Enhancing pyrolysis oil quality through in-situ catalytic pyrolysis of biochar with temperature-driven nitrogen configuration modulation: Mechanistic insights into the catalytic process

Design and development of highly efficient, low-loading, and green non-metal catalysts is critical for advancing biomass sustainable recycling and the derived bio-oil upgrading. This study introduces a nitrogen-doped biochar (NBCs) catalyst that synthesized via a one-step co-pyrolysis of reed straw, which can enabling precise control over nitrogen configurations (pyridinic-N, pyrrolic-N, graphitic-N) for in-situ catalytic pyrolysis with negligible secondary pollution. Product analysis reveals that NBCs significantly improve bio-oil quality using trace catalyst amounts, increasing phenolic compounds from 60.74 % (non-catalytic) to 75.96 % (900NBC) and raising pH from 3.17 to 4.3, indicating effective acidity reduction. Notably, pyrrolic-N dominates oxygen removal by facilitating decarboxylation and other deoxygenation pathways (e.g., C-O bond cleavage), effectively lowering the O/C ratio, while graphitic-N reduces heavy organics (>C20) via enhanced cracking and aromatization. Density functional theory (DFT) calculations elucidate atomic-scale mechanisms: pyrrolic-N exhibits a low hydrogen activation barrier (150.995 kJ/mol at PR25H), facilitating C-O bond cleavage through nucleophilic sites (Fukui function f-=0.1067), whereas graphitic-N activates hydroxyl radicals via water dissociation (energy barrier: 279.298 kJ/mol at GN26OH), promoting aromatic condensation through electrophilic carbon centers (f+=0.0852). The self-catalyzed system achieves efficient reed conversion with minimal catalyst consumption, yielding stable bio-oil (51.94–52.09 wt%). These findings establish a structure-activity model demonstrating that nitrogen-induced electronic effects—rather than physical properties—govern catalytic performance, providing a foundation for designing low-cost, eco-friendly catalysts for sustainable biomass valorization.