**1. Introduction**

Biomass is a potential source of fuel consists of organic materials and considered as renewable and sustainable for energy development. With fossil fuel remaining unstable in terms of cost and sustainability, the need to generate biomass-based energy is becoming more imperative and feasible in economical point of view [1, 2]. One of the main benefits of biomass-based fuel (sometimes called as biofuel) is zero carbon dioxide release as compared to fossil fuel [3]. Furthermore, the dramatic rise of carbon dioxide concentration in the atmosphere has triggered researchers to find viable alternatives to fossil-based fuels.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

One type of biomass which covers mostly of plant molecules is lignocellulosic biomass. Structurally, it composes of cellulose, hemicellulose and lignin. Cellulose and hemicellulose are mainly monomeric sugar linked to form polymer [4]. However, lignin consists of phenylpropane units, which cross-linked with tight and varied chemical bonds [4]. Thus, lignin complexity makes it difficult to be degraded as compared to almost unified type of bonding of cellulose and hemicellulose. Nonetheless, a few researchers have found ways to enhance degradability of lignin through recent pretreatment technologies [4, 5].

Lignin is separated from cellulose and hemicelluloses during kraft process, and only partial is utilised in combustion application as energy source [6]. In addition, through development of bioethanol production, it is expected that more lignin is produced as by-product which is also added to the mass number of lignin. Through advances of pretreatment technology and valorisation, lignin can be used as an alternative source for fine chemicals and raw material for fuel.

In the progress of lignin utilisation for fuel production, hydrogenation of lignin becomes one of viable methods. Lignin contains functional phenolic compounds, but the difficulties of extracting the compounds remain a bottleneck to unlock this potential material for fuel production. For example, the degree of interaction between monomers (phenolic compound) varied due to the heterologous nature of each individual monomers [7]. Another reason would be the feasibility of pretreatment technologies that are needed to be strategised and to fully utilise the monomers present [7]. In addition, the monomers have to be separated from the strong linkages within lignin structure, so that conversion to fuel can be executed efficiently.

In this chapter, lignin will be discussed of its structure and its different functional phenolic compounds. In addition, lignin depolymerisation or valorisation process to obtain individual monomers will also be presented. Further discussion will also include hydrogenation of lignin and the mechanism involved in the process. At the final part of this chapter, the future perspective of lignin hydrogenation that may lead to more innovative applications is discussed.
