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Article

Catalytic Oxidative/Extractive Desulfurization of Model Oil using Transition Metal Substituted Phosphomolybdates-Based Ionic Liquids

by
Yunlei Li
,
Yanjie Zhang
,
Panfeng Wu
,
Caiting Feng
and
Ganglin Xue
*
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), College of Chemistry & Materials Science, Northwest University, Xi’an 710069, China
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(12), 639; https://doi.org/10.3390/catal8120639
Submission received: 31 October 2018 / Revised: 5 December 2018 / Accepted: 5 December 2018 / Published: 8 December 2018
(This article belongs to the Section Environmental Catalysis)
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Abstract

:
Polyoxometalates based ionic liquids (POM-ILs) exhibit a high catalytic activity in oxidative desulfurization. In this paper, four new POM-IL hybrids based on transition metal mono-substituted Keggin-type phosphomolybdates, [Bmim]5[PMo11M(H2O)O39] (Bmim = 1-butyl 3-methyl imidazolium; M = Co2+, Ni2+, Zn2+, and Mn2+), have been synthesized and used as catalysts for the oxidation/extractive desulfurization of model oil, in which ILs are used as the extraction solvent and H2O2 as an oxidant under very mild conditions. The factors that affected the desulfurization efficiency were studied and the optimal reaction conditions were obtained. The results showed that the [Bmim]5[PMo11Co(H2O)O39] catalyst demonstrated the best catalytic activity, with sulfur-removal of 99.8%, 85%, and 63% for dibenzothiophene (DBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzothiophene (BT), respectively, in the case of extraction combining with a oxidative desulfurization system under optimal reaction conditions (5 mL model oil (S content 500 ppm), n(catalyst) = 4 μmol, n(H2O2)/n(Substrate) = 5, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant). The catalyst can be recycled at least 8 times, and still has stability and high catalytic activity for consecutive desulfurization. Probable reaction mechanisms have been proposed for catalytic oxidative/extractive desulfurization.

1. Introduction

Problems of environmental pollution caused by exhaust emissions are receiving more and more attention worldwide with the development of society. Sulfur-containing compounds can be converted to sulfur oxides during the combustion process, causing serious harm to the environment. Many countries have adopted more stringent environmental regulations to restrict the sulfur level of fuels, limiting the sulfur level to less than 10 ppm [1,2]. Accordingly, deep desulfurization of fuels has become a crucial subject in environmental catalysis study. Conventionally, the hydrodesulfurization process (HDS) has been applied to remove aliphatic and alicyclic sulfur-containing compounds from fuels in a refinery [3,4]. However, the HDS process is less efficient for polyaromatic sulfur-containing compounds, such as dibenzothiophene (DBT) and its derivatives [5,6]. Additionally, the HDS process is carried out under severe operation conditions. Therefore, developing more efficient desulfurization processes is paramount. Some alternative or supplementary processes have been studied, such as extractive desulfurization (EDS) [7,8], biodesulfurization [9], oxidative desulfurization (ODS) [10,11,12,13,14], adsorptive desulfurization [15], ultrasound desulfurization [16], and others [17,18,19]. Among all these processes, ODS has been highlighted with special interest as one of the most promising processes, because it can proceed under mild reaction conditions, and oxidized products can be easily removed by extraction with organic extractants, owing to oxidized compounds being more polar than hydrocarbon molecules [20,21,22,23,24]. Although conventional organic extractants, including acetonitrile, methanol, dimethylsulfoxide (DMSO), dimethylformamide (DMF), sulfolane, and dichloromethane, reveal good extraction, their flammable and toxic properties limit their development and application in industries.
Ionic liquids (ILs), as a class of green solvents, have numerous advantages over conventional organic solvents [25,26]. In 2001, Wasserscheid et al. first reported desulfurization of diesel fuel by extraction with ionic liquids [27]. The results demonstrated that ILs have the potential to play an important role in achieving clean fuel oil. Subsequently, significant literature has become available about desulfurization systems by extraction with various ILs, such as imidazolium-based ionic liquids [28,29,30,31], pyridiniuim-based ionic liquids [32], Lewis and Brösted acidic ionic liquids, and redox ionic liquids [7,33]. However, sulfur removal using only ILs as the extractant is insufficient and hardly meets stringent environmental regulations.
Thus, researchers turned towards the addition of catalysts, combining peroxides with the ILs to achieve enhancement in efficiency. H2O2 is often chosen as an oxidant in oxidative desulfurization, because it only produces water as a byproduct and is environmentally benign. Polyoxometalates (POMs) have received increasing attention as oxidative catalysts, due to their characteristic structures and various functionalities [34,35,36,37]. In particular, Keggin type POMs have received enormous interest as catalysts, due to their good stability, adjustable composition at the atomic level, and unique acidic and redox properties [38,39,40]. In recent years, Keggin type and transition metal substituted POMs-IL phase-separation catalysts have emerged as one of the most promising catalysts in the ODS process [36,41,42,43,44]. However, little work has been reported on the study of transition metal substituted phosphomolybdates for oxidative desulfurization. Recently, our group reported catalytic oxidation desulfurization using cesium salts of the transition metal mono-substituted phosphomolybdates as heterogeneous catalysts, with H2O2 as the oxidant and acetonitrile as the extractant. The results showed that the transition metal mono-substituted phosphomolybdates exhibited higher catalytic activity than their parent (Cs3PMo12O40), and Cs5[PCo(H2O)Mo11O39] was found to be the best catalyst, with the removal of nearly all DBT at optimal reaction conditions [45]. For more effective catalysts of desulfurization under very mild conditions, herein four new POM-IL hybrid materials, [Bmim]5[PMo11M(H2O)O39] (M = Co2+, Ni2+, Zn2+, and Mn2+), have been used as catalysts for the extractive and catalytic oxidation desulfurization (ECODS) of model oil, with ILs used as an extraction solvent and H2O2 as an oxidant.

