Asphaltene radicals and their interaction with molecular oxygen: an EPR probe of their molecular characteristics and tendency to aggregate

Abstract

The paramagnetic species nature of different geological o¡ asphaltenes are discussed on the basis of Electron Paramagnetic Resonance (EPR) results. Free organic radicals are present in petroleum asphaltenes but their molecular nature is poorly known owing to the multiplicity of their molecular structures which causes the appearance of a single unresolved EPR signal with a linewidth of 4-6 Gauss. In spite of the poorly resolved signals the microwave power dependence of EPR line intensities gives some insights into the nature of the aromatic rings condensation. More aromatic asphaltenes show higher saturation power of the EPR lines: this fact has been explained by a spin exchange mechanism between n-n electronic clouds of adjacent molecules, supported also by some XRD evidences. Moreover, asphaltenes in argon (not paramagnetic gas) show a maximum intensity at much lower microwave power than in oxygen atmosphere (paramagnetic gas); besides, the differences in the saturation behavior for the different asphaltenes are much more evident in oxygen atmosphere. This phenomenon is suggested to be a consequence of a weak spatial complex between aromatic moieties and oxygen molecules that deeply contribute to the relaxation pathway. Synchronous fluorescence spectroscopy confirms the difference in the size of aromatic cores in asphaltenes molecules as supposed on the basis of the saturation mechanism of EPR line intensities.

In Situ Electron Spin Resonance Study of Molecular Dynamics of Asphaltenes at Elevated Temperature and Pressure

“…The temperature was controlled with two thermocouples located in the upper and lower parts of the sample. This single line is traditionally associated with the absorption of carbon π-systems in asphaltenes [43]. Figure 1 shows the ESR spectrum of asphaltenes extracted from the heavy oil ( Figure 1a) recorded at room temperature.…”

Temperature Dependence of the Electron Paramagnetic Resonance Spectrum of Asphaltenes from Venezuelan Crude Oils and Their Vacuum Residues

“…Also, asphaltenes structure is characterized by the presence of aliphatic chains with different lengths, free radicals (FR), and vanadyl ions. In spite of advances in analytical techniques, Electron Paramagnetic Resonance (EPR) is still applied in studies of paramagnetic properties of crude oils and their fractions, mainly asphaltenes [8,9], vacuum residues [10,11,12,13], as well as, in determining the nature of the metal complexes in this type of compounds. The majority of metals in oil have been located in the polar fractions (resins and asphaltenes), most concentrated in the asphaltenes though a part of vanadium (IV) complexes, which is not trapped there but remains in the crude oils bulk fraction [14].…”

Molecular Structure and Intermolecular Interaction of Asphaltenes by FT-IR, NMR, EPR

“…It is therefore particularly important to verify the consistency of the experimental data in order to understand if a parameter is really meaningful. So the same information has been obtained by independent techniques when possible and the results compared, A two-steps approach is followed: 103,104 I, The molecular features which differentiate the asphaltenes from the seven crude oils are obtained mainly by IR and NMR techniques. The internal consistency and the interdependence of the parameters are shown also comparing them with results obtained by other techniques (elemental analysis, fluorescence), Finally a relation between the molecular characteristics and the structural behaviours investigated by powder X-Ray Diffraction (XRD) was found.…”