Fakultät für Physik and Center for Computational Materials Science, Universität Wien, Sensengasse 8, Wien 1090, Austria.
J Chem Phys. 2009 Dec 7;131(21):214508. doi: 10.1063/1.3265715.
The dehydrogenation of propane over acidic chabazite has been studied using ab initio density-functional simulations in combination with static transition-state searches and dynamic transition path sampling (TPS) methods at elevated temperatures. The acidic zeolite has been modeled both using a small cluster and a large periodic model consisting of two unit cells, the TPS simulations allow to account for the effect of temperature and entropy. In agreement with experimental observations we find propene as the dominant reaction product and that the barrier for the dehydrogenation of a methyl group is higher than that for a methylene group. However, whereas all studies based on small cluster models (including the present one) conclude that the reaction proceeds via the formation of an alkoxy intermediate, our TPS studies based on a large periodic model lead to the conclusion that propene formation occurs via the formation of various forms of propyl cations stabilized by entropy, while the formation of an alkoxy species is a relatively rare event. It was observed only in 15% of the reactive trajectories for methyl dehydrogenation and even in only 8% of the methylene dehydrogenation reactions. Our studies demonstrate the importance of entropic effects and the need to account for the structure and flexibility of the zeolitic framework by using large periodic models.
在高温下，使用从头算密度泛函模拟结合静态过渡态搜索和动态过渡路径采样 (TPS) 方法研究了丙烷在酸性 chabazite 上的脱氢反应。使用小簇和由两个单元组成的大周期性模型对酸性沸石进行了建模，TPS 模拟允许考虑温度和熵的影响。与实验观察一致，我们发现丙烯是主要的反应产物，并且甲基脱氢的势垒高于亚甲基脱氢的势垒。然而，尽管所有基于小簇模型的研究（包括本研究）都得出结论认为反应是通过形成烷氧基中间体进行的，但我们基于大周期性模型的 TPS 研究得出的结论是，丙烯的形成是通过熵稳定的各种形式的丙基阳离子的形成发生的，而形成烷氧基物种是一个相对罕见的事件。仅在 15%的甲基脱氢反应和甚至仅在 8%的亚甲基脱氢反应的反应轨迹中观察到。我们的研究表明了熵效应的重要性，以及通过使用大周期性模型考虑沸石骨架的结构和灵活性的必要性。
