Synthesis and investigation of Ag catalysts for ethylene epoxidation Maximilian Lamoth, Leon Zwiener, Rania Hanna, Andrey Tarasov, Elias Frei

Ag supported on, mainly, α-Al2O3 is used as a catalyst for industrial ethylene epoxidation. The careful addition of promoters allows epoxide selectivities of up to 90% to be reached for the direct conversion of oxygen and ethylene. These promoters influence the complicated Ag-oxygen interaction.1 Our institute has contributed significantly to the understanding of these interactions by studying interplay between various oxygen species in and on the surface model catalysts (Ag foils) for the ethylene epoxidation.2 However, for the development of new industrially relevant catalysts, the knowledge gained from such model studies has to be transferred into new synthesis concepts. For this purpose new materials and model systems have to be investigated and new methods must be applied.

A new promising class of catalysts, α-AgAlO2 Delafossite systems, were identified and a novel synthesis was established. These systems allow a smart catalyst approach where, the catalytically active Ag component is segregated in-situ. The mechanism of segregation (exothermic or endothermic event) and the temperature range (100-800 °C) for Ag decoration is adjustable by the reductive potential of the reactants, leading to a controlled Ag content with particle sizes between 5-50 nm. These catalysts showed not only high activities and selectivities in ethylene epoxidation, but also in probe reactions for the character of the activated oxygen species and surface acidity, CO- and iso-Propanol-oxidation, respectively.

The impact of small (˂ 5 nm) Ag-nanoparticles on oxidation reactions is still under debate.3 To further clarify this point, Ag supported on SiO2 (Aerosil) with narrow size distributions of 3 nm were synthesized and tested in ethylene epoxidation and CO-oxidation. Under industrially relevant conditions these promoter-free catalysts show EO selectivity of 80 % at comparably low-temperatures (˂ 200 °C). This is in agreement with CO-oxidation results, that indicate oxygen activation already at 50 °C.

Due to the high oxygen/oxidic content of the supports/catalysts used in the aforementioned systems O1s XPS analysis is impossible. However, to identify different kinds of oxygen species in and on the Ag-catalysts a TDS setup was constructed, which enables us to quantitatively analyze the oxygen fingerprints of powdered samples. In combination with in-situ Raman spectroscopy all the main oxygen species on Ag powder and various supported catalysts were identified and correlated with their relevance in ethylene epoxidation and CO-oxidation.

References 1. SHELL, Ethylene oxide processes, SHELL (2010), U.S. Pat. No. 4,766,105

2. T. C. R. Rocha, M. Hävecker, A. Knop-Gericke, R. Schlögl, J. of Cat., 312, 12 (2014)

3. Z. Qu, M. Cheng, W. Huang, X. Bao, J. of Cat., 229, 446 (2005)