Organic–inorganic polyoxometalate based salts as thermoregulated phase-separable catalysts for selective oxidation of thioethers and thiophenes and deep desulfurization of model fuels

Abstract

Synthesis of various organic–inorganic polyoxometalate based salts composed of sulfonated pyridinum cations and the Keggin tungstophosphate anion (PW12O403−) and their subsequent application in the oxidation of methyl phenyl sulfide with hydrogen peroxide in aqueous media was reported. Special attention was paid to the temperature-dependent solubility of the salts as a function of the organic cation in water. Cyclic voltammetry was further used for the investigation of the redox behavior of these catalysts. Based on these properties, a novel thermoregulated phase-separable catalyst was introduced and successfully used for the selective oxidation of thioethers and thiophenes to the corresponding sulfoxides or sulfones. Catalytic oxidation of the sulfur-containing model oil (methyl phenyl sulfide, dibenzothiophene and thiophene) was also studied under optimized conditions. In both cases, excellent conversion and selectivity were obtained in relatively short reaction times. This novel catalyst showed the characteristic of homogeneous reaction at ambient temperatures and phase-separation at lower temperatures. By simple decantation, the catalyst could be easily separated from the products upon cooling of the reaction mixture and could be reused several times with high recycling efficiency.