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Special Issue "Lignin for Energy, Chemicals and Materials"

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

Deadline for manuscript submissions: 31 December 2018

Special Issue Editors

Guest Editor
Prof. Dr. Charles Xu

Institute for Chemicals and Fuels from Alternative Resources (ICFAR), Department of Chemical and Biochemical Engineering, Western University, Ontario, Canada
Website | E-Mail
Phone: 519-661-2111 ext. 86414
Fax: 519-661-4016
Interests: Biorefining technologies; Bio-fuels; Bio-based chemicals; Bio-based materials; Thermochemical conversion; Hydrothermal liquefaction; Pyrolysis; Combustion; Gasification; Lignocellulosic biomass; Forestry residues; Agricultural residues; Sugars; Starch; Cellulose; Lignin; Municipal solid wastes; Wastewater sludge; Catalysis; Catalysts; Chemical reaction engineering; Green process engineering
Guest Editor
Dr. Michael Paleologou

Research Leader, Lignin Products, Biorefinery Program, FPInnovations, 570 boul. Saint-Jean, Pointe-Claire(QC) H9R 3J9, Canada
E-Mail
Interests: forest biorefinery; lignin recovery; lignin characterization; lignin products; hemicellulose recovery; hemicellulose products; methanol recovery; biomass processing operations; process integration and economics; black and red liquor characterization; chemical recovery; chemical separation and regeneration technologies for kraft, sulphite and BCTMP mills; system closure

Special Issue Information

Dear Colleagues,

Lignin is the second most abundant natural renewable polymer after cellulose. Natural lignin is a phenolic polymer of three monolignols with an amorphous macromolecular structure. Lignin is currently being produced in large quantities as a by-product of chemical pulping and cellulosic ethanol processes. According to the International Lignin Institute, about 40–50 million tonnes of kraft lignin (KL) are generated each year, globally, in the form of “black liquor”. While combustion of black liquor to regenerate pulping chemicals and to produce steam and power is an integral part of the kraft process, a small portion of the lignin can be removed without compromising mill material and energy balances. Meanwhile, the production of ethanol, butanol and platform chemicals (e.g., lactic, succinic and other organic acids) from cellulosic sugars is growing. For this to achieve extensive commercial success on a worldwide basis, value-added applications are needed for the hydrolysis lignin by-products that are generated from lignocellulose hydrolysis processes.

Many studies have been conducted on lignin utilization. Similar to other carbonaceous solid fuels, lignin can be a source for energy and fuels (e.g., combustion/co-combustion of lignin for energy, pyrolysis or hydrothermal liquefaction of lignin for bio-oils/liquid bio-fuels, or gasification of lignin for syngas/hydrogen, etc.). The presence of various functional groups (aromatic ring free positions and hydroxyl groups) on lignin structure, biodegradability, antioxidant, flame retardant and reinforcing capability make it as a potential candidate for the production of bio-aromatic chemicals (e.g., vanillin, phenols and antioxidants), bio-based polymeric materials (e.g., resins and polymers), and carbon fibers for use as reinforcement fillers in thermoplastic polymers, light-weight composite materials, as well as graphene for use in supercapacitors for energy storage. Direct use of lignin for chemical synthesis and materials can be challenging because the molecular weight is too high and because reactivity is reduced due to steric hindrance effects. The reactivity of lignin could be enhanced through some chemical modifications and thermochemical de-polymerization processes.

This Special Issue aims to cover recent progress and trends in the utilization of lignin or modified/de-polymerized lignin in chemical synthesis, materials and energy. Submissions are welcome but not limited to the topics listed below. Types of contributions to this Special Isssue can be full research articles, short communications, and reviews focusing on the utilization of lignin for energy/fuels, chemical and materials.

  • Extraction of lignin from pulping processes or cellulosic ethanol processes;
  • Chemical modification/de-polymerization of lignin;
  • Combustion/co-combustion of lignin for energy;
  • Pyrolysis or hydrothermal liquefaction of lignin for bio-oils/liquid bio-fuels;
  • Gasification of lignin for syngas/hydrogen;
  • Production of bio-aromatic chemicals from lignin (e.g., vanillin, phenols and antioxidants);
  • Synthesis of bio-based polymeric materials from lignin (e.g., resins and polymers)
  • Production of carbon fibers as reinforcement fillers in thermoplastic polymers or light-weight composite materials
  • Production of graphene for use in supercapacitors for energy storage.

