Mechanistic insights into nitrogen transformation during ethanol liquefaction of microalgae in the presence of residual Fe3O4 nanoparticles.

Ethanol-based thermochemical liquefaction is a promising route for converting microalgae into liquid fuels; however, nitrogen enrichment in the bio-oil severely limits its fuel quality and downstream upgrading. This study investigated how residual Fe3O4 NPs from upstream magnetic harvesting regulate nitrogen transformation during ethanol liquefaction of microalgae. Liquefaction experiments were conducted at 220-260 °C with a fixed Fe3O4 NP loading of 5 wt%. Product distributions, ethanol incorporation, and molecular compositions of the oil phase were analyzed by GC-MS and FT-ICR MS, complemented by DFT calculations. The bio-oil yield ranged from 55.29 to 78.08 wt%, increasing from 68.93 to 78.08 wt% at 260 °C with Fe3O4. Carbon balance analysis showed that up to 4.97 g of ethanol was incorporated into the products, contributing to both increased oil yield and active hydrogen supply. DFT calculations confirm that Fe3O4 promotes ethanol dehydrogenation, while FT-ICR MS reveals decreased abundance and unsaturation of nitrogen-containing compounds after Fe3O4 addition. Fe3O4 primarily influences nitrogen transformation by facilitating the generation of reactive hydrogen species from ethanol, thereby promoting hydrogenation of nitrogen-containing compounds. However, its contribution to overall nitrogen removal was limited, with the relative nitrogen content decreasing from 7.72 wt% to 6.74 wt% at 260 °C. Overall, residual Fe3O4 was compatible with ethanol liquefaction and influenced both bio-oil formation and nitrogen transformation, offering insight into integrating upstream magnetic harvesting with downstream thermochemical conversion.