Technological Advancements of Ionic Liquids in the Process of Hydrogen Desulfurization of Fuels: A Review

Abstract

Hydrogen desulfurization (HDS) is a critical process in petroleum refining aimed at reducing the sulfur content in fuels to meet stringent environmental regulations and minimize harmful emissions. Traditional HDS methods are effective but often require high temperatures and pressures to remove refractory sulfur compounds like dibenzothiophene and its derivatives. In recent years, ionic liquids (ILs) have gained attention as innovative alternatives due to their unique physicochemical properties, including low volatility, thermal stability, and customizable selectivity. This review summarizes recent developments over the past five years in IL-assisted HDS, focusing on the mechanisms of sulfur removal, the types of ILs used, and their dual role as solvents and catalysts. It highlights the advantages ILs offer in achieving cleaner fuel production under milder conditions, along with current limitations such as recyclability, cost, and compatibility with industrial systems. The paper concludes by outlining future research directions aimed at improving the sustainability and scalability of IL-based desulfurization processes.

Keywords

Introduction

The increasing global demand for cleaner fuels has intensified the need for advanced desulfurization technologies to minimize the environmental impact of sulfur emissions. Sulfur-containing compounds in fuels, including thiophenes, benzothiophenes, and dibenzothiophenes (DBT), are significant contributors to atmospheric pollution. During combustion, these compounds undergo oxidation, forming sulfur oxides (SOx), which serve as precursors to acid rain and fine particulate matter, both of which pose severe environmental and health risks ^[1]. Conventional hydrodesulfurization (HDS) remains the primary industrial method for sulfur removal, employing catalytic hydrogenation under high temperatures (300–400°C) and pressures (30–130 atm) ^[2]. This process efficiently eliminates low-molecular-weight sulfur compounds such as thiols and sulfides. However, it exhibits limited effectiveness against refractory sulfur species, particularly DBT and its alkylated derivatives, due to their sterically hindered structures and low reactivity ^[3]. The presence of electron-donating alkyl groups further reduces the electrophilicity of the sulfur atom, making cleavage of the C–S bond thermodynamically and kinetically challenging ^[4]. Consequently, overcoming these limitations requires either harsher operating conditions or alternative desulfurization strategies that enhance selectivity and efficiency while maintaining economic viability and environmental sustainability ^[5]. Ionic liquids (ILs), a class of molten salts with melting points typically below 100°C, have garnered significant interest as environmentally friendly solvents and catalysts in desulfurization processes ^[6]. Unlike conventional molecular solvents, ILs exhibit negligible vapor pressure, reducing volatile organic compound (VOC) emissions and enhancing process safety [^7]. Their high thermal and chemical stability, coupled with their customizable physicochemical properties through judicious selection of cations and anions, enables precise tuning for specific desulfurization applications ^[8]. ILs play a multifaceted role in hydrogen desulfurization (HDS), acting as both extractive solvents and catalysts to enhance sulfur removal efficiency under milder operating conditions ^[9]. Their ability to selectively dissolve and extract sulfur-containing compounds, such as thiophenes, benzothiophenes, and dibenzothiophenes (DBT), stems from strong solvation interactions, including hydrogen bonding, π-π interactions, and electrostatic forces ^[10]. Additionally, functionalized ILs with acidic or redox-active moieties can facilitate oxidative desulfurization (ODS) by promoting selective oxidation of refractory sulfur species into more polar sulfoxides and sulfones, which can be readily separated ^[11]. This review consolidates recent advancements in IL-based desulfurization, analyzing their mechanisms, catalytic efficiency, and synergies with existing HDS techniques. Furthermore, it highlights the advantages of ILs, including reduced energy consumption, recyclability, and potential integration into hybrid desulfurization systems ^[12]. Despite these benefits, challenges such as IL stability, regeneration efficiency, and process scalability remain key hurdles to widespread industrial adoption ^[13]. By examining current limitations and future research directions, this review aims to provide a comprehensive understanding of ILs' role in advancing sustainable and efficient fuel desulfurization technologies.

Mechanisms of desulfurization using ionic liquids:

Ionic liquids facilitate desulfurization through several mechanisms, including extraction, oxidative desulfurization (ODS), and catalytic desulfurization.

Extraction Desulfurization:

ILs can selectively extract sulfur compounds from fuels due to their high affinity for aromatic sulfur species. The extraction efficiency depends on the structure of both the IL and the sulfur compound. For instance, imidazolium-based ILs have shown high extraction efficiency for dibenzothiophene (DBT) due to π–π interactions between the aromatic rings of the IL and DBT ^[10]. The extraction process is typically carried out at ambient temperatures and pressures, making it energy-efficient compared to conventional HDS. Recent studies have explored the use of functionalized ILs for enhanced extraction. For example, ILs with longer alkyl chains or aromatic groups have shown higher extraction efficiencies for DBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) due to increased van der Waals interactions ^[13]. Additionally, the use of ILs in combination with co-solvents, such as methanol or acetonitrile, has been shown to improve extraction efficiency ^[9].

