Gate Adsorption Research Articles

Slacking of Gate Adsorption Behavior on Metal-Organic Frameworks under an External Force.

As flexible metal-organic frameworks (MOFs) and their gate adsorption behaviors are increasingly expected to be used in gas storage and separation systems, evaluating their performance by considering their usage patterns in actual processes is becoming increasingly important. Herein, we show that the shaping of the elastic layer-structured MOF-11 (ELM-11; [Cu(BF4)2(4,4'-bipyridine)2]) into pellet forms using polymer binders smears its stepwise uptake associated with the CO2 gate adsorption. This is a critical problem because the superior adsorption properties of flexible MOFs are highly dependent on the sharpness of the step. Free energy analysis by molecular simulations revealed that the slacking of the gate adsorption is natural from a thermodynamic point of view. In other words, the external force exerted by the polymer binders, which prevents the expansion of MOF particles upon the gate opening, changes the free energy landscape of the system. This causes the flexible motifs within the MOF particles to undergo a structural transition at slightly different pressures from each other. The force profile dependence of the slacking phenomenon on both adsorption and desorption isotherms was also investigated. It was revealed that controlling the force profile applied to MOF particles is important to mold MOF pellets that satisfy the robustness and sharpness of the gate adsorption. Finally, we examined the coating of pellets to verify the relationship between the force profile and the degree of slacking and discussed possible strategies to improve the sharpness of the gate adsorption on MOF pellets considering the revealed mechanism.

Control of gate adsorption characteristics of flexible metal-organic frameworks by crystal defect

Abstract Flexible metal-organic frameworks (flexible MOFs) are known to exhibit a step-wise adsorption behavior induced by structural transition. This adsorption behavior is referred to as the “gate” adsorption behavior as it resembles the opening and closing of a gate. Gate adsorption behavior has been reported to originate from the balance between stabilization owing to the guest adsorption and destabilization that occurs due to structural transition. Although the peculiar adsorption of flexible MOFs is expected to have a wide range of applications, control of the gate adsorption behavior is essential. Here, we focus on CID-4, a member of flexible MOFs with an interdigitated structure. The study's approach to control the gate adsorption behavior of CID-4 is to create the crystal defect through coexistence with mono/bidentate organic ligands during synthesis. Depending on the extent of the crystal defect, the degree of destabilization owing to structural transition can be varied, resulting in different gate pressures. Essentially, by adding a mono organic ligand during the synthesis of CID-4 particles, the crystal defect due to the switch from bidentate to mono organic ligands was confirmed via X-ray diffraction (XRD) and thermal gravimetric analysis (TGA) measurements. Moreover, the gate adsorption behavior of CID-4 with crystal defects dramatically shifted to a higher relative pressure. The free energy analysis based on the crystal structure of interdigitated-type MOFs also supported our strategy to control the gate adsorption behavior by creating the crystal defect.

Flowability improvement of soft metal-organic framework particles by wet granulation

Abstract Owing to their step-wise adsorption behavior induced by structural transition, soft metal-organic frameworks (soft MOFs) are used in a wide range of applications. However, they suffer from poor handling performance because of their submicron size. Hence, it is imperative to improve their flowability in order to render them suitable for industrial applications. In this study, we used wet granulation to improve the flowability of zeolitic imidazolate framework-8 (ZIF-8), which consists of zinc ions and 2-methylimidazolate linkers and exhibits intra-framework flexibility. A high-shear mixing granulator was employed to carry out the wet granulation of the ZIF-8 particles using water and hydroxypropyl cellulose as the binder. The binder and liquid binder concentrations controlled the granule size from 100 to 400 μm, regardless of the primary diameters of the ZIF-8 particles. In addition, the larger granules showed significant improvement in the flowability. The adsorption capacity of the ZIF-8 granules decreased slightly with an increase in the binder concentration. On the other hand, the gate adsorption behavior of ZIF-8 remained almost unchanged even after the granulation process.

Characterization of mixing performance in a microreactor and its application to the synthesis of porous coordination polymer particles

Here we quantitatively evaluated the mixing performance of a tailor-made microreactor with central-collision type through the iodide/iodate chemical test reaction, and applied the microreactor to the synthesis of zeolitic imidazolate framework-8 (ZIF-8), which is a subclass of porous coordination polymers (PCPs) or metal organic frameworks (MOFs). The chemical test reaction demonstrated excellent mixing performance of the microreactor with a characteristic mixing time shorter than 1ms, which is approximately 100 times faster than those of a batch reactor and a millimeter-sized Y-shaped mixer. Taking advantage of the rapid and uniform mixing, the microreactor successfully produced ZIF-8 particles with high reproducibility by simply mixing aqueous solutions of zinc nitrate and 2-methylimidazole. The synthesis at room temperature resulted in ZIF-8 particles with chamfered cube shape, while a lower temperature of 5°C produced raspberry-type spherical particles. We confirmed that prepared ZIF-8 particles have BET surface area of ∼1500m2/g and exhibit the gate adsorption behavior caused by the structural transition of the ZIF-8 framework.

