Fabricating series of controllable-porosity carbon nanofibers-based palladium nanoparticles catalyst with enhanced performances and reusability

Journal of Molecular Catalysis A: Chemical 400 (2015) 95–103

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Fabricating series of controllable-porosity carbon nanofibers-based palladium nanoparticles catalyst with enhanced performances and reusability Liping Guo, Jie Bai ∗ , Junzhong Wang, Haiou Liang, Chunping Li, Weiyan Sun, Qingrun Meng Chemical Engineering College, Inner Mongolia University of Technology, Hohhot 010051, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 13 November 2014 Received in revised form 7 February 2015 Accepted 9 February 2015 Available online 11 February 2015 Keywords: Electrospinning Heck coupling reaction Porous carbon nanofibers Pd nanoparticles

a b s t r a c t The porous carbon nanofibers-supported palladium nanoparticles hybrid catalyst was put forward. Highly porous, large specific surface area and uniform distribution without aggregating of palladium nanoparticles in the carbon matrix were achieved by combining eletrospinning, chemical reduction and subsequent calcination methods. Polystyrene and polyacrylonitrile acted as the thermal degradable polymer and the carbon precursor polymer, respectively. A series of characterization of catalyst were carried out to investigate the morphology and materials properties of the precursors and the final porous carbon nanofibers, which including simultaneous thermal analyzer, Brunauer–Emmett–Teller measurements, UV–vis diffuses reflectance spectra, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, the field emission scanning electron microscope and the field emission transmission electron microscope. Foremost, when it was tested as catalyst for the Heck coupling reaction, the porous carbon nanofibers-loaded palladium nanoparticles catalyst exhibited enhanced activity, excellent stabilization and recyclability. Moreover, PS has the possibility of recycling which is important to develop green chemistry. Overall, the as-made carbon nanofibers-supported palladium nanoparticles catalyst with large porosity was prepared in an easy way and showed enhanced activity in comparision with the pure carbon nanofibers-supported palladium nanoparticles catalyst. © 2015 Elsevier B.V. All rights reserved.

1. Introduction As being known for the most prospect materials of the 21st century, nanomaterials with their superior functions and the extensive usages became the most investigative point in novel material science. Inorganic/organic composite nanomaterials, such as metals, metallic oxide, inorganic oxide, semiconductor and carbon nanotubes, were doped to polymer substrates to obtain composite nanomaterials with special functions [1–7]. They promised to access to a wide range of potential applications in diverse areas of material science and industry, such as electronics, photonics, mechanics, bioengineering, sensing, and so on [8]. Among many available and advanced multifunctional materials, one dimensional carbon nanofibers (CNFs) have attracted much attention in recent years, because nanostructured carbon provided

∗ Corresponding author. Tel.: +86 471 6575722; fax: +86 471 6575722. E-mail address: [email protected] (J. Bai). http://dx.doi.org/10.1016/j.molcata.2015.02.009 1381-1169/© 2015 Elsevier B.V. All rights reserved.

high chemical stability, large specific surface area and excellent mechanical properties, which benefited that electrodes were used in high-power supercapacitors, composites, energy storages, water treatment system and catalyst support [9,10]. Nanostructure carbon-based fibers obviously offered better performance and more sustainable host materials for metal nanoparticles (NPs) (copper, platinum, Pd, lithium, stannum, ruthenium and so on) than traditional and most reported zeolite, molecular sieve and activated carbon, because they were able to be made directly out of biomass and their porosity [11,12]. Pd NPs have the reputation of being one of the most efficient metals in many fields due to their high catalytic activities to different types of reaction, especially for the formation of C C bonds, for example, Suzuki and Heck coupling reactions, excellent economic benefits, good electrical conductivity and optical properties [13–16]. To date, Pd NPs with the size below 10 nm possessed remarkable properties because of their precise and ultrafine dimensions. Nevertheless, few reports involved Pd NPs with very small sizes; The most were larger than 10 nm. So far,

