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.


Introduction
With increasing urbanization, humankind has experienced an escalating dependence on energy. As shown in Figure 1, for EU-28, total energy supply amounted to 1.58M toe (tonnes of oil equivalent) in 2019, and oil and petroleum products represented the highest contribution (32.8%), followed by natural gas (25%), renewables and biofuels (15.8%), nuclear heat (13.2%), and solid fossil fuels (11%). In particular, the contribution of oil and petroleum products in energy supplies has kept with no relevant changes (31.4-33.0%) in the last decade (Figure 1a), with values ranging from 510k (2014) to 569k (2010) toe. Oil and petroleum products are extensively used in our society for transportation, electricity, and construction materials. Transportation may be the sector that most relies on petroleum-based fuels, from ground transportation to planes and ships. In EU-28, the final consumption of energy in the transport sector amounted to 330.8k toe in 2019, being 209.8k toe of gas oil and diesel, 79.1k toe of motor gasoline, 17.4k toe of renewables and biodiesel, and 3.7k toe of natural gas (Figure 1b). In the last decade, gasoline consumption has decreased from 28.5% to 23.9% of the total energy consumed in the transportation sector, whereas gas oil and diesel have increased from 60.7% to 63.4% (Figure 1b). Some heteroatoms are inherently found in petroleum and petroleum-based fuels. The atom found in the highest concentration after carbon and hydrogen is sulfur, followed by nitrogen. Their presence is associated with a series of environmental and health-related issues due to the formation of toxic gaseous products (NO x and SO 2 ) during combustion. Those issues have driven legislative bodies to regulate the content and related emissions of S-and N-based compounds, increasing restrictions. Hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are the technologies applied at the industrial level. However, HDS and HDN fail to achieve the required-by-law content of sulfur without compromising relevant combustion parameters of the fuels (such as diesel and gasoline). Alternative technologies have been studied in the last few years and, among them, oxidative treatments can be highlighted, namely oxidative desulfurization (ODS) and denitrogenation (ODN). As with any catalytic process, the development and selection of catalysts are critical in ODS and ODN, and carbon-based materials have revealed exciting results in the oxidative removal of S-and N-based compounds. This review aims to discuss the state-of-the-art application of carbon-based materials for ODS and ODN purposes, emphasizing the role displayed by those materials, either in hybrid forms or as carbocatalysts.

Sulfur and Nitrogen in Crude Oils: Content and Type of Compounds
Petroleum is composed mostly of hydrocarbons, although impurities such as nitrogen, sulfur, oxygen, and metal compounds (vanadium, nickel, and iron, among others) are found [2]. Sulfur is the component that varies the most between different petroleum samples [2] and is the third most abundant element in crude oils, after carbon and hydrogen [3]. Sulfur content can vary between <0.1 to 8 wt.% [2], leading to the classifications of sweet crude oils (S content <1 wt.%) or sour oils (S content >1 wt.%) [3]. Nitrogen content can vary between <0.1 to 1.8 wt.% [2], and crudes can be classified as high-nitrogen oils (>0.25 wt.%) or nitrogen-poor oils (<0.25 wt.%) [4]. Table 1 displays some examples of the content of sulfur and nitrogen in different crude oil samples. There is no correlation between the content of sulfur and nitrogen. The content of those impurities depends mostly on the area of exploration [4,5]. S-containing compounds found in petroleum can be divided into four main groups: mercaptans (R-SH), sulfides (R 2 S), disulfides (RSSR), and thiophenes. Some examples of these compounds can be seen in Figure 2. Other compounds may include H 2 S and elemental sulfur, although in trace quantities [7]. N-containing compounds are classified into two major groups: basic (derived from pyridine) and non-basic or neutral (derived from pyrrole) [4]. Figure 3 displays some N-containing compounds commonly found in petroleum. There is also evidence of the presence of primary amines (R-NH 2 ), porphyrins structures, or compounds that contain nitrogen and oxygen simultaneously [4].  Heavier boiling fractions contain higher amounts of sulfur compounds (main boiling fractions are displayed in Figure 4). In the gasoline boiling range, mercaptan, sulfide, disulfide, and thiophene groups are found. In contrast, jet fuel and diesel primarily include thiophenes and derivatives [8]. Similarly, the nitrogen content increases for heavier fractions. Most N-compounds occur in the residual boiling fractions [4].

Health, Environmental and Technological Effects of Sulfur-and Nitrogen-Containing Compounds
Upon combustion of petroleum-based fuels, hazardous gases (SO 2 and NO x ) and particulate matter (PM) are released [10]. PM can be generated directly during combustion (primary PM) or from the chemical reaction of exhaust gases, including SO 2 and NO x (secondary PM). The exact composition of the exhaust gas depends on a series of factors, mainly fuel composition and combustion conditions. However, PM and NO x emissions can be controlled by augmenting the cetane number and lowering the number of aromatic compounds in diesel fuels [10], besides reducing the content of N-containing compounds. Reducing the content of sulfur diminishes SO 2 and sulfur-based PM [10].
Exposure to SO 2 and NO x is related to several health issues in humans. NO x and SO 2 are both irritants of the respiratory system and are associated with worsening existing respiratory illnesses, such as asthma, and increased susceptibility to lung infections [11]. Most of the PM emitted in diesel engines (>90%) is smaller than 1 µm in diameter [10]. PM 10 (aerodynamic diameter <10 µm) is a nose and throat irritant, causing lung damage, bronchitis, and risk of cardiac arrest, which is even associated with early death [12]. Studies have shown that long-term exposure to diesel combustion products is associated with a 20-50% increase in the likelihood of developing lung cancer due to genotoxicity [10,13]. Similarly, exposition to diesel exhausts is related to a 4-fold increase in the risk of dying of noncancerous diseases, such as cardiovascular diseases [10]. A study done in 2003 found a positive correlation between exposure to an increased daily concentration of SO 2 in the atmosphere and hospital admissions for cardiovascular diseases and ischemic heart diseases in major European cities [14]. Mortality and morbidity rates due to SO 2 exposure result in relevant monetary costs, as estimated by Wu et al. (2020) (35.76 and 441.47 million RMB Yuan, respectively, for Beijing, China, in 2016) [15]. NO x and SO 2 also provoke adverse environmental effects. NO x and SO 2 from exhaust engines combine with air moisture and contribute to acid rain [10]. Acid rain is linked to the depletion of essential soil nutrients, affecting soil fertility [7]. Soil pH is also altered, destroying some microbial cultures [7]. Soil alterations threaten forest vegetation and crop production. It contributes to dissolving harmful substances (such as heavy metals), leaching to water bodies. It is also associated with lake acidification and water nitrification [16,17], impacting microorganisms' reproduction and growth. Acid rain also affects human-made structures, diminishing the lifespan of buildings, bridges, statues, among others [7,17]. Furthermore, both primary and secondary PM reduce atmospheric visibility between 8 and 25% [10]. Sulfur compounds, mainly H 2 S and mercaptans, are related to a strong odor in sour petroleum [7]. Furthermore, lower aliphatic sulfides and mercaptans are highly reactive towards metals and cause corrosion, damaging storage tanks, pipelines, and equipment [7]. However, the main issue with S-containing compounds during refining is the deactivation of metal catalysts in downstream units [7]. In hydrogenation, reforming, isomerization, and alkylation processes, sulfur is adsorbed in the catalyst surface. Sulfur adsorption then hinders the adsorption of any other species. The deactivation degree depends on the extension of the active sites that were obstructed. Depending on the metal used as catalyst, irreversible poisoning can happen, and thus the regeneration of the catalyst becomes a difficult task [7]. Additionally, sulfur chemisorbs on the catalysts used in the catalytic converter (responsible for reducing CO and NO x emissions in passengers' vehicles), causing catalyst deactivation and reducing engine efficiency [5,18].
N-containing compounds pose issues in catalyst selection and process development and operation in oil upgrading and refining [4]. The main problems associated with nitrogen are: (i) 5-membered rings containing nitrogen compounds, such as pyrrolic compounds, are prone to free radical addition reactions, forming heavier products or gums that highly compromise storage stability [4]; (ii) the presence of basic nitrogen compounds leads to the deactivation of acid catalysts through acid-base reactions [4]; (iii) the presence of basic nitrogen compounds leads to salt precipitation (due to acid-base reactions with carboxylic acids), resulting in equipment fouling and even corrosion if water is present under high-temperature operation [4]; (iv) metal coordination is indirectly controlled by nitrogen, and nitrogen is therefore indirectly responsible for metal deposition, leading to equipment fouling [4]; (v) nitrogen competes with other compounds for active sites on catalyzed processes, such as the HDS process. Thus, the presence of nitrogen compounds hinders the removal of sulfur compounds [4]. Therefore, it is preferable to remove nitrogen compounds, especially before the refining processes.