Prof. Chunbao (Charles) Xu
Dr. Michael Paleologou
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Lignin
  • Chemical characterization
  • Chemical modification
  • De-polymerization
  • Combustion
  • Energy
  • Pyrolysis
  • Hydrothermal liquefaction
  • Bio-oils
  • Phenols
  • Bio-aromatic chemicals
  • Synthesis
  • Resins
  • Polymers
  • Carbon fibers
  • Composites
  • Graphene
  • Supercapacitors

Published Papers (15 papers)

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Research

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Open AccessArticle Lignocellulosic Biomass as Source for Lignin-Based Environmentally Benign Antioxidants
Molecules 2018, 23(10), 2664; https://doi.org/10.3390/molecules23102664
Received: 14 September 2018 / Revised: 8 October 2018 / Accepted: 13 October 2018 / Published: 16 October 2018
PDF Full-text (6691 KB) | HTML Full-text | XML Full-text
Abstract
Antioxidant activity is an essential aspect of oxygen-sensitive merchandise and goods, such as food and corresponding packaging, cosmetics, and biomedicine. Technical lignin has not yet been applied as a natural antioxidant, mainly due to the complex heterogeneous structure and polydispersity of lignin. This
[...] Read more.
Antioxidant activity is an essential aspect of oxygen-sensitive merchandise and goods, such as food and corresponding packaging, cosmetics, and biomedicine. Technical lignin has not yet been applied as a natural antioxidant, mainly due to the complex heterogeneous structure and polydispersity of lignin. This report presents antioxidant capacity studies completed using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The influence of purification on lignin structure and activity was investigated. The purification procedure showed that double-fold selective extraction is the most efficient (confirmed by ultraviolet-visible (UV/Vis), Fourier transform infrared (FTIR), heteronuclear single quantum coherence (HSQC) and 31P nuclear magnetic resonance spectroscopy, size exclusion chromatography, and X-ray diffraction), resulting in fractions of very narrow polydispersity (3.2–1.6), up to four distinct absorption bands in UV/Vis spectroscopy. Due to differential scanning calorimetry measurements, the glass transition temperature increased from 123 to 185 °C for the purest fraction. Antioxidant capacity is discussed regarding the biomass source, pulping process, and degree of purification. Lignin obtained from industrial black liquor are compared with beech wood samples: antioxidant activity (DPPH inhibition) of kraft lignin fractions were 62–68%, whereas beech and spruce/pine-mixed lignin showed values of 42% and 64%, respectively. Total phenol content (TPC) of the isolated kraft lignin fractions varied between 26 and 35%, whereas beech and spruce/pine lignin were 33% and 34%, respectively. Storage decreased the TPC values but increased the DPPH inhibition. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessFeature PaperArticle Effects of Process Parameters on Hydrolytic Treatment of Black Liquor for the Production of Low-Molecular-Weight Depolymerized Kraft Lignin
Molecules 2018, 23(10), 2464; https://doi.org/10.3390/molecules23102464
Received: 31 July 2018 / Revised: 12 September 2018 / Accepted: 13 September 2018 / Published: 26 September 2018
PDF Full-text (2187 KB) | HTML Full-text | XML Full-text
Abstract
The present research work aimed at hydrolytic treatment of kraft black liquor (KBL) at 200–300 °C for the production of low-molecular-weight depolymerized kraft lignin (DKL). Various process conditions such as reaction temperature, reaction time, initial kraft lignin (KL) substrate concentration, presence of a
[...] Read more.
The present research work aimed at hydrolytic treatment of kraft black liquor (KBL) at 200–300 °C for the production of low-molecular-weight depolymerized kraft lignin (DKL). Various process conditions such as reaction temperature, reaction time, initial kraft lignin (KL) substrate concentration, presence of a catalyst (NaOH), capping agent (phenol) or co-solvent (methanol) were evaluated. The research demonstrated effective depolymerization of KL in KBL at 250–300 °C with NaOH as a catalyst at a NaOH/lignin ratio of about 0.3 (w/w) using diluted KBL (with 9 wt. % KL). Treatment of the diluted KBL at 250 °C for 2 h with 5% addition of methanol co-solvent produced DKL with a weight-average molecular weight (Mw) of 2340 Da, at approx. 