Ionic Liquids as Green and Efficient Desulfurization Media Aiming at Clean Fuel

With increasingly stringent emission limits on sulfur and sulfur-containing substances, the reduction and removal of sulfur compounds from fuels has become an urgent task. Emissions of sulfur-containing compounds pose a significant threat to the environment and human health. Ionic liquids (ILs) have attracted much attention in recent years as green solvents and functional materials, and their unique properties make them useful alternatives to conventional desulfurization organic solvents. This paper reviews the advantages and disadvantages of traditional desulfurization technologies such as hydrodesulfurization, oxidative desulfurization, biological desulfurization, adsorptive desulfurization, extractive desulfurization, etc. It focuses on the synthesis of ionic liquids and their applications in oxidative desulfurization, extractive desulfurization, extractive oxidative desulfurization, and catalytic oxidative desulfurization, and it analyzes the problems of ionic liquids that need to be solved urgently in desulfurization, looking forward to the development of sulfuric compounds as a kind of new and emerging green solvent in the field of desulfurization.

Hydrocarbon Extraction with Ionic Liquids

Separation and reaction processes are key components employed in the modern chemical industry, and the former accounts for the majority of the energy consumption therein. In particular, hydrocarbon separation and purification processes, such as aromatics extraction, desulfurization, and denitrification, are challenging in petroleum refinement, an industrial cornerstone that provides raw materials for products used in human activities. The major technical shortcomings in solvent extraction are volatile solvent loss, product entrainment leading to secondary pollution, low separation efficiency, and high regeneration energy consumption due to the use of traditional organic solvents with high boiling points as extraction agents. Ionic liquids (ILs), a class of designable functional solvents or materials, have been widely used in chemical separation processes to replace conventional organic solvents after nearly 30 years of rapid development. Herein, we provide a systematic and comprehensive review of the state-of-the-art progress in ILs in the field of extractive hydrocarbon separation (i.e., aromatics extraction, desulfurization, and denitrification) including (i) molecular thermodynamic models of IL systems that enable rapid large-scale screening of IL candidates and phase equilibrium prediction of extraction processes; (ii) structure-property relationships between anionic and cationic structures of ILs and their separation performance (i.e., selectivity and distribution coefficients); (iii) IL-related extractive separation mechanisms (e.g., the magnitude, strength, and sites of intermolecular interactions depending on the separation system and IL structure); and (iv) process simulation and design of IL-related extraction at the industrial scale based on validated thermodynamic models. In short, this Review provides an easy-to-read exhaustive reference on IL-related extractive separation of hydrocarbon mixtures from the multiscale perspective of molecules, thermodynamics, and processes. It also extends to progress in IL analogs, deep eutectic solvents (DESs) in this research area, and discusses the current challenges faced by ILs in related separation fields as well as future directions and opportunities.

A density functional theory study on the adsorption of different organic sulfides on boron nitride nanosheet

The adsorption of methanethiol (MT), thiophene (T), benzothiophene (BT), dibenzothiophene (DBT) on hexagonal boron nitride (h-BN) has been investigated by the framework of the density functional theory (DFT) calculations in this work. The prefer adsorption sites and interfacial angles of different sulfur compounds on the surface of the h-BN are investigated and analyzed. The adsorption energy results indicated that the adsorption of MT (Ead ≈ -6 kcal mol-1), T (Ead ≈ -10 kcal mol-1), BT (Ead ≈ -15 kcal mol-1), and DBT (Ead ≈ -21 kcal mol-1) on monolayer h-BN is physical interaction, and the value of Ead on bilayer h-BN is more than that on monolayer h-BN 0.05%. Adsorptive conformations show that sulfides prefer to be adsorbed on center B atoms rather than N atoms. Meanwhile, thiophene and its analogues tend to be adsorbed parallel on h-BN plane. Energy decomposition, natural population analysis (NPA), and electrostatic potential (ESP) analysis used to better understand the nature of adsorption on h-BN. van der Waals force plays a dominant role in adsorption process. Due to the π-π interactions, T, BT, and DBT have larger van der Waals forces than MT and the value of adsorption energy is negative correlated to the number of benzene rings. These findings are helpful for deeper understanding the adsorptive desulfurization mechanism and help develop better adsorbents for desulfurization in the future.

