Chemical and structural changes of asphaltenes during oxygen chemisorption at low and high-pressure

• Published in: Fuel (Guildford) 2025-01, Vol.379, p.133000, Article 133000
• Main Authors: Medina, Oscar E., Moncayo-Riascos, Ivan, Heidari, Samira, Nassar, Nashaat N., Pérez-Cadenas, Agustín F., Carrasco-Marín, Francisco, Corteś, Farid B., Franco, Camilo A.
• Format: Article
• Language: English
• Subjects: Asphaltenes; Chemical changes; High-pressure thermogravimetric analysis; Molecular dynamic simulation; Oxygen chemisorption
• ISSN: 0016-2361
• DOI: 10.1016/j.fuel.2024.133000

[Display omitted] •Oxygen chemisorption is influenced by the asphaltene chemical structure.•The formation of carboxyl structures undergoes important compositional changes that determine asphaltene reactivity.•Fused aromatic rings are primary site for oxygen and is affected by pressure.•Asphaltene B forms carboxyl or COO–, whereas the asphaltene E generate simple oxygen–oxygen bonds. This study investigates the changes in asphaltene chemical properties during oxygen chemisorption (OC) to understand its thermal behavior at different pressures and temperatures. To this aim, two n-C7 asphaltene samples (B and E) were subjected to non-isothermal heating using a heating ramp of 10 °C min−1 from 100 °C to 150, 180, and 210 °C under two different pressures: 0.084 MPa and 6.0 MPa, in an oxygen environment. The resulting sample residues were characterized using elemental analysis, x-ray photoelectron spectroscopy, and nuclear magnetic resonance. The experimental observations suggest that oxygen chemisorption is influenced by the asphaltene chemical structure including the C, N, S, and O species and the aromatic size core. In parallel, molecular dynamics simulations (MD) were carried out using a reactive force field to obtain further insights into the chemisorption mechanism. The MD results accurately describe the amount of oxygen chemisorbed for both systems at high pressure (6.0 MPa), confirming that the chemisorption sites are strongly associated with the molecular characteristics of the asphaltene samples. The radial distribution function (RDF) analysis revealed the key pairwise interactions, showing a well-defined first coordination peak between oxygen, nitrogen, and carbon for asphaltene B and between oxygen, hydrogen, and nitrogen for asphaltene E. Consequently, the formation of carboxyl or COO– groups is expected for asphaltene B, while the formation of oxygen–oxygen bonds through phenolic oxygen and ether oxygen is anticipated for asphaltene E. Furthermore, the molecular size of the aromatic core emerged as an important feature for oxygen chemisorption, according to the MD simulations. This study’s findings will contribute to optimizing thermal enhanced oil recovery methods, enabling oil producers to extract and utilize heavy and extra-heavy crude oil resources effectively.