Photooxidation of benzyl alcohols and photodegradation of cationic dyes by Fe₃O₄@sulfur/reduced graphene oxide as catalyst

Abstract

Wide sheets of graphene oxide (GO) were prepared by the Hummers' method. Raman spectroscopy and AFM images were used to characterize the prepared GO. A one-step green chemistry method was used to prepare sulfur/reduced graphene oxide (S/RGO). Magnetic nanoparticles were added to S/RGO to prepare Fe₃O₄@S/RGO composite. The prepared composites were investigated by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) mapping, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) imaging. The magnetism of the composite was measured using a vibrating sample magnetometer (VSM). Diffuse reflectance spectroscopy (DRS) was used to find band gaps of samples. Experimental tests distinguished kinetic photocatalyst degradation of methylene blue (MB) and crystal violet (CV) in presence of the prepared catalysts under visible light irradiation. The results demonstrated that Fe₃O₄@S/RGO has excellent degradation activity in comparison to GO, pure sulfur, and S/RGO. Furthermore, photodegradation of MB is more efficient than CV. Fe₃O₄@S/RGO also was used for photooxidation of substituted benzyl alcohols to their corresponding benzaldehydes. Selectivity and conversion of the oxidation reaction was demonstrated by TLC and GC.

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Introduction

Graphene, a glamorous two-dimensional carbon, has been attracting considerable attention because of its advantageous material properties, including good thermal properties, excellent charge mobility, transparency, high mechanical strength, and flexibility.^1–3 These exclusive traits of graphene are already used for several applications, such as solar cells, sensors to diagnose diseases, electrodes with very high surface area, very low electrical resistance composite materials, and base materials in clean energy.^4,5 Because a desirable graphene sheet consists totally of sp²-hybridized carbon atoms, it has emerged as a “shining star” in the scientific world.^6,7 Very high tensile strength in covalent bonds exists between carbon atoms in graphene; also they have the capability to bond to a fourth atom to saturate the valence bond of carbon atoms (sp³-hybridization). This capability creates extraordinary mechanical tensile and other fascinating properties in material science.⁸

Moreover, doping sulfur into reduced graphene oxide (S/RGO) can open a band gap of graphene for applications as sensors.^9–11 Sulfur has excellent properties such as inexpensive cost, high accessibility, and low perniciousness. Some works have been reported on photocatalytic activity for S/RGO degradation of organic dyes under visible light by photosensitization.¹² Moreover, doping aluminum, silicon, phosphorus, and sulfur on graphene has been reported, showing that functionalized graphene with above mentioned elements can increase electron transfer. Therefore, it can improve photodegradation and oxidation reactions.^13–16 To the best of our knowledge, S/RGO has been used as a photocatalyst for degradation of organic dyes such as methyl orange.¹² This composite of graphene has major potential due to its easy and facile procurement, good photosensitivity with a narrow band-gap, and high stability.^17–19

Benzaldehyde is the most effective and industrially specific significant aromatic aldehyde because it plays a key role as an organic solvent. Furthermore, in the Reformatsky reaction, Claisen–Schmidt condensation, and the Perkin condensation, benzaldehyde plays a vital role as a raw material. Many routes have been applied to generate benzaldehyde; oxidation of benzyl alcohols is the most important method to produce it. Selectivity in this reaction is very significant because the oxidation of benzyl alcohols reaction usually tends to generate benzoic acid, therefore, stopping at the benzaldehyde step is very attractive.^20–25

On the other hand, magnetic particles are very fascinating for scientists because the magnetic particles can be easily collected from the environment. Nanoparticles of Fe₃O₄ have acceptable stability in the environment, they are non-toxic, and can be incorporated in a non-carcinogenic semiconductor which can be used as a magnetically effective photocatalyst. Magnetic particles have substantial morphology and size and so are attractive to researchers; however, one of the disadvantages of magnetic particle is they are very vulnerable to oxidation when exposed to oxygen in the atmosphere.^26–28 In the past decade, many scientists synthesized graphene–Fe₃O₄. In addition, compared with other magnetic materials, nano sheets of graphene demonstrated significantly increased electromagnetic absorption properties due to good separation, high surface areas, and interfacial polarizations.

