An effective approach to preparing MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs for styrene epoxidation action

Abstract

Two kinds of metal oxide–silver nanoparticles (Ag NPs) embedded in carbon nanofibers (CNFs) were prepared by electrospinning followed by calcination. The resulted nanofibers were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It indicated that MgO–Ag NPs and Al₂O₃–Ag NPs were well-distributed in the CNFs. This effective synthesis method can be used to prepare other composite nanofibers with functionality. MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs served as supported catalysts were used in the styrene epoxidation by TBHP. The Al₂O₃–Ag NPs–CNFs catalyst showed its high catalytic activity for the epoxidation of styrene (conversion: 46.45%, styrene oxide (SO) selectivity: 34.45%), as compared with the MgO–Ag NPs–CNFs catalyst. The addition of MgO or Al₂O₃ had the effect of a promoter in the catalyst system. These kinds of composite nanofiber membranes have proven effective catalytic activity and recyclability in the styrene epoxidation.

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1. Introduction

With the increasing attention to nanotechnology, the application of this technology gradually turned to the preparation of inorganic–organic nanocomposites.^1–3 For example, metal and metal oxide nanoparticles were added to the polymer matrix in order to obtain functional composite nanofibers.^4–7 Electrospinning is a highly effective and low consumption technology synthesizing one-dimensional nanocomposite materials.^8–11 Currently, electrospinning technology is used to fabricate composite CNFs with metal or metal oxides, such as Pd–CNFs, Ni–CNFs, Co–CNFs, Pt–Pd–CNFs, Pt–Ru–CNFs, ZnO–CNFs, SiO₂–CNFs, TiO₂–CNFs, Fe₃O₄–CNFs and LiFePO₄–CNFs, by adding the metal, metal oxide or the precursor of theirs to the electrospinning solution directly.^12–21 Metal and metal oxide nanoparticles are able to endow the novel performance of CNFs and enlarge their application field.^22,23 Frey et al. studied the catalytic performance of nano-sized alkaline earth metal oxides supported on carbon nanofibers.²⁴ Dumbre et al. used gold nano-particles supported on MgO (or CaO) and other metal oxides for Suzuki–Miyaura cross-coupling reactions.²⁵

Silver is the efficient catalyst for a styrene epoxidation reaction.^26,27 In 2011, Pan et al. studied the Ag–KOH-γ-Fe₂O₃ composite catalyst, confirming the Ag composite catalyst is a very good and effective catalyst for styrene epoxidation reaction.²⁸ The first to use silver as an ethylene epoxidation catalyst was Lefort in 1931.²⁹ So far, silver remains the main component in the industrial production of ethylene epoxidation catalysts. The main catalyst in composite material prepared is silver, so that the catalyst we prepared for epoxidation is feasible. Styrene oxide acts as an important intermediate for fine chemicals and pharmaceuticals. Choudhary et al. have reported transition metal oxides (NiO, CoO or MoO₃) as highly active catalysts for styrene epoxidation by TBHP, with high conversion (52%) and yield (45%).³⁰ Patil and coworkers deposited gold nanoparticles on MgO as highly active and reusable catalysts in the epoxidation of styrene (conversion: 44.6%, SO selectivity: 36.1%).³¹

In this article, carbon nanofibers decorated with MgO–Ag NPs or Al₂O₃–Ag NPs were successfully fabricated by an electrospinning and calcination process. MgO–Ag NPs or Al₂O₃–Ag NPs nanoparticles uniformly dispersed in the obtained composite nanofibers. We examined the performance of the catalysts MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs in the styrene epoxidation reaction, respectively. The composite nanofibers exhibited high catalytic activity, which might be attributed to the high surface areas of Ag NPs and synergistic effect on delivery of electrons between Ag NPs and CNFs.³² In this article, we reported that the addition of metal oxides MgO or Al₂O₃ in the Ag NPs–CNFs showed high catalytic activity in the styrene epoxidation (Fig. 1). To the best of our knowledge, this method is very novel that the heterogeneous nanofiber membranes were prepared by electrospinning technology.

