Efficient Diesel Desulfurization by Novel Amphiphilic Polyoxometalate-Based Hybrid Catalyst at Room Temperature

Amphiphilic hybrid catalysts were prepared by modifying [SMo12O40]2− with tetrabutylammonium bromide (TBAB), 1-butyl-3-methylimidazole bromide (BMIMBr) and octadecyl trimethyl ammonium bromide (ODAB), respectively. The prepared catalysts were characterized by IR, XRD, SEM, TG and XPS. The desulfurization performance of the catalysts was investigated in model oil and actual diesel using hydrogen peroxide (H2O2) as an oxidant and acetonitrile as an extractant. All catalysts exhibited favorable activity for removing sulfur compounds at room temperature. Dibenzothiophene (DBT) can be nearly completely removed using SMo12O402−-organic catalysts within a short reaction time. For different sulfur compounds, the [TBA]2SMo12O40 catalyst showed a better removal effect than the [BMIM]2SMo12O40 and [ODA]2SMo12O40 catalyst. The [TBA]2SMo12O40 dissolved in extraction solvent could be reused up to five times in an oxidative desulfurization (ODS) cycle with no significant loss of activity. The [BMIM]2SMo12O40 performed as a heterogeneous catalyst able to be recycled from the ODS system and maintained excellent catalytic activity. The catalysts showed a positive desulfurization effect in real diesel treatment. Finally, we described the ODS desulfurization mechanism of DBT using SMo12O402−-organic hybrid catalysts. The amphiphilic hybrid catalyst cation captures DBT, while SMo12O402− reacts with the oxidant H2O2 to produce peroxy-active species. DBT can be oxidized to its sulfone by the action of peroxy-active species to achieve ODS desulfurization.