2. Results and Discussion

2.1. Characterization of the Catalysts

Fourier transform-infrared spectroscopy (FT-IR) are useful to study the framework structure of Keggin anions and the organic cation in the POM-based IL hybrids. The FT-IR spectra of the parent [Bmim]3PMo12O40 and the corresponding transition metal mono-substituted phosphomolybdates-based IL hybrids are compared in Figure 1. [Bmim]3PMo12O40 shows the characteristic bands of the Keggin-type structure: P–O stretching mode at 1062 cm−1, Mo–Ot at 959 cm−1, Mo–Ob–Mo at 883 cm−1, and Mo–Oc–Mo at 798 cm−1, which are in agreement with those of [Bmim]3PMo12O40 reported in the literature [46]. The bands observed between 3200 and 2820 cm−1, and between 1660 and 1350 cm−1, are attributed to the IL cation (alkyl and imidazole ring C–H stretching). When the transition metal was introduced into the framework, the P–O band stretching at 1062 cm−1 in [Bmim]3PMo12O40 shifted to 1048, 1050, 1045, and 1042 cm−1 for [Bmim]5PMo11Co(H2O)O39, [Bmim]5PMo11Ni(H2O)O39, [Bmim]5PMo11Zn(H2O)O39, and [Bmim]5PMo11Mn(H2O)O39, respectively. This is comparable to the P−O band at 1052, 1049, 1051, and 1043 cm−1 for Cs5[PM(H2O)Mo11O39] (M = Co2+, Ni2+, Zn2+, and Mn2+), respectively [46]. Mo–Od band stretching at 959 cm−1 shifted to 941, 943, 939, and 937 cm−1 for those of [Bmim]5PMo11M(H2O)O39 (M = Co2+, Ni2+, Zn2+, and Mn2+), respectively. The shifts in these bands compared to [Bmim]3PMo12O40 indicate that the transition metal was introduced into the framework of Keggin ions successfully.
The UV−vis diffuse reflectance spectra(DRS) (Figure 2a) of [Bmim]Br showed the absorption bands between 200 and 300 nm, which are attributed to the π–π* electron transition of the imidazole ring in the Bmim+ cation. The characteristic bands (Figure 2(b)) of [Bmim]3PMo12O40 appeared in the range of 200–450 nm, and are ascribed to O→Mo charge transfer of Keggin ions and the π–π* electron transition of the imidazole ring [47,48]. The spectra (Figure 2c–f) of [Bmim]5PMo11M(H2O)O39 (M = Co2+, Ni2+, Zn2+, and Mn2+) showed stronger absorptions in the range of 200–450 nm, and an obvious red shift can be observed compared to those of [Bmim]3PMo12O40, due to substitution of the transition metal. In addition, a new band for [Bmim]5PMo11M(H2O)O39 (M = Co2+, Ni2+ and Mn2+) was observed in the visible regions of 550–630, 650–780, and 450–600 nm, respectively, corresponding to the d–d electronic transition of transition metal ions in octahedral coordination. For [Bmim]5PMo11Zn(H2O)O39, no d–d band was observed.
The thermal gravity-derivative thermogravimetric (TG-DTG) curves of four POM-IL hybrid materials (see Figure S1) are very similar. Their thermal decomposition process is approximately divided into three steps. A first weight loss of 0.8% occurs before 150 °C and is in accord with the loss of one coordinated water molecule (calcd. 0.73%). The second weight loss is 28.0–29.0%, occurring from 310 to 500 °C, and is assigned to the loss of all five Bmim molecules (calcd. 28.5%). The last weight loss occurs after about 500 °C, and is attributed to the framework decomposition of polyanions.
