Pyrolysis bio-oils comprise tens of thousands of polyfunctional molecules with ultra-high oxygen content. Therefore, their chemistry is highly dissimilar to fossil fuels. However, upgrading processes generate species that are similar to fossil fuels. Thus, tracking compositional changes of pyrolysis oils through common upgrading processes poses unique analytical challenges. Electrospray ionization (ESI) coupled to high-field (21-tesla) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is uniquely suited to understand bio-oil composition as it reveals the elemental composition of thousands of detected peaks. However, discrepancies between atomic ratios (e.g., H/C and O/C) from FT-ICR MS analysis to that of combustion elemental analysis strongly suggest unwanted bias in the desired uniform ionization of bio-oil species due to selective ionization of low-molecular-weight species with high oxygen content (greater than O₆). The work herein presents separation methods to extend the MS analysis of pyrolysis bio-oils and understand reaction pathways in biorefinery processes. The results indicate that direct analyses of whole pyrolysis bio-oils reveal moderate complexity, up to ~14,000 unique elemental compositions, and a wide range of heteroatom content, up to twenty-two oxygen atoms per molecule (O₂₂ class). Fractionation methods have been developed based on separations previously used for traditional fossil fuels. However, the new methods employ an uncommon elutropic series and additives to solve challenges presented by high oxygen content. Several bio-oil samples were separated via extrography with silica gel and solvents such as acetone and water. Another separation method employed solid phase extraction with a polymeric stationary phase and eluents doped with “displacing” additives. Fractionation of bio-oils and hydrotreated effluents revealed two-fold more molecular formulas, diminished effects of selective ionization, and helped determine optimal pyrolysis and hydrotreating conditions. The SPE method provided bio-oil separation based on aromaticity and oxygen content, which currently helps understand complex reaction pathways for biomass pyrolysis and upgrading processes in a pilot plant-scale unit.

Pierre Giusti

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