Preparation of Rh–SiO₂ fiber catalyst with superior activity and reusability by electrospinning

Abstract

A rhodium (Rh)–SiO₂ fiber catalyst was prepared by electrospinning, calcination, and reduction in that order. The as-prepared Rh–SiO₂ fiber catalyst was applied in the catalytic hydrogenation of alkenes. This catalyst allowed the hydrogenation reaction to be carried out at room temperature with excellent catalytic activity and could be reused nine times without obvious loss of catalytic activity. The excellent mechanical strength, thermal stability, and chemical stability of SiO₂ and the uniform dispersion of the Rh nanoparticles in the fibers are the reasons for the superior activity and reusability of the catalyst.

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1. Introduction

Noble metal catalysts are the most widely studied and used catalysts due to their extraordinary catalytic properties. Many reactions were carried out by homogeneous catalysts with high activity under mild conditions.¹ However, the high price of noble metals and the environmental contamination caused by the residual noble metal catalysts limit the use of homogeneous catalysts, and the recycling of noble metal catalysts remains a significant challenge. Due to the advantages of easier separation and reusability, larger exposed surface area, and stronger resistance to poisons over homogeneous catalysts, heterogeneous catalysts have been extensively used in numerous industrial applications.^2–4 Recently, with the development of nanotechnology, different nanomaterials such as carbon nanotubes,^5,6 graphene,⁷ magnetic nanoparticles,^8,9 and metal–organic frameworks¹⁰ were used as supports to prepare heterogeneous catalysts, but impediments such as complex preparation process, aggregation caused by the nanoscale supports, and high cost limited the practical applications of these catalysts. Therefore, a simple, economical method is needed for the large scale fabrication of heterogeneous catalysts with superior catalytic activity and reusability.

Electrospinning has been the most extensively studied and applied method to prepare one-dimensional nanostructures due to its facility, flexibility, controllability, versatility, and low cost.^11–15 Various kinds of nanofibers, such as polymer fibers,¹⁶ ceramic fibers,^17,18 and carbon fibers¹⁹ have been prepared by electrospinning and widely used in applications like electronics, sensors, catalysis, environmental engineering, energy science, and biomedical engineering. Recently, combining functional nanoparticles (NPs) with nanofibers to fabricate hybrid nanofibers has emerged as one of the most exciting research areas in the field of electrospinning.^20–25 For the hybrid nanofibers, the NPs enhances the properties of the nanofibers, while the nanofibers prevent the NPs from corrosion and aggregation. Thus, the applications of the electrospun nanofibers are expanded and promoted, especially in the field of catalysis, where uniform dispersion and recycling of the noble metal NPs are vital to increase the catalytic activity and reduce the cost.²⁵ Polymer nanofibers such as polyvinylpyrrolidone (PVP) fibers²⁶ and polyethyleneimine/polyvinyl alcohol fibers^27,28 have been prepared by electrospinning and used as catalyst supports. Because of their excellent mechanical strength, thermal stability, and chemical stability, inorganic nanofibers are ideal catalyst supports. Inorganic nanofibers such as carbon fibers,^29–32 SiO₂ fibers,^33–35 CeO₂ fibers,³⁶ and TiO₂ fibers³⁷ have been used as supports for metal NPs in heterogeneous catalysts for various kinds of reactions including reduction, oxidation, hydrolysis, and the Heck coupling reaction.

In our previous study, palladium (Pd)–SiO₂ fibers were prepared by electrospinning, and used as catalyst in the hydrogenation of acrylic acid.³⁵ To the best of our knowledge, there is scarcely any report about the electrospun nanofibers with rhodium (Rh) NPs,²⁸ especially in the catalytic hydrogenation. In this work, we fabricated an Rh–SiO₂ fiber catalyst by electrospinning and applied it in the catalytic hydrogenation of alkenes with different structures. The as-prepared Rh–SiO₂ fiber catalyst exhibited high catalytic activity and superior reusability due to the excellent mechanical properties, thermal stability, and chemical stability of SiO₂ and the uniform dispersion of the Rh NPs.

