Elimination of dibenzothiophene from transportation fuel by combined photocatalytic and adsorptive method

Materials Science in Semiconductor Processing 87 (2018) 110–118

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Elimination of dibenzothiophene from transportation fuel by combined photocatalytic and adsorptive method

T

⁎

Asma Hosseinia, Hossein Faghihiana, , Ali Mohammad Sanatib a b

Department of Chemistry, Islamic Azad University, Shahreza Branch, Shahreza, Iran Department of Environmental Science, Persian Gulf Research Institute, Persian Gulf University, Bushehr 75169, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Dibenzothiophene Transportation fuel Aromatic organosulfur compounds Photocatalyst ZnO, FSM-16 MCM-41

Photocatalytic degradation of dibenzothiophene was studied by use of ZnO/FSM-16 photocatalyst under UV and visible light irradiations. The photocatalyst was prepared by incorporation of ZnO into the FSM-16 nano-sized particles through reaction with zinc acetate solution followed by calcination process. The diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) analysis indicated that immobilization of ZnO on the surface of catalyst support lowered its bang gap energy to visible region and enhanced its degradation efficiency by lowering electron/hole recombination. The nanocomposite was characterized by XRD, FTIR, and BET techniques. It was concluded that the surface area of the synthesized photocatalyst was sufficiently high to promote the adsorption of the pollutant and subsequently enhanced the degradation process. The effect of different experimental parameters and the presence of H2O2 and some organic scavenger on degradation efficiency was studied. The results showed that under optimized conditions 80% of dibenzothiophene was degraded. The catalyst retained its activity after six regeneration cycles. The degradation products including the sulfur containing compounds were efficiently removed by use of MCM-41 adsorbent.

1. Introduction Sulfur compounds of transportation fuels are one of the main sources of atmospheric pollution sources [1]. These compounds spontaneously convert to sulfide oxides in the exhaust gases of diesel engines which eventually react with atmosphere damp producing H2SO4 [2]. Based on the environmental concerns, recent fuel standards obliges the fuel producers to reduce the sulfur content of gasoline to lower than 30mgL−1 and to lower than 15 mg L−1for diesels [3]. Hydrodesulphurization (HDS) technique is the most frequently used process that is enable to efficiently eliminate aliphatic and acyclic sulfur-containing compounds from the fuels. But benzothiophene, dibenzothiophene and their derivatives cannot be easily removed by HDS method. Additionally, high temperatures and high hydrogen pressures needed for the process causes numerous problems, including, high investment, high operating expenditure and limited length of catalysts life [4,5]. In recent years, advanced oxidation processes (AOPs) have been emerged as contemporary oxidative techniques for degradation of detrimental organic compounds both in industrial pretreatment and in full-scale treatment. Different Photocatalysts have been synthesized and used for degradation of various organic pollutants. Degradation of disperse Orange 25 (DO25) as one of the strong azo dyes was conducted

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by use of surface modified iron doped TiO2 nanoparticles under sunlight [6]. Photodegradation of Direct Blue 71 under irradiation by sunlight was performed by use of Fe: ZnO nanomaterials. The precursors were Fe2O3 as dopant, n-butylamine as surface modifier [7]. Optimization of solar degradation efficiency of bio-composting leachate by use of Nd: ZnO nanoparticles has been reported [8]. Fabrication and application of FeSexTe1−x structure has been reported by Sanghan Lee et al. [9]. Photocatalytic treatment of oil and grease spills in wastewater using coated N-doped TiO2 polyscales under sunlight was reported by Shivaraju et al. [10]. Bi2O3 rods/RGO composite has been synthesized by a simple precipitation and calcination method. The crystallinity, structural, and morphological features were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high resolution transmission electron microscopy (HR-TEM) techniques. The photocatalytic activity of the synthesized composites was evaluated by photocatalytic degradation of methylene blue (MB) dye under visiblelight irradiation [11]. Photocatalytic degradation of methylene blue by use of MnFe2O4 and MnFe2O4-CCM photocatalysts has been reported by Silambarasu et al. [12]. Preparation and characterization of nanostructured CdO thin films for photocatalytic decomposition of the methylene blue was conducted by Asath Bahadur et al. [13]. Enhanced photocatalytic activity of spinel CuxMn1–xFe2O4 nanocatalysts for

Corresponding author. E-mail address: [email protected] (H. Faghihian).

https://doi.org/10.1016/j.mssp.2018.07.017 Received 2 March 2018; Received in revised form 8 July 2018; Accepted 9 July 2018 1369-8001/ © 2018 Published by Elsevier Ltd.

