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WIREs Energy Environ.
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Advanced analytical techniques for bio‐oil characterization

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Bio‐oil (pyrolysis oil) is the liquid product of biomass thermochemical conversion. It is a dark, viscous liquid that contains the depolymerization products of hemicellulose, cellulose, and lignin. The physicochemical properties of bio‐oils are determined by employing the conventional methods for fuels analysis with proper adaptations. However, the detailed composition of bio‐oils in terms of analytes as well as their concentration remains ambiguous and is a challenging task for analytical chemistry. The compounds in the bio‐oil range from nonpolar (e.g., hydrocarbons) to highly polar (e.g., phenolics) and from volatile (e.g., organic acids) to nonvolatile (e.g., sugar derivatives), covering a molecular weight (MW) range of about 50–2000 Da. Hence a combination of analytical techniques such as high pressure liquid chromatography, gas chromatography (GC), gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), and Fourier transform infrared spectroscopy (FTIR) are required to determine the bio‐oil's composition. Despite the significant breakthroughs of these techniques, they face limitations regarding the sample pretreatment, the incomplete separation and determination of components, and the need of multiple analyses with each method for more complete results. The development of sophisticated, comprehensive, and hyphenated chromatographic and spectrometric techniques such as GC × GC, LC × LC, high‐resolution mass spectrometry (HRMS), and 2D‐NMR has brought actual advancement in the field of bio‐oil analysis. GC × GC and LC × LC have allowed the development of qualitative and quantitative methods for the individual determination of lower MW compounds. However, HRMS and 2D‐NMR have facilitated the elucidation of the structure of the higher MW components, offering insight in the effect of pyrolysis conditions on biomass depolymerization and the possibilities for further upgrading of bio‐oils. WIREs Energy Environ 2016, 5:614–639. doi: 10.1002/wene.208 This article is categorized under: Bioenergy > Science and Materials Bioenergy > Climate and Environment Energy and Climate > Climate and Environment
GC × GC‐ToFMS chromatogram of a pine thermal bio‐oil with an orthogonal system.
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Superposition of the 2D 1H–13C HSQC NMR spectra of HTL oil (blue peaks) and upgraded oil following hydrotreatment (red peaks). The aliphatic region is shown in panel (a) and the aromatic region is shown in panel (b). Peaks and regions of the spectra corresponding to specific functional groups of interest are labeled. Notable changes in the spectra following hydrotreatment are circled or indicated with arrows, respectively, for peaks that go away or appear following hydrotreatment.
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(a) Van Krevelen plot of H/C versus N/C obtained from ESI, APCI, and APPI. (b) Unique assigned molecular formulas for ESI, APCI, and APPI.
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GC × GC‐ToFMS chromatogram of a pine catalytic bio‐oil with an orthogonal system.
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Nonorthogonal separation on GC × GC‐FID of an organic phase from a typical catalytic solvolysis experiment (Areas: 1 = cyclic alkanes, 2 = linear alkanes, 3 + 4 = aromatics, 5 = ketones/alcohols, 6 = acids, 7 = guaiacols, 8 = alkylphenolics, 9 = catechols. Also a = IS, b = BHT (stabilizer in THF solvent), c = aldol condensation products (MIBK, mesityl oxide), d = IPA and acetone).
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