Advances in high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enable molecular-level characterization of ultracomplex asphaltene samples. Such analyses most often reveal compounds that are highly aromatic but alkyl-deficient in nature and, thus, support the classical “island” model of asphaltene architecture. However, recent works that combine chromatographic separations with mass spectrometry for the analysis of crude oils have shown that differences in ionization may greatly affect the analysis of complex mixtures (known as the matrix effect). Simply, compounds that ionize with greater efficiency are preferentially observed and mask the detection of poorly ionized compounds. Asphaltenes are not immune to this phenomenon. In the first of this series (10.1021/acs.energyfuels.7b02873), it was demonstrated that asphaltenes generated by different precipitants showed greatly varied monomer ion yields (ionization efficiencies). This work focuses on the development of an extrography fractionation method that selectively targets the removal of asphaltene species that exhibit high monomer ion yields and, thus, restrict mass spectral characterization of less efficiently ionized species. Silica gel was used as the stationary phase, and a unique solvent series separated asphaltenes based on their interaction with the silica surface, which was later determined to depend heavily upon the structure as well as monomer ion yield. The first two solvents (acetone and acetonitrile) isolated compounds that most efficiently produce monomeric asphaltene ions and, thus, cause bias in mass spectrometric analyses of whole asphaltenes. A solvent polarity gradient was then used, with n-heptane, toluene, tetrahydrofuran, and methanol, to separate remnant asphaltene compounds on the basis of polarity and structure. Our results demonstrate that mass spectrometry of whole asphaltenes does not reveal the complete molecular composition but rather preferentially exposes highly aromatic, alkyl-deficient, island-type structures. Early eluting fractions are shown to resemble the composition of the whole asphaltene and are enriched in island structures, whereas the analysis of later-eluting fractions reveals archipelago structural motifs as well as species with atypical asphaltene molecular compositions. We also demonstrate that, as molecular weight increases, the asphaltenes exhibit increased contributions of archipelago structural motifs. Higher mass ions (m/z > 550), even from asphaltene fractions enriched in island structures, exhibit fragmentation pathways that originate from archipelago structures. Thus, positive-ion atmospheric pressure photoionization (APPI) FT-ICR MS provides molecular-level data that suggest that the island model is not the dominant structure of asphaltenes. It coexists with abundant archipelago structures, and the ratios of each are sample-dependent.

For decades, discussion of asphaltene structure focused primarily on molecular weight. Now that it is widely accepted that asphaltene monomers are between ∼250 and 1200 g/mol, disagreement has turned to asphaltene architecture. The classic island model depicts asphaltenes as single core aromatic molecules with peripheral alkyl side chains, whereas the less widely accepted archipelago model, includes multiple aromatic cores that are alkyl-bridged with multiple polar functionalities. Here, we analyze asphaltene samples by positive-ion atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry and perform infrared multiphoton dissociation to identify their aromatic core structures to shed light on the abundance of island and archipelago structural motifs. Our results indicate that island and archipelago motifs coexist in petroleum asphaltenes, and unlike readily accessible island motifs, asphaltene purification is required to detect and characterize archipelago species by mass spectrometry. Moreover, we demonstrate that mass spectrometry analysis of asphaltenic samples is biased toward the preferential ionization/detection of island structural motifs and that this bias explains the overwhelming mass spectral support of the island model. We demonstrate that the asphaltene structure is a continuum of island and archipelago motifs and hypothesize that the dominant structure (island or archipelago) depends upon the asphaltene sample.

