Controlling Reaction Routes in Noble-Metal-Catalyzed Conversion of Aryl Ethers.

Hydrogenolysis and hydrolysis of aryl ethers in the liquid phase are important reactions for accessing functionalized cyclic compounds from renewable feedstocks. On supported noble metals, hydrogenolysis is initiated by a hydrogen addition to the aromatic ring followed by C-O bond cleavage. In water, hydrolysis and hydrogenolysis proceed by partial hydrogenation of the aromatic ring prior to water or hydrogen insertion. The mechanisms are common for the studied metals, but the selectivity to hydrogenolysis increases in the order Pd<Rh<Ir<Ru≈Pt in decalin and water; the inverse was observed for the selectivity to hydrolysis in water. Hydrogenolysis selectivity correlates with the Gibbs free energy of hydrogen adsorption. Hydrogenolysis has the highest standard free energy of activation and a weak dependence on H pressure, thus, the selectivity to hydrogenolysis is maximized by increasing temperature and decreasing H pressure. Selectivity to C-O bond cleavage reaches >95 % in water and alkaline conditions.