Introduction
Diesel is an important energy source and plays a vital role in industrial production [1,2]. Sulfur-containing compounds are one of the main pollutants in diesel, and its existence will bring many adverse effects. The combustion of sulfur-containing compounds produces SO x , which can cause air pollution and acid rain [3][4][5]. In addition, the presence of sulfur compounds in diesel can also poison the catalytic converter and corrode internal refinery components. As a result, countries around the world have implemented and proposed more stringent fuel standards [6]. Most countries require that the sulfur content in fuel cannot be higher than 10 ppm [7][8][9][10]. At present, fuel desulfurization technology is mainly divided into two categories: hydrodesulfurization (HDS) and non-hydrodesulfurization. HDS is the most widely used desulfurization technology in the industry [11,12]. HDS requires hydrogen consumption and high temperature and pressure operating conditions, which increase equipment input and economic costs [13,14]. The conventional catalytic HDS process has the least efficiency for the catalytic hydrodesulfurization of thiophene sulfides [15]. Therefore, the high input cost and poor removal effect of thiophene sulfides limit the development and application of HDS technology. Non 2 by using modified phosphomolybdic acid (HPMo) with tetramethylammonium chloride (TMAC), dodecyltrimethylammonium chloride (DTAC), and cetyltrimethylammonium chloride (HTAC). Under the condition of H 2 O 2 as the oxidant, the catalytic activity was compared in the ODS system. When [HPMo][HTAC] 2 was used as a catalyst, the conversion rate of dibenzothiophene (DBT) reached 96% at 60 • C for 180 min. The reason that [HPMo][HTAC] 2 has better catalytic performance is that the longer hexadecyl chain in the structure is not only more beneficial to the wrapping of DBT, but is also beneficial to the formation of a stable emulsion system containing a high DBT concentration [41]. Susana et al. reported three organic-inorganic hybrid catalysts-based [PW 11 Zn(H 2 O)O 39 ] 5− [42]. As a phase transfer agent, the quaternary ammonium cation can promote the effective mass transfer between the oil phase and the extraction phase, and improve the desulfurization rate. Under the action of the catalyst processing tetrabutylammonium bromide cation, the reaction has a favorable removal effect on different sulfur compounds. Li et al. studied amphiphilic catalysts with a core-shell structure [15]. The hydrophilic core is composed of phosphotungstste (PWO) clusters as the catalytic center, and the lipophilic shell consisted of long chain alkyl-imidazole or pyridine cations. DBT could be completely oxidized at 40 • C within 40 min under the catalysis of [C 16 MIM] 3 PWO. This method is useful for actual diesel desulfurization. Amphiphilic ODS catalysts are frequently reported [37,43]. The design of an amphiphilic ODS catalyst with high activity, selectivity, and recyclability is a research hotspot in the field of fuel desulfurization.
In this paper, we studied three kinds of amphiphilic hybrid catalysts based on [SMo 12 O 40 ] 2− . The polyacid anion [SMo 12 O 40 ] 2− is a saturated α-Keggin type POM. In the POM, all of the Mo exhibit the geometric environment of the {MoO 6 } octahedron. The central S atom is surrounded by eight oxygen atoms connected to it, forming a small cube at the center of the entire polyacid. The whole α-Keggin POM is composed of 12 {MoO 6 } octahedrons connected by common edges and common angles, and then connected with the center {SO 4 } unit at common angles. The S element in [SMo 12 O 40 ] 2− anion appears as the sulfate species. POM with the S 6+ heteroatom is rare. The S 6+ heteroatom helps to disperse the negative charges on the surface of the POM and makes the POM more stable. Different hybrid catalysts were prepared by modifying [SMo 12 O 40 ] 2− with a tetrabutylammonium (TBA), 1-butyl-3-methylimidazolium (BMIM), and octadecyldimethulammonium (ODA) cation. The catalytic performance of catalysts was evaluated in model oil and actual diesel using H 2 O 2 as the oxidant and acetonitrile as the extractant. Various reaction conditions that affect desulfurization efficiency are optimized in the ODS process and the recovery performance of catalysts was investigated. To the best of our knowledge, this is the first report on the application and optimization of the SMo 12 O 40 2− -organic hybrid catalysts in the ODS system.  [45,46]. In the [BMIM] 2 SMo 12 O 40 catalyst, the peaks observed at 3144 and 3108 cm −1 are attributed to the imidazole ring's hydrogen. The stretching absorption peak of n-butyl's hydrogenate can be assigned as 2957 and 2926 cm −1 . The characteristic peaks of the imidazole ring are observed at 1566 and 1465 cm −1 [43,47]. In the [ODA] 2 SMo 12 O 40 catalyst, the vibration peaks at 2918, 2851 and 1469 cm −1 are characteristic of an ODA cation structure [27]. Figure 2 Molecules 2023, 28 [BMIM]2SMo12O40 catalyst, the peaks observed at 3144 and 3108 cm −1 are attributed to the imidazole ring's hydrogen. The stretching absorption peak of n-butyl's hydrogenate can be assigned as 2957 and 2926 cm −1 . The characteristic peaks of the imidazole ring are observed at 1566 and 1465 cm −1 [43,47]. In the [ODA]2SMo12O40 catalyst, the vibration peaks at 2918, 2851 and 1469 cm −1 are characteristic of an ODA cation structure [27]. Figure 2 shows the IR spectra of the [BMIM]2SMo12O40 catalyst before and after recovery. The IR of the recovered [BMIM]2SMo12O40 catalyst showed that no obvious characteristic peak was destroyed, indicating that the structure of the recovered catalyst remained intact.  The XRD patterns of catalysts are shown in Figure 3. The strong characteristic peak appeared in the 2θ range of 6.5°-10°, indicating the ordering of the Keggin bulk structure of the polyoxoanion [48]. Due to the introduction of different organic cations, some weak characteristic peaks appeared in different 2θ range.  [BMIM]2SMo12O40 catalyst, the peaks observed at 3144 and 3108 cm −1 are attributed to the imidazole ring's hydrogen. The stretching absorption peak of n-butyl's hydrogenate can be assigned as 2957 and 2926 cm −1 . The characteristic peaks of the imidazole ring are observed at 1566 and 1465 cm −1 [43,47]. In the [ODA]2SMo12O40 catalyst, the vibration peaks at 2918, 2851 and 1469 cm −1 are characteristic of an ODA cation structure [27]. Figure 2 shows the IR spectra of the [BMIM]2SMo12O40 catalyst before and after recovery. The IR of the recovered [BMIM]2SMo12O40 catalyst showed that no obvious characteristic peak was destroyed, indicating that the structure of the recovered catalyst remained intact.  The XRD patterns of catalysts are shown in Figure 3. The strong characteristic peak appeared in the 2θ range of 6.5°-10°, indicating the ordering of the Keggin bulk structure of the polyoxoanion [48]. Due to the introduction of different organic cations, some weak characteristic peaks appeared in different 2θ range.  The XRD patterns of catalysts are shown in Figure 3. The strong characteristic peak appeared in the 2θ range of 6.5 • -10 • , indicating the ordering of the Keggin bulk structure of the polyoxoanion [48]. Due to the introduction of different organic cations, some weak characteristic peaks appeared in different 2θ range.  SEM images of the prepared catalysts at different magnifications are shown in Figure  4. It can be seen from the SEM image that [SMo12O40] 2− performs as a small spherical particle with an irregular surface. The TBA cation component is in the shape of a short rod and is connected with the polyacid anion. The overall structure of the amphiphilic [TBA]2SMo12O40 catalyst is relatively loose.

