EPR as a complementary tool for the analysis of low-temperature oxidation reactions of crude oils

Integrative Investigation of Low-Temperature Oxidation Characteristics and Mechanisms of Heavy Crude Oil

“…Also, the most obvious increase in the free radical concentration appeared as the temperature was increased to 160 from 120 °C, which might be explained as follows. The formed free radicals experienced coupling reactions to form new molecules or tended to attach to the high quantity of π-expanded systems such as resins, asphaltenes, and aromatic fractions to attain higher stability. , The free radicals generated by these π-expanded systems had a longer lifetime than those generated by saturates . Therefore, the free radicals of oil samples oxidized at higher temperature had higher stability, as revealed by their lower content of saturates and higher amount of π-expanded systems relative to oil samples oxidized at lower temperature.…”

“…The microwave power was set to be extremely weak (several microwatts) to avert the saturation effect of signals of free radicals. EPR spectra were acquired under the next conditions: (1) The magnetic field sweep width was 150 G; (2) the center of the magnetic field sweep was set near g factor 2; (3) the recording time was 3 min; and (4) the amplitude of the second magnetic field modulation was 0.2 mT at a frequency of 100 kHz . Each test was repeated at least three times to guarantee the repeatability and accuracy of experimental data.…”

“…Significant parameters, including the free radical concentration, g factor, and line width, could be determined from EPR spectra. The free radical concentrations of oil samples served as a quantified reference to the standard profile of 1,1-diphenyl-2-picrylhydrazyl (DPPH, g = 2.0036) with a known radical concentration and mass. ,, …”

“…Nuclear magnetic resonance (NMR) has been employed for several years to evaluate the structures of complicated organic compounds such as crude oil, coal, and asphaltenes. − Electron paramagnetic resonance (EPR) can monitor the signals of free radicals, which has been widely used to investigate the pyrolysis of coal, oil shale, and biomass tar. − In our latest research, EPR and NMR were introduced to analyze the LTO reactions of crude oils . The results evidenced that the combination of EPR and NMR contributed to an understanding of LTO mechanisms.…”

High-Field (3.4 T) ENDOR Investigation of Asphaltenes in Native Oil and Vanadyl Complexes by Asphaltene Adsorption on Alumina Surface

“…It agrees with the previously revealed trend of inverse dependence of the concentration of FR on the content of vanadyl complexes (see papers [19,21,22] and references therein). Alternatively (or additionally), a formation/disappearing of new stable radicals, in which EPR spectra are indistinguishable from the initial ones, as a result, for example, of chemical bond opening/chemical bonding and oxidation processes upon adsorption [60,61], could be supposed.…”