One-pot synthesis of ordered mesoporous silica encapsulated polyoxometalate-based ionic liquids induced efficient desulfurization of organosulfur in fuel

Abstract

In this work, a novel hybrid material of an ordered mesoporous silica (OMS) encapsulated polyoxometalate-based ionic liquid [C₄mim]₃PW₁₂O₄₀ (C₄-IL) was successfully fabricated through a one-pot hydrothermal process, and firstly employed in the oxidative desulfurization of organosulfur. The as-prepared material OMS encapsulated C₄-IL (C₄-IL@OMS) was systematically characterized by Fourier transform infrared, Raman, X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen absorption–desorption isotherms, scanning electron microscopy and transmission electron microscopy. The designed material exhibited robust activity for removing sulfur-containing compounds under mild conditions due to the high dispersion of C₄-IL in OMS and lager specific surface area of the catalyst. ³¹P nuclear magnetic resonance and ultraviolet-visible spectrum were employed to evaluate the stability of the catalyst after the reaction. Additionally, the sulfur removal of the hybrid material C₄-IL@OMS could still reach 93% after recycling seven times without obvious decrease in activity.

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1. Introduction

Nowadays, removal of sulfur-containing compounds in fuels is considered as one of the most important and challenging procedures in the petroleum industry. The combustion of these sulfur-containing compounds will bring large amounts of SO_x, which is a major source of air pollution.^1–3 At present, hydrodesulfurization (HDS) is widely applied in removing sulfur-containing compounds around the world. However, aromatic sulfur compounds, such as dibenzothiophene (DBT) and its derivatives were hardly removed by HDS due to their steric hindrance.^4,5 Therefore, considerable attentions have been directed to create other low-cost alternatives to reach ultra-desulfurization under mild conditions, such as adsorption,^6–8 oxidization,^9–14 extraction^15,16 and photocatalysis, etc.^17–19 In particular, oxidative desulfurization (ODS) is regarded as one of the most promising ways to achieve deep desulfurization since aromatic sulfides could be easily removed under moderate conditions.

Polyoxometalates (POMs) have been widely used as active species in various organic transformations because of their unique physical and chemical properties, such as thermal stability, acidic and good redox properties.²⁰ Recently, to enlarge the scope of the application of polyoxometalates, POMs-based catalysts coupled with ionic liquids were developed and exhibited excellent catalytic activity upon epoxidation of olefins,²¹ selective adsorption of dyes,²² and rearrangement reactions,²³ etc. However, owing to negative dissolution of POMs coupled with inorganic/organic counterparts in the common solvent, the extensive applications in various reactions were limited.

In recent years, ionic liquids (ILs) as a novel “green solvent” and catalyst, have attracted wide attention owing to their unique properties, such as good thermal stability, wide liquid range and easy to design.²⁴ Based on these advantages, the catalysts could be dissolved into the ILs phase to provide a liquid–liquid biphasic system, which has been successfully applied in the esterification reaction,²⁵ the dehydration of fructose,²⁶ and oxidation of sulfides.^27,28 However, high dosage of ILs is required, and the separation of catalysts used is difficult in these processes, inhibiting their further application in organic reactions. Hence, the heterogenization of homogeneous ILs provided a better way to tackle these limitations. For this purpose, various attempts have been paid to fabricate “supported ionic liquid catalysts” (SILCs) that fulfill these requirements through the immobilization of homogeneous ILs on suitable host, such as silica materials,^29–31 polymer,³² MOF^7,33,34 and CNT,³⁵ etc.

