Ari Meral U, Ahunbay M Göktuğ, Yurtsever Mine, Erdem-Senatalar Ayşe
Institute of Informatics, Chemistry Department, Istanbul Technical University, Maslak, Istanbul, Turkey.
J Phys Chem B. 2009 Jun 11;113(23):8073-9. doi: 10.1021/jp901986s.
Molecular dynamics simulation using COMPASS force field has been employed to understand the dynamics of water diffusion and structuring in silicalite-1 and Na-ZSM-5 (Si/Al = 95 and 191) samples at three different temperatures, 297, 354, and 393 K, at a water loading of 8 molecules per unit cell, in canonical ensemble. Diffusion coefficients were significantly reduced upon the introduction of aluminum atoms into the framework, together with charge balancing cations placed in their vicinity, since the ion-dipole interactions dominant in ZSM-5 samples are stronger than the H-bond interactions in silicalite-1. The activation energy of diffusion increased with decreasing Si/Al ratio. In the silicalite-1 and ZSM-5 samples, straight channels were observed to be preferred than the sinusoidal ones and the channel preference was not observed to be a strong function of either temperature or the Si/Al ratio. The ordered structures of the water molecules, forming clusters in the channels of silicalite-1 at low temperature was observed to be broken to some extent by increased temperatures, and decreased Si/Al ratio, resulting in less ordered structures. The positions of the water molecules in the straight and sinusoidal channels for the ZSM-5 samples were mainly determined by the location of the charge compensating cation(s) in the structure, as was shown by the concentration profiles.
已采用使用COMPASS力场的分子动力学模拟，来理解在297、354和393 K这三个不同温度下，在正则系综中，每单位晶胞含8个水分子的条件下，水在硅沸石-1和Na-ZSM-5（硅铝比分别为95和191）样品中的扩散和结构动力学。将铝原子引入骨架并在其附近放置电荷平衡阳离子后，扩散系数显著降低，因为ZSM-5样品中占主导的离子-偶极相互作用比硅沸石-1中的氢键相互作用更强。扩散的活化能随硅铝比的降低而增加。在硅沸石-1和ZSM-5样品中，观察到直通道比正弦通道更受青睐，且通道偏好未被观察到是温度或硅铝比的强函数。在低温下，观察到在硅沸石-1通道中形成簇的水分子有序结构在一定程度上因温度升高和硅铝比降低而被破坏，导致结构有序性降低。如浓度分布所示，ZSM-5样品中直通道和正弦通道内水分子的位置主要由结构中电荷补偿阳离子的位置决定。