Characterization of Catalysts
[BMIM]2SMo12O40 is composed of larger particles and has long rod-shaped ionic liquid components. The overall structure of [ODA]2SMo12O40 is relatively compact, and its surface is covered with a waxy layer because of the long-chain alkyl substitutes.
[ODA]2SMo12O40 is a lamellar structure, and the active center [SMo12O40] 2− is attached to its surface.   The thermal stability of hybrid catalysts was studied by thermogravimetric analysis. The obtained results are shown in Figure 5. [TBA]2SMo12O40 and [BMIM]2SMo12O40 have weight loss regions between 240-600 °C and 220-600 °C, respectively. This demonstrates  The thermal stability of hybrid catalysts was studied by thermogravimetric analysis. The obtained results are shown in Figure 5 O 40 catalyst showed two weight loss areas. The first small weight loss of 2.5% observed between 36 and 78 • C is attributed to the removal of physically adsorbed water. The weight loss region (~50.7%) between 220-600 • C is assigned to the [OTA] 2 SMo 12 O 40 that is beginning to decompose. The results of thermogravimetric analysis show that the decomposition temperature of the catalysts was higher than the reaction temperature of this experiment. In order to determine the state of elements in the catalyst, an XPS characterization was performed. The results obtained are shown in Figure 6. Through the spectroscopy of Mo element in the [TBA]2SMo12O40 catalyst, it can be found that the binding energy of Mo 3d5/2 appears at 232.7 eV, indicating that the small peak is attributed to Mo 6+ [49]. In the S 2P spectra of the [TBA]2SMo12O40 catalyst, absorption peaks of S 2p3/2 and S 2P1/2 were observed at 169.18 and 170.36 eV, and are attributed to the sulfate species contribution [50,51]. The [SMo12O40] 2− with S 6+ heteroatom has an oxidizing ability. In order to determine the state of elements in the catalyst, an XPS characterization was performed. The results obtained are shown in Figure 6. Through the spectroscopy of Mo element in the [TBA] 2 SMo 12 O 40 catalyst, it can be found that the binding energy of Mo 3d 5/2 appears at 232.7 eV, indicating that the small peak is attributed to Mo 6+ [49]. In the S 2P spectra of the [TBA] 2 SMo 12 O 40 catalyst, absorption peaks of S 2p 3/2 and S 2P 1/2 were observed at 169. 18

Optimization of ODS System
The optimization in the ODS system was performed with the [TBA]2SMo12O40 catalyst. Various factors affecting desulfurization efficiency were investigated, including catalyst dosage, oxygen/sulfur molar ratio, and reaction temperature.
The effect of different catalyst dosages (catalyst dosage to the mass ratio of model oil) on the ODS efficiency were shown in Figure 7. The experimental conditions were set to an initial sulfur content of 500 ppm, an O/S molar ratio of 10, and a temperature of 60 °C. As shown in Figure 8, when no catalyst is added, the DBT removal rate is only 58.62%, depending on the extraction of acetonitrile and the oxidation capacity of H2O2. In the presence of the catalyst, the desulfurization rate exceeded 91% when the reaction only proceeded for 3 min. In the initial stage of the reaction, the increase in the catalyst dosage is beneficial in improving the reaction rate. When the amount of catalyst increased from 0.21 g to 0.63 g, the desulfurization rate gradually increased. When the catalyst dosage was 0.84 g, the desulfurization rate decreased compared with that of 0.63 g. The desulfurization reaction basically reached a stable desulfurization effect in 10 min. When the catalyst dosage was 0.63 g, the best desulfurization rate (96.1%) could be obtained within 10 min. Therefore, the optimal catalyst dosage was determined to be 0.63 g (the catalyst dosage accounted for 1.5 wt% of the model oil quality).

