# Metal Carbides for Biomass Valorization
## 3 Application in Biomass Transformation
### Second-Generation Biomass
#### 3.2.3 Lignin  
Lignin is an important component of lignocellulosic biomass after harvest [66] and its composition varies according to the biomass source. Kraft lignin is waste produced during the utilization of lignocellulosic biomass in paper production [67]. Lignin is a complex and irregular macromolecule, constituting of mainly hydroxyphenyl propane and aliphatic chains [5; 68]. Lignin has a lower oxygen content than that of polysaccharides and is thus a promising candidate to produce renewable petroleum-derived products. Vapor phase hydrodeoxygenation of a lignin-derived phenolic compound mixture to aromatics over a Mo\({}_{2}\)C catalyst was investigated by [PERSON] et al. [69]. The authors carried out the vapor phase HDO of a mixture of phenolic compounds such as m-cresol, anisole, 1,2-dimethoxybenzene (DMB) and guaiacol (GUA) over a Mo\({}_{2}\)C catalyst at 260-280 \({}^{\circ}\)C and at ambient pressure. The x-ray diffraction pattern identified \(\beta\)-Mo\({}_{2}\)C as the major carbide phase. At 280 \({}^{\circ}\)C, 95% conversion was observed with a 95% yield of benzene and toluene, the deoxygenate aromatics. The successive hydrogenation reaction of the HDO products to cyclohexane and methylcyclohexane were limited to less than 10% selectivity (molar basis) at 280 \({}^{\circ}\)C with a high hydrogen to oxygenate molar ratio (\(\sim\)3300). The authors further investigated the in situ titration of a mixture of toluene and benzene over Mo\({}_{2}\)C in the presence of methanol or water at 80 \({}^{\circ}\)C. The hydrogenation of the aromatic rings was inhibited. The results suggest that the introduction of the oxygen-containing compounds that constitute lignin to Mo\({}_{2}\)C under HDO reaction modifies the surface catalyst and therefore impedes the hydrogenation property of the molybdenum carbide catalyst. [PERSON] et al. reported on the transformation of Kraft lignin into valuable chemicals over a molybdenum carbide catalyst in supercritical ethanol [67]. The products of reaction were a mixture of aliphatic alcohols, esters, monophenols, benzyl alcohols and arenes, with ester the predominant product (yield of 947 mg/g of lignin). Kraft lignin was also converted by [PERSON] and coauthors into C\({}_{6}\)-C\({}_{10}\) chemicals at high-yield without any tar or char formation over a nanostructured \(\alpha\)-molybdenum carbide catalyst [33]. Ethanolysis of the Kraft lignin in supercritical ethanol without the addition of gaseous hydrogen took place over an activated carbon-supported \(\alpha\)-molybdenum carbide (\(\alpha\)-MoC\({}_{1-x}\)/AC) catalyst. At the end of the reaction, aliphatic compounds in C\({}_{6}\) alcohols and C\({}_{8}\)-C\({}_{10}\) esters, as well as aromatic compounds in the form of C\({}_{8}\)-C\({}_{10}\) arenes, phenols and benzyl alcohols were produced. 25 products were identified and quantified and their overall total yield was 1.64 g per gram of lignin; almost 15 times more than the yield obtained with noble metal-based catalysts [70]. Due to the fact that the obtained yield is over 100%, it can be assumed that ethanol participates in the reaction. Indeed, the solvent here and the catalyst are both protagonists and play a role in the molecular yields and product composition. Hydrodeoxygenation of guaiacol, a widely used product model resulting from lignin depolymerization, was performed on molybdenum carbide supported on carbon catalysts. Understanding the influence of the carbon support on the Mo phase justified the choice of activated carbon, multi-walled carbon nanotubes and carbon nanofibers. The supports were impregnated either with ammonium molybate labeled Am or molybic acid labeled Ac, and had varying loading (7.5 wt % or 20 wt %). On activated carbon, an amorphous Mo phase was obtained while on CNT, large Mo\({}_{2}\)C or MoC particles (\(\sim\)2 \(\upmu\)m) were revealed in TEM images. The Carbon Nanofibers (CNF)-supported catalysts contained molybdenum carbide in nanoparticle form.