The phosphorus-31 nuclear magnetic resonance (31P NMR) spectra of the parent compound [Bmim]3[PMo12O40] and the transition metal mono-substituted Keggin-type phosphomolybdates [Bmim]5[PMo11M(H2O)O39] (M = Co2+, Ni2+, Zn2+, and Mn2+) are shown in Figure S2. For [Bmim]3[PMo12O40], the main peak at –3.49 ppm is assigned to the Bmim salt of PMo12O403−, which is comparable to −3.4 ppm of the TBA((TBA =(C4H9)4N)) salt of [α-PMo12O40]3− [49]. A very weak peak at −0.76 ppm should be assigned to traces of the monolacunary polyanion [PMo11O39]7− formed during the synthesis. The spectra of [Bmim]5PMo11Zn(H2O)O39 showed a main peak at −2.31 ppm and two weak peaks at −1.63 ppm and −1.82 ppm, probably attributed to the presence of a different number of Bmim cations associated with PMo11Zn(H2O)O395− [50]. Only one single peak was found for [Bmim]5PMo11Ni(H2O)O39 at −1.95 ppm. The 31P NMR spectra of [Bmim]5PMo11(CoH2O)O39 and [Bmim]5PMo11Mn(H2O)O39 showed no significant signals, due to the presence of paramagnetic species (Co2+ and Mn2+) [48]. These results further confirmed the successful preparation of the substituted Keggin-type phosphomolybdates associated to 1-butyl 3-methyl imidazolium.

2.2. Extractive and Catalytic Oxidation Desulfurization (ECODS) of Model Sulfur Compounds

The studies of ECODS were performed using a model oil (500 ppm DBT in n-octane) with 30% H2O2 as an oxidant and the traditional IL (1 mL) as the extraction solvent, in the presence of POM-ILs as catalysts. Model oil was immiscible with the traditional ILs, while the catalysts could dissolve in ILs and hardly dissolved in the model oil. Table 1 shows the results obtained for the desulfurization of DBT in different desulfurization systems. The sulfur removal of S-containing compounds with the ionic liquids is affected by the structure and size of the anion and cation of the ionic liquid [28,29]. It can be seen from Table 1 that the ability of [Omim]BF4 (31.5%) to remove DBT was better than that of [Omim]PF6 (28.3%). The same trends were found in the other two systems. It is obvious that the system of extraction combining with catalytic oxidation is much more effective than the pure extraction and the extraction combining with chemical oxidation when the different desulfurization systems were in the same IL extraction. In the case of the extraction combining with the oxidative desulfurization system in [Omim]BF4, the removal of DBT reached 99.8%, higher than that of pure extraction (31.5%) and extraction combining with chemical oxidation (35.6%). Therefore, the catalyst played a very significant role in the desulfurization system.

