N-doped fiber anchoring PdNi nanoparticles and catalyzing Suzuki reaction

ABSTRACT In this work, polydopamine-modified carbon fiber-supported PdNi nanoparticles were facially prepared and used in the Suzuki cross-coupling reaction with a high catalytic activity. The introduction of nickel increases the electron density around palladium and the catalytic activity. At the same time, comparing the effect of high-temperature carbonization, it is found that high temperature can enhance the electron transfer between the metal and the carrier and make the metal and the carrier bond more firmly. Provide new ideas for fixing nanoparticles in the future. GRAPHICAL ABSTRACT


Introduction
As one of the most important C-C coupling reactions, the Suzuki reaction plays an important role in the synthesis of drugs and organic compounds [1]. Great progress has been made in the development of the Suzuki reaction, but the catalytic conversion of high-activation-energy substrate still remains a major challenge. In recent decades, researchers have been focusing on the development of catalysts to solve the problem fundamentally. Palladium-based catalyst is an important category of metal catalysts [2], which has been generally used to promote various chemical transformations, especially reactions that non-noble metals cannot effectively catalyze, such as Suzuki and Heck reactions. Homogeneous Pd catalysts are usually adopted together with the co-catalysts and toxic phosphine ligands [3], and their recovery is cumbersome and reuse is limited [4]. Considering the importance of these reactions, the wide range of applications, and the high cost of palladium, many research groups aimed to develop recyclable heterogeneous catalysts with low Pd loadings [5], which could fix the Pd species on a suitable support so that they could efficiently catalyze without ligands.
In recent years, Pd has been successfully loaded on Polystyrene [6], covalent organic framework [7], metal organic framework [8,9], carbon nanofibers [10], SiO 2 [11] and other carriers [12][13][14] for catalyzing Suzuki reactions. However, the simple supported palladium metal particles are easy to fall off, resulting in a decrease in the stability of the catalyst, so the modification of the carrier is a major concern of researchers. Through the modification of the carrier, the goal of better restraining metal particles has been achieved, so that the cycle life of the catalyst is longer. Pure palladium metal catalysts cannot meet the actual needs due to economic costs, and the introduction of some heteroatoms [15][16][17] or metal [18][19][20] doping can make precious metal particles more effective.
In order to improve the performance of metal palladium, various synthesis strategies are adopted [21,22], such as reducing the size of nanoparticles (NPs), using unique carriers and introducing other metals to electronically modify Pd NPs [23]. Although there have been reports of non-noble metals [24], nickel [25] copper [26,27] and iron [28] for catalyzing cross-coupling reactions, which are usually considered to be inferior to Pd's catalytic ability. Interestingly, compared with pure palladium metal, the introduction of these metals could achieve a higher catalytic performance since the non-noble metal can donate electrons to the catalytic center, thereby reducing the activation energy of the C-C coupling reaction oxidation addition step [29]. Moreover, the bimetallic catalysts are more cost-effective as they reduce the Pd content. In order to further advance these bimetallic catalysts and make them feasible, precise control of the size and dispersibility of nanoparticles, adjustable chemical composition, and strong interaction with the support are required to prevent metal leaching and particle sintering.
Doping heteroatoms is an important means to improve the properties of metals, especially the use of N atoms, which has applications in many fields. Some researchers use N atoms to improve the anti-sintering performance of catalysts [30][31][32], and some researchers use N atoms to improve the electrochemical performance of metals. The N atom also contributes to the utilization and conversion of carbon dioxide, and N is a good active site for carbon dioxide [33]. At the same time, N atoms can also form coordination with metals to fix metal nanoparticles and improve the loading capacity of metals [34]. There are many sources of N, such as urea, melamine, dopamine, etc. As a high-quality source of N, dopamine is also used to immobilize metal particles [35].
In the following of our research on design and synthesis of polydopamine-modified polyacrylonitrile (pDA-PAN), fiber membrane supported palladium nickel nanoparticles and N-doped carbon fiber loaded with palladium-nickel nanoparticles to catalyze the Suzuki reaction. Previously, we published a research report on loading palladium nickel nanoparticles on carbon fibers and used to catalyze Suzuki. In this research, we hope to continue to play the synergistic catalytic effect of metal nickel and change the preparation method of metal nanoparticles. We sputter the metal nanoparticles onto the polydopamine modified PAN fiber membrane by the method of electric spark sputtering and obtained nitrogen-doped carbon fibers to immobilize PdNi metal nanoparticles through high-temperature carbonization. It is then used to catalyze the Suzuki reaction.

