Facile synthesis of amphiphilic polyoxometalate-based ionic liquid supported silica induced efficient performance in oxidative desulfurization

Accepted Manuscript Title: Facile synthesis of amphiphilic polyoxometalate-based ionic liquid supported silica induced efficient performance in oxidative desulfurization Author: Meng Li Ming Zhang Aimin Wei Wenshuai Zhu Suhang Xun Yanan Li Hongping Li Huaming Li PII: DOI: Reference:

S1381-1169(15)00180-6 http://dx.doi.org/doi:10.1016/j.molcata.2015.05.007 MOLCAA 9493

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

3-2-2015 5-5-2015 7-5-2015

Please cite this article as: Meng Li, Ming Zhang, Aimin Wei, Wenshuai Zhu, Suhang Xun, Yanan Li, Hongping Li, Huaming Li, Facile synthesis of amphiphilic polyoxometalate-based ionic liquid supported silica induced efficient performance in oxidative desulfurization, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of amphiphilic polyoxometalate-based ionic liquid supported silica induced efficient performance in oxidative desulfurization Meng Lia,b, Ming Zhangb, Aimin Weia, Wenshuai Zhua*, Suhang Xuna, Yanan Lia, Hongping Lia, Huaming Lia,b* a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P. R. China b

Institute for Energy Research , Jiangsu University, Zhenjiang 212013, P. R. China

*Corresponding author: Tel.:+86-511-88791800; Fax: +86-511-88791708; E-mail address: [email protected] (H. M. Li), [email protected] (W. S. Zhu)

Graphical

abstractFacile

synthesis

of

amphiphilic

polyoxometalate-based ionic liquid supported silica induced efficient performance in oxidative desulfurization Meng Lia,b, Ming Zhangb, Aimin Weia, Wenshuai Zhua*, Suhang Xuna, Yanan Lia, Hongping Lia, Huaming Lia,b* a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P. R. China b

Institute for Energy Research , Jiangsu University, Zhenjiang 212013, P. R. China

*Corresponding author: Tel.:+86-511-88791800; Fax: +86-511-88791708; E-mail address: [email protected] (H. M. Li), [email protected] (W. S. Zhu) Dibenzothiophene was firstly adsorbed into the polyoxometalate-based ionic liquid layer by hydrophobic-hydrophobic interaction between DBT molecule and the alkyl

chain in ionic liquid, and then oxidized into its corresponding sulfone in the presence of hydrogen peroxide.

Abstract

As strict regulations of fuel specifications implemented, oxidative desulfurization process (ODS) is considered as one of the promising technologies to achieve deep desulfurization under mild conditions. Herein, amphiphilic polyoxometalate-based supported silica [C4mim]3PW12O40/SiO2 was successfully synthesized by a facile hydrothermal process and employed in ODS process. The compositions and structure of obtained hybrid samples were characterized by means of FT-IR, XPS, Raman,

31

P

MAS NMR, UV-vis, wide-angle XRD, N2 adsorption-desorption and SEM. The experimental results indicated that the as-synthesized materials maintained the integrity of architecture of polyoxometalate-based ionic liquids (POM-based ILs), which were finely dispersed into the silica matrix. Besides, the sample [C4mim]3PW12O40/SiO2 possessed moderate hydrophilic-hydrophobic balanced surface leading to higher sulfur removal. Moreover, the effect of the amount of catalyst, H2O2/DBT molar ratio, temperature, type of sulfur-containing substrate on the sulfur removal was also investigated in detail. Under optimal conditions, dibenzothiophene (DBT) could be completely removed in 30 min. Keywords:

Amphiphilic

hybrid

materials;

polyoxometalate;

ionic

liquid;

desulfurization 1. Introduction Stricter legislations on the sulfur content in fuels have been enacted in order to

prevent the environmental pollution from the emission of SOx. In US and Europe, the sulfur content in fuels should be less than 15 ppmw and 10 ppmw since the 2006 and 2009, respectively. To date, hydrodesulfurization (HDS) has been widely used to remove the sulfur compounds, because thiols, sulfides, and disulfides can be effectively removed in this process. However, this approach is not efficient in removing aromatic sulfur-containing compounds such as thiophene and its derivatives due to steric hindrance [1-4]. To achieve deep desulfurization target, high temperature (300-400oC), high hydrogen pressure (2-10 MPa), and noble catalysts are required. Under this situation, other alternative desulfurization strategies under relatively mild conditions,

e.g.

