Tổng quan về lignin: Cấu trúc, phương pháp tổng hợp và ứng dụng

L.V. Thuan, T.T.K. Ngan, T.N. Tien / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 81-86 81 
 A mini review on lignin: Structures, preparations, and applications 
Tổng quan về lignin: Cấu trúc, phương pháp tổng hợp và ứng dụng 
Le Van Thuana,b, Tran Thi Kieu Nganb, Tran Nguyen Tiena,b* 
Lê Văn Thuậna,b, Trần Thị Kiều Ngânb, Trần Nguyên Tiếna,b* 
aCenter for Advanced Chemistry, Institute of Research and Development, Duy Tan University, Da Nang City, 550000, 
Vietnam 
aTrung tâm Hóa học Tiên tiến, Viện Nghiên cứu và Phát triển Công nghệ Cao, Đại học Duy Tân, Đà Nẵng, Việt Nam 
bThe Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang City, 550000, Vietnam 
bKhoa Môi trường và Công nghệ Hóa, Đại học Duy Tân, Đà Nẵng, Việt Nam 
 (Ngày nhận bài: 03/03/2021, ngày phản biện xong: 09/03/2021, ngày chấp nhận đăng: 05/03/2021) 
Abstract 
Lignin is the second most abundant natural renewable biopolymer after cellulose on the earth. It is commonly generated 
as a by-product from the paper and ethanol industry. The complexity and richness of its functional groups make lignin 
attractive for converting into a variety of high-value products such as syngas, carbon fiber, phenolic resin, various 
oxidized products, and multifunctional hydro-carbons. This work intends to provide a comprehensive overview of 
structures and different types of lignin, as well as the recent progress in its preparation techniques. Besides, the 
extensive range of applications and opportunities of lignin were also discussed in detail. 
Keywords: Lignin, lignocellulosic biomass, extraction methods; lignin applications. 
Tóm tắt 
Lignin là polymer tự nhiên phong phú thứ hai trên thế giới, sau cellulose. Nó thường được tạo ra từ sản phẩm phụ của 
các ngành công nghiệp sản xuất giấy và ethanol. Với sự phức tạp và đa dạng của các nhóm chức trong cấu trúc, lignin 
trở nên hấp dẫn để được chuyển đổi thành nhiều loại sản phẩm có giá trị cao như khí tổng hợp, sợi carbon, nhựa 
phenolic, các sản phẩm oxy hóa và hydro-cacbon đa chức năng. Mục đích của nghiên cứu này là cung cấp một cái nhìn 
tổng quan toàn diện về cấu trúc và các loại lignin khác nhau, cũng như những tiến bộ gần đây trong kỹ thuật thu nhận 
lignin. Bên cạnh đó, những ứng dụng tiềm năng, định hướng phát triển trong tương lai của lignin cũng được thảo luận 
chi tiết. 
Từ khóa: Lignin, sinh khối lignocellulose, phương pháp tách chiết, ứng dụng của lignin 
1. Introduction 
 The lignin (lignum) is a component of 
lignocellulose which consists also of cellulose 
and hemicellulose. This material is the second 
most abundant natural polymer on earth, which 
plays an important role in plants, including 
providing mechanical supports, transporting 
water and minerals, and protecting plants or 
02(45) (2021) 81-86
* Corresponding Author: Tran Nguyen Tien; Center for Advanced Chemistry, Institute of Research and Development, 
Duy Tan University, Da Nang City, 550000, Vietnam; The Faculty of Environmental and Chemical Engineering, Duy 
Tan University, Da Nang City, 550000, Vietnam 
Email: trannguyentien@duytan.edu.vn 
L.V. Thuan, T.T.K. Ngan, T.N. Tien / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 81-86 82 
wood from chemical or microbial attacks [1].. 
Traditionally, lignin is considered as low-value 
waste product. However, it has been studied 
that lignin can be used to make high-value 
products such as syngas, carbon fiber, phenolic 
compounds, and multifunctional hydro-carbons 
[2]. In addition, due to its diverse reaction sites, 
high carbon content and low content of oxygen, 
lignin can become a potential sustainable 
alternative source of energy and chemicals [3]. 
The molecular structure of lignin is highly 
dependent on the resources and extraction 
processes. Different types of lignin contain 
different functional groups and show different 
molecular weight and elemental composition. 
Therefore, the structure of lignin is extremely 
complicated and difficult to determine. 
