Kinetics and mechanism of autohydrolysis of hardwoods

Xylan structural diversity, biosynthesis, and functional regulation in plants

Xylan is a vital component of plant cell walls, contributing to their structural integrity and flexibility through interactions with other polymers. Its structure varies among plant species, influencing the mechanical properties of cell walls. Xylan also has significant industrial potential, including in biofuels, biomaterials, food, and pharmaceuticals, due to its ability to be converted into valuable bioproducts. However, key aspects of xylan biosynthesis, regulation, and structural impact on plant growth and structures remain unclear. This review highlights current researches on xylan biosynthesis, modification, and applications, identifying critical gaps in knowledge. Meanwhile the review proposes new approaches to regulate xylan synthesis and understand its role in cell wall assembly and interactions with other polymers. Addressing these gaps could unlock the full industrial potential of xylan, leading to more sustainable applications.

Catalyst-recirculating system in steam explosion pretreatment for producing high-yield of xylooligosaccharides from oat husk

We propose a closed-loop pretreatment process, wherein volatiles produced during steam explosion pretreatment were recovered and reintroduced as acid catalysts into the pretreatment system. The volatiles were separated through a drastic decompression process followed by a steam explosion process and recovered as a liquified catalyst (LFC) through a heat exchanger. The LFC effectively served as an acid catalyst for hemicellulose hydrolysis, significantly decreasing residence time from 90 min to 30 min to achieve 80 % conversion yield at 170 °C. Hydrolysates with high content of lower molecular weight oligomeric sugars were obtained using LFC, and were considered advantageous for application as prebiotics. These results are attributed to the complementary features of acetic acid and furfural contained within the LFC. Computational simulation using Aspen Plus was used to investigate the effects of recycling on LFC, and it demonstrated the feasibility of the catalyst-recirculating system. A validation study was conducted based on simulation results to predict the actual performance of the proposed pretreatment system. Based on these results, the recirculating system was predicted to improve the conversion yield and low-molecular weight oligomers yield by 1.5-fold and 1.6-fold, respectively.

Combination of Autohydrolysis and Catalytic Hydrolysis of Biomass for the Production of Hemicellulose Oligosaccharides and Sugars

Three different types of biomass sourced from forestry waste (eucalyptus residues), agricultural waste (wheat straw), and energy crop (miscanthus) were used as starting materials to produce hemicellulosic sugars, furans (furfural and hydroxymethylfurfural), and oligosaccharides. A two-step hybrid process was implemented; biomass was first autohydrolysed without any additive to extract hemicelluloses and dissolve it in water. Then, the hydrolysate was treated with a solid acid catalyst, TiO2-WOx, in order to achieve hydrolysis and produce monomeric sugars and furans. This article investigates the role of the biomass type, autohydrolysis experimental conditions, polymerisation degree and composition of hemicelluloses on the performance of the process coupling autohydrolysis and catalytic hydrolysis. The highest global yields of both oligosaccharides and monomeric sugars were obtained from Eucalyptus (37% and 18%, respectively).

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.

Effects on Lignin Redistribution in Eucalyptus globulus Fibres Pre-Treated by Steam Explosion: A Microscale Study to Cellulose Accessibility

The objective of this study was to investigate structural changes and lignin redistribution in Eucalyptus globulus pre-treated by steam explosion under different degrees of severity (S₀), in order to evaluate their effect on cellulose accessibility by enzymatic hydrolysis. Approximately 87.7% to 98.5% of original glucans were retained in the pre-treated material. Glucose yields after the enzymatic hydrolysis of pre-treated material improved from 19.4% to 85.1% when S₀ was increased from 8.53 to 10.42. One of the main reasons for the increase in glucose yield was the redistribution of lignin as micro-particles were deposited on the surface and interior of the fibre cell wall. This information was confirmed by laser scanning confocal fluorescence and FT-IR imaging; these microscopic techniques show changes in the physical and chemical characteristics of pre-treated fibres. In addition, the results allowed the construction of an explanatory model for microscale understanding of the enzymatic accessibility mechanism in the pre-treated lignocellulose.

