Electron Paramagnetic Resonance Study of Rotational Mobility of Vanadyl Porphyrin Complexes in Crude Oil Asphaltenes: Probing the Effect of Thermal Treatment of Heavy Oils

Abstract

The structural properties of crude oils and asphaltenes, especially related to the dynamics of their aggregation, have been investigated by different experimental and theoretical methods during the last few decades. However, there are only a few works devoted to study the dynamics of asphaltenes in a native hydrocarbon environment. In this report, we illustrate a possibility to use electron paramagnetic resonance (EPR) spectra of vanadyl porphyrins in asphaltenes for the qualitative and quantitative analysis of their rotational mobility in the crude oil samples. On the basis of the simulation of the EPR spectra, a simple semi-empirical parameter sensitive to the transition between motional regimes is proposed. This mobility parameter can be potentially useful for the prediction and analysis of the thermal influence on heavy oil reservoirs during hydrocarbon production. It is found that the rotational correlation time of the complexes in heavy oil samples changes discontinuously with the temperature. The observed jump could be attributed to a disaggregation of supramolecular complexes of asphaltenes in the close vicinity of a phase transition.

In Situ Identification of Various Structural Features of Vanadyl Porphyrins in Crude Oil by High-Field (3.4 T) Electron–Nuclear Double Resonance Spectroscopy Combined with Density Functional Theory Calculations

“…The obtained experimental and calculated data are in a good correspondence with those for different vanadyls model systems [24,28,34]. The second signal with g-factor of 2.004 is attributed to stable organic "free" radical(s) [15,35]. The EPR spectrum for sample #2 is very similar to the described above and is not shown.…”

Probing the surface of synthetic opals with the vanadyl containing crude oil by using EPR and ENDOR techniques

“…In our notations the values of (g, A)|| correspond to the orientation perpendicular to VO 2+ plane (out of plane), along the direction c and (g, A) correspond to the orientation in the VO 2+ plane, ab plane. Comprehensive description of VO spectra in X and W-bands, their spectra simulations and spectra changes with external treatment are given in papers [26,37,38].…”

Asphaltenes: Separations, structural analysis and applications

“…Compared with other metals in asphaltenes, V and Ni are the richest and extensively researched. Usually, the metal ions are chelated with organic groups, and some are located at the defect centers of an imperfect aromatic sheet as porphyrins [30][31][32] , and these possible structures were always hypothesis [32,33] . Meanwhile, amount of experiments confirmed metal compound to be nonporphyrin structure, existing as free molecule, or captured one in asphaltene nanostructure, or a component of asphaltene [34] .…”

“…There has been an inspiring research in paramagnetic centers of petroleum derivatives for last five years by EPR and ESR, especially for vanadyl porphyrin (VO 2 + ). The high sensitivity of EPR and ESR could detect low concentrations of vanadium species in asphaltenes, provide in situ information of sizes of vanadium molecules and their aggregates, distinguish different types of environments of vanadium species, and monitor environmental dynamics [31,[149][150][151] . The mechanisms of hydrodemetallization were also investigated using EPR [149,152] .…”