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Article

Heteropolyacid-Based Poly(Ionic Liquid) Catalyst for Ultra-Deep and Recyclable Oxidative Desulfurization of Fuels

by
Mengyue Chen
,
Tianqi Huang
,
Shuang Tong
,
Chao Wang
and
Ming Zhang
*
Institute for Energy Research & School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(7), 622; https://doi.org/10.3390/catal15070622
Submission received: 14 April 2025 / Revised: 9 June 2025 / Accepted: 22 June 2025 / Published: 24 June 2025
(This article belongs to the Special Issue Ionic Liquids and Deep Eutectic Solvents in Catalysis)
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Abstract

To address the challenge of ultra-deep desulfurization in fuels, a series of heteropolyacid-based poly(ionic liquid) catalysts (C4-PIL@PW, C8-PIL@PW, and C16-PIL@PW) were synthesized via radical polymerization and anion exchange methods. The prepared catalysts were characterized via FT-IR, XRD pattern, and Raman spectroscopy. Optimal reaction parameters (e.g., temperature, catalyst dosage, and O/S molar ratio) were systematically investigated, as well as the catalytic mechanism. The typical sample C8-PIL@PW exhibited exceptional oxidative desulfurization (ODS) performance, achieving a sulfur removal of 99.2% for dibenzothiophene (DBT) without any organic solvent as extractant. Remarkably, the sulfur removal could still retain 89% after recycling five times without regeneration. This study provides a sustainable and high-efficiency catalyst for ODS, offering insights into fuel purification strategies.

1. Introduction

With the increasing consumption of diesel fuel, the resulting emissions of sulfur oxides (SOX) inevitably lead to serious environmental issues, including acid rain and corrosion, threatening both human health and ecological safety [1,2]. Therefore, the exploration for ultra-deep desulfurization of fuels has garnered widespread attention [3,4,5]. Stringent global regulations, such as the Euro VI and China VI standards, mandate sulfur content limits of less than 10 ppm in commercial fuels [6,7,8,9]. Currently, conventional hydrodesulfurization (HDS) technology exhibits high efficacy in eliminating small molecular sulfides, such as mercaptans and thioether [10,11]. However, HDS faces inherent limitations in removing refractory aromatic sulfur compounds, such as DBT and its derivatives (4-MDBT, 4,6-DMDBT), due to their steric hindrance and low electron density on sulfur atoms [2]. Moreover, HDS requires harsh operating conditions (300–400 °C, 2–10 MPa) [12], leading to high energy consumption and production costs. Thus, alternative technologies operating under milder conditions are urgently required.
Oxidative desulfurization (ODS) is considered as a promising approach, which can convert aromatic sulfur compounds into polar sulfones under ambient conditions [13]. Various oxidants are applied in the ODS process, such as molecular oxygen [14,15], hydrogen peroxide [16,17], and tert-butyl hydrogen peroxide [18,19]. Among these oxidants, hydrogen peroxide (H2O2) [16,17,20] is widely adopted due to its high efficiency [21] and eco-friendly byproduct (water) [7,22]. In the ODS process, the efficiency depends on catalysts capable of activating H2O2 to generate reactive oxygen species.
Various samples have been applied in the ODS process, including polyoxometalates [23], metal oxides [24], ILs [25], and metal-organic frameworks (MOFs) [26]. Among them, polyoxometalates (POMs) exhibit a variety of advances, such as structural tunability, strong redox capacity, and excellent electron transfer capability [8]. In particular, phosphotungstic acid (HPW) with a Keggin-type structure has demonstrated remarkable ODS activity [27]. For instance, Ding reported on a phosphotungstic reaction-controlled phase transfer catalyst (Q9PW9), which could achieve deep desulfurization under optimal conditions, as well as recyclability for three cycles [28]. However, pure POMs often suffer from low surface area (<10 m2/g), inherent hydrophilicity, and poor cyclic performance, impeding their performance in biphasic oil–water systems [19,29,30]. The immobilization of POMs on a solid support has been explored to enhance surface accessibility [31,32]. Although these inorganic frameworks can improve surface area, their hydrophilic nature reduces interactions with hydrophobic reactants in oil [33,34].
Ionic liquids (ILs) and poly(ionic liquid)s (PILs) have emerged as multifunctional catalysts or carriers due to their designable structures, tunable hydrophilicity–hydrophobicity, and mass transfer properties [35,36,37,38] in various fields, including porous materials [39], sorbents materials [40], and electrochemical materials [41]. Although IL-based homogeneous systems can enhance desulfurization efficiency [42,43,44,45,46], their high cost and difficult separation would hinder practical applications in industry [45]. On the other hand, PILs can offer additional advantages such as high specific surface area, mechanical stability, and the ability to fabricate hierarchical porous structures [39,40,41], making them ideal carriers for heterogeneous catalysis [23]. The versatility of PILs has been demonstrated across various catalytic systems. Mayuri et al. integrated hexacyanoferrate (III) ions (Fe(CN)63−) into a kind of PIL-graphitic composite through a facile anion exchange strategy [47], highlighting the potential of PILs to stabilize functional anions. However, combining POMs with PILs while preserving the Keggin structure and redox activity of POMs remains challenging. Precise synthetic strategies, such as anion exchange or in-situ polymerization, are necessary to achieve this integration. Such hybrid systems could leverage the synergistic effects of the redox capability of POMs and the tunable amphiphilicity of PILs, offering a promising pathway to develop highly efficient ODS catalysts.
Herein, a series of HPW-based PIL catalysts (C4-PIL@PW, C8-PIL@PW, and C16-PIL@PW) was prepared via the radical polymerization and anion exchange methods. By modulating the alkyl chain length of PILs, we optimized the hydrophilic–hydrophobic balance at the oil–water–catalyst interface, thereby enhancing oxidative efficiency. Additionally, the retained Keggin structure of HPW and the mesoporous properties of PILs can facilitate reactant diffusion. The prepared catalysts were characterized by FT-IR, XRD, and Raman spectroscopy, and reaction parameters (temperature, catalyst dosage, and oxidant ratio) were systematically optimized. Notably, the typical sample C8-PIL@PW can achieve sulfur removal of 99.2% and retain 89% removal over five cycles without regeneration.