Compared with the nonporous silica gel originated from the hydrolysis of silica precursors, ordered mesoporous silica materials have attracted great interest in catalysis,³⁶ drug delivery,³⁷ and adsorption,³⁸ owing to its relative large surface area, adjustable pore size, nontoxic and large pore volume. To fully utilize the advantages of the mesoporous silica, considerable efforts have been devoted to prepare mesoporous silica-supported catalyst through various methods. Impregnation and grafting method were widely applied in the synthetic process of SILCs. However, the surface area of support decreased remarkably after the introduction of ILs, and the leaching of active components was serious during reaction.^39,40 To overcome these limitations, ILs with some functional groups was introduced into the matrix of the supports by the grafting method, which could improve the stability of ILs in the reaction process.^41,42 Unfortunately, the properties and the freedom of ILs may be confined. Hence, the encapsulation of ILs into the architecture of support by one-pot hydrothermal process is of great interest to prepare the SILCs.^33,43 The as-synthesized heterogeneous materials could not only reduce the amount of ILs used and increase the stability of ILs, but also benefit for the separation of catalysts after reaction. Besides, these materials could also possess high surface area by removing the templates in the subsequent treatment. Inspired by these advantages, the SILCs achieved from the encapsulation of ILs have been explored in various reactions and exhibited excellent catalytic performance in comparison to the pure ILs and the heterogeneous catalysts obtained from the impregnation and grafting method.

Herein, polyoxometalate-based ionic liquids ([C_xmim]₃PW₁₂O₄₀, x = 4, 8, and 16) were successfully encapsulated into the ordered mesoporous silica (OMS) matrix by a facile one-pot hydrothermal procedure (Scheme 1). The designed ordered mesoporous silica encapsulated POMs-based IL catalyst [C₄mim]₃PW₁₂O₄₀@OMS exhibited excellent activity, high stability and good recyclability in the removal of DBT with an ultra-low dosage of oxidant, and no organic solvent was added as the extractant. The stability of the as-synthesized hybrid material was investigated by ³¹P NMR and UV-vis analysis. In addition, the possible desulfurization mechanism was evaluated by GC-MS analysis.

Scheme 1 Illustration of the preparation procedure of ordered mesoporous silica encapsulated POMs-based IL.

2. Experimental section

2.1. Materials

Benzothiophene (BT), 3-methylbenzothiphene (3-MBT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiphene (4,6-DMDBT) were purchased from Aladdin Chemical Co., Ltd. Pluronic P123 (EO₂₀PO₇₀EO₂₀, M_avg = 5800) was supplied by Sigma-Aldrich. 1-Butyl-3-methylimidazole chloride ([C₄mim]Cl), 1-octyl-3-methylimidazole chloride ([C₈mim]Cl), 1-cetyl-3-methylimidazole chloride ([C₁₆mim]Cl) were obtained from Shanghai Chenjie Chemical Co., Ltd. Phosphotungstic acid (H₃PW₁₂O₄₀·14H₂O, AR grade), hydrogen peroxide (H₂O₂, 30 wt%, AR grade), acetonitrile (CH₃CN, AR grade), tetraethyl orthosilicate (TEOS) and n-octane (AR grade) were marketed from Sinopharm Chemical Reagents Co., Ltd.

2.2. Sample preparation

A series of POMs-based ionic liquids [C_xmim]₃PW₁₂O₄₀ (C_x-ILs, x = 4, 8, 16) were prepared according to the study reported,⁴⁴ and ordered mesoporous silica encapsulated C_x-ILs were synthesized according to the previous report with modification.⁴⁵ In a typical procedure, 2.668 g of P123 was dissolved in 63 g of HCl solution (1.9 M). After the surfactant was completely dissolved, 4 mL of acetonitrile containing 0.01 mmol of C₄-IL was dropped into the above clear solution. Then, 4 mL of TEOS was gently added to the above solution under vigorous stirring for 24 h. The resultant was transferred to an autoclave and hydrothermally treated at 100 °C for 24 h. Then, the obtained products were separated and washed with deionized water and ethanol for three times and dried at 80 °C overnight. Finally, the mesoporous silica was obtained by removing template using ethanol for 72 h, and named as C₄-IL@OMS. The obtained sample was with a Si/W molar ratio of 15 in the gel. The ordered mesoporous silica encapsulated POMs-based ILs [C₈mim]₃PW₁₂O₄₀ and [C₁₆mim]₃PW₁₂O₄₀ were prepared with the same Si/W molar ratio through a similar method, and named as C₈-IL@OMS and C₁₆-IL@OMS respectively.

