The Influence of Intrinsic Framework Flexibility on Adsorption in Nanoporous Materials

Abstract

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.

Flexible Zirconium MOFs as Bromine‐Nanocontainers for Bromination Reactions under Ambient Conditions

“…Meanwhile, Smit et al. also studied the predicted flexibility of MOFs based on their crystal topologies . Accordingly, a high degree of flexibility is expected if Zr‐MOFs adopt bcu , flu , or scu topologies.…”

Extension of the QuickFF force field protocol for an improved accuracy of structural, vibrational, mechanical and thermal properties of metal–organic frameworks

“…This is especially true for some newer generation materials such as metal–organic frameworks (MOFs), which are hybrid materials consisting of inorganic building blocks connected by organic linkers . For such materials, force fields are derived with the aim to reproduce various properties such as equilibrium structure, vibrational density of states, thermal expansion, bulk modulus as well as adsorption and diffusion of guest molecules in the pores of the material. An overview of the advances that has been made in this topic is given in refs.…”

Implementing Metal-Organic Frameworks for Natural Gas Storage

“…With computational work predicting the effect of natural gas impurities, it is critical for experimental work to continue because of its paramount importance in identifying limitations for real-world applications. Fundamental understanding missed by some computational studies for real-world applications is needed, such as identifying extra-adsorption sites, phase change processes, MOF structural phase changes, and structure flexibility [119] during adsorption, which have been partially explored experimentally.…”