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Article

The Green Preparation of Mesoporous WO3/SiO2 and Its Application in Oxidative Desulfurization

by
Yinhai Zhang
1,
Xiaoxue Liu
2,*,
Ruyu Zhao
1,
Jingwei Zhang
1,
Lanfen Zhang
1,
Wei Zhang
1,
Jian Hu
1 and
Hao Li
1,*
1
College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, China
2
College of Agriculture, Yangtze University, Jingzhou 434000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(2), 103; https://doi.org/10.3390/catal14020103
Submission received: 29 December 2023 / Revised: 20 January 2024 / Accepted: 24 January 2024 / Published: 26 January 2024
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
Recently, supported WO3-based catalysts have been widely used in oxidative desulfurization (ODS) due to their advantages of easy separation, high activity, and being environment-friendly. In this work, supported mesoporous WO3/SiO2 catalysts have been prepared using an incipient-wetness impregnation method with agricultural waste rice husks as both a silicon source and mesoporous template, and phosphotungstic acid as a tungsten source. The effects of different calcination temperatures and WO3 loadings on the ODS performance of samples are studied, and the appropriate calcination temperature and WO3 loading are 923 K and 15.0 wt.%, respectively. The relevant characterization results show that, compared with pure WO3, the specific surface area and mesopore volume of WO3/SiO2 samples are greatly increased. Due to (a) high WO3 loading, (b) high specific surface area, and (c) nanoscale WO3 grains uniformly dispersed on the surface of the mesoporous SiO2 carrier, active sites of WO3/SiO2 catalysts are greatly increased, and their catalytic activities are improved. After the sixth and eighth runs in the ODS of dibenzothiophene and 4,6-dimethyldibenzothiophene, respectively, the WO3/SiO2 catalyst still maintains high catalytic activity (>99.0%) despite the presence of a partial loss of WO3. In addition, with the aid of the UV-Vis technique, the tungsten-peroxo species, the active intermediates in the ODS reaction catalyzed by the WO3/SiO2 catalyst, are captured. Finally, a possible mechanism for the ODS of bulky organic sulfides using the WO3/SiO2 catalyst is proposed.

Graphical Abstract

1. Introduction

With increasing awareness of the importance of environmental protection among people, countries have successively issued strict sulfur limit standards for fuel oil [1,2]. Today, hydrodesulfurization technology is mainly used in industries to remove organic sulfides in fuel oil. It can efficiently remove thiols, thioethers, and disulfides. However, this technology has drawbacks such as harsh operating conditions, high energy costs, and difficulty in removing thiophene sulfides [3], which cannot meet the requirements for deep desulfurization of fuel oil. Therefore, it is necessary to develop new desulfurization technologies to replace the traditional hydrodesulfurization technology, such as oxidative desulfurization (ODS) [1,2], adsorption desulfurization [4], extraction desulfurization [5], and biodesulfurization [6]. Among them, ODS has attracted much attention due to its advantages of mild reaction conditions, low operating cost, and effective removal of thiophene sulfides.
The catalysts for ODS mainly include polyoxometalates [7] and their composite materials [8], titanium-containing molecular sieves [2,9], transition metal oxides [3,9,10,11,12,13], and so on. Among them, WO3-based heterogeneous catalysts [3,12,13] are favored by researchers for their excellent desulfurization performance. For example, Li et al. [12] have prepared WO3-SBA-15 using the hydrothermal crystallization method using H2WO4 as a tungsten source, tetraethyl orthosilicate as a silicon source, and a mixture of P123 tri-block copolymer and cetyltrimethylammonium bromide as the structure-directing agent. Under optimum conditions for reaction and subsequent extraction, the WO3-SBA-15 can reduce the sulfur content of gasoline from 540 to 46 ppm (the desulfurization rate is 91.3%). Zhao et al. [13] have prepared a series of WO3/g-C3N4 composites with different WO3 contents by directly calcining of a mixture of WO3 and g-C3N4. The 36%WO3/g-C3N4 composite shows a desulfurization rate of 91.2% in the ODS of dibenzothiophene (DBT) model oil, and its activity does not significantly decrease after five cycles. Qin et al. [3] have prepared WO3/TiO2 catalysts using the hydrothermal crystallization method using a functional ionic liquid [C18H37N(CH3)3]3PW12O40 as both the template and tungsten source, and tetrabutylorthotitanate as the titanium source. The 550-WO3/TiO2 gives a desulfurization rate of 100% in the ODS of DBT model oil, and the desulfurization rate decreased to 96.7% after six cycles. Unfortunately, the 550-WO3/TiO2 is not very active in the ODS of 4,6-dimethyldibenzothiophene (4,6-DMDBT). The above WO3-based heterogeneous catalysts are more effective in the ODS of DBT, however, less effective in the ODS of 4,6-DMDBT. In addition, the expensive carrier cost also limits the industrial application of WO3-based heterogeneous catalysts.
Rice husks (RH) are a large and inexpensive renewable biomass resource, mainly containing cellulose, hemicellulose, lignin, and ash. Specifically, RH contain about 20% amorphous silica, which is an ideal raw material for the preparation of silicon-containing catalytic materials [14]. In recent years, the use of RH as a catalyst carrier has attracted extensive attention from researchers [15,16,17,18,19]. Shinde et al. [17] have prepared supported metal chlorides and metal oxides by impregnating RH silica with aqueous or organic solutions of FeCl3, SbCl3, BiCl3, and AlCl3. Among them, the Sil-Fe-O-120 sample shows good catalytic activity and stability in the Friedel–Crafts benzylation of monosubstituted aromatic compounds. Hindryawati et al. [18] have prepared alkali metal silicates by impregnating RH silica with aqueous solutions of LiOH, NaOH, and KOH. These catalysts show high catalytic activity in the transesterification of used cooking oil with methanol, and the contents of methyl ester are still around 80% after six cycles. Priya et al. [19] have prepared MoO3-Si and La2O3-Si catalysts using the impregnation method using RH silica as a carrier, and ammonium heptamolybdate and lanthanum nitrate as metal sources. Among them, MoO3-Si showed high catalytic activity (95.6%) and selectivity of dibenzyl malonate (96.8%) in the transesterification of diethyl malonate with benzyl alcohol. The above studies on the preparation of supported metal oxides using RH silica as a carrier have achieved good catalytic results. To the best of our knowledge, there are currently few reports on the preparation of supported metal oxides using RH as both silicon source and mesoporous template, as well as their application in the ODS.
Here, using acid-treated RH as both the silicon source and mesoporous template, a series of WO3/SiO2 catalysts were prepared by an incipient-wetness impregnation method. The effects of different calcination temperatures and WO3 loadings on the catalytic performance of samples in the ODS of DBT and 4,6-DMDBT are demonstrated, and the appropriate calcination temperature and WO3 loading are obtained. The recycling performances of the WO3/SiO2 are investigated, and the catalytic mechanism of the ODS process is studied.