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the facile and economical preparation of Pd NPs smaller than 10 nm is still challenging [17]. Novel and improved porous morphology polymeric materials have received an increased level of research interest, because they were in great demand for a wide range of applications such as adsorption and ultra-filtration materials, ion-exchange resins as well as supports or carriers for catalysts and reagents [18–20], especially, the preparation of highly porous carbon nanofibers (PCNFs) received significant attention because of their large surface area, well-defined porosity and relatively high electrical conductivity, which were useful to electrodes applied in high-power supercapacitors. The traditional preparation approaches included the phase separation technique (selective dissolution or solvent evaporation) [21–23], template methods [24,25], sol–gel synthesis and thermal treatment or photo degradation one component of blending polymers, for example, Demir et al. have demonstrated the use of this facile thermal treatment method in their fabrication of high specific surface area PCNFs with silicon-containing compounds [26]. Recently, electrospinning attracts immense attention as a versatile and an easy way to prepare a variety of functional polymers, metal oxides and CNFs successfully [27–30]. Porous or hollow fibers can be obtained employing electrospinning with highly volatile solvents or through special treatment following electrospinning [31]. Several research groups attempted to produce PCNFs by blending different components into electrospun fibers. For example, Peng et al. obtained submicrometer CNFs with a nanoporous structure [32]. Wang et al. obtained porous composite nanofibers by electrospinning polyacrylonitrile/polystyrene (PAN/PS) solutions, followed by stabilization and carbonization [33]. Consequently, if Pd NPs catalyst was incorporated into the electrospun PCNFs, the resulting of composite PCNFs would be exhibited the excellent performance and the important application values in future. Most of all, doping carbon with titanium, silicon, chromium, nickel, copper, magnesium, and/or iron initiates resulted in catalytic graphitization of carbon, and hence, highly graphitic structures were able to be produced at lower temperatures. Pd, another metallic element, was able to be used as a dopant for carbon to enhanced catalytic properties [34]. In our observation, there was no report about the preparation and application of PCNFs-supported Pd NPs composite catalyst by combining efficient and low-powered electrospinning and thermal treatment method. In our work, PS and PAN as thermal degradable polymer (TDP) and carbon precursor polymer (CPP) were determined, respectively, and electrospun composite nanofibers based on PAN, different PS contents and palladium chloride (PdCl2 ) precursors. Based on the previous research work of our research group, some factors affecting the size and shape of Pd NPs were investigated, and the optimum reducing agent, dosage of PdCl2 and the optimum carbonized temperature were determined. When they were employed in nanofabrication, thermal decomposition provided a simple one-step strategy for the creation of porous nanostructures with controlled porosity, which remained a challenge with other methods [35]. The nanopores/nanochannels in the fibers were continuous at about several tens of nanometers in widths and lengths. Most of all, the PCNFs-supported Pd NPs catalyst was almost completely conserved and easily separated from a reaction mixture system, which revealed that the PCNFssupported Pd NPs catalyst showed excellent stability, good retrieval and reusability because the stable Pd NPs were able to be anchored both on the surface and in the interior of the PCNFs by electrospinning a mixture of polymer matrix and synergy between the metal nanoparticles and the supporting nanofibers in nanoscale. We believe this heterogeneous composite nanopores/nanochannels catalyst can be applied in many fields widely, such as reaction engineering, electrochemistry and aerospace industry.

