Abstract
Stabilization of reactive intermediates is an enabling concept in biomass fractionation and depolymerization. Deep eutectic solvents (DES) are intriguing green reaction media for biomass processing; however undesired lignin condensation is a typical drawback for most acid-based DES fractionation processes. Here we describe ternary DES systems composed of choline chloride and oxalic acid, additionally incorporating ethylene glycol (or other diols) that provide the desired ‘stabilization’ function for efficient lignocellulose fractionation, preserving the quality of all lignocellulose constituents. The obtained ethylene-glycol protected lignin displays high β-O-4 content (up to 53 per 100 aromatic units) and can be readily depolymerized to distinct monophenolic products. The cellulose residues, free from condensed lignin particles, deliver up to 95.9 ± 2.12% glucose yield upon enzymatic digestion. The DES can be recovered with high yield and purity and re-used with good efficiency. Notably, we have shown that the reactivity of the β-O-4 linkage in model compounds can be steered towards either cleavage or stabilization, depending on DES composition, demonstrating the advantage of the modular DES composition.
Highlights
Stabilization of reactive intermediates is an enabling concept in biomass fractionation and depolymerization
We present the design of inexpensive and potentially biobased ternary deep eutectic solvent systems consisting of choline chloride (ChCl) as hydrogen bond acceptors (HBA) as well as oxalic acid (OA) and a varying amount of diols as hydrogen bond donors (HBD), which allow for tuning the reactivity of lignin β-O-4 model compounds (Fig. 1c) either to bond cleavage/stabilization or selective derivatization/protection by careful selection of the type and relative ratio of Deep eutectic solvents (DES) components
We have previously shown that unstable 5a undergoes recondensation reactions that can be suppressed by trapping in the form of cyclic acetals by a variety of diols[7]
Summary
Stabilization of reactive intermediates is an enabling concept in biomass fractionation and depolymerization. Recognizing the excellent solvent strength and ability to break the strong interactions between lignin and cellulose, many binary and ternary DES compositions were applied for lignocellulose fractionation with promising results[15,16,17,18,19,20,21,22] These systems may comprise choline chloride ChCl as HBD and organic acids or alcohols as HBA or incorporate mineral or Lewis acid additives. A detailed discussion on compositions and uses of relevant systems is included in Supplementary Note 1 While these systems generally deliver good quality cellulose, the lignins obtained this way typically suffer undesired structural modification (affected by the acid content of the DES) via significant loss of their valuable arylether structure and the formation of recalcitrant C–C bonds, which renders them poorly suitable for further catalytic depolymerization (Fig. 1a)[16,18,23]
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