Base-free aqueous-phase oxidation of non-activated alcohols with molecular oxygen on soluble Pt nanoparticles

Abstract

Seven soluble metal nanoparticle catalysts including Pt, Ru, Rh, Pd, Ir, Ag and Au were synthesized and studied for the aqueous-phase selective oxidation of non-activated alcohols under atmospheric pressure of O₂. The effects of particle size were examined on the Pt catalysts with mean diameters of 1.5–4.9 nm. Pt nanoparticles efficiently catalyze the aerobic oxidation of alicyclic and aliphatic alcohols, in particular, primary aliphatic alcohols in the absence of any base. The particle sizes of the Pt catalysts strongly influence their activities, and the one of 1.5 nm exhibits much higher turnover frequencies. In comparison with the other metals examined in this work, it is concluded that Pt is the best metal of choice for the aerobic alcohol oxidation. Aliphatic primary alcohols reacts on the Pt catalysts more preferentially over their isomeric secondary alcohols with increasing their chain length or as they coexist. These steric effects, and the observed kinetic isotope effects with 1-C₄H₉OD and 1-C₄D₉OD are consistent with the general alcohol oxidation mechanism, which includes a sequence of elementary steps involving the formation of the alcoholate intermediates in quasi-equilibrated 1-C₄H₉OH dissociation on the Pt surfaces and the rate-determining hydrogen abstraction from the alcoholates. The inhibiting effects of hydroquinone, a typical radical scavenger, are indicative of the formation of radical intermediates in the H-abstraction step.