Creative biological lignin conversion routes toward lignin valorization

Photothermal-responsive lignin-based polyurethane with mechanically robust, fast self-healing, solid-state plasticity and shape-memory performance

In light of the depletion of petrochemical resources and increase in environmental pollution, there has been a significant focus on utilizing natural biomass, specifically lignin, to develop sustainable and functional materials. This research presents the development of a lignin-based polyurethane (DLPU) with photothermal-responsiveness by incorporating lignin and oxime-carbamate bonds into polyurethane network. The abundant hydrogen bonds between lignin and the polyurethane matrix, along with its cross-linked structure, contribute to DLPU's excellent mechanical strength (30.2 MPa) and toughness (118.7 MJ·m-3). Moreover, the excellent photothermal conversion ability of DLPU (54.4 %) activates dynamic reversible behavior of oxime-carbamate bonds and hydrogen bonds, thereby endowing DLPU with exceptional self-healing performance. After 15 min of near-infrared irradiation, DLPU achieves self-healing efficiencies of 96.0 % for tensile strength and 96.3 % for elongation at break. Additionally, DLPU exhibits photocontrolled solid-state plasticity as well as an excellent phototriggered shape-memory effect (70 s), with shape fixity and recovery ratios reaching 98.8 % and 95.3 %, respectively. By exploiting the spatial controllability and photothermal-responsiveness of DLPU, we demonstrate multi-dimensional responsive materials with self-healing and shape-shifting properties. This work not only promotes the development of multi-functional polyurethanes but also provides a pathway for the high-value utilization of lignin.

Perspective on Lignin Conversion Strategies That Enable Next Generation Biorefineries

The valorization of lignin, a currently underutilized component of lignocellulosic biomass, has attracted attention to promote a stable and circular bioeconomy. Successful approaches including thermochemical, biological, and catalytic lignin depolymerization have been demonstrated, enabling opportunities for lignino-refineries and lignocellulosic biorefineries. Although significant progress in lignin valorization has been made, this review describes unexplored opportunities in chemical and biological routes for lignin depolymerization and thereby contributes to economically and environmentally sustainable lignin-utilizing biorefineries. This review also highlights the integration of chemical and biological lignin depolymerization and identifies research gaps while also recommending future directions for scaling processes to establish a lignino-chemical industry.

Eco-friendly regeneration of lignin with acidic deep eutectic solvent for adsorption of pollutant dyes for water cleanup

In this study, a simple and eco-friendly method was used to treat alkaline lignin with an acidic deep eutectic solvent (DES) to obtain regenerated lignin for the efficient adsorption of pollutant dyes from aqueous environment. Based on the yield and adsorption capacity of the sorbent for these dyes, conditions such as the type and concentration of DES component, solid-to-liquid ratio, reaction time, and temperature were optimized. By characterizing and comparing alkali lignin with regenerated lignin, a series of reactions were demonstrated to occur during the DES treatment process. The performance and mechanism of methylene blue and rhodamine B adsorption on regenerated lignin were studied systematically, and the maximum adsorbed amounts were 348.29 and 551.05 mg/g at 323 K, respectively. This study provides a new strategy for the green preparation of functionalized lignin and its use in the water pollutant treatment.

Nature stays natural: two novel chemo-enzymatic one-pot cascades for the synthesis of fragrance and flavor aldehydes

