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

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.

Quantitative visualization of subcellular lignocellulose revealing the mechanism of alkali pretreatment to promote methane production of rice straw

“…This may be due to the hydrolysis of lignin being greatly dependent on access to the alkali solution; therefore, high accessibility between the epidermis and alkali solution lead to a significant degradation of lignin, as well as to less lignin degradation in the middle of cross sections along with less accessibility to the alkali solution. This result was consistent with former research [59], in which the lignin distribution change in fiber cells of Eucalyptus was illustrated by Raman images with the integration of band intensity (1547-1707 cm −1 ), however, with the exception of fiber cells, other types of cells had not been studied. Furthermore, a distribution change of polysaccharides, the core components of lignocellulosic biomass transformation, had also not been studied [59].…”

“…This result was consistent with former research [59], in which the lignin distribution change in fiber cells of Eucalyptus was illustrated by Raman images with the integration of band intensity (1547-1707 cm −1 ), however, with the exception of fiber cells, other types of cells had not been studied. Furthermore, a distribution change of polysaccharides, the core components of lignocellulosic biomass transformation, had also not been studied [59]. From the histogram of lignocellulose contents of each pixel in chemical images (Fig.…”

Effect of lignin fractions isolated from different biomass sources on cellulose oxidation by fungal lytic polysaccharide monooxygenases

“…A higher amount of aromatic –OH groups indicates higher reactivity of lignin, while a lower amount of aliphatic –OH groups denotes a much intense pretreatment [ 45 ]. The increase in phenolic –OH content is a result of β- O -4 linkages cleavage and leads to a greater fragmentation of the lignin structure [ 46 ], as verified by the molecular weight reduction in the above results. It was also observed that the lignins from the liquid fractions of acid-catalyzed organosolv pretreatment (OS-A-S-LF and OS-A-B-LF) had the highest amount of free –OH groups (Table 4 ), which would make them the most reactive among other lignin fractions.…”

Determination of hydroxyl groups in biorefinery resources via quantitative 31P NMR spectroscopy

“…A large variety of depolymerization or condensation reactions typically occur within the lignin structure under such conditions. Quantitative 31 P NMR analyses of lignins isolated from various biomass pretreatment technologies have been highlighted in recent studies 101,[108][109][110][111][112][113][114][115][116][117] . As lignin has become a key genetic engineering target for the enhancement of wood quality and biofuel production 118,119 , quantitative 31 P NMR analyses also have been carried out on transgenic lignin 97,120 .…”