Legislation
The maximum sulfur content allowed for transportation fuels (gasoline and diesel) has become stricter in the last years worldwide. Currently, the EU allows only 10 ppm of sulfur in diesel and gasoline [19]. In contrast, the US limits of sulfur content for gasoline and diesel are 10 [20] and 15 ppm [21], respectively. Figure 5 shows the evolution of the allowed content of sulfur in gasoline ( Figure 5a) and diesel (Figure 5b) in the last decades. As observed, EU was the first to regulate the content of sulfur in gasoline to 150 (Figure 5b), Japan was also the first to limit the S-content in 10 ppm in 2007, followed by EU and Australia (2009), Brazil (2013), Russia (2016), China (2017), and India (2020) (the US and Canada establish the value of 15 ppm). Nitrogen is not directly regulated in permitted content but indirectly by allowed NO x emissions, gum content, storage, and thermal stability [4].  [22], TransportPolicy.net [23][24][25][26], United States Environmental Protection Agency [20], Australian Government [27], Euro V [28], Miller, and Jin (2019) [29].

Processes for Removal of Sulfur and Nitrogen from Fuels
Environmental issues and restrictive legislation have spiked the interest desulfurization and denitrogenation technologies since 1951 ( Figure 6). The most studied strategies are classified in conventional processes, as HDS and HDN, and in more novel approaches, mainly ODS and ODN. . Papers published on HDS, HDN, ODS, and ODN over the years (source: SCOPUS with the keywords "hydrodesulfurization," "hydrodenitrogenation," "oxidative desulfurization," and "oxidative denitrogenation," respectively, dated August 2021).

Hydrodesulfurization (HDS) and Hydrodenitrogenation (HDN)
Several processes based on the reaction between hydrocarbons and hydrogen are used in the petroleum industry, such as hydrotreating, hydrocracking, and hydrogenation [30]. Hydrotreating refers to removing undesired species, such as sulfur, nitrogen, oxygen, and metal impurities, from petroleum fractions [30] by hydrogenolysis of C-X bonds, where X can be S, N, O, or metal. The net result is the formation of C-H bonds (hydrogenation) and H-X bonds [30]. The chemical reaction of fixating a hydrogen atom onto a hydrocarbon double bond, known as hydrogenation, using nickel catalysts, was first described in 1897. However, only in the mid-50s, the first noble metal catalyst was developed, and the HDS process commercialized [31]. Several HDS processes have been commercialized from then on, and all petroleum refineries worldwide have at least one hydrotreating unit [31].
The hydrotreatment of nitrogen-, sulfur-and oxygen-containing compounds will lead to ammonia (NH 3 ), hydrogen sulfide (H 2 S), and water, respectively [31]. Oxygencontaining compounds are much more reactive than nitrogen-and sulfur-containing compounds towards hydrotreatment, and thus hydrotreating is not specifically developed to remove O-compounds [32]. Most processes are designed to deal with S-compounds due to their hazardous effect on downstream processes. The removal of S, N, and other impurities co-occur in refinery conditions [33]. Thus, N-containing impurities are also removed during hydrotreatment processes designed to remove sulfur [32], although nitrogen compounds are much less reactive and hard to remove than sulfur [4].
Different sulfur compounds have different rates of HDS [32]. Mercaptans, sulfides, and disulfides are highly reactive towards HDS under mild conditions. On the other hand, 5-membered ring compounds containing sulfur (thiophenes) are very little reactive towards HDS, and the reactivity decreases as the number of rings (in the range 1-3 rings) increases [32]. However, the tendency reverses for structures containing four or more rings, and the reactivity increases as the number of rings increases [32]. The distinct behavior is that there is more than one pathway to remove sulfur from a molecule. Depending on the sulfur compound, one pathway may be more relevant than the other [32]. The order of reactivity of common organosulfur compounds towards HDS can be seen in Figure 7. Analogous to thiophenes, nitrogen-containing compounds require a step of hydrogenation before removal of N, and thus, the H 2 partial pressure is crucial [33]. Nitrogen removal for pyridinic structures is more complex than for thiophene structures. One single bond and one double bond require to be broken [4]. In contrast, thiophenes only require two single bonds to be broken [4]. The presence of alkyl groups tends to decrease the reactivity of the S-compounds due to steric hindrance. The closer the alkyl group is to the sulfur or nitrogen atom, the stronger the effect on the reactivity [33]. Further information regarding the effect of temperature, catalysts, and other parameters in HDS and HDN processes may be found elsewhere [35].
Thus, different operating conditions may be required depending on the compounds present in the petroleum fraction. Table 2 shows some common hydrotreatment operational conditions for different fractions of petroleum. Heavier fractions tend to be richer in thiophene and N-containing compounds, and harsher conditions are required to remove those compounds. Increasing operational conditions affects a series of parameters in diesel and gasoline. Oxygen, benzene, aromatics and olefin contents, vapor pressure, and boiling range can be compromised during gasoline range hydrotreatment. In contrast, the compromised parameters for diesel are cetane number, density, and content on polynuclear aromatics and distillation 95% point for diesel [36]. Besides that, hydrogen is a very expensive reactant, and process efficiency is highly limited by the reactivity of certain sulfur compounds, by the deactivation of catalysts due to coke formation, and by the presence of H 2 S [7]. Thus, alternatives to HDS and HDN processes need to be sought.

Alternative Routes to Remove Sulfur and Nitrogen
There is a wide range of alternatives to remove sulfur and nitrogen from liquid fuels. There are approaches based on liquid-liquid extraction (extractive desulfurization-EDS) using traditional solvents, deep eutectic solvents (DES) or ionic liquids (ILs) [37][38][39], adsorption (adsorptive desulfurization-ADS) [40], microorganisms (biodesulfurization-BDS) [41], and the oxidation of the undesired compounds (e.g., oxidative desulfurization-ODS). ODS is a promising technology since it operates under milder temperature and pressure conditions (25-140 • C, 1-2 bar) than HDS (260-425 • C, 5-160 bar). Thus, ODS does not require specialized reactors to operate at high pressure and temperature conditions. Catalyst and solvents can be mostly recycled, and the refractory-towards-HDS sulfur compounds are easily oxidized, leading to deep desulfurization [7]. Finally, ODS does not cause olefins saturation, nor does it result in loss of octane content [42] or other fuel parameters as those affected during HDS under harsher conditions.

