Zeolite-Encaged Isolated\nPalladium Redox Centers toward\nSustainable Wacker-Type Oxidations

The selective oxidation of olefins by molecular oxygen\nholds great\nimportance in the chemical industry due to its remarkable adaptability\nin constructing carbonyl compounds. Classical homogeneous Wacker oxidation\nwith a complex system of PdCl<sub>2</sub>–CuCl<sub>2</sub>–H<sub>2</sub>O is currently employed in the industrial production of acetaldehyde,\nwhich suffers from several key drawbacks. The development of alternative\nheterogeneous catalytic systems for Wacker-type oxidations has been\nhotly pursued for decades. Herein, we report a novel heterogeneous\ncatalyst, namely Pd@FAU containing exclusive singular Pd sites confined\nin zeolite, showing remarkable performance in the Wacker-type oxidation\nof light olefins to the corresponding carbonyl compounds. Typically,\nstable propylene conversion rates of 2.3–3.5 mol/mol<sub>Pd</sub>/min and an acetone selectivity of 75–89% can be achieved\nsimultaneously, surpassing the state-of-the-art homogeneous Wacker\noxidation systems. <i>In situ</i> spectroscopic investigations\ndisclose the spontaneous redox cycle of Pd<sup>+</sup>-Pd<sup>2+</sup>-Pd<sup>+</sup> in Pd@FAU during the reaction, in significant contrast\nto the known Pd<sup>2+</sup>-Pd<sup>0</sup>-Pd<sup>2+</sup> redox\ncycle. Theoretical calculations reveal the unique reaction pathway\nand mechanism of Wacker-type oxidation over Pd@FAU, without the participation\nof water as the nucleophile. Overall, a novel heterogeneous catalyst\nof Pd@FAU has been developed for Wacker-type oxidations with the unique\nreaction mechanism fully interpreted. This study will contribute to\nmore sustainable Wacker-type oxidations and further improve the current\nunderstanding of Pd redox catalysis.