45 wt. % yield, and a solid residue at a yield of ≤1 wt. %. A longer reaction time favored the process by reducing the Mw of the DKL products. Adding a capping agent (phenol) helped reduce repolymerization/condensation reactions thereby reducing the Mw of the DKL products, enhancing DKL yield and increasing the hydroxyl group content of the lignin. For the treatment of diluted KBL (with 9 wt. % KL) at 250 °C for 2 h, with 5% addition of methanol co-solvent in the presence of NaOH/lignin ≈ 0.3 (w/w), followed by acidification to recover the DKL, the overall mass balances for C, Na and S were measured to be approx. 74%, 90% and 77%, respectively. These results represent an important step towards developing a cost-effective approach for valorization of KBL for chemicals. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Ball Milling’s Effect on Pine Milled Wood Lignin’s Structure and Molar Mass
Molecules 2018, 23(9), 2223; https://doi.org/10.3390/molecules23092223
Received: 3 August 2018 / Revised: 25 August 2018 / Accepted: 29 August 2018 / Published: 1 September 2018
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Abstract
The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e.,
[...] Read more.
The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20–75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (31P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the Mz and Mw of the extracted MWLc with an increase in the yield of MWLc, but has only a very subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle All-lignocellulosic Fiberboard from Steam Exploded Arundo Donax L.
Molecules 2018, 23(9), 2088; https://doi.org/10.3390/molecules23092088
Received: 15 July 2018 / Revised: 10 August 2018 / Accepted: 15 August 2018 / Published: 21 August 2018
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Abstract
This paper explores the possibility of producing all-lignocellulosic fiberboards from Arundo donax L. as a source of lignocellulosic fibers with no synthetic binders. This raw material was steam exploded with a thermomechanical aqueous vapor process in a batch reactor. The Arundo donax raw
[...] Read more.
This paper explores the possibility of producing all-lignocellulosic fiberboards from Arundo donax L. as a source of lignocellulosic fibers with no synthetic binders. This raw material was steam exploded with a thermomechanical aqueous vapor process in a batch reactor. The Arundo donax raw material and its obtained pulp were characterized in terms of chemical composition and the results were compared to other lignocellulosic materials. The chemical composition of steam exploded Arundo fibers showed high cellulose and a moderate lignin content suggesting it was a good raw material for fiberboard production. The all-lignocellulosic fiberboards were produced on laboratory scale; using the steam exploded Arundo donax by means of a wet process. The effects of pressing pressure on physical and mechanical properties were evaluated and the conditions that optimize the responses were found. The analyzed properties were density (d); water absorption (WA); thickness swelling (TS); modulus of elasticity (MOE); modulus of rupture (MOR); and internal bond strength (IB). The tested levels of the pressing pressure range from 0.35 to 15 MPa. The optimum IB; MOE; MOR; WA and TS were 1.28 MPa, 7439 MPa, 40.4 MPa, 17.6% and 13.3%, respectively. The obtained fiberboards were of very good quality and more than satisfy the requirements of the relevant standard specifications. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Cationic High Molecular Weight Lignin Polymer: A Flocculant for the Removal of Anionic Azo-Dyes from Simulated Wastewater
Molecules 2018, 23(8), 2005; https://doi.org/10.3390/molecules23082005
Received: 31 July 2018 / Revised: 5 August 2018 / Accepted: 7 August 2018 / Published: 11 August 2018
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Abstract
The presence of dyes in wastewater effluents made from the textile industry is a major environmental problem due to their complex structure and poor biodegradability. In this study, a cationic lignin polymer was synthesized via the free radical polymerization of lignin with [2-(methacryloyloxy)
[...] Read more.
