Lignin linkages

As seen, lignin structure and composition differs from plant to plant, means that lignin obtained from one source may need to be handled differently than lignin from another.

Plant lignins can be broadly divided into three classes: softwood (gymnosperm),  hardwood (angiosperm) and grass or annual plant (graminaceous) lignin.

Lignin abundance generally decreasing in the order of softwood, hardwoods and grasses

Lignins have in commons lots of different linkages. The linkages of lignin, individually depicted in Figure 1, include b-O-4, 5-5, b-5, 4-O-5, b-1, dibenzodioxocin, spiridienone, a-O-4 and b-b linkages, of which the b-O-4 linkages is dominant, consisting of more than half of the linkage structure of lignin. The relative abundance of the various linkages in softwoods (i.e. spruce) and hardwoods (i.e. birch), are given in Table 1

Fig. 1 Bonds in Lignin

Table 1. Linkages found in softwood and hardwood lignin 

References:

W. Boerjan et al., Annu. Rev. Plant Biol.54 (2003) 519

J. Zakzeski, P.C.A. Bruijnincx et al., Chem. Rev. 110 (2010) 355

2P. T. Patil et al., Energy Fuels, 25 (2011) 4713

Lignin structure

The word lignin is derived from the Latin word lignum meaning wood. It is a main component of vascular plants. Indeed, lignin is second only to polysaccharides in natural abundance, contributing 24–33% and 19–28%, respectively, to dry wood weights of normal softwoods and temperate-zone hardwoods [8]. Although the lignin exact structure found in plants is uncertain, we know that lignin is a three dimensional amorphous polymer consisting of methoxylated phenylpropane structures. The three monolignol monomers that form lignin structure are: p-coumaryl, coniferyl alcohol, sinapyl alcohol.

Fig.  Lignin structure and  three building blocks of lignin

These lignols are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaicacyl (G) and syringyl (S) respectively. Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of dicotyledonous angiosperms is more often than not mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.

References:

J. Zakzeski, P.C.A. Bruijnincx et al., Chem. Rev. 110 (2010) 3552

W. Boerjan et al., Annu. Rev. Plant Biol.54 (2003) 519

What is lignin?

Before talking about lignin, maybe we should start talking about biomass, which is defined by Biomass Energy center as "biological material derived from living, or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material."

As all knows, in the last few years the cost of fossil fuels has increased as well as the concern of their environmental impact and greenhouse gas effects. By what, has been carried out extensive research and development programs to find a new resource that could help to solve these problems.

Biomass, viewed as a feedstock for the future of Green Chemistry, is the most abundant renewable resource and one of the possible solution to the aformentioned problems.

Biomass plays an important role not only in providing energy for cooking or heating, but also is a unique source of renewable energy as it can be provided as solid, gaseous or liquids fuels.

Biomass based chemicals and fuels used today are produced by sugar and strach. These products are containing in crops such as beet, corn or grain. They are called first generation biomass. These crops are also used for foods and animal feed, resulting an inapropiate competitive situation on the crops. Because of that, it is important to develop second generation biomass, where waste products are used as raw material.

Second generation biofuels and chemicals are made from lignocelullosic biomass, which is composed of three main fractions; cellulose, hemicellulose and lignin.

Fig 1. Structure of lignocellulosic biomass with cellulose, hemicellulose and lignin represented.(undbiomass) 

Lignin represents about 40% of the biomass energy content and furthermore is the obvious candidate to serve as an avaible feedstock for the production of basic aromatic chemicals such as benzene, xylene, toluene.

References:

 P. T. Patil et al., Energy Fuels, 25 (2011) 4713

 R.J.A. Gosselink et al., Bioresour. Technol., 106 (2012) 173

 J. Zakzeski, P.C.A. Bruijnincx et al., Chem. Rev. 110 (2010) 3552