Pyrolytic characteristics of amino acid–glucose mixtures and the formation of nitrogen-containing heterocycles

Nitrogen-containing compounds in bio-oil, particularly nitrogen-containing heterocycles (N-heterocycles), play a crucial role in both fuel properties and chemical value, yet their formation mechanisms during biomass pyrolysis remain unclear. This study systematically investigates the pyrolysis of glucose mixed with 20 α -amino acids at a 1:1 mass ratio as models for nitrogen-rich biomass, aiming to elucidate how amino-acid structure and sugar interactions are associated with N-heterocycle formation during co-pyrolysis. Pyrolysis-Fourier-Transform infrared spectroscopy (Py-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analyses reveal that volatile release peaks of most mixtures occur at 340–400 °C, which is 40–100 °C lower than that of pure glucose, and five-membered N-heterocycles dominate the nitrogen-containing products, with proline, asparagine, and glutamine showing the highest N-heterocycle relative contents of 59.84%, 57.09%, and 47.46%, respectively. Asparagine results in the highest pyrrole proportion of 48.97%. No obvious diketopiperazine products were detected in any of the binary systems. The Maillard reaction between amino acids and glucose is likely to generate short-chain carbonyl intermediates that may facilitate intramolecular cyclization and thus contribute to the enrichment of N-heterocycles. Aspartic acid and its amide derivative asparagine appear to be important contributors to N-heterocycle composition because of their abundance in biomass and their cyclization characteristics. These findings provide mechanistic insight into nitrogen transformation and offer guidance for optimizing bio-oil quality from nitrogen-rich biomass.