Uncovering the Critical Factors that Enable Extractive Desulfurization of Fuels in Ionic Liquids and Deep Eutectic Solvents from Simulations

Environmental regulatory agencies have implemented stringent restrictions on the permissible levels of sulfur compounds in fuel to reduce harmful emissions and improve air quality. Problematically, traditional desulfurization methods have shown low effectiveness in the removal of refractory sulfur compounds, e.g., thiophene (TS), dibenzothiophene (DBT), and 4-methyldibenzothiophene (MDBT). In this work, molecular dynamics (MD) simulations and free energy perturbation (FEP) have been applied to investigate the use of ionic liquids (ILs) and deep eutectic solvents (DESs) as efficient TS/DBT/MDBT extractants. For the IL simulations, the selected cation was 1-butyl-3-methylimidazolium [BMIM] and the anions included chloride [Cl], thiocyanate [SCN], tetrafluoroborate [BF₄], hexafluorophosphate [PF₆], and bis(trifluoromethylsulfonyl)amide [NTf₂]. The DESs were composed of choline chloride with ethylene glycol (CCEtg) or with glycerol (CCGly). Calculation of excess chemical potentials predicted the ILs to be more promising extractants with energies lower by 1-3 kcal/mol compared to DESs. Increasing IL anion size was positively correlated to enhanced solvation of S-compounds, which was influenced by energetically dominant solute-anion interactions and favorable solute-[BMIM] π-π stacking. For the DESs, the solvent components offered a range of synergistic, yet comparatively weaker, electrostatic interactions that included hydrogen bonding and cation-π interactions. An in-depth analysis of the structure of IL and DES systems is presented, along with a discussion of the critical factors behind experimental trends of S-compound extraction efficiency.

The Role of the Anion in Imidazolium-Based Ionic Liquids for Fuel and Terpenes Processing

The potentialities of methylimidazolium-based ionic liquids (ILs) as solvents were evaluated for some relevant separation problems—terpene fractionation and fuel processing—studying selectivities, capacities, and solvent performance indices. The activity coefficients at infinite dilution of the solute (1) in the IL (3), γ13∞, of 52 organic solutes were measured by inverse gas chromatography over a temperature range of 333.2–453.2 K. The selected ILs are 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], and the equimolar mixture of [C4mim][PF6] and 1-butyl-3-methylimidazolium chloride, [C4mim]Cl. Generally, low polar solutes follow γ1,C4mimCl∞ > γ1,C4mimPF6+C4mimCl∞ > γ1,C4mimPF6∞ while the opposite behavior is observed for alcohols and water. For citrus essential oil deterpenation, the results suggest that cations with long alkyl chains, such as C12mim+, promote capacity, while selectivity depends on the solute polarity. Promising results were obtained for the separation of several model mixtures relevant to fuel industries using the equimolar mixture of [C4mim][PF6] and [C4mim]Cl. This work demonstrates the importance of tailoring the polarity of the solvents, suggesting the use of ILs with mixed anions as alternative solvents for the removal of aliphatic hydrocarbons and contaminants from fuels.

Dibenzothiophene Removal from Fuel Oil by Metal-Organic Frameworks: Performance and Kinetics