In this study, sulfur was embedded on graphene via an environmentally friendly method, then the Fe₃O₄@S/RGO composite was synthesized by immobilizing Fe₃O₄ nanoparticles on S/RGO as in previously reported work.²⁹ The prepared catalysts were investigated for photodegradation of MB and CV as cationic dyes under LED visible light irradiation. In addition, Fe₃O₄@S/RGO was used as a photocatalyst for oxidation of benzyl alcohols. The presence of Fe₃O₄ helps collection from the environment and reusability of a catalyst. In addition, nanoparticles of Fe₃O₄ can help photodegradation and photooxidation processes according to a possible mechanism discussed in a later section.

Results and discussion

Characterization of Fe₃O₄@S/RGO

The preparation of graphene oxide from graphite powder was proven using Raman spectroscopy and AFM imaging. Raman spectra of GO and graphite is shown in Fig. 1. The peak at 1580 cm⁻¹ was designated “G” band; it corresponds to an E_2g mode and showed that the hybridization is sp². The peak at 1290 cm⁻¹ was designated “D” band and is dependent on the vibrations of carbon atoms with sp³. The surface morphology and height profile of the GO was observed by AFM (inset of Fig. 1); individual GO sheets with 200–1000 nm lateral width can be clearly observed.

Fig. 1 The Raman spectroscopy of graphite and graphene oxide. Inset: the AFM images of graphene oxide.

SEM images of GO, S/RGO, and Fe₃O₄@S/RGO and TEM images of GO and Fe₃O₄@S/RGO are shown in Fig. 2. The SEM image of GO shows the sheet structures (Fig. 2A) while in SEM images of S/RGO (Fig. 2B and C) it can be observed that S particles are doped on the surface of graphene sheets. The SEM image of Fe₃O₄ illustrates nanoparticles morphology (Fig. 2D). In the SEM image of Fe₃O₄@S/RGO (Fig. 2E), the nanoparticles of Fe₃O₄ are clearly on the surface of S/RGO. Fig. 2F shows the TEM image of GO and indicates that sheets of GO are transparent. TEM images of Fe₃O₄@S/RGO composite (Fig. 2G–I) demonstrate that the magnetic nanoparticles and sulfur have grown on the graphene sheets with a narrow size distribution and identical dispersion of nanoparticles. Typical particle sizes are in the range of 8–20 nm.

Fig. 2 The SEM images of (A) GO, (B and C) S/RGO, (D) Fe₃O₄, (E) Fe₃O₄@S/RGO. The TEM of (F) GO and (G–I) Fe₃O₄@S/RGO.

In addition, EDX elemental mapping and XPS analysis were done to investigate elements in the Fe₃O₄@S/RGO composite. The EDX mapping images reveal that Fe, C, S, and O components were distributed uniformly in the Fe₃O₄@S/RGO composite (Fig. 3) and show that most of the particles are Fe₃O₄.

Fig. 3 (A–C) EDX mapping images of Fe₃O₄@S/RGO composite for C, S, and Fe elements and (D) percentage of elements in the Fe₃O₄@S/RGO composite.

The percentage of Fe in the composite is 71.5%. The elemental percentage of S/RGO and Fe₃O₄@S/RGO obtained from EDX analysis are listed in Table 1.

Table 1 Element analysis by EDX

Compound	Element
	C	S	O	Fe
S/RGO	44.1	6.4	45.3	—
Fe3O4@S/RGO	27.3	1.5	40.7	71.5

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Complete characterization was further investigated by X-ray photoelectron spectroscopy (Fig. 4). The XPS spectra of Fe₃O₄@S/RGO confirmed the results of EDX analysis. The peaks about 291.1 and 537.1 eV indicate the presence of C and O, respectively. In addition, XPS spectra of sulfur shows very poor peaks at 169 and 165 eV that are related to S 2p_1/2 and S 2p_3/2, respectively. Due to the low amount of sulfur in the composite, these peaks were very weak. The peaks corresponding to Fe₃O₄ are very clear in this spectrum; 710 and 725 eV are related to Fe 2p_3/2 and Fe 2p_1/2, respectively.^30–33

Fig. 4 XPS spectra of Fe₃O₄@S/RGO.

