Fabrication of dual-mesoporous silica by triblock copolymers and metal-based ionic liquid: efficient and durable catalyst for oxidative desulfurization in fuel

Abstract

Dual-mesoporous silica was synthesized using the ionic liquid [C₁₆mim]₃PMo₁₂O₄₀ (1-hexadecyl-3-methylimidazolium phosphomolybdate) and the triblock copolymer P123 as co-templates. In this strategy, the ionic liquid [C₁₆mim]₃PMo₁₂O₄₀ was not only employed as the template, but also provided the metal source. The obtained catalysts were characterized by FTIR, Raman, XRD, XPS, N₂ adsorption–desorption and utilized as catalysts to remove organic sulfur compounds from the model oil. The results demonstrated that compared with single channel silica, the catalysts with two uniform sets of mesopores showed higher catalytic performance for oxidative desulfurization. Dibenzothiophene (DBT) could be completely removed at 60 °C in 50 min. With regard to the refractory sulfur compound 4,6-dimethyldibenzothiophene (4,6-DMDBT), deep desulfurization could also be achieved. Moreover, the dual-mesoporous catalyst exhibited outstanding stability. It could be recycled for at least seven times without any activity decrease.

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1. Introduction

One of the main challenges for present petroleum is to remove organic sulfur compounds in the fuel.¹ Sulfur-containing compounds in fuel can be converted to SO_x during combustion in car engines, which not only leads to acid rain, but also causes poisoning of the catalysts in automobile exhausts.^2,3 Therefore, countries have implemented stringent standards for reducing the sulfur content in fuels.⁴ Hydrodesulfurization (HDS) is the main process for desulfurization in conventional petroleum refineries. But it still requires high temperature and high hydrogen pressure, and is less effective for removing aromatic organosulfur compounds including dibenzothiophene (DBT), benzothiophene (BT) and their alkyl derivatives.⁵ Hence, developing alternative and cost-effective technologies for desulfurization is highly desired.

Currently, many other non-hydrodesulfurization processes, such as oxidative desulfurization (ODS),^6–12 photocatalytic desulfurization,^13–15 extractive desulfurization,^16–19 and adsorptive desulfurization,^3,20–23 have already emerged. Among these alternative desulfurization processes, ODS is a promising desulfurization technology because of its mild operating conditions (low temperature, atmospheric pressure), no hydrogen requirement, and high efficiency to remove aromatic organic sulfur compounds.^24–26

Compared with homogeneous catalytic oxidative desulfurization systems, such as organic acids and a metallic catalyst,^27,28 great interest has arisen in heterogeneous catalysis, due to outstanding properties in catalyst separation, selectivity, recovery and reuse. In previous work, many attempts have been made to immobilize catalytically active species on various supports. Mesoporous materials, owing to their high surface area, large pore volume, and easily modifiable surface, have attracted considerable interest and broad application in adsorption, catalysis, and carriers.^29,30 Up to now, various kinds of mesoporous materials have been reported as supports for oxidative desulfurization catalysts, including MCM-41,^6,31,32 SBA-15,^1,33–36 mesoporous silica pillared clay,^37,38 mesoporous silica,^39–41 well-ordered hexagonal mesoporous silicate (HMS),⁴² mesoporous titania–silica,⁴³ and mesoporous silica nanowires.⁴⁴ The active species were usually supported on these mesoporous materials by surface functionalization, impregnation, and encapsulation of active moieties in the channels of the mesoporous materials. However, these methods have some drawbacks. Random distribution of active moieties may block the pores, an extra step is necessary to prepare the supported catalysts, and the single porous structure can result in poor mass transfer.

Dual-mesoporous materials with highly interconnected pore channels showed an enhanced accessibility and improved catalytic activity in reactions suffering from steric and/or diffusional limitations.⁴⁵ Up to now, several different strategies have been developed for the synthesis of dual-mesoporous materials, such as the removal of framework atoms, dual templating with surfactants, and hard-templating methods.^46–48 In this study, the triblock copolymer P123 and ionic liquid [C₁₆mim]₃PMo₁₂O₄₀ were used as co-templates for the fabrication of dual-mesoporous materials. The strategy is to build a dual-mesoporous material and use an in situ method to introduce the active species. As we know, ionic liquids (ILs) have been widely used for various fields. However, they have rarely been used as a co-template. In this procedure, the ionic liquid [C₁₆mim]₃PMo₁₂O₄₀ not only acted as a cationic surfactant, but also provided the metal source.