Novel synthetic strategies for the production of high-value chemicals based on the 12 principles of green chemistry are highly desired. Herein, we present a proof of concept for two novel chemo-enzymatic one-pot cascades allowing for the production of valuable fragrance and flavor aldehydes. We utilized renewable phenylpropenes, such as eugenol from cloves or estragole from estragon, as starting materials. For the first strategy, Pd-catalyzed isomerization of the allylic double bond and subsequent enzyme-mediated (aromatic dioxygenase, ADO) alkene cleavage were performed to obtain the desired aldehydes. In the second route, the double bond was oxidized to the corresponding ketone via a copper-free Wacker oxidation protocol followed by enzymatic Baeyer-Villiger oxidation (phenylacetone monooxygenase from Thermobifida fusca), esterase-mediated (esterase from Pseudomonas fluorescens, PfeI) hydrolysis and subsequent oxidation of the primary alcohol (alcohol dehydrogenase from Pseudomonas putida, AlkJ) to the respective aldehyde products. Eight different phenylpropene derivatives were subjected to these reaction sequences, allowing for the synthesis of seven aldehydes in up to 55% yield after 4 reaction steps (86% for each step).

Genome editing of wood for sustainable pulping

Wood is an abundant and renewable feedstock for pulping and biorefining, but the aromatic polymer lignin greatly limits its efficient use. Sulis et al. recently reported a multiplex CRISPR editing strategy targeting multiple lignin biosynthetic genes to achieve combined lignin modifications, improve wood properties, and make pulping more sustainable.

Bacterial transformation of lignin: key enzymes and high-value products

Lignin, a natural organic polymer that is recyclable and inexpensive, serves as one of the most abundant green resources in nature. With the increasing consumption of fossil fuels and the deterioration of the environment, the development and utilization of renewable resources have attracted considerable attention. Therefore, the effective and comprehensive utilization of lignin has become an important global research topic, with the goal of environmental protection and economic development. This review focused on the bacteria and enzymes that can bio-transform lignin, focusing on the main ways that lignin can be utilized to produce high-value chemical products. Bacillus has demonstrated the most prominent effect on lignin degradation, with 89% lignin degradation by Bacillus cereus. Furthermore, several bacterial enzymes were discussed that can act on lignin, with the main enzymes consisting of dye-decolorizing peroxidases and laccase. Finally, low-molecular-weight lignin compounds were converted into value-added products through specific reaction pathways. These bacteria and enzymes may become potential candidates for efficient lignin degradation in the future, providing a method for lignin high-value conversion. In addition, the bacterial metabolic pathways convert lignin-derived aromatics into intermediates through the "biological funnel", achieving the biosynthesis of value-added products. The utilization of this "biological funnel" of aromatic compounds may address the heterogeneous issue of the aromatic products obtained via lignin depolymerization. This may also simplify the separation of downstream target products and provide avenues for the commercial application of lignin conversion into high-value products.

Unleashing the capacity of Rhodococcus for converting lignin into lipids

Bioconversion of bioresources/wastes (e.g., lignin, chemical pulping byproducts) represents a promising approach for developing a bioeconomy to help address growing energy and materials demands. Rhodococcus, a promising microbial strain, utilizes numerous carbon sources to produce lipids, which are precursors for synthesizing biodiesel and aviation fuels. However, compared to chemical conversion, bioconversion involves living cells, which is a more complex system that needs further understanding and upgrading. Various wastes amenable to bioconversion are reviewed herein to highlight the potential of Rhodococci for producing lipid-derived bioproducts. In light of the abundant availability of these substrates, Rhodococcus' metabolic pathways converting them to lipids are analyzed from a "beginning-to-end" view. Based on an in-depth understanding of microbial metabolic routes, genetic modifications of Rhodococcus by employing emerging tools (e.g., multiplex genome editing, biosensors, and genome-scale metabolic models) are presented for promoting the bioconversion. Co-solvent enhanced lignocellulose fractionation (CELF) strategy facilitates the generation of a lignin-derived aromatic stream suitable for the Rhodococcus' utilization. Novel alkali sterilization (AS) and elimination of thermal sterilization (ETS) approaches can significantly enhance the bioaccessibility of lignin and its derived aromatics in aqueous fermentation media, which promotes lipid titer significantly. In order to achieve value-added utilization of lignin, biodiesel and aviation fuel synthesis from lignin and lipids are further discussed. The possible directions for unleashing the capacity of Rhodococcus through synergistically modifying microbial strains, substrates, and fermentation processes are proposed toward a sustainable biological lignin valorization.

