For almost 60 years, petroleum scientists have described asphaltenes as nanoaggregates of aromatic cores surrounded by aliphatic chains. This view started in the early 1960’s with the work of professor Teh-Fu Yen, who used X-ray diffraction to propose a molecular representation for asphaltenes. The Yen model included both island and archipelago motifs, in which the aromatic cores were the primary site of interaction. The notion of π–π stacking as the dominant molecular force that determines the degree of aggregation has persisted to the present day, but it is inconsistent with solubility, aggregation, adsorption, emulsifying, and upgrading behavior of asphaltenes. In this work, we use separations, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and gas-phase fragmentation via infrared multiphoton dissociation (IRMPD) to demonstrate that oxygen-containing functionalities, hence hydrogen-bonding interactions, have a fundamental role in asphaltene chemistry. Virgin and thermally modified asphaltenes were subjected to an extrography fractionation that separates samples based on ionization efficiency. FT-ICR MS and IRMPD results demonstrate that island and archipelago motifs coexist and comprise a structural continuum that is increasingly enriched with archipelago species as a function of increasing molecular weight, oxygen content, and aggregation tendency. The island/archipelago ratio is sample-dependent. Asphaltene fractions enriched with island and archipelago motifs were further separated by differential precipitation. In this method, asphaltenes are dispersed in toluene and sequentially precipitated by the stepwise addition of n-heptane. MS results demonstrate that species with multiple oxygen-containing moieties dominate the composition of the most unstable fractions (precipitated with the least amount of n-heptane), whereas highly aromatic/sulfur-containing hydrocarbons concentrate in the most stable fractions. IRMPD and additional separations demonstrate that higher content of species with multiple oxygen-containing moieties correlates with stronger adsorption, interfacial activity, and increased reactivity in gas-phase fragmentation. The results of this work support the previous attempts by Gray, Yarranton, and co-workers to describe asphaltene aggregation by supramolecular chemistry.

Advances in asphaltene petroleomics: Determining the role of heteroatoms in asphaltene chemistry

2019 Spring National Meeting

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