Methane pyrolysis and carbon formation mechanisms in molten manganese chloride mixtures

Molten manganese chloride (MnCl₂) is attractive as a high-temperature liquid catalyst for methane pyrolysis (CH₄ → C + 2H₂). Nevertheless, the mechanism of this reaction for continuously producing high-purity hydrogen and graphitic carbon from CH₄ has not been fully elucidated. In this work, we investigated the reaction kinetics and mechanism of CH₄ pyrolysis in mixtures of molten MnCl₂ with various monovalent or divalent chlorides. The apparent activation energies of the molten MnCl₂ mixtures for CH₄ pyrolysis (<230 kJ mol⁻¹) were much lower than that of the uncatalyzed reaction (400 kJ mol⁻¹). Among the MnCl₂ mixtures, only MnCl₂–KCl showed a lower apparent activation energy than pure MnCl₂ (152 and 172 kJ mol⁻¹, respectively). In addition, MnCl₂–KCl produced the largest amount of CH₂* and the final solid carbon product with the highest crystallinity, suggesting that this system has unique CH₄ dehydrogenation and carbon formation pathways. Density functional theory calculations also predicted high concentrations of CH₂* in MnCl₂–KCl, as confirmed by CH₂ formation being more thermodynamically favorable in MnCl₂–KCl than in MnCl₂. Furthermore, the generation of C₂₊ unsaturated intermediates from CH_x* (x < 3) was thermodynamically favored, suggesting a more facile pathway for the formation of graphitic carbon layers in molten MnCl₂–KCl. In addition, an NVT ab initio molecular dynamics simulation suggested that gas-phase C–C coupling was facilitated in MnCl₂–KCl by the frequent reversible desorption and adsorption of CH_x* intermediates. These fundamental insights into the origins of the enhanced CH₄ pyrolysis performance in MnCl₂–KCl could aid in the development of molten salt catalysts with enhanced reactivity.