# Metal Carbides for Biomass Valorization
## 3 Application in Biomass Transformation
### Second-Generation Biomass
#### 3.2.2 Hemicellulose  
The effective use of biomass is also directly associated with the transformation of sugars (xylose) within hemicellulose, into more valuable products (furfurral, furfuryl alcohol, methylfuran, etc.). Mo\({}_{2}\)C, which was demonstrated as active in the cellulose transformation, is also reported to be a highly selective catalyst in the HDO of furfural. Furfural, a C\({}_{5}\)H\({}_{4}\)O\({}_{2}\) molecule, is an important intermediate for the formation of important fine chemicals, such as 2-methylfuran, furfuryl alcohol and tetrahydrofurfuryl alcohol or the corresponding ethers, such as isopropyl furfuryl ether (1) and isopropyl tetrahydrofurryl ether (2), when isopropanol is used as the solvent (Figure 8). A 60% selectivity of 2-methylfuran results from the furfural HDO at 150 \({}^{\circ}\)C in a flow reactor [56]. 2-methylfuran, which has a high research octane number (RON \(\sim\)131) and a high energy density, together with low water solubility, is considered a promising biofuel [57; 58]. Through a combined approach of density function theory (DFT) and ultra-high vacuum (UHV) studies on model surfaces, [PERSON] and coauthors investigated on the HDO of furfural on Mo\({}_{2}\)C [56]. The DFT calculations showed a strong interaction between the C=O (aldehyde) bond of furfural and the Mo\({}_{2}\)C(001) surface. For comparison, they also studied the interaction between Pt(111) and Mo\({}_{2}\)C. The Mo\({}_{2}\)C surface, compared to Pt(111), exhibited stronger affinity to the C=O (aldehyde) bond. This strong interaction was confirmed by high-resolution electron energy loss spectroscopy (HREELS) experiments on model Mo\({}_{2}\)C surfaces prepared on a Mo(110) single crystal. Surface experiments in UHV of furfural and furfuryl alcohol suggest that Mo\({}_{2}\)C could selectively cleave the C=O (aldehyde) bond of furfural to produce 2-methylfuran as the main product. The HREELS of furfuryl alcohol and 2-methylfuran on Mo\({}_{2}\)C are compared (Figure 9) in order to identify possible reaction intermediates during the HDO of furfural. At 27 \({}^{\circ}\)C, a methylfuran-like intermediate was observed for both furfural and furfuryl alcohol reaction on Mo\({}_{2}\)C at 27 \({}^{\circ}\)C and \(-\)23 \({}^{\circ}\)C, respectively. Figure 8: Schematic representation of reaction pathways during furfural hydrogenation using isopropanol as solvent. [PERSON] and [PERSON] used a model catalyst that consisted of a Mo\({}_{2}\)C thin film on a Mo(100) single crystal for the deoxygenation of glycolaldehyde and furfural [59]. Deoxygenation products were detected by temperature-programmed desorption (TPD). Indeed, ethanol and ethylene were detected in the case of glycolaldehyde, and methylfuran in the case of furfural. For a better insight into the bonding configuration, the authors performed HREELS in order to identify any stable reaction intermediates. The authors concluded that both molecules interact with the Mo\({}_{2}\)C/Mo(100) surface through both carbon and oxygen atoms, i.e., the aldehyde functional group. Moreover, Mo\({}_{2}\)C/Mo(100) is active in the production of 2-methylfuran, but inactive in the hydrogenation of the furan ring, since tetrahydromethylfuran and tetrahydrofuranyl alcohol were not produced. The ability of the Mo\({}_{2}\)C surface to selectively deoxygenate was also investigated by [PERSON] et al. with furfural and furfuryl alcohol [60]. They were able to form 2-methylfuran through a 2-methylfuran-like intermediate from both substrates. Investigations on both substrates contribute to a better understanding of the impact of the furan ring on the reaction pathways. Indeed, together with the production of 2-methylfuran, a reforming reaction, as well as the complete decomposition of the furfural, could occur. In the case of furfural, 2-methylfuran (selectivity 38%) was detected by TPD between 7 \({}^{\circ}\)C and 127 \({}^{\circ}\)C. This suggests that Mo\({}_{2}\)C could selectively cleave the C=O (aldehyde) of furfural, while not affecting the furan ring.