Asphaltenes characterization by nuclear magnetic resonance spectroscopy

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Ratna, Yeasmin
University of Lethbridge. Faculty of Arts and Science
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Lethbridge, Alta. : University of Lethbridge, Dept. of Chemistry and Biochemistry
Asphaltenes from various sources and processed in different ways were studied by nuclear magnetic resonance (NMR) relaxation and diffusion ordered spectroscopy (DOSY) methods. The study investigating samples obtained from various depths of an oil well from a Saudi Arabian deposit found those samples to exhibit increasing aggregate sizes with depths, with the molecular structure remaining essentially unchanged. DOSY experiments reveal three distinct asphaltene aggregation states in different solvents, with increasing particle sizes at greater depths. However, hardware limitations were observed in resolving particle sizes beyond 4 nm. Asphaltenes from in-situ pyrolyzed American oil shale were characterized with respect to maturity, showing that aggregate size increases with maturity. As part of an international effort to characterize one particular crude and sonicated sample from Colombia (PetroPhase 2017), this study found that sonication reduces the aggregate size. Three fluorinated probes were tested to study a Kuwaiti asphaltene sample by 19F NMR spectroscopy and the results were compared to those obtained using the 1H signals of the sample; perfluorooctanoic acid (PFOA) was found to be an effective probe for asphaltene aggregation, mirroring the results of the minor component of the 1H relaxation dispersion analysis. In relaxation dispersion measurements of all four asphaltene types, two mobility regimes were observed with percentage contributions varying between asphaltene origins but being unchanged with respect to processing and sample depth. The behaviour of the contributions of the two mobility regimes in all samples cannot be explained by attributing them to different aggregation states, but a core-shell model is proposed with the more mobile component being on the outside ‘shell’ and the more rigid component constituting the core of a nanoparticle.
Asphaltenes , NMR spectroscopy , Nuclear magnetic resonance , Diffusion ordered spectroscopy , Aggregation behavior