Directional Structure Modification of Poplar Biomass-Inspired High Efficacy of Enzymatic Hydrolysis by Sequential Dilute Acid-Alkali Treatment

A major challenge in converting lignocellulose to biofuel is overcoming the resistance of the biomass structure. Herein, sequential dilute acid-alkali/aqueous ammonia treatment was evaluated to enhance enzymatic hydrolysis of poplar biomass by removing hemicellulose first and then removing lignin with acid and base, respectively. The results show that glucose release in sequential dilute acid-alkali treatments (61.4-71.4 mg/g) was 7.3-24.8% higher than sequential dilute acid-aqueous ammonia treatments (57.2-61.8 mg/g) and 283.8-346.3% higher than control (16.0 mg/g), respectively. Dilute acid treatment removed most hemicellulose (84.9%) of the biomass, followed by alkaline treatment with 27.5% removal of lignin. Roughness, surface area, and micropore volume of the biomass were crucial for the enzymatic hydrolysis. Furthermore, the ultrastructure changes observed using crystallinity, Fourier transform infrared spectroscopy, thermogravimetric analysis, and pyrolysis gas chromatography/mass spectrometry support the effects of sequential dilute acid-alkali treatment. The results provide an efficient approach to facilitate a better enzymatic hydrolysis of the poplar samples.

Unrevealing model compounds of soil conditioners impacts on the wheat straw autohydrolysis efficiency and enzymatic hydrolysis

BACKGROUND

Soil-derived exogenous ash (EA) poses a challenge toward lignocellulosic autohydrolysis due to its buffering capacity. Previous works focusing on this phenomenon have failed to also investigate the role that soluble salts, and organic matter plays in this system. Herein, sodium phosphate and sodium humate were employed as model buffering compounds representing soluble salts and organic matter and dosed into a de-ashed wheat straw (DWS) autohydrolysis process to show the potential impacts of WS attached soil conditioners on the WS autohydrolysis efficiency which would further affect the enzymatic digestibility of autohydrolyzed WS.

RESULTS

Results showed that with the increasing loadings of sodium phosphate and sodium humate resulted in elevated pH values (from 4.0 to 5.1 and from 4.1 to 4.7, respectively). Meanwhile, the reductions of xylan removal yields from ~ 84.3-61.4% to 72.3-53.0% by loading (1-30 g/L) sodium phosphate and sodium humate during WS autohydrolysis lead to a significant decrease of cellulose accessibilities which finally lead to a reduction of the enzymatic digestibility of autohydrolyzed WS from ~ 75.4-77.2% to 47.3-57.7%.

CONCLUSION

The existence of different types soil conditioner model compounds results in various component fractions from autohydrolyzed WS in the process of autohydrolysis. A lack of sufficient xylan removal was found to drive the significant decrease in enzymatic accessibility. The results demonstrated the various effects of two typical tested soil conditioners on WS autohydrolysis and enzymatic hydrolysis.

Green biorefinery - the ultra-high hydrolysis rate and behavior of Populus tomentosa hemicellulose autohydrolysis under moderate subcritical water conditions

A high monosaccharide conversion rate of hemicellulose in a green solvent and under moderate reaction conditions for industrialization is one of the most important keys in a lignocellulosic biorefinery. The behavior of Populus tomentosa hemicellulose polysaccharides, crystallinity and the furfural formation in the autohydrolysis process under moderate subcritical water conditions (160-180 °C, 0.618-1.002 MPa) were studied. The results have shown that the hemicellulose was converted to corresponding monosaccharides at an ultra-high hydrolysis rate. Factor analysis indicates that the temperature is the most important factor affecting hemicellulose autohydrolysis. When the autohydrolysis temperature increased from 160 to 180 °C for 2 h, the hydrolysis rate of xylose, rhamnose, galactose, mannose, and glucose from hemicellulose increased from 70% to 91%, 71% to 100%, 82% to 95%, 42% to 58%, and 34% to 37%, respectively. Arabinose was completely dissolved in 30 min. The xylose, rhamnose, galactose, and arabinose from hemicellulose could be almost completely removed under the conditions. The hemicellulose removal rate obtained herein exceeded the values reported for most acid, alkali, ionic liquid, or deep eutectic solvent treatments. It is notable that almost all glucose in hemicellulose was dissolved and the glucose in cellulose was partially hydrolyzed. An analysis of the sugar composition and the crystallinity change in the process at 180 °C demonstrate that hydrolysis reaction started to shift from amorphous regions to crystalline regions, due to the partial hydrolysis of crystalline cellulose after 90 min at 180 °C. Overall, these results show that the moderate subcritical water autohydrolysis of hemicellulose in Populus tomentosa may be a potential bio-refinery process.