Desulfurization of organic sulfur in the fuel oil is essential to cut down the emission of sulfur dioxide, which is a major precursor of the acid rain and PM2.5. Currently, hydrodesulfurization is regarded as a state-of-art technology for the desulfurization of fuel oil. However, due to the stringent legislation of the fuel oil, the deep desulfurization technology is urgent to be developed. Adsorptive desulfurization method is promising due to the high selectivity and easy operation. The development of efficient adsorbent is important to advance this technology into industrial application. In this work, the five types of metal-organic frameworks (MOFs), including Cu-BTC, UMCM-150, MIL-101(Cr), UIO-66, and Cu-ABTC were synthesized for the adsorption of dibenzothiophene (DBT), a typical organic sulfur compound in the fuel oil. The experimental results revealed that the adsorption capacity of the five MOFs followed the order of Cu-ABTC, UMCM-150, Cu-BTC, MIL-101(Cr), and UIO-66, which adsorption capacities were 46.2, 34.2, 28.3, 26.3, and 22.0 mgS/g, respectively. The three types of Cu-based MOFs such as Cu-ABTC, UMCM-150, and Cu-BTC outperformed the Cr-based MOFs, MIL-101, and Zr-based MOFs, UIO-66. Since the surface area and pore volumes of the Cu-based MOFs were not the greatest among the tested five MOFs, the physical properties of the MOFs were not the only limited factor for the DBT adsorption. The π-complexation between DBT and linkers/metal in the MOFs was also important. Kinetic analysis showed that the DBT adsorption onto the five tested MOFs follows the pseudo-second-order kinetics, confirming that the chemical π-complexation was also contributed to the DBT adsorption. Furthermore, the operation parameters such as oil-adsorbent ratio, initial sulfur concentration and adsorption temperature for the DBT adsorption onto Cu-ABTC were optimized to be 100:1 g/g, 1000 mgS/L and 30 °C, respectively. This work can provide some insights into the development of efficient adsorbent for the organic sulfur adsorption.

Benzothiophene Adsorptive Desulfurization onto trihexYl(tetradecyl)phosphonium Dicyanamide Ionic-Liquid-Modified Renewable Carbon: Kinetic, Equilibrium and UV Spectroscopy Investigations

The negative environmental and industrial impacts of the presence of sulfur compounds such as benzothiophene in fuels have led to a greater interest in desulfurization research. In this work, carbon from palm waste sources was modified with trihexYl(tetradecyl)phosphonium dicyanamide-ionic liquid and characterized by SEM, EDS, XRD and FTIR to assess surface properties. Then, the prepared carbon and carbon modified with ionic liquid were evaluated for the adsorption of benzothiophene by investigating the effects of time. The equilibrium occurred after 120 min, recording adsorption capacities of 192 and 238 mg/g for carbon and carbon modified with ionic liquid, respectively. The effect of the adsorbent dose on the adsorption of benzothiophene was evaluated, indicating that the maximum adsorption capacities were obtained using a dose of 1 g/L for both carbon and carbon modified with ionic liquid. The kinetic investigation for the adsorption of benzothiophene onto carbon and carbon modified with ionic liquid indicated that the second-order kinetic model is well fitted with the adsorption data rather than the first-order kinetic model. The equilibrium investigations for the adsorption of benzothiophene onto carbon and carbon modified with ionic liquid with Langmuir and Freundlich isotherm models reveals that the Freundlich model is the most suitable for describing the adsorption process, suggesting a multilayer adsorption mechanism. The desulfurization process showed a high impact on environmental safety due to the possibility of regenerating and reusing the prepared adsorbents with promising results up to five cycles.

The Use of Polydialkylsiloxanes/Triflic Acid as Derivatization Agents in the Analysis of Sulfur-Containing Aromatics by "Soft"-Ionization Mass Spectrometry

Polycyclic aromatic sulfur-containing compounds are widely distributed in oil, especially in its low-volatile and heavy fractions (resins, asphaltenes), and this dictates the need for their determination when reliable methods for sulfur removing, cleaning and processing oil are developed. In these cases, "soft" ionization mass spectrometry methods, based on electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), are particularly effective. However, aromatic sulfur-containing compounds have low polarity and cannot be readily ionized by these methods. To overcome the problem, their preliminary conversion into sulfonium salts by the action of alkyl iodides and a silver-containing agent is widely used. In the process of developing more economical derivatization methods, we found a rather unexpected possibility of implementing S-alkylation of organic sulfides with commercial polydialkylsiloxanes (alkyl = CH₃ or C₂H₅) in the presence of triflic acid (CF₃SO₃H) as a superacid co-alkylating agent. For homologous dibenzothiophenes as a typical model representative of petroleum sulfur-containing aromatic compounds, ESI and MALDI mass spectra exhibited the signals of corresponding S-alkylsulfonium salts with a high signal-to-noise ratio. A rational mechanism for the described chemical transformation is proposed, including the indispensable activation by triflic acid and the cleavage of the Si-C bond. Specific collision-induced dissociation of corresponding S-alkylated sulfonium cations is considered. The applicability of the derivatization approach to the analysis of petroleum products by high-resolution mass spectrometry is demonstrated.