Experimental section
Yields were analyzed by a Shimadzu GC 2010 plus chromatograph. A scanning electron microscope (SEM and FE-SEM Phenom) was used to characterize the morphology of the supports. The distribution of the nanoparticles and morphology were further observed by a transmission electron microscope (FEITEM, F30 S-TWIN). Fourier-transform infrared spectra (FT-IR) were recorded from the KBr pellet method in a Nicolet Nexus 670 spectrophotometer with a range of 400-4000 cm −1 . The X-ray source of X-ray photoelectron spectroscopy (XPS, Escalab 250, Thermo Fisher Scientific, USA) was monochromatic Al Kα at 150 W. The crystal structure of the catalysts was tested by X-ray diffraction (×RD, Rigaku Ultima IV, Japan) in the range of 2θ from 10° to 90° with a scan rate of 1° min −1 . The 1 H NMR and 13 C NMR spectra were obtained in DMSO-d6 on an Agilent 500 MHz DD2 spectrometer. A TG spectrum was obtained by STA449F3.
Generally, 0.5 g of PAN was added to 4.5 g DMF and then stirred continuously until it completely dissolved. The homogeneous precursor solution was electrospun onto a copper plate at a distance of 18 cm and a voltage of 16 kV get PAN membrace (PAN). Meanwhile, 0.05 g of dopamine hydrochloride was dissolved in a mixed solution of ethanol and water (v/v = 75 ml/25 ml), and then ammonium hydroxide was added to adjust the pH of solution to 8.5; thereafter, a 12 cm 2 membrane was placed in and reacted for 6 hours in the dark. The resulting membrane (pDA-PAN) was washed and freezedried for further use.
A spark ablation nanoparticle generator was used to synthesize PdNi NP with a voltage of 1 kV and a current of 5 mA [36]. During this process, a pair of cylindrical PdNi electrodes was placed in a holder. Then, the carrier gas (N 2 ) at a flow rate of 5 L/min passed through the gap between the electrodes. The pulsed spark formed by electrical gas breakdown ablated the PdNi electrode, and then, the metallic nanoparticles were produced, condensed and deposited on the dopamine-modified PAN nanofibrous membrane. After an hour, the final membrane was collected and placed in an oven at 100°C for drying for 1 h to remove unpolymerized dopamine and name the catalyst PdNi-O/ pDA-PAN.
Then, put the obtained PdNi-O/pDA-PAN in a tube furnace, in a nitrogen atmosphere, linearly heat up to 250℃ at 5℃ per minute, and heat it for 2 h, then heat it up to 550℃ at 2℃ per minute, and then heat it for 2 h, and the temperature was lowered to room temperature to obtain N-PdNi/CNFs. Except for the absence of polydopamine, the other treatment methods are the same to obtain PdNi/CNFs (Scheme 1).