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to 8.5 and the mixture was stirred at 70 °C for 3 h. The solid product (FSM-16) was filtered, washed with deionized water, dried in air, and calcined at 550 °C for 12 h to remove the remaining surfactant [25].

degradation of methylene blue dye was investigated by Mathubala et al. [14]. The (AOPs) process is the most prominent methods for elimination of organic pollutants by converting them into CO2 and H2O [15]. AOPs technique require an adequate heterogeneous photocatalyst enable to produce electron–hole pairs by UV or visible light irradiations. The electron-holes eventually produce hydroxyl radicals which destroy the pollutants in a nonselective manner [16,17]. ZnO as a cost-effective, stable and nontoxic semiconductor with the direct band gap of 3.37 eV is a promising photocatalyst for degradation of organic pollutants [18,19]. The capability of ZnO to absorb higher fractions of UV spectrum is higher than TiO2which is very advantageous for heterogeneous photocatalysis [20,21]. Also, immobilization of photocatalysts onto the adequate catalyst supports provides substantial improvement on their activity [22]. Folded sheet mesoporous materials (FSM-16) have surface areas greater than 1000 m2 g−1, large pore sizes, and one-dimensional pores with uniformly mesoporous size [23]. Moreover, the higher thermal stability of FSM-16 compared to MCM-41 promotes their increasing application in catalysis preparations [24]. The aim of the present study is to prepare and to characterize a new photocatalyst based on immobilization of ZnO on the FSM-16 catalyst support. Application of the synthesized photocatalyst for degradation of dibenzothiophene under different experimental conditions is also investigated. Adsorption of the degradation products by MCM-41 is also studied.

2.3. Synthesis of ZnO/FSM-16

2. Experimental

FSM-16 was impregnated with zinc oxide by ion exchange process. 3.0 g of the synthesized FSM-16 was addedto20 mL of zinc acetate (0.08 mol/L)dissolved in water/ethanol solvent (50%). The mixture was shaken for 24hat room temperature. The zinc loaded photocatalyst (ZnO/FSM-16) was separated by filtration, washed with ethanol to remove the remaining Zn2+ from the surface of photocatalyst, and dried at 100 °C for 2 h and then calcined at 380 °C for 4 h [26]. In the same manner, several photocatalysts containing (3–25% of ZnO) were prepared by using different concentration of zinc acetate solutions. The samples were designated as ZnO/FSM-16 (3%) to ZnO/FSM-16 (25%) according to their ZnO contents. To measure the Zn content of the synthesized photocatalysts, in a polyethylene vessel, 100 mg of the samples was dissolved in 5.0 mL of HNO3(65% w/v) and 1 mL of HClO4 (60% w/v) was added and the mixture was gently heated until proper evaporation of the acids. The procedure was repeated for three times and then 2.0 mL of hydrofluoric acid (30%w/v) was added and gently heated until proper dissolution. Then 10.0 mL of diluted HCl (10%v/v) was added and heated for 20 min. The solution was filtered and diluted to 25 mL in a volumetric flask and the zinc concentration of the solution was measured by AAS.

2.1. Materials and methods

2.4. Photodegradation of the pollutant

All chemicals; silicon dioxide(SiO2), cetyltrimethylammonium bromide(CTAB), ethanol (C2H6O),hydrochloric acid(HCl), sodium hydroxide (NaOH), zinc acetate(Zn(CH3COO)2 2H2O), nitric acid(HNO3), perchloric acid(HClO4), hydrofluoric acid(HF) and dibenzothiophene (C12H8S) were purchased from Merck company (Germany). The synthesized photocatalysts were characterized by X-ray diffraction method (XRD,Philips,PW1730) with CuKα = 1.5406A˚ radiation in 2θ range of 0.5–70 °. Fourier transformation infrared spectroscopy (FTIR) spectra were recorded by a Nicolet single beam FT-IR Impact 400D spectrophotometer by use of KBr pellets. The band gap energy of photocatalyst was determined using diffuse reflectance spectroscopy (DRS) by use of a JASC V 670 instrument (Japan) equipped with an integrating sphere in the wavelength range of 300–900 nm. Barium sulphate was used as the reference material. Nitrogen adsorption-desorption experiments were conducted by a BET, Micrometric model ASAP2020 instrument (USA). To study the surface morphology and chemical composition of the samples, the SEM studies and EDAX analysis were performed by scanning electron microscope, MIRA3LMU model, TESCAN Company. Photoluminescence (PL) spectra were taken by a Cary Eclipse (FL0906M003) instrument. Zinc content of the catalysts was measured by atomic absorption spectrometer (AAS), PG instrument AA500. Dibenzothiophene concentration was determined by a UV–visible spectrophotometer, Perkin Elmer model Lambda 25. The degradation products were identified by GC-MS (Model; Agilent GC 6890 N, Column; HP-5).

