  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.
