Abstract
BackgroundEffective enzymatic hydrolysis of lignocellulosic biomass benefits from lignin removal, relocation, and/or modification during hydrothermal pretreatment. Phase transition, depolymerization/repolymerization, and solubility effects may all influence these lignin changes. To better understand how lignin is altered, Populus trichocarpa x P. deltoides wood samples and cellulolytic enzyme lignin (CEL) isolated from P. trichocarpa x P. deltoides were subjected to batch and flowthrough pretreatments. The residual solids and liquid hydrolysate were characterized by gel permeation chromatography, heteronuclear single quantum coherence NMR, compositional analysis, and gas chromatography–mass spectrometry.ResultsChanges in the structure of the solids recovered after the pretreatment of CEL and the production of aromatic monomers point strongly to depolymerization and condensation being primary mechanisms for lignin extraction and redeposition. The differences in lignin removal and phenolic compound production from native P. trichocarpa x P. deltoides and CEL suggested that lignin-carbohydrate interactions increased lignin extraction and the extractability of syringyl groups relative to guaiacyl groups.ConclusionsThese insights into delignification during hydrothermal pretreatment point to desirable pretreatment strategies and plant modifications. Because depolymerization followed by repolymerization appears to be the dominant mode of lignin modification, limiting the residence time of depolymerized lignin moieties in the bulk liquid phase should reduce lignin content in pretreated biomass. In addition, the increase in lignin removal in the presence of polysaccharides suggests that increasing lignin-carbohydrate cross-links in biomass would increase delignification during pretreatment.
Highlights
Effective enzymatic hydrolysis of lignocellulosic biomass benefits from lignin removal, relocation, and/or modification during hydrothermal pretreatment
Since the weight average molecular weight is more sensitive to the presence of large polymers, these results indicate more long chain polymers in the batch pretreated cellulolytic enzyme lignin (CEL) than flow pretreated CEL
The residual solids and liquid hydrolysate were characterized by a wide range of analytical techniques
Summary
Effective enzymatic hydrolysis of lignocellulosic biomass benefits from lignin removal, relocation, and/or modification during hydrothermal pretreatment. The primary monomeric structural units of lignin are p-coumaryl, coniferyl, and sinapyl alcohol (Figure 1) [3,5] Hardwoods, such as Populus, typically contain syringyl and guaiacyl lignin synthesized from sinapyl and coniferyl alcohol, respectively [5], and β-O-4 (β aryl ether) linkages account for approximately 80% of the linkages involving syringyl units [6]. Noncovalent interactions may link lignin and hemicellulose, but there are few interactions between lignin and cellulose in native biomass [6]
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