Overview of Research Status and Development Trends in Diesel Desulfurization Technology

Abstract

Diesel desulfurization is a critical process for reducing the sulfur content in diesel fuel and mitigating the negative impact of sulfur-containing exhaust gases for the environment. As a cornerstone of the refining industry, desulfurization has garnered significant attention for producing cleaner fuels and reducing pollution. Currently, the primary desulfurization technologies include hydrodesulfurization (HDS), oxidative desulfurization (ODS), biodesulfurization (BDS), adsorptive desulfurization (ADS), and electrochemical desulfurization (ECDS). With the development of global economic competition and the advancement of technological innovation, diesel desulfurization technologies are evolving toward higher efficiency, lower costs, and resource-oriented utilization. This article provides a detailed account of the various desulfurization technologies under investigation and offers an overview of the emerging ultra-deep desulfurization techniques aimed at producing ultra-low-sulfur fuels.

1. Introduction

Sulfur is an inherent component of crude oil and is widely present in diesel fuel, primarily in the form of organic sulfur compounds. These compounds mainly include mercaptans, thiophenes (Ts), benzothiophenes (BTs), and dibenzothiophenes (DBTs), as illustrated in Figure 1 [1]. During diesel combustion, sulfur-containing compounds are converted into various harmful emissions, which can be categorized into gaseous, liquid, and solid components, as summarized in

Table 1. Emissions from diesel engines.

Gaseous	Liquid	Solid
N2 CO2 H2 NO/NO2 SO2/SO3 HC(C2–C15) Oxygenates Organic nitrogen and sulfur compounds	H2O H2SO4 HC(C2–C15) Oxygenates Polyaromatics	Soot Metals Inorganic compounds Sulfates Solid hydrocarbons

. Among these, sulfur oxides (SO_xs) and sulfate particulate matter (SPM) are particularly concerning [2]. SO_x gases, upon their release into the atmosphere, can react with water vapor to form acidic solutions, which not only contribute to acid rain but also corrode vehicle components, increasing the risk of mechanical failure and compromising driving safety [3,4]. Moreover, SO_x emissions can poison the catalysts in vehicle exhaust treatment systems, significantly reducing the catalyzed efficiency, leading to the exhaust not meeting stringent emission standards, and increasing the contents of CO_x, NO_x, and SO_x in the atmosphere. Such emissions exacerbate air pollution and contribute to global environmental challenges, including climate change and urban smog formation. Furthermore, SO_x gases can harm the human respiratory system, increasing cancer risk, exacerbating heart disease, and triggering asthma [5,6,7]. Given the significant environmental and health hazards associated with sulfur emissions from diesel combustion, reducing the sulfur content in diesel fuel has become a critical priority.

To reduce the hazards caused by diesel combustion, countries around the world have implemented more stringent standards for the sulfur content in diesel fuel. In the United States, the sulfur content standard for diesel was reduced from 500 ppm in 2000 to 15 ppm in 2006, a limit that remains in effect today [8,9]. In 2009, the European Union introduced the Euro V diesel standard, which set a maximum sulfur content of 10 ppm. Similarly, China implemented the National V standard, with a 10 ppm sulfur limit in 2017. In the context of the growing global emphasis on energy transition and the efficient utilization of fossil fuels, meeting the increasingly stringent sulfur content requirements for diesel products has become a formidable challenge for the refining industry. Consequently, achieving ultra-deep desulfurization to produce ultra-low-sulfur diesel (ULSD) has emerged as a critical goal and remains a prominent focus of international research efforts. The current diesel desulfurization technologies, which are continually being refined and improved, include hydrodesulfurization, oxidative desulfurization, adsorptive desulfurization, electrochemical desulfurization, catalytic desulfurization, and biodesulfurization [10,11].

2. Hydrodesulfurization

2.1. Introduction to Hydrodesulfurization

Hydrodesulfurization is the most widely used desulfurization technology in the refining industry. High-capacity HDS units have been extensively used in industrial-scale applications, playing a pivotal role in improving diesel quality. This process involves the utilization of a catalyst under high-temperature, high-pressure, and hydrogen-rich conditions to convert the sulfur-containing compounds in diesel into H₂S [12]. The collected H₂S is then converted into elemental sulfur through the Claus method [13,14]. The research efforts on HDS technology have focused on the design and development of efficient HDS catalysts [15]. At present, numerous researchers are conducting extensive studies on both supported and unsupported HDS catalysts. Among the various catalysts being investigated are Al/Co/Mo-based catalysts, as well as the catalysts supported by these metals, which are widely used in HDS. Extensive research has been conducted on the chemical composition, structure, state, and activity of the above-mentioned catalysts in hydrodesulfurization reactions [16]. Overall, the continuous innovation in HDS catalyst development is critical for addressing the limitations of conventional systems and meeting the growing demand for ULSD.

2.2. Mechanism of Hydrodesulfurization

The desulfurization mechanism of the HDS process primarily involves two pathways: direct desulfurization (DDS) and hydrogenation desulfurization (HYD). Sulfur compounds with relatively simple structures, such as mercaptans and sulfides, are typically removed completely under mild reaction conditions through the DDS pathway. In contrast, more complex sulfur compounds, such as thiophenes, benzothiophenes, and dibenzothiophenes, which contain aromatic rings, require harsher reaction conditions for effective desulfurization. For these aromatic sulfur compounds, desulfurization occurs via a combination of the DDS and HYD pathways.

Thiophene is a common cyclic organosulfur compound found in various fossil fuels and is frequently used as a model compound for evaluating catalyst activity in desulfurization studies. Thiophene can be removed via two primary pathways: direct desulfurization and hydrogenation desulfurization [15]. In the DDS pathway, the C–S bond in thiophene is directly cleaved, producing H₂S and butadiene, which is subsequently hydrogenated to form n-butane. Alternatively, in the HYD pathway, thiophene undergoes hydrogenation to form tetrahydrothiophene (THT), which is then desulfurized through C–S bond cleavage to yield H₂S and n-butane (Figure 2).

The desulfurization pathway of thiophene is strongly influenced by the type of catalyst used. Studies by Salnikov and Wang et al. [17,18] have revealed that thiophene primarily follows the HYD pathway with MoS₂/Al₂O₃ catalysts, sequentially forming dihydrothiophene (DHT) and tetrahydrothiophene (THT) before its final desulfurization to produce butane. In contrast, with Pt/Al₂O₃ catalysts, thiophene predominantly follows the DDS pathway, leading to the formation of 1,3-butadiene, which is further hydrogenated to butane. In another study, Huang et al. [19] discovered that thiophene undergoes desulfurization with ReS₂ via two pathways: HYD and DDS. They also concluded that the DDS pathway leads to the formation of 1,3-butadiene, which kinetically hinders the desulfurization process. The HYD pathway, on the other hand, produces three products: butane, 1-butene, and 2-butene, with 2-butene being the primary product. Zheng et al. [20] studied the hydrogenation desulfurization of thiophene with MoS₂ and co-promoted MoS₂ catalysts, and reported that both the DDS and HYD pathways produced four final products. These findings highlight that the type of catalyst and the operating conditions significantly influence the intermediates and products formed during thiophene desulfurization. Understanding the interplay between the DDS and HYD pathways is critical for optimizing catalyst design and process conditions to enhance desulfurization efficiency, particularly for refractory sulfur compounds like thiophene.

