Catalytic Oxidative Desulfurization of Dibenzothiophene Compounds Over Tungsten Oxide Catalysts Supported on Spherical Mesoporous TiO 2

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1 Copyright 2017 by American Scientific Publishers All rights reserved. Printed in the United States of America Science of Advanced Materials Vol. 9, pp , Catalytic Oxidative Desulfurization of Dibenzothiophene Compounds Over Tungsten Oxide Catalysts Supported on Spherical Mesoporous TiO 2 Zhenghua Li 1, Gyoung Hee Hong 1,JinSeaPark 1, Kumarsrinivasan Sivaranjani 1, Chengbin Li 1, Su Bin Park 1, Byung Jin Song 1, 2, Jong-Sun Yoon 1, Chul Wee Lee 2, and Ji Man Kim 1, 1 Department of Chemistry, Sungkyunkwan University, Suwon , South Korea 2 Division of Green Chemistry and Engineering Research, Korea Research Institute of Chemical Technology (KRICT), Daejeon , South Korea ABSTRACT AseriesofWO x /TiO 2 catalysts with different WO x loadings (5 20 wt%) were prepared by incipient wetness impregnation method. The synthesized samples were characterized using various physicochemical characterization techniques, such as X-ray diffraction, nitrogen adsorption desorption, scanning electron microscopy, and Raman spectroscopy. Catalytic performance of all the catalysts was evaluated in oxidative desulfurization of model oil under very mild conditions of atmospheric pressure and 50 C in a biphasic system using hydrogen peroxide as an oxidant and acetonitrile as an extraction solvent. Excellent catalytic activity for the removal of the sulfur-containing compounds from the model oil was observed with 20 wt% WO x /TiO 2 catalyst, mainly due to the porosity associated with the TiO 2 support, fine dispersed active tungsten oxide species on the mesoporous TiO 2 support, and strong interaction between the WO x and the TiO 2 surface. Moreover, without any regeneration process, the catalytic activity of the catalyst was maintained after several recycle tests. KEYWORDS: Mesoporous TiO 2,WO x /TiO 2, Oxidative Desulfurization, Dibenzothiophene. 1. INTRODUCTION Environmental problems associated with harmful pollutants have received much attention in past decades. Sulphur is a natural component in crude oil that ends up in gasoline and diesel unless removed. The combustion of sulfur contained in fuels such as gasoline and diesel produces sulfur dioxide and sulfuric acid, which is a major environmental pollutant. 1 2 This acid rain has been responsible for destroying forests, lakes, streams and also causes health problems. Hence, the removal of sulfur from motor fuels has become an important research area. 3 Hydrodesulfurization (HDS) is highly efficient for the removal of thiols, sulfides, and disulfides. However, it is difficult to reduce refractory sulfur-containing compounds such as dibenzothiophene (DBT) and its derivatives especially 4,6-dimethyldibenzothiophene (4,6-DMDBT) to an ultra-low level using only conventional HDS. Hence, it is necessary to develop alternative ultra-deep desulfurization Author to whom correspondence should be addressed. jimankim@skku.edu Received: 21 August 2015 Accepted: 18 March 2016 processes such as adsorption, 4 extraction, 5 oxidation 6 and bioprocess. 