Polyoxometalate-based silica-supported ionic liquids for heterogeneous oxidative desulfurization in fuels
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With the aim of deep desulfurization, silica-supported polyoxometalate-based ionic liquids were successfully prepared by a one-pot hydrothermal process and employed in heterogeneous oxidative desulfurization of various sulfur compounds. The compositions and structures of the hybrid samples were characterized by various methods such as FT-IR, XPS, Raman, UV–Vis, wide-angle XRD and N2 adsorption–desorption. The experimental results indicated that the hybrid materials presented a high dispersion of tungsten species and excellent catalytic activity for the removal of 4,6-dimethyldibenzothiophene without any organic solvent as extractant, and the sulfur removal could reach 100.0% under mild conditions. The catalytic performance on various substrates was also investigated in detail. After cycling seven cycles, the sulfur removal of the heterogeneous system still reached 93.0%. The GC-MS analysis results demonstrated that the sulfur compound was first adsorbed by the catalyst and subsequently oxidized to its corresponding sulfone.
KeywordsPolyoxometalate Silica-supported ionic liquid Heterogeneous oxidative desulfurization
In the past years, new energy sources such as solar power and hydrogen energy in the auto industry have been springing up. However, fuel oil still occupies a dominant position. Sulfur compounds in fuel oil generate sulfur oxides (SOx) after combustion, and this is a main source of acid rain, haze and other environmental issues (Zhang et al. 2017a; Ibrahim et al. 2017). Thus, most countries have implemented strict standards to limit the sulfur content in fuel oil to below 10 ppm. Up to now, conventional hydrodesulfurization (HDS) is efficient in removing thiols, sulfides and disulfides, but not efficient for the removal of heterocyclic sulfur compounds such as dibenzothiophene and its derivatives, especially 4,6-dimethyldibenzothiophene (Zhang et al. 2017b; Li et al. 2016). Accordingly, the development of alternative desulfurization progress is in the ascendant, including biodesulfurization (Soleimani et al. 2007), adsorption desulfurization (Ren et al. 2018; Yang et al. 2018), extraction desulfurization (Rafiee et al. 2016) and oxidative desulfurization (Akopyan et al. 2015; Zhang et al. 2014). Among these, oxidative desulfurization is considered to be the most promising technology due to its mild operating conditions and high removal efficiency for the heterocyclic sulfur compounds mentioned above (Zhang et al. 2013).
Ionic liquids (ILs) with unique physicochemical properties have been widely employed as solvents, extractants, templates and precursors (Zhou and Qu 2017; Dai et al. 2017). In the desulfurization process, ionic liquids are often used as extractants (Paduszyński et al. 2017; Moghadam et al. 2017), but the sulfur removal is not so satisfactory. Thus, polyoxometalates (POMs), a family of transition metal–oxide clusters with specific physical properties and controllable redox and properties, are often introduced as anions to the family of specific ILs (Zhao et al. 2012; Zhu et al. 2013). In the oxidative desulfurization, POM-based ILs could not only capture the sulfur compounds from oil but also activate the oxidant (e.g., hydrogen peroxide) to oxidize the sulfur compounds to sulfones. The desulfurization performance of POM-based ILs was desirable, but the catalytic systems often suffered from some problems such as high dosage and low specific area of ILs, difficulty in the separation of catalyst (Xun et al. 2016; Li et al. 2015). Thus, immobilization of POM-based ILs on a suitable carrier (e.g., silica) is a good method to solve the above problems.
Hence, in this work, a series of molybdenum-containing silica-supported ionic liquids [C4mim]3PMo12O40/SiO2 were designed to achieve the advantages of both the POM materials and heterogeneous catalysis. The hybrid materials [C4mim]3PMo12O40/SiO2 from a one-pot hydrothermal method were highly efficient in the oxidative desulfurization of 4,6-dimethyldibenzothiophene and other sulfur compounds, where no organic solvents were added as extractants. Moreover, the samples were also systematically characterized by XRD, XPS, FT-Raman, FT-IR, UV–Vis and N2 adsorption–desorption analysis.
