Performance of three delignifying pretreatments on hardwoods: hydrolysis yields, comprehensive mass balances, and lignin properties

Abstract

Background: In this work, three pretreatments under investigation at the DOE Bioenergy Research Centers (BRCs) were subjected to a side-by-side comparison to assess their performance on model bioenergy hardwoods (a eucalyptus and a hybrid poplar). These include co-solvent-enhanced lignocellulosic fractionation (CELF), pretreatment with an ionic liquid using potentially biomass-derived components (cholinium lysinate or [Ch][Lys]), and two-stage Cu-catalyzed alkaline hydrogen peroxide pretreatment (Cu-AHP). For each of the feedstocks, the pretreatments were assessed for their impact on lignin and xylan solubilization and enzymatic hydrolysis yields as a function of enzyme loading. Lignins recovered from the pretreatments were characterized for polysaccharide content, molar mass distributions, β-aryl ether content, and response to depolymerization by thioacidolysis.Results: All three pretreatments resulted in significant solubilization of lignin and xylan, with the CELF pretreatment solubilizing the majority of both biopolymer categories. Enzymatic hydrolysis yields were shown to exhibit a strong, positive correlation with the lignin solubilized for the low enzyme loadings. The pretreatment-derived solubles in the [Ch][Lys]-pretreated biomass were presumed to contribute to inhibition of enzymatic hydrolysis in the eucalyptus as a substantial fraction of the pretreatment liquor was carried forward into hydrolysis for this pretreatment. The pretreatment-solubilized lignins exhibited significant differences in polysaccharide content, molar mass distributions, aromatic monomer yield by thioacidolysis, and β-aryl ether content. Key trends include a substantially higher polysaccharide content in the lignins recovered from the [Ch][Lys] pretreatment and high β-aryl ether contents and aromatic monomer yields from the Cu-AHP pretreatment. For all lignins, the 13 C NMR-determined β-aryl ether content was shown to be correlated with the monomer yield with a second-order functionality. Conclusions:Overall, it was demonstrated that the three pretreatments highlighted in this study demonstrated uniquely different functionalities in reducing biomass recalcitrance and achieving higher enzymatic hydrolysis yields for the hybrid poplar while yielding a lignin-rich stream that may be suitable for valorization. Furthermore, modification of lignin during pretreatment, particularly cleavage of β-aryl ether bonds, is shown to be detrimental to subsequent depolymerization.

Unlocking Structure–Reactivity Relationships for Catalytic Hydrogenolysis of Lignin into Phenolic Monomers

“…This shows the diverse structural properties of lignin are fundamentally correlated with and affect the depolymerization reaction . Therefore, pretreatments or fractionation processes that preserve lignin β‐O‐4 aryl ether bonds are desirable approaches if monoaromatic compounds are targeted …”

Organosolv pretreatment assisted by carbocation scavenger to mitigate surface barrier effect of lignin for improving biomass saccharification and utilization

“…However, the presence of lignin in lignocellulosic biomass and the resulting complex lignocellulosic matrix make it highly resistant towards saccharification by microorganisms or enzymes to produce fermentable sugars [ 5 ]. Thereby, a pretreatment stage is always essential to open up lignocellulosic matrix by fractionating lignin and/or partial hemicellulose components, making cellulose more available to enzymatic attack [ 6 , 7 ].…”

“…However, during acid EOS pretreatment, repolymerization reactions of lignin often occur, through carbocation-induced condensation [ 7 , 15 ] and radical-induced coupling of lignin fragments [ 1 ]. Repolymerization reactions cause a form of more condensed and less hydrophilic lignin structure, which aggravates the hydrophobic interactions between lignin and enzymes in subsequent enzymatic hydrolysis [ 16 ].…”

Determination of Alpha-, Beta- and Gamma- Cellulose in Bagasse and Wheat Straw: Lignin Recovery, Characterization and Depolymerization

“…To recover lignin from lignocellulosic biomass, several methods were reported using Kraft, soda, two-stage alkaline oxidative treatment, lignosulphate, dilute acid, enzymatic, ionic liquids, organosolv process and more (Schutyser et al, 2018;Sandip K. Singh et al, 2019;Bhalla et al, 2019;Sandip K. Singh, 2020). These processes operated at a range of reaction conditions, including temperature, time, chemical, organic solvents, ionic liquids, enzymes, pH, and alkali loadings (Mosier et al, 2005;Sandip K. Singh, 2020).…”

“…Ionic liquid (IL) is considered as Green solvents or catalysts due to their specific properties, including low vapor pressure, non-corrosive relative to mineral acids or bases, good thermal stability, and more properties (Sandip K. Singh and Savoy, 2020). Considering the specific properties of ILs, they have been applied in various reactions, including lignocellulosic biomass deconstruction, lignin depolymerization to aromatic monomers and more (Sandip K. Singh and Dhepe, 2016a;2019;Bhalla et al, 2019).…”