Hydrothermal liquefaction of protein-containing biomass: study of model compounds for Maillard reactions

Abstract

The potential application of bio-oil production from nitrogen-containing biomass via hydrothermal liquefaction (HTL) may be limited due to high nitrogen content, making this product oil unsuitable for fuel-related uses. The Maillard reaction is expected to play a most significant role in the interaction between proteins and carbohydrates during the hydrothermal treatment. To evaluate the Maillard reaction network in this process, lactose, maltose, and lysine were employed as model substances and tested individually and in binary mixtures. HTL experiments were conducted at temperatures between 250 and 350 °C and at 20 min reaction time. When treated individually, conversion of lysine leads to higher bio-oil yields (5–17 wt.%) than the model carbohydrates (6–10 wt.%) during HTL. In mixtures with carbohydrates, the measured bio-oil yields exceeded those obtained from conversion of the single substances (10–39 wt.%). Both yields and the relative nitrogen content of the bio-oil, increase with rising reaction temperature. The composition of the bio-oils obtained through HTL experiments was investigated in more detail: cyclopentenes and furfurals were obtained from disaccharide decomposition, piperidines and quinolines in the bio-oil originate from lysine, pyrazine and its derivatives are obtained from the mixture of lysine and disaccharides. A reaction scheme based on key chemical compounds accompanied with functional groups identified by FT-IR and NMR was developed to provide a better understanding of the Maillard reaction and its impact during HTL of protein-containing biomass.