By Bridget Tang
Fast-pyrolysis bio-oil produced from lignocellulose biomass is believed to one of the most economically feasible carbon-neutral energy sources for replacing fossil fuel oils [1]. The main disadvantage of bio-oils, which makes them incompatible with current infrastructures, is their acidity [2]. Bio-oil is made up of hundreds of small organic molecules. Oxygen-containing compounds, such as aldehydes, ketones and quinones, are the main contributors to a fuel’s acidity.
Nuclear Magnetic Resonance (NMR) spectroscopy is a versatile tool that has been widely used to analyse complex mixtures. However, 1H NMR analysis of bio-oil samples is limited by severe overlap of the signals of the many species present in the sample. The use of other nuclei, such as 19F, offers a promising alternative as their spectra are often much sparser. By derivatising the bio-oil sample with a fluorine-containing compound such as 4-(trifluoromethyl)-phenylhydrazine, 19F NMR provides quantitative information about the carbonyl-containing compounds present [3].
The technique has been translated from traditional high-field NMR to benchtop, or low field, NMR. The aldehyde and ketone content of bio-oil produced from various biomass feedstocks has been successfully analysed. These measurements were verified using both an oximation titration and high-field NMR methods. Benchtop NMR equipment is cheaper, smaller, uses weaker magnets that require no cryogens and samples do not require deuterated solvents. Benchtop NMR offers a low-maintenance, low-cost alternative to traditional NMR spectroscopy [4]. This will make NMR methods more accessible for a wide range of users, from industry to academic institutions.
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2. Bertero, M., de la Puente, G. and Sedran, U. Fuels from bio-oils: Bio-oil production from different residual sources, characterization and thermal conditioning. Fuel, 2012, 95, pp.263-271.
3. Huang, F., Pan, S., Pu, Y., Ben, H. and Ragauskas, A.J. 19 F NMR spectroscopy for the quantitative analysis of carbonyl groups in bio-oils. RSC Advances, 2014, 4(34), pp.17743-17747.
4. Gracia-Vitoria, J., Rubens, M., Feghali, E., Adriaensens, P., Vanbroekhoven, K. and Vendamme, R. Low-field benchtop versus high-field NMR for routine 31P analysis of lignin, a comparative study. Ind. Crops Prod., 2022, 176, p.114405.