Mechanistic insights into the role of N/O-doped biochar in enhanced phenolics production during biomass pyrolysis

Phenolic compounds are vital chemicals that can be converted into high-density jet fuel components, such as aromatic hydrocarbons or cycloalkanes. Pyrolysis of biomass waste for producing high-value phenolic compounds offers a sustainable approach to waste management and energy conversion. While N/O-doped biochar has demonstrated potential in enhancing phenolics production, its application faces challenges such as complex preparation, high activation temperatures, and unclear catalytic mechanisms. This study addresses these issues by developing a single-step sodium amide (NaNH₂) activation method at mild temperatures (<500℃) to produce N/O-doped biochar with optimized catalytic properties. Characterization identified graphitic-N (-GN) and oxidized-N (-ON) configurations, along with aldehyde-O (-CHO) and carboxyl-O (-COOH) groups, as key active sites that enhance catalytic performance. Experimentally, the N/O-doped biochar achieved a phenolics yield of 57.87% at an activation temperature of 400℃, representing a 19.18% increase over non-catalytic conditions. Density functional theory (DFT) calculations further elucidated the role of N and O groups, showing that -GN and -ON in N groups and -CHO and -COOH in O groups lower energy barriers in radical-induced demethoxylation which promotes phenolic product formation. Machine learning analysis identified nucleophilicity and local softness as critical descriptors, indicating that these configurations effectively modulate electron density at active sites. These findings provide a comprehensive mechanistic understanding of how specific N and O functional groups in biochar enhance catalytic efficiency in targeted phenolic production.