The mechanism of xylans removal during hydrothermal pretreatment of poplar fibers investigated by immunogold labeling

Selective degradation of hemicellulose and lignin for improving enzymolysis efficiency via pretreatment using deep eutectic solvents

Deep eutectic solvents (DESs) with different acidity and alkalinity were applied for biomass pretreatment, and the conditions were optimized by response surface methodology. The results showed that lactic acid/betaine hydrochloride had the optimal pretreatment efficiency, where the removal rates of hemicellulose and lignin came up to 89% and 73%, and the enzymolysis efficiency was as high as 92%. Furthermore, eight types of chloride salts with different valence states were introduced into the DESs as the third component. The chloride salts could improve the pretreatment efficiency and positively correlated with the metal valence state. Specifically, AlCl₃ was significantly superior in improving the pretreatment efficiency, where the enzymolysis efficiency reached 96% due to the destruction of crystalline region and the esterification of partial cellulose. Therefore, it is proposed that adding highly valent metal salts to acidic DESs has higher pretreatment and enzymatic efficiency.

Chemical Characteristics of Wood Cell Wall with an Emphasis on Ultrastructure: A Mini-Review

Wood is complex in its chemical composition that has an important influence on its chemical behavior and mechanical strength. The complexity is reflected in the ultrastructure of the wood cell wall. In particular, the concentration of main components (cellulose, hemicelluloses and lignin) changes depending on many factors such as the different type or parts of wood, and varies in different cell wall layers. From an ultrastructural standpoint, we describe the current level of knowledge about chemical characteristics of the wood cell walls. The information of distribution of main components in the cell walls of normal wood, reaction wood and water-logged archaeological wood, the cellulose microfibrils orientation, and the interactions between main components were presented based on the use of advanced techniques including transmission electron microscopy, scanning electron microscopy, spectral imaging and nuclear magnetic resonance. In addition, the chemical changes of the wood cell wall during pretreatment are discussed. This mini-review not only provides a better understanding of wood chemistry, but also brings new insights into cell wall recalcitrance.

Optimized production of xylooligosaccharides from poplar: A biorefinery strategy with sequential acetic acid/sodium acetate hydrolysis followed by xylanase hydrolysis

The preparation of xylooligosaccharides (XOS) from lignocelluloses by organic acid hydrolysis has the advantages of high efficiency and simplicity, but reducing the production of by-products, especially xylose, is a prerequisite for commercial preparation of XOS using organic acid. In this work, to reduce the production of by-products, the acetic acid/sodium acetate conjugate system (AC/SA) was used to prepare XOS from poplar. Under the optimal conditions (0.15 M AC/SA, molar ratio of 3.0, 175 °C, 60 min), the maximum XOS yield was 33.6% with a low xylose/XOS ratio of 0.19. Xylanase hydrolysis effectively converted XOS with DP above 6 in the AC/SA hydrolysate to X2-X6 with little xylose produced. The XOS yield increased to 42.1%, with a xylose/XOS ratio was only 0.17. This work shows that AC/SA in combination with xylanase hydrolysis of poplar successfully achieved high XOS yield with low by-products yields without the extraction of xylan from the substrate.

Co-production of xylooligosaccharides and glucose from birch sawdust by hot water pretreatment and enzymatic hydrolysis

A green method for co-production of value-added xylooligosaccharides (XOS) and glucose from birch was demonstrated using hot water pretreatment. Effects of pretreatment severity factor (Log R₀) on XOS production and enzymatic hydrolysis were investigated. At Log R₀ of 4.05 (180 °C, 50 min), the maximum hydrolysis yield (80.8%) was obtained. At Log R₀ of 3.91 (170 °C, 70 min), the maximum XOS yield (46.1%) was obtained, however the hydrolysis yield decreased to 70.3%. To achieve both the high XOS yield and high glucose output, Tween 80 addition (0.075 g/g cellulose) was employed, leading to an improvement in hydrolysis yield from 70.3% to 89.4%. From a mass balance perspective, 104.6 g of XOS and 372.9 g of glucose could be produced from 1000 g birch. These results demonstrated that birch sawdust is a promising lignocellulosic material for co-production of XOS and glucose.