2.3. Characterization

Fourier transform infrared (FTIR) spectra of as-synthesis material were recorded with a Nicolet FTIR apparatus (Nexus 470, Thermo Electron Corporation) using KBr pellets. Raman spectroscopy was carried out on a DXR Raman spectrometer using a 532 nm excitation. Wide-angle X-ray diffraction (XRD) of samples range from 10 to 80° was collected on a Bruker D8 X-ray diffractometer equipped with Cu Kα radiation. Diffuse reflectance spectra (DSR) were performed on a UV-vis spectrometer (UV-2450, Shimadzu). Small-angle X-ray scattering (SAXS) patterns were obtained using a Nanostar U small-angle X-ray scattering system (Bruker, Germany) equipped with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was collected on a PHI5300 with Mg Kα source. Nitrogen absorption–desorption isotherms were measured using a TriStar II 3020 surface area and porosity analyzer (USA). Transmission electron microscopy (TEM) measurements of samples were recorded with a JEM 2010 (Japan) operated at 200 kV. ³¹P MAS NMR spectra were collected on a Bruker AVANCE III 400WB with 8 kHz.

2.4. Catalytic test

A desired amount of BT, 3-MBT, DBT, 4-MDBT and 4,6-DMDBT was dissolved in n-octane to yield model oil with sulfur content of 250, 500, 500, 500 and 250 ppm, respectively. Desulfurization performance of catalyst was employed in a two-necked flask coupled with thermostatic water bath. In a typical run, 5 mL of model oil was firstly mixed with the catalyst, and then desired amount of H₂O₂ was injected under stirring. The content of sulfur-containing left in upper oil phase was monitored by the gas chromatography (GC7890A, Agilent). Gas chromatography-mass spectrometry (GC-MS7890/5975C-GC/MSD, Agilent) was used to identify the product of catalytic oxidation reaction.

3. Results and discussion

3.1. Characterization of selected samples

FTIR spectra of the pure POMs-based ILs and the OMS encapsulated POMs-based ILs were shown in Fig. 1. For the neat POMs-based ILs (Fig. 1a–c), the characteristic bands of the Keggin units at 1081, 980, 898 and 810 cm⁻¹ could be obviously observed, which were attributed to the P–O_a (central oxygen), W=O (terminal oxygen), W–O–W (corner-sharing oxygen) and W–O–W (edge-sharing oxygen) respectively.^46,47 The absorption peaks around 1500 cm⁻¹ could be ascribed to the stretching vibration of the imidalium ring. Meanwhile, the peaks in the region of 2900–3100 cm⁻¹ were caused by the vibration of the C–H bond.⁴⁸ FTIR spectra of the OMS encapsulated POMs-based ILs was displayed in Fig. 1d–f. The bands of Keggin units could also be found. However, the bands at 1081 and 810 cm⁻¹ could not be detected due to the effect of the vibration of the Si–O–Si. In addition, the characteristic peaks of ν_as(W=O) and ν_as(W–O_c–W) shifted to 979 and 893 cm⁻¹ respectively, which may arise from the interaction of Keggin units with the host OMS.^49,50

Fig. 1 FTIR spectra of (a) C₄-IL; (b) C₈-IL; (c) C₁₆-IL; (d) C₄-IL@OMS; (e) C₈-IL@OMS; (f) C₁₆-IL@OMS.

Fig. 2 exhibits the Raman spectra for the synthesized materials. For C₄-IL@OMS (Fig. 2a), the characteristic peaks of the Keggin unit at 1008 and 993 cm⁻¹, corresponding to vibration of ν_s(W=O) and ν_as(W=O), respectively.^45,51 As shown in Fig. 2b and c, the vibrations of ν_s(W–O) and ν_as(W=O) locating at 1005, 990 cm⁻¹ and 1007, 992 cm⁻¹ were found for C₈-IL@OMS and C₁₆-IL@OMS respectively. The small difference about vibration bands of Keggin units for various OMS encapsulated ILs may be assigned to the interaction between the POM anion and the imidazole cation.

Fig. 2 Raman spectra of (a) C₄-IL@OMS; (b) C₈-IL@OMS; (c) C₁₆-IL@OMS.