The molecular structure of BT can be regarded as the combination of a thiophene molecule and a benzene ring, with its desulfurization process being similar to that of thiophene (Figure 3). In the HYD pathway, BT is first converted to dihydrobenzothiophene (DHBT), which undergoes further hydrogenation to yield ethylbenzene (EB) as the final product. In contrast, the DDS route results in the production of styrene. With the presence of high-pressure hydrogen, styrene is immediately hydrogenated to ethylbenzene, so little styrene is observed during the BT HDS process [21,22]. Escobar et al. [23] utilized NiMo/Al₂O₃ as the HDS catalyst and demonstrated that EB is the primary product of the HYD pathway for benzothiophene. Ahmed et al. [24] found that the final products for the HYD and DDS pathways of BT are both EB.

It has been demonstrated that DBT with two benzene rings exhibits relatively low activity in terms of removing the S atom during the HDS process. Under the conditions of 300 °C and 102 atm, the HDS reaction of DBT can proceed via two parallel pathways, as illustrated in Figure 4. In the HYD pathway, DBT undergoes sequential hydrogenation to form tetrahydrodibenzothiophene (THDBT) and/or hexahydrodibenzothiophene (HHDBT). Due to their high reactivity, THDBT and HHDBT are rapidly desulfurized to produce cyclohexylbenzene (CHB), so that they are difficult to isolate or detect during the reaction. However, in the DDS pathway, the C–S bond in DBT is directly cleaved via hydrogenolysis, resulting in the formation of biphenyl (BP). A small fraction of BP may subsequently undergo hydrogenation to form CHB. Sharifvaghefi et al. [25] demonstrated that the properties of the active catalytic materials, the co-catalyst, and the supporting materials play a critical role in determining the dominant pathway. In a separate study, C. Valdes et al. reported that the HDS reaction of DBT over conventional catalysts predominantly follows the DDS pathway, with BP being the primary reaction product [26,27,28,29,30]. Bicyclohexyl (BCH) is a tertiary product formed in trace amounts through the slow hydrogenation of CHB, which can occur via either of the two pathways [31].

HDS heavily relies on ring hydrogenation to alleviate the steric hindrance and weaken the C–S bond [32]. For the 4,6-DMDBT molecule, the presence of two methyl groups near the sulfur atom results in significant steric hindrance, increasing the difficulty of desulfurization [33,34]. Figure 5 illustrates the simplest mechanism for the HDS of 4,6-DMDBT. The final product of the DDS pathway is dimethylbiphenyl (DMBP). In contrast, the HYD pathway involves sulfur-containing intermediates, such as dimethyl-tetrahydrodibenzothiophene (DMTHDBT), dimethyl-hexahydrodibenzothiophene (DMHHDBT), and dimethyl-pentahydrodibenzothiophene (DMPHDBT). Further hydrodesulfurization of these intermediates yields two common final products: dimethylbicyclohexyl (DMBCH) and dimethylcyclohexylbenzene (DMCHB). Zou et al. [35] prepared FTA-supported NiMo bimetallic catalysts (NiMo/FTA) and applied them to the HDS of 4,6-DMDBT. The results indicated that, compared to the DDS pathway, the HYD and isomerization routes are more effective at reducing the steric hindrance of 4,6-DMDBT, thereby facilitating the HDS reaction. Similarly, Farag et al. [36] conducted a kinetic analysis of the two pathways for 4,6-DMDBT and concluded that the HYD pathway predominates during the desulfurization process.

2.3. Catalysts for Hydrodesulfurization

The conventional industrial HDS catalysts are primarily CoMo(W)S/Al₂O₃ and NiMo(W)S/Al₂O₃, which can remove most of the sulfur from diesel. These catalysts belong to the supported catalysts characterized by high mechanical strength and large specific surface areas [33,37,38]. To enhance the HDS performance of CoMo- and NiMo-based catalysts, numerous modifications have been explored by scholars. One approach has involved the addition of additives, such as phosphorus, fluorine, and boron.

Matheus et al. [39] demonstrated that NiMo/alumina catalysts modified with phosphorus increased the formation of DDS products by twofold compared to HYD products during the HDS of DBT. Tawfik et al. [40] investigated the effect of the boron content on HDS activity. The results showed that the introduction of boron improved the structural properties of the prepared catalysts, enhanced particle dispersion, and increased the catalytic activity of MoCo catalysts in sulfur removal. In the study by Nazanin et al. [41], phosphorus, boron, and fluorine were used as promoters to optimize γ-Al₂O₃ supports and NiMo/γ-Al₂O₃ catalysts via the wet impregnation method. The results showed that boron–fluorine interactions primarily influenced the acidity of the catalyst, while phosphorus–fluorine interactions significantly affected the BET surface area. The optimized catalyst exhibited excellent performance on diesel fuel with a sulfur content of 12,100 ppm, achieving a desulfurization efficiency of 81% and a coke of catalyst of only 3%.

Another approach is the utilization of new support materials with high specific surface areas and excellent support properties, including TiO₂, ZrO₂, SiO₂, zeolites, and molecular sieves. ZrO₂-modified SBA-15 supports have been shown to significantly improve the activity of Mo and NiMo catalysts in HDS reactions, particularly for the desulfurization of 4,6-DMDBT [42]. A third approach is the addition of chelating agents. Some common chelating agents include ethylenediaminetetraacetic acid (EDTA); citric acid (CA), nitrilotriacetic acid (NTA); 1, 2-cyclohexanediamine-N, N, N, N-tetraacetic acid (CyDTA); and ethylenediamine (EN). These complexes inhibit Mo–support interactions, thereby forming highly active (Ni)CoMoS (Type II) species with exceptional selectivity and efficiency. Bao et al. [43] found that during the sulfidation process, CA lowered the reduction temperature of cobalt species, allowing for more cobalt promoters to anchor at the edge sites of WS₂ clusters. This facilitated the formation of abundant CoWS active phases, maximizing the bimetallic synergistic effect in an HDS reaction.

The HDS catalyst is be deactivated during the desulfurization process, and the degree of deactivation depends on the properties of the raw materials. Heavier feedstocks lead to faster catalyst deactivation [31]. Wang et al. [14] mentioned in their article that one of the reasons for the deactivation of unloaded catalysts is the loss of specific surface area due to the collapse of the pore structure and agglomeration of the MoS₂ particles. And, in the absence of catalyst carriers, a large number of unused active sites are embedded in the catalyst body, resulting in the active sites not being able to participate in the reaction effectively, and therefore reducing the catalyst activity. Sulfided catalysts (CoMo/Al₂O₃ and NiMo/Al₂O₃) have been shown to retain their activity as long as they are not reduced to their metallic or oxide forms. The presence of sulfur compounds (mainly in the form of H₂S) has been demonstrated to help maintain catalyst activity [44,45].