2. Results and Discussion

2.1. Structural Characterizations

For the convenience of readers, the designation of catalysts is first explained. For example: ‘WS-923-15R’, where ‘WS’ is short for WO3/SiO2 catalyst, ‘923’ stands for the calcination temperature, ‘15’ stands for the loading of WO3, and ‘r’ stands for the regenerated sample (see Section 3.2.2 and Section 3.4.1 for details).
Figure 1 shows the XRD patterns of SiO2, pure WO3, WS-923-15, and WS-923-15r samples. The SiO2 has a broad and diffuse diffraction peak near 23°, indicating that the sample has an amorphous structure. Pure WO3 displays diffraction peaks at 23.1°, 23.7°, 24.1°, 28.8°, 33.3°, and 34.0°, which correspond to the (001), (020), (200), (111), (021), and (220) crystal planes of WO3 with the orthorhombic phase (JCPDS no. 20-1324) [20]. The WS-923-15 sample shows diffraction peaks at 23.1°, 23.6°, 24.4°, 28.6°, 33.3°, and 34.2°, which correspond to the (002), (020), (200), (−112), (022), and (220) crystal planes of WO3 with the monoclinic phase (JCPDS no. 43-1035), respectively [21]. After recycle tests, the WS-923-15r samples do not show obvious WO3 diffraction peaks, suggesting that the loss of the WO3 active component should have occurred.
Figure 2 shows the UV-Vis spectra of samples. The pure WO3 shows absorption peaks at about 208, 265, and 395 nm, which are attributed to the octahedral polytungstate species and WO3 crystallites, respectively [22,23]. The WS-923-15 sample shows absorption peaks at about 220, 260, and 375 nm, which are attributed to the tetrahedral monomeric tungstate species, octahedral polytungstate species, and WO3 crystallites, respectively [22]. It is generally believed that the ODS activity of different tungstate species is in the order of octahedral polytungstate species > tetrahedral monomeric tungstate species > WO3 crystallites [24,25]. Compared with spectra of the oxidation products, one can see that the WS-923-15r samples do not contain oxidation products of DBT and 4,6-DMDBT after regeneration by calcinating. In addition, compared with fresh WS-923-15, the regenerated samples show weakened peak intensities at about 220, 260, and 375 nm, especially at about 375 nm, which shows the greatest reduction of peak intensity. The results indicate that the contents of the tetrahedral monomeric tungstate species, octahedral polytungstate species, and WO3 crystallites in the regenerated samples decrease, among which the decrease of WO3 crystallite content is the most obvious.
Figure 3 shows the N2 adsorption-desorption isotherms and pore size distributions of samples. Both SiO2 and WS-923-15 show typical IV-type isotherms, with obvious H4 hysteresis loops at P/P0 > 0.40 and basically the same shapes, suggesting that the two samples have similar mesoporous/macroporous structures. The mesoporous and macroporous pore size of the SiO2 carrier are concentrated in the range of 1.9–78 nm, and the most probable pore size is 3.7 nm (Figure 3b). Compared with SiO2, the pore distribution and the most probable pore size of WS-923-15 do not change much, and the SBET and Vmeso of WS-923-15 decrease slightly (Table 1). These results indicate that WO3 crystals are well-dispersed on the surface/pores of the SiO2 and do not block pores of the carrier. Compared with WO3, the SBET and Vmeso of WS-923-15 are much larger (Table 1), which is conducive to the easy access of bulky reactants to the active sites, overcoming the diffusion effect, and thus improving the catalytic performance of WS-923-15 [12]. Moreover, compared with commercially available silica gel, SiO2 has a larger specific surface area, but its mesoporous volume is smaller (Table 1). Compared with 15%WO3/Silica gel, WS-923-15 has a larger specific surface area, but its mesoporous volume is smaller (Table 1).
As shown in Figure 3 and Table 1, compared with fresh WS-923-15, the WS-923-15r regenerated from the seventh ODS reaction of DBT shows a slight increase in the SBET and Vmeso. The WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT shows little change in the SBET, a slight decrease in the Vmeso, and a slight narrowing of the pore distribution. These results indicate that the pore structure of regenerated samples is well-maintained.
Figure 4 reveals the SEM images of samples. The carrier SiO2 is composed of particles of different sizes without specific shapes, and its maximum particle size is about 6.3 μm (Figure 4a). After loading WO3, the morphology of the sample has little change (Figure 4b). Compared with WS-923-15, the morphology of regenerated samples also has little change after recycle tests (Figure 4c,d). As can be seen from Figure 5a, there are a lot of mesopores and macropores in SiO2 samples, which is consistent with the results of the N2 adsorption-desorption isotherm. After loading WO3, nanoscale WO3 grains are uniformly dispersed on the surface of the carrier SiO2 with a crystal size of about 5.0 nm (Figure 5b, yellow circles). After recycle tests, nanoscale WO3 grains can still be seen on the carrier SiO2 (Figure 5c,d, yellow circles).

