- Email: [email protected]
g-C3N4 composites and their application in oxidative desulfurization
Accepted Manuscript Title: Preparation of WO3 /g-C3 N4 Composites and Their Application in Oxidative Desulfurization Author: Rongxiang Zhao PhD Xiuping Li PhD Jianxun Su Xiaohan Gao PII: DOI: Reference:
S0169-4332(16)31763-9 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.120 APSUSC 33869
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-3-2016 19-8-2016 22-8-2016
Please cite this article as: Rongxiang Zhao, Xiuping Li, Jianxun Su, Xiaohan Gao, Preparation of WO3/g-C3N4 Composites and Their Application in Oxidative Desulfurization, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of WO3/g-C3N4 Composites and Their Application in Oxidative Desulfurization Rongxiang Zhao1, Xiuping Li, Jianxun Su, Xiaohan Gao ( College of Chemistry, Chemical Engineering and Environment Engineering, Liaoning Shihua University, 113001, China) 1
Author introduction: Rongxiang hao (1971), male, PhD.
Email:[email protected]
Telephone:+8613470542149 Corresponding author: Xiuping Li(1975-),female,ph D, email: [email protected]
Graphical abstract The WO3/g-C3N4 composite was successfully synthesized through directly calcining phosphotungstic and melamine. The WO3 is one of several good catalyst for oxidation
desulfurization. But, the surface area of WO3 is small and short of active site. Taking into account the catalyst with large surface area generally has a high catalytic activity. The surface area of WO3/g-C3N4 composite is ten times as big as the WO3.The crystalline of WO3/g-C3N4 composites was obviously improved. Desulfurization experiment showed that the desulfurization rate of simulated oil can reach 91.2% under optimal conditions. Meanwhile, the activity of the catalyst was not significantly decreased after the 5 recycles. The result show that stability of catalyst is high.
Highlights
The WO3/g-C3N4 was successfully synthesized through simple calcination. The process is simple and the cost is cheap. The WO3/g-C3N4 firstly applied to ODS. The desulpurization rate of WO3/g-C3N4 may attach to 91.2%. Five recycles of WO3/g-C3N4 still attach to 89.5%.
1
The crystalline and surface area of composite are obviously improved by g-C3N4.
Abstract: WO3/graphitic carbon nitride (g-C3N4) composites were successfully synthesized through direct calcining of a mixture of WO3 and g-C3N4 at 400°C for 2 h. The WO3 was prepared by calcination of phosphotungstic acid at 550°C for 4 h, and the g-C3N4 was obtained by calcination of melamine at 520°C for 4 h. The WO3/g-C3N4 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and Brunner−Emmett−Teller analysis (BET). The WO3/g-C3N4 composites exhibited stronger XRD peaks of WO3 and g-C3N4 than the WO3 and pure g-C3N4. In addition, two WO3 peaks at 25.7° and 26.6° emerged for the 36% -WO3/g-C3N4 composite. This finding indicated that WO3 was highly dispersed on the surface of the g-C3N4 nanosheets and interacted with the nanosheets, which resulted in the appearance of (012) and (022) planes of WO3. The WO3/g-C3N4 composite also exhibited a larger specific surface area and higher degree of crystallization than WO3 or pure g-C3N4, which resulted in high catalytic activity of the catalyst. Desulfurization experiments demonstrated that the desulfurization rate of dibenzothiophene (DBT) in model oil reached 91.2% under optimal conditions. Moreover, the activity of the catalyst was not significantly decreased after five recycles. Keyword: WO3/g-C3N4; Composites; Oxidation Desulfurization;
Introduction WO3 has been applied to the oxidative desulfurization of fuel oil because of its thermal stability and good crystallinity[1]. However, WO3 has a small surface area, which results in a shortage of active sites. Therefore, scholars have investigated heterogeneous catalysts containing tungsten oxide, such as WO3–SBA–15[2], WOx/ZrO2[3], SiO2–WO3[4], and WO3–Al2O3[5], and applied them to the oxidation desulfurization of fuel oil. However, expensive raw materials and the complex preparation process of the catalysts limit the industrial development of tungsten oxide. 2
Therefore, there is an urgent need to develop a simple preparation process for a heterogeneous catalyst containing tungsten oxide using inexpensive raw materials. Graphitice carbon nitride (g-C3N4) is a well-known polymer that has the advantages of low density, high chemical stability, good bio-compatibility, and high wear resistance; therefore, it shows potential for a wide range of high-performance applications such as wear resistant coatings, membrane materials, catalysts, and catalyst carriers[6]. Recently, g-C3N4 has been increasingly applied to the degradation of organic pollutants by photocatalysis[7-11]. Researchers have found that mixtures of metal oxides and g-C3N4 have greater surface areas than metal oxides or pure g-C3N4 alone after grinding and calcination, which clearly indicates higher catalytic activity. Recently, Wang et al.[12] synthesized TiO2/g-C3N4 and applied it to oxidation desulfurization under UV irradiation. The result indicated that the removal rate of dibenzothiophene (DBT) could reach 98.9%. In addition, phosphotungstic acid (HPW)/g-C3N4 was synthesized by Zhu et al.[13] via phosphorus acid loading on the carbon nitride. The HPW/g-C3N4 catalyst exhibited a high catalytic activity in the oxidation desulfurization process. The DBT could be completely removed under optimal reaction conditions, and no significant decrease in the catalytic activity of the catalyst was observed after 15 recycles. Zhu W. et al[14-19] reported a lot of desulfurization using ionic liquid and composite of Tungsten. Inspired by the above method, in this work, WO3/g-C3N4 composites were prepared and applied to the oxidation desulfurization of fuel oil. To the best of our knowledge, no previous studies on the application of WO3/g-C3N4 in the oxidation desulfurization process have been reported. In this study, WO3/g-C3N4 was prepared by direct calcination of inexpensive HPW and melamine, and the desulfurization performance of the catalyst was investigated in detail. The catalyst using simple preparation method exhibits higher desulfurization performance than that of other WO3 heterogeneous catalysts[2-5].
1 Experiment 1.1 Materials
3
Dibenzothiophene(DBT), thiophene(TH) and benzothiophene(BT) were purchased from Sigma-Aldrich. H2O2, melamine, phosphotungstic acid and n-octane were purchased
from
Sinopharm
Chemical
Reagent.
1-ethyl-3-methylimidazolium
diethylsulfate was purchased from Shanghai Chengjie Chemical Co. LTD. 1.2 Preparation of WO3/g-C3N4 catalyst The WO3 was synthesized via a decomposition reaction of solid-state HPW at 550°C for 4 h. The g-C3N4 powders were prepared by directly heating melamine at 520°C in a muffle furnace for 4 h under air conditions. The synthesis of the WO3/g-C3N4 composite was as follows: WO3 and g-C3N4 at a certain mass ratio were mixed and ground for 20 min using a mortar and pestle. The mixed powder was placed in a crucible with a cover and heated at 400°C in a muffle furnace for 4 h. Finally, composites containing 12%, 24%, 36%, and 50% WO3 (mass%) were obtained. 1.3 Characterization X-ray diffraction (XRD) patterns were recorded on an XRD7000 (Shimadzu, Japan). The diffractometer was equipped with a Ni-filtered Cu-Kα radiation source (λ=1.5406 Å) at 40 kV and 40 mA, step 0.02 degrees, the scanning range: 10°~70°. Scanning electron microscopy (SEM, JEOL 6701F) were used to observed the morphology of samples. The FT-IR spectra were recorded with a Nicolet Nexus 470 FT-IR instrument using KBr pellets. The surface area analysis was performed from the nitrogen adsorption isotherms at 77 K with a Micromeritics Model ASAP 2020 instrument. The average pore diameter and pore volume of samples were calculated based on the Barrett−Joyner−Halenda (BJH) method. 1.4 Oxidation desulfurization The model oil (500 ppm) was prepared by dissolving 2.9018 g DBT in 1 L of n-octane. The oxidation desulfurization reaction was performed in a conical flask with a condensing pipe. The oxidation desulfurization system, which contained 5 mL of model oil, a certain amount of ionic liquid, 30% H2O2, and the catalyst were stirred using magnetic stirring at a certain temperature. After the reaction, the mixture was
4