Free Energy Analysis for Adsorption-Induced Structural Transition of Colloidal Zeolitic Imidazolate Framework-8 Particles

Particle size and shape of flexible metal–organic frameworks have been reported to change the gate adsorption phenomenon which is characterized by an abrupt increase in the adsorbed amount induced by structural transitions of a host framework, although a detailed mechanism has not yet been clarified. Here, we focus on zeolitic imidazolate framework-8 (ZIF-8) whose structural flexibility stems from the rotation of 2-methylimidazole linkers. We perform free energy analyses with the aid of adsorption simulations to understand the dependence of the gate adsorption phenomenon on the particle size and shape of ZIF-8. For the adsorption simulations, we construct a simulation cell referred to as the nanoparticle model, which has a ZIF-8 slab structure and gas phase regions. Our simulations demonstrate that a decrease in the slab width results in a reduction of the average adsorbed amount. This can be explained by the smaller amount adsorbed in the region close to the surface of the ZIF-8 structure, which reaches ...

Liquid/vapor-induced reversible dynamic structural transformation of a three-dimensional Cu-based MOF to a one-dimensional MOF showing gate adsorption.

A new 3D metal-organic framework (MOF), in which 2D layers are interlaced to form a 3D architecture, was synthesized by a reaction of Cu(BF4)2 and 1,3-bis(4-pyridyl)propane (bpp) in a water/1-hexanol solvent system, and the crystal structure of the MOF was successfully solved. The MOF is reversibly transformed to a 1D chain MOF, which shows gate adsorption properties. The dynamic transformation gives crystal size reduction resulting in a slight change in CO2 adsorption isotherms. The 1D MOF shows selective adsorption/separation properties on benzene and its analogues with similar sizes and shapes (benzene, toluene, and cyclohexane).

Synthesis of zeolitic imidazolate framework-8 particles of controlled sizes, shapes, and gate adsorption characteristics using a central collision-type microreactor

Controlling the particle size and shape of soft metal-organic framework compounds is crucial for understanding and regulating the adsorption-induced structural transitions that lead to the unique stepwise adsorption behavior of these materials. The present study demonstrates that the intensive mixing facilitated by a microreactor enables the synthesis of monodisperse zeolitic imidazolate framework-8 (ZIF-8) particles with high reproducibility. In the flow microreactor process used in this study, the particle size is predominantly determined by the reaction conditions during the initial mixing in the microreactor. On the other hand, the particle shape depends upon the temperature during the subsequent aging process. The technique proposed in this study is additive-free and allows control of both particle size from 50nm to 2μm and particle shape from cube to chamfered cube to rhombic dodecahedron, simply by adjusting the concentration of feed solutions and reaction temperature. Furthermore, we demonstrate that both particle size and particle shape are key physical parameters determining the adsorption characteristics of ZIF-8 particles.

Gate Opening, Diffusion, and Adsorption of CO2 and N2 Mixtures in ZIF-8

Diffusion and adsorption of CO2/N2 mixtures in the zeolitic imidazolate framework ZIF-8 are investigated by molecular dynamics (MD) and Gibbs ensemble Monte Carlo (GEMC) simulations. Structural changes called “gate opening” could be found for the adsorbed single-component gases and for the mixture. The gate opening appears for the mixture at a total number of guest molecules per cavity between that for the pure CO2 and that for the pure N2 but closer to that of N2 which is lower. Due to the stronger dependence of CO2 adsorption upon the temperature in comparison with N2, the adsorption selectivity is predicted to be higher at lower temperatures, which is in accordance with experimental findings.

Understanding gate adsorption behaviour of CO2 on elastic layer-structured metal-organic framework-11.