Asphaltene structure is one of the most controversial topics in petroleum chemistry. The controversy is centered on the organization of aromatic cores within asphaltene molecules (single aromatic core, island and multiple aromatic core, archipelago) and specifically the inconsistency between the island model and the composition of the products derived from asphaltene pyrolysis/thermal cracking. Such products are consistent with the coexistence of island and archipelago asphaltene structural motifs. However, the archipelago model continues to lack the widespread acceptance of the petroleum community, in part due to mass spectrometry results in support of the island model. In the first and second part of this series, we demonstrated that the disproportionally high atmospheric pressure photoionization (APPI) ionization efficiency (monomer ion yield) of island species is due to weak nanoaggregation of large aromatic cores which do not extensively aggregate in toluene, whereas more archipelago-dominant fractions were shown to have low monomer ion yield due to a greater propensity for aggregation. The discrepancy leads to bias toward the selective ionization of island compounds and thus the erroneous mass spectrometry support of the predominance of the island structural model. A separation method based on aggregation trends and therefore the efficiency of monomeric ion production is critical to access archipelago structures. In the work presented herein, we demonstrate that dominance of island or archipelago structural motif is sample dependent. We present the positive-ion APPI Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) characterization of asphaltenes and asphaltene extrography fractions derived from Wyoming Deposit (island dominant) and Athabasca Bitumen (archipelago dominant) C7 asphaltenes. Wyoming Deposit asphaltenes resemble the “classical” island-type asphaltene structure: they exhibit a high concentration of highly aromatic/alkyl-deficient species with a compositional space close to the polycyclic aromatic hydrocarbon (PAH) limit. Fragmentation results from infrared multiphoton dissociation (IRMPD) confirm that island is the dominant structural motif in Wyoming Deposit C7 asphaltenes; the predominant fragmentation pathway for all extrography fractions consists of loss of CH2 units (or dealkylation), without significant loss of aromaticity. Conversely, Athabasca Bitumen C7 asphaltenes exhibit an “atypical” molecular composition. More than 40 wt % of the sample is extracted in the latest extrography fractions, which are composed of difficult-to-ionize species, a fraction of which exhibit atypically low double bond equivalent (DBE = 5–12) and extended homologous series with carbon numbers up to 60. The fragmentation behavior of all Athabasca Bitumen-derived fractions demonstrates a predominant contribution of archipelago motifs. Our results suggest that the Yen-Mullins molecular definition of asphaltenes cannot be used to describe all asphaltene samples. Island and archipelago structura...

In this contribution, we use high-resolution mass spectrometry to unveil the molecular composition of occluded compounds inside Colombian asphaltenes macrostructures. We use Soxhlet extraction, with n-heptane, coupled with asphaltene maceration to obtain four fractions enriched with chemical compounds occluded inside asphaltene networks. We focused our efforts on the fraction enriched with compounds interacting with asphaltenes via strong intermolecular forces, and used normal phase column chromatography to fractionate it and atmospheric pressure photoionization coupled to Fourier transform ion cyclotron resonance mass spectrometry to obtain a detailed molecular description. Our results indicate that the occluded compounds obtained in the last stage of the washing process are by themselves a complex mixture, consisting mostly of saturated compounds including molecular formulas corresponding to biomarkers, alkyl aromatics with high heteroatom content (up to four heteroatoms), vanadyl porphyrins, and highly aromatic species, which we believe are low-molecular weight asphaltenes transferred to the n-heptane during the extraction process. We consider this information valuable because analysis of occluded compounds gives us a more thorough molecular description of asphaltenes; besides, knowledge of compounds closely related to asphaltenes could not only improve deasphalting processes in pilot plants, but also will help to find new geochemical biomarkers occluded within asphaltenes.

Asphaltenes have traditionally been conceived as highly aromatic, alkyl-deficient compounds enriched in pericondensed aromatic “island” motifs. This structural definition evolved into the general notion that aromatic core-dominated interactions (π-stacking) drive asphaltene aggregation, and heteroatom-based intermolecular forces have no significant effect on the overall solubility and aggregation behavior. However, the exclusion of heteroatoms in asphaltene chemistry is inconsistent with the Boduszynski continuum and known asphaltene properties, such as increased heteroatom content relative to maltenes, interfacial activity, and strong adsorption to polar stationary phases. Thus, to determine whether or not heteroatoms are involved in solubility, we have separated asphaltene fractions enriched in single-core (island) or multicore motifs (archipelago) according to their partitioning in n-heptane by two fractionation methods. In the first separation procedure, the acetone fraction from Wyoming deposit n-heptane asphaltenes (island-enriched) was adsorbed on polytetrafluoroethylene powder and Soxhlet extracted with n-heptane. Subfractions were collected after one day, one week, one month, and three months of extraction, and the residue, n-heptane insoluble material, was recovered with a mixture of toluene and dichloromethane. In the second method, the acetone fraction from Athabasca bitumen n-heptane asphaltenes (archipelago-enriched) was fractionated by differential precipitation in mixtures of n-heptane and toluene. The molecular composition of the asphaltene subfractions was accessed by positive-ion atmospheric pressure photoionization coupled to 9.4 T Fourier transform ion cyclotron resonance mass spectrometry and structures were determined by infrared multiphoton dissociation. The compositional trends for heteroatom content, double bond equivalents, and alkyl substitution suggest that the Boduszynski continuum can be extended to asphaltenes. In particular, the compositional range of polyoxygenated asphaltene compounds shifts toward lower aromaticity, whereas oxygen-depleted species are more aromatic. Moreover, the results demonstrate that polyoxygenated species (e.g., O3 and S2O3 classes) are pivotal in asphaltene solubility, as they concentrate in the most polarizable and insoluble asphaltene subfractions. Therefore, the results support the existence of atypical asphaltene species with remarkably low aromaticity that reside in the most insoluble asphaltene subfractions because of their high heteroatom content. Such asphaltene compounds preferentially ionize as protonated molecules rather than radical cations and overlap the compositional range of interfacially active species, consistent with their tendency to participate in hydrogen bonding. Collectively, the results highlight the need for an asphaltene molecular model based on the existence of polyfunctional species capable of interacting with neighboring asphaltene molecules through several intermolecular forces, including London dispersion forces between ali...