The presence of dyes in wastewater effluents made from the textile industry is a major environmental problem due to their complex structure and poor biodegradability. In this study, a cationic lignin polymer was synthesized via the free radical polymerization of lignin with [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC) and used to remove anionic azo-dyes (reactive black 5, RB5, and reactive orange 16, RO16) from simulated wastewater. The effects of pH, salt, and concentration of dyes, as well as the charge density and molecular weight of lignin-METAC polymer on dye removal were examined. Results demonstrated that lignin-METAC was an effective flocculant for the removal of dye via charge neutralization and bridging mechanisms. The dye removal efficiency of lignin-METAC polymer was independent of pH. The dosage of the lignin polymer required for reaching the maximum removal had a linear relationship with the dye concentration. The presence of inorganic salts including NaCl, NaNO3, and Na2SO4 had a marginal effect on the dye removal. Under the optimized conditions, greater than 98% of RB5 and 94% of RO16 were removed at lignin-METAC concentrations of 120 mg/L and 105 mg/L in the dye solutions, respectively. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessCommunication Optimization of Lignin-Based Biocatalyst Production from Pine Sawdust and Wheat Straw
Molecules 2018, 23(8), 1877; https://doi.org/10.3390/molecules23081877
Received: 28 June 2018 / Revised: 20 July 2018 / Accepted: 24 July 2018 / Published: 27 July 2018
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Abstract
Pine sawdust and wheat straw are abundant lignocellulosic wastes that have been recently converted into bioethanol under a biochemical platform scheme whose main waste is lignin. Lignin can be transformed into a wide variety of high added-value products, including its functionalization as a
[...] Read more.
Pine sawdust and wheat straw are abundant lignocellulosic wastes that have been recently converted into bioethanol under a biochemical platform scheme whose main waste is lignin. Lignin can be transformed into a wide variety of high added-value products, including its functionalization as a catalyst. A key step in the synthesis of a lignin-based catalyst is the sulfonation reaction, whose operating conditions, namely, H2SO4 to lignin ratio (mL/g), temperature and time, have been arbitrarily chosen. In this contribution, an optimization methodology (i.e., Box-Behnken) is applied in order to found the operating conditions during the sulfonation reaction that maximizes the total acid sites density of lignin-based catalysts from pine sawdust and wheat straw. The optimization results show that the time in sulfonation reactions can be significantly reduced, compared to those previously reported, without affecting the performance of both catalysts in esterification reactions. These results could be further considered for energy and costs reduction purposes during the conceptual design engineering of the sulfonation reaction. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Formation of Lignin Nanoparticles by Combining Organosolv Pretreatment of Birch Biomass and Homogenization Processes
Molecules 2018, 23(7), 1822; https://doi.org/10.3390/molecules23071822
Received: 27 June 2018 / Revised: 18 July 2018 / Accepted: 20 July 2018 / Published: 23 July 2018
PDF Full-text (6086 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Valorization of lignocellulosic biomass into a biorefinery scheme requires the use of all biomass components; in this, the lignin fraction is often underutilized. Conversion of lignin to nanoparticles is an attractive solution. Here, we investigated the effect of different lignin isolation processes and
[...] Read more.
Valorization of lignocellulosic biomass into a biorefinery scheme requires the use of all biomass components; in this, the lignin fraction is often underutilized. Conversion of lignin to nanoparticles is an attractive solution. Here, we investigated the effect of different lignin isolation processes and a post-treatment homogenization step on particle formation. Lignin was isolated from birch chips by using two organosolv processes, traditional organosolv (OS) and hybrid organosolv-steam explosion (HOS-SE) at various ethanol contents. For post-treatment, lignin was homogenized at 500 bar using different ethanol:water ratios. Isolation of lignin with OS resulted in unshaped lignin particles, whereas after HOS-SE, lignin micro-particles were formed directly. Addition of an acidic catalyst during HOS-SE had a negative impact on the particle formation, and the optimal ethanol content was 50–60% v/v. Homogenization had a positive effect as it transformed initially unshaped lignin into spherical nanoparticles and reduced the size of the micro-particles isolated by HOS-SE. Ethanol content during homogenization affected the size of the particles, with the optimal results obtained at 75% v/v. We demonstrate that organosolv lignin can be used as an excellent starting material for nanoparticle preparation, with a simple method without the need for extensive chemical modification. It was also demonstrated that tuning of the operational parameters results in nanoparticles of smaller size and with better size homogeneity. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Production of Micro- and Nanoscale Lignin from Wheat Straw Using Different Precipitation Setups
