Quantification of the Free Radical Content of Oilsands Bitumen Fractions

Electron spin resonance (ESR) spectroscopy was employed to perform quantitative analysis of the free radical content of oilsands bitumen, asphaltenes, deasphalted oil, vacuum residue, and vacuum gas oil fractions, as well as thermally converted product fractions. Calibration standards for ESR were compared, and 2,2-diphenyl-1-picrylhydrazyl was selected. The heaviest fractions, including asphaltenes, had free radical concentrations in the range 10¹⁷-10¹⁸ spins/g, whereas lighter fractions such as the lighter gas oil fractions had free radical concentrations in the range 10¹⁶-10¹⁷ spins/g. It was found that the bulk liquid properties affected the measured free radical concentration even after compensating for effects that could affect the spectroscopy. These differences were not analytical artifacts and could be explained with reference to the literature in terms of the "equilibrium" composition resulting from dimerization and decomposition of free radical pairs. Reported free radical concentrations must consequently be interpreted by considering the nature of the bulk liquid that was analyzed. Practically, the results have implications for thermal conversion of bitumen. It appears that the free radical concentration and availability of reactive free radicals can be independently manipulated through temperature and the bulk liquid properties.