The Influence of Intrinsic Framework Flexibility on Adsorption in Nanoporous Materials.
The Influence of Intrinsic Framework Flexibility on Adsorption in Nanoporous Materials.
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DOI：
10.1021/jacs.7b01688
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发表时间：
2017-04-19
影响因子：
15
通讯作者：
Smit B
中科院分区：
文献类型：
--
作者：
Witman M;Ling S;Jawahery S;Boyd PG;Haranczyk M;Slater B;Smit B
对于金属有机骨架(MOF)的应用，如气体储存和分离，柔性通常被视为可以调整材料性能的参数。在这项工作中，我们的目标是确定相似大小的分子(例如Xe/Kr混合物)的形状选择性分离的最佳灵活性。为了系统地了解灵活性如何影响这种类型的分离，我们开发了一个简单的分析模型，该模型预测了材料的亨利区域吸附和选择性作为灵活性的函数。我们阐明了选择性对骨架固有柔韧性的复杂依赖，即根据材料的孔径特征，随着柔韧性的增加，性能得到改善或降低。然而，在刚性近似下，具有最大选择性的孔径和化学成分的材料的选择性随着灵活性的增加而不断降低，这表明全局最优分离存在于完全刚性的孔内。分子模拟表明，我们的简单模型预测了在筛选柔性MOF的吸附行为时观察到的性能趋势。这些灵活的模拟与高性能材料中的实验吸附数据提供了更好的一致性，当将该框架建模为刚性时，该数据不会被捕获，这是在高通量筛选研究中通常进行的近似。我们的结论是，对于形状选择性吸附应用，全局最优的材料将具有最优的孔径/化学和最小的固有柔度，尽管其他非最优材料的选择性实际上可以通过柔韧性来提高。同样重要的是，我们发现灵活的模拟对于正确模拟这些类型的系统中的吸附是至关重要的。
For applications of metal–organic frameworks (MOFs) such as gas storage and separation, flexibility is often seen as a parameter that can tune material performance. In this work we aim to determine the optimal flexibility for the shape selective separation of similarly sized molecules (e.g., Xe/Kr mixtures). To obtain systematic insight into how the flexibility impacts this type of separation, we develop a simple analytical model that predicts a material’s Henry regime adsorption and selectivity as a function of flexibility. We elucidate the complex dependence of selectivity on a framework’s intrinsic flexibility whereby performance is either improved or reduced with increasing flexibility, depending on the material’s pore size characteristics. However, the selectivity of a material with the pore size and chemistry that already maximizes selectivity in the rigid approximation is continuously diminished with increasing flexibility, demonstrating that the globally optimal separation exists within an entirely rigid pore. Molecular simulations show that our simple model predicts performance trends that are observed when screening the adsorption behavior of flexible MOFs. These flexible simulations provide better agreement with experimental adsorption data in a high-performance material that is not captured when modeling this framework as rigid, an approximation typically made in high-throughput screening studies. We conclude that, for shape selective adsorption applications, the globally optimal material will have the optimal pore size/chemistry and minimal intrinsic flexibility even though other nonoptimal materials’ selectivity can actually be improved by flexibility. Equally important, we find that flexible simulations can be critical for correctly modeling adsorption in these types of systems.