2. Experimental

2.1. Materials

Tetraethyl orthosilicate (TEOS, reagent grade) and PVP (M_w = 40 000, M_w = 1 300 000, reagent grade) were purchased from Sigma Aldrich Co., Ltd. Rhodium trichloride hydrate (RhCl₃·3H₂O, ≥99.9%, Rh content ≥ 39.0%) was purchased from Shanghai Jiuling Chemical Co., Ltd, China. N,N-Dimethylformamide (DMF), dimethylsulfoxide (DMSO), hydrochloric acid solution (37.5 v/v%), ethanol, chloroform, styrene, cyclohexene, 1-octene, 1-dodecene, and acrylic acid of analytical grade were purchased from Beijing Chemical Company, China. Nitrogen (>99.999%) and hydrogen (>99.999%) were purchased from Beijing Beiwen Special Gases Factory, China. All the reagents and solvents were used as received without further purification.

2.2. Preparation of Rh–SiO₂ fiber catalyst

The preparation of Rh–SiO₂ fiber catalyst is shown in Scheme 1. In a typical procedure, a pregelation of TEOS (5.2 g) with hydrochloric acid (1.0 g) and ethanol (1.5 g), a solution of PVP (M_w = 1 300 000, 1.2 g) in DMF (5.0 g) and DMSO (2.5 g), and a solution of RhCl₃·3H₂O (320 mg) and PVP (M_w = 40 000, 670 mg) in deionized water (5 mL) were prepared under magnetic stirring for 12 h at room temperature. The spin dope was achieved by mixing the above three solutions under magnetic stirring for 2 h at room temperature.

Scheme 1 Preparation of Rh–SiO₂ fiber catalyst.

The electrospinning was carried out on an electrospinning setup that is composed of a syringe pump (KDS-200), a high voltage power supply (BGG6-351, High Voltage Technology Institute, BEMI Co., Ltd, China), and a rotating drum (5 inches in diameter). The plastic syringe was filled with the spin dope, and a high positive voltage of 15 kV was applied to the needle, which is 20 cm from the drum. The feed rate was maintained at 1.0 mL h⁻¹ by the syringe pump, and the rotating rate of the drum was 200–300 rpm min⁻¹. An Al foil was used to collect the electrospun nanofibers.

The collected nanofiber mats were calcinated in air in a heavy duty tube furnace (Lindberg 54453) at 350 °C for 6 h and then at 800 °C for 1 h for the removal of all organic components and the condensation of silanol. During this process, RhCl₃ was oxidized to Rh₂O₃. Rh–SiO₂ fibers were obtained by the reduction of the Rh₂O₃–SiO₂ fibers in H₂ at 300 °C for 3 h.

2.3. Catalytic hydrogenation of alkenes by Rh–SiO₂ fiber catalyst

The Rh–SiO₂ fiber catalyst and the substrate were placed into a 100 mL autoclave. The autoclave was purged with N₂ and H₂ three times each. Then, the reaction was initiated by heating the autoclave to the reaction temperature, flushing H₂ to the reaction pressure, and adjusting the agitation rate to 1000 rpm. During the reaction, the reaction temperature and H₂ pressure were kept constant. After the reaction, the catalyst was separated from the product by centrifugation. The separated catalyst was washed with chloroform and ethanol 2 times each. After it was dried in vacuum at 40 °C for 2 h, the recycled catalyst was used in the next reaction.

2.4. Characterizations

An ESCALAB 250 X-ray photoelectron spectroscopy (XPS) system (Thermo Electron Corporation, USA) was used to carry out the XPS measurements. Thermogravimetric analysis (TGA) was performed on a STARe system from 40 to 800 °C at a heating rate of 10 °C min⁻¹ under a steady air flow (50 mL min⁻¹). A Bruker D8 Advance diffractometer (Germany) was used to obtain the X-ray diffraction (XRD) patterns. The specific surface area, pore diameter, pore volume, and adsorption–desorption isotherm of the samples were measured by N₂ physical adsorption at 77 K (ASAP2020M Micromeritics). A JEOL JEM-3010 instrument (Hitachi, Japan) was used to take the high resolution transmission electron microscopy (HRTEM) photographs. An S-4800 instrument (Hitachi) was used to take the scanning electron microscopy (SEM) photographs. Energy dispersive spectrometry (EDS) was performed on a GENESIS 307 system equipped with the S-4800 SEM instrument. Inductive coupled plasma mass spectroscopy (ICP-MS) was used to determine the Rh content of the Rh–SiO₂ fiber catalyst. In a typical ICP-MS measurement, sample of 200 mg were dissolved in a mixture of 2 mL of HNO₃ and 8 mL of HF in a CEM MARS6 microwave system at 180 °C for 30 min. The product of the hydrogenation reaction was sampled to measure its degree of hydrogenation (HD) by ¹H-NMR spectra recorded on a Bruker AV 400 MHz spectrometer, with CDCl₃ as the solvent. The HD is given by

