Theoretical and Experimental Studies on Reaction Mechanism for Aerobic Alcohol Oxidation by Supported Ruthenium Hydroxide Catalysts

The experimentally proposed reaction mechanism for the aerobic alcohol oxidation by supported ruthenium hydroxide catalysts (Ru(OH)(x)/support, support = TiO2 or Al2O3) is theoretically investigated by means of ab initio quantum chemistry calculations with model catalysts "Ru(OH)(3)(OH2)(3)" and "RuCl3(OH2)(3)" for Ru(OH)(x)/support and RuClx/support, respectively. The experimentally proposed alcoholate formation and beta-hydride elimination steps can be verified. In the case of 2-butanol (as a model substrate), the calculated activation energy for the alcoholate formation step with Ru(OH)(3)(OH2)(3) (27.7 kJ mol(-1)) is much smaller than that with RuCl3(OH2)(3) (123.2 kJ mol(-1)), showing that the alcoholate formation with Ru(OH)(x)/support much more easily proceeds than that with RuCx/support. The Ru(OH)(x)/support catalysts possess both Lewis acid (Ru center) and Bronsted base (OH- species) sites on the same metal site. Therefore, the alcoholate formation step can be promoted by the "concerted activation" of an alcohol by the Lewis acid (electron transfer from an alcohol to Ru) and Bronsted base (electron transfer from OH- to a hydroxyl proton) sites on Ru(OH)(x)/support. For the reaction of the hydride species with O-2, the coordination of the electron-donating ligands (in particular, an alcohol and OH2) to form the six-coordinated ruthenium monohydride (Ru-H) species is a key to promote the O-2 insertion to the hydride species. The electron donation from the ligands to the hydride species can make the Ru-H bond weaker, resulting in lowering the activation energy for the O-2 insertion step. Finally, the alcoholate or hydroxide species is regenerated with the formation of H2O2, and the catalytic cycle is completed.