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Synthesis of sulfonic acid-functionalized Fe3O4@C nanoparticles as magnetically recyclable solid acid catalysts for acetalization reaction† Fang-Cai Zheng,a Qian-Wang Chen,*a,b Lin Hu,b Nan Yana and Xiang-Kai Konga The Fe3O4@C core–shell magnetic nanoparticles with an average size of about 190 nm were synthesized via a one-pot solvothermal process using ferrocene as a single reactant. The sulfonic acid-functionalized Fe3O4@C magnetic nanoparticles were obtained by grafting the sulfonic groups on the surface of Fe3O4@C nanoparticles to produce magnetically recyclable solid acid catalysts. The as-prepared products were characterized by X-ray diffraction and transmission electron microscopy. The catalytic performance
Received 2nd August 2013, Accepted 12th October 2013 DOI: 10.1039/c3dt52098f www.rsc.org/dalton
1.
of the as-prepared catalysts was examined through the condensation reaction of benzaldehyde and ethylene glycol. The results showed that the catalysts exhibited high catalytic activity with a conversion rate of 88.3% under mild conditions. Furthermore, catalysts with a magnetization saturation of 53.5 emu g−1 at room temperature were easily separated from the reaction mixture by using a 0.2 T permanent magnet and were reused 8 times without any significant decrease in catalytic activity.
Introduction
Homogeneous acid catalysts such as Lewis and Brønsted acids, a very important class of catalysts, are commonly used in the large-scale synthesis of industrial bulk chemicals as well as in the production of fine chemicals.1–3 However, major drawbacks of these catalysts include the separation and recovery of spent catalysts from the reaction solution and hazardousness of the practical process.4,5 Besides, some of the catalysts are very moisture sensitive, which necessitates specialized reaction equipment and increases operating difficulty. Therefore, the development of novel, nontoxic, low cost, eco-friendly, recyclable solid catalysts with high efficiency can replace highly corrosive, hazardous and polluting acid catalysts and can also meet green chemistry demands, which is a challenge of green chemistry and green engineering.6 Solid acid catalysts have received a great deal of attention because they meet the requirements of eco-friendly and efficient catalysts.7,8 In comparison with liquid acid catalysts, solid acid catalysts are environmentally friendly catalysts in most chemical processes, due to reasons such as non-corrosiveness, safety, less waste
a Hefei National Laboratory for Physical Science at Microscale and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected] b High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt52098f
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production through recycling, and easy separation from the reaction mixtures.9,10 Therefore, more and more chemists and engineers have been paying much attention to the substitution of hazardous Lewis and Brønsted acid catalysts by heterogeneous analogues. As an efficient solid acid catalyst, there is an urgent need for excellent support materials which can possess an extremely high surface area and well-defined nanopores with a narrow pore size distribution.11–14 These mesoporous materials could provide more platforms for containing acid groups and could also be applied to the acid-catalyzed reactions, many of which are currently operated in the presence of liquid acids such as H2SO4, HF, and H3PO4. Moreover, these heterogeneous solid acid catalysts were prepared by incorporating acid sites such as sulfonic acid groups and aluminum cations through direct synthesis, post-grafting or functionalization mainly into silicabased mesoporous silicas such as MCM-41,15,16 SBA-15,17 and organosilica hybrid materials.18 As mentioned, these materials have been widely studied and successfully applied to various acid-catalyzed reactions,19,20 for example, condensation of benzaldehyde with ethylene glycol, Beckmann rearrangement, esterification, and other reactions. Wan et al. designed a sulfonated SBA-15, which showed excellent catalytic activity for catalytic condensation of phenol with acetone to form bisphenol-A.21 Wilson and co-workers synthesized pore-expanded SBA-15 sulfonic acid silicas for biodiesel synthesis.22 However, the method for the preparation of these sulfonic group functionalized solid acid catalysts contained two steps: grafting or co-condensation of –SH from 3-(methacryloyloxy)-propyl
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trimethoxysilane (MPTMS) and subsequent oxidation in the presence of H2O2, which was somewhat complicated and toxic. Zhao and co-workers fabricated a family of mesoporous silica and carbon via an inorganic–organic self-assembly approach.23 After a careful sulfonation, a sulfonic acid group functionalized mesoporous material was prepared. In order to overcome the problems encountered in conventional processes, Hara et al. synthesized a sulfonated activated carbon for the degradation of cellulose in ionic liquids which can dissolve the cellulose to form a homogeneous solution.24 However, difficulties in recovering the catalysts efficiently from the reaction mixture severely limited their wide applications despite their distinct catalytic activity. Aldehydes and ketones are commonly used in carbohydrate synthesis and are frequently protected as acetals in the course of organic synthesis.25 1,3-Dioxolane is one of the most popular protecting groups for this purpose.26 Currently, although synthesis of 1,3-dioxolane generally proceeds with the use of ethylene glycol in the presence of sulfonated silica or carbon solid acid catalysts, these solid acid catalysts do not meet the demands of green chemistry because these catalysts can be dispersed into solvents to form stable dispersions, while the separation of these catalysts from the reaction mixture is very difficult. Therefore, the development of novel, recyclable and magnetic catalysts with high catalytic efficiency has attracted a great deal of research attention in organic synthesis for economic and environmental reasons.27 Catalysts supported on magnetic nanoparticles (MNPs) can be quickly and easily recovered and reused since their paramagnetic property enables easy separation of the catalysts from the reaction mixture using an external magnet. In addition, the surface of MNPs can be modified to form a protective shell which contains carbon, polymer or inorganic materials.28 The shell can not only avoid aggregation and deterioration of the magnetic cores, but also can provide the support with a high surface area which can be functionalized to accommodate a wide variety of organic and inorganic catalysts. For example, Lu and co-workers used a multi-step method to synthesize sulfonic group functionalized magnetically separable spheres consisting of magnetite cores and highly cross-linked poly(styrene-codivinylbenzene) (PSD) protecting shells, and found that the catalysts showed high activity and good reusability in the condensation reaction of benzaldehyde with ethylene glycol.29 However, these materials with low surface areas are nonporous and non-ordered. Fu et al. prepared a magnetically separable solid acid catalyst, which was based on the formation of magnetic nanoparticles (MNPs) and mesoporous silica via the surfactant–template sol–gel method.30 Though the as-obtained catalysts could be easily separated from the reaction mixture and showed high catalytic activity for the hydrolysis of β-1,4glucan and carbohydrate dehydration, the process for the preparation of this catalyst by a multi-step synthesis method is extremely complicated and time-consuming. In addition, the use of toxic solvents and organic ligands in the synthesis of these solid acid catalysts is still a crucial issue. Therefore, a green and facile synthesis method for magnetically separable
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solid acid catalysts with mesoporous surface structure and high catalytic efficiency is desired. Very recently, our group developed a one-pot process to prepare superparamagnetic magnetite nanoparticles coated with a mesoporous carbon layer.31 These core–shell structured magnetic nanoparticles (denoted as Fe3O4@C MNPs) with abundant carboxyl groups on the surface of carbon shells can be well dispersed in several types of solutions such as water, acetone, alcohol and DMF. The Pd-Fe3O4/C catalysts for Suzuki coupling reactions were further prepared by deposition of Pd nanoparticles on the surface of Fe3O4/C MNPs.32 The cores of the core–shell nanoparticles can be easily dissolved in acid solution; moreover, the Fe2+ can also be oxidized to Fe3+ by oxidizing agents. The design of a facile method for the sulfonation of the nanoparticles without any damage to the core–shell structure is worthy of investigation. In this article, we report a route for preparing highly efficient, stable, and magnetically recyclable solid acid catalysts via grafting sulfonic groups on the surface of Fe3O4@C MNPs. The as-prepared sulfonic group functionalized Fe3O4@C magnetic nanoparticles (denoted as Fe3O4@C–SO3H MNPs) show a high efficiency in acetalization reaction of benzaldehyde with ethylene glycol. More importantly, it can be easily recycled and used repetitively 8 times without a distinct loss of catalytic efficiency.
