1 Introduction

Catalytic oxidation of styrene is of great interest because styrene oxide and benzaldehyde are important and versatile synthetic intermediates in chemical industry [1]. Especially, benzaldehyde is a very important fine chemical product and can be widely used in many fields, such as medicine, dyes, flavors and resin additives. It is also a very important intermediate in synthesis of other aroma compounds [2, 3]. Great efforts have been made to develop novel catalytic systems for oxidation of styrene in the past years [4–7]. Among the reported systems, heterogeneous ones using green oxidants and solvents are particularly desirable in both economical and environmental aspects [8, 9]. Metal oxides, especially magnetic nanocrystals of complex oxides such as spinel ferrites become more important in recent years due to both their unique properties and broad range of applications in diverse areas such as magnetic recording and separation, ferrofluid, magnetic resonance imaging (MRI), biomedicine, catalysts, gas sensors, high quality ceramics and superparamagnetic materials [10–13]. A great advantage of using magnetic ferrite nanocrystals as catalysts in liquid-phase reactions is that the catalysts are not only thermally and chemically stable in the solution medium, but also easy to be recovered because of their magnetic property. As a matter of fact, these catalysts can be separated from the reaction medium by simply placing a magnetic field on the surface of the flask. Many examples for catalytic oxidation of styrene using spinel ferrites as catalysts were reported, such as MgFe2O4 [14], SrFe2O4 [15], CaFe2O4 [16], nickel and zinc ferrites [17]. Ni–Gd ferrites [18] and ferrites supported silver [19]. However, most of these oxidation reactions have been carried out in organic solvents with low efficiency.

The properties of ferrites are highly related to their shapes, sizes and structures, which can be adjusted through the synthesizing processes. Significant number of methods have been developed to prepare the ferrite nanomaterials with various properties, for instance, sonochemical reactions [20, 21], mechanochemical synthesis [22, 23], hydrolysis of precursors [24], flow injection synthesis [25], aqueous co-precipitation [26], hydrothermal method [27] and sol–gel auto-combustion method [28]. Among these techniques, sol–gel auto-combustion synthesis has been proved to be a simple and economical way to prepare nanopowders [28, 29]. Combining the advantages of chemical sol–gel and combustion processes, sol–gel auto-combustion synthesis gives rise to a thermally induced anionic redox reaction. The energy released from the reaction between oxidant and reductant is adequate to form a desirable phase within very short time. The process exhibits the advantages of inexpensive precursors, a simple preparation process, and can produce highly reactive nano-sized powder [30].

In our previous work [31], CoFe2O4 nanocrystal synthesized by sol–gel auto-combustion method was proved to be highly active and easily recovered catalyst for the oxidation of cyclohexane by molecular oxygen without addition of solvents or reductants. As part of our interest in hydrocarbon oxidation catalyzed by spinel ferrites, we are reporting here Mg–Cu spinel ferrites synthesized by sol–gel auto-combustion method can efficiently catalyze the oxidation of styrene to produce benzaldehyde in water using hydroperoxide as green oxidant.

2 Experimental

2.1 Materials and equipments

All reagents are of analytical grade and were used as received. FT-IR spectra were measured on a Nexus 870 FT-IR spectrophotometer by diffused reflectance accessory in the 4,000–400 cm−1 range. XRD patterns of the samples were collected using a PANalytical X,Pert Pro diffractometer with Cu Kα radiation in the 2θ range of 10°–80° with a scanning rate of 5°/min and a voltage and current of 40 kV and 30 mA. Visible Raman spectra were recorded on a Jobin–Yvon U1000 scanning double monochromator in the range of 300–1,000 cm−1 with a spectral resolution of 4 cm−1. The line at 532 nm from a DPSS 532 Model 200 532 nm single-frequency laser was used as the excitation source. SEM and TEM micrographs were obtained using a JSM-5600LV and Hitachi H-600 microscope using 10 and 120 kV acceleration voltages respectively. The oxidation products were determined by an HP 6890/5973 GC/MS instrument and quantified by a Shimadzu GC-2010 gas chromatograph.