Molecules 2018, 23(3), 633; https://doi.org/10.3390/molecules23030633
Received: 30 January 2018 / Revised: 7 March 2018 / Accepted: 9 March 2018 / Published: 11 March 2018
Cited by 1 | PDF Full-text (5203 KB) | HTML Full-text | XML Full-text
Abstract
Micro- and nanosize lignin has recently gained interest due to its improved properties compared to standard lignin available today. As the second most abundant biopolymer after cellulose, lignin is readily available but used for rather low-value applications. Applications for lignin in micro- to
[...] Read more.
Micro- and nanosize lignin has recently gained interest due to its improved properties compared to standard lignin available today. As the second most abundant biopolymer after cellulose, lignin is readily available but used for rather low-value applications. Applications for lignin in micro- to nanoscale however, ranging from improvement of mechanical properties of polymer nanocomposites, have bactericidal and antioxidant properties and impregnations to hollow lignin drug carriers for hydrophobic and hydrophilic substances. This research represents a whole biorefinery process chain and compares different precipitation setups to produce submicron lignin particles from lignin containing an organosolv pretreatment extract from wheat straw. A batch precipitation in a stirred vessel was compared with continuous mixing of extract and antisolvent in a T-fitting and mixing in a T-fitting followed by a static mixer. The precipitation in the combination of T-fitting and static mixer with improved precipitation parameters yields the smallest particle size of around 100 nm. Furthermore, drying of particles did not influence the particle sizes negatively by showing decreased particle diameters after the separation process. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Lignin from Hardwood and Softwood Biomass as a Lubricating Additive to Ethylene Glycol
Molecules 2018, 23(3), 537; https://doi.org/10.3390/molecules23030537
Received: 30 January 2018 / Revised: 22 February 2018 / Accepted: 23 February 2018 / Published: 28 February 2018
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Abstract
Ethylene glycol (EG)-based lubricant was prepared with dissolved organosolv lignin from birch wood (BL) and softwood (SL) biomass. The effects of different lignin types on the rheological, thermal, and tribological properties of the lignin/EG lubricants were comprehensively investigated by various characterization techniques. Dissolving
[...] Read more.
Ethylene glycol (EG)-based lubricant was prepared with dissolved organosolv lignin from birch wood (BL) and softwood (SL) biomass. The effects of different lignin types on the rheological, thermal, and tribological properties of the lignin/EG lubricants were comprehensively investigated by various characterization techniques. Dissolving organosolv lignin in EG results in outstanding lubricating properties. Specifically, the wear volume of the disc by EG-44BL is only 8.9% of that lubricated by pure EG. The enhanced anti-wear property of the EG/lignin system could be attributed to the formation of a robust lubrication film and the strong adhesion of the lubricant on the contacting metal surface due to the presence of a dense hydrogen bonding (H-bonding) network. The lubricating performance of EG-BL outperforms EG-SL, which could be attributed to the denser H-bonding sites in BL and its broader molecular weight distribution. The disc wear loss of EG-44BL is only 45.7% of that lubricated by EG-44SL. Overall, H-bonding is the major contributor to the different tribological properties of BL and SL in EG-based lubricants. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessFeature PaperArticle Quantification and Variability Analysis of Lignin Optical Properties for Colour-Dependent Industrial Applications
Molecules 2018, 23(2), 377; https://doi.org/10.3390/molecules23020377
Received: 11 January 2018 / Revised: 4 February 2018 / Accepted: 6 February 2018 / Published: 10 February 2018
PDF Full-text (4349 KB) | HTML Full-text | XML Full-text
Abstract
Lignin availability has increased significantly due to the commercialization of several processes for recovery and further development of alternatives for integration into Kraft pulp mills. Also, progress in lignin characterization, understanding of its chemistry as well as processing methods have resulted in the
[...] Read more.