2. Results

Before discussing the characterization and performance of the catalysts, the structural definitions of C4-PIL@PW, C8-PIL@PW, and C16-PIL@PW are provided for clarity. These notations denote heteropolyacid-based poly(ionic liquid) catalysts, where Cn denotes the alkyl chain length (n = 4, 8, or 16 carbon atoms) on the imidazole ring, PIL stands for the poly(ionic liquid) framework, and PW refers to the HPW anion incorporated via anion exchange. Additionally, Cn-IL@PW refers to catalysts using ionic liquid (IL) monomers (rather than polymers) as the support, with “n” indicating the alkyl chain length on the imidazole ring. The synthetic schematic illustration of both Cn-IL@PW and Cn-PIL@PW is displayed in Scheme 1.

2.1. Characterization of Materials

The successful synthesis of the catalyst was initially confirmed using FT-IR spectra. Figure 1 shows the FT-IR spectra of C4-PIL@PW, C8-PIL@PW, C16-PIL@PW, and HPW. The absorption peaks of the imidazole ring were observed at 1660, 1566, and 1544 cm−1, corresponding to the N-H bending vibration, C=N and C=C stretching bonds, respectively [48]. The peaks at 1374 cm−1, 1461 cm−1, 2950 cm−1, and 2850 cm−1 represent the stretching vibration of C-H on the alkyl chain of ILs [49], with increasing intensity as the alkyl chain lengthens. Additionally, pure HPW exhibits characteristic peaks at 1008 cm−1 (P-O stretching vibration), 978 cm−1 (W=O stretching vibration), 898 cm−1 (W-O-W bridging stretching vibration), and 808 cm−1 (W-O stretching vibration) (Figure 1d) [50], which is attributed to the Keggin units, indicating successful introduction of the HPW anion into the PILs. Notably, a blue-shift of the W-O peak to higher wavenumbers in Cn-PIL@PW indicated electrostatic interactions between HPW anions and PIL cations. This structural balance, combined with the optimal alkyl chain length in C8-PIL@PW, could enhance interfacial mass transfer and catalytic activity for ultra-deep desulfurization.
Raman spectroscopy was performed to further confirm the successful introduction of the HPW anion (Figure 2). Two distinct absorption peaks were observed at 990 cm−1 and 1005 cm−1. The former was attributed to the symmetric stretching vibration (Vs) of W=O, whereas the latter was ascribed to the asymmetric stretching vibration (Vas) of W=O within the Keggin structure [51]. The above results demonstrate that the active center structure of HPW anions remains unchanged after their introduction into PILs.
XRD spectra are presented in Figure 3. A series of sharp diffraction peaks ranging from 15–40° is observed in Figure 3d, indicating the presence of the Keggin structure of HPW. In Figure 3a–c, the broad diffraction peak in the range of 15–25° was ascribed to the amorphous structures of the PILs. However, the peaks attributed to the HPW disappear, suggesting the uniform dispersion of HPW in PILs. This uniform dispersion is considered one of the key factors contributing to the enhanced catalytic performance of tungsten-based PILs in the ODS process.
N2 adsorption–desorption isotherms were employed to analyze the pore structure and pore size distribution of catalysts with varying carbon chain lengths. As shown in Figure 4, C4-PIL@PW exhibited a Type IV isotherm with an H4 hysteresis loop, indicating the presence of micro-mesopores with narrow slit-like pores. In contrast, C8-PIL@PW and C16-PIL@PW displayed Type IV isotherms with H3 hysteresis loops, suggesting that these samples consisted of plate-like particles while retaining micro-mesoporous structures. Notably, the pore structure of the catalysts underwent significant changes as the carbon chain length increased. When the chain length reached C16, pore blockage occurred due to the stacking of long carbon chains. The relatively lower specific surface area of C16-PIL@PW (0.6891 m2/g) arose from pore blockage by long alkyl chain stacking [52,53], whereas its large average pore size (35.8 nm) likely reflects non-uniform interparticle voids rather than structured mesopores, as indicated by the H3 hysteresis loop and consistent with semi-quantitative BET interpretation for heterogeneous polymeric materials. From Figure 4a and the pore structure data in Table 1, it was observed that C16-PIL@PW exhibited a substantially reduced nitrogen adsorption capacity compared to catalysts with shorter carbon chains, along with inferior pore parameters (specific surface area, pore size). The pore size distribution curves (Figure 4b) further confirmed the coexistence of micropores and mesopores in the catalysts. The mesopore ranges of C4-PIL@PW and C8-PIL@PW were 3–30 nm and 3–50 nm, respectively. Specifically, the mesopores of C4-PIL@PW were predominantly concentrated around 10 nm, which might significantly hinder mass transfer efficiency. In contrast, C8-PIL@PW showed a more uniform mesopore distribution (10–50 nm), facilitating better adsorption of the reactant (DBT) and subsequent interaction with active sites. According to the BET results (Table 1), the specific surface areas and pore volumes of all samples were summarized. Remarkably, C8-PIL@PW demonstrated the highest specific surface area (10.8744 m2g−1) and the largest pore volume, suggesting its superior potential for catalytic oxidation reactions.
Cyclic voltammetry (CV) was employed to evaluate the oxidative capability of various samples (Figure 5). The anode current was generated in three catalyst electrodes, and C8-PIL@PW exhibited the highest current and the best conductivity, indicating a relatively higher electron transfer rate. According to previous studies [13,54,55,56], the electron transfer rate has an evident effect on catalytic ability, further proving that C8-PIL@PW has better catalytic performance, which is consistent with the catalytic results in Table 2.

