Fabrication of oxygen-defective tungsten oxide nanorods for deep oxidative desulfurization of fuel
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Owing to the increasingly serious environmental issues caused by the sulfur burnt in fuel, desulfurization has become an important topic. In this work, an amphiphilic oxygen-defective tungsten oxide was synthesized by a colloidal chemistry method. The amphiphilic property and oxygen defects were well characterized, and the structure of the oxygen-defective tungsten oxide catalyst was investigated. In addition, the catalyst was employed in oxidative desulfurization system of fuel, and deep desulfurization was achieved. It was found that the very high oxidative desulfurization performance of oxygen-defective tungsten oxide catalyst resulted from both the amphiphilic property and oxygen defects. This work can provide a strategy for preparation of highly active metal oxide catalysts with oxygen defects in oxidative desulfurization reaction of fuel.
KeywordsTungsten oxide Oxygen vacancies Oxidative desulfurization Amphiphilic catalyst
Emission of SOx, originating from the combustion of sulfur compounds in fuel, has become an increasingly serious issue (Wu et al. 2016b; Zhao et al. 2017; Xiao et al. 2016; Lu et al. 2017; Gu et al. 2017; He et al. 2017). Thus, increasingly stringent regulations have been proposed (Xiao et al. 2014; Jiang et al. 2016; Zhu et al. 2015; Zhang et al. 2014a; Xu et al. 2012; Zhu et al. 2013c; Wu et al. 2016a). Generally, the sulfur contents in fuel are limited to <10 ppm in many countries. To realize such a goal, higher requirements are needed in industrial hydrodesulfurization (HDS), such as higher operating temperature and reaction pressure. However, the conventional HDS process is less efficient for aromatic sulfur compounds, making it a choke point for production of ultra-low sulfur fuel oil. Therefore, numerous substitute technologies have been developed, including extractive desulfurization (EDS) (Gao et al. 2013; Li et al. 2012; Chen et al. 2012; Zhao et al. 2016), adsorptive desulfurization (ADS) (Xiao et al. 2015; Khan et al. 2014; Xiong et al. 2016; Xiao et al. 2013), biodesulfurization (BDS) (Ferreira et al. 2017; Aksoy et al. 2014; Zhang et al. 2017a), oxidative desulfurization (ODS) (Wu et al. 2016a, 2017a, b, 2018; Jiang et al. 2017; Xun et al. 2016), etc. Among all the developed substitutes, ODS is a potential one because of its mild reaction conditions and high activity to aromatic sulfur compounds. The above advantages make ODS a promising complementary method to the HDS process (McNamara et al. 2013; Jiang et al. 2016, 2017; Li et al. 2016; Miao et al. 2016).
To achieve a high ODS efficiency, an important issue is finding a highly active catalyst. Currently, task-specific ionic liquids (TSILs) (Zhu et al. 2013d; Kulkarni and Afonso 2010; Zhang et al. 2013; Wishart 2009; Zhu et al. 2013c; Jiang et al. 2015b), metal oxides (Xiao et al. 2016; He et al. 2015; Gonzalez et al. 2017; Rodriguez-Gattorno et al. 2009), polyoxometalates (Zhang et al. 2017b; Yang et al. 2017), titanium silicalites (Kong et al. 2006; Feng et al. 2017; Du et al. 2017; Shen et al. 2015), and so on have been widely employed in ODS. Among all the reported catalysts, a metal oxide is an eye-catching one, which is especially true when it comes to tungsten oxide. However, based on a previous report, stoichiometric tungsten oxide rarely shows excellent catalytic performance (He et al. 2017). For non-stoichiometric tungsten oxide, because of the variable valences of W, the tungsten oxide can be readily reacted with an oxidant to form a reactive intermediate for the further oxidation process. Nevertheless, for tungsten oxide catalysts, bulk tungsten oxide rarely has satisfying catalytic performance because of the poor exposure of catalytically active sites. Thus, numerous strategies have been adopted to improve this situation. For example, our group employed graphene-like hexagonal boron nitride as a support for dispersion of tungsten oxide to prepare the WOx in nanoparticle form (Wu et al. 2015b). However, such strategy is less universally applicable and involves high-temperature treatment. On the other hand, the poor catalytic activity of the bulk tungsten oxide in ODS has mainly originated from the strong hydrophilic property of tungsten oxide, making the catalyst difficult to contact with the fuel oil.
Recently, preparation of metal/metal oxides by colloidal preparation methods has attracted increasing attention because of the successful synthesis of nano-sized materials and a controllable process (Guo et al. 2013; Jiang et al. 2015a; Wu et al. 2015a; Zhang et al. 2014b; Zhu et al. 2013a, b). Moreover, such a process is usually realized in an oil phase, making organic ligands abundant on the surfaces of metal/metal oxide. This property allows the catalyst to be easily dispersed in fuel oil. Thus, when a colloidal chemistry method in an oil phase is employed for the preparation of tungsten oxide materials, there will be not only a catalyst with nanosize, but also it will drive the catalyst to be easily dispersed in fuel oil, which gives rise to an excellent desulfurization performance.