Photocatalytic performance of the synthesized photocatalysts for degradation of dibenzothiophene was evaluated under UV and visible light irradiations in a light tight chamber equipped with a pressure Hg lamp (60 W, Philips), and a fluorescent 60 W lamp located 10 cm above the degradation cell. Known amount of photocatalyst (0.05–0.4 g) was added into a Pyrex-glass cell containing 10 mL of dibenzothiophene dissolved in n-hexane. The mixture was shaken for 10 min in dark and then irradiated for known period (0.0–550 min)while agitated to ensure homogeneity. The reactor was then taken out, the solid was separated by centrifugation and the concentration of dibenzothiophene in the remaining solution was measured by UV–visible spectrophotometer at λmax= 237 nm. The degradation efficiency was calculated by the following equation:

% Degradation = ((A 0 − At)/A 0) × 100 Where (A0) and (At) are the absorbance of the solution before and after irradiation respectively related to the initial and final concentration of dibenzothiophene. To study the effect of surface adsorption, photolysis, catalyst dose, ZnO content of the catalyst, irradiation time, initial dibenzothiophene concentration on the degradation efficiency several series of experiment were accordingly performed. The effect of H2O2and some organic scavengers on the degradation efficiency was also evaluated. 3. Results and discussion

2.2. Synthesis of FSM-16

3.1. Characterization of the synthesized photocatalysts

For the synthesis of FSM-16, the quaternary ammonium cetyltrimethylammonium bromide (CTAB) was used as template.6.0 g of sodium silicate was added to 120 mL of sodium hydroxide (0.27 M) and the mixture was stirred for 3hat room temperature. The solid was separated, dried at100°C in a vacuum oven and then calcined at 700 °C for 6 h to obtain layered sodium silicate (kanemite, Na2Si2O5).3.0 g of the product was dispersed in 30mLof deionized water and stirred for 3 h. The suspension was filtered and the damp kanemite paste was dispersed into 112mLof aqueous solution of CTAB (0.125 M), the pH was adjusted

3.1.1. XRD patterns The XRD patterns of FSM-16, ZnO, and ZnO/FSM-16 (20%) are represented in Fig. 1. In the low angle XRD pattern of FSM-16 (2θ < 10◦), four characteristic diffraction lines belonged to (100), (110), (200), and (210) diffraction planes of two-dimensional hexagonal structure was observed (Fig. 1.a) [27]. In the XRD pattern of ZnO, the diffraction line located in the 2θ range of 30–65° were indexed for hexagonal phase of ZnO as indicated in (JCPDS 36-1451) [28]. In the XRD pattern of the synthesized 111

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Fig. 1. XRD patterns of FSM-16 (a), ZnO (b), and ZnO/FSM-16 (20%)(c). Fig. 3. EDAX analysis of ZnO/FSM-16 sample.

photocatalyst (ZnO/FSM-16 (20%)), beside the diffraction lines of FSM16 located at low angle regions, the characteristic diffraction lines of ZnO were also observed without any changes in their position indicating that after immobilization of ZnO on the FSM-16 support, the structure of photocatalyst and the support remained intact. The average particle size of the samples was determined according to Scherrer's equation; D = 0.9λ/βcosθ, where λ and β are the X-ray wavelength and the width of the line at half maximum intensity respectively. The FSM16 and ZnO/FSM-16 particle size were approximately 21and 49 nm indicating that the synthesized photocatalyst was nano-sized. 3.1.2. SEM image and EDAX analysis of the photocatalyst To study the surface morphology of the ZnO/FSM-16 sample, its SEM images were prepared (Fig. 2). The surface chemical composition of the sample was determined by EDAX analysis (Fig. 3). In the SEM image of ZnO/FSM-16 uniform hexagonal crystals were observed. The average particle size measured by the SEM image was close to the values obtained by XRD pattern. The EDAX spectra of ZnO/FSM-16 revealed that the surface of sample was composed of Si, O, Zn, and the corresponding peaks of Zn and O confirmed the formation of ZnO on the surface of FSM-16.

Fig. 4. FT-IR spectra of FSM-16 (a), and ZnO/FSM-16 (20%) (b).

the support [29]. The absorption band at 1635 cm−1 was attributed to the bending vibration of absorbed water molecules. The strong peak at 823 cm−1 belonged to symmetrical tensile vibrations of (Si‒O‒Si) and the band at 1060 cm−1 referred to the asymmetric tensile vibrations of the siloxane group(Si‒O‒Si). The vibrational-bending of (‒Si‒O‒Si‒) appeared at 465 cm−1. The absorption bands observed in the spectra of the synthesized sample was in good agreement with the results reported by Tiago Borrego᾽ [30].

