Lignocellulosic Pretreatment Research Articles

Recent Advances in Application of Polyoxometalates in Lignocellulose Pretreatment and Transformation.

Lignocellulose, composed of cellulose, hemicellulose, and lignin, holds immense promise as a renewable resource for the production of sustainable chemicals and fuels. Unlocking the full potential of lignocellulose requires efficient pretreatment strategies. In this comprehensive review, efforts were taken to survey the latest developments in polyoxometalates (POMs)-assisted pretreatment and conversion of lignocellulosic biomass. An outstanding finding highlighted in this review is that the deformation of the cellulose structure from I to II accompanied by the removal of xylan/lignin through the synergistic effect of ionic liquids (ILs) and POMs resulted in a significant increase in glucose yield and improved cellulose digestibility. Furthermore, successful integration of POMs with deep eutectic solvents (DES) or γ-valerolactone/water (GVL/water) systems has demonstrated efficient lignin removal, opening avenues for advanced biomass utilization. This review not only presents the key findings and novel approaches in POMs-based pretreatment but also addresses the current challenges and prospects for large-scale industrial implementation. By offering a comprehensive assessment of the progress in this field, this review serves as a valuable resource for researchers and industry professionals aiming to harness the potential of lignocellulosic biomass for sustainable chemical and fuel production.

Renewable deep eutectic solvents pretreatment improved the efficiency of anaerobic digestion by lignin extraction from willow

Lignin extraction from lignocellulosic biomass can enhance its bioconversion efficiency, whilst the recovered lignin can provide added economic value. This study comprehensively investigated the impacts of short-chain carboxylic acid-based deep eutectic solvents (DESs) pretreatments on lignin extraction from willow and assessed subsequent biomethane production through anaerobic digestion. Process parameters (including the DESs type, molar ratios of DESs components, temperature and reaction time) in the DESs pretreatments of willow for lignin extraction were optimized using the central composite surface response methodology. Results showed that lactic acid-based DES pretreatment outperformed acetic acid and propionic acid-based DES pretreatments in terms of lignin removal efficiency and methane production. Under the optimal conditions (choline chloride:lactic acid with a molar ratio of 1:10 at 160 °C for 15 min) lactic acid-based DES pretreatment retained over 94% of the glucan content in the raw willow whilst achieved the highest lignin removal of 80%. The recovered lignin showed a purity of above 81%. Compared with the biomethane production of 89.9 mL/g total solid from raw willow, the biomethane production significantly increased by 36.3% after the lactic acid-based DES pretreatment. The optimal condition reduced the digestion time from 22 to 10 days. The overall energy conversion efficiency of 62.7% demonstrated that lactic acid-based DES pretreatment of lignocellulose could be a promising method to co-produce renewable gaseous fuels and lignin in a sustainable approach.

Significantly enhanced enzymatic hydrolysis of waste rice hull through a novel surfactant-based deep eutectic solvent pretreatment

The potential of green solvents, specifically deep eutectic solvents (DESs), has piqued the interest of researchers in the field of lignocellulose pretreatment. To enhance the enzymatic digestion efficiency of waste rice hull (RCH), an effective pretreatment approach was developed using the DES [AA][CATB], which was made with acetic acid (AA) and cetyltrimethylammonium bromide (CTAB). The results showed that [AA][CATB] improved enzymatic saccharification by 3.7 times compared to raw RCH and efficiently eliminated lignin and removed xylan. The improvement in enzymatic hydrolysis efficiency was then interpreted by a series of characterizations that showed a great morphological changed RCH with an obvious accessibility increase and a lignin surface area and hydrophobicity reduction. This work demonstrates that functional, and easily recoverable DESs have potential for improving the efficiency of lignocellulose pretreatment in biorefineries, providing a promising approach for developing green solvents and achieving more sustainable and efficient biorefinery processes.