2.2. Catalytic Performance on Oxidative Desulfurization

Table 1 shows the effects of different calcination temperatures and WO3 loadings on the catalytic performance of samples. It can be seen that with the increase of the calcination temperature from 823 to 1023 K, conversions of DBT and 4,6-DMDBT first increase and then decrease. So, the appropriate calcination temperature is 923 K. With the increase of WO3 loading from 5.0 to 25.0%, conversions of DBT and 4,6-DMDBT also show a trend of first increasing and then decreasing. Therefore, the proper WO3 loading is 15.0 wt.%. These results are basically consistent with the variation trends of the Vmeso of samples (Table 1).
Figure 6 shows the influence of reaction time on the catalytic performance of WS-923-15. As one can see from Figure 6, with the extension of reaction time from 15 min to 1 h, the conversion of DBT increases from 98.1 to 99.8%. With the further extending of the reaction time, the conversion of DBT keeps constant. With the extension of reaction time from 15 min to 2 h, the conversion of 4,6-DMDBT increased from 81.7 to 100.0%. Therefore, the more suitable reaction times for the ODS of DBT and 4,6-DMDBT are 1 and 2 h, respectively.
Several blank reactions were designed and carried out to investigate the reasons for the good catalytic performance of the WS-923-15 in ODS of DBT and 4,6-DMDBT. As one can see from Table 2, in the absence of catalysts and oxidants, 60.5% of DBT and 31.7% of 4,6-DMDBT are extracted with acetonitrile. When the catalyst was not involved in the reaction, the conversions of DBT and 4,6-DMDBT did not change much, indicating that the contribution of H2O2 itself can be ignored. When SiO2 was used as the catalyst, conversions of DBT and 4,6-DMDBT were similar to the results of the blank reaction, indicating that SiO2 does not have catalytic activity. When pure WO3 was used as the catalyst, conversions of DBT and 4,6-DMDBT were slightly increased compared to the results of the blank reaction, suggesting that the catalytic activities of pure WO3 are very poor. When WS-923-15 was the catalyst and no oxidant was added, conversions of DBT and 4,6-DMDBT increased slightly, indicating that the adsorptive removal of DBT and 4,6-DMDBT by WS-923-15 was not significant. Based on the results of Figure 3 and Figure 5, Table 1, and the blank experiments mentioned above, the good catalytic performance of WS-923-15 is mainly attributed to (a) high WO3 loading, (b) high SBET, and (c) nanoscale WO3 grains uniformly dispersed on the surface of the mesoporous SiO2 carrier. These factors greatly increase active sites of samples, thus improving their catalytic activity. Moreover, compared with 15%WO3/Silica gel (Table 2), WS-923-15 gave high conversions of DBT and 4,6-DMDBT (Figure 6), indicating the obvious superiority of using rice husks as both the silicon source and mesoporous template.
Figure 7 shows the FT-IR spectra of DBT, 4,6-DMDBT, and their oxidation products. Compared with DBT, the oxidation product of DBT exhibits two new absorption peaks at 1164 and 1288 cm−1, which can be attributed to the symmetric and asymmetric stretching vibration of sulfone (O=S=O), respectively [26,27]. Similarly, the new absorption peaks of the 4,6-DMDBT oxidation product at 1153 and 1284 cm−1 are attributed to the symmetric and asymmetric stretching vibration of sulfone (O=S=O), respectively [26,27]. These results indicate that the oxidation products of DBT and 4,6-DMDBT should be dibenzothiophene sulfone and 4,6-dimethyldibenzothiophene sulfone, respectively.