HD = (1 − [C=C]_t/[C=C]₀) × 100%, (1)

where [C=C]_t is the molar content of C=C bonds at reaction time t and [C=C]₀ is the initial molar content of C=C bonds.

3. Results and discussion

3.1. Preparation of Rh–SiO₂ fiber catalyst

As mentioned above and shown in Scheme 1, the preparation of the Rh–SiO₂ fiber catalyst consists of preparation of the spin dope, electrospinning, calcination, and reduction. The as-spun fibers before calcination were labelled as PVP–RhCl₃–SiO₂ fibers, the fibers after calcination were labelled as Rh₂O₃–SiO₂ fibers, and the fibers after reduction were labelled as Rh–SiO₂ fibers. By using the same method but without the use of RhCl₃·3H₂O, pure SiO₂ fibers were obtained and were labelled as SiO₂ fibers.

An SEM and a TEM micrograph of the PVP–RhCl₃–SiO₂ fibers are shown in Fig. 1(a) and (c), respectively. As shown in Fig. 1(a), the PVP–RhCl₃–SiO₂ fibers have a smooth surface and a mean diameter of about 600 nm (see Fig. 1(b)), which was determined from the measurements of 100 fibers. From the TEM image shown in Fig. 1(c), we can see that the PVP–RhCl₃–SiO₂ fiber has a core–shell structure consisting of a core of SiO₂ pregelation and a shell of PVP.^38,39 As mentioned in the Experimental section, the spin dope consisted of three different solutions. In the first solution, TEOS reacted with hydrochloric acid in ethanol to form a pregelation. In the second solution, RhCl₃ was capped by PVP (M_w = 40 000) through chemisorption to form Rh NPs. The third solution, a solution of PVP (M_w = 1 300 000) in DMF and DMSO, acted as a lubricant between the first two solutions.⁴⁰ As the spin dope is spun, the SiO₂ pregelation will diffuse to the core of the fibers, while the PVP-capped Rh NPs will diffuse to the shell of the fibers due to the phase separation resulting from the high solubility of TEOS in ethanol and the poor compatibility of TEOS with PVP.^38,39 Therefore, we can suppose that in the PVP–RhCl₃–SiO₂ fibers, the majority of Rh will be distributed in the shell of the fibers. This supposition can be confirmed by the higher surface Rh content of the PVP–RhCl₃–SiO₂ fibers as measured by XPS (Table 1) than the overall Rh content of the PVP–RhCl₃–SiO₂ fibers as measured by EDS (Table 2). From the XRD patterns shown in Fig. 1(d), we can see that the PVP–RhCl₃–SiO₂ fibers exhibit the characteristic diffraction peaks of both SiO₂ and PVP, indicating the exposure of PVP on the surface of the PVP–RhCl₃–SiO₂ fibers.

Fig. 1 (a) SEM image of PVP–RhCl₃–SiO₂ fibers, (b) diameter distribution of PVP–RhCl₃–SiO₂ fibers, (c) TEM image of PVP–RhCl₃–SiO₂ fiber, and (d) XRD spectra of SiO₂ fibers, PVP powder, and PVP–RhCl₃–SiO₂ fibers.