static until the solution were layered and cooled in air. A small amount of the oil phase was collected, and the sulfur concentration was determined using a WK-2D micro–coulomb analyzer. 2 Results and Discussion 2.1 X-ray diffraction characterization The X-ray diffraction (XRD) patterns of the WO3/g-C3N4 composites with different WO3 contents, pure g-C3N4, and WO3 are presented in Fig.1. The characteristic diffraction peaks of the pure g-C3N4 appearing at 27.4° and 13.1° can be indexed to the (002) and (100) planes, which are attributed to the characteristic interlayer structure and interplanar stacking peaks of aromatic systems[20], respectively. The characteristic diffraction peaks of the WO3 can be attributed to the monoclinic structure (JCPDF 43-1035). The characteristic diffraction peaks of WO3 and g-C3N4 also appeared in the XRD patterns of the WO3/g-C3N4 composites with different WO3 contents, which indicates that the structures of WO3 and g-C3N4 were not damaged during the calcination. The WO3 peaks became stronger and the C3N4 peaks became weaker upon increasing the WO3 content in the composites. In addition, two WO3 peaks at 25.7° and 26.6° emerged in the 36% WO3/g-C3N4 composite, which are attributed to the (012) and (022) planes of WO3[21], respectively. This finding indicates that WO3 was highly dispersed on the surface of the g-C3N4 nanosheets and interacted with the nanosheets. The g-C3N4 peak at 27.4° shifted slightly toward higher diffraction angles with increasing WO3 content, which indicates that the entering of WO3 into the interlayers of pure g-C3N4 resulted in a slight expansion of the interlayer[20]. These results demonstrate that the interaction between WO3 and g-C3N4 can enhance their performance, which resulted in the 36%WO3/g-C3N4 composite exhibiting better crystallinity and performance. These results may also lead to the 36%WO3/g-C3N4 composite exhibiting better desulfurization performance, which will be discussed in Section 2.5. 2.2 SEM analysis The SEM images of WO3, g-C3N4 and the WO3/g-C3N4 with different concentration of WO3 are shown in Fig.2. As shown in Fig.2 (f, g, h), WO3 is a 5
smaller particle size with 100-150 nm. However, there was a serious agglomeration of nanoparticles, which is caused by the high temperature calcination. The morphology of the g-C3N4 is observed in Figure 2 (a, e). From Fig. 2(a, e), it can be observed that g-C3N4 has an irregular layer-structure with powder. The distribution of WO3 nanoparticles in g-C3N4 was seen in Figure 2 (b, c, d). Some nanoparticles of WO3 can be found in the picture, they own the same size as WO3 in Fig.2(f, g, h). The 36% WO3/g-C3N4 show an obvious breaking up comparing to WO3. Connecting with the results of XRD, it can be further confirmed that WO3 can be dispersed on the g-C3N4 nanosheets. 2.3 Fourier-transform infrared spectroscopy analysis The Fourier-transform infrared (FT-IR) spectra of WO3, g-C3N4, and the WO3/g-C3N4 composites with different WO3 contents are presented in Fig. 3. For the pure g-C3N4, the peaks at 1236, 1309, 1435, and 1570 cm−1 can be attributed to aromatic C–NH–C stretching[22]. The peaks at 1645 cm−1 can be attributed to C=N stretching vibration modes[23]. For the FT-IR spectrum of pure WO3, a broad peak appears at 750–1000 cm-1, which corresponds to O–W–O stretching vibration in a monoclinic-type WO3 crystal[20]. For WO3/g-C3N4 with different WO3 contents, all the characteristic peaks of g-C3N4 are displayed in the spectra. However, the characteristic peaks of WO3 were not observed, which may due to the WO3 entering the interlattice of g-C3N4 or overlapping peaks. The peak at 3420 cm−1 is attributed to stretching vibration of –OH in water adsorbed on the surface of WO3 and g-C3N4. This result indicates that WO3 and g-C3N4 easily absorb water molecules in air because their charge is not balanced. However, the shift of the –OH peaks of the composites from 3420 to 3216 cm-1 is attributed to from the intermolecular association change into intramolecular association of –OH. The above results imply that WO3 may enter the interlattice of the g-C3N4 nanosheets and interact with triazine units, leading to intramolecular association of –OH. 2.4 BET analysis The specific surface areas of the WO3, pure g-C3N4, and WO3/g-C3N4 with different WO3 contents are listed in Table 1. The surface areas of WO3 and g-C3N4 were 4.3051 6
and 18.2056 m2/g, respectively. The specific surface area of the WO3/g-C3N4 composites decreased with increasing WO3 content with values of 42.5364, 41.5262, 31.5564, and 24.0876 m2/g for the 12%, 24%, 36%, and 50% WO3 (mass%) composites, respectively. The specific surface areas of the composites increased after calcination of the mixtures of WO3 and g-C3N4. This finding may be related to the WO3 being well dispersed on the g-C3N4 nanosheets, entering the interlayer, and interacting with triazine units of the nanosheets, resulting in expansion of the interlayer, which further leads to an increase of the surface areas of the composites[24]. The 12%WO3/g-C3N4 composite exhibited the largest surface area because the g-C3N4 peak for this composite is the highest in Fig. 1 with the maximum dispersion and exposure of g-C3N4. It is generally believed that a larger catalyst surface area can provide more active sites for the reaction system, which can further enhance the activity of the composite. However, g-C3N4 cannot catalyze and oxide DBT into DBTO2. The N2 adsorption–desorption curve of the composites, WO3, and g-C3N4 are presented in Fig. 4. The surface areas of the composites are greater and their N2 adsorption–desorption curves are higher than those of WO3 and g-C3N4. These results indicate that the larger surface areas of the catalysts yield stronger absorption abilities of the organic compounds. However, the WO3 content is also an important factor because WO3 is catalyst for the formation of the peroxide (DBTO2). In addition, the improved crystallinity and emergence of additional planes for 36%WO3/g-C3N4 resulted in enhanced desulfurization performance. 2.5 Investigation of desulfurization 2.5.1 Effect of catalyst on desulfurization The effect of the different catalysts on desulfurization are shown in Fig. 5. The desulfurization rate was only 14% with g-C3N4 because of the absorption of g-C3N4. The desulfurization rate of the composites increased from 48.4% to 65.4% upon increasing the WO3 content from 12% to 36%, respectively. The 36%WO3/g-C3N4 composite exhibited the highest desulfurization performance, with the desulfurization rate decreasing to 59.6% for the 50%WO3/g-C3N4 catalyst. Based on these results, it is apparent that the surface area of the sample is not the only factor that affects the 7
desulfurization rate. For example, the 12% WO3/g-C3N4 catalyst exhibited the largest surface area; however, the desulfurization rate was less than 50%. The specific surface area of pure WO3 was the smallest of all the samples, whereas the desulfurization rate was the same as that of 12%WO3/g-C3N4. In addition, the reaction velocity of g-C3N4 was faster than that of WO3. Thus, it can be observed that the WO3 content, surface area, and crystallinity of the products have important effects on the desulfurization performance of catalyst. Based on the analytical results, the 36%WO3/g-C3N4 composite was selected as the catalyst for the oxidative desulfurization process. 2.5.2 Effect of temperature on desulfurization The reaction temperature is a key parameter for the desulfurization system. Oxidative desulfurization experiments were performed at 50°C, 60°C, 70°C, and 80°C, and the results are presented in Fig. 6. Increasing the reaction temperature resulted in an obvious increase of the desulfurization rate of the system. With extension of the reaction time, the desulfurization rate of the system reached 74% at 60°C in 180 min. The sulfur removal rate was not enhanced at 70°C and 80°C after a certain reaction time. Although increasing the reaction temperature is beneficial for the oxidative desulfurization process, H2O2 is decomposed at high temperatures[25], and the desulfurization rate of the system no longer increases because of the absence of an oxide agent. Therefore, 60°C was selected as the optimal temperature of the catalytic system as the catalyst exhibited lower activity at 50°C. 2.5.3 Effect of H2O2 dose on desulfurization The amount of H2O2 is other important factor for the removal rate of sulfur. Fig. 7 shows that upon increasing the H2O2 amount from 0.1 to 0.3 mL, the removal rate of DBT increased from 48.9% to 84.2% in 180 min, respectively. However, when the amount of H2O2 was further increased to 0.4 mL, the removal rate of DBT decreased to 78.6% in 180 min. This result can be explained by the presence of two reactions in the oxidative desulfurization system, the oxidation of DBT and decomposition of H2O2[26-27]. Excessive H2O2 was added to the oxidative desulfurization system for certain DBT completely oxidized and H2O2 was surplus. Therefore, the concentration of the ionic liquid in the system is decreased and the extraction ability of the ionic 8