Optimization of ODS System
The optimization in the ODS system was performed with the [TBA] 2 SMo 12 O 40 catalyst. Various factors affecting desulfurization efficiency were investigated, including catalyst dosage, oxygen/sulfur molar ratio, and reaction temperature.
The effect of different catalyst dosages (catalyst dosage to the mass ratio of model oil) on the ODS efficiency were shown in Figure 7. The experimental conditions were set to an initial sulfur content of 500 ppm, an O/S molar ratio of 10, and a temperature of 60 • C. As shown in Figure 8, when no catalyst is added, the DBT removal rate is only 58.62%, depending on the extraction of acetonitrile and the oxidation capacity of H 2 O 2. In the presence of the catalyst, the desulfurization rate exceeded 91% when the reaction only proceeded for 3 min. In the initial stage of the reaction, the increase in the catalyst dosage is beneficial in improving the reaction rate. When the amount of catalyst increased from 0.21 g to 0.63 g, the desulfurization rate gradually increased. When the catalyst dosage was 0.84 g, the desulfurization rate decreased compared with that of 0.63 g. The desulfurization reaction basically reached a stable desulfurization effect in 10 min. When the catalyst dosage was 0.63 g, the best desulfurization rate (96.1%) could be obtained within 10 min. Therefore, the optimal catalyst dosage was determined to be 0.63 g (the catalyst dosage accounted for 1.5 wt% of the model oil quality).  The O/S molar ratio plays an important role in the desulfurization effect. Reactive oxygen molecules come from H2O2. According to the chemical equation of the DBT oxidation reaction, 1 mol DBT can be oxidized to the corresponding sulfone by 2 mol H2O2. However, due to the side reaction of self-decomposition of H2O2 in the reaction process, a larger amount of oxidant is often needed in the experiment. The influence of the O/S molar ratio was studied at 60 °C, using the model oil with an initial sulfur content of 500 ppm and a catalyst dosage of 0.63 g. The DBT conversion rates at different O/S molar ratios are shown in Figure 8. In the absence of oxidant, a desulfurization rate of 65.94% could be obtained within 20 min. With the addition of the oxidant, the desulfurization rate increased rapidly in a short time. The O/S molar ratio is directly proportional to the desulfurization rate and the reaction rate. When the O/S molar ratio was 20, the maximum desulfurization rate reached 98.40%, which was basically consistent with the desulfurization effect when the O/S molar ratio was 15 (desulfurization rate: 98.39%). Considering economic costs and energy utilization, the optimal O/S molar ratio was set to 15 for the following experiments.  The O/S molar ratio plays an important role in the desulfurization effect. Reactive oxygen molecules come from H2O2. According to the chemical equation of the DBT oxidation reaction, 1 mol DBT can be oxidized to the corresponding sulfone by 2 mol H2O2. However, due to the side reaction of self-decomposition of H2O2 in the reaction process, a larger amount of oxidant is often needed in the experiment. The influence of the O/S molar ratio was studied at 60 °C, using the model oil with an initial sulfur content of 500 ppm and a catalyst dosage of 0.63 g. The DBT conversion rates at different O/S molar ratios are shown in Figure 8. In the absence of oxidant, a desulfurization rate of 65.94% could be obtained within 20 min. With the addition of the oxidant, the desulfurization rate increased rapidly in a short time. The O/S molar ratio is directly proportional to the desulfurization rate and the reaction rate. When the O/S molar ratio was 20, the maximum desulfurization rate reached 98.40%, which was basically consistent with the desulfurization effect when the O/S molar ratio was 15 (desulfurization rate: 98.39%). Considering economic costs and energy utilization, the optimal O/S molar ratio was set to 15 for the following experiments. The O/S molar ratio plays an important role in the desulfurization effect. Reactive oxygen molecules come from H 2 O 2. According to the chemical equation of the DBT oxidation reaction, 1 mol DBT can be oxidized to the corresponding sulfone by 2 mol H 2 O 2 . However, due to the side reaction of self-decomposition of H 2 O 2 in the reaction process, a larger amount of oxidant is often needed in the experiment. The influence of the O/S molar ratio was studied at 60 • C, using the model oil with an initial sulfur content of 500 ppm and a catalyst dosage of 0.63 g. The DBT conversion rates at different O/S molar ratios are shown in Figure 8. In the absence of oxidant, a desulfurization rate of 65.94% could be obtained within 20 min. With the addition of the oxidant, the desulfurization rate increased rapidly in a short time. The O/S molar ratio is directly proportional to the desulfurization rate and the reaction rate. When the O/S molar ratio was 20, the maximum desulfurization rate reached 98.40%, which was basically consistent with the desulfurization effect when the O/S molar ratio was 15 (desulfurization rate: 98.39%). Considering economic costs and energy utilization, the optimal O/S molar ratio was set to 15 for the following experiments. Figure 9 shows the conversion rate of DBT at different temperatures, maintaining the other experimental conditions (the model oil with an initial sulfur content of 500 ppm, a catalyst dosage of 0.63 g and an O/S molar ratio of 15). The temperature increase is beneficial to increasing the reaction rate within 5 min. The DBT oxidation reaction at different temperatures reached equilibrium in 10 min. It is worth noting that a lower temperature can often achieve a higher desulfurization efficiency. The reason is that the rising temperature accelerates the decomposition of H 2 O 2 , which is not conducive to the continued progress of the DBT oxidation reaction, leading to less sulfur removal. Under the condition that the initial sulfur content is 500 ppm, O/S = 15, and the dosage of catalyst accounts for 1.5 wt% of the model oil quality, the DBT conversion rate reached 99.89% within 10 min at room temperature, achieving an ultra-fast and efficient desulfurization process. Desulfurization at room temperature can greatly reduce the economic cost and ensure fuel quality. Hence, the optimal reaction temperature is room temperature.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 18 Figure 9 shows the conversion rate of DBT at different temperatures, maintaining the other experimental conditions (the model oil with an initial sulfur content of 500 ppm, a catalyst dosage of 0.63 g and an O/S molar ratio of 15). The temperature increase is beneficial to increasing the reaction rate within 5 min. The DBT oxidation reaction at different temperatures reached equilibrium in 10 min. It is worth noting that a lower temperature can often achieve a higher desulfurization efficiency. The reason is that the rising temperature accelerates the decomposition of H2O2, which is not conducive to the continued progress of the DBT oxidation reaction, leading to less sulfur removal. Under the condition that the initial sulfur content is 500 ppm, O/S = 15, and the dosage of catalyst accounts for 1.5 wt% of the model oil quality, the DBT conversion rate reached 99.89% within 10 min at room temperature, achieving an ultra-fast and efficient desulfurization process. Desulfurization at room temperature can greatly reduce the economic cost and ensure fuel quality. Hence, the optimal reaction temperature is room temperature.

