Zhao Rongxiang; Li Xiuping; Su Jianxun; Shi Weiwei; Gao Xiaohan

(College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001)

Preparation of WO3/C Composite and Its Application in Oxidative Desulfurization of Fuel

Zhao Rongxiang; Li Xiuping; Su Jianxun; Shi Weiwei; Gao Xiaohan

(College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001)

The WO3/C composite was successfully prepared by calcination of a mixture of WO3and g-C3N4at 520oC. The as-synthesized samples were analyzed by X-ray diffraction (XRD), electronic differential system (EDS), scanning electron microscopy (SEM), infrared spectrometry (IR) and the Brunner?Emmet?Teller (BET) techniques. The WO3/C composite, in comparison with the WO3and C3N4, features smaller particle size, bigger surface area and higher desulphurization performance. The infuence of the reaction temperature, the catalyst dosage, the reaction time, the oxidant dosage, the sulfde type and the extractant dose on desulfurization reaction was studied. The results showed that the WO3/C composite revealed a higher desulfurization activity than the WO3. The desulfurization rate could reach up to 95.8% under optimal conditions covering a catalyst dosage of 0.02 g, a H2O2amount of 0.2 mL, a 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) amount of 1.0 mL, a reaction temperature of 70oC and a reaction time of 180 min. After fve recycles, the desulfurization activity of catalyst did not signifcantly decline.

WO3/C; ionic liquid; 1-ethyl-3-methylimidazolium ethyl sulfate; oxidative desulfurization

1 Introduction

In recent years, environmental pollution has been arousing considerable public concerns. Ultra-deep desulfurization of fuel has been an important research subject across the world because of environmental awareness and legal requirements[1-2]. To date, the conventional hydrodesulfurization process is an effective method for removing the organ-sulfur compounds in petroleum products, and the removal of aliphatic and acyclic sulfurcontaining compounds can be easily achieved during the hydrogenation process. But, this process aiming at removal of thiophenic sulfdes, such as dibenzothiophene (DBT) and its derivatives, is less effective due to the steric hindrance effect of these molecules. In addition, the operating conditions of hydrogenation process are more severe (high temperature and high pressure). In this case, extractive desulfurization[3-4], adsorptive desulfurization[5-6], biological desulfurization[7-8], oxidative desulfurization[9-10], and other non-hydrogenation desulfurization processes have gradually been developed. The oxidative desulfurization owing to its advantages of small investment, mild reaction conditions, higher removal rate of thiophene compounds, is considered to be favorable to the development of deep desulfurization technology[11]. WO3can be used as the catalyst because of its thermal stability and good crystalline structure[12]. But WO3has small surface area, resulting in shortage of active sites. Therefore, scholars try to construct a heterogeneous catalyst containing tungsten which can be involved in the oxidative desulfurization of oil, such as WO3-SBA-15[13], WOx/ZrO2[14], SiO2-WO3[15], and WO3-Al2O3[16]. However, expensive raw materials and complex process for preparation of these catalysts would constrain the industrialized development of tungsten oxide. Therefore, an urgent need is to develop a simple process for preparation of heterogeneous catalyst containing tungsten with cheap raw materials.

The graphite phase carbon nitride (g-C3N4) is a kind of old polymer, which, featuring low density, high chemical stability, good biocompatibility, and high wear resistance, has a wide application prospect in the field of highperformance wear-resistant coating, membrane material, catalyst and catalyst carrier[17]. Recently, metal oxides and g-C3N4after being ground and calcined have been used to prepare the composite is applied in the field of photo-catalysis serving as the hot spots of research[18-20]. The composite thanks to its larger surface area has better photo-catalytic activity as compared to the metal oxides and g-C3N4. The g-C3N4was at first used as a carbon source to synthesize the WO3/C complex serving as a catalyst for oxidative desulfurization by a simple synthesized method. The surface area of WO3/C became bigger by incorporation of the carbon composite. The active sites were increased because the surface area of catalyst was enlarged to improve its catalytic properties. The catalyst with large surface areas and more pore volume could display higher dispersion of the active sites. So the oxidation activity of WO3and absorption of g-C3N4were enhanced. The desulfurization activity of WO3/C was improved.

In this work, the WO3, the g-C3N4and WO3/C composite were directly synthesized by the calcination method. The WO3/C composite had smaller particle size, larger specifc surface area as compared to WO3and C3N4. Then the removal of DBT in a simulated oil was studied using 1-ethyl-3-methylimidazolium ethyl sulfate as the extraction agent, hydrogen peroxide as the oxidant, and WO3/C as the catalyst. The effect of reaction temperature, catalyst dosage, oxidant dosage, extractant dosage and sulfides types on the desulfurization rate was studied, and meanwhile the desulfurization mechanism of WO3/C was discussed.

