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Lignin – A Natural Resource with Huge Potential

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Green Chemistry".

Deadline for manuscript submissions: closed (30 June 2021) | Viewed by 60266

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Prof. Dr. Margit Schulze

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Department of Natural Sciences, Bonn-Rhein-Sieg University of Applied Sciences, Sankt Augustin, Germany
Interests: biorefinery concepts; biomass exploitation; biopolymer; isolation; characterization and utilization for construction; packaging, and biomedicine
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Prof. Dr. Birgit Kamm

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Faculty of Environment and Natural Sciences, Brandenburg University of Technology BTU Cottbus-Senftenberg, Platz der Deutschen Einheit 1, D-03046 Cottbus, Germany
Interests: biorefinery technology; lignin degradation (Organosolv, Lignoboost, Bagasse lignin); heterogenic catalysts; radical scavengers; platform chemicals

Special Issue Information

Dear Colleagues,

Numerous studies have been published discussing lignin isolation, characterization, and valorization. However, there are still essential aspects to be discovered (e.g., the influence of biomass (and/or waste) sources, plant genotype, and harvesting conditions on lignin biosynthesis and monolignol linkage formation). Even after decades of lignin research, new linkages and fragments are still being analysed and reported (e.g., tricin was found to be incorporated into grass lignin polymers in 2018). Furthermore, the mechanisms regarding antioxidant and antimicrobial activities of lignins are still not fully understood. The activity differences of lignins have been explored depending on the biomass source (hard versus soft wood versus grasses) and/or depending on the isolation/pulping method (Kraft versus Organosolv), but detailed structure–property relationships have yet to be developed.

In addition, the question remains if depolymerization is required or if “unmodified” lignins could be used (at least partially) to replace phenol and phenol derivatives, for example, in polymer preparation. Most critical for industrial exploitation of lignin as a raw material is fast, cost-effective, and reliable quality control/quality assurance. Here, chemometric (multivariate data analysis) methods such as principal component analyses could become a useful tool as recently reported in different studies.

Potential applications include lignin-derived polymers such as polyurethanes (foams and coatings), phenol-based resins, or additives (e.g., antioxidants). However, industrial applications of lignin and/or lignin-derived products so far are limited, so far, due to missing specifications, especially if applications in food packaging or biomedicine are the focus. In principle, first studies confirmed that lignin-derived composites could be used as environmentally benign drug release and/or scaffold materials for tissue engineering (e.g., bone regeneration). Overall, there are very attractive targets for further efforts in lignin research to be pursued.

You are kindly invited to contribute new ideas, recent data, and studies that try to give answers to these unsolved issues.

Prof. Dr. Margit Schulze
Prof. Dr. Birgit Kamm
Guest Editor

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Keywords

lignin biosynthesis;
monolignol linkages;
lignin structure analysis;
structure–property relationships;
antioxidant capacity;
antimicrobial activity;
lignin depolymerization;
lignin-derived materials (polymers, composites)

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Published Papers (12 papers)

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Research

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21 pages, 2953 KiB

Open AccessArticle

Chemical, Thermal and Antioxidant Properties of Lignins Solubilized during Soda/AQ Pulping of Orange and Olive Tree Pruning Residues

by María E. Eugenio, Raquel Martín-Sampedro, José I. Santos, Bernd Wicklein and David Ibarra

Molecules 2021, 26(13), 3819; https://doi.org/10.3390/molecules26133819 - 23 Jun 2021

Cited by 19 | Viewed by 2607

Abstract

Some agroforestry residues such as orange and olive tree pruning have been extensively evaluated for their valorization due to its high carbohydrates content. However, lignin-enriched residues generated during carbohydrates valorization are normally incinerated to produce energy. In order to find alternative high added-value applications for these lignins, a depth characterization of them is required. In this study, lignins isolated from the black liquors produced during soda/anthraquinone (soda/AQ) pulping of orange and olive tree pruning residues were analyzed by analytical standard methods and Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (solid state ¹³C NMR and 2D NMR) and size exclusion chromatography (SEC). Thermal analysis (thermogravimetric analysis (TGA), differential scanning calorimetry (DSC)) and antioxidant capacity (Trolox equivalent antioxidant capacity) were also evaluated. Both lignins showed a high OH phenolic content as consequence of a wide breakdown of β-aryl ether linkages. This extensive degradation yielded lignins with low molecular weights and polydispersity values. Moreover, both lignins exhibited an enrichment of syringyl units together with different native as well as soda/AQ lignin derived units. Based on these chemical properties, orange and olive lignins showed relatively high thermal stability and good antioxidant activities. These results make them potential additives to enhance the thermo-oxidation stability of synthetic polymers. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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19 pages, 3340 KiB