Figure 8 shows results of cyclic experiments on WS-923-15. After the sixth run, the conversion of DBT is still as high as 99.1%. As the number of cyclic experiments continues to increase, the conversion of DBT begins to decrease. After the eighth run, the conversion of 4,6-DMDBT is still as high as 99.5%. As the number of cyclic experiments continues to increase, the conversion of 4,6-DMDBT starts to decrease. The WS-923-15r samples were further characterized by XRD, UV-Vis, N2 adsorption-desorption isotherms, TEM, and ICP-OES techniques. The results of XRD (Figure 1) showed that the loss of the WO3 active component should have occurred during the recycle tests. The results of UV-Vis (Figure 2) indicated that the contents of tetrahedral monomeric tungstate species, octahedral polytungstate species, and WO3 crystallites in the regenerated samples decreased, among which the decrease of the WO3 crystallite content was the most obvious. The results of N2 adsorption-desorption isotherm (Figure 3 and Table 1) indicated that the pore structure of regenerated samples is well-maintained after recycle tests. The results of TEM (Figure 5) showed that nanoscale WO3 grains remain uniformly dispersed on the carrier SiO2 after recycle tests. The results of ICP-OES showed that WS-923-15, WS-923-15r regenerated from the seventh ODS reaction of DBT, and WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT were 13.3, 2.1, and 2.6 wt.%, respectively. The results indicate that the WO3 in the samples undergoes some loss during recycle tests. The above results show that WS-923-15 exhibits good recycling performances despite the loss of WO3 during recycle tests. In a future research paper, the reasons for the loss of WO3 in the samples and how to suppress the loss of WO3 in recycle tests will be further explored. Moreover, as shown in Table 3, compared with WO3-based catalysts reported in the literature [3,12,13,21,25,28,29], the recycling performances of WS-923-15 prepared in this work are equivalent to or better than those of the reported samples.

2.3. Reaction Mechanism of ODS Process

In general, when samples with WO3 as the active component are used as catalysts, H2O2 as the oxidant, and acetonitrile as the solvent to catalyze the ODS of organic sulfides, the tungsten-peroxo species generated by the reaction of the tungsten active sites with H2O2 are considered to be active intermediate [21,24,28,30]. In order to capture the tungsten-peroxo species, samples were treated with a H2O2 aqueous solution and detected using the UV-Vis technique (Figure 9). After the H2O2 aqueous solution treatment, the WS-923-15 shows a distinct shoulder peak at about 360 nm, which can be attributed to the formation of tungsten-peroxo species from the interaction between tungsten active sites and H2O2 [28]. In addition, after the H2O2 aqueous solution treatment, regenerated samples also show the obvious shoulder peaks at about 380 nm, indicating that regenerated samples still have a strong ability to bind H2O2 to form tungsten-peroxo species, which are consistent with the results of cyclic experiments.
Based on the above results, the ODS reaction mechanism of DBT and 4,6-DMDBT on WS-923-15 with H2O2 is proposed and depicted in Scheme 1. First, DBT (or 4,6-DMDBT) in the oil phase is extracted into the acetonitrile phase and adsorbed on the surface of the catalyst together with H2O2. Then, the tungsten active sites combine with H2O2 to form the tungsten-peroxo intermediates with the loss of one molecule of H2O. Then, the sulfur atoms in DBT (or 4,6-DMDBT) nucleophilically attack the tungsten-peroxo intermediates to form sulfoxides, simultaneously releasing the catalyst. Sulfoxides are very unstable and are rapidly oxidized to sulfones by the tungsten-peroxo intermediates [7,8,31,32]. In addition, acetonitrile is not only an extractant for DBT and 4,6-DMDBT, but also a solvent with strong solubility that can dissolve the sulfoxides and sulfones on the catalyst surface in time and re-expose the active sites. Therefore, after the reaction is complete, the desulfurized model oil can be obtained by simple filtration and liquid separation.

