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In this study, Populus trichocarpa was subjected to dilute acid pretreatment at varying pretreatment times. The three major components of lignocellulosic biomass, namely cellulose, hemicellulose and lignin, were isolated from the starting and dilute acid pretreated poplar. Gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) techniques were utilized to elucidate structural transformations of poplar during dilute acid pretreatment. The results demonstrated that the pretreatment dissolved hemicelluloses and disrupted structural features of lignin and polysaccharides. As revealed by NMR, the aryl-O-ether linkage (b-O-4) of lignin was extensively cleaved and lignin repolymerization occurred during pretreatment. The lignin was also observed to have a decrease in S/G ratio and methoxyl group content and these changes were accompanied with an increase in condensed lignin. The dilute acid pretreatment resulted in a reduction in molecular weight of cellulose and hemicellulose, while no prominent change of molecular weight was observed for lignin. The polydispersity index of cellulose appeared to increase initially within a short time of pretreatment (0.3-1 min) and start to decrease with longer pretreatment time during the bulk phase of chain scission (5.4-26.8 min). The DA pretreatment demonstrated no significant impact on the crystalline index (CrI) of cellulose particularly within the short time range of pretreatments examined in this study, with CrI remaining almost unchanged during the pretreatment time of 0.3-5.4 min and a slight increase observed as the pretreatment time extended to 8.5 and 26.8 min.

An anti-freezing and moisturizing conductive hydrogel, capable of harvesting energy from moisture, was developed by incorporating tannic acid and carbon nanotubes into polyvinyl alcohol containing a water–glycerol dispersion.

Transparent, UV-filtered, anti-freezing, and moisture-retention organohydrogel-based sensors are prepared by incorporating ions/microparticles into a binary solvent system.

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BackgroundHydrothermal pretreatment using liquid hot water (LHW) is capable of substantially reducing the cell wall recalcitrance of lignocellulosic biomass. It enhances the saccharification of polysaccharides, particularly cellulose, into glucose with relatively low capital required. Due to the close association with biomass recalcitrance, the structural change of the components of lignocellulosic materials during the pretreatment is crucial to understand pretreatment chemistry and advance the bio-economy. Although the LHW pretreatment has been extensively applied and studied, the molecular structural alteration during pretreatment and its significance to reduced recalcitrance have not been well understood.ResultsWe investigated the effects of LHW pretreatment with different severity factors (log R 0) on the structural changes of fast-grown poplar (Populus trichocarpa). With the severity factor ranging from 3.6 to 4.2, LHW pretreatment resulted in a substantial xylan solubilization by 50–77% (w/w, dry matter). The molecular weights of the remained hemicellulose in pretreated solids also have been significantly reduced by 63–75% corresponding to LHW severity factor from 3.6 to 4.2. In addition, LHW had a considerable impact on the cellulose structure. The cellulose crystallinity increased 6–9%, whereas its degree of polymerization decreased 35–65% after pretreatment. We found that the pretreatment severity had an empirical linear correlation with the xylan solubilization (R 2 = 0.98, r = + 0.99), hemicellulose molecular weight reduction (R 2 = 0.97, r = − 0.96 and R 2 = 0.93, r = − 0.98 for number-average and weight-average degree of polymerization, respectively), and cellulose crystallinity index increase (R 2 = 0.98, r = + 0.99). The LHW pretreatment also resulted in small changes in lignin structure such as decrease of β-O-4′ ether linkages and removal of cinnamyl alcohol end group and acetyl group, while the S/G ratio of lignin in LHW pretreated poplar residue remained no significant change compared with the untreated poplar.ConclusionsThis study revealed that the solubilization of xylan, the reduction of hemicellulose molecular weights and cellulose degree of polymerization, and the cleavage of alkyl–aryl ether bonds in lignin resulted from LHW pretreatment are critical factors associated with reduced cell wall recalcitrance. The chemical–structural changes of the three major components, cellulose, lignin, and hemicellulose, during LHW pretreatment provide useful and fundamental information of factors governing feedstock recalcitrance during hydrothermal pretreatment.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0926-6) contains supplementary material, which is available to authorized users.