Oxidative Desulfurization (ODS):

ODS involves the oxidation of sulfur compounds to sulfoxides or sulfones, which are more polar and easier to remove. ILs can act as both solvents and catalysts in ODS processes. For example, when combined with oxidants like hydrogen peroxide (H?O?), ILs such as [BMIM][PF?] can oxidize DBT to dibenzothiophene sulfone (DBTO?), which is then extracted into the IL phase ^[11]. Recent advancements in ODS have focused on the development of catalytic ILs that can enhance the oxidation process. For instance, ILs functionalized with metal ions, such as vanadium or molybdenum, have shown high catalytic activity in the oxidation of sulfur compounds [15]. Additionally, the use of ILs in combination with solid catalysts, such as TiO? or MoO?, has been shown to improve the efficiency of ODS processes ^[12].

Catalytic Desulfurization:

Functionalized ILs can act as catalysts or co-catalysts in desulfurization reactions. For example, acidic ILs, such as those containing sulfonic acid groups, can enhance the catalytic activity of metal catalysts in HDS processes ^[14]. The acidic sites in these ILs can facilitate the protonation of sulfur compounds, making them more reactive toward hydrogenation. Recent studies have explored the use of ILs as co-catalysts in conventional HDS processes. For instance, ILs containing imidazolium or pyridinium cations have been shown to enhance the activity of Co-Mo/Al?O? catalysts in the desulfurization of DBT ^[14]. The use of ILs as co-catalysts can reduce the operating temperature and pressure of HDS processes, making them more energy-efficient.

Thermodynamic and computational insights of desulfurization using ionic liquids

Desulfurization of petroleum products, especially diesel and gasoline, is crucial due to stringent environmental regulations and the detrimental effects of sulfur oxides on human health and ecosystems. Traditional hydrodesulfurization (HDS) techniques are effective for thiols and sulfides but inefficient for refractory sulfur compounds such as dibenzothiophene (DBT) and its derivatives. Ionic liquids (ILs), due to their tunable structures and remarkable physicochemical properties, offer a green alternative through extractive and oxidative desulfurization (ECODS). Thermodynamic and computational studies, particularly density functional theory (DFT) and COSMO-RS (Conductor-like Screening Model for Real Solvents), provide deep insights into the mechanisms driving these processes. ^{[15, 16]}

COSMO-RS: Predictive power for IL design: ^{[2, 17]}

COSMO-RS integrates quantum chemical data with statistical thermodynamics to predict thermodynamic properties such as solvation energies, activity coefficients, and partition coefficients. In the context of ECODS, COSMO-RS aids in screening ILs by evaluating their ability to solubilize sulfur-containing compounds over hydrocarbons. It uses molecular surface charge distributions (sigma-profiles) to model molecular interactions, allowing researchers to predict selectivity and capacity of ILs at infinite dilution without extensive experimental work.

Non-covalent interactions driving selectivity: ^{[18, 19]}

One of the core findings from DFT and COSMO-RS studies is that noncovalent interactions such as π-π stacking, hydrogen bonding, and van der Waals forces govern the selective solvation of DBT and its oxidized forms in ILs. Imidazolium-based cations, for example, can engage in π-π interactions with the aromatic ring of DBT. Simultaneously, anions like [AlCl4]? or SCN? form hydrogen bonds with DBT-sulfone, stabilizing it in the IL phase. These interactions reduce the activity coefficients of the sulfur species in the IL phase, increasing their extraction efficiency.

Superior solvation of sulfones: ^[20]

Sulfones such as DBT-sulfone exhibit significantly higher polarity compared to their parent DBT molecules due to the presence of sulfoxide (S=O) bonds. This polarity leads to more favorable solvation in ILs, as evidenced by lower solvation free energies (ΔG_solv) computed via COSMO-RS. Consequently, sulfones show higher affinity for ILs, resulting in their effective retention in the IL phase post-oxidation. The strong solvation of sulfones is critical for the success of ECODS since it allows efficient separation of oxidized sulfur species from the fuel matrix.

DFT-Based mechanistic insights: ^{[21, 22]}

DFT studies further elucidate the nature of IL-sulfur compound interactions at the molecular level. Calculations reveal that both the cation and anion contribute synergistically to the stabilization of sulfur species. For instance, in [bmim][AlCl₄], the imidazolium ring engages in π-stacking [23] while the [AlCl₄] anion forms electrostatic interactions and hydrogen bonds with sulfoxide groups. Electrostatic potential maps confirm complementary charge distributions that enhance binding affinity. DFT also shows that excessive Lewis acidity, such as that introduced by [Al₂Cl₇?], can disrupt optimal interactions, decreasing desulfurization performance.

