ReaxFF-MD揭示木质素热解反应机制的分子动力学研究

doi:10.11949/0438-1157.20240794

摘要/Abstract

摘要：

通过ReaxFF-MD反应分子动力学模拟，深入探讨了木质素热解产物的演化规律及氧元素的迁移机制，重点分析了木质素的热解反应机理。结果表明，2000 K是木质素热解不同反应机制的转折点，低温度条件下一次分解反应起主要作用，主要涉及大分子网络结构的分解及弱桥键的断裂。并且温度的升高有助于氧元素向生物油和热解气迁移。高温度条件下木质素热解过程既存在一次分解反应也包含热解产物的二次反应。反应温度越高，一次反应的时间越短。二次反应过程中绝大部分生物油通过重组或缩聚转化为焦炭，仅有小部分生物油分解生成热解气。高温条件下，氧元素主要存在于热解气中，H₂O、CO₂、CH₂O等含氧气体的产生与木质素结构中含有丰富的羟基、羧基、羰基官能团有关。

关键词: 木质素, 热解, 反应分子动力学, 热解机理

Abstract:

The evolution of lignin pyrolysis products and the migration mechanism of oxygen were discussed in depth by ReaxFF-MD simulation, and the pyrolysis reaction mechanism of lignin was emphatically analyzed. The results showed that 2000 K was the turning point of different pyrolysis mechanisms of lignin, and the primary decomposition reaction was dominant at low temperatures, which principally involved the decomposition of lignin macromolecular network structure and the cleavage of weak bridge bonds. Additionally, an increase in temperature facilitated the migration of oxygen into bio-oil and pyrolysis gas. In the higher temperature range, the lignin pyrolysis process included both the initial decomposition reaction and the subsequent secondary reaction of the pyrolysis products at high temperatures. The higher the reaction temperature, the shorter the time of the primary reaction. During the secondary reaction, most of the bio-oil is converted into coke through recombination or polycondensation, and only a small part of the bio-oil is decomposed to generate pyrolysis gas. Oxygen mainly existed in the pyrolysis gas at high temperatures. The production of oxygen-containing gases such as H₂O, CO₂, and CH₂O were related to the abundant hydroxyl, carboxyl, and carbonyl in the lignin structure.

Key words: lignin, pyrolysis, ReaxFF molecular dynamics, pyrolysis mechanism

中图分类号:

TK 6

徐芳, 张锐, 崔达, 王擎. ReaxFF-MD揭示木质素热解反应机制的分子动力学研究[J]. 化工学报, 2025, 76(3): 1253-1263.

Fang XU, Rui ZHANG, Da CUI, Qing WANG. Study of pyrolysis reaction mechanism of lignin revealed by ReaxFF-MD simulation[J]. CIESC Journal, 2025, 76(3): 1253-1263.

图/表 18

图1 最优Adler木质素三维分子模型

Fig.1 Optimal 3D molecular model of Adler lignin

图2 木质素大分子结构体系

Fig.2 Lignin macromolecular structure system

图3 木质素在热解过程中的产物分布情况

Fig.3 Distribution of lignin pyrolysis products during the thermal decomposition process

图4 焦炭分子数量及其平均分子量变化规律[1 u=(1.6605402±0.0000010)×10-27 kg]

Fig. 4 Variation rule of molecular number and average molecular weight of char

图5 重质生物油和轻质生物油质量分数的演化趋势

Fig.5 Evolution trend of weight of heavy bio-oil and light bio-oil

图6 重质生物油和轻质生物油氢碳原子比（H/C）演化趋势

Fig.6 Evolution trend of H/C of heavy bio-oil and light bio-oil

图7 重质生物油和轻质生物油氧碳原子比（O/C）演化趋势

Fig.7 Evolution trend of O/C of heavy bio-oil and light bio-oil

图8 氧元素在热解产物中的分布

Fig.8 Distribution of oxygen element in pyrolysis products

图9 1200~2000 K温度范围内热解产物中C—O键演化趋势

Fig.9 Evolution trend of C—O of pyrolysis products in the temperature range of 1200—2000 K

图10 主要含氧气体分子数量的演化趋势

Fig.10 Evolution trend of the number of main oxygen-containing gas

图11 CH2O的产生路径

Fig.11 Formation pathways of CH2O

图12 生物油中含氧化合物分子数量的演化趋势

Fig.12 Evolution trend of the number of oxygen-containing compounds in bio-oil

图13 VMD软件得到的不同热解温度的最终产物构型

Fig.13 The final product configurations of different pyrolysis temperatures obtained by VMD

图14 1200~1800 K范围内热解产物随时间的变化趋势

Fig.14 Evolution trend of pyrolysis products with time in the range of 1200—1800 K

表1 低温条件下木质素一次分解反应情况

Table 1 Degree of primary decomposition reaction of lignin at low temperatures

温度/K	一次反应率/%	生物油产率/%	热解气产率/%
1200	1.77	1.23	0.54
1400	30.22	25.74	4.48
1600	68.31	60.45	7.86
1800	92.18	79.52	12.66

图15 2200~2800 K范围内热解产物随时间的变化规律

Fig.15 Variation rule of pyrolysis products with time in the range of 2200—2800 K

表2 高温条件下木质素一次分解反应情况

Table 2 Degree of primary decomposition reaction of lignin at high temperatures

温度/K	12.5 ps一次反应率/%	25 ps一次反应率/%	一次反应大约时间/ps
2200	67.76	82.48	200
2400	88.34	92.34	50
2600	91.06	97.40	25
2800	95.88	98.77	25

表3 高温条件下生物油二次反应情况

Table 3 Degree of secondary reaction of bio-oil at high temperatures

温度/K	二次反应率/%	焦炭转化率/%	热解气产率/%
2200	12.34	78.77	21.23
2400	21.99	68.34	31.66
2600	37.50	62.36	37.64
2800	69.54	69.87	30.13

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