The motivation and aim of this work was the preparation of a dual-mesoporous material containing metal species with the IL [C₁₆mim]₃PMo₁₂O₄₀ as co-template and metal source. The as-prepared Mo containing dual-mesoporous material was applied as a catalyst for the removal of sulfur compounds in model oil. Compared with the single-mesoporous material, the dual-mesoporous material exhibited excellent catalytic activity. This synthesis procedure can be extended to a general strategy for synthesis of dual-mesoporous materials containing active centers and make heterogeneous catalysts with high activity.

2. Experimental

2.1. Materials

Dibenzothiophene (DBT), benzothiophene (BT), 4,6-dimethyldibenzothiophene (4,6-DMDBT) and the triblock copolymer P123 were purchased from Sigma-Aldrich. Commercially available tetraethyl orthosilicate (TEOS), 30 wt% H₂O₂, n-octane, para-xylene, cyclohexene, acetonitrile, tetradecane and H₃PMo₁₂O₄₀·26H₂O were bought from Sinopharm Chemical Reagent Co., Ltd. [C₁₆mim]Cl was purchased from Shanghai Chengjie Chemical Co., Ltd.

2.2. Catalyst preparation

2.2.1. Synthesis of [C₁₆mim]₃PMo₁₂O₄₀. [C₁₆mim]₃PMo₁₂O₄₀ was prepared by the reported literature method.⁴⁹ [C₁₆mim]Cl and H₃PMo₁₂O₄₀·26H₂O were mixed in ultrapure water with a molar ratio of 3 : 1 and stirred at 40 °C for 12 h. The sediment was washed by purified water till the filtrate was neutral and dried. Then, the production [C₁₆mim]₃PMo₁₂O₄₀ was achieved and characterized by the ¹³C solid state NMR (Fig. S1†).

2.2.2. Synthesis of dual-mesoporous MoO₃–SiO₂. 0.23 g of the pre-prepared [C₁₆mim]₃PMo₁₂O₄₀, 5 mL of acetonitrile, 0.23 g of P123 and 37.5 g of 1.6 M HCl were added to a 100 mL round bottom flask. The mixture was stirred at 40 °C for 4 h and then 2.27 mL of TEOS was added and reacted for 24 h. The resulting solution was transferred to a Teflon-lined autoclave and heated at 100 °C for 24 h. After washing with deionized water, drying at 100 °C for 1 h and calcining at 550 °C for 6 h, the target product dual-mesoporous MoO₃–SiO₂ was obtained and labeled as D-MoO₃–SiO₂.

For comparison, single-mesoporous MoO₃–SiO₂ was prepared under similar conditions as above with the amount of P123 as 2 g and the final product was denoted as S-MoO₃–SiO₂.

2.3. Characterization

Fourier transfer infrared spectra (FTIR) were recorded with a Nicolet Model Nexus 470 spectrometer using KBr pellets. Powder X-ray diffraction (XRD) was used for analyzing the crystalline phases of the catalysts using a Bruker D8 diffractometer with Cu Kα (λ = 1.54 Å) radiation. Raman spectra were obtained using a DXR Raman microscope with a 532 nm laser source. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5600 multi-technique system with a monochromatic Al K R X-ray source (Physical Electronics). The surface area, pore volume and pore diameter of the samples were measured using a Brunauer–Emmett–Teller (BET) multipoint method and the adsorption branch of the isotherm. NMR spectra were obtained on a Bruker Advance 400 spectrometer. The morphology and structure of the samples were analyzed by field-emission scanning electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM, JEOL-JEM-2010) running at 200 kV.

