Artie McFerrin Department of Chemical Engineering , Texas A&M University , College Station , Texas 77843-3122 , United States.
Department of Materials Science and Engineering , Texas A&M University , College Station , Texas 77843-3003 , United States.
ACS Appl Mater Interfaces. 2018 Nov 21;10(46):39631-39644. doi: 10.1021/acsami.8b12605. Epub 2018 Nov 12.
A recently reported modification of the zeolitic-imidazolate framework-8 (ZIF-8) with partial replacement of the 2-methylimidazolate (mIm) linker with benzimidazolate (bIm), namely ZIF-7-8, is investigated with molecular simulations using a first-time reported force field. The size of the ZIF-7-8 aperture, which governs the gas-separation efficiency of this material and which has not been estimated before for this modification, is smaller than that of the original ZIF-8. The diffusivities of CO, N, and CH estimated through transition state theory calculations result in remarkably high diffusion selectivities for CO/CH and CO/N mixtures. This performance enhancement is investigated in terms of structural flexibility in the form of the aperture motion through extensive estimation of the effective diameter, the total effective area, and the motion of the aperture linkers, of both ZIF-8 and ZIF-7-8. Both apertures exhibit an oscillation through the rotation of the linkers, which are adjusted according to the size of the penetrant molecules the moment they pass through it. Finally, a subsequent analysis reveals that there is strong dependency of the separation performance on the bIm-to-mIm ratio: below 33% bIm incorporation, the appearance of ZIF-8-alike wide apertures decreases dramatically the size-based selectivity of the mixtures in ZIF-7-8.
最近有报道称，沸石咪唑酯骨架-8（ZIF-8）进行了部分修饰，用苯并咪唑（bIm）取代部分 2-甲基咪唑（mIm）连接体，得到 ZIF-7-8。本研究使用首次报道的力场进行分子模拟，对其进行了研究。该修饰物的 ZIF-7-8 孔径大小（控制该材料的气体分离效率）以前没有进行过估算，结果表明其孔径小于原始 ZIF-8 的孔径。通过过渡态理论计算得到的 CO、N 和 CH 的扩散系数，使得 CO/CH 和 CO/N 混合物具有非常高的扩散选择性。通过对 ZIF-8 和 ZIF-7-8 的有效直径、总有效面积以及孔径连接体的运动进行广泛估计，研究了这种性能增强的结构灵活性。两个孔径都通过连接体的旋转产生振荡，当它们通过时，连接体根据渗透分子的大小进行调整。最后，进一步的分析表明，分离性能对 bIm 与 mIm 比例的依赖性很强：在 bIm 掺入量低于 33%时，ZIF-8 型宽孔径的出现极大地降低了 ZIF-7-8 中混合物的基于尺寸的选择性。