2.2.1. Desulfurization of Model Oil with [Bmim]5[PMo11M(H2O)O39] (M = Co2+, Ni2+, Zn2+, and Mn2+) as Catalysts

The catalytic activities of different transition metal substituted-phosphomolybdates were evaluated for oxidation desulfurization of sulfides. Plots of the conversion (%) and ln(Ct/C0) against the reaction time are constructed in Figure 3, where C0 and Ct correspond to the DBT concentration at the beginning and at time t, respectively. It can be seen from Figure 3 that the four kinds of transition metal substituted-phosphomolybdates as catalysts exhibited high catalytic efficiency in the ECODS of model oil, and the catalytic activity was found in the order of [Bmim]5PMo11Co(H2O)O39 > [Bmim]5PMo11Zn(H2O)O39 > [Bmim]5PMo11Ni(H2O)O39 = [Bmim]5PMo11Mn(H2O)O39. The cobalt substituted-phosphomolybdate could reach nearly 100% removal of DBT under the same conditions, and more rapidly than the other three transition metal substituted-catalysts. The linear relationship for all the catalytic reaction systems indicates that all catalytic reactions conform to pseudo-first-order kinetics, with the reaction rate constants k = 0.1014, 0.075, 0.062, and 0.0566 for [Bmim]5PMo11Co(H2O)O39, [Bmim]5PMo11Zn(H2O)O39, [Bmim]5PMo11Ni(H2O)O39, and [Bmim]5PMo11Mn(H2O)O39, respectively. The catalytic system of the cobalt substituted-phosphomolybdate as the catalyst exhibits the highest catalytic efficiency for oxidation desulfurization of sulfide. Thus, the cobalt substituted-phosphomolybdate catalyst was selected to explore the various parameters, such as catalyst quantity, oxidant quantity, the reaction temperature, and reaction time.

2.2.2. Influence of the Amount of Catalyst on Desulfurization

Different amounts of catalyst were added in the ECODS process by changing the molar ratio of DBT and the catalyst (Figure 4). It can be observed that as the molar ratio of DBT and the catalyst was changed from 100 to 20, the removal of DBT increased from 35.6% to 99.8%. With the increase in the amount of catalyst, more active species were produced, and therefore the conversion increased. When the molar ratio reached 20, the sulfur removal remained unchanged.

2.2.3. Influence of the H2O2/DBT Molar Ratio on Desulfurization

In the catalytic oxidative system, the amount of H2O2 was one of the main factors. As shown in Figure 5, when the O/S molar ratio increased from 2:1 to 5:1, the removal of DBT from the model oil increased from 53.4% to 99.8%. However, with the molar ratio up to 8:1, the sulfur removal fell to 87.2%, because an excess amount of H2O2 could be decomposed by the catalyst into water, which has a negative effect on the desulfurization systems. Therefore, we chose H2O2/DBT = 5:1 as the optimal ratio in the present research.

2.2.4. Influence of Temperature and Reaction Time on Desulfurization

The influence of temperature and reaction time on oxidative desulfurization was investigated at 30, 40, and 50 °C, and different times. As shown in Figure 6, the sulfur removal of model oil increased with increasing reaction time at all temperatures (30, 40, and 50 °C), and the sulfur removal reached 82.5% (30 °C), 91.2% (40 °C), and 99.2% (50 °C) after 50 min.

2.2.5. Catalytic Oxidation Results for 4,6-Dimethyldibenzothiophene (DMDBT) and Benzothiophene (BT)

The desulfurization of other sulfur-containing compounds involving 4,6-DMDBT and BT was also evaluated. As shown in Figure 7, the removal of 4,6-DMDBT and BT could reach 87% and 65%, respectively, at 50 °C after 80 min. Of the three sulfur-containing compounds, the reactivity was found in the order of DBT > 4,6-DMDBT > BT. The reason for this was that the reactivity of sulfur compounds is determined by steric hindrance and electron density around the sulfur atoms. With the increase of the electron density around the sulfur compounds (DBT (5.758), 4,6-DMDBT (5.760), and BT (5.739)), the reactivity is increased [51].

2.2.6. Recyclability of the Catalytic System

Recyclability is very important for industrial application. The ECODS processes were performed in an immiscible liquid–liquid phase system formed by the model oil phase and the traditional IL-catalyst phase. Model oil was decanted from the reactor after the reaction, and then fresh model oil and H2O2 were directly added into the original reactor for the next run. Though some white DBTO2 residue was produced in the system after every run, this had little influence on the sulfur removal ability of the IL-catalyst system. The obtained results are shown in Figure 8. It can be seen that oxidation desulfurization efficiency did not decline after the IL-catalyst system was run 8 times, and the sulfur removal could still reach 98.3%.