Oxidative Desulfurization (ODS) and Denitrogenation (ODN)
The ODS process is based on the oxidation of sulfur-containing compounds towards their corresponding sulfoxides and sulfones (as shown in Figure 8) [43], followed by the extraction or adsorption of the oxidized compound. Thus, ODS is, in fact, a two-step process: (i) oxidation and (ii) extraction (or adsorption) [7,43] of the oxidized compounds. In some cases, especially with ILs, which can act as catalysts and extractants in ODS, an alternative system, extractive-catalytic oxidative desulfurization (ECODS), is introduced [44]. Due to the strong affinity of sulfur for oxygen, oxidation of hydrocarbons is not expected to occur in the usual operating conditions of ODS, thus promoting the selective oxidation of organosulfur compounds without affecting C-C bonds [34]. The reactivity of a sulfur compound towards oxidation depends on the electron density around the S atom. The higher the electron density, the more prone the electrophilic attack on the oxygen is [7] and the higher is the reaction rate towards the formation of sulfones [45]. However, the order of reactivity seems to depend significantly on the type of oxidant, catalyst, and operating conditions. Nevertheless, there is no organosulfur category that is entirely unreactive towards ODS [34].
The ODN process, in turn, has not been as extensively studied as ODS (as demonstrated in Figure 6). There is a broader range of possible reaction products than ODS during the oxidation of N-containing compounds due to the several bonds of N-compounds that are prone to oxidation [46]. For instance, quinoline (QN) oxidation gives reaction intermediates: 5-hydroxyquinoline and 8-hydroxyquinoline are the major products. However, trace quantities of 3-, 6-and 7-hydroxyquinoline are also observed [47]. Selectivity towards one product or the other may be tuned by controlling the reaction conditions [46].
Several oxidants have been reported in the literature for ODS and ODN, such as hydrogen peroxide, organic peracids, oxygen, and ozone, among others [7]. The oxidized compounds have an increased relative polarity compared to their parent compounds, and thus their extraction is facilitated [5]. The oxidized compounds are more commonly removed through liquid-liquid extraction rather than through adsorption. Adequate extractants should fulfill a few criteria: they should be immiscible with the oil phase, they should be able to dissolve sulfones and sulfoxides, differences in density may exist to facilitate the separation between oil and extractant, and not impact the fuel properties, among others [34]. Some extractants found in the literature are acetonitrile (ACN), methanol (MeOH), ethanol (EtOH), diethyl ether (Et 2 O), dimethylformamide (DMF), methyl cyanide, carbon tetrachloride (CCl 4 ), and ILs. Aqueous solutions of H 2 O 2 (used as an oxidant) also are applied as extractants on their own. As previously mentioned, both ODS and ODN are primarily conducted in a two-step process. However, in order to cut costs and accelerate the processes, one-pot approaches are increasingly sought. In one-pot approaches, extraction occurs concomitantly to oxidation and, hence, the extractant must be present during the oxidation step. Proper extractants are immiscible with the oil phase and, thus, one-pot approaches are commonly conducted in a biphasic medium. Biphasic systems have some advantages over traditional homogeneous systems. Biphasic systems allow conducting reactions with reactants with distinct solubility behaviors. They allow an easier separation between reactants, products, and catalysts, and the reactions to be conducted with incompatible substances, avoiding interferences [48,49].
However, biphasic systems display some issues. The main issue is the low interfacial area between the phases, hindering reaction [48,50]. An alternative is emulsifying the biphasic medium [48], as emulsion systems significantly improve the interfacial area. Emulsifying requires the use of a surfactant or of solid particles that allow the stabilization of Pickering emulsions. Pickering emulsions are an exciting template since the solid particles act as stabilizers and can also assume the role of catalysts. This approach is often referred to as Pickering Interfacial Catalysis (PIC) [50,51].
A wide range of solid particles can be used in the stabilization of Pickering emulsions, such as silica [52], magnetic particles [53], and carbon-based materials [54,55], among others. Pickering emulsions are affected by a range of conditions (pH, type of oil, ratio between the 2 phases, and concentration of particles, among others). One of the most interesting strategies to adjust emulsion formation and stability is to tune the amphiphilic character of the solid particles [56]. Amphiphilic materials display both hydrophilic and hydrophobic character and thus are wetted by more than one phase. As expected, particles wetted by more than one phase should allow easier emulsion formation and more stable emulsions [56]. Thus, amphiphilic materials should be advantageous when working under biphasic systems, especially in PIC-like approaches under emulsified systems.
Numerous catalysts have been applied for ODS and ODN purposes. However, discussing possible catalysts rather than carbon materials is not the aim of this review. Further information on other ODS catalysts may be found elsewhere [34,42,44,57]. Carbon-based materials, i.e., the focus of this review, have been widely used in ODS and ODN, either by composing hybrid materials or as catalysts on their own. Carbon-based materials display several advantages in oxidative processes. Their ability to form oxidizing species (such as hydroxyl radicals) from oxidant sources (such as hydrogen peroxide) has been widely studied [58][59][60][61][62]. They are proven catalysts for oxidation reactions in the liquid phase, either as catalysts on their own or in hybrid forms, such as in water and wastewater remediation [63][64][65][66][67]. Thus, applying those materials in the oxidative removal of sulfur and nitrogen from liquid fuels is a natural extension. Carbon-based materials can be synthesized from various precursors, including waste materials [68][69][70], and through several synthesis procedures [68][69][70][71][72][73][74][75]. Table 3 summarizes the main carbon-based materials that will be discussed in the following sections, along with some common carbon sources and synthesis routes. The column with the reference will lead the reader to the appropriate literature regarding the preparation of the mentioned materials.
In the following section, papers related to ODS and ODN of liquid fuels (simulated or real matrix) using strictly carbon-based materials in hybrid forms or as catalysts on their own will be presented. Furthermore, when appropriate, a distinction between the papers that use a two-step process, a biphasic system, and a PIC-type approach will also be made. [ [70][71][72] Graphene Oxide (GO) and Reduced-GO (rGO) Plastic waste, hydrocarbon gases.

Pyrolysis followed by chemical or physical activation
High surface area (usually s bet >500 m 2 g −1 ), highly developed internal pore structure, presence of functional groups, especially oxygenated, wide variety of ACS, usually low cost. [69]

Carbon Materials in ODS
The following sections will present the studies related to carbon materials in ODS systems, breaking them into three main sections: (i) hybrid materials containing carbonaceous structures, (ii) carbon-based materials as catalysts on their own, and (iii) systems applying amphiphilic catalysts containing carbon in their structure. As seen in Figure 9, most papers (73%) reported in the literature are related to applying carbon-based materials forming either hybrid materials or as supports for active centers. Fewer papers deal with amphiphilic materials that contain carbon in their structure (15%) or as catalysts on their own (12%).

Carbon Supports and Hybrid Catalysts
One of the advantages of carbon materials is that they can be combined with other materials to prepare composites or hybrid catalysts. Thus, most of the papers related to ODS with carbon materials have applied them in hybrid forms. As discussed before, ODS can be conducted in various systems, such as two-step processes, biphasic systems, under the emulsified form, and solvent-free, among others. Hence, the different systems are separated into different sections.