Development of optimal steam explosion pretreatment and highly effective cell factory for bioconversion of grain vinegar residue to butanol

BACKGROUND

The industrial vinegar residue (VR) from solid-state fermentation, mainly cereals and their bran, will be a potential feedstock for future biofuels because of their low cost and easy availability. However, utilization of VR for butanol production has not been as much optimized as other sources of lignocellulose, which mainly stem from two key elements: (i) high biomass recalcitrance to enzymatic sugar release; (ii) lacking of suitable industrial biobutanol production strain. Though steam explosion has been proved effective for bio-refinery, few studies report SE for VR pretreatment. Much of the relevant knowledge remains unknown. Meanwhile, recent efforts on rational metabolic engineering approaches to increase butanol production in Clostridium strain are quite limited. In this study, we assessed the impact of SE pretreatment, enzymatic hydrolysis kinetics, overall sugar recovery and applied atmospheric and room temperature plasma (ARTP) mutant method for the Clostridium strain development to solve the long-standing problem.

RESULTS

SE pretreatment was first performed. At the optimal condition, 29.47% of glucan, 71.62% of xylan and 22.21% of arabinan were depolymerized and obtained in the water extraction. In the sequential enzymatic hydrolysis process, enzymatic hydrolysis rate was increased by 13-fold compared to the VR without pretreatment and 19.60 g glucose, 15.21 g xylose and 5.63 g arabinose can be obtained after the two-step treatment from 100 g VR. Porous properties analysis indicated that steam explosion can effectively generate holes with diameter within 10-20 nm. Statistical analysis proved that enzymatic hydrolysis rate of VR followed the Pseudop-second-order kinetics equation and the relationship between SE severity and enzymatic hydrolysis rate can be well revealed by Boltzmann model. Finally, a superior inhibitor-tolerant strain, Clostridium acetobutylicum Tust-001, was generated with ARTP treatment. The water extraction and enzymolysis liquid gathered were successfully fermented, resulting in butanol titer of 7.98 g/L and 12.59 g/L of ABE.

CONCLUSIONS

SE proved to be quite effective for VR due to high fermentable sugar recovery and enzymatic hydrolysate fermentability. Inverse strategy employing ARTP and repetitive domestication for strain breeding is quite feasible, providing us with a new tool for solving the problem in the biofuel fields.

The effects of exogenous ash on the autohydrolysis and enzymatic hydrolysis of wheat straw

The effects of exogenous ash (EA) from harvest wheat straw and its internal components on wheat straw autohydrolysis efficiency and subsequent enzymatic hydrolysis were investigated. Results showed that when EA and its insoluble mineral components were included in the autohydrolysis, the enzymatic efficiencies of pretreated residues were significantly reduced from 84.9% to 66.3% and 58.4%, respectively. This was found to be largely attributable to the buffering of free H⁺ in the pretreatment medium which took place due to the ash. Specifically, the insoluble mineral fraction of said ash exerted strongest buffering capacity in EA. Furthermore, this decrease was found to linearly correlate with decreases to substrate enzymatic accessibility and hydrophobicity. These results demonstrate that the penalties of ash upon autohydrolysis are borne of specific fractions comprising the ash, making the case for ash removal processes or supplementation of processes with additives that will counter the negative effects of ash.