2.2.7. The Possible Mechanism

The ECODS process of the model oil occurred in two distinct stages: The initial extraction of the DBT from the oil phase to the IL phase, and then the catalytic oxidation. The initial extraction is favored by the formation of strong π–π interactions between aromatic sulfur compounds and the imidazolium rings of ILs [2]. The subsequent catalytic oxidation process was always related with the formation of peroxometal intermediates, according to previous works [52,53,54,55]. There were two possible catalytic oxidation mechanisms for the ODS process by H2O2 in the presence of the transition metal mono-substituted phosphomolybdates as catalysts. One pathway was that the phosphomolybdate could be oxidized to peroxophosphomolybdates [56] by H2O2, then the peroxophosphomolybdate could react with the S atom of DBT to form the intermediate, which could further react to form sulfone and [Bmim]5PMo11M (shown in Scheme 1).
Alternatively, oxygen could be transferred from H2O2 to MII of [Bmim]5PMo11M(H2O)O39 (M = Mn2+, Co2+, or Ni2+), leading to a higher valent transition metal-oxo site (MIII = O). Then the M-peroxocomplex (M–O2) could be formed under the interaction with another H2O2. The MIII-peroxocomplex species could oxidize DBT to sulfone. Simultaneously, the MIII-peroxocomplex could be reduced back to [Bmim]5PMo11M(H2O)O39 (shown in Scheme 2).

2.2.8. Comparison of the Catalytic Desulfurization Efficiency with some Related, Reported Catalysts

Table 2 shows the desulfurization efficiency of some reported POMs catalyzed H2O2-based sulfoxidation reactions. It can be seen that a good desulfurization result can be achieved under milder conditions [36,41,43]. Compared with the system using the corresponding cesium salts as catalysts [45], the ECODS process in this system shows higher desulfurization efficiency at lower temperature and in a shorter time. Therefore, it provides an alternative approach for more efficient deep desulfurization.

3. Experimental Section

3.1. Materials and Characterization

The chemical reagents in this work, Na2HPO4·12H2O, Na2MoO4·2H2O, Co(CH3COO)2·4H2O, Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, and NaOH were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). All chemical reagents were analytical grade and used without further purification. Dibenzothiophene (DBT, 98%), benzothiophene (BT, 98+%), 4,6-dimethyldibenzothiophene (4,6-DMDBT, 98%), 1-butyl-3-methylimidazolium bromide ([Bmim]Br), and the conventional ionic liquids, [Bmim]BF4, [Bmim]PF6, [Omim]BF4, and [Omim]PF6, were obtained from Adamas (Shanghai, China) and were used without further treatment. H3PMo12O40 and [Bmim]3PMo12O40 were synthesized according to the published procedures [61].
Elemental analyses (C, H, and N) were performed on a Perkin–Elmer 2400 CHN elemental analyzer (PerkinElmer Inc., Waltham, MA, USA). FT-IR spectra were recorded in the range of 400–4000 cm−1 on an EQUINOX55 FT-IR spectrophotometer (Bruker, Billerica, MA, USA) using KBr pellets. The UV-Vis diffuse reflectance spectra were obtained using a Shimadzu UV-2550 UV-Vis spectrophotometer (Kyoto, Kyoto Prefecture, Japan), and BaSO4 was used as a reflectance standard. TGA-DSC analyses were performed on a NETZSCH STA449C TGA instrument (Netzsch, Selb, Germany) in flowing N2 with a heating rate of 10 °C/min. The 31P NMR measurements were collected for liquid solutions (20 mg samples dissolved in 0.5 mL of DMSO) using a JEOL JNM-ECZ 600R spectrometer (Tokyo, Japan), and chemical shifts were given with respect to external 85% H3PO4.

