Chemical Titration of Promoted Ag/α‐Al2O3 Ethylene Epoxidation Catalysts

Ethylene epoxidation: The introduction of trifluoroethanol into the reactant stream over a promoted Ag/α-Al₂O₃ catalyst inhibits ethylene epoxidation rates and enables in-situ enumeration of catalytic site densities (52–69 μmol_TFE g_cat ⁻¹) that differ from the value measured by ex-situ N₂O adsorption-decomposition (12.3 μmol_N2O g_cat ⁻¹) which likely reflects changes in the number of accessible Ag sites under ethylene epoxidation reaction conditions.

Abstract

We enumerate the active site density for a promoted Ag/α-Al₂O₃ ethylene epoxidation catalyst (35 wt.% Ag) by using trifluoroethanol as a chemical titrant. Trifluoroethanol, when introduced into the reactant stream, deprotonates to deposit stable trifluoroethoxy or trifluoromethyl groups on the Ag surface and inhibits ethylene oxide (EO) rates, thus enabling us to assess catalytic site densities. Active site densities vary between 52 and 69 μmol_TFE  g_cat ⁻¹ across different chlorine coverages (0–0.4 monolayers) and different oxygen concentrations (2.5–7.7 mol%) in the reactor feed. Site densities measured by in situ trifluoroethanol chemical titration are distinct compared to those evaluated using ex-situ N₂O decomposition (12.3 μmol_N2O g_cat ⁻¹) suggesting that dynamic restructuring of the Ag surface under reaction conditions may render pre-reaction chemisorption measurements insufficient to accurately enumerate the active sites relevant for ethylene epoxidation.

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