7 Among these, oxidative desulfurization (ODS) combined with extraction or adsorption is considered to be one of the most promising processes. In the ODS process, the sulfur compounds present in diesel are oxidized to the corresponding sulfones or sulfoxides, then removed from the fuel oils by solvent extraction. As catalysts used in the ODS reaction, there are many WO 3 and MoO 3 based catalysts have been investigated, such as WO 3 -SBA-15, 8 WO x /-ZrO 2, 9 and WO 3 /MoO 3 /Al 2 O In this work, mesoporous, high surface area WO x supported TiO 2 catalyst was used for the removal of sulfurcontaining compounds from the model oil. The oxidative desulfurization reaction was carried out with H 2 O 2 as an oxidant, acetonitrile as an extraction solvent. The WO x / TiO 2 catalyst showed excellent catalytic activity for the removal of sulfur compounds, such as dibenzothiophene (DBT), benzothiophene (BT) and 4,6- dimethyldibenzothiophene (4,6-DMDBT), due to the highly dispersed WO x on the surface of mesoporous TiO 2 and the strong interaction between tungsten species and TiO Sci. Adv. Mater. 2017, Vol. 9, No /2017/9/1236/005 doi: /sam

2 Li et al. Catalytic Oxidative Desulfurization Over WO x Catalysts Supported on Spherical Mesoporous TiO 2 2. EXPERIMENTAL DETAILS 2.1. Synthesis of Mesoporous Spherical TiO 2 Mesoporous spherical TiO 2 was synthesized by reflux of titanium glycolate precursor in deionized water. Detailed synthesis of titanium glycolate precursor was given in our previous paper. 11 In order to prepare mesoporous TiO 2 support, 1.5 g of the titanium glycolate precursor was mixed with deionized water, and the mixture was heated under reflux conditions for 1 h with vigorous stirring. The white precipitate was collected by centrifugation, washed with deionized water three times, and finally dried at 80 C for 24 h, producing a spherical mesoporous TiO 2 support Synthesis of WO x /TiO 2 Catalysts WO x /TiO 2 catalysts (5 20 wt% WO x /TiO 2 were synthesized by incipient wetness impregnation method using (NH 4 6 H 2 W 12 O 40 xh 2 O (Sigma-Aldrich) as the W precursor. After impregnation, the catalysts were dried at 80 C overnight, and then calcined at 400 C. The pure TiO 2 support also calcined at 400 C, and namely TiO 2 T Characterization X-ray diffraction (XRD) patterns were obtained from a Rigaku D/MAX-2200 diffractometer employing Cu K radiation at 30 kv and 40 ma. Transmission electron microscopy (TEM) images were obtained using a G2 FE- TEM at an accelerating voltage of 200 kv. N 2 adsorption desorption isotherms were collected on a Micromeritics Tristar system at liquid N 2 temperature. Raman spectra were recorded under ambient conditions at room temperature using WITEC Alpha Catalytic Performance Sulfur-containing compound (DBT, BT, or 4,6-DMDBT) was dissolved in n-heptane with a corresponding S-content of 2000 ppm as model oil. A typical oxidative desulfurization of model oil was carried out in a 100 ml two-necked flask. 0.1 g catalyst, 0.27 ml of 34.5% H 2 O 2 aqueous solution, 14 ml of acetonitrile, and 14 ml of the model oil were added successively into the flask and stirred vigorously with a magnetic stirrer at 50 C for 2 h under atmospheric pressure. Reaction products (upper oil layer) were analyzed by a gas chromatography-flame ionization detector (GC-FID) equipped with a HP-5 capillary column (30 m 0.32 mm inner diameter 1.0 m film thickness). Fig. 1. Wide-angle X-ray diffraction patterns: (a) mesoporous TiO 2 support, (b) TiO 2 T400, (c) 5 wt% WO x /TiO 2, (d) 10 wt% WO x /TiO 2, (e) 15 wt% WO x /TiO 2, (f) 20 wt% WO x /TiO 2. phase of WO 3.GrainsizeofWO x /TiO 2 catalysts decreased with the increase of the WO x loading, which was calculated by the Debye Scherrer equation and shown in Table I. It indicated that the WO x on the surface of TiO 2 can suppress the grain growth of TiO 2 particles. Figure 2 (left) shows N 2 adsorption desorption results and the corresponding BJH pore size distribution curves (inset) of the WO x /TiO 2 catalysts. Type-IV isotherms with H 2 hysteresis loops are observed with all the catalysts, which are typical characteristics of mesoporous materials. 