丁醇醛和丁醇酸热解形成CO和CO_2机理的密度泛函理论研究

Density functional theory studies on the formation mechanism of CO and CO_2 in pyrolysis of hydroxyl butyraldehyde and butyric acid

采用密度泛函理论方法B3LYP/cc-pVTZ,对模型化合物2,3,4-羟基丁醛的脱羰基和2,3,4-羟基丁酸的脱羧基反应机理进行了量子化学理论研究。对两种模型化合物分别设计了三种热解反应途径,计算了不同温度下各热解反应途径的标准热力学及动力学参数。计算结果表明,纤维素热解过程中CO2和CO的逸出分别与脱羧基和脱羰基反应相对应,脱羧基和脱羰基反应均为分子内氢原子转移的协同过程。脱羰基反应是吸热反应,而脱羧基反应是放热反应。饱和丁醇醛的脱羰基反应反应能垒为288.8 kJ/mol,脱水后的不饱和烯醇醛的脱羰基反应能垒增大;饱和丁醇酸的脱羧基反应能垒较高,为303.4 kJ/mol,脱水后的不饱和烯醇酸的脱羧基反应能垒明显减小,这说明脱水有利于CO2的生成。

The pyrolysis of 2,3,4-hydroxyl-butyraldehyde and 2,3,4-hydroxyl-butyric acid as model compounds was investigated by using B3LYP/cc-pVTZ methods to reveal the decarbonyl and decarboxyl mechanism.Three possible pathways for the pyrolysis of each model compound were designed and the standard thermodynamic and kinetic parameters of each reaction path at different temperatures were determined.The results showed that the release of CO and CO2 during the cellulose pyrolysis is related to the decarbonyl and decarboxyl reactions,respectively;both involve a concerted process via intra-molecular hydrogen transfer.Decarboxyl reaction is endothermic while decarbonyl reaction is exothermic.The activation energy of decarbonyl reaction of 2,3,4-hydroxyl-butyraldehyde is 288.8 kJ/mol,while the activation energy of decarbonyl reaction of undersaturated olefine aldehyde after dehydration is higher than that for saturated aldehyde.The activation energy of decarboxyl reaction of 2,3,4-hydroxyl-butyric acid is 303.4 kJ/mol,while the activation energy of decarboxyl reaction of undersaturated olefine acid after dehydration is much lower,indicating that the dehydration favors the release of CO2.