Oxidative Desulfurization Of Aromatic Sulfur Compounds From Fuel Oil Over Ti-Containing Molecular Sieves

The level for sulfur content in liquid fuels has become more and more stringent all over the world, given the increasing seriously environmental problems. 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, efficient and especially friendly to environment alternative to desulfurization.The level for sulfur content in liquid fuels has become more and more stringent all over the world, given the increasing seriously environmental problems. 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, efficient and especially friendly to environment alternative to desulfurization.At the present, oxidative desulfurization (ODS) has received increasing attention due to its mild reaction condition, simple equipment design, and highly efficiency. In addition, noticed the good performance played by the titanium-containing molecular series in the catalytic oxidation reactions, it might be the excellent solid catalyst for this new desulfurization process alternative to the HDS. In our previous experiments thiophenes have been successfully oxidized and removed with H₂O₂ over TS-1. While TS-1 demonstrated low activity due to restricted access of reactants into the porosity, this work extends the earlier studies to the oxidative desulphurization process of bulky sulfur-containing compounds (especially 4,6-dimethyldibenzothiophene (DMDBT) with H₂O₂ over titanium-containing molecule series, in order to solve the problems which still exist in the ODS process. The major content of this paper is composed of four parts.1. Oxidative desulfurization of sulfur containing compounds in model oil overTi-containing catalystsBenzothiophene (BT) and dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene were solved in octane as the model oil, respectively. The oxidative desulfurization of sulfur compounds in oil was carried out with H₂O₂ over Ti-containing molecular sieves got the results: 1. the oxidative activity of BT and DBT could not occur with TS-1 as catalyst due to restricted access of reactants into the porosity. It can be efficiently solved over Ti-HMS attribute to its larger pore size. BT can be eliminated on Ti-P or Ti-HMS; DBT and DMDBT can be effectively oxidized over Ti-HMS. They were converted into sulfides and sulfones. 2. Ti-HMS was used as the catalyst and H₂O₂ was used as the oxidant to oxidize and remove the sulfur compounds in the model fuel composed with Th, BT, DBT and DMDBT in octane, respectively. It has been showed that the removal of sulfur compounds in oxidation over Ti-HMS is as following: DBT > BT > DMDBT > Th. It is the common effect of electric density on sulfur atom and stetric hindrance. It is in pores of the catalyst that the thiophenes were oxidized and the active sites are framework titanium atoms. The removal of DMDBT increases with the decrease of Si/Ti mole ratio and the increase of relative crystallinity ofAt the present, oxidative desulfurization (ODS) has received increasing attention due to its mild reaction condition, simple equipment design, and highly efficiency. In addition, noticed the good performance played by the titanium-containing molecular series in the catalytic oxidation reactions, it might be the excellent solid catalyst for this new desulfurization process alternative to the HDS. In our previous experiments thiophenes have been successfully oxidized and removed with H₂O₂ over TS-1. While TS-1 demonstrated low activity due to restricted access of reactants into the porosity, this work extends the earlier studies to the oxidative desulphurization process of bulky sulfur-containing compounds (especially 4,6-dimethyldibenzothiophene (DMDBT) with H₂O₂ over titanium-containing molecule series, in order to solve the problems which still exist in the ODS process. The major content of this paper is composed of four parts.1. Oxidative desulfurization of sulfur containing compounds in model oil overTi-containing catalystsBenzothiophene (BT) and dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene were solved in octane as the model oil, respectively. The oxidative desulfurization of sulfur compounds in oil was carried out with H₂O₂ over Ti-containing molecular sieves got the results: 1. the oxidative activity of BT and DBT could not occur with TS-1 as catalyst due to restricted access of reactants into the porosity. It can be efficiently solved over Ti-HMS attribute to its larger pore size. BT can be eliminated on Ti-P or Ti-HMS; DBT and DMDBT can be effectively oxidized over Ti-HMS. They were converted into sulfides and sulfones. 