Cyclic Compound Formation Mechanisms during Pyrolysis of Typical Aliphatic Acidic Amino Acids

This work selected two typical acidic amino acids to study the formation mechanism of nitrogen-containing polycyclic compounds using density functional theory methods at the ωB97XD/6-311G (d, p) theoretical level. According to the energy profile, intramolecular dehydration was easier in comparison to intermolecular dehydration. It suggested heterocyclic compounds as the main components in tar. For glutamic acid, the formation of five-member cyclic compounds tends to occur at 300 °C. Six-member cyclic compounds were generated through multiple molecule reaction steps, while for aspartic acid, the lower temperature (300 °C) was beneficial to promote condensation. At 625 °C, the shorter peptide chains were more likely to be further cracked. The production of succinimide and maleimide could be related to multistep intermolecular C–N dehydration and bond scission. The calculation results were consistent with the experimental results. Furthermore, the proposed pathways for the pyrolysis products were confirmed.