(28c) Reductive Catalytic Fractionation: A Closer Look at the Mechanism

Reductive Catalytic Fractionation (RCF) is a lignin-first strategy to extract and depolymerize the lignin through catalytic hydrogenolysis. Hydrogenolysis of lignin generates a portfolio of phenolic monomers, dimers, and oligomers.

Generally, the yield of the lignin depolymerization is reported based on phenolic monomers, however the accuracy of the yield is also dependent on the lignin composition. A subset of lignins form with gamma-substituted esters, for example poplar lignins have p-hydroxybenzoate groups on 1-15% of the sinapyl subunits. Under hydrogenolysis it is generally assumed that the p-hydroxybenzoate is cleaved from the lignin and released as p-hydroxybenzoic acid (or the ester) and 4-propylsyringol.

In this study, we show that hydrogenolysis of lignins with g-esters using Pd/C in methanol cleaves the b-aryl ether bonds and release saturated phenolic propyl esters, which subsequently degrade to corresponding phenolic propanol and trans-esterified methyl esters. As the reaction time is increased the amount of the phenolic propyl esters decreases, indicating early in the hydrogenolysis the b-aryl ethers with g-esters rapidly release indicating these subunits are likely more readily accessible to hydrogenolysis than portions of the lignin without b-aryl ethers with g-esters. This can be attributed to better solubility of lignins with g-esters branches than lignins without g-esters.

Optimization of lignin depolymerization by heterogenous catalysis requires the inclusion products that retain the g-esters. Current optimization of product distributions does not account for non-catalytic decomposition of reaction intermediates, like the propyl esters reported here. Knowing that these ester bonds are semi-stable under hydrogenolysis reaction conditions provides researchers a new set of variables to consider targeting the conservation or degradation of these transient species.