Unusual adsorption behavior of a highly flexible copper-based MOF

Due to their network flexibility, metal-organic frameworks (MOFs) may respond to external stimuli such as guest molecules, heat, pressure, humidity and so on by a dynamical transformation of their structure. In this work we present experimental studies by means of nitrogen and argon physisorption at cryogenic temperature concerning the influence of handling and storage on the recently synthesized highly flexible copper-based MOF ∞3[(Cu4(μ4-O)(μ2-OH)2(Me2trzpba)4]∞3[(Cu4(μ4-O)(μ2-OH)2(Me2trzpba)4]. Unusual adsorption isotherms with up to three distinct steps in the adsorbed volume have been found substantiating once more: (i) the high sensitivity of flexible network structures for environmental influences, (ii) the high diversity of physisorption isotherm types for flexible materials including isotherms with several steps and large hysteresis loops, and last but not least (iii) that theoretical methods assuming inert solids cannot be applied to highly flexible materials for calculating pore sizes and other solid state parameters.

The influence of storage and handling on a recently synthesized highly flexible Cu-MOF was studied by means of nitrogen and argon physisorption. Unusual adsorption isotherms with distinct steps in the adsorbed volume are found, which are interpreted as guest fluid-induced transition phenomena. It is shown that network flexibility and dynamics represent a new challenge for the interpretation of gas adsorption isotherms concerning solid parameters and that flexible materials have to be handled with the utmost caution in order to preserve their original characteristics.Figure optionsDownload as PowerPoint slideResearch highlights
► Unusual adsorption isotherms for Cu-MOF are found.
► The steps are interpreted as guest fluid-induced transition phenomena.
► No correlation between the relative pressure of gas uptake and pore size is given.
► The high diversity of physisorption isotherm types for flexible materials is shown.
► Flexible network structures are highly sensitive for environmental influences.