Lignin availability has increased significantly due to the commercialization of several processes for recovery and further development of alternatives for integration into Kraft pulp mills. Also, progress in lignin characterization, understanding of its chemistry as well as processing methods have resulted in the identification of novel lignin-based products and potential derivatives, which can serve as building block chemicals. However, all these have not led to the successful commercialization of lignin-based chemicals and materials. This is because most analyses and characterizations focus only on the technical suitability and quantify only the composition, functional groups present, size and morphology. Optical properties, such as the colour, which influences the uptake by users for diverse applications, are neither taken into consideration nor analysed. This paper investigates the quantification of lignin optical properties and how they can be influenced by process operating conditions. Lignin extraction conditions were also successfully correlated to the powder colour. About 120 lignin samples were collected and the variability of their colours quantified with the CIE L*a*b* colour space. In addition, a robust and reproducible colour measurement method was developed. This work lays the foundation for identifying chromophore molecules in lignin, as a step towards correlating the colour to the functional groups and the purity. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Sustainable Bio-Based Phenol-Formaldehyde Resoles Using Hydrolytically Depolymerized Kraft Lignin
Molecules 2017, 22(11), 1850; https://doi.org/10.3390/molecules22111850
Received: 11 September 2017 / Revised: 26 October 2017 / Accepted: 27 October 2017 / Published: 28 October 2017
Cited by 3 | PDF Full-text (11454 KB) | HTML Full-text | XML Full-text
Abstract
In this study bio-based bio-phenol-formaldehyde (BPF) resoles were prepared using hydrolytically depolymerized Kraft lignin (DKL) as bio-phenol to partially substitute phenol. The effects of phenol substitution ratio, weight-average molecular weight (Mw) of DKL and formaldehyde-to-phenol (F/P) ratio were also investigated
[...] Read more.
In this study bio-based bio-phenol-formaldehyde (BPF) resoles were prepared using hydrolytically depolymerized Kraft lignin (DKL) as bio-phenol to partially substitute phenol. The effects of phenol substitution ratio, weight-average molecular weight (Mw) of DKL and formaldehyde-to-phenol (F/P) ratio were also investigated to find the optimum curing temperature for BPF resoles. The results indicated that DKL with Mw ~ 1200 g/mol provides a curing temperature of less than 180 °C for any substitution level, provided that F/P ratios are controlled. Incorporation of lignin reduced the curing temperature of the resin, however, higher Mw DKL negatively affected the curing process. For any level of lignin Mw, the curing temperature was found to increase with lower F/P ratios at lower phenol substitution levels. At 25% and 50% phenol substitution, increasing the F/P ratio allows for synthesis of resoles with lower curing temperatures. Increasing the phenol substitution from 50% to 75% allows for a broader range of lignin Mw to attain low curing temperatures. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessArticle Valorization of Lignin by Partial Wet Oxidation Using Sustainable Heteropoly Acid Catalysts
Molecules 2017, 22(10), 1625; https://doi.org/10.3390/molecules22101625
Received: 12 September 2017 / Revised: 25 September 2017 / Accepted: 27 September 2017 / Published: 28 September 2017
Cited by 6 | PDF Full-text (3263 KB) | HTML Full-text | XML Full-text
Abstract
The production of carboxylic acids by partial wet oxidation of alkali lignin at elevated temperatures and pressures was studied experimentally. Two different heteropoly acids, phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40),
[...] Read more.
The production of carboxylic acids by partial wet oxidation of alkali lignin at elevated temperatures and pressures was studied experimentally. Two different heteropoly acids, phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40), were used to catalyze the oxidation of lignin under hydrothermal conditions. Factors influencing the total yield of carboxylic acids formed during the partial oxidation of lignin were investigated. Formic, acetic and succinic acids were the major products identified. Of the two catalysts used, phosphomolybdic acid gave the most promising results, with carboxylic acid yields and lignin conversions of up to 45% and 95%, respectively. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Review