Two-Step Systems
The papers related to ODS reactions using carbon hybrid materials under a two-step process are gathered in Table 4. As observed, several combinations of hybrid materials consisting of phosphotungstic acid (H 3 PW 12 O 40 , HPW), iron, palladium, molybdenum, and titanium active metal phases with g-C 3 N 4 , GO and its reduced derivative (rGO), CNTs, and AC as supports/hybrids. The catalytic activities of these hybrid materials have been assessed in ODS of solutions with initial sulfur concentrations ([S] 0 ) ranging from 100 to 9400 ppm using H 2 O 2 as oxidant under mild conditions (25-80 • C) and achieving sulfur removals higher than 90%. In general, most authors have not justified the role of the carbon structure in these runs [76][77][78][79][80][81][82], with a few exceptions.
Huang et al. (2017) [83] performed ODS of a simulated fuel using dibenzothiophene (DBT) dissolved in n-octane ([S] 0 = 100 ppm) using an N-doped onion-like (NOLC) supported HPW. They have obtained high conversions (>90%), which were maintained for up to 10 reuse cycles. They attributed the excellent results to the excellent dispersion of the active phase (HPW) in the NOLC structure. The surface of NOLC displayed a high content of oxygenated groups, which can interact with HPW, strengthening the HPWsupport interaction. In turn, the HPW dispersion is improved, boosting the reaction [75]. Rafiee, Shahbazirad, and Khodayari (2017) also found that an increased dispersion of HPW over TiO 2 /g-C 3 N 4 resulted in higher desulfurization rates of a simulated fuel (DBT (C 0 = 500 ppm), BT (C 0 = 250 ppm) and Th (C 0 = 250 ppm) in EtOH/n-heptane) and for a real oil ([S] 0 = 500 or 1900 ppm) [84]. Abdelrahman et al. (2018) [85] also highlighted the high dispersion of Mo 132 as one of the reasons for the high activity of a hybrid catalyst supported on GO in the ODS of DBT, benzothiophene (BT) or 4,6-dimethyldibenzothiophene (4,6-DMDBT) as model compounds dissolved in dodecane ([S] 0 = 500 ppm). They obtained conversions of sulfur compounds higher than 94% regardless of the model compound and were able to maintain the activity up to 10 cycles. Besides the high dispersion of the active phase, GO was also found to increase the adsorption of sulfur compounds due to the Lewis acidic character of its defect sites. The adsorption of sulfur compounds in the catalyst structure approximates the S compound and the oxidant, thus enhancing the oxidation reaction [85]. Tables 5-8 show the studies related to the application of carbon-based hybrid materials in the ODS of a range of sulfur compounds in the biphasic system, i.e., systems where oxidation and extraction occur simultaneously. Each table shows studies using different allotropes of carbon. Not all authors have studied the influence of the carbon structure in the proposed systems [86][87][88][89]. Rongxiang et al. (2017) performed ODS of a simulated fuel (DBT in n-octane) using a hybrid catalyst of WO 3 supported over g-C 3 N 4 . They have found that the most relevant contribution of the carbon structure is related to its high surface area, which in turn allows for a higher dispersion of the active phase [90]. The higher surface area increases the adsorption of sulfur compounds, whereas the higher dispersion impacts the desulfurization performance [90]. The high surface area of AC is also suggested by Xiao et al. (2014) as one of the reasons for the increased activity of HPW supported over AC compared to unsupported HPW in the ODS of thiophene (Th) in a mixed solvent simulating a gasoline matrix [91]. Other authors have also pointed out the high surface area and related high dispersion as relevant contributions of carbonaceous structures to the catalytic activity [92][93][94][95][96]. In contrast, the agglomeration of active centers was linked to a decrease in catalytic activity [97]. Hou et al. (2018) [98], on the other hand, reported that the surface area of the catalyst is not the only relevant parameter for ODS of a simulated fuel (DBT in n-octane) using a g-C 3 N 4 supported MoO 2 catalyst. They also observed that the active phase concentration, or loading, is essential information. While increasing the amount of loaded MoO 2 increases the number of active sites, it significantly reduces the surface area of the catalysts, resulting in poor dispersion of the active sites. On the other hand, low loading of active center results in a catalyst with a high surface area but a low number of active sites. Thus, finding the optimum combination between loading of active phase and maintaining high surface area is a crucial step [98].

L-L Biphasic Systems
In 3D-carbonaceous structures, the surface area was also pointed out as a critical parameter. Ye et al. (2020) have confined MoO x in a 3D-macroporous carbon structure and applied these hybrid materials in ODS of a simulated fuel (BT, DBT, and 4,6-DMDBT in n-octane). One of the reasons for the high activity of the materials (conversions higher than 85% in 75 min of reaction) was attributed to the high surface area of the 3D-carbon structure, which resulted in a fast mass transfer to the active sites confined within the structure [99]. Zhu et al. (2020) also have observed that the 3D structure of carbon spheres (3D-CS) is beneficial to an increased mass transfer during oxidative desulfurization [97]. Bhadra et al. (2017) also highlighted a fast mass transfer resultant from mesoporosity of the catalyst as an important contributor to the high activity of a TiO 2 -C in ODS of a simulated fuel (Th, BT, DBT or 4,6-DMDBT in n-octane, [S] 0 = 1000 ppm) [92].
Other authors have pointed out that carbon structures play an important role through adequate adsorption of sulfur compounds, which can be ascribed to an increase in DBT concentration in the vicinity of active catalytic centers [100]. Wang et al. (2010) observed that the main role of CNT in a hybrid catalyst consisting of CNT and a phosphotungstic acid was to enhance the transfer of DBT to the active center by adsorption, facilitating the oxidation of DBT [101]. Xing et al. (2017) observed that increasing the calcination temperature of a CoWO 4 /g-C 3 N 4 resulted in an enhanced surface area and adsorption ability towards DBT [102]. Other authors have also highlighted the adsorption capacity of carbonaceous materials, either towards DBT and other sulfur compounds [90,99,100,103] or towards the oxidation products [104]. Yu et al. (2010) also observed that the adequate adsorption of DBT into the carbon structure was an essential contribution of the carbon-based support in an HPW/AC catalyst [105]. Their reasoning is that increased adsorption of DBT leads to a higher collision probability between DBT and active oxygen species from the decomposition of H 2 O 2 that takes place in the active center (HPW). The carbon surface also influences the resonancestabilization of these active oxygen species, boosting oxidation. Moreover, due to the hydrophobicity of AC, it can be adequately dispersed in the oily phase or placed in the interface between the oily and polar phases, acting as a phase transfer agent [105]. Kermani et al. (2018) also found that the combination between adsorption capacity towards DBT and adequate lipophilicity of AC structure was an essential feature of the composite Mo 132 /AC [106].
In an ultrasound-assisted ODS (UAODS) of a simulated fuel with an HPW/AC as the catalyst, Liu et al. (2014) also identified the increased adsorption of DBT and the ability of the catalyst to act as a phase transfer agent as important features arising from the AC support [107]. However, they have also pointed out that the AC structure plays another relevant role in UAODS: the high surface area was also correlated to a higher cavitation activity in UAODS, boosting reaction rates [107,108].
Functional groups on the carbon material can impact the coordination of active species (metallic centers) [109][110][111]. Dini, Afsharpour, and Tabar-Heydar (2019) argued that the oxidation of DBT undergoes an electrophilic attack of the peroxo group (LUMO) on the catalyst surface to the sulfur compound (HOMO) [110]. Thus, efficient oxidation needs to overcome the LUMO-HOMO barrier. The functional groups on the support surface (in this case, CNTs) affect the coordination of the metal-active species (Mo). This geometry boosts the electron density of peroxo groups and increases the HOMO energy level. By increasing the energy level of HOMO, the gap between HOMO-LUMO decreases, and oxidation is facilitated [110,111].
Other authors have correlated the activity of hybrid materials to an increased electron transfer between carbon structure and active phase, especially in doped carbon-based materials. Bhadra, Khan, and Jhung (2019) observed that N-doped graphitic carbon-supported Co resulted in a more effective electron transfer, which could be helpful for oxidation and reduction reactions [112]. Liao et al. (2019) observed that the strong interaction between HPW and rGO enhanced electron mobility and, consequently, the reaction rate [113].   [95] and Zhao et al. (2017) [96] have proposed that g-C 3 N 4 was able to activate H 2 O 2 and form hydroxyl radicals (HO • ). Moreover, the carbon structure was also found to avoid the leaching of active species [113]. Table 9 displays a miscellaneous of other ODS systems, whereas Table 10 displays photocatalytic ODS systems. Similar to what was observed earlier, the developed surface area and the structure of pores in carbon materials, leading to well-dispersed active phases, were found as a critical contribution of carbon structures [114][115][116][117][118].   [119] also highlighted the dispersion of TiO 2 over a graphite carbon (GC) as a relevant factor for the ODS of a simulated fuel (DBT, 4-methyldibenzothiophene (4-MDBT) and 4,6-DMDBT) dissolved in n-octane, [S] 0 = 500 ppm). However, they also observed that the dispersion is related to the amount of active phase dispersed on the support. Upon increasing TiO 2 content over 10 wt.%, the catalytic activity of the hybrid catalyst decreased due to the agglomeration of TiO 2 particles. Thus, loading the proper amount of the active phase is an important step. The synergistic effect between the support and the active centers is the main reason a catalyst displays outstanding properties in ODS systems [119]. Gao et al. (2020) [120] found that the increased surface area was also associated with a higher collision probability between sulfur compounds and the catalyst. They observed that the activity of the polyoxometalate-based IL supported over a series of carbon materials decreased accordingly to the surface area of the support (P[ [120]. This observation highlights the importance of a developed surface area to a well-dispersed active phase and high desulfurization rates.

