Comparison of Coal-Derived and Petroleum Asphaltenes by ¹³C Nuclear Magnetic Resonance, DEPT, and XRS

Abstract

The molecular architecture of asphaltenes is still a matter of debate. Some literature reports provide evidence that the contrast of petroleum asphaltenes versus coal-derived asphaltenes is useful for understanding the governing principles of asphaltene identity. Coal-derived asphaltenes provide an excellent test for understanding the relationship of asphaltene molecular architecture with asphaltene properties. Diffusion measurements have shown that coal-derived asphaltenes are half the size of many crude oil asphaltenes, but there are relatively few studies comparing coal-derived and petroleum asphaltenes using liquid state 13C NMR. 13C NMR confirms that the molecular sizes of these coal-derived asphaltenes are smaller than virgin petroleum asphaltenes. DEPT-45 experiments were performed in order to determine the relative amount of nonprotonated and protonated carbon in the aromatic region of the spectrum. In contrast to previous NMR work on asphaltenes that ignored interior bridgehead carbon, we show this is an important component of asphaltenes and that correctly accounting for this carbon enables proper determination of the number of fused rings. XRS data supports interpreting the NMR data with a model that weighs circularly condensed structures more heavily than linearly condensed structures. Significantly more carbon exists in chains at least 9 carbons long in petroleum asphaltenes (≥7%) compared to coal-derived asphaltenes (≥1%).

Advances in Asphaltene Science and the Yen–Mullins Model

“…yielding ∼7 PAHs in virgin petroleum asphaltenes, with ∼6 PAHs in coal-derived asphaltenes. 59 This NMR study provided direct interrogation of aromatic carbon bonded to hydrogen as opposed to assigning a spectra cutoff. The corresponding method is distortionless enhancement by polarization transfer (DEPT) NMR.…”

“…Thus, coal-derived asphaltenes are much lower in molecular weight and also have smaller PAHs than petroleum asphaltenes. 29,50,59 Petroleum resid asphaltene with its reduced alkane content also exhibits reduced PAH size. 63 Initial asphaltene molecules that are subject to alkane removal by cracking in distillation become less soluble forming coke.…”

“…29,59 ■ NANOAGGREGATES AND CLUSTERS Many of the techniques that are sensitive to the colloidal properties of asphaltenes provide information on both nanoaggregates and clusters. Relevant issues for these nanocolloidal particles are the concentration of formation and aggregation number (or size).…”

Elucidation of structural information achievable for asphaltenes via collision-activated dissociation of their molecular ions in MSn experiments: A model compound study

“…They comprise 63 the fraction of petroleum that is soluble in toluene but insoluble 64 in n-alkane [4] and can correspond to up to 20% of crude oil [5]. 65 The elucidation of the structures of the molecules in asphaltenes 66 has far reaching implications [3] since asphaltenes clog pipes used 67 for transportation of crude oil and poison catalysts used in refining 68 [1,[6][7][8][9][10][11]. Further, only few uses exist for asphaltenes, including 69 expensive upgrading to distillates [12], road construction [13] 70 and recently proposed use in organic electronic materials [14,15] [9] to be especially well suited for evaporation and ionization of 114 asphaltenes since it has no bias to more polar analytes and it pro-115 duces only one ion type, molecular ion (M +Å ), for all asphaltene 116 model compounds studied [9].…”

The dynamic Flory-Huggins-Zuo equation of state

“…In conceptually simple, robust, surfacetension studies, the PAH (polycyclic aromatic hydrocarbon) size for several asphaltene types has been confirmed [33,34]. NMR studies have also confirmed the asphaltene PAH sizes, [35,36] given the asphaltene orientation at the interface [37]. Alicyclic rings fused to aromatic rings have been confirmed by heteronuclear single quantum coherence NMR studies [36].…”