Oxidative desulfurization of dibenzothiophene with dioxygen and reverse micellar peroxotitanium under mild conditions

Applied Catalysis B: Environmental 106 (2011) 343–349

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Oxidative desulfurization of dibenzothiophene with dioxygen and reverse micellar peroxotitanium under mild conditions Caiyun Jiang a , Jingjing Wang a , Shengtian Wang a , Hong yu Guan a , Xiaohong Wang a,∗ , Minxin Huo b,∗ a b

Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, Jilin Province, PR China School of Urban and Environmental Sciences, Northeast Normal University, Changchun 130024, PR China

a r t i c l e

i n f o

Article history: Received 3 March 2011 Received in revised form 5 May 2011 Accepted 24 May 2011 Available online 31 May 2011 Keywords: Desulfurization Dibenzothiophene Polyoxometalates Reverse-micelles Oxidation

a b s t r a c t The reverse micellar peroxotitanium-containing catalyst [C18 H37 N(CH3 )3 ]7 [PW10 Ti2 O38 (O2 )2 ] was assembled in the organic solvent and the structure was characterized by Fourier transform infrared spectroscopy(FT-IR), Diffuse reflectance UV–vis spectrum (DR-UV–vis), X-ray photoelectron spectrum (XPS), transmission electron microscopy (TEM), Energy dispersive X-ray analysis (EDAX), and Matrixassisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Direct oxidation of dibenzothiophene (DBT) using O2 was performed by this reverse micellar catalyst with ∼100% selectivity in the oxidation of DBT to sulfone. By this catalyst, it could catalytically decrease sulfur level in diesel from 500 ppm to 1.0 ppm at ambient pressure and moderate temperature. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Deep desulfurization from fuel oils has been paid more attention due to urgently environmental problem. More and more stringent regulation limits the sulfur content to be about 10 ppm in diesel in many countries by 2010. The conventional method for removing sulfur in industry is catalytic hydrodesulfurization (HDS) process, which can desulfurize aliphatic and acyclic sulfur compounds. This process, however, is limited for treating some refractory sulfur compounds such as dibenzothiophene (DBT) or its derivatives. In order to achieve deep desulfurization, severe operating conditions such as high temperature (T > 623), high pressure (3–10 Mpa) and high hydrogen consumption are required, leading to high instrumental cost [1]. Therefore, an essential demand for developing alternative non-HDS methods to achieve deep desulfurization is to produce clean diesel containing low sulfur concentration. Among them, oxidative desulfurization (ODS) combined with extraction is considered to be one of the most promising process, because it can remove the refractory sulfur compounds [2–4] under mild conditions. In ODS, the choice of oxidant is one of the key factors for deep desulfurization. In aqueous solution, hydrogen peroxide (H2 O2 ) and peroxyacids are very known oxidants for ODS [5–7] when using different catalysts such as polyoxometalates (POMs) [5,6,8], metal oxide [9] or Fenton-like reagents in ionic liquids [10].

∗ Corresponding authors. Tel.: +86 431 85099667; fax: +86 431 85099759. E-mail address: [email protected] (X. Wang). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.05.038

Besides peroxides, molecular oxygen is the most economical and environmental benign oxidant which might be used in desulfurization of fuels [11–13]. In these processes, the sulfur compounds can be oxidized to corresponding sulfones using O2 and heterogeneous catalysts involving aldehydes as co-oxidants, which transfer O2 , from gas-phase to liquid-phase, to peroxyacids by itself oxidation. The two major disadvantages are the cost of co-oxidants and the loss of fuel during the separation of organic acids. To date, there are only few reports [14] on directly catalytic oxidation of DBT using O2 in non-polar hydrocarbon solvent without a sacrificial agent at 100 ◦ C and 0.3 MPa for 2 h, in which 500 ␮g/g of DBT could be decreased to less than 4 ␮g/g. And more recently, Li reported that aerobic oxidative desulfurization could be catalyzed by Andersontype polyoxometalates under 80 ◦ C and 10 h [15]. The oxidation of DBT by oxygen instead of H2 O2 under rather mild conditions would be of great significance in industry due to its lower costs and green chemical advantages. In order to achieve this, the main task is to seek an active and durable catalyst under mild temperature and atmospheric pressure. Polyoxometalates (POMs), represent an increasingly important class of environmentally benign catalysts that can be used even under rather mild conditions for the oxidation of a number of organic substrates in the presence of either O2 or other donors [16–18]. The properties of titanium-containing POMs are of great interest that can lead to interesting applications in catalysis and medicine [19,20]. Especially in H2 O2 -based oxidation catalysis, titanium-containing POMs could react with hydrogen peroxide for forming peroxo complexes—actual active species toward organic

