The conversion of technical lignin into these monoaromatic chemicals is assumed to be a long-term application (Holladay et al., 2007). Increased worldwide research activities can be observed in this area where predominantly thermochemical approaches are under study to convert lignin model compounds and depoly — merize technical lignins into the desired aromatic com­pounds. In general, lignin depolymerization can not only be performed in aqueous and organic phases, but also in dry form. Complex mixtures are the result in which the individual mass yields barely exceeds few percent. Mostly, C—O—C bonds are cleaved, while the C—C linkages in the lignin structures are very resistant to cleavage. The use of catalysts seems to be a necessity and these activities have been recently reviewed (Zakzeski et al., 2010; Gallezot, 2012; Azadi et al., 2013) showing the following main routes for technical lignin depolymerization in (mono)aromatic chemicals.

Base-catalyzed Depolymerization

Most work related to base-catalyzed depolymeriza­tion (BCD) originates from the pulp and paper industry where these alkaline processes are used to depolymerize and liberate lignin from the lignocellulosic matrix as described in the previous sections. Besides extensive cleavage of the b-O-4 linkages under BCD conditions the methoxyl contents in lignin decrease with the severity of alkaline conditions. However, repolymeriza­tion of liberated lignin fragments to condensation products may occur. Alcell organosolv lignin depolymer­ization in alkali (0—4%) yielded 7—30% liquid products. The maximum concentration of identified phe­nols was 4.4%, mostly syringol (2.4%) and a limited amount of guaiacol when less severe conditions were applied. Catechol was found at higher pH and tempera­tures (Thring, 1994). More recently, Yuan et al. (2010) studied the base-catalyzed degradation of kraft lignin in water—ethanol at 220—300 °C, with phenol as the capping agent into oligomers with a negligible char and gas production. Under the conditions applied lignin could not be degraded completely into lignin monomers.

Base-catalyzed lignin depolymerization with the addition of boric acid greatly facilitates the depolymer­ization of lignin in water, increase product selectivity and boric acid acts as a capping agent to suppress addi­tion and condensation reactions (Roberts et al., 2011).