Fine-Tuning Simulation of the ESR Spectrum─Sensitive Tool to Identify the Local Environment of Asphaltenes In Situ

Abstract

An original method to study the local environment of vanadyl-containing components of heavy oils based on finetuning simulation of their electron spin resonance (ESR) spectra in situ is developed. The approach is backgrounded on precise computation of anisotropic ESR spectra of individual porphyrin complexes in specific solvents modeling a particular local environment of certain polarity using specific g and A tensor components, their dispersion, and linewidth determined by unresolved super-hyperfine interaction with the closest nitrogen. The high level of simulation accuracy allowed us to differentiate the values of the g and A tensor components related to individual vanadyl complexes in different solvents with certain polarity in reference systems and attribute the observed variation to the interaction of the vanadyl complex with surrounding molecules located in its axial direction. The method developed for the model systems is successfully applied to the real oil-based systems where different vanadyl complexes with different local environments are presented, forming the experimentally observed ESR spectrum. The Monte Carlo procedure was suggested to define the relative contribution (weight) of the individual spectra having tiny peculiarities related to vanadyl fragments in different local environments and finally mapping the g and A tensor components distribution which is characteristic for the particular oil-based sample. Extraordinary sensitivity to the localization and interaction with surrounding molecules of vanadyl-containing complexes which are present in heavy oil as a natural spin probe or can be specially introduced give us a unique opportunity to in situ study the local environment of asphaltenes and their intermolecular interactions with other oil components.

Resin–Paraffin Aggregation Processes in Waxy Oil Revealed by Vanadyl Porphyrin Spin Probe

“…The experimental data for individual solvents clearly demonstrate that the axial structure of the VOOEP spin probe makes it possible to follow the polarity changes in the axial environment of vanadyl due to weak polar bonds formed at low temperatures in polar solvents. 49,65 S3. Similar to polar solvents, in the V-oil-W sample, an increase in the A iso parameter by 9.9 MHz is observed when the temperature changes from 130 to 400 K, which indicates the coordination of most of the VOOEP molecules to the polar fragments of the oil components when the solution is frozen.…”

“…Currently, a number of methods are being developed that allow obtaining information about the structure, dynamics, and local environment of heavy components in oils at the atomic and molecular level, among which spectroscopic (FTIR spectroscopy, ,, small-angle scattering (SAS), − and dynamic light scattering (DLS)) , and resonance (NMR and electron spin resonance (ESR)) ,− methods can be distinguished. Each of these methods has advantages and limitations.…”

“…We have shown that the simulation of ESR spectra of slowly rotating vanadium-containing molecules present in real oils as a natural (intrinsic) spin probe has opened up new possibilities for quantifying the size distribution of asphaltenes and their aggregates in crude oil. , Notably, due to the vanadyl complex stability, this approach can be applied in situ at elevated temperatures and pressures, which is important for understanding the asphaltene aggregation process and the conditions under which fouling occurs. Recently, we have shown that the precise modeling of experimental ESR spectra of vanadyl containing frozen solution allows one to obtain information about the polarity of the local environment of the vanadyl complex in real oils . Unfortunately, the presence of a natural vanadium-containing spin probe in oils, being an advantage in some cases, is clouded by the existence of many different porphyrin complexes in the system.…”

From Components to Phase-Dependent Dynamics of Diffusivity in Wax Solutions Subjected to Fluid–Solid Phase Transition: Insights from Pulsed Field Gradient NMR

“…Measurements of longitudinal relaxation times, T 1 , in model samples showed that the wax concentration has virtually no effect on T 1 . This is likely due to the lack of active paramagnetic centers in model wax solution such as the VO 2+ (vanadyl) fragment of asphaltenes 63 or highly dispersed rock particles, both abundantly presented in the natural oils. Nevertheless, the relatively large molecular size of long-chain paraffins favors the slow reorientation (tumbling) motion, which give rise to quite long T 1 values due to the modulation of correlation times, τ c , of the fluctuating local magnetic fields responsible for the dipole−dipole mechanism of NMR relaxation (according to the Bloembergen−Purcell−Pound theory, 64 the T 1 reaches the minimum at ω 0 τ c ∼ 1).…”

A Simulation Independent Analysis of Single- and Multi-Component cw ESR Spectra

“…The low-intensity signal components are difficult to analyze through standard spectral simulations in a variety of cases [17][18][19]. Accurate spectral analysis in such cases require quantum chemical computations or special techniques [20][21][22], but without sufficient expertise and knowledge in the field, the interpretations rely heavily on the researchers' experience and intuition than the robustness of the method. An even more challenging problem, which occurs frequently, is the presence of more than one structurally similar molecules in a system.…”