Catalytic oxidative desulfurization of fuels in acidic deep eutectic solvents with [(C6H13)3P(C14H29)]3PMo12O40 as a catalyst
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Deep eutectic solvents (DESs) are a new class of green solvents analogous to ionic liquids due to their biodegradable capacity and low cost. However, the direct extractive desulfurization of diesel oil by DESs cannot meet the government’s standard. In this work, amphiphilic polyoxometalates were synthesized and characterized by FT-IR and mass spectrometry. The oxidative desulfurization results showed that benzothiophene (BT) could be completely removed by employing a [(C6H13)3P(C14H29)]3PMo12O40, DES (ChCl/2Ac) and H2O2 system. It was also found that the organic cation of catalysts played a positive role in oxidative desulfurization. The reaction conditions, such as reaction temperature and time, the amount of catalyst and DES and H2O2/S (O/S) molar ratio, were optimized. Different sulfides were tested to determine the desulfurization selectivity of the optimal reaction system, and it was found that 97.2% of dibenzothiophene (DBT) could be removed followed by 80.7% of 4-MDBT and 76.0% of 4,6-DMDBT. After reaction, the IR spectra showed that the catalyst [(C6H13)3P(C14H29)]3PMo12O40 was stable during the reaction process and the oxidative product was dibenzothiophene sulfone (DBTO2). Furthermore, the catalyst can be regenerated and recycled for four runs with little loss of activity.
KeywordsDiesel Oxidative desulfurization Deep eutectic solvents Polyoxometalates H2O2
Deep desulfurization of fuels is a worldwide topical issue because of the air pollution caused by SOx. Thus, increasingly strict regulations have been implemented to limit sulfur content in fuels to 10 ppm or even lower (Xiao et al. 2014; Zhang et al. 2018; Li et al. 2018; Liu et al. 2013; Wu et al. 2017). For example, since 2017 the sulfur content in diesel fuel and gasoline must be not higher than 10 ppm according to the latest standard in China. Hydrodesulfurization (HDS) is a classic technology that can remove most of the sulfides in fuels but can barely remove aromatic sulfides or needs harsh conditions such as temperature >370 °C and pressure >40 atm (Zeng et al. 2017; Zhang et al. 2016; Wu et al. 2016; Rafiee et al. 2016; Kianpour and Azizian 2016). Therefore, exploiting alternative or supplementary desulfurization approaches is highly desirable for petroleum refineries (Rezvani et al. 2018; Ja’fari et al. 2018; Zhang et al. 2017; Li et al. 2017a, b; Jiang et al. 2014a, b).
Extractive desulfurization is a feasible approach because of its simple and mild conditions (Nejad and Beigi 2015). The common extractants are organic solvents, ionic liquids and deep eutectic solvents (DESs) (Jiang et al. 2015; Li et al. 2016a, b, c; Zhao et al. 2016; Jiang et al. 2016; Zhang et al. 2018). Among the three types of solvent, DESs, a eutectic mixture of two or more components, are the most promising extractants due to their low cost and biodegradability. However, the extraction efficiency is still not very high in a one-step extraction. Li et al. (2016a, b, c) reported several tetrabutyl ammonium chloride-based DESs for extractive desulfurization and tetrabutyl ammonium chloride/polyethylene glycol (TBAC/PEG) showed 82.8% of desulfurization efficiency in one cycle. Hence, it is essential to extract for five cycles to reach deep desulfurization (Li et al. 2016a, b, c). Another sample using carboxylic acid-based DESs for extractive desulfurization was also faced with the obstacle of low desulfurization efficiency (Li et al. 2016a, b, c). Some researchers tried to add an oxidant to acidic DESs to establish a new desulfurization system, that is, extraction and catalytic oxidative desulfurization (ECODS) (Yin et al. 2015; Lu et al. 2015; Liu et al. 2016; Hao et al. 2017). Two p-toluenesulfonic acid-based DESs were employed as extractants and catalysts and H2O2 was used as oxidant for oxidation of benzothiophene (BT) and the desulfurization efficiency could reach up to 99.99% (Yin et al. 2015). Lu and his group (Lu et al. 2015) found that an oxalate-based DES showed good catalytic activity with H2O2 as an oxidant and 91% of dibenzothiophene (DBT) could be removed. Therefore, compared with extractive desulfurization, the oxidative desulfurization with DESs may be more promising due to its high activity. However, the reported DESs in oxidative desulfurization are very few probably owing to the difficulty of exploiting efficient DESs.
Polyoxometalates are commonly used in the field of catalysis because of their adjustable acid–base and redox properties over a wide range (Omwoma et al. 2015; Li et al. 2017a, b; Ma et al. 2017; Lu et al. 2017; Bertleff et al. 2017; Jiang et al. 2017; Chen et al. 2013). In our previous work, an oxidative desulfurization system with DESs as extractants was developed and commercial phosphotungstic acid (PTA) showed superior catalytic activity (Liu et al. 2016). However, it is difficult to separate and recycle the catalyst from the reaction system, and this may hinder the application of the PTA/DESs system. Thus, development of easily separable catalysts in the DESs system is necessary. In this work, amphiphilic polyoxometalates were introduced to acidic DESs to evaluate their catalytic performance. The main advantage is that these catalysts can be separated from the reaction system due to the fact that the DESs can be dissolved in water, but the catalysts amphiphilic polyoxometalates are insoluble. Then different DESs, catalysts with different cations and reaction conditions, with model oils with different aromatic sulfides, olefins and aromatics, were investigated.