2. Experimental 2.1. Materials All the chemicals are of analytical grade and without further purification. Polyacrylonitrile (PAN, Mw = 80,000) was purchased from Kunshan hongyu plastics Co., Ltd. Polystyrene (PS, Mw = 110,000) was purchased from Xindahui chemical company in Tianjin. N,N-Dimethylformamide (DMF, C3 H7 NO, AR, 99.5%) was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. Absolute ethyl alcohol (C2 H6 O, AR, 99.7%) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Palladium chloride (PdCl2 , AR), iodobenzene (C6 H5 I, CP, 97%), methyl acrylate (C4 H6 O2 , CP, 98%), ethyl acrylate (C5 H8 O2 , CP, 98%), n-butyl acrylate (C7 H12 O2 , CP, 98%) and triethylamine (C6 H15 N, AR, 99%) were purchased from Sinopharm. 2.2. Fabrication of Pd NPs/PS/PAN composite nanofibers In our experiments, the preparation process consisted of three steps. First, PAN was dissolved in DMF solution at a concentration of 8 wt% by stirring at room temperature for 12 h. Then, PAN/DMF and PdCl2 powder were blended at mole ratio of 50 (nAN :nPdCl2 ) intensively stirred 24 h. In the end, the different contents doping of PS were mixed with PdCl2 /PAN/DMF to form kinds of blending solution, as well as wt% of PS was respected to PAN in the solution, and the molar ratio of PAN and PS was 10, 20, 30 and 40, respectively. The blending solution was loaded into a jet nozzle. The positive voltage was applied to the tip was 16 kV and the distance between the needle tip and the collector was 18 cm. The as-spun PAN fibers were collected on aluminum foil. The obtained PdCl2 /PS/PAN nanofibers were put into hydrogenated kettle and reduced by the hydrogen under 100 ◦ C and 2.5 MPa after nitrogen and hydrogen gas were exchanged three times, respectively. After 5 h, the samples were taken out and characterized by UV–vis, FTIR, FESEM and etc. 2.3. An easy procedure for the preparation of Pd NPs/PCNFs catalyst For stabilization process, all electrospun nanofibers substrates were placed in a tube furnace and stabilized in air at 250 ◦ C for 2 h for the preoxidation of PAN in the carbonization process and the removing of PS. Then samples were annealed at 450 ◦ C for 2 h under N2 atmosphere to allow the forming of optimal Pd NPs on/in the carbonized nanofibers. Finally, the substrates were carbonized at 500 ◦ C for 4 h under N2 atmosphere (2◦ /min heating rate). 2.4. Characterization UV–vis diffuses reflectance spectra (DRS) (UV-3600, Shimadzu Corporation) with a variable wavelength from 190 to 600 nm to detect if bivalent Pd was reduced to zerovalent Pd completely or not. Fourier transform infrared spectroscopy (FTIR, 670, Thermo Nicolet Corporation) confirmed the characteristic functional groups of as-spun PS/PAN, PdCl2 /PS/PAN, Pd NPs/PS/PAN and Pd NPs/PCNFs. Field emission scanning electron microscope (FESEM, FEG 650, Quanta) and scanning electron microscope (SEM, S-3400, Hitachi Ltd.) characterized the morphology of nanofibers of electrospun on aluminum foil and PCNFs. Field emission transmission electron microscope (FEITEM, F20 S-TWIN, Tecnai) observed the morphology and distribution of Pd NPs in/on the surface of PCNFs and porous structure which distributed in ethanol solution, and then they were dropped upon carbon-coated copper grids. Thermal performances of as-spun PS/PAN nanofibers were evaluated using a simultaneous thermal analyzer (STA PT1600, Linseis). Brunauer–Emmett–Teller measurements (BET,

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Fig. 1. The schematic of the preparation of Pd NPs/PCNFs catalyst in the paper.

Quadrasorb SI-KR/MP, Quantachrome Instruments) confirmed surface area (SBET ), pore volume (Vpore ) and pore radius (dpore ) of Pd NPs/PCNFs. The sample of X-ray diffraction (XRD, D/Max-2500/PC, Rigaku Japan) was prepared by grinding into powder, it was carried out by using Cu K␣ (18 Kv) radiation, diffraction angle ranged from 10◦ to 100◦ at a rate of 3◦ /min. The X-ray source of X-ray photoelectron spectroscopy (XPS, Escalab 250, ThermoFisher Scientific USA) was monochromatic Al k␣ at 150 W. 2.5. The testing for activity and reusability of Pd NPs/PCNFs Catalyst