extraction

[5,6],

adsorption

[7-11],

oxidation

[12-18],

biodesulfurization [19,20], and photocatalytic desulfurization [21,22] have been extensively investigated. At present, oxidative desulfurization (ODS) is one of the most promising approaches in which aromatic sulfur compounds are conveniently oxidized its corresponding sulfones, subsequently decomposed the formed sulfone and recover biphenyl [23] or extracted with proper solvents [24]. Ionic liquids (ILs) as an immense family of molten salts , have been attracted wide attention as solvents [25,26], extractants [27,28], templates [29,30] and precursors [31,32] due to their unique physicochemical properties. Bosmann firstly developed an extractive desulfurization process using conventional ILs with different cations and anions as extractants [33]. However, it is difficult to reach deep desulfurization with simple extraction process. Hence, Lo et al. reported a novel approach combining the oxidation with the extraction with ILs could achieve deep desulfurization. This

process is much more efficient compared to a simple extraction by ILs [34]. In the past years, various ILs have been developed [35-37]. Our group has engaged in developing specific ILs and employing them in extractive catalytic oxidative desulfurization (ECODS) [38,39]. Though these catalysts displayed good activity, the dosage of ILs in this process was relatively large. Moreover, the recovery of ILs after reaction was difficult. To solve these problems, many endeavours have been directed to create “supported ionic liquid materials” using solid supports, such as SiO2 [40,41], TiO2 [42], mesoporous materials [43], CNT [44] and MOF [31]. Noteworthily, these hybrid catalysts have been applied in various reactions with an ample prospect. Polyoxometalates (POMs) are a family of transition metal-oxide clusters with unique structure diversity, specific physical properties, controllable redox and acidic properties. For these advantages, POMs have attracted wide interests in a variety of organic transformations, such as epoxidation of alkenes [45], selective oxidation of alcohols [46], esterification of acid with alcohol [47] and oxidation of sulfides [48] . However, POM clusters were hardly dispersed in the common organic solvents due to the crystalline feature and high lattice energy [49]. To confront this limitation, a series of POM-anions tethered with inorganic/organic functional groups have been developed, which obtained better catalytic performance in many types of organic reaction [50-54]. Recently, a series of W/IL emulsion systems with POM-based ILs were formed [55,56]. These systems were also proven to be more active compared to the previous works. However, these catalytic systems often suffered from slow reaction rate between organic substrates and catalyst. To overcome the obstacle, other

attention has been paid to immobilization of POMs onto suitable support [57], which can not only provide large surface area to enhance the mass transfer rate but also benefit for the recovery of catalysts. In the present work, to integrate the advantages of POMs-based ILs and heterogeneous materials, amphiphilic hybrid materials [C4mim]3PW12O40/SiO2 was synthesized over a facile hydrothermal process and evaluated in ODS system, where no other solvent was added. These hybrid materials characterized in detail were highly efficient on the removal of DBT and easily separated for recycling under mild conditions, which could be potentially applied in industry. Besides, the oxidation products in the reaction were also investigated by GC-MS analysis. 2. Experimental section 2.1. Materials H3PW12O40·14H2O (AR grade), NH3·H2O (aqueous solution, 25%), acetonitrile (CH3CN, AR grade), Tetraethylorthosilicate (TEOS, AR grade)，hydrogen peroxide (H2O2, 30 wt%), benzothiophene (BT, 99%), dibenzothiophene (DBT, 98%), 4,6-dimethyldibenthiophene (4,6-DMDBT, 99%) were obtained from Sigma-Aldrich. n-octane (CP grade) and tetradecane (AR grade) were purchased from SinopharmChemical Reagent Co, Ltd. [C4mim]Cl (99%), [C8mim]Cl (99%) and [C16mim]Cl (99%) were gain from Shanghai Chenjie Chemical Co, Ltd. All the reagents were used directly without further purification. 2.2. Catalyst preparation [Cnmim]3PW12O40 (n = 4, 8, 16) was synthesized according to the previous

literature [58]. The hybrid materials [Cnmim]3PW12O40/SiO2 were synthesized by a facile hydrothermal method. The synthesis of [C4mim]3PW12O40/SiO2 as an example. In detail, a 0.25 g （0.076 mmol）portion of [C4mim]3PW12O40 was dissolved in 4 mL of acetonitrile at 50oC to form solution A . Afterward, solution A was added to 26 mL of deionized water drop by drop. After then, 2 mL of Tetraethylorthosilicate (TEOS) was transferred to the above solution, followed by addition of 0.5 mL of 25% aqueous ammonia. After stirring for 3 h at room temperature, the mixture was transferred into a Teflon-lined stainless-steel autoclave and maintained at 120oC for 24 h. Then resultant was filtered and washed several times with deionized water and was dried in electric oven at 80oC overnight. The solid materials were collected and handled at 200oC for 3 h at a heating rate of 2oC/min. Finally, the white catalyst was obtained. In this reaction, the molar ratio of TEOS : [C4mim]3PW12O40 : H2O : NH3·H2O = 1.0:0.008:160:1.5. Other hybrid materials with different alkyl chain were synthesized with the equal molar ratio through the above method. 2.3. Characterization FT-IR spectra of various samples were collected on a Nicolet Model Nexus 470 FT-IR apparatus using KBr pellets. SEM analysis was measured on JEOL JSM-7001F field-emission microscope. XPS of sample was performed on PHI530 with a monochromatic Mg Kα source. Raman spectroscopy was carried out on a DXR Raman microscope using a 532 nm excitation laser power. UV-vis spectra were conducted with a UV-vis spectrometer (UV-2450, Shimadzu). XRD patterns were obtained by using a Bruker D8 X- ray diffractometer using Cu Kα radiation (λ =