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substrates [21], which are widely used to promote the epoxidation of the oleinic substrates [22] and are responsible for sulfide oxidation [23]. On the basis of these findings, we proposed that the peroxo-titanium complexes could provide reactive peroxo species, and oxygen could be transferred from Ti(O2 ) species to the substrates, leading to the oxidation of organic substrates with O2 as oxidant instead of H2 O2 . In addition, the most often used strategy is to utilize surfactants to form micelles or reverse micelles for significantly accelerating the reaction rate in a micellar medium [24]. Amphiphilic quaternary ammonium micelles of POMs, with a surfactant surface and a POM core, can form supramolecular reverse micellar assemblies ranging from nanoscale to microscale sizes in non-polar solvents [25]. This structure could possibly afford high local reactant concentrations near the catalysts, helping to enhance the catalytic rate. Amphiphilic POM catalysts have been successfully used in various organic synthesis reactions with hydrogen peroxide [5,6,26–29] as an oxidant in an emulsion or microemulsion system. Thus far, only Li and co-workers [13] have reported on the oxidative desulfurization of dibenzothiophene with O2 using amphiphilic POM emulsion catalysts and aldehyde as the sacrificial agent at 60 ◦ C for 8 h. Few studies have been conducted to develop peroxotitanium POM reverse-micellar catalysts for O2 desulfurization of organic sulfur by now [15]. Here, we report the oxidation of DBT with O2 as the oxidant using reverse micellar system under natural daylight, where an amphiphilic catalyst [C18 H37 N(CH3 )3 ]7 [PW10 Ti2 O38 (O2 )2 ] (abbreviated as C18 PW10 Ti2 (O2 )2 ) acts as both the surfactant and the catalyst. The catalyst C18 PW10 Ti2 (O2 )2 in decalin could form an reverse micelle with POM core and surfactant surface. As result, sulfur-containing molecules could assemble on surfactant ends of reverse-micellar droplets and oxygen molecules could assemble inside the reverse-micellar droplets. So the oxidation rate of DBT could accelerate and could be completely oxidized by O2 into sulfone in this reverse micellar system under mild conditions within short time. 2. Experimental 2.1. Materials All reagents were of AR grade and used without further purification. K7 PW10 Ti2 O40 and K7 PW10 Ti2 O38 (O2 )2 were prepared according to the literature methods [30,31], and were characterized by IR spectroscopy. 2.2. Physical measurements Elemental analysis was carried out using a Leeman Plasma Spec (I) ICP-ES and a P-E 2400 CHN elemental analyzer. IR spectra (4000–400 cm−1 ) were recorded in KBr discs on a NicoletMagna 560 IR spectrometer. DR-UV–vis spectra (200–600 nm) were recorded on a Cary 500 UV–vis-NIR spectrophotometer. XPS were recorded on an Escalab-MK II photoelectronic spectrometer with Al K␣ (1200 eV). Dynamic light scattering (DLS) was employed in order to study the sizes of the reverse-micelles in the Microtrac S3500 Particle Analyzer in terms of the hydrodynamic radius. TEM image was determined by JEM-2100F instrument. Energy dispersive X-ray analysis (EDAX) was performed to take into account of the C, P, W and Ti elements. The identification and quantification of DBT in decalin were performed by Gas Chromatography (GC) equipped with an OV1701 capillary column (30 m × 0.32 mm × 0.25 ␮m) using Flame Ionization Detector (SHIMADZU GC-14C). Analysis conditions were as follows: injection port temperature, 280 ◦ C; detector temperature, 250 ◦ C; oven temperature program, 280 ◦ C, hold for 8 min; split ratio, 1/100; carrier

