Oxidation reactivities of dibenzothiophenes in polyoxometalate/H2O2 and formic acid/H2O2 systems

Dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophen e are typical thiophenic sulfur compounds that exist in diesel fuels. Using toluene solutions of the model compounds, experiments were carried out to compare the reactivity of the different dibenzothiophenes in oxidation reac tions, a key step for oxidative desulfurizations. A series of polyoxometala te/H2O2 systems were evaluated for dibenzothiophene oxidation. The H2O2 sol utions of phosphotungstic acid and its salt were very active catalyst syste ms for the model compound oxidation, while their molybdenum counterpart sys tems were much less active. The H2O2 solutions of silicotungstic and silico molybdic compounds were the least active catalyst systems for the reaction. Oxidation reactivities decreased in the order of dibenzothiophene > 4-meth yldibenzothiophene > 4,6-dimethyldibenzothiophene, the same reactivity tren d that exists in HDS. However, the oxidation of the dibenzothiophenes was a chieved under mild reaction conditions and it was easy to increase reaction temperature or reaction time to achieve high oxidation conversions, even f or the least reactive 4,6-dimethyldibenzothiophene. Apparent activation ene rgies of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenz othiophene oxidation were 53.8, 56.0, and 58.7 kJ/mol, respectively. These activation energies indicated a decrease in reactivity of dibenzothiophenes as methyl substitutes increased at the 4 and 6 positions on dibenzothiophe ne rings. Interestingly, in a formic acid/H2O2 system, the oxidation reacti vity of the dibenzothiophenes showed the reverse trend, suggesting that ste ric hindrance might play a role when bulky polyoxoperoxo species, which lik ely form in a hydrogen peroxide solution, act as catalysts. Crown Copyright (C) 2001 Published by Elsevier Science B.V. All rights reserved.