3.1.3. FTIRs Spectra of the photocatalysts The FTIR spectra ofFSM-16 and ZnO/FSM-16 (20%) are represented in Fig. 4. In the spectrum ofFSM-16,the wide adsorption band appeared in the region of 3084 cm−1–3800 cm−1 with maximum absorption at 3417 cm−1 was related to water molecules adsorbed on the surface of

Fig. 2. SEM patterns of ZnO/FSM-16 sample. 112

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In the spectrum of ZnO/FSM-16 (Fig. 4.b), the absorption bands belonged to FSM-16 were observed at the same positions. The extra absorption band appeared at 475 cm−1was attributed to Zn‒O vibrations [31]. The broadening of the bands in the region of 475–798 cm−1was attributed to the combination of the stretching Zn‒O bands with the original vibrational bands of the FSM-16 framework [32].

Table 1 Direct band gaps of the photocatalysts (eV). n 1/2

photocatalysts ZnO 3.37

ZnO/FSM-16 2.91

3.1.4. Influence of support on the band gap energy of ZnO 3.1.4.1. Study by DRS analysis. Diffuse reflectance spectra of ZnO and ZnO/FSM-16 were obtained by the Kubelka-Munk and Tauc plots and the band gap energy was calculated by the following equations [33,34]:

K = (1 − R)2 /2R

(1)

Where, K is the reflectance transformed and R is the reflectance percentage [35]. Direct band gaps of the photocatalysts (eV) were calculated according to Tauc equations:

αh ν = k(h ν − Eg)n

(2)

Where αhν is the absorption coefficient, h is Planck's constant, υ is photon's frequency). The plot of (αhν)1/2 versus hv was drawn to calculate the direct transitions band gaps of ZnO and ZnO/FSM-16 (Fig. 5). The results are given in Table 1 [36]. The results indicated that a significant reduction in the band gap energy of ZnO/FSM-16 compared to bulk ZnO was occurred. The shift from 3.37 to 2.91 eV is extremely beneficial for degradation of dibenzothiophene by visible light irradiation [37]. As indicated in the figure, the relative absorption intensity of ZnO/FSM-16 was significantly higher than the value observed for bulk ZnO indicating that photocatalytic activity was enhanced after loading of ZnO on the catalyst support [33].

Fig. 6. PL spectra of ZnO(a), ZnO/FSM-16 (b).

3.1.4.2. Study by photoluminescence analysis. The photoluminescence spectra were recorded for bulk ZnO and ZnO-FSM-16 nanocomposite (Fig. 6). The excitation wavelength was 290 nm and the emission peaks of ZnO were emerged at 377 and 420 nm. In the ZnO/FSM-16 spectra, the intensity of the peaks at 377, 420 belonged to ZnO were significantly decreased indicating that immobilization of ZnO on the ZnO/FSM-16 support lowered electron/hole recombination. From the results of this experiments and the DRS analysis the following conclusions were withdrawn: -By using of catalyst support he band gap of ZnO shifted to the lower energy facilitating the degradation of the pollutant by use of visible light. -By using of catalyst support, the degradation efficiency was enhanced by lowering electron/hole recombination. Similar observations was made by Torki et.al who immobilized NiS, and NiS photocatalyst on Fe3O4 @PPY for degradation of cephalexin [38].

Fig. 7. Nitrogen adsorption-desorption isotherms of FSM-16 (a) ، ZnO/FSM-16 (b) and BJH plots (inset).

3.1.5. BET analysis The information concerning pore size, surface area, and pore volume of the samples were provided by nitrogen adsorption-desorption isotherms performed by BET and BJH methods (Fig. 7). ForFSM-16 sample, the adsorption isotherm was almost coincides with the desorption branch which is the indication of high mesoporous uniformity of the sample. Similar observation was made for ZnO/FSM-16 indicating that the mesoporous uniformity of FSM-16 did not change after incorporation of ZnO onto the structure of FSM-16 [39]. From the results given in Table 2, it was concluded that the specific

Fig. 5. DRS spectra of ZnO and ZnO/FSM-16 samples. 113

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Table 2 Specific surface area, pore diameter and pore volume of the photocatalysts.

