正癸烷与二甲苯在超临界压力下的热裂解

采用连续流动装置对正癸烷和二甲苯在超临界压力下的热裂解对比研究. 用气相色谱和色质联用仪对其气相产物和液相产物进行分析, 计算气相产物产率和裂解转化率, 并运用计算化学方法获得正癸烷和二甲苯不同化学键的键能, 从实验和理论上分析其裂解反应的难易程度和裂解规律. 实验结果表明, 在4 MPa和650、700、750 ℃条件下, 正癸烷比二甲苯更容易裂解, 正癸烷裂解产物以C₁-C₃小分子的烃类和氢气为主, 而二甲苯裂解产物主要为乙苯、甲苯和其它芳香类化合物; 键能计算结果表明, 正癸烷碳链骨架的C－C键能和C－H键能均较小, 裂解反应的诱发步骤应该是C－C键断裂, 而二甲苯苯环上C－C和C－H键能均较大, 裂解诱发步骤应该是侧链甲基脱氢反应. 因此正癸烷裂解反应以C－C键断裂和脱氢反应为主, 二甲苯裂解主要发生侧链甲基C－C键断裂和脱氢反应, 而芳环则比较稳定, 理论计算键能分析与裂解实验结果一致.

Pyrolysis of n-Decane and Dimethylbenzene under Supercritical Pressure

The pyrolysis of n-decane and dimethylbenzene under supercritical pressure was studied using a continuous flow reactor. Samples were heated to a temperature of 650, 700, or 750 ℃ under a pressure of 4 MPa without oxygen. n-Decane pyrolyzed more easily than dimethylbenzene. We analyzed gaseous products by online gas chromatography, and liquid products by gas chromatography-mass spectrometry, allowing us to calculate the cracking gas yield and cracking conversion of these systems. A quantum chemistry computation was used to evaluate the binding energies of C－C and C－H bonds in n-decane and dimethylbenzene. Both experimental and theoretical results were also used to analyze the cracking reactivity of these species. Analysis of the components in the products indicated that the main products of n-decane were C₁-C₃ hydrocarbons and hydrogen, whereas ethylbenzene, toluene and other aromatic compounds were the main products of dimethylbezene after pyrolysis. Binding energy calculations showed that both C－C and C－H bonds in n-decane possessed lower binding energies than those in dimethylbezene, and a C－C bond was the weakest. In dimethylbenzene, a C－H bond in the methyl groups was the weakest, and its binding energy was much smaller than those of the C－C and CH bonds in the benzene ring. Therefore, the main reactions in the cracking process of n-decane are breakage of a C－C bond and dehydrogenation. However, the cracking process in dimethylbenzene mainly involves the fracture and dehydrogenation of methyl groups. The theoretical calculations reasonably explained the experimental phenomena.