3.2. Synthesis of [Bmim]5[PMo11M(H2O)O39] (Bmim = 1-Butyl 3-Methyl Imidazolium; M = Co2+, Ni2+, Zn2+, and Mn2+)

H3PMo12O40 (1.825 g, 1 mmol) was dissolved in 20 mL deionized water, then the pH of the solution was adjusted to 4.3 with 1 M NaOH aqueous solution. Co(CH3COO)2·4H2O (0.249 g, 1 mmol) was added and stirred at 80 °C for 1.5 h and filtered hot. Then, an aqueous solution of [Bmim]Br (1.096 g, 5 mmol) was added dropwise to the obtained filtrate, yielding a brown precipitate. The resulting suspension was stirred for 2 h at room temperature, and the solid product was separated by filtration, washed with deionized water, and then dried overnight at 60 °C. The other three transition metal (M = Ni, Zn, and Mn) substituted phosphomolybdates [Bmim]5PMo11M(H2O)O39 were also synthesized according to a similar method by taking the corresponding metal-acetate salt. Elemental analysis shows that the found values are in good agreement with the analytical values. The results are given as follows: Calcd. for [Bmim]5PMo11Co(H2O)O39: C, 19.35; H, 3.12; N, 5.64. Found: C, 19.55; H, 3.07; N, 5.61. 1H NMR (400 MHz, CD3CN) δ 8.96 (s, 1H), 6.79 (s, 2H), 3.83 (s, 2H), 3.45 (s, 3H), 1.86 (s, 2H), 1.37–1.39 (d, 2H), 0.73–0.55 (t, 3H). Calcd. for [Bmim]5PMo11Ni(H2O)O39: C, 19.34; H, 3.12; N, 5.64. Found: C, 19.40; H, 3.01; N, 5.62. 1H NMR (400 MHz, CD3CN) δ 8.96 (s, 1H), 6.79 (s, 2H), 3.83 (s, 2H), 3.43 (s, 3H), 1.87 (s, 2H), 1.37–1.39 (d, 2H), 0.73–0.55 (t, 3H). Calcd. for [Bmim]5PMo11Zn(H2O)O39: C, 19.30; H, 3.12; N, 5.63. Found: C, 19.50; H, 3.05; N, 5.69. 1H NMR (400 MHz, CD3CN) δ 8.96 (s, 1H), 6.79–6.81 (s, 2H), 3.86 (s, 2H), 3.45 (s, 3H), 1.84 (s, 2H), 1.33–1.36 (d, 2H), 0.75–0.54 (t, 3H). Calcd. for [Bmim]5PMo11Mn(H2O)O39: C, 19.37; H, 3.13; N, 5.65. Found: C, 19.57; H, 3.07; N, 5.70. 1H NMR (400 MHz, CD3CN) δ 8.96 (s, 1H), 6.79 (s, 2H), 3.85 (s, 2H), 3.47 (s, 3H), 1.86 (s, 2H), 1.35–1.38 (d, 2H), 0.73–0.55 (t, 3H).

3.3. ECODS Process

The ECODS experiments were carried out in a closed Pyrex cell, equipped with a magnetic stirrer and a thermostatic oil bath. The model oil was prepared following the procedure described in the literature [45]. ECODS experiments were performed in the presence of ILs as the extraction solvent. The catalysts, ILs, and 30% H2O2 were added into the reactor, and then 5 mL of model oil was injected. The mixture was heated to the appropriate reaction temperature in the oil bath. Liquid samples from model oil (the upper) were taken from the reactor at an interval of 10 min and analyzed by microcoulometry (Jiangsu national innovation Instrument Co., Ltd. Jiangyan, China; detection limit, 0.2 ppm) to determine the concentration variation of the sulfur-compounds with time.

4. Conclusions

In this study, four POM-IL hybrid materials based on transition metal mono-substituted Keggin-type phosphomolybdates [Bmim]5[PMo11M(H2O)O39] (M = Co2+, Ni2+, Zn2+, and Mn2+) have been synthesized and used as catalysts for ECODS, with traditional ILs as an extraction solvent and H2O2 as an oxidant. It was found that the substituted transition metal in the polyanion [PMo11M(H2O)O39], ionic liquid cations, and the type of extract can greatly affect the desulfurization efficiency. The [Bmim]PMo11Co(H2O)O39 catalyst showed excellent catalytic activity for the oxidation of model oil in the immiscible liquid–liquid phase system formed by the model oil and traditional ILs as an extraction solvent, with H2O2 as an oxidant under very mild conditions. Our reaction can achieve nearly 100% desulfurization efficiency at a relatively small amount of catalyst, shorter reaction time, and lower temperature, as compared to some reported systems with POMs catalyzed H2O2-based sulfoxidation reactions. Additionally, the oxidation desulfurization efficiency is not reduced after the IL-catalyst system has been recycled 8 times, and it is harmless to the environment.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/8/12/639/s1, Figure S1. TG and DTG curves of (a) [Bmim]5PMo11Co(H2O)O39, (b) [Bmim]5PMo11Ni(H2O)O39, (c) [Bmim]5PMo11Zn(H2O)O39, (d) [Bmim]5PMo11Mn(H2O)O39; Figure S2. 31P NMR spectra of (a) [Bmim]3PMo12O40, (b) [Bmim]5PMo11Co(H2O)O39, (c) [Bmim]5PMo11Ni(H2O)O39, (d) [Bmim]5PMo11Zn(H2O)O39 and (e) [Bmim]5PMo11Mn(H2O)O39.

