The overall chloriding effectiveness factor (Z*), defined as the ratio of ethyl chloride concentration in parts per million to the sum of ethylene and ethane concentration in mole percent multiplied by a weighting factor to account for their efficacy in removing chlorine-adatoms from the surface, was used as a parameter to account for the effects of chlorine on the kinetics of ethylene epoxidation on a highly promoted 35 wt % Ag/α-Al2O3 catalyst. An increase in O2 order (≈0.7 to 1) and a decrease in C2H4 order (≈0.5 to <0) with increasing Z* (Z*=2.1, 3.4, 5.2, and 8.9) was observed implicating kinetic relevance of O2 activation on chloride-promoted silver catalysts. Carbon dioxide co-feed (1–5 mol %) was found to promote ethylene oxide selectivity as CO2 co-feed reversibly inhibits CO2 synthesis rates (−0.6 order) more than ethylene oxide synthesis rates (−0.49 order) at all Z* values. Ethylene oxide and CO2 rates were found to be invariant with ethylene oxide (0–0.5 mol %) and acetaldehyde (0–1.7 ppm) co-feeds, suggesting that there is minimal product inhibition under reaction conditions. A model involving a common reaction intermediate for ethylene oxide and carbon dioxide synthesis and two types of atomically adsorbed oxygen species—nucleophilic and electrophilic oxygen—is proposed to plausibly describe the observed reaction rate dependencies and selectivity trends as a function of the chloriding effectiveness.
Bibliographical noteFunding Information:
We acknowledge financial support from The Dow Chemical Company. We thank Mr. Linh Bui for assistance with N2O experiments.
Aditya Bhan received his Bachelor of Technol- ogy (B. Tech.) in Chemical Engineering from IIT Kanpur in 2000 and his Ph.D. in Chemical Engineering from Purdue University in 2005. From January 2005 to August 2007, he was a postdoctoral scholar at the University of Cali- fornia at Berkeley and since then he has been on the Chemical Engineering and Materials Science faculty at the University of Minnesota. Prof. Bhan leads a research group that focus- es on mechanistic characterization of cata- lysts useful in energy conversion and petro- chemical synthesis. His group at the University of Minnesota has been recognized with the Young Researcher Award from the Acid–Base Catalysis Society, the Ipatieff Prize from the American Chemical Society, the US Department of Energy Early Career Award, the US National Science Foundation Career Award, the 3M Non-tenured Faculty Award, and the McKnight Land Grant Professorship from the University of Minnesota. He serves as Associate Editor for Journal of Catalysis and as President-Elect of the ACS Catalysis Division.
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- homogeneous catalysis
- product inhibition