2. Experimental

2.1 Materials

Silver nitrate (AgNO₃, AR, 99.8%) was provided by Tianjin Yingda Sparseness & Noble Reagent Chemical Factory. Magnesium nitrate (Mg(NO₃)₂·6H₂O, AR, 99.0%) Tianjin Regent chemicals Co. Ltd. Polyacrylonitrile (PAN, average M_w = 90 000) was purchased from Kunshan Hongyu Plastic Co. Ltd. Aluminium nitrate (Al(NO₃)₃·9H₂O, AR, 99.0%), nitrogen–nitrogen dimethylformamide (DMF, AR, 99.5%) and dichloromethane (CH₂Cl₂, AR, 99.5%) were purchased from Tianjin Fengchuan Chemical Technology Co. Ltd. Hydrazine hydrate (N₂H₄·H₂O, AR, 50.0%) was purchased from Beijing Chemical Factory. Styrene (C₈H₈, AR, 99.0%) was purchased from Sinopharm Chemical Reagent Co. Ltd. Isopropanol ((CH₃)₂CHOH, AR, 99.5%) was purchased from Tianjin Beichen Founder Reagent Plant. tert-Butyl hydroperoxide ((CH₃)₃COOH, AR, 70%) was purchased from Tianjin Alfa Aesar Chemical Co. Ltd. All the chemicals were used as received without further purification.

2.2 Preparation of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs

First, the solution of PAN–DMF was prepared by dissolving PAN in DMF for 12 h at room temperature with continuous stirring, in which the concentration of PAN was 6 wt%. 0.256 g AgNO₃ was added into 10 g PAN–DMF solution (the molar ratio of AgNO₃/PAN = 1/10) and then stirred for 2 h in the opaque background. Then, the solution was heated to 80 °C in a water bath for about 30 min. The Ag NPs–PAN–DMF solution was prepared by using DMF as a reducing agent with an in situ reduction. Afterwards, quantitative magnesium nitrate or aluminium nitrate was added into the above solution. The molar ratio of precursors was Mg(NO₃)₂ or Al(NO₃)₃ : AgNO₃ : PAN = 1 : 10 : 100. The mixed solution was stirred for 2 h. Then, Mg(NO₃)₂–Ag NPs–PAN–DMF and Al(NO₃)₃–Ag NPs–PAN–DMF solutions were obtained.

Next, the schematic setup of the electrospinning process we used in this study was assembled as in a previous study.³³ The prepared Mg(NO₃)₂–Ag NPs–PAN–DMF and Al(NO₃)₃–Ag NPs–PAN–DMF electrospun solution was loaded into a glass tube with a sharp nozzle (the inner diameter was 1 mm). A high-voltage power supply was used to generate the electric field of 17 kV. An aluminum foil served as the counter electrode and the collector. The distance between the needle and the substrate electrode was 17 cm. All of the electrospinning operations were performed in air atmosphere at room temperature.

Finally, the Mg(NO₃)₂–Ag NPs–PAN or Al(NO₃)₃–Ag NPs–PAN nanofiber membranes were calcined in the vacuum tube furnace. The tube furnace was used to stabilize and carbonize the precursor PAN nanofibers. During the stabilization phase, the nanofiber membranes were heated to 250 °C at a rate of 5 °C min⁻¹ under air atmosphere and kept at 250 °C for 2 h. After stabilization, carbonization was carried out under nitrogen gas flow while raising the temperature at a rate of 3 °C min⁻¹ to 900 °C. The samples were kept at 900 °C for 2 h. Finally, the samples were allowed to cool down (below 50 °C) before being removed from the furnace. Then, MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs were obtained.

2.3 Characterization of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs

The samples were investigated by a scanning electron microscope (SEM, Hitachi, S-3400N, Japan) which was equipped with an energy dispersive X-ray analyzer (EDX) and transmission electron microscope (TEM, Jeol, JEM-2010, Japan and Fei, F20 S-TWIN, Tecnai). The samples for TEM were dispersed in ethanol by ultrasonic treatment; a drop of the dispersion was deposited on a TEM carbon-coated Cu grid and dried at room temperature. The phase and crystalline structures of Ag NPs were characterized by an X-ray diffractometer (XRD, PIGAKV, D/MAX-2500/PC, Japan) over a range of 2θ angles from 10° to 90°. The measurements of the X-ray photoelectron spectra (XPS) were performed by using a Thermo fisher Scientific ESCALAB 250 spectrometer.