2.4. Oxidative desulfurization of model oil

Three types of model oil were prepared by dissolving DBT, BT and 4,6-DMDBT in n-octane with S-contents of 500, 500 and 216 ppm, respectively. The reaction of catalytic oxidative desulfurization was performed in a 20 mL two-necked flask with the experimental conditions of 0.01 g of catalyst, 0.234 mmol of H₂O₂ and 5 mL of the model oil at 60 °C. After the reaction, the remaining sulfur-containing compounds in the model oil were detected by a gas chromatography-flame ionization detector (GC-FID) with tetradecane as the internal standard (Agilent 7890A; HP-5, 30 m × 0.32 mm i.d. × 0.25 μm; FID: Agilent). The sulfur removal from the model oil was calculated by the following formula:
where C_o and C_t (mg L⁻¹) were the initial sulfur concentration and the sulfur concentration at time t, respectively.

3. Results and discussion

3.1. Catalyst characterization

The FTIR spectra of the as-prepared samples are shown in Fig. 1. For D-MoO₃–SiO₂ and S-MoO₃–SiO₂, the absorption peaks at about 460, 805, 1086 cm⁻¹ are attributed to the bending vibration, symmetric and asymmetric stretching vibration of Si–O–Si, respectively.³³ The bands at 1632 cm⁻¹ are ascribed to the bending of the hydroxyl stretch and physisorbed water.⁵⁰ The stretching vibrations of Mo=O and Mo–O–Mo at 962, 882 cm⁻¹ can also be seen,⁵¹ indicating that the Mo species were successfully introduced the mesoporous silica.

Fig. 1 FTIR spectra of D-MoO₃–SiO₂ (a) and S-MoO₃–SiO₂ (b).

The low-angle XRD patterns of D-MoO₃–SiO₂ and S-MoO₃–SiO₂ are presented in Fig. 2. Both D-MoO₃–SiO₂ and S-MoO₃–SiO₂ showed diffraction peaks in the range of 2θ = 1–3°, which suggests that the as-prepared materials were mesostructural. Fig. S2† shows the wide-angle XRD patterns of D-MoO₃–SiO₂ and S-MoO₃–SiO₂. As can be seen from the figure, except the characteristic amorphous SiO₂ peak (wider diffraction peak at 25°), no remarkable other peaks were found on the wide-angle XRD patterns of the as-prepared materials. The results indicate that the Mo species was highly dispersed on the SiO₂ matrix.

Fig. 2 Low-angle XRD patterns of D-MoO₃–SiO₂ (a) and S-MoO₃–SiO₂ (b).

The chemical state of Mo was confirmed by Raman analysis. From the Raman spectra (Fig. 3), an intensive peak at 991 cm⁻¹ corresponding to the Mo=O bond and a weak peak at approximately 824 cm⁻¹ assigned to the O–Mo–O bond can be observed.⁵² Within the range of 200–400 cm⁻¹, some characteristic peaks at 239, 338, and 374 cm⁻¹ are visible and can be attributed to the different bending vibrational modes of MoO₃.⁵³ The results imply that the final condition of Mo was MoO₃.

Fig. 3 Raman spectra of D-MoO₃–SiO₂ (a), and S-MoO₃–SiO₂ (b).

The surface composition and chemical states of the as-prepared D-MoO₃–SiO₂ were investigated by XPS analysis. Fig. 4a shows the survey spectrum of the material. The presence of elemental Mo, O, Si can be confirmed from the peaks at binding energies corresponding to Mo 3d, O 1s, Si 2p. The peak of C 1s was from carbon contamination. Fig. 4b presents the high resolution Mo 3d XPS spectrum of the sample. The two classic peaks of Mo 3d_5/2 at 232.7 eV and Mo 3d_3/2 at 235.7 eV ascribed to Mo⁶⁺ were observed.⁵⁴ The Mo 3d and O 1s peaks indicated the existence of MoO₃ in the sample. The UV-vis diffuse reflectance analysis is also provided to further determine the composition (Fig. 5). The samples showed two absorption bands at 233 and 314 nm, which is in agreement with the reported data on MoO₃.⁵⁵ The results of XPS and UV-vis diffuse reflectance analysis were consistent with the Raman characterization.

