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.

中文翻译：

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Pd 位点结构变化对 PdCu/分子筛上乙烯瓦克氧化的影响


PdCu/分子筛上的非均相瓦克氧化可以替代腐蚀性的 PdCu-氯化物，将烯烃选择性氧化成羰基化合物。传统上认为瓦克氧化发生在 Pd 离子上，而 PdO 簇被认为是无活性的。在这里，我们将 Pd 离子滴定、X 射线吸收光谱 （XAS） 和 PdCu/Faujasite 材料（1 wt% Pd、4 wt% Cu、Si/Al 2.6）的反应动力学测量相结合，以研究高温空气处理如何影响 Pd 位点的结构及其对瓦克乙烯氧化为乙醛的反应性。离子 Pd 的分数是通过 NH ₄ ⁺ -离子反向交换评估的，并得到 XAS 的证实。合成的催化剂仅包含 Pd ²⁺ 离子，这些离子在高温（573 K、773 K）的空气中煅烧后逐渐转化为小的 PdO 簇，从而产生一系列具有不同离子 Pd 分数的材料。瓦克速率（每 Pd、378 K、3 kPa H ₂ O）在这些材料中是不变的，这与直觉相反，表明 Pd 离子和 PdO 簇是本文研究的条件下瓦克氧化的相似活性位点前体。反应后 XAS 证实，暴露于 Wacker 反应条件下后，PdO 簇和 Pd 离子之间没有明显的重组。我们进一步在还原和氧化瞬变期间进行了原位 XAS，以了解氧化还原活性 Pd 和 Cu 的分数。虽然大多数 Pd ²⁺ 和 Cu ²⁺ 离子在乙烯 + H ₂ O 中可还原为亚纳米 Pd ⁰ 簇和 Cu ⁺ 离子，但 Pd ⁰ 簇不耐 O ₂ （378 K） 的再氧化，这意味着需要快速的 Pd 再氧化以避免烧结。 当催化剂在反应过程中或还原条件下失活时，煅烧去除焦炭并再生活性 Pd 离子和 PdO 簇。这项工作为了解 Pd 活性位点及其对 PdCu 分子筛上 Wacker 氧化的稳定性提供了新的见解。