Efficient delignification of wheat straw for microbial lipid production enabled by a novel ternary deep eutectic solvent containing ethylene glycol

Lignocellulose pretreatment often faces the problem of lignin condensation with less attention to biofuel production from total sugars, which significantly hinders biomass fractionation and energy utilization efficiency. The aim of this work was to design a novel ternary deep eutectic solvent (DES) (citric acid: choline chloride: ethylene glycol in a molar ratio of 1:1:2) for pretreatment of wheat straw with a solid/liquid ratio (S/L) of 1:20 (w/w) to achieve non-condensed lignin and high-yield fermentable sugars at 100 °C for 12 h. The introduction of ethylene glycol into ternary DES could capture the formed benzyl carbocation to protect lignin structure, inhibit excessive degradation of hemicellulose but to fermentable sugars (e.g., xylose), and improve the enzymatic digestibility of cellulose (98.73%, w/w). The mechanisms of both lignin fractionation and the in-situ lignin protection during the pretreatment of wheat straw enabled by ternary DES were systematically elucidated. Importantly, Trichosporon cutaneum cultivated in undetoxified DES waste liquid (hemicellulose-rich) and residue hydrolysate (cellulose-rich) could totally afford 8.7 g microbial lipid per 100 g wheat straw, which is superior to previous reports. This green in-situ lignin protection and fractionation strategy provides a new avenue for upgrading lignocellulose to microbial lipids.

Poplar Wood Pretreatment Using Deep Eutectic Solvents for Promoting Enzymatic Hydrolysis

Bioethanol produced from lignocellulose is a renewable energy substitute for traditional fossil fuels. Poplar wood as forest waste is popular in bioethanol production. Nonetheless, the complex structure of lignocellulose leads to low reducing sugar and ethanol yields. Thus, lignocellulose pretreatment is necessary to promote enzymatic hydrolysis. Deep eutectic solvents (DESs) have good dissolution capacity, low vapor pressure, a simple synthesis procedure, low synthesis cost and low toxicity. More and more researchers have begun paying attention to the application of DESs in lignocellulose pretreatment. In this work, poplar wood was pretreated using a series of basic DESs based on diol. The effects of the DES species, the basicity of the solvents, the pretreatment temperature and the pretreatment time on the effectiveness of pretreatment and enzyme hydrolysis for poplar wood were investigated, and characterization analysis (Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy) of poplar wood was carried out to reveal the pretreatment mechanism. The best pretreatment effect was obtained from K: 1, 2-PG, which removed 89.2% and 71.6% of the lignin and hemicellulose, respectively, while preserving 97.5% of the cellulose at 130 °C for 7 h. This enhanced the reducing sugar yield to 82.5% relative to that of the raw sample (3.3%) after 72 h of hydrolysis. The results of the characterization analysis demonstrated that lignin and hemicellulose were removed. Therefore, the DES based on K: 1, 2-PG is a promising solvent for poplar wood pretreatment, and could improve the industrial production of reducing sugar and bioethanol.

Modeling the microbial pretreatment of camelina straw and switchgrass by Trametes versicolor and Phanerochaete chrysosporium via solid-state fermentation process: A growth kinetic sub-model in the context of biomass-based biorefineries.

Advancing microbial pretreatment of lignocellulose has the potential not only to reduce the carbon footprint and environmental impacts of the pretreatment processes from cradle-to-grave, but also increase biomass valorization, support agricultural growers, and boost the bioeconomy. Mathematical modeling of microbial pretreatment of lignocellulose provides insights into the metabolic activities of the microorganisms as responses to substrate and environment and provides baseline targets for the design, development, and optimization of solid-state-fermentation (SSF) bioreactors, including substrate concentrations, heat and mass transfer. In this study, the growth of Trametes versicolor 52J (TV52J), Trametes versicolor m4D (TVm4D), and Phanerochaete chrysosporium (PC) on camelina straw (CS) and switchgrass (SG) during an SSF process was examined. While TV52J illustrated the highest specific growth rate and maximum cell concentration, a mutant strain deficient in cellulose catabolism, TVm4D, performed best in terms of holocellulose preservation and delignification. The hybrid logistic-Monod equation along with holocellulose consumption and delignification models described well the growth kinetics. The oxygen uptake rate and carbon dioxide production rate were directly correlated to the fungal biomass concentration; however, a more sophisticated non-linear relationship might explain those correlations better than a linear model. This study provides an informative baseline for developing SSF systems to integrate fungal pretreatment into a large-scale, on-farm, wet-storage process for the utilization of agricultural residues as feedstocks for biofuel production.