2 Experimental

2.1 Reagent and instrument

Dibenzothiophene (98% pure), benzothiophene (98% pure) and thiophene (98% pure) were purchased from the Aladdin Reagent Company; octane (98% pure), phospho-wolframic acid (98% pure), H2O2(with a concentration of 30%) and melamine (99% pure) were purchased from the Sinopharm Chemical Reagent Co., Ltd; 1-ethyl-3-methylimidazolium ethyl sulfate was purchased from the Shanghai Chengjie Chemical Co., Ltd. Alumina was purchased from the Fushun Research Institute of Petroleum and Petrochemicals (FRIPP). The experimental equipment included a rotary evaporator (type RE-52, made by the Gongyi Yuhua Instrument Co. Ltd.), a TDL-40B desktop electric centrifuge (made by the Tianjin Kexing Instrument Factory), and a magnetic stirrer (made by the Gongyi Yuhua Instrument Limited Liability Company). The S-content of the upper oil phase was tested with a WK-2D micro-coulomb comprehensive analyzer (made by the Jiangsu Electric Analysis Instrument Co., Ltd.).

2.2 Preparation of WO3

The phospho-tungstic acid was placed into a ceramic crucible equipped with a cover, which was then preheated at a temperature increase rate of 5oC/min in a muffle furnace until the temperature reached 550oC, and was kept at that temperature for 3 h. Then the samples were taken out to be cooled down to room temperature. Finally, the product was collected and ground.

2.3 Preparation of g-C3N4

The g-C3N4power was prepared by directly heating melamine in a muffe furnace at 520oC for 3 h after being preheated at a temperature increase rate of 15oC/min before reaching the specifed temperature of 520oC. The product was washed with distilled water three times and dried at 60oC for 6 h.

2.4 Preparation of WO3/C composites

A mixture of g-C3N4(0.76g) and WO3(0.24g) was ground in an agate mortar for 30 minutes. Then, the mixture was calcined in a muffe furnace at 520oC for 4 h. The samples were taken out, cooled and milled. As a contrast, the WO3/ Al2O3sample was prepared using the same method.

2.5 Characterization

The X-ray diffraction (XRD) patterns were recorded on an XRD7000 diffractometer (Shimadzu, Japan). The diffractometer was equipped with a Ni-filtered Cu Kαradiation source (λ=1.5406 ?) operating at 40 kV and 40 mA. The scanning range covered 10°—80° with a scanning speed of 2 (°)/min. The scanning electron microscopy (SEM) and the energy dispersive X-ray spectroscopy (EDS) of samples were performed using a scanning electron microscope (SEM, JEOL 6701). The FT-IR spectra were recorded with a Nicolet Nexus 470 FT-IR instrument using KBr pellets. The surface area analysis was recorded from the nitrogen adsorptionisotherms at 77 K with a Micromeritics Model ASAP 2020 instrument. All samples were degassed at 383 K under vacuum for 6 h. The average pore diameter and pore volume were calculated based on the Barrett?Joyner?Halenda (BJH) method.

2.6 Oxidative desulfurization experiment

Preparation of 500-μg/g-S model oil: 1.436 g of dibenzothiophene (DBT) were dissolved into 500 mL of octane solvent. The oxidative desulfurization experiment was carried out in an Erlenmeyer flask equipped with a refux condenser. In a typical experiment, 5 mL of model oil, EMIES, catalyst and 30% H2O2were placed into the reactor under stirring at a specified temperature. After the reaction, the mixture remained in the reactor until the solution was separated into two layers and was cooled to room temperature. A small amount of the upper oil phase was collected and the sulfur concentration was determined by a WK-2D micro coulomb analyzer.

3 Results and Discussion

3.1 Characteristic of catalyst

3.1.1 Analysis of X-ray diffraction

The XRD patterns of WO3/C, pure g-C3N4and WO3samples are shown in Figure 1. The results showed that the pattern peaks of the WO3and WO3/C were consistent with the standard JCPDS card (JCPDS 43-1035). The peaks of 2θ=23.04°, and 33.90° did not appear in WO3formed via directive calcination of the phospho-tungstic acid. Surprisingly, the diffraction peaks of g-C3N4did not appear in the XRD patterns of WO3/C. These results indicated that some of the characteristic functional groups were burned away during the preparation of the catalyst. Similarly, the diffraction peaks of C were not found in Figure 1. This phenomenon might be derived from the decomposition of a large amount of carbon nitride, which could hardly result in the formation of carbon species. The peaks of 2θ=23.04° and 33.90o, which obviously appeared in the XRD patterns of WO3/C, were attributed to crystal planes (002) and (121) of WO3. The results demonstrated that the structure and crystalline structure of WO3happened to change due to the composite of g-C3N4. The activity of WO3/C could change too.

