Amides in Bio-oil by Hydrothermal Liquefaction of Organic Wastes: A Mass Spectrometric Study of the Thermochemical Reaction Products of Binary Mixtures of Amino Acids and Fatty Acids

Abstract

Among biofuels, the bio-oil produced by hydrothermal liquefaction of waste biomass can be considered an alternative to fossil fuels in industry as well as transport and heating compartments. The bio-oil complex composition is directly dependent upon the specific biomass used as feedstock and the process used for the chemical conversion. The coexistence of proteins and lipids can explain, in principle, the high percentage of fatty acid amides found in the produced bio-oil. In the present study, the amides in a sample of bio-oil have been separated by gas chromatography and identified at first on the basis of their electron impact (EI) mass spectra. To distinguish between N-alkyl isomers, standard amides have been synthesized and analyzed. Because the most reasonable origin of fatty acid amides in hydrothermal bio-oils is the condensation reaction between fatty acids and the decarboxylation products of amino acids, a series of model experiments have been carried out by reacting hexadecanoic acid, at high temperature and pressure, with each of the 20 amino acids constitutive of proteins, looking for the formation of fatty acid amides. Remarkably, by such experiments, all of the amides present in the bio-oil have been recognized as hydrothermal coupling compounds of the decomposition products of amino acids with fatty acids, thus allowing for their structural elucidation and, also important, confirming their (bio)chemical origin.