**5. Future trends**

**4.3. Examples of lignin hydrogenation**

298 New Advances in Hydrogenation Processes - Fundamentals and Applications

O/CH<sup>3</sup>

2 h TOS. The step by step precipitated Ni/SiO<sup>2</sup>

tive method for hydrogenation of lignin.

and mole ratio of H<sup>2</sup>

Consumption of fossil fuels would decrease significantly by implementing this approach of hydrogenating lignin to obtain the desirable products. Nevertheless, factors such as product selectivity, cost and the conversion efficiency of using commercially available catalysts are still unsatisfactory

**Figure 6.** Summary of lignin conversion process (note: the abscissa represents the typical temperature range of the lignin

Yu et al. [52] proposed an in situ catalytic hydrogenation system as relative to conventional method for converting lignin depolymerisation compounds to alcohols. In this work, The Raney Ni has been used for hydrogenation process. They found guaiacol conversion and cyclohexanol selectivity to be 99% and 93.74%, respectively, for 7 h time on stream (TOS). These results were obtained under the optimal conditions of 220°C, initial pressure of 3.0 MPa

The recent work of Shu et al. [53] utilised a highly efficient and selective hydrogenation process

Almost a complete conversion of guaiacol to cyclohexanol was obtained at 120°C, 2.0 MPa for

the conversion of the guaiacol (c.f. **Figure 7**). The structure of catalysts has been significantly modified by increasing the specific surface area and high Ni metal dispersion on the support that translated into high catalytic activity. Furthermore, this method also provides an appropriate acidity of catalyst and, hence, improves the catalytic performance significantly. Interestingly,

for phenolic compounds at a mild condition over step by step precipitated Ni/SiO<sup>2</sup>

this method also improves the longevity of the catalyst with an excellent recyclability.

OH/feedstock = 20:5:0.8. Thus, this technique offers a new alterna-

preparation method thus significantly improved

catalyst.

[51]. Hence, endless research has been carried out in order to overcome these difficulties.

conversion processes). Reprinted with permission Ref. [47]. Copyright (2015) American Chemical Society.

The development of lignin valorisation or degradation remains open for new ideas and approaches. The energy and environmental crises which modern world is experiencing are forcing to re-evaluate the efficient utilisation or finding alternative uses for natural, renewable resources and using clean technologies. In this regard, lignocellulosic biomass holds considerable potential to meet the current energy demand and to overcome the excessive dependence on fossil fuels. Further advanced biotechnologies are crucial for discovery and produce biofuels and bio-chemicals. In current scenario, future trends are being directed to lignocellulosic biotechnology and genetic engineering for improved processes and products [55]. To overcome the current energy problems, it is predicted that lignocellulosic biomass in addition of green biotechnology will be the main focus of the future research [18].

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 for lignin conversion [57].

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

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 pretreatment and increased sugar recoveries [67].