Open AccessArticle

Strategies for the Removal of Polysaccharides from Biorefinery Lignins: Process Optimization and Techno Economic Evaluation

by Sandra Corderi, Tom Renders, Kelly Servaes, Karolien Vanbroekhoven, Tony De Roo and Kathy Elst

Molecules 2021, 26(11), 3324; https://doi.org/10.3390/molecules26113324 - 1 Jun 2021

Cited by 5 | Viewed by 2842

Abstract

The utilization of biorefinery lignins as a renewable resource for the production of bio-based chemicals and materials remain a challenge because of the high polysaccharide content of this variety of lignins. This study provides two simple methods; (i) the alkaline hydrolysis-acid precipitation method and (ii) the acid hydrolysis method for the removal of polysaccharides from polymeric biorefinery lignin samples. Both purification strategies are optimized for two different hardwood hydrolysis lignins, HL1 and HL2, containing 15.1% and 10.1% of polysaccharides, respectively. The treated lignins are characterized by polysaccharide content, molecular weight, hydroxyl content, and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). Preliminary techno-economic calculations are also carried out for both purification processes to assess the economic potential of these technologies. The results indicate that both protocols could be used for the purification of HL1 and HL2 hydrolysis lignins because of the minimal polysaccharide content obtained in the treated lignins. Nevertheless, from an industrial and economic perspective the acid hydrolysis technology using low acid concentrations and high temperatures is favored over the alkaline hydrolysis-acid precipitation strategy. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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19 pages, 3569 KiB

Open AccessArticle

Preparation and Characterization of Biobased Lignin-Co-Polyester/Amide Thermoplastics

by Eric L. Young and Armando G. McDonald

Molecules 2021, 26(9), 2437; https://doi.org/10.3390/molecules26092437 - 22 Apr 2021

Cited by 13 | Viewed by 3256

Abstract

More than 23 million tonnes of lignin are produced annually in the US from wood pulping and 98% of this lignin is burnt. Therefore, creating products from lignin, such as plastics, offers an approach for obtaining sustainable materials in a circular economy. Lignin-based copolymers were synthesized using a single pot, solvent free, melt condensation reaction. The synthesis occurred in two stages. In the first stage, a biobased prepolymer consisting of butanediol (BD, 0.8–1 molar content) and a diacid (succinic (SA), adipic (AA) and suberic acids (SuA), with varying amounts of diaminobutane (DAB, 0–0.2 molar content) was heated under vacuum and monitored by Fourier transform infra-red (FTIR) spectroscopy and electrospray ionization-mass spectrometry (ESI-MS). In the second stage, prepolymer was mixed with a softwood kraft lignin (0–50 wt.%) and further reacted under vacuum at elevated temperature. Progression of the polymerization reaction was monitored using FTIR spectroscopy. The lignin-copolyester/amide properties were characterized using tensile testing, X-ray diffraction (XRD), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. Lignin co-polymer tensile (strength 0.1–2.1 MPa and modulus 2 to 338 MPa) properties were found to be influenced by the diacid chain length, lignin, and DAB contents. The lignin-copolymers were shown to be semi-crystalline polymer and have thermoplastic behavior. The SA based copolyesters/amides were relatively stiff and brittle materials while the AA based copolyesters/amides were flexible and the SuA based copolyesters/amides fell in-between. Additionally, > 30 wt.% lignin the lignin- copolyesters/amides did not exhibit melt behavior. Lignin-co-polyester/amides can be generated using green synthesis methods from biobased building blocks. The lignin- copolyesters/amides properties could be tuned based on the lignin content, DAB content and diacid chain length. This approach shows that undervalued lignin can be used in as a macromonomer in producing thermoplastic materials. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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12 pages, 2885 KiB

Open AccessArticle

Reactivity of Aliphatic and Phenolic Hydroxyl Groups in Kraft Lignin towards 4,4′ MDI

by Leonardo Dalseno Antonino, Júlia Rocha Gouveia, Rogério Ramos de Sousa Júnior, Guilherme Elias Saltarelli Garcia, Luara Carneiro Gobbo, Lara Basílio Tavares and Demetrio Jackson dos Santos

Molecules 2021, 26(8), 2131; https://doi.org/10.3390/molecules26082131 - 7 Apr 2021

Cited by 58 | Viewed by 5169

Abstract

Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. ³¹P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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11 pages, 4982 KiB