Trihexyl tetradecyl phosphonium bromide as an effective catalyst/extractant in ultrasound-assisted extractive/oxidative desulfurization

Phosphonium-based ionic liquid (PIL) has been used as a catalyst and extractant. Here, the PIL, trihexyl tetradecyl phosphonium bromide ([THTDP]Br) was utilized for the S-removal of model oil (MO) and acted as the reaction-induced self-separation catalyst. The influence of oxidant to sulfur molar ratio (n(O/S)), mass ratio of model oil to ionic liquid (m(MO/IL)), sonication time, and temperature was observed to investigate the optimal conditions for the ultrasound-assisted extractive/oxidative desulfurization (UEODS) catalyzed by [THTDP]Br. A kinetic study was performed, and the reaction rate constant and half-life were calculated as the oxidation reaction was following pseudo-first-order reaction kinetics. Moreover, the oxidation reactivity and selectivity of various sulfur substrates were in the following order: DBT > BT > TH > 3-MT. The DBT removal with various initial S-content was observed to be constant, which makes it feasible for practical application. The interaction energy between [THTDP]Br and S-compounds was examined using Density Functional Theory. The sulfur removal of base oil (BO) was also examined using various desulfurization systems at DBT optimized conditions. The highest desulfurization efficiency of BO was obtained during the UEODS process, which made it industrially feasible. [THTDP]Br was regenerated and recycled six times with a slight variation in efficiency.

Extractive–oxidative desulfurization of model fuels using imidazole-based dicationic ionic liquids as extractants

Desulfurization efficiency of bis-imidazole-based ionic liquids with different alkyl chain lengths.

Dimensionless evaluation and kinetics of rapid and ultradeep desulfurization of diesel fuel in an oscillatory baffled reactor

The oxidative desulfurization (ODS) of dibenzothiophene in diesel fuel cut using a homogeneous liquid catalytic system in a novel reactor is presented.

Carbon-Based Materials for Oxidative Desulfurization and Denitrogenation of Fuels: A Review

Sulfur (S) and nitrogen (N) are elements naturally found in petroleum-based fuels. S- and N-based compounds in liquid fuels are associated with a series of health and environmental issues. Thus, legislation has become stricter worldwide regarding their content and related emissions. Traditional treatment systems (namely hydrodesulfurization and hydrodenitrogenation) fail to achieve the desired levels of S and N contents in fuels without compromising combustion parameters. Thus, oxidative treatments (oxidative desulfurization–ODS, and oxidative denitrogenation-ODN) are emerging as alternatives to producing ultra-low-sulfur and nitrogen fuels. This paper presents a thorough review of ODS and ODN processes applying carbon-based materials, either in hybrid forms or as catalysts on their own. Focus is brought to the role of the carbonaceous structure in oxidative treatments. Furthermore, a special section related to the use of amphiphilic carbon-based catalysts, which have some advantages related to a closer interaction with the oily and aqueous phases, is discussed.

Assessment of polyethylene/Zn-ionic as a diesel fuel sulfur adsorbent: gamma radiation effect and response surface methodology

Irradiated waste high-density polyethylene@Zn/ionic liquid novel composite well-fabricated via coacervation method was irradiated by gamma-irradiation and studied the effect of that radiation on the desulfurization process. The prepared composites were characterized by various analytical techniques as follows: X-ray diffraction (XRD), Fourier-Transform infrared (FT-IR), X-ray photoelectron spectrometer (XPS), scanning electron microscope (SEM), High Resolution Transmission Electron Microscopy (HRTEM), N₂-adsorption-desorption isotherm, and thermal gravimetric analysis (TG/DTA). The adsorptive desulfurization process of benzothiophene (BT) and dibenzothiophene (DBT) which are harmful compounds in diesel model fuel was investigating using the irradiated and unirradiated composite. The results illustrated that the unirradiated and irradiated composites exhibit an adequate adsorption capacity reached (50-75 mg S/g) and (60-85 mg S/g) for BT and DBT, respectively. The adsorption process over the prepared adsorbents follows the pseudo-second-order kinetic models. The irradiated composite exhibited more adsorption capacity than the unirradiated one due to the radiation generated more surface area and created proton-bond donor sites in the composite surface, which increases the interaction between the surface and sulfur species. The adsorption capacity and adsorption percentage for irradiated and unirradiated composites towards (SCCs) were studied using response surface methodology based on the central composite design (CCD). The thermodynamic factors (∆H°, ∆G°, and ∆S°) reveal that these processes are endothermic adsorption processes. The irradiated PEt @Zn/IL was re-used without significant loss of adsorption activity. This novel irradiated PEt @Zn/IL is the first time used as an adsorbent with an advantage that includes its excellent adsorption capacity, which ensures the product will be efficient in a real process such as the petrochemical industry.

