Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides

Described are the selectivities observed for reactions of lignin model compounds with modifications of the copper-doped porous metal oxide (CuPMO) system previously shown to be a catalyst for lignin disassembly in supercritical methanol (Matson et al., J. Amer. Chem. Soc. 2011, 133, 14090-14097). The models studied are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether, biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics of the alpha-O-4, beta-O-4, 4-O-5, 5-5, and beta-S linkages characteristic of lignin. Also, briefly investigated as a substrate is poplar organosolv lignin. The catalyst modifications included added samarium(III) (both homogeneous and heterogeneous) or formic acid. The highest activity for the hydrogenolysis of aryl ether linkages was noted for catalysts with Sm(III) incorporated into the solid matrix of the PMO structure. In contrast, simply adding Sm3+ salts to the solution suppressed the hydrogenolysis activity. Added formic acid suppressed aryl ether hydrogenolysis, presumably by neutralizing base sites on the PMO surface but at the same time improved the selectivity toward aromatic products. Acetic acid induced similar reactivity changes. While these materials were variously successful in catalyzing the hydrogenolysis of the different ethers, there was very little activity toward the cleavage of the 5-5 and beta-5 C-C bonds that represent a small, but significant, percentage of the linkages between monolignol units in lignins.