liquid is weakened. In addition, water from the decomposition of surplus H2O2 hinders the oxidation and extraction of DBT in the system. Therefore, too much H2O2 results in a decrease in the desulfurization rate. 2.5.4 Effect of catalyst dose on desulfurization The significant effect of the catalyst dose on the desulfurization rate is demonstrated in Fig. 8. Upon increasing the catalyst dose from 0.01 to 0.03 g, the desulfurization rate clearly increased with the reaction time extended. The optimal catalyst dose was 0.03 g in 5 mL of model oil, and the desulfurization rate of oxidative desulfurization system reached 86.7% in 180 min. Further increase of the catalyst dose to 0.04 g had no effect on the desulfurization rate for a reaction time of 180 min. 2.5.5 Effect of ionic liquid dose on desulfurization In this experiment, ionic liquid was the extraction agent of the oxidative desulfurization system, and the ionic liquid dose had a significant effect on the desulfurization rate. Ionic liquid doses of 0, 0.25, 0.50, and 0.75 mL were used, and the experimental results are listed in Table 2. Without the addition of the ionic liquid, the desulfurization rate of the system was 40.1%. Upon increasing the dose to 0.25 mL, the desulfurization rate reached 91.2%. However, when an excessive amount of ionic liquid was added to the system, the desulfurization rate decreased to 87.2% (0.5 mL) and 86.5% (0.75 mL). This finding may be due to the decrease of the ratio of oxygen to sulfur in the system because more DBT can be extracted into the ionic liquid phase upon increasing the ionic liquid content, whereas the amount of H2O2 is unchanged, similar to previous findings in the literature[28]. 2.5.6 Effect of substrate on desulfurization To study the effect of 36%WO3/g-C3N4 on different sulfur compounds, benzothiophene (BT), DBT, and thiophene (TH) were selected as model oils in the oxidative desulfurization process under the same experimental conditions, and the results are presented in Fig. 9. The order of the desulfurization rate was DBT > BT > TH, which is consistent with previous results in the literature
[29]
. The oxidative
desulfurization activity of the model oil depends on the electron cloud density of 9
sulfur atoms in the organic sulfur compounds. In general, a lower electron density of sulfur atoms results in a lower desulfurization rate. The electron densities of the sulfur atoms are 5.785 (DBT), 5.739 (BT), and 5.696 (TH). In the oxidative desulfurization system, the sulfide is first absorbed by the catalyst. The adsorption capacity of the catalyst is related to the structure of the catalyst, including its specific surface area, pore volume, and pore size. Among the three sulfides, the space structure and steric hindrance of TH are the smallest and those of DBT are the largest. Therefore, TH is the most easily absorbed by the catalyst, whereas DBT is the most difficult to absorb. The removal rate of TH is thus the highest, that of DBT is the lowest, and that of BT is in between. In addition, the TH and ionic liquid have similar polarity and TH easily was extracted[29] too. However, with the extension of the reaction time, the oxidation desulfurization is dominant. Therefore, the order of the desulfurization rate still follows DBT > BT > TH. 2.5.7 Repeated use of catalyst and ionic liquids The repeated use of the catalyst and recovery of ionic liquid holds great significance for industrial application. Thus, the repeatability of use of the catalyst and ionic liquids were investigated, and the results are presented in Fig. 10. The repeated use of the catalyst and ionic liquids were tested as the following procedures: the ionic liquid phase and oil phase were separated by centrifugation from the reaction system after each run. Extraction of the sulfur compounds in the ionic liquid phase was performed three times with a certain amount of CCl4. Then, fresh H2O2, the model oil, and ionic liquid containing the catalyst (recovered ionic liquid and catalyst) were directly added to the next run under the same conditions. The results indicated that the sulfur removal rate remained as high as 87.5% after five recycles. To verify the stability of the 36%WO3/g-C3N4, the XRD pattern of the recovered catalyst was compared with that of the fresh catalyst, as shown in Fig. 11. The structure of the recovered catalyst did not exhibit any obvious change. The diffraction peaks of some impurities in the range of 10°–23° can be attributed to oxidation products of organic sulfur adsorbed on the catalyst surface. The results indicate that the oxidative desulfurization system containing ionic liquid and 36%WO3/g-C3N4 is a promising 10
reaction system. To determine the type of sulfur oxidation products, a reverse extraction experiment was performed using CCl4 as the extraction agent. First, the model oil from the oxidation desulfurization system was separated using a separating funnel. Then, the water was evaporated by rotary evaporation. Next, the ionic liquid was extracted by the CCl4, and a white solid was obtained and characterized using FT-IR spectroscopy. Compared with DBT, the white solid exhibited absorption peaks of sulfone at 1288 and 1166 cm-1 and absorption peaks of sulfoxide at 1047 cm-1, which are consistent with results in the literature[30]. This finding suggests that DBT was oxidized to DBTO and DBTO2 using the 36%WO3/g-C3N4 as a catalyst with the assistance of H2O2. 2.5.8 Mechanism of oxidative desulfurization reaction of WO3/g-C3N4 The mechanism of the oxidative desulfurization reaction from DBT to DBTO2 is illustrated in Fig. 13. Two phases, the ionic liquid and model oil, are presented in the reaction system. The WO3/g-C3N4 and H2O2 in the ionic liquid were immiscible with the model oil. The ionic liquid plays an important role in oxidative desulfurization. In the absence of the ionic liquid, the rate of desulfurization was only 40%. The removal rate of sulfur increased sharply in the presence of the ionic liquid, which demonstrated that DBT was oxidized in the ionic liquid phase. First, WO3 was oxidized to H2WO2(O2)2 under the action of H2O2[24]. The H2WO2(O2)2 exhibited higher oxidation activity than WO3. The sulfur compounds in the model oil were oxidized to their corresponding sulfones by the H2WO2(O2)2 in the ionic liquid under stirring; then, the sulfones were extracted into the ionic liquid phase. Note that g-C3N4 has a special effect on the desulfurization process. The surface area and crystallinity of the catalyst were improved when using the WO3/g-C3N4 composite. In addition, g-C3N4 can activate the H2O2[31] form hydroxyl free radicals. 3. Conclusion WO3/g-C3N4 composites with different WO3 contents were prepared via direct calcination. The specific surface areas of the composites were larger than those of WO3 or pure g-C3N4, and their crystallinities were substantially increased compared 11
with that of WO3. In particular, the 36%WO3/g-C3N4 exhibited XRD peaks for (012) and (022) planes. The crystallinity of the composites had a significant effect on the desulfurization performance. Ionic liquids with various catalysts can oxidize DBT to DBTO2 with assistance of H2O2 for deep desulfurization under moderate conditions. The sulfur removal rate of the model oil containing DBT in reached 91.2% with the 36%WO3/g-C3N4 catalyst and the assistance of H2O2 at 60°C for 180 min. This finding demonstrates the remarkable advantage of this process over desulfurization using WO3 alone as the catalyst. The 36%WO3/g-C3N4 catalyst could also be recovered and reused five times without a significant decrease in activity. Acknowledgements The authors also acknowledge the financial support of the Natural Science Foundation of China (Project no. 21003069) and Liaoning Province Doctoral Fund (Project no.201501105) References [1] Rakhmanov E V, Tarakanova A V, Valieva T, et al. Oxidative desulfurization of diesel fraction with hydrogen peroxide in the presence of catalysts based on transition metals[J]. Petroleum Chemistry, 54(2014): 48-50. [2] Li X, Huang S, Xu Q, et al. Preparation of WO3-SBA-15 mesoporous molecular sieve and its performance as an oxidative desulfurization catalyst[J]. Transition metal chemistry, 34(2009): 943-947. [3] Torres-Garcia E, Canizal G, Velumani S, et al. Influence of surface phenomena in oxidative desulfurization with WOx/ZrO2 catalysts[J]. Applied Physics A, 79(2004): 2037-2040. [4] Song H, Mu J, Wang D, et al. Preparation and Characterization of SiO2-WO3 Mixed Oxides and Its Catalytic Performance in Oxidative Desulfurization of Benzothiophene[J]. Acta Petrolei Sinica (Petroleum Processing Section), 5(2012): 750-756. [5] Li X, Zhu H, Wang A, et al. Oxidative Desulfurization of Dibenzothiophene over Tungsten Oxides Supported on SiO2 and γ-Al2O3[J]. Chemistry Letters, 42(2013): 8-10. [6] Thomas A, Fischer A, Goettmann F, et al. Graphitic carbon nitride materials: variation of
12
