Lignin Valorization: Emerging Approaches: Volume 19 (Energy and Environment Series) - Hardcover

Hardcover

ISBN 10: 1782625542 ISBN 13: 9781782625544

Publisher: Royal Society of Chemistry, 2018

Synopsis

A comprehensive, interdisciplinary picture of how lignocellulosic biorefineries could potentially employ lignin valorization technologies.

About this title

About the Author

Gregg T Beckham is a Senior Engineer in the National Bioenergy Center at the National Renewable Energy Laboratory, USA.

From the Back Cover

Lignin, an aromatic biopolymer found in plant cell walls, is a key component of lignocellulosic biomass and generally utilized for heat and power. However, lignin's chemical composition makes it an attractive source for biological and catalytic conversion to fuels and chemicals. Bringing together experts from biology, catalysis, engineering, analytical chemistry, and techno-economic/life-cycle analysis, Lignin Valorization presents a comprehensive, interdisciplinary picture of how lignocellulosic biorefineries could potentially employ lignin valorization technologies.

Chapters will specifically focus on the production of fuels and chemicals from lignin and topics covered include (i) methods for isolating lignin in the context of the lignocellulosic biorefinery, (ii) thermal, chemo-catalytic, and biological methods for lignin depolymerization, (iii) chemo-catalytic and biological methods for upgrading lignin, (iv) characterization of lignin, and (v) techno-economic and life-cycle analysis of integrated processes to utilize lignin in an integrated biorefinery.

The book provides the latest breakthroughs and challenges in upgrading lignin to fuels and chemicals for graduate students and researchers in academia, governmental laboratories, and industry interested in biomass conversion.

From the Inside Flap

Lignin, an aromatic biopolymer found in plant cell walls, is a key component of lignocellulosic biomass and generally utilized for heat and power. However, lignin s chemical composition makes it an attractive source for biological and catalytic conversion to fuels and chemicals. Bringing together experts from biology, catalysis, engineering, analytical chemistry, and techno-economic/life-cycle analysis, Lignin Valorization presents a comprehensive, interdisciplinary picture of how lignocellulosic biorefineries could potentially employ lignin valorization technologies.

Chapters will specifically focus on the production of fuels and chemicals from lignin and topics covered include (i) methods for isolating lignin in the context of the lignocellulosic biorefinery, (ii) thermal, chemo-catalytic, and biological methods for lignin depolymerization, (iii) chemo-catalytic and biological methods for upgrading lignin, (iv) characterization of lignin, and (v) techno-economic and life-cycle analysis of integrated processes to utilize lignin in an integrated biorefinery.

The book provides the latest breakthroughs and challenges in upgrading lignin to fuels and chemicals for graduate students and researchers in academia, governmental laboratories, and industry interested in biomass conversion.

Excerpt. © Reprinted by permission. All rights reserved.

