(704c) Kinetics of Ethylene Oxidation As a Function of Chlorine Coverage over a Highly-Promoted Ag/?-Al2O3 Catalyst

The kinetics of ethylene epoxidation over a highly promoted 35 wt% Ag/α-Al₂O₃ were studied in the presence of co-fed carbon dioxide, gas-phase ethyl chloride, and ethane at 513 K and 5.3 bar total pressure under excess ethylene conditions (C₂H₄:O₂ 8-15:1). Under these conditions, the kinetics of ethylene oxidation were measured as a function of a “chloriding effectiveness factor,” Z*, defined empirically as the ratio of the gas-phase ethyl chloride content (in volumetric parts per million) to the quantity [ethane mol% + 0.01*(ethylene mol%)], which accounts for the relative efficacy of gas-phase hydrocarbons in removing chlorine adatoms. The rates of ethylene epoxidation and combustion decreased with increasing Z*, while the ethylene oxide selectivity increased as a function of Z*. Common ethylene and oxygen reaction orders and apparent activation energies were measured for both ethylene oxide and carbon dioxide formation at all values of Z*, consistent with an ethylene oxidation mechanism that proceeds via a common intermediate. Increasing Z* from 2.1 to 8.9 resulted in monotonic increases in the oxygen reaction order from ~0.7 to 1.0 accompanied by monotonic decreases in the ethylene reaction order from ~0.5 to sub-zero. The reaction order in carbon dioxide was negative and independent of Z* for both ethylene oxide and carbon dioxide formation, though carbon dioxide inhibits ethylene combustion (-0.6 order) to a greater extent than ethylene epoxidation (-0.49 order) resulting in increased ethylene oxide selectivity with increasing carbon dioxide pressure. Ethylene epoxidation and combustion rates were independent of ethylene oxide and acetaldehyde pressures at all Z*, demonstrating that carbon dioxide is the only product with appreciable surface coverage. Decreased reactivity as a function of Z* suggests increased chlorine coverage eliminates sites required for ethylene oxidation, while variation in apparent kinetic parameters as a function of Z* provides evidence for electronic effects of chlorine-adatoms that result in variation in the surface coverages of O₂ and ethylene. We also discuss our recent efforts to quantify surface chlorine coverage as a function of the steady-state gas-phase composition during ethylene oxidation in an effort to correlate surface chloride coverage to measured changes in rates and selectivity as a function of process conditions.