared from natural, inexpensive and renewable chitosan together with in-situ reduction of copper salt in a one-pot hydrothermal carbonization process. The copper nanoparticles were uniformly dispersed on hydrochar by choosing block copolymer F127 as surfactant. Moreover, maleic acid was introduced to improve the hydrophilicity of hydrochar. The most active copper nanocomposite catalyst, that is, Cu/HCS-MA-F127, exhibited excellent catalytic activity for Ullmann CeN coupling reaction in water. The nature of the Cu/HCS-MA-F127 was characterized by FTIR spectroscopy, TG, XRD, SEM and XPS. Moderate to excellent yields of aimed products were gained by using this catalytic strategy. Moreover, the Cu/HCS-MA-F127 catalyst can be reused by simple centrifugal recovery with a stable performance.
Introduction Enormous efforts have been made for constructing a CeN bond, which constitutes an important structural motif in various drugs, natural products and materials [1–3]. In 1990s, palladium-catalyzed reactions as explored by Buchwald and Hartwig [4,5] achieved a major breakthrough in CeN coupling. After that, quite some protocols have been exploited to improve the CeN coupling processes under mild reaction condition. In particular, copper-catalyzed Ullmann reaction, in which the toxic and expensive palladium is not involved, has proven to be an efficient strategy and continues attracting broad attention [6,7]. For example, Buchwald [8] first disclosed the catalytic system of diamine ligands for N-arylation; afterwards, Cristau and Taillefer [9] employed polydentate ligands like Schiff base and oxime in such reactions; Ma [10] found that amino acids are particularly efficient for CeN coupling. Though various ligands have been developed and used to promote copper-catalyzed Ullmann reactions, these homogeneous systems suffer from well-known disadvantages, such as unrecyclable catalysts, complex ligands and potentially toxic solvents (e.g., DMF, DMSO, dioxane). [11–14] Thus, exploration of heterogeneous catalysts and application of green solvents for Ullmann CeN coupling reaction continue to be demanding. Recently, due to the high catalytic activity of metal nanoparticles
⁎
(MNPs) as found for the organic coupling reactions, the nano-sized metal catalysts on proper supports attract raising attention. [15–17] Also, catalysts based on Cu, CuO and Cu2O nanoparticles have been employed to catalyze the formation of CeN bonds. For example, Bai et al. [18] reported that Cu0 anchored on nanofibers by hydrothermal method for promoting Ullmann CeN coupling at 140 °C in DMF. Hyeon et al. [19] prepared Cu2O coated Cu nanoparticles (NPs) as catalysts for Ullmann type amination coupling in DMSO; S. Rawat et al. [20] reported that copper NPs incorporated on alumina/silica support can promote the cross coupling of aryl chlorides with amines at 150 °C in DMF; Jafarzadeh et al. [21] loaded CuO NPs on MOFs for promoting Ullmann coupling at 110 °C in DMSO. In spite of the above achievements, improvements are still required for the application of MNPs regarding the use of organic ligands, multi-step synthetic process and the emission of wastes. So far as reported, the heterogeneous copper catalysts have been prepared by entrapment and coordination of copper on various kinds of supports, such as carbon [22], alumina [20], zeolites [23], polymers [24,25] and magnetic materials [26,27], all of which exhibited remarkable performance in catalyzing the Ullmann CeN coupling processes. Due to the chemical inertness and economic accessibility of carbon, MNPs fabricated on carbon (MNP/C) is an ideal approach. Hydrothermal carbonization (HTC) processing of biomass turns out to
Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (S. Zhou).
https://doi.org/10.1016/j.mcat.2019.110726 Received 7 August 2019; Received in revised form 25 October 2019; Accepted 25 November 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xin Ge, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110726
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Fig. 1. The prepartion of Cu/hydrochar catalyst.
vacuum at 50 °C overnight to give the CuSO4/CS catalyst. Similarly, the Cu/CS catalyst was prepared [43]: 1 g chitosan and 0.3 g CuSO4·5H2O were added into 30 ml of deionized water and stirred vigorously. Then 2 M NaOH was added dropwise to turn the pH value to 13. Next, 0.227 g NaBH4 was added to the above solution and continuously stirred for 2 h. The solid was separated by filtration, washed with water and ethanol, dried under vacuum at 50 °C overnight.
be an effective strategy to prepare carbonaceous materials (often called hydrochar) and metal-hybrid materials [28–30]. This method involves thermal dehydration and transformation of biomass in water at low temperatures [31,32]. Various carbonaceous structures and metal-hybrid materials (e.g., metal/C core-shell [33], metal/C nanocable [34] and porous carbon [35]) were afforded using HTC; the latter thus exhibits strong potential in many fields, such as adsorption [36], solid fuel [37], energy storage [38] and catalysis [39,40]. On a different background, featured as biodegradable, renewable and inexpensive compounds, polysaccharides have become the most promising biomass [41]. Among these polysaccharides, chitosan (CS) possesses a large amount of amino and hydroxyl groups, which would be retained in the HTC process [42]. CS-derived hydrochar has strong metal-chelating capability and are suitable support for MNP/C. In this work, we presented a one-pot, in-situ HTC process using CS and a solution of metal ions as the raw materials (Fig. 1). CS usually serves as the reducing agents, while in this work it functions as both the carbon source and the reductant. Consequently, the copper nanoparticles dispersed uniformly on hydrochar (Cu/hydrochar). To our delight, the so-prepared Cu/hydrochar showed high catalytic activity for the Ullmann CeN coupling. Moreover, such Cu/hydrochar species is of strong hydrophilicity and thus able to promote the coupling reactions in water. Herein, we describe in detail the preparation of copper NPs on CS-derived hydrochar and its application in catalytic coupling of Ullmann CeN reaction.
