Zeolite-Encaged Isolated Palladium Redox Centers toward Sustainable Wacker-Type Oxidations

Abstract

The selective oxidation of olefins by molecular oxygen holds great importance in the chemical industry due to its remarkable adaptability in constructing carbonyl compounds. Classical homogeneous Wacker oxidation with a complex system of PdCl₂–CuCl₂–H₂O is currently employed in the industrial production of acetaldehyde, which suffers from several key drawbacks. The development of alternative heterogeneous catalytic systems for Wacker-type oxidations has been hotly pursued for decades. Herein, we report a novel heterogeneous catalyst, namely Pd@FAU containing exclusive singular Pd sites confined in zeolite, showing remarkable performance in the Wacker-type oxidation of light olefins to the corresponding carbonyl compounds. Typically, stable propylene conversion rates of 2.3–3.5 mol/mol_Pd/min and an acetone selectivity of 75–89% can be achieved simultaneously, surpassing the state-of-the-art homogeneous Wacker oxidation systems. In situ spectroscopic investigations disclose the spontaneous redox cycle of Pd⁺-Pd²⁺-Pd⁺ in Pd@FAU during the reaction, in significant contrast to the known Pd²⁺-Pd⁰-Pd²⁺ redox cycle. Theoretical calculations reveal the unique reaction pathway and mechanism of Wacker-type oxidation over Pd@FAU, without the participation of water as the nucleophile. Overall, a novel heterogeneous catalyst of Pd@FAU has been developed for Wacker-type oxidations with the unique reaction mechanism fully interpreted. This study will contribute to more sustainable Wacker-type oxidations and further improve the current understanding of Pd redox catalysis.