English

Gas-Phase Mechanism Study of Methane Nonoxidative Conversion by ReaxFF Method

• Liu Yuan ^1,2,3, ,
• Duan Zenghui ^2, ,
• Li Jun ^2,3, ,
• Chang Chunran ^{1, ,}

• 1.

  Shaanxi Key Laboratory of Energy Chemical Process Intensification, School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an 710049, China.

• 2.

  Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.

• 3.

  Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, Guangdong Province, China

Corresponding author: Chang Chunran, changcr@mail.xjtu.edu.cn

• Received Date: 03 November 2020
  Accepted Date: 26 November 2020
  Revised Date: 25 November 2020
  Available Online: 15 November 2021
• Fund Project:

  The project was supported by the National Natural Science Foundation of China (91645203, 22078257), the China Postdoctoral Science Foundation (2018T111034, 2018M630139), the Fundamental Research Funds for the Central Universities (xtr0218016, cxtd2017004), the Shaanxi Creative Talents Promotion Plan-Technological Innovation Team (2019TD-039), and the K. C. Wong Education Foundation

Abstract: With the rapid consumption of petrochemical resources and massive exploitation of shale gas, the use of natural gas instead of petroleum to produce chemical raw materials has attracted significant attention. While converting methane to chemicals, it has long seemed impossible to avoid its oxidation into O-containing species, followed by de-oxygenation. A breakthrough in the nonoxidative conversion of methane was reported by Guo et al. (Science 2014, 344, 616), who found that Fe©SiO₂ catalysts exhibited an outstanding performance in the conversion of methane to ethylene and aromatics. However, the reaction mechanism is still not clear owing to the complex experimental reaction conditions. One view of the reaction mechanism is that methane molecules are first activated on the Fe©SiC₂ active center to form methyl radicals, which then desorb into the gas phase to form the ethylene and aromatics. In this study, ReaxFF methods are applied to five model systems to study the gas-phase reaction mechanism under near-experimental conditions. For the pure gas-phase methyl radical system, the main simulation product is ethane after 10 ns simulation, which is produced by the combination of methyl radicals. Although a small amount of ethylene produced by C₂H₆ dehydrogenation can be detected, it is difficult to explain the high selectivity for ethylene in the experiment. When the methyl radicals are mixed with hydrogen and methane molecules, ethane remains the main product, together with some methane produced by the collision of hydrogen with methyl radicals, while ethylene is still difficult to produce. With the addition of hydrogen radicals to the methane atmosphere, methane activation can be enhanced by hydrogen radical collisions, which produce some methyl radicals and hydrogen molecules, but the methyl radicals eventually combine with the hydrogen species to produce methane molecules again. If some hydrogen molecules and methyl radicals are added to the CH₄/H∙ system, the activation of methane molecules by hydrogen radicals will be weakened. Hydrogen radicals are more likely to combine with themselves or with methyl radicals to form hydrogen and methane molecules, and the high selectivity for ethylene remains difficult to achieve. Thermal cracking of C₁₀H₁₂ at high temperature can produce hydrogen radicals and ethylene at the same time, which can partially explain the enhanced methane conversion and ethylene selectivity in the experiment of Hao et al. (ACS Catal. 2019, 9, 9045). Overall, the selective production of ethylene by nonoxidative conversion of methane over Fe©SiO₂ catalyst appears hard to achieve via a gas-phase mechanism. The catalyst surface may play a key role in the entire process of methane transformation.

Key words:
• Methane activation
•  /  Nonoxidative conversion
•  /  Gas-phase mechanism
•  /  Reactive force field
•  /  Computational simulation
•  /  Thermal cracking reaction

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