2.2. ODS Performance

The ODS performance of different catalysts was investigated under identical conditions (Table 2). The sulfur removal of the original HPW catalyst was only 26.2% (entry five), which was ascribed to its incompatibility with the oil phase. Conversely, C8-IL@PW exhibited superior desulfurization performance, which reached 89.4% sulfur removal under identical reaction conditions (entry four). This improvement was attributed to the introduction of a long carbon chain to enhance phase transfer ability and promote interaction between active sites and sulfur compounds [6]. Notably, C8-PIL@PW demonstrates even higher catalytic activity due to its larger specific surface area [52,53], enabling more efficient contact between catalytic active sites and sulfur-containing substrates. This conclusion is supported by the data in Table 1. Previous work suggests that H2O2 initially reacts with phosphotungstic acid anion to form peroxyactive substances during ODS, followed by the reaction of these substances with thiophene compounds to form sulfone substances [57]. The amphiphilic properties of the catalysts, stemming from the organic cation and HPW anion components, facilitate mass transfer among the three phases [58]. Table 2 suggests that sulfur removal decreased following the order of C8-PIL@PW > C4-PIL@PW > C16-PIL@PW (entry 1–3). C8-PIL@PW appears to strike a balance in permeability between the water and oil phases, thereby leading to significantly enhanced desulfurization efficiency in this study.
The reaction temperature significantly influences the desulfurization process, prompting an investigation into its effect on desulfurization performance. As shown in Figure 6a, sulfur removal reached a remarkable 99.2% when the reaction temperature was raised to 50 °C, meeting the criteria for deep desulfurization. However, sulfur removal marginally decreased with further temperature enhancement, possibly due to the accelerated thermal decomposition of H2O2 at higher temperatures. The impact of catalyst dosage on desulfurization efficiency was also examined (Figure 6b). It is evident that sulfur removal reached 99.2% when the catalyst dosage increased from 30 mg to 50 mg. However, sulfur removal markedly decreased to 70% at a dosage of 100 mg, likely attributed to catalyst aggregation. As depicted in Figure 6c, the O/S ratio significantly affects the catalytic oxidation process. When the O/S ratio increased to 6, desulfurization efficiency reached 99.2%. Nonetheless, desulfurization efficiency slightly decreased with an increasing O/S ratio due to catalyst accumulation, leading to a reduction in active sites.
Various refractory sulfur compounds are known to exist in real fuel, including 4-MDBT and 4,6-DMDBT. Hence, the ODS test of these compounds using the catalyst was further explored (Figure 6d). The results indicate that the sulfur removal of DBT, 4-MDBT, and 4,6-DMDBT reached up to 99.2%, 50%, and 60% after 2 hours of reaction, respectively. According to previous literature, it was indicated that the electron densities of sulfur atoms of DBT, 4-MDBT, and 4,6-DMDBT are 5.758, 5,759, and 5.760 [59]. It has been proposed that catalytic activity may correlate with the electron density of sulfur atoms based on the electrophilic addition mechanism [60]. However, DBT lacks the methyl groups present in 4-MDBT and 4,6-DMDBT, suggesting that DBT has lower steric hindrance. This suggests that the desulfurization activity of catalysts on different sulfur-containing substrates is primarily influenced by sulfide steric hindrance. Additionally, C8-PIL@PW achieved a sulfur removal efficiency of 78.8% for real diesel fuel (initial sulfur content: 496 ppm, containing nitrogen-containing compounds and aromatics) under optimized conditions, further demonstrating its practical potential for industrial applications. Consequently, DBT is more readily oxidized to the corresponding sulfone, demonstrating superior ODS performance.
To compare the prepared sample with previous studies, the ODS performance of other related polyoxometalate catalysts using hydrogen peroxide as the oxidant is presented in Table 3. Comparatively, the prepared sample C8-PIL@PW exhibits higher desulfurization activity without using any organic solvent as an extractant [61,62].

