A novel route with a Cu(II)-MOF-derived structure to synthesize Cu/Cu2O NPs@graphene: the electron transfer leads to the synergistic effect of the Cu(0)–Cu(I) phase for an effective catalysis of the Sonogashira cross-coupling reactions

Wei Sun , Lingfeng Gao *, Xu Sun and Gengxiu Zheng *
School of Chemistry and Chemical Engineering, University of Jinan, No. 336 West Road of Nan Xinzhuang, Jinan 250022, P. R. China. E-mail: chm_zhenggx@ujn.edu.cn; gaolf108@mail.ustc.edu.cn

Received 3rd February 2018 , Accepted 25th March 2018

First published on 26th March 2018


A uniformly dispersed two-phase Cu/Cu2O NPs@graphene was obtained via an innovative “take” and “off” synthesis strategy with a Cu-MOF-derived jacket structure. The electron transfer between the Cu phase and Cu2O phase leads to the synergistic effect of the Cu(0)–Cu(I) phase for an efficient catalysis of the Sonogashira cross-coupling reactions.


The carbon-based materials doped with copper nanoparticles (Cu NPs) are well-accepted as remarkable, important, and promising catalysts.1 Among the different types of Cu NPs, cuprous oxide nanoparticles (Cu2O NPs) have been extensively studied due to their important properties and applications.2 Also, there have been great advances on the synthesis of Cu2O.3 However, there are only few reports in the literature regarding the properties and catalytic activities of other types of Cu NPs, and the studies on the synergistic effect of different types of Cu NPs on a catalytic reaction process are even less. As a result, it is critical to find a purposeful control on the synthesis process of Cu NPs as catalysts with different Cu phases, but it still remains a challenge.

Herein, we communicate a simple, efficient and scientific strategy to synthesize Cu/Cu2O NPs@reduced graphene (Cu/Cu2O-rGO) with two different phases: Cu and Cu2O. The synthesis process mainly consists of a “take” and “off” process with a Cu-MOF-derived jacket structure on the graphene surface. Graphene is selected as the Cu NP supporter because it has high specific surface area, superior mechanical properties, and excellent electrical conductivity.4 Such an exploration of the MOF-derived jacket-structured composites has the potential to produce catalyst materials based on the effectively and uniformly dispersed Cu NPs with two different precisely-controlled phases Cu and Cu2O; this can provide a convincing evidence to study the synergistic effect of different phases of Cu NPs and develop multifunctioning catalysts from Cu NPs with different phases.

Cu/Cu2O-rGO was synthesized via the “take” and “off” steps with a novel MOF jacket structure and an in situ reduction process. The schematic diagram of the synthesis process is shown in Fig. 1. First, CuCl2·2H2O and terephthalic acid (TPA) were added to water. Due to the strong coordination interaction between the Cu(II) ions and TPA, the TPA-Cu jacket structure was successfully fabricated just as “take” the MOF-jacket shell protection on the surface of Cu NPs.5 The jacket structure prevented the serious agglomeration of Cu NPs during the preparation process.6 Then, the TPA-Cu structure could be easily anchored to both sides of GO because of the strong interaction between the TPA-Cu structure and the oxygen-containing functional groups of GO.7 Second, the resulting solution went through a solvothermal reduction process to form MOFs-Cu2O on the surface of graphene (MOFs-Cu2O-rGO). Next, MOFs-Cu2O-rGO via the following annealing operation to “off” the MOF jacket structure was performed to obtain the Cu2O-rGO material. Finally, the obtained Cu2O-rGO was dispersed in a cuprous chloride solution to capture Cu(I) ions in this solution.8 Then, the Cu(I) ions captured on the Cu2O-rGO surface were reduced to Cu(0) NPs by NaBH4, and the latter were deposited on the Cu2O-rGO surface. The uniformly dispersed two-phase Cu/Cu2O-rGO material was successfully obtained from the novel MOF jacket structure by a “take” and “off” synthesis route.


image file: c8dt00465j-f1.tif
Fig. 1 Schematic illustration of the synthesis process of Cu/Cu2O-rGO.

The transmission electron microscopy (TEM) images of the Cu2O-rGO and Cu/Cu2O-rGO composites revealed that Cu2O NPs were uniformly anchored on the surface of the graphene sheets (Fig. 2a and c). The high-resolution TEM (HRTEM) image of Cu2O-rGO showed the presence of crystal lattices of Cu2O (Fig. 2b). The lattice fringes of Cu2O-rGO were observed clearly with an interlayer spacing of 0.25 nm and matched well with the (111), (200), (220) and (311) lattice planes of Cu2O.9 The TEM image of Cu/Cu2O-rGO displayed distinctive differences when compared with that of Cu2O-rGO (Fig. 2c). Numerous NPs were evenly deposited on the graphene sheet, which was ascribed to the formation of Cu NPs on Cu2O-rGO. As shown in Fig. 2d, the HRTEM image revealed that one could even distinguish two different Cu phases (Cu and Cu2O), which explicitly showed the lattice fringes with an interlayer spacing of 0.25 nm and 0.21 nm, and these spacings were ascribed to the (111) lattice plane of Cu2O and (311) lattice plane of Cu, respectively.10


image file: c8dt00465j-f2.tif
Fig. 2 TEM images of (a) Cu2O-rGO and (c) Cu/Cu2O-rGO. HRTEM images of (b) Cu2O-rGO, scale bar of 5 nm and (d) Cu/Cu2O-rGO, scale bar of 2 nm. (e) SEM image and (f) EDS mapping of Cu/Cu2O-rGO.