7890A gas chromatograph (GC) equipped with a FID detector, chromatographic condition as follows according to different reaction substrates, the temperature of injection port and detector ranged from 220 to 280◦ C. The temperature of oven adopted temperature programming, the highest temperature ranged from 220 to 270◦ C; chromatographic column was J&W 113-3032, SE-30. Sample volume was 0.3 ␮l every time. Besides, the catalyst which possessed of superior catalytic properties was further demonstrated with different recycle times and a series of substrates in Heck reaction for iodobenzene. 3. Results and discussion

In a typical procedure, 0.408 g iodobenzene, 0.606 g triethylamine and 0.030 g the as-synthesized Pd NPs/PCNFs catalyst were added into DMF solution (10 ml) under stirring. After 15 min, alkene was added under nitrogen. Then the mixture was heated to 125 ◦ C and kept at that temperature for 12 h with stirring. After reaction, the products were naturally cooled to room temperature, the catalyst was filtered and separated easily, and then it was washed by absolute alcohol and distilled water three times and dried below 80 ◦ C, it was able to be used repeatedly. Products were examined by

PCNFs-supported Pd NPs heterogeneous composite nanofibers catalyst was fabricated by electrospinning a PdCl2 /PS-containing PAN composite followed by gas-phase hydrogenation reduction and subsequent calcinations techniques. The method is schematically depicted in Fig. 1. First, PdCl2 /PS/PAN nanofibers employing different PS contents were prepared according to typical electrospinning. Subsequently, the obtained PdCl2 /PS/PAN nanofibers were reduced by hydrogen, resulting in the formation of Pd NPs/PS/PAN nanofibers. Finally, by the implementation of stabilization and carbonization processes, four PCNFs-supported Pd NPs heterogeneous composite nanofibers were obtained. During this process, PAN was not only able to prevent the agglomeration of Pd NPs, but was also able to be transformed into CNFs coat on Pd NPs surfaces. Meanwhile, a large amount of nanopores/nanochannels throughout the surface and the interior of CNFs were obtained. 3.1. The possible mechanism involved the formation of porous structure

Fig. 2. DSC thermograms of PS/PAN composite precursor nanofibers.

The possible mechanism was discussed. In this experiment, several methodologies were used in fabricating PCNFs with abundant nanopores/nanochannels features. (1) PS was a common thermoplastic polymer made from the aromatic monomer styrene with good formability and it was widely used in many systems. (2) PS was performed as a pore-forming material polymer due to it held a relatively strong endothermic peak centered at 255 ◦ C, which was likely associated with complex chemical reactions, including dehydrogenation, cyclization, C C bond cleavage and so on (see Fig. 2 HDSC method) and chained rigidity which prevented fibers from shrinking and collapsing during the Pd NPs/PS/PAN

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Table 1 Fiber diameter, SBET , Vpore and dpore of Pd NPs/PCNFs (molar ratio of PAN and PS was 40). Sample a

Pd NPs/PCNFs

Particles size (nm)

Fiber diameter (nm)

SBET (m2 /g)

Vpore (cm3 /g)

dpore (nm)

