Synergistic effect between nanostructured catalysts and high-frequency ultrasound: Application in biomass conversion to specialty chemicals

Abstract

Within the context of sustainable chemistry, sonochemistry is now emerging as an alternative unconventional technology in catalysis. While ultrasound-generated radicals can participate in chemical reactions, mastering the reaction selectivity of polyfunctional substrates remains an elusive task. To address this challenge, we designed nanostructured metal oxides with leaf-like morphologies (e.g., CuO) by sonochemical synthesis (20 kHz) and investigated their activity for controlling the selectivity of oxidation reactions under ultrasonic irradiation in aqueous solution. We demonstrated that colocalization of the cavitation event onto the CuO surface active sites enables the direct utilization of radicals generated by cavitation for chemo-selective chemical reactions. In particular, we provided evidence for an alternative reaction pathway in glucose selective oxidation through synergistic cavitation–catalyst interactions at 550 kHz. We showed that the unwanted H• radicals stemming from water sonolysis are trapped by the surface lattice oxygen of CuO, thereby increasing the coverage of •OH radicals on the catalyst surface, and steering the selective oxidation of glucose to glucuronic acid, a valuable chemical whose synthesis remains a formidable challenge in the field of catalysis. This work also highlights that the particle size of the sonocatalyst is a key parameter governing an optimal transfer of radicals from the cavitation bubbles to the catalyst surface.