Other ODS Systems
Carbon-based materials were also found to accelerate the electron transfer for different active phase-support pair materials, such as MoO 2 /g-C 3 N 4 [115], FePc (NO 2 ) 3 -CF [118], V/P-doped g-C 3 N 4 [121], VO-MoO 2 @N-doped CNT [122], considering different oxidants (TBHP [115,121], O 2 [118] and CHP [122]), which enhances oxidative desulfurization rate. Chen, Ren, and Yuan (2020) observed that the rate-determining step in the ODS of DBT using V supported over a P-doped g-C 3 N 4 material is the electron transfer, and thus an enhanced electron transfer due to P doping greatly improved oxidation [121]. They also observed that P-doping g-C 3 N 4 enhanced the electron transfer, resulting in a more efficient ODS process [121].
Some authors have reported that carbonaceous materials favor electron-hole separation [123][124][125][126][127] (cf. Table 10), thus benefiting the photocatalytic activity of those hybrid materials. Li et al. (2017) claimed that the narrower bandgap of g-C 3 N 4 allows the application of visible light [124]. Furthermore, the well-matched band structures between support and active phase (CeO 2 ) favor the ready transfer of electrons generated in the surface of the support to the surface of CeO 2 to avoid the recombination of photo-generated electrons and holes effectively. Thus, the electrons can react with hydrogen peroxide to form hydroxyl radicals. They observed that the holes might also participate in the reaction, although contributing much less than electrons, due to the weak potential of the valence band of g-C 3 N 4 (1.79 eV) compared to the redox potential of the OH − /HO • pair (1.99 eV) [124]. Ma et al. (2017) also found that the use of carbon quantum dots (CQD) improved the efficiency of the use of visible light [125].
In general, most ODS systems require an extractant phase and a catalyst to remove the oxidized intermediates. However, in a few processes, the hybrid material can act as a catalyst to oxidize sulfur compounds and adsorbents for the oxidized products in a solvent-free approach. Jiang et al. (2017) immobilized a polyoxometalate-based IL in graphite carbon and tested this hybrid catalyst in the solvent-free ODS of a simulated fuel. A 100% removal of DBT in the model oil was observed, whereas only DBT sulfone (DBTO 2 ) was detected in the catalyst phase. The high efficiency of the process was ascribed to the hydrophilic characteristics of IL, allowing a closer contact with H 2 O 2 associated with the high hydrophobic character of carbon, which allowed a closer contact with the oil phase [128]. Ma et al. (2019) reported similar results. Only DBTO 2 was detected in the catalyst phase, and no sulfur compounds were observed in the oil phase [114].
Other reasons pointed out by the authors were the hydrophobic characteristics of GC in a polyoxometalate-based IL supported on GC, which allowed the catalyst to be better dispersed in the oil phase in the solvent-free ODS [128]. The protective environment arising from the CNT structure was also an essential feature, especially to avoid leaching of the active phase, thus increasing catalyst stability and reusability [129].

Trends on Hybrid Materials Containing Carbonaceous Structures
Carbon-based materials have proved to be an exciting platform either as supports for active phases or as components in hybrid materials, leading to highly active catalysts in ODS processes. Carbon materials can be used in a wide range of different approaches and systems. Most authors agree that the main contribution of the carbon-based structure is usually related to the high surface area displayed by those types of materials, which leads to a high degree of dispersion of the active phases. Others have also pointed out that the hydrophobic nature of the carbon structure increases the interaction of the hybrid materials with the organic phase and increases the contact between the oxidant and the target S compound. A few authors also highlighted an increased electron transfer between the carbon structure and the active phase as the critical contribution of carbon structures. Specifically for photocatalytic systems, carbon-based materials have the advantage of reducing the recombination of electron-hole pairs, which dramatically boosts reaction rates. Carbon materials can act as catalysts of the process and adsorb the oxidized compounds, allowing solvent-free systems. Furthermore, carbon materials were able to avoid the leaching of active phases.