11 Textural parameters of all the catalysts are summarized in Table I. Interestingly, WO x loaded catalysts show higher surface areas than the pure TiO 2 T400. Moreover, surface area is increased with increasing the amounts of WO x loading. This observation is attributed to the nanopropping effect. 12 Figure 2 (right) shows the Raman spectra of TiO 2 T400 and all the WO x /TiO 2 catalysts. The bands of all the catalysts at around 148 cm 1 (E g ), 407 cm 1 (B 1g ), 520 cm 1 (A 1g and 644 cm 1 (E g ) could be assigned to the anatase TiO After impregnated WO x,anewband Table I. Physical properties of the materials. a S BET b V tot c D p Grain size d Catalyst (m 2 /g) (cm 3 /g) (nm) (nm) 3. RESULTS AND DISCUSSION Figure 1 shows wide angle X-ray diffraction (XRD) patterns of WO x /TiO 2 catalysts. The diffraction peaks of all the materials in the XRD patterns could be indexed to the anatase phase of TiO 2 (JCPDS File: ). Absence of the crystalline WO 3 peaks in all the WO x /TiO 2 catalysts, suggesting that the highly dispersion of the active site (WO x ) on the mesoporous TiO 2 support or amorphous TiO 2 support TiO 2 T wt%WO x /TiO wt% WO x /TiO wt% WO x /TiO wt% WO x /TiO Notes: a BET surface area calculated from the N 2 adsorption. b Total pore volumes measured at p/p 0 = c Calculated BJH method from N 2 adsorption branches. d Calculated by Debye-Scherrer equation from XRD peaks. Sci. Adv. Mater., 9, ,

3 Catalytic Oxidative Desulfurization Over WO x Catalysts Supported on Spherical Mesoporous TiO 2 Li et al. Fig. 2. (Left) N 2 -isotherms and pore size distributions (inset) and (Right) Raman spectra of (a) mesoporous TiO 2 support, (b) TiO 2 T400, (c) 5 wt% WO x /TiO 2, (d) 10 wt% WO x /TiO 2,(e)15wt%WO x /TiO 2, (f) 20 wt% WO x /TiO 2. at around 965 cm 1 appeared in higher W content samples (10 wt% 20 wt%), which is attributed to the W O stretching frequencies of the tetrahedral coordinated tungsten oxide. 14 None of the Raman features of crystalline WO 3 (271, 325, 712 and 813 cm 1 was observed in the Raman spectra, indicated that the strong electronic interaction between the highly dispersed WO x species and the TiO 2 support. Fig. 3. TEM images: (a) TiO 2 T400; (b) 20 wt% WO x /TiO 2 ; (c) HRTEM image of TiO 2 T400; (d), (e) and (f) EDX mapping images of 20 wt% WO x /TiO Sci. Adv. Mater., 9, , 2017

4 Li et al. Catalytic Oxidative Desulfurization Over WO x Catalysts Supported on Spherical Mesoporous TiO 2 Fig. 4. (a) Effect of different catalysts on the performance of DBT removal. (b) Influence of different model sulfur compounds over 20 wt% WO x /TiO 2 catalyst. The TEM images of TiO 2 T400 and 20 wt% WO x /TiO 2 catalysts shows the presence of disordered mesoporosity which arises mainly due to intergrowth of fundamental particles and the same leads to aggregates with significant extra framework void space, as shown in Figures 3(a b). Figure 3(c) shows the high-resolution image of TiO 2 T400 catalyst. The lattice fringe spacing of 0.35 nm and 0.24 nm can be indexed to (101) and (004) planes of anatase TiO 2, respectively. Energy-dispersive X-ray spectroscopy (EDX) mapping of 20 wt% WO x /TiO 2 has been carried out to measure the catalyst composition as well as to find out the extent of homogeneity of the material. It is noteworthy that the tungsten atoms were finely distributed throughout the TiO 2 support, as shown in Figures 3(d f). In order to analyse the catalytic activity of the prepared WO x /TiO 2 catalysts, oxidative desulfurization of DBT with H 2 O 2 was carried out at 50 C under atmospheric pressure as shown in Figure 4(a). Among all the catalysts, 20 wt% WO x /TiO 2 catalyst showed the best catalytic activity, 100% of DBT was removed after 2 h. Higher catalytic activity of 20 wt% WO x /TiO 2 is mainly attributed to the high surface area and the fine dispersion of active WO x species on mesoporous TiO 2, where the active WO x species can be transformed into the corresponding peroxo species by reacting with H 2 O 2, then the DBT was oxidized to its corresponding sulfone through oxidation process. 