New two-stage pretreatment for the fractionation of lignocellulosic components using hydrothermal pretreatment followed by imidazole delignification: Focus on the polysaccharide valorization

Isolation of Highly Crystalline Cellulose via Combined Pretreatment/Fractionation and Extraction Procedures within a Biorefinery Concept

Sustainable production of bio-based materials and chemicals requires integrated approaches which utilize all fractions of lignocellulosic biomass. In this work, highly crystalline cellulose was isolated via combined pretreatment/fractionation and extraction processes from beechwood sawdust. The proposed approach was based on the selective recovery of hemicellulose components in the first step, followed by enhanced delignification in the second step, permitting the efficient recovery of the remaining cellulose via bleaching in the final step. Hydrothermal pretreatment under tailored conditions in neat water or dilute acid resulted in almost complete hemicellulose removal (80-96 wt %) in the liquid fraction. In the second step, the formed surface lignin was isolated via mild extraction while enhanced removal of both native/structural and surface lignin (71 wt %) was achieved by applying the organosolv treatment using dilute sulfuric acid as catalyst. Dilute sulfuric acid pretreatment followed by acid catalyzed organosolv pretreatment proved to be the most efficient combined approach, leading to 80 wt % hemicellulose removal as xylose monomer, and 71 wt % delignification. High crystallinity cellulose (<88%), with an overall cellulose recovery of 68-91 wt % based on native cellulose in parent biomass was isolated in the last step via bleaching of all pretreated biomass solids. The proposed integrated biorefinery procedures that aim to whole "waste" biomass valorization, replacing fossil resources, with the use of green solvents (water, ethanol) at relatively mild temperature/pressure conditions, are in line with the scope of several United Nations Sustainable Development Goals, such as UN SDG 8, 11, 12, and 13.

Lignin Extraction by Using Two-Step Fractionation: A Review

Lignocellulosic biomass represents the most abundant renewable carbon source on earth and is already used for energy and biofuel production. The pivotal step in the conversion process involving lignocellulosic biomass is pretreatment, which aims to disrupt the lignocellulose matrix. For effective pretreatment, a comprehensive understanding of the intricate structure of lignocellulose and its compositional properties during component disintegration and subsequent conversion is essential. The presence of lignin-carbohydrate complexes and covalent interactions between them within the lignocellulosic matrix confers a distinctively labile nature to hemicellulose. Meanwhile, the recalcitrant characteristics of lignin pose challenges in the fractionation process, particularly during delignification. Delignification is a critical step that directly impacts the purity of lignin and facilitates the breakdown of bonds involving lignin and lignin-carbohydrate complexes surrounding cellulose. This article discusses a two-step fractionation approach for efficient lignin extraction, providing viable paths for lignin-based valorization described in the literature. This approach allows for the creation of individual process streams for each component, tailored to extract their corresponding compounds.

Facile degradation of chitosan-sodium alginate-chromium (III) gel in relation to leather re-tanning and filling

Natural polysaccharide hydrogel, exemplified by chitosan‑sodium alginate (CS-SA), has been prevailing in adsorption of chromium (III) (Cr(III)) containing contaminant. However, the traditional desorption of CS-SA-Cr(III) to recycle the adsorbent faces the problems including chemical desorbents secondary pollution, resource waste of the terminal CS-SA adsorbents, and tedious work of reusing the desorbed Cr(III). Herein, the adsorption product, CS-SA-Cr(III) gel, was degraded to CS/SA/Cr(III) sol and applied in leather re-tanning and filling processes directly. To achieve this goal, three degradation methods were used to transform the gel to sol. Due to the excellent overall performance of the CS/SA/Cr(III)-HMD₄ sol (obtained by the hydrothermal-mechanical degradation method for 4 h (HMD₄)), including wide size and distribution range, moderate viscosity (54 ± 3.1 mPa·s), high electronegativity (-38.6 ± 5.8 mV), and good stability, the resultant leather after re-tanning and filling by the sol achieved fascinating properties such as good thermal stability (T_s, 116.8 ± 1.8 °C; T_d, 94.2 ± 1.7 °C), mechanical performance (tensile strength, 6.9 ± 0.52 MPa; elongation at break, 95 ± 3.0 %), and superduper thickening rate (31.8 %). Moreover, the mechanism of good re-tanning and filling effects was deciphered. Therefore, this work intends to overcome the limitation of traditional desorption technology and further realizes the high-valued application of the exhausted CS-SA-Cr(III) in leather re-tanning and filling processes.