Non-Solvent Fractionation of Lignin Enhances Carbon Fiber Performance

Even though lignin carbon fiber has been sought after for several decades, the poor mechanical performance remains to be a major barrier for commercial applications. The low mechanical performance is attributed to the heterogeneity of lignin polymer. Recent advances in fractionation technologies showed the great potential to reduce lignin heterogeneity, but current fractionation methods often depend on costly chemicals and materials such as enzymes, organic solvents, membranes, and dialysis tubes. Here, a new non-solvent strategy was developed to fractionate lignin by autohydrolysis. By using only water, lignin was efficiently fractionated into water-soluble and -insoluble fractions. The latter fraction had increased molecular weight and uniformity and resulted in more β-O-4 interunitary linkages as analyzed by size-exclusion chromatography and 2D heteronuclear single quantum coherence NMR spectroscopy, respectively. In particular, the water-insoluble fraction significantly enhanced the mechanical performances of the resultant carbon fibers. Mechanistic study by differential scanning calorimetry (DSC) revealed that the miscibility of lignin with guest polyacrylonitrile molecules was improved with the reduced lignin heterogeneity. Crystallite analyses by XRD and Raman spectroscopy revealed that the crystallite size and content of the pre-graphitic turbostratic carbon structure were increased. The fundamental understanding revealed how lignin fractionation could modify lignin chemical features to enhance the mechanical performance of resultant carbon fibers. The autohydrolysis fractionation thus represents a green, economic, and efficient methodology to process lignin waste and boost lignin carbon fiber quality, which could open new horizons for lignin valorization.

Pretreatment Processes of Biomass for Biorefineries: Current Status and Prospects

This article seeks to be a handy document for the academy and the industry to get quickly up to speed on the current status and prospects of biomass pretreatment for biorefineries. It is divided into two biomass sources: vegetal and animal. Vegetal biomass is the material produced by plants on land or in water (algae), consuming sunlight, CO2, water, and soil nutrients. This includes residues or main products from, for example, intensive grass crops, forestry, and industrial and agricultural activities. Animal biomass is the residual biomass generated from the production of food from animals (e.g., manure and whey). This review does not mean to include every technology in the area, but it does evaluate physical pretreatments, microwave-assisted extraction, and water treatments for vegetal biomass. A general review is given for animal biomass based in physical, chemical, and biological pretreatments.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Balancing the effect of pretreatment severity on hemicellulose extraction and pulping performance during auto-hydrolysis prior to kraft pulping of acacia wood

When using a combination of pre-extraction and chemical pulping, a high yield of sugar recovery and minimal negative effect on the subsequent pulping step are expected. In this work, the P factor was utilized to investigate the effect of auto-hydrolysis severity on sugar recovery, removal of the main component, and impact on the kraft pulping of acacia wood chips. Using a P factor of 235, 84.34% of the polysaccharides in 14.05 g L^-1 of dissolved sugars could be obtained. In addition, the soluble sugars were easily separated with a recovery yield of 3.54 g ·L^-1 and Mw of 4,690 g mol^-1 by direct precipitation using organic solvents. However, a maximum of 22.14 g L^-1 of dissolved sugars was obtained with approximately 72.53% polysaccharides and Mw of 2,198 g mol^-1 for a P factor of 601. Moreover, nearly 50% of the degraded carbohydrates remained in the auto-hydrolyzed wood chips. The decrease in the mass of pentosan, holocellulose, and klason lignin was 62, 30, and 8.76%, respectively. With intensifying severity, the screened yield and viscosity of pulps decreased markedly, whileas the Kappa number increased. No significant differences were observed in the morphology of the resultant fibers. Moreover, there was a decrease in the physical strength of the pulps due to the loss of the intrinsic strength of the pulp fibers, which in turn resulted from the cellulose damage. The combustion performance of the resultant pulping black liquor is improved due to the higher lignin content.