gas, ultra-purity nitrogen; column flow, 0.9 mL/min; reagent gases, air flow of 100 mL/min, hydrogen flow of 75 mL/min; the injection volume of sample was 1 ␮L. The leaching concentrations of C18 PW10 Ti2 (O2 )2 during the reaction were also measured through analyzing the dissolved concentration of W in solvent using a Leeman Plasma Spec (I) ICP-ES. The positive-ion MALDI-TOF mass spectra of [PW10 Ti2 O38 (O2 )2 ]7− were recorded by using Bruker autoflex III smartbeam MALDI-TOF/TOF (smartbeam laser with 355 nm wavelength, Germany). In the experiments the mass spectrometer was tuned in the linear mode by using delayed extraction of 200 ns. The acceleration voltage was set to +20 kV. The samples were dissolved in CHCl3 and the matrix solution was dithranol. 2.3. Preparation of [C18 H33 N(CH3 )3 ]7 [PW10 Ti2 O38 (O2 )2 ] The preparation of C18 PW10 Ti2 (O2 )2 was as follows: a 50 mL alcohol solution of [(C18 H37 )N(CH3 )3 ]Br(7 mmol) was added dropwise into a 20 mL hydrogen peroxide (30%) solution containing K7 PW10 Ti2 O38 (O2 )2 (1 mmol). A yellow precipitate was formed immediately and was stirred for another 2 h. The yellow precipitate was filtered and dried in nitrogen to produce. The resulting C18 PW10 Ti2 (O2 )2 was obtained with yield 50%. IR(1% KBr pellet, 4000–400 cm−1 ): 1059(as P-Oa), 954(as W-Od), 882(as W-ObW), 804(as W-Oc-W), 650(Ti-O-O)[32]. The other amphiphilic quaternary ammonium salts of peroxotitanium-POMs were prepared in the same procedure except using [(C8 H17 )N(CH3 )3 ]Br, [(C12 H25 )N(CH3 )3 ]Br, or [(C16 H33 )N(CH3 )3 ]Br instead of [(C18 H37 )N(CH3 )3 ]Br. 2.4. Catalytic procedure The oxidative desulfurization experiments were carried out in a 100 mL three-necked flask. The model sulfur-containing compound (DBT) was dissolved in 50 mL decalin and a sulfur concentration of 500 ppm. In this typical reaction run, a water bath was heated to 90 ◦ C. Then, the catalyst (0.05 mmol) was added to model oil and molecular oxygen was bubbled through the reaction solution. Keep stirring at 200 rpm and 90 ◦ C for some time. After the reaction, the samples were placed into an ice chamber to stop the reaction. The sulfur content of the upper clear solution was determined by GC. Finally, the catalyst was separated by centrifugation and reused without any treatment. 2.5. Adsorption experiments Adsorption experiments were carried out to determine the adsorption capacity of catalysts for DBT at 90 ◦ C and atmosphere pressure flowing nitrogen with an agitator. In the simultaneous adsorption experiments, 50 mL of 500 ppm DBT solution and 0.05 mmol of catalyst were loaded in the bottle. At the predetermined time intervals, samples were taken and the concentration of DBT adsorbed on the catalysts were gained by measuring the decrease of the concentration in solution using GC. 3. Results and discussion 3.1. Characterization of the micellar catalyst The IR spectra of C18 PW10 Ti2 (O2 )2 were investigated. Peaks in the range of 600–1100 cm−1 corresponding to Keggin structural vibrations could be easily distinguished. This indicates that C18 PW10 Ti2 (O2 )2 maintains the Keggin structure. From the results of the elemental analyses: W, 38.56; Ti, 2.21; P, 0.58; C, 35.74; H, 6.39; N, 2.43%. Compared with the calculated values W, 38.10; Ti, 1.98; P, 0.64; C, 36.59; H, 6.73; N, 2.03%, the results are satisfactory.

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Fig. 3. Positive-ion MALDI-TOF mass spectrum [C18 H37 N(CH3 )3 ]7 PW10 Ti2 O38 (O2 )2 ([PW10 Ti2 O38 (O2 )2 ]7 - abbr. POM).

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of

within the estimated experimental accuracy of ±1 u in the m/z 3500–5000 range. Thus, the MALDI-TOF results confirm that the polyanionic structure remains intact. 3.2. Oxidative activity of C18 PW10 Ti2 (O2 )2 reverse-micellar catalyst Fig. 1. The TEM image (a) [C18 H37 N(CH3 )3 ]7 PW10 Ti2 O38 (O2 )2 .