Carbon Catalysts on Their Own
Carbon-based materials have been reported as catalysts on their own for ODS reactions, and a summary with the main conditions and results are collected in Table 11. As observed, a series of carbon-based materials (AC, GO, rGO, and CNT) have been assessed in the ODS of model compounds (mainly BT, DBT, or 4,6 DMDBT). Initial sulfur concentrations ranging between 400 and 2189 ppm were studied, considering mild operating conditions (25-150 • C), allowing to achieve removals higher than 95% from 30 min of reaction time. In general, carbon materials have displayed similar activity to those reported earlier considering hybrid or supported materials. The activity of the carbon material can be ascribed to the presence of defect sites on its surface, its developed surface area, or the presence of surface groups. Here, we propose the mechanism displayed in Figure 10 for the ODS of sulfur-containing compounds according to different proposals in the oxidation activity of carbon catalysts [130,131].
Gonzalez et al. (2012) compared a thermally treated AC (SX-1) and an AC treated with phosphoric acid (MW-99). MW-99 led to an increase in sulfur removal in the range 2.8-3.3 × compared to SX-1, depending on the sulfur compound. Those results were ascribed to a more developed surface area by MW-99, a lower content of basic sites and a higher concentration of acidic sites [132]. Haw et al. (2010) also found that the presence of acidic groups in AC, especially oxygenated groups, increased catalytic activity due to the electron-withdrawing characteristic of the acidic groups allied to the electron donor ability of S compounds. This interaction between acidic groups and S atoms increases the physisorption of S compounds in the AC structure [133].  [131] studied the influence of doping GO with nitrogen and oxygen for the aerobic ODS of various sulfur compounds. They compared two GO samples. The first was an N-doped GO subjected to annealing at 1000 • C, leading to PG-15. The other sample was obtained by subjecting PG-15 to thermal treatment in a muffle furnace at 450 • C, leading to PG-450. PG-15 removed approximately 9.3% of DBT, which is mainly ascribed to an adsorption effect. On the other hand, PG-450 could remove 98.5% of DBT in 5 h of reaction. The activity of PG-450 was ascribed to the oxygen groups (19.9% as opposed to 3.6% for PG-15) and to an increase in the content of graphitic-like N structure in the material (47.8% in PG-450 vs. 37.2% in PG-15). The main active sites for the activation of O 2 are the oxygenated groups. However, since this activation requires an electron transfer process, the existence of electron-rich oxygenated groups should facilitate this electron transfer. In N, O-doped materials, the existence of electron-rich oxygen groups is ascribed to the enhanced electron delocalization that arises from the N-doping. To demonstrate the synergistic effect of both N and O doping, a graphite material was calcined at 450 • C (Graphite-450) and tested in the ODS reactions. Graphite-450 is rich in oxygenated groups but poor in nitrogenated groups. In 5 h, only a 40% removal of sulfur compounds was observed, thus reinforcing the synergy between N and O doping [131].
The presence of oxygenated groups was also found as crucial by Gu et al. (2019) [134] in the activity of oxidized CNTs (oCNTs). To identify which oxygenated groups were more relevant, they selectively removed different oxygenated groups (carboxylic, lactone, anhydride, phenolic, or carbonyls) through thermal treatment or selectively blocked some of them. When blocking or removing carbonyl groups, the activity of oCNT decreased drastically (from 100% removal of DBT to 31%), thus, pointing out the relevance of carbonyl groups. Gu et al. (2017) [135] have also found a positive correlation between carbonyl groups and catalytic activity. Other authors have reported that lactones and carboxylic groups also play an essential role [130].
On the other hand, some authors have reported that total oxygen-containing groups could not directly correlate to the catalytic activity in ODS experiments [136]. Zhang et al. (2014) have demonstrated that the change in the content of oxygen-containing groups affects the catalytic activity. However, there was no clear correlation between total content or specific groups in CNTs and desulfurization activity. Nonetheless, they did find a correlation between the increased graphitization degree of CNTs and increased catalytic activity. An increased graphitization is ascribed to a lower degree of defects. They attribute this phenomenon to a higher electron transfer due to higher electrical conductivity in highly graphitic CNTs [136]. In opposition to that, most authors seem to correlate the presence of defect sites to an increase in catalytic activity. Gu et al. (2019) observed an increase in defect sites after acid treatment (D*/G value increased from 1.77 to 1.91 for CNTs and oCNTs, respectively) which were correlated to the increased activity of oCNTs compared to CNTs (along with the oxygenated groups discussed above) [134]. In 2017, Gu et al. [135], by saturating defect sites in rGO through the formation of C-H bonds, have observed a sharp decrease in DBT removal, thus pointing that defect sites also play a significant role in the activity of carbocatalysts. Zeng et al. (2017) [137] also identified zigzag edges and defects in GO as relevant active centers for the catalytic process.
In contrast, Timko et al. (2016) [138] reported that basal plane defects in AC are more relevant than edge defects. They presumed that basal plane defects are oxidized to generate carboxylic or percarboxylic acids on the surface of the material. Those oxygenated groups are then responsible for promoting ODS. The sulfur compounds are also expected to adsorb into the basal planes, increasing local concentration. However, adsorption alone is not sufficient. The surface oxidation allied to an increased local concentration of the sulfur compound in the carbon basal plane is the reason pointed out by the authors that distinguish the activity displayed by distinct ACs [138].

Trends on Carbon-Based Catalysts
In general, there are fewer reports related to the application of carbon-based materials as catalysts on their own in the ODS process as compared to their application in hybrid forms. However, carbon-based materials have proved to be adequate catalysts, and there is still room to study their application for desulfurization purposes.
Overall, the presence of acidic groups on the surface of carbon-based materials was related to higher desulfurization rates due to an acid-base interaction between carbon surface and sulfur molecules. Among the acidic surface groups, oxygen-containing groups are thought to be the active centers by most studies. However, there is still a controversy related to the influence of defect sites in catalytic activity.
Carbon-based materials have some unique advantages, especially their easy manipulation and introduction of surface groups. Nevertheless, there is a lack of reports related to applying doped carbon with heteroatoms other than oxygen and nitrogen. Thus, further investigating the influence of other dopants is an opportunity to apply carbon-based materials as catalysts on their own.

Amphiphilic Carbons
Amphiphilic carbons are materials that simultaneously display hydrophilic and hydrophobic characteristics. Thus, amphiphilic carbons can better interact with aqueous and organic phases. The contact angle of a water drop onto a material is widely used to describe amphiphilicity. Materials with a contact angle of 0 • are completely hydrophilic (high water-wettability), whereas materials with contact angles of 180 • are entirely hydrophobic. Amphiphilicity is an attractive characteristic when considering a biphasic reaction, as is the case of ODS, due to increased contact between an organic phase (fuels) and an extractant phase, which usually presents a more polar nature. Table 12 summarizes the reports related to the application of amphiphilic carbon-based materials for ODS reactions, main operating conditions, and results. Abdi et al. (2017) [139] studied the removal of DBT from a simulated fuel in a biphasic system using GO/COOH as a catalyst. They have attributed the high removal rates of sulfur to the amphiphilicity of GO/COOH, which allowed for better interaction with both hydrophilic H 2 O 2 and hydrophobic sulfur compounds. Similarly, Liu et al. (2020) have observed that POM catalyst alone, due to its hydrophilicity, could not properly contact DBT (which is lipophilic). Thus, much better interaction between DBT and catalyst was observed by covering the POM catalyst with a hydrophobic carbon layer, significantly increasing the reaction rate [140]. In another approach, Yu et al. (2020) immobilized lipophilic C 12 PW on hydrophilic g-C 3 N 4 , thus increasing the utilization of hydrogen peroxide and the contact between DBT and active sites [141]. Li et al. (2019) found that g-C 3 N 4 is not only responsible for improving the dispersion of the active phase (BMPO) but also for creating a heterojunction between support and BMPO, which increases the utilization rate of electrons. Furthermore, the amphiphilic character of the catalyst solves the issue with the mass transfer between oily and aqueous phases [142].
Amphiphilic materials can also be applied in PIC-like approaches that rely on emulsion formation. The higher interfacial area between the apolar and polar phases in emulsion systems should increase the reaction rates. Mambrini et al. (2017) have studied ODS of DBT in emulsion systems (cyclohexane/aqueous H 2 O 2 ) using metallic catalysts (Fe and FeMo) modified with carbon to alter their amphiphilicity. Fe and FeMo alone could not remove any DBT; however, stable emulsions were formed when carbon coating was introduced, resulting in increased contact between organic and water phases, thus increasing reaction rates [143]. They proposed that the materials act in two ways: one, by increasing the interfacial area between phases due to the stabilization of an emulsion system, and two, through the activation of hydroxyl species in a Fenton-like system. The generated hydroxyls radicals were then able to oxidize DBT in the interface between the organic and the aqueous phase. The increased polarity and high interfacial area were also responsible for facilitating the extraction of DBTO 2 [143]. Figure 11 shows the presumed mechanism for emulsified systems, adapted from elsewhere [144][145][146]. Other authors have also reported ODS emulsion systems that rely upon amphiphilic catalysts to increase catalytic activity [147].

