Abstract
Pyrolytic lignin (PL) is the collective name of the water-insoluble fraction of pyrolysis oils produced from the fast pyrolysis of lignocellulosic biomass. As the name suggests, PL is composed by fragments derived from lignin, which is the largest natural source of aromatic carbon. Its valorization is of major importance for the realization of economically competitive biorefineries. Nonetheless, the valorization of PL is hindered by its complex structure, which makes the development of tailored strategies for its deconstruction into valuable compounds challenging. In this work, we provide an in-depth analysis of the structural composition of PL obtained from a commercially available pine-derived pyrolysis oil obtained at 500 °C (Empyro B.V., the Netherlands). Molecular weight distribution and thermal stability were accessed by GPC and TGA, respectively, and the monomers present in the PL (≈ 15 wt%) were identified and quantified by chromatographic analyses (GCxGC–FID, GCxGC/TOF–MS, GC–MS and HPLC). Together with FTIR, Py-GC–MS, TAN, elemental analysis and various advanced NMR techniques (13C NMR, 31P NMR, 19F NMR, HSQC NMR, HMBC NMR), structural features of the PL oligomers were elucidated, revealing a guaiacyl backbone linked by alkyl, ether, ester and carbonyl groups, with none of the typical native lignin linkages (i.e. β–O–4, β–β, β–5) present. Furthermore, 72.3 % of the oxygen content in PL could be assigned to specific motifs by the quantitative analyses performed, and oligomeric models were proposed based on the obtained information. We expect that this characterization work can support future research on the development of valorization pathways for PL, allowing the feasible conversion of this promising feedstock into valuable biobased products with a wide range of possible applications, e.g. fuels, materials and specialty chemicals.
Highlights
Lignocellulosic biomass is a widely available source of renewable carbon and a promising feedstock for the replacement of petro-based fuels, chemicals and materials through the conversion in so-called biorefineries [1,2,3,4]
Pyrolysis oils can be fractionated by the addition of water, which leads to the separation of a sugar fraction and a water-insoluble fraction comprised mostly of lignin-derived aromatic fragments [8], typically referred to as pyrolytic lignin (PL)
The dry yield of PL was calculated to be 29.9 wt% of PL based on pyrolysis oil, which is in the 25−30 wt% range reported for lignin content in pine wood [47,48,49]
Summary
Lignocellulosic biomass is a widely available source of renewable carbon and a promising feedstock for the replacement of petro-based fuels, (intermediate) chemicals and materials through the conversion in so-called biorefineries [1,2,3,4]. Pyrolysis oils can be fractionated by the addition of water, which leads to the separation of a sugar (aqueous) fraction and a water-insoluble fraction comprised mostly of lignin-derived aromatic fragments [8], typically referred to as pyrolytic lignin (PL) This allows for the two fractions, intermediates in a pyrolysis oil biorefinery scheme, to be processed independently by strategies tailored to their nature and inherent properties into a wide range of valuable products, e.g. alkylphenolics [9,10], biofuels [4,11,12], hydroxymethylfurfural (HMF) [13], as well as feedstocks suitable for co-feeding traditional refineries [14,15]
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