Improving Saccharomyces cerevisiae acid and oxidative stress resistance using a prokaryotic gene identified by functional metagenomics

Innovations in obtaining products from lignocellulosic biomass have been largely based on the improvement of microorganisms and enzymes capable of degrading these materials. To complete the whole process, microorganisms must be able to ferment the resulting sugars and tolerate high concentrations of product, osmotic pressure, ion toxicity, temperature, toxic compounds from lignocellulose pretreatment, low pH, and oxidative stress. In this work, we engineered laboratory and industrial Saccharomyces cerevisiae strains by combining a gene (hu) recovered from a metagenomic approach with different native and synthetic promoters to obtain improved acid and oxidative stress resistance. Laboratorial strains harboring hu gene under the control of the synthetic stress responsive PCCW14v5 showed increased survival rates after 2 h exposure to pH 1.5. The hu gene was also able to significantly enhance the tolerance of the industrial strain to high concentrations of H2O2 when combined with PTEF1, PYGP1 or PYGP1v7 after 3 h exposure.

Using a systematic review to develop a cellulose nanocrystals production framework for use as a design baseline and optimization tool

Various cellulose nanocrystals (CNC) production processes are not systematically reviewed. Therefore, there lacks a basis for future CNC process designs and optimizations. To solve this problem, the objective of this study was to perform a systematic review of selected CNC processes using the preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) guidelines to review the process information from each selected literature item to identify the common process segments to create a segmented CNC production framework. The results revealed four major CNC production segments: feed pretreatment, chemical treatment, separation, and product purification. Statistical data analysis indicated that varying forms of mechanical pretreatment comprised 100% of the lignocellulosic feedstock pretreatment. However, the most common chemical treatment, separation, and product purification options are acid hydrolysis (60–75%), mechanical separation (40–60%), and washing (20–50%), respectively. The technology options explored within each segment created a design basis for CNC production improvements to assist the design of new CNC production plants and the optimization of existing ones. The model can also help improve the understanding of the minimum protocol content and improve the competency of process designers.

Thermodynamic analysis, synthesis, characterization, and evaluation of 1-ethyl-3-methylimidazolium chloride: Study of its effect on pretreated rice husk

This work is focused on the determination of the optimal reaction conditions to synthesize the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM][Cl]) and assess its suitability for the pretreatment of rice husk. The modified UNIFAC (UNIversal quasi-chemical Functional-group Activity Coefficients) approach for ionic liquids is used to develop a thermodynamic model that describes the reactive system methylimidazole (MIM), chloroethane (C2H5Cl) and [EMIM][Cl]. The model allows to study the phase equilibria coexistence (vapor–liquid equilibria and solid–liquid equilibria) and yields the theoretically optimal conditions to synthesize the ionic liquid. The model predictions are validated with the available experimental and reported data. By implementing the developed model, a simple way to synthesize ionic liquid [EMIM][Cl] was found allowing to study its influence on the structure and morphology of pretreated rice husk. The lignocellulosic materials involved in this study are characterized by their composition, enzymatic digestibility, scanning electron microscopy, and crystallinity. Compared to untreated material, [EMIM][Cl]-pretreated rice husk produces cellulose that can be efficiently enzymatic hydrolyzed with high sugar yields. This work offers a suitable methodology to include the synthesis and thermodynamics of the solvent media within the design of low-cost ionic liquids for lignocellulosic biomass pretreatment.

Study on mechanism of cellulose hydrolysis during alkali thermal pretreatment of lignocellulose by density functional theory

Alkali thermal pretreatment can effectively destroy the compact three-dimensional aggregated structure of lignocellulose, and further promote the production of high value-added chemicals. Cellulose is an important component of lignocellulose, and in this study the mechanism of cellulose hydrolysis in alkali thermal pretreatment was studied by density functional theory. The hydrolysis was divided into two steps, firstly the hydroxide ion captured a proton from the hydroxyl of which had most positive potential value of 1.23 eV. Subsequently, the glycosidic bond was cleaved to form a new C–O bond, and the hydroxide ion attacked the intermediate forming a glucose residue by a new covalent. The amount of electron transfer of the two steps were natural orbitals for chemical valence eigenvalue 0.31 and 0.34, and the corresponding energy barriers were 147.11 and 101.17 kJ/mol at 298.15 K, respectively. With the increase of pretreatment temperature from 298.15 to 413.15 K, the enthalpy and Gibbs free energy change of cellobiose hydrolysis decreased by 2.97 and 2.34 kJ/mol and the thermodynamic feasibility and reaction rate increased. This study will providing an important guidance for the process control, reactor design and performance enhancement of lignocellulose pretreatment.

