Formation of low molecular weight heterocycles and polycyclic aromatic compounds (PACs) in the pyrolysis of α-amino acids

Abstract

The formation of pyrolysis products, including polycyclic aromatic compounds (PACs), of three α-amino acids acids (asparagine, proline, and tryptophan) was studied in inert atmosphere. The PACs included polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing polycyclic aromatic compounds (N-PACs). A number of low temperature pyrolysis products, primarily single ring heterocyclic compounds, were also identified. The reactor consisted of a quartz tube with two-zones, each heated and controlled separately. The amino acids were pyrolyzed at 300 °C to obtain a low temperature tar (LTT) and a low temperature char (LTC). The LTC was then pyrolyzed at 625 °C to obtain a high temperature tar (HTT) and a high temperature char (HTC). In some cases, the LTT and HTT were subjected to higher temperatures in the range 700–920 °C to force secondary reactions. The various tars were analyzed by gas chromatography/mass spectrometry (GC/MS). In the absence of secondary reactions, asparagine and proline did not form any PACs. Tryptophan formed harman and norharman. After the secondary pyrolysis, all three amino acids formed three- and four-ring N-PACs and the yields generally increased with temperature. Asparagine and proline also formed PAHs in low yields, especially above 820 °C. The results suggest different decomposition pathways such as decarboxylation and deammoniation for the three amino acids leading to the formation of various PACs under different pyrolysis conditions.