Effect of the Fe Catalyst on Formation Pathways of Fractionated Bio-Oils from Solvent-Modulated Co-hydrothermal Liquefaction

Abstract Solvent modulation synergized with Fe catalysis offers a promising strategy for regulating fractionated bio-oil formation during co-hydrothermal liquefaction. Herein, corn stover (CS) and spirulina (SP) were used as feedstocks in this study. An ethanol-water co-solvent system (EWCS) containing 62.5 vol% ethanol was identified as optimal, where the bio-oil yield and energy recovery (ER) peaked at 52.49 and 95.61%, respectively, far above those of pure water (23.00, 45.07%) and pure ethanol (47.95, 81.57%). To avoid overlapping components obscuring the intrinsic catalytic behaviors of Fe, the recovered bio-oil was separated into water-soluble (WSB) and water-insoluble (WISB) fractions. The bio-oil yields, elemental distributions, and chemical compositions were compared under Fe catalysis in pure water, EWCS, and pure ethanol. The solvent-only effect raised the WSB yield to nearly 40% and drove feedstock nitrogen toward the WSB, elevating the nitrogen recovery (NR) of the WSB in EWCS to 55.73% while suppressing that of the WISB to 9.68%. The heating values of the two fractions remained comparable under Fe-catalyzed conditions, ranging from 30.20 to 34.30 MJ/kg. Fe barely changed the bulk bio-oil yield yet markedly reshaped the composition. The catalytic performance of Fe exhibited a clear dependence on the proportion of water within the co-solvent. It promoted hydrodenitrogenation (HDN) and phenol enrichment in pure water, directed ethanol dehydrogenation into aldol condensation that anomalously raised the WISB oxygen content in EWCS, and integrated esterification with hydrodeoxygenation (HDO) in pure ethanol. The regulatory effect of solvent polarity on nitrogen distribution and Fe active species provides innovative insights for the sustainable production of upgraded Co-HTL bio-oils.