Acidic and alkaline deep eutectic solvents (DESs) pretreatment of grapevine: Component analysis, characterization, lignin structural analysis, and antioxidant properties

Deep eutectic solvents (DESs) have been extensively applied to pretreat lignocellulose; however, comparative research on acidic and alkaline DES pretreatment is relatively lacking. Herein, pretreatment of grapevine agricultural by-products with seven DESs were compared in terms of removal of lignin and hemicellulose and component analysis of the pretreated residues. Among the tested DESs, both acidic choline chloride-lactic (CHCl-LA), and alkaline potassium carbonate-ethylene glycol (K2CO3-EG) were effective in delignification. Thereafter, the CHCl-LA and K2CO3-EG extracted lignin was compared by analyzing their physicochemical structure changes and antioxidant properties. The results showed that the thermal stability, molecular weight, and phenol hydroxyl percentage of CHCl-LA lignin were inferior to K2CO3-EG lignin. It was found that the high antioxidant activity of K2CO3-EG lignin was mainly attributed to the abundant phenol hydroxyl, guaiacyl (G), and para-hydroxy-phenyl (H). By comparing acidic and alkaline DES pretreatments and their lignin nuances in biorefining, novel insights are derived for the scheduling and selection of DES for lignocellulosic pretreatment.

The Role of Lignin Structure on Cellulase Adsorption and Enzymatic Hydrolysis

Lignin is one of the important components of lignocellulosic cell walls, which endows plant cell walls with rigidity and strength and protects them from microbial invasion. The presence of lignin is thought to hinder the conversion of biomass to bioenergy, so understanding enzyme-lignin interactions is very important in order to reduce the inhibition of lignin and improve the hydrolysis yields. Conversion of lignocellulosic raw materials into bioethanol is divided into pretreatment, enzymatic hydrolysis, and fermentation. In this paper, both pretreatment and enzymatic hydrolysis of lignocellulose are described in detail. Finally, the reasons why lignin hinders enzymatic hydrolysis efficiency, mainly from forming spatial barriers and interacting with cellulase, are discussed, and the influencing factors and mechanisms of action of cellulase hydrolysis are explored with a view to targeted regulation of lignin structure to improve lignocellulosic saccharification.

Effects of enzymatic hydrolysis and physicochemical properties of lignocellulose waste through different choline based deep eutectic solvents (DESs) pretreatment

DESs are promising green solvents in lignocellulose pretreatment. In this work, four choline chloride based DESs were synthesized and their physicochemical properties were evaluated. Herbaceous lignocellulose (wheat straw, sorghum straw) and woody lignocellulose biomass (poplar wood, pine wood) were selected as pretreated substrates. In order to reveal the mechanism that pretreatment can improve the efficiency of enzymatic hydrolysis, the pretreated lignocellulose and lignin were analyzed. SEM, FT-IR and XRD characterization revealed the microstructural changes of pretreated lignocellulose, including the increase of surface roughness and pore erosion, changes of polysaccharide/lignin chemical groups, the increase of CrI. The efficient delignification increased the accessibility of enzyme to cellulose, the lignin content negatively affected enzymatic hydrolysis conversion. Furthermore, GPC analysis showed that the polydispersity of sulfonated lignin was reduced after pretreatment, lignin had more homogeneous molecular structures. The degree of unsaturation was calculated according to the elemental composition of sorghum straw and lignin. The unsaturation of pretreated sorghum straw decreased, that may be due to the removal of partial lignin. The elemental composition of acid insoluble lignin (separated from sorghum straw) reflects the increase of unsaturation of lignin after pretreatment.

