Gating effect for gas adsorption in microporous materials-mechanisms and applications

In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. cesium, potassium, etc.). These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing.

Visualizing Structural Transformation and Guest Binding in a Flexible Metal-Organic Framework under High Pressure and Room Temperature

Understanding the effect of gas molecules on the framework structures upon gas sorption in porous materials is highly desirable for the development of gas storage and separation technologies. However, this remains challenging for flexible metal-organic frameworks (MOFs) which feature "gate-opening/gate-closing" or "breathing" sorption behaviors under external stimuli. Herein, we report such a flexible Cd-MOF that exhibits "gating effect" upon CO₂ sorption. The ability of the desolvated flexible Cd-MOF to retain crystal singularity under high pressure enables the direct visualization of the reversible closed-/open-pore states before and after the structural transformation as induced by CO₂ adsorption/desorption through in situ single-crystal X-ray diffraction experiments. The binding sites of CO₂ molecules within the flexible MOF under high pressure and room temperature have also been identified via combined in situ single-crystal X-ray diffraction and powder X-ray diffraction studies, facilitating the elucidation of the states observed during gate-opening/gate-closing behaviors. Our work therefore lays a foundation to understand the high-pressure gas sorption within flexible MOFs at ambient temperature, which will help to improve the design efforts of new flexible MOFs for applications in responsive gas sorption and separation.

Giant Hysteretic Sorption of CO2 : In Situ Crystallographic Visualization of Guest Binding within a Breathing Framework at 298 K

A dynamic Zn^II -MOF has been shown to exhibit extreme breathing behavior under gas pressure. The very narrow pore form of the activated framework opens up in the presence of carbon dioxide, thus making it a suitable material for CO₂ capture. Sorption of CO₂ at 298 K and relatively high pressure clearly shows a two-step isotherm with giant hysteresis for the second step. In-situ single-crystal diffraction analysis was carried out under CO₂ gas pressure at 298 K using an environmental gas cell in order to visualize the interaction between CO₂ and the host framework. The results are well supported by pressure-gradient differential scanning calorimetry (P-DSC) and variable-pressure powder X-ray analysis. Theoretical calculations have been carried out in order to further back up the crystallographic data.

High-temperature in situ crystallographic observation of reversible gas sorption in impermeable organic cages

Crystallographic observation of adsorbed gas molecules is a highly difficult task due to their rapid motion. Here, we report the in situ single-crystal and synchrotron powder X-ray observations of reversible CO2 sorption processes in an apparently nonporous organic crystal under varying pressures at high temperatures. The host material is formed by hydrogen bond network between 1,3,5-tris-(4-carboxyphenyl)benzene (H3BTB) and N,N-dimethylformamide (DMF) and by π-π stacking between the H3BTB moieties. The material can be viewed as a well-ordered array of cages, which are tight packed with each other so that the cages are inaccessible from outside. Thus, the host is practically nonporous. Despite the absence of permanent pathways connecting the empty cages, they are permeable to CO2 at high temperatures due to thermally activated molecular gating, and the weakly confined CO2 molecules in the cages allow direct detection by in situ single-crystal X-ray diffraction at 323 K. Variable-temperature in situ synchrotron powder X-ray diffraction studies also show that the CO2 sorption is reversible and driven by temperature increase. Solid-state magic angle spinning NMR defines the interactions of CO2 with the organic framework and dynamic motion of CO2 in cages. The reversible sorption is attributed to the dynamic motion of the DMF molecules combined with the axial motions/angular fluctuations of CO2 (a series of transient opening/closing of compartments enabling CO2 molecule passage), as revealed from NMR and simulations. This temperature-driven transient molecular gating can store gaseous molecules in ordered arrays toward unique collective properties and release them for ready use.

Structurally simple complexes of CO2

A wide range of structurally characterized adducts of CO₂are discussed in this review, from the strongly bound, charge assisted carbamate complexes through the weaker halide and pseudo-halide complexes to the weakest possible inclusion complexes.

A preferable molecular crystal membrane for H₂ gas separation

A novel molecular crystal membrane [Cu(II)2(9-AC)4(pyz)]n (9-AC: 9-anthracenecarboxylate, pyz: pyrazine) shows high permselectivity for H2/CO (79.0), H2/CH4 (137), and H2/CO2 (12.1) due to the high rate of H2 permeation (3820 Barrer) while inhibiting the permeation of such a large gaseous particle even in its wider channel than the Lennard-Jones diameter of each gas.

Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers

This review gives a brief overview of single-crystal X-ray diffraction studies and single-crystal to single-crystal transformations of porous coordination polymers.

Single-crystal membrane for anisotropic and efficient gas permeation

Development of gas separation materials has been one of the basic requirements of industry. Microporous materials have adequate pores for gas separation and have contributed to the advancement of gas purification techniques. Because the simplest and most economical method would be membrane separation, various microporous membranes have been prepared and explored for their separation properties. However, a key issue remains as to how to generate defect-free membranes with practical gas permeance. Here we report the preparation of a well-oriented single-crystal membrane with high permeance by using a flexible single crystal of [Cu(2)(bza)(4)(pyz)](n) possessing one-dimensional (1D) penetration channels; this membrane exhibits anisotropic gas permeation through the 1D channels with high permselectivity for H(2) and CO(2). Although the diameter of the neck of the narrow channels is smaller than the kinetic diameters of the sample gases, various gases pass through the 1D channels. This report provides a new way of developing gas permeation membranes as sophisticated crystal devices for gas purification techniques.