Lignin-carbohydrate complexes: properties, applications, analyses, and methods of extraction: a review

The complexity of lignin and hemicellulose segmentation has been known since the middle of the ninetieth century. Studies confirmed that all lignin units in coniferous species and 47-66% of lignin moieties in deciduous species are bound to hemicelluloses or cellulose molecules in lignin-carbohydrate complexes (LCC). Different types and proportions of lignin and polysaccharides present in biomass lead to the formation of LCC with a great variety of compositions and structures. The nature and amount of LCC linkages and lignin substructures affect the efficiency of pulping, hydrolysis, and digestibility of biomass. This review paper discusses the structures, compositions, and properties of LCC present in biomass and in the products obtained via pretreating biomass. Methods for extracting, fractionating, and analyzing LCC of biomass, pulp, and spent pulping liquors are critically reviewed. The main perspectives and challenges associated with these technologies are extensively discussed. LCC could be extracted from biomass following varied methods, among which dimethyl sulfoxide or dioxane (Björkman's) and acetic acid (LCC-AcOH) processes are the most widely applied. The oxidation and methylation treatments of LCC materials elucidate the locations and frequency of binding sites of hemicelluloses to lignin. The two-dimensional nuclear magnetic resonance analysis allows the identification of the structure and the quantity of lignin-carbohydrate bonds involved in LCC. LCC application seems promising in medicine due to its high anti-HIV, anti-herpes, and anti-microbial activity. In addition, LCC was successfully employed as a precursor for the preparation of spherical biocarriers.

Characterization of Pseudo-Lignin from Steam Exploded Birch

There is a growing interest in a more wholesome utilization of biomass as the need for greener chemistry and non-mineral oil-based products increases. Lignin is the largest renewable resource for aromatic chemicals, which is found in all types of lignocellulosic biomass. Steam-explosion of lignocellulosic biomass is a useful pretreatment technique to make the polymeric material more available for processing. However, this heat-based pretreatment is known to result in the formation of pseudo-lignin, a lignin-like polymer made from carbohydrate degradation products. In this work, we have analyzed steam-exploded birch with a varying severity factor (3.1-5.0) by pyrolysis-gas chromatography-mass spectrometry, 2D-NMR, and Fourier transform infrared spectroscopy. The main results reveal a consumption of acetic acid at higher temperatures, with the increase of furan components in the pyrolyzate. The IR and NMR spectral data support these results, and there is a reason to believe that the conditions for humin formation are accomplished under steam explosion. Pseudo-lignin seems to be a humin-like compound.

Kinetics and mechanism of hemicelluloses removal from cellulosic fibers during the cold caustic extraction process

The effective separation of hemicelluloses and cellulose is desirable for the production of high-purity cellulose, which is a sustainable raw material for many value-added applications. For this purpose, the kinetics and mechanism of hemicelluloses removal from the cold caustic extraction (CCE) were investigated in the present study. The hemicelluloses removal process consists of: 1) the bulk phase, characteristic of significant hemicelluloses removal; 2) the transition phase, hemicelluloses transferring from the inner to the outer region of the fiber wall, with negligible overall hemicelluloses removal; 3) the residual phase, presenting a weak but continuing hemicelluloses removal. Furthermore, the enzymatic peeling method was adopted to study the fundamentals of hemicelluloses removal. The results showed that the molecular weight of hemicelluloses is the main parameter governing their diffusion/dissolution processes, and that the low molecular weight hemicelluloses are preferentially removed.

Structural features of mildly fractionated lignin carbohydrate complexes (LCC) from spruce

Characterization of novel lignin carbohydrate complexes (LCC) unveils structural differences of relevance to fundamental and applied science.

Sequential Fenton oxidation and hydrothermal treatment to improve the effect of pretreatment and enzymatic hydrolysis on mixed hardwood

Sequential Fenton oxidation (FO) and hydrothermal treatment were performed to improve the effect of pretreatment and enzymatic hydrolysis of mixed hardwood. The molar ratio of the Fenton reagent (FeSO4·7H2O and H2O2) was 1:25, and the reaction time was 96h. During the reaction, little or no weight loss of biomass was observed. The concentration of Fe(2+) was determined and was found to increase continuously during FO. Hydrothermal treatment at 190-210°C for 10-80min was performed following FO. Sequential FO and hydrothermal treatment showed positive effects on pretreatment and enzymatic hydrolysis. Xylose concentration in the hydrolysate was as high as 14.16g/L when FO-treated biomass was treated at 190°C, while its concentration in the raw material was 3.72g/L. After 96h of enzymatic hydrolysis, cellulose conversion in the biomass obtained following sequential treatment was 69.58-79.54%. In contrast, the conversion in the raw material (without FO) was 64.41-67.92%.

