Search

Phenolic resins (PRs) are being widely used in many fields such as molding plastics, foams, coatings, and semiconductor packaging owing to their good properties including corrosion resistance, heat resistance, flame resistance, and electrical insulation properties. However, the traditional phenolic resins industry depends very heavily on petroleum as a source of phenol raw material, which is under the influence of petroleum reserves. In recent years, many studies have been conducted to develop cheap, abundant, renewable, and high-performance alternatives to phenol for the production of PR. Lignin, as the most abundant natural polyphenol, has similar chemical structures to that of phenol. Lignin has received increasing attention as a potential feedstock for renewable fuels and chemical production. The substitution of lignin for phenol can not only reduce the costs of PRs but also increase their performance, such as low-toxicity residues and environmental friendliness. This paper reviews the recent progress in lignin activation and modification for lignin-based phenolic resin (LPR) synthesis, highlights the different lignin modification methods, compares the performance of different LPR products, and summarizes their applications in adhesives, foams, molding powders, micro/nano-spheres, and other advanced materials. Finally, this review puts forward the current challenges and potential future prospects for LPR materials. [ABSTRACT FROM AUTHOR]

Due to the similarity in chemical structure, lignin has presented great potential in the synthesis of bio-based phenolic resin. Herein, a facile and scalable synthesis strategy for a novel cage-like lignin-based phenolic resin was proposed through direct spray drying of the mixture of phenolic resin and unmodified lignin. The as-prepared lignin-based phenolic resin was pre-carbonized and carbonized using KOH to produce mesoporous carbon materials (LPRAC) that were further employed as supercapacitor electrodes. The results revealed that the lignin addition led to the noticeable change of the resin shape from perfect sphere to cage-like structure owing to the surfactivity and hydrophilicity of lignin. The cage-like structure promoted the KOH-activated pore generation and expansion in the carbonization process, resulting in the significantly increased specific surface area of the phenolic carbon and the intensified mesoporous formation. The electrochemical performance determination indicated that the specific capacitance of LPRAC-20% (20% lignin addition) was 217.3 F/g at 0.5 A/g, which was considerably higher than that of the lignin-free phenolic carbon (122.6 F/g). Moreover, the LPRAC-20% also exhibited preferable capacitance retention and excellent cycling stability. Accordingly, the present work offers a simple and scalable approach to prepare cage-like mesoporous carbon with excellent electrochemical performance from lignin-based phenolic resin and thus provides a promising route for lignin valorization in energy storage materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Eucalyptus dissolving kraft pulp pre-hydrolysis liquor (PHL) is rich in sugar and other biomass resources, which can produce furfural and may serve as a platform molecule for sustainable fuel production. However, the presence of lignin increases the side reactions during the high-value utilization of PHL. Therefore, a novel process technology, consisting of laccase-induced lignin polymerization, was developed in this paper to enhance the removal of lignin. During the study, the mechanism of the action of laccase on lignin was speculated and it was discovered that the effect of laccase on lignin is to polymerize its small molecules. The main reaction of laccase oxidation is the formation of 5-5′ and 4-O-5′ bonds, and occasionally β-O-4′. If position 5 is blocked, other side reactions will occur, including coupling at position 1 and oxidation at position α. Simultaneously, the –OH content of lignin increases under the activation of laccase. Then, the conversion of furfural from the delignified pre-hydrolysis liquor (LPHL) under the catalysis of 2 wt% sulfuric acid was investigated. With the traditional one-pot reactor system, furfural and the reaction medium cannot be separated in time and the degradation of furfural cannot be prevented. The side reactions caused by this, such as fragmentation, condensation and resinification, which is visible to the naked eye, involve the production of humic substances, which becomes the most important factor in the low furfural yield. These side reactions are verified by GC/MS. According to the results in this study, the biorefinery process towards PHL, which allows the separation of lignin by laccase and the conversion of xylose to furfural, improves the overall application of the PHL biomass. [ABSTRACT FROM AUTHOR]

Utilization of plastic materials (e.g., polyethylene) in mulching film production has inevitably resulted in non-degradable pollutants in soil, which accordingly promotes the development of biodegradable mulching film industry. As a rich renewable, biodegradable aromatic polymer, Lignin exhibits great potential for fabricating functional biodegradable composite mulching films. In recent years, composite mulching films containing lignin have garnered more interest for both practical applications and fundamental research, but comprehensive reviews detailing preparation and application of this rapidly developing composite are still limited. Herein, we begin with a brief description about the processes used to prepare lignin-based biodegradable mulching film. Furthermore, the design and application advances of lignin-based biodegradable mulching films are elaborated based on the polymer materials used, including natural and synthetic biodegradable polymers. Finally, the remaining challenges and future perspectives of lignin-based biodegradable mulching films are summarized. We expect that this finding will shed light on the forthcoming research of lignin-based biodegradable mulching film, and stimulate the development in this research area. • Design and application of lignin-based biodegradable composite mulching film are reviewed. • Film preparation methods including physical and chemical processing are described individually. • Application progresses of lignin-based mulching film are elaborated based on the polymer used. • Enhancements of composite mulching film properties by lignin addition are emphasized. • Challenges and perspectives of lignin-based biodegradable mulching films are summarized. [ABSTRACT FROM AUTHOR]