2. Materials and methods 2.1
Materials
Ferrocene (Fe(C5H5)2, ≥98%), hydrogen peroxide (H2O2, 30%), acetone (C3H6O, 99%), fuming sulfonation acid (50 wt% SO3/ H2SO4), benzaldehyde (PhCHO, ≥98.5%), and ethylene glycol (EG, ≥99%) were of analytical grade and obtained from the Shanghai Chemical Factory, China. All chemicals were used as received without further purification.
2.2
Synthesis of magnetic nanoparticles
The Fe3O4@C MNPs were prepared according to a method developed previously by our group.31 In a typical synthesis, 0.30 g of ferrocene was dissolved in acetone (30 ml) with intense sonication for 30 min. And then 1.50 ml of hydrogen peroxide was slowly added into the above mixture solution, which was then vigorously stirred for 30 min with magnetic stirring. The obtained yellow solution was then transferred and sealed into a Teflon-lined stainless autoclave with a total volume of 50.0 ml. The autoclave was heated at 240 °C for 72 h, and then allowed to cool naturally to room temperature. After intense sonication for 15 min, the product from the Teflon-lined stainless autoclave was magnetized for 10 min using a 0.20 T magnet, and the supernatant was discarded under a magnetic field. The black product was washed with acetone three times to remove excess ferrocene. Finally, the precipitate was dried at room temperature for 6 h in a vacuum oven.
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2.3
Synthesis of the Fe3O4@C–SO3H MNPs
The sulfonation of Fe3O4@C MNPs was completed by the method of vapor-phase sulfonation as described previously.33 The sulfonation was carried out in a Teflon-lined autoclave where the powder of Fe3O4@C MNPs was contacted with the vapor from 5 ml 50% SO3/H2SO4 at 333 K for 48 h. The sulfonated sample was washed with hot distilled water (>353 K) to remove any physically adsorbed species until the sulfate ions were not detected in the upper layer solution. After separation using an external magnet (0.2 T), the sample was dried at 373 K overnight in air. 2.4
Solid acid titration
The acid loading of the sulfonic group functionalized Fe3O4@C MNPs was determined by titration.34 The ionexchange reaction for the catalysts was achieved by treating 50 mg of the sample with 10 ml of a saturated NaCl solution for 24 h at room temperature. The catalysts were separated using an external magnet, and the saturated NaCl solution was decanted and saved. The same procedure was repeated to ensure that all the protons have been exchanged completely. Then, two drops of a phenolphthalein solution were added to the saturated NaCl solution. The solution was titrated to neutrality using a 0.01 M NaOH solution to determine the loading of acid sites of Fe3O4@C–SO3H MNPs. 2.5
Characterization
The powder X-ray diffraction (XRD) patterns of all samples were recorded on an X-ray diffractometer (Japan Rigaku D/MAX-γA) equipped with Cu-Kα radiation (λ = 1.54178 Å) over the 2θ range of 10–70°. Field emission scanning electron microscopy (FE-SEM) images were collected on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kV. The Fourier transform infrared (FT-IR) spectrum was recorded on a Magna-IR-750 spectrometer using the KBr pellet technique in the range of 400–4000 cm−1. The sulfur content of the Fe3O4@C–SO3H MNPs was measured by elemental analysis on a CHN/S Analyzer (Vario EL III) with combustion and reduction temperatures of 1150 °C and 850 °C, respectively. Acid–base titrations were done with 0.01 M aqueous solution of NaOH as the titration base. The distribution of the elements was characterized using scanning transmission electron microscopy (STEM, JEM 2100F) with energy dispersive X-ray (EDX) spectroscopy. The specific surface area was evaluated at 77 K (Micromeritics ASAP 2020) using the Brunauer–Emmett–Teller (BET) method, while the pore volume and pore size were calculated according to the Barrett–Joyner–Halenda (BJH) formula applied to the adsorption branch. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer equipped with an MgKα excitation source (1253.6 eV).
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2.6
Catalytic test
In order to investigate the catalytic performance of the Fe3O4@C–SO3H MNPs, the acetalization reaction of benzaldehyde with ethylene glycol was chosen as a model reaction. Briefly, 0.05 g of Fe3O4@C–SO3H MNPs was added to a mixture containing benzaldehyde (15 mmol), ethylene glycol (45 mmol) and 5 g cyclohexane. The reactions were conducted in a 50 ml three-necked flask with a reflux device under vigorous stirring at 363 K under an N2 atmosphere. On the completion of the initial reaction, the catalysts collected using an external magnet (0.2 T) were washed with ethanol three times to remove any adsorbed residues, dried at 323 K and then tested under the same catalytic reaction conditions repeatedly. The products were analyzed using a gas chromatograph mass spectrometer (GC-MS), and the reaction conversion was monitored by gas chromatography (GC) analysis using toluene as an internal standard.
3. Results and discussion The synthetic procedure for the Fe3O4@C–SO3H solid acid catalysts is schematically illustrated in Scheme 1. Firstly, the Fe3O4@C core–shell structure was prepared with a facile onepot method. The as-synthesized Fe3O4@C MNPs were stable in liquid phase and could be easily separated using an external magnet (0.2 T), which can be used as a carrier for the preparation of magnetically separable catalysts. Moreover, the Fe3O4@C–SO3H MNPs were synthesized with a gas–solid reaction by contacting Fe3O4@C MNPs with the vapor from 50 wt% SO3/H2SO4 in a Teflon-lined autoclave. It was interestingly found that the mesoporous carbon layer on the surface of Fe3O4 cores can not only stabilize the Fe3O4 MNPs against aggregation and prevent oxidation of Fe3O4 MNPs, but also can be coordinated or grafted with –SO3H groups as a Brønsted acid for many practical applications in the chemical industry.35,36 Consequently, as-synthesized Fe3O4@C–SO3H MNPs were used as solid acid catalysts in the acetal formation of benzaldehyde and ethylene glycol with high efficiency under mild conditions. More importantly, the catalysts can be simply recycled and used repetitively 8 times without a distinct loss of catalytic efficiency.
Scheme 1 The synthesis process of the Fe3O4@C–SO3H solid acid catalysts and the recyclable catalytic process for the acetalization reaction of benzaldehyde with ethylene glycol.
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Fig. 1 X-ray diffraction patterns of (a) Fe3O4@C MNPs, (b) Fe3O4@C– SO3H MNPs, and (c) Fe3O4@C–SO3H MNPs after catalysis.
Fig. 1 shows representative X-ray powder diffraction (XRD) patterns of the samples. As shown in Fig. 1(a), characteristic diffraction peaks were observed at 30.3°, 35.4°, 43.1°, 53.4°, 56.9° and 62.5°, which were assigned to the (220), (311), (400), (422), (511), and (440) lattice planes of Fe3O4 (JCPDS file 19-0629, magnetite), respectively. After a series of treatments including functionalization of Fe3O4@C MNPs with –SO3H groups and catalysis of acetalization reactions, the magnetite nanoparticles were stable, as revealed by the XRD patterns in Fig. 1(b) and (c), respectively. And these results also indicate that the sulfonated process can avoid damage of Fe3O4@C MNPs, and Fe3O4@C–SO3H MNPs can be catalytically used in acetalization reactions with high stability. The morphologies of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol were also investigated by scanning electron microscopy (SEM), respectively. As shown in Fig. 2, the Fe3O4@C MNPs consisted of round-shaped particles with diameters of about 190 nm (Fig. 2a), while the Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol kept the same morphologies as those of the Fe3O4@C MNPs (Fig. 2c and d), indicating that the Fe3O4@C MNPs were not damaged in the process of sulfonation and catalysis. Transmission electron microscopy (TEM) was also used to observe the morphologies of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol, as shown in Fig. 2. The results further confirmed that they possessed the same morphology, which was consistent with the results observed by SEM. The structure of Fe3O4@C MNPs was further characterized by high resolution transmission electron microscopy (HRTEM) (ESI Fig. S1†), which reveals clearly a well-defined core–shell structure with a shell thickness of about 25 nm. The SEM and TEM images further confirmed that the Fe3O4@C MNPs were so stable that the sulfonation and catalytic process could not destroy the morphologies and structures of Fe3O4@C MNPs. These factors enabled the Fe3O4@C–SO3H MNPs to meet the demands for
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Fig. 2 SEM and TEM images of (a, b) Fe3O4@C MNPs, (c, d) Fe3O4@C– SO3H MNPs, and (e, f ) Fe3O4@C–SO3H MNPs after catalysis.
