Extractive and oxidative desulfurization of model oil in polyethylene glycol

Abstract

The polyethylene glycol 200 (PEG-200) was explored as a green solvent for extractive and catalytic oxidative desulfurization (ODS) process. ODS experiments were conducted using model oil with SeO₂ as catalyst and H₂O₂ as an oxidant. The desulfurization system was very efficient for the removal of DBTs under mild conditions. The reaction conditions such as the amount of PEG-200, temperature, mole ratio of H₂O₂ and sulfur compound were investigated. The different sulfur substrates removed by this desulfurization system were also investigated and the following order was found: dibenzothiophene (DBT) > 4,6-dimethyl dibenzothiophene (4,6-DMBT) > benzothiophene (BT). The desulfurization system could be used 7 times without significant decrease in activity. The possible catalytic mechanism seems that peroxyseleninic acid may be considered as the true oxidative agent formed from hydrogen peroxide and selenium dioxide, which translated the DBT to the corresponding sulfone.

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Introduction

Sulfur compounds in fuels are converted to SO_x during combustion, which is one of the major sources of air pollution. SO_x not only results in air pollution but also poisons catalytic converters in cars. Primarily for environmental concerns, governments worldwide have issued increasingly stringent regulations to limit sulfur levels in fuels. The demand for producing low-sulfur fuels imposes significant challenges to current desulfurization methods and the development of new technologies.¹

In the petroleum refining industry, the conventional method for reducing sulfur is catalytic hydrodesulfurization (HDS). It is efficient for the removal of thiols, sulfides and thiophenes, but less effective for removing refractory sulfur compounds such as dibenzothiophene, and their alkyl derivatives.^2,3 Severe hydrodesulfurization conditions such as very high temperatures (300–400 °C) and pressures (2–10 Mpa of H₂) are required in order to remove such compounds.⁴ These operating conditions result in large hydrogen consumption and a significant increase in the operating expenses.^36,37 Therefore, extensive research is carried out to propose alternative technologies to obtain low-sulfur fuels such as extraction, adsorption, biodesulfurization, oxidation and supercritical water desulfurization.^{5–15,38–40}

The extraction desulfurization process can be carried out at ambient conditions, for example, standard temperature and pressure, especially without H₂. Different organic solvents, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and 1-methyl-2-pyrrolidinone (NMP) have been used as extractive solvents in desulfurization.^16,32 Nevertheless, the commonly used solvents are volatile, flammable, toxic and they have serious consequences for the environment. Ionic liquids (ILs) as “green solvent” are receiving much attention because some of their properties make them excellent choices as reaction media and extraction solvents and in some applications. The first attempt of deep desulfurization using ILs was made by the groups of Wasserscheid and Jess in 2001, which was followed by several publications.⁶ The desulfurization of fuels oil by extraction has been described during past few years. But, the efficiencies of sulfur removal with various kinds of ILs used as extractants are rather low. To obtain deep desulfurization, multistage extraction must be operated or extraction should be integrated with oxidative desulfurization process. Various oxidants have been investigated in the previous studies, such as hydrogen peroxide (H₂O₂), NO₂, O₂, O₃, K₂FeO₄, and organic peroxides.^17–30 Among these oxidants, H₂O₂ is the most widely employed.

Compared with the expensive ILs, PEG-200 has several advantages such as low viscosity, non-toxicity (which approved by the Food and Drug Administration (FDA), USA), corrosion inhibition, cheap and easily obtained.³¹ Saeid Azizian et al. and other researchers reported PEG-200 had high desulfurization ratio (about 75%) with single time, when the volume ratio of PEG/fuel is 1 : 1 at room temperature.^31,32 PEG-200 as a proton donor solvent with high extraction capability can be related to the possibility of bond formation between hydrogen of PEG and sulfur atom in DBT.³¹ However, either large volumes of PEG-200 or multistage extraction for an efficient liquid–liquid extraction process must be used to obtain deep desulfurization.

