194 research outputs found
Computational characterization and prediction of metal-organic framework properties
In this introductory review, we give an overview of the computational
chemistry methods commonly used in the field of metal-organic frameworks
(MOFs), to describe or predict the structures themselves and characterize their
various properties, either at the quantum chemical level or through classical
molecular simulation. We discuss the methods for the prediction of crystal
structures, geometrical properties and large-scale screening of hypothetical
MOFs, as well as their thermal and mechanical properties. A separate section
deals with the simulation of adsorption of fluids and fluid mixtures in MOFs
On the use of the IAST method for gas separation studies in porous materials with gate-opening behavior
Highly flexible nanoporous materials, exhibiting for instance gate opening or
breathing behavior, are often presented as candidates for separation processes
due to their supposed high adsorption selectivity. But this view, based on
"classical" considerations of rigid materials and the use of the Ideal Adsorbed
Solution Theory (IAST), does not necessarily hold in the presence of framework
deformations. Here, we revisit some results from the published literature and
show how proper inclusion of framework flexibility in the osmotic thermodynamic
ensemble drastically changes the conclusions, in contrast to what intuition and
standard IAST would yield. In all cases, the IAST method does not reproduce the
gate-opening behavior in the adsorption of mixtures, and may overestimates the
selectivity by up to two orders of magnitude
Stress-Based Model for the Breathing of MetalâOrganic Frameworks
International audienceGas adsorption in pores of flexible metalâorganic frameworks (MOF) induces elastic deformation and structural transitions associated with stepwise expansion and contraction of the material, known as breathing transitions between large pore (lp) and narrow pore (np) phases. We present here a simple yet instructive model for the physical mechanism of this enigmatic phenomenon considering the adsorption-induced stress exerted on the material as a stimulus that triggers breathing transitions. The proposed model implies that the structural transitions in MOFs occur when the stress reaches a certain critical threshold. We showcase this model by drawing on the example of Xe adsorption in MIL-53 (Al) at 220 K, which exhibits two consecutive hysteretic breathing transitions between lp and np phases. We also propose an explanation for the experimentally observed coexistence of np and lp phases in MIL-53 materials
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