Table 1 Surface element compositions (wt%) of SiO₂ fibers, PVP–RhCl₃–SiO₂ fibers, Rh₂O₃–SiO₂ fibers, and Rh–SiO₂ fibers as measured by XPS^a

Element	C	O	N	Rh	Si	S	Cl
a —: Not detected.
SiO2 fibers	5.44	45.48	—	—	49.08	—	—
PVP–RhCl3–SiO2 fibers	37.3	28.42	5.12	5.39	19.8	1.97	2.00
Rh2O3–SiO2 fibers	8.08	52.58	0.17	8.92	29.94	—	0.31
Rh–SiO2 fibers	6.73	58.39	—	8.29	26.59	—	—

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Table 2 Element compositions (wt%) of SiO₂ fibers, PVP–RhCl₃–SiO₂ fibers, Rh₂O₃–SiO₂ fibers, and Rh–SiO₂ fibers as measured by EDS^a

Element	C	O	N	Rh	Si	S	Cl
a —: Not detected.
SiO2 fibers	2.11	60.34	—	—	37.55	—	—
PVP–RhCl3–SiO2 fibers	31.15	33.67	4.36	2.55	24.18	2.35	1.74
Rh2O3–SiO2 fibers	5.30	61.00	—	4.16	29.54	—	—
Rh–SiO2 fibers	5.44	59.80	—	4.38	30.38	—	—

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The mechanism of the interaction between PVP and RhCl₃ was studied by investigating the surface chemical composition of the PVP–RhCl₃–SiO₂ fibers by XPS. As shown in Fig. 2, the binding energies of the Rh 3d_5/2 core-level and Rh 3d_3/2 core-level of the PVP–RhCl₃–SiO₂ fibers are 308.25 eV and 313.15 eV, respectively, while those of RhCl₃ are 310.02 eV and 314.75 eV, respectively. Meanwhile, we can see from Fig. 3 and 4 that the binding energies of the O 1s core-level and N 1s core-level of the PVP–RhCl₃–SiO₂ fibers are 532.56 eV and 400.48 eV, respectively, while those of PVP are 531.21 eV and 399.54 eV, respectively. In the PVP–RhCl₃–SiO₂ fibers, the Rh ions are coordinated with the O and N atoms of PVP, and charge transfer occurs from the O and N atoms to the Rh ions, leading to the decrease of the binding energy of Rh and the increase of the binding energies of O and N in the PVP–RhCl₃–SiO₂ fibers.^40–42 FTIR was also used to investigate the interaction between PVP and RhCl₃ in the PVP–RhCl₃–SiO₂ fibers. As shown in Fig. S1 (ESI†), the FTIR spectrum of PVP shows characteristic peaks of about 1658 cm⁻¹, 1300–1500 cm⁻¹, and 1290 cm⁻¹ for C=O stretching, C–H bending, and C–N stretching, respectively. However, in the spectrum of the PVP–RhCl₃–SiO₂ fibers, the peak for C=O stretching red shift from 1658 cm⁻¹ to 1647 cm⁻¹, and the relative intensity of the peaks between 1300–1500 cm⁻¹ for C–H bending decrease, indicating the chemisorption of Rh ions on the O atoms of PVP. The relative intensity of the peak 1290 cm⁻¹ also decrease, suggesting that the chemisorption of Rh on O atoms should greatly weaken or even break the N–C bond of PVP.⁴⁰ The peaks of 1658 cm⁻¹, 1300–1500 cm⁻¹, and 1290 cm⁻¹ disappear after calcination, suggesting the removal of organic components after calcination. Therefore, the Rh NPs are formed in the aqueous solution of RhCl₃ and PVP (M_w = 40 000) by the chemisorption between the Rh ions and the N and O atoms of PVP before the preparation of the spin dope. After the electrospinning, the PVP-capped Rh NPs diffuse to the shell of the fibers, leading to the high content of Rh in the shell of the PVP–RhCl₃–SiO₂ fibers.

Fig. 2 XPS Rh 3d core-level spectra of (a) RhCl₃ and (b) PVP–RhCl₃–SiO₂ fibers.

Fig. 3 XPS O 1s core-level spectra of (a) PVP powder and (b) PVP–RhCl₃–SiO₂ fibers.

Fig. 4 XPS N 1s core-level spectra of (a) PVP powder and (b) PVP–RhCl₃–SiO₂ fibers.

