Abstract
Current lignin fractionation methods use harsh conditions that alter the native lignin structure, resulting in a recalcitrant material which is undesired for downstream processing. Milder fractionation processes allow for the isolation of lignins that are high in β-aryl ether (β-O-4) content, however, at reduced extraction efficiency. The development of improved lignin extraction methods using mild conditions is therefore desired. For this reason, a flow-through setup for mild ethanosolv extraction (120 °C) was developed. The influence of acid concentration, ethanol/water ratio, and the use of other linear alcohol co-solvents on the delignification efficiency and the β-O-4 content were evaluated. With walnut shells as model feedstock, extraction efficiencies of over 55% were achieved, yielding lignin with a good structural quality in terms of β-O-4 linking motifs (typically over 60 per 100 aromatic units). For example, lignin containing 66 β-O-4 linking motifs was obtained with an 80:20 n-propanol/water ratio, 0.18 M H2SO4 with overall a good extraction efficiency of 57% after 5 h. The majority of the lignin was extracted in the first 2 hours and this lignin showed the best structural quality. Compared to batch extractions, both higher lignin extraction efficiency and higher β-O-4 content were obtained using the flow setup.
Highlights
Lignin is the most abundant aromatic biopolymer and, as such, has inspired much research aiming to unlock its potential to serve as feedstock for both bulk and high-value aromatic compounds [1,2,3].valorization of lignin is deemed essential for the development of economically competitive biorefineries [4,5]
100cylindrical mL cylindrical reactor lignocellulosic biomass in which lignin extraction of the stationary biomass source took place. It was with lignocellulosic biomass in which lignin extraction of the stationary biomass source took place
A total β-O-4 content of 65–70 linking motifs was obtained in the middle fractions (2–4 h) of this extraction and an average β-O-4 content of 64 linking motifs. This is sharp contrast with the 35 β-O-4 linking motifs that we reported for the batch extraction with identical reaction conditions [46], showing the superior performance of this extraction setup in terms of structure preservation, while showing only a small decrease in extraction efficiency
Summary
Lignin is the most abundant aromatic biopolymer and, as such, has inspired much research aiming to unlock its potential to serve as feedstock for both bulk and high-value aromatic compounds [1,2,3].valorization of lignin is deemed essential for the development of economically competitive biorefineries [4,5]. The heterogeneous structure of lignin (Figure 1A) and the structural deviation among biomass sources has severely hampered the development of efficient catalytic routes towards selective product formation in combination with high yields [6,7]. There are several recurring linking motifs in the native lignin structure of which the β-aryl ether motif (β-O-4) is by far the most abundant. Many of the developed depolymerization methods focus on selective breakdown of this C–O bonded linking motif, but as the majority cannot be implemented during fractionation [8], the development of methods to extract native-like lignin high in β-O-4 linking motifs is essential. Most of the applied lignin fractionation methods apply harsh extraction conditions that break a significant amount of β-O-4 linking motif [9,10,11,12,13], resulting in a more recalcitrant, condensed
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