Sustainably sourced lignin presents great potential as a green feedstock for fertilizer production but commercial fulfillment is still challenging owing to the mediocre fertilizer activity of lignin. To address this issue, an effective strategy to enhance the activity of lignin-based potassium fertilizer (LPF) is proposed through lignin fractionation. Three lignin fractions subdivided from enzymatic hydrolysis lignin (EHL) were adopted as the feedstock for LPF preparation, and the effect of lignin fractionation on wheat seed germination and seedling growth was investigated. Compared with the potassium fertilizer from unfractionated lignin, LPF-F1 showed significantly improved effects on promoting seed germination and seedling growth, which can be attributed to the high potassium content resulted from its abundant phenolic hydroxyl and carboxyl contents. Under the optimal treatment concentration (100 mg/L), LPF-F1 showed comparable promotion effect to commercial fulvic acid potassium on wheat seedling growth, suggesting the potential of LPF-F1 as commercial potassium fertilizer. Overall, this work reveals that lignin heterogeneity presents critical effects on the wheat seed germination and seedling growth of LPF, and the fertilizer activity of LPF can be substantially improved using fractionated lignin with low molecular weight as the raw material. [Display omitted] [ABSTRACT FROM AUTHOR]

As the most abundant aromatic biopolymer, lignin presents great potential to produce valuable materials and chemicals. However, its large-scale value-added application is still facing many practical challenges and one of them is the unstable properties caused by lignin heterogeneity. Herein, we developed a novel "one-step-three-fractions" fractionation strategy to reduce lignin heterogeneity using aqueous two-phase system (ATPS) composed of (NH 4) 2 SO 4 and ethanol. In contrast to conventional step-wise fractionation processes, the proposed process subdivided heterogeneous lignin into three homogeneous fractions in only one step: the first fraction (F1) dissolved in the ethanol-rich top layer; the second fraction (F2) dissolved in the salt-rich bottom layer and the last fraction (F3) insoluble in both two layers. F2 presented the lowest molecular weight followed by F1 while F3 showed the highest molecular weight. With the increase of molecular weight, the contents of guaiacyl unit and β-O-4 linkage increased while the content of hydrophilic groups (carboxyl and aromatic hydroxyl) decreased significantly. Moreover, the ATPS exhibited satisfactory recyclability and the fractionation approach could be applied to different types/sources of lignin. Consequently, the work indicates that ATPS is a novel and effective way to fractionate lignin and reduce its molecular weight polydispersity and structural heterogeneity in one step. [Display omitted] • Aqueous two-phase system (ATPS) consisting of (NH 4) 2 SO 4 and ethanol was used for lignin fractionation. • Enzymatic hydrolysis lignin was subdivided into three fractions in only one dissolution step. • The three fractions showed reduced heterogeneity in molecular weight and chemical structure. • The universal fractionation approach can be applied to different types/sources of lignin. • The fractionation using regenerated ATPS demonstrated the satisfactory recyclability. [ABSTRACT FROM AUTHOR]

Different dimensional (0D–3D) carbon materials with tailorable compositions and nanostructures have attracted widespread attention and are promising for a wide range of applications in energy storage, adsorption, and catalysis. Lignin, as a typical renewable, low-cost, highly aromatic, and high carbon content bio-component, presents compelling potential in the preparation of multifunctional carbon materials. Currently, numerous lignin-derived carbon materials in different dimensions have been developed successfully. Moreover, by structural design and surface modification, lignin-derived carbon materials with desirable properties can be obtained. This work presents a comprehensive overview on the preparation techniques employed for lignin-derived carbon materials as well as the configurations and applications of the obtained lignin-derived carbon materials. First, advances in cutting-edge research on the preparation of lignin-derived carbon materials (pyrolysis-activation and template methods) are summarized. When lignin is used as a carbon precursor, carbon materials with different dimensions such as 0D, 1D, 2D, or 3D with complex compositions (e.g., heteroatom doping or metal loading) can be prepared. Additionally, the applications of lignin-derived carbon materials for energy storage, adsorption and catalysis are reviewed. Finally, prospects and potential challenges in designing and developing lignin-derived carbon materials are discussed in depth. [Display omitted] • The main components of lignin are briefly introduced. • The preparation of different dimensions of lignin-derived carbon materials are elaborated. • The prospects and challenges of lignin-derived carbon materials for high-value applications are presented. [ABSTRACT FROM AUTHOR]

Sustainably sourced lignin is a desired precursor for carbon materials due to its cross-clinked aromatic structure and high carbon content. However, the heterogeneity of lignin feedstock presents non-negligible effects on its physicochemical properties and thereby substantially influences its applications in carbon materials fabrication. In this work, fractionation of corn stalk enzymatic hydrolysis lignin via a simple two-step ordinal dissolution was adapted to reduce lignin heterogeneity and further improve the microstructure and electrochemical performances of lignin-derived heteroatom-doped carbon materials. The results revealed that the fractionation process successfully obtained three lignin fractions with sequentially increasing molecular weight and specific surface area. When the three lignin fractions were subjected to carbonization/activation to fabricate N/P/S co-doped carbon, the F3-derived carbon material (LC-F3) showed the highest specific surface area (2022.4 m2/g) and the best heteroatom doping effect due to the improved activation process resulted from the largest specific surface area of F3. Moreover, the good thermal stability of F3 also contributed to the maintenance of disordered structure during carbonization. Therefore, LC-F3 exhibited excellent specific capacitance (337.2 F/g, 0.5 A/g) and cycling stability (96.5% after 10,000 cycles) as a consequence of its high disordered level, heteroatom doping density and specific surface area. Consequently, this work systematically evaluates the effects of lignin fractionation on the physicochemical structural and electrochemical performances of the carbon materials and further proposes a simple strategy to enhance electrochemical properties of the heteroatom-doped carbon materials through lignin fractionation. [Display omitted] • Lignin was subdivided into three fractions in two sequential dissolution step. • Fractionation contributes to the formation of pores in lignin. • Heteroatom-doped improves the wettability and introduces the pseudocapacitance. • LC-F3 showed high specific capacitance due to its disordered structure and high S BET. [ABSTRACT FROM AUTHOR]