The element composition of the PVP–RhCl₃–SiO₂ fibers shows the presence of C, O, N, S, Si, Rh, and Cl (Table 2; Fig. S1, ESI†). C, N, and part of O come from PVP, DMSO, and DMF; S comes DMSO; and Cl comes from RhCl₃. To obtain pure inorganic Rh–SiO₂ fibers, we calcinated the PVP–RhCl₃–SiO₂ fiber mats in air to remove all the organic components. As shown in Fig. 5, five weight loss stages are shown in the TGA-DTG thermograms of the PVP–RhCl₃–SiO₂ fibers. In the first stage, the DTG peak at 49 °C is attributed to the evaporation of ethanol. In the second stage, the DTG peak at 125 °C is ascribed to the evaporation of trapped solvents (DMF and DMSO). In the third stage, the DTG peak at 207 °C results from the decomposition of the side chains of PVP. The DTG peak at 342 °C in the fourth stage and the DTG peak at 376 °C in the fifth stage correspond to the complete decomposition of PVP,^35,43 and the weight loss between 400 °C and 800 °C is assigned to the release of water formed by the condensation of hydrolyzed TEOS in the PVP–RhCl₃–SiO₂ fibers.^35,43 The element composition of the obtained Rh₂O₃–SiO₂ fibers (Table 2; Fig. S2, ESI†) shows that the elements of N, S, and Cl are completely removed during calcination, while C still remains due to the carbonization of part of the organic components.

Fig. 5 TG-DTG thermograms of PVP–RhCl₃–SiO₂ fibers.

RhCl₃ was oxidized to Rh₂O₃, and the obtained Rh₂O₃–SiO₂ fibers were reduced in a H₂ atmosphere at 300 °C for 3 h to form Rh–SiO₂ fibers. The surface element compositions of the Rh₂O₃–SiO₂ fibers and Rh–SiO₂ fibers was investigated by XPS. The Rh 3d spectra in Fig. 6 shows peaks at binding energies of 308.61 eV and 313.4 eV for Rh₂O₃–SiO₂ fibers and at binding energies of 307.1 eV and 311.85 eV for Rh–SiO₂ fibers. These results are consistent with the standard binding energies of Rh₂O₃ and Rh, indicating that Rh³⁺ has been reduced to Rh⁰ successfully through the reduction in a H₂ atmosphere.

Fig. 6 XPS Rh 3d core-level spectra of (a) Rh₂O₃–SiO₂ fibers and (b) Rh–SiO₂ fibers.

XRD was used to characterize the structures of the Rh₂O₃–SiO₂ fibers and the Rh–SiO₂ fibers, and the results are shown in Fig. 7. The XRD pattern of the Rh₂O₃–SiO₂ fibers exhibits diffraction peaks at 2θ angles of 23.8°, 32.8°, 35.0°, 39.0°, 55.0°, 61.3°, and 64.9°, representing the (012), (104), (110), (006), (211), (214), and (125) planes of Rh₂O₃, respectively. At the same time, the XRD pattern of the Rh–SiO₂ fibers exhibits diffraction peaks at 2θ angles of 41.1°, 47.8°, 69.8°, and 84.4°, representing the (111), (200), (220), and (311) planes of Rh, respectively. These results are consistent with the standard peaks of Rh₂O₃ (JCPDS 43-1025) and Rh (JCPDS 05-0685), demonstrating the oxidation of RhCl₃ to Rh₂O₃ and the reduction of Rh₂O₃ to Rh.

Fig. 7 XRD patterns of (a) Rh₂O₃–SiO₂ fibers and (b) Rh–SiO₂ fibers.

The morphology and size of the Rh–SiO₂ fibers were examined by SEM and HRTEM, and representative electron microscopy images of the Rh–SiO₂ fibers are shown in Fig. 8. From the SEM images of the Rh–SiO₂ fibers shown in Fig. 8(a) and (b), we can see that the Rh NPs, which are spherical, are supported on the SiO₂ fibers and the average diameter of these Rh NPs is about 20 nm. However, from the HRTEM images of the Rh–SiO₂ fiber shown in Fig. 8(c) and (d), we can see that much smaller Rh NPs are supported on the SiO₂ fibers and these spherical Rh NPs have an average diameter of about 2.5 nm. The d-spacing for adjacent lattice fringes measured at different locations and the fast Fourier-transform (FFT) pattern shown in the inset of Fig. 8(d) reveal clear lattice fringes with regular distances of 0.219 nm and 0.191 nm, indicating the (111) and (200) planes of a face-centred cubic (fcc) Rh crystal structure. Therefore, the majority of the supported Rh NPs have a small size of about 2.5 nm, and the small number of larger Rh NPs (about 20 nm) are probably local aggregates of the smaller Rh NPs.