2.3. Catalyst Recycling Capacity

The stability and recyclability of a catalyst is also an important criterion for industrial applications. The recycling process proceeded as follows: the upper oil phase was directly separated after the reaction, and the lower catalyst phase was dried at 80 °C overnight. Under identical operational conditions, fresh model oil and H2O2 were added for the subsequent cycle. The sulfur content of the oil was determined using GC-MS. As shown in Figure 7, even after five cycles, the sulfur removal efficiency of the catalyst remained high at 89%, demonstrating that the C8-PIL@PW catalyst exhibits good durability. Following the circulation experiment, the recycled catalyst was separated from the oil by centrifugation and was dried at 80 °C. The collected catalyst was then tested using FT-IR analysis. As illustrated in Figure 8, it is evident that there were no structural changes observed in the recycled catalyst. Overall, these results confirm that C8-PIL@PW is not only highly stable but also possesses excellent recyclability properties. To further verify the absence of phosphotungstic acid (HPW) leaching into the oil phase, both FT-IR and inductively coupled plasma (ICP) analyses were conducted. Additionally, ICP was used to measure the concentration of tungsten (W) in the oil phase after reaction with the W content below the detection limit (<0.1 ppm). It can be observed from Figure 9 that the typical characteristic peak of HPW did not appear in the oil after the reaction, which indicated that C8-PIL@PW was not dissolved into the oil during the reaction process. The above results show that its structure is stable.

2.4. Catalytic Mechanism

In order to explore the mechanism of the catalytic reaction in the ODS process, radical quenching experiments were conducted. The formation of peroxyl radicals was confirmed through the use of para-benzoquinone (BQ), a superoxide radical quencher, and tert-butyl alcohol (TBA), a hydroxyl radical quencher [68] (Figure 10). In the presence of TBA, the sulfur removal decreased to 78% after 2 h, indicating the involvement of hydroxyl radicals (•OH) as oxidative species. Meanwhile, upon addition of BQ, a sharp reduction in sulfur removal was observed, suggesting the influence of superoxide radicals (•O2−) on the catalytic reaction. These findings collectively indicate a synergistic effect between •O2− and •OH in the desulfurization process [5].
The structural advantages of C8-PIL@PW further facilitate this process. As shown in Table 1, C8-PIL@PW exhibits the highest specific surface area (10.8744 m2/g) and optimal mesoporous distribution (10–50 nm), with a critical average pore size of 22.5 nm. This structural feature significantly enhances mass transfer efficiency, allowing the rapid diffusion of DBT molecules to active sites. In contrast, C16-PIL@PW suffers from severely reduced surface area (0.6891 m2/g) and pore blockage due to the stacking of long alkyl chains, which explains its poor desulfurization efficiency (9.4%). For the control sample C8-IL@PW, although its N2 adsorption isotherms exhibit a Type IV curve with an H3 hysteresis loop (indicating mesoporous structures), its surface area (1.1102 m2/g) and pore size (9.8 nm) are significantly inferior to those of C8-PIL@PW. This comparison confirms that PIL polymerization is critical for constructing hierarchical porosity—the polymeric framework not only increases specific surface area but also creates larger mesopores (22.5 nm in C8-PIL@PW) to overcome mass transfer limitations, a key factor in its superior activity (99.2% sulfur removal, Table 2). These findings align with literature studies on mesoporous catalysts in ODS, which emphasize the decisive role of pore size and surface area in optimizing reactant accessibility [31,69].
The reaction products of the heterogeneous catalyst C8-PIL@PW in ODS were systematically characterized by GC-MS analysis. As illustrated in Figure 11a, characteristic peaks corresponding to tetradecane (4.2 min) and dibenzothiophene (DBT, m/z= 184) at 7.1 min were clearly identified in the model oil phase prior to the reaction. Analysis of the oil during the reaction (Figure 11b) revealed a significant reduction in DBT concentration accompanied by the emergence of a minor peak for dibenzothiophene sulfone (DBTO2, m/z= 216). Notably, the extracted catalyst phase (Figure 11d) displayed a prominent DBTO2 signal at 11.6 min of retention time, confirming successful oxidation of the sulfur-containing compound. Critical evidence for complete conversion was obtained from Figure 11c, where the complete disappearance of the DBT peak in the post-reaction oil phase demonstrated near-total oxidation of the substrate within the designated reaction period. The absence of residual DBT signals in both catalytic and oil phases, coupled with the exclusive detection of DBTO2 in the catalyst phase, provides conclusive evidence for the effective transformation pathway from DBT to its sulfone derivative through this ODS process.