In this work, a method for colloidal preparation of non-stoichiometric tungsten oxide (W18O49) nanorods was employed. The obtained catalyst was characterized as having abundant organic ligands on its surface. Because of the existence of organic ligands on the surface, the obtained W18O49 nanorods show excellent lipophilicity, making the catalyst easy to contact with fuel oil and lead to a high desulfurization performance. Additionally, effects of oxidant amount and reaction temperature were investigated in detail. The current work may provide a strategy for preparation of highly active and amphipathic catalyst for deep oxidative desulfurization.
Oleylamine (OAM, 90%), oleic acid (OAC, AR), tungsten hexachloride (WCl6, AR), dibenzothiophene (DBT, 99%), and tetradecane (AR) were purchased from Sigma-Aldrich and used without further purification. Ethanol (C2H5OH, AR), n-hexane (AR), and hydrogen peroxide (H2O2, 30 wt%) were obtained from Sinopharm Chemical Reagent Co., Ltd. China.
2.2 Preparation of catalyst
2.2.1 Preparation of W18O49-MC
OAM- and OAC-modified W18O49 was prepared as follows: typically, 0.2 g of WCl6 was dissolved in 10 mL of ethanol and stirred for 10 min under N2. Afterward, a mixture of 0.5 mL of OAM and 0.5 mL of OAC was added to the solution drop by drop and stirred for another 10 min. Then, the mixed solution was transferred to a 20-mL autoclave. Subsequently, the autoclave was placed in an oven at 180 °C for 24 h followed by natural cooling to room temperature. The obtained product was separated by centrifugation at a speed of 10,000 rpm for 10 min. Then, the product was washed with ethanol and re-dispersed in cyclohexane for three times. Finally, the product was placed in a vacuum oven at 50 °C for 10 h to obtain the product. The obtained black-blue catalyst was denoted as W18O49-MC.
2.2.2 Preparation of W18O49-E
The preparation process for W18O49-E is similar to that of W18O49-MC, except without adding OAM and OAC during the preparation process.
A Nicolet Nexus 470 Fourier transform infrared spectrometer was used to collect the Fourier transform infrared spectra (FT-IR). They were recorded at room temperature with KBr pellets. X-ray diffraction (XRD) patterns were from a D8 Advance with Cu Kα radiation and ranged from 10° to 80° with a scanning rate of 7°/min. A Hitachi H-700 was used for transmission electron microscopy (TEM). Scanning electron microscopy (SEM) images were recorded with a JSM-7001F thermal field emission scanning electron microscope at 2–15 keV accelerating voltage. Ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) were recorded by a Shimadzu UV-2450 spectrophotometer with a spherical diffuse reflectance accessory. The scanning range was 200–800 nm and BaSO4 was employed as background. The XPS results were collected on a PHI 3056 spectrometer with an Al anode source.
2.4 Oxidative desulfurization experiments
2.4.1 Preparation of model oil
The DBT-containing model oil was prepared by the following process: DBT was dissolved in n-octane to make the sulfur concentration of 500 ppm. Afterward, additional tetradecane was added to a concentration of 4000 ppm as the internal standard.
2.4.2 Catalysis experiments
The catalytic oxidative desulfurization was carried out as follows: firstly, a certain amount of catalyst was added to a 40-mL homemade two-neck flask. Afterward, 5 mL of DBT-containing model oil was added, followed by a certain amount of H2O2. Then, the flask was placed in an oil bath at the required reaction temperature and magnetically stirred. After required reaction times, the oil was allowed to stand for 1 min to allow the separation of the oil and catalyst phases, and 1 µL of the upper oil phase was taken for further analysis.
2.4.3 Analysis of sulfur content
3 Results and discussion
3.1 Raman characterization
3.2 N2 adsorption–desorption isotherm analysis and SEM
3.3 XRD characterization of W18O49-MC catalyst
3.4 UV–vis DRS characterization of W18O49-MC catalyst
3.5 FT-IR characterization of the catalyst
3.6 TEM image of the W18O49-MC catalyst
3.7 Catalytic oxidative desulfurization performance
3.8 Effect of n(O)/n(S) on sulfur removal
3.9 Effect of reaction temperature on sulfur removal
3.10 Recycling performance of the catalyst
A ligand-covered tungsten oxide with oxygen defects was successfully obtained by a colloidal chemistry method. The amphipathic property and oxygen defects were well characterized, and the structure of the catalyst was determined by a series of characterization methods. It was found that with the presence of oleylamine and oleic acid, the obtained tungsten oxide was in the form of nanorods, which is beneficial to the contact between catalyst and substrates. Furthermore, the obtained catalyst was employed as an efficient catalyst in oxidative desulfurization. With optimized reaction conditions, a deep desulfurization performance was obtained, making the S content lower than 10 ppm. The very excellent oxidative desulfurization performance mainly originates from the amphipathic property and oxygen defects of the catalyst. This work may provide a strategy for the preparation of highly active oxidative desulfurization metal oxide catalysts.
The authors are grateful for financial support by Students' Platform for innovation and entrepreneurship training of Jiangsu Province (201810299008z), the National Nature Science Foundation of China (Nos. 21576122, 21722604, 21766007), Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Hainan Normal University (20150376), and the Natural Science Foundation of Jiangsu Provincial Department of Education (17KJA150002).
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