Open AccessArticle

Preparation and Characterization of Size-Controlled Lignin Nanoparticles with Deep Eutectic Solvents by Nanoprecipitation

by Tong Luo, Chao Wang, Xingxiang Ji, Guihua Yang, Jiachuan Chen, Srinivas Janaswamy and Gaojin Lyu

Molecules 2021, 26(1), 218; https://doi.org/10.3390/molecules26010218 - 4 Jan 2021

Cited by 46 | Viewed by 6777

Abstract

Lignin nanomaterials have wide application prospects in the fields of cosmetics delivery, energy storage, and environmental governance. In this study, we developed a simple and sustainable synthesis approach to produce uniform lignin nanoparticles (LNPs) by dissolving industrial lignin in deep eutectic solvents (DESs) followed by a self-assembling process. LNPs with high yield could be obtained through nanoprecipitation. The LNPs were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). Distinct LNPs could be produced by changing the type of DES, lignin sources, pre-dropping lignin concentration, and the pH of the system. Their diameter is in the range of 20–200 nm and they show excellent dispersibility and superior long-term stability. The method of preparing LNPs from lignin–DES with water as an anti-solvent is simple, rapid, and environmentally friendly. The outcome aids to further the advancement of lignin-based nanotechnology. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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16 pages, 3284 KiB

Open AccessArticle

Valorization of Lignin via Oxidative Depolymerization with Hydrogen Peroxide: Towards Carboxyl-Rich Oligomeric Lignin Fragments

by Ulrike Junghans, Justin J. Bernhardt, Ronja Wollnik, Dominik Triebert, Gerd Unkelbach and Daniela Pufky-Heinrich

Molecules 2020, 25(11), 2717; https://doi.org/10.3390/molecules25112717 - 11 Jun 2020

Cited by 19 | Viewed by 7426

Abstract

The extraction and characterization of defined and carboxyl-rich oligomeric lignin fragments with narrow molecular weight distribution is presented herein. With regard to the well-known pulp bleaching process, oxidative lignin depolymerization was investigated using hydrogen peroxide in an aqueous alkaline solution (i.e., at T = 318 K, t = 1 h) and subsequent selective fractionation with a 10/90 (v/v) acetone/water mixture. While the weight average molecular weight (M_W) of lignin in comparison to the starting material was reduced by 82% after oxidation (T = 318 K, t = 1 h, c_lignin = 40 g L⁻¹, c_H2O2 = 80 g L⁻¹, c_NaOH = 2 mol L⁻¹) and subsequent solvent fractionation (T = 298 K, t = 18 h, c_{cleavage product} = 20 g L⁻¹), the carboxyl group (–COOH) content increased from 1.29 mmol g⁻¹ up to 2.66 mmol g⁻¹. Finally, the successful scale-up of this whole process to 3 L scale led to gram amounts (14% yield) of oligomeric lignin fragments with a M_W of 1607 g mol⁻¹, a number average molecular weight (M_N) of 646 g mol⁻¹, a narrow polydispersity index of 3.0, and a high –COOH content of 2.96 mmol g⁻¹. Application of these oligomeric lignin fragments in epoxy resins or as adsorbents is conceivable without further functionalization. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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12 pages, 2331 KiB

Open AccessArticle

Characterization of Ionic Liquid Lignins Isolated from Spruce Wood with 1-Butyl-3-methylimidazolium Acetate and Methyl Sulfate and Their Binary Mixtures with DMSO

by Artyom V. Belesov, Anton V. Ladesov, Ilya I. Pikovskoi, Anna V. Faleva and Dmitry S. Kosyakov

Molecules 2020, 25(11), 2479; https://doi.org/10.3390/molecules25112479 - 27 May 2020

Cited by 18 | Viewed by 3302

Abstract

Ionic liquids (ILs) based on 1-butyl-3-methylimidazolium (bmim) cation have proved to be promising solvents for the fractionation of plant biomass with the production of cellulose and lignin. This study deals with the characterization of lignins isolated from coniferous (spruce) wood using [bmim]OAc and [bmim]MeSO₄ ionic liquids and their binary mixtures with DMSO (80:20). Molecular weight distributions, functional composition, and structural features of IL lignins were studied by size-exclusion chromatography, NMR spectroscopy (1D and 2D) and atmospheric pressure photoionization high-resolution mass spectrometry. It was shown that the interaction of ILs with lignin leads to significant chemical changes in the biopolymer; a decrease in the degree of polymerization and in the content of free phenolic hydroxyl groups due to alkylation, the disappearance (in the case of [bmim]OAc) of carbonyl groups and a significant destruction of β-O-4 bonds. The chemical reactions between lignin and 1-butyl-3-methylidazolium cation with covalent binding of ionic liquids or products of their decomposition is evidenced by the presence of a large number of nitrogen-containing oligomers in IL lignins. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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13 pages, 3589 KiB