The synthesis of Cu/HCS-MA-F127 Typically, 0.3 g CuSO4·5H2O and 0.2 g F127 were added into 30 ml of deionized water. The mixture was stirred at room temperature for 30 min. Then 1 g chitosan and 0.2 g maleic acid were added to the above mixture, and stirred for 30 min. Finally, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 180 °C for 10 h. After cooling naturally, the black brown carbonaceous material was obtained. The material was washed with water and ethanol several times, and dried in a vacuum at 80 °C overnight. General procedure for the Ullmann reaction catalyzed by Cu/HCS-MA-F127 To a stirred solution of H2O (4 mL), aryl halide (1.0 mmol), nucleophile (1.2 mmol), Cu/HCS-MA-F127 and K2CO3 (2 mmol) were added at room temperature. Next, the reaction mixture was heated to 100 °C in air and stirred for 24 h. After cooling down to room temperature, the catalyst Cu@HCS-MA-F127 was separated by centrifugation. The reaction mixture was partitioned by adding ethyl acetate (20 mL) and water (20 mL). Subsequently, the organic phase was separated and the aqueous phase was extracted with ethyl acetate (20 mL) twice. The combined organic phases were washed with brine, dried over Na2SO4, and concentrated in vacuo. Finally, the crude product was purified by column chromatography with silica gel, eluting with a petroleum ether/ethyl acetate solvent mixture, to give the pure product.
Experimental section Materials and methods Chitosan powder (MW: 10000–50000, deacetylation degree 95 %, purchased from Aladdin reagent (Shanghai) Co., Ltd) was used without further purification. Maleic acid (MA), acetic acid, sulfuric acid and CuSO4·5H2O was purchased from Sinopharm Chemical Reagent Co. Ltd, and Pluronic F127 was purchased from Energy Chemical. Aryl halides and imidazole were purchased from Alfa Aesar. Nuclear magnetic resonance (NMR) spectra were measured at 400 MHz (1H) or at 100 MHz (13C) with CDCl3 as the solvent on a Bruker Avance DRX-400 spectrometer. All reactions were monitored by analytical thin-layer chromatography (TLC) from Merck with detection by UV. The products were purified by column chromatography through silica gel (300–400 mesh). All reagents and solvents were general reagent grade unless stated otherwise.
The characterization of catalysts The X-ray diffraction (XRD) pattern was recorded with a diffractometer (Bruker D8 Advance) using Cu Kα radiation. The catalysts were examined at room temperature and a range of 5-80° on 2θ. The scanning electron microscopic (SEM) was equipped with a field-emission scanning electron microscope (S-4800, Hitachi) at 30 kV accelerating voltage to investigate the morphology of the catalysts. The transmission electron microscopic (TEM) images were taken using a JEM-2100plus at an acceleration voltage of 100 kV. The TEM samples were prepared by dropping the catalyst suspension directly onto a copper grid and allowed to dry. The fourier transform infrared spectra (FTIR) were recorded in a range of 7800-350 cm−1 at a resolution of 0.09 cm-1 using a “Nicolet 6700” spectrometer (ThermoFisher Scientific, America). The thermogravimetric analysis (TGA) was
The synthesis of CuSO4/CS and Cu/CS Typically, 1 g chitosan and 0.3 g CuSO4·5H2O were added into 30 ml of deionized water. The mixture was continuously stirred at room temperature for 3 h. After adsorption of the copper, the solid was separated by filtration, washed with water and ethanol, dried under 2
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Table 1 Optimization of reaction conditionsa.
Entry
Catalyst
Base
Solvent
Temperature/oC
Yieldb/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
CuSO4/CS Cu/CS Cu/HCS Cu/HCS-MA Cu/HCS-AC Cu/HCS-SA Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127c Cu/HCS-MA-F127d
K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 NaOAc KOAc NaHCO3 KHCO3 Na2CO3 Cs2CO3 K2CO3 K2CO3 K2CO3 K2CO3
DMSO H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
110 110 100 100 100 100 100 100 100 100 100 100 100 90 80 100 100
70 60 75 88 80 80 98 60 70 80 86 90 92 90 70 80 95
a b c d
Reaction conditions: 4-iodoanisole 1a (1.0 mmol), imidazole 2a (1.2 mmol), base (2.0 mmol) and the catalyst (100 mg) in the solvent (4 mL) for 24 h. Isolated yield. Hot-filtration test of the model reaction conducted under optimized conditions for 12 h. Cu/HCS-MA-F127 was pretreated in water at 100 °C for 10 h.