2. Ti-HMS was used as the catalyst and H₂O₂ was used as the oxidant to oxidize and remove the sulfur compounds in the model fuel composed with Th, BT, DBT and DMDBT in octane, respectively. It has been showed that the removal of sulfur compounds in oxidation over Ti-HMS is as following: DBT > BT > DMDBT > Th. It is the common effect of electric density on sulfur atom and stetric hindrance. It is in pores of the catalyst that the thiophenes were oxidized and the active sites are framework titanium atoms. The removal of DMDBT increases with the decrease of Si/Ti mole ratio and the increase of relative crystallinity of Ti-HMS. Both the meso-pore framework and framework titanium atoms are necessary to the oxidation of DMDBT.The kinetics of the oxidation of DMDBT with H₂O₂ over Ti-HMS was also investigated. The obvious first reaction order was oBTained both for DMDBT and the catalyst, as well as H₂O₂. The reaction rate can be expressed as:And the obvious reaction constantOxidative desulphurization of DMDBT in octane has been investigated with hydrogen peroxide (H₂O₂) over Ti-HMS under mild conditions. The reaction rate of the oxidative desulphurization was promoted with the increase of the amount of the Ti-HMS catalyst, the mole ratio of H₂O₂ and the S-containing molecule (H₂O₂/S), as well as the concentration of DMDBT and the temperature on the reaction rate. The Arrhenius apparent activity energy of the oxidative reaction of DMDBT with H₂O₂ over Ti-HMS was determined as 38.54kJmol^-1.2. Deactivation and modification of the Ti-HMS catalystIn this part, the performance of the fresh Ti-HMS catalyst and the spent catalyst, the cause of the deactivation of the spent catalyst, the method to regenerate the deactivated catalyst and the modification of the fresh catalyst were investigated. More than 90% of DMDBT was removed from the model oil by oxidation over Ti-HMS, but the spent catalyst was deactivated, mainly due to the strong adsorption of the oxidized products of DMDBT on the catalyst. The catalyst can not be regenerated by washing using solvent such as methanol and acetonitrile, while calcination can efficiently regenerate the deactivated catalyst. Modification of the catalyst with tetraethyl orthosilicate (TEOS) and methyltrichlorsilicane as pretreatment can avoid the deactivation of the catalyst in the oxidation of DMDBT.After the . fresh catalyst was modified by tetraethylorthosilicane (TEOS) to support 1.0wt%～3.0wt% SiO₂ on the catalyst with dip-molding or gas sedimentation, the oxidative activation of the spent catalyst after desulfurization remained still high. Proper loading 2.0 wt % of the SiO₂ was optimum quantity. 3. The influence of the aromatic compounds and nitrogen compounds on desulfurization of the model oilThe oxidation of sulfur compound was also investigated in the presence of various hydrocarbons and nitrogen compounds. Considering other compounds in real diesel, hexadecane, toluene, cumin, naphthalene, 2-methylnaphthalene, decalin, styrene were added into the model oil respectively, in order to study the effect of these compounds on the desulfurization. The following results can be oBTained: 1. the saturated hydrocarbon does not influence benzothiophene oxidation. 2. Benzothiophene can also be oxidized in the presence of aromatics. However, the reaction rate for benzothiophene oxidation became lower in present of the aromatics, while the final removal rate was not changed in reaction for 3h. 3. The final desulfurization yield of benzothiophene was influenced by alkenes. We proposed that the carbon-carbon dibond of the alkenes were more intend to also act with the active site of the catalyst, and thus hindered the oxidation of benzothiophene.What is more, the influence of nitrogen containing compounds such as indole, quinoline, carbazole, and pyridine on the desulfurization was investigated, as following the order: indole styrene > carbazole > quinoline > pyridine. The effect of nitrogen compounds on the desulfurization shows: the desulfurization rate decreased mainly due to the scramble of H₂O₂ between denitrification and desulfurization. The products of styrene oxidation are the main retardation to desulfurizion.4. Oxidative desulfurization of real diesel in the H₂O₂/Ti-HMS systemThe oxidative desulfurization of No.0 diesel purchased from market with H₂O₂ over Ti-HMS was investigated. There are several findings as results: (1) the efficiency of desulfurization of real diesel has not been high as expected, due to the different compositions of real diesel from the model oil. The presence of olefins and aromatic compounds as well as nitrogen-containing compounds leads to hindrance for desulfurization. (2) Although the removal rate of sulfur-containing compounds in diesel increased with the amount of H₂O₂ added increasing, the desulfurization could no more enhance when the H₂O₂/S mole ratio is higher than 6, which is attributed to the alkyl DBTs and BTs' sulfones remaining in oil. The addition of acetonitrile can remove the sulfones by solvent extraction, as a result, aBTaning clean oil.