The structural information of C₄-IL@OMS, C₈-IL@OMS and C₁₆-IL@OMS mesoporous catalysts was performed by wide-angle XRD (Fig. 3). For the pure OMS (Fig. 3a), the broad peak around 22° was caused by amorphous silica.⁵² XRD patterns of different ILs were presented in Fig. 3b–d. It was clearly observed that the diffraction peaks of Keggin unit ranged from 15–35°. However, after the encapsulation of ILs into the OMS structure, the diffraction peaks ascribed to the Keggin anion could not be found, suggesting that the ILs were uniformly dispersed into the OMS matrix.

Fig. 3 XRD patterns of (a) OMS; (b) C₄-IL; (c) C₈-IL; (d) C₁₆-IL; (e) C₄-IL@OMS; (f) C₈-IL@OMS; (g) C₁₆-IL@OMS.

Fig. 4 displays the UV-vis spectra of OMS, C₄-IL@OMS, C₈-IL@OMS and C₁₆-IL@OMS. For support OMS (Fig. 4a), no obvious absorption peak could be found ranging from 200 to 800 nm. Compared with the OMS, the as-synthesized materials encapsulated ILs exhibited strong peaks around 266 nm (Fig. 4b and c), which was ascribed to the charge transfer from O²⁻ to W⁶⁺.⁵³ These results demonstrated that the ILs were successfully encapsulated into the ordered mesoporous silica structure and maintained the structural integrity of Keggin unit, agreeing well with the information from wide-angle XRD analysis.

Fig. 4 UV-vis spectra of (a) OMS; (b) C₄-IL@OMS; (c) C₈-IL@OMS; (d) C₁₆-IL@OMS.

To further study the chemical features of the designed catalyst C₄-IL@OMS, XPS spectrum of C₄-IL@OMS and W4f XPS analysis were performed (Fig. 5). As shown in Fig. 5A, all elements C, N, P and W for the hybrid material C₄-IL@OMS could be found. In addition, Fig. 5B presents peaks of W4f emerged in 36.2 and 38.1 eV were assigned to the W^VI of the composite C₄-IL@OMS.^54,55 These results indicated that the C₄-IL remained the structure integrity after encapsulating into the OMS, which was well consistent with the results of UV-vis analysis.

Fig. 5 XPS spectrum of (A) C₄-IL@OMS and (B) W4f spectrum of C₄-IL@OMS.

SEM and TEM images of the OMS and C₄-IL@OMS prepared via hydrothermal procedure were shown in Fig. 6A–E. Fig. 6A reveals that the as-prepared material OMS possessed rod-like morphologies and relatively uniform size. As shown in Fig. 6B, compared with the parent mesoporous OMS, the rod-like morphologies was maintained after the encapsulation of C₄-IL, demonstrating that the encapsulation of the C₄-IL had little influence on the formation of the morphologies of catalysts. The similar phenomenon could be found for C₈-IL@OMS and C₁₆-IL@OMS (Fig. S1†). TEM images of the rod-like OMS viewed as parallel and perpendicular to the ordered channels were presented in Fig. 6C and D, respectively. The results revealed that the silica rods possessed the one-dimensional (1D) mesoporous channels, long-range order and a hexagonal structure.⁵⁶ For C₄-IL@OMS (Fig. 6E), the ordered mesostructure of the resultant material was still retained, and no aggregation of ILs could be found. The observations obtained above could be further conformed by small-angle XRD (Fig. 6F). For the parent OMS (Fig. 6F(a)), three well-resolved reflections ascribed to (100), (110) and (200) planes for space group P6mm could be found, implying that the two-dimensional (2D) hexagonal mesostructured with long range order. Small-angle XRD patterns of various IL@OMS shows similar reflections compared with the parent OMS, indicating that all IL@OMS possessed an ordered mesoporous structure. It was worth noted that the (100) reflection of the as-synthesized materials IL@OMS shifted to the low angle, and the reflections peaks broadened, which was due to a lattice expansion during the encapsulation of ILs.⁵⁷ N₂ adsorption–desorption isotherms of the neat OMS and the IL@OMS encapsulated with different length alkyl chain POM-based ILs all presented typical type IV curves with a H1 hysteresis loop appeared in 0.6 < p/p_o <0.9, which suggested that the various catalysts possessed uniform meso-structure. After the encapsulation of ILs into the OMS, the specific surface area of C₄-IL@OMS was 630 m² g⁻¹ (Table 1), slightly smaller than that of the parent OMS (647 m² g⁻¹), but larger than that of C₈-IL@OMS and C₁₆-IL@OMS (590 m² g⁻¹ and 540 m² g⁻¹, respectively). Notably, it could be observed that the sorption steps of the as-prepared materials IL@OMS shifted to lower relative pressure as the length of alkyl chain in IL increased, which was ascribed to the pore volume packed with ILs.⁵⁸ In addition, the various hybrid materials presented narrow pore size distributions around 7 nm obtained from Barrett–Joyner–Halenda (BJH) (Fig. 6H).