1.5418 Å). A TriStar II 3020 surface area and porosity analyzer was used to collect the N2 absorption-desorption isotherms at 77 K. The oxidation products of DBT were studied

by

GC-MS

(Agilent

7890/5975C-GC/MSD;

temperature

program:

100ºC-temperature rising 15 ºC/ min-200 ºC for 10 min; HP-5 MS column, 30 m ×250 µm i.d. × 0.25 µm.). 2.4. Catalytic activity test Model oil was obtained by dissolving desired amount of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in n-octane with a corresponding s-content of 250, 500 and 250 ppm, respectively. Oxidation of sulfur compounds of fuels were carried out by mixing 0.03 g of catalyst and 5 mL of model oil in a two-necked kettle equipped with a magnetic stirrer and a condenser. After then, a desired amount of H2O2 was rapidly added into above mixture under vigorous stirring at 60oC. After the reaction, the residual sulfur content was monitored via gas chromatography (GC, Agilrnent-7890A) equipped with a capillary column (HP-5, 30 m× 0.32 mm × 0.25 µm). 3. Results and discussion 3.1. FT-IR spectroscopy Figure 1 illustrates the FT-IR spectra of POM-based ILs [Cnmim]3PW12O40 (n = 4, 8, 16) and ILs supported silica. For the POM-based ILs (Fig. 1a, b, c), the peak around 3427 cm-1 is accounted for the characteristic band of surface silanols (Si-OH) of the hybrid catalysts. The bands between 2950 cm-1 and 2850 cm -1 are assigned to the stretching vibration of C-H of alkyl chain, and the bands ranging from 1560 cm-1

to 1460 cm-1 were attributed to the stretching vibration of C=N and scissoring vibration of C-H, respectively [59]. In addition, all ILs exhibited several characteristic bands related to Keggin units at νas(P-Oa) = 1080 cm-1, νas (W=Ot) = 978 cm-1, νas(W-Oc-W) = 898 cm-1 and νas(W-Oe-W) = 808 cm-1. For the as-synthesized materials (Fig. 1d, e, f), the stretching vibrations of the C-H of IL cation from 2900 cm-1 to 2800 cm-1 were detected. These results verified that the various POM-based ILs were successfully incorporated into the silicate matrix. Moreover, the absorption peaks of the Keggin anion could not be found, which was overlapped by the characteristic peaks of silica.

a 2980 2933 2873

Transmittance (a.u.)

b c

2927

d

2923 2982

e

2942

f 3427

2930

2855

1565

1464

1566

1458

1570

1459

2852

2931 2822 2854

4000 3600 3200 2800 2400 2000 1600 1200

800

400

Wavenumber (cm-1)

Fig. 1. FT-IR spectra of various samples (a) [C4mim]3PW12O40; (b) [C8mim]3PW12O40; (c) [C16mim]3PW12O40; (d) [C4mim]3PW12O40/SiO2; (e) [C8mim]3PW12O40/SiO2; (f) [C16mim]3PW12O40/SiO2。

3.2. XPS spectra

In order to obtain the additional information about the elemental composition and the chemical nature of the active species in the hybrid sample, XPS spectra of the hybrid material [C4mim]3PW12O40/SiO2 was performed (Fig. 2). As shown in Fig. 2A, the peaks corresponding to C 1s, N 1s, Si 2s, Si 2p and P 2p appeared at 285.2 eV, 401.2 eV, 176.4 eV, 104.2 eV and 134.4 eV, respectively, which indicated the existence of POM-based IL. Moreover, the core level spectrum shows two peaks concerning the W4f7/2 and 4f5/2 with the bonding energies of 35.7 eV and 37.8 eV (Fig. 2B), which further verified the structure of POMs [42]. These results also demonstrated that the POM-based IL was successfully embedded into the architecture of supporter and retained the integrity of structure.

A

Si 2p

Intensity (a.u.)

Si 2s

C 1s W 4f

0

N 1s

P 2p

100

200

300

Bonding energy (eV)

400

B

4f7/2

Intensity (a.u.)

4f5/2

32

34

36

38

40

Bonding energy (eV)

Fig. 2. XPS spectra survey (A) and peak deconvolutions for W 4f (B) of

hybrid sample [C4mim]3PW12O40/SiO2.

3.3. Raman spectroscopy Additional structural information concerning the existence of POM anion in the hybrid catalysts was achieved from Raman scattering spectroscopy (Fig. 3). The bands ranging from 950 cm -1 to 1050 cm -1 could be obviously observed, which could assign to the νs (W=O) = 1005cm -1 and νas (W=O) = 990 cm -1 in the Keggin unit [57]. This phenomenon also demonstrated the neat ILs were successfully introduced into silica matrix as well as maintained the integrity of Keggin anion.

Intensity (a.u.)

a

b

c

1200

1100

1000

900

800

-1

Raman shift (cm )

Fig. 3. Raman spectra of samples (a) [C4mim]3PW12O40/SiO2, (b) [C8mim]3PW12O40/SiO2,

(c) [ C16mim]3PW12O40/SiO2.

