Molecular Sieving Properties of Nanoporous Mixed-Linker ZIF-62: Associated Structural Changes upon Gas Adsorption Application

The evaluation of the flexibility in zeolitic imidazolate frameworks (ZIFs) has been very useful to understand their performance in gas adsorption and separation applications. Here, we have evaluated the adsorption properties of a nanoporous mixed-linker ZIF-62 using a combination of gas adsorption measurements, grand canonical Monte Carlo simulations, and synchrotron X-ray powder diffraction under operando conditions. While adsorption studies in nanoporous ZIF-62 at 77 K and atmospheric pressure predict a large O2/N2 separation ability, computational studies anticipate that the observed differences must be attributed to kinetic restrictions of N2 to access the internal porosity at cryogenic temperatures. Interestingly, upon a small increase in the adsorption temperature (90 K vs 77 K), both N2 and O2 are able to access the inner porous structure through the promotion of a phase transition (ca. 3.8% volume expansion) upon gas adsorption. This narrow phase (np) to expanded phase (ep) structural transition in ZIF-62 is completely suppressed above 150 K. Based on the excellent molecular sieve properties of nanoporous ZIF-62 for O2/N2 at cryogenic temperatures, we extended our study to the adsorption of linear and branched hydrocarbons. This study predicts the preferential adsorption of alkanes over alkenes in ZIF-62 for small hydrocarbons (C2), while in the case of C3 hydrocarbons and above, the adsorption process is mainly defined by kinetic restrictions.

评估沸石咪唑酯骨架材料（zeolitic imidazolate frameworks, ZIFs）的柔性，对于理解其在气体吸附与分离应用中的性能极具实用价值。本研究结合气体吸附测试、巨正则蒙特卡洛（grand canonical Monte Carlo）模拟以及原位同步辐射X射线粉末衍射技术，对混合配体纳米多孔ZIF-62的吸附性能展开了评估。此前在77 K、常压条件下针对纳米多孔ZIF-62开展的吸附研究曾预测其具备优异的O₂/N₂分离能力，但计算研究表明，实验观测到的差异应归因于低温下N₂难以进入内部孔道的动力学限制。有趣的是，当吸附温度从77 K小幅提升至90 K时，N₂与O₂可通过气体吸附诱导的结构相变（体积膨胀约3.8%）进入内部多孔结构。ZIF-62中窄相（narrow phase, np）向膨胀相（expanded phase, ep）的结构相变在150 K以上会被完全抑制。基于纳米多孔ZIF-62在低温下对O₂/N₂的优异分子筛分性能，本研究进一步拓展至直链与支链烃类的吸附研究。该研究预测，对于C₂小分子烃类，ZIF-62对烷烃的吸附优先性高于烯烃；而对于C₃及以上烃类，吸附过程主要由动力学限制决定。