FSM-16 ZnO/FSM-16

Specific surface area (m2/g) 1001.44 823.14

Average pore diameter (nm) 4.83 4.52

Degradation efficiency(%)

Samples

FSM-16

Total pore volume (cm3/g) 1.21 0.93

60

ZnO ZnO/FSM-16

50

photolysis adsorption

40 30 20 10

surface area of ZnO/FSM-16 was slightly lowered after incorporation of ZnO on to the structure of catalyst support. This was attributed to the partial engagement of the pores by the host molecules. This assumption was supported by the lower pore diameters and pore volumes obtained for ZnO/FSM-16 as indicated in Table 2. However the surface area of the synthesized photocatalyst was sufficiently high and very advantageous for the catalyst performance. The specific surface area of ZnO/ FSM-16 was 834.14 m2/g and much higher than the values reported for previously studied photocatalysts. H.G. Chen used BET techniques for determination of surface area and pore volume of the synthesized photocatalyst prepared by immobilization of ZnO on MCM-41. They reported that after immobilization of ZnO on the catalyst support the surface area was significantly decreased [40]. Kenichi Shimizu et al. synthesized FSM-16 and Ir/FSM-16 catalyst for hydrogenation of linoleic acid. The catalyst, Ir/FSM-16 was prepared by impregnation method and was analyzed by BET method. The results indicated that after introduction of Ir into the catalyst support, the surface area was reduced from 1043 to 912 m2g−1 [41]. To estimate the nature of the pores of the photocatalyst, the BJH plot was prepared and given in Fig. 7. It was concluded that the nature of the pore was mesoporous.

0

0

100

200

300 Time (min)

400

500

600

a 60

Degradation efficiency(%)

FSM-16 50

ZnO ZnO/FSM-16

40

Photolysis

30 20 10 0

0

100

200

300 Time (min)

400

500

600

b

Fig. 8. Degradation of dibenzothiophene versus times under UV (a) and visible light (b) irradiations.

3.2. Photocatalysts performance and the equilibrium was established within 400 and 450 min respectively for UV and visible light irradiation and at half equilibrium time respectively 66% and 44% of degradation were obtained. The equilibrium time of this work was shorter than the value reported for previously reported photocatalysts. Degradation of dibenzothiophene studied by TiO2needed 600 min time to achieve equilibration [43]. Zhang et al. used magnetic polyoxometalate in the presence of hydrogen peroxide for degradation of dibenzothiophene and obtained 92% degradation after 10 h [44].

3.2.1. Effect of catalyst support To evaluate the photocatalyts performance for degradation of dibenzothiophene, the amount of adsorbed pollutant and the photolysis process were measured prior to degradation experiments. The results indicated that the adsorption and direct photolysis of dibenzothiophene was insignificant (Fig. 8a and b). The degradation efficiency ofFSM-16, bulk ZnO, and ZnO/FSM-16, was then measured under UV and visible light irradiations. The results indicated that the degradation efficiency obtained by FSM-16 under UV and visible light irradiations was very limited. By the bulk ZnO respectively 45% and 35% of the pollutant was decomposed under UV and visible lights. The degradation efficiency of ZnO/FSM-16 was much higher and the efficiencies of 60% and 50% was respectively obtained by UV and visible light irradiations. The higher degradation efficiency of ZnO/FSM-16 compared to bulk ZnO was justified by the fact that by using of the catalyst support, the ZnO particles were homogenously distributed on the surface of the support, helping the direct interaction of generated radicals with the pollutant molecules. Moreover as discussed in Section 3.1.4.2, the support hampered the electron/hole recombination which leading to enhanced degradation efficiency. The higher photocatalytic activity of ZnO/FSM-16 was also attributed to the higher surface area of ZnO/FSM-16 compared to ZnO. The ZnO/FSM-16 surface area was 823.15 m2 g−1 while the value for ZnO was reported to be 79.13 m2 g−1(Table 2) [42].

3.2.3. Optimized catalyst dose The degradation efficiency of dibenzothiophene was studied in the catalyst concentration range of 0.05–0.4 g L−1under UV and visible light irradiations (Fig. 9). By increasing of catalyst dose, the presence of more active sites enhanced the number of generated electron/holepairs. The optimized amount of catalyst dose was 0.3 g L−1and then degradation efficiency was slowly decreased. Beyond the optimum concentration, aggregation of photocatalyst particles reduced the surface area of the photocatalyst leading to lower degradation efficiency. Moreover, with the aggregated particles, scattering of the incident photon was higher and hence lower electron/hole pairs were generated [45,46]. Naeemi et al. used clinoptilolite impregnated with titanium oxide for degradation of dibenzothiophene and concluded that the optimal degradation was obtained with 0.4 g L−1 of the photocatalyst [47]. Arellano et al. used CuO/GC photocatalyst for degradation of dibenzothiophene and reported the maximal degradation efficiency with 0.3 g L−1of catalyst [48].