Fig. 4 X-ray photoelectron spectroscopy of D-MoO₃–SiO₂: (a) survey spectrum, (b) Mo 3d spectrum.

Fig. 5 UV-vis diffuse reflectance spectra of D-MoO₃–SiO₂ (a), and S-MoO₃–SiO₂ (b).

N₂ adsorption–desorption isotherms were used to identify the structure of the catalysts (Fig. 6). The two as-prepared materials exhibited type-IV curves, indicating characteristic mesopores. From Fig. 6a, it can be observed that S-MoO₃–SiO₂ showed a distinct H2 type hysteresis loop, suggesting a uniform mesopore size distribution. While the desorption curve of D-MoO₃–SiO₂ showed two rapid drop at P/P₀ = 0.4–0.6, indicating a bimodal-mesopore structure.⁵⁶ The pore size distribution (PSD) was calculated using the Barrett–Joyner–Halenda model. S-MoO₃–SiO₂ displayed a very sharp PSD centered at 3.8 nm (Fig. 6b). The PSD curve of D-MoO₃–SiO₂ reflected sharp bimodal-mesopores with uniform sizes of 3.9 and 4.9 nm. The specific BET surface area and total pore volume of S-MoO₃–SiO₂ were 584 m² g⁻¹ and 0.64 cm³ g⁻¹, respectively. D-MoO₃–SiO₂ had a bimodal-mesopore structure but with a relatively small specific BET surface area (478 m² g⁻¹) and total pore volume (0.56 cm³ g⁻¹). It has been widely recognized that catalysts with larger specific surface areas can provide more active centers for contacting with reactants, which can lead to higher catalytic activity. In this system, the specific BET surface area of D-MoO₃–SiO₂ was lower than S-MoO₃–SiO₂, suggesting that the specific BET surface area was not the main factor influencing the catalytic performance of the catalysts.

Fig. 6 N₂ adsorption–desorption isotherms (a) and pore size distribution curves (b) of the samples.

SEM and TEM analysis were further performed to confirm the morphology and structure of the two materials. From the SEM images (Fig. S3†), it could be seen that the two materials all showed bulk texture without obvious difference. The TEM images are shown in Fig. 7. It can be seen that the two samples have mesoporous structures. However, D-MoO₃–SiO₂ showed two kinds of mesopores. One was ordered, the other was wormhole mesoporous. The result was consistent with the N₂ adsorption–desorption isotherms analysis. As for S-MoO₃–SiO₂, it only showed the wormhole mesopores. In addition, it was noteworthy that the MoO₃ particles could not be found in the TEM images, which proved the high dispersion of the active species. The results accorded well with the wide-angle XRD analysis.

Fig. 7 TEM images of D-MoO₃–SiO₂ (a and b) and S-MoO₃–SiO₂ (c and d).

3.2. Catalysts for oxidative desulfurization

To explore their applicability, the as-prepared materials were used as catalysts for oxidative desulfurization. As shown in Fig. 8, D-MoO₃–SiO₂ exhibited excellent catalytic activity. The desulfurization efficiency could reach 100% in 50 min. S-MoO₃–SiO₂ was less active with 86.9% desulfurization efficiency within the same reaction period. When the time was prolonged to 90 min, the DBT was still not completely removed. Apparently, though both catalysts possessed the same chemical composition, D-MoO₃–SiO₂ revealed better catalytic activity than S-MoO₃–SiO₂. It was understood that a larger catalyst surface area could enhance the catalytic performance. S-MoO₃–SiO₂ had the larger surface area, while the poorer catalytic activity was observed. The result implied that a dual-mesoporous structure made a significant contribution to the catalytic oxidative desulfurization. Except that at 3.9 nm, D-MoO₃–SiO₂ possessed a larger pore size at 4.9 nm, which was beneficial to mass transfer for the larger molecules, like DBT. Therefore, the excellent pore accessibility of D-MoO₃–SiO₂ in the catalytic oxidative desulfurization system was proven. Moreover, a comparison of the most recently reported materials and D-MoO₃–SiO₂ used in oxidative desulfurization are listed in Table S1.† It was found that all of the catalysts show high catalytic performance for oxidative desulfurization in certain conditions. However, D-MoO₃–SiO₂ has advantages in the consumption of H₂O₂ and reaction time. Thus, D-MoO₃–SiO₂ should have potential in the field of oxidative desulfurization.