Fig. 3 FTIR spectra of (a) Fe3O4@C MNPs, (b) Fe3O4@C–SO3H MNPs, and (c) Fe3O4@C–SO3H MNPs after catalysis.
magnetically separable, highly efficient and stable solid acid catalysts. The FTIR spectra of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol are shown in Fig. 3. In comparison with the parent samples, the distinguishing features of the Fe3O4@C–SO3H MNPs are the presence of new absorption bands at 1070 cm−1 and 1040 cm−1 in the spectra, which are assigned to the typical symmetric stretching of the SvO bond.37 The SvO peak intensity is relatively weak in the IR spectrogram, simply because the S content (0.627 wt%) is relatively low in the total particle. The strong peak at 580 cm−1 indicates the existence of magnetite in the products, which further proves the results obtained by XRD. To avoid the influence of iron oxide cores, they were removed from Fe3O4@C–SO3H MNPs using a dilute hydrochloric acid solution. The FTIR spectrum of the residual carbon shell (ESI Fig. S2†) clearly
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Fig. 4 (a) Dark-field STEM image of Fe3O4@C–SO3H MNPs. (b–e) Elemental mapping of the same nanoparticles, indicating the spatial distribution of C (b), O (c), Fe (d) and S (e) respectively.
shows peaks at 1070 and 1040 cm−1 assigned to the SO3H groups, which further confirms successful introduction of sulfonic acid groups. For Fe3O4@C MNPs, the peak at 1604 cm−1 is assigned to CvC bond stretching, which indicates that the carbon layer outside the Fe3O4 core is amorphous carbon.38 It was reported that the amorphous carbon layer could provide platforms for grafting the sulfonic groups as Brønsted acid catalysts for acid-catalyzed reactions.39 Moreover, the peaks at 1668 and 3411 cm−1 indicate the existence of hydrophilic carboxyl groups on the surface of Fe3O4@C MNPs, which can also be of benefit for acid-catalyzed reactions. An annular dark-field scanning transmission electron microscopy (STEM) analysis of Fe3O4@C–SO3H MNPs was performed to obtain more detailed information about the structure. As shown in Fig. 4, the dark-field STEM (Fig. 4(a)) clearly demonstrates that the as-synthesized Fe3O4@C–SO3H MNPs have a typical core–shell structure, and EDX element mapping of the same particles (Fig. 4(b–e)) further shows the spatial distributions of C, O, Fe and S in nanoparticles of Fe3O4@C– SO3H MNPs. The diameter of the element C (Fig. 4(b)) is larger than that of elements Fe and O (Fig. 4(c,d)), which confirms that the Fe3O4 nanoparticles are coated with carbon shells. However, the carbon layer is abundant in carboxyl and hydroxyl groups, which does not contribute to the grafting of sulfonic groups onto the surface of the catalyst nanoparticles due to electrostatic repulsion between sulfonic groups and carboxyl/hydroxyl groups. During the sulfonation process, the –SO3H groups would be successfully grafted on the hole surfaces of inner carbon layers. Consequently, the diameter of the element S contour is a bit smaller than that of C. In addition, the surface element composition and chemical state of the sample were further confirmed by XPS results, and the corresponding experimental results are shown in Fig. 5. The S species were present in the +6 oxidation state, corresponding to the –SO3H group with the binding energy around 168.7 eV in the S 2p level.40 This result also indicates that magnetically separable solid acid catalysts can be successfully prepared with a facile method as mentioned in the Experimental section. The features of the carbon shells, especially porosity, determine the properties of core–shell nanoparticles. Fe3O4@C– SO3H MNPs were characterized by Brunauer–Emmett–Teller
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Fig. 5 XPS of Fe3O4@C–SO3H MNPs and S 2p spectra (inset) over the Fe3O4@C–SO3H MNPs.
Fig. 6 N2 adsorption–desorption isotherms and pore size distribution of (a) Fe3O4@C MNP (●○) and (b) Fe3O4@C–SO3H MNP samples (▲△).
(BET) analysis. As shown in Fig. 6, it shows the corresponding N2 adsorption–desorption isotherms and the Barrett–Joyner– Halenda (BJH) pore size distributions of the core–shell nanoparticles. These samples exhibit typical IV type isotherms with a sharp capillary condensation step in the relative pressure range of 0.5–0.75, indicating the presence of mesoporosity. The BET surface area, pore volume, and average pore size of Fe3O4@C MNPs were calculated to be 89.31 m2 g−1, 0.10 m3 g−1, and 4.53 nm, respectively. After the sulfonation, the obtained Fe3O4@C–SO3H sample shows N2 adsorption– desorption isotherms similar to those of the sample Fe3O4@C, confirming that the sulfonation treatment has no substantial effect on the Fe3O4@C MNPs. However, the BET surface area, pore volume, and average pore size of Fe3O4@C–SO3H MNPs were calculated to be 70.36 m2 g−1, 0.08 m3 g−1, and 4.70 nm, respectively. The BET surface area and pore volume decrease after the sulfonation treatment, which may be attributed to the presence of a large number of –SO3H groups. The above results suggest that the porous structure and large surface area
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could be responsible for grafting –SO3H groups and for making reactants come into contact with acid sites with high efficiency. In addition, the N2 adsorption–desorption hysteresis loop was not closed, which is possibly attributed to the aggregation of these nanoparticles due to condensation of SO3H groups during drying.41 Element analysis shows that the sulfur content of the Fe3O4@C–SO3H MNPs is about 0.627 wt%, which corresponds to a sulfuric acid concentration of 0.2 mmol g−1. Moreover, it is assumed that sulfur only takes the form of –SO3H in the sulfonated sample of the Fe3O4@C–SO3H MNPs. However, the number of H+ determined by our acid–base titration was 0.25 mmol g−1. This value is slightly higher than that obtained from elemental analysis, which is probably due to the presence of a small amount of carboxyl groups in the sulfonated samples. Therefore, the amount of H+ contributed by carboxyl groups was 0.05 mmol g−1. The findings further verified that the –SO3H groups were grafted on the surface of the Fe3O4@C MNPs and they formed a considerable amount of Brønsted acid sites. This observed phenomenon is consistent with that of FTIR, and these carboxyl groups could also be of benefit to acid-catalyzed reactions. The concentration of the sulfonic acid sites (0.2 mmol g−1) is lower than that of the related sulfonated hybrid materials reported previously, which can be attributed to the Fe3O4 cores contributing the main mass to our prepared Fe3O4@C–SO3H MNPs. Our group previously modified this method for the Fe3O4@C MNPs to prepare the hollow structure Fe3O4@C MNPs, and the weight percentage of Fe3O4 was about 79%.42 If the weight of the Fe3O4 cores was removed from the Fe3O4@C–SO3H MNPs, the concentration of the sulfonic acid sites would be increased to 1 mmol g−1. In the syntheses of the fine chemical and pharmaceutical industries, acetals are generally applied in carbohydrate synthesis and organic reactions for carbonyl protection.43 In the general industrial process, sulfonic acid or fuming sulfonic acid are used as homogeneous catalysts for the acetalization reaction of benzaldehyde with ethylene glycol under severe conditions. Therefore, these homogeneous catalysts used in the practical process can cause problems such as corrosion of the reactor, difficulty of product separation and environmental pollution. To solve these problems, the solid phase catalytic system as a substitute would be more promising. The Fe3O4@C–SO3H MNPs obtained by our group can not only be effectively and stably used as Brønsted acid catalysts for acid-catalyzed reactions, but also can be easily separated from the reaction mixture using an external magnet (0.2 T). In addition, it was reported that the sulfonated mesoporous materials can serve as nanoreactors for catalytic reactions.44 Moreover, as the N2 adsorption–desorption isotherms showed that the carbon layers covered on Fe3O4@C MNPs contain lots of mesopores, sulfonated Fe3O4@C–SO3H MNPs can also serve as a novel nanoreactor. In this study, to evaluate the catalytic ability of Fe3O4@C–SO3H MNPs, the acetalization reaction was carried out as a model reaction. The catalytic activity of these novel functional mesoporous Fe3O4@C–SO3H MNPs was investigated by the acetalization reaction (Scheme 1) of
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Fig. 7 The catalytic conversion of benzaldehyde as a function of reaction time during the acetalization reaction of benzaldehyde with ethylene glycol using (a) Fe3O4@C MNPs and (b) Fe3O4@C–SO3H MNPs, respectively.