Ultrastructural change in lignocellulosic biomass during hydrothermal pretreatment

In recent years, visualization and characterization of lignocellulose at different scales elucidate the modifications of its ultrastructural and chemical features during hydrothermal pretreatment which include degradation and dissolving of hemicelluloses, swelling and partial hydrolysis of cellulose, melting and redepositing a part of lignin in the surface. As a result, cell walls are swollen, deformed and de-laminated from the adjacent layer, lead to a range of revealed droplets that appear on and within cell walls. Moreover, the certain extent morphological changes significantly promote the downstream processing steps, especially for enzymatic hydrolysis and anaerobic fermentation to bioethanol by increasing the contact area with enzymes. However, the formation of pseudo-lignin hinders the accessibility of cellulase to cellulose, which decreases the efficiency of enzymatic hydrolysis. This review is intended to bridge the gap between the microstructure studies and value-added applications of lignocellulose while inspiring more research prospects to enhance the hydrothermal pretreatment process.

Multiscale investigation on the chemical and anatomical changes of lignocellulosic biomass for different severities of hydrothermal treatment

The chemical changes sustained by lignocellulosic biomass during hydrothermal treatment are reflected at multiple scales. This study proposes to benefit from this multiscale nature in order to provide a global understanding of biomass alterations during hydrothermal treatment. For this purpose, complementary imaging techniques-confocal Raman microscopy and X-ray nano-tomography-analysed by image processing and coupled to chemical measurements were used. This unique combination of analyses provided valuable information on topochemical and morphological changes of poplar samples, without the artefacts of sample preparation. At the cell wall level, holocellulose hydrolysis and lignin modifications were observed, which corresponded to anatomical modifications observed at higher scales. Overall, after treatment, samples shrank and had thinner cell walls. When subjected to more severe pre-treatments, cells were disrupted and detached from adjacent cells. Anatomical changes were then used to obtain quantitative indicators of the treatment severity. The effects of treatment at different scales can thus be quantitatively connected in both directions, from micro to macro and from macro to micro.

Engineering aspects of hydrothermal pretreatment: From batch to continuous operation, scale-up and pilot reactor under biorefinery concept

Different pretreatments strategies have been developed over the years mainly to enhance enzymatic cellulose degradation. In the new biorefinery era, a more holistic view on pretreatment is required to secure optimal use of the whole biomass. Hydrothermal pretreatment technology is regarded as very promising for lignocellulose biomass fractionation biorefinery and to be implemented at the industrial scale for biorefineries of second generation and circular bioeconomy, since it does not require no chemical inputs other than liquid water or steam and heat. This review focuses on the fundamentals of hydrothermal pretreatment, structure changes of biomass during this pretreatment, multiproduct strategies in terms of biorefinery, reactor technology and engineering aspects from batch to continuous operation. The treatise includes a case study of hydrothermal biomass pretreatment at pilot plant scale and integrated process design.

Fractionation of lignocellulosic biopolymers from sugarcane bagasse using formic acid-catalyzed organosolv process

A one-step formic acid-catalyzed organosolv process using a low-boiling point acid-solvent system was studied for fractionation of sugarcane bagasse. Compared to H₂SO₄, the use of formic acid as a promoter resulted in higher efficiency and selectivity on removals of hemicellulose and lignin with increased enzymatic digestibility of the cellulose-enriched solid fraction. The optimal condition from central composite design analysis was determined as 40 min residence time at 159 °C using water/ethanol/ethyl acetate/formic acid in the respective ratios of 43:20:16:21%v/v. Under this condition, a 94.6% recovery of cellulose was obtained in the solid with 80.2% cellulose content while 91.4 and 80.4% of hemicellulose and lignin were removed to the aqueous-alcohol-acid and ethyl acetate phases, respectively. Enzymatic hydrolysis of the solid yielded 84.5% glucose recovery compared to available glucan in the raw material. Physicochemical analysis revealed intact cellulose fibers with decreased crystallinity while the hemicellulose was partially recovered as mono- and oligomeric sugars. High-purity organosolv lignin with < 1% sugar cross-contamination was obtained with no major structural modification according to Fourier-transform infrared spectroscopy. The work represents an alternative process for efficient fractionation of lignocellulosic biomass in biorefineries.

Probing and visualizing the heterogeneity of fiber cell wall deconstruction in sugar maple (Acer saccharum) during liquid hot water pretreatment

The S2 layer was differentiated into heavy-damaged region with more polysaccharides removed and relatively intact light-damaged region after LHW pretreatment.