Study on pyrolysis mechanism and kinetics of two of spiro [4,5] decane and spiro [5,6] dodecane
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摘要: 采用B3LYP/6-311++G(d,p)和反应性分子动力学方法，对螺[4,5]-癸烷（C10H18）和螺[5,6]-十二烷（C10H22）的热解机理进行研究，揭示不同碳环结构和尺寸效应对燃料初始分解反应活性及小分子和芳烃产物生成行为的影响。结果表明，两种螺环烷烃燃料初始分解路径相似，均通过单分子碳碳键解离发生开环异构反应和小分子自由基进攻燃料母体的氢提取反应而消耗。相较于C10H18，C12H22中分子张力更大的七元环使速控步碳碳键及碳氢键能更低，导致燃料呈现出更低的初始分解温度和更高的反应活性。两种螺环燃料初始分解产生的自由基进一步影响了C1−C7小分子烃类和环烯产物的生成。其中，乙烯的生成始终占据主导地位。由于螺环尺寸效应的影响，链烃和环烯烃的生成表现出明显的结构差异性。对于C10H18分解而言，可能生成大量的五元环烯产物，包括环戊二烯、环戊烯、富烯、甲基环戊二烯和甲基环戊烯；而C12H22中更大的七元环结构，将生成对应的七元环烯产物（环庚烯、亚甲基环庚烷）。Abstract: Active cooling with high-energy-density liquid hydrocarbon fuel is one of promising techniques for the thermal protection of hypersonic aircrafts. To extend the flight range and increase the payload of volume-limited aerospace vehicles, high-energy-density liquid hydrocarbon fuel is directly used as an energetic additive or liquid propellant. Among them, biomass derived spiro-fuels, shown high density, low freezing point and high net heat of combustion due to compact molecular structures, are a kind of significant high-density fuel. The investigation of thermal pyrolysis of these spiro-fuels not only significantly improves the heat sink through the endothermic reaction, but also brings the new challenges of ignition and combustion of the cracked hydrocarbon fuel. The detailed theoretical calculations and molecular dynamics simulations of spiro [4,5] decane (C10H18) and spiro [5,6] dodecane (C12H22) initial pyrolysis were performed to explore the ring-size effect on the consumption of fuels and formation of primary products. The results show that the initial decomposition paths of the two spiro-fuels are very similar, mainly consumed by the open-ring isomerization via the unimolecular C−C dissociations of the six membered ring structure and H-abstractions by small radicals attacking the fuel parents. Due to the lower C−C and C−H bond dissociation energies caused by large tension of the seven membered ring structure, C12H22 may exhibit the lower initial decomposition temperature and higher reaction activity than C10H18. The differences of simultaneously formed fuel radicals during C10H18 and C12H22 initial pyrolysis further affect the formation pathways of C1−C7 small products, cycloalkenes and monocyclic aromatics. Among them, ethylene are the most important products. Due to the presence of inherent six membered rings in two spiro-fuels, monocyclic aromatics mainly originate from multi-step dehydrogenation reactions of fuel radicals, involving benzene, toluene, styrene and ethylbenzene. Notably, the size effect of spiro-ring in two fuels leads the obvious structural differences of the formation of chain hydrocarbons and cycloalkenes. For C10H18, a large number of penta- cycloalkenes may be generated, including cyclopentadiene, cyclopentene, fulvene, methylcyclopentadiene and methylcyclopentene, whereas the seven-membered ring structure of C12H22 may produce corresponding seven-membered products (cyclohexene and methylene cycloheptane). Moreover, the pyrolysis behaviors of these two spiro-fuels at 2000 K based on the ReaxFF molecular dynamics simulation were explored and show well consistent with the main products derived from DFT theoretical calculations. This work performs the DFT theoretical calculations and ReaxFF molecular dynamics simulation on the pyrolysis kinetic mechanisms of two representative high-density biomass fuels of spiro fuels, providing a possible initial pyrolysis path and laying a theoretical foundation for their practical application in engines. However, the complex working environment of the cooling channel poses more challenges to the actual pyrolysis process of the new high-density hydrocarbon fuels. In future research, pyrolysis experiments will be conducted under high temperature and high pressure conditions, and the detailed pyrolysis kinetics models with excellent predictive performance over a wide operating range will be constructed. At the same time, research will be conducted on the subsequent ignition and combustion process of the engine combustion chamber, exploring the impact mechanism of catalysts and fuel additives on this process, assisting in the practical application of fuel, improving fuel combustion efficiency, and effectively controlling pollutant emissions.
