Mild Organosolv Lignin Extraction with Alcohols: The Importance of Benzylic Alkoxylation

Abstract

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, αsubstitution with not only bulky secondary alcohols and tertiary alcohols but also chloride was 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.

Multiscale analysis of lignocellulose recalcitrance towards OrganoCat pretreatment and fractionation

“…The proportion of β– O –4 linkages has only a minor effect in this, which is different for the lignin yield in the organic phase. The positive coefficient indicates the effectiveness of OrganoCat, similar to other organosolv pretreatments, to be partially dependent on the cleavage of β-aryl ether linkages [ 51 , 52 ]. As expected, biomass with higher content of β– O –4 linkages yielded a higher amount of extracted lignin.…”

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review

“…In 2019, Ragauskas and co‐workers developed acid‐catalyzed diol pretreatment of eucalyptus lignin and 1,4‐butanediol (BDO) pretreated lignin retained higher amount of β‐O‐4 linkages than ethanol pretreated lignin, indicating that 1,4‐BDO quenched the benzyl carbocation species and formed ether linkages with a hydroxyl tail at the C α position of side‐chain . Recently, the group of Deuss applied four primary alcohols (ethanol, n ‐propanol, n ‐butanol, n ‐pentanol) during organosolv fractionation process and observed etherified structure at benzylic C α position, which could not only protect β‐O‐4 motif but also enhance delignification degree under pretreatment condition …”

Reductive catalytic fractionation of pine wood: elucidating and quantifying the molecular structures in the lignin oil

“…24 In this respect, lignin-rst approaches are currently under development, focusing primarily on the valorisation of lignin. [24][25][26][27] One particular lignin-rst rening technology, Reductive Catalytic Fractionation (RCF), has emerged notably in recent years. 24,28 RCF targets a maximum carbon efficiency, by focusing primarily on lignin extraction, leaving most of the carbohydrates unaltered, mainly as pulp.…”