**Author details**

Currently, lignocellulose is processed to product through three steps that include pretreatment, saccharification and fermentation [56]. The chemical pretreatment process has shown it important in the subsequent enzymatic hydrolysis and conversion of cellulosic feedstock to valuable products in the process of fermentation. An analysis of the chemical pretreatment method result shows that the composition of biomass such as hardwood, softwood or grass is the main factor in the selection of pretreatment method [55]. However, any chemical pretreatment that requires lower-cost chemical reagents and conditions and yield more sugar is preferable. The chemical pretreatment process can be divided into six types: acidic pretreatment, alkaline pretreatment, wet oxidation, ionic liquids, oxidative delignification and organosolv [55]. However, it has been reported that chemical pretreatment process adds significantly to the cost of feedstock hydrolysates by consuming energy, expensive catalysts and chemicals while potentially hindering the downstream bioprocess [56, 57], thus requires more research effort, such as the hydrogenolysis of lignin in methanol, however, produced mostly phenols. This clearly shows that the solvent plays an essential role in directing catalytic selectivity and, thus, it must be taken into consideration in the design of catalytic systems

300 New Advances in Hydrogenation Processes - Fundamentals and Applications

The necessity of chemical pretreatment of lignin-containing biomass represents a major barrier to downstream fermentation [56] especially involving physical and thermochemical processes that alter the physiochemical recalcitrance of biomass that enhances downstream enzyme digestibility [58–62]. In addition, it has been reported that pretreatment processes modify the polysaccharide matrix reducing overall yield of fermentable sugar or generating

Alternatively, it has been reported that microbes with tolerance to the inhibitory compounds produced during pretreatment are of industrial interest as fermenters of sources of saccharolytic enzymes and enhances downstream fermentation capabilities, thus potentially eliminate thermochemical pretreatment steps [18]. Indeed, biological pretreatment processes are an environmental-friendly alternative to thermochemical pretreatments, improving biorefinery economics by reducing pretreatment costs, easing inhibitor formation and increasing downstream fermentation [65–67], as reported that a few fungi suitable for wheat straw biological

In a nutshell, the hydrogenation of lignin can potentially become a breakthrough route for production of biofuel if we can unlock the lignin functionality. With the recent findings, it is interesting to observe that the exploitation of lignin valorisation shall make this route become viable and efficient to produce biofuel via hydrogenation of lignin. Further advanced biotechnologies are crucial for discovery such as more efficient biological pretreatment system, bioprospecting and development of stable genetically engineering microorganisms, improved gene cloning and sequencing technologies, characterisation of new enzymes and production in homologous or heterologous systems and ultimately lead

by-products that inhibit enzyme hydrolysis and fermentation [63, 64].

pretreatment and increased sugar recoveries [67].

for lignin conversion [57].

**6. Conclusion**

Bawadi Abdullah1,\* , Syed Anuar Faua Ad Syed Muhammad<sup>2</sup> and Nik Azmi Nik Mahmood<sup>2</sup>

\*Address all correspondence to: bawadi\_abdullah@petronas.com.my

1 Chemical Engineering Department, Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia

2 Universiti Teknologi Malaysia, Skudai, Johor, Malaysia