Reaction characteristics of metal-salt coordinated deep eutectic solvents during lignocellulosic pretreatment

A binary deep eutectic solvent consisting of choline chloride and glycerol (ChCl/Gly) is proven to be ineffective in lignocellulosic pretreatment, and arbitrarily screening coordination agents makes it difficult to directionally construct a valid solvent. This study investigated the designable diversity of ternary ChCl/Gly coordinated with various metal-salts by elucidating solvent properties and reaction characteristics. Alkaline/acidic strengths and hydrogen-bonding forces were key factors for component fractionation. Pretreatment performance was typically poor for solvents with few acidic sites and active protons (i.e., ChCl/Gly-KCl and MgCl2), as well as those with close proximity of H-bond donor and acceptor abilities (ChCl/Gly-ZnCl2). By contrast, strong alkaline ChCl/Gly-K2CO3 could achieved 72 % of delignification with a remarkable holo-cellulose preservation (70.8–80.9 %). Delignification underwent an independent rate-control of solvent diffusion and surface reaction with an endothermic nature (17.1 kJ/mol of ΔH and 20.3 kJ/mol of apparent activation energy). High temperature was beneficial for mitigating diffusion constraint at the initial pretreatment stage, maximally increasing the reaction rate by 7.7-fold. Moreover, Lewis acidic ChCl/Gly-FeCl3 (or AlCl3) removed 76–84.4 % of hemicellulose and 47.6–61.9 % of lignin while significantly deconstructing the aryl-ether bonds and β-O-4 linkages. But for acidic ChCl/Gly solvents, biomass dissolution was inconformity with the independent diffusion/reaction-controlling process, owing to the excessive hemicellulose degradation and hydrolytic by-product formation (maximally 91 g/kg xylose and 4 g/kg furans). Based on the lignin structural characterization, the potential reaction mechanism related to solvent properties was proposed to develop efficient ternary coordinated deep eutectic solvents.

Biomass degradation: Challenges and strategies in extraction and fractionation of hemicellulose

Extraction and fractionation of hemicellulose from complex lignocellulosic biomass are crucial steps to acquiring purified hemicellulose that can probably used in biofuel and biochemical sectors. Yet, several challenges have impeded for obtaining the hemicellulose fraction as it involves the complexity of extracting and fractioning hemicellulose. In this study, the authors have discussed the outline of the most prospective and effective method for separation, extraction, and purification of hemicellulose-rich biomass. A current overview of challenges and problems in hemicellulose separation was addressed. Subsequently, the influence of heterogeneous biomass types, cultivation and harvesting times, accessible surface areas, acetyl and other functional groups on purity was observed. On the other hand, several proper strategies efficiently employed in hemicellulose extraction and fractionation are provided. The biochemical process gives a highly positive contribution to sustaining hemicellulose-based bioproducts. Biological pre-treatment of lignocellulose using enzymes and bacteria is convinced as the most effective and efficient method to reduce the lignin content and to improve the purity due to less inhibitory and minimal coke formation. Saccharomyces cerevisiae has been widely used in the hemicellulose-based bioprocess due to resulting in high conversion (>80%) and yield (>70%). The hydrothermal method followed by multistep ultrafiltration treatment generated the highest hemicellulose purity (100%) and by-product removal (94.4–99.2 wt%). Comprehensive strategies for hemicellulose extraction and fractionation involving biological and non-biological treatments are also the highlights of this article.

Screening of Gluconobacter oxydans in xylonic acid fermentation for tolerance of the inhibitors formed dilute acid pretreatment.

Pre-hydrolysate liquor, as an inevitable by-product, contains a large amount of xylose, and is therefore an inexpensive feedstock that can be upgraded to value-added chemical xylonic acid. However, inhibitors, simultaneously formed in lignocellulose pretreatment process, are regarded as the major obstacle for effectively bio-converting xylose in pre-hydrolysate into xylonic acid. In this study, Gluconobacter oxydans, with highly selective and efficient, was employed for xylonic acid production; the impacts of five typical toxic inhibitory compounds on xylonic acid productivity and the activity of the membrane-bound dehydrogenase were evaluated. The results revealed that the inhibitors showed different degrees of influence toward xylonic acid production, and the order of inhibitory effect for acidic inhibitors was formic acid > acetic acid > levulinic acid; the inhibitory effect of aldehyde inhibitors was furfural > 5-hydroxymethyl-furfural. This study provides an important basis of metabolic modification and detoxification process for enhancing inhibitor tolerance and xylonic acid productivity.