A careful survey of literature showed that there were few detailed studies about PEG-200 extractive coupling with oxidative desulfurization of fuel oil. Therefore, in consideration of the property of PEG-200 such as green, cheap and easily obtained, the oxidation is coupled with extraction for the deep desulfurization of model fuel. In the present work, a mild and effective catalyst SeO₂ was introduced and PEG-200 was utilized as extractant for extractive and oxidative desulfurization of model fuel oil with H₂O₂. The catalyst is efficient for catalytic oxidative DBTs. The catalytic factors and mechanism were systematically investigated in the desulfurization of model fuel.

Experimental

Materials

BT (>98%), DBT (>98%) were purchased from Alfa Aesar, USA. 4,6-DMDBT (>97%) was obtained from ABCR, Germany. PEG-200 (AR grade), n-octane (AR grade), hydrogen peroxide (aqueous solution, 30%), were purchased from Shanghai Sinopharm Chemical Co., Ltd.

Desulfurization of model oil

Model oil was prepared by dissolving BT, DBT or 4,6-DMDBT in n-octane giving a corresponding sulfur content 500 μg mL⁻¹, separately. All the oxidative and extractive desulfurization experiments were conducted in 50 mL round bottom flask equipped with stirrer under the different conditions. In a typical run, the PEG-200 (0.7 mL), catalyst (SeO₂) 10 mg and 48 μL of H₂O₂ were added to the round bottom flask. Then, the model oil (DBT, 5 mL, 500 μg mL⁻¹) was transferred into the flask. The mixture was stirred and heated to 60 °C in an oil bath. The upper oil phase was periodically withdrawn and analyzed by gas chromatography coupled with flame photometric detection (GC-FPD). A 30 m × 0.25 mm inner diameter × 0.33 mm film thickness Se-30 capillary column was used for separation. Analysis conditions were as follows: injection temperature, 280 °C; detector temperature, 250 °C; oven temperature, 240 °C. The sulfur content in the model fuel was detected with ultraviolet fluorescence sulfur analyzer (TS-3000, Jiangsu Jiangfen Electroanalytical instrument Co., Ltd.). The oxidation product of DBT was detected by gas chromatography-mass spectroscopy (GC-MS, Agilent 7890A GC coupled with an Agilent 5975C MSD).

Results and discussion

Effect of reaction conditions on DBT removal

The extractive and oxidative desulfurization of model oil was investigated firstly with SeO₂ as the catalyst and H₂O₂ as an oxidant in the PEG-200. DBT is chosen as a model thiophenic compound because it constitutes the majority of the refractory thiophenic compounds remaining in fuels after the HDS process. The different desulfurization systems with and without H₂O₂ were employed to investigate whether the catalyst is efficient. The results are shown in Table 1. The sulfur removal of DBT was only 20.5% and 19.6% without catalyst (entry 1, 2), respectively. But the sulfur removal of DBT increased sharply and reached 100% with catalyst at the same reaction conditions (entry 3). The results show that catalyst is efficient for catalytic oxidative DBT. Fig. S1† shows the GC-FPD analysis before and after the catalytic oxidation of DBT. DBT in n-octane could not be detected after extraction and catalytic oxidation at 60 °C in 5 h. The corresponding oxides did not exist in the oil phase.

Table 1 Influence of different desulfurization systems^a

Entry	Desulfurization systems	Sulfur removal (%)
a Conditions: DBT, 500 μg mL^−1, 5 mL; PEG-200, 0.7 mL; reaction temperature, 60 °C; reaction time, 5 h.b O/S = 6.c Catalyst, 10 mg.
1	PEG-200^a	20.5
2	PEG-200 + H2O2^b	19.6
3	PEG-200 + H2O2 + catalyst^c	100

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The temperature plays an important role on the desulfurization of DBT from the model oil. As shown in Fig. 1, the sulfur removal increases from 54.1% to 90.7% and 100% in 5 h when the temperature is enhanced from 25 °C to 45 °C and 60 °C, respectively. The temperature increase can accelerate the reaction of the catalyst with H₂O₂ and more DBT is oxidized. It is interesting that the DBT can be removed to 99% at 25 °C with the reaction over time to 25 h. The results also show that SeO₂ is a mild and effective catalyst for oxidative DBT with H₂O₂ under the room temperature.