The FT-IR spectra of GO, S/RGO, Fe₃O₄, and Fe₃O₄@S/RGO are shown in Fig. 5A. The FT-IR of GO displayed several characteristic bands at 1110 cm⁻¹ (C–O bond, carbonyl), 1375 cm⁻¹ (O–H bond, hydroxyl), 1714 cm⁻¹ (C–O bond, carbonyl and carboxylic acid), 1614 cm⁻¹ (H–O–H in adsorbed water) and broad band at 3100–3500 cm⁻¹ (O–H bond, hydroxyl). In the S/RGO spectrum, the absence of a carbonyl band at 1710 cm⁻¹ obviously demonstrated loss of oxygen. The bands at 1050 cm⁻¹ and 1200 cm⁻¹ in the spectrum of S/RGO belong to C–S stretching and C=S stretching, respectively, and showed that S particles are chemically bonded to the graphene. In FT-IR spectra of bare Fe₃O₄, the Fe–O bond was shown by a strong absorption band at around 570 cm⁻¹ and the peak at ∼3400.4 cm⁻¹ is ascribed to the stretching vibrations of OH, which is from OH absorbed on the surface of Fe₃O₄ particles. In the FT-IR spectrum of Fe₃O₄@S/RGO, the presence of a Fe–O band at around 570 cm⁻¹ and Fe–S stretching bands at ∼274 and ∼311 cm⁻¹ demonstrate that Fe₃O₄ particles are embedded on S/RSO.

Fig. 5 (A) The FT-IR spectroscopy of GO, S/RGO, Fe₃O₄, and Fe₃O₄@S/RGO. (B) The XRD pattern of S/RGO, Fe₃O₄, and Fe₃O₄@S/RGO. (C) The UV-vis DRS of GO, S/RGO, and Fe₃O₄@S/RGO.

The XRD patterns of bare S/RGO, Fe₃O₄, and Fe₃O₄@S/RGO composite are shown in Fig. 5B. As can be seen in the XRD pattern of S/RGO, typical reflections of α-octa-sulfur (S8, JCPDS#08-0247) were clearly observed for the pure S particles.¹² There are additional peaks at 2θ = 24/5° and 27/8° that can be indexed to (−2 4 1) and (−3 1 3) corresponding to reduced graphene oxide, respectively. The other peaks can be indexed to the structure and the lack of any peaks relating to GO at lower angles could be attributed to complete reduction of graphene oxide to graphene during synthesis. The characteristic peaks in the XRD pattern of Fe₃O₄ are related to a phase of cubic magnetite, which is in a close agreement with the reference pattern of JCPDS 01-088-0315. In the XRD pattern of Fe₃O₄@S/RGO composite, most of the considerable diffraction peaks can be indexed to Fe₃O₄. In addition, a few peaks appeared that belong to R/SGO. In conclusion, these patterns confirm the presence of sulfur and Fe₃O₄ nanoparticles on graphene sheets.

The UV-vis DRS measurements of GO, S/RGO, and Fe₃O₄@S/RGO are shown in Fig. 5C. As seen in this figure, the presence of S atoms is confirmed with absorbance in UV and visible range. Furthermore, immobilizing Fe₃O₄ on the surface of S/RGO increases absorbance. Therefore, the presence of S atoms and Fe₃O₄ nanoparticles can enhance the photocatalytic activity of graphene.

The magnetic hysteresis loops for the synthesized bare Fe₃O₄ and Fe₃O₄@S/RGO composite are shown in Fig. 6. The magnetic properties of the samples were analyzed at room temperature using a vibrant sample magnetometer (VSM). For bare Fe₃O₄, the value of saturation magnetization (M_s) and the remnant magnetization (M_r) are 76.5 and 17.79 emu g⁻¹, respectively. It can be inferred from the hysteresis loops that Fe₃O₄ nanoparticles are magnetically soft at room temperature. The value of saturation magnetization (M_s) for the Fe₃O₄@S/RGO composite is 58.1 emu g⁻¹. In other words, with presence of S/RGO, the magnetite saturation was slightly decreased.

