Constant Volume Gate-Opening by Freezing Rotational Dynamics in Microporous Organically Pillared Layered Silicates

Abstract

Microporous organically pillared layered silicates (MOPS) are a class of microporous hybrid materials that, by varying pillar density, allows for optimization of guest recognition without the need to explore different framework topologies. MOPS are found to be capable of discriminating two very similar gases, carbon dioxide and acetylene, by selective gate-opening solely through quenching pillar dynamics. Contrary to conventional gate-opening in metal organic frameworks, the additional adsorption capacity is realized without macroscopic volume changes, thus avoiding mechanical stress on the framework. Of the two gases studied, only CO can accomplish freezing of pillar dynamics. Moreover, the shape of the slit-type micropores in MOPS can easily be fine-tuned by reducing the charge density of the silicate layers. This concomitantly reduces the Coulomb attraction of cationic interlayer space and anionic host layers. Surprisingly, we found that reducing the charge density then alters the gate-opening mechanism to a conventional structural gate-opening involving an increase in volume.

Pseudo‐Gated Adsorption with Negligible Volume Change Evoked by Halogen‐Bond Interaction in the Nanospace of MOFs

“…The volume expansion in the adsorbent solid is disadvantageous as it may damage the storage tank, decompose the moldings, and also may result in slower diffusion of gases in the tank. Consequently, design of new porous materials that can show abrupt guest adsorption and desorption without significant volume change is highly desirable …”

Microporous Organically Pillared Layered Silicates (MOPS): A Versatile Class of Functional Porous Materials

“…In conventional gate‐opening, the additional adsorption capacity is associated with macroscopic volume changes exerting significant mechanical stress on the framework. By contrast, we showed that MOPS are able of discriminating two very similar gases (CO 2 and C 2 H 2 ) by a new fully reversible and selective gate‐opening at constant volume which is triggered solely by freezing pillar dynamics without any macroscopic displacements of the framework (Figure ) . The underlying mechanism was unraveled by in situ physisorption/X‐ray diffraction in combination with 13 C solid‐state NMR spectroscopy.…”

“…Prior to gate‐opening, CO 2 is adsorbed in a pseudohexagonal packing in the voids between three adjacent pillars. At gate‐opening pressure CO 2 is capable to squeeze into the gaps along the pseudohexagonal a ‐axis, which are too small for CO 2 with rotating pillars, concomitantly blocking the rotation of the pillars (Figure ) . According to the classification of Schneemann et al.…”

“…Tuning the electrostatic interaction of pillars and silicate layers by reducing the charge density as described above will also lower the pillar density and thus increase the permanent micropore volume. Since charge density of the silicate layers modulates concomitantly Coulomb attraction, polarization, and micropore volume at the same time, a meticulous balance is established resulting for any given combination of adsorbate and layer charge in its particular type of gate opening:…”

“…For MOPS‐6 and CO 2 adsorption, the mechanism can thus be classified as interdigitated layer type of gate‐opening according to Schneemann et al …”