2.2 Synthesis of the catalysts

The spinel Mg–Cu ferrite catalysts Mg1−xCuxFe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) were prepared by the sol–gel auto-combustion route under optimized conditions reported in our previous work [32]. In a typical procedure, stoichiometric amounts of Mg(NO3)2·6H2O, Cu(NO3)2·3H2O, Fe(NO3)3·9H2O and citric acid were completely dissolved in distilled water with 1:1 molar ratio of metals to citric acid and 0.1 mol L−1 of metals concentration. Concentrated ammonia (25–28 %) was added slowly under constant stir to adjust the solution to neutral. The solution was evaporated in an oil bath at 80 °C under continuous stirring until a brown gel formed. After the reaction, the formed gel was dried at 120 °C until a spumous xerogel was obtained. Then the produced xerogel was ignited at 650 °C, a self-propagating combustion process occurred and an olive brown product was obtained after it combusted completely. Seven samples with different ratios of Mg to Cu were prepared as described above and designated as cat.1–7, respectively. The samples were ground finely and then used to catalyze the oxidation of styrene with hydrogen peroxide.

2.3 Oxidation of styrene

The selective oxidation of styrene was carried out in a 25 mL round bottom flask equipped with a Teflon coated magnetic stirrer and a reflux condenser. In a typical procedure, 10 mg of catalyst, 2 mL (17.4 mmol) of styrene, 5 mL of water and 4.5 mL of hydrogen peroxide (30 %), molar ratio of styrene/H2O2 = 2:5, were added successively into the flask. The flask is then immersed in an oil bath at desired temperature and time with stirring. The products were identified by GC–MS and quantified by GC using toluene as internal standard.

3 Results and discussion

3.1 Characterization of catalysts

The XRD patterns of the samples cat.1–6 confirm existing of crystalline spinel (Fig. 1). The seven peaks at 18.3, 30.2, 35.4, 43.2, 53.1, 57.2 and 62.7° can be ascribed to the reflection of (111), (220), (311), (400), (422), (511) and (440) diffractions of MgFe2O4 (JCPDS NO. 88-1936) and CuFe2O4 (JCPDS NO. 77-0010) spinels. The XRD pattern of the sample cat.7 is in agreement with the standard one for body-centered tetragonal CuFe2O4, the diffraction peaks at 2θ values of 18.3°, 29.8°, 34.6°, 35.9°, 37.1°, 41.6°, 43.8°, 58.0°, 62.0°, 63.8° and 74.7° can be ascribed to the reflection of (101), (112), (103), (211), (202), (004), (220), (321), (224), (400) and (413) diffractions of the CuFe2O4 (JCPDS NO. 34-0425), respectively (Fig. 1). The mean particle sizes of the samples based on the Sherrer equation are 28, 20, 22, 27, 23, 18 and 26 nm for cat.1–7, respectively.

Fig. 1
figure 1

The XRD patterns of the samples cats. 1–7

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Figure 2 shows the FT-IR spectra of the seven samples. A strong band at around 570 cm−1 for cat.1–7 is presented. It is associated with the Fe–O stretching vibrations in magnesium ferrite and copper ferrite phase [33]. Two weak absorption bands around 3,420 and 1,630 cm−1 can be attributed to stretching and bending vibrations of O–H in adsorbed water. No characteristic bands corresponding to citric acid or NO3− appeared, and this indicates that no citric acid or NO3− is residual in the samples.

Fig. 2
figure 2

The FT-IR spectra of cats. 1–7

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The Raman spectra of the samples are shown in Fig. 3. The low-frequency vibrations (below 600 cm−1) are attributed to motion of oxygen around the octahedral lattice sites whereas the higher frequencies are attributed to oxygen around tetrahedral sites [34]. Thus, the intense band in the Raman spectra of the samples at 696 cm−1 can be assigned to the A 1g symmetry (tetrahedral) [35], and the 479 cm−1 band is characteristic of the octahedral sites [36].

Fig. 3
figure 3

The Raman spectra of cats. 1–7

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The morphologies of the samples were also characterized by SEM and TEM. The SEM and TEM images of cat.4 are shown in Fig. 4 as representations. The sample cat.4 features nanoparticles with an irregular morphology and a broad particle size distribution ranging from 10–20 to 30–50 nm, with a high percentage of small particles (25–30 nm). It is clear that the sample have particle agglomeration forms. Other samples showed similar morphologies with cat.4 except for different particle sizes as calculated from the Sherrer equation based on the XRD patterns.

