Catalytic Mechanism of Liquid-Metal Indium for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons

There is a great interest in direct conversion of methane to valuable chemicals. Recently, we reported that silica-supported liquid-metal indium catalysts (In/SiO₂) were effective for direct dehydrogenative conversion of methane to higher hydrocarbons. However, the catalytic mechanism of liquid-metal indium has not been clear. Here, we show the catalytic mechanism of the In/SiO₂ catalyst in terms of both experiments and calculations in detail. Kinetic studies clearly show that liquid-metal indium activates a C-H bond of methane and converts methane to ethane. The apparent activation energy of the In/SiO₂ catalyst is 170 kJ mol^-1, which is much lower than that of SiO₂, 365 kJ mol^-1. Temperature-programmed reactions in CH₄, C₂H₆, and C₂H₄ and reactivity of C₂H₆ for the In/SiO₂ catalyst indicate that indium selectively activates methane among hydrocarbons. In addition, density functional theory calculations and first-principles molecular dynamics calculations were performed to evaluate activation free energy for methane activation, its reverse reaction, CH₃-CH₃ coupling via Langmuir-Hinshelwood (LH) and Eley-Rideal mechanisms, and other side reactions. A qualitative level of interpretation is as follows. CH₃-In and H-In species form after the activation of methane. The CH₃-In species wander on liquid-metal indium surfaces and couple each other with ethane via the LH mechanism. The solubility of H species into the bulk phase of In is important to enhance the coupling of CH₃-In species to C₂H₆ by decreasing the formation of CH₄ though the coupling of CH₃-In species and H-In species. Results of isotope experiments by combinations of CD₄, CH₄, D₂, and H₂ corresponded to the LH mechanism.