However, it is generally accepted that lignin is 
three- dimensional macromolecule formed by 
the coupling of three phenylpropane units 
(empirical formula of C31H34O11): guaiacyl, 
syringyl, and p-hydroxyphenyl alcohol, which 
are formed through Shikimate and Cinnamate 
pathways [4]. 
Lignin can be obtained from a variety of 
natural sources, including woody biomass, 
agricultural residues, and energy crops. 
Lignocellulosic biomass is primarily comprised 
of cellulose (38–50%), hemicellulose (23–32%) 
and lignin (12–25%) components. The biomass 
reserves on the earth have been estimated to be 
approximately 1.85–2.4×1012 tons and about 
20% of this amount of biomass is lignin. 
Besides the natural abundance, lignin is also 
present as a major byproduct of the pulp and 
paper industry. About 50–70 million tons of 
lignin is produced annually at pulp and paper 
facilities world-wide. It is estimated by 2030, 
this number will increase by 225 million tons 
per year as the annual production of lignin [2]. 
However, the majority is discarded as waste or 
burnt to recover heat and electricity, causing 
serious environmental pollution and resource 
waste. Only approximately 2% of the produced 
lignin is isolated and effectively used for various 
products such as dispersants, adhesives, 
surfactants, and fuel [1]. Finding new and high 
value-added applications of lignin recovered from 
pulping waste liquor is imperative, which has both 
economic and environmental benefit. Currently, 
increasing research focuses on developing 
different approaches to lignin extraction and 
converting it into value-added products. 
In this review, we provide a comprehensive 
overview of the structures, preparation methods 
of lignin, and their applications in different 
commercial fields. In addition, the challenges, 
and opportunities of lignin applications were 
discussed in this study. 
2. Structures of lignin 
Lignin is a complex, amorphous, branched 
polyphenolic macromolecule with aromatic 
polymeric structure [5]. The structure of lignin 
varies based on the extraction process, and the 
presence of various functional groups. Lignin 
exhibits a high recalcitrance to chemical and 
biochemical depolymerization due to the 
existence of phenylpropanoid polymers, ether 
linkages (β–O–4) and a range of functional 
groups namely methoxy, aliphatic and aromatic 
hydroxy, benzyl alcohol, ether and non–cyclic 
benzyl ether and carbonyls [6]. Lignin has 
different functional groups such as: hydroxyl, 
methoxyl, carbonyl, and carboxyl, etc. Lignin 
has three basic types of monomers; coniferyl 
alcohol, sinapyl alcohol, and p-coumaryl 
alcohol, also known as monolignols (Figure 1). 
Peroxidase and laccase enzymes in the plant 
can cause the dehydrogenation of phenolic OH 
groups and generate intermediate free radicals 
from these lignin precursors [7]. The exact 
structure of lignin in its native form in plants is 
still unclear, as studies have concluded that the 
structure is modified during its isolation and 
L.V. Thuan, T.T.K. Ngan, T.N. Tien / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 81-86 83 
differs from that of native lignin. Composition 
and amount of lignin varies from species to 
species, tree to tree, and even in woods from 
different parts of the same tree [5]. 
Figure 1. Three phenylpropanoid units of lignin structure [8] 
3. Preparation methods and types of lignin 
Lignin can be extracted from the 
lignocellulosic feedstock by a variety of 
methods involving chemical, physical, 
physicochemical, and biological treatments 
(Figure 2). Depending on the process 
employed, the properties of the resulting 
isolated lignin differ. The physical pretreatment 
involves increments in temperature or pressure 
leading to a change in the structure of 
lignocellulosic material and facilitating biomass 
destruction. Meanwhile, the chemical treatment 
involves using organic or inorganic substrates 
that cause the structure disruption in the 
lignocellulosic materials by interacting with 
inter and intrapolymer bonds of cellulose, 
lignin, and hemicellulose. These chemical and 
physical processes could be utilized separately. 
However, the combination of physical and 
chemical methods could remarkably increase 
the biomass digestibility, leading to an increase 
in the yield of the desired products [9]. 
Biological pretreatment, like use of fungi, 
offers the benefit of low chemical and energy 
use, but time taking process. Part of lignin 
could be removed from other biomass by 
producing digestible cellulose via passing hot 
water. But, the process is energy intensive and 
not suitable in sustainability aspects [10]. 
Figure 2. Different approaches to isolate lignin from lignocellulosic biomass [9] 
L.V. Thuan, T.T.K. Ngan, T.N. Tien / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 81-86 84 
Two major processes popular in the pulp and 
paper industries to separate cellulose from 
lignin, commonly known as kraft (generating 
kraft lignin, sometimes also called alkali lignin) 
and sulfite pulping (generating lignosulfonate). 