Fig. 4
figure 4

The SEM and TEM images of the cats. 4

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3.2 Catalysis tests

The results of styrene oxidation over the as-prepared samples cat.1–7 are listed in Table 1. In order to check the role of the composite ferrites towards selective oxidation of styrene, a blank reaction was carried out under the same conditions. As shown in Table 1, in the absence of catalyst, almost no reaction took place as seen from GC/MS analysis (entry 1). The samples cat.1–7 can efficiently catalyze the oxidation of styrene and benzaldehyde was found to be the major product in all cases. As expected, composite oxides of Mg–Cu ferrites showed better performances than pure one of either MgFe2O4 (entry 2) or CuFe2O4 (entry 8). Especially, Mg0.5Cu0.5Fe2O4 (cat.4) has obtained the highest styrene conversion of 16.7 % with 86.3 % of selectivity for benzaldehyde. A free radical mechanism may be involved in selective oxidation of styrene over ferrite catalysts [15]. As a fact, when the same reaction was carried out in the presence of tertiary butyl alcohol as a scavenger, only 9.2 % of styrene conversion was obtained (entry 9). It has been noted that the catalytic effectiveness of such system is due to the ability of metallic ions to migrate between the sub-lattices [14, 20, 21]. So before discovery of new evidences, it is reasonable to believe that appropriate molar ratio of MgII–CuII in cat.4 is in favor of migration of metallic ions between the sub-lattices consequently. This possibility maybe account for optimum catalytic activity of Mg0.5Cu0.5Fe2O4 (cat.4). In order to optimize the reaction conditions, the sample Mg0.5Cu0.5Fe2O4 was chosen as catalyst to optimize the reaction conditions and investigate the recyclable performance of the catalysts.

Table 1 Oxidation of styrene over different catalysts
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3.3 Effect of the solvents

Table 2 shows effects of different solvents on styrene conversion and products distribution over Mg0.5Cu0.5Fe2O4. It is clear that aprotic solvents like acetonitrile and acetone are more favorable for the conversion of styrene while protic ones are more favorable for formation of benzaldehyde, Which is consistent with the reported results on MgxFe3−xO4 and AFe2O4(A = Fe, Ni, Zn) catalysts [14, 17]. As a result, the highest styrene conversion of 40.5 % and benzaldehyde selectivity of 86.3 % were obtained in acetonitrile and water, respectively. The Mg0.5Cu0.5Fe2O4 showed much higher efficiency than analogous catalysts reported (entry 5 and 6) and much higher turnover number can be obtained in less reaction time. Although acetonitrile is most favorable for styrene conversion, water is selected as the most environmentally friendly solvent for following investigations in pursuit of higher benzaldehyde selectivity.

Table 2 The effect of styrene oxidation with different solvents
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3.4 Effect of styrene/H2O2 molar ratio

The reaction was carried out in the presence of different styrene/H2O2 molar ratios such as 2:1, 1:1, 2:3, 1:2 and 2:5, for which 10 mg of catalyst were taken in 5 mL of water at 70 °C for 9 h reaction. The results were listed in Table 3. It can be seen clearly that, as the molar ratio of H2O2 to styrene increases, styrene conversion also increases. As a result, when H2O2 concentration increased from 2:1 to 2:5, styrene conversion increased from 7.4 to 16.7 %. In respect of the products distribution, the selectivity for benzaldehyde varied in the range of 85.2–88.5 %, the selectivity for either phenylacetaldehyde or styrene oxide showed a little decrease, while the selectivity for other by-products increased. Considering conversion of styrene and selectivity for benzaldehyde, 2:5 was selected as the optimum molar ratio of styrene to H2O2.