2.4. Outlook for Hydrodesulfurization

HDS is the core technology in petroleum refining for removing sulfur compounds. It is achieved by passing the heated feedstock and hydrogen over an active catalyst. HDS technology is widely applied to the fractions derived from crude distillation and conversion units (such as fluid catalytic cracking and hydrocracking). HDS is typically performed prior to catalytic reforming due to the toxicity of the sulfur-to-platinum catalysts. In an HDS reactor, sulfur is reduced to H₂S, which is subsequently removed from the tail gas via amine scrubbing. Although caustic washing is sometimes employed to remove the low-molecular-weight mercaptans, HDS remains the primary desulfurization technology, and is also capable of removing some nitrogen compounds and metal impurities [46].

Despite the widespread application of HDS technology for removing sulfur from diesel, several challenges and limitations remain, particularly for achieving ULSD standards. These challenges are closely tied to the nature of the sulfur compounds in diesel and the performance of the catalysts. Sulfur compounds, such as 4,6-DMDBT, are sterically hindered, making them difficult to desulfurize under conventional HDS conditions. As shown in

Table 2. Comparison of different catalysts’ performance in HDS.

HDS Catalyst	Organic Sulfur Compounds	Desulfurization Rate	Ref.
Ni1Mo1-200	DBT	78%	[47]
AlCNTMoNi	DBT	99%	[48]
Ni2P/GaAlOx-0.50	4,6-DMDBT	70.1%	[49]
NiMo/Al2O3-600	4,6-DMDBT	94.5%	[50]
Pt-Ni2P/Al2O3	4,6-DMDBT	97.1%	[51]
Beta-DMSNs	4,6-DMDBT	98.3%	[52]

, different catalysts exhibit significant variations in their performance for the removal of sulfur compounds. For instance, catalysts, such as Beta-DMSNs and AlCNTMoNi, achieve desulfurization rates of 98.3% and 99%, respectively, for sulfur compounds like DBT and 4,6-DMDBT, demonstrating excellent efficiency. However, other catalysts, such as Ni₁Mo₁-200, achieve only 78% desulfurization efficiency for DBT, while Ni₂P/GaAlOx-0.50 achieves a desulfurization rate of merely 70.1% for 4,6-DMDBT. Refineries need to expand the number of HDS units and require higher temperatures and pressures to remove the refractory sulfur compounds, thereby raising their operational costs. Moreover, simply increasing the temperature and pressure are insufficient to remove the final traces of sulfur without compromising the octane ratings.

Future research on HDS should prioritize the optimization of the composition, structure, and dispersion of the catalytic active sites to improve their activity and selectivity for removing refractory sulfur compounds. In addition, the development of advanced catalysts capable of operating efficiently under milder conditions, such as lower temperatures and pressures, is imperative to minimize energy consumption and reduce operational costs. Looking ahead, the HDS process is poised to evolve toward greater efficiency and sustainability, underpinned by advancements in catalyst design, process intensification, and the integration of renewable hydrogen sources. These innovations will be critical for meeting increasingly stringent environmental regulations and supporting the global transition toward cleaner energy systems.

3. Oxidative Desulfurization

3.1. Introduction of Oxidative Desulfurization

In recent years, ODS technology has garnered substantial attention as an alternative desulfurization method [53,54,55,56]. Operating under relatively mild conditions (30–120 °C, atmospheric pressure) [57,58], ODS offers unique advantages over HDS. The ODS process has been shown to oxidize refractory-substituted sulfur-containing organics at low temperatures and pressures, obviating the necessity for expensive hydrogen. This hydrogen-free process not only reduces the operational complexity but also significantly lowers the capital investment requirements compared to deep HDS units, rendering them particularly well suited for isolated small and medium-sized refineries.

Given the comparable electronegativity of sulfur and carbon, the sulfur–carbon bond is regarded as relatively nonpolar, and the properties of sulfur-containing compounds are closely related to their corresponding organic compounds. This phenomenon explains why sulfur-containing compounds and hydrocarbons have almost the same solubility in polar and nonpolar solvents. The electronic d-orbital of sulfur facilitates the oxidation of sulfur-containing compounds by oxidizing agents. During this process, the sulfur-containing compounds are converted into sulfoxides or sulfones, which exhibit enhanced solubility in polar solvents as their polarity increases. Therefore, desulfurization by selective oxidation generally comprises two primary steps. Firstly, the sulfur-containing compounds present in diesel are oxidized to their corresponding sulfoxides and sulfones. Subsequently, these oxidation products are removed from the diesel matrix using techniques, such as extraction, adsorption, or distillation.

The effectiveness of this process, based on the characteristics of the ODS, largely depends on the selection of the oxidants and catalysts. H₂O₂ has become a widely used oxidant due to its efficiency, environmental friendliness, and ability to selectively oxidize sulfur-containing compounds. When combined with suitable catalysts or co-oxidants (such as organic acids, heteropoly acids, or transition metal oxides), H₂O₂-based systems can achieve significant desulfurization performance under mild conditions.

3.2. Oxidative Desulfurization Systems

3.2.1. H₂O₂/Organic Acid

In 1996, Petro Star Inc. pioneered a combined conversion and extraction-based ODS process for the removal of sulfur from diesel fuel [59]. The fuel is mixed with H₂O₂/acetic acid (peroxyacetic acid) and the oxidative reaction takes place below 100 °C under atmospheric pressure. The reaction is followed by liquid–liquid extraction, yielding a low-sulfur fuel and a sulfur-rich extract. The extraction solvent is then removed from the extract for re-use and the concentrated extract is made available for further processing to remove the sulfur and to produce hydrocarbon. Barham et al. [60] investigated the ODS of real diesel samples using hydrogen peroxide activated by dicarboxylic acids, including malonic acid, succinic acid, and glutaric acid. The study demonstrated that the oxidation of the sulfur compounds to sulfones occurred efficiently, followed by acetonitrile extraction of the sulfones. The desulfurization rates reached 90.9%, 88.9%, and 93% for malonic acid, succinic acid, and glutaric acid, respectively, under optimized conditions (95 °C, 6 h, 10 mL H₂O₂ and 0.6 g organic acid). In another study by Barham et al. [61], an ODS system for high-sulfur diesel fuel was investigated using citric acid, pimelic acid, and α-ketoglutaric acid as the catalysts to activate H₂O₂. The aim of their study was to evaluate the potential of these carboxylic acids as H₂O₂ activators and to optimize the reaction conditions for maximum desulfurization. Under the optimized conditions (95 °C, 1 h, 10 mL H₂O₂ and 0.6 g acid dosage), the desulfurization efficiencies of citric acid, α-ketoglutaric acid, and octanedioic acid reached 27, 34, and 84.57%, respectively, for diesel fuel samples with an initial sulfur content of 2568 mg/L.