structure and morphology and their use as metal-free catalysts[J]. Journal of Materials Chemistry, 18(2008): 4893-4908. [7] Zang Y, Li L, Zuo Y, et al. Facile synthesis of composite g-C3N4/WO3: a nontoxic photocatalyst with excellent catalytic activity under visible light[J]. RSC Advances, 3(2013): 13646-13650. [8] Huang L, Li Y, Xu H, et al. Synthesis and characterization of CeO2/gC3N4 composites with enhanced visible-light photocatatalytic activity[J]. RSC Advances, 3(2013): 22269-22279. [9] Chen L Y, Zhang W D. In2O3/gC3N4 composite photocatalysts with enhanced visible light driven activity[J]. Applied Surface Science, 301(2014): 428-435. [10] Sun J X, Yuan Y P, Qiu L G, et al. Fabrication of composite photocatalyst g-C3N4–ZnO and enhancement of photocatalytic activity under visible light[J]. Dalton Transactions, 41(2012): 6756-6763. [11] Huang L, Xu H, Zhang R, et al. Synthesis and characterization of g-C 3N4/MoO3 photocatalyst with improved visible-light photoactivity[J]. Applied Surface Science, 2013, 283: 25-32. [12] Wang C, Zhu W, Xu Y, et al. Preparation of TiO2 /g-C3N4 composites and their application in photocatalytic oxidative desulfurization[J]. Ceramics International, 2014, 40(8): 11627-11635. doi:10.1016/j.ceramint.2014.03.156 [13] Zhu Y, Zhu M, Kang L, et al. Phosphotungstic acid supported on mesoporous graphitic carbon nitride as catalyst for oxidative desulfurization of fuel[J]. Industrial & Engineering Chemistry Research, 2015, 54(7): 2040-2047. [14] Zhu W., Dai B. , Wu P, et al.Graphene-Analogue Hexagonal BN Supported with Tungsten-based Ionic Liquid for Oxidative Desulfurization of Fuels[J].ACS Sustainable Chem. Eng., 2015, 3 (1):186–194. DOI: 10.1021/sc5006928 [15] P Wu, W Zhu, A Wei, et al.Controllable Fabrication of Tungsten Oxide Nanoparticles Confined in Graphene-Analogous Boron Nitride as an Efficient Desulfurization Catalyst[J].Chem. Europ. J., 2015, 21(43): 15421-15427. DOI: 10.1002/chem.201501413 [16] Q Gu, W Zhu, S Xun, et al. Preparation of highly dispersed tungsten species within mesoporous silica by ionic liquid and their enhanced catalytic activity for oxidative desulfurization [J].Fuel, 2014, 117, 667-673.doi:10.1016/j.fuel.2013.08.082
13
[17] Zhang M, Zhu W, Li H, et al. One-pot synthesis, characterization and desulfurization of functional mesoporous W-MCM-41 from POM-based ionic liquids[J]. Chemical Engineering Journal, 2014, 243: 386-393.doi:10.1016/j.cej.2013.12.093 [18] Zhu W, Li H, Jiang X, et al. Commercially available molybdic compound-catalyzed ultra-deep desulfurization of fuels in ionic liquids[J]. Green chemistry, 2008, 10(6): 641-646. DOI: 10.1039/B801185K [19]Zhu W. , Li H. , Jiang X. et al. Oxidative Desulfurization of Fuels Catalyzed by Peroxotungsten
and
Peroxomolybdenum
Complexes
in
Ionic
Liquids[J].Energy
Fuels, 2007, 21 (5):2514–2516 DOI: 10.1021/ef700310r [20] Ding J, Liu Q, Zhang Z, et al. Carbon nitride nanosheets decorated with WO3 nanorods: Ultrasonic-assisted facile synthesis and catalytic application in the green manufacture of dialdehydes[J]. Applied Catalysis B: Environmental, 165(2015): 511-518. [21] Mo Ruofei, Jin Guoqiang, Guo Xiaongyun.Hydrothermal Synthesis of Tungsten Trioxides Using Citric Acid as Controlling Agent,Chinese Journal of Inorganic Chemistry, 23(2007): 1615-1620. [22] Khabashesku V N, Zimmerman J L, Margrave J L. Powder synthesis and characterization of amorphous carbon nitride[J]. Chemistry of materials, 12(2000): 3264-3270. [23] Li X, Zhang J, Shen L, et al. Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine[J]. Applied Physics A, 94(2009): 387-392. [24] He Y, Zhang L, Wang X, et al. Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3-g-C3N4 composite under visible light irradiation[J]. RSC Advances, 4(2014): 13610-13619. [25] Li F, Wu B, Liu R, et al. An inexpensive N-methyl-2-pyrrolidone-based ionic liquid as efficient extractant and catalyst for desulfurization of dibenzothiophene[J]. Chemical Engineering Journal,274( 2015): 192-199. [26] Chen X, Song D, Asumana C, et al. Deep oxidative desulfurization of diesel fuels by Lewis acidic ionic liquids based on 1-n-butyl-3-methylimidazolium metal chloride[J]. Journal of Molecular Catalysis A: Chemical, 359(2012): 8-13. [27] Zhou Q, Fu S, Zou M, et al. Deep oxidative desulfurization of model oil catalyzed by magnetic MoO 3/Fe 3 O 4[J]. RSC Advances,5(2015): 69388-69393. 14
[28] Chen X, Song D, Asumana C, et al. Deep oxidative desulfurization of diesel fuels by Lewis acidic ionic liquids based on 1-n-butyl-3-methylimidazolium metal chloride[J]. Journal of Molecular Catalysis A: Chemical, 2012, 359: 8-13. [29] Li HP, Zhu WS,Zhu SW, et al. The selectivity for sulfur removal from oils: An insight from conceptual density functional theory[J].AIChE J.2016,62(6):2087-2100.DOI: 10.1002/aic.15161 [30] Seubert A. A critical comparison of on-line coupling IC-ICP-(AES, MS) with competing analytical methods for ultra trace analysis of microelectronic materials[J]. Fresenius journal of analytical chemistry, 364(1999): 404-409. [31] Cui Y, Ding Z, Liu P, et al. Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants[J]. Physical Chemistry Chemical Physics, 14(2012 ): 1455-1462.
15
Figure caption Fig.1 XRD pattern of g-C3N4,WO3 and the WO3/g-C3N4 with different concentration of WO3. Fig.2 SEM images of (a,e) g-C3N4, (b,c,d) 36%-WO3/g-C3N4 and (f,g,h) WO3. Fig.3 FT-IR spectra of g-C3N4,WO3 and WO3/g-C3N4 with different concentration of WO3. Fig.4 The Nitrogen adsorption-desorption curve of the composite, WO3 and g-C3N4 Fig.5 influence of catalytic type on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL, 0.02g of catalysts, VH2O2 = 0.2mL, t= 180 min,T= 343 K, Fig.6 the influence of temperature on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL, 0.02g of 36%-WO3/g-C3N4, VH2O2 = 0.2mL, t= 180 min Fig.7 the influence of H2O2 dose on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL,0.02g of 36%-WO3/g-C3N4, t= 180 min, T=333K Fig.8 influence of catalyst dose on desulphurization activity Conditions: Voil=5 ml,VIL= 0.75 mL, VH2O2 = 0.3mL, Fig.9
t= 180 min, T=333K
influence of substrate on sulfur removal
Fig.10 Recycle of the reaction system Fig.11 XRD of catalysts before use and after use. Fig.12 IR spectra of DBT (A) and its oxidation products (B). Fig.13 Oxidation desulfurization mechanism of reaction system containing WO3/g-C3N4 catalyst
16
WO3
28.7
12%-WO3/g-C3N4 28.7
Intensity(a.u)
24%-WO3/g-C3N4 26.6 28.7
36%-WO3/g-C3N4 28.7
50%-WO3/g-C3N4
(100)
(002)
25.7
10
20
30
g-C3N4 40
50
60
70
O
2θ/( ) Fig.1 Fig.1
a2.2 SEM analysis
b
c
d
g
h
Fig.2
e
f
Fig.2
17
1545
12%-WO3/g-C3N4
1684
24%-WO3/g-C3N4 880 1463 1309
Transmittance(%)
810
36%-WO3/g-C3N4
12361435 3100-3300
50%-WO3/g-C3N4 g-C3N4 13091570
WO3 3300-3600
1645 1236
805 500
1000
1435
1500
2000
2500
3000
3500
-1
Wavenumber(cm )
Fig.3
120
3
□ ○
●
Volume adsorbed(cm g-1)
■
100 80
△ ▽ ☆
WO3 12%-WO3/C3N4 24%-WO3/C3N4 36%-WO3/C3N4 50%-WO3/C3N4 C3N4
60 40 20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
relative pressure(P/P0) Fig.4
18
4000
12%-WO3/g-C3N4
70
24%-WO3/g-C3N4 36%-WO3/g-C3N4
60
sulfur removal(%)
50%-WO3/g-C3N4 WO3
50
g-C3N4
40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
t/min
Fig.5
80 70
sulfur removal(%)
60 50 40 30 o
50 C o 60 C o 70 C o 80 C
20 10 0 0
20
40
60
80
100
t/min
Fig.6
19
120
140
160
180
200
90
0.1mL H2O2
sulfur removal(%)
80
0.2mL H2O2
70
0.3mL H2O2
60
0.4mL H2O2
50 40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
t/min
Fig.7 90
0.01g 0.02g 0.03g 0.04g
80
sulfur removal(%)
70 60 50 40 30 20 10 0
20
40
60
80
100
t/min
Fig.8
20
120
140
160
180
200
100 90
DBT BT TH
sulfur removal(%)
80 70 60 50 40 30 20 10 0
20
40
60
80
100
120
140
160
180
t/min
Fig.9
90
sulfur removal(%)
80 70 60 50 40 30 20 10 0 1
2
3
Reaction recycle
Fig.10
21
4
5
200
intensity/(a.u)
36%-WO3/g-C3N4
Used 36%-WO3/g-C3N4
10
20
30
40
50
60
70
O
2q/( )
Transmittance
Fig.11
A
B
1047.43 1288.77
1500
1400
1166.74 1300
1200 -1
Wavenumber(cm )
Fig.12
22
1100
1000
S
oil phase IL phase H 2O 2
H 2O
WO3/g-C3N 4 S
S O
Fig.13
23
O
Table caption Table 1. Specific surface area and average pore size of sample
Table 2 Effect of adding amount of ionic liquid on the desulfurization rate Conditions: Voil=5 ml, VH2O2 = 0.3mL, 0.03g of 36%-WO3/g-C3N4, t= 180 min, T=333K
Table 1. Sample
Surface(m2/g)
Pore Volume(cm3/g)
Pore size(nm)
WO3
4.3051
0.0172
1.9393
12%-WO3/C3N4
42.5364
0.1275
11.9974
24%-WO3/C3N4
41.5262
0.1266
12.1953
36%-WO3/C3N4
31.5544
0.09975
11.8920
50%-WO3/C3N4
24.0876
0.08229
13.6651
C3N4
18.2056
0.07819
17.1803
Table 2 entry
1
2
3
4
IL(mL)
0
0.25
0.5
0.75
Sulfur removal(%)
4.01
91.2
87.2
86.5
Conditions: Voil=5 ml, VH2O2 = 0.3mL, 0.03g of 36%-WO3/g-C3N4, t= 180 min, T=333K
24
S0169-4332(16)31763-9 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.120 APSUSC 33869
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-3-2016 19-8-2016 22-8-2016
Please cite this article as: Rongxiang Zhao, Xiuping Li, Jianxun Su, Xiaohan Gao, Preparation of WO3/g-C3N4 Composites and Their Application in Oxidative Desulfurization, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of WO3/g-C3N4 Composites and Their Application in Oxidative Desulfurization Rongxiang Zhao1, Xiuping Li, Jianxun Su, Xiaohan Gao ( College of Chemistry, Chemical Engineering and Environment Engineering, Liaoning Shihua University, 113001, China) 1
Author introduction: Rongxiang hao (1971), male, PhD.