Results and discussion
To observe the morphologies of polymer nanofibers and the corresponding composite nanofibers, SEM images of the polymer nanofibers are obtained and shown in Figure S1. Compared with the smooth fibers before dopamine modification, the surface of the modified fibers is rather rough. The palladium-nickel nanoparticles on the surface of the fibers can be observed by a transmission electron microscope (TEM).  ( Figure 1(e,f)) show the even distribution of Pd and Ni species supported on PdNi-O/ pDA-PAN and N-PdNi-CNFs. The XRD pattern (Figure 2) shows that the peak at 2θ = 43.2° is the (200) crystal plane of NiO (JCPDS 47-1049). Analyzing the morphology of N-PdNi-CNF, it is found that there are metal particles on the surface of the fiber. The lattice spacing of Pd and Ni NPs in HRTEM is 0.24 nm and 0.32 nm, respectively, corresponding to the (111) plane of PdO and the (111) plane of NiO, respectively. The ICP results show that the content of metallic palladium in PdNi-O/pDA-PAN is 0.47 wt %, the content of metallic nickel is 1.27 wt%, the content of metallic palladium in N-PdNi-CNFs is 2.25 wt%, and the content of metallic nickel is 5.66 wt%. The reason for the increase in metal content is that high temperature decomposes polydopaminemodified fibers. Figure 3 shows the FT-IR spectra of PAN, pDA-PAN and PdNi-O/pDA-PAN samples. 3337 cm −1 (stretching vibration of N-H), 1616, 1512 cm −1 (skeleton vibration of benzene ring) and 871,814 cm −1 (trisubstituted structure of benzene ring) are observed for confirming that the dopamine was successfully modified on PAN nanofibers [37]. After high-temperature carbonization, the main functional groups of the polymer. The absorption peak disappeared, indicating the complete carbonization of the fiber. From the TG curves Figure S2, it can be found that the decomposition temperature of the fibrous sample increases by 30°C after dopamine modified, indicating the successful dopamine modification as well [38].s XPS analysis was performed to obtain the surface electronic properties of the samples. The Ni 3d XPS spectra (Figure 4(a)) show two doublet peaks. The two peaks at 872.6 eV and 855.9 eV are attributed to Ni 2p 1/2 and 2p 3/2 of Ni(II) species, respectively [39]. The other two peaks at 878.9 eV and 861.3 eV are the shake-up satellite peaks of Ni(II). By comparing with or without the introduction of polydopamine, it is found that the introduction of polydopamine makes 2p 3/2 of Ni have a higher electronic binding energy and 2p 1/2 has a lower electronic binding energy. It shows that part of the  in Ni 2p 3/2 transfer to Pd, so that the palladium metal surrounding the formation of electron-rich state. The Pd 3d XPS spectrum (Figure 4(b)) of PdNi-O/pDA-PAN, PdNi/CNFs and N-PdNi/CNFs shows two peaks at 337.2 eV and 341.2 eV, which are attributed to Pd 3d 3/2 and 3d 5/2 of Pd(II) [5]. The introduction of N causes the 3d 3/2 peak of Pd to move to the low field, indicating that the introduction of N makes the  metal palladium gain electrons. The N 1s peaks (Figure 4(c)) at 398.5 and 399.8 eV are attribute to pyridine-like and pyrrole-like nitrogen, respectively [40]. Please note that there is a significant difference between N before and after carbonization. Uncarbonized N is more pyridine-like nitrogen, and after carbonization, the proportion of pyrrole-like nitrogen increases significantly. More importantly, carrier allows electrons to flow from the metal to the N in the carrier, so that the metal nanoparticles and the carrier are more closely combined, which also provides support for N-PdNi/CNFs is higher reuse rate.
To reveal the structure-activity relationship (SAR) of the prepared PdNi-O/ pDA-PAN catalyst and N-PdNi-CNFs catalyst, it was used for catalyzing Suzuki coupling reactions with the presence of various reaction substrates. The reaction result data are summarized in Table 1 and Table 2. It can be found that aryl iodobenzene is effectively converted into the corresponding coupling product with a high reactivity by using PdNi-O/pDA-PAN and N-PdNi/CNFs and finally provide a satisfactory yield.
Moreover, when -CH 3 , -OCH 3 and -NO 2 group-induced aryl iodide is used as a substrate, the yield is as high as the aryl iodobenzene system. By using iodobenzene and phenylboronic acid as model Suzuki reaction reactants, the reusability of the catalyst is tested ( Figure 5). The results show that the palladium-nickel nanoparticles directly loaded on the surface of the fiber modified with dopamine are easy to fall off due to the weak force with the carrier. After high-temperature carbonization, it was found that the force between the PdNi metal nanoparticles and the carrier was enhanced, which ensured the stable and efficient use of the catalyst multiple times. In addition, compared with the catalysts in the previous work, the introduction of nickel can indeed improve the catalytic activity of the sole palladium catalyst (Table S1).

Conclusions
In summary, we report a new type of electric spark technology to prepare palladium-nickel oxide nanoparticles and load them on polydopamine-modified nanofibers. At the same time, through high-temperature carbonization, N-doped carbon fiber metal materials are obtained for catalysis Suzuki reaction. The use of electric spark technology can realize the industrial-scale preparation of the catalyst. The catalyst exhibits good catalytic performance and has excellent catalytic performance for substrates with different inducing groups. In addition, the catalyst exhibits good activity stability after the reaction cycle, and after high-temperature carbonization, the metal particles and the carrier can be more closely combined. The immobilization of the nanoparticles provides a new method. Reaction conditions: aryl halides or their derivatives (0.5 mmol), phenylboronic acid or the various derivatives of it (0.55 mmol), K 2 CO 3 (0.5 mmol) and catalyst (5 mg) were added into the solvent mixture. Air atmosphere, 80°C and without any stirring during the reaction.