Author Contributions

G.X. conceived and designed the experiments; Y.Z., Y.L., P.W. and C.F. performed the experiments; Y.Z., Y.L. and G.X. wrote the paper.

Funding

This work was funded by the National Natural Science Foundation of China (21673176).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FT-IR spectra of hybrid materials: (a) [Bmim]3PMo12O40, (b) [Bmim]5PMo11Co(H2O)O39, (c) [Bmim]5PMo11Ni(H2O)O39, (d) [Bmim]5PMo11Zn(H2O)O39, (e) [Bmim]5PMo11Mn(H2O)O39.
Figure 1. FT-IR spectra of hybrid materials: (a) [Bmim]3PMo12O40, (b) [Bmim]5PMo11Co(H2O)O39, (c) [Bmim]5PMo11Ni(H2O)O39, (d) [Bmim]5PMo11Zn(H2O)O39, (e) [Bmim]5PMo11Mn(H2O)O39.
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Figure 2. UV–Vis DRS spectra of (a) [Bmim]Br ionic liquid, and hybrid materials (b) [Bmim]3PMo12O40, (c) [Bmim]5PMo11Co(H2O)O39, (d) [Bmim]5PMo11Ni(H2O)O39, (e) [Bmim]5PMo11Zn(H2O)O39, (f) [Bmim]5PMo11Mn(H2O)O39.
Figure 2. UV–Vis DRS spectra of (a) [Bmim]Br ionic liquid, and hybrid materials (b) [Bmim]3PMo12O40, (c) [Bmim]5PMo11Co(H2O)O39, (d) [Bmim]5PMo11Ni(H2O)O39, (e) [Bmim]5PMo11Zn(H2O)O39, (f) [Bmim]5PMo11Mn(H2O)O39.
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Figure 3. (a) Diagram of reaction conversion. (b) Plot of ln(Ct/C0) versus the reaction time for the oxidation of DBT. Conditions: Model oil = 5 mL, n(catalyst) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
Figure 3. (a) Diagram of reaction conversion. (b) Plot of ln(Ct/C0) versus the reaction time for the oxidation of DBT. Conditions: Model oil = 5 mL, n(catalyst) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
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Figure 4. Influence of the amount of catalyst on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Bmim]5PMo11Co(H2O)O39 as the catalyst and [Omim]BF4 (1 mL) as the extractant.
Figure 4. Influence of the amount of catalyst on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Bmim]5PMo11Co(H2O)O39 as the catalyst and [Omim]BF4 (1 mL) as the extractant.
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Figure 5. Influence of the H2O2/DBT molar ratio on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
Figure 5. Influence of the H2O2/DBT molar ratio on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
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Figure 6. Influence of temperature and reaction time on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, with [Omim]BF4 (1 mL) as the extractant.
Figure 6. Influence of temperature and reaction time on the oxidative desulfurization of DBT. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, with [Omim]BF4 (1 mL) as the extractant.
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Figure 7. The catalytic oxidation results for DBT, 4,6-dimethyldibenzothiophene (DMDBT), and benzothiophene (BT). Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C, with [Omim]BF4 (1 mL) as the extractant.
Figure 7. The catalytic oxidation results for DBT, 4,6-dimethyldibenzothiophene (DMDBT), and benzothiophene (BT). Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C, with [Omim]BF4 (1 mL) as the extractant.
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Figure 8. Recyclability of catalyst. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