Lignin Valorization

Emerging Approaches

By Gregg T. Beckham

The Royal Society of Chemistry

Copyright © 2018 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-554-4

Contents

Chapter 1 A Brief Introduction to Lignin Structure Rui Katahira, Thomas J. Elder and Gregg T. Beckham, 1, 
Chapter 2 Lignin Isolation Methodology for Biorefining, Pretreatment and Analysis Joseph J. Bozell, Stephen E. Chmely, William Hartwig, Rebecca Key, Nicole Labbé, Preenaa Venugopal and Ernesto Zuleta, 21, 
Chapter 3 Lessons Learned from 150 Years of Pulping Wood Carl Houtman, 62, 
Chapter 4 Thermal and Solvolytic Depolymerization Approaches for Lignin Depolymerization and Upgrading Anders Jensen, Joachim Bachmann Nielsen, Anker Degn Jensen and Claus Felby, 74, 
Chapter 5 Early-stage Conversion of Lignin over Hydrogenation Catalysts Roberto Rinaldi, 108, 
Chapter 6 Oxidative Valorization of Lignin Ruoshui Ma, Mond Guo and Xiao Zhang, 128, 
Chapter 7 Catalytic Conversion of Lignin-derived Aromatic Compounds into Chemicals Christopher S. Lancefield, Bert. M. Weckhuysen and Pieter C. A. Bruijnincx, 159, 
Chapter 8 Biological Lignin Degradation A. T. Martínez, S. Camarero, F. J. Ruiz-Dueñas and M. J. Martínez, 199, 
Chapter 9 Bacterial Enzymes for the Cleavage of Lignin ß-Aryl Ether Bonds: Properties and Applications Yudai Higuchi, Kenji Takahashi, Naofumi Kamimura and Eiji Masai, 226, 
Chapter 10 Using Aerobic Pathways for Aromatic Compound Degradation to Engineer Lignin Metabolism Sarah C. Seaton and Ellen L. Neidle, 252, 
Chapter 11 Biological Funneling as a Means of Transforming Lignin-derived Aromatic Compounds into Value-added Chemicals Lindsay D. Eltis and Rahul Singh, 290, 
Chapter 12 Systems Biology Analyses of Lignin Conversion Zhi-Hua Liu and Joshua S. Yuan, 314, 
Chapter 13 Anaerobic Pathways for the Catabolism of Aromatic Compounds G. Durante-Rodríguez, H. Gómez-Álvarez, B. Blázquez, H. Fernández-Llamosas, Z. Martín-Moldes, D. Sanz, J. Nogales, M. Carmona and E. Díaz, 333, 
Chapter 14 Biogas Production from Lignin via Anaerobic Digestion Daniel Girma Mulat and Svein Jarle Horn, 391, 
Chapter 15 Lignin Analytics Heiko Lange, Paola Giannì and Claudia Crestini, 413, 
Chapter 16 Lignin Visualization: Advanced Microscopy Techniques for Lignin Characterization Yining Zeng and Bryon S. Donohoe, 477, 
Chapter 17 Adding Value to the Biorefinery with Lignin: An Engineer's Perspective Mary J. Biddy, 499, 
Subject Index, 519, 

CHAPTER 1

A Brief Introduction to Lignin Structure

RUI KATAHIRA, THOMAS J. ELDER AND GREGG T. BECKHAM

1.1 Introduction

Lignocellulosic biomass is a vast resource for the sustainable production of renewable fuels, chemicals, and materials for mankind. Biomass, especially wood, has been used for millennia as a building and construction material for myriad applications and a source for heat as a fuel. The majority of mass in plants is in the cell walls, which are primarily composed of the polysaccharides cellulose, hemicellulose, and pectin along with the alkylaromatic heteropolymer lignin. The three basic building blocks of lignin, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, are synthesized via the phenylpropanoid pathway in plants and differ in their extent of methoxylation (0, 1, and 2, respectively). Lignin is synthesized via enzymatic dehydrogenation of these monomers, which form both C–O and C–C bonds, leading to a heterogeneous structure and a three-dimensional structure. As discussed briefly below, additional components of lignin such as hydroxycinnamic acids and flavonoids further complicate the structure and decorate the aromatic heteropolymer with additional linkages and chemical functionality.

Although under development for at least the last century, the conversion of biomass polysaccharides into fuels and chemicals has especially gained substantial momentum in the past several decades, primarily motivated by the potential to offset petroleum usage with a renewable, sustainable feedstock and to reduce associated global greenhouse gas emissions. For fuels production, the primary biorefinery models examined to date have adopted a strategy to utilize thermochemical pretreatment and enzymatic hydrolysis to produce pentose and hexose sugars for subsequent fermentation to ethanol using natural or engineered yeast or bacterial strains. Enormous technical diversity exists around this biomass deconstruction paradigm with many different thermochemical deconstruction/pretreatment strategies being pursued, including (but not limited to) the use of acid, base, hot water, steam, organic solvents, and ionic liquids. Industrial enzyme systems to date have primarily focused on the use of carbohydrate-active enzymes from cellulolytic fungi and anaerobic rumen bacteria, but the rise of the (meta)genomics-enabled science has rapidly accelerated the discovery of new polysaccharide deconstruction paradigms and individual enzymes. Both thermochemical and enzymatic polysaccharide deconstruction approaches remain highly pursued areas of research.