Fig. 6 The VSM of bare Fe₃O₄ and Fe₃O₄@S/RGO composite.

Study of photocatalytic degradation reactions

Photocatalytic activities were evaluated by the degradation of organic cationic dyes (methylene blue and crystal violet) in aqueous solution under visible LED lamp (7 W) irradiation. Decomposition of both of dyes was done under the same conditions (20 mL MB and CV solutions with initial concentrations of 10 and 20 mg mL⁻¹ in presence of 2 mg catalyst). Before reaction, the catalyst was dispersed in the mixture by sonication. Then the mixture was stirred in the dark for 30 min to allow MB or CV adsorption-desorption by the catalysts. Finally, the mixture was exposed to light irradiation. Absorption spectra of the dye solutions (with concentrations of 10 mg L⁻¹) after irradiation at different times are shown in Fig. 7A and B for MB and CV, respectively.

Fig. 7 The temporal evolution of the absorption spectra of (A) MB and (B) CV solutions (20 mL, initial concentration: 10 mg L⁻¹) in presence of Fe₃O₄@S/RGO catalyst (2 mg) under 7 W LED visible light irradiation at 3 h. The degradation efficiency of (C) MB and (D) CV in presence of different catalysts in the dark and under LED visible light irradiation.

Absorption at wavelengths 664 nm (maximum peak of MB) and 580 nm (maximum peak of CV) was chosen to monitor the photocatalysts degradation process of MB and CV solutions, respectively. The removal efficiency of MB and CV by Fe₃O₄@S/RGO, S/RGO, pure graphene, and pure sulfur are illustrated in Fig. 7C and D, respectively. Comparison between these catalysts showed that Fe₃O₄@S/RGO has the highest capability for degrading the dyes. In addition, the compounds have been used to degrade anionic dye (MO),¹² but it has been demonstrated that Fe₃O₄@S/RGO is better for cationic dyes rather than anionic dyes.

The kinetic studies of the photocatalytic degradation of MB and CV under visible light were demonstrated in Fig. 8A and B, respectively. The photodegradation reaction of MB and CV by Fe₃O₄@S/RGO exhibited a Pseudo-first-order kinetic model which is expressed as follows:

where C₀ is the initial concentration of a pollutant, C is MB or CV concentration at time t and k is the rate constant of first-order reaction.

Fig. 8 The kinetics of photocatalytic degradation of (A) MB and (B) CV (10 and 20 mg L⁻¹) in presence of Fe₃O₄@S/RGO catalyst under 7 W LED visible light irradiation.

Proposed mechanism of photocatalytic degradation

The S/RGO composite was used as a photocatalyst for decomposition of MB and CV by hydroxyl radicals formed at their interface. Using loaded sulfur atoms can promote opening of a band gap in the structure of graphene. The other role of sulfur loaded on the surface of graphene for degradation of MB and CV is photo-generated electrons from S to graphene sheets as electron acceptors.

Fe₃O₄ acts as a magnetization function with the composite; moreover, due to the low band gap, it can be easily excited by visible light irradiation and the electrons promoted from their ground state to the excited state. However, electrons may return from CB to VB. The graphene sheets act as an excellent electron collector. With immobilizing of Fe₃O₄ on graphene, electrons can be injected into graphene sheets and the probability of electron–hole recombination can be reduced. Therefore, this composite makes better electron-conducting surfaces and increases photocatalytic activities, essentially due to the effective prevention of photo-generated electron–hole pair recombination.^34–36 The schematic for the excitation and charge transfer process from S and Fe₃O₄ to graphene sheets under light irradiation is shown in Scheme 1. The holes can directly oxidize MB and CV molecules adsorbed on the catalyst surface. In addition, the graphene sheets can convey photo-generated electrons to dissolved oxygen molecules leading to the production of oxygen peroxide radicals (O₂˙). The positive charged hole also can react with OH⁻ derived from H₂O to generate hydroxyl radicals (OH˙). The MB and CV molecules then can be photocatalytically degraded by the O₂˙ and OH˙ radicals to CO₂, H₂O, and other mineralization products.^37,38

Scheme 1 Proposed mechanism of photocatalytic degradation of MB and CV by the Fe₃O₄@S/RGO composite.

