Hydrolytic cleavage of C-O linkages in lignin model compounds catalyzed by water-tolerant Lewis acids

We have explored C-O bond cleavage in different lignin model compounds in high-temperature water using catalytic amounts of water-tolerant Lewis acids. Experiments comparing indium triflate, scandium triflate, ytterbium triflate, and indium chloride indicate that indium triflate is the most active catalyst for this cleavage in guaiacol. Reactions of guaiacol at 225, 250, and 275 C were consistent with first-order kinetics for guaiacol disappearance. The conversion reached 96% at 275 C after 2 h using indium triflate. Milder temperatures (175, 200, and 225 C) were sufficient for selective cleavage of the weaker C-O bond in benzyl phenyl ether. The conversion reached 99% at 225 C after 3 h using indium triflate. The activation energies for guaiacol and benzyl phenyl ether hydrolysis were found to be 134 ± 5 and 98 ± 5 kJ·mol ^-1, respectively. As expected, diphenyl ether was more resistant to C-O cleavage, but even its very strong aryl-O-aryl bond can be hydrolyzed to form phenol at 330 C with indium triflate as the catalyst. These results point to the potential application of water-tolerant Lewis acids as catalysts for lignin depolymerization.