3.4. 31P MAS NMR 31

P MAS NMR spectra of various samples were performed in order to evaluate the

structural integrity after [C4mim]3PW12O40 introduced to the silica matrix (Fig. S1). For neat [C4mim]3PW12O40 (Fig. S1a), only one intense peak around -15.7 ppm was detected, which assigned to

31

P chemical shift of Keggin unit [60,61].

31

P NMR

resonance signal of [C4mim]3PW12O40/SiO2 was presented in Fig. S1b. The peak at -15.7 ppm could be still observed, and additional two peaks around -13.2 ppm and 0.5 ppm were found, which was attributed to the interaction of Keggin structure with the host material and phosphate groups of the heterogeneous material, respectively [62,63].

31

P NMR spectra for [C4mim]3PW12O40/SiO2 after catalytic of DBT also

exhibited the typical peak of Keggin species, which demonstrated that the active specie [C4mim]3PW12O40 was stable in the composite after reaction.

3.5. UV-vis analysis Fig. 4 shown UV-vis spectra of various hybrid materials, which was an appropriately method to evaluate the charge transfer behavior for selected products. Two major adsorption bands could be observed distinctly in the region of 200-400 nm, which were attributed to the electronic properties of the center-metal in the structure of Keggin anion. The absorption band located at 212 nm was assigned to O-P transition, and the absorption peak located at 260 nm was mainly accounted for the ligand to metal atom charge transfer (O2-→ W6+), which was consistent well with

Absorbance (a.u.)

previous study [64].

[C4mim]3PW12O40/SiO2 [C8mim]3PW12O40/SiO2 [C16mim]3PW12O40/SiO2

200

300

400

500

600

700

Wavelength (nm)

Fig. 4. UV-vis spectra of various hybrid samples.

800

3.6. XRD patterns Fig. 5 exhibited the wide-angle XRD patterns of the bare ILs and hybrid samples. For POM-based ILs (Fig. 5a, b, c), the diffraction patterns ranging from 15° to 40° (2θ) were caused by the POM structure in the IL. For the IL supported materials (Fig. 5d, e, f), XRD patterns presented only a broad peak at around 2θ = 22.5o corresponding to the characteristic peak of amorphous silica, which implied that the ILs were finely dispersed into the silica matrix. This phenomenon was mainly responsible for the better catalytic performance compared to the bare ILs.

f

Intensity (a.u)

e

d

c b

a

10

20

30

40

50

60

70

80

2 theta (degree)

Fig. 5. XRD patterns of various hybrids (a) [C16mim]3PW12O40; (b)

[C8mim]3PW12O40; (c) [C4mim]3PW12O40; (d) [C16mim]3PW12O40/SiO2; (e) [C8mim]3PW12O40/SiO2; (f) [C4mim]3PW12O40/SiO2. 3.7. Porous structure analysis N2 absorption-desorption isotherms (A) and BJH pore size distribution curves (B) of various samples are shown in Fig. 6. In Fig 6A, N2 absorption-desorption isotherms of various samples (Fig. 6A) could be classified as type IV with a clear H1-type

hysteresis loop according to the IUPAC classification, indicating the presence of mesoporosity [46,65]. In Fig. 6B, pore size distributions curves of various materials presents the probable pore size centered at 18.1 nm, 17.8 nm and 22.7 nm for [C4mim]3PW12O40/SiO2,

[C8mim]3PW12O40/SiO2

and

[C16mim]3PW12O40/SiO2,

respectively, which also indicated mesoporosity in the supported materials. Table 1 displayed the textural properties of heterogeneous samples. The results demonstrated that all the samples have BET surface ranging from 88 cm3 g-1 to 155 cm3 g-1 and pore volume from 0.76 cm3 g-1 to 1.03 cm3 g-1. It was noteworthy that the hybrid

material [C4mim]3PW12O40/SiO2 had a surface area of 155 m2 g-1, which was larger than that of [C8mim]3PW12O40/SiO2 and [C16mim]3PW12O40/SiO2. These results could be responsible for the different catalytic performance of various hybrid materials in the desulfurization system (Table 2).

B

0.12

-1

dV/dD (cm3g .nm)

0.10 0.08

[C4mim]3PW12O40/SiO2 0.06

[C8mim]3PW12O40/SiO2 [C16mim]3PW12O40/SiO2

0.04 0.02 0.00

0

25

50

Pore diameter (nm)

75

100

Fig. 6. Nitrogen adsorption-desorption isotherms (A) and pore size

distributions of hybrid materials (B).

3.8. SEM analysis To further evaluate the morphology of the as-synthesized material, the SEM and EDS analysis of the hybrid material were evaluated (Fig. S2). The SEM image of the hybrid material [C4mim]3PW12O40/SiO2 exhibited irregular massive congeries with relatively even surface. From EDS analysis, the elements of C, N, P and W existed in the hybrid material [C4mim]3PW12O40/SiO2, demonstrating that POM-based IL [C4mim]3PW12O40 was successfully embedded into the silica matrix. The results agreed well with the results of FT-IR spectra. 3.9. Contact angle test The hydrophilic and hydrophobicity test of various hybrid samples are shown in Fig. 7. When a water droplet contacts the sheet of [C4mim]3PW12O40/SiO2, an approximately

contact

angle

of

22o

was

obtained

(Fig.