Comparison of Desulfurization Performance of Three Hybrid Catalysts Based [SMo12O40] 2−
Under the above optimized reaction conditions, the desulfurization performance of the [TBA]2SMo12O40 catalyst for different sulfur compounds (DBT, BT and 4,6-DMDBT) was investigated (Figure 10). For the removal of DBT, a high conversion rate of 99.89% was achieved when the reaction was carried out for 10 min. BT and 4,6-DMDBT are sulfur compounds that are more difficult to oxidize than DBT. Under the catalysis of [TBA]2SMo12O40, the removal rates of BT and 4,6-DMDBT reached 97.06% and 95.20%, respectively, when the reactions were carried out for 90 min. In a comparison of the ODS activity of different catalysts in a previous work and in this study (    [23]. They pointed out that the increase of the carbon chain length of the quaternary ammonium cation was beneficial to the enhancement of the activity of the catalyst. Zhuang et al. synthesized three molybdovanadophosphoric POM-based catalysts by contacting H5PMo10V2O40 with different ionic liquids [57]. They also reached a conclusion: The removal efficiency of the sulfur compound improved with the increase of the alkyl chain length of the catalyst. However, Lu et al. mentioned that quaternary ammonium cations with too long carbon chains may cause steric effects, reducing catalytic activity [49]. In our work, we observed differences in the solubility of three hybrid catalysts containing different cations in the acetonitrile.
[TBA]2SMo12O40 is fully soluble in acetonitrile, allowing more effective contact between sulfur-containing compounds and oxidizing active substances, thereby obtaining a high desulfurization efficiency. Under the catalysis of [TBA]2SMo12O40 and [BMIM]2SMo12O40, the activities of the three sulfur compounds all followed this sequence: DBT > BT > 4,6-DMDBT, which is consistent with the report by Li et al. [58]. However, 4,6-DMDBT showed a higher oxidation activity than BT when [ODA]2SMo12O40 acted as a catalyst. The activity of sulfur compounds is mainly related to electron density around the sulfur atom and steric hindrance.   [23]. They pointed out that the increase of the carbon chain length of the quaternary ammonium cation was beneficial to the enhancement of the activity of the catalyst. Zhuang et al. synthesized three molybdovanadophosphoric POM-based catalysts by contacting H 5 PMo 10 V 2 O 40 with different ionic liquids [57]. They also reached a conclusion: The removal efficiency of the sulfur compound improved with the increase of the alkyl chain length of the catalyst. However, Lu et al. mentioned that quaternary ammonium cations with too long carbon chains may cause steric effects, reducing catalytic activity [49]. In our work, we observed differences in the solubility of three hybrid catalysts containing different cations in the acetonitrile. [ 40 , the activities of the three sulfur compounds all followed this sequence: DBT > BT > 4,6-DMDBT, which is consistent with the report by Li et al. [58]. However, 4,6-DMDBT showed a higher oxidation activity than BT when [ODA] 2 SMo 12 O 40 acted as a catalyst. The activity of sulfur compounds is mainly related to electron density around the sulfur atom and steric hindrance.
According to Otsuki's report [59], the electron densities of sulfur atoms in DBT, BT and 4,6-DMDBT are 5.758, 5.739 and 5.760, respectively. The higher the electron density of sulfur atoms in sulfur compounds, the easier it is apt to be oxidized, so DBT is easier to remove than BT. However, the structure of 4,6-DMDBT contains two methyl groups. The effect of steric hindrance makes the removal process of 4,6-DMDBT more difficult. The [ODA] 2 SMo 12 O 40 catalyst with the long alkyl chain has greater ease in wrapping up the larger molecule of 4,6-DMDBT close to the active center [41]. Hence, 4,6-DMDBT had a better removal effect than BT when using the [ODA] 2 SMo 12 O 40 catalyst.