Use of lignin-based crude carbon dots as effective antioxidant for natural rubber

Rubber is widely recognized as an important material, whose irreplaceable applications range from damping materials to tires. Generally, rubber is vulnerable to oxidative degradation, leading to a deterioration in the material's performance. Therefore, antioxidants are often added to extend the service life of rubber. In this study, crude lignin-based carbon dots (CLCDs) were prepared by a simple hydrothermal treatment of lignin with H2O2 and triethylenetetramine. The thus prepared CLCDs exhibit excellent radical scavenging capability, and were incorporated into natural rubber with vinyl pyridine-styrene-butadiene terpolymer (VPR) as coupling agent. The results revealed that CLCDs could endow NR with excellent antioxidative performance. Interestingly, CLCDs even show superior antioxidant effect towards rubber compared to purified lignin-based carbon dots (PLCDs). This work provides a unique source of inspiration for the preparation of low-cost, highly effective CLCDs from plant biomass waste, most of lignin being used to produce steam and energy, with excellent antioxidant capability for rubber, which is beneficial for a green and sustainable world.

Low-temperature pretreatment by AlCl3-catalyzed 1,4-butanediol solution for producing 'ideal' lignin with super-high content of β-O-4 linkages

High contents of internal β-O-4 linkages in lignin are critical for high-yield production of high-value aromatic monomers by depolymerization. However, it remains great challenge due to lack of suitable protection strategy. In this work, a very effective lignin-first strategy was developed to produce ideal lignin with a super high content of β-O-4 linkages (up to 72 %) from poplar, in which the pretreatment was undertaken at low temperatures of 90-130 °C with the use of AlCl3-catalyzed 1, 4-butanediol solution. 2D-HSQC NMR spectra revealed that lignin β-O-4 linkages were protected from etherification of the OH group by 1, 4-butanediol at the α position of lignin aliphatic chains. Besides, the OH groups at the γ position of lignin was also etherified, leading the formation of a structure of Ph-CH=CHCH2O(CH2)4OH. Interestingly, structure protection facilitated the formation of lignin nanoparticles via self-assembly (<100 nm). In addition, it was observed from pyrolysis results that addition of 1, 4-butanediol remarkably protected the structure of lignin by avoiding condensation, promoting the production of aromatics. The cellulose-rich fraction possessed a high cellulose digestibility of 91.64 % by enzymatic hydrolysis at a cellulase dosage of 15 FPU/g cellulose, approximately 6-fold untreated poplar (15.91 %). This low-temperature lignin-first strategy was of great importance for multi-products biorefining lignocellulose because it leads to the production of both lignin with super high content of β-O-4 linkages for depolymerization and highly digestible cellulose for sugar production.

Evaluation of a dye-decolorizing peroxidase from Comamonas serinivorans for lignin valorization potentials

Although dye-decolourising peroxidases (DyPs) are well-known for lignin degradation, a comprehensive understanding of their mechanism remains unclear. Therefore, studying the mechanism of lignin degradation by DyPs is necessary for industrial applications and enzyme engineering. In this study, a dye-decolourising peroxidase (CsDyP) gene from C. serinivorans was heterologously expressed and studied for its lignin degradation potential. Molecular docking analysis predicted the binding of 2, 2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), veratryl alcohol (VA), 2, 6-dimethylphenol (2, 6- DMP), guaiacol (GUA), and lignin to the substrate-binding pocket of CsDyP. Evaluation of the enzymatic properties showed that CsDyP requires pH 4.0 and 30 °C for optimal activity and has a high affinity for ABTS. In addition, CsDyP is stable over a wide range of temperatures and pH and can tolerate 5.0 mM organic solvents. Low NaCl concentrations promoted CsDyP activity. Further, CsDyP significantly reduced the chemical oxygen demand decolourised alkali lignin (AL) and milled wood lignin (MWL). CsDyP targets the β-O-4, CO, and CC bonds linking lignin's G, S, and H units to depolymerize and produce aromatic compounds. Overall, this study delivers valuable insights into the lignin degradation mechanism of CsDyP, which can benefit its industrial applications and lignin valorization.

