HDO is a promising upgrading technology to remove the oxygen from biomass-derived streams, for example obtained after pyrolysis. Strong emphasis is put on finding selective catalysts to minimize the use of hydrogen while maintaining the aromatic function­ality of lignin. HDO of lignin model compounds can be efficiently performed over a copper chromite catalyst (Deutsch and Shanks, 2012). The hydroxymethyl group of benzyl alcohol is highly reactive to HDO. Demethox — ylation of anisole is the primary reaction pathway in contrast to demethylation and transalkylation. The latter are more prevalent for conventional hydrotreating catalysts. The hydroxyl group of phenol strongly acti­vated the aromatic ring toward cyclohexanol and cyclohexane.

When applied directly to isolated technical lignin a wide range of chemical reactions occur at 380—430 °C including cleavage of interunit linkages, deoxygenation, ring hydrogenation, and removal of alkyl and methoxyl moieties. A complex bio-oil is the result, but the oxygen content of this hydropyrolysis oil is lower compared to pyrolysis oil and therefore this HDO bio-oil is chemi­cally more stable. The hydrogen pressure, typically 50—150 bar, strongly influences the oil yield. Ideal cata­lysts should have high activity for hydrogenolysis and/or cracking of C—O—C and C—C linkages; low ac­tivity for ring hydrogenation; meaningful selectivity to­ward a certain aromatic compound or class of compounds to allow effective product isolation; high resistance against coke formation and easy regeneration; high sulfur resistance for processing sulfur-containing lignins. Bifunctional catalysts comprise an active hydro­genation metal (e. g. NiMo-Cr2O3, Pd, Co-Mo) and an acidic support such as zeolites to selectively open some C—C bonds. By using catalysts the yield of HDO bio-oil has been improved from 15% up to 81% (Azadi et al., 2013). For development of viable catalytic HDO bio-oil upgrading technologies to produce trans­portation fuel include (1) improved catalysts, (2) alterna­tive hydrogen source, (3) detailed kinetics study and

(4) optimizing the HDO reactions conditions suitable for existing refinery infrastructure (Bu et al., 2012).