Promotional Role of Acid Sites on Aluminosilicate-Supported PdAu for Vinyl Acetate Synthesis

PdAu catalysts supported on both pure silica and aluminosilicates and promoted with KOAc are the most active and selective compositions for the industrial-scale acetoxylation of ethylene with acetic acid (AcOH) to form vinyl acetate. However, the effect of surface acidity on this reaction has not been systematically studied. In this work, we prepared PdAu (8:4 wt % Pd/Au) and KOAc-promoted PdAu (8:4:3 wt % Pd/Au/K) model catalysts using pure silica and aluminosilicate (10 wt % Al₂O₃, “Siral90”) as supports. XRD showed that the four fresh catalysts (PdAu/SiO₂, PdAu/KOAc/SiO₂, PdAu/Siral90, and PdAu/KOAc/Siral90) contained two distinct phase compositions: a nearly pure Au phase and a Pd-rich alloy phase. XRD grain sizes were noticeably larger for the Pd-rich alloy phase on the aluminosilicate materials compared with the silica-supported ones. The overall Pd surface atom content was lower and more dispersed on the aluminosilicate-supported catalysts based on XPS, XAS, and CO chemisorption analyses. In the presence of KOAc, the overall Pd surface atom content decreased and became more dispersed on both the silica- and aluminosilicate-supported catalysts. DRIFTS of fresh PdAu/KOAc/Siral90 showed the formation of Pd₃(OAc)₆ and surface-adsorbed acetate species, which were absent on PdAu/KOAc/SiO₂. Operando DRIFTS-GC of PdAu/KOAc/Siral90 under reactant gas flow showed higher activity and selectivity to vinyl acetate compared to the silica-only case, which correlated to increased amounts of Pd₃(OAc)₆ and surface-adsorbed acetate. In situ DRIFTS revealed the presence of K₂Pd₂(OAc)₆ on PdAu/KOAc/Siral90 during exposure to inert, oxygen, or AcOH gas conditions. In situ XRD showed the formation of a third crystal phase (∼Pd₅₀Au₅₀) under different gas conditions. We find that acid sites alter the catalyst nanostructure and promote the formation of Pd-acetate surface species, leading to greater activity and selectivity and illustrating how surface acidity contributes to PdAu-catalyzed vinyl acetate production.