(113e) Effects of Surface Acetate Coverage on Selectivity in Vinyl Acetate Synthesis

Vinyl acetate (VA) is a high-value chemical produced by vapor-phase oxidative coupling of ethylene and acetic acid over Pd-based catalysts at high acetate coverages. The high cost and low stability of industrial PdAu catalysts motivates the development of alternative VA synthesis catalysts. Understanding the effects of surface acetate (CH₃COO*) coverage on VA formation rate and selectivity, and of how different coverage dependencies arise from changing metal identities due to varying surface-adsorbate and adsorbate-adsorbate interactions, can help develop better catalysts. We present density functional theory (DFT) calculations and experimental validations that reveal the elementary steps mediating undesired CO₂ production on monometallic Pd and describe how the energetics of such steps change with coverage in comparison to the desired VA formation steps. VA synthesis requires kinetically coupled steps for CH₃COO* formation (from acetic acid and O₂) and consumption (by coupling with C₂H₄* to form VA) with a narrow coverage range where both can occur at equal rates for steady-state turnovers. At low acetate coverages around the accessible ranges, the dominant side reaction is combustion of ethylidyne (CCH₃*) formed by rate-limiting dehydrogenation of strongly bound ethylene. Increasing acetate coverage weakens ethylene adsorption and promotes selective C–O coupling to produce VA, but high acetate coverage makes ethylene adsorption prohibitively weak. Acetate combustion takes place via CH₃COO* dehydrogenation to form readily oxidized intermediates and is relatively insensitive to surface coverage, with both oxidative and non-oxidative steps being kinetically relevant. Rate and selectivity equations derived from these steps agree with experimental measurements on SiO₂-supported Pd catalysts, which show C₂H₄* decomposition as the dominant side pathway at lower coverage and CH₃COO* decomposition and oxidation dominating at higher coverage. These insights provide a mechanistic framework and critical steps and coverage parameters that must be assessed over different catalyst compositions for theory-led design of new catalysts.