Study On Deep Desulfurization Of Fuel Oil With Polyoxometalates As Catalysts

A large amount of sulfur dioxide (SO₂) ,which is one major air pollutants and the precursors of acid rain , are generated in the fuel oil consumption process. Therefore, strict standards have been issued by many countries to control the sulfur content in the fuel oil, and desulfurization technology has received much attention. The desulfurization technologies are divided into two main categories: hydrodesulfurization (HDS) and non-hydrodesulfurization(NHDS). HDS is one kind of matural technology, and has been widely applied commercially, but it has several disadvantages, such as high investmentin one lump sum, high operating costs, large consumption of hydrogen,and it cannot reach the goal of deep desulfurization. NHDS does not require hydrogen and is in accordance with the requirements of the deep desulfurization, so NHDS has becoming the focus of worldwide researches. In the last decade, oxidative desulfurization (ODS) has been studied intensively. ODS is operated at atmospheric pressure and a reaction temperature of below 100℃, the organic sulfides are oxidized to stronger polarity matters by oxidatants, and the products can be seperated by extraction or absorption. However, catalysts are necessary in ODS, therefore, good catalysts are particularly important. Heteropoly compounds (HPCs) are functional material which are unharmful to the environment. Using heteropoly compounds as catalysts for oxidative desulfurization has shown good prospect of research and process development.In this paper, several kinds of HPCs and supported HPCs were prepared and evaluated concerning their desulfurization efficiencies to two kind of sulfides that are difficult to remove: thiophene(TH) and dibenzothiophene(DBT), among which excellent catalysts for ODS are selected. The mechanism of the process of ODS was also revealed in this paper.The paper is composed of three sections.In the first section, the desulfurization of TH by heteropoly acid and their salts are evaluated. Results showed that H₃PMo₆W₆O₄₀ is the best heteropoly acid catalyst for the desufuriztion of TH, and the optimum conditions were as follows: reaction temperature, 60℃; oxygen to sulfur ratio, 15; the sulfur content, 500ppm; the reaction time, 210min; thiophene conversion rate was of 94.3%. When the cesium salts of heteropoly acid were used as the desulfurization catalyst, Cs_2.5H_0.5PMo₁₂O₄₀ achieves the highest thiophene conversion rate of 86.4%, the process conditions were as follows: sulfur content, 800ppm; reaction temperature, 65℃; oxygen to sulfur ratio, 20, reaction time, 40min; corresponding to a thiophene conversion rate of 73.8%.In the second section, the desulfurization of DBT by heteropoly acid and their supported salts were evaluated, results showed that H₃PMo₆W₆O₄₀ is the best heteropoly acid catalyst for the DBT desufuriztion , and the optimum conditions were as follows: the amount of catalyst, 1% the mass of the simulation fuel oil; reaction temperature, 60℃; oxygen to sulfur ratio, 15; sulfur content, 500ppm; reaction lasted time 90min; corresponding to a DBT conversion rate of nearly 100%. When Cs_2.5H_0.5PW₁₂O₄₀/CNT was chosen as the desulfurization catalyst, the highest conversion rate of DBT was achieved at the following conditons: sulfur content, 500ppm; reaction temperature, 60℃; oxygen to sulfur ratio, 20; reaction time, 150min. When 10%H₃PMo₁₂O₄₀-NaY was selected as catalyst, the optimum conditions were as follows: the amount of catalyst, 1% the mass of the simulated fuel oil, reaction temperature, 60℃; sulfur to oxygen ratio, 20; sulfur content, 500ppm; reaction time, 180min; the corresponding DBT conversion rate, 84.1%.In the third section,the products from the oxidation of the simulated fuel oil were analysed, results showed that the oxidation products of thiophene were mostly SO₄^2-, and also trace amount of styrene was generated; and the oxidation products of DBT was the corresponding sulfone species only.