Email:[email protected]
Telephone:+8613470542149 Corresponding author: Xiuping Li(1975-),female,ph D, email: [email protected]
Graphical abstract The WO3/g-C3N4 composite was successfully synthesized through directly calcining phosphotungstic and melamine. The WO3 is one of several good catalyst for oxidation
desulfurization. But, the surface area of WO3 is small and short of active site. Taking into account the catalyst with large surface area generally has a high catalytic activity. The surface area of WO3/g-C3N4 composite is ten times as big as the WO3.The crystalline of WO3/g-C3N4 composites was obviously improved. Desulfurization experiment showed that the desulfurization rate of simulated oil can reach 91.2% under optimal conditions. Meanwhile, the activity of the catalyst was not significantly decreased after the 5 recycles. The result show that stability of catalyst is high.
Highlights
The WO3/g-C3N4 was successfully synthesized through simple calcination. The process is simple and the cost is cheap. The WO3/g-C3N4 firstly applied to ODS. The desulpurization rate of WO3/g-C3N4 may attach to 91.2%. Five recycles of WO3/g-C3N4 still attach to 89.5%.
1
The crystalline and surface area of composite are obviously improved by g-C3N4.
Abstract: WO3/graphitic carbon nitride (g-C3N4) composites were successfully synthesized through direct calcining of a mixture of WO3 and g-C3N4 at 400°C for 2 h. The WO3 was prepared by calcination of phosphotungstic acid at 550°C for 4 h, and the g-C3N4 was obtained by calcination of melamine at 520°C for 4 h. The WO3/g-C3N4 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and Brunner−Emmett−Teller analysis (BET). The WO3/g-C3N4 composites exhibited stronger XRD peaks of WO3 and g-C3N4 than the WO3 and pure g-C3N4. In addition, two WO3 peaks at 25.7° and 26.6° emerged for the 36% -WO3/g-C3N4 composite. This finding indicated that WO3 was highly dispersed on the surface of the g-C3N4 nanosheets and interacted with the nanosheets, which resulted in the appearance of (012) and (022) planes of WO3. The WO3/g-C3N4 composite also exhibited a larger specific surface area and higher degree of crystallization than WO3 or pure g-C3N4, which resulted in high catalytic activity of the catalyst. Desulfurization experiments demonstrated that the desulfurization rate of dibenzothiophene (DBT) in model oil reached 91.2% under optimal conditions. Moreover, the activity of the catalyst was not significantly decreased after five recycles. Keyword: WO3/g-C3N4; Composites; Oxidation Desulfurization;
Introduction WO3 has been applied to the oxidative desulfurization of fuel oil because of its thermal stability and good crystallinity[1]. However, WO3 has a small surface area, which results in a shortage of active sites. Therefore, scholars have investigated heterogeneous catalysts containing tungsten oxide, such as WO3–SBA–15[2], WOx/ZrO2[3], SiO2–WO3[4], and WO3–Al2O3[5], and applied them to the oxidation desulfurization of fuel oil. However, expensive raw materials and the complex preparation process of the catalysts limit the industrial development of tungsten oxide. 2
Therefore, there is an urgent need to develop a simple preparation process for a heterogeneous catalyst containing tungsten oxide using inexpensive raw materials. Graphitice carbon nitride (g-C3N4) is a well-known polymer that has the advantages of low density, high chemical stability, good bio-compatibility, and high wear resistance; therefore, it shows potential for a wide range of high-performance applications such as wear resistant coatings, membrane materials, catalysts, and catalyst carriers[6]. Recently, g-C3N4 has been increasingly applied to the degradation of organic pollutants by photocatalysis[7-11]. Researchers have found that mixtures of metal oxides and g-C3N4 have greater surface areas than metal oxides or pure g-C3N4 alone after grinding and calcination, which clearly indicates higher catalytic activity. Recently, Wang et al.[12] synthesized TiO2/g-C3N4 and applied it to oxidation desulfurization under UV irradiation. The result indicated that the removal rate of dibenzothiophene (DBT) could reach 98.9%. In addition, phosphotungstic acid (HPW)/g-C3N4 was synthesized by Zhu et al.[13] via phosphorus acid loading on the carbon nitride. The HPW/g-C3N4 catalyst exhibited a high catalytic activity in the oxidation desulfurization process. The DBT could be completely removed under optimal reaction conditions, and no significant decrease in the catalytic activity of the catalyst was observed after 15 recycles. Zhu W. et al[14-19] reported a lot of desulfurization using ionic liquid and composite of Tungsten. Inspired by the above method, in this work, WO3/g-C3N4 composites were prepared and applied to the oxidation desulfurization of fuel oil. To the best of our knowledge, no previous studies on the application of WO3/g-C3N4 in the oxidation desulfurization process have been reported. In this study, WO3/g-C3N4 was prepared by direct calcination of inexpensive HPW and melamine, and the desulfurization performance of the catalyst was investigated in detail. The catalyst using simple preparation method exhibits higher desulfurization performance than that of other WO3 heterogeneous catalysts[2-5].
1 Experiment 1.1 Materials
3
Dibenzothiophene(DBT), thiophene(TH) and benzothiophene(BT) were purchased from Sigma-Aldrich. H2O2, melamine, phosphotungstic acid and n-octane were purchased
from
Sinopharm
Chemical
Reagent.
1-ethyl-3-methylimidazolium
diethylsulfate was purchased from Shanghai Chengjie Chemical Co. LTD. 1.2 Preparation of WO3/g-C3N4 catalyst The WO3 was synthesized via a decomposition reaction of solid-state HPW at 550°C for 4 h. The g-C3N4 powders were prepared by directly heating melamine at 520°C in a muffle furnace for 4 h under air conditions. The synthesis of the WO3/g-C3N4 composite was as follows: WO3 and g-C3N4 at a certain mass ratio were mixed and ground for 20 min using a mortar and pestle. The mixed powder was placed in a crucible with a cover and heated at 400°C in a muffle furnace for 4 h. Finally, composites containing 12%, 24%, 36%, and 50% WO3 (mass%) were obtained. 1.3 Characterization X-ray diffraction (XRD) patterns were recorded on an XRD7000 (Shimadzu, Japan). The diffractometer was equipped with a Ni-filtered Cu-Kα radiation source (λ=1.5406 Å) at 40 kV and 40 mA, step 0.02 degrees, the scanning range: 10°~70°. Scanning electron microscopy (SEM, JEOL 6701F) were used to observed the morphology of samples. The FT-IR spectra were recorded with a Nicolet Nexus 470 FT-IR instrument using KBr pellets. The surface area analysis was performed from the nitrogen adsorption isotherms at 77 K with a Micromeritics Model ASAP 2020 instrument. The average pore diameter and pore volume of samples were calculated based on the Barrett−Joyner−Halenda (BJH) method. 1.4 Oxidation desulfurization The model oil (500 ppm) was prepared by dissolving 2.9018 g DBT in 1 L of n-octane. The oxidation desulfurization reaction was performed in a conical flask with a condensing pipe. The oxidation desulfurization system, which contained 5 mL of model oil, a certain amount of ionic liquid, 30% H2O2, and the catalyst were stirred using magnetic stirring at a certain temperature. After the reaction, the mixture was
4
static until the solution were layered and cooled in air. A small amount of the oil phase was collected, and the sulfur concentration was determined using a WK-2D micro–coulomb analyzer. 2 Results and Discussion 2.1 X-ray diffraction characterization The X-ray diffraction (XRD) patterns of the WO3/g-C3N4 composites with different WO3 contents, pure g-C3N4, and WO3 are presented in Fig.1. The characteristic diffraction peaks of the pure g-C3N4 appearing at 27.4° and 13.1° can be indexed to the (002) and (100) planes, which are attributed to the characteristic interlayer structure and interplanar stacking peaks of aromatic systems[20], respectively. The characteristic diffraction peaks of the WO3 can be attributed to the monoclinic structure (JCPDF 43-1035). The characteristic diffraction peaks of WO3 and g-C3N4 also appeared in the XRD patterns of the WO3/g-C3N4 composites with different WO3 contents, which indicates that the structures of WO3 and g-C3N4 were not damaged during the calcination. The WO3 peaks became stronger and the C3N4 peaks became weaker upon increasing the WO3 content in the composites. In addition, two WO3 peaks at 25.7° and 26.6° emerged in the 36% WO3/g-C3N4 composite, which are attributed to the (012) and (022) planes of WO3[21], respectively. This finding indicates that WO3 was highly dispersed on the surface of the g-C3N4 nanosheets and interacted with the nanosheets. The g-C3N4 peak at 27.4° shifted slightly toward higher diffraction angles with increasing WO3 content, which indicates that the entering of WO3 into the interlayers of pure g-C3N4 resulted in a slight expansion of the interlayer[20]. These results demonstrate that the interaction between WO3 and g-C3N4 can enhance their performance, which resulted in the 36%WO3/g-C3N4 composite exhibiting better crystallinity and performance. These results may also lead to the 36%WO3/g-C3N4 composite exhibiting better desulfurization performance, which will be discussed in Section 2.5. 2.2 SEM analysis The SEM images of WO3, g-C3N4 and the WO3/g-C3N4 with different concentration of WO3 are shown in Fig.2. As shown in Fig.2 (f, g, h), WO3 is a 5