Trends on the Application of Amphiphilic Materials
Amphiphilic materials have been applied in an extensive range of different ODS systems, including processes considering two-steps, biphasic systems, and PIC-like approaches. Regardless of the system, amphiphilic materials were attractive options due to their ability to interact with aqueous oxidant phases, lipophilic sulfur compounds, and organic phases simulating fuels. Regardless of the initial concentration, complete desulfurization was achieved in 120-180 min. For fuel with [DBT] 0 = 1000 ppm, the presence of N compounds slightly hindered the removal of DBT, reducing the overall removal to~90-95%.  100% removal of S compounds in 3 h of reaction was observed. In the presence of aromatic or N compounds, 100% removal was also possible but with longer reaction times (4-6 h). Increasing the catalyst dosage from 2.5 to 8.5 g mL −1 resulted in a sharp increase in S removal (25 to 90%, respectively). [77] Naphtha (C 0 = NM) MnO x /CNT ACN C cat = 5 mg mL −1 , V H2O2 = NM, T = 25-30 • C, t = 30 min.
After 30 min, comparing EDS and ODS process, desulfurization of 9 and 83% were achieved, respectively. Under optimum conditions, 92% desulfurization was observed, maintained for 4 cycles. A DoE was used to determine the importance of some parameters, which were found to be: time > temperature > catalyst dosage. The optimum desulfurization (92%) was predicted at 225 min of reaction, 2.5 g of catalyst, and 60 • C.  Different systems were compared. When using only H 2 O 2 and no co-catalyst, 10-12% desulfurization rates were achieved. By adding acetic acid and Na 2 CO 3 , 71 and 66% desulfurization efficiency was observed for Fe 2 O 3 /AC and Pd/AC, respectively.  DBT, BT or 4,6-DMDBT in n-Octane ([S] 0 = 800 µg g −1 ) WO 3 /CNT ACN V fuel = 5 mL, V ACN = 1 mL, m cat = 0.03 g, H 2 O 2 /S = 5, T = 50 • C, t = 60 min.
Removals of~78, 90, and 85% were observed for BT, DBT, and 4,6-DMDBT, respectively. dbt conversion was maintained for 4 cycles, in the 5th run decreasing to 80%. With no extraction solvent, removal rates were low. The best desulfurization was observed at 60 • C, 7.5 mg of Catalyst, H 2 O 2 /S ratio of 2, Resulting in 99% desulfurization. Increasing sulfur concentration slightly decreased conversion to 98%. At room temperature, a conversion higher than 95% was obtained. After the 4th recycle run, conversion dropped to 25% due to poisoning of active sites. Using CNT as support increased sulfur removal from 90 to 100%, Compared to AC support. A 100% conversion of dbt was obtained and 100% yield towards the sulfones. Three cycles were run with only a slight decline in conversion.  DBT and 4,6-DMDBT removal achieved 100%, whereas BT was only 70%. In mixed fuel, overall S removal was 99%. Removal of S higher than 90% for 8 cycles.  Decreasing particle size of the AC support improves desulfurization due to an increase in surface area. Increasing HPW loading up to 10% increases desulfurization; however, further increasing reduces AC surface area and interferes with the ac structure effect in the ultrasound-assisted process. Ultrasound power over 70 W did not improve desulfurization. DBT removal was 100%. [108] Th in n-Octane (C 0 = 656 ppm) Pd/AC Aqueous Solution Containing 2 g NaOH, 5 g Water, and 2. After 20 min at 70 • C, a model fuel with sulfur content lower than 1 ppm is obtained. Obtaining fuels with ultra-low sulfur at lower temperatures is possible; however, more time is required. >99% conversion was maintained for 5 cycles. Upon increasing HPW loading, the reaction rate increased up to a 10 wt.% loading. increasing catalyst concentration also improves desulfurization (up to 1.25 wt.%), whereas higher catalyst loading hinders the propagation of ultrasound waves. Comparing the ultrasound-assisted process to a regularly mixed process, ultrasound resulted in a 30% improvement. Temperature played a very significant role in increasing the reaction rate. At 40 • C, 70% of DBT is oxidized in 30 min, whereas at 70 • C, 100% is oxidized in 9 min. The proper amount of oxidant was important to fully wet the carbon surface and to permit emulsion formation. Desulfurization decreased to 90% and~75% in the 2nd and 3rd runs, respectively. HPW Loading of 10% was found as optimal. Increasing the H 2 O 2 /S Ratio and temperature improves desulfurization. At lower temperatures, a higher amount of oxidant is necessary to achieve full desulfurization. A total of 100% removal of DBT obtained.     The optimal loading of HPW was found to be 7 wt.%. A total of 100% removal of DBT maintained for 3 cycles, slight decrease on the 4th and 5th cycles (to 93%). the desulfurization rate was very similar regardless of the model sulfur compound. One hundred percent desulfurization was achieved for all model oils in 2.5 h. Increasing the chain length of the catalyst (n = 2-6) increases desulfurization. For n > 6, the steric hindrance of the material increases, and desulfurization is hindered. The maximum removal observed was 99.6%.   With the addition of specific scavengers for • O 2 − , h + , e − and HO • , the authors have observed that HO • was the most important radical. desulfurization rates were: >96% removal for 6 cycles (DBT), >85% for 4,6-DMDBT and 60% for BT. Increasing loading of MoO 2 to 3 wt.% resulted in higher desulfurization. A 5 wt.% loading negatively affected the S removal. Temperatures higher than 60 • C, catalyst dosage over 0.02 g, and IL volume higher than 1 mL resulted in lower desulfurization performances. Maximum removal was observed with 0.2 mL of oxidant. Maximum conversions were 95% (DBT, 120 min), 60% (4,6-DMDBT, 140 min) and 35% (Th, 140 min). Increasing WO 3 loading to 36% results in an increase in desulfurization. With 50% loading, the desulfurization was negatively affected. A 90% conversion for DBT was observed (150 min) and maintained for 5 cycles. Th Removal of 60% was observed regardless of reaction time (20-180 min), and for BT a maximum of 43% was obtained in 180 min. [96] Legend: IL = ionic liquid. MeOH = methanol. V fuel = 100 mL, C cat = 1 g L −1 , 1.5 L min −1 of O 2 , T = 70 • C.
Using a DoE, the temperature was the most relevant variable, followed by catalyst mass and oxygen quantity. Conversion of DBT was maintained above 95% up to 8 cycles. In 6 h of reaction, >95% of S was removed, regardless of the S compound tested, leading to fuel with <10 Ppm S content (for 5 cycles). Adding 5 Wt.% of Cyclohexane and P-Xylene slowed the reaction rate; however, it was still possible to achieve deep desulfurization in 8 h of reaction. >93% of removal of DBT and >90% of 4,6-DMDBT maintained for 6 cycles. [116] DBT, 4,6-DMDBT or BT in n-Octane (C 0 = 500 ppm) V/P-doped g-C 3 N 4 TBHP N.M.

