熊果酸和咖啡酸的热降解机理及其分解动力学研究(英文)

Thermal Decomposition Mechanism and Kinetics of Ursolic Acid and Caffeic Acid

采用热重-傅里叶红外光谱联用技术（ TG-FT-IR）对杜仲中的两种天然有机酸熊果酸和咖啡酸进行了非等温热分析。结合量子化学GAMESS软件分子模拟计算、热失重和热解逸出气体红外光谱分析,对两种有机酸化学键的断裂情况进行了推断和验证。使用积分Coats-Redfern法、微分Achar法以及Malek法等3种热分析动力学方法对热重实验数据进行了分析,推断了各步分解最概然机理函数,得到相应的动力学参数---表观活化能（Ea）和指前因子（A）,并推断其贮存期。研究表明,各步失重、分子模拟推断和逸出气体红外光谱解析,这三者能够对热分解过程的化学键断裂情况进行判断；熊果酸的热分解自236.8℃至431.2℃,且一步完成,为二维扩散控制机制（圆柱形对称型）,符合Valensi方程,Ea为228.26 kJ/mol,lnA为39.56；咖啡酸在153.0℃后发生了两步分解,第一步热分解为二维扩散控制机制,符合Jander方程,Ea1为111.28 kJ/mol, lnA1为25.78,而第二步热分解为三维扩散控制机制,符合 Z.-L.-T.方程, Ea2为231.75 kJ/mol, lnA2为43.50；熊果酸和咖啡酸在室温（25℃）的贮存期分别为4-5年和3年。

Two kinds of natural organic acid, ursolic acid and caffeic acid were analyzed under the nitrogen atmosphere by thermogravimetry combined fourier transform infrared spectroscopy （ TG-FT-IR） . Experimental data of TG were computationally processed by three kinds of thermal analysis kinetics methods （ Coats-Redfern, Achar and Malek methods） , through which the most probable mechanisms and functions of each stage were inferred and the values of apparent activation energy Ea and pre-exponential factor A were also obtained. The results showed that according to percentage of weight loss, molecular simulation and infrared spectrum of evolved gases, the cracking of chemical bonds was confirmed. Thermal decomposition of ursolic acid began at 236. 8 ℃ and ended at 431. 2 ℃ with a complete mass loss while after 153. 0 ℃ caffeic acid underwent the two-stage thermal decomposition. The most probable mechanisms of ursolic acid and caffeic acid at the first stage and the second stage were two-dimension diffusion, two-dimension diffusion and three-dimension diffusion, corresponding with Valensi equation, Jander equation and Z.-L.-T. equation, respectively. Ea of ursolic acid and caffeic acid at first stage and second stage were 228. 26, 111. 28 and 231. 75 kJ/mol while their lnA were 39. 56, 25. 78 and 43.50, respectively. According to kinetic parameters, shelflives of ursolic acid and caffeic acid at 25 ℃ were 4-5 years and 3 years, respectively.