Effects of Pd site structural changes on Wacker oxidation of ethylene over PdCu/zeolites

Abstract

Heterogeneous Wacker oxidation over PdCu/zeolites could replace corrosive PdCu-chlorides for selective oxidation of olefins to carbonyl compounds. Wacker oxidation is traditionally thought to occur on Pd ions, while PdO clusters are assumed to be inactive. Here, we combined Pd ion titrations, X-ray absorption spectroscopy (XAS), and reaction kinetics measurements on a PdCu/Faujasite material (1 wt% Pd, 4 wt% Cu, Si/Al 2.6) to investigate how high-temperature air treatments impact the structure of Pd sites and their reactivity for Wacker oxidation of ethylene to acetaldehyde. The fraction of ionic Pd was assessed through NH₄⁺-ion back-exchange corroborated by XAS. As-synthesized catalysts contain solely Pd²⁺ ions, which are progressively converted to small PdO clusters upon calcination in air at elevated temperature (573 K, 773 K), creating a series of materials with varying fractions of ionic Pd. Wacker rates (per Pd, 378 K, 3 kPa H₂O) are invariant across these materials, showing, counterintuitively, that Pd ions and PdO clusters are similar active site precursors for Wacker oxidation at the conditions studied here. Post-reaction XAS confirms the absence of significant restructuring between PdO clusters and Pd ions following exposure to Wacker reaction conditions. We further performed in situ XAS during reduction and oxidation transients to understand the fraction of redox-active Pd and Cu. While most Pd²⁺ and Cu²⁺ ions are reducible in ethylene + H₂O to sub-nanometer Pd⁰ clusters and Cu⁺ ions, Pd⁰ clusters are recalcitrant to re-oxidation by O₂ (378 K), implying that rapid Pd re-oxidation is needed to avoid sintering. While catalysts deactivate during reaction or under reducing conditions, calcination removes coke and regenerates an active pool of Pd ions and PdO clusters. This work provides new insights into Pd active sites and their stability for Wacker oxidation over PdCu-zeolites.