title: 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.
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California , Berkeley, California 94720, United States.
authors.forename: Matthew
authors.identifier: 0000-0001-6263-5114
authors.initials: M
authors.lastname: Witman
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, United Kingdom.
authors.forename: Sanliang
authors.identifier: 0000-0003-1574-7476
authors.initials: S
authors.lastname: Ling
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California , Berkeley, California 94720, United States.
authors.forename: Sudi
authors.identifier: 0000-0001-9950-6199
authors.initials: S
authors.lastname: Jawahery
authors.affiliation: Laboratory of Molecular Simulation, Institut des Sciences et Ingénierie Chimiques, Valais, Ecole Polytechnique Fédérale de Lausanne (EPFL) , Rue de l'Industrie 17, CH-1951 Sion, Switzerland.
authors.forename: Peter G
authors.identifier: 0000-0001-6541-0594
authors.initials: PG
authors.lastname: Boyd
authors.affiliation: Computational Research Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|IMDEA Materials Institute , C/Eric Kandel 2, 28906 Getafe, Madrid, Spain.
authors.forename: Maciej
authors.initials: M
authors.lastname: Haranczyk
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, United Kingdom.
authors.forename: Ben
authors.initials: B
authors.lastname: Slater
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of California , Berkeley, California 94720, United States.|Laboratory of Molecular Simulation, Institut des Sciences et Ingénierie Chimiques, Valais, Ecole Polytechnique Fédérale de Lausanne (EPFL) , Rue de l'Industrie 17, CH-1951 Sion, Switzerland.
authors.forename: Berend
authors.identifier: 0000-0003-4653-8562
authors.initials: B
authors.lastname: Smit