Abstract
Nitrogen compounds from proteins challenge hydrothermal liquefaction (HTL) due to their negative effects on biocrude quality and post-processing needs. Understanding amino acid behavior is essential to elucidate nitrogen migration, hence, HTL of glutamic acid (GA) was investigated to clarify the reaction mechanism and nitrogen redistribution. High-pressure differential scanning calorimetry (HP-DSC) was applied considering 16 temperature-heating rate conditions. Subsequently, products were analyzed via two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-ToF/MS). DSC showed endothermic behavior (ΔH = 0.2–0.3 MJ/kgdry basis) with a shift at higher heating rates linked to enhanced molecular activity. The apparent activation energies varied from 110.92 to 112.61 kJ/mol with pre-exponential factors from 5.35 to 8.77 × 1012 min−1, and reaction orders from 1.26 to 3.01, indicating a transition from unimolecular to bimolecular or multistep mechanisms. GC×GC-ToF/MS identified diverse compounds, such as 2-pyrrolidinone, alkylated pyrrolidinediones, the relative abundance of which is influenced by both temperature and heating rate. By correlating thermo-kinetic data with product distributions, three mechanistic regimes were mapped: initial cyclization at 250 °C, water assisted branching at 270 and 290 °C, and more thermally induced fragmentation at 310 °C. Water acts as reactant and catalyst, enabling nitrogen retention via proton-shuttle and hydrolysis. The production of polar compounds, and less-polar species at higher temperatures resulting from alkylation, indicates that the underlying reactions could be exploited to divert the nitrogen into stable aqueous-phase products, thereby reducing its incorporation in the biocrude by selective operation conditions. These findings clarify nitrogen transformation in HTL and offer methodological guidance for nitrogen mitigation.