Understanding of pH value and its effect on autohydrolysis pretreatment prior to poplar chemi-thermomechanical pulping

Autohydrolysis pretreatment with different severity factors was performed on poplar chips prior to chemi-thermomechanical pulping (CTMP) in order to investigate the change in pH value and its effect on the autohydrolysis pretreatment. The results showed that the dissolution amount of acetic acid increased with raising the severity factor of the pretreatment and declining the size of poplar chips, respectively. Besides, a logarithmic relationship between the amount of acetic acid released in the autohydrolysis liquor (AHL) and pH value of the AHL was observed. The amounts of glucose and xylose (including those in the form of monomers, oligomers, and polysaccharides) as well as furfural and hydroxymethylfurfural (HMF) also depended on the pH value of the AHL to some extent.

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

Hydrothermal pretreatment initially removed the lignin-free xylan from the middle layer of secondary wall, followed by the lignin-bound xylan, but the cellulose-bound xylan was seldom removed by this pretreatment. An in-depth understanding of the mechanism of xylan removal during hydrothermal pretreatment (HTP) of wood is critical for cost-effective conversion of lignocellulosic biomass to biofuels. Several studies demonstrated the kinetics and mechanism of xylan removal during HTP on molecular scale, but the dissolution mechanism of xylan during HTP remains unclear at ultra-structural level. Our study investigated changes in the micro-distribution of xylan in poplar fiber cell walls during HTP by transmission electron microscopy (TEM) in combination with immunogold labeling. The study revealed that HTP caused greater decline in the density of xylan labeling in the S2 layer of fiber wall than in the S1 layer. There was a greater loss in the density of xylan labeling during HTP in the delignified and enzymatically treated fibers compared to untreated fibers. We propose that in the initial stages of HTP lignin-free xylan in the S2 layer was more readily hydrolyzed than in the S1 layer by hydronium ions. With increasing pretreatment time, the xylan covalently bound to lignin was also removed from the S2 layer due to the dissolution of lignin. The xylan tightly bound to cellulose was seldom removed during HTP, but was hydrolyzed in subsequent enzymatic treatment. This TEM-immunolabeling investigation reveals the manner in which different xylan fractions are removed from fiber cell wall during HTP, and we expect the information to be helpful in developing processes tailored for more effective conversion of cellulosic biomass into fermentable sugars.

Xylan hydrolysis in Populus trichocarpa × P. deltoides and model substrates during hydrothermal pretreatment

Previous studies defined easy and difficult to hydrolyze fractions of hemicellulose that may result from bonds among cellulose, hemicellulose, and lignin. To understand how such bonds affect hydrolysis, Populus trichocarpa × Populus deltoides, holocellulose isolated from P. trichocarpa × P. deltoides and birchwood xylan were subjected to hydrothermal flow-through pretreatment. Samples were characterized by glycome profiling, HPLC, and UPLC-MS. Glycome profiling revealed steady fragmentation and removal of glycans from solids during hydrolysis. The extent of polysaccharide fragmentation, hydrolysis rate, and total xylose yield were lowest for P. trichocarpa × P. deltoides and greatest for birchwood xylan. Comparison of results from P. trichocarpa × P. deltoides and holocellulose suggested that lignin-carbohydrate complexes reduce hydrolysis rates and limit release of large xylooligomers. Smaller differences between results with holocellulose and birchwood xylan suggest xylan-cellulose hydrogen bonds limited hydrolysis, but to a lesser extent. These findings imply cell wall structure strongly influences hydrolysis.

Visualization of Miscanthus × giganteus cell wall deconstruction subjected to dilute acid pretreatment for enhanced enzymatic digestibility

BACKGROUND

The natural recalcitrance of lignocellulosic plant cell walls resulting from complex arrangement and distribution of heterogeneous components impedes deconstruction of such cell walls. Dilute acid pretreatment (DAP) is an attractive method to overcome the recalcitrant barriers for rendering enzymatic conversion of polysaccharides. In this study, the internodes of Miscanthus × giganteus, a model bioenergy crop, were subjected to DAP to yield a range of samples with altered cell wall structure and chemistry. The consequent morphological and compositional changes and their possible impact on saccharification efficiency were comprehensively investigated. The use of a series of microscopic and microspectroscopic techniques including fluorescence microscopy (FM), transmission electron microscopy (TEM) and confocal Raman microscopy (CRM)) enabled correlative cell wall structural and chemical information to be obtained.