5–17

450–900

250.972

0.058

18.966

stabilization process. (3) PAN and PS matrixes were incompatible, phase separation happened between PAN and PS in the bicomponent fibers during the Pd NPs/PS/PAN heat treatment processes and we removed the PS in the composite polymer nanofibers completely. As PAN converted to carbon matrix and the sacrificial polymer eliminated to create intra-fiber porous structure. Extremely high surface area (250.9720 m2 /g, see Table 1) with a majority of meso- or nanopores/nanochannels in the range of several to tens of nanometer was found. 3.2. The characterization of Pd element and the structure of PCNFs A comparison of the UV–vis spectra of PdCl2 /PS/PAN (Fig. 3A) and Pd NPs/PS/PAN nanofibers (Fig. 3(B–E)) was displayed in Fig. 3. Fig. 3B–E shows Pd NPs/PS/PAN nanofibers which containing different PS contents, the molar ratio of PAN and PS was 10, 20, 30 and 40, respectively. As shown in Fig. 3, the characteristic absorption peak of Pd2+ or PdCl4 2− ions at 247.37 nm (Fig. 3A) disappeared after reducing by the hydrogen in the reaction progress (Fig. 3(B–E)), revealing that bivalent Pd was reduced to zerovalent Pd completely in/on nanofibers. The absorption in the visible region due to the band structure of metal nanoparticles increased, indicating the Pd NPs were formed. Next, to further obtain the chemical state of palladium, the fine scans of Pd 3d peaks were further confirmed by the XPS analysis (Fig. 4), Fig. 4A–D represents Pd NPs/PCNFs catalysts which derived from Pd NPs/PS/PAN nanofibers containing different PS contents, the molar ratio of PAN and PS was 10, 20, 30 and 40, respectively. All data showed that the Pd NPs were present as zero-valent state consistent with the observed binding energies of 334.54–335.67 eV (Pd (0) 3d5/2 ) and 339.85–340.76 eV (Pd (0) 3d3/2 ) with 3d spin–orbital splitting of 5.15–5.31 eV which were agreement with literature (5.2 eV). The energy values correlated well with the literature for the positions of metallic palladium, Pd (0). It evidently indicated that the thermal treatment and porous structure were able to change the Pd electron binding energy (3d5/2

and 3d3/2 ) slightly, the chemical state and environment of the Pd element changed [36,37]. The XPS signal therefore indicated that the Pd (0) formed by this process was completely reduced to its zero-valent state. XRD was conducted to study the crystalline phase of Pd NPs/PCNFs catalyst that carbonization temperature was 500 ◦ C (the right inset of Fig. 4D). Based on the right inset of Fig. 4D, it can be seen the strong peaks at 2Â values of 39.9◦ , 46.7◦ , 67.9◦ , 82.0◦ and 86.4 ◦ can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) plane of Pd (0) crystals phase with face-centered-cubic structure, respectively [38]. The XRD peaks broadened because of the narrow size distribution of Pd NPs; it showed that the structures of Pd crystals formed as a consequence of the 4 h heat treatment of Pd NPs/PCNFs at 500 ◦ C. A wide diffraction peaks at about 23.8◦ was the representative (0 0 2) plane of the stacked graphite layers of PCNFs (JCPDS75-1621), due to the crystal structure of PCNFs was not perfect with turbostratic graphite structure, so XRD diffraction peak looked wider than the perfect crystalline graphite crystal structure [39]. Impurity peak were not observed, it suggested that Pd NPs/PCNFs were highly pure and clean. In the end, FTIR spectra of precursor PAN, PS/PAN, PdCl2 /PS/PAN, Pd NPs/PS/PAN and of Pd NPs/PCNFs which derived from Pd NPs/PS/PAN nanofibers containing different PS contents (the molar ratio of PAN and PS was 10, 20, 30 and 40, respectively) were recorded in the 400–4000 cm−1 wavelength range and shown in Fig. 5. The prominent peaks observed at 697 cm−1 are assigned to C H out-of-plane bending vibration of mono-substituted benzene derivatives PS (Fig. 5(B–D)). The peaks disappeared after carbonizing 4 h at 500◦ C due to the pyrolyzation of PS (Fig. 5(E–H)). The absorption peak at 1581 cm−1 (Fig. 5(E–H)) corresponded to the stretching vibration of double bonds (C C), because the graphite planar network layer structure gradually formed following the process of the high temperature carbonization of PAN nanofibers. The stretching vibration of C O bonds formed broad peaks at 1346 cm−1 (Fig. 5(E–H)). PS and most of the organic functional groups in carbon nanofibers were removed, for example, the stretching vibration peak of CN at 2237 cm−1 (Fig. 5(A–D)) almost vanished in the process of carbonization, so the PCNFs was welltolerated to majority of solvents. Meanwhile, they had more perfect graphite layers structure which were beneficial for the transmission of electrons and was able to promote catalytic performance according to literatures [40]. It also proved that the removing of TDP and the cause of forming nanopores/nanochannels structure. 3.3. The morphology characterization of Pd NPs/PCNFs

Fig. 3. UV–vis spectra of PdCl2 /PS/PAN nanofibers films (A) and Pd NPs/PS/PAN nanofibers films (B–E), which molar ratio of PAN and PS was 10, 20, 30 and 40, respectively.

