Forman Evan M, Baniani Amineh, Fan Lei, Ziegler Kirk J, Zhou Erkang, Zhang Fengyi, Lively Ryan P, Vasenkov Sergey
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA.
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA.
J Memb Sci. 2020 Jan 1;593. doi: 10.1016/j.memsci.2019.117440. Epub 2019 Sep 3.
Self-diffusivities of ethane were measured by multinuclear pulsed field gradient (PFG) NMR inside zeolitic imidazolate framework-11 (ZIF-11) crystals dispersed in several selected polymers to form mixed-matrix membranes (MMMs). These diffusivities were compared with the corresponding intracrystalline self-diffusivities in ZIF-11 crystal beds. It was observed that the confinement of ZIF-11 crystals in ZIF-11 / Torlon MMM can lead to a decrease in the ethane intracrystalline self-diffusivity. Such diffusivity decrease was observed at different temperatures used in this work. PFG NMR measurements of the temperature dependence of the intracrystalline self-diffusivity of ethylene in the same ZIF-11 / Torlon MMM revealed similar diffusivity decrease as well as an increase in the diffusion activation energy in comparison to those in unconfined ZIF-11 crystals in a crystal bed. These observations for ethane and ethylene were attributed to the reduction of the flexibility of the ZIF-11 framework due to the confinement in Torlon leading to a smaller effective aperture size of ZIF-11 crystals. Surprisingly, the intra-ZIF diffusion selectivity for ethane and ethylene was not changed appreciably by the confinement of ZIF-11 crystals in Torlon in comparison to the selectivity in a bed of ZIF-11 crystals. No ZIF-11 confinement effects leading to a reduction in the intracrystalline self-diffusivity of ethane and ethylene were observed for the other two studied MMM systems: ZIF-11 / Matrimid and ZIF-11 / 6FDA-DAM. The absence of the confinement effect in the latter MMMs can be related to the lower values of the polymer bulk modulus in these MMMs in comparison to that in ZIF-11 / Torlon MMM. In addition, there may be a contribution from possible differences in the ZIF-11/polymer adhesion in different MMM types.
通过多核脉冲场梯度（PFG）核磁共振法测量了分散在几种选定聚合物中以形成混合基质膜（MMM）的沸石咪唑酯骨架-11（ZIF-11）晶体内部乙烷的自扩散系数。将这些扩散系数与ZIF-11晶体床中相应的晶体内自扩散系数进行了比较。观察到ZIF-11晶体在ZIF-11/Torlon混合基质膜中的受限会导致乙烷晶体内自扩散系数降低。在本工作中使用的不同温度下均观察到了这种扩散系数降低现象。对同一ZIF-11/Torlon混合基质膜中乙烯晶体内自扩散系数的温度依赖性进行的PFG NMR测量显示，与晶体床中未受限的ZIF-11晶体相比，扩散系数也有类似降低以及扩散活化能增加。对乙烷和乙烯的这些观察结果归因于由于在Torlon中的受限导致ZIF-11骨架柔性降低，从而使ZIF-11晶体的有效孔径变小。令人惊讶的是，与ZIF-11晶体床中的选择性相比，ZIF-11晶体在Torlon中的受限对乙烷和乙烯的ZIF内扩散选择性没有明显改变。对于其他两个研究的混合基质膜体系：ZIF-11/Matrimid和ZIF-11/6FDA-DAM，未观察到导致乙烷和乙烯晶体内自扩散系数降低的ZIF-11受限效应。后一种混合基质膜中不存在受限效应可能与这些混合基质膜中聚合物体积模量的值低于ZIF-11/Torlon混合基质膜中的值有关。此外，不同类型混合基质膜中ZIF-11/聚合物粘附力的可能差异也可能起了作用。