Fig. 8 Oxidative removal of DBT on the different catalysts D-MoO₃–SiO₂ and S-MoO₃–SiO₂. Experimental conditions: m (catalyst) = 0.01 g, n (H₂O₂) = 0.234 mmol, V (oil) = 5 mL, T = 60 °C.

Besides DBT, there are also many different sulfur-containing compounds in real fuel, such as BT and 4,6-DMDBT. Thus, the catalytic performance of D-MoO₃–SiO₂ for BT and 4,6-DMDBT was also studied as shown in Fig. 9. It can be seen that the sulfur removal of the three sulfur-containing compounds decreased in the order of DBT > 4,6-DMDBT > BT under the same conditions. It has been reported that desulfurization performance decreased with the reduction of electron density on sulfur atoms.⁵⁷ Among BT, DBT and 4,6-DMDBT, the electron density on the sulfur atom is 5.739, 5.758 and 5.760, respectively.⁶ The lowest electron density on the sulfur atom of BT resulted in the worst desulfurization efficiency. For DBT and 4,6-DMDBT, the electron density on the sulfur atom of 4,6-DMDBT (5.760) is slightly higher than that of DBT (5.758), but the sulfur removal of 4,6-DMDBT is lower than that of DBT owing to the effect of the steric hindrance of two methyl. Due to the steric hindrance of two methyl, 4,6-DMDBT is well recognized as a refractory sulfur compound and is difficult to remove efficiently in most reported literature.^34,58 It was noted that 4,6-DMDBT could be removed completely in 70 min in this system. The enhanced catalytic oxidation activity for the removal of 4,6-DMDBT can be ascribed to the dual-mesoporous structure of the catalyst with interconnected pore channels, which can improve catalytic activity in reactions suffering from steric and/or diffusional limitations.

Fig. 9 Effect of sulfur-containing compounds on sulfur removal. Experimental conditions: m (D-MoO₃–SiO₂) = 0.01 g, n (H₂O₂) = 0.234 mmol, V (oil) = 5 mL, T = 60 °C.

It is worth considering that the components in real fuel oil are complicated, such as aromatics, alkenes, etc. To investigate the effect of aromatics and alkenes on sulfur removal, 10 wt% para-xylene and 10 wt% cyclohexene as representative compounds were added to the model oil, respectively (Fig. 10). Compared with the model oil containing none, the desulfurization efficiency decreased slightly in the same conditions as the para-xylene was added, while a drastic reduction happened with the addition of cyclohexene. After 50 min of reaction, the final sulfur removal of DBT was 100%, 99.3% and 76.3% corresponding to the model oil containing none, 10 wt% para-xylene and 10 wt% cyclohexene, respectively. For cyclohexene, the exposed pi electrons are more sensitive to being attacked by H₂O₂ to form cycloalkane, and thus the cyclohexene played a strong negative effect on the removal of DBT. On the other hand, though the pi electrons of para-xylene exhibited relative stability, the α-hydrogen on the side-chain methyl groups is sensitive to being oxidized.⁵ Hence, the addition of a certain amount of para-xylene had a slightly negative impact on the removal of DBT. The results suggested that aromatics and alkenes in fuel oil have an inhibiting effect on ODS, owing to the competitive oxidation of these compounds and DBT on the D-MoO₃–SiO₂ catalyst with H₂O₂. At the same time, it was also indicated that alkene compounds may compete with DBT more intensively for catalytic oxidation sites on the D-MoO₃–SiO₂ catalyst than aromatic compounds.

Fig. 10 Effects of fuel composition on sulfur removal. Experimental conditions: m (D-MoO₃–SiO₂) = 0.01 g, n (H₂O₂) = 0.234 mmol, V (oil) = 5 mL, T = 60 °C.