7A).

Besides,

[C16mim]3PW12O40/SiO2 displayed contact angle of 145o (Fig. 7B), indicating that the interaction of the [C4mim]3PW12O40/SiO2 with H2O2 was facile compared to that of the [C16mim]3PW12O40/SiO2. Moreover, the hydrophobicity investigations of heterogeneous samples [C4mim]3PW12O40/SiO2 and [C16mim]3PW12O40/SiO2 were presented in Fig. 7C and D, respectively. With n-octane as the testing droplet, similar yield of 20o and 18o of the two samples was achieved, demonstrating that the hybrid samples have good surface wettability for n-octane. These results illustrated that the

hybrid [C4mim]3PW12O40/SiO2 shown good wettability for both H2O2 and n-octane, which was benefit for the oxidation reactions below.

Fig. 7. Contact angles of water droplets on the surface of (A) [C4mim]3PW12O40/SiO2, (B) [C16mim]3PW12O40/SiO2; contact angles of n-octane droplets on the surface of (C) [C4mim]3PW12O40/SiO2, (D) [C16mim]3PW12O40/SiO2.

3.10. Catalytic activity for oxidation of sulfur-containing substrates Table 2 presents the catalytic performances of various hybrid samples in the oxidation of DBT in model oil. Obviously, all of the POM-based ILs exhibit sulfur removal less than 13% (Table 2, entries 1-3). However, when the POM-based ILs were introduced into the silica matrix as heterogeneous catalysts, much higher

desulfurization performance could be achieved, ranging from 80% to 100%, in the various desulfurization systems (Table 2, entries 4-6). Noteworthily, the hybrid catalyst [C4mim]3PW12O40/SiO2 displayed the highest catalytic activity compared to the other hybrid materials. This results were mainly attributed to the heterogeneous material [C4mim]3PW12O40/SiO2 possessed the moderate amphiphilic property leading to the fast interaction with H2O2 and model oil [56], as well as bigger BET surface area, which could accelerate the mass transfer process.

Reaction conditions: m (catalyst) = 0.03 g, T = 60oC, t = 30 min, n (H2O2) / n (DBT) = 3. The feature of the substrates, such as electron density and steric hindrance, plays a vital role in the desulfurization system. Herein, 4,6-DMDBT and BT, as other substrates, were evaluated in this desulfurization. As shown in Fig. 8, it is distinctly observed that the oxidation of different sulfur-containing substrates decreased in the order of DBT > 4,6-DMDBT > BT under the same conditions. DBT and 4,6-DMDBT can be completely removed at 60oC in 30 min, while BT was only 85.1%. These results could be attributed to synthetic effects of the electron density on the sulfur atom of different substrates and the steric hindrance of alkyl groups located on aromatic ring. In this heterogeneous oxidative desulfurization reaction system, the difference in electron density on sulfur atom of DBT (5.758) and 4,6-DMDBT (5.760) was tiny [66]. Then, the catalytic performance of the hybrid catalysis for DBT and 4,6-DMDBT was mainly influenced by the steric hindrance. However, compared to the DBT and 4,6-DMDBT, the electron density of BT was lowest (5.739), leading to

negatively attacked efficient by the peroxide species and exhibiting lowest reaction performance. The results were consistent with the previous work [67].

Sulfur removal ( % )

100

80 DBT 4,6-DMDBT BT

60

40 10

20

30

40

50

60

Time (min)

Fig. 8. Effect of feature of different sulfur-containing compounds on the

sulfur removal. Reaction conditions: m ([C4mim]3PW12O40/SiO2) = 0.03 g, T= 60oC, O/S = 3.

3.11. GC-MS analysis To understand the oxidation products of DBT after catalytic oxidative reaction, GC-MS analysis was performed as follows (Fig. 9). In a typical run, the upper oil phase was directly decanted and the used catalyst was extracted by carbon tetrachloride for the GC-MS analysis. As shown in Fig. 9A, only one peak for n-tetradecane at 3.6 min could be observed, indicating that DBT were completely removal from the model oil. On the other hand, in Fig. 9B, the peak at 12.1 min (Fig. 9B insert) was caused by DBTO2. Besides, no other peaks were detected. The results

demonstrated that DBT was entirely oxidized to DBTO2. To further verify the oxidation product, FT-IR spectrum of used hybrid material [C4mim]3PW12O40/SiO2 was measured. As shown in Fig. S6, the appearance of characteristic peaks at 1290 cm-1 and 1160 cm-1 indicated that DBTO2 was only product, consisting well with the results of GC-MS [68]. Hence, it was concluded that the DBT was adsorbed to the structure of amphiphilic hybrid materials, and then oxidized to its corresponding sulfone.