and

the

EDAX

spectrum

(b)

of

The catalyst C18 PW10 Ti2 (O2 )2 in decalin forming an reverse micelle with POM core and surfactant surface was determined by TEM image(Fig. 1(a)). The TEM image of C18 PW10 Ti2 (O2 )2 indicates that it can form relatively uniform-sized (about 20 nm) reversemicellar droplets. The EDAX measurement results showed that the catalysts were present in the reverse-micelles (Fig. 1(b)) and the molar ratio of C:P:W:Ti was 147:1:10:2. The two characteristic absorbance bands at 260 and 383 nm (Fig. 2) of the DR-UV–vis spectrum corresponded to the charge transfer of oxygen to tungsten and titanium O → W and ␩2 –O2 → Ti, respectively [33,34]. The positive-ion MALDI-TOF mass spectrum of catalyst displayed the same peaks in the region of m/z 3500–5000, regularly spaced with 311.6 u, corresponding to the mass of the [(C18 H37 )N(CH3 )3 ]+ (S) counterion (Fig. 3). The experimental values are consistent with the calculated mass for the different cations,

Fig. 2. The UV–vis pattern of [C18 H37 N(CH3 )3 ]7 PW10 Ti2 O38 (O2 )2 .

3.2.1. Influence of the catalyst Oxidation of DBT in decalin with O2 using C18 PW10 Ti2 (O2 )2 as a catalyst was investigated. Sulfur-containing compounds in decalin were analyzed by GC-FID periodically. As shown in Fig. 4(a), no DBT oxidation is detected without any catalyst, showing that the oxidation ability of O2 at experimental conditions is limited. Using [(C18 H37 )N(CH3 )3 ]Br as a catalyst, the decrease of DBT reaches a maximum value 12.1% for about 90 min, which could be attributed to the adsorption effect, since no corresponding sulfoxide and sulfone obtained. DBT can be oxidized to the sulfone by C18 PW10 Ti2 (O2 )2 with molar ratio of DBT to catalyst 4.86. The conversion of DBT increased with the increasing of reaction time, and the conversion of DBT reached 100% at 360 min (Fig. 4(b)). Compared to the catalytic activity of C18 PW10 Ti2 , the catalytic activity of PW10 Ti2 O38 (O2 )2 7− was enhanced by peroxo species. Five catalysts with different quaternary ammonium cations were synthesized for studying the effects of amphiphilic POM catalyst on the formation of the reverse micelles and the catalytic activity (Fig. 5). KPW10 Ti2 (O2 )2 without a hydrophobic (lipophilic) tail is heterogeneous one resulting in lower oxidation effect of DBT (46.1%). The hydrophobic chain of a surfactant must have a certain length (>C10 ) to enable successful micelle or reverse-micelle formation [24,25]. With the enhancement of the length from C8 to C18 , the catalytic activity increases significantly, from 57.8% for C8 PW10 Ti2 (O2 )2 to 100% for C18 PW10 Ti2 (O2 )2 , respectively. The different catalytic performances confirm that the length of the carbolic chain of cations plays an important role in the formation of reversemicelles. As shown in Fig. 6, DLS measurements indicates that the aggregates have a hydrodynamic radius (Rh ) centered at 11.00 nm, 15.55 nm and 18.50 nm for C12 PW10 Ti2 (O2 )2 , C16 PW10 Ti2 (O2 )2 and C18 PW10 Ti2 (O2 )2 , respectively. The catalyst C8 PW10 Ti2 (O2 )2 with a short carbon chain can not form reverse-micellar droplets and C12 PW10 Ti2 (O2 )2 form small reverse-micellar droplets, which can not supply enough oxidative agent. The catalysts C16 PW10 Ti2 (O2 )2 and C18 PW10 Ti2 (O2 )2 , with a longer chain have high catalytic activity, since it can form relatively uniform reverse-micellar droplets suspending in decalin. Therefore, the surfactant-POMs act as both a catalyst and a surfactant to assemble reverse-micelles in non-polar solvent.

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Fig. 4. (a) Oxidation efficiency of DBT by different catalysts (0.05 mmol) at 90 ◦ C, 500 ppm of DBT in 50 mL decalin, under atmospheric pressure, with the O2 flowing rate 0.04 m3 /h. (b) Sulfur specific GC-FID chromatograms for the oxidation of DBT with O2 in decalin. (Conditions: C18 PW10 Ti2 (O2 )2 (0.05 mmol), 500 ppm of DBT in 50 mL decalin, under atmospheric pressure, reaction temperature 90 ◦ C, O2 flowing rate 0.04 m3 /h)

Fig. 5. Influence of length of chain of Cn PW10 Ti2 (O2 )2 (0.05 mmol) on oxidation of DBT at 90 ◦ C, 500 ppm of DBT in 50 mL decalin, under atmospheric pressure, with the O2 flowing rate 0.04 m3 /h.