Fig. 8 (a) SEM image of Rh–SiO₂ fibers; (b) SEM image of Rh–SiO₂ fiber (inset: particle size distribution of the larger Rh NPs); (c) HRTEM image of Rh–SiO₂ fiber (inset: particle size distribution of the smaller Rh NPs); (d) HRTEM image of Rh NPs (inset: Fourier-transform (FFT) pattern).

The Rh content of the Rh–SiO₂ fibers was measured by ICP-MS to be 3.96 wt%, a value close to that (4.38 wt%) measured by EDS (Table 2). Tables 1 and 2 show that the Rh₂O₃–SiO₂ fibers have a surface Rh content (as measured by XPS) of 8.92% and an overall Rh content (as measured by EDS) of 4.16%, while the Rh–SiO₂ fibers have a surface Rh content of 8.29% and an overall Rh content of 4.38%. These results suggest that in the Rh–SiO₂ fibers, the majority of Rh NPs is distributed in the shell of the fibers and only a small amount of Rh NPs is distributed inside the fibers.

N₂ absorption measurements were used to characterize the porosity of the Rh–SiO₂ fibers, and the N₂ adsorption–desorption isotherm is shown in Fig. 9. A typical type I isotherm is observed, indicating the micropore structure of the Rh–SiO₂ fibers. The BET surface area was 321.5 m² g⁻¹, and the micropore volume and average pore diameter calculated from the adsorption isotherm by the Horvath–Kawazoe method were 0.15 cm³ g⁻¹ and 0.52 nm, respectively. According to our previous study, pure SiO₂ fibers had a BET surface area of 4.14 m² g⁻¹, and the BET surface area increased from 4.14 m² g⁻¹ to 345.07 m² g⁻¹ with increasing content of SiO₂ NPs, which leads to the porous structure of the SiO₂ NPs–SiO₂ hybrid fibers.³⁹ Therefore, in the present case, the presence of Rh NPs may be the reason for the formation of the micropore structure of the Rh–SiO₂ fibers. These results suggest that the Rh–SiO₂ fibers have sufficient micropores and a high surface area, both of which are of great importance for catalytic hydrogenation applications, where the efficient interaction between the catalytic active centres and the substrate is vital to the increase in catalytic activity.

Fig. 9 N₂ adsorption–desorption isotherm of the Rh–SiO₂ fibers.

3.2. Catalytic hydrogenation of alkenes by Rh–SiO₂ fiber catalyst

To evaluate its catalytic activity, we used the Rh–SiO₂ fiber catalyst in the catalytic hydrogenation of alkenes with different structures (Fig. S3, ESI†). The results in Table 3 show that the Rh–SiO₂ fiber catalyst exhibits excellent catalytic activity in the catalytic hydrogenation of alkenes with different structures at mild reaction conditions. From the corresponding ¹H-NMR spectra in Fig. S4–S8 (ESI†), we can see that the proton peaks for the double bonds of the alkenes disappear after the hydrogenation reaction and the intensity of the alkyl peaks increases, suggesting the successful hydrogenation of the alkenes. In our previous study, the Pd–SiO₂ fiber catalyst, on which the Pd NPs with an average diameter of 20–30 nm supported, exhibited excellent activity in the hydrogenation of acrylic acid at the reaction conditions of 70 °C, hydrogen pressure of 2.5 MPa, and 5 h.³⁵ However, the present Rh–SiO₂ fiber catalyst exhibited excellent activity even at the reaction conditions of 30 °C, hydrogen pressure of 1 MPa, and 30 min, suggesting the superior activity of the Rh–SiO₂ fiber catalyst. During the hydrogenation, the hydrogen molecules and the alkenes molecules react with each other on the catalytic active centres. The efficient contact between the catalytic active centres and the reactants is of utmost importance to acquire high catalytic activity. The superior catalytic activity of the Rh–SiO₂ fiber catalyst can be attributed to the high surface area afforded by the micropore structure of the Rh–SiO₂ fibers, the small size of the supported Rh NPs, and the uniform dispersion of the supported Rh NPs.

