Chemical speciation of Sulphur in petroleum asphaltenes using XANES spectroscopies

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

Campinas

Otro; Annual Users Meeting LNLS/CNPEM; 2017

LNLS/CNPEM

Asphalthenes are complexmixture of heteroatom rich aromatic hydrocarbons that are present as a solidsuspension in crude oils. The asphalthenes are the fraction of oil insoluble inheptane (or pentane) but soluble in toluene. All asphaltene chemical proprieties,excepting elemental composition, had been the subject of debate, specially theirmolecular weight and structure. The most probable composition of asphaltenesare a complex mixtures of polycyclic aromatic hydrocarbons groups (PAHs) ofseven fused aromatic rings (7FAR) with a width of three to ten fused aromaticrings (3FAR-10FAR) with substituted alkyl chains with heteroatoms functionalgroups. Sulphur is the most abundant heteroatom in the oil, present in theheavier fractions: asphaltenes (about 0.5 to 7 wt%,) and resins. Thus, theknowledge of the sulphur chemistry in petroleum helps in the mitigation ofsulphur induced problems in resource utilization. Sulfur speciation variesgreatly in asphaltenes from different locations representing different sourceand reservoir rocks. Besides sulfur is a good marker of the alteration of oilsbecause it is very sensitive to oxidation, and exhibits a range of oxidationstates from -2 to +6. [1, 2] The XANES spectroscopy is an appropriatedtechnique for the speciation of chemical species and provide qualitative andquantitative information about the sulphur chemistry in asphaltenes. [3, 4]Three samples of asphalteneswere extracted from different Argentinian crude oils by precipitation with n-pentane,n-hexane and n-heptane, using a solvent to the bitumen ratio of 40:1volume/weight. The precipitated asphaltenes were separated from malthenes by filtrationand then the resins coprecipitated were removed with a Soxhlet extraction with therespective n-alkane. The XANES measurements were performed in the SXS beamline atLNLS. The commercial standard models used for the experiment were sodiumsulfide, pyrite, dibenzyl sulfide, dibenzyl disulfide, diphenyl disulfide,dibenzothiophene, dibenzothiophene sulfona, thianthrene, tetraphenylthiophene,dimethyl sulfone, diphenyl sulfoxide, diphenyl sulfone, and potassium sulfate. Thespectra of asphalthenes can be describe in terms of three different sulfurstructures as sulfide, thiophene and sulfate. It has been possible to observe amajor contribution of sulfide structure in asphalthenes coprecipitated withresins compare to the spectra of asphantenes. It is well know that theprecipitation in n-heptano is selective for asphalthenes of highestmolecular weight, and in this case, the XANES spectra show a different shapeaccording to the solvent. An increase in the contribution of more oxidated sulphur form as thiophenicandsulfone group were observed. [1] Mullins, O. C.; Sheu,E. Y.; Hammami, A.; Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics; 2007.[2] Mullins, O. C. Annu.Rev. Anal. Chem. 2011, 4, 393?418.[3] Pomerantz, A. E.; Bake,K. D.; Craddock, P. R.; Kurzenhauser, K. W.; Kodalen, B. G.; Mitra-kirtley, S.;Bolin, T. B. Org. Geochem. 2014, 68, 5?12.[4] George, G. N.; Gorbaty,M. L. J. Am. Chem. Soc 1989, 111, 3182?3186. Acknowledgements:This work has beensupported by LNLS under Proposals SXS-20160690. The authors thank Facultadde Ciencias Exactas, Universidad Nacional de La Plata, CONICET.