Mechanistic Insights into Alkylated and Polycyclic N-Heterocycle Formation from Aromatic Amino Acids during Hydrothermal Liquefaction

The kinetics and thermodynamics of the Pictet–Spengler cyclization (PSC) and electrophilic aromatic substitution (EAS) were investigated to elucidate the complete reaction pathways involved in the hydrothermal liquefaction (HTL) of aromatic amino acids, including the formation of alkylated and polycyclic N-Heterocycles. A series of multilevel factorial HTL experiments were conducted on pure l-phenylalanine, l-tyrosine, and L-tryptophan (first stage) and mixtures of key intermediate compounds under noncatalytic and H₃PO₄-catalyzed conditions (second stage) at 250–350 °C for 20–100 min. The first-stage experiments revealed the endothermic nature of biocrude, hydrochar, and gaseous coproduct formation from aqueous organics generated during HTL. The second stage identified decarboxylation products (e.g., 2-phenylethan-1-amine, 4-(2-aminoethyl)phenol, and 2-(1H-indol-3-yl)ethan-1-amine) as key intermediates that reacted with aqueous aldehydes (formaldehyde and acetaldehyde) via PSC, forming polycyclic N-heterocycles (e.g., isoquinolines and carbolines) in biocrude and hydrochar. Similarly, aliphatic carbon–carbon cleavage products (e.g., 1H-indole and benzene) reacted with aldehydes and pyrazine to form alkylated N-heterocycles via EAS, e.g., 2-ethyl pyrazine, 2,2’-(pyrazine-2,3-diyl)diphenol, 2,5-diphenylpyrazine, and 3,5,7-trimethyl-1H-indole. Mechanistic analysis indicated that the endothermic dehydration was the rate-limiting step in the PSC reaction, while EAS involved multiple endothermic steps, making both reactions favor higher reaction temperatures for selective formation of the products. The presence of a Bro̷nsted acid catalyst was beneficial, as protonation played a crucial role in both mechanisms. This study demonstrated that in addition to side chain cleavage and decarboxylation, PSC and EAS reactions were fundamental in the HTL of aromatic amino acids.