Matching diverse feedstocks to conversion processes for the future bioeconomy

A wide variety of wasted or underutilized organic feedstocks can be leveraged to build a sustainable bioeconomy, ranging from crop residues to food processor residues and municipal wastes. Leveraging these feedstocks is both high-risk and high-reward. Converting mixed, variable, and/or highly contaminated feedstocks can pose engineering and economic challenges. However, converting these materials to fuels and chemicals can divert waste from landfills, reduce fugitive methane emissions, and enable more responsible forest management to reduce the frequency and severity of wildfires. Historically, low-value components, including ash and lignin, are poised to become valuable coproducts capable of supplementing cement and valuable chemicals. Here, we evaluate the challenges and opportunities associated with converting a range of feedstocks to renewable fuels and chemicals.

Recent Advances in Tailored Fabrication and Properties of Biobased Self-Healing Polyurethane

With the emergence of challenges in the environmental degradation and resource scarcity fields, the research of biobased self-healing polyurethane (BSPU) has become a prevailing trend in the technology of the polyurethane industry and a promising direction for developing biomass resources. Here, the production of BSPU from lignocellulose, vegetable oil, chitosan, collagen, and coumarin is classified, and the principles of designing polyurethane based on compelling examples using the latest methods and current research are summarized. Moreover, the impact of biomass materials on self-healing and mechanical properties, as well as the tailored performance method, are presented in detail. Finally, the applications of BSPU in biomedicine, sensors, coatings, etc. are also summarized, and the possible challenges and development prospects are explored to helpfully make progress in the development of BSPU. These findings demonstrate valuable references and practical significance for future BSPU research.

Ethylenediamine pretreatment simultaneously improved carbohydrate hydrolysis and lignin valorization

Lignocellulosic biomass (LCB) is the promising feedstock for value-added products, which would contribute to the bioeconomy and sustainable development. The efficient pretreatment is still required in the biorefinery of LCB. To make a simultaneous utilization of carbohydrates and lignin, a novel easy-recycled ethylenediamine (EDA) pretreatment was designed and evaluated in the present study. The results highlighted that this pretreatment yielded 96% glucose and 70% xylose in enzymatic hydrolysis. It simultaneously promoted the depolymerization of lignin into small molecules and functionalized the yielded lignin with Schiff base and amide structures. These animated-lignins showed a pH-responsive behavior and the excellent flocculation capacity by reducing more than 90% turbidity of kaolin suspensions. Therefore, easy-recycled EDA pretreatment hold the promise to simultaneously enhance the enzymatic hydrolysis of carbohydrates and endowed the new functionality of lignin toward downstream valorization, which improved the process feasibility and potentially enable the sustainability of LCB utilization.

Bioconversion of Homogeneous Linear C-Lignin to Polyhydroxyalkanoates

The bioconversion of homogeneous linear catechyl lignin (C-lignin) to polyhydroxyalkanoates (PHA) was examined for the first time in this study. C-lignins from vanilla, euphorbia, and candlenut seed coats (denoted as C1, C2, and C3, respectively) varied in their molecular structures, which showed different molecular weight distributions, etherification degrees, and contents of hydroxyl groups. A notable amount of nonetherified catechol units existed within C1 and C2 lignins, and these catechol units were consumed during fermentation. These results suggested that the nonetherified catechol structure was readily converted by Pseudomonas putida KT2440. Since the weight-average molecular weight of C2 raw lignin was 26.7% lower than that of C1, the bioconversion performance of C2 lignin was more outstanding. The P. putida KT2440 cell amount reached the maximum of 9.3 × 107 CFU/mL in the C2 medium, which was 37.9 and 82.4% higher than that in the C1 and C3 medium, respectively. Accordingly, PHA concentration reached 137 mg/L within the C2 medium, which was 41.2 and 149.1% higher than the C1 and C3 medium, respectively. Overall, C-lignin, with a nonetherified catechol structure and low molecular weight, benefits its microbial conversion significantly.