3. Materials and Methods

3.1. Materials

Dibenzothiophene (DBT, 98%), 4,6-dimethyldibenzothiophene (4,6-DMDBT, 97%), and 4-methyldiben-zothiophene (4-MDBT, 97%) were all purchased from Sigma-Aldrich (St. Louis, MO, USA). Phosphotungstic acid (HPW) was purchased from Wokai Chemical Reagent Co., Ltd. (Shanghai, China). Anhydrous alcohol and H2O2 (30 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Hebei, China). The following reagents were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China): 1-vinyl-3-octylimidazolium bromide (99%), 1-Vinyl-3-butylimidazole bromide (99%), 1-Vinyl-3-cetylimidazole bromide (99%), 2,2′-azobisisobutyronitrile (AIBN, 97%), and Octane (98%). All reagents and solvents are of analytical grade and were used directly without further purification.

3.2. Catalyst Preparation

  • Polymerization of ionic liquid
The poly(ionic liquid)s were prepared as follows: 1-vinyl-3-octylimidazolium bromide (11.5 mmol), which is denoted as C8-IL, was dissolved into a single neck flask with 15 mL ethanol. Then, AIBN (0.0575 g) was put into the above solution and further stirred in a nitrogen environment for 24 h at 70 °C. After the reaction, the solution was cooled to room temperature and dropped slowly into cyclohexane, and the precipitate was dried at 70 °C to obtain poly (1-vinyl-3-octylimidazolium bromide, denoted as C8-PIL).
  • Synthesis of PIL@PW and IL@PW
The tungsten-based poly(ionic liquid), denoted as C8-PIL@PW, was synthesized according to the literature [6,70]. In short, C8-PIL (2.5 mmol) was dispersed into 50 mL of ethanol, HPW (0.85 mmol) was dissolved in 10 mL of deionized water, and the solution was dropped slowly into the above solution with vigorous stirring, maintaining stirring at room temperature for 24 h. The white precipitate was centrifuged and washed with deionized water and anhydrous ethanol three times, and the product was obtained by drying in a vacuum at 60 °C for 12 h. For C8-IL@PW, C8-IL (the monomer, 11.5 mmol) and HPW, with a molar ratio of 3:1, was mixed in 50 mL of deionized water with continuous stirring at room temperature for 24 h. The same synthetic method was applied to prepare C4-PIL@PW, C16-PIL@PW, C4-IL@PW, and C16-IL@PW, using their corresponding monomers or polymers. The catalyst was named as follows: in Cn-PIL@PW, n = 4, 8, or 16 indicates the number of carbon atoms in the alkyl chain on the imidazole ring. PIL stands for poly(ionic liquid) carrier, and PW represents phosphotungstic acid.

3.3. Characterization

Fourier transform infrared (FT-IR) spectra were operated in the 4000–400 cm−1 regions with KBr pellets as the media on a Nicolet Nexus 470 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). X-ray powder diffraction (XRD) patterns were recorded on an XRD-6100 (Shimadzu, Japan) using Cu Kα radiation (λ = 1.5406 Å), with a scanning rate of 7° min−1 over a 2θ range of 10° to 80°. The specific surface area was measured using a TriStar II 3020 analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA) by the Brunauer–Emmett–Teller (BET) method, and the materials were degassed at 393 K for 3 h under a vacuum of 10−3 Torr before analysis. Raman spectra were collected on a DXR Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA), using a 532 nm excitation laser power. Inductively coupled plasma (ICP) analysis for tungsten leaching was performed using an ICP-OES 5110 spectrometer (Agilent Technologies, Santa Clara, CA, USA) with a detection limit of 0.1 ppm. CV curves were measured using an ITO working electrode, an Ag/AgCl reference electrode, and a Pt counter electrode in a 0.1 mol/L KOH electrolyte. The sulfur content of real oil was determined using a UV fluorescence sulfur detector (GCTS-3000 model) (AMETEK, Berwyn, IL, USA) under the experimental conditions of a magnification factor of 10 and an injection volume of 10 μL.