2.1 Preparation of catalysts and DESs
According to the method in the literature (Zhu et al. 2015a, b), a solution of trihexyltetradecyl phosphonium chloride ([(C6H13)3P(C14H29)]Cl; 1, 2, 3 mmol) in 20 mL of anhydrous ethanol was added dropwise into 50 mL of ethanol solution of H3PMo12O40·26H2O (1 mmol) while stirring for 4 h at room temperature. A bright yellow precipitate was formed, filtered and washed several times with anhydrous ethanol. Then, the catalysts, [(C6H13)3P(C14H29)]H2PMo12O40 (PH2Mo), [(C6H13)3P(C14H29)]2HPMo12O40 (PHMo) and [(C6H13)3P(C14H29)]3PMo12O40 (PMo), can be obtained by drying at 50 °C in vacuum for 24 h.
The DES ChCl/2Ac was simply prepared by mixing choline chloride (ChCl) with acetic acid (Ac) at a molar ratio of 1:2. The mixture was then warmed to 80 °C and stirred for 3 h. DES ChCl/2Fo and TBAC/2Ac were obtained according to the same process with formic acid (Fo) as a hydrogen bond donor and tetrabutylammonium chloride (TBAC) as a quaternary ammonium salt.
2.2 Desulfurization procedure
Preparation of model oils: The sulfur compounds including BT, DBT, 3-MBT, 4-MDBT and 4,6-DMDBT were dissolved in n-octane, respectively, with tetradecane as an internal standard. The sulfur contents of these model oils are 500 mg L−1.
The desulfurization tests were performed in a self-made two-necked flask, to which a certain amount of catalysts, DES, model oil and H2O2 were added in turn. The reaction mixture was stirred in a water bath at a set temperature for 3 h. The model oil in the upper layer was periodically withdrawn and analyzed by gas chromatography–flame ionization detection (GC-FID) with the analytical conditions listed in our previous work (Zheng et al. 2015; Jiang et al. 2014a, b).
3 Results and discussion
3.1 Characterization of catalysts
3.2 Effect of the cation on sulfur removal
3.3 Selection of a suitable DES
3.4 Optimization of reaction conditions for sulfur removal
In the above experiments, PMo combined with ChCl/2Ac showed the best desulfurization performance, and thus these were selected to evaluate a series of parallel experiments to systematically study the effects of different reaction conditions on sulfur removal.
As shown in Fig. 4b, c, the amount of catalyst and extractant greatly affected the removal of DBT. The extractive desulfurization (EDS) rate was only 2.7% at V(DES)/V(model oil) = 1:5 and increased to 11.5% at V(DES)/V(model oil) = 3:5 (Fig. 4b). And the catalytic desulfurization rate was only 5.3% in the absence of DES (Fig. 4b). In order to obtain low-sulfur fuel, the amounts of catalyst and extractant were evaluated. The results showed that the 97.2% of DBT could be removed at V (DES)/V (model oil) = 2.5:5 and n(DBT)/n(catalyst) = 5:1. Thus, both the catalyst and extractant played important roles in sulfur removal. Figure 4c shows that the H2O2/S (O/S) molar ratio has a crucial influence on the sulfur removal. According to a previous report (Jiang et al. 2017), the decomposition of H2O2 is the competitive reaction to its oxidative reaction, resulting in an increase of H2O2 dosage. The sulfur removal increased from 72.8% at O/S = 2 to 97.2% at O/S = 4, and excessive H2O2 cannot promote the oxidation of DBT.
3.5 Oxidative removal of different sulfur compounds from model oil
3.6 Effects of fuel composition on sulfur removal
3.7 Reusability of the catalyst
A deep desulfurization system with DES as extractant was explored. BT could be removed completely with ChCl/2Ac as extractant and [(C6H13)3P(C14H29)]3PMo12O40 as catalyst, followed by DBT, 4-MDBT and 4,6-DMDBT. Two other catalysts [(C6H13)3P(C14H29)]2HPMo12O40 and [(C6H13)3P(C14H29)]H2PMo12O40 showed the lower activity than [(C6H13)3P(C14H29)]3PMo12O40, indicating the positive role of the long carbon chain cation of the catalysts. However, the long carbon chain of DESs exhibited a contrary role for oxidative desulfurization because the sulfur removal was 88.5% and 63.6% with ChCl/2Fo and TBAC/2Ac, respectively, while 95.6% with ChCl/2Ac. After reaction, the catalyst can be regenerated by water and recycled for at least four times. The reaction product was determined by IR spectra as DBTO2.
This work was financially supported by the National Natural Science Foundation of China (Nos. 21506080 and 21766007), Natural Science Foundation of Jiangsu Province (No. BK20150485), Advanced Talents of Jiangsu University (No. 15JDG053) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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