Molecular Structure of Athabasca Asphaltene: Sulfide, Ether, and Ester Linkages

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Abstract

Athabasca n-C5-asphaltene was fractionated into occluded maltene, low and high molar mass (LMA, HMA) asphaltene, and the latter fractions were subjected to Ni2B reduction to cleave the sulfide C−S bonds, basic hydrolysis to cleave the ester C−O bonds, and BBr3 treatment to cleave the ether C−O bonds. Ni2B reduction of asphaltenes yielded 5−18% n-pentane solubles, which were separated into saturates, aromatics, and polars, and the saturates were analyzed for biomarkers. The residual asphaltene underwent 40% desulfurization and a greater than 4-fold drop in the MW of HMA but no change in the MW of LMA. The decrease in the MW is attributed to sulfide-bound core segments in the structure of the asphaltene: [core]-S-[core]-S-[core]-S-[core] + Ni2B → 4[core] + 3H2S. This is an important structural feature of Athabasca asphaltene and is responsible for its upgradability without excessive coke formation. The biomarkers of the asphaltene fractions were also characteristically different with regard to maturity status and composition. Both fractions yielded n-alkanes, cheilanthanes, regular steranes, hopanes, and gammacerane, and the LMA also contained dicyclic terpanes and C21−C25 steranes. Noteworthy was the absence of diasteranes, which are the only steranes in the maltene. In terms of the 20S/(S + R) steranes and 22S/(S + R) hopanes parameters the maturity varies as maltene > LMA > HMA. This difference is a manifestation of the thermocatalytic nature of the maturation process and the protection of the macromolecular nature of the asphaltene against contact with external reagents. Ni2B reduction indicates that (1) the n-alkane products arise from n-alkyl substituted thiolane/thiane and thiophene and (2) C27−C30 steranes are attached to the asphaltene by one S atom, and the C21−C25 steranes and terpanes by two S atoms. Basic and BBr3 hydrolyses of HMA showed that both ester and ether linkages of n-acids and n-alcohols are present and that the esters are of recent origin, whereas the ethers were derived from the original biotic source material of the bitumen.

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