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.
Solid wastes of organic origins are potential feedstocks for the production of liquid biofuels, which could be suitable alternatives to fossil fuels for the transport and heating sectors, as well as for industrial use. By hydrothermal liquefaction, the wet biomass is partially transformed into a water-immiscible, oil-like organic matter called bio-oil. In this study, an integrated NMR spectroscopy/mass spectrometry approach has been developed for the characterization of the hydrothermal liquefaction of bio-oil at the molecular level. (1)H and (13)C NMR spectroscopy were used for the identification of functional groups and gauging the aromatic carbon content in the mixture. GC-MS analysis revealed that the volatile fraction was rich in fatty acids, as well as in amides and esters. High-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) has been applied in a systematic way to fully categorize the bio-oil in terms of different classes of components, according to their molecular formulas. Most importantly, for the first time, by using this technique, and for the liquefaction bio-oil characterization in particular, FT-MS data have been used to develop a methodology for the determination of the aromatic versus aliphatic carbon and nitrogen content. It is well known that, because they resist hydrogenation and represent sources of polluting species, both aromatic molecules and nitrogen-containing species raise concerns for subsequent upgrading of bio-oil into a diesel-like fuel.
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