Table 3 Hydrogenation of alkenes catalyzed by Rh–SiO₂ fiber catalyst^a

Substrate	Product	HD (%)	TOF (h^−1)
a Reaction conditions: 0.2 mol of substrate, 0.1 mmol of catalyst (based on Rh content) measured by ICP-MS, Rhmol : substratemol = 1 : 2000, 30 °C, PH2 = 1 MPa, 1000 rpm (rotational speed of autoclave), and 30 min (reaction time). Degree of hydrogenation (HD, %) was determined by ^1H-NMR. Turnover frequency (TOF) was measured in [molproduct][molRh]^−1 h^−1.
Styrene	Ethylbenzene	100	≥4000
1-Octene	Octane	100	≥4000
1-Dodecene	Dodecane	100	≥4000
Acrylic acid	Propanoic acid	100	≥4000
Cyclohexene	Cyclohexane	82.61	3304.4

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Reusability is the most significant advantage of supported catalysts over homogeneous catalysts. Styrene was used as a substrate to evaluate the reusability of the Rh–SiO₂ fiber catalyst, and the results are shown in Fig. 10.

Fig. 10 Reusability of Rh–SiO₂ fiber catalyst in hydrogenation of styrene. Reaction conditions: 0.2 mol styrene, 0.1 mmol catalyst (based on Rh content measured by ICP-MS), Rh_mol : styrene_mol = 1 : 2000, 30 °C, P_H2 = 1 MPa, 1000 rpm, and 30 min.

Fig. 10 shows that the Rh–SiO₂ fiber catalyst also exhibits superior reusability in the hydrogenation of styrene. A high HD of 95.3% is obtained even after the catalyst has been reused for nine times, and the HD drops to 75.8% after the 10^th reuse. To determine the reason for the loss of catalytic activity, we characterized the Rh–SiO₂ fiber catalyst after 10 reuses by XRD, XPS, SEM, and ICP-MS, and the results are shown in Fig. 11.

Fig. 11 (a) XRD pattern of Rh–SiO₂ fibers after 10 reuses; (b) XPS Rh 3d spectrum of Rh–SiO₂ fibers after 10 reuses; (c) and (d) SEM images of Rh–SiO₂ fibers after 10 reuses.

The XRD pattern of the reused Rh–SiO₂ fibers shown in Fig. 11(a) exhibits the same diffraction peaks as the XRD pattern of the fresh Rh–SiO₂ fibers, while the XPS Rh 3d spectrum shown in Fig. 11(b) exhibits peaks at 307.0 eV and 311.7 eV, corresponding to Rh⁰ 3d_5/2 and Rh⁰ 3d_3/2, respectively. These results demonstrate that the chemical valence of the supported Rh NPs does not change even after 10 reuses. Fig. 11(c) and (d) show the SEM images of the recycled Rh–SiO₂ fibers. We can see that the reused Rh–SiO₂ fibers are not broken due to the excellent mechanical strength, thermal stability, and chemical stability of SiO₂. Meanwhile, the supported Rh NPs are still uniformly dispersed, spherical, but larger, suggesting that aggregation of the supported Rh NPs happens during the hydrogenation. The Rh content of the recycled Rh–SiO₂ fibers was measured by ICP-MS to be 2.73 wt%, which is lower than the 3.96% of the fresh Rh–SiO₂ fibers, indicating that corrosion of the supported Rh NPs occurs during the hydrogenation. Therefore, we can conclude that the micropore structure, high surface area, and uniform dispersion of the Rh NPs are the reasons for the high catalytic activity of the Rh–SiO₂ fiber catalyst, and the excellent reusability of the Rh–SiO₂ fiber catalyst is due to the excellent mechanical strength, thermal stability, and chemical stability of SiO₂. The aggregation and corrosion of the Rh NPs during the hydrogenation is the reason for the loss of catalytic activity.

4. Conclusions

An Rh–SiO₂ fiber catalyst was fabricated by electrospinning, calcination, and reduction. The as-prepared Rh–SiO₂ fiber catalyst exhibited high catalytic activity in the catalytic hydrogenation of alkenes and could be reused nine times without obvious decrease of activity. The excellent mechanical strength, thermal stability, and chemical stability of SiO₂, the high surface area resulting from the micropore structure, and the uniform dispersion of the Rh NPs are the reasons for the high activity and superior reusability of the Rh–SiO₂ fiber catalyst.

Acknowledgements

The financial support of the National Natural Science Foundation of China under Grant no. 20774009 is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20473a

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