3.2.2. Effect of irradiation time To evaluate the kinetic of the degradation process, the degradation efficiency was measured at different time interval (0.0–550 mints). The degradation solution was prepared by adding 0.2 g of the photocatalyst to 25 mL of dibenzothiophene solution (100 mg L−1). The mixture was shaken for the known period of time under UV or visible irradiations. The photocatalyst was removed and dibenzothiophene concentration was measured in the remaining solutions. As indicated in Fig. 8, the degradation efficiency was increased with increasing of irradiation time

3.2.4. Effect of initial dibenzothiophene concentration The effect of initial concentration of dibenzothiophene on the degradation efficiency was studied on concentration range of 100–600 mg L−1 under UV and visible irradiations (Fig. 10). It was concluded that by increasing of dibenzothiophene concentration from 100 to 200 mg L−1, the degradation efficiency was increased and thereafter the degradation extent was decreased. The maximal 114

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Degradation efficiensy(%)

Degradation efficiency(%)

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30

200mg/L

20

400mg/L

10

600mg/L

0

0

100

200 300 Time (min)

400

ZnO/FSM-16(3%) ZnO/FSM-16(5%) ZnO/FSM-16(10%) ZnO/FSM-16(15%) ZnO/FSM-16(20%) ZnO/FSM-16(25%)

60 50 40 30 20 10

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a

0

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200 300 Time (min)

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a 200 mg/L

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400mg/L

60 Degradation efficiency(%)

Degradation efficiency(%)

100 mg/L

600 mg/L

50 40 30 20 10 0

Time (min)

b

ZnO/FSM-16(3%) ZnO/FSM-16(5%) ZnO/FSM-16(10%) ZnO/FSM-16(15%) ZnO/FSM-16(20%) ZnO/FSM-16(25%)

0

100

200

300

400

500

Time(min)

Fig. 9. Effect of catalyst dose on dibenzothiophene degradation under UV (a) and visible light (b) irradiations.

b Fig. 11. Effect of ZnO loading on dibenzothiophene degradation under UV (a) and visible light (b) irradiations.

Degradation efficiency(%)

80 70

degradation of 70% observed at concentration of 200 mg L−1, can be justified by the fact that by increasing of pollutant concentration, the diffusion of dibenzothiophene molecules to the catalyst surface where the hydroxyl radicals were generated was facilitated. Since the hydroxyl radicals’ lifetime is very short, they will be deactivated if the pollutant concentration close to the surface is not sufficient. By increasing of dibenzothiophene concentration beyond 200 mg L−1, the incident photons are partially absorbed by dibenzothiophene molecules causing g lower generated OH radicals [49]. Faghihian et al. used silicate mesoporous material impregnated with titanium dioxide for elimination of dibenzothiophene and reported that the optimized degradation was obtained with 200 mg L−1 solution [50]. Moradi et al. studied the photocatalytic degradation of dibenzothiophene by use of La/PEG catalyst modified with TiO2 and reported that the maximal efficiency was obtained in 250 mg L−1 solution [51].

60 50 40 100mg/L

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0

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Degradation efficiency(%)

100 mg/L 200 mg/L 400mg/L

3.2.5. Optimized ZnO loading To study the effect of zinc oxide loading on the degradation efficiency of dibenzothiophene, a series of degradation experiments were conducted with the photocatalyst containing different amount of ZnO (3–25%). The degradation solution was irradiated at the optimized conditions under UV or visible lights. The results are represented in Fig. 11. It was concluded that by increasing of photocatalyst loading, the degradation efficiency increased and the optimized value was obtained with the catalyst loading of 20%. The lower efficiency observed at higher ZnO loading was attributed to the tendency of the particle for aggregation which reduced interfacial surface between reaction solution and the photocatalyst particles reducing the number of active sites [52]. This also increased recombination of photo-generated electron/ hole, reducing the adsorption capacity of the support, and lastly decreased the photodegradation extent [53]. Park et al. who used carbon mesoporous materials impregnated

600 mg/L

Time (min)

b Fig. 10. Effect of initial dibenzothiophene concentration on degradation efficiency under UV (a) and visible light (b) irradiations.

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Degradation efficiency(%)

Degradation efficiency(%)

90 70 60 50 40 30 20 10

60 50 40 30 20 10

0

0

3% 10% H2O2 Concentration(%)

20%

0

Fig. 12. Effect of H2O2 addition on dibenzothiophene degradation.