Carbon Materials in ODN
Denitrogenation reactions are not as widely studied as desulfurization, and thus there is a limited number of papers regarding denitrogenation with carbon-based materials. In general, the roles of the carbon-based materials in ODS are similar in ODN reactions. Many authors highlighted the amphiphilicity of carbon structures as beneficial in ODN reactions. Table 13 summarizes the papers related to ODN reactions. As observed, most of the works deal with ODN of QN (model component most extensively studied). ODN is mainly conducted with H 2 O 2 using iron-containing carbon materials under mild conditions (25-80 • C) for reaction times up to 3 h (achieving complete oxidation of the model molecule at lower times depending on the selected catalyst and operating conditions). The selection of iron catalyst with H 2 O 2 as oxidant may be ascribed to the reactive iron-H 2 O 2 pair, also known as Fenton-reagent [162,163]. Guimarães et al. (2013) have tested magnetite covered by carbon as the catalyst for the simultaneous oxidation of DBT and QN in a biphasic system (toluene-acetonitrile, [QN] 0 = 50 ppm) [159]. DBT conversion was higher than QN conversion (100% instead of~60% for QN). The presence of DBT reduced QN oxidation by about 50%. The authors have highlighted the amphiphilicity of the catalyst and its ability to remain in the interface between the two solvents as one of the critical conditions to display high catalytic activity [159]. Mambrini et al. (2017) studied the removal of QN in cyclohexane ([N] 0 = 50 ppm) [143] in a PIC-like system using a FeMo/C as the catalyst. Due to the amphiphilic character of the catalyst, it can interact with aqueous and oily phases, stabilizing an emulsion cyclohexaneaqueous H 2 O 2 . The emulsified system increases the interfacial area and enhances the reaction rate. The high interfacial area resulted in a very fast conversion of QN: almost complete conversion in less than 10 min [143]. FeMo/C revealed a higher catalytic activity than Fe/C, which was ascribed to the higher content of carbon on FeMo/C. Higher carbon content allowed a better interaction between the oily and the aqueous phase, favoring emulsion stabilization [143]. Oliveira et al. (2014) prepared amphiphilic catalysts through the modification of red mud by chemical vapor deposition (CVD) of ethanol [145]. The materials were tested in the PIC-like oxidation of QN in cyclohexane with H 2 O 2 . Within 60 min, QN was removed entirely, and this activity was maintained for 5 cycles. They ascribed the exciting results to the amphiphilicity resulting from the carbon deposits after CVD, which allowed the stabilization of the emulsified system, increasing the contact between the organic and the aqueous phases [145]. Similar materials were synthesized by Teixeira et al. (2013) [146] and applied in the biphasic oxidation of QN in a PIC-like system. One hundred percent oxidation of QN was possible in less than 30 min of reaction. The efficiency of the reaction system was maintained for 5 cycles [146].
Souza, Pereira, and Oliveira (2012) produced amphiphilic catalysts based on onionlike carbon (OLC) over iron oxide and applied them in the biphasic oxidation of QN in toluene [164]. Ninety three percent removal of QN was obtained. The activity of the material was ascribed to increased contact between the aqueous oily phases due to the presence of a hydrophobic carbon layer on the catalyst [164]. Oliveira et al. (2020) prepared N-doped CNTs over red mud and applied them in the biphasic oxidation of QN in cyclohexane ([N] 0 of 30 ppm) with H 2 O 2 [165]. The catalysts were synthesized sequentially feeding ethylene (as carbon source) and acetonitrile (as carbon and nitrogen sources), leading to 4 different samples. (i) A completely undoped (synthesized using only ethylene); (ii) a completely N-doped (synthesized using only acetonitrile); and (iii) two samples synthesized using both precursors during different times, leading to two partially N-doped samples. QN oxidation varied according to the content of nitrogenated groups on the material: higher N contents led to increased removal of QN (80% versus 44%) [165]. A study with similar materials was previously done by Purceno et al. (2015) [166]. However, the selectively N-doped CNTs acted only as emulsion stabilizers in their case, and a homogeneous catalyst (Fe +2 ) was applied. Without the presence of the CNTs, only 10% of QN was removed. On the other hand, by adding amphiphilic CNTs, a removal of QN in the range 85-100% was observed in 15-40 min of reaction, depending on the CNT sample. The results were directly ascribed to the ability of amphiphilic CNTs to stabilize Pickering emulsions, increasing interfacial area [166].
Ammar, Kareem, and Mohammed (2020) studied exclusively the ultrasound-assisted catalytic oxidative-adsorptive denitrogenation (COADN) of pyrrole (PYR) and indole (IND) (dissolved in n-nonane, [PYR] 0 and [IND] 0 of 200 ppm) [167] using a PMo-Fe 2 O 3 /rGO as the catalyst. Upon increasing sonication power and time, oxidation of N compounds increased. The best results were obtained with 200 W ultrasound power for 240 min, leading to 86 and 90% conversion for PYR and IND, respectively. Compared to the silent reaction (no ultrasound), an increase in 50 and 58% removals for PYR and IND, respectively, were observed. This high conversion was maintained for up to 5 cycles [167]. The authors did not highlight the role of the carbon structure.
Raffie and Khodayari (2017) added N compounds to a simulated fuel containing DBT in order to investigate the influence of the presence of N compounds (IND, QN, pyridine, and pyrrole, C 0 = 60 ppm) in the removal of DBT using a catalyst containing PMoV/Fe 3 O 4 /g-C 3 N 4 [76]. All nitrogenated compounds were oxidized in very short reaction times (20 min). Although the presence of N compounds slowed DBT oxidation, the effect was minimal. Thus, proving that desulfurization and denitrogenation can be conducted simultaneously for the proposed system [76]. A similar study was performed previously by the same group [84] using an HPW/TiO 2 /GC catalyst. They have also concluded that the addition of nitrogenated compounds slightly impacts DBT removal. However, they have not reported the conversion of those nitrogenated compounds in the system. However, further increasing any of those parameters did not have a significant impact on IND removal. The removal of the remaining N compounds was also investigated. In 120 min of reaction,~90% of IND was removed (maintained for 4 cycles), followed closely by 2-Me-IND. Removals of 1-Me-IND and 3-Me-IND were~70%, whereas PYR and CBZ removals were 40% and <20%, respectively. QN was not oxidized in the studied conditions. The distinction between the oxidation rate of different compounds was ascribed to the electron density around the N atom, similar to that observed with S compounds. However, the authors have highlighted that further studies are necessary to truly understand the relation between electron density in N compounds and reaction rate [168]. The authors did not highlight the role of the carbon structure. EtOH V fuel = 5 mL, m cat = 0.03 g, n H2O2 = 10 mmol, T = 80 • C.

Final Considerations and Perspectives
Carbon-based materials have proved to be interesting materials that can be used in ODS and ODN processes under different systems. The facility to tune their properties and combine with other materials to form hybrid materials makes them attractive when designing effective catalysts for desulfurization and denitrogenation. Especially in biphasic systems, their developed surface areas, which impact the dispersion of active phases and an increased adsorption capacity towards S pollutants, have been pointed out as a key contribution of those materials, mainly for hybrid catalysts. Besides that, carbon materials were found to benefit ultrasonic cavitation, boosting reaction in ultrasoundassisted processes. Their usual hydrophobic nature increases the interaction with fuels and lipophilic pollutants, such as sulfur compounds, allowing them to act as phase transfer agents to extract these S compounds. Finally, they were found to accelerate electron mobility, and in photocatalytic systems, they avoid electron-hole recombination. When considering carbon-based materials as catalysts on their own, their activity is mainly ascribed to the presence of oxygenated groups and defect sites. However, there is still some controversy on how each influences the desulfurization capacity. In amphiphilic materials, carbon-based materials were again used as catalysts on their own and in hybrid forms. Their main contribution comes from stabilizing PIC-like systems, where the material acts simultaneously as a catalyst and as an emulsifier.
However, a few points must be raised. (i) There are much more reports related to the application of carbon-based hybrid materials and much less related to carbonaceous structures as catalysts on their own. However, the results reported in this review point out that carbon materials have allowed similar removals to those observed with hybrid materials. Thus, there is still the opportunity to study carbon-based materials as catalysts on their own for desulfurization and denitrogenation. (ii) There is also a lack of reports testing carbon-based materials as catalysts on their own containing dopants other than oxygen. Nitrogen, phosphorus, and sulfur are all possible dopants that have been shown to increase catalytic activity in aqueous phase oxidation reactions [64,[170][171][172]. (iii) Denitrogenation approaches are much less common, and there is still the opportunity to study denitrogenation reactions, mainly aiming at simultaneous high removals of N and S-containing compounds. (iv) There are very few works dealing with the use of catalysts prepared from renewable or waste sources. Environmental aspects related to the catalyst (metal leaching species and waste generation, among others), economic issues, or life cycle assessment (LCA) are not reported by any papers. (v) Most authors fail to explain the role of the carbonaceous structure in the process, or the advantages of using carbon-based materials, mainly when considering carbon-based composites for ODS and ODN purposes. Understanding the role of each component of the catalyst is critical to developing outstanding catalysts.

Conflicts of Interest:
The authors declare no conflict of interest.