Aromatic natural products synthesis from aromatic lignin monomers using Acinetobacter baylyi ADP1

Achieving sustainable chemical synthesis and a circular economy will require process innovation to minimize or recover existing waste streams. Valorization of lignin biomass has the ability to advance this goal. While lignin has proved a recalcitrant feedstock for upgrading, biological approaches can leverage native microbial metabolism to simplify complex and heterogeneous feedstocks to tractable starting points for biochemical upgrading. Recently, we demonstrated that one microbe with lignin relevant metabolism, Acinetobacter baylyi ADP1, is both highly engineerable and capable of undergoing rapid design-build-test-learn cycles, making it an ideal candidate for these applications. Here, we utilize these genetic traits and ADP1's native β-ketoadipate metabolism to convert mock alkali pretreated liquor lignin (APL) to two valuable natural products, vanillin-glucoside and resveratrol. En route, we create strains with up to 22 genetic modifications, including up to 8 heterologously expressed enzymes. Our approach takes advantage of preexisting aromatic species in APL (vanillate, ferulate, and p-coumarate) to create shortened biochemical routes to end products. Together, this work demonstrates ADP1's potential as a platform for upgrading lignin waste streams and highlights the potential for biosynthetic methods to maximize the existing chemical potential of lignin aromatic monomers.

Exoproteomic Study and Transcriptional Responses of Laccase and Ligninolytic Peroxidase Genes of White-Rot Fungus Trametes hirsuta LE-BIN 072 Grown in the Presence of Monolignol-Related Phenolic Compounds

Being an abundant renewable source of aromatic compounds, lignin is an important component of future bio-based economy. Currently, biotechnological processing of lignin through low molecular weight compounds is one of the conceptually promising ways for its valorization. To obtain lignin fragments suitable for further inclusion into microbial metabolism, it is proposed to use a ligninolytic system of white-rot fungi, which mainly comprises laccases and peroxidases. However, laccase and peroxidase genes are almost always represented by many non-allelic copies that form multigene families within the genome of white-rot fungi, and the contributions of exact family members to the overall process of lignin degradation has not yet been determined. In this article, the response of the Trametes hirsuta LE-BIN 072 ligninolytic system to the presence of various monolignol-related phenolic compounds (veratryl alcohol, p-coumaric acid, vanillic acid, and syringic acid) in culture media was monitored at the level of gene transcription and protein secretion. By showing which isozymes contribute to the overall functioning of the ligninolytic system of the T. hirsuta LE-BIN 072, the data obtained in this study will greatly contribute to the possible application of this fungus and its ligninolytic enzymes in lignin depolymerization processes.

Biological valorization of lignin to flavonoids

Lignin is the most affluent natural aromatic biopolymer on the earth, which is the promising renewable source for valuable products to promote the sustainability of biorefinery. Flavonoids are a class of plant polyphenolic secondary metabolites containing the benzene ring structure with various biological activities, which are largely applied in health food, pharmaceutical, and medical fields. Due to the aromatic similarity, microbial conversion of lignin derived aromatics to flavonoids could facilitate flavonoid biosynthesis and promote the lignin valorization. This review thereby prospects a novel valorization route of lignin to high-value natural products and demonstrates the potential advantages of microbial bioconversion of lignin to flavonoids. The biodegradation of lignin polymers is summarized to identify aromatic monomers as momentous precursors for flavonoid synthesis. The biosynthesis pathways of flavonoids in both plants and strains are introduced and compared. After that, the key branch points and important intermediates are clearly discussed in the biosynthesis pathways of flavonoids. Moreover, the most significant enzyme reactions including Claisen condensation, cyclization and hydroxylation are demonstrated in the biosynthesis pathways of flavonoids. Finally, current challenges and potential future strategies are also discussed for transforming lignin into various flavonoids. The holistic microbial conversion routes of lignin to flavonoids could make a sustainable production of flavonoids and improve the feasibility of lignin valorization.