Study of photocatalytic oxidation reactions

Application of the synthesized Fe₃O₄@S/RGO composite was investigated for oxidation of benzyl alcohols using H₂O₂ as a green oxidant. Initially, the oxidation of benzyl alcohol was used as a model reaction in the presence of prepared Fe₃O₄@S/RGO as catalyst in order to determine optimized conditions. The effect of different conditions (H₂O₂ amount, catalyst loading, solvent, and light irradiation) in the oxidation of benzyl alcohol in the presence of the Fe₃O₄@S/RGO composite was studied. The results, listed in Table 2, show that the best circumstance for oxidation of benzyl alcohol to produce benzaldehyde is entry 8 with 99% conversion and 100% selectivity. The free solvent condition (entry 1) was the worst condition. In addition, acetonitrile had enormous efficiency of conversion rather than water as solvent. The result of entry 3 compared to entry 7 shows that increasing the catalyst loading up to 30 mg did not caused improvement in the conversion. In an additional investigation, entry 5 versus 4 shows that increasing H₂O₂ did not improve efficiency. By using light irradiation during the oxidation reaction (entry 8), conversion of benzyl alcohol increased considerably. So, the conditions of entry 8 were selected as the best option for oxidation of other benzyl alcohols.

Table 2 The effect of H₂O₂ amount, catalyst loading, solvent, and light irradiation in the oxidation of benzyl alcohol in the presence of Fe₃O₄@S/RGO composite^a

Entry	H2O2 (mL)	Catalyst loading (mg)	Solvent	Visible light	Conversion^b (%)	Selectivity^b (%)
a Reaction conditions: alcohol (1 mmol), 80 °C, 90 min, reflux.b Conversion and selectivity were determined by GC.
1	0.2	20	—	—	10	100
2	0.2	20	H2O	—	20	100
3	0.2	20	CH3CN	—	88	100
4	0.1	20	CH3CN	—	80	100
5	0.4	20	CH3CN	—	85	100
6	—	20	CH3CN	—	70	100
7	0.2	30	CH3CN	—	91	100
8	0.2	20	CH3CN	Was used	99	100

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In order for a catalytic evaluation of the Fe₃O₄@S/RGO composite to selectively convert benzyl alcohol to benzaldehyde, other substituted derivatives in presence of this catalyst were carried out under the optimal conditions (Table 3).

Table 3 Catalytic evaluation of Fe₃O₄@S/RGO composite in oxidation of benzyl alcohols^a

Entry	Structure of alcohol	Name of alcohols	Time (min)	Conversion^b (%)	Selectivity^b (%)
a Reaction conditions: alcohol (1 mmol), catalyst (20 mg), H2O2 (2 mmol), CH3CN (6 mL), reflux, 80 °C, under visible LED lamp irradiations.b Conversion and selectivity were determined by GC-Ms.
1		C6H5CH2OH	90	99	100
2		2-MeC6H5CH2OH	120	35	100
3		4-MeC6H5CH2OH	120	56	100
4		2-ClC6H5CH2OH	120	56	100
5		4-ClC6H5CH2O	100	65	100
6		2-N2OC6H5CH2OH	120	45	80
7		4-N2OC6H5CH2OH	120	55	67
8		3,4-Di-OMeC6H5CH2OH	100	30	100

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The best conversion belongs to benzyl alcohol because of no substituted positions and thus easier oxidizing. Both electron deficient and electron rich derivatives of benzyl alcohol demonstrated good reactivity and excellent selectivity. On the other hand, para derivatives have excellent conversion relative to other derivatives (in ortho or meta positions), because they have minimum steric hindrance in comparison to ortho substituted positions. As seen at Table 3, the conversion of 3,4-di-OMeC₆H₅CH₂OH (entry 8) with two electron donor substituents (methoxy groups) was only 30%, while in entry 4 and entry 5, with electron acceptor substituents (chloride) in ortho and para locations were 56% and 60%, respectively. Therefore, benzyl alcohols with electron acceptor substituents were oxidized more efficiency than benzyl alcohols with electron donor substituents. In addition, para substituents (entry 3, entry 5, and entry 7) were better than ortho substitutes (entry 2, entry 4, and entry 6) because para substituents have less steric hindrance than ortho substituents.