In order to investigate the stability and recyclability of D-MoO₃–SiO₂, the reusability of the catalyst was tested and the results are shown in Fig. 11. After each reaction, the catalyst was separated easily by decanting the model oil from a two-neck flask, dried in a vacuum oven at 80 °C overnight, and then used for a subsequent reaction by adding fresh H₂O₂ and model oil. As shown in Fig. 11, the catalyst could be reused seven times without any obvious reduction of catalytic activity. In addition, N₂ adsorption–desorption analysis was used again to check the presence of the dual pores after sulfur removal. It could be found the bimodal-mesopore structure of D-MoO₃–SiO₂ still remained from the PSD curves (Fig. S4†). The results suggested that D-MoO₃–SiO₂ exhibited excellent stability and remarkable recyclability.

Fig. 11 Recycle times on removal of DBT with D-MoO₃–SiO₂ as catalyst. Experimental conditions: m (catalyst) = 0.01 g, n (H₂O₂) = 0.234 mmol, V (oil) = 5 mL, T = 60 °C, t = 60 min.

4. Conclusions

In summary, a novel dual-mesoporous material D-MoO₃–SiO₂ was synthesized successfully using the ionic liquid [C₁₆mim]₃PMo₁₂O₄₀ and triblock copolymer P123 as co-templates. In this route, [C₁₆mim]₃PMo₁₂O₄₀ worked as the soft template and active component source at the same time which was superior to the impregnation method. The synthesized dual-mesoporous material was first applied to oxidative desulfurization exhibiting excellent catalytic performance due to its two uniform sets of mesopores facilitating mass transfer. The catalyst possessed favorable stability and could be recycled and reused at least seven times with barely any decrease in performance because of the unique dual-mesoporous structure. In addition, it is expected that many more dual-mesoporous materials like WO_x/SiO₂, MoO₃/TiO₂ and WO_x/TiO₂ can be explored for the application of oxidative desulfurization by the current strategy.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. 21276117, 21376111, 21406092), the State Key Laboratory of Heavy Oil Processing (SKLOP201402004) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