3.2.2. H₂O₂/Heteropolyacid

In 1997, Collins et al. [55] investigated the oxidation of DBT by H₂O₂ in a biphasic solvent system comprising water and toluene, using phosphotungstic acid (PWA) as the catalyst and tetraoctylammonium bromide as the phase transfer agent. Their findings demonstrate that, under optimized conditions, the oxidation of DBT can be achieved with nearly 100% selectivity. However, the decomposition of H₂O₂ competes with the DBT oxidation process. The article further notes that ODS technology is complementary to conventional HDS technology, and that it produces superior results for sulfides that are challenging to treat with HDS. The study proposes an integrated HDS-ODS hybrid system, whereby the oxidized oil is treated and regenerated by using two silica gel adsorption columns alternately, and the resulting sulfur dioxide is converted into elemental sulfur. Te et al. [62] compared the oxidation reactivity of DBT, 4-methyldibenzothiophene (4-MDBT) and 4,6-DMDBT in two distinct catalytic systems: polymetallic oxalate/hydrogen peroxide and formic acid/hydrogen peroxide. The results revealed that catalysts based on PWA and its sodium salt exhibit significantly higher catalytic activity than those derived from phosphomolybdic acid. In contrast, silicotungstic acid and silicomolybdic acid-based catalysts displayed the lowest activity. This variation in catalytic performance was attributed to the stability of the polymetallic oxygen peroxide species formed and the structural properties of the active species. In the polymetallic oxygenate/hydrogen peroxide system, the oxidation activity of DBT decreased with the increase in methyl substituents; the opposite was true for the formic acid/hydrogen peroxide system.

3.2.3. H₂O₂/MoO_x

H₂O₂/MoOx is an ODS system with the addition of an active phase [63,64,65,66,67,68]. This system integrates the positive effects of the support and the active phase to further improve desulfurization efficiency. Molybdenum, as a transition metal, is readily available and exhibits significant activity, making it commonly used as the active phase in ODS processes. Jin et al. [69] studied the efficiency of a Ca/MoO₃/Al₂O₃ catalyst for the ODS of 4,6-DMDBT, DBT, and BT under mild conditions (55 °C). The desulfurization reactivity followed the order DBT > 4,6-DMDBT > BT, achieving nearly 100% sulfur removal within 10, 8, and 16 min, respectively. The catalyst also exhibited excellent recyclability, maintaining over an 87% desulfurization efficiency for all the compounds after eight cycles. Similarly, Yang et al. [70] employed molybdenum supported on a 4A molecular sieve for the removal of DBT, and oil-soluble cyclohexanone peroxide (CYHPO) was chosen as the oxidant. At 373 K, the removal rate of the DBT and the residual sulfur content reached 99.0% and 5 ppmw under the optimal experimental conditions (Mo loading of 6 wt.%, molar ratio of CYHPO/DBT of 2.5, volume/mass ratio of model gasoline to catalyst of 100) over 30 min.

3.2.4. H₂O₂/Ti-Zeolite

Titanium-containing zeolite materials have garnered significant research attention due to their strong oxidative properties, environmental compatibility, and cost-effectiveness [71,72,73,74,75,76,77]. Kong et al. [78] synthesized an Ag/TS-1 catalyst via the iso-volumetric impregnation method. The ODS of fluid catalytic cracking (FCC) gasoline was conducted over the Ag/TS-1 catalyst, employing H₂O₂ as the oxidant and water as the reaction solvent. Remarkably, the sulfur content of the FCC gasoline was reduced from 136.5 μg/g to 18.8 μg/g within 4 h. In a separate study, Park et al. [79] prepared mesoporous TS-1 catalysts by the nanocasting method using two different carbon template sources, CMK-3 and commercial carbon black. The catalyst exhibited excellent ODS catalytic activity compared to conventional microporous TS-1.

3.3. Outlook for Oxidative Desulfurization

In contrast to HDS, ODS operates under mild conditions, and does not necessitate elevated temperatures or pressures or expensive hydrogen. Moreover, ODS preserves the integrity of aromatics and olefins, avoiding saturation and octane loss, suggesting that ODS has great potential to realize deep desulfurization as a complementary process to traditional HDS. Nevertheless, the practical implementation of ODS is not without challenges. As shown in

Table 3. Comparison of different catalysts’ performance in ODS.

ODS Oxidant	Organic Sulfur Compounds	Desulfurization Rate	Ref.
GB-W18O49	DBT	97.7%	[80]
Mo-INFs (H2O2)	DBT	97.9%	[81]
MoO3-Fe3O4 (H2O2)	DBT	98%	[82]
VO-MoO3@NPC (H2O2)	DBT	99.1%	[83]
SiW12@ZSTU-10	DBT	99.8%	[84]
MoOX/C-750-4 (H2O2)	DBT	100%	[85]

, a limitation that is critical to the desulfurization efficiency is the need to identify selective oxidants to minimize undesired side reactions. Additionally, the choice of solvent plays a pivotal role in preventing the loss of aromatic and olefinic compounds. Exploring new oxidizing agents (e.g., organic peroxides or green oxidizing agents) to reduce costs while improving oxidation selectivity and safety is a top priority for future ODS research. In addition, the recycling technology for the oxidizers is also an important direction. By studying the reaction kinetics; simulating the process; and optimizing the process parameters, such as the reaction temperature, time, and oxidant dosage; a more efficient and economical desulfurization process is expected to be achieved.

4. Adsorptive Desulfurization

4.1. Introduction of Adsorptive Desulfurization

ADS has emerged as a widely utilized strategy for the removal of sulfur-containing compounds from liquid hydrocarbon fuels. This approach is favored due to its low operational costs, mild process conditions (typically ranging from ambient temperature to approximately 100 °C), and high adaptability across a variety of desulfurization systems.

The mechanisms underlying adsorptive desulfurization involve several key interactions, including π-complexation, sulfur–adsorbent (S-M) interactions, chemical reaction-based adsorption, and other effects (such as acid–base interactions, hydrogen bonding, etc.). π-complexation is characterized by the coordination of metal ions and sulfur centers in organic sulfur-containing compounds. The process of π-complexation entails the formation of conventional σ bonds between the cations and their unoccupied s-orbitals. Concurrently, their d-orbitals can donate electron density to the antibonding π* orbitals of the sulfur rings, thereby contributing to the stabilization of the complex structure. This mechanism is prevalent in metal ion-modified zeolite adsorbents. For instance, Cu(I)-Y zeolite prepared by the vapor-phase ion exchange method can undergo π-complexation with thiophene, and its adsorption capacity is higher than that prepared by the liquid-phase ion exchange method [86]. S-M interactions can be regarded as an acid–base reaction between the metal and sulfur. Some metals (such as Ce⁴⁺, Ni²⁺, and air-regenerable metal oxides, etc.) can achieve selective adsorption through their interaction with sulfur. For chemically reactive adsorption, the sulfur-containing fuel interacts chemically with the adsorbent. The mechanism of chemical reaction adsorption combines the characteristics of hydrodesulfurization and adsorptive desulfurization and can be carried out at low or ambient temperatures, as well as at high temperatures (in a reaction medium using hydrogen). The adsorbent can be regenerated by separating the sulfur precursors (such as H₂S, SO₂, and elemental sulfur). In some other adsorption systems, the acid–base interactions play a significant role in the adsorption process. For example, the incorporation of PWA onto Cu-BTC significantly improved the removal of BT due to the PWA functioning as an acid center that interacted with the BT via an acid–base mechanism [87]. Furthermore, for some structurally flexible MOFs (such as MIL-53(Al)), the adsorption amount of the organic sulfur compounds increases with an increase in the content of diethyl ether (where hydrogen bonding exists).