15 Figure 4(b) shows the ODS performances of the 20 wt% WO x /TiO 2 catalyst for the removal of different sulfur-containing organic substrates such as DBT, BT and 4,6-DMDBT. The ODS conversions of various sulfur compounds follow the order: DBT > BT > 4,6-DMDBT. Recyclability of the catalyst is mandatory requirement in many industrial reactions. Interestingly our catalyst can be recycled more than 5 times without loss of its initial efficiency as shown in Figure 5(a). More importantly washing or calcinations are not required to reuse the catalyst which Fig. 5. Recycle tests of 20 wt% WO x /TiO 2 catalyst. (b) Raman spectra of the fresh 20 wt% WO x /TiO 2 and the used 20 wt% WO x /TiO 2 catalysts. Sci. Adv. Mater., 9, ,

5 Catalytic Oxidative Desulfurization Over WO x Catalysts Supported on Spherical Mesoporous TiO 2 Li et al. greatly reduces the production cost. Raman spectrum of used catalyst (Fig. 5(b)) is similar to fresh catalyst, indicating the structural stability of our catalyst during ODS reaction. 4. CONCLUSION The WO x /TiO 2 catalysts were prepared by an impregnation method, using mesoporous TiO 2 as the support and (NH 4 6 H 2 W 12 O 40 xh 2 O as the tungsten precursor. The catalytic performance on oxidative desulfurization of DBT was increased by increasing the amount of tungsten oxide species, and 20 wt% WO x /TiO 2 catalyst showings the best catalytic activity, due to the porosity, strong interaction of active WO x species with support material, and fine dispersion of the active sites. There was no decrease in catalytic activity even after 5 recycletest repetitions, which indicates the reusability of the catalyst. Acknowledgments: This work was supported by the cooperative R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea (Grant B ). References and Notes 1. T. W. Kim, M. J. Kim, F. Kleitz, M. M. Nair, R. Guillet-Nicolas, K. E. Jeong, H. J. Chae, C. U. Kim, and S. Y. Jeong, ChemCatChem. 4, 687 (2012). 2. K. E. Jeong, C. S. Cho, H. J. Chae, C. U. Kim, and S. Y. Jeong, J. Nanosci. Nanotechol. 10, 3547 (2010). 3. N. Y. Chan, T. Y. Lin, and T. F. Yen, Energy Fuels 22, 3326 (2008). 4. G. S. He, L. B. Sun, X. L. Song, X. Q. Liu, Y. Yin, and Y. C. Wang, Energy Fuels 25, 3506 (2011). 5. D. Xu, W. Zhu, H. Li, J. Zhang, F. Zou, H. Shi, and Y. Yan, Energy Fuels 23, 5929 (2009). 6. W. Zhang, H. Zhang, J. Xiao, Z. Zhao, M. Yu, and Z. Li, Green Chem. 16, 211 (2014). 7. L. He, H. Li, W. Zhu, J. Guo, X. Jiang, J. Lu, and Y. Yan, Ind. Eng. Chem.Res. 47, 6890 (2008). 8. X. Li, S. Huang, Q. Xu, and Y. Yang, Transition Met. Chem. 34, 943 (2009). 9. Z. Hasan, J. Jeon, and S. H. Jhung, J. Hazard. Mater. 205, 216 (2012). 10. W. A. W. A. Bakar, R. Ali, A. A. A. Kadir, and W. N. A. W. Mokhtar, Fuel Process. Technol. 101, 78 (2012). 11. S.B.Park,K.Sivaranjani,H.J.Na,Z.H.Li,Y.J.Choi,Y.J.Yang, and J. M. Kim, Chem. Lett. 44, 61 (2015). 12. J. K. Shon, H. Kim, S. S. Kong, S. H. Hwang, T. H. Han, J. M. Kim, C. Pak, S. Doo, and H. Chang, J. Mater. Chem. 19, 6727 (2009). 13. J. K.Yang, X. T. Zhang, H. Liu, C. H. Wang, S. P. Liu, and P. P. Sun, Catal. Today 201, 195 (2013). 14. G. Lu, X. Li, Z. Qu, Q. Zhao, H. Li, Y. Shen, and G. Chen, Chem. Eng. J. 159, 242 (2010). 15. Z. H. Li, H. J. Jeong, K. Sivaranjani, B. J. Song, S. B. Park, D. H. Li,C.W.Lee,M.S.Jin,andJ.M.Kim,Nano 10, 5 (2015) Sci. Adv. Mater., 9, , 2017