Figure 1 XRD patterns of g-C3N4, WO3and WO3/C.

3.1.2 SEM analysis

The SEM images of WO3and WO3/C composite are shown in Figure 2. Figure 2 (a) shows the granular structure of WO3samples. The average particle size of catalyst was from 300 nm to 400 nm, which demonstrated a certain degree of reunion phenomenon[21]. Figure 2 (b) shows that the structure of WO3/C had revealed signifcant changes since the layered and porous structure emerged in the composite, and its particle size became smaller. The particle size of WO3/C ranged from 100 nm to 150 nm. It can be seen from the enlarged SEM image of WO3/C in Figure 2(c) that the WO3/C sample exhibited a large number of porous structures. The appearance of the pores, which could represent the adsorption ability of WO3/C, was strengthened and the desulfurization ability of the WO3/C sample was improved.

Figure 2 SEM images of the WO3and WO3/C samples

3.1.3 Energy dispersive spectroscopic analysis

In order to further determine the elemental composition of WO3/C, the WO3/C sample was characterized by the energy dispersive spectroscopy, with the data shown in Figure 3. The EDS spectra clearly indicated the presence of W, O and C species. The results showed that the composite contained the elements of W, O and C, which demonstrated that the composite was WO3/C.

Figure 3 EDS spectra of WO3/C

3.1.4 FT-IR analysis

Figure 4 shows the FT-IR spectra of g-C3N4, WO3, WO3/g-C3N4and WO3/C samples. It can be seen from Figure 4 that a strong broad absorption peak at around 3 450 cm-1was attributed to the vibration of O-H absorbed on samples. It shows that the water in air was absorbed on the surface of sample[22]. The strong absorption peak at 2 200 cm-1was the vibration absorption peak of C≡N bonds and the characteristic peaks at 1 200—1 650 cm-1represented the stretching vibration of C≡N bonds in Figure 4(c). The peak at 808 cm-1was ascribed to the bending vibration of absorption peak of sym-triazine structure[23]. All these data indicate that the sample (a) contained the carbon and nitrogen containing structure. However, the spectral types of Figure 4 (b) and Figure 4 (d) are completely similar. It can be seen that the strong absorption peak at 815 cm-1corresponded to the stretching vibration of O-W-O structure[24]. The result show that g-C3N4had been decomposed in the course of preparation of the catalyst. However, the WO3/g-C3N4is yellow and WO3/C is black in the appearance. The results showed that the black C product was formed during calcination of WO3and g-C3N4. The same conclusion can be obtained from the EDS analysis. The absorption peak of C species was not shown in the IR spectra of WO3/C, which might be resulted from decomposition of functional groups during calcination.

Figure 4 FT-IR spectra of g-C3N4, WO3and WO3/C composites.

3.1.5 Surface area analysis

The specific surface area, average pore diameter and pore volume of the samples are given in Table 1. The surface area of WO3, WO3/C, and g-C3N4is found to be 4.3051 m2/g, 27.8003 m2/g, and 18.2056 m2/g, respectively. The result suggests that tungsten oxide species can be well dispersed by forming composite with C species to get more reaction active sites. Therefore, the catalytic performance of composite with bigger surface area and more active sites is improved.

Table 1 Surface structural characteristics of different catalysts

3.2 Oxidative desulfurization performance of WO3/C

3.2.1 Effect of different catalyst system on desulfurization rate

The desulfurization rate of WO3and WO3/C was investigated in order to obtain an optimal catalytic system, with the results shown in Figure 5. It can be seen from Figure 5 that the desulfurization rate achieved by WO3was merely 67.3%, while the desulfurization rate achieved by WO3/C could reach 95.1% under the same reaction conditions because of the bigger surface area and more active sites of WO3/C, which were beneficial to the absorption and desulfurization reaction[25]. As a comparison, the oxidative desulfurization activity ofWO3/Al2O3was investigated. The results showed that the desulfurization activity of WO3/Al2O3could only reach 58% in 180 min. Therefore, WO3/C was identifed as a more efficient catalyst used in the oxidative desulfurization experiments.