3. Materials and Methods

3.1. Materials

The RH originated from the College of Agriculture of Yangtze University. Phosphotungstic acid hydrate (HPW), ammonium metatungstate hydrate (99.5%), dibenzothiophene (DBT, 99%), 4,6-dimethyldibenzothiophene (4,6-DMDBT, 97%), and hydrogen peroxide (H2O2, 30 wt.%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Silica gel for thin-layer chromatography (HG/T2354, C.P.) was purchased from Qingdao Haiyang Chemical Co., Ltd. (Qingdao, China). Isopropanol, hydrochloric acid, n-octane, acetonitrile, and other reagents (A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Synthesis

3.2.1. Pretreatment of RH

The collected RH were rinsed with water, dried naturally, and then dried at 323 K for 24 h. Then, the RH were pulverized using a pulverizer to collect the powder that had passed through a 200-mesh sieve. The RH powder was acidified with diluted hydrochloric acid to remove the metal compounds contained in the RH [33]. A typical acidification treatment of RH is as follows: 30 g of RH powder was put into a 1 L beaker, and 600 mL of hydrochloric acid aqueous solution (0.5 mol/L) was added and stirred evenly. Then, the mixture was treated for 24 h in a 323 K water bath. After the treatment, the RH powder was filtered and washed with deionized water until neutral, and then dried at 323 K for 24 h. The contents of metal ions in the RH powder before and after acidification were determined by ICP-OES. The contents of Fe3+, Mn2+, Zn2+, Ca2+, Na+, Mg2+, and K+ in RH powder were 0.069, 0.016, 0.011, 0.039, 0.029, 0.041, and 0.045 wt.%, respectively. After acidification treatment, the contents of the above ions were 0.008, 0, 0.006, 0.012, 0.008, 0.006, and 0.007 wt.%, respectively. The results indicate that a hydrochloric acid acidification treatment can basically remove metal compounds from the RH powder [18,33].

3.2.2. Synthesis of WO3/SiO2

The pretreated RH powder (about 15.0 g) was dried at 383 K for 4 h to obtain an absolutely dry RH powder. The SiO2 in the RH was obtained by calcining 5.0 g of absolutely dried RH powder at 873 K for 5 h. Accordingly, the SiO2 content in the absolutely dried RH powder can be calculated as 19.6 wt.%. In a typical synthesis, 5.0 g of absolutely dried RH powder was fed into a 100 mL beaker. Also, 0.175 g of HPW was dissolved in 4.9 mL of isopropanol to obtain a clear solution. The clear solution was added dropwise to the RH powder under stirring with a glass rod until it reached a saturated impregnation state. After aging at room temperature for 24 h, the mixture was dried at 383 K for 8 h, ground and calcined at 923 K for 5 h (heating rate: 1 K/min). Finally, a light green solid was obtained and named as WS-923-15, where ‘923’ and ‘15’ represent the calcination temperature and WO3 loading, respectively. Moreover, using a similar preparation method, the 15%WO3/silica gel catalyst was prepared using thin-layer chromatography silica gel as a carrier. For comparison, pure WO3 was obtained by calcining ammonium metatungstate at 873 K for 5 h.

3.2.3. Treatment of Samples with Hydrogen Peroxide

In a typical treatment, 60 mg of WO3/SiO2, 2.5 mL of acetonitrile and 370 μL of H2O2 were added into a 50 mL two-neck glass flask equipped with a condenser. The mixture was then stirred, heated to 333 K, and kept at this temperature for 1 h. Finally, the mixture was concentrated by rotary evaporation, and the residue was dried in a vacuum oven at room temperature for 12 h to obtain a light green-yellowish solid.