The ADS processes of materials, including zeolite-based materials, metal–organic framework MOFs, mesoporous molecular sieve composites, and clay-based adsorbents, have been summarized and generalized. By exploring the structural features, adsorption mechanisms, and performances of the various ones, their potential applications and limitations for achieving efficient desulfurization can be better understood.

4.2. Adsorbents

4.2.1. Zeolite-Based Adsorbents

The combination of high surface area, shape selectivity, stability, and tunable properties makes zeolite-based adsorbents highly effective for ADS processes. Their ability to selectively remove refractory sulfur compounds, coupled with their renewability and adaptability, positions them as a promising solution for achieving ultra-low-sulfur fuel standards.

Xiao et al. [88] developed ZSM-5 zeolite nanocrystals using a steam-assisted gel crystallization strategy. By controlling the particle size to within 70–160 nm, the nanocrystalline ZSM-5 exhibited reduced diffusion pathways, an increased specific surface area, and enhanced exposure of the adsorption sites, leading to improved desulfurization performance. ZSM-5 nanocrystals with a SiO₂/Al₂O₃ mole ratio of 200 achieved a breakthrough capacity of 0.0509 mmol S/g adsorbent. Tian et al. [89] prepared graded β-zeolite with varying pore sizes through an alkali treatment and evaluated the ADS performance on a range of model fuels. The findings of this study indicated that the alkali treatment could form a mesoporous structure without compromising the crystallinity of the β-zeolite, thereby significantly enhancing its desulfurization performance. Moreover, the introduction of cerium ions was found to mitigate the adverse impact of toluene on the desulfurization performance to a certain extent. Similarly, Wang et al. [90] investigated the adsorption properties of Ce/Ni-loaded Y zeolite on DBT. The NiCeY zeolite, synthesized via aqueous-phase ion exchange, exhibited superior adsorption selectivity for organosulfur compounds in aromatic-rich solutions compared to the NiY and CeY, achieving an adsorption capacity of 7.8 mg/g. The enhanced performance was attributed to the synergistic interaction between the Ni and Ce, which rendered the DBT adsorption less susceptible to the presence of aromatics, such as toluene. The coexistence of toluene in the model fuel was detrimental to DBT adsorption, while the presence of Ce as a co-cation in the NiCeY promoted sulfur adsorption. Furthermore, the adsorption of DBT on NiCeY conformed to the Langmuir and Freundlich isothermal model.

4.2.2. Metal–Organic Framework-Based Adsorbents

Metal–organic frameworks (MOFs) represent a versatile class of materials characterized by diverse structural architectures, featuring coordinatively unsaturated (CUS) open metal sites. Their unique structural characteristics endow MOFs with tunable properties and favorable traits, rendering them highly adaptable for diverse applications, including adsorption, catalysis, and reaction engineering [91].

Li et al. [92] synthesized five MOF materials (MOF-5, HKUST-1, MIL-53 (Fe), MIL-53 (Cr), and MIL-101 (Cr)) according to the established literature method. These materials were subsequently evaluated for their adsorption and desulfurization performance against three distinct aromatic sulfur compounds. The experimental findings revealed that the adsorption capacities of the different MOFs of these compounds exhibited significant variations. For thiophene, the adsorption capacity followed the trend MIL-53 (Cr) > HKUST-1 > MOF-5 > MIL-53 (Fe) > MIL-101 (Cr). Notably, the adsorption behavior of aromatic sulfur compounds varied across the MOFs. For instance, the adsorption capacity of MOF-5, HKUST-1, and MIL-53 (Fe) for sulfur compounds was in the order of thiophene > BT > DMDBT; whereas for MIL-53 (Cr), it was BT > thiophene > DMDBT; and for MIL-101 (Cr), it was DMDBT > BT > thiophene. These findings underscore the pivotal role of the adsorbent–adsorbate interactions in adsorptive desulfurization, modulated by the metal centers, framework structure, and chemical characteristics of the aromatic sulfur compounds. Furthermore, it is noteworthy that the window diameter exerts a substantial influence when the molecular size of the sulfur compounds is commensurate with the window diameter of the MOFs.

Qin et al. [93] devised a vapor-induced selective reduction (VISR) strategy to prepare Cu(I)-containing sites on three MOFs: MIL-101(Cr), MIL-100(Fe), and HKUST-1. The desulfurization performance of these materials was systematically evaluated in tert-butylbenzene-containing systems using model sulfur compounds, including thiophene and BT. Among the tested materials, Cu(I)-functionalized MIL-101(Cr) with a copper loading of 3 mmol/g (denoted as Cu(I) M-3) exhibited the highest adsorption and desulfurization efficiency. Specifically, the adsorption capacity of Cu(I) M-3 for thiophene reached 0.291 mmol/g at penetration and 0.373 mmol/g at saturation, significantly outperforming the pristine MIL-101(Cr). Furthermore, Cu(I) M-3 demonstrated an enhanced adsorption capacity for larger sulfur-containing molecules, such as BT and DMDBT, albeit with a modest improvement, likely constrained by pore clogging. In systems containing 10 wt% tert-butylbenzene, Cu(I) M-3 can maintain high thiophene selectivity, and the adsorption activity remains stable over four cycles of recycling, underscoring the robust recyclability of Cu(I) M-3.

Khan et al. [94] synthesized Cu-BTC via microwave-assisted synthesis and subsequently loaded with varying contents of PWA by the impregnation method to yield PWA (0.5)/Cu-BTC, PWA (1.0)/Cu-BTC, and PWA (2.0)/Cu-BTC, respectively. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) demonstrated that lower PWA loadings (W/Cu wt./wt. ≤ 1.0) had a negligible impact on the crystal structure and morphology of the Cu-BTC. However, a higher PWA loading (W/Cu wt./wt. = 2.0) resulted in partial structural degradation. The nitrogen adsorption isotherms confirmed that all the samples retained a microporous structure, but the PWA incorporation reduced both the specific surface area and pore volume. The PWA (2.0)/Cu-BTC exhibited a significantly diminished adsorption capacity attributed to the structural disruption. In contrast, the PWA (1.0)/Cu-BTC displayed a slightly lower specific surface area and pore volume than the pristine Cu-BTC, yet demonstrated superior adsorption performance, indicating that porosity alone is not the dominant factor governing adsorption. The enhanced adsorption was hypothesized to arise from the acid–base interactions between the acidic PWA and the weakly basic Cu-BTC framework. This hypothesis was further supported by the observed increase in adsorption with a higher PWA loading, underscoring the critical role of acid–base interactions in adsorption enhancement.

