Ethylene epoxide (EO) is an important chemical, produced from ethylene and used for the production of glycols, ethers, ethanolamines and ethoxylates. It is produced on doped silver catalysts as a direct, selective oxidation of ethylene. For better selectivity, silver is doped with antimony, bismuth, cesium, chlorine, copper, or noble metals. Industrially, Ag/Al2O3 doped with Re/Cs (in excess ethylene) or with Na, Cs (in excess oxygen) is preferred. Although silver is known to be the best catalyst, there is much controversy regarding the microscopic origins of the beneficial effects of the dopants used. The reactive O* species seems to play a crucial role. It then reacts with any adsorbed ethylene, forming a ring-like intermediate called oxametallocycle (OMC), which can convert into either epoxide or acetaldehyde (AA). The latter quickly burns to CO2, representing carbon loss. We use first-principles computational screening of the periodic table to (i) understand this reaction better, (ii) predict a better catalyst (alloy), (iii) explain the favorable effects of the known dopants. We calculate the exact reaction mechanism on nine late-transition metals (fcc(111) of Ag, Au, Cu, Ni, Pd, Pt, Rh and hcp(0001) metals (Co, Ru)) using DFT. The reaction rates are calculated from the DFT data using the transition state theory. Then, scaling relations and the BEP (Brønsted–Evans–Polanyi) relations are used to find correlations between the calculated reaction parameters and a unique descriptor of each catalyst (adsorption energies of O and C). A microkinetic model is built to calculate the macroscopic efficiency of the catalyst (kinetics). Then, a vast screening campaign is undertaken to calculate the descriptors of each metal in the periodic table, which explains the use of several dopants, predicts some high-performing alloys (silver with copper, cadmium, zinc, and thallium) and provides a universal framework for future screening campaigns. Based on the simulations, a volcano plot was constructed. Ag is closest to the maximum. On the other side of the volcano, Cu, Tl and Zn lie, which explains why alloying with them improves the performance as it moves the catalyst closer to the top. The alkali metals are unique in the respect of having high EadsC values and low EadsO values. This allows for the catalyst to “move horizontally” in the volcano plot, staying near the optimum region. This explains why alkali metals are often used in industrial settings as co-dopants.

Explaining ethylene epoxidation with first-principles multiscale modelling

ACS Fall 2021

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