Oxidative desulfurization strategies using Keggin-type polyoxometalate catalysts: Biphasic versus solvent-free systems

[EN] Strategic polyoxometalate Keggin-type structural modification was performed to increase the oxidative catalytic performance to desulfurize model and real diesels. The most active lacunar structure [PWO] (PW) showed to complete desulfurize a simulated diesel after 60 min at 70 °C. Its application as homogeneous catalyst using a biphasic system 1:1 diesel/acetonitrile needed to use an excess of oxidant (ratio HO/S = 8). The immobilization of the PW on amine-functionalized SBA-15 supports originated two heterogeneous catalysts PW@aptesSBA-15 and PW@tbaSBA-15. The best results were attained with the PW@aptesSBA-15 catalyst showing identical oxidative desulfurization performance as the homogeneous analogue. As advantage, this heterogeneous catalyst promotes the complete desulfurization of simulated diesel using a solvent-free system, i.e. without the need of acetonitrile use. On the other hand, the same desulfurization efficiency could be achieved using half the amount of oxidant (HO/S = 4). The oxidative desulfurization of the real diesel achieved a remarkable 83.4% of efficiency after just 2 h. The recycling capacity of PW@aptesSBA-15 catalyst was confirmed for eight consecutive cycles using the biphasic and the solvent-free systems. Its stability investigation demonstrates to be higher under the solvent-free system than the biphasic system, without practically any occurrence of PW leaching in the first case. On the other hand, the Venturello peroxocomplex [PO{W(O)} ], recognized as active intermediate in the homogeneous biphasic system, was not identified in the heterogeneous catalytic systems.