Figure 5 Effect of different catalyst system on desulfurization rate

3.2.2 Influence of catalytic dosage on desulfurization rate

In order to investigate the influence of catalyst dosage on the desulfurization rate, different catalyst dosage was investigated, with the results shown in Figure 6. It can be seen from Figure 6 that when the amount of the catalyst was 0, the desulfurization rate of the system was only 17.7%. This shows that the extractive desulfurization effect was very poor. However, the rate of removal of DBT increased gradually with an increasing catalyst dosage. Because higher catalyst/oil molar ratio could provide more opportunity for the sulfides to be readily oxidized and removed[26]. When the catalyst dosage reached 0.02 g, the desulfurization rate of DBT surged to 95.1% after 180 min, while a further increase in the amount of catalyst could not further accelerate the removal of DBT. Upon taking into account the cost of the catalyst, the dosage of the catalyst added was specifed at 0.02 g.

Figure 6 Influence of catalyst dosage on desulfurization rate

3.2.3 Effect of amount of H2O2on desulfurization rate

To investigate the influence of the oxidant amount on desulphurization rate, the oxidation of DBT in the H2O2?WO3/C system with various H2O2amount was carried out, with the results shown in Figure 7. The removal rate of DBT increased with an increasing H2O2dosage. When the quantity of hydrogen peroxide was set to 0.2 mL, the removal rate of DBT increased to 95.8%. A further increase in H2O2dosage resulted in a decline of desulfurization rate. The explanation for this phenomenon is that the hydrogen peroxide was decomposed into water during the oxidative desulfurization process, and the water formed thereby could hinder the oxidation reaction[27]. In addition, the ionic liquid was diluted because of the excess water, which could affect the ability of the ionic liquid to extract DBT. So the optimal amount of hydrogen peroxide was 0.2 mL.

Figure 7 Effect of amount of H2O2on desulfurization rate

3.2.4 Effect of the temperature on desulfurization rate

The effect of the temperature on the desulfurization rate was studied, with the results shown in Figure 8. The test results showed that the desulfurization rate gradually increased with the increase of temperature. The desulfurization rate reached a highest value at 70oC.

Thereafter, the desulfurization rate decreased when the temperature continued to increase. This could occur because the elevated temperature was advantageous to the activity of the catalyst. However, the desulfurization rates did decrease due to the partial decomposition of hydrogen peroxide at a too high temperature, leading to rapid decrease in the oxidant concentration[28-29]. So the optimum reaction temperature was set as 70oC.

Figure 8 Effect of temperature on desulfurization rate

3.2.5 Effect of ionic liquid dosage on desulfurization rate

The ionic liquids played an important role in the oxidative desulfurization reaction. The effect of ionic liquids dosage on desulfurization rate is shown in Figure 9, indicating that with the increase of the ionic liquid dosage, the desulfurization rate was increased too. When the amount of ionic liquid was equal to 1 mL, the desulfurization rate reached 95.8%. As the amount of ionic liquid further increased to 2.5 mL, the desulfurization rate of DBT also reached 97.8% after 180 min. Upon considering the cost of ionic liquid, the amount of ionic liquid was chosen as 1.0 mL.

Figure 9 Effect of amount of ionic liquids on desulfurization rate

3.2.6 Influence of sulfur species on desulfurization rate

The sulfur species are an important factor affecting the oxidation desulfurization process. In order to investigate the catalytic performance of the catalysts for converting different sulfur species contained in the model oil, thiophene (TH), benzothiophene (BT) and dibenzothiophene (DBT) were chosen as the substrates. The oxidative desulfurization reaction of different model oil (BT, DBT, and TH) was carried out under the same conditions, with the results presented in Figure 10. The removal of different sulfides decreased in the following order: DBT > BT > TH. The rate for removal of DBT and BT was higher, which reached 95.8% and 72.6%, respectively. The rate for removal of TH was only 53.2%. The capability of oxidative desulfurization is related to the electron cloud density of sulfur atoms in its molecular structure. The higher the electron cloud density is, the easier the oxidative desulfurization would be. The density of sulfur atoms in DBT, BT, and TH molecules is 5.758, 5.739 and 5.696, respectively[30]. So DBT was the most vulnerable to oxidation, and BT is the second, while TH was the most oxidation-resistant.

