Bio-oil emerges as a potent green energy to halt planet warming. However, its direct utilization as a biofuel is impeded by substantial challenges resulting from its complex composition, leading to undesirable properties. This research delves into elucidating and proposing structures of the unknown sugar oligomers in bio-oil by integrating experimental and modeling approaches. Additionally, it aims to upgrade the heavy bio-oil fractions, containing both sugar and lignin oligomers, and investigate the mechanism of oxygen removal during hydrotreatment.Experimental techniques and computational modeling were employed to achieve these goals. Density functional theory (DFT) was used to study the dehydration and fragmentation reactions to propose structures of the unknown oligomeric sugars and investigate their reaction mechanisms during upgrading. Chromatographic techniques were applied to fractionate sugar compounds in bio-oil, followed by comprehensive characterization to identify their specific components. Bio-oil upgrading was conducted by co-hydrotreating the heavy bio-oil fractions with waste cooking oil in a batch-type reactor under defined operating conditions.The results put forward potential structures for the unidentified sugar oligomers. DFT results showed that water, hydroxyacetaldehyde, and hydroxyacetone are most favorably formed from the non-reducing end of the sugar oligomers. These modeling yields were integrated with the experimental data acquired from chromatographic techniques to gain a deeper understanding of the oligomers. Moreover, the upgrading study showed that the heavy bio-oil, containing both sugar and lignin oligomers, is viable. The behavior of the hydrotreated heavy bio-oil demonstrated comparable coke yields to those obtained when solely using a pyrolytic fraction. An in-depth understanding of the structure of the unknown compounds and their behavior during hydrotreatment will contribute toward the development and advancement of biomass pyrolysis techniques and product upgrading strategies
