Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy Study of Carbohydrate Decomposition by Hydrothermal Liquefaction Treatment: A Modeling Approach on Bio-oil Production from Organic Wastes

Glucose and cellulose as model compounds were treated under hydrothermal liquefaction (HTL) conditions to describe the main reaction pathways that are involved in the process. The HTL-derived phases (gas phase, bio-oil, aqueous phase, and solid residue) were fully characterized by a combination of analytical techniques [i.e., elemental analysis (EA), gas chromatography mass spectrometry (GC-MS), electrospray ionization/atmospheric pressure photoionization Fourier transform ion cyclotron resonance-mass spectrometry (ESI/APPI FTICR-MS), and C-13 cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS NMR)], and a comprehensive HTL degradation mechanism was proposed. A wide range of different reactions (dehydration, decarboxylation, retro-aldol, aromatization, condensation, oxidation, and reduction) were found to be involved in the formation of the different compounds detected in the four phases. The main identified products in both glucose and cellulose HTL bio-oils were furfural derivatives, which further react leading to several phenolic and aliphatic compounds. Oligomers arising from the condensation of furfural derivatives were also found, and their polymerization finally results in a solid residue whose characterization confirmed the presence of polyfuranic networks together with graphite-like domains. Finally, glucose and cellulose showed a similar behavior considering the product yields and phase composition, suggesting that the polymerization degree does not significantly affect the HTL process.