Jump to: Research, Other

Open AccessFeature PaperReview Lignin-Derived Biomaterials for Drug Release and Tissue Engineering
Molecules 2018, 23(8), 1885; https://doi.org/10.3390/molecules23081885
Received: 6 July 2018 / Revised: 23 July 2018 / Accepted: 24 July 2018 / Published: 27 July 2018
Cited by 1 | PDF Full-text (3424 KB) | HTML Full-text | XML Full-text
Abstract
Renewable resources are gaining increasing interest as a source for environmentally benign biomaterials, such as drug encapsulation/release compounds, and scaffolds for tissue engineering in regenerative medicine. Being the second largest naturally abundant polymer, the interest in lignin valorization for biomedical utilization is rapidly
[...] Read more.
Renewable resources are gaining increasing interest as a source for environmentally benign biomaterials, such as drug encapsulation/release compounds, and scaffolds for tissue engineering in regenerative medicine. Being the second largest naturally abundant polymer, the interest in lignin valorization for biomedical utilization is rapidly growing. Depending on its resource and isolation procedure, lignin shows specific antioxidant and antimicrobial activity. Today, efforts in research and industry are directed toward lignin utilization as a renewable macromolecular building block for the preparation of polymeric drug encapsulation and scaffold materials. Within the last five years, remarkable progress has been made in isolation, functionalization and modification of lignin and lignin-derived compounds. However, the literature so far mainly focuses lignin-derived fuels, lubricants and resins. The purpose of this review is to summarize the current state of the art and to highlight the most important results in the field of lignin-based materials for potential use in biomedicine (reported in 2014–2018). Special focus is placed on lignin-derived nanomaterials for drug encapsulation and release as well as lignin hybrid materials used as scaffolds for guided bone regeneration in stem cell-based therapies. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessReview Production of Flocculants, Adsorbents, and Dispersants from Lignin
Molecules 2018, 23(4), 868; https://doi.org/10.3390/molecules23040868
Received: 11 March 2018 / Revised: 3 April 2018 / Accepted: 6 April 2018 / Published: 10 April 2018
Cited by 1 | PDF Full-text (5701 KB) | HTML Full-text | XML Full-text
Abstract
Currently, lignin is mainly produced in pulping processes, but it is considered as an under-utilized chemical since it is being mainly used as a fuel source. Lignin contains many hydroxyl groups that can participate in chemical reactions to produce value-added products. Flocculants, adsorbents,
[...] Read more.
Currently, lignin is mainly produced in pulping processes, but it is considered as an under-utilized chemical since it is being mainly used as a fuel source. Lignin contains many hydroxyl groups that can participate in chemical reactions to produce value-added products. Flocculants, adsorbents, and dispersants have a wide range of applications in industry, but they are mainly oil-based chemicals and expensive. This paper reviews the pathways to produce water soluble lignin-based flocculants, adsorbents, and dispersants. It provides information on the recent progress in the possible use of these lignin-based flocculants, adsorbents, and dispersants. It also critically discusses the advantages and disadvantages of various approaches to produce such products. The challenges present in the production of lignin-based flocculants, adsorbents, and dispersants and possible scenarios to overcome these challenges for commercial use of these products in industry are discussed. Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Open AccessErratum Erratum: Demesa, A.G.; et al. Valorization of Lignin by Partial Wet Oxidation Using Sustainable Heteropoly Acid Catalysts. Molecules 2017, 22, 1625
Molecules 2018, 23(7), 1625; https://doi.org/10.3390/molecules23071625
Received: 15 June 2018 / Accepted: 23 June 2018 / Published: 4 July 2018
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Abstract
The authors would like to make the following correction to their published paper [1]. [...] Full article
(This article belongs to the Special Issue Lignin for Energy, Chemicals and Materials)
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Author: Abayneh Demesa
Affiliation: Laboratory of Process and Product Development, LUT School of Engineering Science, Lappeenranta University of Technology, Skinnarilankatu 34, FI-53850 Lappeenranta, Finland
Tentative title: Lignin valorization for chemical production

