CaO催化PE热解及H2O对催化过程影响的ReaxFF MD研究与机理分析

doi:10.11949/0438-1157.20210244

摘要/Abstract

摘要：

结合反应分子动力学（ReaxFF MD）模拟方法和自行开发的全自动ReaxFF反应机理分析软件（automatic reaction mechanism analyzer， AutoRMA）深入探究了CaO催化聚乙烯（PE）热解及H₂O对催化过程的影响，通过热解产物分析、热解过程反应路径追踪和C―C键及C―H键时序变化分析来揭示反应机理。结果显示，CaO作为催化剂提高了PE的热解反应速率，有效降低了PE热解过程中C―C键和C―H键断裂的活化能，使其分别从316.88 kJ/mol和430.13 kJ/mol降低到22.24 kJ/mol和30.87 kJ/mol，并促进了PE向轻质油和气体分子的转化；CaO对自由基和碳链上的非饱和碳原子的吸附及解离可以有效促进PE的热解；但H₂O存在时由于其自身与CaO结合会抑制CaO的催化作用，同时H₂O主要通过取代反应以羟基的形式存在于烃类产品中，水分含量（质量分数）为PE的50%时导致10%的油品中含有氧元素，降低产品质量，也会导致液相产品中存在大量的水分，因此应避免热解的废PE携带水分。研究表明，ReaxFF MD结合AutoRMA有助于对PE等聚合物催化热解机理的深入理解，进而优化反应体系。

关键词: 热解, 聚乙烯, 催化, ReaxFF, AutoRMA, 动力学

Abstract:

CaO-catalyzed polyethylene (PE) pyrolysis and influence of H₂O on this process was explored by using ReaxFF MD simulation and an automatic reaction mechanism analysis software (AutoRMA), the pyrolysis products were tracked to find the reaction paths and the changes of C―C bond and C―H bond numbers were analyzed. The results showed that CaO promoted the pyrolysis of PE and effectively reduced the activation energies of the C―C bond and C―H bond cleavages from 316.88 kJ/mol and 430.13 kJ/mol to 22.24 kJ/mol and 30.87 kJ/mol respectively, and promoted the conversion of PE to light oil and gas molecules. CaO adsorption and dissociation of free radicals and unsaturated carbon atoms on the carbon chain can effectively promote the heat of PE solution. But presence of H₂O will inhibit its catalysis due to the combination of H₂O with CaO. At the same time, H₂O mainly exists in hydrocarbon products in form of hydroxyl through substitution reaction, resulting in 10% (mass) of oil containing oxygen elements when the initial mass of H₂O is 50%(mass) of PE, and the residue water is mixed with the liquid products, degrading the oil quality. Therefore, appearance of H₂O in the pyrolysis system should be avoided. Studies have shown that ReaxFF MD combined with AutoRMA helps to establish an in-depth understanding of the catalytic pyrolysis mechanism of PE and other polymers, and then optimize the reaction system.

Key words: pyrolysis, polyethylene, catalytic, ReaxFF, AutoRMA, kinetics

中图分类号:

TQ 325.1

贺兴处,陈德珍,梅振飞,阿迪力·巴吐尔null,安青. CaO催化PE热解及H₂O对催化过程影响的ReaxFF MD研究与机理分析[J]. 化工学报, 2021, 72(9): 4665-4674.

Xingchu HE,Dezhen CHEN,Zhenfei MEI,Batuer ADILI,Qing AN. ReaxFF MD study on the pyrolysis of PE catalyzed by CaO and the effect of H₂O on the catalytic process and mechanism analysis[J]. CIESC Journal, 2021, 72(9): 4665-4674.

图/表 13

表1 模型体系分子构成

Table 1 Molecular composition of model system

体系	PE(C₃₀₀H₆₀₂)单链/条	CaO超晶胞(Ca₅₄₅O₅₄₄)/个	H₂O分子/个
PE	8	—	—
PE-H₂O	8	—	900
PE-CaO	8	1	—
PE-CaO-H₂O	8	1	900

图1 PE-CaO-H2O体系初始构象

Fig.1 Initial snapshot of PE-CaO-H2O model system

图2 AutoRMA分析策略流程（以PE热解模拟为例）

Fig.2 Process of analysis strategy of AutoRMA

图3 分子信息探测深度优先搜索算法实现流程

Fig.3 Process of depth first search algorithm for molecular information detection

图4 化学反应搜索深度优先算法实现流程

Fig.4 Process of depth first search algorithm for chemical reaction search

图5 3000 K时热解产物分布

Fig.5 Distribution of products of PE pyrolysis at 3000 K

图6 3000 K时热解气体分子分布

Fig.6 Distribution of gas molecules from PE pyrolysis (3000 K, 250 ps)

图7 不同温度下热解过程整个体系C―C键和C―H键的数量变化及动力学计算

Fig.7 The number change of C―C bond and C―H bond in the whole pyrolysis system at different temperatures and kinetics calculation

图8 不同温度下热解过程炭产物中C―C键和C―H键的数量变化及动力学计算

Fig.8 The number change of C―C bond and C―H bond in char product during pyrolysis at different temperatures and kinetics calculation

图9 分子片段断裂过程中CaO的吸附和解离过程样例

Fig.9 Adsorption and dissociation of CaO in the process of fracture of molecular fragments

图10 CaO和H2O作用下PE热解过程H2的形成机理

Fig.10 H2 formation mechanism during pyrolysis of PE-CaO-H2O system

图11 体系中H2O的主要反应路径

Fig.11 Main reaction path of H2O in the system

图12 H2O中O和H在产物中的分布

Fig.12 Distribution of O and H of H2O in the products

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CaO催化PE热解及H₂O对催化过程影响的ReaxFF MD研究与机理分析

ReaxFF MD study on the pyrolysis of PE catalyzed by CaO and the effect of H₂O on the catalytic process and mechanism analysis

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