Intraporous Liquid Water Stabilizes PdCu/Zeolite Catalysts for Wacker Oxidation of Ethylene

Stable heterogeneous PdCu-zeolite catalysts could replace homogeneous Pd–Cu-chloride catalysts for the industrially relevant Wacker oxidation of olefins to carbonyl compounds, avoiding the use of corrosive chlorides. Within zeolite nanopores, the condensation of water vapor is known to affect catalytic function, but the impact of intrapore H2O condensation on Wacker oxidation catalysis is poorly understood. Here, we combine kinetic measurements, ab initio molecular modeling, and X-ray absorption spectroscopy on a PdCu/Faujasite material to elucidate the catalytic consequences of intrapore H2O condensation on reactivity, selectivity, and stability for Wacker oxidation of ethylene. While Wacker rates (per Pd, 378 K) decrease over time-on-stream at lower H2O pressures due to carbonaceous deposit formation, we show that the extent of catalyst deactivation decreases to undetectable values at elevated H2O pressures (at or above 14 kPa H2O), corresponding to the regime where FAU nanopores become filled by H2O. These observations correlate with decreases in the formation of carbonaceous deposits on the catalyst surface by suppressing oligomerization side-reactions and alleviating inhibition by product acetaldehyde, as confirmed by computed ab initio Pd-ligand phase diagrams at reaction conditions. Catalyst deactivation observed at lower H2O pressures can be reversed at elevated H2O pressures, recovering initial Wacker reactivity over time scales much longer than catalytic turnovers. Maintaining stable Wacker catalysis also requires operating under oxidizing conditions (O2/C2H4 > 40), as reducing conditions induce irreversible Pd sintering even at elevated H2O pressures. This work reveals the mechanistic origins of enhancements in Wacker catalyst performance at elevated H2O pressures, informing the design of metal-zeolite catalysts for industrially relevant selective oxidations and redox reactions in the presence of solvent vapors.