Abstract
BackgroundBiorefining of lignocellulosic biomass has become one of the most valuable alternatives for the production of multi-products such as biofuels. Pretreatment is a prerequisite to increase the enzymatic conversion of the recalcitrant lignocellulose. However, there is still considerable debate regarding the key features of biomass impacting the cellulase accessibility. In this study, we evaluate the structural and chemical features of three different representative biomasses (Miscanthus × giganteus, poplar and wheat straw), before and after steam explosion pretreatment at increasing severities, by monitoring chemical analysis, SEM, FTIR and 2D NMR.ResultsRegardless the biomass type, combined steam explosion pretreatment with dilute sulfuric acid impregnation resulted in significant improvement of the cellulose conversion. Chemical analyses revealed that the pretreatment selectively degraded the hemicellulosic fraction and associated cross-linking ferulic acids. As a result, the pretreated residues contained mostly cellulosic glucose and lignin. In addition, the pretreatment directly affected the cellulose crystallinity but these variations were dependent upon the biomass type. Important chemical modifications also occurred in lignin since the β-O-4′ aryl-ether linkages were found to be homolytically cleaved, followed by some recoupling/recondensation to β-β′ and β-5′ linkages, regardless the biomass type. Finally, 2D NMR analysis of the whole biomass showed that the pretreatment preferentially degraded the syringyl-type lignin fractions in miscanthus and wheat straw while it was not affected in the pretreated poplar samples.ConclusionsOur findings provide an enhanced understanding of parameters impacting biomass recalcitrance, which can be easily generalized to both woody and non-woody biomass species. Results indeed suggest that the hemicellulose removal accompanied by the significant reduction in the cross-linking phenolic acids and the redistribution of lignin are strongly correlated with the enzymatic saccharification, by loosening the cell wall structure thus allowing easier cellulase accessibility. By contrast, we have shown that the changes in the syringyl/guaiacyl ratio and the cellulose crystallinity do not seem to be relevant factors in assessing the enzymatic digestibility. Some biomass type-dependent and easily measurable FTIR factors are highly correlated to saccharification.
Highlights
Biorefining of lignocellulosic biomass has become one of the most valuable alternatives for the production of multi-products such as biofuels
The present study highlights some key structural features impacting the enzymatic saccharification of steam exploded lignocellulosic biomass (LCB) varying both in pretreatment severity and in biomass type, i.e. Miscanthus × giganteus, poplar
All the values are expressed as mean ± standard deviation (SD) of triplicate a Represents the ester-linked fraction released during 2 M NaOH hydrolysis at 35 °C for 2 h b Represents the difference between the ester-linked fraction and the total fraction released during NaOH hydrolysis c Represents the ester- and the ether-linked fraction released during 4 M NaOH hydrolysis at 170 °C for 2 h
Summary
Biorefining of lignocellulosic biomass has become one of the most valuable alternatives for the production of multi-products such as biofuels. The pretreatment step aims to reduce the lignin and/or the hemicellulose content, simultaneously disrupting the plant cell wall architecture increasing surface area and cellulose accessibility [15]. Among these pretreatments, steam explosion has been increasingly considered as one of the most efficient, environmentally friendly and cost-effective processes for industrial application and, has been widely tested at the pilot scale for various LCBs [16,17,18]. Steam explosion involves exposing LCB to high-pressure saturated steam and reducing pressure swiftly, making the materials undergoing an explosive decompression This results in the breakdown of the lignocellulosic matrix, partial removal and/or redistribution of lignin and significant enhancement of the cellulose accessibility [16]. The dilute acid presoaked steam explosion process is considered to be close to industrial application [19]
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