One interesting approach to reduce the consumption of molecular hydrogen during the HDO of bio-oils is to use hydrogen donating solvents. For instance, Elliott has reported that when the bio-oil upgrading is carried out in the presence of a hydrogen donor solvent (tetra — lin, 1-1 ratio with bio-oil feedstock) the oxygen removal increases from 70 to 85% and less deactivation of the catalyst was observed. Some of the components already present in the bio-oil, such as alcohols or acids, may also provide hydrogen for the deoxygenation reac­tions [10]. Traditional catalysts active in hydrogen transfer reactions, such as Pd, Ni or Cu should be used in this process [73].

Another attractive option is to use hydrogen donating solvents during the hydrotreating of biomass. The idea is to obtain a bio-oil with a lower oxygen content, and therefore, easier to upgrade. This concept has been mainly applied in the pyrolysis of lignin. If a hydrogen do­nor molecule is added during the pyrolysis, both depolymerization and hydrogenation oc­cur simultaneously. Remarkable results have been obtained using hydrogen-donating solvents, such as tetralin or 9,10-dihydroanthracene [74]. However, a major drawback is the need for large quantities of these solvents. At this point, formic acid appears to be a promis­ing donor molecule, as it can be obtained together with levulinic acid from the hydrolysis of biomass. On heating, formic acid decomposes completely into CO2 and two active hydrogen atoms, which are efficient scavengers of any radical species formed in the lignin. By succes­sive homolytic cleavage of the covalent linkages of the lignin, including aromatic rings, most of the oxygen is removed as water and hydrocarbons are formed (Figure 13). When pyroly­sis is carried out with formic acid, lignin can be converted into hydrogen-rich, oxygen de­pleted products with no added catalyst [75].

4.1. Hydrogenolysis with in-situ generation of hydrogen