Greenhouse gas emissions in the chemical industry commonly originate from reactions involving hydrocarbons or organics with oxygen that release CO2 as a byproduct. Vinyl acetate monomer (VAM), which is a key intermediate in the production of vinyl-based polymers, is industrially synthesized by the gas-phase reaction between ethylene and acetic acid (AcOH) in the presence of oxygen, generating CO2 as a primary byproduct. Although a potassium-promoted, aluminosilicate-supported bimetallic PdAu catalyst has been successfully implemented in industry for decades, its complexity has left many questions unresolved regarding the underlying catalytic mechanisms and dynamics. The primary goal of this work is to uncover relationships between catalyst properties (e.g., nanostructure and surface species) and performance (e.g., activity and selectivity) by leveraging in situ and operando crystallographic and spectroscopic capabilities developed within our research group. New insights into heterogeneous VAM chemistry gained from this work will allow for the development of catalysts with enhanced activity and selectivity for sustainable VAM production.

Silica (SiO2)-supported PdAu catalysts are commonly promoted with potassium acetate (KOAc) to improve long-term catalytic activity and selectivity for VAM. Although it is generally assumed that this catalyst preparation step has minimal effect on catalyst structure, we report evidence to the contrary. A PdAu/SiO2 model catalyst was synthesized and then impregnated with varying loadings of KOAc using wet impregnation solutions with varying AcOH concentrations. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that high loadings of KOAc and AcOH caused substantial leaching of Pd as K2Pd2(OAc)6 dimers and Pd3(OAc)6 trimers, which can ultimately affect catalyst performance. Multivariate analysis further confirmed that the effects from KOAc and AcOH were statistically significant. These results suggest care should be taken during wet impregnation of salt solutions as we demonstrate that structural modifications can occur.

Although industrial VAM catalysts are typically supported on aluminosilicate materials, the effect of surface acidity has yet to be systematically studied for this reaction. Therefore, we sought to uncover the role of acid sites by comparing model catalysts supported on either pure SiO2 or aluminosilicate powders. Operando DRIFTS studies of the aluminosilicate-supported catalysts under simulated VAM reaction conditions showed significantly higher activity and selectivity to VAM compared to the SiO2-supported catalysts. The increased activity correlated strongly with the presence of substantial quantities Pd-acetate species detected by DRIFTS, while the increased selectivity correlated strongly with a greater degree of isolated surface Pd determined from CO chemisorption. These results led us to propose that acid sites on aluminosilicate supports induce the formation of positively charged PdxAuy-H+ adducts, promoting acetate adsorption and enhanced catalyst performance compared to pure SiO2 supports.

Pd-acetate trimers and dimers are known to form on VAM catalysts. However, their role in heterogenous VAM chemistry remains unclear. To elucidate potential roles, we prepared two catalysts by physically mixing Pd3(OAc)6, KOAc, and SiO2 powders. The KOAc-free catalyst initially contained trimeric Pd3(OAc)6 and dimeric Pd2(OAc)4 species, whereas K2Pd2(OAc)6 dimers formed after KOAc addition. Although operando X-ray diffraction (XRD) revealed the reaction-induced formation of Pd and PdCx phases for both catalysts, particle sizes were significantly larger for the KOAc-free catalyst. Consequently, the KOAc-free catalyst was essentially inactive, whereas the KOAc-impregnated catalyst was nearly as active as a conventional KOAc-promoted PdAu catalyst. From the combined crystallographic and spectroscopic results, we proposed that Pd-acetate trimers and dimers actively participate in a redox cycle consisting of the formation and re-oxidation of Pd and PdCx phases. Although Pd-acetate trimers and dimers themselves may not necessarily be active or inactive species, they are nevertheless strong indicators of catalytic performance.