Lignin, conversely, is typically relegated for heat and power due to its inherent heterogeneity and recalcitrance. However, techno-economic analysis of lignocellulosic biorefineries is revealing that lignin utilization is a crucial component of integrated biorefineries, and thus new strategies for lignin must be developed. As such, many new discoveries and developments are emerging in the past decade regarding lignin utilization, especially given significant government and industrial funding in large consortia and centers throughout the world. This book brings together world-leading experts in lignin utilization to present and review the most recent discoveries in lignin valorization to highlight opportunities going forward to utilize lignin more efficiently and sustainably. Emphasis is placed on very recent, emerging topics in chemical and biological catalysis for lignin valorization. This introductory chapter primarily focuses on the chemical aspects of lignin structure, as a preface to the subsequent chapters.

1.2 Lignin Structure

Lignin is a polyphenolic material and one of the main components in the plant cell wall. Its biosynthesis occurs through enzymatic dehydrogenation of three phenylpropanoid monomers, p-coumaryl alcohol (2), coniferyl alcohol (3), and sinapyl alcohol (4) (Figure 1.1). Phenoxyl radials generated from these three monolignols are randomly polymerized to produce a biopolymer with a three-dimensional network. The weight average molecular weight (Mw) of isolated lignin (milled wood lignin) is 6700, 14 900, and 23 500 Da from Eucalyptus globulus, Southern pine, and Norway spruce, respectively, with the molecular weight of lignin varying widely with isolation method. Lignin contents, as measured by the Klason method, are 25–35% in softwood, 20–25% in hardwood, and 15–25% in herbaceous plants. In the cell wall of a hardwood, lignin deposition starts from the middle lamella, then in a primary wall and S1 layer in the secondary wall. Subsequently, lignin is located in the S2 and S3 layers.

An understanding of the chemical structure of lignin is critical for developing robust and effective lignin valorization processes. In past decades, many lignin chemists have studied and developed methods for the determination, isolation from biomass, destructive/non-destructive analytical methods, and the degradation of lignin, which is reviewed in more detail in Chapter 15. The main quantitative lignin determination methods are the original Klason acid hydrolysis method, the modified Klason method, and the acetyl bromide method. Various lignin isolation methods have also been developed with less structural changes. Among these are milled wood lignin (MWL), cellulolytic enzymatic lignin (CEL), kraft lignin, soda lignin, and organosolv lignin, with MWL and CEL considered to retain the native lignin structure. Isolated lignin has been characterized using numerous methods such as solution- and solid-state NMR, SEC, GC-MSn, LC-MSn, UV-Vis, FTIR, Raman, SEM/TEM, and EPR. NMR, especially solution-state NMR, which is a non-destructive method, has provided analytical breakthroughs for insights into the interunit linkages of whole lignin. In addition, several methods have been developed for understanding the chemical structure of lignin based on its degradation products. These include acidolysis, thioacidolysis, DFRC, nitrobenzene oxidation, hydrogenolysis, ozonation, permanganate oxidation, and nucleus exchange reaction. While a large number of such studies have focused on lignin chemical structure, to date the whole lignin chemical structure has still not been determined. It is obvious, however, that the large amount of information based on these instrumental and degradation methods has provided considerable insights into the structure of the polymer. In the next section, insights about the type and frequency of interunit linkages, their side chain composition, and functional groups are described.

The three phenylpropane building blocks of lignin correspond to p-hydroxyphenyl (1H), guaiacyl (1G), and syringyl (1S) structures in lignin, respectively (Figure 1.1). Softwood lignin is composed of mainly G-units with a small amount of H-unit lignin, while hardwood lignin consists of both G- and S-units. Herbaceous plant lignin contains all three monolignol units of G, S, and H units, and p-coumarate and ferulate, which are incorporated with normal G- and S-units. G-units and S-units with a Ca=O (1G', 1S') also exist.