Nanocatalysts for Oxidative Desulfurization of Liquid Fuel: Modern Solutions and the Perspectives of Application in Hybrid Chemical-Biocatalytic Processes

In this paper, the current advantages and disadvantages of using metal-containing nanocatalysts (NCs) for deep chemical oxidative desulfurization (ODS) of liquid fuels are reviewed. A similar analysis is performed for the oxidative biodesulfurization of oil along the 4S-pathway, catalyzed by various aerobic bacterial cells of microorganisms. The preferences of using NCs for the oxidation of organic sulfur-containing compounds in various oil fractions seem obvious. The text discusses the development of new chemical and biocatalytic approaches to ODS, including the use of both heterogeneous NCs and anaerobic microbial biocatalysts that catalyze the reduction of chemically oxidized sulfur-containing compounds in the framework of methanogenesis. The addition of anaerobic biocatalytic stages to the ODS of liquid fuel based on NCs leads to the emergence of hybrid technologies that improve both the environmental characteristics and the economic efficiency of the overall process. The bioconversion of sulfur-containing extracts from fuels with accompanying hydrocarbon residues into biogas containing valuable components for the implementation of C-1 green chemistry processes, such as CH4, CO2, or H2, looks attractive for the implementation of such a hybrid process.

Preparation of TiO2/porous glass-H with the coupling of photocatalysis oxidation-adsorption system in the initial position and its desulfurization performance on model fuel

TiO₂/porous glass-H as composite catalysts were synthesized hydrothermally in the presence of H₂O₂ using porous glass microspheres as carriers. The photocatalytic-adsorptive desulfurization of model fuel by composite catalysts was investigated under UV irradiation. The structure and morphology of the composite catalysts were characterized via scanning electron microscopy (SEM), N₂ adsorption, X-ray diffraction (XRD) and ultraviolet-visible spectroscopy (UV-vis). The results showed that TiO₂/porous glass-H exhibited a significantly enhanced photocatalytic-adsorption desulfurization performance due to its enhanced surface area, highly enhanced light absorption, and reduced recombination of photogenerated electron pairs compared with TiO₂/porous glass synthesized in the absence of H₂O₂. The optimized TiO₂ loading was 20% and the reaction temperature was 303.15 K, which could achieve almost 100% sulfur removal when 0.1 g catalyst was applied to a sulfide concentration of 300 mg L⁻¹. Based on the kinetic fitting of the obtained data, it was found that the rate-controlling step of sulfide adsorption on the catalyst was a molecular diffusion process and the adsorption intensity and adsorption capacity of the composite catalyst were significantly improved compared with the porous glass-H in the adsorption thermodynamic curve, and ΔS, ΔH and ΔG of the adsorption process were calculated. In addition, TiO₂/porous glass-H could be regenerated via simple heat treatment, exhibiting similar efficiency as the original TiO₂/porous glass-H after three regeneration cycles.

TiO₂/porous glass-H as composite catalysts were synthesized hydrothermally in the presence of H₂O₂ using porous glass microspheres as carriers.

