# Metal Carbides for Biomass Valorization
## 3 Application in Biomass Transformation
### Second-Generation Biomass
#### 3.2.3 Lignin
The authors based their study on the yield of phenol obtained after the HDO of guaiacol in dodecane as the solvent at 300 \({}^{\circ}\)C. The catalysts, 20% Ac/CNF, 7.5% Am/CNF, 20% Am/CNF and 20% Am/CNT, which showed relevant yield (\(\sim\)35%) to phenol, all contained crystalline Mo\({}_{2}\)C. By increasing the reaction temperature to 350 \({}^{\circ}\)C, almost 98% guaiacol conversion and about 50% selectivity to phenol were reported, suggesting that the crystalline Mo\({}_{2}\)C is selective to phenol. The durability of 20% Am/CNF was tested at 300 \({}^{\circ}\)C: over three runs, the conversion to guaiacol and the yield to phenol decreased, mainly due to coke formation. However, the authors emphasized that both the guaiacol and the amount of coke decrease continuously. This result hints that not only coke formation is responsiblefor the observed deactivation, but also probably the encapsulation of carbide particles into the carbon support, as suggested by [PERSON] et al. [37]. Altering the carbon support to hollow carbon spheres (HCS), [PERSON] and coauthors studied the HDO of phenol over Mo2C and more specifically the direct deoxygenation (DDO) and the hydrogenation (HYD), the two mechanisms of HDO (Figure 11) [71]. In the phenol DDO on MoC\({}_{\mathrm{x}}\)/HCS, the authors reported a preferred pathway in the DDO both from experimental and computational investigations. A high selectivity to benzene (85%) was obtained at 350 \({}^{\circ}\)C. They explained this predilection to direct deoxygenation by the affinity of phenol to adsorb on the hydrogenated Mo2C surface. The negative effect of water during the phenol HDO was also examined. Both molecules, phenol and water, are in competition for the Mo adsorption sites. When the water partial pressure increases, the Mo active sites are covered by water, thus obstructing the access of phenol to the Mo surface atoms. However, the effect is reversible. Deactivation of the catalyst is therefore due to, first, the competition between water and phenol and, second, to the irreversible formation of MoO\({}_{2}\). Due to its similar catalytic properties compared to Pt-group metals in diverse reactions, including hydrogenation, tungsten carbide was utilized as a catalyst for the transformation of lignin [72; 73]. The C-O bond cleavage of a model compound of lignin, 2-(2-Methoxyphenyloxy)-1-phenylethanol a common \(\beta\)-O-4 dimeric model, on W2C/AC was investigated [72]. The authors investigated the effect of the reaction temperature and solvent on the activity and selectivity. 2-(2-Methoxyphenyloxy)-1-phenylethanol converts to mainly guaiacol, followed randomly by styrene and ethylbenzene. At 200 \({}^{\circ}\)C, low conversion (23.3%) and low selectivity to the products of reaction were reported, with an almost similar selectivity to styrene (16%) and guaiacol (17.4%). At 260 \({}^{\circ}\)C the initial substrate was practically fully converted after 2 h with ethylbenzene and guaiacol yields of 88.4% and 93.0%, respectively. W2C/AC is thus a promising catalyst for the cleavage of \(\beta\)-O-4, without altering the aromatic rings of the lignin compounds. The study was extended to other molecules and W2C/AC was active in destroying more resistant linkages in lignin, namely \(\alpha\)-O-4 and \(\beta\)-\(\beta\). It is worth mentioning, that at higher reaction temperature, the selectivity to ethylbenzene (88.4%) is higher than the selectivity of styrene (8.5%). At lower temperature, the selectivity to ethylbenzene was 2.3% and the selectivity of styrene was 16.0%. At higher reaction temperature, hydrogenation of the alkene group into styrene is favored. Reaction in methanol is preferable, as the solvent allows the in situ hydrogen-transfer reaction from methanol to the substrate on W2C/AC. The catalyst was not only active and selective, but also stable along five runs with a nearly comparable aptitude in the C-O bond cleavage. Moving away from the model compound, [PERSON] and coworkers studied the transformation of corn stalk lignin over W2C/AC in methanol [73].