**4.3. Examples of lignin hydrogenation**

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

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 and mole ratio of H<sup>2</sup> O/CH<sup>3</sup> OH/feedstock = 20:5:0.8. Thus, this technique offers a new alternative method for hydrogenation of lignin.

The recent work of Shu et al. [53] utilised a highly efficient and selective hydrogenation process for phenolic compounds at a mild condition over step by step precipitated Ni/SiO<sup>2</sup> catalyst. Almost a complete conversion of guaiacol to cyclohexanol was obtained at 120°C, 2.0 MPa for 2 h TOS. The step by step precipitated Ni/SiO<sup>2</sup> preparation method thus significantly improved 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, this method also improves the longevity of the catalyst with an excellent recyclability.

**Figure 7.** Conversion of guaiacol hydrogenation for different Ni/SiO<sup>2</sup> catalysts under 120°C, 2 MPa H<sup>2</sup> atmosphere for 2 h. Adapted from Ref. [52] with permission of The Royal Society of Chemistry.

Pd also exhibits a good metal to be doped with Al<sup>2</sup> O3 as a catalyst for hydrogenation process using wet impregnation technique. The work of Yi et al. (2016) [54] employed 3 wt.%Pd/ Al<sup>2</sup> O3 to convert 4-ethylphenol to cyclohexanols under mild reaction conditions in an aqueous phase. The reaction exhibits 100% conversion at 60°C for 12 h TOS. The catalysts also exhibit water resistance and stability even after recycling four times. This catalyst may provide a new catalyst's formulation to selectively produce cyclohexanol at mild reaction conditions.

Overall, the choice of catalysts is very imperative steps in determining the product selectivity. For example, Raney Ni has been extensively used in hydrogenation of lignin in past decades and regarded as the most common catalysts in hydrogenation process. However, there is not much study in bimetallic catalysts that might be worth to be investigated in order to enhance the product conversion and catalyst's longevity.
