Catalytic depolymerisation of isolated lignin to fine chemicals: depolymerisation of Kraft lignin

Kraft lignin depolymerisation over Pt/alumina, Rh/alumina and solvolysis was studied under a range of solvent solutions (ethanol/water, isopropanol/water, acetone/water 25:75, 50:50, 75:25 v/v), high temperature (573 K) and pressure (20 barg). The reactions generated mainly non-alkylated phenolic monomers, especially 2-methoxyphenol. In terms of overall yield, solvolysis was shown to be as effective as the catalysed reactions in some experiments. Nevertheless, it was found that for individual molecules a suitable combination of solvent mixture and catalyst resulted in the possibility of target compound generation. To contribute to an understanding of the mechanism of Kraft lignin depolymerisation, kinetic isotopic effects (KIE) were evaluated using fully deuterated and partially deuterated experiments. This revealed hydrogen exchange between the molecules and solvents. In addition, a variety of positive and inverse KIE showed the complexity of lignin depolymerisation and the direct involvement of solvents in those reactions.