3.4. Catalytic Activity Test

The model oil was composed of n-octane as the main solvent, 1.4659 g of DBT, and 2.0202 g of n-tetradecane as the internal standard. The n-tetradecane and DBT were firstly dissolved in a beaker, and then the mixture was transferred into a 500 mL volumetric flask. Finally, n-octane was added as the solvent and the volume was fixed with a glass rod, making the sulfur content of the model oil reach 500 ppm with the internal standard of 4000 ppm. Typically, a certain amount of catalyst and 5 mL of model oil were poured into the self-made bottle equipped with a condenser tube and magnetic stirrer. The mixture was stirred vigorously at 700 r/min in a water bath at the preset temperature for 2 h. Then, 30 wt% H2O2 was quickly injected into the bottle using a syringe at t = 0. The upper samples were taken from the oil phase at 30-minute intervals, and the sulfur content was determined by gas chromatography (GC, Agilent-7890A).

4. Conclusions

In this work, a series of heterogeneous heteropolyacid-based poly(ionic liquid) catalysts (C4-PIL@PW, C8-PIL@PW, and C16-PIL@PW) were successfully synthesized and employed as catalysts in the ODS process. Notably, C8-PIL@PW demonstrated outstanding catalytic performance in the ODS process of DBT, achieving 99.2% sulfur removal within 2 h under optimized conditions without adding any organic solvent as an extractant. Additionally, even after recycling five times without any treatment, the sulfur removal remained at 89%. Furthermore, the characterization results revealed that the catalyst maintained its original structure, leading to remarkable stability. These results suggest that C8-PIL@PW has potential as a promising catalyst for ODS applications.