The potential of imidazole as a new solvent in the pretreatment of agro-industrial lignocellulosic biomass

Lignocellulosic biomass is a renewable material of great abundance. However, its recalcitrant characteristic requires the application of pretreatments. Sugarcane bagasse (SB), soybean hulls (SH), cocoa pod husks (CPH) and oil palm empty fruit bunches (OPEFB) were subjected to imidazole pretreatment in order to evaluate chemical composition variations and influence over enzymatic hydrolysis efficiency. Non-treated SH, SB and OPEFB have higher content of holocellulose, while CPH is rich in lignin polymers (31.2%). After imidazole-pretreatment, all biomasses presented structural disorganization of lignocellulosic fibres and enrichment in the percentage of cellulose. Levels of up to 72% delignification were obtained, which allowed an enzymatic conversion greater than 95% for SB, SH and OPEFB, while only 83% was reached for CPH. Imidazole is then emerging as a potential catalyst for the pretreatment of agro-industrial by-products, allowing the valorisation of these residues and their reinsertion into the production chain under a biorefinery concept.

Lignocellulosic Biorefinery Technologies: A Perception into Recent Advances in Biomass Fractionation, Biorefineries, Economic Hurdles and Market Outlook

Lignocellulosic biomasses (LCB) are sustainable and abundantly available feedstocks for the production of biofuel and biochemicals via suitable bioconversion processing. The main aim of this review is to focus on strategies needed for the progression of viable lignocellulosic biomass-based biorefineries (integrated approaches) to generate biofuels and biochemicals. Processing biomass in a sustainable manner is a major challenge that demands the accomplishment of basic requirements relating to cost effectiveness and environmental sustainability. The challenges associated with biomass availability and the bioconversion process have been explained in detail in this review. Limitations associated with biomass structural composition can obstruct the feasibility of biofuel production, especially in mono-process approaches. In such cases, biorefinery approaches and integrated systems certainly lead to improved biofuel conversion. This review paper provides a summary of mono and integrated approaches, their limitations and advantages in LCB bioconversion to biofuel and biochemicals.

Towards integrated sustainable biofuel and chemical production: An application of banana pseudostem ash in the production of biodiesel and recovery of lignin from bamboo leaves

This study investigated an integrated approach to the biowaste transformation and valorization of byproducts. Biochar obtained from the banana pseudostem was calcined to synthesize a heterogeneous catalyst and sustainably prepare a highly alkaline solution. The ash was utilized directly as a heterogeneous catalyst in biodiesel production from waste cooking oil. At the same time, an alkaline solution prepared from the ash was used for delignification and recovery of lignin from bamboo leaves by the hydrothermal reaction. Techniques like Fourier-transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET), Transmission electron microscopy (TEM), and Energy dispersive X-ray (EDX) were applied to characterized the catalyst. The alkaline solution was analyzed with Atomic absorption spectroscopy (AAS). The Response surface methodology (RSM) technique was considered for the optimization of different parameters in the transesterification and hydrothermal reaction. Under the optimized condition, waste cooking oil (WCO) to Fatty acid methyl ester (FAME) conversion was 97.56 ± 0.11%, and lignin recovery was 43.20 ± 0.45%. While at the best operating pyrolysis temperature, the liquid fraction yield from the banana pseudostem (500 °C) was 38.10 ± 0.31 wt%. This integrated study approach encourages the inexpensive, sustainable, and environment-friendly pathway for synthesizing catalysts and preparing a highly alkaline solution for the valorization of biowaste into biofuel and biochemicals.

