乙炔热解为富勒烯的分子动力学模拟研究

doi:10.6023/A23020026

摘要/Abstract

通过分子动力学模拟, 探讨了乙炔高温热解制备富勒烯的可能条件. 结果显示, 乙炔裂解可以形成富勒烯. 在裂解过程中, 乙炔热解产生的氢原子和氢分子能起到球磨碳团簇和阻止其尺寸过度增长的作用, 并促使碳笼更趋近于经典富勒烯, 暗示从炔烃制备富勒烯不需要惰性气体的协助, 从而可显著降低富勒烯的生产成本. 模拟显示, 以乙炔为反应物时, 主要有两种途径形成富勒烯. 第一种是碳链→团簇→碳笼, 多发生于低密度条件下; 第二种是碳链→团簇→碳片→碳笼, 多发生于高密度条件下. 在富勒烯形成过程中, 六元环的增长是由碳笼缺陷和键的旋转导致的. 这些结果有助于理解从炔烃热解制备富勒烯的过程, 对开发富勒烯的低成本生产方法具有重要启示意义.

关键词: 乙炔, 热解, 富勒烯, 分子动力学, 形成机理

Acetylene (C₂H₂) is one of the main intermediate species in the industrial conversion of natural gas to many high-value chemicals. In this work, molecular dynamics simulation was carried out to study the possibility of producing fullerenes by high temperature pyrolysis of acetylene. The effect of atomic and molecular hydrogen, reaction temperature and carbon density were discussed according to the results obtained from reactive force filed molecular dynamics simulation. The results show that C₂H₂ can form fullerenes at suitable temperature and density. In the early stage, hydrogen in the system is conducive to the aggregation of small carbon clusters and the formation of carbon chains is faster. Interestingly, in the later stage, it is not conducive to further aggregation reaction, but the atomic and molecular hydrogen produced by C₂H₂ pyrolysis can make the carbon clusters rounder and prevent the excessive growth of the carbon clusters, and can make the carbon cage more similar to classical fullerenes, which suggest that the production of fullerenes from C₂H₂ does not require the assistance of inert gases, thus significantly reducing the production cost of fullerenes in principle. Two main pathways of fullerenes formation were observed in simulations under different conditions. The first pathway is from carbon chain, cluster to carbon cage, most occurring in low density conditions. The other pathway is from carbon chain, cluster, graphene and carbon cage, most occurring in high density conditions. The growth of six-membered rings during fullerene formation is realized mainly via carbon cage defects and bond rotation, and the general trend is to make the carbon cage closer to the classical fullerene. The preparation of fullerenes by C₂H₂ pyrolysis is worthy of further study. These results are helpful to understand the process of producing fullerenes from the pyrolysis of C₂H₂ and have important implications for the development of industrial production methods of fullerenes.

Key words: acetylene, pyrolysis, fullerene, molecular dynamics, formation mechanism

引用此文

刘祯钰, 甘利华. 乙炔热解为富勒烯的分子动力学模拟研究[J]. 化学学报, 2023, 81(5): 502-510.

Liu Zhenyu, Gan Li-Hua. Molecular Dynamics Simulation of Acetylene Pyrolysis into Fullerenes[J]. Acta Chimica Sinica, 2023, 81(5): 502-510.

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图/表 15

图1. 3500 K 5 kg/m3条件下(a) 150 C2H2和(b) 300 C环数随时间的变化(3次平均)

Figure 1. Evolution of rings of (a) 150 C2H2 and (b) 300 C at 3500 K, 5 kg/m3 (averaged 3 times)

图2. 3500 K, 同一碳密度的不同初始输入体系中物种数的变化(3次平均) (a) 150 C2H2; (b) 300 C; (c) 300 C&150 H2; (d) 300 C&300 H; (e) 300 C&300 H2; (f) 300 C&600 H

Figure 2. Evolution of components in different initial systems with the same carbon density at 3500 K (averaged 3 times) (a) 150 C2H2; (b) 300 C; (c) 300 C&150 H2; (d) 300 C&300 H; (e) 300 C&300 H2; (f) 300 C&600 H

图3. (a) 3500 K, 5 kg/m3, 150 C2H2及300 C中C6的形成过程(该C6之后是碳笼的一部分)及(b) 3500 K, 同一碳密度的不同初始输入体系的碳分子含氢数随时间的变化(碳分子含氢数的计算方法为体系中总氢数减去H和H2数量, 已进行3次平均)

Figure 3. (a) Formation process of C6 at 3500 K, 5 kg/m3, 150 C2H2 and 300 C (C6 is part of the carbon cage) and (b) evolution of H in carbon molecules in different initial input systems with the same carbon density at 3500 K (the number of H in carbon molecules is calculated as the total hydrogen number in the system minus the amount of H and H2, the data has been averaged 3 times)

图4. 150 C2H2和300 C前1000 ps的快照

Figure 4. Snapshots of 150 C2H2 and 300 C in 1000 ps

图5. 3500 K, 同一碳密度的不同初始输入体系(a) sp2-C数随时间的变化(3次平均)及(b) 3 ns时的快照及三条轨迹的碳笼平均碳数

Figure 5. (a) Evolution of sp2-C (averaged 3 times) and (b) snapshots and the average number of carbon in the carbon cage of the three trajectories at 3 ns in different initial input systems with the same carbon density at 3500 K

