Oxidative Desulfurization of Liquid Fuels Using Polyoxometalate-Based Catalysts: A Review

Nowadays, desulfurization of liquid fuels is inevitable because of strict environmental and industrial regulations on liquid fuels specifications. Hydrodesulfurization (HDS) technology has been used widely to produce fuels with ultralow sulfur content (S ≤ 10 ppm). However, this method is not effective enough for removing refractory sulfur compounds (e.g., benzothiophene, dibenzothiophene, and their derivatives). Consequently, alternative or supplementary desulfurization methods have been rapidly developed in recent years. The oxidative desulfurization (ODS) process is a promising method with high selectivity, low cost, mild reaction conditions, and high efficiency. In the past few decades, the ODS process of fuels by polyoxometalates (POMs) as catalysts have attracted considerable attention, resulting in different works have been published due to their strong acidity, fast and reversible multielectron redox properties, tunable redox properties, as well as thermal, hydrolytic, and oxidative stability. In this review, the removal of S-compounds from fuel oils is investigated via the ODS process using homogeneous and heterogeneous polyoxometalate catalysts. Moreover, the advantages and problems of each system are discussed. Various techniques for reducing their noticeable drawbacks are also presented. Finally, the regeneration of POM catalysts, as an important step in industrial applications, is examined.