Lignin nanosizing has received much interest as it offers new potential for value-added applications of the currently under-utilized lignin biopolymers. However, conventional lignin nanosizing technologies often rely on the use of large amounts of toxic organic solvents, and a time-consuming dialysis process that is required to remove the solvents. Herein, we present a novel and effective approach using a binary system consisting of green γ-valerolactone (GVL) and glycerol solvents to prepare lignin nanospheres (LNS) without lignin modification and additional dialysis processes. The rationale of LNS formation lies in the emulsification of uniform lignin-containing GVL droplets in glycerol by a process consisting of (1) heating to 80 °C, and (2) cooling to room temperature. Through this simple process, we obtained very high LNS yield (over 90%), with narrow size distribution (about 275 nm) by using maple kraft lignin as the raw material. This lignin nanosizing approach is universal when applied to different sources/types of lignins. The as-prepared LNS were further applied as a green reducing agent and carrier for the synthesis of Pd nanoparticles (NPs) in a facile in situ reduction process. Pd@LNS exhibited significantly enhanced catalytic capacity in the hydrogen evolution from formic acid and in the reduction of Cr(VI) to Cr(III) compared with bare Pd NPs. The Pd@LNS catalyst demonstrated high recyclability owing to the good chemical stability of LNS and robust loading of Pd NPs on LNS. Consequently, this work offers a green, universal and effective approach for LNS fabrication and presents a promising application of LNS as metal NP carriers for catalysis purposes. [ABSTRACT FROM AUTHOR]

As the most abundant aromatic biopolymer, lignin has great potential to replace petroleum-based phenol to prepare phenolic resins (e.g. phenolic adhesive and foam). In this work, a novel lignin-functionalized phenolic resin (LPR) nanosphere was synthesized using a simple hydrothermal curing process and was further used as both a green reducing agent and a support for the formation of Ag nanoparticles (NPs). Interestingly, it was found that the incorporation of lignin into the phenolic resin (PR) decreased its size from microscale to nanoscale due to the surface activity of lignin. Besides, a part of lignin (the uncombined lignin) acted as a sacrificial template to increase the surface roughness of nanospheres. A decrease in the support size and an increase in the surface roughness enhanced the specific surface area of LPR nanospheres and the aqueous dispersion stability of LPR@Ag nanocomposites. Owing to the excellent reducing activity of lignin and the enhancement of the specific surface area, the loading amount Ag NPs on LPR nanospheres was considerably higher than on the lignin-free PR microspheres. Accordingly, the LPR@Ag nanocomposites showed excellent capabilities in the catalytic reduction of two typical organic pollutants (anionic methyl orange and cationic methylene blue). Particularly, due to the adsorption of cationic methylene blue by lignin, a synergistic effect of adsorption and catalysis was observed, which considerably improved the catalytic performance of LPR@Ag nanocomposites. LPR@Ag nanocomposites also exhibited good recyclability owing to the stable loading of Ag NPs on the nanosphere support. Finally, this work demonstrates the multifunctionality of lignin as a surfactant, sacrificial template, and green reducing agent in the synthesis of LPR@Ag nanocomposites and therefore, provides a new frontier in the development of lignin-based polymer materials focusing on the synergistic effect of multiple functionalities of lignin. [ABSTRACT FROM AUTHOR]

Over the recent years, the exploitation of antibacterial performance of lignin and its subsequent usage as bacteriostatic additives has gained increasing interests. However, due to the restriction from structural heterogeneity, the antibacterial activity of lignin is always modest and unstable (especially against Gram-negative bacteria). In this regard, we proposed a facile one-step ethanol fractionation process to decrease the heterogeneity of lignin and, therefore, improve its antibacterial activity. Two fractions (Fs and Fi, 95% ethanol soluble and insoluble fraction, respectively) were obtained from bamboo kraft lignin (BKL) and their antibacterial effectiveness was compared. The results showed that the antibacterial activity of Fs increased significantly compared to that of BKL. On the contrary, Fi barely exhibited inhibitory effect on the growth of two Gram-positive bacteria and even promoted the growth of two Gram-negative bacteria. The much lower phenolic-OH content in Fi was an important reason for the absence of antibacterial activity. Besides, the growth promotion of Gram-negative bacteria by Fi was possibly caused by the formation of insoluble carriers for bacteria growth due to the poor water-solubility of Fi. Accordingly, after the elimination of Fi, the one-step fractionation significantly enhanced the antibacterial activity of lignin against both Gram-positive and Gram-negative bacteria. • Bamboo kraft lignin was subdivided into Fs and Fi by one-step ethanol fractionation. • Fs showed enhanced antibacterial activity compared to bamboo kraft lignin. • Fi barely showed inhibition on bacteria growth, and even promoted bacterial growth. • The antibacterial activity differences between Fs and Fi were elucidated. • One-step ethanol fractionation of lignin is effective to improve antibacterial activity. [ABSTRACT FROM AUTHOR]