3.3. Characterization

The XRD patterns were recorded with Cu-Kα radiation on a Panalytical Empyrea X–ray diffractometer with a scanning angle (2θ) of 10–80°. The UV-Vis spectra were measured using a PerkinElmer Lambda 650S spectrometer with a spectral range of 190–800 nm. The FT-IR spectra were measured by a Nicolet 6700 infrared spectrometer with a pure KBr background. Measurements were made over a wavelength range of 400–4000 cm−1. The SEM images were taken using a TESCAN MIRA3 field emission scanning electron microscope. The TEM images were collected on a FEI TECNAI F20 transmission electron microscope. The metal content of samples was determined using a PerkinElmer Optima 8000 inductively coupled with a plasma emission spectrometer. The N2 adsorption–desorption isotherms were carried out on a Micromeritics ASAP 2020HD88 physical adsorption instrument at 77 K. Prior to analysis, all of the samples were degassed at 573 K for 4 h. The specific surface area (SBET) was determined using the BET equation, and the micropore volume (Vmicro) was evaluated using the t-plot method. The cumulative pore volume (Vmeso) and pore size distribution were calculated using the Barrett–Joyner–Halenda (BJH) model from the desorption branch of N2 physisorption isotherms. The absorption spectra and standard curves of DBT and 4,6-DMDBT model oils were determined using a TU–1900 UV-Vis spectrophotometer (Beijing Puxi general instrument Co., Ltd, Beijing, China).

3.4. Catalytic Activity and Recycle Tests

3.4.1. Catalytic Activity

DBT and 4,6-DMDBT model oils with a sulfur content of 1000 ppmw in both cases were prepared using n-octane as a solvent. In a typical reaction, 0.08 g of WS-923-15, 10 mL of model oil, and 10 mL of acetonitrile were sequentially added to a 50 mL two-neck glass flask equipped with a condenser. The mixture was heated to 333 K with stirring, and the reaction was started by adding 135.0 μL of H2O2. When the set time was reached, the reaction was stopped. The reaction mixture was cooled and centrifuged to give an upper oil phase, an acetonitrile phase, and the solid. The absorbance of the upper oil phase at the set wavelength (236 nm for DBT, and 241 nm for 4,6-DMDBT) was determined by the UV-Vis Spectrophotometer, and the residual amounts of DBT and 4,6-DMDBT in the upper oil phase were calculated using the standard curve method. The absorption spectra and standard curves of DBT and 4,6-DMDBT model oils are shown in Figure 10 and Figure 11, respectively. As can be seen from Figure 10 and Figure 11, the linear regression equation of DBT model oil was y = 1.26991 x, with a correlation coefficient of R2 of 0.99987, and a linear range of 0–0.475 ppmw. The linear regression equation of 4,6-DMDBT model oil was y = 1.05888 x, with a correlation coefficient of R2 of 0.99804, and a linear range of 0–0.550 ppmw. The conversion of DBT or 4,6-DMDBT (X) was defined as the amount of sulfide reacted divided by the initial amount of sulfide added. The acetonitrile phase was concentrated by rotary evaporation to obtain a white solid, which was dried under vacuum at room temperature for 24 h to obtain the oxidized product of DBT (or 4,6-DMDBT). The solid was centrifuged, washed with acetonitrile, dried overnight, and then calcined at 823 K for 3 h (1 K/min) to obtain the regenerated sample. The regenerated sample was labeled as WS-923-15r.

3.4.2. Recycle Tests

The cyclic experiments of regenerated samples were carried out in a 250 mL two-necked flask equipped with a condenser. The operational and analytical conditions were similar to the above procedures, except that the conditions for the first reaction were as follows: 80 mL of 1000 ppmw model oil, 80 mL of acetonitrile, 0.640 g of catalyst, H2O2/S molar ratio of 6, a reaction temperature of 333 K, and a reaction time of 1 h for the DBT model oil (2 h for 4,6-DMDBT). In each run, the dosage of model oil, H2O2, and acetonitrile was calculated based on the feed ratio of the first reaction and the mass of the regenerated catalyst obtained in the previous run.

4. Conclusions

In summary, a series of mesoporous WO3/SiO2 catalysts were successfully prepared using the incipient-wetness impregnation method using RH as both the silicon source and mesoporous template. The samples were characterized systematically by relevant characterization techniques. The results show that the samples contain tetrahedral monomeric tungstate species, octahedral polytungstate species, and WO3 crystallites. Compared with pure WO3, the WO3/SiO2 samples have higher SBET and Vmeso. Due to (a) high WO3 loading, (b) high SBET, and (c) nanoscale WO3 grains uniformly dispersed on the surface of the mesoporous SiO2 carrier, the active sites of WO3/SiO2 samples are greatly increased. Therefore, the WO3/SiO2 samples exhibit high catalytic activity in the ODS of DBT and 4,6-DMDBT. The effects of the calcination temperature and WO3 loading on the catalytic properties of samples were investigated, and the appropriate calcination temperature and WO3 loading are 923 K and 15.0 wt.%, respectively. After the sixth and eighth runs in the ODS of DBT and 4,6-DMDBT, respectively, WS-923-15 still maintains high conversions of DBT (99.1%) and 4,6-DMDBT (99.5%). The related characterization results indicate that WO3 in the WS-923-15 had undergone some loss during recycle tests. The WS-923-15 and WS-923-15r samples were treated with H2O2 and the active intermediates of the ODS reaction, tungsten-peroxo species, were captured using the UV-Vis technique. Moreover, a possible mechanism for the ODS of bulky organic sulfides using WS-923-15 is proposed. In the next study, the causes of WO3 loss in the samples will be further investigated as well as how to suppress WO3 loss in recycle tests.

