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

Max bio-oil yield reaches 78.08% with N reduced to 6.74 wt%. • Fe 3 O 4 NPs increase ethanol incorporation to 4.97 g at 260 °C. • Fe 3 O 4 synergizes with ethanol to enhance esterification and hydrogen supply. • Nitrogen species shift toward less condensed and more hydrogenated forms. 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 Fe 3 O 4 NPs from upstream magnetic harvesting regulate nitrogen transformation during ethanol liquefaction of microalgae. Liquefaction experiments were conducted at 220–260 °C with a fixed Fe 3 O 4 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 Fe 3 O 4 . 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 Fe 3 O 4 promotes ethanol dehydrogenation, while FT-ICR MS reveals decreased abundance and unsaturation of nitrogen-containing compounds after Fe 3 O 4 addition. Fe 3 O 4 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 Fe 3 O 4 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. Download: Download high-res image (141KB) Download: Download full-size image