With heavy crude oil refining on the rise, upgrading strategies are fundamental to yield high-value products. Hydroconversion and thermal cracking are well-established and widely used upgrading processes for heavy oils' distillation cuts and residues. Recognizing molecular changes in these fractions after upgrading, particularly of asphaltenic compounds, is fundamental to understand and optimize the processes. In this work, we follow compositional changes in the asphaltene fraction of a Colombian heavy crude, after hydroconversion and thermal cracking, using high-resolution mass spectrometry. The liquid products from the upgrading processes were fractionated into maltenes and residual asphaltenes, with yields between 33% and 38% in maltenes from the original asphaltene feedstock. Contoured plots of double bond equivalents versus carbon number and van Krevelen diagrams show maltenic fractions exhibiting lower aromaticity, smaller molecular size, fewer heteroatomic species, and higher content of alkyl side chains than the starting asphaltenic material. Residual asphaltenes, on the other hand, consist of compounds with lower H/C ratios and reduced content of alkyl groups than the feedstock. In addition, structural information about the feedstock, such as archipelago or island structures, can be derived from the plots. This information is useful to establish trends between compound class reactivity and the suitability to produce valuable maltenic compounds through upgrading technologies.

Road asphalt is comprised of aggregate (rocks) mixed with a binder composed of high-boiling petroleum-derived compounds, which have been thought to be relatively inert (unreactive) and thus leach small amounts of polyaromatic hydrocarbons (PAHs) into water from the built environment. However, recent studies have demonstrated that petroleum readily undergoes photooxidation and generates water-soluble oxygencontaining hydrocarbons. Therefore, here, we investigate the effects of solar irradiation on an asphalt binder. Upon irradiation in a photooxidation microcosm, thin films of the asphalt binder produce abundant oil-and water-soluble oxygenated hydrocarbons, which we hypothesize are also leached from roads and highways through photooxidation reactions. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enables extensive compositional characterization of the virgin asphalt binder, irradiated asphalt binder, and the water-soluble photoproducts. The results reveal the production of water-soluble species that resemble the molecular composition of petroleum-derived dissolved organic matter, including abundant hydrocarbons and Scontaining species with up to 18 oxygen atoms. The results also confirm photo-induced oxidation, fragmentation, and potentially polymerization as active processes involved in the production of water-soluble organic pollutants from asphalt.

Despite extensive research, the molecular-level chemical characterization of asphaltenes, a highly aromatic solubility fraction of petroleum, remains an analytical challenge. This fraction is related to diverse problems in crude oil exploration, transportation, and refining. Two asphaltene architecture motifs are commonly discussed in the literature, "island" (single-core)-and "archipelago" (multicore)-type structures. The thermal desorption and pyrolysis behavior of island-and archipelago-enriched asphaltenes and their extrography fractions was investigated. For this purpose, the evolved chemical pattern was investigated by thermal analysis coupled with ultrahigh-resolution mass spectrometry (Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)). Soft atmospheric pressure chemical ionization preserved the molecular information of the thermal emission profile. Time-/temperature-resolved analysis allowed the chemical characterization of the occluded material as well as of asphaltene building blocks during pyrolysis. Regarding the thermogravimetric information, the island-type enriched sample (Wyoming asphaltenes) revealed a significantly higher coke residue after the pyrolysis process compared to the archipelago-type enriched sample (Athabasca asphaltenes). In contrast to whole asphaltenes, extrographic fractions revealed that occluded material evolved during the desorption phase. For the acetone fraction, this effect was the most abundant and suggests cooperative aggregation, which persists at high temperatures. Pyrolysis revealed a bimodal behavior for most of the compound classes, suggesting the presence of both architecture motifs in each asphaltene. double-bond equivalent (DBE) vs #C diagrams of the pyrolysis molecular profile revealed specific compositional trends: compounds with high DBE values and short alkylation are likely to be originated from island-type asphaltenes, whereas species with low DBE values and high carbon numbers likely derive from archipelago-type asphaltenes. In the asphaltene structural debate, thermal analysis ultrahigh-resolution mass spectrometry serves as an additional technique and supplements results obtained by other techniques, such as direct infusion approaches. Consistent results on the structural motifs are indicated by the molecular fingerprint visualized by DBE vs #C diagrams and serve as a measure for the dominance of a structural motif.