MA-F127 species; as a result, the latter exhibited even higher catalytic activity in aqueous medium (Table 1, entry 7). Further, several bases including K2CO3, NaOAc, KOAc, NaHCO3, KHCO3, Na2CO3 and Cs2CO3 were tested, and it turned out that K2CO3 performed best (Table 1, entries 7–12). Then, as to an appropriate temperature (Table 1, entries 7, 14–15); the yield of the final product is up to 98 % at 100 °C. Finally, to verify the heterogeneous nature of Cu/HCS-MA-F127, a hot filtration test was performed. When the reaction proceeded for 12 h and the yield was up to 80 % (Table 1, entry 16), the particles of Cu/HCS-MA-F127 were centrifuged out of the reaction mixture and the reaction result did not change next 12 h. It turned out that the Cu is stable on the surface of hydrochar and the catalyst is truly heterogeneous. To conclude, the optional conditions for the CeN coupling reaction in water are: using Cu/HCS-MA-F127 as catalyst, K2CO3 as base source, and controlling the reaction temperature at 100 °C. To further probe the nature of metal-hybrid carbonaceous materials in a catalytic cycle, chitosan-derived hydrochar (HCS), Cu/HCS and Cu/ HCS-MA-F127 were characterized by X-ray diffraction (XRD). As shown in Fig. 2, all carbonaceous materials had a broad diffraction peak at around 20°, which is assigned to the (002) planes of carbon. Meanwhile, both Cu/HCS-MA-F127 and Cu/HCS presented typical diffraction peaks of 43.2°, 50.4° and 74.1° at 2θ, corresponding to the (111), (200) and (220) planes of elemental copper, respectively (JCPDS File no.03-1018) [45]. It was thus indicated that copper sulfate was reduced to elemental copper, resulted from the strong reducibility of chitosan under the hydrothermal condition [46]. More details about the morphology and the microstructure of Cu/HCS-MA-F127 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 3a showed the well dispersion of copper nanoparticles on HCS. As shown in Fig. 3c, the particle size of the copper NPs ranges from 50 to 100 nm with a spherical morphology, which is due to the structure-directing agent (F127) tightly wrapping [47,48]. The average size of spherical copper NPs is 61.11 nm (in Fig. 3d). Moreover, the copper of the fresh Cu/HCS-MA-F127 was measured by ICP-AES and a content of 1.1 mmol g−1 was detected. The FTIR spectra of Cu/HCS and Cu/HCS-MA-F127 were shown in
performed on a METTLER instruments (TGA/DSC1/1100SF). TG curves were recorded from 40 to 800 °C under a N2 atmosphere with a heating rate of 10 °C min−1. The X-ray photoelectron spectroscopic (XPS) measurements were conducted on an AXIS Supra instrument. Inductively coupled plasma atomic emission spectrometry (ICP-AES, ThermoFisher IRIS Intrepid II, America) was used to determine the elemental content of copper. Results and discussion Our initial experimental investigation focused on the influence of metal-hybrid carbonaceous materials on the catalytic activity. The coupling of 4-iodoanisole with imidazole was selected as the model reaction to benchmark the performance of the metal-hybrid carbonaceous catalyst and to optimize the reaction condition; more details are provided in Table 1. CuSO4 supported on chitosan (CuSO4/CS) was prepared first as a reference to catalyze the model reaction in DMSO (Table 1, entry 1), and the result was consistent with the previous study [11]. The Cu particles supported on chitosan (Cu/CS) promote the model reaction in H2O to give a yield of 60 % (Table 1, entry 2). When we attempted one-pot, in-situ HTC of the precursor CS with CuSO4 solution, copper NPs were dispersed on the hydrochar (Cu/HCS). Surprisingly, the so-prepared Cu/HCS exhibited good catalytic activity for the model reaction with 75 % yield (Table 1, entry 3); note that the green and sustainable solvent, i.e. water, was employed here. Actually, it has reported that different water-soluble vinyl monomers can be effectively combined with polymers under the HTC process to provide larger surface and excellent mechanical properties, which implies a cycloaddition mechanism between maleic acid and carbon framework. [42,44] As introducing acid monomer into the system improves the hydrophilicity of hydrochar, various acid monomers like. maleic acid, acetic acid and sulfuric acid, were examined (Table 1, entries 4–6). All acids were proven to improve the catalytic activity of metal-hybrid carbonaceous materials, while obviously maleic acid outperformed the others. Next, to enhance the uniformity of copper nanoparticle, block copolymer F127 was chosen as the surfactant to prepare the Cu/HCS3
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Fig. 4. FTIR spectra of Cu/HCS and Cu/HCS-MA-F127.
Fig. 2. X-ray diffraction patterns of Cu/HCS-MA-F127 (a), Cu/HCS (b) and HCS (c).
range of 50−100 °C, which might be caused by the evaporation of adsorbed water; significant weight loss occur from 250 °C to 350 °C, which can be attributed to the thermal decomposition of the materials. Importantly, Cu/HCS-MA-F127 benefits from high thermal stability that it can be used for reactions carried out at a temperature up to 200 °C. X-ray photoelectron spectroscopy (XPS) experiments were carried out to further analyze the elemental composition and chemical state of the catalysts (Fig. 6). Fig. 6a is the full spectra of HCS and Cu/HCS-MAF127 in which the indication of copper elements is absent. This may be caused by tight wrapping of the copper surface by F127 such that XPS cannot detect the presence of copper. Moreover, the absence of 2p(Cu) peak for fresh Cu/HCS-MA-F127 was consistent with the above
Fig. 4. For Cu/HCS, the characteristic absorption appears at 3400 cm−1 (O-H stretching vibration), 2873 cm−1 (C-H stretching vibration), 1575 cm−1 (N-H bending vibration), 1375 cm−1 (bending vibration of C-H), 1150 cm−1 (C-O-C stretching vibration) and 1025 cm−1 (C-O stretching vibration). Compared with Cu/HCS, a new carboxyl peak emerges at 1700 cm−1 in the FTIR spectra of Cu/HCS-MA-F127. The new peak implied that carboxyl groups were successfully incorporated into the carbonaceous skeleton of the hydrochar. Moreover, the presence of a large amount of carboxyl and hydroxyl groups constitutes the hydrophilic surface of the hydrochar. Further, Fig. 5 exhibits the results for TG analysis of HCS, Cu/HCS and Cu/HCS-MA-F127. Thus, all three materials have two stages of mass loss: the initial weight loss is in the
Fig. 3. SEM images of Cu/HCS-MA-F127 (a), recovered Cu/HCS-MA-F127 after the second run (b), TEM images of Cu/HCS-MA-F127 (c) and particle size distribution of Cu/HCS-MA-F127 (d). 4
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(Table 1, entry 17). After filtration and drying, the catalyst was examined and the yield of the final product was still up to 95 %. As shown in Fig. 6c and d, the 1 s(N) peak of HCS and recycled Cu/HCS-MA-F127 after the second round were presented. Without chelation of copper, the peaks of HCS at 398.2 eV and 399.5 eV (in Fig. 6c) can be attributed to the N and -N-H- moieties of pyridine. When Cu/HCS-MA-F127 was recylced after the second round, the 1 s(N) resonance can be divided into three independent peaks at 398.4, 399.5 and 400.6 eV (in Fig. 6d), owing to the N, -N-H and -N-Cu moieties of pyridine, respectively [50–52]. As shown in Fig. 7, the C 1s and O 1s spectra of XPS were used to further investigate the structural information of hydrochar. Fig. 7a is the C 1s spectra of Cu/HCS, in which was fitted into two peaks at 284.4 and 286.8 eV, corresponding to the C-C, C-N and C-O bonds, respectively [49,53,54]. In Fig. 7b, the C 1s spectrum of Cu/HCS-MA-F127 consists of various types of C functionalities, including C-C (284.5 eV), C-N/C-O (285.9 eV), and C=O (287.4 eV) bonds, indicating that the carboxyl groups were successfully incorporated into the carbon framework. As shown in Fig. 7c and d, the O 1s pattern of Cu/HCS and Cu/ HCS-MA-F127 were presented, The O 1s spectrum of Cu/HCS could be deconvoluted into one peak, corresponding to C-O/O-H (532.6 eV). The O 1s signal of Cu/HCS-MA-F127 at 530.8 eV and 532.3 eV, which were contributed by C=O and C-O/C-H, respectively. Further, in order to reveal the generality of this protocol, various substrates were employed in Cu/HCS-MA-F127 catalyzed Ullmann CeN coupling reaction in water; the associated details are summarized in Table 2. Firstly, the catalytic activity was assessed with different aryl halides using imidazole as the substrate. Substrates with all substitution groups (such as ortho-, meta-, and para-substituted aryl halides, 3a-m)
Fig. 5. TG curves of HCS, Cu/HCS and Cu/HCS-MA-F127.