Membrane-Supported Layered Coordination Polymer as an Advanced Sustainable Catalyst for Desulfurization

The application of a catalytic membrane in the oxidative desulfurization of a multicomponent model diesel formed by most refractory sulfur compounds present in fuel is reported here for the first time. The catalytic membrane was prepared by the impregnation of the active lamellar [Gd(H₄nmp)(H₂O)₂]Cl·2H₂O (UAV-59) coordination polymer (CP) into a polymethyl methacrylate (PMMA, acrylic glass) supporting membrane. The use of the catalytic membrane in the liquid–liquid system instead of a powder catalyst arises as an enormous advantage associated with the facility of catalyst handling while avoiding catalyst mass loss. The optimization of various parameters allowed to achieve a near complete desulfurization after 3 h under sustainable conditions, i.e., using an aqueous H₂O₂ as oxidant and an ionic liquid as extraction solvent ([BMIM]PF₆, 1:0.5 ratio diesel:[BMIM]PF₆). The performance of the catalytic membrane and of the powdered UAV-59 catalyst was comparable, with the advantage that the former could be recycled successfully for a higher number of desulfurization cycles without the need of washing and drying procedures between reaction cycles, turning the catalytic membrane process more cost-efficient and suitable for future industrial application.

Deep desulfurization performance of thiophene with deep eutectic solvents loaded carbon nanotube composites

One source of air pollution is the combustion of sulfur compounds in fuel oil. Reducing sulfur content in fuel oil has become a hot issue demanding timely solutions. Using ionic liquids and deep eutectic solvents (DESs) to remove sulfides in fuel oil has achieved good results presently. However, since DESs are liquid and their transportation and separation are inconvenient, a new way is proposed that the DESs are loaded on the carbon nanotubes (CNTs) with large specific surface area and good chemical stability. A series of composites materials (DESs/CNTs) were prepared. Finally, they are applied to the removal of sulfides in fuel oil. This loading method, which imparts introduced unique physico-chemical properties of the DESs to the carrier materials, preserves both advantages while overcoming some of the problems with DESs. The interaction between DESs and CNTs is mutual promotion. Therefore, this study has important theoretical significance and industrial application value. Under optimal conditions, when the reagent ChCl/p-TsOH (1 : 2) was loaded on multi-walled CNTs (OD = 30-60 nm) to prepare the composite material (ChCl/p-TsOH)/CNTs, the single desulfurization rate of the composite material was 95.8%. Finally, the catalytic/oxidation mechanism was studied systematically and this work would provide a green route for the desulfurization of fuels.

Metal-Based Ionic Liquids in Oxidative Desulfurization: A Critical Review

Ionic liquids (ILs) as novel functional desulfurization materials have attracted increasing attentions. Metal-based ionic liquids (MILs) are classified into three types of metal chloride ILs, metal oxide ILs, and metal complex ILs based on the definition and basic structure of MILs in this critical review. On the basis of the properties of ILs such as structure designability, super dissolution performance, good thermal and chemical stability, nonflammability, and wide electrochemical window, MILs exhibit unique advantages on hydrophobicity, oxidation performance, and Brönsted-Lewis acidity. Therefore, MILs possess both the absorption and oxidation centers for the intramolecular adsorption and oxidation to improve the oxidative desulfurization (ODS) process. During the novel nonaqueous wet oxidative desulfurization process (Nasil), H₂S can be oxidized into elemental sulfur with hydrophobic MILs, which can be regenerated by oxygen for recycle, to solve the problems of low sulfur capacity, low sulfur quality, and severe secondary pollution in the aqueous Lo-Cat wet oxidative desulfurization process. Another outstanding feature of MILs in ODS is biomimetic catalysis, which has the function of activating molecular oxygen and improving the oxidation performance. Metal oxide ILs and metal complex ILs are used in combination with hydrogen peroxide or oxygen with the existing water to generate a Fenton-like reaction to convert hydrophobic organic sulfur or SO₂ into hydrophilic sulfoxide/sulfone or sulfur acid, respectively. However, the corrosion of Cl^- to the equipment and emulsification phenomenon in the extraction process of sulfoxide/sulfone separation still need further study. Furthermore, the promising strategies to construct highly efficient and green desulfurization processes for large-scale applications are provided.