The high activity of reverse-micellar POM catalysts is attributed to DBT accumulation on the part of C18 PW10 Ti2 (O2 )2 . Aromatic compounds are also often found in both the head-group region and the core of micelles [24]. The IR spectra of C18 PW10 Ti2 (O2 )2 (Fig. 7(a)) and its adsorbed DBT (Fig. 7(b)) confirm the assembly of DBT inside of reverse-micellar droplets. The spectrum of C18 PW10 Ti2 (O2 )2 adsorbed DBT gives new peaks at 1639 and 1163 cm−1 corresponding to the benzene vibration, showing the adsorption of DBT inside reverse-micellar catalyst occurred. The test of DBT adsorption on catalysts also confirms the accumulation of DBT on the POM system (Fig. 8). Nitrogen was passed over a DBT decalin solution (500 ppm, 50 mL) in the presence of catalysts to ensure that no O2 enter into the experimental system. DBT adsorption on C18 PW10 Ti2 (O2 )2 reaches saturate after 30 min and the concentration of DBT did not change after that. In addition, during 30 min, small certain oxidation of DBT was found. This result indicated that DBT could be oxidized by C18 PW10 Ti2 (O2 )2 and without any O2 , the oxidation of DBT is weak. The influence of C18 PW10 Ti2 (O2 )2 on DBT degradation is presented in Table 1. The initial rate shows an increasing trend as the concentration of C18 PW10 Ti2 (O2 )2 increases. To study the effect of the amount of DBT, experiments were performed using different concentrations (300, 500 and 1000 ppm), while other variables were kept constant (Fig. 9). The rate of degradation decreases when the DBT concentration increases, indicating that DBT degradation

Fig. 6. DLS patterns of [Cn H2n+1 N(CH3 )3 ]7 PW10 Ti2 O38 (O2 )2 in 0.1 mg mL−1 decalin solution.

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Fig. 7. The IR spectra of pure C18 PW10 Ti2 (O2 )2 (a), C18 PW10 Ti2 (O2 )2 after adsorbing DBT (b) and C18 PW10 Ti2 (O2 )2 after the oxidation of DBT(c).

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Fig. 10. The conversion of DBT vs. reaction temperature with the O2 flowing rate 0.04 m3 /h, the catalyst 0.05 mmol and DBT 500 ppm in 50 mL decalin at 90 ◦ C.

depends on its initial concentration. This phenomenon may be due to the decrease of catalytic active sites. The increase of the initial concentration of DBT leads to more DBT molecules absorbed on the catalyst. Thus, an increase in the amount of substrates accommodating the catalyst inhibits the action of the catalyst with O2 . In this way, the degradation efficiency decreases. 3.2.2. The influence of reaction temperature Fig. 10 shows the trend of DBT conversion with temperature and reaction time. Temperature has a significant effect on DBT degradation. The catalytic activity of C18 PW10 Ti2 (O2 )2 is high even at lower temperature, such as at 60 ◦ C, where the degradation efficiency is 86.5% for a 360 min reaction. With an increase in temperature, degradation efficiency increases significantly, and DBT conversion reaches 100% at 90 ◦ C. Therefore, higher temperatures can considerably promote DBT degradation. Fig. 8. The adsorption of DBT on C18 PW10 Ti2 O38 and [C18 H37 N(CH3 )3 ]Br(0.05 mmol) at 90 ◦ C.

3.2.3. The effect of solvent The solvent effect was also studied for the oxidation of DBT using O2 as the oxidant (Table 1). When acetonitrile were used, the activity of C18 PW10 Ti2 (O2 )2 decreased sharply. This showed that acetonitrile is not suitable for the oxidation of DBT under experimental conditions. When water was introduced into decalin system, the amphiphic catalyst could form emulsion and promote the reaction rate [5,7].

Table 1 Oxidation of DBT by O2 with C18 PW10 Ti2 (O2 )2 catalysta .

Fig. 9. The conversion of DBT vs. substrate’s concentrations with the O2 flowing rate 0.04 m3 /h, the catalyst 0.05 mmol at 90 ◦ C.

Entry

Catalyst (mmol)

Solvent (mL)

Time (h)

Conversion of DBT (%)

TOF (h−1 )

1 2 3 4 5 6 7 8 9 10

0.05 0.05 0.05 0.05 0.025 0.01 0.05 0.05 0.05 0.05

0 0 0 0 0 0 CH3 CN (50) H2 O (5) H2 O (5) H2 O (5)

6 5 4 3 6 6 6 5 4 3

100 98.4 96.5 94.8 76.1 43.8 36.2 100 93.4 83.1

0.81 0.96 1.17 1.54 1.23 1.77 − − − −

a

Conditions: DBT (500 ppm) in 50 mL decalin, reaction temperature 90 ◦ C and O2 .