conjecture, as shown in Fig. 6b. Interestingly, the 2p(Cu) peak was observed after the second round of cycling and the peaks of 2p1/2(Cu) and 2p3/2(Cu) were 952.9 eV and 933.1 eV, respectively. In addition, compared with Fig. 3a and b, the copper NPs are more clearly monodispersed and immobilized on carbonaceous materials after the second round of cycling. This is due to the fact that F127 was washed away and the elemental copper was exposed after reaction [49]. In order to study the stability of copper nanoparticles without F127, the Cu/HCS-MAF127 was pretreated in water at 100 °C for 10 h to get F127 wash away
Fig. 6. XPS spectra of HCS and Cu/HCS (a), Cu 2p peaks of Cu/HCS-MA-F127 and recovered Cu/HCS-MA-F127 after the second run (b), N 1s pattern of HCS (c) and recovered Cu/HCS-MA-F127 after the second run (d). 5
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Fig. 7. C 1s pattern of Cu/HCS (a) and Cu/HCS-MA-F127 (b), O1s pattern of Cu/HCS (c) and Cu/HCS-MA-F127 (d).
Conclusions
were efficiently coupled with imidazole to afford the corresponding Naryl compounds with moderate to good yields (50–90 %). Moreover, both aryl iodide and aryl bromide gave rise to good yields of desired products (98 and 70 %), respectively. And the reactivity of different aryl halides follows the order PhI > PhBr > PhCl (3a). As to the influence of substitution groups, the electron-donating ones, such as 4OMe (3b), 4-Me (3c), 4-OEt (3d) and 4-OCF (3e), result in slightly higher yields than that given by the electron-withdrawing ones, such as 4-F (3 h), 4-Cl (3i), 4-Ac (3 j) and 4-NO2 (3k). Next, various nitride substrates were examined during the reaction. All employed nitride substrates (3n-t) could be efficiently coupled with 4-iodoanisole in moderate to good yields (40–85 %). Finally, we checked the recyclability of Cu/HCS-MA-F127 for aqueous Ullmann CeN coupling systems. The separation of Cu/HCS-MAF127 from the aqueous medium was achieved by centrifugal filtration. After that, the separated catalyst was washed several times with deionized water to remove the base and dried at 50 °C under vacuum. The catalyst was thus recycled and reused subsequently for the next round. The recyclability of Cu/HCS-MA-F127 catalyst was examined with the model reaction as mentioned in Table 1; the results were summarized in Fig. 8. It was found that after five round the Cu/HCS-MA-F127 catalyst kept its excellent catalytic activity and stability. Moreover, the SEM image of Cu/HCS-MA-F127 after the second round of cycling was shown in Fig. 3b, which clearly shows that copper NPs are monodispersed and immobilized on carbonaceous materials. The copper content of this recovered Cu/HCS-MA-F127 was determined as 0.8 mmol g−1 by ICP-AES.
In summary, we developed a facile method for fabrication of copper NPs on the hydrochar by using natural, inexpensive and renewable chitosan as the carbon source and reductant combined with in-situ reduction of copper salt in a one-pot hydrothermal carbonization process. The copper NPs were uniformly dispersed on hydrochar, exhibiting excellent catalytic activity for Ullmann CeN coupling reaction. Moreover, Cu/HCS-MA-F127 possesses strong hydrophilicity and are thus able to promote the coupling reaction in water with moderate to excellent yields. Notably, the Cu/HCS-MA-F127 catalyst can be recycled by simple centrifugal filtration and reused several times without obvious decrease of activity.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors are grateful for the financial support from the Natural Science Foundation of China (21606104) and the National Key Research and Development Program of China (2016YFB0301800). 6
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Table 2 Substrate scope for Ullmann CeN coupling reactiona.
a
Reaction conditions: aryl halides 1 (1.0 mmol), nitrogen nucleophiles 2 (1.2 mmol), K2CO3 (2.0 mmol) and Cu/HCS-MA-F127 (100 mg) in H2O (4 mL) at 100 °C for 24 h.
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Fig. 8. Recyclability of the Cu/HCS-MA-F127.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110726.
