Sodium Conversion of Oilsands Bitumen-Derived Asphaltenes

Abstract

Alkali metals are strong electron donors and can form electron donor–acceptor ion pairs with multinuclear aromatics. Single electron donation converts the aromatic into a radial ion, and two electron donations convert the aromatic into a dianion. The anionic aromatic species are more susceptible to reductive addition reactions, such as hydrogen transfer from more hydrogen-rich molecules, such as employed in Birch reduction. The conversion of heavy oils in the presence of alkali metals is claimed to be capable of bulk desulfurization with little hydrogen consumption. In this work, the conversion of oilsands bitumen asphaltenes with sodium was investigated over the temperature range of 60–250 °C under an inert atmosphere to limit the contribution of thermal conversion. Control experiments conducted with asphaltenes without sodium revealed that the asphaltenes were reactive on their own, and the nature of products was affected by the phase behavior of the asphaltenes that gradually changed from a solid to liquid between 124 and 142 °C. When fluidity was limited, intermolecular hydrogen transfer was restricted; intramolecular hydrogen disproportionation and “in cage” intermolecular reactions resulted in the formation of more condensed and potentially more aromatic products. Sodium markedly affected the natural hydrogen transfer and disproportionation. Conversion of the asphaltenes with sodium did not result in an increased maltene yield, but the maltenes had a higher hydrogen/carbon ratio, less sulfur, and less nitrogen. Model compound reactions were employed to study the influence of the reaction atmosphere (N2 or H2) and hydrogen donor properties of the organic matrix. In the presence of H2, the formation and reaction of NaH appeared to influence selectivity and the reaction network. In the absence of H2, desulfurization of thiophenic compounds by sodium proceeded by hydrogenolysis and hydrogenation pathways, with the hydrogenolysis pathway being favored by the lack of hydrogen donor molecules. It was also found that hydrogenolysis of the carbon–carbon bond in the thiophenic ring of dibenzothiophene is reversible.