Insights to the oxidative desulfurization process of fossil fuels over organic and inorganic heterogeneous catalysts: advantages and issues

Strict environmental laws have been put in place around the world to reduce the amount of sulfur in the fuel to reduce the emissions of harmful gases from fuel combustion and improve air quality. Therefore, extensive researches have been undertaken to devise effective processes or to improve the desulfurization processes. Among the desulfurization processes, the oxidative desulfurization (ODS) process is a promising method to achieve very low and near-zero sulfur content of the fuel. In this process, sulfur compounds are converted to the corresponding sulfone by a catalyst and in the presence of an oxidant. The obtained compounds by polar solvents or adsorbents are removed from the fuel. In recent decades, extensive studies have been carried out on the catalysts used in the oxidative desulfurization process. In this review, a comprehensive survey has been performed on heterogeneous catalysts used in the oxidative desulfurization process. According to the reported researches, the heterogeneous catalysts used can be divided into five groups: ionic liquids, carbon materials, polyoxometalates, transition metal oxides stabilized on porous solid substrates, and metal-organic frameworks. The proposed mechanisms with different catalysts have also been studied in this work.

Effect of Acid Treatment on the Functionalization of Surface, Structural and Textural Properties of Carbon Nanotubes Taunit

The role of acid treatment of Taunit carbon nanotubes in the formation of oxygen-containing functional groups on its surface as well as morphological and textural properties was studied. Acid treatment was carried out in an HNO3 solution or its mixture with H2SO4 under mild conditions (85 °C/1 h) with subsequent washing with distilled water or without washing. Properties of the initial and oxidized samples were investigated using elemental carbon, hydrogen, nitrogen, oxygen (CHNO) analysis, BET (Brunauer-Emmett-Teller) determination of surface area, X-ray diffraction, Raman and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy, and hydrogen temperature-programmed reduction. Treatment with HNO3 and HNO3/H2SO4 mixture was shown to be efficient for the formation of various oxygen-containing groups on the Taunit surface; therewith, the water washing step also contributed to functionalization of the surface. Depending on the oxidant, acid treatment increased graphite and oxygen content in the samples by a factor of 3‒4.5. Treatment with HNO3 without water washing exerted a weak effect on the graphite structure ordering, the concentration of aliphatic groups was high as compared to other oxidation conditions. Treatment of Taunit with the HNO3/H2SO4 mixture, on the contrary, increased the number of defects in graphite layers and decreased the concentration of aliphatic structures.

A Comprehensive Review on Catalytic Oxidative Desulfurization of Liquid Fuel Oil

The production of green fuel oil is of the utmost importance for maintaining a healthy life and environment in the current world. Effective and complete removal of sulfur refractory compounds (such as 4,6-dimethyldibenzothiophene and other alkyl-substituted thiophene derivatives) from fuel oil is essential to meet the new requirements of sulfur standards. Several techniques have been proposed for desulfurization of fuel oil, such as hydrodesulfurization (HDS), selective adsorption, extractive distillation, biodesulfurization, and oxidative desulfurization (ODS). The removal of sulfur by the HDS process requires higher investment costs, high reaction temperature (up to 400 °C), and high pressure (up to 100 atm) reactors. On the other hand, studies have shown that the ODS process is remarkably successful in the removal of sulfur under mild reaction conditions. This review article presents a comparative analysis of various existing catalytic oxidation techniques: acetic acid/formic acid catalytic oxidation, heteropolyacid (HPA) catalytic oxidation, ionic liquid catalytic oxidation, molecular sieve catalytic oxidation, polyoxometalates catalytic oxidation, titanium catalytic oxidation, and ultrasound-assisted oxidation systems, as well as discusses research gaps, and proposes important recommendations for future challenges.