Fig. 6 shows the typical SEM, FESEM imaged these composite nanofibers. Fig. 6A–D represents different molar ratio of PAN and PS, they was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers; (1) PdCl2 /PS/PAN nanofibers; (2) Pd NPs/PS/PAN nanofibers; (3) Pd NPs/PCNFs carbonization temperature was 500 ◦ C and (4) Pd NPs/PCNFs carbonization temperature was 500 ◦ C, which taken at higher magnification; (5) Pd NPs/PCNFs participated in the reaction for iodobenzene and n-butyl acrylate after recycled five times (Molar ratio of PAN and PS was 40) and (6) Pd NPs/PCNFs participated in the reaction for iodobenzene and n-butyl acrylate after recycled five times (Molar ratio of PAN and PS was 40) (high magnification). From these images, a series of uniform and smooth fibers without crosslinking and node among fibers and diameter range from 450 to 900 nm can be observed.

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Fig. 4. XPS patterns of Pd NPs/PCNFs catalysts that carbonization temperature was 500◦ C; (A) molar ratio of PAN and PS was 10 in pecursor Pd NPs/PS/PAN nanofibers; (B) molar ratio of PAN and PS was 20 in pecursor Pd NPs/PS/PAN nanofibers; (C) molar ratio of PAN and PS was 30 in pecursor Pd NPs/PS/PAN nanofibers; (D) molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers and the right inset shows XRD pattern of this Pd NPs/PCNFs catalyst.

Fig. 5. FTIR images of (A) PAN nanofibers; (B) PS/PAN nanofibers; (C) PdCl2 /PS/PAN nanofibers; (D) Pd NPs/PS/PAN nanofibers; (E–H) Pd NPs/PCNFs carbonization temperature was 500◦ C which molar ratio of PAN and PS was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers.

The average nanofiber diameter of the carbonized Pd NPs/PCNFs was observed to increase slightly with increasing PS content in the precursor nanofibers. This phenomenon maybe concerned with the formation of porosity of PCNFs, due to the creation of voids leads to an increase of total volume of PCNFs. Moreover, with the process of carbonization, the fibers stack closely, PCNFs became more regular and smooth, which were able to be attributed to the thermal relaxation of PAN macromolecules and a preferential shrinkage along the longitudinal axis of the fibers. SEM images (Fig. 6D-3 and 5) and FESEM images (Fig. 6D-4 and 6) of Pd NPs/PCNFs were compared before and after the reaction. Obviously, the uniform fibers morphology was still unchanged, and their diameters were still unchanged. To further observe the morphology and particle size of Pd NPs, FETEM was carried out (Fig. 7). Fig. 7A–D FETEM imaged and all right insets (each inset corresponded to a FETEM micrograph to allow careful observation of each porous nanostructure) represented different molar ratio of PAN and PS, and they was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers. They showed obviously when carbonization temperature was 500◦ C, Pd NPs with particles size range from 5 to 17 nm evenly distributed in/on the surface of PCNFs without obvious agglomeration and PCNFs had great specific surface area due to the nanopores/nanochannels in the fibers, nanopores/nanochannels exhibit uniform

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Fig. 6. SEM and FESEM images of (A–D) molar ratio of PAN and PS was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers; (1) PdCl2 /PS/PAN nanofibers; (2) Pd NPs/PS/PAN nanofibers; (3) and (4) Pd NPs/PCNFs carbonization temperature was 500◦ C; (5) and (6) Pd NPs/PCNFs participated in the reaction for iodobenzene and n-butyl acrylate after recycled five times (molar ratio of PAN and PS was 40). Figs. 4 and 6 are FESEM micrographs that taken at higher magnification to allow careful observation of each nanostructure.

