# Metal Carbides for Biomass Valorization
## 3 Application in Biomass Transformation
### Second-Generation Biomass
#### 3.2.2 Hemicellulose
The simultaneous detection of H\({}_{2}\) and CO at 44 \({}^{\circ}\)C, together with carbon accumulation, revealed an additional reforming reaction (17% selectivity) pathway. The complete decomposition (45% selectivity) pathway was suggested by the detection of extra surface oxygen by Auger Electron Spectroscopy. In the case of furfuryl alcohol conversion over Mo\({}_{2}\)C, 2-methylfuran was mainly detected between \(-\)33 \({}^{\circ}\)C and 127 \({}^{\circ}\)C, hinting at a selective deoxygenation of furfuryl alcohol on the C-O (alcohol). However, together with the deoxygenation, neither hydrogenation of furfural nor dehydrogenation of furfuryl alcohol were observed in TPD and HREELS. For the same range of temperatures, the authors concluded that hydrogenation-dehydrogenation reactions of the two substrates would not interfere with the deoxygenation reaction. This is in agreement with [PERSON] et al. who reported that the furan ring weakened the C-O bond of the furfuryl alcohol [61]. Performing kinetic measurements of vapor-phase furfural HDO over a Mo\({}_{2}\)C catalyst, [PERSON] and coauthors [32] converted furfural to 2-methylfuran with 50-60% selectivity, C\({}_{10}\)\({}^{+}\) products (such as decane and 2-furan-2-ylmethyl)-5-methylfuran) and furfuryl alcohol at 150 \({}^{\circ}\)C. Selective C=O  
Figure 9: HREELS of furfural (red), and 2-methylfuran (black) and furfuryl alcohol (blue) adsorbed onto Mo\({}_{2}\)C after annealing the surface to various temperatures. Adapted with permission from [56]. Copyright WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, 2014.  
bond cleavage, instead of C-C bond cleavage, was suggested by the low amount of furan (<1%) produced. Moreover, consecutive hydrogenation of 2-methylfuran did not occur, as established by the absence of 2-methyltetrahydrofuran, indicating an efficient hydrogen usage during the reaction. Gas chromatography mass spectroscopy analysis of the reactor effluents confirmed that not only 2-methylfuran was produced during the HDO of furfural, but also C\({}_{10}\)\({}^{+}\) products (\(\sim\)30%). The authors inferred that the C\({}_{10}\)\({}^{+}\) products result from the dimerization of furfural and/or furfuryl alcohol through acid-catalyzed reactions [62; 63]. Probe reaction studies, as well as NH\({}_{3}\) TPD experiments performed by [PERSON] et al. highlighted the existence of acidic sites on Mo\({}_{2}\)C catalysts [64]. The Mo\({}_{2}\)C catalyst suffers, however, from deactivation, mainly due to a loss of active sites (CO chemisorption analysis), which is a consequence of carbonaceous species deposition on the surface of the catalyst. This is confirmed by High resolution transmission electron microscopy (HRTEM) images. Furfural conversion, as well as product selectivity, were reestablished under H\({}_{2}\) flow at 477 \({}^{\circ}\)C for one hour after two regeneration cycles (Figure 10). Based on DFT calculations with a correction for the long-range dispersion interaction (PBE-D3), [PERSON] et al. also studied the furfural conversion to 2-methylfuran and furan on the clean and 4H pre-covered Mo\({}_{2}\)C(101) surfaces [65]. The Mo\({}_{2}\)C(101) surface has two distinct surface sites, the unsaturated C\({}_{A}\) atoms for H adsorption and unsaturated Mo\({}_{A}\) atoms for furfural adsorption. C\({}_{A}\) represents the 4-coordinated (two surface Mo atoms and two bulky phase Mo atoms) C atom; while Mo\({}_{A}\) stands for 10-coordinated (three surface Mo atoms, three surface C atoms and four bulky phase Mo atoms) surface Mo. The adsorption energy value of furfural at the Mo\({}_{A}\) compared to those of H at the Mo\({}_{A}\) and C\({}_{A}\) sites indicates that the surface will be principally covered by furfural. From their study, furfural is converted according to two parallel and competitive reaction paths. The first one leads to the formation of 2-methylfuran through first the furfural hydrogenation into furfuryl oxide, followed by the dissociation of the later into furfuryl and finally the formation of 2-methylfuran.