Separation of Xe from Kr with Record Selectivity and Productivity in Anion‐Pillared Ultramicroporous Materials by Inverse Size‐Sieving

The separation of xenon/krypton (Xe/Kr) mixture is of great importance to industry, but the available porous materials allow the adsorption of both, Xe and Kr only with limited selectivity. Herein we report an anion‐pillared ultramicroporous material NbOFFIVE‐2‐Cu‐i (ZU‐62) with finely tuned pore aperture size and structure flexibility, which for the first time enables an inverse size‐sieving effect in separation along with record Xe/Kr selectivity and ultrahigh Xe capacity. Evidenced by single‐crystal X‐ray diffraction, the rotation of anions and pyridine rings upon contact of larger‐size Xe atoms adapts cavities to the shape/size of Xe and allows strong host‐Xe interaction, while the smaller‐size Kr is excluded. Breakthrough experiments confirmed that ZU‐62 has a real practical potential for producing high‐purity Kr and Xe from air‐separation byproducts, showing record Kr productivity (206 mL g^?1) and Xe productivity (42 mL g^?1, in desorption) as well as good recyclability.     

Tuning the Gate‐Opening Pressure in a Switching pcu Coordination Network,X‐pcu‐5‐Zn,by Pillar‐Ligand Substitution

Coordination networks that reversibly switch between closed and open phases are of topical interest since their stepped isotherms can offer higher working capacities for gas‐storage applications than the related rigid porous coordination networks. To be of practical utility, the pressures at which switching occurs, the gate‐opening and gate‐closing pressures, must lie between the storage and delivery pressures. Here we study the effect of linker substitution to fine‐tune gate‐opening and gate‐closing pressure. Specifically, three variants of a previously reported pcu ‐topology MOF, X‐pcu‐5‐Zn , have been prepared: X‐pcu‐6‐Zn , 6 =1,2‐bis(4‐pyridyl)ethane (bpe), X‐pcu‐7‐Zn , 7 =1,2‐bis(4‐pyridyl)acetylene (bpa), and X‐pcu‐8‐Zn , 8 =4,4′‐azopyridine (apy). Each exhibited switching isotherms but at different gate‐opening pressures. The N₂, CO₂, C₂H₂, and C₂H₄ adsorption isotherms consistently indicated that the most flexible dipyridyl organic linker, 6 , afforded lower gate‐opening and gate‐closing pressures. This simple design principle enables a rational control of the switching behavior in adsorbent materials.     

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

The design of microporous hybrid materials, tailored for diverse applications, is a key to address our modern society's imperative of sustainable technologies. Prerequisites are flexible customization of host–guest interactions by incorporating various types of functionality and by adjusting the pore structure. On that score, metal–organic frameworks (MOFs) have been the reference in the past decades. More recently, a new class of microporous hybrid materials emerged, microporous organically pillared layered silicates (MOPS). MOPS are synthesized by simple ion exchange of organic or metal complex cations in synthetic layered silicates. MOFs and MOPSs share the features of “component modularity” and “functional porosity”. While both, MOFs and MOPS maintain the intrinsic characteristics of their building blocks, new distinctive properties arise from their assemblage. MOPS are unique since allowing for simultaneous and continuous tuning of micropores in the sub-Ångström range. Consequently, with MOPS the adsorbent recognition may be optimized without the need to explore different framework topologies. Similar to the third generation of MOFs (also termed soft porous crystals), MOPS are structurally ordered, permanently microporous solids that may also show a reversible structural flexibility above a distinct threshold pressure of certain adsorbents. This structural dynamism of MOPS can be utilized by meticulously adjusting the charge density of the silicate layers to the polarizability of the adsorbent leading to different gate opening mechanisms. The potential of MOPS is far from being fully explored. This Concept article highlights the main features of MOPS and illustrates promising directions for further research.     