Table 3 The effect of styrene oxidation with different styrene/H2O2 molar ratio
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3.5 Effect of reaction temperature

Keeping styrene:H2O2 with molar ratio of 2:5, the influence of reaction temperature on styrene oxidation was evaluated in the range of 50–90 °C for 9 h reaction, and the results were listed in Table 4. It can be found that high temperature is in favor of styrene conversion. As a result, when temperature was improved from 50 to 90 °C, styrene conversion increased from 4.5 to 22.3 %. As for products distribution, with increase of temperature, selectivity for benzaldehyde increased at first and then decreased slightly, the highest value of 86.3 % was obtained at 70 °C. The selectivities for phenylacetaldehyde and styrene oxide showed a tendency of decrease with the increase of the temperature while total selectivity for other by-products increased a little. This indicates that more deeply oxidized products have formed at higher temperature.

Table 4 The effect of styrene oxidation under different reaction temperature
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3.6 Effect of reaction time

The influence of reaction time on styrene oxidation was investigated at 80 °C in the range of 0.5–9 h with styrene:H2O2 molar ratio of 2:5. As shown in Table 5, styrene conversion increased expectedly when reaction time was prolonged. In the first 2 hours, only about 5.6 % conversion was obtained. As the reaction time was increased from 3 to 8 h,the conversion of styrene increased from 5.6 to 21.2 %. The selectivity for benzaldehyde increased at first and then dropped when reaction time was over 6 h, the highest value of 87.6 % was obtained for 6 h reaction; the selectivity for phenylacetaldehyde hardly changed within the first 3 h and then dropped rapidly from 10.9 to 2.9 % when reaction time increased from 4 to 8 h; the selectivity for styrene oxide showed a tendency of decrease while the selectivity for total by-products dropped slowly in the first 5 h, but then increased greatly from 1.8 to 15.3 % when reaction time was prolonged from 6 to 8 h. In pursuit of higher selectivity for main product benzaldehyde, 6 h was selected as optimum reaction time.

Table 5 The effect of styrene oxidation with different reaction time
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3.7 Effect of catalyst amount

The effect of catalyst amount on styrene oxidation was investigated in the range of 5–20 mg at 80 °C for 6 h reaction with styrene:H2O2 molar ratio of 2:5. The results were shown in Table 6. With increase of the catalyst amount, the styrene conversion and benzaldehyde selectivity showed similar profile, that is, increased firstly and then decreased. The highest value of 17.2 % for styrene conversion was obtained when 10 mg catalyst was used. However, with further increase of catalyst amount to 20 mg, styrene conversion decreased to 12.6 % possibly due to adsorption or chemisorptions of two reactants on separate catalyst particles, thereby reducing the chance to interact. Similar observations were noted by Sharma and Pardeshi [9, 15]. Selectivity for benzaldehyde increased from 76.9 to 87.6 % when catalyst amount increased from 5 to 10 mg, and then dropped to 76.6 % when catalyst amount increased to 20 mg due to formation of more by-products. Based on above results, 10 mg has been considered as optimum catalyst amount.

Table 6 The effect of styrene oxidation with different catalyst amounts
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3.8 Reuse of the catalyst

After the reaction, the catalyst can be seen being adsorbed on the magnet. The catalyst together with the magnet can be easily separated by simple decantation after applying a magnetic field on the surface of the flask, and then subjected to the second run under the same conditions. The results are shown in Table 7. The selectivity for benzaldehyde changed slightly after five runs, but the conversion of styrene decreased from 16.5 to 12.4 %. The decrease in the activity could be mainly attributed to unavoidable loss of the catalyst during the process of collection. The results confirm that the nanocrystalline spinel cat.4 has good stability and recyclable applicability for the oxidation of styrene with 30 % H2O2.

Table 7 Reuse of the catalyst
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4 Conclusions

Nanosized spinel-type Mg–Cu ferrite complex oxides were prepared by a simple and effective route of sol–gel auto-combustion using cheap precursors. The complex ferrite catalysts are more active and easily reusable catalysts for styrene oxidation in a green catalytic system and benzaldehyde is the main product. Protic solvent water is favorable for increasing the selectivity for benzaldehyde. The catalytic performances of the samples are highly related to their components and the Mg0.5Cu0.5Fe2O4 spinel has shown the highest catalytic activity. This work, along with other published ones, indicates that ferrite complex oxides have a considerable potential for becoming a kind of tunable catalysts in respect of catalytic performances and components. Based on this purpose, more efforts have been and will be focused on investigating structure-activity relationships of the ferrite complex oxides in catalytic oxidation reactions in our present and future work.