Alternatively, instead of the use of metal catalysts and hydrogen for hydrogenation, solvolytic depolymeriza­tion reactions were performed in the presence of hydrogen donors such as tetralin or anthracene deriva­tives (Dorrestijn et al., 1999). However, the high costs of these solvents that are consumed during the process prevent practical implementation. A solution to this problem could be the use of formic acid or 2-propanol as hydrogen donors (Kleinert and Barth, 2008; Kleinert et al., 2009). In the presence of relatively large amounts of formic acid and a low chain alcohol the resulting phenolic oil contains substantial amounts of aliphatic hydrocarbons, indicating that extensive hydrogenation of the resulting depolymerization products occurs (Gellerstedt et al., 2008). Another advantage of this pro­cess is the negligible formation of char. Xu et al. (2012) used this approach to depolymerize lignin with a combi­nation of formic acid and a Pt/C catalyst in ethanol to further promote the production of lower molar mass fractions. After 4 h all lignin has been completely solubi­lized. The highest H/C and lowest O/C molar ratios were obtained with prolonged reaction times.

Lignin depolymerization in aqueous ethanol leads to a reduced formation of char, which might be attributed to the solubility power of ethanol and the hydrogen donation capability of ethanol to stabilize generated free lignin radicals (Ye et al., 2012).

Zakzeski et al. 2012 used ethanol/water mixtures that greatly enhanced the solubility of different technical lig­nins (e. g. kraft, organosolv and sugarcane bagasse lignin) and consequently led to higher yields of monoar­omatics in one-pot lignin liquid phase reforming (LPR) reactions. During solubilization extensive cleavage of various ether linkages in the macromolecule occurred. The Pt/Al2O3-catalyzed LPR reactions yielded up to 17% of monomeric guaiacol-type products for kraft lignin in the presence of H2SO4. Depending on the lignin source and the used cocatalyst, different product distri­butions and light gases such as hydrogen and methane were formed. Char formation was not observed in any of the reactions. HDO reduction of solubilized lignin us­ing transition metal catalysts led to the formation of alkyl-substituted guaiacol-type molecules with isolated yields of up to 6% for Pt/Al2O3.

Toledano et al. 2012 used a microwave-assisted bifunctional catalytic process using tetralin or formic acid as in situ hydrogen donating solvents lead to over 30% bio-oil yield mostly enriched in monomeric and dimeric phenolic compounds. However, the amount of biochar and residual lignin still needs to be reduced.

Organosolv and kraft lignin were depolymerized using a silica-alumina catalyst in a water/1-butanol mixture to a yield of 85—88 C-mol%. In a second step the lignin-derived slurry was cracked over a ZrO2— Al2O3—FeOx catalyst in water/1-butanol Total recovered phenols is 6.6—8.6% and the conversion of methoxy phenol reached 92—94% to phenol and cresol (Yoshi — kawa et al., 2013).