A

Intensity (a.u.)

n-tetradecane

4

6

8

Time (min)

10

12

14

B S

Abundance

O O

216.0

DBTO2

6

8

10

12

14

16

Retention time (min)

50

100

150

200

250

m/z

Fig. 9. GC analysis of the upper oil after reaction (A); GC-MS analysis of sulfur compounds in the catalyst after reaction (B).

3.12. Recycling ability To obtain the wide utilization of amphiphilic hybrid catalysts in the practical process, the recycle capability of hybrid catalysts [C4mim]3PW12O40/SiO2 in the desulfurization system was tested (Fig. 10). After the first run, the lower hybrid material was removed from the reaction system by decantation immediately. Then, the residual was dried in an oven at 50oC overnight to be used in next reaction under the same conditions. It was worth noting that the removal of DBT could still obtained 96.6% sulfur removal after 13-times recycling without significant decrease, which would be potential used in industry .

100

Sulfur removel (%)

80

60

40

20

0 1

2

3

4

5

6

7

8

9

10 11 12 13

Recycle times

Fig. 10. Recycle performance of hybrid catalyst [C4mim]3PW12O40/SiO2. Reaction conditions: m ([C4mim]3PW12O40/SiO2) = 0.03 g, T = 60oC, t = 30 min, O/S = 3.

3.13. Reaction mechanism It is necessary to propose a mechanism of catalytic oxidative desulfurization of DBT using hybrid catalyst [C4mim]3PW12O40/SiO2. In scheme 1, the DBT molecules were firstly adsorbed into the IL layer by hydrophobic-hydrophobic interaction between DBT molecule and the alkyl chain in ILs. Then, DBT molecules were oxidized to its corresponding sulfone by active peroxide species, which was formed by the reaction of POM and H2O2. In addition, the oxidized product DBTO2 accumulated in the catalyst phase after reaction due to its nature of polarity, which was of benefit to the application in the industry.

oil phase S

IL layer H2O 2

S

O

O O ¦Ì O

SiO2 :

N

N

n

n = 1, 3, 13

,

3: [PW12O 40]

Scheme 1. Proposed mechanism of catalytic oxidative desulfurization process using hybrid [C4mim]3PW12O40/SiO2 in the presence of H2O2

4. Conclusion In summary, amphiphilic POM-based supported silica were successfully synthesized by a facile hydrothermal process and employed in the desulfurization system. The characterization results that the POM-based ILs were embedded into the silica matrix uniformly and maintained the structure integrity. The contact angle tests demonstrated that the hybrid materials possessed a moderate amphiphilic property of good wettability for both H2O2 and n-octane. In this desulfurization system, the hybrid material [C4mim]3PW12O40/SiO2 displayed excellent catalytic activity on the removal of sulfur compounds without other organic solvents, and still reached 96.6% after thirteen cycles. The oxidative efficiency of different substrates decreased in the order of DBT> 4,6-DMDBT> BT. Besides, the amphiphilic heterogeneous catalyst could be easily separated after reaction, which was of benefit to potential using in the wide

fields. Acknowledgements This work was financially supported by the National NatureScienceFoundation of China (Nos. 21276117, 21376111, 21406092), The Natural Science Foundation of Jiangsu

Province

(Nos.BK2012697,

BK20131207),

Advanced

Talents

of

JiangsuUniversity (No. 13JDG080), Postdoctoral Foundation of China (No. 2014M551516), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

References [1] A. Rothlisberger, R. Prins, J.Catal., 235 (2005) 229-240. [2] R. Javadli, A. de Klerk, Energy Fuels, 26 (2012) 594-602. [3] A.K. Tuxen, H.G. Fuchtbauer, B. Temel, B. Hinnemann, H. Topsoe, K.G. Knudsen, F. Besenbacher, J.V. Lauritsen, J. Catal., 295 (2012) 146-154. [4] X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res., 35 (1996) 2487-2494. [5] H. Gao, C. Guo, J. Xing, J. Zhao, H. Liu, Green Chem., 12 (2010) 1220-1224. [6] Z. Li, C. Li, Y. Chi, A. Wang, Z. Zhang, H. Li, Q. Liu, U. Welz Biermann, Energy Fuels, 26 (2012) 3723-3727. [7] M. Seredych, T.J. Bandosz, Energy Fuels, 24 (2010) 3352-3360. [8] W. Huang, H. Li, W. Xu, W. Zhou, Z. Zhou, W. Yang, Adsorpt. Sci. Technol., 30 (2012) 331-343. [9] H.X. Zhang, H.L. Huang, C.X. Li, H. Meng, Y.Z. Lu, C.L. Zhong, D.H. Liu, Q.Y. Yang, Ind. Eng. Chem. Res., 51 (2012) 12449-12455.