Figure 10 Removal efficiency of different sulfur compounds

3.2.7 Re-usability of catalyst

After the desulfurization reaction, the catalyst was recovered and reused. Firstly, the catalyst was collected and dried in a vacuum oven at 90oC for 6 h. Then, the recovered catalyst, the new H2O2solution, the model oil and the ionic liquids were added into the reactor. Underthe optimal experimental conditions, the experiments using the recycled catalyst were carried out, with the outcome shown in Figure 11. The rate for removal of DBT was reduced from 95.8% to 85.5% after fve recycle use, which indicated that the desulfurization stability of the catalyst was still high.

Figure 11 Recycled use of WO3/C

3.2.8 Desulfurization mechanism of WO3/C

Figure 12 shows the mechanism of oxidative desulfurization. The DBT frstly were extracted from the model oil into the ionic liquid phase. The WO3in the supported WO3/C was oxidized by H2O2to form complex H2[W2O3(O2)4(H2O)2]2in the ionic liquids phase, then DBT was oxidized to DBTO2by H2[W2O3(O2)4(H2O)2]2due to its strong oxidation ability[31-32]. Then, DBTO2was removed from the oil phase. During the reaction, the oxidation activity of WO3was maintained with the help of H2O2until it was completely consumed.

Figure 12 Desulfurization mechenism of DBT in the presence of WO3/C

4 Conclusions

The WO3/C composite were prepared by the simple calcination method. The catalyst was characterized via XRD, BET, EDS, SEM and IR techniques. The structure and surface area of WO3were improved due to the addition of carbon species. The desulfurization experiment was carried out using the WO3/C as the catalyst, the H2O2as the oxidation agent, and the ionic liquid as the extractant. The test results showed that the removal rate of DBT could reach 95.8% under optimal conditions. The optimal reaction conditions covered: a hydrogen peroxide dosage of 0.2 mL, a WO3/C amount of 0.02 g, an ionic liquid amount of 1.0 mL, a reaction temperature of 70oC, and a reaction time of 180 min. In addition, the catalyst recycling experiments showed that the desulfurization rate still reach 85.5% after five recycles of the catalyst. The desulfurization system exhibited higher activity and stability for model oil.

Acknowledgements: The authors acknowledge the financial support of the Natural Science Foundation of China (Project No. 21003069) and the Liaoning Province Doctoral Fund (Project No.201501105).

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Successful Preparation of Magnetic Titanium-Silicate Based Catalyst at North University of China

The North University of China (NUC) has made breakthroughs in the preparation of magnetic Ti-silicate based catalyst and in the research on magnetic separation reaction after having developed a composite catalyst with catalytic activity and magnetic separation function. NUC has also developed an integrated magnetic separation unit to tackle the tough problems for solid-liquid separation of fine catalyst particles, which can allow for axing the succeeding solid-liquid separation unit and associated facilities.

This research achievement of NUC can be used in these domains, including the manufacture of cyclohexanoneoxime through the magnetic titanium-silicate catalyzed cyclohexanone oximation, the photocatalytic degradation of organic wastewater in the presence of magnetic titanium oxide, the magnetic copper-silicate catalyzed hydrogenation of furfural to furfuryl alcohol, and the manufacture of polyolefins via magnetic solid acid catalyzed olefn polymerization. The integrated reactionseparation unit involving the magnetic catalyst and separation reaction can make the solid-liquid separation of ultrafne catalyst particles easier to become a genuine energy-saving, consumption-reducing and emissionreducing high-tech and green process.

First in World Syngas-to-Ethanol Demonstration Project Cranks out Qualified Product

On January 11, 2017 the 100 kt/a syngas-to-ethanol commercial demonstration project at the Yanchang Petroleum Group, to which much public attention has been paid, had been put on stream at the first attempt, while cranking out qualified anhydrous ethanol with a purity of 99.71%. This fact has symbolized the full success of the first in the world syngas-to-ethanol commercial demonstration unit to make an important breakthrough in application of China’s novel coal chemical industry technology.

The syngas-to-ethanol commercial demonstration project has been jointly developed by the Yanchang Petroleum Group and the CAS Dalian Institute of Chemical Physics. The investment in this project totals 743 million RMB, and the construction work was started in October 2015, with the engineering and construction work, the equipment installation work and the pre-commissioning work all successfully completed within one year. At present, the pre-commissioning process of the production unit proceeds smoothly, while delivering products that can meet the quality standard with all process indicators complying with the norms. The production staffs of this unit are striving without cease to realize the set production targets as soon as possible.

date: 2016-09-28; Accepted date: 2016-11-29.

Dr. Li Xiuping, Telephone: +86-13942361930; E-mail: lilili_171717@126.com.