Small-scale OrganoCat processing to screen rapeseed straw for efficient lignocellulose fractionation

Agricultural residues such as rapeseed straw can be a valuable source of cellulose, sugars, and aromatic molecules like lignin. Understanding its composition is crucial in order to develop suitable processing technology for the production of biofuel or biochemicals from rapeseed straw. Here, we developed a small-scale OrganoCat system to screen multiple technical conditions and different samples at higher throughput and utilize this system to analyze straw samples from a set of 14 genetically different Brassica lines on their processability. Correlation analysis was performed to investigate the effects of cell wall polymer features on rapeseed biomass disintegration. At comparably mild reaction conditions, the differences in recalcitrance towards OrganoCat fractionation within the set were especially associated with parameters such as pectic polysaccharide content, acetylation, and hemicellulose composition. These findings can subsequently be used to optimize and scale up the pretreatment and fractionation of lignocellulose derived from rapeseed straw.

Effect of Alkaline and Mechanical Pretreatment of Wheat Straw on Enrichment Cultures from Pachnoda marginata Larva Gut

In order to partially mimic the efficient lignocellulose pretreatment process performed naturally in the gut system of Pachnoda marginata larvae, two wheat straw pretreatments were evaluated: a mechanical pretreatment via cutting the straw into two different sizes and an alkaline pretreatment with calcium hydroxide. After pretreatment, gut enrichment cultures on wheat straw at alkaline pH were inoculated and kept at mesophilic conditions over 45 days. The methanogenic community was composed mainly of the Methanomicrobiaceae and Methanosarcinaceae families. The combined pretreatment, size reduction and alkaline pretreatment, was the best condition for methane production. The positive effect of the straw pretreatment was higher in the midgut cultures, increasing the methane production by 192%, while for hindgut cultures the methane production increased only by 149% when compared to non-pretreated straw. Scanning electron microscopy (SEM) showed that the alkaline pretreatment modified the surface of the wheat straw fibers, which promoted biofilm formation and microbial growth. The enrichment cultures derived from larva gut microbiome were able to degrade larger 1 mm alkaline treated and smaller 250 µm but non-pretreated straw at the same efficiency. The combination of mechanical and alkaline pretreatments resulted in increased, yet not superimposed, methane yield.

New concept for conversion of lignocellulose to ethanol and furfural.

Studies on the chemical mechanisms of furfural formation showed the possibility to apply a new differential catalysis of hemicellulose - its depolymerisation and pentose dehydration to furfural. This change led to the increase in furfural yield and essential decrease of cellulose destruction. The lignocellulose residue that remains after the production of furfural may be subjected to enzymatic hydrolysis to glucose and the subsequent fermentation to ethanol. The remaining lignin appeared to be suitable for the production of additional various value-added products including medicinal mushrooms and laccase-containing enzyme complexes. Based on these developments, an innovative concept is proposed for the waste-free use of lignocellulose to obtain various valuable products. KEY POINTS: • New chemical mechanism of furfural production. • New lignocellulose pretreatment does not damage cellulose and lignin. • Lignocellulose may be processed using waste-free technology.

Facile and sustainable preparation of hierarchical porous carbon material from lignin/DES mixture after pretreatment

Deep eutectic solvent (DES) pretreatment of lignocellulose is an appealing and green approach for efficient lignin extraction. However, the process of lignin regeneration and DES recovery was time-consuming and labor-intensive. Herein, without any separation and purification after the DES pretreatment of Eucalyptus, the lignin/DES mixture was firstly used to fabricate hierarchical porous carbon materials through DES-thermal carbonization (DTC) and activation. Results revealed that the DTC processing involved degradation, condensation, aromatization, and aggregation, which significantly improved the carbonization degree and thermostability of carbon materials. X-ray photoelectron spectroscopic results confirmed the introduction of nitrogen atom from the choline chloride/lactic acid DES into the fabricated carbon materials during the DTC processing. This phenomenon can generate extra pseudocapacitance and thus increase the specific capacitance of supercapacitors. The capacitance retention of the resultant porous carbon materials (AC-200–12) reached above 95% after 10,000 charge-discharge cycles, which was much superior to commercial activated carbon. This facile and sustainable methodology overcomes traditional complex and expensive methods for preparing carbon materials from hardwood, which show promising potential to be used as electrode materials in a supercapacitor.