Revealing the structure and distribution changes of Eucalyptus lignin during the hydrothermal and alkaline pretreatments

Jing Bian,Hanyin Li,Chenzhou Wang,Mingfei Li,Runcang Sun

doi:10.1038/s41598-017-00711-w

Jing Bian, Hanyin Li + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-00711-w

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Abstract

An integrated pretreatment process based on hydrothermal pretreatment (HTP) followed by alkaline pretreatment has been applied to treat Eucalyptus. The chemical composition and structure changes of lignin during the pretreatment were comprehensively characterized. The surface morphology of the cell walls and lignin distribution of the pretreated Eucalyptus were detected by scanning electron and confocal Raman microscopies. It was found that the chemical bonds between lignin and hemicelluloses were cleaved during the pretreatment. The results also indicated that the contents of β-O-4′, β-β′, and β-5′ linkages were decreased with the increase of hydrothermal pretreatment temperature and the cleavage of β-O-4′ linkages in lignin was accompanied with repolymerization reactions. 31P NMR analysis showed that the content of aliphatic OH was reduced as the temperature increased and the total phenolic OH was elevated and then declined with the increase of temperature. Raman spectra analysis revealed that the dissolution rate of lignin in the secondary wall regions was faster than that in cell corner middle lamella regions during the pretreatment. These results will enhance the understanding of the cell wall deconstruction during the pretreatment and the mechanism of the integrated pretreatment process acting on Eucalyptus.

Highlights

With the ever-increasing energy consumption and deterioration of environment around the world, the exploitation of renewable resources for the production of fuels and materials has garnered much attention
The HTP residue yields were affected by the pretreatment severity and decreased from 73.2 to 61.2%, which was mainly due to the degradation of hemicelluloses and the solubilization of amorphous cellulose during hydrothermal pretreatment
The yields of the lignin fractions increased from 5.1% to 7.5% as the temperature increased from 180 to 200 °C, indicating that the chemical bonds between lignin and hemicelluloses were cleaved to some extent

Summary

Introduction

With the ever-increasing energy consumption and deterioration of environment around the world, the exploitation of renewable resources for the production of fuels and materials has garnered much attention. A series of pretreatment technologies including physical, chemical, physicochemical and biological approaches have been proposed to overcome the biomass recalcitrance[4, 8]. Among these pretreatments, hydrothermal pretreatment (HTP) is an economical and environmentally friendly pretreatment technology for lignocelluloses, since it uses only feedstock and water as the medium, avoiding corrosion problems and the formation of neutralization sludges[9]. It has been reported that alkaline pretreatment is a promising technology for the disruption of cell wall by solubilizing hemicelluloses and lignin effectively[13]. Due to the development of laser and detector technologies, confocal Raman microscopy (CRM) has been widely applied to give insights into both the chemical and structural information of lignocellulosic materials in situ on the subcellular level[17, 18]

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