RESULTS

DAP of M. × giganteus resulted in solubilization of arabinoxylan and cross-linking hydroxycinnamic acids in a temperature-dependent manner. The optimized pretreatment (1% H2SO4, 170°C for 30 min) resulted in significant enhancement in the saccharification efficiency (51.20%) of treated samples in 72 h, which amounted to 4.4-fold increase in sugar yield over untreated samples (11.80%). The remarkable improvement could be correlated to a sequence of changes occurring in plant cell walls due to their pretreatment-induced deconstruction, namely, loss in the matrix between neighboring cell walls, selective removal of hemicelluloses, redistribution of phenolic polymers and increased exposure of cellulose. The consequently occurred changes in inner cell wall structure including damaging, increase of porosity and loss of mechanical resistance were also found to enhance enzyme access to cellulose and further sugar yield.

CONCLUSIONS

DAP is a highly effective process for improving bioconversion of cellulose to glucose by breaking down the rigidity and resistance of cell walls. The combination of the most relevant microscopic and microanalytical techniques employed in this work provided information crucial for evaluating the influence of anatomical and compositional changes on enhanced enzymatic digestibility.

Degradation and dissolution of hemicelluloses during bamboo hydrothermal pretreatment

To elucidate the hemicelluloses degradation and dissolution during hydrothermal pretreatment, hemicelluloses separated from both hydrolysate and pretreated substrate were investigated. Along with the pretreatment proceeding, some hemicelluloses fractions dissolved and diffused into the bulk liquor; MW (molecular weight) of these hemicelluloses fractions increased first and then decreased as well as amount of the fractions. Based on the definition of MW of the soluble hemicellulose, it has been concluded that some insoluble hemicellulose fractions appeared in the hydrolysate. In contrast, the hemicellulose degradation occurred continually and had been observed by the gradual decrease of MW of the hemicellulose isolated from pretreated substrate. Lingering dissolution at the later stage might attribute to the facts that some soluble fractions were still entrapped in the substrate. 5-15% hemicellulose fractions remained in the pretreated substrate at the later stage were composed of soluble species.

Potential of hot water extraction of birch wood to produce high-purity dissolving pulp after alkaline pulping

The potential of hot water extraction of birch wood to produce highly purified dissolving pulp in a subsequent soda-anthraquinone pulping process was evaluated. After intermediate extraction intensities, pulps with low xylan content (3-5%) and high cellulose yield were successfully produced. Increasing extraction intensity further decreased the xylan content in pulp. However, below a xylan content of 3%, the cellulose yield dramatically decreased. This is believed to be due to cleavage of glycosidic bonds in cellulose during severe hot water extractions, followed by peeling reactions during alkaline pulping. Addition of sodium borohydride as well as increased anthraquinone concentration in the pulping liquor increased the cellulose yield, but had no clear effects on pulp purity and viscosity. The low intrinsic viscosity of pulps produced after severe extraction intensities and soda-anthraquinone pulping corresponded to the viscosity at the leveling-off degree of polymerization, suggesting that nearly all amorphous cellulose had been degraded.

Assessing the molecular structure basis for biomass recalcitrance during dilute acid and hydrothermal pretreatments

The production of cellulosic ethanol from biomass is considered a promising alternative to reliance on diminishing supplies of fossil fuels, providing a sustainable option for fuels production in an environmentally compatible manner. The conversion of lignocellulosic biomass to biofuels through a biological route usually suffers from the intrinsic recalcitrance of biomass owing to the complicated structure of plant cell walls. Currently, a pretreatment step that can effectively reduce biomass recalcitrance is generally required to make the polysaccharide fractions locked in the intricacy of plant cell walls to become more accessible and amenable to enzymatic hydrolysis. Dilute acid and hydrothermal pretreatments are attractive and among the most promising pretreatment technologies that enhance sugar release performance. This review highlights our recent understanding on molecular structure basis for recalcitrance, with emphasis on structural transformation of major biomass biopolymers (i.e., cellulose, hemicellulose, and lignin) related to the reduction of recalcitrance during dilute acid and hydrothermal pretreatments. The effects of these two pretreatments on biomass porosity as well as its contribution on reduced recalcitrance are also discussed.