smaller particle size with 100-150 nm. However, there was a serious agglomeration of nanoparticles, which is caused by the high temperature calcination. The morphology of the g-C3N4 is observed in Figure 2 (a, e). From Fig. 2(a, e), it can be observed that g-C3N4 has an irregular layer-structure with powder. The distribution of WO3 nanoparticles in g-C3N4 was seen in Figure 2 (b, c, d). Some nanoparticles of WO3 can be found in the picture, they own the same size as WO3 in Fig.2(f, g, h). The 36% WO3/g-C3N4 show an obvious breaking up comparing to WO3. Connecting with the results of XRD, it can be further confirmed that WO3 can be dispersed on the g-C3N4 nanosheets. 2.3 Fourier-transform infrared spectroscopy analysis The Fourier-transform infrared (FT-IR) spectra of WO3, g-C3N4, and the WO3/g-C3N4 composites with different WO3 contents are presented in Fig. 3. For the pure g-C3N4, the peaks at 1236, 1309, 1435, and 1570 cm−1 can be attributed to aromatic C–NH–C stretching[22]. The peaks at 1645 cm−1 can be attributed to C=N stretching vibration modes[23]. For the FT-IR spectrum of pure WO3, a broad peak appears at 750–1000 cm-1, which corresponds to O–W–O stretching vibration in a monoclinic-type WO3 crystal[20]. For WO3/g-C3N4 with different WO3 contents, all the characteristic peaks of g-C3N4 are displayed in the spectra. However, the characteristic peaks of WO3 were not observed, which may due to the WO3 entering the interlattice of g-C3N4 or overlapping peaks. The peak at 3420 cm−1 is attributed to stretching vibration of –OH in water adsorbed on the surface of WO3 and g-C3N4. This result indicates that WO3 and g-C3N4 easily absorb water molecules in air because their charge is not balanced. However, the shift of the –OH peaks of the composites from 3420 to 3216 cm-1 is attributed to from the intermolecular association change into intramolecular association of –OH. The above results imply that WO3 may enter the interlattice of the g-C3N4 nanosheets and interact with triazine units, leading to intramolecular association of –OH. 2.4 BET analysis The specific surface areas of the WO3, pure g-C3N4, and WO3/g-C3N4 with different WO3 contents are listed in Table 1. The surface areas of WO3 and g-C3N4 were 4.3051 6
and 18.2056 m2/g, respectively. The specific surface area of the WO3/g-C3N4 composites decreased with increasing WO3 content with values of 42.5364, 41.5262, 31.5564, and 24.0876 m2/g for the 12%, 24%, 36%, and 50% WO3 (mass%) composites, respectively. The specific surface areas of the composites increased after calcination of the mixtures of WO3 and g-C3N4. This finding may be related to the WO3 being well dispersed on the g-C3N4 nanosheets, entering the interlayer, and interacting with triazine units of the nanosheets, resulting in expansion of the interlayer, which further leads to an increase of the surface areas of the composites[24]. The 12%WO3/g-C3N4 composite exhibited the largest surface area because the g-C3N4 peak for this composite is the highest in Fig. 1 with the maximum dispersion and exposure of g-C3N4. It is generally believed that a larger catalyst surface area can provide more active sites for the reaction system, which can further enhance the activity of the composite. However, g-C3N4 cannot catalyze and oxide DBT into DBTO2. The N2 adsorption–desorption curve of the composites, WO3, and g-C3N4 are presented in Fig. 4. The surface areas of the composites are greater and their N2 adsorption–desorption curves are higher than those of WO3 and g-C3N4. These results indicate that the larger surface areas of the catalysts yield stronger absorption abilities of the organic compounds. However, the WO3 content is also an important factor because WO3 is catalyst for the formation of the peroxide (DBTO2). In addition, the improved crystallinity and emergence of additional planes for 36%WO3/g-C3N4 resulted in enhanced desulfurization performance. 2.5 Investigation of desulfurization 2.5.1 Effect of catalyst on desulfurization The effect of the different catalysts on desulfurization are shown in Fig. 5. The desulfurization rate was only 14% with g-C3N4 because of the absorption of g-C3N4. The desulfurization rate of the composites increased from 48.4% to 65.4% upon increasing the WO3 content from 12% to 36%, respectively. The 36%WO3/g-C3N4 composite exhibited the highest desulfurization performance, with the desulfurization rate decreasing to 59.6% for the 50%WO3/g-C3N4 catalyst. Based on these results, it is apparent that the surface area of the sample is not the only factor that affects the 7
desulfurization rate. For example, the 12% WO3/g-C3N4 catalyst exhibited the largest surface area; however, the desulfurization rate was less than 50%. The specific surface area of pure WO3 was the smallest of all the samples, whereas the desulfurization rate was the same as that of 12%WO3/g-C3N4. In addition, the reaction velocity of g-C3N4 was faster than that of WO3. Thus, it can be observed that the WO3 content, surface area, and crystallinity of the products have important effects on the desulfurization performance of catalyst. Based on the analytical results, the 36%WO3/g-C3N4 composite was selected as the catalyst for the oxidative desulfurization process. 2.5.2 Effect of temperature on desulfurization The reaction temperature is a key parameter for the desulfurization system. Oxidative desulfurization experiments were performed at 50°C, 60°C, 70°C, and 80°C, and the results are presented in Fig. 6. Increasing the reaction temperature resulted in an obvious increase of the desulfurization rate of the system. With extension of the reaction time, the desulfurization rate of the system reached 74% at 60°C in 180 min. The sulfur removal rate was not enhanced at 70°C and 80°C after a certain reaction time. Although increasing the reaction temperature is beneficial for the oxidative desulfurization process, H2O2 is decomposed at high temperatures[25], and the desulfurization rate of the system no longer increases because of the absence of an oxide agent. Therefore, 60°C was selected as the optimal temperature of the catalytic system as the catalyst exhibited lower activity at 50°C. 2.5.3 Effect of H2O2 dose on desulfurization The amount of H2O2 is other important factor for the removal rate of sulfur. Fig. 7 shows that upon increasing the H2O2 amount from 0.1 to 0.3 mL, the removal rate of DBT increased from 48.9% to 84.2% in 180 min, respectively. However, when the amount of H2O2 was further increased to 0.4 mL, the removal rate of DBT decreased to 78.6% in 180 min. This result can be explained by the presence of two reactions in the oxidative desulfurization system, the oxidation of DBT and decomposition of H2O2[26-27]. Excessive H2O2 was added to the oxidative desulfurization system for certain DBT completely oxidized and H2O2 was surplus. Therefore, the concentration of the ionic liquid in the system is decreased and the extraction ability of the ionic 8
