Catalytic aerobic desulfurization of fuels in the presence of nanosized mixed carbide FeWC

Highlights

• Mixed carbide (FeWC) was successfully applied in aerobic desulfurization.

• Synthesis by microwave irradiation allows to obtain a nanoscale catalyst in 15 min.

• Dibenzothiophene can be completely oxidized in 1 h at 130 °C, 6 atm air pressure.

• Total sulfur content in straight-run gasoline can be reduced from 996 to 6 ppm.

• The catalyst retains its activity for at least 5 cycles of oxidation-regeneration.

Abstract

Nanosized mixed tungsten-iron carbide (FeWC) was successfully applied in aerobic oxidative desulfurization. The combination of catalytically active centers responsible for the activation of oxygen and the oxidation of sulfur-containing compounds made it possible to obtain a highly efficient catalyst. The catalyst was synthesized by microwave irradiation, allowing to obtain a nanoscale catalyst in just 15 min. The catalyst was characterized in detail by a variety of methods: XRD, HRTEM, EDX, SEM, XPS, and low-temperature nitrogen adsorption/desorption. The key factors influencing the dibenzothiophene (DBT) oxidation were investigated. Under optimized conditions DBT conversion was 100% in 1 h at 130 °C, 6 atm. The possible mechanisms including oxygen activation, alkyl peroxide formation, and substrate oxidation by tungsten peroxo-complexes were discussed. The catalyst retains its activity for at least 5 cycles of oxidation-regeneration. Aerobic oxidative desulfurization of straight-run gasoline in the presence of FeWC was performed and sulfur content was reduced from 995 to 6 ppm.

Introduction

With the increase in energy consumption, the extraction of fossil fuels is continuously growing. At the same time, in recent years, the boost in the production of hydrocarbon raw materials has been accompanied by a deterioration of their quality. This is especially reflected in the viscosity of the produced oil and the content of heteroatomic compounds therein [1], [2]. Among the latter, sulfur-containing compounds are of particular interest are sulfur-containing compounds, which have a because of their strong corrosive activity and their negative impact on the quality of the resulting fuels. Sulfur oxides (SO_x) as products of combustion of sulfur-containing fuel cause grave environmental problems, being sources of acid rain [3]. In addition, toxic sulfur oxides released by transport can form sulfonated smog particles, which are extremely dangerous for human health [4]. It is also known that heterocyclic sulfur derivatives present in car exhaust gases are carcinogens and mutagens [5]. Therefore, strict rules have been introduced around the world since 2015, which strictly limit the sulfur content in the fuels to below 10 ppm [6], [7].

Existing approaches to reduce the sulfur content in hydrocarbon feedstock include hydrogen-based and hydrogen-free methods [8], [9]. Up to now, only the hydrogen desulfurization method, the so-called hydrodesulfurization process, has been implemented in industry. This process produces ultra-low sulfur hydrocarbon fuel (ULSD), mainly exhibiting high efficiency with respect to thiols, sulfides, and disulfides. It should be noted that gasoline and diesel fuel can also contain a large amount of sulfur compounds of the thiophene series. The difficulty of their removal by the hydrodesulfurization process is due to the stable conjugated π-system of the thiophene ring [10]. In addition, substituted condensed thiophene derivatives are highly resistant to hydrogenation, which is mainly due to steric hindrance [11]. Solutions to the indicated series of problems can be achieved using hydrogen-free desulfurization methods, such as extraction [12], [13], adsorption [14], [15], oxidative desulfurization [16], [17], [18], [19], and biodesulfurization [20], [21].

The most promising method is oxidative desulfurization, which ensures the oxidation of low-activity aromatic sulfides to sulfoxides and sulfones under fairly mild conditions. The polar oxidation products can then be easily removed by extraction or adsorption. The most common oxidizing agents used in oxidative desulfurization include hydrogen peroxide [22], [23], organic peroxides [24], peroxy acids [25], and gaseous oxidants: ozone, molecular oxygen, and air [26], [27]. Among the variety of oxidizing agents, air is the cheapest and most environment-friendly one. However, air oxygen is a rather inert oxidizing agent, which requires high temperature and pressure to activate. The development of a highly efficient catalyst providing the possibility of oxygen activation under mild conditions (below 150 degrees of Celsius) remains an important task to this day.

To date, various catalytic systems based on metal-containing catalysts, such as Fe [28], [29], Co [30], [31], Mn [32], [33], Mo [34], [35], W [36], [37], [38], V [39], [40], as well as bi- and trimetallic systems based on Co-Mo, Mn-Mo, Fe-Mo, Co-Fe-Mo [41], [42], [43], [44], [45], Mo-W, and V-Mo [46], [47] have been studied (Table 1S, Supporting Information). It is reported that the combination of various metals, in particular, the metals of groups VI and VIII, improves the efficiency of the oxidation of sulfur compounds by enhancing the “electronic effects” due to the transfer of electrons that facilitate the activation of O₂ [45]. It was also found that the introduction of transition metals favors the formation of oxygen vacancies, which subsequently lead to surface defects, which also improve the catalytic properties [42], [43]. However, the above-described catalysts require complex synthesis procedures that result in long catalyst preparation times. Besides, the suggested approaches involve the addition of sacrificial agents (aldehydes, hydroperoxides) and have also the drawback of requiring increased reaction temperature and pressure due to the relatively low activity of these catalysts under milder conditions. It should also be noted that there are no studies in the literature on the use of Fe-W as bimetallic catalysts for aerobic desulfurization.

In our earlier study, we showed the effective use of tungsten carbide synthesized in just 15 min by a microwave method for aerobic oxidation of dibenzothiophene (DBT) [48]. It was found that the catalytic activity of tungsten carbide depends on the composition of the surface layer and the presence of oxides in a variable oxidation state (in particular, W⁴⁺). It has been shown that oxygen reacts with the carbide phase to form WO₂ and then WO₃ without being selectively consumed in the oxidation of particles on the catalyst surface. Considering this factor, an additional activation procedure was carried out, which resulted in a dramatic increase of the efficiency of aerobic desulfurization increased dramatically. In order to avoid uncontrolled oxygen consumption for the formation of active particles and additional time spent on the activation procedure, it was proposed to introduce iron particles, which provide oxygen activation due to electron transfer [49].

In this work a nanosized mixed tungsten-iron carbide catalyst was developed and applied in the process of aerobic desulfurization for the first time. The resulting FeWC nanocatalyst has a high catalytic performance in the oxidation of dibenzothiophene under mild oxidation conditions (120 °C, 2 h), does not require an additional activation procedure, and has a nonporous nanosized structure. The synthesis of the catalyst includes one stage using the method of microwave irradiation for an ultrashort time (15 min). The influence of temperature, amount of catalyst, pressure, nature of the substrate on the conversion of a sulfur-containing compound was studied. The proposed mechanism for the oxidation of DBT with atmospheric oxygen is discussed. It was found that the addition of iron particles contributes to the generation of oxygen-containing radicals through electron transfer. FeWC catalyst is capable of at least 5 cycles of reuse without an essential loss of catalytic efficiency. High stability of the catalyst was shown using oxine complex and UV/Visible spectroscopy. Aerobic desulfurization of straight-run gasoline fraction was performed and a detailed analysis of the hydrocarbon composition was carried out using 2D gas chromatography with TOFMS detection.