Catalyst Recovery
As excellent ODS catalysts, the recovery performance of [TBA] 2 SMo 12 O 40 and [BMI] 2 SMo 12 O 4 was investigated.
After the desulfurization reaction, [TBA] 2 SMo 12 O 40 was completely dissolved in the acetonitrile phase and could not be separated from the ODS system as a solid. However, the acetonitrile extraction solution containing the dissolved catalyst could be reused for the removal of sulfur compounds. After each reaction, the upper low-sulfur oil was removed, and the fresh model oil with an initial sulfur content of 500 ppm and H 2 O 2 (O/S = 15) were added to start a new round of ODS. The reaction time was set to 10 min. After five consecutive cycles of this, there was no apparent loss of catalyst activity. Figure 11 shows the desulfurization effect of the recovered [ According to Otsuki's report [59], the electron densities of sulfur atoms in DBT, BT and 4,6-DMDBT are 5.758, 5.739 and 5.760, respectively. The higher the electron density of sulfur atoms in sulfur compounds, the easier it is apt to be oxidized, so DBT is easier to remove than BT. However, the structure of 4,6-DMDBT contains two methyl groups. The effect of steric hindrance makes the removal process of 4,6-DMDBT more difficult. The [ODA]2SMo12O40 catalyst with the long alkyl chain has greater ease in wrapping up the larger molecule of 4,6-DMDBT close to the active center [41]. Hence, 4,6-DMDBT had a better removal effect than BT when using the [ODA]2SMo12O40 catalyst.

Catalyst Recovery
As excellent ODS catalysts, the recovery performance of [TBA]2SMo12O40 and [BMI]2SMo12O4 was investigated.
After the desulfurization reaction, [TBA]2SMo12O40 was completely dissolved in the acetonitrile phase and could not be separated from the ODS system as a solid. However, the acetonitrile extraction solution containing the dissolved catalyst could be reused for the removal of sulfur compounds. After each reaction, the upper low-sulfur oil was removed, and the fresh model oil with an initial sulfur content of 500 ppm and H2O2 (O/S = 15) were added to start a new round of ODS. The reaction time was set to 10 min. After five consecutive cycles of this, there was no apparent loss of catalyst activity. Figure 11 shows the desulfurization effect of the recovered [TBA]2SMo12O40 catalyst (99.89%, 99.89%, 99.89%, 99.32% and 99.01% of DBT conversion rate after 10 min for the first, to the fifth ODS cycle, respectively). The result showed that the [TBA]2SMo12O40 catalyst dissolved in acetonitrile still had an excellent catalytic activity after repeated use. [BMIM]2SMo12O40 performs heterogeneous catalysis in an ODS system. At the end of the reaction, the catalyst deposited on the bottom of the container was filtered, washed with distilled water, and dried in an oven. The desulfurization performance of the recovered [BMI]2SMo12O40 catalyst was evaluated in the optimized desulfurization system. After 15 min of reaction, the DBT conversion rate only reached 87%. This desulfurization result is lower than the desulfurization rate that was obtained during the initial catalysis. However, when the reaction time was extended to 45 min, the desulfurization rates obtained in five ODS recovery experiments were 99.81%, 99.81%, 99.78%, 99.07% and 98.87%, respectively ( Figure 12). This is not significantly different from the initial desulfurization rate. Moreover, the IR of the catalyst before and after the recovery were similar, which indicated that the structure of the catalyst was not destroyed during the catalytic desulfurization reaction. Therefore, the [BMIM]2SMo12O40 catalyst has excellent recovery activity. [BMIM] 2 SMo 12 O 40 performs heterogeneous catalysis in an ODS system. At the end of the reaction, the catalyst deposited on the bottom of the container was filtered, washed with distilled water, and dried in an oven. The desulfurization performance of the recovered [BMI] 2 SMo 12 O 40 catalyst was evaluated in the optimized desulfurization system. After 15 min of reaction, the DBT conversion rate only reached 87%. This desulfurization result is lower than the desulfurization rate that was obtained during the initial catalysis. However, when the reaction time was extended to 45 min, the desulfurization rates obtained in five ODS recovery experiments were 99.81%, 99.81%, 99.78%, 99.07% and 98.87%, respectively ( Figure 12). This is not significantly different from the initial desulfurization rate. Moreover, the IR of the catalyst before and after the recovery were similar, which indicated that the structure of the catalyst was not destroyed during the catalytic desulfurization reaction. Therefore, the [BMIM] 2 SMo 12 O 40 catalyst has excellent recovery activity. Molecules 2023, 28, x FOR PEER REVIEW 13 of 18