benzaldehyde with ethylene glycol in cyclohexane under mild conditions. From Fig. 7(b), it can be seen that the conversion of benzaldehyde quickly reaches 88.3% after reaction for 2 h (TOF = 662.3 h−1), indicating an excellent catalytic performance due to the high concentration of sulfonic groups of the Fe3O4@C–SO3H MNPs which can provide enough acid sites for this acid-catalyzed reaction. The selectivity of the catalyst is 100% for benzaldehyde acetalization reactions under mild conditions. As a result, the yield of product for the reaction is 88.3%. For comparison purposes, Fe3O4@C MNPs that contain a small number of carboxyl groups were also used as the heterogeneous acid catalysts for the acid-catalyzed reaction. The result shows that Fe3O4@C MNPs without loading of sulfonic groups can also catalyze the acetalization reaction of benzaldehyde with ethylene glycol, but the activity of Fe3O4@C MNPs is much lower (44%) than that of Fe3O4@C–SO3H MNPs for this catalytic reaction, as shown in Fig. 7(a). The catalytic activity of the Fe3O4@C MNPs can be attributed to carboxyl groups from the amorphous carbon layer which can provide acid sites for the acid-catalyzed reactions. As the Fe3O4@C–SO3H MNPs exhibited much higher catalytic activity than the Fe3O4@C MNPs, it is believed that the anchored sulfonic acid groups are the main catalytic active sites of the Fe3O4@C–SO3H MNPs. Therefore, it is suggested that Fe3O4@C–SO3H MNPs with high activity for the conversion of benzaldehyde with ethylene glycol can also be used as magnetically separable solid acid catalysts for other acid-catalyzed reactions. Reusability and isolation of the catalysts are important factors for any practical application.45 The magnetic properties of Fe3O4@C–SO3H MNPs were measured at room temperature (300 K) in an applied magnetic field up to 20 000 Oe. As shown in Fig. 9, the saturated magnetization values of Fe3O4@C– SO3H MNPs before and after catalysis are 53.6 and 43.7 emu g−1, respectively. The saturated magnetization dropped, possibly due to the partial oxidation of the Fe3O4 cores in the drying
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Fig. 8 The reusability of Fe3O4@C–SO3H MNPs in the acetalization reaction of benzaldehyde with ethylene glycol.
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sulfonic acid concentration for Fe3O4@C–SO3H MNPs after the catalysis, the CHN/S analyzer was used to detect the content of the sulfur element in the samples, and the result showed that the sulfur content of Fe3O4@C–SO3H MNPs was about 0.55 wt%, which corresponds to a sulfuric acid concentration of 0.17 mmol g−1. And the volatility of the conversion of benzaldehyde with ethylene glycol after different running times can be attributed to the level of cleaning after washing in different recycling processes. It is well known that magnetically separable solid acid catalysts are superior to the traditional solid acid catalysts which are separated from the reaction mixtures with complicated procedures, such as centrifugation and filtration. For example, though sulfonated silica or carbon solid acid catalysts with the conversion rate of about 90% for this acidcatalyzed reaction were reported by other groups, the preparation of these solid acid catalysts and the separation of the catalysts from the reaction mixtures are very complicated and time-consuming.23,32 In addition, the outstanding catalytic performance of the Fe3O4@C–SO3H MNPs is attributed to their unique porous carbon structure which can provide more platforms to coordinate or accommodate sulfonic groups efficiently, and the Fe3O4 cores can be of benefit for the easy separation of catalysts from the reaction mixtures and can enable the catalysts to meet the demand of environmental protection.
4.
Fig. 9 Field-dependent magnetization of Fe3O4@C–SO3H MNPs before and after catalysis. The inset shows a photograph of Fe3O4@C–SO3H MNPs: (a) dispersed in the reaction mixture and (b) magnetic separation of the nanoparticles from the reaction medium.
process in air and residual water molecules in the pores of carbon shells, which can be avoided by drying the sample in a vacuum. In addition, the Fe3O4@C–SO3H MNPs after the acidcatalyzed reaction can also be easily collected using an external magnet (0.2 T) as shown in Fig. 9. In order to test the reusability of the catalysts, the catalysts were separated using a permanent magnet at the end of the reaction and reused after thoroughly washing with ethanol and drying in air at 373 K overnight. In order to investigate the stability of the catalyst in the catalytic process, the amount of leached Fe in a filtrate was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the mass ratio of leached Fe to the used catalyst is about 0.1%, which suggests that the catalyst is stable in the catalytic reaction. As shown in Fig. 8, the conversion rate of benzaldehyde can be maintained above 70% except for the 5th run even after running 8 times, indicating excellent reusability of the catalysts. In order to measure the
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Conclusions
In summary, we have demonstrated that magnetically separable solid acid catalysts, Fe3O4@C–SO3H MNPs, were prepared by a novel and simple approach, and the sulfonic acid group loading is up to 0.2 mmol g−1. The mesoporous carbon layer covered on the surface of Fe3O4 cores can not only stabilize the Fe3O4 MNPs against aggregation and prevent oxidation of the Fe3O4 MNPs, but also can be coordinated or grafted with –SO3H groups as a Brønsted acid for many practical applications in the chemical industry. The catalysts exhibited excellent activity for the acetalization reaction of benzaldehyde with ethylene glycol with a high conversion rate (88.3%) under mild conditions. Furthermore, solid acid catalysts could be easily separated using an external magnet, and could be used repeatedly 8 times without a distinct loss of catalytic activity. It is believed that these magnetically separable Fe3O4@C–SO3H MNPs could also serve as active, selective and stable catalysts for a number of acid-catalyzed reactions.
Acknowledgements This work was supported by the National Natural Science Foundation (NSFC; 21071137 and U1232211).
Notes and references 1 J. A. Melero, R. Grieken and G. Morales, Chem. Rev., 2006, 106, 3790.
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2 M. Hara, T. Yoshida, A. Takagaki, T. Takata, J. N. Kondo, S. Hayashi and K. Domen, Angew. Chem., Int. Ed., 2004, 43, 2955. 3 M. D. González, Y. Cesteros, J. Llorca and P. Salagre, J. Catal., 2012, 290, 202. 4 M. Toda, A. Takagaki, M. Okamura, J. N. Kondo, S. Hayashi, K. Domen and M. Hara, Nature, 2005, 438, 178. 5 F. X. Zhu, W. Wang and H. X. Li, J. Am. Chem. Soc., 2011, 133, 11632. 6 S. Shylesh, J. Schweizer, S. Demeshko, V. Schunemann, S. Ernst and W. R. Thiela, Adv. Synth. Catal., 2009, 351, 1789. 7 K. Kandel, S. M. Althaus, C. Peeraphatdit, T. Kobayashi, B. G. Trewyn, M. Pruski and I. I. Slowing, J. Catal., 2012, 291, 63. 8 H. I. Ryoo, L. Y. Hong, S. H. Jung and D. P. Kim, J. Mater. Chem., 2010, 20, 6419. 9 F. J. Liu, X. J. Meng, Y. L. Zhang, L. M. Ren, F. Nawaz and F. S. Xiao, J. Catal., 2010, 271, 52. 10 vander M. Werner, Environ. Sci. Technol., 2010, 44, 1806. 11 Y. Ide, G. Ozaki and M. Ogawa, Langmuir, 2009, 25, 5276. 12 K. Nakajima, I. Tomita, M. Hara, S. Hayashi, K. Domen and J. N. Kondo, Adv. Mater., 2005, 17, 1839. 13 K. Mertins, I. Iovel, J. Kischel, A. Zapf and M. Beller, Angew. Chem., Int. Ed., 2005, 44, 238. 14 B. M. Choudary, R. S. Mulukutla and K. J. Klabunde, J. Am. Chem. Soc., 2003, 125, 2020. 15 R. Marschall, I. Bannat, A. Feldhoff, L. Wang, G. Q. Lu and M. Wark, Small, 2009, 5, 854. 16 A. Stein, B. J. Melde and R. C. Schroden, Adv. Mater., 2000, 12, 1403. 17 J. A. Melero, R. V. Grieken, G. Morales and V. Nuño, Catal. Commun., 2004, 5, 131. 18 Q. Yang, J. Liu, J. Yang, M. P. Kapoor, S. Inagaki and C. Li, J. Catal., 2004, 228, 265. 19 B. Karimi and M. Vafaeezadeh, Chem. Commun., 2012, 48, 3327. 20 A. Karam, Y. L. Gu, F. Jérôme, J. P. Douliez and J. Barrault, Chem. Commun., 2007, 2222. 21 W. Wang, X. Zhuang, Q. F. Zhao and Y. Wan, J. Mater. Chem., 2012, 22, 15874. 22 J. P. Dacquin, A. F. Lee, C. Pirez and K. Wilson, Chem. Commun., 2012, 48, 212. 23 Q. Yue, M. H. Wang, J. Wei, Y. H. Deng, T. Y. Liu, R. C. Che, B. Tu and D. Y. Zhao, Angew. Chem., Int. Ed., 2012, 51, 10368. 24 S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2008, 130, 12787.