Author Contributions

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

Funding

This paper was funded by the National Natural Science Foundation of China (22108105).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00622 sch001
Scheme 1. Schematic Diagram of the Catalyst Synthesis Process.
Scheme 1. Schematic Diagram of the Catalyst Synthesis Process.
Catalysts 15 00622 sch001
Catalysts 15 00622 g001
Figure 1. FT-IR spectra of various samples: (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
Figure 1. FT-IR spectra of various samples: (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
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Catalysts 15 00622 g002
Figure 2. Raman spectra of various samples: (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
Figure 2. Raman spectra of various samples: (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
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Catalysts 15 00622 g003
Figure 3. XRD patterns of (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
Figure 3. XRD patterns of (a) C4-PIL@PW; (b) C8-PIL@PW; (c) C16-PIL@PW; and (d) HPW.
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Catalysts 15 00622 g004
Figure 4. (a) Nitrogen adsorption–desorption isotherms; (b) pore size distribution.
Figure 4. (a) Nitrogen adsorption–desorption isotherms; (b) pore size distribution.
Catalysts 15 00622 g004
Catalysts 15 00622 g005
Figure 5. Cyclic voltammetry curves of the as-prepared materials in 0.1 M KOH solutions.
Figure 5. Cyclic voltammetry curves of the as-prepared materials in 0.1 M KOH solutions.
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Catalysts 15 00622 g006
Figure 6. Effect of the reaction conditions of C8-PIL@PW. (a) m(catalyst) = 50 mg, O/S = 6; (b) T = 50 °C, O/S = 6; (c) m(catalyst) = 50 mg, T = 50 °C; and (d) m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
Figure 6. Effect of the reaction conditions of C8-PIL@PW. (a) m(catalyst) = 50 mg, O/S = 6; (b) T = 50 °C, O/S = 6; (c) m(catalyst) = 50 mg, T = 50 °C; and (d) m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
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Catalysts 15 00622 g007
Figure 7. Recycling performance of catalyst C8-PIL@PW. Reaction condition: m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
Figure 7. Recycling performance of catalyst C8-PIL@PW. Reaction condition: m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
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Catalysts 15 00622 g008
Figure 8. The FT-IR spectra of the fresh and recycled catalyst materials.
Figure 8. The FT-IR spectra of the fresh and recycled catalyst materials.
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Catalysts 15 00622 g009
Figure 9. The FT-IR spectra of the fresh catalyst and the model oil after the reaction.
Figure 9. The FT-IR spectra of the fresh catalyst and the model oil after the reaction.
