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

Abstract

The Electron Paramagnetic Resonance (EPR) technique is used to evaluate the temperature dependence of the aggregation processes in asphaltenes of heavy and extra heavy crude oils and its vacuum residues from Venezuelan Oil Belt. All the EPR spectra of the samples studied show an intense central signal assigned to Free radicals (FR) and a multiplet due to the presence of a V +4 porphyrin. From the analyses of the inverse of the normalized area as a function of the temperature for the FR signal and the most intense peak in the vanadium

Quantifying Crude Oil Contamination in Sand and Soil by EPR Spectroscopy

“…These particular features provide an opportunity to identify crude oil within a background of natural soil or sand. While by no means all types of oil contain significant amounts of free radicals, most heavy oils have indeed been found to show an EPR signal [34]; see also [35][36][37][38][39] for more recent studies on oil composition. It is commonly assumed that radicals are located in the large polycyclic aromatic hydrocarbons (PAHs), predominantly in asphaltenes, but possibly also in resins [40].…”

Electron Paramagnetic Resonance Study of the Fractions and Trapped Compounds in Asphaltenes of Merey Heavy Crude Oils and Its Vacuum Residue

“…A typical spectrum for crude oils 22−24 is shown, in which a very intense and symmetrical signal around g = 2.0052 ± 0.0005, attributed to the average of all contributions of the different stable carbon-centered "free radicals" (FR) in the sample, is observed, and a less intense set of signals corresponding to V +4 in accord with a nuclear spin I = 7/2 for 51 V (in the inset of Figure 5, an extension of this line octet is shown). 7 Temperature dependence of the inverse of the normalized area of the EPR spectrum (1/A N ) for FRs in asphaltenes from Merey heavy crude oil and its fractions is shown in Figure 6. Linear behavior is observed for the 1/A N vs T curve for the FRs in A 1 -ASCM asphaltenes (Figure 6a), in the temperature range 90 ≤ T ≤ 450 K, in accord with the Curie−Weiss law for paramagnetic compounds:…”

“…The meaning of T v1 , T c2 , and cracking temperatures were described earlier. 7 The increase in the FR concentration occurs up to a temperature named valley temperature 1 (T v1 ), where the recombination of FRs starts to lower its concentration. This phenomenon takes place until reaching a cusp temperature named T c2 , in which mass losses or structural changes may occur and then a cracking temperature appears.…”

“…In this way, the free radical will remain "caged" and preserved against chemical reactions. 2 Although crude oils studied by us 7 have been subjected to thermal and distillation treatments, free radicals persist; it could be interesting to fractionate the samples obtained from the distillation processes and their vacuum residues to evaluate the behavior of the free radicals in these terms. It is desirable to determine if fractionation can expose free radicals from asphaltenes samples, or if on the contrary, it is necessary to apply more aggressive processes to be able to strip them.…”

“…This information could shed light on the role played by free radicals in the aggregation processes of asphaltenes. In our electron paramagnetic resonance (EPR) study on asphaltenes of heavy and extra-heavy crude oils, 7 the Merey heavy crude oil asphaltenes, labeled as ASCM, and their vacuum residue asphaltenes, labeled as ARVM, who have the highest values of API gravity (16 °API and 3.3 °API, respectively) of all samples studied, showed the highest spin density concentration (ASCM highest between crude oils asphaltenes, and ARVM lowest between vacuum residues asphaltenes and highest than ASCM). However, one would expect the free radical content in the asphaltenes to be maintained even when the vacuum residues to crude oils ratio changes, maintaining similar tendencies in all the samples, but this has not happened, on the contrary, the free radical content of asphaltenes from Merey vacuum residue was considerably smaller than the rest of vacuum residue samples; also, the temperature dependence of the EPR line intensity of free radicals from ASCM showed the highest valley (Tv 1 ) and cusp (Tc 2 ) temperatures in comparison with the rest of samples.…”