Open AccessArticle

Direct Precipitation of Lignin Nanoparticles from Wheat Straw Organosolv Liquors Using a Static Mixer

by Stefan Beisl, Johannes Adamcyk and Anton Friedl

Molecules 2020, 25(6), 1388; https://doi.org/10.3390/molecules25061388 - 18 Mar 2020

Cited by 32 | Viewed by 4448

Abstract

Micro- and nanosize lignin shows improved properties compared to standard lignin available today and has been gaining interest in recent years. Lignin is the largest renewable resource with an aromatic skeleton on earth but it is used for relatively low-value applications. Lignin in micro- to nanoscale; however, could facilitate rather valuable applications. Current production methods consume high amounts of solvents for purification and precipitation. The process investigated in this work uses the direct precipitation of lignin nanoparticles from organosolv pretreatment extract in a static mixer and can reduce solvent consumption drastically. The pH value, ratio of antisolvent to organosolv extract and flowrate in the mixer were investigated as precipitation parameters in terms of the resulting particle properties. Particles with dimensions ranging from 97.3 to 219.3 nm could be produced, and at certain precipitation parameters, carbohydrate impurities reach values as low as in purified lignin particles. Yields were found independent of the precipitation parameters with 48.2 ± 4.99%. Results presented in this work can be used to optimize precipitation parameters with emphasis on particle size, carbohydrate impurities or the solvent consumption. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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15 pages, 2769 KiB

Open AccessArticle

Effect of Heat Treatment on the Chemical Structure and Thermal Properties of Softwood-Derived Glycol Lignin

by Thi Thi Nge, Yuki Tobimatsu, Masaomi Yamamura, Shiho Takahashi, Eri Takata, Toshiaki Umezawa and Tatsuhiko Yamada

Molecules 2020, 25(5), 1167; https://doi.org/10.3390/molecules25051167 - 5 Mar 2020

Cited by 12 | Viewed by 3839

Abstract

A large-scale glycol lignin (GL) production process (50 kg wood meal per batch) based on acid-catalyzed polyethylene glycol (PEG) solvolysis of Japanese cedar (JC) was developed at the Forestry and Forest Products Research Institute (FFPRI), Tsukuba, Japan. JC wood meal with various particle size distributions (JC-S < JC-M < JC-L) (average meal size, JC-S (0.4 mm) < JC-M (0.8 mm) < JC-L (1.6 mm)) and liquid PEG with various molecular masses are used as starting materials to produce PEG-modified lignin derivatives, namely, GLs, with various physicochemical and thermal properties. Because GLs are considered a potential feedstock for industrial applications, the effect of heat treatment on GL properties is an important issue for GL-based material production. In this study, GLs obtained from PEG400 solvolysis of JC-S, JC-M, and JC-L were subjected to heating in a constant-temperature drying oven at temperatures ranging from 100 to 220 °C for 1 h. All heat-treated GL series were thermally stable, as determined from the Klason lignin content, TMA, and TGA analyses. SEC analysis suggests the possibility of condensation among lignin fragments during heat treatment. ATR-FTIR spectroscopy, thioacidolysis, and 2D HSQC NMR demonstrated that a structural rearrangement occurs in the heat-treated GL400 samples, in which the content of α–PEG-β–O-4 linkages decreases along with the proportional enrichments of β–5 and β–β linkages, particularly at treatment temperatures above 160 °C. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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16 pages, 1687 KiB

Open AccessFeature PaperArticle

Lignins from Agroindustrial by-Products as Natural Ingredients for Cosmetics: Chemical Structure and In Vitro Sunscreen and Cytotoxic Activities

by Oihana Gordobil, Paula Olaizola, Jesus M. Banales and Jalel Labidi

Molecules 2020, 25(5), 1131; https://doi.org/10.3390/molecules25051131 - 3 Mar 2020