[10] H. Song, X. Wan, M. Dai, J. Zhang, F. Li, H. Song, Fuel Process. Technol., 116 (2013) 52-62. [11] N.A. Khan, Z. Hasan, S.H. Jhung, Chem. Eur. J., 20 (2014) 376-380. [12] L. Cedeno Caero, M. Ramos Luna, M. Mendez Cruz, J. Ramirez Solis, Catal. Today, 172 (2011) 189-194. [13] B. Li, W. Ma, J. Liu, C. Han, S. Zuo, X. Li, Catal. Commun., 13 (2011) 101-105. [14] J. Zhang, A. Wang, X. Li, X. Ma, J. Catal., 279 (2011) 269-275. [15] J. Dou, H.C. Zeng, ACS Catal., 4 (2014) 566-576. [16] S. Ribeiro, A.D.S. Barbosa, A.C. Gomes, M. Pillinger, I.S. Goncalves, L. Cunha-Silva, S.S. Balula, Fuel Process. Technol., 116 (2013) 350-357. [17] Y. Zhu, M. Zhu, L. Kang, F. Yu, B. Dai, Ind. Eng. Chem. Res., 54 (2015) 2040-2047. [18] W. Zhu, B. Dai, P. Wu, Y. Chao, J. Xiong, S. Xun, H. Li, H. Li, ACS Sustainable Chem. Eng., 3 (2015) 186-194. [19] M. Alejandro Dinamarca, L. Orellana, J. Aguirre, P. Baeza, G. Espinoza, C. Canales, J. Ojeda, Biotechnol. Lett, 36 (2014) 1649-1652. [20] M. Alejandro Dinamarca, A. Rojas, P. Baeza, G. Espinoza, C. Ibacache Quiroga, J. Ojeda, Fuel, 116 (2014) 237-241. [21] X.J. Wang, F.T. Li, J.X. Liu, C.G. Kou, Y. Zhao, Y.J. Hao, D. Zhao, Energy Fuels, 26 (2012) 6777-6782. [22] F. Lin, Y. Zhang, L. Wang, Y. Zhang, D. Wang, M. Yang, J. Yang, B. Zhang, Z. Jiang, C. Li, Appl. Catal. B: Environ., 127 (2012) 363-370. [23] Y.K. Park, S.Y. Kim, H.J. Kim, K.Y. Jung, K.E. Jeong, S.Y. Jeong, J.K. Jeon, Korean J. Chem. Eng., 27 (2010) 459-464.

[24] F.T. Li, R.H. Liu, J.H. Wen, D.S. Zhao, Z.M. Sun, Y. Liu, Green Chem., 11 (2009) 883-888. [25] P. Migowski, J. Dupont, Chem. Eur. J., 13 (2007) 32-39. [26] C. Shu, T. Sun, Q. Guo, J. Jia, Z. Lou, Green Chem., 16 (2014) 3881-3889. [27] C. Li, D. Li, S. Zou, Z. Li, J. Yin, A. Wang, Y. Cui, Z. Yao, Q. Zhao, Green Chem., 15 (2013) 2793-2799. [28] B. RodriguezCabo, H. Rodriguez, E. Rodil, A. Arce, A. Soto, Fuel, 117 (2014) 882-889. [29] Q. Gu, W. Zhu, S. Xun, Y. Chang, J. Xiong, M. Zhang, W. Jiang, F. Zhu, H. Li, Fuel, 117 (2014) 667-673. [30] J. Xiong, W. Zhu, W. Ding, P. Wang, Y. Chao, M. Zhang, F. Zhu, H. Li, RSC Adv., 4 (2014) 40588-40594. [31] K. Fujie, T. Yamada, R. Ikeda, H. Kitagawa, Angew. Chem. Int. Ed., 53 (2014) 11302-11305. [32] M. Zhang, W. Zhu, H. Li, S. Xun, W. Ding, J. Liu, Z. Zhao, Q. Wang, Chem. Eng. J., 243 (2014) 386-393. [33] A. Bosmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz, P. Wasserscheid, Chem. Commun., (2001) 2494-2495. [34] W.H. Lo, H.Y. Yang, G.T. Wei, Green Chem., 5 (2003) 639-642. [35] H.Y. Lu, J.B. Gao, Z.X. Jiang, F. Jing, Y.X. Yang, G. Wang, C. Li, J. Catal., 239 (2006) 369-375. [36] J. Zhang, W. Zhu, H. Li, W. Jiang, Y. Jiang, W. Huang, Y. Yan, Green Chem., 11 (2009) 1801-1807. [37] W. Jiang, W. Zhu, H. Li, J. Xue, J. Xiong, Y. Chang, H. Liu, Z. Zhao, Chem. Eng. Technol., 37 (2014) 36-42. [38] J. Xiong, W. Zhu, H. Li, Y. Xu, W. Jiang, S. Xun, H. Liu, Z. Zhao, AlChE J., 59 (2013) 4696-4704.

