Catalytic Oxidative Desulfurization Of Gasoline And Diesel Over Ti-containing Molecular Sieves

With the development of world economy, there are more and more liquid fuels needed. There seems no other energy to replace it in a short time. Organic sulfur compounds are undesirable in fuels, because they're potential to cause pollution. Sulfur also results in corrosion of refining equipment and poisoning of many precious metal catalysts in the petrochemical industry and fuel cell processor, and increased seriously environment problems. So the regulations for the sulfur content in light oil are more and more strict. However, the hydrodesulfurization (HDS) used at present can't reach such low sulfur standard, unless elevating temperature, pressure, using more active metal catalysts and larger reactors. These is because sulfur compounds in liquid fuels: benzothiophene (BT), dibenzothiophene (DBT), especially DBT having alkyl substituents on its 4- and/or 6- positions can not be efficiently removed in HDS owing to their steric hindrance. To overcome the limitations of HDS and gain ultra-low sulfur fuels, many new desulfurization processes was investigated.In these new processes, the oxidative desulfurization (ODS) was considered one of the promising processes for its mild reaction conditions, simple equipment design, and highly efficiently. In addition, as an excellent, green solid catalyst, easily recycled and reused from reaction mixtures, the Ti-containing molecular sieves had been used in the oxidative desulfurization of liquid fuels. It has reported that TS-1 was an efficient catalyst for thiophene oxidation, but with low activity for BTs and DBTs owing to its restriction of size of main channel. Ti-beta, Ti-MCM-41 and Ti-HMS were efficiently to BTs and DBTs oxidation. However, owing to intrinsic property of these catalysts, which have not gained good results in desulfurization of real light oils. This thesis studied oxidative desulfurization for gasoline and diesel over Ti-MWW—a novel titanosilicate.At the third part of this thesis, we studied the oxidation of organic sulfur compounds (TH, BT, DBT and 4, 6-DMDBT) over four different Ti-containing molecular sieves, Ti-MWW demonstrated higher activity than TS-1, Ti-beta and Ti-MCM-41, especially to oxidation for 4, 6-DMDBT. Effects of solvent, temperature, catalyst amount and reactant ratios (H₂O₂/S) were also studied in Ti-MWW/H₂O₂ systems. The oxidation of sulfur compounds demonstrated higher activity in acetonitrile than in water and methanol. The optimal reaction temperature is 50-70℃, the optimal reactant ratios(H₂O₂/S) is 4.In the fourth part of this thesis, we studied the oxidation of sulfur compounds in the presence of olefins and nitrogen compounds over Ti-MWW. At the same reaction conditions, when the weight ratio of cyclohexene or 1-octene in model light increased from 0 % to 30 %, the oxidation conversion of thiophene was decreasing from 78 % to 48 % or 29 %; the oxidation conversion of BT was decreasing from 80 % to 25 % or 45 %. When the weight ratios of 1-hexadecene in model light oil increased from 0 % to 30 %, the oxidation conversion of BT decreased from 80 % to 55 %, the oxidation conversion of DBT decreased from 80 % to 57 %, the oxidation conversion of 4, 6-DMDBT decreased from 87 % to 67 %. When organic nitrogen compound existed in model light oil, the oxidation conversion of sulfur compound is lower than absence of nitrogen compound. This is because the nitrogen compound can be oxidized and compete with the oxidation of sulfur compound, and they can also poison the catalyst.In the fifth chapter of this paper, we investigated oxidative desulfurization of real light oil in Ti-MWW/H₂O₂ systems. The sulfur compound in gasoline can't be oxidized effectively, owing to there are too much olefins in gasoline, which can compete with the oxidation of sulfur compound and their polymer can be poison the catalyst. The sulfur compounds in commercial diesel can be oxidized efficiently. Results showed that with Ti-MWW, the sulfur content of diesel oil was decreased successfully from 1015μg/mL to 140μg/mL under mild conditions, and the desulfurization yield was 86 %.In the sixth part of this thesis, we investigated ODS of model light oil and commercial diesel in Ti-MWW / t-BuOOH system. The results showed that BT, DBT and 4, 6-DMDBT can be oxidized efficiently, but thiophene can't be oxidized efficiently. The sulfur compound in catalytic diesel can't be oxidized efficiently, owing to there are too much nitrogen compounds in catalytic diesel, which can compete with the oxidation of sulfur compound and poison the catalyst. The sulfur compound in commercial diesel can be oxidized efficiently. The sulfur content of diesel oil was decreased successfully from 1015μg/mL to 11μg/mL under mild conditions, and the desulfurization yield was 99 %.In conclusion, Ti-MWW demonstrated high activity for oxidation of BT, DBT and 4, 6-DMDBT. Ti-MWW has application foreground in ODS of liquid fuels especially to commercial diesel.