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

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

Slow CO2 Diffusion Governed by Steric Hindrance of Rotatory Ligands in Small Pores of a Metal-Organic Framework

Understanding the adsorption and diffusional dynamics of CO₂ in metal-organic frameworks (MOFs) is essential in the application of these materials to CO₂ capture and separation. We show that the dynamics of adsorbed CO₂ is related to the rotational motion of ligands located in the narrow pore windows of a MOF using solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR analyses of local dynamics reveal that CO₂ adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO₂ monitored by ¹³C NMR is much less than that in the larger pores of MOFs and decreases cooperatively with ligand mobility, which indicates that the rate of diffusion is influenced by the steric hindrance of the rotatory ligands. Adsorbed CH₄ also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.

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.

Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exemplified by most metal-organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These "soft" or "stimuli-responsive" CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from first principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These "switching CNs" are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure-function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.

Designer Metal-Organic Frameworks for Size-Exclusion-Based Hydrocarbon Separations: Progress and Challenges

The separation of hydrocarbons is of primary importance in the petrochemical industry but remains a challenging process. Hydrocarbon separations have traditionally relied predominantly on costly and energy-intensive heat-driven procedures such as low-temperature distillations. Adsorptive separation based on porous solids represents an alternative technology that is potentially more energy efficient for the separation of some hydrocarbons. Great efforts have been made recently not only in the development of adsorbents with optimal separation performance but also toward the subsequent implementation of adsorption-based separation technology. Emerging as a relatively new class of multifunctional porous materials, metal-organic frameworks (MOFs) hold substantial promise as adsorbents for highly efficient separation of hydrocarbons. This is because of their exceptional and intrinsic porosity tunability, which enables size-exclusion-based separations that render the highest possible separation selectivity. In this review, recent advances in the development of MOFs for separation of selected groups of hydrocarbons are reviewed, including methane/C₂ hydrocarbons, normal alkanes, alkane isomers, and alkane/alkene/alkyne and C₈ alkylaromatics, with a particular focus on separations based on the size-exclusion mechanism. Insights into tailor-made structures, material design strategies, and structure-property relationships will be elucidated. In addition, the existing challenges and possible future directions of this important research field will be discussed.

Structural Basis of CO₂ Adsorption in a Flexible Metal-Organic Framework Material

This paper reports on the structural basis of CO₂ adsorption in a representative model of flexible metal-organic framework (MOF) material, Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)₄] (NiBpene or PICNIC-60). NiBpene exhibits a CO₂ sorption isotherm with characteristic hysteresis and features on the desorption branch that can be associated with discrete structural changes. Various gas adsorption effects on the structure are demonstrated for CO₂ with respect to N₂, CH₄ and H₂ under static and flowing gas pressure conditions. For this complex material, a combination of crystal structure determination and density functional theory (DFT) is needed to make any real progress in explaining the observed structural transitions during adsorption/desorption. Possible enhancements of CO₂ gas adsorption under supercritical pressure conditions are considered, together with the implications for future exploitation. In situ operando small-angle neutron and X-ray scattering, neutron diffraction and X-ray diffraction under relevant gas pressure and flow conditions are discussed with respect to previous studies, including ex situ, a priori single-crystal X-ray diffraction structure determination. The results show how this flexible MOF material responds structurally during CO₂ adsorption; single or dual gas flow results for structural change remain similar to the static (Sieverts) adsorption case, and supercritical CO₂ adsorption results in enhanced gas uptake. Insights are drawn for this representative flexible MOF with implications for future flexible MOF sorbent design.

Rotational Dynamics of Linkers in Metal⁻Organic Frameworks

Among the numerous fascinating properties of metal⁻organic frameworks (MOFs), their rotational dynamics is perhaps one of the most intriguing, with clear consequences for adsorption and separation of molecules, as well as for optical and mechanical properties. A closer look at the rotational mobility in MOF linkers reveals that it is not only a considerably widespread phenomenon, but also a fairly diverse one. Still, the impact of these dynamics is often understated. In this review, we address the various mechanisms of linker rotation reported in the growing collection of literature, followed by a highlight of the methods currently used in their study, and we conclude with the impacts that such dynamics have on existing and future applications.

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.

Statistical mechanical model of gas adsorption in porous crystals with dynamic moieties

Some nanoporous, crystalline materials possess dynamic constituents, for example, rotatable moieties. These moieties can undergo a conformation change in response to the adsorption of guest molecules, which qualitatively impacts adsorption behavior. We pose and solve a statistical mechanical model of gas adsorption in a porous crystal whose cages share a common ligand that can adopt two distinct rotational conformations. Guest molecules incentivize the ligands to adopt a different rotational configuration than maintained in the empty host. Our model captures inflections, steps, and hysteresis that can arise in the adsorption isotherm as a signature of the rotating ligands. The insights disclosed by our simple model contribute a more intimate understanding of the response and consequence of rotating ligands integrated into porous materials to harness them for gas storage and separations, chemical sensing, drug delivery, catalysis, and nanoscale devices. Particularly, our model reveals design strategies to exploit these moving constituents and engineer improved adsorbents with intrinsic thermal management for pressure-swing adsorption processes.

Gate-opening upon CO2 adsorption on a metal–organic framework that mimics a natural stimuli-response system

A dynamic d-champhorate-based protuberant-grid-type framework, undergoes gate opening and closing processes that were triggered by the stimuli of the adsorption or desorption of CO₂. It is able to specifically recognize CO₂ over than N₂ and H₂ and shows a high CO₂ uptake of 90 mg g⁻¹ under 35 bar at 298 K.

Separation of C2/C1 hydrocarbons through a gate-opening effect in a microporous metal–organic framework

Here we report highly selective C2 separation from methane in a porous metal–organic framework.