H2O2 + e → OH+OH

H2O2+O-•2 →OH• + OH-+O2 •

H2O2+OH →H2O

+OH•2

Isopropanol

TBHP

Table 3 Scavenging effect of some organic scavengers. •

OH and e-aq scavenger OH and e-aq scavenger O2•− scavenger

Iso-propanol TBHP Benzoquinone

•

classified to h+ and •OH scavenger, e-aq and •OH scavenger,h+ scavenger, O2•− scavenger and °OH scavenger as indicated in Table 3 [60]. It is suggested that isopropanol reacts more quickly than dibenzothiophene with OH and e-according to Eqs. (5) and (6) generating more stable radicals and causes a significant decrease on °OH and econcentration [38].

3.2.6. Addition of H2O2 In presence of H2O2, due to generation of more •OH radicals, higher degradation rates has been reported. In this research, the effect of hydrogen peroxide on degradation of dibenzothiophene was studied by adding different aliquots of H2O2 to 10 mL of pollutant solution and the degradation solution was irradiated under optimized conditions (Fig. 12). By addition of hydrogen peroxide additional OH radicals were initially yielded by reaction between hydrogen peroxide and e- or O•−2 (Eqs. (3) and (4)). The extra amount of hydrogen peroxide will trap OH• radicals and convert them to weaker HO•2 radicals which restrained the degradation of dibenzothiophene [57]. The optimized degradation yield was obtained with addition of 3% of hydrogen peroxide. -

Benzoquinone

Fig. 13. Effect of organic scavengers on dibenzothiophene degradation.

with TiO2 for degradation of rodamine observed that the optimized photocatalyst loading was 10% [54]. X.M.Yan et al. studied the degradation of dibenzothiophene by a photocatalyst prepared by immobilization of TiO2 on phosphotangestic acid and reported that the maximal degradation was achieved with 20% of catalyst loading [55]. Ettireddy et al. employed TiO2-loaded MCM-41 catalysts for visible and UV light-driven photodegradation of aqueous organic pollutants and reported that the catalyst with 25% ofTiO2 loading had the maximal degradation efficiency [56]. From the results obtained by different investigation, it was concluded that the optimized catalyst loading depended on the photocatalyst type and catalyst support.

•

0

•

OH + i-prOH → H2O + •i-prOH+ i-prO• •

e + i-prOH → H + i-prO -

-

(6) (7)

(O2-°)

Benzoquinone is a super oxide radical quencher and according to Eqs. (8)–(11) prevents the generation of °OH radicals [38]. It is noteworthy that in the presence of benzoquinone, determination of dibenzothiophene concentration was impossible because of spectra overlapping, therefore the measurement was performed by HPLC technique. O2 + e- →O2•-

(8)

(3)

O2•- + h+→ H2O2 + O2

(4)

O2•-

(5)

H2O2 + e → OH + OH

+e → H2O2 + H -

+

-

Similar results was reported in the previously published papers. Matsuzawa et al. observed that in the photocatalytic oxidation of dibenzothiophene by addition of hydrogen peroxide (3%) the degradation rate was significantly increased and beyond this concentration remarkable decrease was obtained [58]. Saadati et al. report degradation of tetracycline by using TiO2 under UV irradiation. The results showed that by increasing of H2O2 concentration in the range of 50–100 mg L_1, the rate constant of degradation was increased [59].

-

(9) (10)

•

(11)

TBHP reacts more rapidly than dibenzothiophene with HO°and e-causing asignificant decrease on their concentration according to Eqs. (12) and (13) [35]. TBHP + •OH → H2O + TBHP• + TBP• •

TBHP + e → H + TBP -

-

(12) (13)

Similar effect has been reported in previously published works. Torki et al. studied the effect of benzoquinone on the degradation of cephalexin by use of NiS-PPY-Fe3O4 nanophotocatalyst and reported that benzoquinone acting as an organic scavenger [38].

3.2.7. Addition of organic scavenger To examine the effect of organic scavengers on the degradation of dibenzothiophene, the process was conducted in the presence of isopropanol, benzoquinone and tert-butyl hydroperoxide solutions (TBHP) (0.1 M) under optimized conditions. The scavengers were added to the degradation solution and the degradation yield was calculated following irradiation step. The results are given in Fig. 13. In advanced oxidation processes, after generation of electron/hole, the hydroxyl radicals (•OH), super-oxide radical (•O2−) are produced and acting as the effective species in the degradation process. These process can be scavenged by different compounds. The scavengers are

3.3. Removal of degradation products from degradation solution The degradation products was analyzed by Gas-Mass Spectrometry using a (Model; Agilent GC 6890 N),equipped with 30 m fused silica capillary column (HP-5MS),0.25 mm i.d. and 0.25 µm film thickness, by use of helium as carrier. The degradation products produced by the sample irradiated by UV are given in Table 4 and those produced by the sample irradiated by 116

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100

Table 4 Identified degradation products produced (UV irradiation) before adsorption and after adsorption.