Highlights • Solid acid catalytic phenolation (SACP) is applied to treat enzymatic hydrolysis lignin (EHL). • EHL is depolymerized significantly during SACP treatment without condensation. • Phenol is incorporated into EHL by a nucleophilic reaction that stops lignin condensation. • Treated EHL shows increased phenolic hydroxyl content and reactivity with formaldehyde. • 50% phenol is substituted by treated EHL to prepare phenolic foam with satisfactory properties. Abstract Lignin valorization offers a significant opportunity for the commercial operation of a cellulosic ethanol-derived biorefinery. In this work, a novel solid acid catalytic phenolation (SACP) process was employed to treat enzymatic hydrolysis lignin (EHL) and the obtained product was further applied to prepare lightweight phenolic foams (ρ = 30–40 kg/m3). The changes of the molecular weight and structure of lignin under the SACP treatment were investigated by GPC, Py-GCMS, and 2D-NMR analyses. The results indicated that under the optimized SACP conditions (140 °C, 4% HZSM-5, and 2 h), the molecular weight (M ¯ w) of EHL sharply decreased from 6695 to 2418 g/mol without condensation. The Py-GCMS and 2D-NMR analyses suggested that phenol was incorporated at the C α position through a nucleophilic reaction and this reaction inhibited the lignin condensation during the acidic treatment. Due to the simultaneous depolymerization and phenolation, the treated lignin showed noticeable increase in phenolic hydroxyl content and reactivity with formaldehyde. The further characterization of lignin-based phenolic foams showed that the treated lignin could replace 50% phenol to prepare phenolic foams with satisfactory thermal insulation and compression strength properties. Consequently, this work demonstrates that SACP is effective to depolymerize and activate lignin for high lignin content phenolic foam preparation and offers a value-added way of lignin application as heat-insulating foam materials. [ABSTRACT FROM AUTHOR]

Abstract: BACKGROUND: With the emergence of lignocellulose refining, there is an increasing interest in exploring the valuable applications for lignin due to its abundant and underused features. Originating from the physiological function of lignin in plants, this work attempted to evaluate the antimicrobial potential of enzymatic hydrolysis lignin (EHL) from a bioethanol‐derived biorefinery. RESULTS: Successive ethanol–water dissolution was employed to fractionate EHL and the antimicrobial activities of three obtained lignin fractions (F1, F2 and F3) were compared by the agar diffusion method and minimum inhibitory concentration (MIC) assays. Interestingly, the results showed that the antimicrobial activities of EHL were centralized in F1 which exhibited obviously growth inhibitory effects against two Gram‐positive (Staphylococcus aureus and Bacillus subtilis) and two Gram‐negative (Escherichia coli and Salmonella enterica) bacteria with MICs of 1, 2, 2 and 2 mg mL−1, respectively. The further characterization suggested that the better solubility and higher phenolic hydroxyl content of F1 caused by lower molecular weight were possibly responsible for the improvement of antimicrobial performance. CONCLUSIONS: Consequently, this work suggested that successive ethanol–water fractionation was an effective way to enhance the antimicrobial activity of EHL. © 2018 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The conversion of renewable lignin with low-cost and high carbon content properties into porous carbon materials for supercapacitor applications has caught considerable interest. Herein, two dimensional lignin-derived carbon nanosheets (N-LHPC) with hierarchically porous structures were facilely synthesized via a novel metal-lignin assembly strategy and their performances for supercapacitor applications were investigated. During the carbonization process, the uniformly distributed Zn facilitates the coordinating development of micropores structure and the generated MgO embedded in the carbon matrix acts as a template to produce mesoporous structure after acid washing. Moreover, the melamine addition promotes the development of mesopores by formation of lamellae structure and realizes the N doping in the carbon materials. Therefore, the obtained N-LHPC presents an excellent specific capacitance of 235.75 F/g at 0.5 A/g owing to its hierarchical pore structure as well as the N/O functional groups. Moreover, at the power density of 450 W/kg, the N-LHPC achieves a maximum energy density of 14.75 Wh/kg, showing great application potential in energy storage. The metal-lignin assembly strategy followed by N-doping proposed in this paper provides N-LHPC materials with hierarchical nanostructure, good electron/ion transfer properties, and abundant pseudocapacitive active species, which improve the capacitance performances of the N-LHPC. [Display omitted] • N-LHPC was synthesized by one-pot pyrolysis of metal-lignin complex with melamine. • Metal lignin complexes contribute to the formation of homogeneous pores in carbon. • N/O functional groups improve the wettability and introduce the pseudocapacitance. • N-LHPC showed high specific capacitance due to its nanosheet structure and high S BET. [ABSTRACT FROM AUTHOR]

The heterogeneity of lignin chemical structure and molecular weight results in the lignin inhomogeneous properties which also covers the antioxidant performance. In order to evaluate the effects of lignin heterogeneity on its antioxidant activity, four lignin fractions from enzymatic hydrolysis lignin were classified by sequential organic solvent extraction and further evaluated by DPPH (1,1-Diphenyl-2-Picrylhydrazyl) free radical scavenging capacity and reducing power analysis. The characterization including FTIR, 1 H NMR and GPC showed that the fractionation process could effectively separate lignin fractions with distinctly different molecular weight and weaken the heterogeneity of unfractionated lignin. The antioxidant performance comparison of lignin fractions indicated that the dichloromethane fraction (F1) with lowest molecular weight (4585 g/mol) and highest total phenolics content (246.13 mg GAE/g) exhibited the highest antioxidant activity whose value was close to commercial antioxidant BHT (butylated hydroxytoluene). Moreover, the relationship between the antioxidant activity and the structure of lignin was further discussed to elucidate the mechanism of antioxidant activity improvement of lignin fractionation. Consequently, this study suggested that the sequential extraction was an effective way to obtain relatively homogeneous enzymatic hydrolysis lignin fractions which showed the potential for the value-added antioxidant application. [ABSTRACT FROM AUTHOR]

