Oxidative Desulfurization Of Sulfur In Gasoline Over Titanium Silicalite

Concerning the increasing seriously environmental problems, the world has seen a more and more stringent level for sulfur content in liquid fuels proposed in many countries. The traditional hydrodesulfurization process (HDS) becomes economically insufficient with the decreasing lower sulfur in fuels. It has now attracted more research groups in finding a more economical and efficient process to obtain low sulfur fuels.Currently, oxidative desulfurization (ODS) was considered one of the promising processes for its mild reaction condition, simple equipment design, and highly efficiency. In addition, noticed the good performance played by the titanium silicalite in the catalytic oxidation reactions, it might be the excellent solid catalyst for this new desulfurization process alternative to the HDS. Therefore, we do basic study intending to solve the problems which still exist in the ODS process. The major content of this paper is composed of four parts.1. The oxidation of sulfide, mercaptans and benzothiophenesMethyl sulfide (DMS), butyl mercaptan (BM), benzothiophene (BT), dibenzothiophene (DBT) and 4,6dimethyl dibenzothiophene (4,6DMDBT) in octane was studied using the titanium silicalite as the catalyst and hydrogen peroxide as the oxidant. The result shows that DMS and BM can be very easily oxidized over TS-1 and TS-1/TiO2. For their steric hindrance, the oxidation of benzothiophens can not proceed over TS-1 but those molecular sieves with larger pore size, such as Ti- β, Ti-HMS or Ti-MCM-41. The solvents for the reaction could be methanol, acetonitrile or water. The main role played by the solvent in this process was to remove the products efficiently in order to maintain the activity of the solid catalyst.2. The oxidation of thiophene and 2-methyl thiopheneThe system for effetive oxidation of thiophene was settled by changing the solvent in the process. It is a triphase system notices as TS-1/H2O/oil. It can be seen from the results that thiophene can be effectively oxidized to form sulfuric acid by H2O2. The above reaction can also proceed in solvent butanol although in a relatively low reation rate,comparing with that in solvent water. The oxidation of thiophene does not occure in solvent methanol and acetonitrile. Using Ti-HMS as catalyst, the oxidation of thiophene can only proceed in solvent water, we proposed that the active site of titanium silicalite can interact with the n -electrons of thiophene. Once the stability of the molecular has been destroyed, it becomes very easy to undergo oxidation in the reaction condition. The interaction between the active site and thiophene must not be disturbed. In solvent water, this could achieve because of the hydrophobicity of the structure of TS-1 and Ti-HMS. However, in solvent methanol and acetonitrile, the small organic molecular can get also close to the active site and thus inhibit the interaction between catalyst and thiophene. The reaction does not happen. When using t-butanol, thiophene can also be oxidized over TS-1 for steric hindrance of t-butanol getting close to the active site. But the reaction was also inhibited over Ti-HMS, because in this case, the steric hindrance for butanol now dissappeard for it larger pore size.The kinetics of thiophene oxidation was also studied. The obvious first reaction order was obtained both for thiophene and the catalyst. It was zero for H2O2. The reation rate can be expressed as:v = 2.05 x 1 IT5 [catal.r [Th] [H2O2 ]° °7T=333 K; P= 101 kPa. And the obvious reaction constantk = 4.45 x 102 exp(-29.9/RT)2-methyl thiophene can also be oxidized to sulfuric acid over TS-1 by hydrogen peroxide under mild condition. The reaction condition was the same as that for thiophene. However, 2-methyl thiophene does not undergo exactly the same reaction mechanism as that for thiophene oxidation because of the methyl substituent. After activation, the sulfur atom in thiophene is first oxidized while it is the carbon atom in thiophene ring for 2-methyl thiophene.3. The oxidation of thiophene in various hydrocarbon compoundsThe oxidation of thiophene was also investigated in the presence of various hydrocarbons. The following results can be obtained: 1. The saturated hydrocarbon does not influence thiophene oxidation. 2. Thiophene can also be oxidized in the presence of aromatics. However, the reaction rate for thiophene oxidation was greatly influenced by the aromatics, especially those with larger substituents. 3. Thiophene cannot be oxidized in the presence of alkenes. We proposed that the carbon-carbon di-bond of the alkenes weremore intend to also act with the active site of the catalyst, and thus hindered the activation of thiophene. After modificated with metal ions, the catalyst shows activity for thiophene oxidation even in the presence of alkene. The oxidation reaction rate could also be influenced in the presence of alkane over the metal ion modified TS-1. The reaction rate does not change obviously in the presence of aromatics. The result also shows that the Ag modified catalyst performce better than the Cu modified one. Proper loading of the Ag was important. Too much Ag loading could over layed the active site, while too little will result in the lose of Ag species. Both will lead to poor catalyst performance.The oxidation of organic of sulfur compounds in gasoline was also investigated. In the batch reactor, the following results were obtained: the organic sulfur in Fu Shun FCC gasoline could be reduced from the initial 136.5 U g/g to 18.8 V g/g over Ag/TS-1, while no sulfur could be removed over TS-1 catalyst. However, TS-1 shows activity of desulfurization for FCC gasoline after olefin reduction treatment of the above gasoline. In the fix-bed reactor, the TS-1 catalyst also shows activity for sulfur removal. Ag/TS-1 performs better than the TS-1 catalyst, but the catalyst deactivated faster. The unstable components in gasoline tend to polymerize under the reaction condition and cover the surface of catalyst. This prevents the interaction between the organic sulfur and the active site, and thus results in the deactivation of both TS-1 and Ag/TS-1 catalyst.4. Study on the active site of the catalyst for thiophene oxidationTitanium silicalite was characterized in this paper by FT-IR, UV-vis, TEM, STEM/EDX, etc. to obtain information how thiophene oxidation happened over the catalyst. The results show: 1. The framework Ti speices was the active site for thiophene oxidation reaction. Very quickly it reacted with hydrogen peroxide and form the active species for thiophene oxidation. It could disterb the n -complex of thiophene ring and make it easier for thiophene to be oxidized; 2. The none framework Ti species does not play a role as active site. Instead, its presence can only influence the utilization of the framework Ti species and stericaly hinder the reaction diversly; 3. The crystal size of TS-1 also influences the activity obviously. The catalyst with small crystal size shows better activity for thiophene oxidation because of the relatively shorter dispersion path; 4. The catalytic performance of Ti-HMS and Ti-MCM-41 is lower than TS-1, because their structure hydrophobisity is poorer than that of TS-1.STEM/EDX charaterization of Ag modified TS-1 shows that the Ag species were highly dispersed around the Ti species of titanium silicalite. The catalyst shows sulfur selectivity in the presence of alkene because of the introduction of little amount of Ag onto...