Abstract
The heterogenization of Wacker catalysts using chloride-free systems can potentially be a good alternative for the commercial homogeneous Wacker oxidation of ethylene, which utilizes excessive aqueous chloride solvents. However, the mechanism of the heterogeneous system has not been clarified, preventing the rational design of better catalysts. Here, we report a transient X-ray absorption spectroscopic (XAS) investigation of the heterogeneous Wacker oxidation over Pd-Cu/zeolite Y coupled with kinetic studies and chemometric analysis. Insight is obtained by operando quickXAS allowing the quantitative determination of rates and thereby revealing a rapid redox reaction involving copper. Our work demonstrates that copper is not only the site of oxygen activation, but is also involved in the formation of undesired carbon dioxide. Without detecting the presence of Cu(0) and Pd(I), our results suggest that two one-electron transfers to two Cu(II) ions to reoxidize Pd(0) is at work in this heterogeneous Wacker catalyst.
Highlights
The heterogenization of Wacker catalysts using chloride-free systems can potentially be a good alternative for the commercial homogeneous Wacker oxidation of ethylene, which utilizes excessive aqueous chloride solvents
We investigate the mechanism of the heterogeneous Wacker oxidation of ethylene to acetaldehyde over PdCu/zeolite Y through the synergistic combination of kinetic, spectroscopic and chemometric studies
The materials utilized in this work were prepared by aqueous ion exchange of the sodium form of zeolite Y and denoted as Pd(a)Cu(b)-Y [a,b=wt%; compositional (Supplementary Table 1) and physicochemical characterization (Supplementary Figs. 1, 2; Supplementary Table 2)]
Summary
The heterogenization of Wacker catalysts using chloride-free systems can potentially be a good alternative for the commercial homogeneous Wacker oxidation of ethylene, which utilizes excessive aqueous chloride solvents. Based on the results of these kinetic studies, transient XAS experiments were designed to monitor structural changes in the catalyst in each of the kinetic regimes by varying the oxygen partial pressure.
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