In this work, the alkaline extraction processes of lignin from untreated and steam-exploded corn stalk were compared and the enhanced extraction process was investigated by neutral solvent extraction and structural analysis of lignin. It was found that a first order model could readily simulate the lignin extraction from untreated stalk while it was helpless to describe the enhanced process after steam explosion due to the intense dissolution of lignin at the beginning stage of extraction. The lignin extracted by neutral solvent from ball-milled corn stalk and its structural characterization demonstrated that the poor description of extraction process by a first order model was mainly attributed to the considerable depolymerization during steam explosion. Due to the depolymerization of lignin, the alkaline extraction after steam explosion became the direct dissolution of depolymerized lignin and was successfully simulated by a second-order model. Finally, the activation energy of lignin alkali-extraction decreased 37.78% after steam explosion treatment with 1.8 MPa and 5 min, which indicated that milder extraction conditions were available to extract lignin from steam-exploded corn stalk. This study also provides a basic guidance for delignification approaches of lignocelluloses after steam explosion. [ABSTRACT FROM AUTHOR]

Lignin heterogeneity, including molecular weight polydispersity and structural complexity, causes undesirable properties of lignin in its valorization. In this regard, the current work focuses on the effect of lignin heterogeneity on the green synthesis of lignin nanosphere (LNS) supported Pd nanoparticles (Pd@LNS) and their catalytic performance. Three lignin fractions with reduced heterogeneity (F1, F2, and F3) were used for LNS fabrication and the as-prepared LNSs were further applied as carriers for Pd nanoparticles (Pd NPs) synthesized using lignin as an in-situ reducer. It was found that the three LNSs exhibited a gradually decreased particle size from F1 to F3, while all of them revealed improved particle size uniformity compared to that from the initial lignin. Owing to the superior reducing power of F1, the LNS prepared from F1 presented the highest loading amount of Pd NPs among the three LNSs. Moreover, these Pd NPs had the smallest particle size and they were dispersed in a much more uniform way than those on the LNSs from F2 and F3. Therefore, the Pd@LNS prepared from F1 exhibited significantly increased catalytic activity in the Cr(VI) reduction compared with those from F2 and F3, as well as the initial lignin. Consequently, the work implies that lignin heterogeneity has considerable influences on the green formation of Pd NPs on LNS, and the catalytic performance of Pd@LNS can be substantially improved using low molecular weight lignin fraction as the feedstock. [Display omitted] • Effect of lignin heterogeneity on lignin nanosphere supported Pd NPs (Pd@LNS) preparation was studied. • Three fractions with reduced heterogeneity (F1, F2 and F3) were obtained by fractionation of initial lignin. • LNS-F1 showed the highest Pd NPs loading amount among the three LNSs due to high reducing power of F1. • Pd NPs on LNS-F1 showed decreased particle size and their dispersion on LNS-F1 was much more uniform. • Pd@LNS-F1 had markedly higher catalytic activity for Cr(VI) reduction than Pd@LNS-F2 and Pd@LNS-F3. [ABSTRACT FROM AUTHOR]

Highlights: [•] Steam explosion decreased carbohydrate content in alkali-extracted lignin solution. [•] Lignin liquor without purification was methylolated under optimized conditions. [•] Methylolated lignin liquor substituted 50% phenolic adhesive with approved properties. [•] FTIR indicated formaldehyde was incorporated to aromatic C5 of G unit by methylolation. [•] Phenolic substitution by lignin from steam-explosion-derived biorefinery is feasible. [ABSTRACT FROM AUTHOR]

Highlights: [•] Sequential dissolution in ethanol–water solvent is applied to fractionate and purify lignin. [•] Different molecular weight lignin shows different solubility in ethanol–water solvent. [•] Three fractions with low, medium and high molecular weight were obtained. [•] Due to absence of harmful and pricy solvent, the work provides a potential lignin fractionation process. [ABSTRACT FROM AUTHOR]

Abstract: The heterogeneity of lignin, especially the wide distribution of molecular weight, restricts the high value-added application of lignin. This work provided a convenient and efficient method to fractionate lignin by gradient acid precipitation. To investigate the acid precipitation performance of lignin with different molecular weight, four alkali-extracted lignin fractions from steam-exploded stalk were obtained from ultrafiltration and subsequently precipitated by acid addition. Results showed that molecular weight had a significant effect on the acid precipitation behaviors of lignin. High molecular weight lignin fractions (20kDa retentate) were precipitated completely at higher pH and had narrow precipitation pH range compared with low molecular weight lignin fractions. The determination of Zeta potential and the particle size of lignin colloid revealed that higher Zeta potential and larger particle size formed by high molecular weight lignin accelerated the precipitation. According to the acid precipitation behaviors of different molecular weight lignin, gradient acid precipitation of lignin at pH 5.3, 4.0 and 2.0 was established and four lignin fractions were obtained. FT-IR and UV analysis showed similar chemical structures among the four fractions and increasing content of acidic groups as the molecular weight decreased. The GPC results indicated that the lignin fractions obtained from gradient acid precipitation showed gradually decreasing molecular weight from 15,099–3053g/mol and significantly decreasing polydispersity. Consequently, the study indicated gradient acid precipitation a convenient and successful technique to fractionate alkali-extracted lignin from steam-exploded stalk. [Copyright &y& Elsevier]