distribution, same size apertures in principle and throughout the whole nanofibers. As shown by viewing Fig. 7, the porosity of PCNFs slightly increased with increasing PS content in the precursor Pd NPs/PS/PAN nanofibers, because PS was thermal removed from nanofibers leaving behind a porous, nanolamellar or nanotubular structure. Most of the organic functional groups in carbon nanofibers were removed, so the PCNFs were welltolerated to majority of solvents; Meanwhile, they had partial graphite layers structure which was benefit to the transmission of electros and could promote catalytic performance [40]. Subsequently, FETEM images of Pd NPs/PCNFs catalyst (carbonization temperature was 500◦ C) which was applied to Heck coupling reaction of iodobenzene and n-butyl acrylate after used five times (Fig. 7E) were also investigated. Compared it with Fig. 7D, no deactivation of catalyst was found, which showed the Pd NPs were still

uniform distribution without aggregating after reaction five times and nanopores/nanochannels structure was not changed. Furthermore, the morphology of PCNFs also retained and Pd NPs almost had no leaching, because PCNFs was able to effectively anchor Pd NPs thus forming a strong force between metal nanoparticles and the composite carrier in nanoscale. Hence, this catalyst was very stability and reusability. 3.4. The characterization of catalytic performance and catalytic recycling for Pd NPs/PCNFs In this paper, four catalysts were prepared, and the molar ratio of PAN and PS was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers and carbonization temperature were 500 ◦ C. For comparison, different Pd NPs/PCNFs catalysts were

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Fig. 7. FETEM images of Pd NPs/PCNFs carbonization temperature was 500◦ C; (A–D) molar ratio of PAN and PS was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers, the right inset shows cross section of a single fiber; (E) Pd NPs/PCNFs catalyst participated in the reaction for iodobenzene and n-butyl acrylate and it had been applied in Heck reaction after five times (molar ratio of PAN and PS was 40).

applied in Heck reaction in parallel (see Table 2). Obviously, the conversion rate of PhI significantly was improved by decreasing the PS content in the fibers. Maximum yield (PhI conversion rate: 100%; n-butyl cinnamate esters selectivity: 98.33%) was obtained

from the carbonized Pd NPs/PCNFs sample which molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers. The catalytic reaction occured on the surface of the Pd nanoparticles, so the yield of Heck reaction was a consequence of several factors including a

Table 2 Comparative product yields for iodobenzene with n-butyl acrylate under series of catalysts; carbonization temperature of Pd NPs/PCNFs catalysts were 500◦ C. Molar ratio of PAN and PS was 10, 20, 30 and 40, respectively, in pecursor Pd NPs/PS/PAN nanofibers. nPAN :nPS

DMF (g)

PhI (g)

N-Butyl acrylate (g)

Base (g)

Catalyst (g)

Conversion (%)

Selectivity (%)

10 20 30 40

10.8338 10.3572 10.8231 10.8499

0.4344 0.4448 0.4368 0.4262

0.4615 0.4535 0.4540 0.4593

0.616 0.6213 0.6211 0.6216

0.0303 0.0302 0.0303 0.0305

99.65 99.69 99.84 100.00

97.96 98.33 96.12 98.33

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Table 3 Results of Pd NPs/PCNFs catalyst catalysing the Heck coupling reaction for iodobenzene with various substrates; carbonization temperature of Pd NPs/PCNFs catalyst was 500◦ C. Molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers. Entry

PhI (g)

Alkene (g)

Catalyst (g)

Products

Conversion (%)

Selectivity (%)

1

0.4030

Methyl acrylate (0.2600)

0.0300

Methyl cinanmate

100.00

93.65

2

0.4030

Ethyl acrylate (0.3000)

0.0300

Ethyl cinnamate

100.00

97.28

3

0.4030

N-Butyl acrylate (0.4300)

0.0300

N-Butyl cinnamate

100.00

95.38

Table 4 The recycling experiments using Pd NPs/PCNFs catalyst in the Heck coupling reaction of iodobenzene and n-butyl acrylate, carbonization temperature of Pd NPs/PCNFs catalyst was 500◦ C. Molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers. Run

PhI (g)

