Crystal Size Dependent Flexibility in ZIF-7: From Macro- to Nanoscale

Flexible metal–organic frameworks (MOFs) are highly
desirable
materials for gas separation, but most of them become rigid when the
particle size is reduced toward nanoscale. We aim to comprehend the
effect of textural properties such as crystal size, its distribution,
and morphology on the gate-opening behavior stimulated by the adsorption
of guest molecules in ZIF-7. The synthesis conditions are varied to
obtain ZIF-7 batches with crystal sizes ranging between 0.05 and 15
μm with various size distributions. We report for the first
time a CO2-filled open pore phase of ZIF-7 at 195 K (OP2)
derived from in situ powder X-ray diffraction (PXRD)
data measured in parallel to CO2 physisorption. The adsorption
of CO2 on ZIF-7 indicates persisting flexibility for all
particle size regimes; with the crystal size, its distribution, and
morphology having a significant impact on both gate-opening and gate-closing
pressures and slope of CO2 adsorption isotherms. In situ PXRD measurement indicated further expansion of
the ZIF-7 framework in the presence of methanol as guest species.
The capability of ZIF-7 to accommodate molecules larger than its 0.3
nm window diameter signifies the importance of intermolecular interactions
to overcome the energy barrier for linker movement/gating of the framework.

柔性金属有机框架（metal–organic frameworks，MOFs）是气体分离领域极具应用前景的材料，但绝大多数该类材料在粒径被缩减至纳米级时会丧失柔性，转变为刚性结构。本研究旨在探究晶体尺寸、尺寸分布及形貌等织构性质，对客体分子吸附诱导沸石咪唑酯骨架-7（Zeolitic Imidazolate Framework-7，ZIF-7）发生开门行为的影响。通过调控合成参数，我们制备得到了一系列ZIF-7批次样品，其晶体尺寸分布于0.05至15 μm区间，且具备各异的尺寸分布特征。本研究首次通过与CO₂物理吸附同步采集的原位粉末X射线衍射（in situ powder X-ray diffraction，PXRD）数据，在195 K下观测到了ZIF-7被CO₂填充的开放孔相（OP2）。CO₂在ZIF-7上的吸附实验表明，所有粒径区间的ZIF-7均保持柔性；晶体尺寸、尺寸分布及形貌对开门压力、关门压力以及CO₂吸附等温线的斜率均存在显著影响。原位PXRD测试结果显示，当以甲醇作为客体物种时，ZIF-7的骨架会发生进一步膨胀。ZIF-7能够容纳尺寸大于其0.3 nm窗口直径的分子，这一特性印证了分子间相互作用在克服配体运动/骨架门控所需能垒过程中的重要作用。