Oxidative Desulfurization of Real Diesel
The desulfurization performance of [TBA]2SMo12O40 and [BMIM]2SMo12O40 catalysts were evaluated in real diesel. Due to the complex and diverse components of real diesel, the removal effect of sulfur compounds is affected. The initial sulfur content of real diesel is 514.53 ppm. The reaction conditions are set as the amount of catalyst being 0.63 g (the amount of catalyst accounts for 1.5 wt% of the simulated oil quality), O/S = 15, and at room temperature. Using [TBA]2SMo12O40 as a catalyst, the sulfur content of real diesel was reduced from 514.53 ppm to 93.17 ppm after 150 min of reaction (Table 2). In order to further remove the sulfur compounds, the oil after the reaction was collected for the second ODS treatment, and the sulfur content in the oil was reduced to 20.18 ppm after 15 min. The total desulfurization rate of the [TBA]2SMo12O40 catalyst reached 96.08% after two ODS treatments. Under the catalysis of [BMIM]2SMo12O40, the sulfur content in the oil was decreased from 514.53 ppm to 41.52 ppm by two ODS treatments, maintaining the same reaction conditions (the reaction time of the first and second ODS process is 150 min and 60 min, respectively) (   Figure 13 shows the ODS process of DBT using SMo12O40 2− -organic hybrid catalysts. In the ODS system, DBT in model oil was extracted into the acetonitrile phase. The cation of the amphiphilic hybrid catalyst is lipophilic, which can capture the DBT close to the oxidation active center, thereby increasing the contact area between the oxidant and sulfur compound and improving the desulfurization rate. When the SMo12O40 2− reacts with the oxidant H2O2, it will generate the peroxy-active species with a stronger oxidizing ability. This peroxy-active species is the real oxidant in the ODS reaction. DBT can be oxidized to DBT sulfone by the action of the oxidant. The oxidation product sulfone is dissolved in the acetonitrile phase to achieve separation from the oil.  (Table 2). In order to further remove the sulfur compounds, the oil after the reaction was collected for the second ODS treatment, and the sulfur content in the oil was reduced to 20. 18 Figure 13 shows the ODS process of DBT using SMo 12 O 40 2− -organic hybrid catalysts. In the ODS system, DBT in model oil was extracted into the acetonitrile phase. The cation of the amphiphilic hybrid catalyst is lipophilic, which can capture the DBT close to the oxidation active center, thereby increasing the contact area between the oxidant and sulfur compound and improving the desulfurization rate. When the SMo 12 O 40 2− reacts with the oxidant H 2 O 2 , it will generate the peroxy-active species with a stronger oxidizing ability. This peroxy-active species is the real oxidant in the ODS reaction. DBT can be oxidized to DBT sulfone by the action of the oxidant. The oxidation product sulfone is dissolved in the acetonitrile phase to achieve separation from the oil.

Synthesis of Catalysts
[TBA]2SMo12O40 was prepared by the method described in the literature [44]. A certain amount of Na2MoO4·2H2O (6.05 g, 25 mmol) was dissolved in 200 mL distilled water and stirred for 5 min at room temperature. NH4VO3 (0.6 g, 5.1 mmol) dissolved in H2SO4 (50 mL, 2 mol/L) was added and stirred for 5 min. Next, CH3COCH3 (250 mL) was added to the aforementioned system. After the mixture was stirred for 60 min at room temperature, tetrabutylammonium bromide (abbreviated as TBAB, 10 g, 31 mmol) was added. In order to complete metathesis, the above mixture was stirred continuously for 30 min. The precipitation was filtered and washed by distilled water and ethanol. The obtained solid catalyst was dried in a vacuum oven. The synthesis method of [BMIM]2SMo12O40 (6.8 g, 31 mmol) and [ODA]2SMo12O40 (12.2 g, 31 mmol) is similar to the above method, and only requires the replacement of the addition of TBAB with 1-butyl-3-methylimidazolium bromide (BMIMBr) or octadecyl trimethyl ammonium bromide (ODAB) in the reaction system.

Characterization
Infrared absorption spectra (IR) were performed for the 400-4000 cm −1 region on a Nicolet iS10 spectrometer (Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) with a resolution of 4 cm −1 and 32 scans. X-ray diffraction (XRD) patterns were recorded on a Brooke D8 advance power diffractometer (Brucker AXS Co., Ltd, Karlsruhe, Germany) in the range of 2θ from 5° to 60°. The SEM images were acquired by a 450 FEG electron microscope (United States FEI Co., Ltd., Hillsboro, OR, USA). Thermogravimetric analysis was carried out using a TG 209F3 thermal analyzer (Germany NETZSCH Co., Ltd., Selb, Germany) in the temperature range between 30 °C and 600 °C with a heating rate of 10 °C min −1 under nitrogen. X ray photoelectron spectrometric (XPS) were analyzed by a Thermo Fischer Escalab 250Xi spectrometer (Thermo Fisher Scientific Co., Ltd.).

