Tang Yu-Bo, Xie Shi-Jie
Center for Membrane Separation and Water Science & Technology, Zhejiang University of Technology, Hangzhou 310014, P. R. China.
Phys Chem Chem Phys. 2022 Feb 23;24(8):5220-5232. doi: 10.1039/d1cp05571b.
A classical atomistic simulation study is reported for the microscopic structure and dynamics of a water/methanol mixture confined in flexible nanoporous zeolitic imidazolate framework ZIF-8. Both the radial density distribution and vivid two-dimensional density profile demonstrate that methanol molecules can roughly be viewed as "embedded" between two layers of water molecules to form a "sandwich" structure. The reason for the formation of such a specific structure is explained based on the hydrogen-bonding state and the strength of various hydrogen bonds. The investigation of guest molecular diffusion shows that the self-diffusion coefficient of confined water is generally one to two orders of magnitude smaller than that of bulk water. In addition, the dependence of the self-diffusion coefficient on loading is non-monotonic: the self-diffusion coefficient firstly shows a significant increase and then decreases at higher loading. Moreover, both the structure and dynamics of the hydrogen bond (HB) network of confined water molecules are investigated in a spatially resolved manner. The results indicate that both the HB structure and dynamics of water molecules near the ZIF-8 surface deviate significantly from those of bulk water. However, while water molecules located at the pore center are relatively similar to bulk water molecules with respect to the HB structure, they exhibit strong slowdown in HB dynamics when compared with bulk water. This simulation study elucidates in detail the structural and dynamical properties of a water/methanol mixture in nanoscopic ZIF-8 confinement, which is expected to provide a deep insight into the role of porous fillers, such as ZIF-8, in improving the performance of the dehydration of alcohols pervaporation and other related processes.
报道了一项经典的原子模拟研究，该研究针对限制在柔性纳米多孔沸石咪唑酯骨架ZIF-8中的水/甲醇混合物的微观结构和动力学。径向密度分布和生动的二维密度剖面均表明，甲醇分子大致可视为“嵌入”在两层水分子之间，形成“三明治”结构。基于氢键状态和各种氢键的强度解释了形成这种特定结构的原因。客体分子扩散的研究表明，受限水的自扩散系数通常比本体水的自扩散系数小一到两个数量级。此外，自扩散系数对负载的依赖性是非单调的：自扩散系数首先显著增加，然后在较高负载下降低。此外，还以空间分辨的方式研究了受限水分子氢键（HB）网络的结构和动力学。结果表明，ZIF-8表面附近水分子的HB结构和动力学均与本体水有显著偏差。然而，虽然位于孔中心的水分子在HB结构方面与本体水分子相对相似，但与本体水相比，它们在HB动力学方面表现出强烈的减慢。该模拟研究详细阐明了纳米级ZIF-8限制下水/甲醇混合物的结构和动力学性质，有望深入了解多孔填料（如ZIF-8）在提高醇类渗透汽化脱水和其他相关过程性能中的作用。
