Balyan Sonit, Saini Shikha, Khan Tuhin S, Pant K K, Gupta Puneet, Bhattacharya Saswata, Haider M Ali
Renewable Energy and Chemicals Lab, Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India.
Nanoscale. 2021 Feb 28;13(8):4451-4466. doi: 10.1039/d0nr07044k. Epub 2021 Jan 6.
C-H bond activation steps in non-oxidative methane dehydroaromatization (MDA), constitute a key functionalization of the reactant and adsorbed species to form aromatics. Previous studies have focused on studying the energetics of these steps at the most stable active sites involving molybdenum carbide species. Herein, a different paradigm is presented via studying the reactivity of a metastable molybdenum carbide (MoC) nanocluster for the C-H bond activation of methane, ethane, and ethylene and comparing it with the reactivity of the lowest energy MoC nanocluster. Interestingly, the metastable nanocluster is observed to result in a consistent reduction (by half) in the C-H bond activation barrier of the respective alkane and alkene molecules compared to the global minimum isomer. This specific metastable form of the nanocluster is identified from a cascade genetic algorithm search, which facilitated a rigorous scan of the potential energy surface. We attribute this significant lowering of the C-H bond activation barrier to unique co-planar orbital overlap between the reactant molecule and active centers on the metastable nanocluster. Based on geometrical and orbital analysis of the transition states arising during the C-H bond activation of methane, ethane, and ethylene, a proton-coupled electron transfer mechanism is proposed that facilitated C-H bond cleavage. Motivated by the high reactivity for C-H bond activation observed on the metastable species, a contrasting framework to analyze the elementary-step rate contributions is presented. This is based on the statistical ensemble analysis of nanocluster isomers, where the calculated rates on respective isomers are normalized with respect to the Boltzmann probability distribution. From this framework, the metastable isomer is observed to provide significant contributions to the ensemble average representations of the rate constants calculated for C-H bond activation during the MDA reaction.
非氧化甲烷脱氢芳构化（MDA）中的C-H键活化步骤，是反应物和吸附物种形成芳烃的关键官能化过程。先前的研究集中于在涉及碳化钼物种的最稳定活性位点上研究这些步骤的能量学。在此，通过研究亚稳态碳化钼（MoC）纳米团簇对甲烷、乙烷和乙烯的C-H键活化反应性，并将其与最低能量MoC纳米团簇的反应性进行比较，提出了一种不同的范式。有趣的是，与全局最小异构体相比，观察到亚稳态纳米团簇会使相应烷烃和烯烃分子的C-H键活化势垒一致降低（减半）。这种纳米团簇的特定亚稳态形式是通过级联遗传算法搜索确定的，该算法有助于对势能面进行严格扫描。我们将C-H键活化势垒的显著降低归因于反应物分子与亚稳态纳米团簇上活性中心之间独特的共平面轨道重叠。基于甲烷、乙烷和乙烯C-H键活化过程中产生的过渡态的几何和轨道分析，提出了一种促进C-H键断裂的质子耦合电子转移机制。受亚稳态物种上观察到的高C-H键活化反应性的启发，提出了一个对比框架来分析基元步骤的速率贡献。这是基于纳米团簇异构体的统计系综分析，其中各异构体上计算的速率相对于玻尔兹曼概率分布进行归一化。从这个框架中可以观察到，亚稳态异构体对MDA反应中C-H键活化计算的速率常数的系综平均表示有显著贡献。
