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. 2015 Nov 10;10(11):e0142452.
doi: 10.1371/journal.pone.0142452. eCollection 2015.

Probing the Carbonyl Functionality of a Petroleum Resin and Asphaltene Through Oximation and Schiff Base Formation in Conjunction With N-15 NMR

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Probing the Carbonyl Functionality of a Petroleum Resin and Asphaltene Through Oximation and Schiff Base Formation in Conjunction With N-15 NMR

Kevin A Thorn et al. PLoS One. .
Free PMC article

Abstract

Despite recent advances in spectroscopic techniques, there is uncertainty regarding the nature of the carbonyl groups in the asphaltene and resin fractions of crude oil, information necessary for an understanding of the physical properties and environmental fate of these materials. Carbonyl and hydroxyl group functionalities are not observed in natural abundance 13C nuclear magnetic resonance (NMR) spectra of asphaltenes and resins and therefore require spin labeling techniques for detection. In this study, the carbonyl functionalities of the resin and asphaltene fractions from a light aliphatic crude oil that is the source of groundwater contamination at the long term USGS study site near Bemidji, Minnesota, have been examined through reaction with 15N-labeled hydroxylamine and aniline in conjunction with analysis by solid and liquid state 15N NMR. Ketone groups were revealed through 15N NMR detection of their oxime and Schiff base derivatives, and esters through their hydroxamic acid derivatives. Anilinohydroquinone adducts provided evidence for quinones. Some possible configurations of the ketone groups in the resin and asphaltene fractions can be inferred from a consideration of the likely reactions that lead to heterocyclic condensation products with aniline and to the Beckmann reaction products from the initially formed oximes. These include aromatic ketones and ketones adjacent to quaternary carbon centers, β-hydroxyketones, β-diketones, and β-ketoesters. In a solid state cross polarization/magic angle spinning (CP/MAS) 15N NMR spectrum recorded on the underivatized asphaltene as a control, carbazole and pyrrole-like nitrogens were the major naturally abundant nitrogens detected.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Quantitative liquid-state C-13 NMR spectra of asphaltene before and after PTC-methylation with 13CH3I.
LB = line broadening. For the underivatized asphaltene, carbon aromaticity(fa) = 0.63.
Fig 2
Fig 2. Quantitative liquid-state C-13 NMR spectra of resin before and after PTC-methylation with 13CH3I.
LB = line broadening. For the underivatized resin, carbon aromaticity(fa) = 0.33.
Fig 3
Fig 3. Liquid-state DEPTGL C-13 NMR methyl subspectra of asphaltene before and after PTC-methylation with 13CH3I.
LB = line broadening.
Fig 4
Fig 4. Liquid-state DEPTGL C-13 NMR spectra showing all protonated carbons of asphaltene before and after 13C-diazomethylation.
LB = line broadening.
Fig 5
Fig 5. Liquid-state DEPTGL C-13 NMR methyl subspectra of resin before and after PTC-methylation with 13CH3I and 13C-diazomethylation.
LB = line broadening.
Fig 6
Fig 6. Carbon-13 NMR chemical shift ranges and examples of O-CH3, N-CH3, C-CH3 and S-CH3 methyl carbons resulting from methylation of acidic oxygen, carbon, nitrogen and sulfur groups.
Fig 7
Fig 7. Solid-state CP/MAS N-15 NMR spectrum of naturally abundant asphaltene nitrogen.
LB = line broadening. ct = contact time. Spinning speed = 5 kHz.
Fig 8
Fig 8. Solid-state CP/MAS N-15 NMR spectra of asphaltene and resin derivatized with 15N-labeled hydroxylamine.
LB = line broadening. ct = contact time. Spinning speed = 5 kHz for resin and 6 kHz for asphaltene. Asterisks denote spinning sidebands.
Fig 9
Fig 9. Reactions of carbonyl groups with hydroxylamine.
Fig 10
Fig 10. Nitrogen-15 NMR chemical shifts in ppm for oximes and Beckmann reaction products.
From reference {Thorn, 1992 #523}. Separate resonances are observed for the Z and E isomers of ketoximes. In general, the E isomers of ketoximes are deshielded with respect to the Z isomers.
Fig 11
Fig 11. Solid-state CP/MAS N-15 NMR spectrum of asphaltene derivatized with 15N-labeled aniline.
LB = line broadening. ct = contact time. Spinning speed = 6 kHz.
Fig 12
Fig 12. Reactions of carbonyl groups with aniline.
Fig 13
Fig 13. Nitrogen-15 NMR chemical shifts in ppm for Schiff Bases and condensation products of aniline with carbonyl compounds.
From reference {Thorn, 1996 #277}. (a) Determined in CDCl3, referenced to neat aniline as 55.7 ppm. (b) Determined in CD3OD, referenced to neat formamide as 112.4 ppm. (b) Determined in DMSO-d6, referenced to neat formamide as 112.4 ppm.
Fig 14
Fig 14. Liquid-state DEPT N-15 NMR spectra of asphaltene and resin derivatized with 15N-labeled aniline.
LB = line broadening.

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Grant support

This work was funded by the United States Geological Survey Toxics Substances Hydrology Program.
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