The comparison between various catalysts is shown in Table 4. As can be seen, results in the presence of Fe₃O₄@S/RGO catalyst with less time and less power consumption and H₂O₂ are the best (96% conversion, 100% selectivity).

Table 4 Comparison of the results for the oxidation of benzyl alcohol by other catalysts

Entry	Catalyst conditions	Catalyst loading (mg)	Time (min)	Conversion (%)	Selectivity (%)	Ref.
a Conditions of reactions: 0.5 mL benzyl alcohol, 20 mL H2O plus 5 mL Na2CO3–NaHCO3 buffer solution (pH = 9), T = 100 °C, 3 atm O2.b 16.7 mmol benzyl alcohol, 50.1 mmol H2O2, T = 100 °C.c 0.1 mmol alcohol, 80 mL H2O, 1 atm O2, T = ​343 K, 1300 rpm.d 0.1 mmol alcohol, room temperature, atmospheric pressure.e magnetic stirrer under argon environment, 2 mL acetonitrile solvent, 2.0 mL benzyl alcohol, 1 mL 30% H2O2, room temperature.f 1 mmol alcohol, 0.5 mL CH3CN·H2O, 5 equiv. 30% H2O2, T = 90 °C.g 1 mmol of substrate, 5 mL of CH3CN, room temperature.h 1 mmol benzyl alcohol, 1.25 mmol NaOCl, pH = 8.6 in the presence of 1 mol% TEMPO, 1 mol% porphyrins or 1 mol% metalloporphyrins, 10 mol% KBr, 8 mL CH2Cl2/H2O = 1 : 1, T = ​0 °C, 30 min.
1	Au/RGO	75	480	65	93	41^a
2	GO–N–PW	100	360	76	99	42^b
5	Au/NG	30	360	67	40	43^c
6	CdS/RGO	8	240	35	71	44^d
7	ZnFe2O4	24.1	10	6.9	95.9	45^e
8	WO4^2−@PMO-IL	230	12	75	100	46^f
9	FeBr3	59	24	70	100	47^g
10	Porphyrin/Mn(TDCPP)Cl	520	0.5	48	99	48^h
11	Fe3O4@S/RGO	20	90	96	99	This work

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Proposed mechanism of photocatalytic oxidation

The mechanism of photooxidation is similar to a photodegradation process. After exposure, an electron can transfer from a valance band to a conduction band. Therefore, electrons (e⁻) can promote the photoreduction, while holes (h⁺) participate in the photooxidation.¹⁶

In the first step, after the collision, with visible light to Fe₃O₄@S/RGO, it can produce holes and electrons and then h⁺ can react with benzyl alcohol to produce free radicals of PhCHOH.

Step 1:

Fe₃O₄@S/RGO + hν → Fe₃O₄@S/RGO (h⁺ + e⁻)

PhCH₂OH + h⁺ → PhCHOH + H⁺

On the other hand, H₂O₂ is decomposed into oxygen and water by heat. Oxygen can react with e⁻ to O₂⁻. In the next step, some of the free radicals of PhCHOH can react with h⁺ and some of them react with O₂⁻ to generate benzaldehyde. Therefore, benzaldehyde can be formed in two reactions (step 2).

Step 2:

H₂O₂ + heat → H₂O + ½O₂

O₂ + e⁻ → O₂⁻

PhCHOH + h⁺ → PhCHO + H⁺

PhCHOH + O₂⁻ + H⁺ → PhCHO + H₂O

In order to investigate active species in the experiment, isopropanol was used as the scavenger of hydroxide radicals, KI as the scavenger for holes, and N₂ perch as scavenger for superoxide. When KI was added into the reaction, strong low oxidation activity was observed. Therefore, h⁺ plays an obligatory role in the oxidation process. However, when isopropanol was added to the reaction, reaction activity declined poorly; therefore, radical hydroxide doesn't have a key role in the reaction. In addition, when N₂ perch was added, the activity only partly decreased, showing that O₂⁻ also takes part in the reaction, but that it is not necessary.

