(630a) Catalytic Methane Decomposition in Molten KCl-MnCl2 for the Co-Production of Hydrogen and Separable Graphitic Carbon

The co-production of molecular hydrogen and separable graphitic carbon from methane was achieved in bubble columns containing molten KCl:MnCl₂ mixtures as catalysts. Methane was decomposed to hydrogen by contact with the catalytic molten KCl:MnCl₂ mixtures at 700 - 1050 °C. The solid co-product, graphitic carbon, was accumulated on the surface of molten KCl:MnCl₂ melt. While the apparent activation energy of pure KCl and MnCl₂ was up to 300 and 175 kJ/mole respectively, it was minimized to 150 kJ/mole in a 50:50 mol % mixture of KCl:MnCl₂ where the relative concentration of tetrahedral MnCl₄^2- to octahedral MnCl₆^4- was maximized in the melt. There seems to be a synergistic effect between the KCl and MnCl₂ to provide catalytic activity for methane pyrolysis by forming ion complexes in the melt mixture. Hydrogen selectivity of the molten KCl:MnCl₂ mixture approached to 99 % at 1050 °C in comparison to pure molten KCl where it was 90%. The hydrocarbon byproducts suggested that the mechanism in pure molten KCl was predominately gas phase proceeding through methyl radicals. H-D exchange between methane and deuterium showed the highest concentration of CH₂D₂, in molten KCl:MnCl₂, whereas in the pure KCl melt CH₃D was the predominant intermediate. Solid carbon product from the KCl:MnCl₂ melt was highly graphitic in comparison to amorphous structure of carbon product from the pure KCl melt. Taken together, the high hydrogen selectivity, greater deuterium substitution of methane, and the graphitic carbon product from the molten KCl:MnCl₂ mixture, suggest the active site for C-H bond activation facilitates multiple dehydrogenation steps of methane.