University College London, Department of Chemistry, Materials Chemistry, Third Floor, Kathleen Lonsdale Building, Gower Street, London WC1E 6BT, UK.
Phys Chem Chem Phys. 2013 Nov 21;15(43):19024-30. doi: 10.1039/c3cp52653d.
The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore 'breathing' assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.
采用经典分子动力学模拟研究了长度为 n-C8 至 n-C20 的正构烷烃在 silicalite 沸石中的扩散。使用刚性沸石骨架的统一原子烃模型、刚性沸石骨架的全原子烃模型和柔性沸石骨架的全原子烃模型进行了不同的模拟，所有模拟均在 300 K 下进行。后两种模型以前从未用于模拟 silicalite 中的更长链烷烃。使用刚性沸石骨架测量的扩散系数在 [010] 方向上表现出对链长的周期性依赖性，与先前观察到的共振扩散现象一致，而与所使用的烃模型无关。从 silicalite 结构内低能陷阱的位置来解释扩散的阻碍，提出了扩散势垒。然而，在用全原子烃模型和柔性骨架时，观察到扩散系数与链长呈单调依赖性，这归因于孔“呼吸”辅助扩散的发生。还注意到，当使用后一种模型时，计算出的扩散系数低了一个数量级，并且实验扩散系数更接近。