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Facile synthesis of hydrochar supported copper nanocatalyst for Ullmann CeN coupling reaction in water Xin Gea, Meng Gea, Xinzhi Chenb, Chao Qianb, Xuemin Liua,*, Shaodong Zhoub,* a
School of Chemical and Material Engineering, Jiangnan University, Wuxi, PR China Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrochar Nanocatalyst Chitosan Copper eCN coupling reaction In water
The exploration of inexpensive and stable heterogeneous catalysts and application of green solvents for Ullmann CeN coupling reaction remain challenging. We present a facile fabrication of copper nanoparticles on hydrochar as prepared from natural, inexpensive and renewable chitosan together with in-situ reduction of copper salt in a one-pot hydrothermal carbonization process. The copper nanoparticles were uniformly dispersed on hydrochar by choosing block copolymer F127 as surfactant. Moreover, maleic acid was introduced to improve the hydrophilicity of hydrochar. The most active copper nanocomposite catalyst, that is, Cu/HCS-MA-F127, exhibited excellent catalytic activity for Ullmann CeN coupling reaction in water. The nature of the Cu/HCS-MA-F127 was characterized by FTIR spectroscopy, TG, XRD, SEM and XPS. Moderate to excellent yields of aimed products were gained by using this catalytic strategy. Moreover, the Cu/HCS-MA-F127 catalyst can be reused by simple centrifugal recovery with a stable performance.
Introduction Enormous efforts have been made for constructing a CeN bond, which constitutes an important structural motif in various drugs, natural products and materials [1–3]. In 1990s, palladium-catalyzed reactions as explored by Buchwald and Hartwig [4,5] achieved a major breakthrough in CeN coupling. After that, quite some protocols have been exploited to improve the CeN coupling processes under mild reaction condition. In particular, copper-catalyzed Ullmann reaction, in which the toxic and expensive palladium is not involved, has proven to be an efficient strategy and continues attracting broad attention [6,7]. For example, Buchwald [8] first disclosed the catalytic system of diamine ligands for N-arylation; afterwards, Cristau and Taillefer [9] employed polydentate ligands like Schiff base and oxime in such reactions; Ma [10] found that amino acids are particularly efficient for CeN coupling. Though various ligands have been developed and used to promote copper-catalyzed Ullmann reactions, these homogeneous systems suffer from well-known disadvantages, such as unrecyclable catalysts, complex ligands and potentially toxic solvents (e.g., DMF, DMSO, dioxane). [11–14] Thus, exploration of heterogeneous catalysts and application of green solvents for Ullmann CeN coupling reaction continue to be demanding. Recently, due to the high catalytic activity of metal nanoparticles
⁎
(MNPs) as found for the organic coupling reactions, the nano-sized metal catalysts on proper supports attract raising attention. [15–17] Also, catalysts based on Cu, CuO and Cu2O nanoparticles have been employed to catalyze the formation of CeN bonds. For example, Bai et al. [18] reported that Cu0 anchored on nanofibers by hydrothermal method for promoting Ullmann CeN coupling at 140 °C in DMF. Hyeon et al. [19] prepared Cu2O coated Cu nanoparticles (NPs) as catalysts for Ullmann type amination coupling in DMSO; S. Rawat et al. [20] reported that copper NPs incorporated on alumina/silica support can promote the cross coupling of aryl chlorides with amines at 150 °C in DMF; Jafarzadeh et al. [21] loaded CuO NPs on MOFs for promoting Ullmann coupling at 110 °C in DMSO. In spite of the above achievements, improvements are still required for the application of MNPs regarding the use of organic ligands, multi-step synthetic process and the emission of wastes. So far as reported, the heterogeneous copper catalysts have been prepared by entrapment and coordination of copper on various kinds of supports, such as carbon [22], alumina [20], zeolites [23], polymers [24,25] and magnetic materials [26,27], all of which exhibited remarkable performance in catalyzing the Ullmann CeN coupling processes. Due to the chemical inertness and economic accessibility of carbon, MNPs fabricated on carbon (MNP/C) is an ideal approach. Hydrothermal carbonization (HTC) processing of biomass turns out to
Corresponding authors. E-mail addresses: [email protected] (X. Liu), [email protected] (S. Zhou).
https://doi.org/10.1016/j.mcat.2019.110726 Received 7 August 2019; Received in revised form 25 October 2019; Accepted 25 November 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xin Ge, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110726
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Fig. 1. The prepartion of Cu/hydrochar catalyst.
vacuum at 50 °C overnight to give the CuSO4/CS catalyst. Similarly, the Cu/CS catalyst was prepared [43]: 1 g chitosan and 0.3 g CuSO4·5H2O were added into 30 ml of deionized water and stirred vigorously. Then 2 M NaOH was added dropwise to turn the pH value to 13. Next, 0.227 g NaBH4 was added to the above solution and continuously stirred for 2 h. The solid was separated by filtration, washed with water and ethanol, dried under vacuum at 50 °C overnight.
be an effective strategy to prepare carbonaceous materials (often called hydrochar) and metal-hybrid materials [28–30]. This method involves thermal dehydration and transformation of biomass in water at low temperatures [31,32]. Various carbonaceous structures and metal-hybrid materials (e.g., metal/C core-shell [33], metal/C nanocable [34] and porous carbon [35]) were afforded using HTC; the latter thus exhibits strong potential in many fields, such as adsorption [36], solid fuel [37], energy storage [38] and catalysis [39,40]. On a different background, featured as biodegradable, renewable and inexpensive compounds, polysaccharides have become the most promising biomass [41]. Among these polysaccharides, chitosan (CS) possesses a large amount of amino and hydroxyl groups, which would be retained in the HTC process [42]. CS-derived hydrochar has strong metal-chelating capability and are suitable support for MNP/C. In this work, we presented a one-pot, in-situ HTC process using CS and a solution of metal ions as the raw materials (Fig. 1). CS usually serves as the reducing agents, while in this work it functions as both the carbon source and the reductant. Consequently, the copper nanoparticles dispersed uniformly on hydrochar (Cu/hydrochar). To our delight, the so-prepared Cu/hydrochar showed high catalytic activity for the Ullmann CeN coupling. Moreover, such Cu/hydrochar species is of strong hydrophilicity and thus able to promote the coupling reactions in water. Herein, we describe in detail the preparation of copper NPs on CS-derived hydrochar and its application in catalytic coupling of Ullmann CeN reaction.