Efficient and reusable ordered mesoporous WOx/SnO2 catalyst for oxidative desulfurization of dibenzothiophene

The oxidative desulfurization (ODS) of organic sulfur compounds over tungsten oxide supported on highly ordered mesoporous SnO₂ (WO_x/meso-SnO₂) was investigated. A series of WO_x/meso-SnO₂ with WO_x contents from 10 wt% to 30 wt%, were prepared by conventional wet impregnation. The physico-chemical properties of the WO_x/meso-SnO₂ catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption isotherms, electron microscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and the temperature-programmed reduction of hydrogen (H₂-TPR). The characterization results indicated that these catalysts possessed mesoporous structures with uniform pores, high specific surface areas, and well-dispersed polyoxotungstate species on the surface of meso-SnO₂ support. The ODS performances were evaluated in a biphasic system (model oil/acetonitrile, S_initial = 2000 ppm), using H₂O₂ as an oxidant, and acetonitrile as an extractant. Dibenzothiophene (DBT) in the model oil was removed completely within 60 min at 50 °C using 20 wt% WO_x/meso-SnO₂ catalyst. Additionally, the effect of reaction temperature, H₂O₂/DBT molar ratio, amount of catalyst and different sulfur-containing substrates on the catalytic performances were also investigated in detail. More importantly, the 20 wt% WO_x/meso-SnO₂ catalyst exhibited 100% surfur-removal efficiency without any regeneration process, even after six times recycling.

The highly ordered mesoporous WO_x/meso-SnO₂ showed excellent catalytic activity and reusability in removing dibenzothiophene (DBT).

Effect of Cation Structure in Quinolinium-Based Ionic Liquids on the Solubility in Aromatic Sulfur Compounds or Heptane: Thermodynamic Study on Phase Diagrams

Experimental and theoretical studies on thermodynamic properties of quinolinium-based ionic liquids (ILs) based on bis(trifluoromethylsulfonyl)imide anion (namely N-butyl-quinoloinium bis(trifluoromethylsulfonyl)imide, [BQuin][NTf₂], N-hexylquinoloinium bis(trifluoromethyl-sulfonyl)imide, [HQuin][NTf₂], and N-octylquinoloinium bis(trifluoromethyl-sulfonyl)imide, [OQuin][NTf₂]) with aromatic sulfur compounds and heptane, as a model compound of fuel were examined in order to assess the applicability of the studied ionic liquids for desulfurization of fuels. With this aim, the temperature-composition phase diagrams of 13 binary mixtures composed of organic sulfur compounds (thiophene, benzothiophene, or 2-methylthiophene) or heptane and ionic liquid (IL) were investigated at ambient pressure. A dynamic method was used to determine the (solid-liquid) equilibrium phase diagrams in binary systems over a wide composition range and temperature range from T = 255.15 to 365.15 K up to the fusion temperature of ILs. The immiscibility gap with an upper critical solution temperature (UCST) was observed for each binary system under study. The influence of the alkane chain length of the substituent on the IL cation and of the sulfur compounds (the aromaticity of the solvent) was described. The experimental (solid + liquid) phase equilibrium dataset were successfully correlated using the well-known NRTL equation.

Insights to the oxidative desulfurization process of fossil fuels over organic and inorganic heterogeneous catalysts: advantages and issues

Strict environmental laws have been put in place around the world to reduce the amount of sulfur in the fuel to reduce the emissions of harmful gases from fuel combustion and improve air quality. Therefore, extensive researches have been undertaken to devise effective processes or to improve the desulfurization processes. Among the desulfurization processes, the oxidative desulfurization (ODS) process is a promising method to achieve very low and near-zero sulfur content of the fuel. In this process, sulfur compounds are converted to the corresponding sulfone by a catalyst and in the presence of an oxidant. The obtained compounds by polar solvents or adsorbents are removed from the fuel. In recent decades, extensive studies have been carried out on the catalysts used in the oxidative desulfurization process. In this review, a comprehensive survey has been performed on heterogeneous catalysts used in the oxidative desulfurization process. According to the reported researches, the heterogeneous catalysts used can be divided into five groups: ionic liquids, carbon materials, polyoxometalates, transition metal oxides stabilized on porous solid substrates, and metal-organic frameworks. The proposed mechanisms with different catalysts have also been studied in this work.