Desulfurization of liquid fuels using air‐assisted performic acid oxidation and emulsion catalyst

CATEGORY: DESULFURIZATION
Chinese Journal of Catalysis 34 (2013) 1839–1847
Desulfurization of liquid fuels using air‐assisted performic acid oxidation and emulsion catalyst
Ahmad Imtiaz, Ahmad Waqas *, Ishaq Muhammad
Institute of Chemical Sciences, University of Peshawar, 25120, Khyber Pukhtunkhwa, Pakistan
Abstract
Catalytic oxidative desulfurization (ODS) of model oil and commercial oil samples was investigated using an air‐assisted performic acid oxidation system with a phase transfer or emulsion catalyst comprising a quaternary ammonium salt‐based heteropolyoxometalate. Different emulsion catalysts with a Keggin type heteroployoxometalate anion (containing W, Mo, and V) and cetyltrimethylammonium bromide cation were prepared and characterized by X‐ray fluorescence, Fourier transform infrared spectroscopy, and scanning electron microscopy. [C16H33N(CH3)3]3[PW9Mo3O40] was the most effective catalyst in the current oxidation system, which reduced the sulfur content of the model oil from 1275 μg/g to 57 μg/g. The reactivity order of different model sulfur compounds was thiophene < dibenzothiophene < 4,6‐dimethyldibenzothiophene. The ODS of model sulfur compounds followed first order kinetics with apparent activation energy from 29 to 27 kJ/mol. The catalysts also performed efficiently in the ODS of the industrial oil samples, including untreated naphtha, light gas oil, heavy gas oil, and Athabasca oil sands derived bitumen, for which sulfur removal rates were 83%, 85%, 68% and 64%, respectively.
A new approach to phase transfer catalysis uses surfactant‐based or amphiphilic catalysts, which combine together a heteropolyoxometalate anion and a quaternarnary ammonium cation. These have shown remarkable selectivity and activity for the oxidation of organosulfur compounds in the presence of H2O2. Emulsion catalysts like [(C18H37)2N(CH3)2]3 [PW12O40]  and [C18H37N(CH3)3]4 [H2NaPW10O36]  have been found to reduce the sulfur contents of diesel oil from more than 500 μg/g to less than 30 μg/g under mild conditions, and they can be recycled with 100% selectivity. Tetrabutylammonium salts of [V(VW11)O40]4– were found to decrease the sulfur contents of gas oil by up to 90% in the presence of H2O2. A number of studies revealed that hexadecyl tri‐ and dimethyl ammonium cations, i.e., (C18H37)2N+(CH3)2 and (C18H37)N+‐ (CH3)3, are good quaternary ammonium cations for emulsion catalysts [13,16–20]. The reactivity order of different sulfur compounds in the emulsion system was found to be benzo[ b]thiophene < 5‐methylbenzothiophene < dibenzothiophene < 4,6‐dimethyldibenzothiophene. The order is the same as that found in the H2O2/formic acid system.
In the present work, ODS of commercial oil fractions with high viscosity and high sulfur contents were investigated using the dual oxidation system of air‐assisted performic acid and a quaternary ammonium‐based heteropolyoxometalate as a phase transfer catalyst. The extraction of oxidized sulfur compounds was studied using an economical extraction system. The oxidation reactivity of different sulfur compounds was also investigated with model oils.
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