The synthesis of Cu/HCS-MA-F127 Typically, 0.3 g CuSO4·5H2O and 0.2 g F127 were added into 30 ml of deionized water. The mixture was stirred at room temperature for 30 min. Then 1 g chitosan and 0.2 g maleic acid were added to the above mixture, and stirred for 30 min. Finally, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 180 °C for 10 h. After cooling naturally, the black brown carbonaceous material was obtained. The material was washed with water and ethanol several times, and dried in a vacuum at 80 °C overnight. General procedure for the Ullmann reaction catalyzed by Cu/HCS-MA-F127 To a stirred solution of H2O (4 mL), aryl halide (1.0 mmol), nucleophile (1.2 mmol), Cu/HCS-MA-F127 and K2CO3 (2 mmol) were added at room temperature. Next, the reaction mixture was heated to 100 °C in air and stirred for 24 h. After cooling down to room temperature, the catalyst Cu@HCS-MA-F127 was separated by centrifugation. The reaction mixture was partitioned by adding ethyl acetate (20 mL) and water (20 mL). Subsequently, the organic phase was separated and the aqueous phase was extracted with ethyl acetate (20 mL) twice. The combined organic phases were washed with brine, dried over Na2SO4, and concentrated in vacuo. Finally, the crude product was purified by column chromatography with silica gel, eluting with a petroleum ether/ethyl acetate solvent mixture, to give the pure product.
Experimental section Materials and methods Chitosan powder (MW: 10000–50000, deacetylation degree 95 %, purchased from Aladdin reagent (Shanghai) Co., Ltd) was used without further purification. Maleic acid (MA), acetic acid, sulfuric acid and CuSO4·5H2O was purchased from Sinopharm Chemical Reagent Co. Ltd, and Pluronic F127 was purchased from Energy Chemical. Aryl halides and imidazole were purchased from Alfa Aesar. Nuclear magnetic resonance (NMR) spectra were measured at 400 MHz (1H) or at 100 MHz (13C) with CDCl3 as the solvent on a Bruker Avance DRX-400 spectrometer. All reactions were monitored by analytical thin-layer chromatography (TLC) from Merck with detection by UV. The products were purified by column chromatography through silica gel (300–400 mesh). All reagents and solvents were general reagent grade unless stated otherwise.
The characterization of catalysts The X-ray diffraction (XRD) pattern was recorded with a diffractometer (Bruker D8 Advance) using Cu Kα radiation. The catalysts were examined at room temperature and a range of 5-80° on 2θ. The scanning electron microscopic (SEM) was equipped with a field-emission scanning electron microscope (S-4800, Hitachi) at 30 kV accelerating voltage to investigate the morphology of the catalysts. The transmission electron microscopic (TEM) images were taken using a JEM-2100plus at an acceleration voltage of 100 kV. The TEM samples were prepared by dropping the catalyst suspension directly onto a copper grid and allowed to dry. The fourier transform infrared spectra (FTIR) were recorded in a range of 7800-350 cm−1 at a resolution of 0.09 cm-1 using a “Nicolet 6700” spectrometer (ThermoFisher Scientific, America). The thermogravimetric analysis (TGA) was
The synthesis of CuSO4/CS and Cu/CS Typically, 1 g chitosan and 0.3 g CuSO4·5H2O were added into 30 ml of deionized water. The mixture was continuously stirred at room temperature for 3 h. After adsorption of the copper, the solid was separated by filtration, washed with water and ethanol, dried under 2
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Table 1 Optimization of reaction conditionsa.
Entry
Catalyst
Base
Solvent
Temperature/oC
Yieldb/%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
CuSO4/CS Cu/CS Cu/HCS Cu/HCS-MA Cu/HCS-AC Cu/HCS-SA Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127 Cu/HCS-MA-F127c Cu/HCS-MA-F127d
K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 NaOAc KOAc NaHCO3 KHCO3 Na2CO3 Cs2CO3 K2CO3 K2CO3 K2CO3 K2CO3
DMSO H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
110 110 100 100 100 100 100 100 100 100 100 100 100 90 80 100 100
70 60 75 88 80 80 98 60 70 80 86 90 92 90 70 80 95
a b c d
Reaction conditions: 4-iodoanisole 1a (1.0 mmol), imidazole 2a (1.2 mmol), base (2.0 mmol) and the catalyst (100 mg) in the solvent (4 mL) for 24 h. Isolated yield. Hot-filtration test of the model reaction conducted under optimized conditions for 12 h. Cu/HCS-MA-F127 was pretreated in water at 100 °C for 10 h.
