Effective biomass fractionation and lignin stabilization using a diol DES system

Practical, high-yield lignin depolymerization methods could greatly increase biorefinery productivity and profitability. However, development of these methods is limited by the presence of interunit carbon-carbon bonds within native lignin, and further by formation of such linkages during lignin extraction. We report that adding formaldehyde during biomass pretreatment produces a soluble lignin fraction that can be converted to guaiacyl and syringyl monomers at near theoretical yields during subsequent hydrogenolysis (47 mole % of Klason lignin for beech and 78 mole % for a high-syringyl transgenic poplar). These yields were three to seven times those obtained without formaldehyde, which prevented lignin condensation by forming 1,3-dioxane structures with lignin side-chain hydroxyl groups. By depolymerizing cellulose, hemicelluloses, and lignin separately, monomer yields were between 76 and 90 mole % for these three major biomass fractions. Show more

A recyclable and green biomass-derived deep eutectic solvent (DES) pretreatment was developed to deconstruct the recalcitrant structure of Eucalyptus for further cellulose enzymatic saccharification and lignin valorization. A low-cost and green biorefinery will increase the economy and revenue from lignocellulosic biomass. Herein, a biomass-derived deep eutectic solvent (DES) pretreatment was developed to deconstruct the recalcitrant structure of Eucalyptus for further cellulose enzymatic hydrolysis and lignin valorization. The DES consisted of biomass-derived chemicals (lactic acid and choline chloride). The results showed that DES pretreatment resulted in notable removal of hemicelluloses and lignin, and drastically reduced “biomass recalcitrance”. Under the optimum conditions (DES ratio: 10 : 1, temperature: 110 °C, time: 6 h), the glucose yield by enzymatic hydrolysis reached 94.3%, which was significantly enhanced 9.8 times compared to that of the original biomass without DES pretreatment. The state-of-the-art analysis indicated that the regenerated lignin exhibited well-preserved structures ( i.e. , β- O -4, β–β linkages) without contaminated carbohydrates, and it had a relatively low and homogeneous molecular weight. All these structural characteristics suggested that lignin has great potential application in its conversion into bio-based chemicals and materials. Besides, it is urgent to develop low-cost recycled DESs as green solvents for sustainable biomass pretreatment. The lifetime and recyclability experiment of the DES solution showed that the recovery yield of the DES was at least 90% and the fundamental structural properties of the recycled DES were almost unchanged throughout the recycling cycles. More importantly, the pretreatment efficiency (delignification and enzymatic saccharification) was still largely maintained after the recycling process. Overall, this work demonstrated that biomass pretreatment with the recycled DES was promising for a low-cost biorefinery to achieve an efficient fractionation of lignocellulosic biomass into fermentable glucose and high-quality lignin with tailored chemical structures. Show more

With the aid of DFT calculation, deep eutectic solvents can be designed more powerful for the pretreatment of lignocellulose and the production of biochemicals. As an emerging generation of green solvents, deep eutectic solvents (DESs) are promising for the pretreatment of lignocellulose and the production of biochemicals. However, not all DESs are effective for the cleavage of lignin–carbohydrate complexes (LCCs) in lignocellulose and the fractionation of lignin. In this study, we analyzed the nature of complex molecular interactions between choline chloride (ChCl) and glycerol in ChCl/glycerol (1 : 2) DES using density functional theory and the Kamlet–Taft solvatochromic method. The ChCl–glycerol DES exhibited weak competing interactions towards the linkages in the LCC network because the intramolecular hydrogen bonds (H-bonds) in ChCl–glycerol were constrained by mutually anionic H-bonds ([Cl(glycerol)] − ) and cationic H-bonds ([Ch(glycerol)] + ). Furthermore, because of the absence of active protons and acidic sites, the DES was unable to cleave ether bond linkages in the LCCs. Accordingly, we designed a three-constituent DES (3c-DES) by coordinating AlCl 3 ·6H 2 O in ChCl/glycerol DES based on an acidic multisite coordination theory. The competition of anionic H-bonds and unidentate aluminum ligands was synchronized to form supramolecular complexes, allowing the multisite bridging ligands to cleave both the H-bonds and ether bonds in LCCs. Consequently, the lignin fractionation efficiency was significantly improved from 3.61% to 95.46%, and the lignin purity reached 94 ± 0.45%. Show more