(308b) Mechanistic and Kinetic Role of Pd in Ethane Oxidation to Acetic Acid over Pd-Mov Oxides

Acetic acid is a valuable commodity chemical that is typically produced through methanol carbonylation. This process requires homogeneous Rh/Ir catalysts with halogen cocatalysts making downstream separation difficult. Ethane oxidative dehydrogenation (ODH) over heterogeneous MoV oxide catalysts has been shown to produce acetic acid as a byproduct during ethylene formation. Pd has been introduced to these MoV oxides to enhance acetic acid formation under ODH conditions. This allows for the simultaneous production of acetic acid and ethylene, another highly-produced commodity chemical typically produced via the energy intensive steam-cracking of ethane. To investigate the role of Pd, MoV oxide catalysts containing no Pd, supported Pd, and Pd dispersed throughout the material were synthesized. These catalysts with varying amounts of surface Pd were tested for the simultaneous formation of ethylene and acetic acid. Figures 1A and 1B demonstrate the product distributions as a function of ethane conversion for the catalysts tested. The inclusion of even a small amount of Pd on the surface decreases selectivity towards ethylene while increasing the selectivity towards acetic acid. Product co-feed experiments were also performed to identify a single reaction mechanism for ethane to acetic acid and ethylene. Lattice oxygen activates the C-H bond of ethane in the rate-determining step to form an ethoxide surface intermediate. This intermediate can undergo further dehydrogenation to form ethylene or can be oxidized to form acetal and acetate surface intermediates. The acetates can then react with a surface hydroxyl to form acetic acid. This work demonstrates the utility of Pd-MoV oxide catalysts for the co-production of acetic acid and ethylene from ethane. The amount of surface Pd can be tuned to tailor the product distribution towards either ethylene or acetic acid.