Figure 8. Recyclability of catalyst. Conditions: Model oil = 5 mL, n([Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with [Omim]BF4 (1 mL) as the extractant.
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Scheme 1. The possible mechanism of extractive and catalytic oxidation desulfurization (ECODS) via pathway one.
Scheme 1. The possible mechanism of extractive and catalytic oxidation desulfurization (ECODS) via pathway one.
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Scheme 2. The possible mechanism of ECODS via pathway two.
Scheme 2. The possible mechanism of ECODS via pathway two.
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Table
Table 1. Oxidation of dibenzothiophene (DBT) in different desulfurization systems.
Table 1. Oxidation of dibenzothiophene (DBT) in different desulfurization systems.
Sulfur Removal (%)
EntryType of ILsILIL + H2O2IL + H2O2 + catalyst
1[Bmim]BF419.628.594.4
2[Omim]BF431.535.699.8
3[Bmim]PF615.417.276.1
4[Omim]PF628.330.790.5
Conditions: Model oil = 5 mL, n(catalyst, [Bmim]5PMo11Co(H2O)O39) = 4 μmol, n(H2O2)/n(DBT) = 5, T = 50 °C for 60 min with ILs (1 mL) as the extractant.
Table
Table 2. Comparison with some related catalysis systems for the removal of DBT.
Table 2. Comparison with some related catalysis systems for the removal of DBT.
Substrates/ReferenceCatalystsReaction ConditionsRemoval of DBT (%)
DBT(1000 ppm)/[57][PhPyPS]PWH2O2, H2O/EtOH, 60 °C, 2 h93.6
DBT(500 ppm)/[58]H3PW12O40/TiO2H2O2, CH3CN, 60 °C, 2 h95.2
DBT(500 ppm)/[59][Bmim]3PMo12O40/SiO2H2O2, CH3CN, 60 °C, 2 h100
DBT(500 ppm)/[60]H3PW6Mo6O40H2O2, CH3CN, 60 °C, 60 min99.7
DBT(1000 ppm)/[35]HPW-TiO2-SiO2 (1:3)H2O2, petroleum ether, 70 °C, 2 h100
DBT(640 ppm)/[20]Na2HPW12O40H2O2, CH3CN, 60 °C, 150 min95
DBT(500 ppm)/[43][Bmim]5PW11Zn(H2O)O39H2O2, CH3CN, 50 °C, 60 min10
DBT(500 ppm)/[43][Bmim]5PW11Zn(H2O)O39H2O2, CH3CN, 50 °C, 3 h100
DBT(500 ppm)/[36][PSPy]3PW12O40·2H2OH2O2, [Omim]BF4, 30 °C, 60 min77.1
DBT(500 ppm)/[41][(CH3)N(n-C8H17)3]2[Mo2O11]H2O2, [Omim]BF4, 30 °C, 2 h97.8
DBT(500 ppm)/[41][(CH3)N(n-C8H17)3]2[W2O11]H2O2, [Omim]BF4, 30 °C, 2 h85.9
DBT(500 ppm)/[45]CsPMo11M (M = Co, Ni, Mn, Zn)H2O2, CH3CN, 60 °C, 100 min>97
DBT(500 pm)/This work[Bmim]5PMo11Co(H2O)O39H2O2, [Omim]BF4, 50 °C, 60 min99.8

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Li, Y.; Zhang, Y.; Wu, P.; Feng, C.; Xue, G. Catalytic Oxidative/Extractive Desulfurization of Model Oil using Transition Metal Substituted Phosphomolybdates-Based Ionic Liquids. Catalysts 2018, 8, 639. https://doi.org/10.3390/catal8120639

AMA Style

Li Y, Zhang Y, Wu P, Feng C, Xue G. Catalytic Oxidative/Extractive Desulfurization of Model Oil using Transition Metal Substituted Phosphomolybdates-Based Ionic Liquids. Catalysts. 2018; 8(12):639. https://doi.org/10.3390/catal8120639

Chicago/Turabian Style

Li, Yunlei, Yanjie Zhang, Panfeng Wu, Caiting Feng, and Ganglin Xue. 2018. "Catalytic Oxidative/Extractive Desulfurization of Model Oil using Transition Metal Substituted Phosphomolybdates-Based Ionic Liquids" Catalysts 8, no. 12: 639. https://doi.org/10.3390/catal8120639

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