Molecular Structures of Asphaltenes Based on the Dissociation Reactions of Their Ions in Mass Spectrometry

Abstract

We report here an examination of the mass spectrometric fragmentation behavior of molecular ions generated (and excited) by electron ionization (EI) from several asphaltene model compounds simulating both the island and archipelago structural models. This behavior is compared to that of protonated molecules generated from the same compounds by atmospheric pressure chemical ionization (APCI) and excited by collision-activated dissociation (CAD). The fragmentation behavior of the protonated molecules and molecular ions is surprisingly similar. Both types of ions yielded distinct fragmentation patterns for both types of model compounds. Ions derived from the island-type model compounds fragment predominantly by losing their alkyl chains (with either all carbons or all but one), one after another, which allows for the identification of the chain lengths and counting the number of chains. Increasing the length of the alkyl chains reduces the extent of spontaneous fragmentation occurring upon EI, likely because of more efficient cooling of the fragmenting ions via emission of infrared (IR) light made possible by the reduced fragmentation rates of the longer chains. Ions derived from the archipelago model compounds with ethylene bridges connecting two or three aromatic cores (without alkyl side chains) readily undergo cleavages in these bridges. Increasing the length of the alkyl chain between the aromatic cores reduces the extent of fragmentation caused by EI. Similarly, the addition of long external alkyl chains to archipelago model compounds with an ethylene bridging two aromatic cores greatly hinders fragmentation upon EI. When these molecules are protonated and subjected to high-energy CAD, they appear to fragment almost randomly but, nevertheless, indicating some preference for cleavages of the bonds in the chain connecting the aromatic cores. A comparison of these findings to the fragmentation patterns observed for protonated asphaltenes indicates that the asphaltene molecules studied are likely composed of many isomeric and isobaric molecules. Each may contain several aromatic rings and a distribution of mostly aliphatic alkyl chains (and possibly naphthenic rings) ranging in size from 1 to at least 14 carbons, several containing methyl branching at the α carbons. The results do not allow for the unambiguous differentiation between island- and archipelago-type structures, although they are in a better agreement with the island model.

The dynamic Flory-Huggins-Zuo equation of state

“…Most importantly, L2MS studies have established the ability to disaggregate asphaltene nanoaggregates, [30] a known problem with some mass spectral techniques. Related mass spectral studies corroborate island molecular architecture [31,32]. In conceptually simple, robust, surfacetension studies, the PAH (polycyclic aromatic hydrocarbon) size for several asphaltene types has been confirmed [33,34].…”

Fractionation of Asphaltenes in Understanding Their Role in Petroleum Emulsion Stability and Fouling

“…According to Yen-Mullins model, asphaltene molecules are dominated by island architecture with most probable molecular weight of 750 Da, as supported by recent mass spectral analysis. [39][40][41] Approximately six asphaltene molecules form a nanoaggregate via -stacking of their PAH cores. The asphaltene nanoaggregates can further associate to form clusters with aggregation numbers of approximately eight.…”

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].…”

“…These methods 120 allow for the determination of the molecular weight and elemental 121 composition of all analytes present. However, structural informa-122 tion can only be obtained via tandem mass spectrometry experi-123 ments wherein the molecular ions are isolated and subjected to 124 collision-activated dissociation (CAD) or other reactions [8,28]. 125 Previous studies on CAD of protonated asphaltene model com- 126 pounds have revealed useful structural information [8].…”

“…However, structural informa-122 tion can only be obtained via tandem mass spectrometry experi-123 ments wherein the molecular ions are isolated and subjected to 124 collision-activated dissociation (CAD) or other reactions [8,28]. 125 Previous studies on CAD of protonated asphaltene model com- 126 pounds have revealed useful structural information [8]. In sharp 127 contrast, CAD of the molecular ions of asphaltene model com- 128 pounds has not been explored.…”