(7b) Effects of Pd Site Structure on Heterogeneous Wacker Oxidation of Olefins over PdCu-Zeolite Catalysts

Wacker oxidation is an industrial process to oxidize olefins to carbonyl compounds, utilizing interactions between liquid-phase Cu and Pd ions charge-balanced by chloride ligands. The use of corrosive chlorides requires corrosion-resistant reactors and abating toxic byproducts. Earlier studies have shown that Cu and Pd ions stabilized on Faujasite (FAU) zeolite can facilitate Wacker oxidation through homogeneous-like mechanism without chloride and X-ray absorption spectroscopy (XAS) data suggested that Pd ion sintering causes deactivation. Thus suggesting that Pd ions are the sole active sites for Wacker oxidation, and the reactivity of PdO clusters remains unclear.

We synthesized PdCu/FAU zeolite (1 wt.% Pd, 4 wt.% Cu, Si/Al = 3) and combined characterization and kinetics to investigate how calcination pretreatments impact the structure of Pd sites and their reactivity for Wacker oxidation. The fraction of ionic Pd relative to Pd/PdO clusters was assessed through NH₄⁺-ion back-exchange, corroborated by XAS. As-synthesized catalysts contain solely ionic Pd(NH₃)₄²⁺, which are progressively converted to PdO upon calcination in air at increasing temperature (573 K, 773 K), creating a series of materials with varying fractions of ionic Pd. Steady-state kinetics (Fig. 1) shows that ethylene oxidation rates (per total Pd) are invariant with Pd structure across these series of materials, implying that both Pd ions and PdO clusters are equivalent active site precursors for Wacker oxidation catalysis. This finding stands in sharp contrast to the mechanism of homogeneous Wacker oxidation. While catalysts deactivate with time-on-stream due to coking, they are regenerable upon calcination in air, even though calcination treatments convert ionic Pd to PdO clusters. Our findings indicate either that Pd ions and PdO clusters are equally active, or that these species interconvert under reaction conditions, providing new insights into active site requirements for Wacker oxidation over CuPd-zeolites.

(1) Imbao et al. Chem. Commun. 2020, 56 (9), 1377–1380.