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/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.