2 Experimental section
2.1 Chemicals and materials
n-octane (CP grade), H3PMo12O40·26H2O (AR grade), acetonitrile (CH3CN, AR grade), tetraethylorthosilicate (TEOS, AR grade), hydrogen peroxide (H2O2, 30 wt%) and ammonia (NH3·H2O, 25 wt%) were acquired from Sinopharm Chemical Reagent Co. Ltd., China. Benzothiophene (BT, 99%), dibenzothiophene (DBT, 98%), 4-methyldibenzothiophene (4-MDBT, 98%) and 4,6-dimethyldibenzothiophene(4,6-DMDBT, 99%) were purchased from Sigma-Aldrich. [C4mim]Cl (99%) was obtained from Shanghai Chenjie Chemical Co. Ltd., China. All the reagents were used directly as received.
2.2 Synthesis of the catalysts
The ionic liquid [C4mim]3PMo12O40 was synthesized according to a previous study (Rajkumar and Rao 2008). Then, silica-supported ionic liquid was synthesized by a one-pot hydrothermal method as follows: Firstly, 0.17 g of [C4mim]3PMo12O40 was dissolved in 4 mL of acetonitrile at 50 °C under continuous stirring. After that, the solution above was added dropwise to 26 mL of deionized water under continuous stirring for 10 min and followed by addition of 2 mL of TEOS. Afterward, 0.5 mL of 25% aqueous ammonia was added to the mixture. After stirring for 3 h at room temperature, the mixture was transferred into the reactor and kept at 120 °C for 24 h. Then, the resultant was filtered and washed with deionized water three times and treated at 150 °C for 3 h. For comparison, other materials with different Mo:Si molar ratios and different treating temperatures were prepared by a similar method and denoted as x-[C4mim]3PMo/SiO2-y (x = 0.05, 0.1 and 0.2, and y = 150, 200 and 250 °C).
FT-IR spectra of the catalysts were recorded on a Nicolet Nexus 470 spectrometer using KBr pellets (Thermo Electron Corporation, USA). SEM imaging was performed on a JEOL JEM-7001F field-emission microscope (JEOL Corporation, Japan). XPS was collected on a PHI 530 with a monochromatic Mg Kα source (Ulvac-Phi Corporation, Japan). Raman spectroscopy was studied on a DXR Raman microscope (Thermo Fisher Scientific, USA) with a 532-nm excitation laser power. UV–visible spectra were recorded on a UV–Vis spectrometer (UV-2450, Shimadzu, Japan). X-ray diffraction patterns were recorded with a Bruker D8 diffractometer using Cu Kα radiation (\(\lambda\) = 1.5418 Å). The N2 absorption–desorption isotherms were collected on a TriStar II 3020 surface area and porosity analyzer (Micromeritics Corporation, USA). The oxidation products of 4,6-DMDBT were studied by GC–MS on an Agilent 7890A (Agilent, USA).
2.4 Catalytic activity test
3 Results and discussion
3.1 Characterization of samples
Structural properties of various samples
SBET, m2 g−1
Pore volume, cm3 g−1
3.2 Catalytic performance
3.3 Analysis of oxidation product
3.4 Reusability of the catalyst
Silica-supported ionic liquid was successfully prepared by a one-pot hydrothermal method and employed in the heterogeneous oxidative desulfurization of various sulfur compounds. The characterization results indicated that the POM-based IL was uniformly dispersed on the silica matrix and maintained its structural integrity. The sample 0.1[C4mim]3PMo/SiO2-250 had excellent catalytic activity for the removal of various sulfur compounds without any solvents and could still reach sulfur removal of 93% after seven cycles. The oxidative efficiency of different substrates decreased in the order of 4,6-DMDBT > 4-MDBT > DBT > BT.
This work was financially supported by the National Nature Science Foundation of China (Nos. 21776116, 21576122, 21722604), Postdoctoral Foundation of China (No. 2017M621646), Postdoctoral Foundation of Jiangsu Province (No. 2018K083C) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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