2.4 Catalysis performance of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs

The styrene epoxidation by TBHP over the composite catalyst was carried at atmospheric pressure, using a reaction mixture containing 1 ml styrene, 5 ml TBHP, 5 ml solution and 0.03 g catalyst in a 25 ml round bottom flask, under reflux (at 80 °C) and vigorously stirring for a period of 8 h. The catalysts of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs were used in the styrene epoxidation. The catalyst was separated from the reaction mixture by centrifugation. The reaction products and unconverted reactants were analysised by an Agilent (7890A) gas chromatograph (GC) and Agilent (5975C) gas chromatography mass spectrometer (GC-MS). The GC equips with FID using SE-30 column and N₂ as carrier gas. Using an area normalization method calculates the content of each component in the products.

Fig. 1 Scheme of the MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs preparation process and chemical equation.

3. Results and discussion

3.1 Characterization of catalysts

Fig. 2 shows the SEM images of Mg(NO₃)₂–Ag NPs–PAN nanofibers (Fig. 2A), MgO–Ag NPs–CNFs (Fig. 2B), Al(NO₃)₃–Ag NPs–PAN nanofibers (Fig. 2D) and Al₂O₃–Ag NPs–CNFs (Fig. 2E). The images indicated smooth surface and uniform diameter nanofibers had been prepared successfully. Compared with Fig. 2A, B, D and E, the diameter of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs had decreased after calcined. The diameter of nanofibers became thinner which is due to the removal of some organics in the nanofibers in the calcined process. But, CNFs maintained the original morphology of the nanofibers. This showed that the addition of MgO–Ag NPs and Al₂O₃–Ag NPs did not affect the formation of CNFs. Ag NPs can be seen in the form of small white particulates in the nanofibers (Fig. 2B and E). The morphology of original inorganic Ag NPs had not changed so it can be easily seen in the SEM images. EDX equipped SEM was performed on the MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs. EDX analysis (Fig. 2C and F) revealed that the obtained nanofibers contain C, O, Ag, Mg or Al elements.

Fig. 2 The SEM images of the composite nanofibers Mg(NO₃)₂–Ag NPs–PAN (A) and MgO–Ag NPs–CNFs (B); EDX spectrum of MgO–Ag NPs–CNFs (C); Al(NO₃)₃–Ag NPs–PAN (D) and Al₂O₃–Ag NPs–CNFs (E); EDS spectrum of Al₂O₃–Ag NPs–CNFs (F).

The TEM technique exhibited the detection and analysis of nano-size particles directly and efficiently. The images further investigated that the composite nanofibers indeed have small nanoparticles embedded in the CNFs. It could be seen in the Fig. 3 that the obtained MgO–Ag NPs and Al₂O₃–Ag NPs were loaded in carbon nanofibers after calcination with a diameter of about 300–400 nm. The surfaces of the composite nanofibers were covered with Ag nanoparticles. There was no obvious aggregation. It was found out that the Ag catalysts exhibited fine dispersion with the Ag nanoparticle sizes ranging from 10–40 nm in the MgO–Ag NPs–CNFs or Al₂O₃–Ag NPs–CNFs.

Fig. 3 TEM images of MgO–Ag NPs–CNFs (A); Al₂O₃–Ag NPs–CNFs (B and C).

The XRD patterns of MgO–Ag NPs–CNFs (A) and Al₂O₃–Ag NPs–CNFs (B) are shown in Fig. 4. The five major peaks were detected on the XRD pattern between 20° and 90°. These peaks around 2θ = 38.4°, 44.24°, 64.36°, 77.3° and 81.4°, which were assigned to the (111), (200), (220), (311) and (222) crystal planes, respectively. The positions of these peaks were in excellent agreement with the cubic crystal silver (JCPDS files no. 04-0783). The mean size of Ag nanoparticles was calculated using the Scherer equation (D = 0.89λ/β cos θ). The average crystallite size of Ag nanoparticles was 34.4 nm in the MgO–Ag NPs–CNFs and 44.0 nm in the Al₂O₃–Ag NPs–CNFs catalysts. These digitals were consistent with the results from TEM tests generally. The diffraction peak located at a 2θ value of about 24° was referred to carbon (002). The existing form of C is C=C graphite flat network layer in CNFs. In Fig. 4 the diffraction peak located at a 2θ value of about 43.1° was referred to magnesium oxide (200).

