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 10 17 −10 18 spins/g, whereas lighter fractions such as the lighter gas oil fractions had free radical concentrations in the range 10 16 −10 17 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.
It was speculated that hydrogen transfer was not the only type of transfer reaction that could take place during thermal conversion, and that methyl transfer could also take place. There was a large body of literature on intramolecular methyl migration, but little mention of intermolecular methyl transfer as a potentially important reaction type in free radical systems. Methyl transfer during thermal conversion at 400 °C was investigated using indene, 2-methylindene and α-methylstyrene. It was demonstrated that methyl transfer took place. Reaction products that could be explained only in terms of intermolecular methyl transfer were identified, including products that were formed by transfer of two methyl groups. Thermal conversion that considered both intermolecular methyl transfer and hydrogen transfer highlighted potential implications for industrial practice. Products from methyl transfer were more reactive for thermally induced free radical initiation and hydrogen/methyl transfer reactions. Methyl transfer during molecule-induced homolysis provided a plausible pathway for release of methane that does not require a reaction involving hydrogen and methyl radicals. This work not only showed the importance of intermolecular methyl transfer in free radical systems, but also indicated that methyl transfer as opposed to hydrogen transfer affected the composition and bulk properties of the reaction product.
The impact of a solvent environment on persistent free radical concentrations at ambient conditions was studied by electron spin resonance spectrometry. The analyte selected was Canadian oil-sand-derived bitumen due to its high persistent free radical content. The ability of 54 different solvents to produce a homogeneous 5 wt % solution of bitumen was evaluated. The influence of solvents on the free radical content in bitumen was determined exclusively for solvents that were capable of quantitatively dissolving the bitumen. These were compounds in the classes of alkynes, mono-and bicyclic benzene-derivatives, and heteroatom-containing compounds containing nitrogen, oxygen, sulfur, and chlorine. It was found that a shift in the g-factor of bitumen occurred when the solvent was changed. The shift was attributed to the radical−solvent interaction that is affected by the polarity of the solvent and reflected in the solvent dipole moment property. The change in the free radical concentration was independent of changes in g-factor and was not correlated with any of the following solvent properties: molecular weight, dipole moment, dielectric constant, refractive index, density, and viscosity. There was a relationship between the free radical concentration in bitumen and the ionization potential of sulfur-containing and diaromatic hydrocarbon solvents. It was concluded that the bulk liquid properties that affected the electronic environment of the free radical species, resulting in a shift in g-factor, were not related to the bulk liquid properties that affected the dissociation equilibrium and resulted in a change in the free radical concentration.
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