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Catalysts 15 00622 g010
Figure 10. Radical quenching experiment with BQ and TBA. Reaction condition: m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
Figure 10. Radical quenching experiment with BQ and TBA. Reaction condition: m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
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Catalysts 15 00622 g011
Figure 11. The GC-MS analysis of (a) the model oil; (b) the oil phase in reaction; (c) the oil phase after the reaction; and (d) the extraction phase of C8-PIL@PW.
Figure 11. The GC-MS analysis of (a) the model oil; (b) the oil phase in reaction; (c) the oil phase after the reaction; and (d) the extraction phase of C8-PIL@PW.
Catalysts 15 00622 g011
Table 1. BET surface area and pore volumes of materials.
Table 1. BET surface area and pore volumes of materials.
SampleSBET (m2g−1)Pore Volume (cm3g−1)Average Pore Size (nm)
C4-PIL@PW 8.2420 0.0208 4.1
C8-PIL@PW 10.8744 0.0819 22.5
C16-PIL@PW 0.6891 0.0114 35.8
C8-IL@PW 1.1102 0.0019 9.8
Table 2. Catalytic performance of various catalysts.
Table 2. Catalytic performance of various catalysts.
EntrySamplesSulfur Removal (%)
1 C4-PIL@PW 41.0
2 C8-PIL@PW 99.2
3 C16-PIL@PW 9.4
4 C8-IL@PW89.3
5C4-IL@PW24.9
6C16-IL@PW69.2
Reaction condition: m(catalyst) = 50 mg, T = 50 °C, O/S = 6.
Table 3. Desulfurization performance of various samples.
Table 3. Desulfurization performance of various samples.
EntryCatalystSolventsT (°C)O/SSulfur Removal (%)Literature
1 [C2(MIM)2]PW12O40acetonitrile50698.35[61]
2 PMo12O40@MnFe2O4acetonitrile353-mL oxidant (H2O2/CH3COOH)98[62]
3 [CTA]11P2W13V5O64-70690[63]
4 [THTDP]Cl-MoO-30698.37[64]
5PW12@TM–SBA-15[BMIM] PF6, acetonitrile708100[65]
6[PyPS]3(NH4)3Mo7O24[Omim]BF470599[66]
7[(C4H9)4N]3{PO4[MoO(O2)2]4[Bmim]BF470697.3[67]
8C8-PIL@PW-50699.2This work
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Chen, M.; Huang, T.; Tong, S.; Wang, C.; Zhang, M. Heteropolyacid-Based Poly(Ionic Liquid) Catalyst for Ultra-Deep and Recyclable Oxidative Desulfurization of Fuels. Catalysts 2025, 15, 622. https://doi.org/10.3390/catal15070622

AMA Style

Chen M, Huang T, Tong S, Wang C, Zhang M. Heteropolyacid-Based Poly(Ionic Liquid) Catalyst for Ultra-Deep and Recyclable Oxidative Desulfurization of Fuels. Catalysts. 2025; 15(7):622. https://doi.org/10.3390/catal15070622

Chicago/Turabian Style

Chen, Mengyue, Tianqi Huang, Shuang Tong, Chao Wang, and Ming Zhang. 2025. "Heteropolyacid-Based Poly(Ionic Liquid) Catalyst for Ultra-Deep and Recyclable Oxidative Desulfurization of Fuels" Catalysts 15, no. 7: 622. https://doi.org/10.3390/catal15070622

APA Style

Chen, M., Huang, T., Tong, S., Wang, C., & Zhang, M. (2025). Heteropolyacid-Based Poly(Ionic Liquid) Catalyst for Ultra-Deep and Recyclable Oxidative Desulfurization of Fuels. Catalysts, 15(7), 622. https://doi.org/10.3390/catal15070622

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