(708h) First Principles Analysis of Oxidative Dehydrogenation of Ethane on Iron Sulfide Catalysts Using Sulfur As a Soft Oxidant

Oxidative dehydrogenation of ethane (ODHE) to produce ethylene is a potentially useful process for small-scale processing units suitable for the widely distributed shale reserves in the United States. ODHE, however, is prone to over-oxidize ethane to carbon dioxide. In a recent development in the ODHE literature, Marks et al. have shown that substituting gaseous oxygen with gaseous sulfur can reduce non-selective reactions to provide better selectivity and yield to ethylene over sulfurized metal oxide catalysts. Indeed, a maximum ethylene yield of 76% has been reported in ODHE reaction over a sulfurized Fe₃O₄ catalyst¹, which compares favorably to the industry benchmark of the steam cracking process.

In the present study, we use periodic plane-wave Density Functional Theory (DFT) calculations to elucidate the atomic-level features of the metal sulfide catalysts, including defected and sulfur-terminated surfaces, that control the mechanism and selectivity of ethane conversion to ethylene. We begin by predicting the thermodynamically stable and catalytically relevant single crystal surfaces of FeS₂, which is the active bulk phase observed after sulfurization of Fe₃O₄². The stoichiometric 001 surface facet of FeS₂, terminated by singly coordinated sulfur atoms, is the energetically most favorable FeS₂ surface termination at high reaction temperatures, while sulfur-richer surfaces can predominate at lower temperatures. Next, we evaluate the energetics of selective and non-selective reaction pathways over various active sites on the iron sulfide surfaces. The role of the undercoordinated sulfur defects and adsorbed sulfur moieties, as well as traditional terrace sites, are also investigated, and a combined free energy/microkinetic analysis is finally used to estimate the activity and selectivity of the different surface features for ethylene production.

References:

1. Liu, Arinaga, Lohr, Marks, ChemCatChem (2020)

2. Liu, Udyavara, Zhang, Peter, Lohr, Dravid, Neurock, Marks, PNAS (2021)