The Dilemma of Reproducibility of Gating Isotherms for Flexible MOFs

Porous materials receive a high level of scientific and
technological
interest due to their applications in various fields such as adsorption,
separation and storage, catalysis, ion exchange, nanotechnology, etc.
Gas adsorption is a well-established tool for the characterization
of the texture of porous solids. Physisorption isotherms are generally
expected to be well reproducible for rigid adsorbents, but this is
not always the case for nonrigid (flexible) materials. The presence
of a metastability region and sensitivity of the activation barriers
to the material’s texture often influence the isotherms’
run. Here, we address the complexity that arises in terms of reproducibility
and sample handling for flexible metal–organic frameworks,
with the example of DUT-8­(Ni). It belongs to the group of “gate
opening” metal–organic frameworks and is a typical representative
of the pillared layer compounds. We propose characteristic parameters
for the analysis and comparison of adsorption isotherms, showing the
“gate opening” step, associated with the adsorption-induced
solid-state phase transition. A set of 50 nitrogen physisorption isotherms
measured at 77 K were analyzed and correlated with the synthetic and
outgassing conditions. The study highlights the importance of accurate
descriptions and record-keeping of experimental details and their
role in the replication of scientific results.