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Scheme 1. Proposed mechanism for the oxidation of DBT by O2 in the presence of the [C18 H37 N(CH3 )3 ]7 [PW10 Ti2 O38 (O2 )2 ].

3.3. The possible mechanism The oxidation of DBT to sulfone with O2 and C18 PW10 Ti2 (O2 )2 might proceed via the following steps (Scheme 1): the oxidative process includes two consecutive stages: the first stage includes one DBT molecule obtained one oxygen atom from peroxy group of C18 PW10 Ti2 (O2 )2 yielding sulfoxide (Fig. 4); second, one sulfoxide oxidized to sulfone by C18 PW10 Ti2 (O2 )2 . And then the peroxy bonds could be regenerated by O2 under visible-light illumination. Without illumination, the oxidation of DBT by O2 catalyzed by C18 PW10 Ti2 (O2 )2 only achieved 78.9% efficiency. Therefore, it can be concluded that without light, the regeneration of peroxy to C18 PW10 Ti2 is difficult. The catalytic cycle could not be achieved. This phenomenon was observed in peroxy-Zr/Hf POMs [35]. The peroxo functions of the catalyst before and after the reaction were readily reduced by iodide and titrated by a standard thiosulfate solution, confirming the presence of two active oxygen atoms per molecule [35] and peroxy bonds in the catalyst do not change after the reaction. The MALDI-TOF mass spectrum of catalyst after the reaction was in accordance with that before the reaction, thus, the polyanionic structure ([PW10 Ti2 O38 (O2 )2 ]7− ) remained intact. The XPS of the catalyst before and after the reaction also confirms the stability of peroxotitanium in POM catalysts. Based on XPS analysis, the catalyst structure does not change after six repeated experiments (Fig. 11). Compared with the fresh catalyst before (Fig. 11 before), the bonding energies of Ti 2p, P 2p, W 4p, W 4d, W 4f and O 1s do not shift after the reaction (Fig. 11 after), which means that the oxidation states of W, P, O and Ti do not change and this catalyst is deemed stable during the reaction. In addition, the O 1s XPS (Fig. 11(b)) features at 530.1 eV with two very small shoulders at ∼531.0 eV and ∼533.0 eV were attributed to the oxide and peroxide [36–38], respectively. Therefore, the peroxo bond could be regenerated during the reaction.

Fig. 11. Survey XPS patterns before and after the reaction (a) and the O 1s XPS (b) of [C18 H37 N(CH3 )3 ]7 PW10 Ti2 (O2 )2 catalyst.

needs no treatment for reuse. The catalytic activity of DBT oxidation remains efficiently after six repeated experiments, showing only a slight decrease in activity (Fig. 12). Moreover, the total amount of C18 PW10 Ti2 (O2 )2 leaching through six runs of the reaction is only 4.8% of the starting amount.

3.4. The regeneration of the catalyst The life span of a catalyst is a more important parameter for its evaluation. The regeneration of C18 PW10 Ti2 (O2 )2 is of practical and economic importance. It can be easily achieved by the centrifugation of the yellow species from the reaction system. In addition, the catalyst C18 PW10 Ti2 (O2 )2 was identified by IR spectra after the oxidation of DBT at 90 ◦ C for 6 h. The result showed that no peaks corresponding to DBT, showing that DBT was totally oxidized by O2 under our reaction condition (Fig. 7(c)). According to the fact that no DBT molecules attaching to C18 PW10 Ti2 (O2 )2 , the catalyst

Fig. 12. Cycling runs in the oxidation of DBT (500 ppm, 50 mL) in the presence of C18 PW10 Ti2 O38 (O2 )2 (0.05 mmol) at 90 ◦ C with the O2 flowing rate 0.04 m3 /h.

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4. Conclusion The catalyst C18 PW10 Ti2 (O2 )2 assembled in reverse-micellar droplets could completely oxidize DBT to sulfone using O2 as the oxidant under mild conditions such as 90 ◦ C and atmospheric pressure. And the reverse-micellar POM catalysts can be separated and recycled easily. These results demonstrate that oxidation of DBT via above process is a possible process. In addition, the sulfones can be removed directly from diesel by extraction of MeCN.

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