Chemical speciation of Sulphur in petroleum asphaltenes using XANES spectroscopies

BAVA YANINA BELÉN; GERONÉS MARIANA; ANDRINI LEANDRO; GIOVANETTI LISANDRO; ERBEN MAURICIO

Campinas, San Pablo

Congreso; 27th RAU; 2017

Laboratorio Nacional de Luz Sincrotrón

Asphalthenes are complex mixture of heteroatom rich aromatic hydrocarbons that are present as a solid suspension in crude oils. The asphalthenes are the fraction of oil insoluble in heptane (or pentane) but soluble in toluene. All asphaltene chemical proprieties, excepting elemental composition, had been the subject of debate, specially their molecular weight and structure. The most probable composition of asphaltenes are a complex mixtures of polycyclic aromatic hydrocarbons groups (PAHs) of seven fused aromatic rings (7FAR) with a width of three to ten fused aromatic rings (3FAR-10FAR) with substituted alkyl chains with heteroatoms functional groups. Sulphur is the most abundant heteroatom in the oil, present in the heavier fractions: asphaltenes (about 0.5 to 7 wt%,) and resins. Thus, the knowledge of the sulphur chemistry in petroleum helps in the mitigation of sulphur induced problems in resource utilization. Sulfur speciation varies greatly in asphaltenes from different locations representing different source and reservoir rocks. Besides sulfur is a good marker of the alteration of oils because it is very sensitive to oxidation, and exhibits a range of oxidation states from -2 to +6. [1, 2] The XANES spectroscopy is an appropriated technique for the speciation of chemical species and provide qualitative and quantitative information about the sulphur chemistry in asphaltenes. [3, 4]Three samples of asphaltenes were extracted from different Argentinian crude oils by precipitation with n-pentane, n-hexane and n-heptane, using a solvent to the bitumen ratio of 40:1 volume/weight. The precipitated asphaltenes were separated from malthenes by filtration and then the resins coprecipitated were removed with a Soxhlet extraction with the respective n-alkane. The XANES measurements were performed in the SXS beamline at LNLS. The commercial standard models used for the experiment were sodium sulfide, pyrite, dibenzyl sulfide, dibenzyl disulfide, diphenyl disulfide, dibenzothiophene, dibenzothiophene sulfona, thianthrene, tetraphenylthiophene, dimethyl sulfone, diphenyl sulfoxide, diphenyl sulfone, and potassium sulfate. The spectra of asphalthenes can be describe in terms of three different sulfur structures as sulfide, thiophene and sulfate. It has been possible to observe a major contribution of sulfide structure in asphalthenes coprecipitated with resins compare to the spectra of asphantenes. It is well know that the precipitation in n-heptano is selective for asphalthenes of highest molecular weight, and in this case, the XANES spectra show a different shape according to the solvent. An increase in the contribution of more oxidated sulphur form as thiophenic and sulfone group were observed.