[39] W. Zhu, J. Zhang, H. Li, Y. Chao, W. Jiang, S. Yin, H. Liu, RSC Adv., 2 (2012) 658-664. [40] N. Li, F. Wang, Z. Zhang, H. Nie, C. Ren, M. Cui, Ind. Eng. Chem. Res., 53 (2014) 16664-16671. [41] P. Sharma, M. Gupta, Green Chem., 17 (2015) 1100-1106. [42] J. Xiao, X. Wang, Y. Chen, M. Fujii, C. Song, Ind. Eng. Chem. Res., 52 (2013) 15746-15755. [43] D. Elhamifar, M. Nasr-Esfahani, B. Karimi, R. Moshkelgosha, A. Shabani, Chemcatchem, 6 (2014) 2593-2599. [44] Y. Ding, B. Zhang, N. Gupta, D.S. Su, Green Chem., 17 (2015) 1107-1112. [45] S.P. Maradur, C. Jo, D.-H. Choi, K. Kim, R. Ryoo, Chemcatchem, 3 (2011) 1435-1438. [46] G. Chen, Y. Zhou, Z. Long, X. Wang, J. Li, J. Wang, ACS Appl. Mat. Interfaces, 6 (2014) 4438-4446. [47] Y. Leng, J. Wang, D. Zhu, X. Ren, H. Ge, L. Shen, Angew. Chem. Int. Ed., 48 (2009) 168-171. [48] X.L. Hao, Y.Y. Ma, H.Y. Zang, Y.H. Wang, Y.G. Li, E.B. Wang, Chem. Eur. J., 21 (2015) 3778-3784. [49] A. Nisar, J. Zhuang, X. Wang, Adv. Mater., 23 (2011) 1130-1135. [50] H. Lü, W. Ren, P. Liu, S. Qi, W. Wang, Y. Feng, F. Sun, Y. Wang, Appl. Catal. A: Gen., 441-442 (2012) 136-141. [51] H. Lü, W. Ren, W. Liao, W. Chen, Y. Li, Z. Suo, Appl. Catal. B: Environ., 138-139 (2013) 79-83. [52] P. Zhao, M. Zhang, Y. Wu, J. Wang, Ind. Eng. Chem. Res., 51 (2012) 6641-6647. [53] X. Cui, D. Yao, H. Li, J. Yang, D. Hu, J. Hazard. Mater., 205 (2012) 17-23. [54] Y. Chen, S. Zhao, Y. Song, Appl. Catal. A: Gen., 466 (2013) 307-314. [55] Y. Ding, W. Zhu, H. Li, W. Jiang, M. Zhang, Y. Duan, Y. Chang, Green Chem., 13 (2011) 1210-1216.

[56] J. Xu, S. Zhao, Y. Ji, Y.-F. Song, Chem. Eur. J., 19 (2013) 708-714. [57] M. Zhang, W. Zhu, S. Xun, H. Li, Q. Gu, Z. Zhao, Q. Wang, Chem. Eng. J., 220 (2013) 328-336. [58] T. Rajkumar, G.R. Rao, Mater. Chem. Phys., 112 (2008) 853-857. [59] S. Xun, W. Zhu, D. Zheng, L. Zhang, H. Liu, S. Yin, M. Zhang, H. Li, Fuel, 136 (2014) 358-365. [60] L.H. Wee, F. Bonino, C. Lamberti, S. Bordiga, J.A. Martens, Green Chem., 16 (2014) 1351-1357. [61] D. Nedumaran, A. Pandurangan, Mater. Res. Bull., 61 (2015) 1-8. [62] B.J.S. Johnson, A. Stein, Inorg. Chem., 40 (2001) 801-808. [63] A. Ghanbari-Siahkali, A. Philippou, J. Dwyer, M.W. Anderson, Appl. Catal. A: Gen., 192 (2000) 57-69. [64] W. Huang, W. Zhu, H. Li, H. Shi, G. Zhu, H. Liu, G. Chen, Ind. Eng. Chem. Res., 49 (2010) 8998-9003. [65] F. Liu, W. Kong, C. Qi, L. Zhu, F.-S. Xiao, ACS Catal., 2 (2012) 565-572. [66] S. Otsuki, T. Nonaka, N. Takashima, W.H. Qian, A. Ishihara, T. Imai, T. Kabe, Energy Fuels, 14 (2000) 1232-1239. [67] L.S. Nogueira, S. Ribeiro, C.M. Granadeiro, E. Pereira, G. Feio, L. Cunha Silva, S.S. Balula, Dalton Trans., 43 (2014) 9518-9528. [68] A. Nisar, Y. Lu, J. Zhuang, X. Wang, Angew. Chem. Int. Ed., 50 (2011) 3187-3192.

Table1. Texture properties of different supported-IL Samples SBET(cm3 g-1) 155 148 88

Samples [C4mim]3PW12O40/SiO2 [C8mim]3PW12O40/SiO2 [C16mim]3PW12O40/SiO2

Pore volume (cm3 g-1) 0.85 1.03 0.76

Volume adsorbed STP (cm3 g-1)

A 1200

900

600

[C16mim]3PW12O40/SiO2

[C8mim]3PW12O40/SiO2

300 [C4mim]3PW12O40/SiO2 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Table 2. Catalytic performance of various samples. Entry 1 2 3 4 5 6

Type of catalyst [C4mim]3PW12O40 [C8mim]3PW12O40 [C16mim]3PW12O40 [C4mim]3PW12O40/SiO2 [C8mim]3PW12O40/SiO2 [C16mim]3PW12O40/SiO2

S-removal of different system(%) 11.4 10.3 12.5 100 92.1 80.6