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Synthesis of sulfonic acid-functionalized Fe3O4@C nanoparticles as magnetically recyclable solid acid catalysts for acetalization reaction† Fang-Cai Zheng,a Qian-Wang Chen,*a,b Lin Hu,b Nan Yana and Xiang-Kai Konga The Fe3O4@C core–shell magnetic nanoparticles with an average size of about 190 nm were synthesized via a one-pot solvothermal process using ferrocene as a single reactant. The sulfonic acid-functionalized Fe3O4@C magnetic nanoparticles were obtained by grafting the sulfonic groups on the surface of Fe3O4@C nanoparticles to produce magnetically recyclable solid acid catalysts. The as-prepared products were characterized by X-ray diffraction and transmission electron microscopy. The catalytic performance
Received 2nd August 2013, Accepted 12th October 2013 DOI: 10.1039/c3dt52098f www.rsc.org/dalton
1.
of the as-prepared catalysts was examined through the condensation reaction of benzaldehyde and ethylene glycol. The results showed that the catalysts exhibited high catalytic activity with a conversion rate of 88.3% under mild conditions. Furthermore, catalysts with a magnetization saturation of 53.5 emu g−1 at room temperature were easily separated from the reaction mixture by using a 0.2 T permanent magnet and were reused 8 times without any significant decrease in catalytic activity.
Introduction
Homogeneous acid catalysts such as Lewis and Brønsted acids, a very important class of catalysts, are commonly used in the large-scale synthesis of industrial bulk chemicals as well as in the production of fine chemicals.1–3 However, major drawbacks of these catalysts include the separation and recovery of spent catalysts from the reaction solution and hazardousness of the practical process.4,5 Besides, some of the catalysts are very moisture sensitive, which necessitates specialized reaction equipment and increases operating difficulty. Therefore, the development of novel, nontoxic, low cost, eco-friendly, recyclable solid catalysts with high efficiency can replace highly corrosive, hazardous and polluting acid catalysts and can also meet green chemistry demands, which is a challenge of green chemistry and green engineering.6 Solid acid catalysts have received a great deal of attention because they meet the requirements of eco-friendly and efficient catalysts.7,8 In comparison with liquid acid catalysts, solid acid catalysts are environmentally friendly catalysts in most chemical processes, due to reasons such as non-corrosiveness, safety, less waste
a Hefei National Laboratory for Physical Science at Microscale and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected] b High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt52098f
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production through recycling, and easy separation from the reaction mixtures.9,10 Therefore, more and more chemists and engineers have been paying much attention to the substitution of hazardous Lewis and Brønsted acid catalysts by heterogeneous analogues. As an efficient solid acid catalyst, there is an urgent need for excellent support materials which can possess an extremely high surface area and well-defined nanopores with a narrow pore size distribution.11–14 These mesoporous materials could provide more platforms for containing acid groups and could also be applied to the acid-catalyzed reactions, many of which are currently operated in the presence of liquid acids such as H2SO4, HF, and H3PO4. Moreover, these heterogeneous solid acid catalysts were prepared by incorporating acid sites such as sulfonic acid groups and aluminum cations through direct synthesis, post-grafting or functionalization mainly into silicabased mesoporous silicas such as MCM-41,15,16 SBA-15,17 and organosilica hybrid materials.18 As mentioned, these materials have been widely studied and successfully applied to various acid-catalyzed reactions,19,20 for example, condensation of benzaldehyde with ethylene glycol, Beckmann rearrangement, esterification, and other reactions. Wan et al. designed a sulfonated SBA-15, which showed excellent catalytic activity for catalytic condensation of phenol with acetone to form bisphenol-A.21 Wilson and co-workers synthesized pore-expanded SBA-15 sulfonic acid silicas for biodiesel synthesis.22 However, the method for the preparation of these sulfonic group functionalized solid acid catalysts contained two steps: grafting or co-condensation of –SH from 3-(methacryloyloxy)-propyl
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trimethoxysilane (MPTMS) and subsequent oxidation in the presence of H2O2, which was somewhat complicated and toxic. Zhao and co-workers fabricated a family of mesoporous silica and carbon via an inorganic–organic self-assembly approach.23 After a careful sulfonation, a sulfonic acid group functionalized mesoporous material was prepared. In order to overcome the problems encountered in conventional processes, Hara et al. synthesized a sulfonated activated carbon for the degradation of cellulose in ionic liquids which can dissolve the cellulose to form a homogeneous solution.24 However, difficulties in recovering the catalysts efficiently from the reaction mixture severely limited their wide applications despite their distinct catalytic activity. Aldehydes and ketones are commonly used in carbohydrate synthesis and are frequently protected as acetals in the course of organic synthesis.25 1,3-Dioxolane is one of the most popular protecting groups for this purpose.26 Currently, although synthesis of 1,3-dioxolane generally proceeds with the use of ethylene glycol in the presence of sulfonated silica or carbon solid acid catalysts, these solid acid catalysts do not meet the demands of green chemistry because these catalysts can be dispersed into solvents to form stable dispersions, while the separation of these catalysts from the reaction mixture is very difficult. Therefore, the development of novel, recyclable and magnetic catalysts with high catalytic efficiency has attracted a great deal of research attention in organic synthesis for economic and environmental reasons.27 Catalysts supported on magnetic nanoparticles (MNPs) can be quickly and easily recovered and reused since their paramagnetic property enables easy separation of the catalysts from the reaction mixture using an external magnet. In addition, the surface of MNPs can be modified to form a protective shell which contains carbon, polymer or inorganic materials.28 The shell can not only avoid aggregation and deterioration of the magnetic cores, but also can provide the support with a high surface area which can be functionalized to accommodate a wide variety of organic and inorganic catalysts. For example, Lu and co-workers used a multi-step method to synthesize sulfonic group functionalized magnetically separable spheres consisting of magnetite cores and highly cross-linked poly(styrene-codivinylbenzene) (PSD) protecting shells, and found that the catalysts showed high activity and good reusability in the condensation reaction of benzaldehyde with ethylene glycol.29 However, these materials with low surface areas are nonporous and non-ordered. Fu et al. prepared a magnetically separable solid acid catalyst, which was based on the formation of magnetic nanoparticles (MNPs) and mesoporous silica via the surfactant–template sol–gel method.30 Though the as-obtained catalysts could be easily separated from the reaction mixture and showed high catalytic activity for the hydrolysis of β-1,4glucan and carbohydrate dehydration, the process for the preparation of this catalyst by a multi-step synthesis method is extremely complicated and time-consuming. In addition, the use of toxic solvents and organic ligands in the synthesis of these solid acid catalysts is still a crucial issue. Therefore, a green and facile synthesis method for magnetically separable
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solid acid catalysts with mesoporous surface structure and high catalytic efficiency is desired. Very recently, our group developed a one-pot process to prepare superparamagnetic magnetite nanoparticles coated with a mesoporous carbon layer.31 These core–shell structured magnetic nanoparticles (denoted as Fe3O4@C MNPs) with abundant carboxyl groups on the surface of carbon shells can be well dispersed in several types of solutions such as water, acetone, alcohol and DMF. The Pd-Fe3O4/C catalysts for Suzuki coupling reactions were further prepared by deposition of Pd nanoparticles on the surface of Fe3O4/C MNPs.32 The cores of the core–shell nanoparticles can be easily dissolved in acid solution; moreover, the Fe2+ can also be oxidized to Fe3+ by oxidizing agents. The design of a facile method for the sulfonation of the nanoparticles without any damage to the core–shell structure is worthy of investigation. In this article, we report a route for preparing highly efficient, stable, and magnetically recyclable solid acid catalysts via grafting sulfonic groups on the surface of Fe3O4@C MNPs. The as-prepared sulfonic group functionalized Fe3O4@C magnetic nanoparticles (denoted as Fe3O4@C–SO3H MNPs) show a high efficiency in acetalization reaction of benzaldehyde with ethylene glycol. More importantly, it can be easily recycled and used repetitively 8 times without a distinct loss of catalytic efficiency.