[Display omitted] • Lignin heterogeneity causes undesirable properties of lignin and limits its valorization. • Fractionation of lignin into homogeneous fractions is effective to reduce lignin heterogeneity. • Formation of lignin heterogeneity based on molecular weight is elucidated. • Lignin fractionation methods are elaborated and their industrial potentials are discussed. • Upgraded valorization using lignin fractions with different molecular weight is reviewed. Conventional pulping processes and emerging lignocellulosic biorefineries provide a large quantity of lignin with heterogeneous characters, including complex chemical structure and polydisperse molecular weight distribution. The heterogeneity causes undesirable properties for down-stream applications, which limit lignin valorization seriously. Fractionation of heterogeneous lignin into several homogeneous fractions is considered as an effective approach to overcome this limitation and realize upgraded lignin valorization. To date, lignin fractionation approaches have been extensively studied and they can be broadly classified into three categories: gradient acid precipitation, step-wise solvent fractionation, and membrane separation with different cut-offs. After fractionation, the obtained lignin fractions with decreased polydispersity exhibit improved molecular weight dependent properties, and thus using them as raw materials for subsequent applications results in the upgradation of lignin valorization. In this work, the formation of lignin heterogeneity based on molecular weight is firstly elucidated, and lignin fractionation approaches are elaborated and their potentials for industrialization are discussed. Finally, the upgraded valorization using lignin fractions with high and low molecular weight is also reviewed comprehensively in this work. [ABSTRACT FROM AUTHOR]

• Anti-tyrosinase activity of hydrolysate obtained from steam exploded corn stalk is evaluated. • Phenolic compounds, especially lignin H-unit monomers are the main tyrosinase inhibitors. • Ethyl-acetate fractionation is used to enrich phenolic compounds and improve the anti-tyrosinase activity. • Ethyl-acetate soluble fraction exhibits better activity than p -coumaric acid (a typical tyrosinase inhibitor). • The work indicates the potential of corn stalk hydrolysate as a value-added tyrosinase inhibitor. In this work, corn stalk was subjected to steam explosion (SE) pretreatment, and the hydrolysate in post-washing liquor of steam exploded corn stalk was assayed for anti-tyrosinase activity for the first time. The hydrolysate showed strong anti-tyrosinase activity with an IC 50 of 2.17 mg/mL (inhibitor concentration when 50% of tyrosinase activity is inhibited) at SE severity of 4.20 (198 °C, 20 min). Through hydrolysate component profiles and model substance analysis, phenolic compounds (especially lignin-derived H-unit monomers and phenolic acids) were confirmed to be the major tyrosinase inhibitors in hydrolysate. To further improve the inhibitory activity, an enrichment process of phenolic compounds by ethyl-acetate fractionation of hydrolysate was employed. Ethyl-acetate soluble fraction exhibited significantly enhanced anti-tyrosinase activity (IC 50 , 0.29 mg/mL), which was even better than p -coumaric acid (a typical tyrosinase inhibitor with IC 50 of 0.31 mg/mL). Additionally, the mixed-type inhibition of hydrolysate on tyrosinase was illustrated by kinetics study and the interaction between hydrolysate and tyrosinase was confirmed by fluorescence quenching measurement. Consequently, this work aims to evaluate and improve the anti-tyrosinase activity of hydrolysate from steam explosion processing of corn stalk, which opens new perspectives in potential application of currently underused corn stalk hydrolysate as a value-added tyrosinase inhibitor. [ABSTRACT FROM AUTHOR]

In this work, lignin fractionation is proposed as an effective approach to reduce the heterogeneity of lignin and improve the adsorption and recycle performances of lignin as a cationic dye adsorbent. By stepwise dissolution of enzymatic hydrolysis lignin in 95% and 80% ethanol solutions, three lignin subdivisions (95% ethanol-soluble subdivision, 80% ethanol-soluble subdivision, and 80% ethanol-insoluble subdivision) were obtained. The three lignin subdivisions were characterized by gel permeation chromatography (GPC), FTIR, 2D-NMR and scanning electron microscopy (SEM), and their adsorption capacities for methylene blue were compared. The results showed that the 80% ethanol-insoluble subdivision exhibited the highest adsorption capacity and its value (396.85 mg/g) was over 0.4 times higher than that of the unfractionated lignin (281.54 mg/g). The increased adsorption capacity was caused by the enhancement of both specific surface area and negative Zeta potential. The maximum monolayer adsorption capacity of 80% ethanol-insoluble subdivision by adsorption kinetics and isotherm studies was found to be 431.1 mg/g, which was much higher than most of reported lignin-based adsorbents. Moreover, the 80% ethanol-insoluble subdivision had much higher regeneration yield (over 90% after 5 recycles) compared with the other two subdivisions. Consequently, the proposed fractionation method is proved to be a novel and efficient non-chemical modification approach that significantly improves adsorption capacity and recyclability of lignin. [ABSTRACT FROM AUTHOR]