1 2 3 4 5

0.4030 0.4230 0.4300 0.4090 0.4180

N-Butyl acrylate (g)

0.4360 0.4440 0.4400 0.4430 0.4450

Base (g)

Catalyst (g)

Conversion (%)

Selectivity (%)

0.6110 0.6150 0.6130 0.6090 0.6150

0.0306 0.0312 0.0311 0.0309 0.0315

100.00 100.00 100.00 100.00 100.00

95.38 93.67 96.54 95.52 97.64

greater number of active center, fine size and distribution of Pd nanoparticles, numbers of conduction pathways for electro movement in the carrier and large specific surface area of carrier that enable electrons to be scattered from the surfaces. The construction of porous structures of PCNFs was superior to Pd NPs/CNFs those obtained in our previous research work in our research group [41]. Subsequently, the catalytic potential of catalyst which possesed maximum yield in Heck reaction, molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers, carbonization temperature was 500◦ C, dosage of catalyst is 0.030 g and reaction time is 12 h was further demonstrated with a series of substrates in Heck coupling reaction for iodobenzene, the results showed that their yields were over 93 % (Table 3). Table 4 shows the yields of the different recyclable times in Heck coupling reaction of iodobenzene and n-butyl acrylate, the molar ratio of PAN and PS was 40 in pecursor Pd NPs/PS/PAN nanofibers, the carbonization temperature of Pd NPs/PCNFs catalyst was 500◦ C, the dosage of catalyst was 0.03 g, and reaction time was 12 h. Results revealed that product yields were more than 93% in each run time and catalyst possessed good catalyst retrieval from each run time with a retrieval rate of about 98% in a repeat test of 5 runs. Compared with another paper, it was prepared to a robust recyclable polymer complex stabilized Pd NPs catalyst for Heck reactions by simultaneous cross linking of polyvinyl alcohol and polyacrylamide. In the coupling reaction system, the Heck arylation of iodobenzene with alkenesin the solvent of DMF, though conversion and recyclability in this experiment were lower than they were given in the tables [42], but Porous carbon nanofibers yielded a heterogeneous system with the producted and provided stability to palladium nanoparticles avoiding the undesirable formation of palladium black after reaction. These special features enabled easily to product separation and recovery of the hybrid catalyst system, thus allowing its re-use up to 5 times without apparent loss of catalytic activity or selectivity.

Retrieval of catalyst Retrieval (g)

Rate (%)

0.0306 0.0311 0.0309 0.0308 0.0310

100.00 99.68 99.36 99.68 98.41

5. Conclusions In conclusion, various PCNFs-supported Pd NPs catalysts have been prepared by combining the electrospinning, chemical reduction and subsequent calcination processes. The article discussed on the following four aspects. (1) Metal nanoparticles/polymer composite nanomaterials were able to avoid the agglomeration of metal nanoparticles effectively and improved electron transfer and transportation ability of the polymer materials greatly. (2) Pd NPs possessed superior economic value. (3) PCNFs-supported Pd NPs composite catalyst was producted by a novel and facile approach and PCNFs possessed well-defined nanostructures, and were easily tuned micro-/mesoporous properties, large specific surface area and excellent absorption and transmission performance. So the stable Pd NPs were able to be anchored both on the surface and in the interior of the nanofibers by electrospinning of a mixture of polymer matrix and synergy between the metal nanoparticles and the supporting nanofibers in nanoscale were benefit to catalysis. (4) For practical applications, enhanced catalytic properties catalyst Pd NPs/PCNFs was applied in Heck reaction. (5) PS had the possibility in recycling which was important to develop green chemistry. This finding has not been reported, it might open up new opportunities for the design and fabrication of other high-performance, the exceptional novel materials and reusability catalyst which might hold promising potential for economic and application values.

Acknowledgements The authors gratefully acknowledge the support of the National Natural Science Foundation of China (21266016), and the Inner Mongolia Natural Science Foundation (2012MS0202).

L. Guo et al. / Journal of Molecular Catalysis A: Chemical 400 (2015) 95–103

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