2. Materials and methods 2.1
Materials
Ferrocene (Fe(C5H5)2, ≥98%), hydrogen peroxide (H2O2, 30%), acetone (C3H6O, 99%), fuming sulfonation acid (50 wt% SO3/ H2SO4), benzaldehyde (PhCHO, ≥98.5%), and ethylene glycol (EG, ≥99%) were of analytical grade and obtained from the Shanghai Chemical Factory, China. All chemicals were used as received without further purification.
2.2
Synthesis of magnetic nanoparticles
The Fe3O4@C MNPs were prepared according to a method developed previously by our group.31 In a typical synthesis, 0.30 g of ferrocene was dissolved in acetone (30 ml) with intense sonication for 30 min. And then 1.50 ml of hydrogen peroxide was slowly added into the above mixture solution, which was then vigorously stirred for 30 min with magnetic stirring. The obtained yellow solution was then transferred and sealed into a Teflon-lined stainless autoclave with a total volume of 50.0 ml. The autoclave was heated at 240 °C for 72 h, and then allowed to cool naturally to room temperature. After intense sonication for 15 min, the product from the Teflon-lined stainless autoclave was magnetized for 10 min using a 0.20 T magnet, and the supernatant was discarded under a magnetic field. The black product was washed with acetone three times to remove excess ferrocene. Finally, the precipitate was dried at room temperature for 6 h in a vacuum oven.
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2.3
Synthesis of the Fe3O4@C–SO3H MNPs
The sulfonation of Fe3O4@C MNPs was completed by the method of vapor-phase sulfonation as described previously.33 The sulfonation was carried out in a Teflon-lined autoclave where the powder of Fe3O4@C MNPs was contacted with the vapor from 5 ml 50% SO3/H2SO4 at 333 K for 48 h. The sulfonated sample was washed with hot distilled water (>353 K) to remove any physically adsorbed species until the sulfate ions were not detected in the upper layer solution. After separation using an external magnet (0.2 T), the sample was dried at 373 K overnight in air. 2.4
Solid acid titration
The acid loading of the sulfonic group functionalized Fe3O4@C MNPs was determined by titration.34 The ionexchange reaction for the catalysts was achieved by treating 50 mg of the sample with 10 ml of a saturated NaCl solution for 24 h at room temperature. The catalysts were separated using an external magnet, and the saturated NaCl solution was decanted and saved. The same procedure was repeated to ensure that all the protons have been exchanged completely. Then, two drops of a phenolphthalein solution were added to the saturated NaCl solution. The solution was titrated to neutrality using a 0.01 M NaOH solution to determine the loading of acid sites of Fe3O4@C–SO3H MNPs. 2.5
Characterization
The powder X-ray diffraction (XRD) patterns of all samples were recorded on an X-ray diffractometer (Japan Rigaku D/MAX-γA) equipped with Cu-Kα radiation (λ = 1.54178 Å) over the 2θ range of 10–70°. Field emission scanning electron microscopy (FE-SEM) images were collected on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a Hitachi H-800 transmission electron microscope using an accelerating voltage of 200 kV. The Fourier transform infrared (FT-IR) spectrum was recorded on a Magna-IR-750 spectrometer using the KBr pellet technique in the range of 400–4000 cm−1. The sulfur content of the Fe3O4@C–SO3H MNPs was measured by elemental analysis on a CHN/S Analyzer (Vario EL III) with combustion and reduction temperatures of 1150 °C and 850 °C, respectively. Acid–base titrations were done with 0.01 M aqueous solution of NaOH as the titration base. The distribution of the elements was characterized using scanning transmission electron microscopy (STEM, JEM 2100F) with energy dispersive X-ray (EDX) spectroscopy. The specific surface area was evaluated at 77 K (Micromeritics ASAP 2020) using the Brunauer–Emmett–Teller (BET) method, while the pore volume and pore size were calculated according to the Barrett–Joyner–Halenda (BJH) formula applied to the adsorption branch. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer equipped with an MgKα excitation source (1253.6 eV).
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2.6
Catalytic test
In order to investigate the catalytic performance of the Fe3O4@C–SO3H MNPs, the acetalization reaction of benzaldehyde with ethylene glycol was chosen as a model reaction. Briefly, 0.05 g of Fe3O4@C–SO3H MNPs was added to a mixture containing benzaldehyde (15 mmol), ethylene glycol (45 mmol) and 5 g cyclohexane. The reactions were conducted in a 50 ml three-necked flask with a reflux device under vigorous stirring at 363 K under an N2 atmosphere. On the completion of the initial reaction, the catalysts collected using an external magnet (0.2 T) were washed with ethanol three times to remove any adsorbed residues, dried at 323 K and then tested under the same catalytic reaction conditions repeatedly. The products were analyzed using a gas chromatograph mass spectrometer (GC-MS), and the reaction conversion was monitored by gas chromatography (GC) analysis using toluene as an internal standard.
3. Results and discussion The synthetic procedure for the Fe3O4@C–SO3H solid acid catalysts is schematically illustrated in Scheme 1. Firstly, the Fe3O4@C core–shell structure was prepared with a facile onepot method. The as-synthesized Fe3O4@C MNPs were stable in liquid phase and could be easily separated using an external magnet (0.2 T), which can be used as a carrier for the preparation of magnetically separable catalysts. Moreover, the Fe3O4@C–SO3H MNPs were synthesized with a gas–solid reaction by contacting Fe3O4@C MNPs with the vapor from 50 wt% SO3/H2SO4 in a Teflon-lined autoclave. It was interestingly found that the mesoporous carbon layer on the surface of Fe3O4 cores can not only stabilize the Fe3O4 MNPs against aggregation and prevent oxidation of Fe3O4 MNPs, but also can be coordinated or grafted with –SO3H groups as a Brønsted acid for many practical applications in the chemical industry.35,36 Consequently, as-synthesized Fe3O4@C–SO3H MNPs were used as solid acid catalysts in the acetal formation of benzaldehyde and ethylene glycol with high efficiency under mild conditions. More importantly, the catalysts can be simply recycled and used repetitively 8 times without a distinct loss of catalytic efficiency.
Scheme 1 The synthesis process of the Fe3O4@C–SO3H solid acid catalysts and the recyclable catalytic process for the acetalization reaction of benzaldehyde with ethylene glycol.
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Fig. 1 X-ray diffraction patterns of (a) Fe3O4@C MNPs, (b) Fe3O4@C– SO3H MNPs, and (c) Fe3O4@C–SO3H MNPs after catalysis.
