Hydrothermal liquefaction of glutamic acid: DSC kinetics and GC×GC-ToF/MS analysis

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 aqueousphase 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