Oak Ridge Associated Universities, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6164, USA.
J Am Chem Soc. 2013 Apr 24;135(16):6130-41. doi: 10.1021/ja3121845. Epub 2013 Apr 15.
Polyethylene is an emerging precursor material for the production of carbon fibers. Its sulfonated derivative yields ordered carbon when pyrolyzed under inert atmosphere. Here, we investigate its pyrolysis pathways by selecting n-heptane-4-sulfonic acid (H4S) as a model compound. Density functional theory and transition state theory were used to determine the rate constants of pyrolysis for H4S from 300 to 1000 K. Multiple reaction channels from two different mechanisms were explored: (1) internal five-centered elimination (Ei5) and (2) radical chain reaction. The pyrolysis of H4S was simulated with kinetic Monte Carlo (kMC) to obtain thermogravimetric (TGA) plots that compared favorably to experiment. We observed that at temperatures <550 K, the radical mechanism was dominant and yielded the trans-alkene, whereas cis-alkene was formed at higher temperatures from the internal elimination. The maximum rates of % mass loss became independent of initial ȮH radical concentration at 440-480 K. Experimentally, the maximum % mass loss occurred from 440 to 460 K (heating rate dependent). Activation energies derived from the kMC-simulated TGAs of H4S (26-29 kcal/mol) agreed with experiment for sulfonated polyethylene (~31 kcal/mol). The simulations revealed that in this region, decomposition of radical HOSȮ2 became competitive to α-H abstraction by HOSȮ2, making ȮH the carrying radical for the reaction chain. The maximum rate of % mass loss for internal elimination was observed at temperatures >600 K. Low-scale carbonization utilizes temperatures <620 K; thus, internal elimination will not be competitive. E(i)5 elimination has been studied for sulfoxides and sulfones, but this represents the first study of internal elimination in sulfonic acids.
聚乙烯是一种新兴的碳纤维前驱体材料。其磺化衍生物在惰性气氛下热解时会生成有序碳。在这里，我们选择正庚烷-4-磺酸（H4S）作为模型化合物来研究其热解途径。我们使用密度泛函理论和过渡态理论来确定从 300 到 1000 K 下 H4S 的热解速率常数。从两种不同的机制探索了多个反应通道：（1）内部五中心消除（Ei5）和（2）自由基链式反应。我们用动力学蒙特卡罗（kMC）模拟了 H4S 的热解，得到了与实验相当的热重（TGA）图谱。我们观察到，在温度<550 K 时，自由基机制占主导地位，生成反式烯烃，而在较高温度下，内部消除生成顺式烯烃。在 440-480 K 时，最大质量损失率变得与初始ȮH 自由基浓度无关。实验中，最大质量损失率发生在 440 到 460 K（加热速率相关）。从 H4S 的 kMC 模拟 TGA 得出的活化能（26-29 kcal/mol）与磺化聚乙烯的实验值（~31 kcal/mol）相符。模拟表明，在该区域，自由基 HOSȮ2 的分解变得与 HOSȮ2 的α-H 夺取竞争，使ȮH 成为反应链的携带自由基。内部消除的最大质量损失率出现在温度>600 K 时。低温碳化利用温度<620 K；因此，内部消除不会具有竞争性。E(i)5 消除已在亚砜和砜中进行了研究，但这是首次在磺酸中研究内部消除。
