Hierarchical TS-1 zeolite as an efficient catalyst for oxidative desulphurization of hydrocarbon fractions

Abstract

The catalytic oxidative desulphurization (ODS) of S-containing aromatic compounds has been investigated over a hierarchical TS-1 zeolite prepared from silanized protozeolitic units, characterized by having, additionally to the zeolitic micropores, a secondary porosity within the supermicro/mesopore region. A standard TS-1 sample has been employed as reference catalyst. The influence of both the solvent (n-heptane and acetonitrile) and the oxidizing agent (hydrogen peroxide and tert-butylhydroperoxide, TBHP) was first studied using dibenzothiophene (DBT) as model substrate. In all cases, the catalytic activity exhibited by the hierarchical TS-1 was much higher than that obtained with the conventional TS-1 as a consequence of the improved accessibility caused by the presence of the secondary porosity. A very high desulphurization activity was obtained with the combination of heptane and TBHP, as solvent and oxidant, respectively, leading to an almost total DBT conversion. This fact has been attributed to the fully miscibility of the oxidant solution (TBHP in decane) in the organic reaction medium. Moreover, it is noteworthy that, in these conditions, the oxidation product (sulphone) is insoluble in the reaction medium, which facilitates its removal by filtration or centrifugation. Additionally, the oxidation of different aromatic organosulphur compounds [benzothiophene (BT), 2-methylbenzothiophene (2-MBT), 2,5-dimethylthiophene (2,5-DMT) and dibenzothiophene (DBT)] was investigated using the hierarchical TS-1 catalyst. The trend in the conversion values obtained was 2,5-DMT<BT<2-MBT<DBT. These results indicate that the oxidation conversion is determined by the nucleophilic character of the sulphur atom, and not by the molecular size of the S-containing compound, provided that the steric and diffusional limitations are overcome by the presence of the hierarchical TS-1 porosity.