A combined process of activated carbon adsorption, ion exchange resin treatment and membrane concentration for recovery of dissolved organics in pre-hydrolysis liquor of the kraft-based dissolving pulp production process

To recover dissolved organics in pre-hydrolysis liquor (PHL) of the kraft-based dissolving pulp production process, a new combined process concept of sequential steps of activated carbon adsorption, ion exchange resin treatment, and membrane concentration, was proposed. The removal of lignin in the PHL was achieved in the activated carbon adsorption step, which also facilitates the subsequent operations, such as the membrane filtration and ion exchange resin treatment. The ion exchange resin treatment resulted in the removal/concentration of acetic acid, which opens the door for acetic acid recovery. The membrane filtration is to recover/concentrate the dissolved sugars. The combined process resulted in the production of PHL-based concentrate with relatively high concentration of hemicellulosic sugars, i.e., 22.13%.

Effects of pH control with phthalate buffers on hot-water extraction of hemicelluloses from spruce wood

Ground spruce wood was extracted with water at 170 °C at four different pH levels (3.8, 4.0, 4.2 and 4.4) achieved by using phthalate buffers. Static batch extractions were carried out in an accelerated solvent extractor (ASE-300). The extracted non-cellulosic carbohydrates, predominantly galactoglucomannans (GGMs), were characterised mainly by sugar unit analysis and molar mass determination. Compared to extraction with plain water, extractions with phthalate buffer solutions gave similar yields of non-cellulosic carbohydrates, but gave up to 70% less monosaccharides, and consequently higher molar masses of extracted GGMs. Moreover, at these pH levels, the hydrolysis of acetyl groups were decreased by 40% compared to extraction with plain water, thus maintaining the water solubility of GGMs. It is concluded that hot water extraction of hemicelluloses in high-molar-mass form (average Mw about 10 kDa) from wood in good yields (8% of wood) demands appropriate control of pH, to a level of about 4.

Degradation kinetics of the main carbohydrates in birch wood during hot water extraction in a batch reactor at elevated temperatures

Hot water extraction of wood at elevated temperatures may be a suitable method to produce hemicellulose-lean pulps and to recover xylan-derived products from the water extract. In this study, water extractions of birch wood were conducted at temperatures between 180 and 240 °C in a batch reactor. Xylan was extensively removed, whereas cellulose was partly degraded only at temperatures above 180 °C. Under severe extraction conditions, acetic acid content in the water extract was higher than the corresponding amount of acetyl groups in wood. In addition to oligo- and monosaccharides, considerable amounts of furfural and 5-hydroxymethylfurfural (HMF) were recovered from the extracts. After reaching a maximum, the furfural yield remained constant with increasing extraction time. This maximum slightly decreased with increasing extraction temperature, suggesting the preferential formation of secondary degradation products from xylose. Kinetic models fitting experimental data are proposed to explain degradation and conversion reactions of xylan and glucan.

Production of carbohydrates, lignins, and minor components from triticale straw by hydrothermal treatment

The effects of temperature (116 °C, 150 °C, and 183 °C) and flow rate (66, 150, and 234 mL/min) on the fractionation of triticale straw into different products was determined using a flow-through pressurized low polarity water reactor. The greatest concentration of biomass was hydrolyzed and extracted in the first two of eight 600 mL fractions (1.2 L), after which dry matter yield decreased. Carbohydrate, lignin, acetyl group, and uronic acid yield increased with temperature, but there was no effect due to flow rate. Most dry matter extracted at 116 °C was probably associated with the extractives. Xylan yields decrease slightly at the highest flow rate due to a decrease in the residence time of the acids produced in situ. Carbohydrates were extracted mostly as oligosaccharides, and the highest processing temperature resulted in the production of furans from the xylose and arabinose in the liquid extracts.