Advances in physicochemical pretreatment strategies for lignocellulose biomass and their effectiveness in bioconversion for biofuel production

The inherent recalcitrance of lignocellulosic biomass is a significant barrier to efficient lignocellulosic biorefinery owing to its complex structure and the presence of inhibitory components, primarily lignin. Efficient biomass pretreatment strategies are crucial for fragmentation of lignocellulosic biocomponents, increasing the surface area and solubility of cellulose fibers, and removing or extracting lignin. Conventional pretreatment methods have several disadvantages, such as high operational costs, equipment corrosion, and the generation of toxic byproducts and effluents. In recent years, many emerging single-step, multi-step, and/or combined physicochemical pretreatment regimes have been developed, which are simpler in operation, more economical, and environmentally friendly. Furthermore, many of these combined physicochemical methods improve biomass bioaccessibility and effectively fractionate ∼96 % of lignocellulosic biocomponents into cellulose, hemicellulose, and lignin, thereby allowing for highly efficient lignocellulose bioconversion. This review critically discusses the emerging physicochemical pretreatment methods for efficient lignocellulose bioconversion for biofuel production to address the global energy crisis.

Second Generation Bioethanol Production from Soybean Hulls Pretreated with Imidazole as a New Solvent

Soybean hulls (SH) are the main industrial waste from soybean processing, representing 5–8% of the whole grain. Imidazole was employed for the hydrothermal pretreatment of SH and further bioethanol production. Different pretreatment temperatures (120 and 180 °C) and times (1 and 3 h) were tested. Lignin removal and glucose yield were significantly influenced by temperature. After 48 h of enzymatic hydrolysis of imidazole-treated SH (120 °C, 1 h), 32.7 g/L of glucose and 9.4 g/L of xylose were obtained. A maximum bioethanol yield of 78.9% was reached after 12 h of fermentation by Saccharomyces cerevisiae using SH enzymatic hydrolysate. Imidazole appears to be a potential alternative to pretreat lignocellulosic wastes such as SH for the production of second-generation biofuels and other biomolecules.

Promising seawater hydrothermal combining electro-assisted pretreatment for corn stover valorization within a biorefinery concept

In this study, for the first time, seawater hydrothermal (SH) pretreatment combining subsequent electrogenerated alkaline hydrogen peroxide (EAHP) pretreatment was proposed to achieve an effective fractionation of corn stover into high value-added products. During SH pretreatment, complex ions in natural seawater (Mg²⁺, Ca²⁺ and Cl^-) were used to promote depolymerization of xylan into xylo-oligosaccharides with 49.37% yield (190 °C,40 min), 18.52% higher than that of deionized water. Subsequent EAHP treatment not only provided a green and economical way to produce hydrogen peroxide but also synchronously realized satisfied delignification (94.91%). The integrated pretreatment resulted in 91.16% of glucose yield, which was about 5.6 times more than that of unpretreated corn stover. In addition, the recovered lignin fraction which has a potential application in functional materials were investigated by FTIR, 2D-HSQC NMR and GPC. In short, this work provided a novel and environmentally-friendly strategy for biorefinery-based fractionation of corn stover.

The application of green solvent in a biorefinery using lignocellulosic biomass as a feedstock

The high dependence on crude oil for energy utilization leads to a necessity of finding alternative sustainable resources. Solvents are often employed in valorizing the biomass into bioproducts and other value-added chemicals during treatment stages. Unfortunately, despite the effectiveness of conventional solvents, hindrances such as expensive solvents, unfavourable environmental ramifications, and complicated downstream separation systems often occur. Therefore, the scientific community has been actively investigating more cost-effective, environmentally friendly alternatives and possess the excellent dissolving capability for biomass processing. Generally, 'green' solvents are attractive due to their low toxicity, economic value, and biodegradability. Nonetheless, green solvents are not without disadvantages due to their complicated product recovery, recyclability, and high operational cost. This review summarizes and evaluates the recent contributions, including potential advantages, challenges, and drawbacks of green solvents, namely ionic liquids, deep eutectic solvents, water, biomass-derived solvents and carbon dioxide in transforming the lignocellulosic biomass into high-value products. Moreover, research opportunities for future developments and potential upscale implementation of green solvents are also critically discussed.

