Study on pyrolysis mechanism of hexamethyldisiloxane using reactive molecular dynamics simulations

doi:10.11949/0438-1157.20220278

Abstract

Abstract:

Hexamethyldisiloxane (HMDSO) is an important precursor for the combustion synthesis of high-purity silica nanoparticles. The pyrolysis of hexamethyldisiloxane was investigated in this work by using ReaxFF reactive molecular dynamics simulations. The effects of three different reaction force fields on the simulations are evaluated and the reliability of each force field is analyzed. The most suitable force field was selected to investigate the pyrolysis products at different temperatures and pressures. The simulation results were used to reveal the pyrolysis path and mechanism of hexamethyldisiloxane together with the gas chromatography experiments. The results show that the reaction force field has important influences on the results of ReaxFF molecular dynamics simulations and the optimal reaction force field is obtained through the comparative analysis. The initial reaction step for HMDSO pyrolysis is the removal of CH₃ radical induced by Si—C bond breaking. Temperature is a major factor affecting the pyrolysis of HMDSO. The total molecular number of pyrolysis products increases with the temperature increasing and the products tended to be fragmented. The small hydrocarbon molecules (i.e., CH₃, CH₄, C₂ hydrocarbons, H₂, CH₂O, etc.) and Si-containing products (i.e., SiH₄, SiH₂, CH₄Si, etc.) appear in the middle and last stages of the pyrolysis process. The change of pressure will cause the change of the concentration of the pyrolysis system, thus affecting the probability of intermolecular collision and the occurrence of the reaction. The higher the pressure, the easier it is to form a stable pyrolysis product.

Key words: hexamethyldisiloxane, pyrolysis, reactive force field, reactive molecular dynamics simulations, gas chromatography

摘要：

六甲基二硅氧烷是燃烧合成高纯二氧化硅纳米颗粒的重要前体，采用ReaxFF分子动力学模拟方法研究其高温热解过程，讨论了三种不同反应力场对模拟的影响并分析其可靠性，选择其中最合适的力场开展不同温度与压力下的热解产物分析，结合气相色谱实验，揭示六甲基二硅氧烷的热解路径和机理。结果表明，反应力场对ReaxFF分子动力学模拟有重要影响，通过比较分析获得了最佳反应力场，六甲基二硅氧烷的初始热解反应为Si—C键断裂导致的CH₃脱离，温度升高会加剧解热反应的发生且使产物趋向于碎片化，热解的主要产物为CH₃、CH₄、C₂烃、H₂、CH₂O等小分子以及SiH₄、SiH₂、CH₄Si等含硅化合物。压力的改变会造成热解体系浓度的改变，从而影响分子间相互碰撞概率和反应的发生，压力越大则越容易形成稳定的热解产物。

关键词: 六甲基二硅氧烷, 热解, 反应力场, 反应动力学模拟, 气相色谱

CLC Number:

O 643.12

Yugong CHEN, Hao CHEN, Yaosong HUANG. Study on pyrolysis mechanism of hexamethyldisiloxane using reactive molecular dynamics simulations[J]. CIESC Journal, 2022, 73(7): 2844-2857.

陈玉弓, 陈昊, 黄耀松. 基于分子反应动力学模拟的六甲基二硅氧烷热解机理研究[J]. 化工学报, 2022, 73(7): 2844-2857.

Figures/Tables 20

Fig.1 Geometry of HMDSO molecule

Fig.2 The initial structure of HMDSO pyrolysis simulation system

Fig.3 Temperature evolutions in the initial stage of simulations at the target temperatures of 1800 K, 2000 K and 2500 K

Fig.4 Evolution of HMDSO molecular number during the heating simulation with different force fields

Fig.5 Molecular structures for force field A (a), force field B (b) and force field C (c) at 40 ps

Fig.6 Primary pyrolysis pathways for decomposition of unimolecular HMDSO

Fig.7 Evolution of the main products during the simulations with different force fields

Fig.8 Molecular structures of pyrolysis products at the end of the simulation with different force fields

Fig.9 Evolution of some main products during the simulations with different temperatures

Fig.10 Evolution of the main Si-containing products during the simulations with different temperatures

Fig.11 Molecular structures of pyrolysis products at 1800 K and 2500 K

Fig.12 Evolution of the total molecular number of the system for different pressures

Fig.13 Evolution of potential energy during the whole simulation process with different pressures (1 cal=4.18 J)

Fig.14 Evolution of some main small molecule products for different pressures

Fig.15 Evolution of the main Si-containing products during the simulations with different pressures

Fig.16 Molecular structures of pyrolysis products at different pressures

Table 1 The chemical name and structure of the preset compound in the gas chromatograph

Peak entry	Chemical name	Peak entry	Chemical name
1	methane	2	ethane
3	ethylene	4	propane
5	cyclopropane	6	propylene
7	iso-butane	8	n-butane
9	1,2-propadiene	10	1-butene
11	trans-2-butene	12	2-methylpropene
13	iso-pentane	14	cis-2-butene
15	n-pentane	16	1,3-butadiene

Fig.17 Chromatography of pyrolysis at 700℃, 800℃, 900℃

Table 2 The percentages of some major hydrocarbons in the total products obtained by ReaxFF MD simulation at different temperatures

Chemical name	Percentage/%
Chemical name	1600 K	1800 K	2000 K	2500 K
methane	3.70	49.21	48.19	19.83
acetylene	0	3.17	12.05	10.34
ethylene	3.70	7.94	6.02	8.62
ethane	3.70	0	1.20	0.86

Fig.18 Overview diagram of HMDSO pyrolysis

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