1.2.1 Side Chain Structure in the End-group

Based on analyses of degradation products, p-coumarate (5), ferulate (6), hydroxycinnamyl alcohol (7), hydroxycinnamaldehyde (8), and arylglycerol end-units (11) have been identified in lignin (Figure 1.2). Using a color reaction method the abundance of hydroxycinnamaldehyde and hydroxycinnamyl alcohol in spruce MWL were determined to be 0.02–0.04/OAr, and 0.02, respectively. The cinnamyl alcohol (including 5-hydroxyconiferyl alcohol) units are more abundant in hardwoods than softwoods and herbaceous plants as detected by solution-state HSQC NMR, while p-coumarate and ferulate end-units are found in mainly in herbaceous species. Dihydrocinnamyl alcohol end-units (9) are incorporated primarily into softwood lignin. This unit is homo- and cross-coupled with coniferyl alcohol to generate the dibenzodioxocin unit, as described below. Guaiacylpropane-1,3-diol end-units (10) are derived from dihydrocinnamyl alcohol end-units (9), and found in low concentrations in pine and in high concentrations in CAD-deficient pine mutants. The arylglycerol end-unit (11) was detected in the hydrolysates from dioxane–H2O and hot water, but specific methods for its determination have not yet been established.

1.2.2 Acylated End-groups

Various products with acyl groups from incomplete monolignol biosynthesis are generated, such as hydroxycinnamic acids (12; p-coumaric acid, ferulic acid, sinapic acid, and p-hydroxybenzoic acid) and hydroxycinnamaldehydes (8). These molecules incorporate into the lignin polymer through ester linkages to create various acylating end-groups at the ?-OH position in many lignins, such as acetate (14), p-coumarate (15), and p-hydroxybenzoate (16) (Figure 1.3). For instance, over 50% of kenaf bast fiber lignin contains g-acetylate (14), and ?-p-coumarate (15) makes up about 10% of grass lignins. The ?-p-hydroxybenzoate substituents (16) with an aliphatic hydroxyl group is present up to a level of 10% in aspen native lignin. Additionally, ?-p-hydroxybenzoate was also detected in poplar lignin, palm, and willow. Sinapyl p-hydroxybenzoate has also been isolated in lignifing xylem tissue in poplar, and its synthetic mechanism was proposed to be a coupling reaction of sinapyl alcohol (4) and sinapyl p-hydroxybenzoate (16). Recently, it was shown that the p-hydroxybenzoate selectively acylates the g-hydroxyl group in the S unit. More importantly, it was found that these acylating groups are not introduced from acylation of the lignin polymer, but that acylated lignin monomers are generated first and then these monomers are polymerized through radical coupling to incorporate into acylated lignin. This indicates that these acylated compounds are pre-acylated lignin precursors in parallel to the classical three monolignols. With regard to the enzymes that promote the acylated monolignol in the lignin biosynthesis pathway, p-coumaroyl-CoA:monolignol transferases in grasses and feruloyl-CoA:monolignol transferases in engineered poplar promote p-coumaroylation and feruloylation of monolignols, respectively. These results imply the existence of p-hydroxybenzoyl-CoA:monolignol transferase.

1.2.3 Lignin Interunit Linkages

To identify interunit linkages, lignin is generally degraded to low molecular weight products of monomers, dimers, and oligomers, and analyzed using GC-MS, LC-MS, and NMR. Based on these techniques and computational modeling, estimates of the type and relative abundance of interunit linkages have been determined. The common interunit linkages found are summarized in Figure 1.4. Among these, the ß-O-4, ß-ß, ß-5, and 5-50 units are the main structures, with dibenzodioxocin, spirodienone, and benzodioxane representing recently discovered units. Other linkages such as ß-1, 4-O-5 diphenyl, and a-O-4 are relatively uncommon. The relative abundance of each linkage depends on the species of biomass, lignin type, and isolation method.

1.2.3.1 Aryl Ether Unit (ß-O-4)

The most frequent interunit linkage in lignin is the arylglycerol-b-aryl ether (ß-O-4) bond, which is considered to occur at levels of 40–60% in softwood and hardwood. Adler et al. reported that the arylglycerol-ß-aryl ether structure was degraded to Hibbert ketones through acidolysis, and estimated the ß-O-4 content at 0.25–0.3/OAr in spruce lignin. Lapierre et al. found that thioacidolysis could specifically cleave the ß-O-4 structure in lignin and reported both the levels of ß-O-4 units and H/G/S ratio from various lignin samples and biomass species including herbaceous plants. More recently, Ralph et al. developed the derivatization followed by reductive cleavage (DFRC) method, which can also degrade ß-O-4 units using AcBr and is followed by reductive cleavage with zinc. Using spectroscopic analyses, the ß-O-4 linkage can be estimated by 1H NMR of acetylated lignin samples and by quantitative 13C NMR of non-derivatized lignin samples. More recently, it has been demonstrated that the advanced two-dimensional NMR technique HSQC could measure relative amounts of ß-O-4 units in the whole cell wall. Based on these results from chemical degradation and spectroscopic methods, it has been confirmed that the ß-O-4 unit is the predominant interunit linkage in lignin and is critical for depolymerization.