Fig. 6 SEM images of (A) OMS and (B) C₄-IL@OMS; TEM images of (C and D) OMS and (E) C₄-IL@OMS; (F) small-angle XRD; (G) N₂ adsorption–desorption isotherms and (H) pore size distribution of (a) OMS, (b) C₄-IL@OMS, (c) C₈-IL@OMS and (d) C₁₆-IL@OMS.

Table 1 The catalytic activity of different catalyst toward DBT

Entry	Sample	SBET (m^2 g^−1)	Sulfur removal (%)
			Catal.^a	Catal. + H2O2^b
a Reaction conditions: m(catalyst) = 0.01 g, T = 60 °C, t = 60 min.b Reaction conditions: m(catalyst) = 0.01 g, T = 60 °C, t = 60 min, n(H2O2)/n(S) = 3.
1	OMS	647	6.1	12.2
2	C4-IL@OMS	630	4.0	99.5
3	C8-IL@OMS	590	6.3	76.7
4	C16-IL@OMS	540	9.6	30.5

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3.2. Catalytic performance

3.2.1. Desulfurization of different reaction systems. The desulfurization performance of OMS and IL@OMS was summarized in Table 1. The results showed that various composites presented low sulfur removal without H₂O₂ (below 10%). It was worth noted that the sulfur removal of IL@OMS was higher as the length of long alkyl chain in IL increased, which was mainly attributed to the strong interaction between DBT molecule and long alkyl chain.⁵⁹ After the introduction of oxidant H₂O₂, the catalytic performance of various catalysts was obviously improved. The hybrid materials IL@OMS exhibited higher activity than that of the parent OMS, indicating ILs played a virtue role in the reaction. Compared with other IL@OMS, the removal of DBT for the designed material C₄-IL@OMS could reach 99.5% in 60 min, much higher than that of C₈-IL@OMS and C₁₆-IL@OMS (76.7% and 30.5%, respectively). The higher activity of C₄-IL@OMS may be attributed to its larger surface area, which could provide enough the exposure of active site to catalyze the oxidation of DBT. On the other hand, short alkyl chain in IL could decrease interaction resistance between DBT and peroxide species, which was also beneficial for the desulfurization reaction. These results demonstrated that the specific surface area of designed catalyst, and the chemical property in pore channels play a comprehensive role in the catalytic reaction. Besides, the effects of various reaction conditions such as the amount of the catalyst, reaction temperature, O/S molar ratio on the removal of DBT were investigated in detail (Fig. S2–S4†), and the optimal conditions were obtained. Moreover, the catalytic desulfurization performance of POMs-based catalyst reported was summarized in Table 2. Compared to the other POMs-based reaction systems, designed catalyst C₄-IL@OMS presented efficient performance toward the removal of DBT, and the dosage of oxidant was relatively small. Besides, to compare with the activity of C₄-IL@OMS, other oxidative desulfurization systems about C₄-IL were also investigated with the same amount of samples and oxidant. The catalytic activity of C₄-IL was only 12.7% (entry 14, Table 2). After the introduction of acetonitrile (1 mL) as extractant, the sulfur removal increased to 31.6% (entry 15, Table 2). For the hybrid C₄-IL@OMS, the removal could reach 99.5% with the same condition (entry 16, Table 2).

