Nishikawa Yuta, Ohtsuka Yuhki, Ogihara Hitoshi, Rattanawan Rattanawalee, Gao Min, Nakayama Akira, Hasegawa Jun-Ya, Yamanaka Ichiro
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo 152-8552, Japan.
Institute for Catalysis, Hokkaido University, Hokkaido 001-0021, Japan.
ACS Omega. 2020 Oct 22;5(43):28158-28167. doi: 10.1021/acsomega.0c03827. eCollection 2020 Nov 3.
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) 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 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 catalyst is 170 kJ mol, which is much lower than that of SiO, 365 kJ mol. Temperature-programmed reactions in CH, CH, and CH and reactivity of CH for the In/SiO 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-CH coupling via Langmuir-Hinshelwood (LH) and Eley-Rideal mechanisms, and other side reactions. A qualitative level of interpretation is as follows. CH-In and H-In species form after the activation of methane. The CH-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-In species to CH by decreasing the formation of CH though the coupling of CH-In species and H-In species. Results of isotope experiments by combinations of CD, CH, D, and H corresponded to the LH mechanism.
甲烷直接转化为有价值的化学品引起了人们极大的兴趣。最近，我们报道了二氧化硅负载的液态金属铟催化剂（In/SiO）对甲烷直接脱氢转化为高级烃类是有效的。然而，液态金属铟的催化机理尚不清楚。在此，我们通过实验和计算详细展示了In/SiO催化剂的催化机理。动力学研究清楚地表明，液态金属铟激活了甲烷的C-H键并将甲烷转化为乙烷。In/SiO催化剂的表观活化能为170 kJ/mol，远低于SiO的365 kJ/mol。在CH、CH和CH中的程序升温反应以及In/SiO催化剂对CH的反应活性表明，铟在烃类中选择性地激活甲烷。此外，进行了密度泛函理论计算和第一性原理分子动力学计算，以评估甲烷活化、其逆反应、通过朗缪尔-欣谢尔伍德（LH）和埃利-里德机理的CH-CH偶联以及其他副反应的活化自由能。定性解释如下。甲烷活化后形成CH-In和H-In物种。CH-In物种在液态金属铟表面移动，并通过LH机理相互偶联形成乙烷。H物种溶解到In的体相中对于通过减少CH-In物种和H-In物种的偶联形成CH来增强CH-In物种偶联形成CH很重要。CD、CH、D和H组合的同位素实验结果与LH机理一致。
