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

[Image: see text] There is a great interest in direct conversion of methane to valuable chemicals. Recently, we reported that silica-supported liquid-metal indium catalysts (In/SiO(2)) 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/SiO(2) 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/SiO(2) catalyst is 170 kJ mol(–1), which is much lower than that of SiO(2), 365 kJ mol(–1). Temperature-programmed reactions in CH(4), C(2)H(6), and C(2)H(4) and reactivity of C(2)H(6) for the In/SiO(2) 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, CH(3)–CH(3) coupling via Langmuir–Hinshelwood (LH) and Eley–Rideal mechanisms, and other side reactions. A qualitative level of interpretation is as follows. CH(3)–In and H–In species form after the activation of methane. The CH(3)–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 CH(3)–In species to C(2)H(6) by decreasing the formation of CH(4) though the coupling of CH(3)–In species and H–In species. Results of isotope experiments by combinations of CD(4), CH(4), D(2), and H(2) corresponded to the LH mechanism.