Table 2 Catalytic oxidative desulfurization of DBT about POM-based systems reported

Entry	POM-based catalyst	O/S^a	t (min)	S-removal (%)	Ref.
a O/S molar ratio.b m(catalyst) = 0.01 g.
1	HPMo/HKUST-1	6	120	99	60
2	PTA/MSN/AEM	6	1320	100	10
3	HPW/NH2-MCM-41	8	180	100	61
4	[C16H33(CH3)2NOH]3{PO4[WO(O2)2]4}/silica	4	180	100	62
5	HPW/HMS	8	120	98	63
6	(Bu4N)4H3(PW11O39)/MCM-41	4	60	99	64
7	LaW10/NH3^+/SiO2	5	35	99	65
8	TBA3PW12O40/MIL-101	10	60	100	66
9	HPW/mpg-C3N4	8	150	100	67
10	H3PW12O40/MIL-101-(Cr)–NH2	4	60	100	68
11	PTA/MIL-101	50	180	91	69
12	LaW10/IL-SiO2	5	40	100	11
13	[(C6H13)3PC14H29]2W6O19	4	80	99.3	9
14^b	C4-IL	3	60	12.7	This work
15^b	C4-IL + acetonitrile	3	60	31.6	This work
16^b	C4-IL@OMS	3	60	99.5	This work

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3.2.2. Effect of toluene content in model oil on the sulfur removal. It is necessary to investigate the influence of toluene on the sulfur removal, since aromatic compounds widely exist in real oil. As shown in Fig. 7, the desulfurization performance of C₄-IL@OMS in the presence of toluene was limited compared with the pure model oil, which revealed that the presence of toluene played a vital role in the desulfurization performance. However, as the toluene content increased from 5 to 30 wt%, it is interesting that the removal of DBT increased from 83% to 99%, which could be attributed to synthetic action of the competition of adsorption and elimination of oxidized sulfur products.⁷⁰ The competitive adsorption between toluene and DBT was primary when the toluene content was low in model oil, resulting in negative influence on the sulfur removal. Nevertheless, as the toluene content constantly increased, the catalytic activity was promoted, which may be due to high solubility of oxidized sulfur compounds with increasing aromatic property of model oil, thus leading to more active sites to oxidize the DBT than that of the system with 5 wt% of toluene in model oil.⁷¹

Fig. 7 Influence of the toluene content on the sulfur removal. Reaction conditions: m(C₄-IL@OMS) = 0.01 g, O/S = 3, T = 60 °C, t = 60 min.

3.2.3. Effect of different substrates on the sulfur removal. In order to investigate the desulfurization performance of catalyst C₄-IL@OMS toward different organosulfur compounds in model oil, the catalytic test of five sulfur-containing compounds was carried out under the same conditions. As shown in Fig. 8, the removal efficiency of various organic sulfides decreased in the order of DBT > 3-MBT > 4-MDBT > BT > 4,6-DMDBT. The removal of DBT could reach 99.5% in 60 min. However, the removal of BT and 4,6-DMDBT were 76% and 70%, respectively. The electron density of S atom of BT (5.739) was lower than that of DBT (5.758), resulting in negative influence on the desulfurization.⁷² 4,6-DMDBT exhibited the lowest activity compared with other sulfides, which was mainly attributed to the steric hinder effect caused by methyl groups. These results suggested that the electron density around the S atoms and the steric hinder effect played the synthetic role in the sulfur removal.

Fig. 8 Catalytic performance of different sulfur-containing substrates. Reaction conditions: m(C₄-IL@OMS) = 0.01 g, T = 60 °C, O/S = 3.