Fig. 1 shows representative X-ray powder diffraction (XRD) patterns of the samples. As shown in Fig. 1(a), characteristic diffraction peaks were observed at 30.3°, 35.4°, 43.1°, 53.4°, 56.9° and 62.5°, which were assigned to the (220), (311), (400), (422), (511), and (440) lattice planes of Fe3O4 (JCPDS file 19-0629, magnetite), respectively. After a series of treatments including functionalization of Fe3O4@C MNPs with –SO3H groups and catalysis of acetalization reactions, the magnetite nanoparticles were stable, as revealed by the XRD patterns in Fig. 1(b) and (c), respectively. And these results also indicate that the sulfonated process can avoid damage of Fe3O4@C MNPs, and Fe3O4@C–SO3H MNPs can be catalytically used in acetalization reactions with high stability. The morphologies of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol were also investigated by scanning electron microscopy (SEM), respectively. As shown in Fig. 2, the Fe3O4@C MNPs consisted of round-shaped particles with diameters of about 190 nm (Fig. 2a), while the Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol kept the same morphologies as those of the Fe3O4@C MNPs (Fig. 2c and d), indicating that the Fe3O4@C MNPs were not damaged in the process of sulfonation and catalysis. Transmission electron microscopy (TEM) was also used to observe the morphologies of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol, as shown in Fig. 2. The results further confirmed that they possessed the same morphology, which was consistent with the results observed by SEM. The structure of Fe3O4@C MNPs was further characterized by high resolution transmission electron microscopy (HRTEM) (ESI Fig. S1†), which reveals clearly a well-defined core–shell structure with a shell thickness of about 25 nm. The SEM and TEM images further confirmed that the Fe3O4@C MNPs were so stable that the sulfonation and catalytic process could not destroy the morphologies and structures of Fe3O4@C MNPs. These factors enabled the Fe3O4@C–SO3H MNPs to meet the demands for
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Fig. 2 SEM and TEM images of (a, b) Fe3O4@C MNPs, (c, d) Fe3O4@C– SO3H MNPs, and (e, f ) Fe3O4@C–SO3H MNPs after catalysis.
Fig. 3 FTIR spectra of (a) Fe3O4@C MNPs, (b) Fe3O4@C–SO3H MNPs, and (c) Fe3O4@C–SO3H MNPs after catalysis.
magnetically separable, highly efficient and stable solid acid catalysts. The FTIR spectra of Fe3O4@C MNPs and Fe3O4@C–SO3H MNPs before and after the acetalization reaction of benzaldehyde with ethylene glycol are shown in Fig. 3. In comparison with the parent samples, the distinguishing features of the Fe3O4@C–SO3H MNPs are the presence of new absorption bands at 1070 cm−1 and 1040 cm−1 in the spectra, which are assigned to the typical symmetric stretching of the SvO bond.37 The SvO peak intensity is relatively weak in the IR spectrogram, simply because the S content (0.627 wt%) is relatively low in the total particle. The strong peak at 580 cm−1 indicates the existence of magnetite in the products, which further proves the results obtained by XRD. To avoid the influence of iron oxide cores, they were removed from Fe3O4@C–SO3H MNPs using a dilute hydrochloric acid solution. The FTIR spectrum of the residual carbon shell (ESI Fig. S2†) clearly
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Fig. 4 (a) Dark-field STEM image of Fe3O4@C–SO3H MNPs. (b–e) Elemental mapping of the same nanoparticles, indicating the spatial distribution of C (b), O (c), Fe (d) and S (e) respectively.
shows peaks at 1070 and 1040 cm−1 assigned to the SO3H groups, which further confirms successful introduction of sulfonic acid groups. For Fe3O4@C MNPs, the peak at 1604 cm−1 is assigned to CvC bond stretching, which indicates that the carbon layer outside the Fe3O4 core is amorphous carbon.38 It was reported that the amorphous carbon layer could provide platforms for grafting the sulfonic groups as Brønsted acid catalysts for acid-catalyzed reactions.39 Moreover, the peaks at 1668 and 3411 cm−1 indicate the existence of hydrophilic carboxyl groups on the surface of Fe3O4@C MNPs, which can also be of benefit for acid-catalyzed reactions. An annular dark-field scanning transmission electron microscopy (STEM) analysis of Fe3O4@C–SO3H MNPs was performed to obtain more detailed information about the structure. As shown in Fig. 4, the dark-field STEM (Fig. 4(a)) clearly demonstrates that the as-synthesized Fe3O4@C–SO3H MNPs have a typical core–shell structure, and EDX element mapping of the same particles (Fig. 4(b–e)) further shows the spatial distributions of C, O, Fe and S in nanoparticles of Fe3O4@C– SO3H MNPs. The diameter of the element C (Fig. 4(b)) is larger than that of elements Fe and O (Fig. 4(c,d)), which confirms that the Fe3O4 nanoparticles are coated with carbon shells. However, the carbon layer is abundant in carboxyl and hydroxyl groups, which does not contribute to the grafting of sulfonic groups onto the surface of the catalyst nanoparticles due to electrostatic repulsion between sulfonic groups and carboxyl/hydroxyl groups. During the sulfonation process, the –SO3H groups would be successfully grafted on the hole surfaces of inner carbon layers. Consequently, the diameter of the element S contour is a bit smaller than that of C. In addition, the surface element composition and chemical state of the sample were further confirmed by XPS results, and the corresponding experimental results are shown in Fig. 5. The S species were present in the +6 oxidation state, corresponding to the –SO3H group with the binding energy around 168.7 eV in the S 2p level.40 This result also indicates that magnetically separable solid acid catalysts can be successfully prepared with a facile method as mentioned in the Experimental section. The features of the carbon shells, especially porosity, determine the properties of core–shell nanoparticles. Fe3O4@C– SO3H MNPs were characterized by Brunauer–Emmett–Teller
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Fig. 5 XPS of Fe3O4@C–SO3H MNPs and S 2p spectra (inset) over the Fe3O4@C–SO3H MNPs.
Fig. 6 N2 adsorption–desorption isotherms and pore size distribution of (a) Fe3O4@C MNP (●○) and (b) Fe3O4@C–SO3H MNP samples (▲△).
