Synergistic Effect of High-Frequency Ultrasound with Cupric Oxide Catalyst Resulting in a Selectivity Switch in Glucose Oxidation under Argon

We report here, and rationalize, a synergistic effect between a non-noble metal oxide catalyst (CuO) and high-frequency ultrasound (HFUS) on glucose oxidation. While CuO and HFUS are able to independently oxidize glucose to gluconic acid, the combination of CuO with HFUS led to a dramatic change of the reaction selectivity, with glucuronic acid being formed as the major product. By means of density functional theory (DFT) calculations, we show that, under ultrasonic irradiation of water at 550 kHz, the surface lattice oxygen of a CuO catalyst traps H· radicals stemming from the sonolysis of water, making the ring-opening of glucose energetically unfavorable and leaving a high coverage of ·OH radical on the CuO surface, which selectively oxidizes glucose to glucuronic acid. This work also points toward a path to optimize the size of the catalyst particle for an ultrasonic frequency that minimizes the damage to the catalyst, resulting in its successful reuse.