## 1 Introduction  
Nowadays, the emphasis in the oil industry shifts to the extraction and processing of heavy, high-viscosity, high pour point sulfurous oil and oil residues. Most of the world's produced oil is sulfurous and high sulfur oil. Their total sulfur content varies between very wide limits from 0.5 to 10% by weight [1]. Sulfur contained in oils is mostly represented by its organic compounds. To date, more than 250 compounds have been identified [1; 2; 3]. The efforts of researchers to extract sulfur-containing compounds (SCC) and establish their structure are focused on middle distillate fractions, representing the main raw material base for practical use. Organosulfur compounds of most middle distillate fractions of sulphur and high sulfur oils are mainly presented by sulfides and thiophene homologues mixed in various ratios [1]. To date, special attention has been paid to technologies for processing of such high-sulfur oils [4; 5]. The process of hydrotreatment over solid-phase catalysts is extensively applied to remove sulfur impurities from hydrocarbons. Its main disadvantages are the use of catalysts, large volumes of hydrogen, and high pressures. The process is economically irrational due to high cost for small and medium-sized plants. This is one of the reasons for development of alternative approaches to the hydrocarbon feed purification. Currently, the process of extraction desulfurization aimed to reduce the sulfur content of diesel fuels is actively developing [6; 7]. In addition to the fact that the extraction is carried out under mild conditions, the extraction processes cause no changes in the chemical structure of diesel fuel components and allow removing not only sulfur-containing compounds, but also aromatic hydrocarbons. An extraction process is based on the higher solubility of sulfur-containing components and aromatic hydrocarbons compared to that of non-aromatic hydrocarbons in well-suited polar solvents. The effectiveness of desulfurization depends on the correct solvent selection, the chemical activity of the components being removed, and other factors including environmental and toxicological aspects. The most commonly used solvents for the extraction of desulfurization of diesel fuel are \(N\),\(N\)-dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetonitrile [6; 7], \(N\)-methyl-2-pyrrolidone [8], and polyethylene glycol [9]. The extraction of organic sulfides and sulfoxides from high-sulfur diesel distillates, in particular, a diesel distillate of a mixture of oils from the Gazlinskoye oil and gas field and Fergana oil and gas region (boiling point: 140-360 \({}^{\circ}\)C) using methanol, acetonitrile, dimethylformamide, and dimethylsulfoxide as extractants, and also mixtures of these solvents with 5-20% water is reported in [7]. In addition to these solvents, solutions of zinc bromide in methanol, butanol, and sulfuric acid (20-94%) were used. An investigation of a number of aprotic and proton-donating polar solvents and extraction systems, including a nonpolar solvent (hexane) used as extractants for the removal of sulfur-containing compounds from the diesel fraction of high-sulfur oil from South Uzbekistan has shown that effective desulfurization of the fraction is achieved using the phenol-water-hexane extraction system [10]. It is possible to reduce the sulfur content in oil objects by using of modified organic solvents during the extraction desulfurization. For this purpose, metal salts are introduced into coordinating organic solvents. It was found out that an introduction of electron-withdrawing compounds represented by metal chlorides (Cd, Cu, Ni, Co, Mn, Cr, Zn, and Li) into coordinating organic DMF, DMSO, and propylene carbonate (PC) solvents, and the use of these systems in the course of desulfurization results in a decrease in content of sulfur compounds in diesel fuels [8; 9]. Thus, an analysis of the literature data shows that organic solvents may be well suited for the extraction of sulfur-containing compounds from the diesel fraction.