Role of Molecular Structure in the Production of Water-Soluble Species by Photo-oxidation of Petroleum

Abstract

Asphaltenes are high-boiling and recalcitrant compounds that are generally minor components of crude oil (∼0.1–15.0 wt %) but dominate the composition of heavily weathered spilled petroleum. These solid residues exhibit a high structural complexity, comprised of polycyclic aromatic hydrocarbons (PAHs) that are a mixture of single-core (island) and multicore (archipelago) structural motifs. The mass fraction of each motif is sample-dependent. Thus, knowledge of a potential structural dependence (single- versus multicore) on the production of water-soluble species from asphaltene samples is key to understanding the contribution of photochemically generated dissolved organic matter from oil spills. In this work, asphaltene samples with enriched mass fractions of either island (single-core) or archipelago (multicore) structural motifs are photo-oxidized on artificial seawater by the use of a solar simulator. Molecular characterization of oil- and water-soluble photoproducts, conducted by Fourier transform ion cyclotron resonance mass spectrometry, reveals that island motifs exhibit very limited production of water-soluble species, and their oil-soluble products reflect the molecular composition of the starting material. Conversely, archipelago motifs yield a water-soluble compositional continuum of O x , S x O y , and N x O y containing hydrocarbons species that exhibit the typical molecular fingerprint of dissolved organic matter (DOM). The lower carbon number and aromaticity of the archipelago-derived asphaltene photoproducts suggest the occurrence of photofragmentation (or photolysis) reactions. To investigate the possibility of the opposite reaction (photopolymerization), the photo-oxidation of small PAHs isolated from a low-boiling petroleum distillation cut was also performed. It yielded water-soluble compounds with carbon number and aromaticity up to 2-fold higher than the starting material, strongly suggesting that polymerization (addition reactions) occurs. Collectively, the results indicate that the presence of archipelago motifs and the occurrence of cracking/polymerization reactions are central in the production of dissolved organic matter from fossil fuels.

Characterization of an Asphalt Binder and Photoproducts by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Reveals Abundant Water-Soluble Hydrocarbons

“…Furthermore, the oxygenated species in the water-soluble fraction reveal a shift to higher carbon number and aromaticity as more oxygen atoms are incorporated (e.g., from O 3 S 1 to O 16 S 1 ). This phenomenon has been highlighted in previous studies and suggests photopolymerization reactions . Another possible explanation for this shift is that as the number of oxygen atom increases, alkylated (nonpolar) species with higher DBE become increasingly polarizable and thus water soluble.…”

“…Water-soluble products exhibit lower carbon number (<40) and DBE (<20) ranges than the oil-soluble species, which contain compounds with up to 50 carbons and DBE values up to 25. The lower carbon number and aromaticity ranges of water-soluble photoproducts have been previously reported . Furthermore, the oxygenated species in the water-soluble fraction reveal a shift to higher carbon number and aromaticity as more oxygen atoms are incorporated (e.g., from O 3 S 1 to O 16 S 1 ).…”

“…Asphaltenes have been shown to generate water-soluble organic compounds through photooxidation processes that are structure-dependent. High-resolution mass spectrometry has revealed two structural motifs in asphaltenes: an aromatic core with alkyl side chains (single-core) or several covalently linked aromatic cores (multicore) . Asphaltenes with high concentration of multicore motifs were shown to “crack” to produce small water-soluble polyaromatic hydrocarbons (PAHs) through photofragmentation reactions, whereas samples enriched in single-core motifs were not .…”

“…High-resolution mass spectrometry has revealed two structural motifs in asphaltenes: an aromatic core with alkyl side chains (single-core) or several covalently linked aromatic cores (multicore) . Asphaltenes with high concentration of multicore motifs were shown to “crack” to produce small water-soluble polyaromatic hydrocarbons (PAHs) through photofragmentation reactions, whereas samples enriched in single-core motifs were not . The photochemically produced oxidized PAHs can then undergo polymerization reactions to produce water-soluble compounds with up to 18 oxygen atoms and increased carbon number .…”

“…Asphaltenes with high concentration of multicore motifs were shown to “crack” to produce small water-soluble polyaromatic hydrocarbons (PAHs) through photofragmentation reactions, whereas samples enriched in single-core motifs were not . The photochemically produced oxidized PAHs can then undergo polymerization reactions to produce water-soluble compounds with up to 18 oxygen atoms and increased carbon number . Due to the high proportion of asphaltenes in the asphalt binder, we therefore hypothesize that photooxidation of road asphalt could be a potential source of water-soluble, oxidized hydrocarbons, which may be toxic to ecosystems. , Previous work has indicated that photochemically weathered crude oil produces toxic water-soluble species .…”