Thermochemical and Catalytic Conversion Technologies for the Development of Brazilian Biomass Utilization

The social, economic, and environmental impacts of climate change have been shown to affect poorer populations throughout the world disproportionally, and the COVID-19 pandemic of 2020–2021 has only exacerbated the use of less sustainable energy, fuel, and chemical sources. The period of economic and social recovery following the pandemic presents an unprecedented opportunity to invest in biorefineries based on the pyrolysis of agricultural residues. These produce a plethora of sustainable resources while also contributing to the economic valorization of first-sector local economies. However, biomass-derived pyrolysis liquid is highly oxygenated, which hinders its long-term stability and usability. Catalytic hydrogenation is a proposed upgrading method to reduce this hindrance, while recent studies on the use of nickel and niobium as low-cost catalysts, both abundant in Brazil, reinforce the potential synergy between different economic sectors within the country. This review gathers state-of-the-art applications of these technologies with the intent to guide the scientific community and lawmakers alike on yet another alternative for energy and commodities production within an environmentally sustainable paradigm.

Imidazole Processing of Wheat Straw and Eucalyptus Residues-Comparison of Pre-Treatment Conditions and Their Influence on Enzymatic Hydrolysis

Biomass pre-treatment is a key step in achieving the economic competitiveness of biomass conversion. In the present work, an imidazole pre-treatment process was performed and evaluated using wheat straw and eucalyptus residues as model feedstocks for agriculture and forest-origin biomasses, respectively. Results showed that imidazole is an efficient pre-treatment agent; however, better results were obtained for wheat straw due to the recalcitrant behavior of eucalyptus residues. The temperature had a stronger effect than time on wheat straw pre-treatment but at 160 °C and 4 h, similar results were obtained for cellulose and hemicellulose content from both biomasses (ca. 54% and 24%, respectively). Lignin content in the pre-treated solid was higher for eucalyptus residues (16% vs. 4%), as expected. Enzymatic hydrolysis, applied to both biomasses after different pre-treatments, revealed that results improved with increasing temperature/time for wheat straw. However, these conditions had no influence on the results for eucalyptus residues, with very low glucan to glucose enzymatic hydrolysis yield (93% for wheat straw vs. 40% for eucalyptus residues). Imidazole can therefore be considered as a suitable solvent for herbaceous biomass pre-treatment.

Hydrothermal pretreatment of lignocellulosic biomass for hemicellulose recovery

The hemicellulosic fraction recovery is of interest for integrated processes in biorefineries, considering the possibility of high economic value products produced from their structural compounds of this polysaccharide. However, to perform an efficient recovery, it is necessary to use biomass fractionation techniques, and hydrothermal pretreatment is highlighted as a valuable technique in the hemicellulose recovery by applying high temperatures and pressure, causing dissolution of the structure. Considering the possibility of this pretreatment technique for current approaches to hemicellulose recovery, this article aimed to explore the relevance of hydrothermal pretreatment techniques (sub and supercritical water) as a strategy for recovering the hemicellulosic fraction from lignocellulosic biomass. Discussions about potential products to be generated, current market profile, and perspectives and challenges of applying the technique are also addressed.

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.

Challenges and Perspective of Recent Biomass Pretreatment Solvents

The increased demands on renewable and sustainable products require enhancing the current conversion efficiency and expanding the utilization of biomass from a single component (i.e., cellulose) to entire biomass components in the biorefinery concept. Pretreatment solvent plays a critical role in various biorefinery processes. Recent pretreatment solvents such as organic co-solvents, acid hydrotropes, ionic liquids and deep eutectic solvents showed effective biomass fractionation as well as preservation of high-quality cellulose and lignin under mild conditions. Despite these significant enhancements in biomass pretreatment solvent, there are still many challenges, such as feedstock variety, valorization of non-cellulose components, and eco-friendliness of the applied catalyst and solvent. These technical, economic and environmental obstacles should be considered in future biomass pretreatment solvents. In particular, the development of feedstock-agnostic solvent with high fractionation performance for high quality and quantity of all three major components (i.e., cellulose, hemicellulose, and lignin) together would be an ideal direction.

