Amino acid structure-oriented pyrolysis nitrogen evolution mechanism: multi-scale experiment and DFT simulation

Understanding nitrogen transformation mechanisms during pyrolysis of protein-rich biomass is crucial for biofuel, chemical production, and emission control. Using representative amino acids (Aspartic acid, Histidine, Leucine, and Phenylalanine) spanning polar/non-polar types, we pyrolyzed them at key temperatures to establish molecular conformation‑nitrogen partitioning relationships. Polar amino acids (aspartic acid and histidine) preferentially formed char-N (20–38 wt%) through bimolecular cross-linking at 750 °C. Conversely, non-polar leucine and phenylalanine enriched nitrogen in the tar phase due to side-chain inertness, which simultaneously hindered nitrogen migration to char. Significantly, steric hindrance governed reaction pathway selectivity. In phenylalanine, for instance, the phenyl ring reduced the cyclization barrier by 54.35 kcal/mol versus straight-chain dehydrogenation, facilitating preferential formation of nitrogen-containing chemicals such as indole and phenylacetonitrile via dehydrogenation/decarboxylation reactions in the primary pyrolysis of phenylalanine and secondary cracking of its tar. This work establishes molecular-level theoretical foundations for targeted synthesis of nitrogenous chemicals and NOx precursor emission control during nitrogen-rich biomass pyrolysis.