Novel carbon nanofiber-supported Ni(0) nanoparticles catalyse the Heck reaction under ligand-free conditions

Abstract

Nickel, as an inexpensive and abundant transition metal, is widely used in the field of catalysis. A novel one-dimensional carbon nanofiber-supported Ni(0) composite catalyst (Ni(0)/CNFs) was prepared in this work, and its catalytic properties in the Heck reaction were explored. Compared with conventional nickel–ligand catalyst systems, the solid Ni(0)/CNFs composite catalyst has unique advantages. Ni(0)/CNFs does not require ligands or complexes, which results in high efficiency and is beneficial for reuse in the Heck reaction.

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The Heck reaction has attracted great attention in recent years, because it offers a simple way to generate new C–C bonds.^1,2 This cross-coupling reaction has a wide variety of applications in the synthesis of natural products,³ bioorganic chemistry⁴ and industrial applications.⁵ Many catalysts show good catalytic activities in the Heck reaction, including palladium catalysts. Although palladium has dominated the catalyst field and won the 2010 Nobel Prize, there is still much interest in the development of nickel-catalysed cross-coupling reactions. Consequently, we have paid much attention to the key advances in nickel-catalysed Heck reactions due to its wide distribution, low price and efficient catalytic activity. In particular, it has been shown how to take advantage of its characteristic properties to perform innovative and useful transformations.⁶ So far, nickel catalysts incorporating phosphine-based ligands have been widely used in C–C cross-coupling reactions, with ligands such as 1,1′-bis(diphenyphosphino)ferrocene (DPPF),^7–9 tributylphosphane (P(n-Bu)₃),¹⁰ tricyclohexyl phosphine (PCy₃),¹¹ cyclohexyl(diphenyl) phosphine (PCy₂Ph)¹² and 1,3-bis(diphenyphosphino)propane (DPPP).¹³ Muto and co-workers¹⁰ reported that P(n-Bu)₃ was the best ligand out of a range of ligands, surveyed the optimum conditions and carried out theoretical calculations on the cross-coupling reaction of esters. However, there are a number of problems. For example, phosphine ligands are expensive, toxic and sensitive to oxygen and water. Simultaneously, it is very difficult to directly catalyse the Heck reaction using a nickel catalyst, and few groups have managed to achieve this. Thus, much attention has been paid to developing new methods. Winkler and co-workers¹⁴ used the Heck reaction in a very efficient orthogonal method for polymer–polymer conjugation, which was introduced in a modular form. This inspired many researchers to explore better methods and seek more appropriate catalysts.

With the advancement of research, many metallic materials have been widely and efficiently applied to the Heck reaction, especially nickel. A number of complexes have also been applied to the Heck reaction, such as bis-diimidazolylidine,¹⁵ 1,3-bis(2,6-di-isopropylphenyl)-4,5-dihydroimidazol-2-ylidine (SIPr),¹⁶ amido bis(amine)¹⁷ and o-tolyl (TMEDA).¹⁸ But there are some defects in the outflow of the catalyst and non-recyclability. Unlike Pd-catalysed processes, the Ni-catalysed cross-coupling reaction should not be underestimated and it has more challenges due to the defects in Ni complexes.¹⁹ It is necessary to find a new approach to conquer these difficulties. Therefore, due to their outstanding properties, researchers have begun to take full advantage of polymers, such as polyvinyl pyrrolidone (PVP),²⁰ polyvinylalcohol (PVA),²¹ poly(methyl methacrylate) (PMMA)²² and polyacrylonitrile (PAN)²³ as catalyst substrate materials. It is worth mentioning that PAN has high conductivity and chemical stability, and has many applications in chemical reactions. The synthesis and properties of metal/PAN composites have been widely investigated. In terms of the intrinsic properties of polymers, PAN is the most widely researched owing to its low price, high environmental stability²⁴ and superior performance for supercapacitor applications.²⁵ PAN can be carbonized and eventually turned into graphite at high temperature. Carbon is stable and hence the best carrier for the metal. Its adsorption capacity is exploited to increase contact between the reaction substrate and the active center of the catalyst, further promoting the Heck reaction.

Recently, electrospinning has emerged as a useful technique for preparing one-dimensional or multidimensional nanofibers,²⁶ and PAN as an efficient catalyst carrier is convenient for organic reactions. Therefore, combining electrospinning technology and PAN for the fabrication of functional nanoparticles on nanofibers has been the subject of much focus. In the present work, a novel carbon nanofiber-supported Ni(0) composite catalyst (Ni(0)/CNFs) has been fabricated successfully through electrospinning and a high-temperature carbonization process, and was used to efficiently catalyse the Heck cross-coupling reaction. A significant result was obtained for the performance of Ni(0)/CNFs in the coupling reaction.

Scanning electron microscopy (SEM) is normally used to characterize the morphologies of nanocomposites. To examine the nickel nanoparticles supported on carbon nanofibers, transmission electron microscopy (TEM) was used. The typical SEM image in Fig. 1a shows the morphology of the nanofibers obtained from a Ni(acac)₂/PAN solution with a molar rate of 1/20, which demonstrates that the nanofibers have a uniform dispersion and are smooth with diameters in the range of 150–250 nm. As shown in Fig. 1b and c, compared with Fig. 1a, the morphology of the processed nanofibers hardly changed after H₂ reduction and carbonization. To further understand the elemental composition of the CNFs obtained from the calcination of Ni/PAN nanofiber membranes, EDS analysis was performed, which clearly confirmed that the composite nanofibers consisted of Ni, C, N and O elements. More importantly, this result ascertained that nickel was successfully loaded on the nanofibers.