Fig. 4 XRD patterns for the MgO–Ag NPs–CNFs (A); Al₂O₃–Ag NPs–CNFs (B).

XPS was used to test the resulting MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs films. The wide XPS pictures of the products are shown in Fig. 5. Fig. 5A and D show the fully scanned spectra in the range of 0–1350 eV of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs films. The overview spectra demonstrated that C, Ag, O and N exist in the composite nanofibers. Fig. 5B and E show the photoelectron spectra of Ag 3d which peaks at 367.6 eV, 373.6 eV and 368.5 eV, 374.5 eV, and these dates correspond well with Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively. In this work, only Ag⁰ existed in all the tested samples. It indicated that the addition of MgO or Al₂O₃ promoter enhanced oxygen adsorption ability and increased the distribution of surface active sites.³⁴ Fig. 5C indicated the existance of Mg. Fig. 5F indicated that the existance of Al was aluminum oxide. All of these results gave the conclusion that the composite nanofibers were composed of MgO–Ag NPs–CNFs or Al₂O₃–Ag NPs–CNFs.

Fig. 5 XPS patterns for the MgO–Ag NPs–CNFs (A–C); Al₂O₃–Ag NPs–CNFs (D–F).

We investigated the effect of this supported catalyst by selecting six kinds of solvent in styrene epoxidation in Table 1. After the analysis of GC and GC-MS detection, the main products of the reaction were SO and benzaldehyde (BZ). Among the metal oxide catalysts, the Al₂O₃ catalyst showed the best performance, of which the highest conversion was 46.45% and selectivity was 34.45%. The MgO catalyst also showed very good performance with the conversion of 43.00% and selectivity of 33.10%. It was confirmed that synergistic effect existed between Ag NPs and CNFs. According to the Fermi level, electrons left the Ag from a thus depleted region near an Ag–CNF interface into the CNF, which ends up with an electron-enriched region.³² All the catalysts also showed excellent reusability in the epoxidation. The results indicated that isopropanol was the preferential solvent for the styrene epoxidation reaction, compared with the restof the solvents for which the conversion of styrene decreased.

Table 1 Styrene oxidation results with different solvents

	Catalyst	Solvent	Conversion	Selectivity
				SO	BZ
1#	MgO–Ag NPs–CNFs (1/10/100)	Isopropanol	43.00%	33.10%	66.90%
2#		Dichloromethane	14.30%	35.40%	57.00%
3#		Methanol	25.64%	42.09%	57.91%
4#		Dimethylformamide	32.2%	44.1%	56.0%
5#	Al2O3–Ag NPs–CNFs (1/10/100)	Isopropanol	46.45%	34.45%	65.55%
6#		Benzene	25.59%	49.79%	50.21%
7#		Dichloromethane	25.20%	37.33%	62.67%
8#		Methanol	17.78%	33.57%	66.43%
9#		Dimethylformamide	17.57%	50.23%	49.77%
10#		Acetone	17.56%	51.97%	48.03%

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4. Conclusions

We focused on preparing a kind of catalyst involving metal oxide–silver nanoparticles supported on a CNF material. The catalysts of MgO–Ag NPs–CNFs and Al₂O₃–Ag NPs–CNFs were prepared by using electrospinning technology and high temperature calcination technology. They promise to be environmentally friendly (easily separated and reused) catalysts to accomplish styrene oxidation. They had excellent catalytic performance for the styrene epoxidation. The Al₂O₃–Ag NPs–CNFs catalyst showed its high catalytic activity for the epoxidation of styrene (conversion: 46.45%, SO selectivity: 34.45%), as compared with the MgO–Ag NPs–CNFs catalyst (conversion: 43.00%, SO selectivity: 33.10%). It provided a new idea for the preparation of other functional inorganic–organic composite nanofibers and had a certain role in promoting the broad application areas of composites. Nanoparticles as a kind of catalyst not only reduced the amount of catalyst but also improved the efficiency of catalytic reactions. The addition of a metal oxide improved the strength of the support and enhanced the catalytic performance. It showed its uniqueness that optimized the reaction path and increased the reaction rate.

Acknowledgements

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (21166016) and (21366016).

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