Author Contributions

Conceptualization, Y.Z. and X.L.; methodology, Y.Z. and R.Z.; validation, J.Z. and L.Z.; formal analysis, J.Z. and J.H.; investigation, Y.Z. and J.H.; resources, X.L., L.Z., and W.Z.; data curation, R.Z. and J.Z.; writing—original draft preparation, Y.Z. and R.Z.; writing—review and editing, W.Z., X.L., and H.L.; visualization, X.L. and H.L.; supervision, X.L. and H.L.; project administration, H.L.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (21303008) and the Natural Science Foundation of Hubei Province of China (2012FFB00103).

Data Availability Statement

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The XRD patterns of samples with the reported standard JCPDS data: (1) WS-923-15, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (4) SiO2, (5) pure WO3.
Figure 1. The XRD patterns of samples with the reported standard JCPDS data: (1) WS-923-15, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (4) SiO2, (5) pure WO3.
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Figure 2. The UV-Vis spectra of samples: (1) WS-923-15, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (4) oxidation product of DBT, (5) oxidation product of 4,6-DMDBT, (6) WO3.
Figure 2. The UV-Vis spectra of samples: (1) WS-923-15, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (4) oxidation product of DBT, (5) oxidation product of 4,6-DMDBT, (6) WO3.
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Figure 3. N2 adsorption-desorption isotherms (a) and pore size distribution curves (b) of samples: (1) SiO2, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15, (4) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
Figure 3. N2 adsorption-desorption isotherms (a) and pore size distribution curves (b) of samples: (1) SiO2, (2) WS-923-15r regenerated from the seventh ODS reaction of DBT, (3) WS-923-15, (4) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
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Catalysts 14 00103 g004aCatalysts 14 00103 g004b
Figure 4. SEM images of samples: (a) SiO2, (b) WS-923-15, (c) WS-923-15r regenerated from the seventh ODS reaction of DBT, (d) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
Figure 4. SEM images of samples: (a) SiO2, (b) WS-923-15, (c) WS-923-15r regenerated from the seventh ODS reaction of DBT, (d) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
Catalysts 14 00103 g004aCatalysts 14 00103 g004b
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Figure 5. TEM images of samples: (a) SiO2, (b) WS-923-15, (c) WS-923-15r regenerated from the seventh ODS reaction of DBT, (d) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
Figure 5. TEM images of samples: (a) SiO2, (b) WS-923-15, (c) WS-923-15r regenerated from the seventh ODS reaction of DBT, (d) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT.
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Figure 6. Effect of reaction time on the ODS of DBT and 4,6-DMDBT catalyzed by WS-923-15. Reaction conditions: model oil 10 mL, acetonitrile 10 mL, catalyst 0.08 g, molar ratio of H2O2/S 6, temperature 333 K.
Figure 6. Effect of reaction time on the ODS of DBT and 4,6-DMDBT catalyzed by WS-923-15. Reaction conditions: model oil 10 mL, acetonitrile 10 mL, catalyst 0.08 g, molar ratio of H2O2/S 6, temperature 333 K.
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Figure 7. FT-IR spectra of the reactants and their corresponding products: (a) DBT and its oxidation product, (b) 4,6-DMDBT and its oxidation product.
Figure 7. FT-IR spectra of the reactants and their corresponding products: (a) DBT and its oxidation product, (b) 4,6-DMDBT and its oxidation product.
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Figure 8. Recycle tests for the catalytic oxidation of DBT and 4,6-DMDBT over WS-923-15r samples.
Figure 8. Recycle tests for the catalytic oxidation of DBT and 4,6-DMDBT over WS-923-15r samples.
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Figure 9. UV-Vis spectra of fresh WS-923-15 and regenerated WS-923-15r samples: (1) fresh WS-923-15, (2) fresh WS-923-15 treated with H2O2, (3) WS-923-15r regenerated from the seventh ODS reaction of DBT, (4) WS-923-15r regenerated from the seventh ODS reaction of DBT and treated with H2O2, (5) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (6) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT and treated with H2O2.
