Introductory Chapter: Introduction to Structure Properties and Application of Lignin

1. Introduction

Here, the polymeric properties of naturally existing lignin and its potential in the field of many chemical industries have been focused. The structure of lignin is considered as the cross-linking of various polyphenols containing methoxy groups and aldehyde functionality at terminal position. Lignin is the second most naturally existing polymer with full of aromatic functionality. Despite being its abundance in nature lignin, utilization is not seen in relevant areas such as polymers, adhesives, and rubber industries.

With exponential growth of population and their energy demands, there is a strong urge to incline our dependency on sustainable approaches [1]. The recent researches are focusing on the development of methods which are dependent on sustainable or renewable sources. Lignin is a nonedible polymer with carbon-neutral concept and is found in plenty of quantities from agriculture and forestry residues [2]. For the low utilization of these lignin resources and considering its potential in various chemical industries, the researchers are continuously working to develop methods for isolation of lignin (high-quality technical) for further transformation to various value-added chemicals [2].

2. Structure of lignin

As known from centuries, that lignin is found in the cell wall of the plants and is the major component to provide rigidity to cell walls. The cell wall is basically constituted of cellulose, hemicellulose, and lignin part. The utilization of cellulose and hemicellulose can be seen in various forms in literature, but lignin owing to its complex polymeric structure not being utilized much. As mentioned above, lignin is composed of various aromatic units along with methoxy and aldehyde groups as their functionality. The typical composition of these monomer units is different in different plant species dependent on their growth, their environmental condition, type of biomass, etc. [3, 4]. Despite seeing their variation in composition, it is generally considered that every cell wall on an average contains 15–25% of lignin along with 30–40% of cellulose and 15–30% hemicellulose, and few % of other components like starch, pectin, protein, carbohydrate, minerals, etc., exist in the cell wall [5, 6, 7]. Most of the natural polymers are made up of single monomer; on contrary, lignin is composed of 3-D copolymer interconnected via ether linkages of phenylpropanoid units (Figure 1).

Figure 1.

Structure of lignocellulosic biomass.

3. Characterization of lignin

It is very important to know the composition and structure well before planning to use them as precursor for any of the applications. Since lignin can be recovered from various plant resources and can be separated well by numerous methods, identification of its unique characteristics is very important [8, 9]. The functional group analysis in lignin is conducted by using FT-IR spectroscopy and Raman and fluorescence spectroscopy [10]. The stretching frequencies for ether bond, methoxy, and hydroxyl groups can be easily identified using FT-IR spectroscopy [11]. Macromolecular structure of lignin is determined by electron microscopy such as SEM-EDX (scanning electron microscope with energy dispersive X-ray spectroscopy) and TEM (transmission electron microscopy). UV-visible spectroscopy shows good absorption band owing to its aromatic nature. The shape of UV-visible graph is very sensitive to its particular type, pH, and solvent used [12].

Many structural aspects of lignin regarding its composition and reactivity can be assessed by using advanced NMR techniques like 2D-NMR and 2D HSQC-NMR. The basic and rough estimation of lignin structure can be done by [13] C-NMR and [1]H-NMR, but the spectra obtained is much overloaded and complex. For better resolution, hyphenated NMR techniques gives best results [13, 14, 15]. Gel polymer chromatography (GPC) is a type of size exclusion chromatography which is utilized in measuring the molecular weight of lignin polymers. Various aqueous and nonaqueous solvents can be used based on hydrophilic and hydrophobic nature of lignin molecule [16, 17].

4. Application of lignin

Target application is the only thing which defines which lignin molecule (of which particular characteristics) is required. As a general rule, we can state that low-quality lignin is used for the production of lignin-based polymers or plastics, whereas high-quality lignin is used for their transformation to value-added products of biomedical applications [18, 19, 20]. Lignin is used as food packaging material because of its antioxidant properties due to aromatic ring with hydroxyl and methoxy groups functionalities present in it. It has been noticed from the literature that lignin-based food packaging helps to protect the food against UV radiation [21]. High potential of lignin is also observed in biofoaming. Foaming in polymers is introduced by blowing air or chemicals between the polymer matrixes. Here, the lignin has found its importance due to polyhydroxyl groups present in it, as it can substitute natural polyols in polyurethane [22]. Resins are the another important sector where lignin has been found replacing phenol in phenol-formaldehyde resins [23].

Considering the tremendous properties of lignin, it has been an interest of researchers to find out its biomedical and pharma applications in form of hydrogels, aerogels, and sensors. Lignin-based hydrogels have been noticed to show good results in tissue engineering and drug delivery applications owing to their porous nature and high swelling capacity in aqueous media [24]. A review article published in 2000 has shown the application of lignin in biosensing [25]. These lignin-based sensors are designed to measure the biological species quantitatively.

Apart from above-mentioned applications, there are still endless applications of lignin molecule. It is really difficult to task to enlist all the applications here. Few such applications are absorption of heavy metals [26], flocullants [27], fibers [28], batteries [29], and much more.