MA-F127 species; as a result, the latter exhibited even higher catalytic activity in aqueous medium (Table 1, entry 7). Further, several bases including K2CO3, NaOAc, KOAc, NaHCO3, KHCO3, Na2CO3 and Cs2CO3 were tested, and it turned out that K2CO3 performed best (Table 1, entries 7–12). Then, as to an appropriate temperature (Table 1, entries 7, 14–15); the yield of the final product is up to 98 % at 100 °C. Finally, to verify the heterogeneous nature of Cu/HCS-MA-F127, a hot filtration test was performed. When the reaction proceeded for 12 h and the yield was up to 80 % (Table 1, entry 16), the particles of Cu/HCS-MA-F127 were centrifuged out of the reaction mixture and the reaction result did not change next 12 h. It turned out that the Cu is stable on the surface of hydrochar and the catalyst is truly heterogeneous. To conclude, the optional conditions for the CeN coupling reaction in water are: using Cu/HCS-MA-F127 as catalyst, K2CO3 as base source, and controlling the reaction temperature at 100 °C. To further probe the nature of metal-hybrid carbonaceous materials in a catalytic cycle, chitosan-derived hydrochar (HCS), Cu/HCS and Cu/ HCS-MA-F127 were characterized by X-ray diffraction (XRD). As shown in Fig. 2, all carbonaceous materials had a broad diffraction peak at around 20°, which is assigned to the (002) planes of carbon. Meanwhile, both Cu/HCS-MA-F127 and Cu/HCS presented typical diffraction peaks of 43.2°, 50.4° and 74.1° at 2θ, corresponding to the (111), (200) and (220) planes of elemental copper, respectively (JCPDS File no.03-1018) [45]. It was thus indicated that copper sulfate was reduced to elemental copper, resulted from the strong reducibility of chitosan under the hydrothermal condition [46]. More details about the morphology and the microstructure of Cu/HCS-MA-F127 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 3a showed the well dispersion of copper nanoparticles on HCS. As shown in Fig. 3c, the particle size of the copper NPs ranges from 50 to 100 nm with a spherical morphology, which is due to the structure-directing agent (F127) tightly wrapping [47,48]. The average size of spherical copper NPs is 61.11 nm (in Fig. 3d). Moreover, the copper of the fresh Cu/HCS-MA-F127 was measured by ICP-AES and a content of 1.1 mmol g−1 was detected. The FTIR spectra of Cu/HCS and Cu/HCS-MA-F127 were shown in
performed on a METTLER instruments (TGA/DSC1/1100SF). TG curves were recorded from 40 to 800 °C under a N2 atmosphere with a heating rate of 10 °C min−1. The X-ray photoelectron spectroscopic (XPS) measurements were conducted on an AXIS Supra instrument. Inductively coupled plasma atomic emission spectrometry (ICP-AES, ThermoFisher IRIS Intrepid II, America) was used to determine the elemental content of copper. Results and discussion Our initial experimental investigation focused on the influence of metal-hybrid carbonaceous materials on the catalytic activity. The coupling of 4-iodoanisole with imidazole was selected as the model reaction to benchmark the performance of the metal-hybrid carbonaceous catalyst and to optimize the reaction condition; more details are provided in Table 1. CuSO4 supported on chitosan (CuSO4/CS) was prepared first as a reference to catalyze the model reaction in DMSO (Table 1, entry 1), and the result was consistent with the previous study [11]. The Cu particles supported on chitosan (Cu/CS) promote the model reaction in H2O to give a yield of 60 % (Table 1, entry 2). When we attempted one-pot, in-situ HTC of the precursor CS with CuSO4 solution, copper NPs were dispersed on the hydrochar (Cu/HCS). Surprisingly, the so-prepared Cu/HCS exhibited good catalytic activity for the model reaction with 75 % yield (Table 1, entry 3); note that the green and sustainable solvent, i.e. water, was employed here. Actually, it has reported that different water-soluble vinyl monomers can be effectively combined with polymers under the HTC process to provide larger surface and excellent mechanical properties, which implies a cycloaddition mechanism between maleic acid and carbon framework. [42,44] As introducing acid monomer into the system improves the hydrophilicity of hydrochar, various acid monomers like. maleic acid, acetic acid and sulfuric acid, were examined (Table 1, entries 4–6). All acids were proven to improve the catalytic activity of metal-hybrid carbonaceous materials, while obviously maleic acid outperformed the others. Next, to enhance the uniformity of copper nanoparticle, block copolymer F127 was chosen as the surfactant to prepare the Cu/HCS3
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Fig. 4. FTIR spectra of Cu/HCS and Cu/HCS-MA-F127.
Fig. 2. X-ray diffraction patterns of Cu/HCS-MA-F127 (a), Cu/HCS (b) and HCS (c).
range of 50−100 °C, which might be caused by the evaporation of adsorbed water; significant weight loss occur from 250 °C to 350 °C, which can be attributed to the thermal decomposition of the materials. Importantly, Cu/HCS-MA-F127 benefits from high thermal stability that it can be used for reactions carried out at a temperature up to 200 °C. X-ray photoelectron spectroscopy (XPS) experiments were carried out to further analyze the elemental composition and chemical state of the catalysts (Fig. 6). Fig. 6a is the full spectra of HCS and Cu/HCS-MAF127 in which the indication of copper elements is absent. This may be caused by tight wrapping of the copper surface by F127 such that XPS cannot detect the presence of copper. Moreover, the absence of 2p(Cu) peak for fresh Cu/HCS-MA-F127 was consistent with the above
Fig. 4. For Cu/HCS, the characteristic absorption appears at 3400 cm−1 (O-H stretching vibration), 2873 cm−1 (C-H stretching vibration), 1575 cm−1 (N-H bending vibration), 1375 cm−1 (bending vibration of C-H), 1150 cm−1 (C-O-C stretching vibration) and 1025 cm−1 (C-O stretching vibration). Compared with Cu/HCS, a new carboxyl peak emerges at 1700 cm−1 in the FTIR spectra of Cu/HCS-MA-F127. The new peak implied that carboxyl groups were successfully incorporated into the carbonaceous skeleton of the hydrochar. Moreover, the presence of a large amount of carboxyl and hydroxyl groups constitutes the hydrophilic surface of the hydrochar. Further, Fig. 5 exhibits the results for TG analysis of HCS, Cu/HCS and Cu/HCS-MA-F127. Thus, all three materials have two stages of mass loss: the initial weight loss is in the