Thermodynamic favorability and entropy contributions: ^[24]

COSMO-RS not only accounts for enthalpic contributions but also incorporates entropy effects, which are significant in systems involving long alkyl chains and flexible IL structures. These entropic effects explain the enhanced solubility of bulky molecules like DBT and its oxidized derivatives in ILs with extended alkyl chains. Entropy release upon solvation contributes to the overall thermodynamic favorability, making such ILs more effective in practical desulfurization processes.

Practical implications and IL screening: ^[25]

The integration of DFT and COSMO-RS enables the rational design and screening of ILs tailored for specific desulfurization tasks. ILs with aromatic cations and polar, hydrogen-bonding anions exhibit superior performance in ECODS. Screening results guide experimental efforts, allowing the selection of ILs with optimal activity coefficients and partition coefficients for targeted sulfur compounds. This predictive capacity streamlines the development of ILs, reducing trial-and-error in the laboratory.

Present status of ionic liquids in desulfurization process:

Ionic liquids (ILs) offer several advantages in desulfurization, including enhanced efficiency, milder operating conditions, reusability, and environmental benefits. ILs effectively remove refractory sulfur compounds that are challenging for traditional HDS methods]. Their ability to operate under lower temperatures and pressures improves energy efficiency ^[9]. Additionally, ILs can be regenerated and reused multiple times, reducing operational costs ^[10]. Their non-volatile nature minimizes emissions, aligning with green chemistry principles ^[25]. For instance, Zhang et al. demonstrated that [BMIM][PF?] achieved a 98% DBT removal rate from model diesel under mild conditions ^[10], while Wang et al. reported that a combination of [BMIM][PF?] and H?O? in an oxidative desulfurization process achieved 99% DBT removal ^[11].

Current challenges and limitations in the exploration of ILs in desulfurization:

Despite their potential, the application of ionic liquids (ILs) in hydrogen desulfurization faces several challenges, including high cost, viscosity issues, regeneration difficulties, and potential toxicity. The expensive synthesis of ILs limits their large-scale adoption, while their high viscosity can hinder mass transfer and reduce process efficiency ^[10]. Although ILs are reusable, their regeneration processes can be energy-intensive ^[11]. Additionally, some ILs pose environmental and health risks, highlighting the need for biodegradable alternatives ^[9]. For example, phosphonium-based ILs, though effective, are often more costly and toxic than imidazolium-based ILs. Furthermore, the high viscosity of certain ILs restricts their application in continuous flow processes ^[26].

CONCLUSION

Ionic liquids (ILs) have emerged as a promising and versatile alternative to conventional hydrodesulfurization (HDS) techniques, offering numerous advantages including tunable physicochemical properties, low volatility, and the ability to operate under milder conditions. These properties make ILs particularly effective in addressing one of the major limitations of traditional HDS, the removal of refractory sulfur compounds such as dibenzothiophene (DBT) and its alkylated derivatives. Through extractive, catalytic, and oxidative mechanisms, ILs facilitate deep desulfurization while aligning with the principles of green chemistry. However, the widespread industrial application of IL-based desulfurization is still constrained by several challenges, including high synthesis costs, elevated viscosities which hinder mass transfer, and potential environmental toxicity. Overcoming these limitations requires a multidisciplinary approach combining materials science, process engineering, and computational modeling. In this regard, thermodynamic and quantum chemical tools such as COSMO-RS (Conductor-like Screening Model for Real Solvents) and Density Functional Theory (DFT) have played a pivotal role. These methods enable researchers to investigate solvation free energies, partition coefficients, and the nature of non-covalent interactions (such as π–π stacking, hydrogen bonding, and van der Waals forces) that govern the selective solubilization of sulfur compounds in ILs. Computational modeling not only elucidates the mechanistic pathways of extraction and oxidative desulfurization but also provides a predictive framework for designing next-generation ILs with enhanced performance and reduced environmental impact. In particular, COSMO-RS simulations have demonstrated the superior solvation and retention of sulfones (oxidized sulfur species) in ILs, thereby validating the thermodynamic favorability of IL-assisted oxidative desulfurization (ODS). Entropic contributions from flexible IL structures further enhance their performance by promoting favorable molecular interactions and phase behavior. Looking forward, the integration of high-throughput computational screening, machine learning algorithms, and advanced synthetic strategies will be instrumental in tailoring ILs for industrial-scale applications. Efforts should also focus on the development of biodegradable and recyclable ILs, minimizing toxicity and environmental burden. In parallel, experimental validation under real-world refinery conditions, including multi-component fuel matrices and continuous flow systems, will be crucial to assess the practical viability and scalability of IL-based technologies. As environmental regulations continue to tighten and the global demand for ultra-low sulfur fuels grows, ILs are poised to play a transformative role in sustainable fuel purification. Their multifunctional nature, combined with the insights provided by modern computational tools, positions ILs at the forefront of next-generation desulfurization strategies in petroleum refining.

CONFLICT OF INTEREST:

The authors declare no conflict of interest, financial or otherwise.

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