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CHAPTER 1
A Brief Introduction to Lignin Structure
RUI KATAHIRAa, THOMAS J. ELDER,b AND GREGG T. BECKHAM*a
a National Bioenergy Center, National Renewable Energy Laboratory 15013 Denver West Parkway Golden CO 80403 USA;
b USDA-Forest Service Southern Research Station 521 Devall Dr., Auburn, AL 36849 USA
1.1Introduction
Lignocellulosic biomass is a vast resource for the sustainable production of renewable fuels, chemicals, and materials for mankind.1,2 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).3 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.1,4 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.4,5 Industrial enzyme systems to date have primarily focused on the use of carbohydrate-active enzymes from cellulolytic fungi6 and anaerobic rumen bacteria,7 but the rise of the (meta)genomics-enabled science has rapidly accelerated the discovery of new polysaccharide deconstruction paradigms and individual enzymes.8 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.9 However, techno-economic analysis of lignocellulosic biorefineries is revealing that lignin utilization is a crucial component of integrated biorefineries,10 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.
1.2Lignin 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).11–15 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,16 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.17 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.18,19
Figure 1.1 Repeating units in lignin.
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,17,20 and the acetyl bromide method.21 Various lignin isolation methods have also been developed with less structural changes.22 Among these are milled wood lignin (MWL),23,24 cellulolytic enzymatic lignin (CEL),24 kraft lignin, soda lignin, and organosolv lignin,23 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,25–27 SEC,28 GC-MSn, LC-MSn, UV-Vis,29 FTIR,30,31 Raman,32 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 additio...