Fig. 1 SEM images of (a) Ni/NFs, (b) H₂-deoxidized Ni/NFs, and (c) Ni(0)/CNFs maintained at 600 °C for 2 h (insert: EDS); TEM images of Ni(0)/CNFs (d and e) with different scale labels; (f) HRTEM image of Ni(0)/CNFs.

The morphology and distribution of the metal nanoparticles in Ni(0)/CNFs were researched by TEM. Fig. 1d and e display typical TEM pictures with small nanoparticles and uniform dispersion and without agglomeration after carbonization. According to the obtained images, the synthesized Ni nanoparticles were sheathed by a carbon shell. At the same time, it was revealed that the average particle size of the active Ni components was about 14 nm, which was much smaller. The HRTEM image in Fig. 1f shows that the lattice spacing was measured to be 0.2 nm, which could be assigned to the d-spacing for the (111) plane of Ni metal.

Fourier transform infrared spectroscopy (FTIR) analysis was used to examine the individual chemical structures present in the prepared nanocomposites. The FTIR spectrum of Ni-loaded CNF films was also investigated. As shown in Fig. 2, sharp bands occurred at 2931 cm⁻¹ and 1395 cm⁻¹, which were attributed to the stretching vibration and bending vibration of methylene (–CH₂–) in the PAN, respectively. The sharp peak at 2243 cm⁻¹ was assigned to the stretching vibration of the nitrile group (–C≡N).²⁷ Broad absorption bands at 1591 cm⁻¹ and 1300 cm⁻¹ could be corresponded to the stretching vibrations of C=C and C–O bonds, respectively.

Fig. 2 FTIR analysis of (a) Ni/NFs, (b) H₂-deoxidized Ni/NFs, and (c) Ni(0)/CNFs maintained at 600 °C for 2 h under N₂.

Next, the valence of the nickel in the Ni(0)/CNFs was examined. As Fig. 3 shows, no peaks appeared. Temperature-programmed reduction (TPR) as a means to probe the valence of nickel gave a satisfactory result, which was that the nickel was zero-valent.

Fig. 3 TPR analysis of as-prepared Ni(0)/CNFs maintained at 600 °C for 2 h.

X-ray photoelectron spectroscopy (XPS) analysis was applied to examine the chemical states of elements on the surface of the Ni(0)/CNFs. As expected (Fig. 4a), peaks appeared at 853.8 and 871.2 eV, and these were attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively. These values also indicated the presence of metallic nickel.²⁷ The survey spectrum (Fig. 4b) clearly showed the presence of Ni, C, O and N elements from the full spectral survey.

Fig. 4 XPS spectra of the synthesized Ni(0)/CNFs with etching 10 nm deep: (a) spectrum in the Ni 2p reported range and (b) full scanning spectrum.

The optimum conditions for the application of the catalyst in the Heck reaction were obtained by studying different reaction times, temperatures and solvents. Firstly, five different reaction times were explored. As a result, a time of 18 h was found to be the best (Table 1, entries 1–5). Following this, the reaction temperature was considered. According to gas chromatography (GC) results, we concluded that the optimal temperature was 140 °C (entries 4 and 6–8).

Table 1 Effect of different reaction parameters on the Heck reaction of iodobenzene^ab

Entry	Time/h	Temperature/°C	Solvent	Cov (%)	Sel (%)
a Reaction conditions: iodobenzene (1 mmol), n-butyl acrylate (1.7 mmol), Et3N (3 mmol) solvent (5 ml) and Ni(0)/CNFs (0.025 g), at the desired temperature and stirring rate and under a N2 atmosphere.b Yields determined by gas chromatography.
1	4	140	NMP	22.1	100
2	8	140	NMP	36.6	97.7
3	12	140	NMP	57.1	97
4	18	140	NMP	62.3	98
5	24	140	NMP	58.1	97.4
6	18	120	NMP	27.5	79.4
7	18	130	NMP	38.2	90.4
8	18	150	NMP	61	97
9	18	140	DMF	10	82
10	18	140	DMSO	33.7	13.3
11	18	140	DMAC	0	0

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Finally, to consider the influence of the solvent on the reaction, a series of organic solvents were compared. The result obtained was that NMP was the best solvent (entries 4 and 9–11). In conclusion, the best conditions were using Ni(0)/CNFs in NMP at 140 °C for 18 h. According to the results of five cycles, the catalyst still had a high catalytic activity, which indicated the stable properties of Ni(0)/CNFs (Table 2). The above results represent an important advance in carbon–carbon bond-forming reactions using simple catalysts.

Table 2 Yields obtained from recycling the Ni(0)/CNFs catalyst in the Heck reaction^ab

Cycle	Cov (%)	Sel (%)
a Reaction conditions: iodobenzene (1 mmol), n-butyl acrylate (1.7 mmol), Et3N (3 mmol), NMP (5 ml) and Ni(0)/CNFs (0.025 g), 140 °C, 18 h, at the desired stirring rate and under a N2 atmosphere.b Yields determined by gas chromatography.
1	57.6	97.7
2	54.2	95
3	53	95
4	56	91
5	53	91

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In summary, a novel nickel-based carbon nanofiber composite catalyst was prepared through the simple and straightforward technology of electrospinning and high-temperature carbonization. According to SEM and TEM, the Ni(0)/CNFs had a one-dimensional morphology, and the average size of the Ni nanoparticles sheathed by CNFs was 14 nm. The result that nickel existed in zero-valent form could be observed by TPR and XPS. The Ni(0)/CNFs was applied to the C–C cross-coupling reaction, and a significant catalytic effect was determined through a series of experiments. The solid zero-valent nickel/carbon nanofiber composite can catalyse the Heck reaction with high efficiency and can be reused more than once. This novel solid Ni(0)/CNFs catalyst provides a new way to catalyse the C–C cross-coupling reaction efficiently with zero-valent nickel.

Acknowledgements

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (21266016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01918h

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