Unravelling the redox mechanism and kinetics of a highly active and selective Ni-based material for the oxidative dehydrogenation of ethane

Abstract

Ethane oxidative dehydrogenation (ODH-C₂) is a promising alternative for producing ethylene. Even if SnO₂–NiO catalysts are characterized by their long-term stability and high selectivity to ethylene, their kinetic performance has not been investigated in detail yet. In this work, three kinetic models were developed according to redox mechanisms, without assuming a single rate-determining step, for evaluating the ODH-C₂ kinetics over an in-house synthesized SnO₂–NiO catalyst. Reaction mechanism proposals were based on detailed catalyst characterization (XRD, physisorption of N₂, XPS, O₂-TPIE, NH₃-TPD, H₂-TPR, and SEM-EDX) and robust experimental evaluation of the material. Experiments were performed in the kinetic control regime in a micro-reaction unit at temperatures between 360 and 480 °C, inlet oxygen partial pressures between 2 and 15 kPa, inlet ethane partial pressures between 3 and 10 kPa and space times between 9.4 and 45.5 kg_cat s mol_C2H6⁻¹. In a set of additional experiments, feeding ethylene instead of ethane, the conversion ranged from 1.5 to 14% and CO₂ was the only reaction product. Physicochemical and statistical criteria were employed to discriminate between the performance of the considered redox mechanisms. In analyzing the kinetic results, it was found that: outlet molar flow rates were better fitted by the redox mechanism not accounting for the chemisorption of hydrocarbon-based molecules and considering the adsorption of oxygen species on active sites rather than their insertion into the lattice; water was the component with the highest affinity on the active sites; and ethylene and carbon dioxide production is associated with activation energies ranging from 66–94 kJ mol⁻¹ and 80.0–113.0 kJ mol⁻¹, respectively. These findings contribute to the mechanistic understanding of ODH-C₂ and provide a reliable model for the future design and scale-up of the process.