• Novel Pd@LPR catalyst was synthesized by hydrothermal curing and in-situ reduction. • Lignin plays as a surfactant, sacrificial template, and reductant in Pd@LPR synthesis. • Due to multifunction of lignin, more Pd NPs are loaded onto LPR than the lignin-free PR. • Pd@LPR is applied as a highly efficient and reusable catalyst for catalytic reduction of Cr(VI). Lignin, the most abundant aromatic biopolymer, has great potential as a sustainable phenol replacement in phenolic resin synthesis, which has been considered as one of the most promising options for lignin valorization. In this work, kraft lignin from bamboo was used as a phenol substitute for the fabrication of novel lignin-based phenolic resin (LPR) nanosphere using a facile hydrothermal process. The as-prepared LPR nanosphere was then used as a reducing agent and a stabilizing support for synthesis of Pd nanoparticles (NPs). It was found that 40 % replacement of phenol by lignin led to a significant decrease of the nanosphere size and enhancement of the nanosphere surface roughness, which eventually increased the specific surface area of LPR nanosphere. Owing to the enhanced specific surface area and the excellent reducing activity of lignin, the as-prepared LPR nanosphere had considerably higher Pd NPs loading amount compared with the lignin-free PR support. Therefore, the as-prepared Pd@LPR nanocomposites demonstrated highly efficient catalytic ability for the reduction of toxic Cr(VI) and two typical dyes. Moreover, the Pd@LPR nanocomposites exhibited excellent reusability owing to the stable loading of Pd NPs on the lignin-based support. Consequently, the lignin-based nanosphere, prepared by a facile hydrothermal process, is used as a reducing agent and a support for Pd NPs synthesis, which presents a novel approach to develop lignin-functionalized catalyst materials. [ABSTRACT FROM AUTHOR]

Nano-lignin presents great potential in advanced carbon materials preparation since it integrates the advantages of nanomaterials as well the preferable properties of lignin (e.g. high carbon content and highly aromatic structure). Herein, lignin-derived carbon nanosphere supported Pd catalysts (Pd@LCNS) were prepared via a two-step carbonization of Pd2+ adsorbed lignin nanospheres (LNS) and applied in vanillin hydrodeoxygenation. The effect lignin heterogeneity on the synthesis of Pd@LCNS as well as its catalytic performance was further investigated through the synthesis of Pd@LCNS using three lignin fractions with different molecular weight. The results showed that the three Pd@LCNSs exhibited significant differences in the morphology of both carbon support and Pd nanoparticles. Pd@LCNS-3 prepared from high molecular weight lignin fraction (L-3) presented stable carbon nanosphere support with the smallest particle size (∼150 nm) and the highest Pd loading amount (3.78 %) with the smallest Pd NPs size (∼1.6 nm). Therefore, Pd@LCNS-3 displayed superior catalytic activity for vanillin hydrodeoxygenation (99.34 % of vanillin conversion and 99.47 % of 2-methoxy-4-methylphenol selectivity) at 90 °C without H 2. Consequently, this work provides a sustainable strategy to prepare uniformly dispersed lignin-based carbon-supported Pd catalyst using high molecular weight lignin as the feedstock and further demonstrate its superior applicability in the selective transfer hydrogenation of vanillin. [Display omitted] • Pd@LCNS is simply prepared by a two-step carbonization of Pd2+ adsorbed LNS. • Lignin heterogeneity presents critical effect on Pd@LCNS synthesis and catalytic performance. • Pd@LCNS-3 from high molecular weight lignin has the highest Pd loading amount with the smallest size. • Pd@LCNS-3 achieves superior VAN conversion (99.34 %) and MMP selectivity (99.47 %) at mild 90 °C without H 2. • Pd@LCNS-3 showes excellent reusability owing to strong metal-carrier interaction. [ABSTRACT FROM AUTHOR]

The cover image, by Guanhua Wang et al., is based on the Research Article Successive ethanol‐water fractionation of enzymatic hydrolysis lignin to concentrate its antimicrobial activity, DOI: 10.1002/jctb.5656. [ABSTRACT FROM AUTHOR]