Fig. 1 Effect of different temperature on sulfur removal (conditions: sulfur content, 500 μg mL⁻¹; model oil, 5 mL; PEG-200, 0.7 mL; catalyst, 10 mg; O/S = 6).

In the oxidative desulfurization system, one of the main factors is the O/S molar ratio on sulfur removal. In order to investigate the effect of O/S molar ratio on the desulfurization rate, a series of experiments with different dosage of H₂O₂ were carried out under the same reaction conditions. The results were shown in Fig. 2. Stoichiometrically, 2 molar H₂O₂ can transform DBT to DBTO₂. However, the sulfur removal of DBT is 66.1%. One of the reasons is the decomposition of H₂O₂. With the increase of molar ratio of O/S from 2 to 4, the DBT removal ratio enhance from 66.1% to 99.8% in 5 h. The DBT could not be detected in oil phase when the O/S reached 6 in 5 h.

Fig. 2 Effect of different molar ratio of O and S on sulfur removal (conditions: sulfur content, 500 μg mL⁻¹; model oil 5, mL; PEG-200, 0.7 mL; catalyst, 10 mg; reaction temperature, 60 °C).

Fig. 3 illustrates the effect of PEG-200 dosage on sulfur removal. The desulfuration ratio increased with the volume ratio of PEG-200 to oil up to 0.14 : 1 (PEG-200, 0.7 mL) and then decreased beyond this value. Nevertheless more DBT can be extracted from oil phase when the volume ratio of PEG-200 to oil increases to 0.2 : 1 or 0.4 : 1, the DBT removal ratio reduced to 90.9% and 80.4%, respectively. It is different with desulfurization using IL as extractant and polyoxometalate as catalyst.⁴¹ One of reason may be the mild catalyst. The density of peroxyseleninic acid in PEG-200 decreased when the volume ratio of PEG-200 to oil increased (see Fig. 6). It resulted to the less DBT conversion within a certain period of time. The same phenomenon had occurred when methanol was used as extractant under the same reaction conditions. The further research need be carried out. So v_PEG/v_oil = 0.14, O/S = 6 and 60 °C were chosen in the follow experiment.

Fig. 3 Effect of different volume of PEG-200 on sulfur removal (conditions: sulfur content, 500 μg mL⁻¹; model oil, 5 mL; O/S = 6; catalyst, 10 mg; reaction temperature, 60 °C; reaction time, 5 h).

Effect of different sulfur substrates on desulfurization

The features of the other sulfur substrates, such as BT, and 4,6-DMDBT, present in fuel were also investigated in this work. So, the reactivity of different sulfur-containing compounds, including BT, DBT and 4,6-DMDBT which the initial concentration of sulfur in model oil was 500 μg mL⁻¹ respectively, was estimated for oxidation with the desulfurization system at the same conditions. The removal of sulfur-containing compounds versus the reaction times in the system is shown in Fig. 4. The extractive ability of PEG-200 for different sulfur-containing compounds is at the following order: DBT > BT > 4,6-DMBT as investigated with Kianpour.³² Fig. 4 shows that the extractive and catalytic oxidation desulfurization of BT, DBT and 4,6-DMDBT was 100%, 97.3%, and 56.7% after 5 h, respectively. The sulfur removal by this desulfurization system is at the following order: DBT > 4,6-DMBT > BT. DBTs are easier to remove than that of BT. The sulfur removal order of three sulfur compounds by extractive and catalytic oxidation process was not consistent with the previous reports. As calculated by Shiraishi et al. and Otsuki et al., the electron densities on sulfur atoms are 5.739 for BT, 5.758 for DBT and 5.760 for 4,6-DMBT.^33,34 These calculated results indicate that the reaction activity of these model sulfur-containing compounds increase with increasing electron density on the sulfur atoms. The difference of sulfur removal in this desulfurization system with previous reports may be due to the extractive system that is different with previous liquid desulfurization system. Although extractive desulfurization ratio of BT is higher than that of 4,6-DMDBT, the lower electron densities on the sulfur of BT lead to its lower reactivity. The difference in electron density is slight for DBT and 4,6-DMDBT, but the main difference in desulfurization performance was caused by the influence of steric hindrance. The two methyl groups in 4,6-DMDBT which hinder the extractive performance by PEG-200, make the lower desulfurization ratio than that DBT. The results were consistent with the extractive and oxidative desulfurization by ionic liquids.²⁰