4.2.3. Mesoporous Molecular Sieve Composites

Mesoporous molecular sieve composites have a high specific surface area, which can provide a large number of adsorption sites for sulfur-containing compounds to enhance the adsorption capacity. Moreover, their regular and adjustable pore structure enables the selective adsorption of specific sulfur-containing molecules, and the materials are chemically stable and can perform reliably and consistently in complex environments. In addition, mesoporous molecular sieve composites can be easily modified to enhance their adsorption performance in a variety of ways to adapt to different scenarios, and they can be recycled after regeneration, which reduces the costs and is in line with the concept of sustainable development.

Subhan et al. [95] investigated lanthanum-loaded mesoporous MCM-41 and related adsorbents. The specific surface area and pore size of the MCM-41 were shown to vary depending on the synthesis method employed. The incorporation of lower lanthanum loadings had a minimal impact on the structural characteristics, while excessive loadings led to a decrease in the specific surface area and pore structure. A structural characterization confirmed the successful incorporation of aluminum in Al-MCM-41(H), the effective dispersion of lanthanum within the MCM-41, and the tangible impact of the lanthanum loading on the acidity of the adsorbent. It was determined that 5% La/MCM-41(H) demonstrated the optimal thiophene sulfur removal from diesel fuel. This improvement was attributed to the interactions between the HO-La (OSiAl) units and the sulfur in thiophene and its derivatives. The desulfurization efficiency was found to depend not only on the lanthanum content but also significantly on the specific surface area of the carrier. With a moderate lanthanum loading, 1.5–5% La/MCM-41(L) and MCM-41 exhibited intact mesoporous structures and a uniform dispersion of the lanthanum species, while excessive lanthanum loading led to a decrease in the specific surface area and a deterioration of the mesoporous structure, which in turn reduced the desulfurization efficiency.

Subhan et al. [96] prepared nickel-loaded mesoporous MCM-41 and AlMCM-41 adsorbents to evaluate their desulfurization efficacy in model and real diesel fuels. Among these materials, 15% Ni-AlM41(30) exhibited superior sulfur adsorption capacity, driven primarily by the presence of Lewis acidic sites and the well-maintained carrier pore structure. The d10 electronic configuration of Ni0 in the reduced 15% Ni-AlM41 (30) promoted electron transfer, facilitating sulfur adsorption. However, the presence of naphthalene was found to inhibit thiophene adsorption. BET and Fourier transform infrared spectroscopy (FT-IR) analyses confirmed that the structural integrity of the adsorbent remained intact before and after regeneration.

4.2.4. Clay Adsorbents

Natural clay materials have significant advantages in the adsorption desulfurization process, as they are widely available, inexpensive, environmentally benign, non-toxic, and possess a rich pore structure with a large specific surface area, which can provide adsorption space and increase the contact opportunities to enhance the desulfurization efficiency. However, their intrinsic properties often need to be modified to enhance their hydrophobicity and improve their adsorption capacity for the sulfur-containing compounds in diesel fuel [97]. Baia et al. [98] systematically examined the desulfurization performance of three commercial clay materials and alumina, assessing the characteristics of the adsorbents and the adsorption process. The study revealed that bentonite clays (designated as Clay B and Clay C) exhibited a great adsorption capacity for both nitrogen- and sulfur-containing compounds, which was attributed to the presence of Brønsted acid sites and their high specific surface areas. In contrast, bumpy clay (Clay A) displayed selectivity towards nitrogen species, with its adsorption behavior strongly influenced by its unique chemical composition. Overall, the clays demonstrated superior adsorption performance compared to the alumina for the removal of sulfur and nitrogen compounds from diesel fuel. Ha et al. [97] modified bentonite clay with BM, tetramethylammonium chloride (TM), and hexadecyltrimethylammonium chloride (HM). The adsorption and desorption properties of the modified clays on the sulfur compounds in model fuels were investigated. The results showed the efficient adsorption of DBT and BT from the model fuels at ambient temperature, with the BM-modified bentonite achieving the highest adsorption capacity. Interestingly, an increase in the adsorbent dosage resulted in reduced adsorption efficiency, which was attributed to the aggregation of the clay particles. Various adsorption models indicated the existence of a mixed adsorption mechanism and a strong affinity between the adsorbent and adsorbate. Additionally, the partial regeneration of the adsorbent was achieved using 0.1 M HCl as the elution solvent.

Ahmad et al. [99] investigated the use of metal-impregnated montmorillonite (MMT) clay for the adsorptive desulfurization of kerosene and diesel fuels. Their study showed showed that montmorillonite clay can be used as an effective adsorbent for sulfur removal, and metal impregnation significantly enhanced its ADS performance. Specifically, zinc impregnation improved the specific surface area, pore structure, and surface morphology of the clay. The Zn-MMT desulfurization efficiency was higher than that of the original clay, and the best selective adsorption of the sulfur compounds was observed at 1 h of adsorption time, 25 °C, and 1.5 g of adsorbent dosage.

Habimana et al. [87] synthesized Cu-BTC/Mt composite porous materials for the adsorptive removal of thiophene from simulated oils. The 40% Cu-BTC/Mt composites showed the highest adsorption capacity under optimized conditions and demonstrated reusability for at least five cycles. Moreover, the adsorption behavior of thiophene on the 40% Cu-BTC/Mt composite conformed to both quasi-first-order and quasi-second-order kinetic models.

4.3. Outlook for Adsorptive Desulfurization

The integration of metal oxides with carriers, such as zeolites, mesoporous materials, and activated carbons, has substantially enhanced the physicochemical properties and adsorption performance of desulfurization adsorbents. These advancements have yielded remarkable improvements in desulfurization efficiencies under laboratory conditions, as summarized in

Table 4. Comparison of different catalysts’ performance in ADS.

ADS Catalyst	Organic Sulfur Compounds	Desulfurization Rate	Ref.
G0.1CeHY	Thiophene	51%	[100]
Cu(I)-BTC@PAF30	DBT	91%	[101]
Fe3O4@MnO2@AC	DBT Kerosene Diesel fuel	99% 97.6% 90%	[102]
Zn/AC	DBT	92%	[103]
Pt/h-BNNS	DBT	98%	[104]
Ni/ZnO-SiO2	Thiophene	96%	[105]

. However, a critical challenge persists: these studies predominantly relied on simplified model fuels that fail to capture the compositional complexity of real-world fossil fuels.

To address this limitation, future investigations must prioritize evaluating the performance of ADS materials in complex fuel matrices. In particular, the presence of competing substances, such as aromatics, nitrogen compounds, and other impurities, significantly influences adsorption mechanisms and could impede performance. Furthermore, an in-depth analysis of the in situ chemical transformations of the adsorbed sulfur species is crucial to elucidate their impacts on the long-term stability and recyclability of the adsorbents.

