ACS Catal. 2016, 6, 7385-7392.
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 substituents bearing one fewer carbon atom than the original linkages in lignin. The fragmentations of synthetic lignin and several organosolv lignins derived from Miscanthus giganteus and pine tree were conducted. Because the lignins contain alkenes that accept the hydrogen, two procedures involving reduction of the alkenes prior to C–O bond cleavage were developed. The first procedure involves reduction of the alkenes, followed by catalytic cleavage of C–O bonds after saturation of the C–C bonds; a second involves cleavage of lignin samples in the presence of 1 atm of hydrogen to saturate the alkenes before cleavage in situ. These protocols convert solid lignin to monomeric phenolic compounds with 20 mol % catalyst or to an oil (with 5 mol % Pd/C loading) having favorable viscosity parameters upon blending with a renewable organic solvent.