Oxidation of benzothiophene and dibenzothiophene with hydrogen peroxide catalyzed by polyoxometalates

Abstract

In this work various polyoxometalates: Na2H[PMo12O40], Na2H[PW12O40], H3PMo12O40, H3PW12O40, (VO)H[PMo12O40], (VO)H[PW12O40], (n-Bu4N)3[PW12O40], (n-Bu4N)4[PVW11O40], (n-Bu4N)5[PV2W10O40] and (n-Bu4N)6[PV3W9O40] were synthesized. The synthesized catalysts were characterized by XRD, FT-IR, AAS, ICP, UV-Vis and TPR. The oxidation of sulfur model compounds (benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene) with hydrogen peroxide catalyzed by these polyoxometalates was studied. These sulfur compounds are typical refractory sulfur compounds in diesel fuels. The compounds were oxidized to sulfone. The experimental results show that the W catalysts have higher activity than the Mo catalysts. The activity also depends on the countercation: Na2H[PW12O40] ~ H3PW12O40 > (VO)H[PW12O40]. For the V-containing catalysts, the relative activity order is: (n-Bu4N)6[PV3W9O40] > (n-Bu4N)5[PV2W10O40] > (n-Bu4N)4[PVW11O40] > (n-Bu4N)3[PW12O40]. The oxidation reactivity order of sulfur compounds is: dibenzothiophene (DBT)> 4,6-dimethyldibenzothiophene (4,6-DMDBT) > benzothiophene (BT). In the case of the most refractory compound: BT, a higher %yield of sulfone can be obtained at more severe reaction conditions. Addition of phase transfer catalyst can also increase the product yield. The selected catalyst: Na2H[PW12O40], was utilized in the oxidation of a commercial diesel oil. The oxidized product was removed by solvent extraction and alumina adsorption. Sulfur in the diesel oil can be removed up to 99% (from 0.575 %wt to 0.0056 %wt sulfur) at 70 °C in 5 h.