In Situ Tracking of Dynamic NO Capture through a Crystal-to-Crystal Transformation from a Gate-Open-Type Chain Porous Coordination Polymer to a NO-Adducted Discrete Isomer

Optimal control of gas adsorption properties in metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) remains a great challenge in the field of materials science. An efficient strategy to capture electron-acceptor-type gas molecules such as nitrogen monooxide (NO) is to use host–guest interactions by utilizing electron-donor-type MOFs/PCPs as host frameworks. Herein, we focus on a highly electron-donating chain compound by using the paddlewheel-type [Ru₂^II,II] complex [Ru₂(2,4,5-Me₃PhCO₂)₄] (2,4,5-Me₃PhCO₂⁻=2,4,5-trimethylbenzoate) with the phenazine (phz) linker: [Ru₂(2,4,5-Me₃PhCO₂)₄(phz)] ( 1 ). Compound 1 exhibited a specific gated adsorption for NO under gas pressures greater than 60 kPa at 121 K, which finally resulted in approximately seven molar equivalents being taken up at 100 kPa followed by four molar equivalents remaining under vacuum at 121 K; its Rh isomorph ( 2 ) with weaker donation ability was inactive for NO. When the sample of 1 ⊃4 NO was heated to room temperature, the compound underwent a crystal-to-crystal phase transition to give [Ru₂(2,4,5-Me₃PhCO₂)₄(NO)₂](phz) ( 1 -NO), involving a post-synthetic nitrosylation on the [Ru₂] unit, accompanied by an eventful site-exchange with phz. This drastic event, which is dependent on the NO pressure, temperature, and time, was coherently monitored by using several different in situ techniques, revealing that the stabilization of NO molecules in nanosized pores dynamically and stepwisely occurred with the support of strong electronic/magnetic host–guest interactions.     

Direct Structural Identification of Gas Induced Gate‐Opening Coupled with Commensurate Adsorption in a Microporous Metal–Organic Framework

Gate‐opening is a unique and interesting phenomenon commonly observed in flexible porous frameworks, where the pore characteristics and/or crystal structures change in response to external stimuli such as adding or removing guest molecules. For gate‐opening that is induced by gas adsorption, the pore‐opening pressure often varies for different adsorbate molecules and, thus, can be applied to selectively separate a gas mixture. The detailed understanding of this phenomenon is of fundamental importance to the design of industrially applicable gas‐selective sorbents, which remains under investigated due to the lack of direct structural evidence for such systems. We report a mechanistic study of gas‐induced gate‐opening process of a microporous metal–organic framework, [Mn(ina)₂] (ina=isonicotinate) associated with commensurate adsorption, by a combination of several analytical techniques including single crystal X‐ray diffraction, in situ powder X‐ray diffraction coupled with differential scanning calorimetry (XRD‐DSC), and gas adsorption–desorption methods. Our study reveals that the pronounced and reversible gate opening/closing phenomena observed in [Mn(ina)₂] are coupled with a structural transition that involves rotation of the organic linker molecules as a result of interaction of the framework with adsorbed gas molecules including carbon dioxide and propane. The onset pressure to open the gate correlates with the extent of such interaction.     

Purely Physisorption‐Based CO‐Selective Gate‐Opening in Microporous Organically Pillared Layered Silicates

Separation of gas molecules with similar physical and chemical properties is challenging but nevertheless highly relevant for chemical processing. By introducing the elliptically shaped molecule, 1,4‐dimethyl‐1,4‐diazabicyclo[2.2.2]octane, into the interlayer space of a layered silicate, a two‐dimensional microporous network with narrow pore size distribution is generated (MOPS‐5). The regular arrangement of the pillar molecules in MOPS‐5 was confirmed by the occurrence of a 10 band related to a long‐range pseudo‐hexagonal superstructure of pillar molecules in the interlayer space. Whereas with MOPS‐5 for CO₂ adsorption, gate‐opening occurs at constant volume by freezing pillar rotation, for CO the interlayer space is expanded at gate‐opening and a classical interdigitated layer type of gate‐opening is observed. The selective nature of the gate‐opening might be used for separation of CO and N₂ by pressure swing adsorption.     

Tuning Gate-Opening of a Flexible Metal–Organic Framework for Ternary Gas Sieving Separation

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C₂H₄), acetylene (C₂H₂), and carbon dioxide (CO₂) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C₂H₂ and CO₂ through gate-opening and only negligible amount of C₂H₄. Breakthrough experiments demonstrated that this material can simultaneously capture C₂H₂ and CO₂, yielding polymer-grade (>99.99 %) C₂H₄ from single breakthrough separation.