Figure 9. UV-Vis spectra of fresh WS-923-15 and regenerated WS-923-15r samples: (1) fresh WS-923-15, (2) fresh WS-923-15 treated with H2O2, (3) WS-923-15r regenerated from the seventh ODS reaction of DBT, (4) WS-923-15r regenerated from the seventh ODS reaction of DBT and treated with H2O2, (5) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT, (6) WS-923-15r regenerated from the ninth ODS reaction of 4,6-DMDBT and treated with H2O2.
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Scheme 1. A proposed cyclic mechanism for the ODS of DBT and 4,6-DMDBT catalyzed by WS-923-15 in the presence of H2O2 (R represents H or CH3).
Scheme 1. A proposed cyclic mechanism for the ODS of DBT and 4,6-DMDBT catalyzed by WS-923-15 in the presence of H2O2 (R represents H or CH3).
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Figure 10. The absorption spectra (inserted image) and standard curve of DBT model oil.
Figure 10. The absorption spectra (inserted image) and standard curve of DBT model oil.
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Figure 11. The absorption spectra (inserted image) and standard curve of 4,6-DMDBT model oil.
Figure 11. The absorption spectra (inserted image) and standard curve of 4,6-DMDBT model oil.
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Table 1. The pore structure parameters and catalytic performances of samples.
Table 1. The pore structure parameters and catalytic performances of samples.
SampleSBET/(m2/g)Vmicro/(cm3/g)Vmeso/(cm3/g)XDBT (%)X4,6-DMDBT (%)
SiO2294.400.39--
Pure WO30.100.01--
Silica gel184.000.77--
15%WO3/Silica gel157.000.65--
WS-823-15246.800.3398.486.8
WS-923-15227.900.3599.295.6
WS-1023-15149.900.2997.486.5
WS-923-5260.900.3697.691.3
WS-923-25196.100.3095.783.7
WS-923-15r a249.400.38--
WS-923-15r b234.700.32--
Reaction conditions: model oil 10 mL, acetonitrile 10 mL, catalyst 0.08 g, molar ratio of H2O2/S 6, temperature 333 K, reaction time 0.5 h. a Regenerated from the seventh ODS reaction of DBT; b Regenerated from the ninth ODS reaction of 4,6-DMDBT.
Table 2. Blank and comparative reaction results for ODS of DBT and 4,6-DMDBT.
Table 2. Blank and comparative reaction results for ODS of DBT and 4,6-DMDBT.
SampleOxidantXDBT (%)X4,6-DMDBT (%)
--60.531.7
-H2O258.930.4
SiO2H2O261.032.9
Pure WO3H2O268.346.1
WS-923-15-62.232.3
15%WO3/Silica gelH2O284.881.8
Reaction conditions: model oil 10 mL, acetonitrile 10 mL, catalyst 0.08 g, molar ratio of H2O2/S 6, temperature 333 K, reaction time for DBT 1 h (2 h for 4,6-DMDBT).
Table 3. Comparison of the WS-923-15 catalyst in the ODS of DBT and 4,6-DMDBT (or gasoline) with reported similar catalysts.
Table 3. Comparison of the WS-923-15 catalyst in the ODS of DBT and 4,6-DMDBT (or gasoline) with reported similar catalysts.
SampleTemp. (K)Cyclic NumberXDBT a (%)X4,6-DMDBT a (%)Ref.
WO3-SBA-15333485 b-[12]
550-WO3/TiO2323696.7-[3]
36%WO3/g-C3N4333587.5-[13]
15WOx@SnO2333593.1-[21]
WO3/CNT-23235~80-[25]
a-Cr2WO6/WO3343693.1-[28]
CoW (15)/CNT348599.0-[29]
WS-923-15333699.199.5 c
a The catalytic results of the last cyclic reaction of DBT and 4,6-DMDBT, respectively. b Overall desulfurization rate of gasoline (gasoline was diluted with n-octane to a sulfur content of 540 ppm) after catalytic oxidation and 1-methyl-2-pyrrolidone extraction. c The conversion of 4,6-DMDBT after the eighth run.
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Zhang, Y.; Liu, X.; Zhao, R.; Zhang, J.; Zhang, L.; Zhang, W.; Hu, J.; Li, H. The Green Preparation of Mesoporous WO3/SiO2 and Its Application in Oxidative Desulfurization. Catalysts 2024, 14, 103. https://doi.org/10.3390/catal14020103

AMA Style

Zhang Y, Liu X, Zhao R, Zhang J, Zhang L, Zhang W, Hu J, Li H. The Green Preparation of Mesoporous WO3/SiO2 and Its Application in Oxidative Desulfurization. Catalysts. 2024; 14(2):103. https://doi.org/10.3390/catal14020103

Chicago/Turabian Style

Zhang, Yinhai, Xiaoxue Liu, Ruyu Zhao, Jingwei Zhang, Lanfen Zhang, Wei Zhang, Jian Hu, and Hao Li. 2024. "The Green Preparation of Mesoporous WO3/SiO2 and Its Application in Oxidative Desulfurization" Catalysts 14, no. 2: 103. https://doi.org/10.3390/catal14020103

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