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图 2 螺环烷烃单分子C−C键解离及其双自由基分子内歧化反应
Figure 2 The unimolecular C−C bond scissions of two spiro-fuels and corresponding disproportionation reactions of diradicals
图 3 螺环烷烃单分子C−H键解离能
Figure 3 The unimolecular C−H bond scissions of two spiro-fuels (kJ/mol)
图 4 两种螺环烷烃在2000 K下的初始分解速
Figure 4 The fuel decomposition of two spiro-fuels of C10H18 and C12H22 at 2000 K
图 5 H/CH3自由基进攻两种螺环烷烃燃料的氢提取反应势能面
Figure 5 The potential energy surface of H-abstraction reactions of two spiro-fuels by H and CH3 (transition state: italic; kJ/mol)
图 6 螺环烷烃自由基的β−C−C键解离能
Figure 6 The energetics of various initial β-scission channels (transition state: italic; kJ/mol)
图 7 R1的分解路径势能面
Figure 7 The potential energy surface of decomposition of R1 radical from C10H18 (transition state: italic; kJ/mol)
图 8 （a）R2和（b）R3的分解路径势能面
Figure 8 The potential energy surface of decomposition of (a) R2 and (b) R3 radicals from C10H18 (transition state: italic; kJ/mol.)
图 9 （a）R4和（b）R5的分解路径势能面
Figure 9 The potential energy surface of decomposition of (a) R4 and (b) R4 radicals from C10H18 (transition state: italic; kJ/mol.)
图 10 （a）R1'和（b）R2'的分解路径势能面
Figure 10 The potential energy surface of decomposition of (a) R1' and (b) R2' radicals from C12H22 (transition state: italic; kJ/mol.)
图 11 （a）R3'和（b）R4'的分解路径势能面
Figure 11 The potential energy surface of decomposition of (a) R3' and (b) R4' radicals from C12H22 (transition state: italic; kJ/mol.)
图 12 （a）R5'和（b）R6'的分解路径势能面
Figure 12 The potential energy surface of decomposition of (a) R5' and (b) R6' radicals from C12H22 (transition state: italic; kJ/mol.)
图 13 螺环烷烃可能的热解反应路径
Figure 13 Possible primary mechanism for the spiro-fuels decomposition
图 14 两种螺环烷烃在2000 K下主要热解产物分布
Figure 14 Typical time-dependent evolution of main product fragments during two spiro-fuels pyrolysis at 2000 K using reactive MD simulations
表 1 联环与螺环烷烃燃料的结构及理化性质[7]
Table 1 Structure and physicochemical properties of bicyclic and spiro fuels
结构 分子式 密度（g/cm3，20 ℃） 冰点/℃ QHHV/（MJ·kg−1） 粘度（mm2/s，25℃） 
C10H18 0.866 −38.0 42.4 1.62 
C12H22 0.886 1.20 42.5 3.72 
C10H18 0.870 −76.0 42.7 2.12 
C12H22 0.893 −51.0 43.0 4.37 
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表 2 利用B3LYP/6-311++G（d, p）和高精度CBS-QB3理论计算获得的C10H18和C12H22的几何结构参数
Table 2 Comparisons of calculated geometries for C10H18 and C12H22 at B3LYP and CBS-QB3 levels
Geometrical
parameterQC NBD B3LYP/6-311++g(d,p) CBS-QB3 B3LYP/6-311++g(d,p) CBS-QB3 r(C1-C2)/Å 1.549 1.550 1.555 1.555 r(C2-C3)/Å 1.533 1.533 1.535 1.535 r(C3-C4)/Å 1.547 1.547 1.534 1.534 r(C4-C5)/Å 1.545 1.545 1.534 1.533 r(C5-C6)/Å 1.533 1.533 1.538 1.538 r(C6-C1)/Å 1.560 1.560 1.554 1.553 r(C1-C7)/Å 1.548 1.547 1.554 1.554 r(C7-C8)/Å 1.541 1.541 1.541 1.541 r(C8-C9)/Å 1.556 1.557 1.540 1.540 r(C9-C10)/Å 1.550 1.550 1.542 1.542 r(C10-C1)/Å 1.549 1.549 − − r(C10-C11)/Å − − 1.538 1.538 r(C11-C12)/Å − − 1.540 1.541 r(C12-C1)/Å − − 1.551 1.551 θ(C2-C1-C6)/(°) 109.9 109.9 107.9 108.0 θ(C10-C1-C7)/(°) 101.0 100.9 − − θ(C12-C1-C7)/(°) − − 111.9 111.9 θ(C7-C1-C2-C6)/(°) 123.6 123.7 118.2 118.2 θ(C10-C1-C2-C6)/(°) 123.8 123.8 − − θ(C12-C1-C2-C6)/(°) − − 120.8 120.9
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表 3 利用B3LYP/6-311++G（d, p）和CBS-QB3理论计算获得的C10H18分解的键解离能
Table 3 Dissociation enthalpies of C10H18 to free radicals by C−C and C−H bond scissions at B3LYP and CBS-QB3 levels
Reaction △H of C−C bond disociations (kJ·mol−1) Reaction △H of C−H bond disociations (kJ·mol−1) B3LYP/6-311++g(d,p) CBS-QB3 difference B3LYP/6-311++g(d,p) CBS-QB3 difference C10H18→BR1 324 328 −4 C10H18→R1 388 390 −2 C10H18→BR2 326 329 −3 C10H18→R2 391 393 −2 C10H18→BR3 282 285 −3 C10H18→R3 391 393 −2 C10H18→BR4 282 285 −3 C10H18→R4 389 391 −2 C10H18→BR5 319 322 −3 C10H18→R5 390 392 −2 C10H18→BR6 322 326 −4
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