liquid is weakened. In addition, water from the decomposition of surplus H2O2 hinders the oxidation and extraction of DBT in the system. Therefore, too much H2O2 results in a decrease in the desulfurization rate. 2.5.4 Effect of catalyst dose on desulfurization The significant effect of the catalyst dose on the desulfurization rate is demonstrated in Fig. 8. Upon increasing the catalyst dose from 0.01 to 0.03 g, the desulfurization rate clearly increased with the reaction time extended. The optimal catalyst dose was 0.03 g in 5 mL of model oil, and the desulfurization rate of oxidative desulfurization system reached 86.7% in 180 min. Further increase of the catalyst dose to 0.04 g had no effect on the desulfurization rate for a reaction time of 180 min. 2.5.5 Effect of ionic liquid dose on desulfurization In this experiment, ionic liquid was the extraction agent of the oxidative desulfurization system, and the ionic liquid dose had a significant effect on the desulfurization rate. Ionic liquid doses of 0, 0.25, 0.50, and 0.75 mL were used, and the experimental results are listed in Table 2. Without the addition of the ionic liquid, the desulfurization rate of the system was 40.1%. Upon increasing the dose to 0.25 mL, the desulfurization rate reached 91.2%. However, when an excessive amount of ionic liquid was added to the system, the desulfurization rate decreased to 87.2% (0.5 mL) and 86.5% (0.75 mL). This finding may be due to the decrease of the ratio of oxygen to sulfur in the system because more DBT can be extracted into the ionic liquid phase upon increasing the ionic liquid content, whereas the amount of H2O2 is unchanged, similar to previous findings in the literature[28]. 2.5.6 Effect of substrate on desulfurization To study the effect of 36%WO3/g-C3N4 on different sulfur compounds, benzothiophene (BT), DBT, and thiophene (TH) were selected as model oils in the oxidative desulfurization process under the same experimental conditions, and the results are presented in Fig. 9. The order of the desulfurization rate was DBT > BT > TH, which is consistent with previous results in the literature
[29]
. The oxidative
desulfurization activity of the model oil depends on the electron cloud density of 9
sulfur atoms in the organic sulfur compounds. In general, a lower electron density of sulfur atoms results in a lower desulfurization rate. The electron densities of the sulfur atoms are 5.785 (DBT), 5.739 (BT), and 5.696 (TH). In the oxidative desulfurization system, the sulfide is first absorbed by the catalyst. The adsorption capacity of the catalyst is related to the structure of the catalyst, including its specific surface area, pore volume, and pore size. Among the three sulfides, the space structure and steric hindrance of TH are the smallest and those of DBT are the largest. Therefore, TH is the most easily absorbed by the catalyst, whereas DBT is the most difficult to absorb. The removal rate of TH is thus the highest, that of DBT is the lowest, and that of BT is in between. In addition, the TH and ionic liquid have similar polarity and TH easily was extracted[29] too. However, with the extension of the reaction time, the oxidation desulfurization is dominant. Therefore, the order of the desulfurization rate still follows DBT > BT > TH. 2.5.7 Repeated use of catalyst and ionic liquids The repeated use of the catalyst and recovery of ionic liquid holds great significance for industrial application. Thus, the repeatability of use of the catalyst and ionic liquids were investigated, and the results are presented in Fig. 10. The repeated use of the catalyst and ionic liquids were tested as the following procedures: the ionic liquid phase and oil phase were separated by centrifugation from the reaction system after each run. Extraction of the sulfur compounds in the ionic liquid phase was performed three times with a certain amount of CCl4. Then, fresh H2O2, the model oil, and ionic liquid containing the catalyst (recovered ionic liquid and catalyst) were directly added to the next run under the same conditions. The results indicated that the sulfur removal rate remained as high as 87.5% after five recycles. To verify the stability of the 36%WO3/g-C3N4, the XRD pattern of the recovered catalyst was compared with that of the fresh catalyst, as shown in Fig. 11. The structure of the recovered catalyst did not exhibit any obvious change. The diffraction peaks of some impurities in the range of 10°–23° can be attributed to oxidation products of organic sulfur adsorbed on the catalyst surface. The results indicate that the oxidative desulfurization system containing ionic liquid and 36%WO3/g-C3N4 is a promising 10
reaction system. To determine the type of sulfur oxidation products, a reverse extraction experiment was performed using CCl4 as the extraction agent. First, the model oil from the oxidation desulfurization system was separated using a separating funnel. Then, the water was evaporated by rotary evaporation. Next, the ionic liquid was extracted by the CCl4, and a white solid was obtained and characterized using FT-IR spectroscopy. Compared with DBT, the white solid exhibited absorption peaks of sulfone at 1288 and 1166 cm-1 and absorption peaks of sulfoxide at 1047 cm-1, which are consistent with results in the literature[30]. This finding suggests that DBT was oxidized to DBTO and DBTO2 using the 36%WO3/g-C3N4 as a catalyst with the assistance of H2O2. 2.5.8 Mechanism of oxidative desulfurization reaction of WO3/g-C3N4 The mechanism of the oxidative desulfurization reaction from DBT to DBTO2 is illustrated in Fig. 13. Two phases, the ionic liquid and model oil, are presented in the reaction system. The WO3/g-C3N4 and H2O2 in the ionic liquid were immiscible with the model oil. The ionic liquid plays an important role in oxidative desulfurization. In the absence of the ionic liquid, the rate of desulfurization was only 40%. The removal rate of sulfur increased sharply in the presence of the ionic liquid, which demonstrated that DBT was oxidized in the ionic liquid phase. First, WO3 was oxidized to H2WO2(O2)2 under the action of H2O2[24]. The H2WO2(O2)2 exhibited higher oxidation activity than WO3. The sulfur compounds in the model oil were oxidized to their corresponding sulfones by the H2WO2(O2)2 in the ionic liquid under stirring; then, the sulfones were extracted into the ionic liquid phase. Note that g-C3N4 has a special effect on the desulfurization process. The surface area and crystallinity of the catalyst were improved when using the WO3/g-C3N4 composite. In addition, g-C3N4 can activate the H2O2[31] form hydroxyl free radicals. 3. Conclusion WO3/g-C3N4 composites with different WO3 contents were prepared via direct calcination. The specific surface areas of the composites were larger than those of WO3 or pure g-C3N4, and their crystallinities were substantially increased compared 11
with that of WO3. In particular, the 36%WO3/g-C3N4 exhibited XRD peaks for (012) and (022) planes. The crystallinity of the composites had a significant effect on the desulfurization performance. Ionic liquids with various catalysts can oxidize DBT to DBTO2 with assistance of H2O2 for deep desulfurization under moderate conditions. The sulfur removal rate of the model oil containing DBT in reached 91.2% with the 36%WO3/g-C3N4 catalyst and the assistance of H2O2 at 60°C for 180 min. This finding demonstrates the remarkable advantage of this process over desulfurization using WO3 alone as the catalyst. The 36%WO3/g-C3N4 catalyst could also be recovered and reused five times without a significant decrease in activity. Acknowledgements The authors also acknowledge the financial support of the Natural Science Foundation of China (Project no. 21003069) and Liaoning Province Doctoral Fund (Project no.201501105) References [1] Rakhmanov E V, Tarakanova A V, Valieva T, et al. Oxidative desulfurization of diesel fraction with hydrogen peroxide in the presence of catalysts based on transition metals[J]. Petroleum Chemistry, 54(2014): 48-50. [2] Li X, Huang S, Xu Q, et al. Preparation of WO3-SBA-15 mesoporous molecular sieve and its performance as an oxidative desulfurization catalyst[J]. Transition metal chemistry, 34(2009): 943-947. [3] Torres-Garcia E, Canizal G, Velumani S, et al. Influence of surface phenomena in oxidative desulfurization with WOx/ZrO2 catalysts[J]. Applied Physics A, 79(2004): 2037-2040. [4] Song H, Mu J, Wang D, et al. Preparation and Characterization of SiO2-WO3 Mixed Oxides and Its Catalytic Performance in Oxidative Desulfurization of Benzothiophene[J]. Acta Petrolei Sinica (Petroleum Processing Section), 5(2012): 750-756. [5] Li X, Zhu H, Wang A, et al. Oxidative Desulfurization of Dibenzothiophene over Tungsten Oxides Supported on SiO2 and γ-Al2O3[J]. Chemistry Letters, 42(2013): 8-10. [6] Thomas A, Fischer A, Goettmann F, et al. Graphitic carbon nitride materials: variation of
12