(BET) analysis. As shown in Fig. 6, it shows the corresponding N2 adsorption–desorption isotherms and the Barrett–Joyner– Halenda (BJH) pore size distributions of the core–shell nanoparticles. These samples exhibit typical IV type isotherms with a sharp capillary condensation step in the relative pressure range of 0.5–0.75, indicating the presence of mesoporosity. The BET surface area, pore volume, and average pore size of Fe3O4@C MNPs were calculated to be 89.31 m2 g−1, 0.10 m3 g−1, and 4.53 nm, respectively. After the sulfonation, the obtained Fe3O4@C–SO3H sample shows N2 adsorption– desorption isotherms similar to those of the sample Fe3O4@C, confirming that the sulfonation treatment has no substantial effect on the Fe3O4@C MNPs. However, the BET surface area, pore volume, and average pore size of Fe3O4@C–SO3H MNPs were calculated to be 70.36 m2 g−1, 0.08 m3 g−1, and 4.70 nm, respectively. The BET surface area and pore volume decrease after the sulfonation treatment, which may be attributed to the presence of a large number of –SO3H groups. The above results suggest that the porous structure and large surface area
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could be responsible for grafting –SO3H groups and for making reactants come into contact with acid sites with high efficiency. In addition, the N2 adsorption–desorption hysteresis loop was not closed, which is possibly attributed to the aggregation of these nanoparticles due to condensation of SO3H groups during drying.41 Element analysis shows that the sulfur content of the Fe3O4@C–SO3H MNPs is about 0.627 wt%, which corresponds to a sulfuric acid concentration of 0.2 mmol g−1. Moreover, it is assumed that sulfur only takes the form of –SO3H in the sulfonated sample of the Fe3O4@C–SO3H MNPs. However, the number of H+ determined by our acid–base titration was 0.25 mmol g−1. This value is slightly higher than that obtained from elemental analysis, which is probably due to the presence of a small amount of carboxyl groups in the sulfonated samples. Therefore, the amount of H+ contributed by carboxyl groups was 0.05 mmol g−1. The findings further verified that the –SO3H groups were grafted on the surface of the Fe3O4@C MNPs and they formed a considerable amount of Brønsted acid sites. This observed phenomenon is consistent with that of FTIR, and these carboxyl groups could also be of benefit to acid-catalyzed reactions. The concentration of the sulfonic acid sites (0.2 mmol g−1) is lower than that of the related sulfonated hybrid materials reported previously, which can be attributed to the Fe3O4 cores contributing the main mass to our prepared Fe3O4@C–SO3H MNPs. Our group previously modified this method for the Fe3O4@C MNPs to prepare the hollow structure Fe3O4@C MNPs, and the weight percentage of Fe3O4 was about 79%.42 If the weight of the Fe3O4 cores was removed from the Fe3O4@C–SO3H MNPs, the concentration of the sulfonic acid sites would be increased to 1 mmol g−1. In the syntheses of the fine chemical and pharmaceutical industries, acetals are generally applied in carbohydrate synthesis and organic reactions for carbonyl protection.43 In the general industrial process, sulfonic acid or fuming sulfonic acid are used as homogeneous catalysts for the acetalization reaction of benzaldehyde with ethylene glycol under severe conditions. Therefore, these homogeneous catalysts used in the practical process can cause problems such as corrosion of the reactor, difficulty of product separation and environmental pollution. To solve these problems, the solid phase catalytic system as a substitute would be more promising. The Fe3O4@C–SO3H MNPs obtained by our group can not only be effectively and stably used as Brønsted acid catalysts for acid-catalyzed reactions, but also can be easily separated from the reaction mixture using an external magnet (0.2 T). In addition, it was reported that the sulfonated mesoporous materials can serve as nanoreactors for catalytic reactions.44 Moreover, as the N2 adsorption–desorption isotherms showed that the carbon layers covered on Fe3O4@C MNPs contain lots of mesopores, sulfonated Fe3O4@C–SO3H MNPs can also serve as a novel nanoreactor. In this study, to evaluate the catalytic ability of Fe3O4@C–SO3H MNPs, the acetalization reaction was carried out as a model reaction. The catalytic activity of these novel functional mesoporous Fe3O4@C–SO3H MNPs was investigated by the acetalization reaction (Scheme 1) of
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Fig. 7 The catalytic conversion of benzaldehyde as a function of reaction time during the acetalization reaction of benzaldehyde with ethylene glycol using (a) Fe3O4@C MNPs and (b) Fe3O4@C–SO3H MNPs, respectively.
benzaldehyde with ethylene glycol in cyclohexane under mild conditions. From Fig. 7(b), it can be seen that the conversion of benzaldehyde quickly reaches 88.3% after reaction for 2 h (TOF = 662.3 h−1), indicating an excellent catalytic performance due to the high concentration of sulfonic groups of the Fe3O4@C–SO3H MNPs which can provide enough acid sites for this acid-catalyzed reaction. The selectivity of the catalyst is 100% for benzaldehyde acetalization reactions under mild conditions. As a result, the yield of product for the reaction is 88.3%. For comparison purposes, Fe3O4@C MNPs that contain a small number of carboxyl groups were also used as the heterogeneous acid catalysts for the acid-catalyzed reaction. The result shows that Fe3O4@C MNPs without loading of sulfonic groups can also catalyze the acetalization reaction of benzaldehyde with ethylene glycol, but the activity of Fe3O4@C MNPs is much lower (44%) than that of Fe3O4@C–SO3H MNPs for this catalytic reaction, as shown in Fig. 7(a). The catalytic activity of the Fe3O4@C MNPs can be attributed to carboxyl groups from the amorphous carbon layer which can provide acid sites for the acid-catalyzed reactions. As the Fe3O4@C–SO3H MNPs exhibited much higher catalytic activity than the Fe3O4@C MNPs, it is believed that the anchored sulfonic acid groups are the main catalytic active sites of the Fe3O4@C–SO3H MNPs. Therefore, it is suggested that Fe3O4@C–SO3H MNPs with high activity for the conversion of benzaldehyde with ethylene glycol can also be used as magnetically separable solid acid catalysts for other acid-catalyzed reactions. Reusability and isolation of the catalysts are important factors for any practical application.45 The magnetic properties of Fe3O4@C–SO3H MNPs were measured at room temperature (300 K) in an applied magnetic field up to 20 000 Oe. As shown in Fig. 9, the saturated magnetization values of Fe3O4@C– SO3H MNPs before and after catalysis are 53.6 and 43.7 emu g−1, respectively. The saturated magnetization dropped, possibly due to the partial oxidation of the Fe3O4 cores in the drying
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Fig. 8 The reusability of Fe3O4@C–SO3H MNPs in the acetalization reaction of benzaldehyde with ethylene glycol.
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sulfonic acid concentration for Fe3O4@C–SO3H MNPs after the catalysis, the CHN/S analyzer was used to detect the content of the sulfur element in the samples, and the result showed that the sulfur content of Fe3O4@C–SO3H MNPs was about 0.55 wt%, which corresponds to a sulfuric acid concentration of 0.17 mmol g−1. And the volatility of the conversion of benzaldehyde with ethylene glycol after different running times can be attributed to the level of cleaning after washing in different recycling processes. It is well known that magnetically separable solid acid catalysts are superior to the traditional solid acid catalysts which are separated from the reaction mixtures with complicated procedures, such as centrifugation and filtration. For example, though sulfonated silica or carbon solid acid catalysts with the conversion rate of about 90% for this acidcatalyzed reaction were reported by other groups, the preparation of these solid acid catalysts and the separation of the catalysts from the reaction mixtures are very complicated and time-consuming.23,32 In addition, the outstanding catalytic performance of the Fe3O4@C–SO3H MNPs is attributed to their unique porous carbon structure which can provide more platforms to coordinate or accommodate sulfonic groups efficiently, and the Fe3O4 cores can be of benefit for the easy separation of catalysts from the reaction mixtures and can enable the catalysts to meet the demand of environmental protection.
4.
Fig. 9 Field-dependent magnetization of Fe3O4@C–SO3H MNPs before and after catalysis. The inset shows a photograph of Fe3O4@C–SO3H MNPs: (a) dispersed in the reaction mixture and (b) magnetic separation of the nanoparticles from the reaction medium.
process in air and residual water molecules in the pores of carbon shells, which can be avoided by drying the sample in a vacuum. In addition, the Fe3O4@C–SO3H MNPs after the acidcatalyzed reaction can also be easily collected using an external magnet (0.2 T) as shown in Fig. 9. In order to test the reusability of the catalysts, the catalysts were separated using a permanent magnet at the end of the reaction and reused after thoroughly washing with ethanol and drying in air at 373 K overnight. In order to investigate the stability of the catalyst in the catalytic process, the amount of leached Fe in a filtrate was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the mass ratio of leached Fe to the used catalyst is about 0.1%, which suggests that the catalyst is stable in the catalytic reaction. As shown in Fig. 8, the conversion rate of benzaldehyde can be maintained above 70% except for the 5th run even after running 8 times, indicating excellent reusability of the catalysts. In order to measure the
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Conclusions
In summary, we have demonstrated that magnetically separable solid acid catalysts, Fe3O4@C–SO3H MNPs, were prepared by a novel and simple approach, and the sulfonic acid group loading is up to 0.2 mmol g−1. The mesoporous carbon layer covered on the surface of Fe3O4 cores can not only stabilize the Fe3O4 MNPs against aggregation and prevent oxidation of the Fe3O4 MNPs, but also can be coordinated or grafted with –SO3H groups as a Brønsted acid for many practical applications in the chemical industry. The catalysts exhibited excellent activity for the acetalization reaction of benzaldehyde with ethylene glycol with a high conversion rate (88.3%) under mild conditions. Furthermore, solid acid catalysts could be easily separated using an external magnet, and could be used repeatedly 8 times without a distinct loss of catalytic activity. It is believed that these magnetically separable Fe3O4@C–SO3H MNPs could also serve as active, selective and stable catalysts for a number of acid-catalyzed reactions.
Acknowledgements This work was supported by the National Natural Science Foundation (NSFC; 21071137 and U1232211).
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