# Furfural Hydrogenation on Modified Niobia
## 1 Introduction  
Over the last decade the depletion of fossil fuel reserves, combined with the environmental crisis, has led scientists to focus their attention on the production of chemicals and fuels in a greener and more sustainable way. In this respect, biomass is playing a crucial role, since it is the most attractive carbon feedstock due to its abundancy and low production cost [1; 2; 3; 4]. Lignocellulose (non-edible plant-based biomass), in particular, is composed of cellulose, hemicellulose and lignin; all together they represent ca. the 75% of the 170 billion metric tons of the total biomass production per year by photosynthesis [5]. The hydrolysis process of hemicellulose leads to a homogeneous mixture of pentoses and hexoses, such as xylose, arabinose, glucose, mannose and galactose [6; 7; 8; 9]. The further dehydration of pentoses generates furfural, a potential chemical platform for the synthesis of several chemicals and biofuels [10; 11]. Furfural can also be produced directly from cascade hydrolysis and dehydration of hemicellulose; in this respect, solid acid catalysts are typically used in biphasic solvent systems in order to promote the process [12; 13]. The use of solid acid catalysts has been recently studied in the conversion of furfural into high-added value products (Figure 1) [14; 15]. The presence of Bronsted and/or Lewis acid sites, in fact, can change both the activity and selectivity of the reaction. [PERSON] et al., for example, reported a domino reaction catalyzed by zeolites with both Bronsted and Lewis acid sites for the production of \(\gamma\)-valerolactone, where Lewis sites catalyzed the hydrogenation step, while the Bronsted sites catalyzed the ring-opening hydration step [16]. Lewis acidity is responsiblefor acetalization reactions as well, as reported by [PERSON] and co-workers, who observed considerable amount of furfurraldehyde diethyl/dimethyl acetal during the hydrogenation of furfural to furfuryl alcohol over Pt nanoparticles supported on various metal oxides (\(\gamma\)-Al\({}_{2}\)O\({}_{3}\), CeO\({}_{2}\), SiO\({}_{2}\), and ZnO) in alcoholic solvent (either ethanol or methanol) [17]. It is therefore clear that the reaction pathway can be altered through a careful tuning of the acidic properties of the heterogeneous catalysts used as well as the solvent employed. One of the most important solid acid catalyst is niobia (Nb\({}_{2}\)O\({}_{5}\), niobium oxide). Niobia is a metal oxide showing NbO\({}_{6}\) octahedra, with a great structural versatility [18; 19]. The presence of Nb=O bonds in the Nb\({}_{2}\)O\({}_{5}\) structure is in fact associated with Lewis acid sites, while the highly polarised Nb-O bonds generate surface OH groups which function as Bronsted acid sites [19; 20; 21]. This intrinsic acidity, combined with air-stability and water-insolubility, makes niobia a suitable catalyst for reactions in aqueous and polar media and therefore a very attractive material for industrial use [22; 23]. Niobia-based catalysts have in fact been extensively studied for the aforementioned hydrolysis and dehydration of hemicellulose to furfural [14; 23]. However, deactivation phenomena are often reported in these reactions and it is generally attributed to the strong acid nature of the niobia active sites; acid-catalyzed formation of solid products such as coke and humins, in fact, can partially cover the active sites and therefore substantially decrease the overall catalytic activity [23; 24; 25]. One of the most common methods to finely tune the surface acidity of niobia is the addition of a second metal oxide, which can result in the formation of new materials with different chemical and physical features in comparison with the starting materials. Several techniques can be employed in order to obtain the so-called niobia-based mixed oxides, such as hydrothermal synthesis, sol-gel, and co-precipitation techniques [26; 27; 28; 29; 30; 31]. Nb\({}_{2}\)O\({}_{5}\)-MeO\({}_{2}\) (Me = Zr, Ce and Ti) mesoporous mixed oxide were, for example, prepared by [PERSON] et al.