References

1. K. S. Cho and Y. K. Lee, Appl. Catal., B, 2014, 147, 35.
2. P. S. Kulkarni and C. A. M. Afonso, Green Chem., 2010, 12, 1139.
3. J. Xiong, W. S. Zhu, H. P. Li, W. J. Ding, Y. H. Chao, P. W. Wu, S. H. Xun, M. Zhang and H. M. Li, Green Chem., 2015, 17, 1647.
4. F. S. Mjalli, O. U. Ahmed, T. Al-Wahaibi, Y. Al-Wahaibi and I. M. AlNashef, Rev. Chem. Eng., 2014, 30, 337.
5. J. Xiao, L. M. Wu, Y. Wu, B. Liu, L. Dai, Z. Li, Q. B. Xia and H. X. Xi, Appl. Energy, 2014, 113, 78.
6. J. Xiong, W. S. Zhu, H. M. Li, Y. H. Xu, W. Jiang, S. H. Xun, H. Liu and Z. Zhao, AIChE J., 2013, 59, 4696.
7. B. Y. Zhang, Z. X. Jiang, J. Li, Y. N. Zhang, F. Lin, Y. Liu and C. Li, J. Catal., 2012, 287, 5.
8. W. H. Wang, G. Li, W. G. Li and L. P. Liu, Chem. Commun., 2011, 47, 3529.
9. E. Torres-García, A. Galano and G. Rodriguez-Gattorno, J. Catal., 2011, 282, 201.
10. W. H. Lo, H. Y. Yang and G. T. Wei, Green Chem., 2003, 5, 639.
11. H. Y. Lü, P. C. Li, C. L. Deng, W. Z. Ren, S. N. Wang, P. Liu and H. Zhang, Chem. Commun., 2015, 51, 10703.
12. W. S. Zhu, P. W. Wu, L. Yang, Y. H. Chang, Y. H. Chao, H. M. Li, Y. Q. Jiang, W. Jiang and S. H. Xun, Chem. Eng. J., 2013, 229, 250.
13. W. S. Zhu, C. Wang, H. P. Li, P. W. Wu, S. H. Xun, W. Jiang, Z. G. Chen, Z. Zhao and H. M. Li, Green Chem., 2015, 17, 2464.
14. F. Lin, D. E. Wang, Z. X. Jiang, Y. Ma, J. Li, R. G. Li and C. Li, Energy Environ. Sci., 2012, 5, 6400.
15. C. Wang, W. S. Zhu, Y. H. Xu, H. Xu, M. Zhang, Y. H. Chao, S. Yin, H. M. Li and J. G. Wang, Ceram. Int., 2014, 40, 11627.
16. J. J. Gao, H. Meng, Y. Z. Lu, H. X. Zhang and C. X. Li, AIChE J., 2013, 59, 948.
17. A. Bösmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz and P. Wasserscheid, Chem. Commun., 2001, 2494.
18. C. P. Li, D. Li, S. S. Zou, Z. Li, J. M. Yin, A. L. Wang, Y. N. Cui, Z. L. Yao and Q. Zhao, Green Chem., 2013, 15, 2793.
19. W. Jiang, W. S. Zhu, H. P. Li, X. Wang, S. Yin, Y. H. Chang and H. M. Li, Fuel, 2015, 140, 590.
20. J. Xiong, W. S. Zhu, H. P. Li, L. Yang, Y. H. Chao, P. W. Wu, S. H. Xun, W. Jiang, M. Zhang and H. M. Li, J. Mater. Chem. A, 2015, 3, 12738.
21. J. Xiong, W. S. Zhu, W. J. Ding, P. Wang, Y. H. Chao, M. Zhang, F. X. Zhu and H. M. Li, RSC Adv., 2014, 4, 40588.
22. J. Xiao, X. X. Wang, M. Fujii, Q. J. Yang and C. S. Song, AIChE J., 2013, 59, 1441.
23. P. Tan, J. X. Qin, X. Q. Liu, X. Q. Yin and L. B. Sun, J. Mater. Chem. A, 2014, 2, 4698.
24. W. S. Zhu, B. L. Dai, P. W. Wu, Y. H. Chao, J. Xiong, S. H. Xun, H. P. Li and H. M. Li, ACS Sustainable Chem. Eng., 2015, 3, 186.
25. P. W. Wu, W. S. Zhu, A. M. Wei, B. L. Dai, Y. H. Chao, C. F. Li, H. M. Li and S. Dai, Chem.–Eur. J., 2015, 21, 15421.
26. P. W. Wu, W. S. Zhu, Y. H. Chao, J. S. Zhang, P. F. Zhang, H. Y. Zhu, C. F. Li, Z. G. Chen, H. M. Li and S. Dai, Chem. Commun., 2015 10.1039/c5cc07830j.
27. W. H. Lo, H. Y. Yang and G. T. Wei, Green Chem., 2003, 5, 639.
28. W. Trakarnpruk and K. Rujiraworawut, Fuel Process. Technol., 2009, 90, 411.
29. Y. H. Deng, D. W. Qi, C. H. Deng, X. M. Zhang and D. Y. Zhao, J. Am. Chem. Soc., 2008, 130, 28.