Cited by 52 | Viewed by 7428

Abstract

The growing concern about the environmental impact and human health risk related to the excessive use of synthetic ingredients in cosmetics and topical formulations calls for the exploration of safe and sustainable natural alternatives. Lignin-rich lignocellulosic industrial wastes such as hazelnut and walnut shells were used as a lignin polymer source. Agro-derived lignins were evaluated as a potential natural active ingredient for health care products. Aside from the structural characteristics of isolated lignins, which were identified by GPC, Py-GC–MS, and 2D HSQC NMR techniques, functional properties such as antioxidant power and UV absorption ability were investigated. The SPF values found for creams containing 5% of hazelnut and walnut lignin content were 6.9 and 4.5, respectively. Additionally, both lignin types presented appropriate protection against UVA radiation, highly interesting property to block the full ultraviolet spectrum. The biological activity of isolated lignins assessed at different concentrations (0.01–1 mg/mL) and different times (24, 48, and 72 h) on murine fibroblast cell line 3T3 suggested their suitability for cosmetic applications. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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Review

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17 pages, 1937 KiB

Open AccessReview

A Review on the Utilization of Lignin as a Fermentation Substrate to Produce Lignin-Modifying Enzymes and Other Value-Added Products

by Attia Iram, Aydin Berenjian and Ali Demirci

Molecules 2021, 26(10), 2960; https://doi.org/10.3390/molecules26102960 - 16 May 2021

Cited by 48 | Viewed by 8006

Abstract

The lignocellulosic biomass is comprised of three major components: cellulose, hemicellulose, and lignin. Among these three, cellulose and hemicellulose were already used for the generation of simple sugars and subsequent value-added products. However, lignin is the least applied material in this regard because of its complex and highly variable nature. Regardless, lignin is the most abundant material, and it can be used to produce value-added products such as lignin-modifying enzymes (LMEs), polyhydroxyalkanoates (PHAs), microbial lipids, vanillin, muconic acid, and many others. This review explores the potential of lignin as the microbial substrate to produce such products. A special focus was given to the different types of lignin and how each one can be used in different microbial and biochemical pathways to produce intermediate products, which can then be used as the value-added products or base to make other products. This review paper will summarize the effectiveness of lignin as a microbial substrate to produce value-added products through microbial fermentations. First, basic structures of lignin along with its types and chemistry are discussed. The subsequent sections highlight LMEs and how such enzymes can enhance the value of lignin by microbial degradation. A major focus was also given to the value-added products that can be produced from lignin. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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21 pages, 3988 KiB

Open AccessReview

Lignins Isolated via Catalyst-Free Organosolv Pulping from Miscanthus x giganteus, M. sinensis, M. robustus and M. nagara: A Comparative Study

by Michel Bergs, Yulia Monakhova, Bernd W. Diehl, Christopher Konow, Georg Völkering, Ralf Pude and Margit Schulze

Molecules 2021, 26(4), 842; https://doi.org/10.3390/molecules26040842 - 5 Feb 2021

Cited by 6 | Viewed by 3393

Abstract

As a low-input crop, Miscanthus offers numerous advantages that, in addition to agricultural applications, permits its exploitation for energy, fuel, and material production. Depending on the Miscanthus genotype, season, and harvest time as well as plant component (leaf versus stem), correlations between structure and properties of the corresponding isolated lignins differ. Here, a comparative study is presented between lignins isolated from M. x giganteus, M. sinensis, M. robustus and M. nagara using a catalyst-free organosolv pulping process. The lignins from different plant constituents are also compared regarding their similarities and differences regarding monolignol ratio and important linkages. Results showed that the plant genotype has the weakest influence on monolignol content and interunit linkages. In contrast, structural differences are more significant among lignins of different harvest time and/or season. Analyses were performed using fast and simple methods such as nuclear magnetic resonance (NMR) spectroscopy. Data was assigned to four different linkages (A: β-O-4 linkage, B: phenylcoumaran, C: resinol, D: β-unsaturated ester). In conclusion, A content is particularly high in leaf-derived lignins at just under 70% and significantly lower in stem and mixture lignins at around 60% and almost 65%. The second most common linkage pattern is D in all isolated lignins, the proportion of which is also strongly dependent on the crop portion. Both stem and mixture lignins, have a relatively high share of approximately 20% or more (maximum is M. sinensis Sin2 with over 30%). In the leaf-derived lignins, the proportions are significantly lower on average. Stem samples should be chosen if the highest possible lignin content is desired, specifically from the M. x giganteus genotype, which revealed lignin contents up to 27%. Due to the better frost resistance and higher stem stability, M. nagara offers some advantages compared to M. x giganteus. Miscanthus crops are shown to be very attractive lignocellulose feedstock (LCF) for second generation biorefineries and lignin generation in Europe. Full article

(This article belongs to the Special Issue Lignin – A Natural Resource with Huge Potential)

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