5. Electrochemical Desulfurization

5.1. Introduction of Electrochemical Desulfurization

ECDS has emerged as a promising approach for the removal of sulfur compounds from fossil fuels through electrochemical oxidation or reduction mechanisms [106,107]. This method has attracted considerable attention in recent years due to its relatively mild operating conditions, reduced environmental impact, and high efficiency. ECDS has demonstrated broad applicability across diverse sulfur-containing matrices, including gaseous compounds [108], liquid fuels [109], and solid minerals [110]. The flexibility of this technology can be further refined and expanded through the strategic optimization of process parameters (such as reactor configurations and electrolytes), thereby advancing its potential for industrial desulfurization applications.

The desulfurization of organic sulfide model compounds (e.g., thiophene and BT) and heavy hydrocarbons by electrochemical reduction represents a promising approach for desulfurization. During the electroreduction process, sulfur species are reduced to H₂S or S²⁻ at the cathode [111]. However, it has been demonstrated that hydrocarbon feedstocks may also undergo polymerization, with the possibility that sulfur oligomers may be entrained in the products of polymerization. Additionally, the strong electrochemical reducing conditions required for effective desulfurization may result in the deposition of contaminants on the catalyst surface, which can severely deactivate the catalyst over time. In aqueous systems, the evolution of hydrogen at high reducing potentials may partially mitigate catalyst deactivation by cleaning the surface. Nevertheless, this hydrogen evolution reaction (HER) can also compete with the desulfurization process, potentially reducing the overall efficiency. For instance, Vieira et al. [112] demonstrated that during the electrochemical reduction of phenylthiophenol derivatives on Pt, the HER competes with the desulfurization reaction, thereby influencing the efficiency and selectivity of the process.

In contrast to electrochemical reduction desulfurization, which generates H₂S as a byproduct, electrochemical oxidation desulfurization offers a cleaner alternative. Sulfur-containing compounds are removed via two principal mechanisms: (i) direct electrochemical oxidation, where the sulfur-containing compounds exchange electrons directly with the anode surface without the involvement of other substances; and (ii) indirect electrochemical oxidation, where electron transfer occurs through redox-active intermediates generated at the anode surface, rather than through direct interaction with the electrode [10]. As depicted in Figure 6, low-valent elements are oxidized indirectly to high-valent elements. Sulfur-containing compounds are oxidized to soluble sulfates at the anode, while H₂ is precipitated at the cathode. In alkaline media, reactive oxygen species (ROS) generated at the anode, such as hydroxyl radicals (·OH), atomic oxygen (·O), superoxide (O²⁻), and molecular oxygen (O₂), serve as potent oxidizing agents. These active species can oxidize organic sulfides to sulfoxides or sulfones, which subsequently separate from the system. Furthermore, alkaline leaching has been demonstrated to promote desulfurization [113]. The ECDS process in alkaline conditions is illustrated as Equations (1)–(3) below [114].

O₂ + 2R–S–S–R → 2R–S–S(O)R

(1)

2O₂ + R–S–S–R → R–S(O₂)–S(O₂)R

(2)

R–S(O₂)–S(O₂)R + 2H₂O → 2R–OH + H₂ + 2SO₄²⁻

(3)

In acidic systems, transition metals, such as iron, vanadium, manganese, cobalt, nickel, and copper, are commonly employed as oxidizing agents. High-valence metal ions oxidize sulfur-containing compounds, while the resulting lower-valence metal ions are regenerated via electrode reactions. Wang et al. [115] conducted a thermodynamic feasibility analysis and reported that desulfurization reactions are not thermodynamically spontaneous in acidic electrolyte systems. However, the process becomes spontaneous upon the addition of an aqueous cerium nitrate solution containing Ce³⁺ ions as the electrolyte. Tang et al. [116] reported a mixed electrolyte consisting of H₂SO₄ and MnSO₄, in which Mn²⁺ can be oxidized to Mn³⁺. The Mn³⁺ subsequently reacts with water to form MnOOH, which can effectively oxidize the sulfur compounds present in kerosene to sulfoxides or sulfones.

5.2. Reactors

5.2.1. Divided Cells

Divided and undivided cells represent the two principal reactor configurations employed in the electrochemical desulfurization processes. Divided cells leverage solid electrolyte membranes, facilitating the separation of the desulfurization products. However, the use of electrolyte membranes limits the design and operation of such reactors. For example, there are stringent humidity requirements (Nafion membranes) and susceptibility to the chemical composition of the feedstock. More research has been conducted on improving divided cell systems, utilizing polymer electrolyte membranes, metal membranes, and iontophoresis membranes.

In addition to the reactors for separation by membranes, multi-cells have also become a focus of research in recent years. Abdullah et al. [117] developed a segregated trickle-bed electrochemical reactor (TBER) for the in situ generation of H₂O₂, enabling the oxidative desulfurization of DBT in diesel fuel. The modified porous carbon electrochemical drip-bed reactor was able to generate H₂O₂ in situ and oxidize the DBT in diesel fuel. The maximum concentration of H₂O₂ was 18.0 mM when 10% diesel fuel was added to the electrolyte, which was 43% lower than that in the absence of diesel fuel. Despite the low generation of H₂O₂, DBT was successfully oxidized in situ in the TBER with a conversion rate of 97.8% in 6 h. Yang et al. [118] integrated an 8-unit enhanced TBER with a MnO₂ catalyst for the oxidative desulfurization of DBT in model diesel (n-octane). The MnO₂ greatly enhanced the electrochemical oxygen reduction activity in the reactor. Moreover, despite the presence of diesel fuel resulting in inhibitory effects on H₂O₂ production, 71.76 mM of H₂O₂ could still be produced in the presence of 10% diesel fuel. This system achieved the complete oxidation of 500 ppm DBT within 2.5 h, underscoring its potential efficacy for deep desulfurization processes.

5.2.2. Undivided Cells

In undivided cells, there is no separation between the cathode and the anode. These systems have been extensively applied for the desulfurization of various sulfur-containing fuels through electrochemical oxidation or reduction, including hydrocarbon oils, oil shale, sulfur-containing organics, and naphtha. Hu et al. [119] systematically investigated the electrochemical behavior and desulfurization performance of different anode materials during the electrolysis of bauxite slurry (BWS) in an undivided cell. Their findings revealed that nickel exhibited the highest corrosion resistance, while a lead–silver alloy was the most prone to corrosion. The apparent activation energy (AAE) of the nickel and iron electrode reactions was higher than 40 kJ/mol when the potential was lower than 0.8 V, which implies that the oxygen precipitation reaction of the BWS electrolysis was controlled by electrochemical reactions. However, when the electrode potentials were higher than 0.8 V, the apparent activation energies of the nickel and iron electrode reactions entered the diffusion-controlled region (AAE < 40 kJ/mol). Additionally, the nickel anode demonstrated superior oxygen evolution activity compared to the iron anode. Shams et al. [120] employed undivided cells for the electrochemical desulfurization of a model fuel mixed with a polar solvent. This approach achieved complete the desulfurization of a model fuel containing 300 ppm DBT within 5 h under ambient temperature and pressure. Cyclic voltammetry (CV), gas chromatography–mass spectrometry (GC-MS), and FT-IR analyses confirmed that the DBT was primarily oxidized to the corresponding sulfone (DBTO₂) by electrochemical oxidation during the solvent-extracted phase (MeCN-H₂O).