30. J. Wei, Q. Yue, Z. K. Sun, Y. H. Deng and D. Y. Zhao, Angew. Chem., Int. Ed., 2012, 51, 6149.
31. R. Wang, Y. H. Li, B. S. Guo and H. W. Sun, Energy Fuels, 2011, 25, 3940.
32. Z. E. A. Abdalla and B. S. Li, Chem. Eng. J., 2012, 200–202, 113.
33. W. J. Ding, W. S. Zhu, J. Xiong, L. Yang, A. M. Wei, M. Zhang and H. M. Li, Chem. Eng. J., 2015, 266, 213.
34. J. Xiong, W. S. Zhu, W. J. Ding, L. Yang, M. Zhang, W. Jiang, Z. Zhao and H. M. Li, RSC Adv., 2015, 5, 16847.
35. J. Xiong, W. S. Zhu, W. J. Ding, L. Yang, Y. H. Chao, H. P. Li, F. X. Zhu and H. M. Li, Ind. Eng. Chem. Res., 2014, 53, 19895.
36. L. N. Yang, J. Li, X. D. Yuan, J. Shen and Y. T. Qi, J. Mol. Catal. A: Chem., 2007, 262, 114.
37. B. S. Li, Z. X. Liu, C. Y. Han, J. J. Liu, S. L. Zuo, Z. Y. Zhou and X. M. Pang, J. Mol. Catal. A: Chem., 2011, 348, 106.
38. B. S. Li, Z. X. Liu, J. J. Liu, Z. Y. Zhou, X. H. Gao, X. M. Pang and H. T. Sheng, J. Colloid Interface Sci., 2011, 362, 450.
39. Y. Chen and Y. F. Song, ChemPlusChem, 2014, 79, 304.
40. Y. Chen, S. Zhao and Y. F. Song, Appl. Catal., A, 2013, 466, 307.
41. M. Chamack, A. R. Mahjoub and H. Aghayan, Chem. Eng. J., 2014, 255, 686.
42. B. S. Li, W. Ma, J. J. Liu, C. Y. Han, S. L. Zuo and X. F. Li, Catal. Commun., 2011, 13, 101.
43. X. M. Yan, P. Mei, L. Xiong, L. Gao, Q. F. Yang and L. J. Gong, Catal. Sci. Technol., 2013, 3, 1985.
44. J. Dou and H. C. Zeng, ACS Catal., 2014, 4, 566.
45. D. P. Serrano, J. M. Escola and P. Pizarro, Chem. Soc. Rev., 2013, 42, 4004.
46. P. Kortunov, S. Vasenkov, J. Karger, R. Valiullin, P. Gottschalk, M. Fé Elía, M. Perez, M. Stöcker, B. Drescher, G. McElhiney, C. Berger, R. Gläser and J. Weitkamp, J. Am. Chem. Soc., 2005, 127, 13055.
47. K. Möller, B. Yilmaz, U. Müeller and T. Bein, Chem. Mater., 2011, 23, 4301.
48. H. Y. Chen, J. Wydra, X. Y. Zhang, P.-S. Lee, Z. P. Wang, W. Fan and M. Tsapatsis, J. Am. Chem. Soc., 2011, 133, 12390.
49. M. Zhang, W. S. Zhu, H. M. Li, S. H. Xun, W. J. Ding, J. J. Liu, Z. Zhao and Q. Wang, Chem. Eng. J., 2014, 243, 386.
50. X. C. Shao, X. T. Zhang, W. G. Yu, Y. Y. Wu, Y. C. Qin, Z. L. Sun and L. J. Song, Appl. Surf. Sci., 2012, 263, 1.
51. S. L. Wang, Y. Z. Zhang, T. Chen and G. Y. Wang, J. Mol. Catal. A: Chem., 2015, 398, 248.
52. R. Nadimicherla, W. Chen and X. Guo, Mater. Res. Bull., 2015, 66, 140.
53. J. Adamiak, W. Tomaszewski and W. Skupiński, Catal. Commun., 2012, 29, 92.
54. L. L. Sui, Y. M. Xu, X. F. Zhang, X. L. Cheng, S. Gao, H. Zhao, Z. Cai and L. H. Huo, Sens. Actuators, B, 2015, 208, 406.
55. C. Lin, K. Tao, H. B. Yu, D. Y. Hua and S. H. Zhou, Catal. Sci. Technol., 2014, 4, 4010.
56. X. F. Qian, J. M. Du, B. Li, M. Si, Y. S. Yang, Y. Y. Hu, G. X. Niu, Y. H. Zhang, H. L. Xu, B. Tu, Y. Tang and D. Y. Zhao, Chem. Sci., 2011, 2, 2006.
57. W. S. Zhu, G. P. Zhu, H. M. Li, Y. H. Chao, Y. H. Chang, G. Y. Chen and C. R. Han, J. Mol. Catal. A: Chem., 2011, 347, 8.
58. W. S. Zhu, H. M. Li, X. Jiang, Y. S. Yan, J. D. Lu, L. N. He and J. X. Xia, Green Chem., 2008, 10, 641.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17509g

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