The chromatogram of photocatalytic oxidation products of benzyl alcohol (Fig. S1†) shows both benzyl alcohol and benzaldehyde, whereas, there is no peak for benzoic acid or other products from the reaction. Actually, because benzyl alcohol is easier to be adsorbed on catalysts than benzaldehyde, benzaldehyde may desorb from catalysts after formation, inhibiting further oxidation.^39,40

Reusability of Fe₃O₄@S/RGO catalyst

Using a catalyst as slurry has problems of leaching and separation of it from the reaction mixture after a reaction. Fixation of a catalyst on a stationary support could circumvent the need to recover it from the reaction mixture without any leaching.

The reusability of Fe₃O₄@S/RGO catalyst in the oxidation of benzyl alcohols was examined and the results are shown in Fig. 9. All the catalytic tests were conducted under previous experimental conditions. Results show that the benzyl alcohol conversion decreases slowly from 99% to 96% after 3 runs; however, benzaldehyde selectivity remains above 99%. One reason for the negligible decrease in the activity is due to mass loss during the filtration procedure.

Fig. 9 Reusability of Fe₃O₄@S/RGO for the oxidation of benzyl alcohol.

Stability of the Fe₃O₄@S/RGO composite was studied using EDX analysis of it after an oxidation reaction carried out. The percentage of Fe and S elements in the composite remained approximately constant (71.2% and 1.4%, respectively). Because of the anti-leaching feature of the Fe₃O₄@S/RGO composite, it is able to serve as a stable, reusable catalyst with ignorable activity loss for selective oxidation of alcohols and degradation of MB and CV in water.

Experimental

Materials and methods

All materials were of analytical grade and used without further purification. Graphite flakes (60 mesh, 98% pure), potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), sulfuric acid (H₂SO₄, 98%), sodium thiosulfate (Na₂S₂O₃), and sodium hydroxide (NaOH) were purchased from Merck Company (USA). The as-prepared samples were characterized using an X-ray diffractometer (XRD, JEOL diffractometer, Japan, with monochromatic Cu Kα radiation, λ = 1.5418 Å). The particle morphologies of Fe₃O₄@S/RGO powder were determined using an AIS2100 (Seron Technology, South Korea) scanning electron microscopy (SEM). FT-IR analyses were carried out on a Shimadzu FTIR-8400S spectrophotometer (Japan) using a KBr pellet. Atomic force microscopy (AFM, Nano Scope II from Digital Instruments, CA, USA, in contact mode) was used to analyze the surface morphology of the composite. DRS spectra were prepared via a Shimadzu (MPC-2200, Japan) spectrophotometer and we realized that band gap in S/RGO was lower than graphene oxide. The elemental mapping was accomplished with FE-SEM Sigma model (Zeiss, Germany). X-ray photoelectron spectroscopy (XPS) was done using an X-ray 8025-BesTec XPS system (Germany) equipped with an Al Kα X-ray source at energy of 1486.6 eV.

Preparation of graphene oxide

Graphene oxide was prepared from graphite powder and followed the Hummers and Offeman method.⁷ Ascorbic acid was used for reduction of graphene oxide to graphene. In summary, 25 mL H₂SO₄ (98%) in 100 mL H₂O was prepared and 0.5 g NaNO₃ was added and the solution was stirred for 2 h, then 0.5 g graphite was added. After that, 3 g KMnO₄ was slowly added and the mixture was stirred for 24 h at 30 °C. The mixture was diluted by adding 110 mL water at 50 °C and further treated with 10 mL H₂O₂ (20%). Suddenly, the color of the solution changed to bright yellow. After this event the mixture was diluted by adding 40 mL water. Finally, graphene oxide was formed and then filtered and washed with DI water several times to pH = 4.