Lignin hydrogenolysis into monophenols presents a promising route for lignin-based chemical and fuel production but still suffers from harsh reaction conditions (e.g. high temperature and high external H 2 pressure). Herein, we report an ultrafine Ni nanocluster anchored on N-doped carbon nanosheets (Ni/LNC) for efficient catalytic transfer hydrogenolysis of poplar organosolv lignin. The catalyst was fabricated through a simple pyrolysis process of lignin‑nickel complex mixed with melamine, in which the lignin coordination greatly improved the Ni dispersion and the strong Ni N interaction inhibited the Ni nanocluster growth, resulting in the ultrafine particle size (1–2 nm). Under mild conditions (160 °C, 1 h), 22.08% of monophenol yield was obtained over the catalyst, which was significantly higher than those over the two reference catalysts (Ni/AC and Ni/LC) as well as other previously reported Ni-based catalysts. The excellent catalytic activity can be attributed to both the substantial increase in catalytic active sites and the enhancement in H transfer from methanol resulted from the electron-rich N-doped nanosheet support. Finally, magnetically recycled Ni/LNC showed excellent recycling stability. Consequently, this work presents a facile and low-cost approach for the synthesis of N-doped carbon nanosheet supported ultrafine Ni nanocluster and further demonstrates its superior applicability in lignin hydrogenolysis. [Display omitted] • Novel Ni/LNC was prepared by simple pyrolysis of melamine mixed Ni-lignin complex. • Ultrafine Ni nanoclusters (1-2 nm) were formed due to lignin coordination and N-anchoring. • High monophenol yield (22.08%) was obtained over Ni/LNC via CTH process at 160 °C. • Ultrafine Ni nanoclusters and N-doping synergistically enhance Ni/LNC catalytic activity. • Magnetically recycled Ni/LNC shows excellent reusability due to strong Ni N interactions. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • High-molecular-weight lignin was depolymerized via in situ ethanol/acid catalysis. • Structural characterizations indicated breakage of the lignin ether bond during catalysis. • Solubility and total phenolics content of depolymerized lignin were visibly improved. • IC 50 value of lignin depolymerized at 200 °C was reduced from 168.1 to 66.0 μ g/mL. • Depolymerized lignin showed satisfactory antioxidant activity comparable to BHT. Abstract Lignin, a by-product obtained from the pulping industry and cellulosic ethanol production, has received considerable attention for its antioxidant activity owing to its polyphenol structure. In this work, high-molecular-weight lignin obtained by ethanol-water fractionation of enzymatic hydrolysis lignin, was moderately depolymerized via in situ ethanol/acid catalysis to improve its solubility and antioxidant activity. The depolymerized products showed significant reduction in molecular weight and improvement in solubility in an 80% ethanol-water (v/v) solution owing to the cleavage of ether linkages; this was confirmed by gel permeation chromatography (GPC), fourier transform-infrared spectroscopy (FT-IR), pyrolysis-gas chromatography/ mass spectrometer (Py-GC/MS), carbon-13 nuclear magnetic resonance spectrometry (13C NMR), and heteronculear single quantum coherence nuclear magnetic resonance spectrometry (HSQC NMR) analyses. In addition, the total phenolics content of the depolymerized products increased sharply, leading to a significant enhancement in the free radical scavenging capacity of 1,1-diphenyl-2-picrylhydrazyl (DPPH). In particular, D-200 (depolymerized product at 200 °C) with IC 50 = 66.0 ± 2.6 μ g/mL was comparable to the positive control butylated hydroxytoluene (BHT) with IC 50 = 53.3 ± 4.5 μ g/mL, indicating the potential application of lignin as a high-value-added antioxidant. In conclusion, the depolymerization of high-molecular-weight lignin via moderate ethanol/acid catalysis may effectively improve its solubility and antioxidant activity. [ABSTRACT FROM AUTHOR]

Glycerol, as a nontoxic, biodegradable and sustainable solvent, together with biomass-derived ethanol was employed in this work to fractionate and purify enzymatic hydrolysis lignin (EHL) from enzymatic hydrolysis residue (EHR). Detailed characterizations, including GPC, FTIR, 13 C and 2D-HSQC NMR and Py-GC/MS analysis were conducted to provide a broad evaluation of the obtained lignin fractions. The GPC analysis revealed that the proposed method could realize the efficient fractionation of EHL with relatively narrow polydispersity, and three lignin fractions (F1, F2 and F3) with gradually increased molecular weight ( M ¯ n from 8690 to 20110 g mol −1 ) and reduced polydispersity were obtained. The structural characterization (FTIR, NMR and Py-GC/MS) indicated that with the increase of molecular weight, G unit content in lignin fractions increased due to the more stable C C bond formed by C5 in G unit. Besides, F1 contained highest phenolic acids, which indicated the potential application of F1 in bioactive products. The study of glycerol recycle showed that glycerol could be recycled with high recovery rate and re-employed to fractionate lignin efficiently. Consequently, the study indicated that compared with traditional dissolving by multiple organic solvents, the sequential dissolution in glycerol-ethanol solvent was a novel, green, and efficient way to fractionate EHL and reduce its molecular weight and structural heterogeneity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Efficient acid-promoted reductive lignin depolymerization under mild conditions was proposed. • Hydroxybenzyl alcohol was used as capping agent to inhibit acid-induced condensation. • 20.42% of monophenol yield with 31.70% of oligomer yield were obtained at 160 °C without H 2. • Oligomers (mainly dimers/trimers) showed great potential as antioxidants in polymeric blends. • Exceptionally high monophenol yield (32.72%) was achieved when using poplar as the feedstock. Lignin reductive depolymerization into phenolic monomers is a critical step for the scale-conversion of lignin into liquid fuels but its scale-up is still compromised by harsh reaction conditions (e.g. high temperature and high external H 2 pressure) and inevitable condensations. Herein, we present an efficient acid-promoted reductive depolymerization of lignin over Ni/C without external H 2 via a condensation minimizing approach using lignin monomer analogue (p -hydroxybenzyl alcohol, HBA) as the capping agent. With addition of 0.4 % H 2 SO 4 and 0.4 mmol/g HBA, 20.42 % of phenolic monomer yield along with 31.70 % oligomer yield was achieved at 160 °C while the condensated/undepolymerized lignin yield was less than 10 %. HBA inhibited the acid-induced condensation by competitive reaction with the nucleophilic C6 in guaiacyl unit. When the approach was applied in lignin-first reductive fractionation of poplar, 32.72 % of phenolic monomer yield and 76.59 % of delignification were achieved at 160 °C in the presence of acid and HBA. Additionally, xylan was dissociated concurrently owing to the acid-catalyzed hydrolysis, resulting in a cellulose-rich solid residue. Consequently, this work proposes an efficient approach for lignin reductive depolymerization and lignin-first biomass fractionation under mild conditions through the synergism of acid and capping agent. [ABSTRACT FROM AUTHOR]