Fig. 4 Effect of different sulfur species on sulfur removal (conditions: sulfur content, 500 μg mL⁻¹; model oil 5, mL; PEG-200, 0.7 mL; catalyst, 10 mg; O/S = 6; reaction temperature, 60 °C).

Fig. 5 The recycling of desulfurization system of DB model oil (conditions: sulfur content, 500 μg mL⁻¹; model oil, 5 mL; PEG-200 0.7 mL; catalyst 10 mg; O/S = 6; 60 °C).

The reuse and regeneration of desulfurization system

The recycle of this desulfurization system has been investigated in extractive and oxidative desulfurization of the DBT model fuel. After reaction, the oil phase and PEG-200 phase can be separated by decantation. The PEG-200 phased was dealt with rotatory evaporator 3 h at 60 °C. Then, the fresh oil and H₂O₂ were added to repeat the process of desulfurization. As illustrated in Fig. 5, the sulfur removal of the third time of desulfurization decreased to 97.5% because of the massive oxidative products in 0.7 mL PEG-200. So the used PEG-200 phase was washed with deionized water and the precipitation was detected with GC-MS. The water phase was rotatory evaporator 6 h at 60 °C to remove the water and reuse the desulfurization system. As is shown in Fig. 5, it was found that the desulfurization system could be used 7 times without significant decrease in activity.

The possible process and mechanism

The process whereby DBT is extracted from oil phase and oxidized in the PEG-200 phase is as shown in Fig. 6. In a combination of extraction and oxidation, DBT was oxidized in the PEG-200 phase as it was extracted from the oil phase, so a continuous decrease in the concentration of DBT in n-octane was observed for each solvent during the oxidation process. The PEG-200 phase used 3 times was washed with deionized water. The precipitation was filtered and dried in vacuum at 60 °C over night to obtain white crystals. The GC-MS spectrogram of crystals in Fig. S2† indicates that DBT has been oxidized to DBTO₂ in the extractive and oxidative process. In this reaction system, the possible catalytic mechanism seems that peroxyseleninic acid may be considered as the true oxidative agent formed from hydrogen peroxide and selenium dioxide, which was proposed by Drabowicz et al.³⁵ DBT in PEG-200 phase was first translated to sulfoxide and then was oxidated to corresponding sulfone with peroxyseleninic acid. The corresponding sulfone which has high polar was easily dissolved in PEG-200 and was precipitated gradually after 2 runs when they were decreased to room temperature.

Fig. 6 The process of extraction and oxidation of sulfur-containing compounds in PEG–catalyst–H₂O₂ system.

Conclusions

In summary, the desulfurization system with PEG-200 and catalyst, SeO₂, can oxidize BT and DBTs present in n-octane to the corresponding sulfone using H₂O₂ as the oxidant under mild conditions. The non-toxic, inexpensive PEG-200 of low volatility can be used as extractant and the corresponding sulfone exists in PEG phase. The oxidation and extraction can proceed simultaneously. High DBTs removal was achieved under mild conditions. The oxidative reactivity of different sulfur compounds decreased according to the following order: DBT > 4,6-DMDBT > BT. The desulfurization system could be used 7 times without significant decrease in activity.

Acknowledgements

The authors are grateful for financial supported by the National Science Foundations of China (No. 21276265 and 21006122), Shanxi Province Science Foundation for Youths (No. 2010021007-1).

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Footnote

† Electronic supplementary information (ESI) available: The GC chromatogram and GC-MS chromatogram. See DOI: 10.1039/c6ra00996d

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