Fragmentation of Lignin Samples with Commercial Pd/C under Ambient Pressure of Hydrogen

Abstract

We report the reagentless cleavage of prevalent β-O-4 linkages in lignin model compounds, as well as the cleavage of several types of organosolv lignins, catalyzed by commercially available Pd/C. Such lignin fragmentation occurred without added reagent if the indigenous double bonds were reduced first or it occurred under conditions in which just 1 atm of hydrogen was added to the system to reduce C═C bonds of the original lignin sample in situ prior to fragmentation. A detailed view of the sites of cleavage of lignin samples from various sources was gained by HSQC NMR experiments. Complex model compounds were prepared and shown to form simpler arenes and substituted phenols under catalytic conditions without added reagents. The hydrogen generated in situ from alcohol functionalities provides the reductant for concomitant hydrogenolysis of C–O bonds in β aryl ethers. Decarbonylation of primary alcohols also occurred, and this process resulted in significant amounts of aromatic products containing substitu...