Oxidative Desulfurization Process for Model Oil
The model oil is prepared by dissolving a certain amount of DBT (BT or 4,6-DMDBT) in 60 mL of n-octane. The initial sulfur content of the model oil is 500 ppm. The ODS system employs H2O2 as the oxidant and acetonitrile as the extractant. In a typical ODS experiment, 60 mL model oil with a sulfur content of 500 ppm, 60 mL acetonitrile, a certain dosage of catalyst, and H2O2 were added into a 250 mL three-neck flask. The three-neck flask was placed in a water bath with magnetic stirring. In order to reduce the evaporation of oil at different temperatures, the middle port of the three-neck flask was connected with a condenser tube and the left and right ports are plugged with plugs. Samples were taken

Synthesis of Catalysts
[TBA] 2 SMo 12 O 40 was prepared by the method described in the literature [44]. A certain amount of Na 2 MoO 4 ·2H 2 O (6.05 g, 25 mmol) was dissolved in 200 mL distilled water and stirred for 5 min at room temperature. NH 4 VO 3 (0.6 g, 5.1 mmol) dissolved in H 2 SO 4 (50 mL, 2 mol/L) was added and stirred for 5 min. Next, CH 3 COCH 3 (250 mL) was added to the aforementioned system. After the mixture was stirred for 60 min at room temperature, tetrabutylammonium bromide (abbreviated as TBAB, 10 g, 31 mmol) was added. In order to complete metathesis, the above mixture was stirred continuously for 30 min. The precipitation was filtered and washed by distilled water and ethanol. The obtained solid catalyst was dried in a vacuum oven. The synthesis method of [BMIM] 2 SMo 12 O 40 (6.8 g, 31 mmol) and [ODA] 2 SMo 12 O 40 (12.2 g, 31 mmol) is similar to the above method, and only requires the replacement of the addition of TBAB with 1-butyl-3-methylimidazolium bromide (BMIMBr) or octadecyl trimethyl ammonium bromide (ODAB) in the reaction system.

Characterization
Infrared absorption spectra (IR) were performed for the 400-4000 cm −1 region on a Nicolet iS10 spectrometer (Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) with a resolution of 4 cm −1 and 32 scans. X-ray diffraction (XRD) patterns were recorded on a Brooke D8 advance power diffractometer (Brucker AXS Co., Ltd, Karlsruhe, Germany) in the range of 2θ from 5 • to 60 • . The SEM images were acquired by a 450 FEG electron microscope (United States FEI Co., Ltd., Hillsboro, OR, USA). Thermogravimetric analysis was carried out using a TG 209F3 thermal analyzer (Germany NETZSCH Co., Ltd., Selb, Germany) in the temperature range between 30 • C and 600 • C with a heating rate of 10 • C min −1 under nitrogen. X ray photoelectron spectrometric (XPS) were analyzed by a Thermo Fischer Escalab 250Xi spectrometer (Thermo Fisher Scientific Co., Ltd.).

Oxidative Desulfurization Process for Model Oil
The model oil is prepared by dissolving a certain amount of DBT (BT or 4,6-DMDBT) in 60 mL of n-octane. The initial sulfur content of the model oil is 500 ppm. The ODS system employs H 2 O 2 as the oxidant and acetonitrile as the extractant. In a typical ODS experiment, 60 mL model oil with a sulfur content of 500 ppm, 60 mL acetonitrile, a certain dosage of catalyst, and H 2 O 2 were added into a 250 mL three-neck flask. The three-neck flask was placed in a water bath with magnetic stirring. In order to reduce the evaporation of oil at different temperatures, the middle port of the three-neck flask was connected with a condenser tube and the left and right ports are plugged with plugs. Samples were taken from the upper oil phase at a certain time. The sulfur content of the sample was analyzed in a WK-2E microcoulometer. The conversion rate of DBT (BT or 4,6-DMDBT) was calculated using the following formula.
Conversion (%) = (C 0 − C t )/C 0 × 100% where C 0 is the initial sulfur content in oil, and C t is the sulfur content at the different samples.

Conclusions
This paper presented an effective method for the deep desulfurization of model oil using the amphipathic SMo 12  2 SMo 12 O 40 catalyst can make the DBT conversion rate reach 99.47% within 60 min. The catalysts also have an excellent removal effect on BT and 4,6-DMDBT. The oxidation activity of the sulfur compound is affected by the electron density and steric hindrance. The desulfurization results of catalysts show that DBT is more easily oxidized than BT and 4,6-DMDBT. Under the condition of [ODA] 2 SMo 12 O 40 as a catalyst, 4,6-DMDBT is easier to remove than BT, which is different from the desulfurization performance of the other two catalysts. The reason is that it is easier for the long alkyl chain to wrap up the larger molecule of 4,6-DMDBT close to the active center. The recycle performance of catalysts were investigated. The [TBA] 2 SMo 12 O 40 immobilized in the acetonitrile phase was still reactive after the reaction, and can be reused at least five times for ODS. [ 40 2− -based catalyst provides a simple, mild and fast approach for deep desulfurization. However, the preparation cost of an SMo 12 O 40 2− -based catalyst is comparatively approximately 20% higher compared to that of the current commercial catalyst, and further R&D is needed for an SMo 12 O 40 2− -based catalyst with regard to laboratory and industrial applications.