# Metal Carbides for Biomass Valorization
## 1 Introduction  
Carbon-rich fossil feedstocks have been identified as the main resource for obtaining fuels and fine chemicals [1]. However, they are not renewable and have a non-negligible impact on environmental pollution. Human activity, such as the burning of fossil fuels and the clearing of forests, increases the total amount of greenhouse gas emissions, which in turn contributes to climate change. Worldwide demand for energy is increasing with increasing population and the improvement in economic wellbeing. Therefore, new methods for the production of energy from alternative, renewable resources must be developed. Solar, wind, hydropower and biomass are possible solutions that can satisfy the growing worldwide energy demand. Environmentally friendly biomass was demonstrated to be a valid carbon-based alternative source for the production of renewable chemicals and fuels [1; 2; 3; 4; 5]. Biomass can be broadly classified into two categories: first-generation and second-generation biomass. First-generation biomass materials consist of triglycerides, which are the major component of vegetable oils, waste grease, and oil-processing waste [2; 3]. Second-generation biomass consists of lignocellulosic, which has attracted much attention in recent decades [4; 5; 6; 7]. Biomass must undergo several transformations in order to be converted into biofuels and fine chemicals. Systems utilizing heterogeneous catalysts have been demonstrated to be more efficient for the transformation of biomass than homogenous or enzymatic systems. Furthermore, heterogeneous catalysts are easier to recover and have a higher durability [8]. The first step of biomass transformation is depolymerization through hydrolysis to produce a mixture of oxygenase molecules. The removal of oxygen is required in order to produce fuel biomass-derived molecules with the same number of carbon atoms, because these biomass derivatives contain an large amount of oxygen, i.e., a high O/C ratio, deoxygenation is a significant reaction [9]. The products deriving from the pyrolysis of raw biomass contain ketones, aldehydes or acids, i.e., C=O bonds, and they need to be selectively hydrogenated (-C-O-H) to increase the stability of the oils [10; 11]. Selective cleavage of C-OH or C-O bonds by dehydroxylation or deoxygenation resultsin water formation with the elimination of oxygen. Loss of carbon as CO or CO\({}_{2}\) by decarbonylation or decarboxylation, respectively, is therefore avoided and results in the preservation of the number of the initial carbon atoms. One drawback of this strategy lies in the lack of hydrogen before the end of the dehydration process. To be able to further remove oxygen, additional hydrogen should be provided through hydrodeoxygenation (HDO). The desired HDO catalyst should therefore be able to selectively cleave C-O bonds without any C-C cleavage. [PERSON] and co-authors [12] and later [PERSON] et al. [13] reported that a metal/Brensted bifunctional catalyst fulfills the prerequisites. Metal and acid sites are responsible for the hydrogenation and the dehydration, respectively. Up to now, mainly noble metal-based catalysts have been used for the transformation of biomass-based resources [5; 6]. They have demonstrated good activity and stability under the harsh reaction conditions (acidic, basic and aqueous) required for biomass conversion. Nevertheless, noble metals have a significant cost and are they are present in limited quantity. Therefore, suitable substitutes should be found [14; 15]. Transition metal carbides constitute a suitable alternative for noble metal-based catalysts [16; 17]. Indeed, the utilization of these materials as a replacement for noble metal will decrease the final cost of the catalyst. For example, the cost per gram of tungsten (purity 99.5%, Aldrich, Milano, Italy) and molybdenum (Mo\({}_{2}\)C, purity 99.5%, Aldrich, Milano, Italy) carbides is 4 and 1 EUR respectively.