含有典型C—C煤炭模型化合物热解过程的自由基调控机理研究

Construction of a free radical regulatory mechanism for pyrolysis process of coal model compounds containing typical C—C

煤本身是一种复杂的非均质混合物,含有大量的致密环状芳香烃。针对煤结构中各种C—C化学键,采用联苄、二苯甲烷、联苯作为煤C—C结构的模型化合物,分别在600℃,650℃,700℃,750℃下通过Py-GC/MS探究其热解产物分布情况;通过添加供氢溶剂(hydrogen donor solvent,HDS)捕获中间自由基验证其反应路径的存在;利用Gaussian09,Shermo,选取M06-2X泛函、def2-TZVP基组,加上D03(0)色散校正计算化学键解离焓(BDE)。通过实验与模拟相结合的方式印证自由基路径的存在。同时,用Py-GC/MS进行不同温度的模型化合物的热解实验。结果表明:模型化合物的热解均为自由基路径;由于C—C键类型不同,模型化合物的热解程度不同。各个键按能垒由大到小排序依次为C_(ar)—C_(ar),C_(ar)—C_(al),C_(al)—C_(al),因此,热解程度由大到小的化合物依次为联苄、二苯甲烷、联苯。供氢溶剂可能会降低断键能垒;模型化合物热解中间自由基如苯基自由基和苯甲基自由基等可以被供氢溶剂提供的氢自由基稳定;温度不同,断键比率发生变化,说明产生自由基比率发生改变。在700℃和750℃下,联苄两条反应路径的断链速率比分别为4.783×10^(8)和1.503×10^(8),证明温度会改变化学键的断键速率,但不改变主要热解路径。

Coal itself is a complex inhomogeneous mixture containing a substantial amount of condensed polycyclic aromatic hydrocarbons.Bibenzyl,diphenylmethane,and biphenyl were used as model compounds for simulating the C—C structures in coal.Pyrolysis gas chromatography/mass spectrometry(Py-GC/MS)was utilized to investigate the distribution of their pyrolysis products at temperatures of 600℃,650℃,700℃,and 750℃.The existence of reaction pathways was verified by capturing the intermediate radicals by adding hydrogen-donating solvents.Meanwhile,the bond dissociation energies(BDE)were calculated through using Gaussian09 and Shermo software,employing the M06-2X generalized method and def2-TZVP basis group,together with the D03(0)dispersion correction.Hence,the existence of free radical paths is corroborated by a combination of experimental and computational approach.The results show that the pyrolysis of the model compounds follows a free radical pathway,and the pyrolysis degree depends on the type of C—C bond.The energy barriers for the different bonds from large to small follow the order of C_(ar)—C_(ar),C_(ar)—C_(al),C_(al)—C_(al),hence the pyrolysis degree from large to small is in the order of bibenzyl,diphenylmethane,biphenyl.Hydrogen-supplying solvents are found to potentially reduce bond dissociation energy.Intermediate free radicals,such as phenyl radicals and phenylmethyl radicals,produced during the pyrolysis of the model compounds,can be stabilized by hydrogen free radicals provided by the solvent.The variation in bond cleavage ratios at different temperatures indicates a change in the generation rate of free radicals.At 700℃and 750℃,the ratio of bond cleavage for the two pathways of bibenzyl are determined to be 4.783×10^(8) and 1.503×10^(8),respectively,demonstrating that temperature can alter the bond dissociation rate while maintaining the primary pyrolysis paths.