Synthesis of sulfur/reduced graphene oxide (S/RGO)

To prepare S/RGO, 70 mg sodium thiosulfate was added to 140 mL of water and the mixture was sonicated for 1 h. Subsequently, 70 mg graphene oxide was added to the solution, then the mixture was sonicated for 3 h, followed by stirring for 30 min to form a homogenous solution. The pH value of the solution was adjusted to 1 by slowly adding 30% H₂SO₄ after which the color of the solution changed to gray. Then 1 mg of ascorbic acid was added into this solution to totally convert graphene oxide to graphene. While the solution was sonicated by adding 1 M NaOH, the pH was adjusted to 11. Finally, the composite formed and was washed with DI water and dried in an oven at 90 °C.

Synthesis of Fe₃O₄@sulfur/reduced graphene oxide

The synthesis of Fe₃O₄@S/RGO nanoparticles was carried out similar to our previous work²⁷ and via a co-precipitation method; first, 0.4 g FeCl₃·6H₂0 and 0.2 g FeSO₄·4H₂O (mole ratio: 2/1) were added to 60 mL of water while the solution was stirred at 70 °C for 3 h. Then, 0.1 g S/RGO was added to the mixture and kept stirring for 2 h at 80 °C. Finally, 10 mL NaOH (5% wt) was slowly added to the solution to precipitate and create the magnetic S/RGO after which the mixture was heated in an oven at 100 °C.

Photocatalysis process

In a typical process, the catalytic reaction was carried out in a 100 mL photo reactor, which contained 20 mL of MB dye (10 mg L⁻¹) solution and 0.02 g of catalyst (GO, pure sulfur, S/RGO, and Fe₃O₄@S/RGO). Before irradiation, the catalyst was dispersed by sonication (5 min) and then the solution stirred in the dark (15 min) to allow equilibrium of the system. Irradiation was carried out using a 7 W LED lamps as the light source. All photocatalytic experiments were carried out under the same conditions. Samples (3 mL) were collected during the irradiation and the photocatalyst was separated from dye solutions by centrifugation. The degradation was monitored by measuring absorbance using a double beam UV-vis spectrophotometer (Shimadzu UV-1700, Japan) at 664 and 580 nm wavelength for MB and CV, respectively.

Photocatalytic oxidation reaction

The liquid phase oxidation of benzyl alcohol and other benzyl alcohols to their corresponding benzaldehydes in presence of the prepared catalysts was carried out in a reflux flask equipped with a condenser under magnetic stirring and LED visible light irradiation for 2 h. In these experiments, 29 mmol benzyl alcohol (or substituted benzyl alcohols), 0.005 g catalyst and 0.052 mL H₂O₂ were added to 5.0 mL of acetonitrile as solvent. The reaction progress was monitored by TLC and GC. After implementing the reaction, the mixture was cooled to room temperature and the catalyst was separated with a magnet. The products were identified by GC-MS.

Conclusions

In summary, sulfur was successfully loaded onto reduced graphene oxide (S/RGO) by using sodium thiosulfate and graphene oxide. During this reaction, graphene oxide was reduced to graphene via an environmentally friendly method. Then Fe₃O₄ nanoparticles were immobilized on the surface of S/RGO to prepare a Fe₃O₄@S/RGO composite. The photocatalytic and photooxidative activity of synthesized S/RGO and Fe₃O₄@S/RGO was studied. The results showed that Fe₃O₄@S/RGO (with efficiency degradation of 87% and 76% for MB and CV, respectively) had better photocatalyst activity than S/RGO. On the other hand, the Fe₃O₄@S/RGO composite photodegraded MB more efficiently than CV. In the oxidation reaction, Fe₃O₄@S/RGO catalysts showed effective activity for selective oxidation of benzyl alcohols to benzaldehyde with H₂O₂ under reflux conditions. The association of graphene sheets with sulfur atoms and Fe₃O₄ – considerably increases the photocatalytic and photooxidative effects. The graphene functions for the electrons to gather to confront the electron–holes, leading to generating OH˙ and O₂˙ radicals, which results in degrading the dyes and oxidizing the alcohols. This catalytic oxidation reaction can be useful for producing benzaldehydes in industry.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00137h

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