Reduction of CuO into Cu with Guaiacol as a Model Compound of Lignin with a Homogeneous Catalyst of NaOH

The reduction of CuO with a lignin model compound, guaiacol, is investigated, to develop a new and green method for both Cu smelting and the utilization of lignin. The results showed that CuO can be completely reduced into Cu and guaiacol selectively converted to carboxylic acids such as fumaric, maleic, acetic, and formic acids at a mild temperature of 250 °C. The presence of NaOH has a significant effect on enhancing the reduction of CuO and the selective yield of carboxylic acids. Two possible reaction pathways were proposed for the reduction of CuO with guaiacol. One pathway involves the oxidative decomposition of guaiacol into carboxylic acids. The other reaction pathway proceeds with the polymerization of guaiacol into oligomers, which can be reused for CuO reduction due to their easy hydrolyzation to monomers under hydrothermal conditions.