structure and morphology and their use as metal-free catalysts[J]. Journal of Materials Chemistry, 18(2008): 4893-4908. [7] Zang Y, Li L, Zuo Y, et al. Facile synthesis of composite g-C3N4/WO3: a nontoxic photocatalyst with excellent catalytic activity under visible light[J]. RSC Advances, 3(2013): 13646-13650. [8] Huang L, Li Y, Xu H, et al. Synthesis and characterization of CeO2/gC3N4 composites with enhanced visible-light photocatatalytic activity[J]. RSC Advances, 3(2013): 22269-22279. [9] Chen L Y, Zhang W D. In2O3/gC3N4 composite photocatalysts with enhanced visible light driven activity[J]. Applied Surface Science, 301(2014): 428-435. [10] Sun J X, Yuan Y P, Qiu L G, et al. Fabrication of composite photocatalyst g-C3N4–ZnO and enhancement of photocatalytic activity under visible light[J]. Dalton Transactions, 41(2012): 6756-6763. [11] Huang L, Xu H, Zhang R, et al. Synthesis and characterization of g-C 3N4/MoO3 photocatalyst with improved visible-light photoactivity[J]. Applied Surface Science, 2013, 283: 25-32. [12] Wang C, Zhu W, Xu Y, et al. Preparation of TiO2 /g-C3N4 composites and their application in photocatalytic oxidative desulfurization[J]. Ceramics International, 2014, 40(8): 11627-11635. doi:10.1016/j.ceramint.2014.03.156 [13] Zhu Y, Zhu M, Kang L, et al. Phosphotungstic acid supported on mesoporous graphitic carbon nitride as catalyst for oxidative desulfurization of fuel[J]. Industrial & Engineering Chemistry Research, 2015, 54(7): 2040-2047. [14] Zhu W., Dai B. , Wu P, et al.Graphene-Analogue Hexagonal BN Supported with Tungsten-based Ionic Liquid for Oxidative Desulfurization of Fuels[J].ACS Sustainable Chem. Eng., 2015, 3 (1):186–194. DOI: 10.1021/sc5006928 [15] P Wu, W Zhu, A Wei, et al.Controllable Fabrication of Tungsten Oxide Nanoparticles Confined in Graphene-Analogous Boron Nitride as an Efficient Desulfurization Catalyst[J].Chem. Europ. J., 2015, 21(43): 15421-15427. DOI: 10.1002/chem.201501413 [16] Q Gu, W Zhu, S Xun, et al. Preparation of highly dispersed tungsten species within mesoporous silica by ionic liquid and their enhanced catalytic activity for oxidative desulfurization [J].Fuel, 2014, 117, 667-673.doi:10.1016/j.fuel.2013.08.082
13
[17] Zhang M, Zhu W, Li H, et al. One-pot synthesis, characterization and desulfurization of functional mesoporous W-MCM-41 from POM-based ionic liquids[J]. Chemical Engineering Journal, 2014, 243: 386-393.doi:10.1016/j.cej.2013.12.093 [18] Zhu W, Li H, Jiang X, et al. Commercially available molybdic compound-catalyzed ultra-deep desulfurization of fuels in ionic liquids[J]. Green chemistry, 2008, 10(6): 641-646. DOI: 10.1039/B801185K [19]Zhu W. , Li H. , Jiang X. et al. Oxidative Desulfurization of Fuels Catalyzed by Peroxotungsten
and
Peroxomolybdenum
Complexes
in
Ionic
Liquids[J].Energy
Fuels, 2007, 21 (5):2514–2516 DOI: 10.1021/ef700310r [20] Ding J, Liu Q, Zhang Z, et al. Carbon nitride nanosheets decorated with WO3 nanorods: Ultrasonic-assisted facile synthesis and catalytic application in the green manufacture of dialdehydes[J]. Applied Catalysis B: Environmental, 165(2015): 511-518. [21] Mo Ruofei, Jin Guoqiang, Guo Xiaongyun.Hydrothermal Synthesis of Tungsten Trioxides Using Citric Acid as Controlling Agent,Chinese Journal of Inorganic Chemistry, 23(2007): 1615-1620. [22] Khabashesku V N, Zimmerman J L, Margrave J L. Powder synthesis and characterization of amorphous carbon nitride[J]. Chemistry of materials, 12(2000): 3264-3270. [23] Li X, Zhang J, Shen L, et al. Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine[J]. Applied Physics A, 94(2009): 387-392. [24] He Y, Zhang L, Wang X, et al. Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3-g-C3N4 composite under visible light irradiation[J]. RSC Advances, 4(2014): 13610-13619. [25] Li F, Wu B, Liu R, et al. An inexpensive N-methyl-2-pyrrolidone-based ionic liquid as efficient extractant and catalyst for desulfurization of dibenzothiophene[J]. Chemical Engineering Journal,274( 2015): 192-199. [26] Chen X, Song D, Asumana C, et al. Deep oxidative desulfurization of diesel fuels by Lewis acidic ionic liquids based on 1-n-butyl-3-methylimidazolium metal chloride[J]. Journal of Molecular Catalysis A: Chemical, 359(2012): 8-13. [27] Zhou Q, Fu S, Zou M, et al. Deep oxidative desulfurization of model oil catalyzed by magnetic MoO 3/Fe 3 O 4[J]. RSC Advances,5(2015): 69388-69393. 14
[28] Chen X, Song D, Asumana C, et al. Deep oxidative desulfurization of diesel fuels by Lewis acidic ionic liquids based on 1-n-butyl-3-methylimidazolium metal chloride[J]. Journal of Molecular Catalysis A: Chemical, 2012, 359: 8-13. [29] Li HP, Zhu WS,Zhu SW, et al. The selectivity for sulfur removal from oils: An insight from conceptual density functional theory[J].AIChE J.2016,62(6):2087-2100.DOI: 10.1002/aic.15161 [30] Seubert A. A critical comparison of on-line coupling IC-ICP-(AES, MS) with competing analytical methods for ultra trace analysis of microelectronic materials[J]. Fresenius journal of analytical chemistry, 364(1999): 404-409. [31] Cui Y, Ding Z, Liu P, et al. Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants[J]. Physical Chemistry Chemical Physics, 14(2012 ): 1455-1462.
15
Figure caption Fig.1 XRD pattern of g-C3N4,WO3 and the WO3/g-C3N4 with different concentration of WO3. Fig.2 SEM images of (a,e) g-C3N4, (b,c,d) 36%-WO3/g-C3N4 and (f,g,h) WO3. Fig.3 FT-IR spectra of g-C3N4,WO3 and WO3/g-C3N4 with different concentration of WO3. Fig.4 The Nitrogen adsorption-desorption curve of the composite, WO3 and g-C3N4 Fig.5 influence of catalytic type on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL, 0.02g of catalysts, VH2O2 = 0.2mL, t= 180 min,T= 343 K, Fig.6 the influence of temperature on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL, 0.02g of 36%-WO3/g-C3N4, VH2O2 = 0.2mL, t= 180 min Fig.7 the influence of H2O2 dose on desulphurization activity Conditions: Voil=5 ml,VIL=0.75 mL,0.02g of 36%-WO3/g-C3N4, t= 180 min, T=333K Fig.8 influence of catalyst dose on desulphurization activity Conditions: Voil=5 ml,VIL= 0.75 mL, VH2O2 = 0.3mL, Fig.9
t= 180 min, T=333K
influence of substrate on sulfur removal
Fig.10 Recycle of the reaction system Fig.11 XRD of catalysts before use and after use. Fig.12 IR spectra of DBT (A) and its oxidation products (B). Fig.13 Oxidation desulfurization mechanism of reaction system containing WO3/g-C3N4 catalyst
16
WO3
28.7
12%-WO3/g-C3N4 28.7
Intensity(a.u)
24%-WO3/g-C3N4 26.6 28.7
36%-WO3/g-C3N4 28.7
50%-WO3/g-C3N4
(100)
(002)
25.7
10
20
30
g-C3N4 40
50
60
70
O
2θ/( ) Fig.1 Fig.1
a2.2 SEM analysis
b
c
d
g
h
Fig.2
e
f
Fig.2
17
1545
12%-WO3/g-C3N4
1684
24%-WO3/g-C3N4 880 1463 1309
Transmittance(%)
810
36%-WO3/g-C3N4
12361435 3100-3300
50%-WO3/g-C3N4 g-C3N4 13091570
WO3 3300-3600
1645 1236
805 500
1000
1435
1500
2000
2500
3000
3500
-1
Wavenumber(cm )
Fig.3
120
3
□ ○
●
Volume adsorbed(cm g-1)
■
100 80
△ ▽ ☆
WO3 12%-WO3/C3N4 24%-WO3/C3N4 36%-WO3/C3N4 50%-WO3/C3N4 C3N4
60 40 20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
relative pressure(P/P0) Fig.4
18
4000
12%-WO3/g-C3N4
70
24%-WO3/g-C3N4 36%-WO3/g-C3N4
60
sulfur removal(%)
50%-WO3/g-C3N4 WO3
50
g-C3N4
40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
t/min
Fig.5
80 70
sulfur removal(%)
60 50 40 30 o
50 C o 60 C o 70 C o 80 C
20 10 0 0
20
40
60
80
100
t/min
Fig.6
19
120
140
160
180
200
90
0.1mL H2O2
sulfur removal(%)
80
0.2mL H2O2
70
0.3mL H2O2
60
0.4mL H2O2
50 40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
t/min
Fig.7 90
0.01g 0.02g 0.03g 0.04g
80
sulfur removal(%)
70 60 50 40 30 20 10 0
20
40
60
80
100
t/min
Fig.8
20
120
140
160
180
200
100 90
DBT BT TH
sulfur removal(%)
80 70 60 50 40 30 20 10 0
20
40
60
80
100
120
140
160
180
t/min
Fig.9
90
sulfur removal(%)
80 70 60 50 40 30 20 10 0 1
2
3
Reaction recycle
Fig.10
21
4
5
200
intensity/(a.u)
36%-WO3/g-C3N4
Used 36%-WO3/g-C3N4
10
20
30
40
50
60
70
O
2q/( )
Transmittance
Fig.11
A
B
1047.43 1288.77
1500
1400
1166.74 1300
1200 -1
Wavenumber(cm )
Fig.12
22
1100
1000
S
oil phase IL phase H 2O 2
H 2O
WO3/g-C3N 4 S
S O
Fig.13
23
O
Table caption Table 1. Specific surface area and average pore size of sample
Table 2 Effect of adding amount of ionic liquid on the desulfurization rate Conditions: Voil=5 ml, VH2O2 = 0.3mL, 0.03g of 36%-WO3/g-C3N4, t= 180 min, T=333K
Table 1. Sample
Surface(m2/g)
Pore Volume(cm3/g)
Pore size(nm)
WO3
4.3051
0.0172
1.9393
12%-WO3/C3N4
42.5364
0.1275
11.9974
24%-WO3/C3N4
41.5262
0.1266
12.1953
36%-WO3/C3N4
31.5544
0.09975
11.8920
50%-WO3/C3N4
24.0876
0.08229
13.6651
C3N4
18.2056
0.07819
17.1803
Table 2 entry
1
2
3
4
IL(mL)
0
0.25
0.5
0.75
Sulfur removal(%)
4.01
91.2
87.2
86.5
Conditions: Voil=5 ml, VH2O2 = 0.3mL, 0.03g of 36%-WO3/g-C3N4, t= 180 min, T=333K
24