Propane oxidative dehydrogenation catalyzed by molten metal alloys†

The commercial, CO₂-intensive, propylene generation process is challenged by a low equilibrium yield, costly separation processes, and severe carbon deposition. Oxidative dehydrogenation (ODH) of propane offers a promising alternative; however, its potential is hampered by the undesirable over-oxidation of propylene to carbon oxides. Chemical looping approaches have been used, where a metal oxide reacts with propane and/or hydrogen – instead of O₂ – which can avoid over-oxidation of the hydrocarbons to carbon oxides. Oxygen can then react with the reduced solid product to regenerate it to a stoichiometric oxide. However, solid chemical looping catalysts have a low oxygen carrying capacity, often less than 1% in order to avoid cyclic deactivation, arising from changes to the lattice structure in each cycle. Herein, we report the use of molten metals as chemical looping catalysts, where the catalyst completely reduces to metallic form and melts each half-cycle. A high oxygen capacity is possible, and melting reduces lattice strain upon reduction. Thermodynamic calculations indicate that 21 individual metal candidates could be viable, and the most promising 14 are experimentally compared. Bi–Sn had the highest conversion, and 50–50 mol% Bi–Sn was supported in a fixed-bed reactor. This resulted in 22% propylene yield at 600 °C, compared to a 20% yield from the reference using borosilicate beads at the same temperature. After 10 cycles with separate flows of O₂ followed by C₃H₈, totaling 37 hours on stream, no deactivation was observed. Enhanced propylene selectivity was observed using a 50–50 mol% Bi–Sn molten mixture, surpassing the performance of Bi or Sn alone, and leading to reduced carbon oxide formation. This improved selectivity occurred with both co-fed and pre-oxidized catalysts, suggesting the creation of a unique metal oxide selectively generating propylene while minimizing overoxidation to carbon oxides. Additionally, the oxygen conversion over the Bi–Sn alloy was lower than over Bi or Sn separately when co-feeding propane and oxygen. Selectivity to cracked products was high for most of the active melts tested, especially above 550 °C.