The novel MOF jacket structure led to a strong interaction of the Cu species with the support, providing stabilization of the Cu clusters with the mean size of 7.55 ± 2.92 nm (Fig. S4). The scanning electron microscopy (SEM) image of Cu/Cu2O-rGO clearly revealed an ultrathin nanosheet structure with a plicated surface (Fig. 2e). The energy dispersive spectroscopy (EDS) mapping shown in Fig. 2f proved that Cu/Cu2O-rGO had a uniform distribution of Cu, C, and O elements.

The nitrogen adsorption–desorption (BET) curve was obtained to evaluate the surface area and pore size distribution. As shown in Fig. 3a, SBET of Cu/Cu2O-rGO was obtained as 154 m2 g−1, and the average pore diameter of Cu/Cu2O-rGO was 8.76 nm. The curve belonged to the type IV isotherm with distinct hysteresis loops close to those of the H3 type, suggesting that the Cu/Cu2O-rGO composite had a characteristic lamellar stacking. The phase of the catalyst was determined by the X-ray diffraction (XRD) measurements (Fig. 3b). The peaks at 29.6, 36.5, 42.4, 61.6 and 73.7 could be assigned to the (110), (111), (200), (220), and (311) planes of the cubic Cu2O for Cu2O-rGO. Then, after the in situ reduction process, the peaks of the Cu phase appeared. The features at 43.21, 50.41, and 74.11 corresponded to the (111), (200), and (220) planes, respectively, of cubic Cu for Cu/Cu2O-rGO (JCPDS No. 85-1326).


image file: c8dt00465j-f3.tif
Fig. 3 (a) Nitrogen adsorption–desorption isotherms and pore size distribution curve (the inset) of the Cu/Cu2O-rGO composite. (b) XRD patterns of Cu2O-rGO and Cu/Cu2O-rGO. XPS survey spectra of Cu 2p3/2 peaks of (c) Cu2O-rGO and (d) Cu/Cu2O-rGO.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to analyze the Cu 2p3/2 peaks of Cu2O-rGO and Cu/Cu2O-rGO, and the results are shown in Fig. 3c and d, respectively. The binding energy at 932.5 eV corresponding to Cu in the Cu(0) and Cu(I) states revealed the Cu2O and Cu phases in Cu/Cu2O-rGO prepared by us (Fig. 3b), which was in agreement with the results of XRD study. The intensive broad lines at 934.1 eV for Cu2O-rGO and at 934.5 eV for Cu/Cu2O-rGO corresponded to the Cu(II) state. The higher contents of Cu(0) and Cu(I) obtained from Cu/Cu2O-rGO than those obtained from Cu2O-rGO indicated that more active sites on the graphene surface were formed after the in situ reduction. Inductively coupled plasma (ICP) was used to calculate the content of copper in Cu/Cu2O-rGO, and the results showed that the amounts of Cu in the Cu2O-rGO and Cu/Cu2O-rGO catalysts were 10.3% and 14.8%, respectively.

We were interested in studying the synergistic catalytic effect between Cu and Cu2O phases in the Sonogashira cross-coupling reaction.11 Phenylacetylene with aryl iodide was selected as the model substrate to test our Cu/Cu2O-rGO catalyst (Table 1). First, the Cu2O-rGO and Cu-rGO catalysts with a single Cu phase were used in the model reaction; as expected, the product was obtained in 68% and 32% yields, respectively (entries 3 and 4, Table 1). Notably, the newly obtained two-phase Cu catalyst Cu/Cu2O-rGO gave an excellent yield (91%) towards the product 1,2-diphenylethyne (entry 5, Table 1). In the control experiments, the single-metal catalysts Cu2O and Cu were used in the reaction to obtain the yields of 42% and 18%, respectively (entries 6 and 7, Table 1). When a simple mixture of the two metal catalysts was used, the yield became higher (53%, entry 8, Table 1). Hence, it can be concluded that the newly obtained Cu/Cu2O-rGO catalyst allowed the Cu species to be reversibly oxidized and reduced through the electron transfer between Cu and Cu2O and that graphene could promote this electron transfer process. Thus, this was a synergic catalytic process between Cu(0) and Cu(I), and Cu(I) played a direct catalytic role in carrying out this cross-coupling reaction.