5.3. Electrolytes

The ECDS of fossil fuels can proceed under specific conditions even without the use of an electrolyte. For instance, at extremely high applied potentials or elevated temperatures, the reaction may no longer depend on an external electrolyte to sustain the process. Previous studies [121,122,123] have demonstrated that the intrinsic conductivity of heavy hydrocarbon feeds can increase exponentially with temperature. At sufficiently high temperatures, the ionic conductivity of these hydrocarbons may become adequate to facilitate electrochemical desulfurization. However, maintaining the structural and functional integrity of diesel fuel before and after desulfurization necessitates operating at lower temperatures, thereby making the addition of electrolytes indispensable.

5.3.1. Alkaline Electrolytes

Under alkaline conditions, desulfurization has been demonstrated to occur via ·OH, which act as oxidizing agents in a system. Wang et al. [124] prepared β-PbO₂/C particle electrodes for the electrochemical catalytic oxidation and extraction of gasoline in an electrochemical fluidized bed reactor using NaOH as the electrolyte. It was concluded that the desulfurization process proceeded through an indirect electrochemical mechanism: OH- were oxidized at the anode of the β-PbO₂/C particle cluster to generate ·OH, which subsequently oxidized the sulfur compounds into sulfoxides or sulfones. Shen et al. [125] studied coal–water slurry desulfurization by electrochemical reduction with sodium borate in an alkaline system. An increased concentration of SO₄²⁻ in the electrolyte post-reaction was detected, suggesting the release of sulfur from the coal in ionic form. The possible desulfurization mechanism involved the transformation of NaBO₂ to NaBH₄, and then the NaBH₄ reduced the sulfides in the coal to H₂S and S²⁻, followed by the anodic oxidation of S²⁻ into SO₄²⁻. Behrouzifar et al. [106] investigated the electrically reduced desulfurization process of thiophene-containing solutions using cyclic voltammetry square wave potentiometry technique in a KOH solution. The result showed that the optimal adsorption potential of thiophene on a platinum electrode was −0.54 V. Humadi et al. [126] explored the catalytic electrochemical oxidative desulfurization of diesel fuel in a KOH alkaline electrolyte. The results of the study indicated that the sulfur compounds in the diesel were oxidized to sulfoxides and sulfones under alkaline conditions.

5.3.2. Acidic Electrolytes

The development of acidic electrolyte systems has facilitated the better electrochemical catalytic desulfurization of certain specialty electrodes. Tang et al. [127] investigated the electrochemical oxidation of condensate gasoline using H₂SO₄ as the electrolyte with different kinds of inorganic salts and a small amount of phase-transfer catalyst. They proposed that the electrochemical oxidation of H₂O and Cl⁻ at the anode generated a highly active oxidizing medium. The oxidizing medium oxidized sulfur atoms at the interface between the electrolyte and the condensate gasoline phase, increased the polarity of the sulfide molecules, and participated in the next reaction after its own reduction. After undergoing electrochemical oxidation and a three-stage extraction process, the sulfur content in the condensate gasoline was reduced from 3478.4 μg/g to 13.1 μg/g, achieving a desulfurization efficiency of 99.62%. Alipoor et al. [128] investigated the adsorption and oxidation behavior of thiophene on Pt electrode surfaces using cyclic voltammetry and square-wave potential techniques. The study determined that the optimal adsorption potential of thiophene on a platinum electrode was 0.2 V, with the most effective square-wave potential frequency being 50 Hz. Under the above-mentioned conditions, the conversion efficiency for thiophene desulfurization in an aqueous solution reached 100%, while for a thiophene-containing model fuel, the conversion rate was approximately 88%.

5.4. Outlook for Electrochemical Desulfurization

ECDS technology has the potential to be environmentally friendly and efficient, but it still requires further optimization in terms of its selectivity, energy consumption, and material stability. As shown in

Table 5. Comparison of different catalysts’ performance in ECDS.

Electrode	Electrolyte	Desulfurization Rate	Ref.
Pt/Pt	0.5 M H2SO4	48.2%	[129]
Graphite/Graphite	NaCl	92.67%	[130]
Cu/Cu	MnSO4/H2SO4	95.07%	[116]
AAO-CeO2 NTs/Pt	0.1 M Ce(NO3)3	96.82%	[131]
Graphite/ Graphite	NaCl/10% acetic acid	98.71%	[132]

, the selection of electrodes and electrolyte is crucial for an efficient ECDS process. During the electrochemical desulfurization process, the electrode materials are susceptible to corrosion and contamination by the sulfides or by-products, resulting in decreased activity. In addition, the high cost of high-performance electrode materials (e.g., precious metal electrodes) limits their large-scale industrial application. Moreover, electrochemical desulfurization requires additional electrical energy to drive the reaction, which consumes a high amount of energy, especially when treating large-scale fuels, and energy efficiency becomes a key issue. Future research could focus on developing electrode materials with high electrical conductivity, corrosion resistance, and catalytic activity; exploring the reaction kinetics and optimizing the parameters, such as the voltage, current density, and reaction time, to reduce the energy consumption and improve the efficiency of desulfurization; and designing electrodes or catalysts with specific selectivity that could preferentially oxidize or reduce complex sulfur compounds that are difficult to remove by regulating the electrochemical reaction path. At the same time, through its integration with other desulfurization technologies and the resource utilization of its by-products, electrochemical desulfurization is expected to become an important technological means to realize the production of ultra-low-sulfur fuel.

6. Conclusions

The advancement of diesel desulfurization technologies continues to evolve in response to increasingly stringent global environmental regulations. Among the existing approaches, HDS, ODS, ADS, and ECDS each exhibit unique advantages while facing distinct challenges. HDS is widely used; however, its efficiency and cost-effectiveness are hindered when targeting refractory sulfur compounds. ODS offers a mild and cost-efficient alternative, yet its practical application is constrained by limitations in oxidation selectivity and solvent compatibility. ADS has garnered significant attention due to its mild operating conditions and other favorable attributes, though the development of high-performance adsorbents remains a critical bottleneck. ECDS, with its potential for high efficiency and reduced environmental impact, shows promise but requires further advancements in catalyst design and reaction optimization to achieve industrial viability.

The development of diesel desulfurization technology will focus on improving the efficiency of desulfurization, reducing costs, reducing environmental impacts, and achieving the efficient use of resources. The efforts to optimize and upgrade the individual technologies are expected to include the development of more efficient HDS catalysts, the identification of highly selective ODS systems, the refinement of adsorbent materials, and the improvement of current efficiencies in ECDS processes. Beyond individual advancements, the integration of complementary desulfurization techniques is anticipated to emerge as a key strategy for achieving ultra-deep desulfurization, enabling compliance with increasingly stringent sulfur content standards for diesel fuels. Furthermore, interdisciplinary innovations in fields, such as artificial intelligence and materials science, are poised to inject new momentum into the development of desulfurization technologies, paving the way for a greener and more sustainable refining industry.