1.2.3.2 Resinol Unit (ß-ß)

The pinoresinol structure was first detected in the enzymatic dehydrogenation polymer (DHP) of coniferyl alcohol, while other resinol units such as syringaresinol, episyringaresinol, lariciresinol, and dimethoxylariciresinol have also been isolated in percolation hydrolysate and hydrogenation products from hardwood. The amount of resinol has been estimated to be 0.02 and 0.03–0.05/OAr in spruce and birch lignin, respectively. Based on the quantitative data of these resinol units, it was found that hardwood lignin contains more resinol units than softwood, but maize lignin has no resinol structures.

1.2.3.3 Phenylcoumaran Unit (ß-5)

The ß-5 structure was found in dioxane–H2O (1 : 1, v/v) hydrolysate. The ring-opened ß-5 dimer/trimers were isolated after hydrogenolysis. The ß-5 unit was determined to be 0.11/OAr by quantification of phenylcoumarone structure by UV-Vis, which was converted from the phenylcoumaran structure after acidolysis. Based on quantitative analysis of isohemipic acid after potassium permanganate oxidation, the amount of ß-5 units was also estimated to be 0.05 and 0.09–0.12/OAr from birch and spruce lignin, respectively. The solution-state HSQC NMR spectra of whole cell wall samples revealed that poplar had fewer ß-5 units than pine and that maize lignin had no ß-5 structures.

1.2.3.4 Biphenyl Unit (5-5')

The biphenyl structure was confirmed by the isolation of dehydrodivanillin from nitrobenzene oxidation products. Other biphenyl dimers have been also detected in the hydrogenolysis products. The amount of biphenyl was estimated to be 0.045, 0.09–0.11, and 0.023/OAr in birch, spruce, and beech, respectively, calculated from the amount of the corresponding oxidative degradation products.

1.2.3.5 Dibenzodioxocin (5-5'/ßO-4) and Spirodienone (ß-1/a-O-a) Units

The dibenzodioxocin structure was discovered in softwood lignin using several two-dimensional NMR techniques (HMQC and HOHAHA) by Karhunen et. al. This structure has also been found in MWL of hardwood in low amounts. Later, using confocal laser-scanning fluorescence microscopy (CLSM), Kukkola found that dibenzodioxocine was localized in the S3 layer in Norway spruce and silver birch xylem. Spirodienones have also been observed with 0.03/OAr abundance in spruce and with 0.018/OAr abundance in aspen lignin. The unit contains the ß-1 structure and therefore has been considered as a precursor of ß-1 linkage. According to studies using 1D and 2D NMR techniques (quantitative 13C, HSQC, HSQCTOCSY, and HMBC), it has been found that the spirodienone structure is the predominant form of the ß-1 structures present in both softwood and hardwood native lignin, and that a spirodienone structure of the guaiacyl type exist in spruce lignin and the syringyl type is predominant in kenaf and birch lignin at levels of 0.03–0.04/OAr.

1.2.3.6 Benzodioxane Unit (C-Lignin)

Most recently, a catechyl lignin polymer was found in the seed coats of both monocot and dicot plants. This lignin is derived solely from caffeyl alcohol, which was usually not considered as amonolignol, linked in a linear benzodioxane chain. It has been dubbed "C-lignin" and is observed in the seed coat of the vanilla orchid with high concentration. 5-Hydroxyconiferyl alcohol can also be produced by genetic modification of hardwoods that interrupts the formation of sinapyl alcohol. These units have been incorporated into angiosperm lignin. Currently, the 5-hydroxyconiferyl alcohol is also considered to form benzodioxane units.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2018
• Language: English
• ISBN 10: 1782625542
• ISBN 13: 9781782625544
• Binding: Hardcover
• Edition number: 1
• Number of pages: 544
• Editor: Gregg T Beckham