Ammonia and sodium sulfite synergistically pretreat reed to enhance enzymatic saccharification

Pretreatment is important to overcome the structural recalcitrance of reed (a viable energy grass) to produce fermentable sugar. Herein, the study reported the pretreatment of reed using different alkali chemicals (sodium hydroxide/anthraquinone, sodium hydroxide/sodium sulfite, sodium hydroxide/sodium sulfide, ammonia/hydrogen peroxide, triethanolamine, and ammonia/sodium sulfite). The comparative study showed that the pretreatment using ammonia and sodium sulfite (NS) performed the best among them. The NS pretreatment of reed was further optimized using the Response Surface Methodology (RSM). The results showed that about 90.36% lignin was removed when reed was pretreated with 10 wt% of ammonia and 10% of sodium sulfite at 172 °C for 20 min. The excellent lignin removal performance was attributable to the synergistic effects between ammonia and sodium sulfite. The NS pretreated reed achieved 85.6% of enzymatic hydrolysis efficiency and 64.83% of total sugar yield.

Bio-Based Valorization of Lignin-Derived Phenolic Compounds: A Review

Lignins are the most abundant biopolymers that consist of aromatic units. Lignins are obtained by fractionation of lignocellulose in the form of "technical lignins". The depolymerization (conversion) of lignin and the treatment of depolymerized lignin are challenging processes due to the complexity and resistance of lignins. Progress toward mild work-up of lignins has been discussed in numerous reviews. The next step in the valorization of lignin is the conversion of lignin-based monomers, which are limited in number, into a wider range of bulk and fine chemicals. These reactions may need chemicals, catalysts, solvents, or energy from fossil resources. This is counterintuitive to green, sustainable chemistry. Therefore, in this review, we focus on biocatalyzed reactions of lignin monomers, e.g., vanillin, vanillic acid, syringaldehyde, guaiacols, (iso)eugenol, ferulic acid, p-coumaric acid, and alkylphenols. For each monomer, its production from lignin or lignocellulose is summarized, and, mainly, its biotransformations that provide useful chemicals are discussed. The technological maturity of these processes is characterized based on, e.g., scale, volumetric productivities, or isolated yields. The biocatalyzed reactions are compared with their chemically catalyzed counterparts if the latter are available.

A Novel Microbial Consortia Catalysis Strategy for the Production of Hydroxytyrosol from Tyrosine

Hydroxytyrosol, a valuable plant-derived phenolic compound, is increasingly produced from microbial fermentation. However, the promiscuity of the key enzyme HpaBC, the two-component flavin-dependent monooxygenase from Escherichia coli, often leads to low yields. To address this limitation, we developed a novel strategy utilizing microbial consortia catalysis for hydroxytyrosol production. We designed a biosynthetic pathway using tyrosine as the substrate and selected enzymes and overexpressing glutamate dehydrogenase GdhA to realize the cofactor cycling by coupling reactions catalyzed by the transaminase and the reductase. Additionally, the biosynthetic pathway was divided into two parts and performed by separate E. coli strains. Furthermore, we optimized the inoculation time, strain ratio, and pH to maximize the hydroxytyrosol yield. Glycerol and ascorbic acid were added to the co-culture, resulting in a 92% increase in hydroxytyrosol yield. Using this approach, the production of 9.2 mM hydroxytyrosol was achieved from 10 mM tyrosine. This study presents a practical approach for the microbial production of hydroxytyrosol that can be promoted to produce other value-added compounds.