Catalytic transfer hydrogenation (CTH) is a process in which hydrogen is transferred from a hydrogen donor molecule to an acceptor [80]. CTH reactions can be of industrial importance as the renewable production, transportation and storage of hydrogen donors can be cheaper than those for molecular hydrogen. For CTH, it has been reported that adjacent sites may be necessary for donor and acceptor molecules [73]. Therefore, the first criterion to be fulfilled by the selected hydrogen donor molecules is to be soluble in the compound to be hydro­treated. Moreover, in order to improve the yield of desired products, reactions other than dehydrogenation of the donor should be minimized under the operating conditions. The best hydrogen donors for heterogeneous CTH include simple molecules like cyclohexene, hydrazine, formic acid and formates [81]. Alcohols like 2-propanol (2-PO) or methanol can also be used as hydrogen donors; primary alcohols are generally less active than the corre­sponding secondary alcohols due to the smaller electron-releasing inductive effect of one al­kyl group as against two [82]. The most active catalysts for heterogeneous transfer reduction are based on palladium metal. Other noble metals such as Pt and Rh are also widely utiliz­ed. Sometimes, other transition metals such as Ni and Cu have also been reported but for operation at higher temperature [73].

In this area, the most studied process has been the conversion of glycerol into 1,2-PDO. Mu- solino et al. [83] studied glycerol hydrogenolysis by transfer hydrogenation under 5 bar in­ert atmosphere, using ethanol and 2-PO as solvents and hydrogen donor molecules over 10PdFe2O3 catalyst at 453 K. They observed that complete glycerol conversion and high se — lectivities to 1,2-PDO could be obtained when the hydrogen came from the dehydrogenation of the solvent. Formic acid has also been used as a hydrogen donor molecule in the glycerol hydrogenolysis process using Ni-Cu/Al2O3 catalysts [84]. Under the operating conditions used, formic acid was readily converted into CO2 and H2, therefore, a semi-continuous set­up was used to continuously pump formic acid to the glycerol water solution, in order to ensure a constant supply of hydrogen at an appropriate rate [85]. For a constant metal con­tent of 35 wt-% (Ni+Cu), increasing Ni proportion caused an increase in glycerol conversion but also an increase in C-C bond cleavage reactions. Cu is known to be active in the C-O bond cleavage but not in the C-C bond cleavage. The presence of Cu and the creation of a

Ni-Cu alloy notably reduced formation of products <C3. This was related to the fact that C-C bond cleavage reactions are ensemble size sensitive and that the formation of a Cu-Ni alloy causes a decrease in the Ni ensemble size. Therefore, the presence of both metals is required for obtaining high 1,2-PDO yields: Ni to provide high hydrogenolysis activity and Cu to shift the selectivity towards C-O bond cleavage. It was also observed that above a certain metal content, further increments led to a decrease in glycerol conversion. This was correlat­ed to the total acidity of the catalyst that also decreased with increasing metal content. A di­rect glycerol hydrogenolysis mechanism was also proposed (Figure 15).

4. Conclusions

Bio-oils coming from the pyrolysis of biomass feedstocks and biomass based platform chem­icals present a common limiting feature: their high oxygen content. This oxygen can be re­moved by catalytic hydrotreating in the form of H2O. Intensive research is required in this field in order to develop catalytic systems active and stable under the hard operating condi­tions used: high temperatures and pressures, and high concentrations of sub-critical water. The required bifunctional catalysts must have Bronsted acidity to catalyze dehydration reac­tions or/and Lewis acid sites to attract the oxygen ion pair of the target molecule; but also metal sites that show the ability to activate hydrogen molecules. In this sense, the combina­tion of oxophilic metals (Re, Mo or W) with Ni or noble metals has shown to be a promising approach. In the case of bio-oil upgrading, the developed catalysts should promote hydro­deoxygenation reactions against hydrogenation reactions that lead to higher hydrogen con­sumption and reduction in the octane number of the oil. In order to avoid coke formation under the hard operating conditions used, neutral supports appear as an interesting option. In the case of catalysts for platform chemical valorization, C-C bond cleavage reactions should be avoided. Therefore, for some applications, like glycerol hydrogenolysis to 1,2- PDO, Cu based catalysts have to be considered due to the high selectivity of Cu for C-O bond cleavage reactions.

Hydrogenolysis processes for oxygen removal require the use of large amounts of hydro­gen, which is commonly supply by operating under high molecular hydrogen pressures. Nonetheless, this might be a problem because nowadays, most technologies to obtain hydro­gen are energy intensive and non-renewable. An interesting alternative might be to in-situ generate the required hydrogen. Among all the alternatives, the use of hydrogen donor mol­ecules that can be obtained from biomass in a renewable way, such as formic acid, appears as a promising approach.

Author details

Inaki Gandarias* and Pedro Luis Arias

*Address all correspondence to: inaki_gandarias@ehu. es

Department of Chemical and Environmental Engineering, University of the Basque Country (UPV/EHU) Alameda Urquijo s/n, Bilbao, Spain