(654f) Propane Oxidative Dehydrogenation Catalyzed By Iodine, Bromine, and Halide Salts

Propane oxidative dehydrogenation suffers from non-selective carbon oxide formation that results from the direct oxidation of propane, propene, and propyl intermediates. Halogens are also used as soft oxidants for propane dehydrogenation, and are regenerated catalytically using halide salts and oxygen. The choice of halogen requires a trade-off between rates of breaking the C-H bond and regenerating the halide to its halogen. For example, chlorine reacts rapidly with propane in the gas-phase, however hydrogen chloride is slow to convert back to chlorine and requires a catalyst in a separate reactor. Iodine has the opposite trend, and bromine is in between. The two approaches presented here circumvent this trade-off. Specifically, (1) halide salts are used as chemical looping catalysts¹ to regenerate hydrogen halides in-situ­ and (2) combinations of iodine and bromine are used to break the C-H bond with bromine while other steps in the gas-phase radical reactions occur with iodine. The yields to propylene are among the highest reported in the literature.

When combinations of iodine and bromine were studied at low temperatures (350 °C), bromine radicals abstract H and break the C-H bond in propane to form propyl radicals that subsequently react with I₂, Br₂, or IBr to form propyl halides, which in turn, are easy to separate from alkanes and derivatize. At higher temperatures (500 °C), both iodine and bromine activate the C-H bond in propane and methane, forming propyl halides which subsequently undergo dehydrohalogenation to form propylene. Iodine-bromine mixtures significantly increased propane halogenation and dehydrohalogenation rates, compared to iodine alone. The mechanism in the gas-phase was modeled using a microkinetic model to interpret the experimental results. Mixtures of lithium iodide, lithium bromide, and lithium hydroxide were also investigated as catalysts and chemical looping agents for propane oxidative dehydrogenation with gaseous iodine and bromine as intermediates.

1 Upham, D. C., Gordon, M. J., Metiu, H. & McFarland, E. W. Halogen-Mediated Oxidative Dehydrogenation of Propane Using Iodine or Molten Lithium Iodide. Catalysis Letters 146, 744-754, doi:10.1007/s10562-016-1701-1 (2016).