Author: Rehman Javaid, Aqsa Sabir, Nadeem Sheikh, Muhammad Ferhan
Affiliation:  Lignin Valorization & Nanomaterials Lab, Centre for Applied Molecular Biology (CAMB),
University of the Punjab, 87-West Canal Bank Road, Thokar Niaz Baig, Lahore-53700, Pakistan
Institute of Molecular Biology and Biotechnology (IMBB), The University of Lahore, 1- KM Raiwind Road, Lahore
Tentative title: Recent advances in applications of acidophilic fungal microbes for bio-chemicals.
Tentative abstract: Lignocellulosic feedstock (cellulose, hemicellulose and lignin) has been used for a variety of purposes. Among them, lignin can produce value-added chemicals containing a variety of structurally related phenyl propanoid subunits known as core lignin, consisting of either C-C bonds or ether linkages. It can be depolymerized by microbial activity together with certain enzymes (laccases and peroxidases). Both acetic acid and formic acid production by certain fungi contribute significantly to lignin depolymerization to obtain most chemicals substances. Natural organic acids production by fungi has many key roles in nature that are strictly dependent upon organic acid producing fungus type. Enzymatic conversion of lignocellulosic is beneficial over other physio-chemical processes because of enzymatic specificity in reactions. Laccases, the copper containing proteins contribute to oxidize a broad spectrum of inorganic as well as organic compounds but most specifically phenolic compounds by radical catalyzed mechanism. Similarly, lignin peroxidases (LiP), the heme containing proteins perform a vital part in oxidizing a wide variety of aromatic compounds with H2O2. Lignin depolymerization yields polyaromatics, the important of which are BTX (Benzene, Toluene and Xylene). These aromatic complexes are found in several different configurations. However, most modern aromatics complexes enhance the production of para-xylene, benzene and sometimes ortho-xylene respectively. Thus, the aim of this review is to provide a concept that chemical and biological modifications of lignin yield certain value added and environment friendly green biochemicals.

Author: Basma El Khaldi-Hansen, Abla Alzazgameem, Birgit Kamm and Margit Schulze1,*
Tentative title: Lignin-Derived Biomaterials for Drug Release and Tissue Engineering
Tentative abstract: Renewable resources gain increasing interest as source for environmentally benign biomaterials, such as drug encapsulation and release compounds, and scaffolds for tissue engineering in regenerative medicine. Being the second largest naturally abundant polymer, the interest in lignin utilization in biomedicine is rapidly growing. Within the last five years, remarkable progress has been made in isolation, functionalization and modification of lignin and lignin-derived materials. However, literature so far (including review articles) most often focus industrial utilization such as fuels, resins and lubricants. The purpose of this review is to summarize the current state of the art and highlight the most important results of lignin-derived materials for biomedicine (cited references include original paper and patents in 2013-18). Special focus is drawn on guided bone regeneration: 3D scaffolds are known to directly influence differentiation and proliferation of mesenchymal stem cells into bone tissue due to scaffold structure (polarity, porosity, surface topography) and controlled release of osteoinductive and/or osteoconductive drugs. The review highlights recent progresses in guided bone formation, including stem cell-based approaches for future therapies.
Keywords: biomaterial, bone regeneration, drug encapsulation, drug release, lignin, osteogenesis, scaffolds, stem cells, tissue engineering.

Author: Thomas Rosenau
Affiliation: Universitat fur Bodenkultur Wien, Vienna, Austria
Tentative title: Lignin-based foams for insulation - a review
Tentative abstract: The bulk use of renewable polymers is largely limited to cellulose and, less significantly, hemicelluloses. The situation is quite different for lignin which finds application in novel materials only to a very small extent. Lignin, which is the second most abundant biopolymer, is currently only used in low-performance applications or is simply burned for energy ("energetic utilization"). Due to its attractive chemical structure, i.e. different reactive chemical motifs, together with the huge quantities available from pulping processes, technical lignins have indeed potential for innovative applications on larger scale, which hopefully will find access to the markets in the near future. Promising applications for lignin are thermal insulation materials, 3D printing, membranes in separation processes, packaging and adsorbent materials, fertilizers, to name but a few. In this review, the state of the art of foamed lignin-based polymers, so called lignofoams, as high performance insulation materials, is presented. Fundamental foaming principles and influential agents which directly or potentially improve the matrix interactions between lignin/lignosulfonate and a copolymer for foam composites are discussed, and different approaches are critically compared.

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