ABSTRACT
Comprehensive on-line two-dimensional liquid chromatography (on-line LC × LC) was used for the characterization of bio-oils obtained by fast pyrolysis of lignocellulosic biomass. The resulting bio-oil contains a large number of oxygenated chemical families and must therefore be upgraded before being used as drop-in transportation biofuels. The good knowledge of its complex composition is essential for optimizing the mandatory bio-oil upgrading process to biofuels, thereby requiring powerful separation techniques designed to be hyphenated to mass spectrometry detection (LC × LC-MS). In this study, reversed phase conditions were optimized in both dimensions for the RPLC × RPLC separation of the aqueous fraction of bio-oils. The first step of method development consisted in searching for a suitable set of RP-conditions via the screening of a large number of RP-systems (made up of different stationary phases and/or mobile phases and/or temperature). The practical peak capacity and the degree of orthogonality were calculated for a sample of 38 representative compounds, both descriptors having been considered as selection criterion. Two different couplings were chosen and evaluated for the RPLC × RPLC separation of the 38 representative compounds. The best of both, in terms of real practical peak capacity, was further successfully applied to the separation of the aqueous phase of a partially dehydroxygenated bio-oil.
Subject(s)
Biofuels , Chromatography, Reverse-Phase/methods , Biomass , Mass SpectrometryABSTRACT
Biomass fast pyrolysis is considered as a promising route to produce liquid for the transportation field from a renewable resource. However, the derived bio-oils are mainly oxygenated (45-50%w/w O on a wet basis) and contain almost no hydrocarbons. Therefore, upgrading is necessary to obtain a liquid with lower oxygen content and characterization of oxygenated compounds in these products is essential to assist conversion reactions. For this purpose, comprehensive two-dimensional gas chromatography (GC × GC) can be investigated. Oxygen speciation in such matrices is hampered by the large diversity of oxygenated families and the complexity of the hydrocarbon matrix. Moreover, response factors must be taken into account for oxygenate quantification as the Flame Ionisation Detector (FID) response varies when a molecule contains heteroatoms. To conclude, no distillation cuts were accessible and the analysis had to cover a large range of boiling points (30-630 °C). To take up this analytical challenge, a thorough optimization approach was developed. In fact, four GC × GC column sets were investigated to separate oxygenated compounds from the hydrocarbon matrix. Both model mixtures and the upgraded biomass flash pyrolysis oil were injected using GC × GC-FID to reach a suitable chromatographic separation. The advantages and drawbacks of each column combination for oxygen speciation in upgraded bio-oils are highlighted in this study. Among the four sets, an original polar × semi-polar column combination was selected and enabled the identification by GC × GC-ToF/MS of more than 40 compounds belonging to eight chemical families: ketones, furans, alcohols, phenols, carboxylic acids, guaiacols, anisols, and esters. For quantification purpose, the GC × GC-FID chromatogram was divided into more than 60 blobs corresponding to the previously identified analyte and hydrocarbon zones. A database associating each blob to a molecule and its specific response factor (determined by standards injection at different concentrations) was created. A detailed molecular quantification by GC × GC-FID was therefore accessible after integration of the corrected normalized areas. This paper aims to present a detail level in terms of characterization of oxygenated compounds in upgraded bio-oils which to our knowledge has never been reached so far. It is based on an original column set selection and an extremely accurate quantification procedure.
Subject(s)
Biofuels , Chromatography, Gas/methods , Fuel Oils , Flame Ionization , OxygenABSTRACT
Considering the global energetic context, diversifying fuels is of growing importance and many new alternatives are promising. Coal liquefaction products definitely appear among the new generation substitutes. These product's characteristics are very far from fuel specifications as they are mainly composed of naphthenes, aromatics, polycondensed naphthenic and aromatic structures and heteroatomic compounds (nitrogen and oxygen), with a very low paraffin content. Identification and quantification of oxygen-containing species in coal-derived liquids are of considerable importance to understand their behaviors in further processing. However, these species have not been characterized as fully as the predominant hydrocarbon components. Literature shows that these compounds consist mainly in alkylated phenolic and furanic structures. Therefore, comprehensive two-dimensional gas chromatography has been investigated to provide enhanced molecular characterization of these complex samples. Several different configurations involving innovative column configurations were tested. Each of them was optimized by testing different column lengths, modulation periods, and oven conditions. A comparison of the contribution of each column configuration was carried out regarding four main criteria: individual separation of oxygenates, group type separation, resolution, and space occupation. One of them enabled an outstanding separation of paraffins, naphthenes, monoaromatics, diaromatics and targeted O-compounds in a direct coal liquefaction product. It was therefore subjected to further experimentations using a time-of-flight mass spectrometer to validate the identification and unravel more than fifty oxygenated molecular structures. A group-type quantification was also established for four column arrangements and gives the distribution of paraffins, naphthenes and aromatics. It can be concluded from this study that a non-orthogonal arrangement involving a highly polar column in the first dimension was the most adapted one.
