基于ReaxFF的甲烷无氧转化气相机理研究

doi:10.3866/PKU.WHXB202011012

物理化学学报 >> 2021, Vol. 37 >> Issue (11): 2011012.doi: 10.3866/PKU.WHXB202011012

所属专题：能源与材料化学

基于ReaxFF的甲烷无氧转化气相机理研究

刘源¹^,²^,³, 段增晖², 李隽²^,³, 常春然¹^,*()

¹ 西安交通大学化学工程与技术学院，陕西省能源化工过程重点实验室，西安 710049
² 清华大学化学系，有机光电子与分子工程教育部重点实验室，北京 100084
³ 南方科技大学化学系，广东深圳 518055

收稿日期:2020-11-03 录用日期:2020-11-26 发布日期:2020-12-03
通讯作者: 常春然 E-mail:changcr@mail.xjtu.edu.cn
基金资助:
国家自然科学基金(91645203);国家自然科学基金(22078257);中国博士后科学基金(2018T111034);中国博士后科学基金(2018M630139);中央高校基本科研业务费(xtr0218016);中央高校基本科研业务费(cxtd2017004);陕西省科技创新团队支持计划(2019TD-039);王宽诚教育基金会资助项目

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

Yuan Liu¹^,²^,³, Zenghui Duan², Jun Li²^,³, Chunran Chang¹^,*()

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

Received:2020-11-03 Accepted:2020-11-26 Published:2020-12-03
Contact: Chunran Chang E-mail:changcr@mail.xjtu.edu.cn
About author:Chunran Chang, Email: changcr@mail.xjtu.edu.cn
Supported by:
the National Natural Science Foundation of China(91645203);the National Natural Science Foundation of China(22078257);the China Postdoctoral Science Foundation(2018T111034);the China Postdoctoral Science Foundation(2018M630139);the Fundamental Research Funds for the Central Universities(xtr0218016);the Fundamental Research Funds for the Central Universities(cxtd2017004);the Shaanxi Creative Talents Promotion Plan-Technological Innovation Team(2019TD-039);the K. C. Wong Education Foundation

摘要/Abstract

摘要：

晶格限域的Fe©SiO₂催化剂在甲烷无氧直接转化生成乙烯的反应中表现出优异的性能。但由于反应条件苛刻，对该反应的分子机理研究一直存在较大的挑战。本文采用反应力场的方法模拟近反应条件下甲烷无氧直接转化气相机理，发现当气相只有甲基自由基存在时，很难产生高选择性乙烯产物。当在气相中加入氢自由基时，虽能在一定程度上增强甲烷的活化，但同样较难生成乙烯。高温下热裂解C₁₀H₁₂分子能同时产生氢自由基和乙烯分子，能合理地解释实验中加入C₁₀H₁₂分子可以在一定程度上提高乙烯选择性和甲烷转化率的现象。总之，甲烷无氧直接转化高选择性生成乙烯很难通过单纯的气相反应机理来实现，进而推断催化剂表面在甲烷活化和转化的整个过程中起着至关重要的作用。

关键词: 甲烷活化, 无氧转化, 气相机理, 反应力场, 计算模拟, 热裂解反应

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

MSC2000:

O643

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刘源, 段增晖, 李隽, 常春然. 基于ReaxFF的甲烷无氧转化气相机理研究[J]. 物理化学学报, 2021, 37(11): 2011012.

Yuan Liu, Zenghui Duan, Jun Li, Chunran Chang. Gas-Phase Mechanism Study of Methane Nonoxidative Conversion by ReaxFF Method[J]. Acta Phys. -Chim. Sin., 2021, 37(11): 2011012.

图/表 5

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Elementary step	ΔE_CCSD(T)/ΔE_ReaxFF (eV)	E_aCCSD(T)/E_aReaxFF (eV)
2CH₃∙ → C₂H₆	−1.25/−0.85	N/A
CH₃∙ + H∙ → CH₄	−2.45/−2.43	N/A
CH₃∙ + H₂ → CH₄ + H∙	0.48/0.39	2.04/2.07
C₂H₆ + H∙ → C₂H₅∙ + H₂	−0.82/−0.81	1.54/1.82
C₂H₆ + CH₃∙ → C₂H₅∙ + CH₄	−0.35/−0.42	2.09/2.40
C₂H₅∙ → C₂H₄ + H∙	0.36/0.51	1.77/2.03

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基于ReaxFF的甲烷无氧转化气相机理研究

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

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