Decoupling Oxygen Content and Corrosivity in HTL Bio-Oils through Speciation and Structural Embedding

Hydrothermal liquefaction (HTL) products often contain substantial oxygen, which is conventionally associated with high total acid numbers (TAN), corrosion, and chemical instability. However, certain HTL bio-oils exhibit remarkable stability despite comparable or higher oxygen content relative to pyrolysis oils. Here, we investigate the role of oxygen accessibility, speciation, and molecular integration in governing the corrosion behavior and chemical stability of solid and highly viscous HTL products. Bulk elemental analysis reveals that oxygen content alone is a poor predictor of TAN and corrosivity. It is worth noting that thermogravimetry and chromatography analysis specifically demonstrate a scarcity of low-molecular-weight, volatile oxygenates in HTL products, indicating limited chemical accessibility. Further, solvent extraction experiments reveal that only a minor fraction of oxygenates is accessible, which rather limits their participation in corrosion reactions. These findings support a mechanistic framework in which oxygenated intermediates generated during HTL undergo condensation and structural embedding, suppressing the formation of reactive low-molecular-weight acids. Consequently, HTL products retain high oxygen content while exhibiting low TAN, negligible corrosion, and truly enhanced chemical stability. This study emphasizes that oxygen speciation and molecular integration, rather than total oxygen content, govern the stability of HTL bio-oils, providing a rationale for direct functionalization without extensive deoxygenation.