Green solvent pretreatment for enhanced production of sugars and antioxidative lignin from poplar

A promising pretreatment mediated by biomass-derived deep eutectic solvent (DES) with choline chloride to lactic acid was implemented to overcome the recalcitrant structure of poplar sawdust for effective enzymatic hydrolysis and valorized lignin. Results showed the DES applied was strongly selective towards removal of lignin and xylan while preserving cellulose. Under the optimal pretreatment condition (DES ratio: 1:2, temperature: 130 °C, time:1.5 h), the glucose yield from enzymatic hydrolysis was 75.8%. Chemical and structure changes of recovered lignin were evaluated to gauge its valorization potential. It indicated that the recovered lignin possessed molecular weight (4000-6000 g/mol), low polydispersity (PDI < 2.0), low quantity of β-aryl-ethers with no recondensation, and an abundance of phenolic OH groups. The excellent antioxidant capacity of lignin demonstrated its great value as a polyphenolic antioxidant. Overall, this work demonstrated an emerging biorefinery technology method for effective fractionation and valorization of lignocellulosic biomass.

Wheat straw components fractionation, with efficient delignification, by hydrothermal treatment followed by facilitated ethanol extraction

Lignocellulosic biomass fractionaion into its three major components is critically important for efficient feedstock utilization. The hydrothermal-ethanol method has broad application as its first step, hydrothermal treatment, provides high hemicellulose separation efficiency. However, it severely inhibits the delignification on the subsequent ethanol extraction. In this study, the second step, ethanol extraction, was facilitated by the addition of 3% NaOH and 3% H₂O₂, resulting in a significant improvement of lignin separation (by 48.2%). SEM, AFM, XPS, and XRD were used to characterize the surface composition of the remaining solids (crude cellulose) while the structure of isolated lignin was characterized by FT-IR, CP/MAS 13C NMR, GPC and TGA. The lignin samples isolated with both facilitated and non-facilitated ethanol extraction were compared to elucidate the lignin removal mechanism. The results showed that lignin degradation and crosslinking/polymerization occur in parallel during both the hydrothermal treatment and ethanol extraction.

Scale-up of two-step acid-catalysed glycerol pretreatment for production of oleaginous yeast biomass from sugarcane bagasse by Rhodosporidium toruloides

Two-step dilute acid and acid-catalysed glycerol pretreatment was developed to maximise sugar yield from sugarcane bagasse. At the laboratory scale, dilute acid pretreatment at 130 °C followed by acid-catalysed glycerol pretreatment at 170 °C led to a total sugar (C5 + C6) yield of 82%, 31% higher than that from one-step acid-catalysed glycerol pretreatment. At the pilot scale, the two-step dilute acid and acid-catalysed glycerol pretreatment led to a maximum sugar yield of 74%, 13% higher than that from one-step pretreatment with 52% reduction in glycerol usage. The enzymatic hydrolysate containing glucose and residual glycerol were used to produce microbial oils by a Rhodosporidium toruloides strain. A fed-batch cultivation strategy led to the production of 44.8 g/L cell mass, including 26.6 g/L oil, 8.6 g/L protein and 12.7 mg/L carotenoid. The cell mass and oil yields were 19% higher than those from batch cultivation as feedstock inhibition and catabolite repression were alleviated.

Sequential pretreatment with alkaline hydrogen peroxide and choline chloride:copper (II) chloride dihydrate - Synergistic fractionation of oil palm fronds

In this study, a novel Type II deep eutectic solvent (DES) namely, choline chloride:copper(II) chloride dihydrate (ChCl:CuCl₂·2H₂O) was used to pretreat oil palm fronds (OPFs). The sequential pretreatment with alkaline hydrogen peroxide (0.25 vol%, 90 min) at ambient conditions and a Type II DES (90 °C, 3 h) at a later stage resulted in a delignification of 55.14% with high xylan (80.79%) and arabinan (98.02%) removals. The characterizations of pretreated OPFs confirmed the excellent performance of DES in OPF fractionation. Thus, the application of a Type II DES at ambient pressure and relatively lower temperature was able to improve the lignin and hemicellulose removals from OPFs.