Lignin holds the key for maximizing value extraction from lignocellulosic biomass. This is currently hindered by the application of fractionation methods that significantly alter the lignin structure to give highly recalcitrant materials. For this reason, it can be highly beneficial to use less severe fractionation conditions that allow for efficient extraction of lignin with retention of the β-aryl ether (β-O-4) content. Here, we present a detailed study on mild alcohol based organosolv fractionation with the aim of understanding how to achieve a balance between efficiency of lignin extraction and the structure of the resulting lignin polymers, using walnut shells as model biomass. Monitoring different extraction conditions reveals how the structure of the extracted lignin changes depending on the extraction conditions in terms of molecular weight, alcohol incorporation and H/G/S ratios. Moving from ethanol to n-pentanol it was revealed that, in particular, alcohol incorporation at the benzylic α-position of β-aryl ether units not only plays a key role in protecting the β-O-4 linking motif, but more importantly increases the solubility of larger lignin fragments under extraction conditions. This study shows that α-substitution already occurs prior to extraction and is essential for reaching improved extraction efficiencies. Furthermore, α-substitution with not only bulky secondary alcohols and tertiary alcohols, but also chloride were revealed for the first time and the latter could be involved in facilitating α-alkoxylation. Overall, this study demonstrates how by tuning the fractionation setup and conditions, the resulting lignin characteristics can be influenced and potentially tailored to suit downstream demands.

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.

Lignin valorization strategies are a key factor for achieving more economically competitive biorefineries based on lignocellulosic biomass. Most of the emerging elegant procedures to obtain specific aromatic products rely on the lignin substrate having a high content of the readily cleavable β-O-4 linkage as present in the native lignin structure. This provides a miss-match with typical technical lignins that are highly degraded and therefore are low in β-O-4 linkages. Therefore, the extraction yields, and the quality of the obtained lignin are of utmost importance to access new lignin valorization pathways. In this manuscript, a simple protocol is presented to obtain lignins with high β-O-4 content by relatively mild ethanol extraction that can be applied to different lignocellulose sources. Furthermore, analysis procedures to determine the quality of the lignins are presented together with a depolymerization protocol that yields specific phenolic 2-arylmethyl-1,3-dioxolanes, which can be used to evaluate the obtained lignins. The presented results demonstrate the link between lignin quality and potential for the lignins to be depolymerized into specific monomeric aromatic chemicals. Overall, the extraction and depolymerization demonstrates a trade-off between the lignin extraction yield and the retention of the native aryl-ether structure and thus the potential of the lignin to be used as the substrate for the production of chemicals for higher-value applications.

Iridium catalysed acceptorless dehydrogenative decarbonylation of organosolv lignin is exemplified for the combined lignin defunctionalisation and liberation of synthesis gas.

Water addition to α,β‐unsaturated nitriles would give facile access to the β‐hydroxy‐nitriles, which in turn can be hydrogenated to the γ‐amino alcohols. We have previously shown that alcohols readily add in 1,4‐fashion to these substrates using Milstein's Ru(PNN) pincer complex as catalyst. However, attempted water addition to α,β‐unsaturated nitriles gave the 3‐hydroxynitriles in mediocre yields. On the other hand, addition of benzyl alcohol proceeded in excellent yields for a variety of β‐substituted unsaturated nitriles. Subsequent treatment of the benzyl alcohol addition products with TMSCl/FeCl₃ resulted in the formation of 3‐hydroxy‐alkylnitriles. The 3‐benzyloxy‐alkylnitriles obtained from oxa‐Michael addition also could be hydrogenated directly in the presence of acid to give the amino alcohols as their HCl salts in excellent yields. Hydrogenation under neutral conditions gave a mixture of the secondary and tertiary amines. Hydrogenation in the presence of base and Boc‐anhydride gave the orthogonally bis‐protected amino alcohols, in which the benzyl ether can subsequently be cleaved to yield Boc‐protected amino alcohols. Thus, a variety of molecular scaffolds with a 1,3‐relationship between O‐ and N‐functional group is accessible starting from oxa‐Michael addition of benzyl alcohol to α,β‐unsaturated nitriles.