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

Abstract

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.