Products

Dibenzothiophene 2-ethyl, thiophene 2–butyl, thiophene Cyclohexane, 1,1dimethyl 2-Hexenoic acid, 2hexenyl ester 2-isopenthyl, thiophen Cyclohexane Biphenyl Dibenzothiophenesulfone Benzendicarboxylic acid a

After adsorption Retention time (min)

Products

11.20 29.67 23.20 15.11

Dibenzothiophene N.da N.da N.da

12.16

N.da

33.05 6.12 17.89 35.11

N.da Cyclohexane N.da N.da

42.45

Benzendicarboxylic acid

Degradation efficiency(%)

Before adsorption

Retention time (min) 11.30

ZnO/FSM-16(150˚C)

80

ZnO/FSM-16(450˚C)

70 60 50 40 30 20 10

6.13

0

1

2

3

4

5

6

No.Runs

42.46

Fig. 14. Regeneration of photocatalyst.

Nd= Not detected.

The results represented in Fig. 14 showed that the photocatalyst regenerated at 450 °C had higher activity than that regenerated at 150 °C. The capability of the regenerated photocatalyst of this work was superior to the previously studied photocatalysts. Faghihian et al. used TiO2 immobilized on the clinoptilolite surface for degradation of dibenzothiophene and concluded that the application of photocatalyst was limited to four regeneration cycles [47]. Application of phosphotangestic acid modified with silver nanoparticles for degradation of dibenzothiophene indicated that the regeneration of photocatalyst was limited to three cycles [62].

Table 5 Identified degradation products produced (visible light irradiation) before adsorption and after adsorption. Before adsorption

After adsorption

Products

Retention time (min)

Products

Retention time (min)

Dibenzothiophene 2-ethyl thiophene 2–butyl thiophene Cyclohexane, 1, 1dimethyl 2-isopenthyl, thiophen Cyclohexane Biphenyl Dibenzothiophenesulfone Benzendicarboxylic acid

11.20 29.6 23.20 15.11

Dibenzothiophene N.da N.da N.da

11.30

33.05 6.12 17.89 35.11

N.da Cyclohexane N.da N.da

42.45

Benzendicarboxylic acid

a

90

4. Conclusions In this research, FSM-16 mesoporous material with ordered mesoporous structure was synthesized and was used as catalyst support for ZnO. The synthesized photocatalyst was used for degradation of dibenzothiophene under UV and visible light irradiations. The results indicated that by immobilization of ZnO on the FSM-16 support, the activity of photocatalyst was sharply increased. The band gap energy of ZnO after immobilization on the support was shifted to lower energy enhancing the degradation process by visible light-assisted degradation. Addition of hydrogen peroxide enhanced the photodegradation yield while the studied organic scavengers reduced the yield of degradation process. The photocatalyst showed good regeneration ability and retained more that 60% of its initial activity after 6 regeneration cycles. The degradation products were identified by GC-Mass, some contained sulfur in their compositions which were properly removed by the used adsorbent. On the whole, since the HDS technique as the most conventional desulfurization technique is not capable to remove the aromatic sulfur containing compounds, the combined photodegradationadsorption method used in this study remove the drawback of the HDS method and help to deep desulfurization of the transportation fuels.

6.13

42.46

Nd = Not detected.

visible light are given in Table 5. Some of these compounds including 2ethylthiophenes, 2-buthylthiophenes, thiophene, 2-hexyl, Thiophene 2pentyl, 2-isopenthyl thiophene, dibenzothiophene-sulfone contained sulfur in their compositions. For profound desulfurization, these compounds are to be removed from the solution. For this purpose, the adsorption method by use of mesoporous MCM-41 was performed. MCM41 is an ordered mesoporous materials, which exhibit high adsorptive properties and is a potentially interesting adsorbent for the removal of organic materials. [61]. The adsorption process was conducted by mixing of 50 mL of the degradation solution with 0.2 g of MCM-41. The mixture was shaken for 10 h at room temperature. The adsorbent was then separated and the remaining solution was analyzed by GC-Mass technique. The remaining compound were identified and listed in Tables 4, 5 respectively for the sample irradiated by UV and visible light. The result indicated that sulfur containing compounds were properly removed.

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