Fig. 3. SEM images of Cu/HCS-MA-F127 (a), recovered Cu/HCS-MA-F127 after the second run (b), TEM images of Cu/HCS-MA-F127 (c) and particle size distribution of Cu/HCS-MA-F127 (d). 4
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(Table 1, entry 17). After filtration and drying, the catalyst was examined and the yield of the final product was still up to 95 %. As shown in Fig. 6c and d, the 1 s(N) peak of HCS and recycled Cu/HCS-MA-F127 after the second round were presented. Without chelation of copper, the peaks of HCS at 398.2 eV and 399.5 eV (in Fig. 6c) can be attributed to the N and -N-H- moieties of pyridine. When Cu/HCS-MA-F127 was recylced after the second round, the 1 s(N) resonance can be divided into three independent peaks at 398.4, 399.5 and 400.6 eV (in Fig. 6d), owing to the N, -N-H and -N-Cu moieties of pyridine, respectively [50–52]. As shown in Fig. 7, the C 1s and O 1s spectra of XPS were used to further investigate the structural information of hydrochar. Fig. 7a is the C 1s spectra of Cu/HCS, in which was fitted into two peaks at 284.4 and 286.8 eV, corresponding to the C-C, C-N and C-O bonds, respectively [49,53,54]. In Fig. 7b, the C 1s spectrum of Cu/HCS-MA-F127 consists of various types of C functionalities, including C-C (284.5 eV), C-N/C-O (285.9 eV), and C=O (287.4 eV) bonds, indicating that the carboxyl groups were successfully incorporated into the carbon framework. As shown in Fig. 7c and d, the O 1s pattern of Cu/HCS and Cu/ HCS-MA-F127 were presented, The O 1s spectrum of Cu/HCS could be deconvoluted into one peak, corresponding to C-O/O-H (532.6 eV). The O 1s signal of Cu/HCS-MA-F127 at 530.8 eV and 532.3 eV, which were contributed by C=O and C-O/C-H, respectively. Further, in order to reveal the generality of this protocol, various substrates were employed in Cu/HCS-MA-F127 catalyzed Ullmann CeN coupling reaction in water; the associated details are summarized in Table 2. Firstly, the catalytic activity was assessed with different aryl halides using imidazole as the substrate. Substrates with all substitution groups (such as ortho-, meta-, and para-substituted aryl halides, 3a-m)
Fig. 5. TG curves of HCS, Cu/HCS and Cu/HCS-MA-F127.
conjecture, as shown in Fig. 6b. Interestingly, the 2p(Cu) peak was observed after the second round of cycling and the peaks of 2p1/2(Cu) and 2p3/2(Cu) were 952.9 eV and 933.1 eV, respectively. In addition, compared with Fig. 3a and b, the copper NPs are more clearly monodispersed and immobilized on carbonaceous materials after the second round of cycling. This is due to the fact that F127 was washed away and the elemental copper was exposed after reaction [49]. In order to study the stability of copper nanoparticles without F127, the Cu/HCS-MAF127 was pretreated in water at 100 °C for 10 h to get F127 wash away
Fig. 6. XPS spectra of HCS and Cu/HCS (a), Cu 2p peaks of Cu/HCS-MA-F127 and recovered Cu/HCS-MA-F127 after the second run (b), N 1s pattern of HCS (c) and recovered Cu/HCS-MA-F127 after the second run (d). 5
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Fig. 7. C 1s pattern of Cu/HCS (a) and Cu/HCS-MA-F127 (b), O1s pattern of Cu/HCS (c) and Cu/HCS-MA-F127 (d).
Conclusions
were efficiently coupled with imidazole to afford the corresponding Naryl compounds with moderate to good yields (50–90 %). Moreover, both aryl iodide and aryl bromide gave rise to good yields of desired products (98 and 70 %), respectively. And the reactivity of different aryl halides follows the order PhI > PhBr > PhCl (3a). As to the influence of substitution groups, the electron-donating ones, such as 4OMe (3b), 4-Me (3c), 4-OEt (3d) and 4-OCF (3e), result in slightly higher yields than that given by the electron-withdrawing ones, such as 4-F (3 h), 4-Cl (3i), 4-Ac (3 j) and 4-NO2 (3k). Next, various nitride substrates were examined during the reaction. All employed nitride substrates (3n-t) could be efficiently coupled with 4-iodoanisole in moderate to good yields (40–85 %). Finally, we checked the recyclability of Cu/HCS-MA-F127 for aqueous Ullmann CeN coupling systems. The separation of Cu/HCS-MAF127 from the aqueous medium was achieved by centrifugal filtration. After that, the separated catalyst was washed several times with deionized water to remove the base and dried at 50 °C under vacuum. The catalyst was thus recycled and reused subsequently for the next round. The recyclability of Cu/HCS-MA-F127 catalyst was examined with the model reaction as mentioned in Table 1; the results were summarized in Fig. 8. It was found that after five round the Cu/HCS-MA-F127 catalyst kept its excellent catalytic activity and stability. Moreover, the SEM image of Cu/HCS-MA-F127 after the second round of cycling was shown in Fig. 3b, which clearly shows that copper NPs are monodispersed and immobilized on carbonaceous materials. The copper content of this recovered Cu/HCS-MA-F127 was determined as 0.8 mmol g−1 by ICP-AES.
In summary, we developed a facile method for fabrication of copper NPs on the hydrochar by using natural, inexpensive and renewable chitosan as the carbon source and reductant combined with in-situ reduction of copper salt in a one-pot hydrothermal carbonization process. The copper NPs were uniformly dispersed on hydrochar, exhibiting excellent catalytic activity for Ullmann CeN coupling reaction. Moreover, Cu/HCS-MA-F127 possesses strong hydrophilicity and are thus able to promote the coupling reaction in water with moderate to excellent yields. Notably, the Cu/HCS-MA-F127 catalyst can be recycled by simple centrifugal filtration and reused several times without obvious decrease of activity.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors are grateful for the financial support from the Natural Science Foundation of China (21606104) and the National Key Research and Development Program of China (2016YFB0301800). 6
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Table 2 Substrate scope for Ullmann CeN coupling reactiona.
a
Reaction conditions: aryl halides 1 (1.0 mmol), nitrogen nucleophiles 2 (1.2 mmol), K2CO3 (2.0 mmol) and Cu/HCS-MA-F127 (100 mg) in H2O (4 mL) at 100 °C for 24 h.
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Fig. 8. Recyclability of the Cu/HCS-MA-F127.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110726.
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