System	N₅	N₆	N₅＋N₆	N_3-8	N₆/N_3-8	(N₅＋N₆)/N_3-8
150 C₂H₂	24	67	91	108	0.620	0.843
300 C	34	89	123	146	0.610	0.842
300 C&300 H	24	74	98	112	0.661	0.875
300 C&300 H₂	18	47	65	75	0.627	0.867
300 C&600 H	22	65	87	98	0.663	0.888

表1. 3 ns, 3500 K, 相同碳密度的不同体系中五元环数(N5)、六元环数(N6)、三~八元数N3-8及(N5＋N6)/N3-8 (3次平均)

Table 1. Numbers of five-membered rings (N5), six-membered rings (N6), the total numbers of three-membered rings to eight-membered rings (N3-8) and (N5＋N6)/(N3-8) in different systems with the same carbon density at 3 ns and 3500 K (averaged 3 times)

图6. 在不同温度时sp2-C数目随时间的变化(3次平均)

Figure 6. Evolution of sp2-C at different temperatures (averaged 3 times)

图7. 150 C2H2在3700 和3900 K的3 ns快照(分别选自三条轨迹中的第一条轨迹)

Figure 7. Snapshots of 150 C2H2 in 3700 and 3900 K in 3 ns (Choose from the first of the three trajectories)

密度及编号	出现时间	组成	消失时间	组成	存在时长	H/C变化
5-4	1.450	C₉₀H₅	1.542	C₁₂₂H₈	0.092	0.010
25-2	0.540	C₁₂₄H₁₇	0.640	C₁₁₂H₁₁	0.100	－0.039
25-5	0.540	C₂₀₀H₂₄	0.825	C₂₄₇H₃₂	0.285	0.010
5-5	1.300	C₁₀₈H₅	1.850	C₁₈₃H₇	0.550	－0.008
50-5	0.500	C₂₂₉H₃₉	1.260	C₂₈₃H₂₆	0.760	－0.078
50-4	0.550	C₂₃₃H₃₉	1.510	C₂₈₄H₁₇	0.960	－0.108
25-7	0.790	C₁₆₂H₂₁	1.800	C₂₆₂H₁₄	1.010	－0.076
25-3	0.580	C₁₅₀H₂₂	1.680	C₂₇₇H₁₉	1.100	－0.078
50-2	0.620	C₂₀₄H₃₈	2.050	C₂₈₅H₁₈	1.430	－0.123
25-0	0.750	C₂₂₉H₃₆	2.190	C₂₈₃H₁₉	1.440	－0.090
25-4	0.617	C₁₆₂H₂₄	2.065	C₂₇₉H₁₇	1.448	－0.087
50-9	0.540	C₂₁₂H₄₃	1.990	C₂₈₃H₂₄	1.450	－0.118
25-8	0.700	C₁₉₉H₂₉	5.000	C₂₉₅H₁₄	4.300	－0.098

表2. 密度为5、25和50 kg/m3, 150 C2H2, 3500 K条件下出现石墨烯的轨迹统计结果a

Table 2. Statistics on the trajectory of graphene at densities of 5, 25 and 50 kg/m3, 150 C2H2, 3500 K

图8. 密度为5、25和50 kg/m3, 150 C2H2, 3500 K条件下的(a) C2H和(b) C2H2随时间的变化(10次平均)

Figure 8. Evolution of (a) C2H and (b) C2H2 at 3500 K, 150 C2H2 with the density of 5, 25 and 50 kg/m3 (averaged 10 times)

图9. 密度为(a) 5、(b) 25和(c) 50 kg/m3, 150 C2H2, 3500 K条件下500 ps内数量排前十的反应(10次平均)

Figure 9. Number of the top ten reactions within 500 ps at 3500 K, 150 C2H2 with density of (a) 5, (b) 25 and (c) 50 kg/m3 (averaged 10 times)

图10. 缺陷引起的六元环变化快照来自3300 K, 150 C2H2, 5 kg/m3中3条轨迹的第1条. (a)缺陷收缩或扩张, (b)碳链的吸附或脱落

Figure 10. Defects induced the change of six-membered rings Snapshots from the first trajectory of the three trajectories in 3300 K, 150 C2H2, 5 kg/m3. (a) defect shrinkage or expansion, (b) adsorption or detachment of carbon chains

图11. 键旋转引起的六元环变化快照来自3300 K, 150 C2H2, 5 kg/m3中3条轨迹的第1条

Figure 11. Bond rotation induced the change of six-membered rings Snapshots from the first trajectory of the three trajectories in 3300 K, 150 C2H2, 5 kg/m3

Time/ps	δN₆	缺口收缩/扩大	碳链	键旋转
2520~2521	5	5
2521~2522	－2	－2
2523~2524	－1	－1
2524~2525	2	2
2526~2527	－2	－2
2527~2528	2	2
2528~2529	1		1
2529~2530	－1	－1
2530~2531	1	2		－1
2531~2532	0
2532~2533	1	1
2534~2535	－1	－2		1
2535~2536	3	2		1
2536~2537	－2		－2
2537~2538	1
2538~2539	1		2
2539~2540	－1			－1
2540~2541	3			3
总计	10	6	1	3

表3. 3300 K, 150 C2H2, 5 kg/m3中3条轨迹的第1条中2520~2541 ps对六元环环数变化(δN6)及其来源

Table 3. Source of changes in 6-membered ring number (δN6) during 2520~2541 ps from the first trajectory of the three trajectories in 3300 K, 150 C2H2, 5 kg/m3

图12. 3500 K, 150 C2H2, 5 kg/m3成笼过程快照及其代表性反应

Figure 12. Snapshots of cage forming process at 3300 K, 150 C2H2, 5 kg/m3 and representative reactions

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