Kraft pulping is the major chemical pulping 
process, accounting for 85% of the total lignin 
production in the world [11]. The process is 
performed at a high pH, and about 90–95% of 
the lignin is dissolved into the black liquor. 
Kraft lignin is typically precipitated and 
recovered from black liquor by the addition of 
acidifying agents. Predominantly, the 
acidification is carried out by adding either 
mineral acid (e.g., sulfuric acid) or carbon 
dioxide, followed by filtering, washing, and 
drying for the recovery of Kraft lignin. The 
sulfite pulping process is conducted between a 
pH of 2–12, depending on the cationic 
composition of the pulping liquor. The process 
uses a heated aqueous solution of a sulphite or 
bisulfite salt with counter cations such as 
sodium, ammonium, magnesium, or calcium. 
Lignosulfonates, isolated lignins from the 
sulfite process, contain significant amounts of 
sulfur in the form of sulfonate groups. Since 
lignosulfonates are widely available, 
lignosulfonates were used in a wide range of 
applications, such as dispersants, flocculants, 
concrete additives, and composites [9]. 
Organosolv process is one of the most 
relevant methods, which is based on the 
solubilization of lignin using a mixture of 
different organic solvents and water as cooking 
liquor. It is also helpful in extracting highly 
homogeneous lignin, which enables its further 
valorization into value-added products [7]. 
Since the organosolv process is conducted in 
the absence of sulfur, it has recently been 
utilized more so than Kraft and sulfite pulping. 
Furthermore, the large-scale production of 
organosolv lignin is expected from the 
emerging cellulosic ethanol sectors, which 
offers significant opportunities for lignin 
valorization. 
4. Applications 
The efforts in using lignin derivatives in 
more sophisticated applications are currently 
booming. Lack of toxicity and versatility of 
lignin creates several potential industrial 
application routes. Stringent regulations, bulk 
availability, cost efficiency and growing need 
for bio-based and renewable chemicals are 
high-value lignin properties [12]. 
Lignins are being used for the controlled 
release of fertilizers, modified for slow-release 
fertilizers and herbicides in agriculture, as a 
base for different materials application in the 
fields of bioplastics, (nano)composites and 
nanoparticles [13]. By considering unique 
attributes of lignin, such as its binding 
properties, products, such as adhesive for wood, 
pellets, foundry resins, and epoxy resins could 
be produced. Properties, such as 
hydrophobicity, antioxidant and thermal 
resistance, could also facilitate lignin use in 
thermoplastics, composites, and packaging. 
Furthermore, as adsorbents in solution, 
protective UV-absorbents, dispersants, to 
improve the saccharification of lignocelluloses 
in the production of biofuels, in electro-chemical 
applications and in environmentally friendly 
functionalization approach to extend the role of 
lignin for future biomass and biofuel 
applications [14]. By con- trolling the structure 
of lignin, other advanced applications could be 
developed, such as nano/microcapsules, 
nano/microporous materials, and lignin 
nanotubes, as a smart DNA delivery without 
possessing the cytotoxicity related to carbon 
nanotubes [15]. Significant scope for diverse 
applications (Figure 3) principally segmented as 
power/energy, macromolecules and aromatics. 
L.V. Thuan, T.T.K. Ngan, T.N. Tien / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 81-86 85 
Figure 3. Application fields of lignin [12] 
5. Conclusions 
This review discussed the structures, 
extraction methods, and applications of lignin. 
Lignin is available in huge amounts as a by-
product of the pulping process, and this will 
further increase as the lignocellulosic ethanol 
process gets commercialized. Grand amount of 
lignin is produced worldwide. However, only a 
small amount is applicable for further 
applications such as additives, dispersants, 
binders, or surfactants. The remaining part is 
burn and cause environmental problem. Many 
research efforts have been done in developing 
processes that could produce valuable lignin-
derived compound. Recently, there have been 
more investigations conducted on using lignin 
to construct medical materials, electrochemical 
energy materials, and 3D printing composites. 
In spite of the increasing utilization of lignin-
based biomaterials, the high value-added 
applications of lignin still face some challenges 
mainly due to its complicated and changeable 
macromolecular structure. We hope that in the 
future, the advancement of science and 
technology will overcome these obstacles, 
thereby further expanding the valuable 
applications of lignin. 
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