Reductive Catalytic Fractionation of straw digestates for the production of biogenic aromatic monomers

Lignin is hardly degraded and presumably only scarcely structurally modified during the anaerobic digestion of straw and thus accumulating in the digestate remaining after digestion. As available industrial residue stream and due to the accumulation of unmodified lignin, such a digestate appears as promising substrate for the production of biogenic aromatics. One process for recovering monomeric aromatics from structurally unmodified lignin is the Reductive Catalytic Fractionation (RCF). Here, the RCF of straw digestates from a lab fermentation and from an industrial plant was investigated and compared to the RCF of unfermented straw as well as beech wood. It turned out that monomeric aromatics can be recovered from straw digestates by RCF. Removing impurities from the digestates by washing, increasing the hydrogen pressure and extending the reaction time to at least 7 h further increased the monomer yield. The selectivity towards γ-OH groups in the monomers can be controlled by the hydrogen pressure and the choice of hydrogenation catalyst. No yield-reducing influence of hydroxycinnamic acids found in straw got evident, however, only their purely ester-bound fraction could be easily removed and quantified. Compared to the straw digestates, monomer yields on biomass basis were lower from unfermented straw, but significantly higher from beech wood for all tested catalysts. Thus, although having a comparable lignin content, beech wood represents a higher yielding source for monomeric biogenic aromatics compared to straw digestates.