3.2.4. Recycling ability and stability of hybrid material C₄-IL@OMS. The regeneration and recycling ability of used catalyst is of significant importance for its industrial applications. In this desulfurization system, the heterogeneous catalyst C₄-IL@OMS is miscible with the model oil, which is favorable to the separation of the catalyst after reaction. The recycling process of as-synthesized material was performed as follows: the upper oil phase was decanted directly after catalytic reaction, and the wet catalyst was dried in an oven at 50 °C overnight. And then, the next catalytic evaluation was carried out after adding the fresh model oil and oxidant. As shown in Fig. 9, the removal of DBT could still reach 93% after recycling for 7 times, demonstrating that the hybrid catalyst was robust and stable in the desulfurization process.

Fig. 9 Recycling tests of C₄-IL@OMS for the catalytic oxidation of DBT. Reaction conditions: m(C₄-IL@OMS) = 0.01 g, O/S = 3, T = 60 °C, t = 60 min.

The stability of as-synthesized material C₄-IL@OMS in the desulfurization was also performed by ³¹P NMR and UV-vis spectra. In Fig. 10, a main peak around −15.6 ppm could be observed for the parent C₄-IL dissolved in DMSO (Fig. 10a), which was attributed to ³¹P chemical shift of Keggin structure in the fresh C₄-IL.^51,73 However, no obvious ³¹P signal could be observed in the model phase after reaction (Fig. 10b), demonstrating that no leaching of C₄-IL into the reaction solution. On the other hand, UV-vis spectra of fresh catalyst and used catalyst were shown in Fig. 11. Besides the absorbance peak concerning the charge transfer from O²⁻ to W⁶⁺ for the IL in hybrid material (Fig. 11a), the emerging peaks responsible for the oxidant products of DBT were observed, which also indicated the good stability of the hybrid materials.

Fig. 10 ³¹P NMR spectra of the (a) fresh C₄-IL and (b) the model oil phase after reaction.

Fig. 11 UV-vis spectra of (a) fresh C₄-IL@OMS and (b) used C₄-IL@OMS.

3.2.5. The possible mechanism of the conversion of DBT. In this desulfurization system, adsorption and oxidation desulfurization procedure was proposed (Scheme 2). Firstly, DBT and H₂O₂ molecules were adsorbed into the channels of the hybrid material C₄-IL@OMS. Then, DBT molecules were oxidized to its corresponding sulfones by the peroxo species formed by the reaction of C₄-IL with H₂O₂.⁷⁴ In order to further understand the desulfurization process, GC-MS analysis of model oil and the used catalyst after reaction was performed (Fig. 12). Typically, the upper model oil phase was separated by decantation, and the used catalyst was extracted by tetrachloromethane solution. The resultant solution above was analyzed by GC-MS. In Fig. 12A, the peak at 5.4 min could be found, which was assigned to the residual DBT (m/z = 184.1). However, the peaks for both DBT and DBTO₂ (m/z = 216.1) could be detected in Fig. 12B. According to the results above, it could be proposed that the desulfurization of DBT coupled the adsorption with oxidation process.

Scheme 2 The possible reaction mechanism of desulfurization system.

Fig. 12 GC-MS analysis of (A) the upper oil phase after reaction and (B) the used catalyst extracted by tetrachloromethane.

4. Conclusions

In summary, a novel catalyst C₄-IL@OMS with high specific surface area and ordered mesoporous channels was successfully fabricated by one-pot hydrothermal method and employed in the desulfurization of organosulfurs. The as-synthesized material characterized indicated that the POM-based IL was successfully encapsulated with a highly uniform dispersion in the silica architecture. Additionally, the hybrid material presented excellent activity toward various sulfur-containing compounds, which could be assigned to the hexagonal mesoporous silica with large surface area, enough exposed active species to interact with sulfides, and the chemical property in pore channels. Moreover, the hybrid catalyst still shows high activity after recycling for seven times. ³¹P NMR and UV-vis spectra after reaction demonstrated that the as-prepared material C₄-IL@OMS was highly stable during the reaction. Considering of the advantages for the novel catalyst, it would be further applied in other reactions.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. 21406092, 21376111, 21376109), The Natural Science Foundation of Jiangsu Province (No. BK20131207), Advanced Talents of Jiangsu University (No. 13JDG080), Postdoctoral Foundation of China and Jiangsu Province (No. 2014M551516, 1301001A), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13787j

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