Table 1 The catalytic efficiency of different catalysts for the Sonogashira reactiona

image file: c8dt00465j-u1.tif

Entry Cat. Yieldb (%)
a Reaction conditions: Phenylacetylene (2 mmol, 1 equiv.), iodobenzene (2 mmol, 1 equiv.), catalyst (1 wt%), Cs2CO3 (4 mmol, 2 equiv.), DMF (1 mL), 80 °C, and 8 h. b Isolated yields. c Catalyst (0.1 mmol). d Cu[thin space (1/6-em)]:[thin space (1/6-em)]Cu2O = 0.05 mmol[thin space (1/6-em)]:[thin space (1/6-em)]0.05 mmol.
1 Black Trace
2 Graphene Trace
3 Cu2O-rGO 68
4 Cu-rGO 32
5 Cu/Cu2O-rGO 91
6c Cu2O 42
7c Cu 18
8d Cu/Cu2O 53


In addition, the model reaction was also tested to optimize the reaction condition with different solvents (entries 1–9, Table S1) and various bases (entries 1–9, Table S2), and the best yield was 92%. Also, Cu/Cu2O-rGO showed a good catalytic reusability. More excitingly, the activity of the two-phase Cu/Cu2O-rGO catalyst was maintained well after five cycles in the catalytic reaction with a good yield of 81% (Fig. S7).

Based on these investigations, it can be stated that the synergistic association of Cu and Cu2O phases led to an efficient catalytic activation of alkyne C–H. Three roles are generally considered in the catalytic cycle catalysed by Cu/Cu2O-rGO: (a) the Cu phase acts as an authentic heterogeneous catalyst through its surface; (b) the Cu phase gets into a homogeneous catalytic cycle; and (c) an oxidative addition takes place on the surface of the Cu phase with subsequent leaching of Ar–Cu–I species, which start a homogeneous catalytic cycle. In this sense, from the poisoning tests (Table S4), the addition of either mercury (2 equiv.) or CS2 (0.25 equiv.) inhibited the standard reaction,12 which highly proved that the Cu phase on the graphene surface promoted the reaction. Further information was obtained from the negative filtration test, and the results of this test ruled out the mechanistic option (b).

To conclude this section, a proposed reaction mechanism was put forward on the basis of all of the above experimental results (Fig. 4). In cycle A, the Cu2O phase first coordinated with the terminal alkyne to generate the coordination adduct intermediate (I), and the Cu2O phase could increase the acidity of the inactive C–H bond and participated in the C–H activation. Meanwhile, an alkyne carbanion was generated by the deprotonation of the terminal alkyne in the presence of Cs2CO3 to form the intermediate (II).13 On the other hand, it is normally accepted that the alkyne activation occurs by the coordination of Ar–Cu–X species to the C[double bond, length as m-dash]C bond.14 Next, as shown in cycle B, with the presence of the Cu phase, iodide was displaced by the Cu phase to produce the oxidative addition species (R–Cu–I, (III)) and at the same time, (III) reacted with (II) to form the aryl copper acetylide (IV). Also, this was followed by reductive elimination to afford the product with the regeneration of Cu NPs.


image file: c8dt00465j-f4.tif
Fig. 4 Reaction mechanism proposed for the Sonogashira cross-coupling reaction catalysed by Cu/Cu2O-rGO.

Among the two different Cu phases on the graphene of this catalytic system, the Cu2O phase has a stronger coordination effect with terminal alkynes to promote the phenylacetylene activation step and plays the role of an oxidant in the C–C bond formation, which is proved to be a fast step in the reaction.13 All of these clues describe the catalytic cycle: the possibility of the Cu2O phase acting as the catalyst in this catalytic system is more when compared with the possibility of the Cu phase acting as the catalyst. The Cu2O phase coordinates with alkynes to promote the C–H activation, followed by a fast electron-transfer step from the Cu phase, and the graphene promotes the transfer process to give the cross-coupling product (Fig. S7). The overall reaction proceeds with the synergistic association of Cu phase and Cu2O phase on the graphene.

In conclusion, we have demonstrated a simple, efficient and scientific strategy to synthesize uniformly dispersed two-phase Cu/Cu2O-rGO from a Cu-MOF derived structure; the strategy mainly includes a “take” and “off” process with an innovative MOF jacket structure. The newly fabricated Cu/Cu2O-rGO catalyst allows the Cu species to be reversibly oxidized and reduced through the electron transfer between Cu phase and Cu2O phase, and the graphene can promote good electric conduction. Thus, due to the synergistic effect of Cu phase and Cu2O phase, the material works as an excellent heterogeneous noble metal-free catalyst for the Sonogashira cross-coupling reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the Natural Science Foundation of Shandong Province (ZR2017PB005) and the National Natural Science Foundation of China (No. 21601064).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8dt00465j

This journal is © The Royal Society of Chemistry 2018