We demonstrate that CO2 gate adsorption behaviour of elastic layer-structured metal-organic framework-11 (ELM-11: [Cu(BF4)2(4,4'-bipyridine)2]), which is a family of soft porous crystals (SPCs), can be described by a thermodynamic model by free energy analysis with the aid of an adsorption experiment and a molecular simulation. The structures of ELM-11 (closed structure) at 273 K after its evacuation and CO2-encapsulated ELM-11 (open structure) at 195-298 K were determined by the Rietveld analysis using in situ synchrotron X-ray powder diffraction data. We then performed grand canonical Monte Carlo (GCMC) simulations for CO2 adsorption on the open host framework structures of ELM-11 from the Rietveld analysis. The temperature dependence of the Helmholtz free energy change of host ΔF(host) from the closed structure to the open structure was obtained by the free energy analysis using the GCMC data. We show that there is a linear correlation between ΔF(host) and temperature, and thus, the internal energy and entropy changes of host, ΔU(host) and ΔS(host), respectively, can be obtained. The obtained ΔU(host) value is in good agreement with that obtained from the quantum chemical calculations using the closed and open host framework structures, which demonstrates that the thermodynamic model for gate adsorption is highly appropriate. Moreover, our result suggests that the gate adsorption pressure depends on not only the guest-host interaction and the internal energy change of host, but also the entropy change of host, which should be one of the key factors for the tailored synthesis of SPCs.

Potential theory for gate adsorption on soft porous crystals

We demonstrate that an adsorption potential at the gate adsorption pressure of soft porous crystals (SPCs) based on the Polanyi's potential theory of adsorption shows a constancy to temperature. This was done using grand canonical Monte Carlo simulations and free energy analysis, which were carried out with a simplified stacked-layer SPC model. This finding implies that the characteristic curve obtained from an experimental gate adsorption isotherm on SPCs can be used to predict the temperature dependence of the gate-opening pressure, even though the potential theory of adsorption does not take into account the deformation of porous solids during the adsorption. We develop a modified potential theory for gate adsorption and show that the derived relation has a form that the Gibbs free energy change due to the host framework deformation per guest molecule, − ΔGhost/N, and a correction term, C, are added to the expression of the original potential theory of adsorption. The term C is not an empirical correction factor but is the difference of intermolecular interaction potential and entropy between the bulk liquid phase at the saturated state and the adsorbed phase, originating from spatial constraint of adsorbed guest molecules in the host. By evaluating the modified expression for gate adsorption using the simulation results, we demonstrate that the constancy of the adsorption potential to temperature results from a compensation effect between three terms: guest–host interaction potential per guest molecule, − ΔGhost/N and C, which have a temperature dependence.

Modeling and Visualization of CO2 Adsorption on Elastic Layer-Structured Metal–Organic Framework-11: Toward a Better Understanding of Gate Adsorption Behavior

Elastic-layer-structured metal–organic framework-11 (ELM-11) is a soft porous crystal (SPC) with a 2D square-grid framework [Cu(BF4)2(bpy)2] (bpy = 4,4′-bipyridine) that has attracted attention as a material for CO2 capture and storage because of its gate adsorption properties. Herein, we demonstrate that the structure of CO2-encapsulated ELM-11 at 100 kPa and 273 K can be precisely modeled and visualized by our new structure refinement method, which combines Rietveld analysis of in situ synchrotron X-ray powder diffraction data with molecular simulations. We believe that this is the first study in which the structure of a guest–SPC system that exhibits an extensive, complex structural transformation by the gate adsorption was successfully refined by such an approach. The crystallographic data of the open framework structure of ELM-11 enable grand canonical Monte Carlo (GCMC) simulations of CO2 adsorption. The free energy analysis of the gate adsorption phenomenon with the resulting GCMC adsorption isothe...

マイクロリアクタの混合特性評価と多孔性錯体微粒子合成への適用

Abstract Here we quantitatively evaluated the mixing performance of a tailor-made microreactor with central-collision type through the iodide/iodate chemical test reaction, and applied the microreactor to the synthesis of zeolitic imidazolate framework-8 (ZIF-8), which is a subclass of porous coordination polymers (PCPs) or metal organic frameworks (MOFs). The chemical test reaction demonstrated excellent mixing performance of the microreactor with a characteristic mixing time shorter than 1 ms, which is approximately 100 times faster than those of a batch reactor and a millimeter-sized Y-shaped mixer. Taking advantage of the rapid and uniform mixing, the microreactor successfully produced ZIF-8 particles with high reproducibility by simply mixing aqueous solutions of zinc nitrate and 2-methylimidazole. The synthesis at room temperature resulted in ZIF-8 particles with chamfered cube shape, while a lower temperature of 5 °C produced raspberry-type spherical particles. We confirmed that prepared ZIF-8 particles have BET surface area of ∼1500 m 2 /g and exhibit the gate adsorption behavior caused by the structural transition of the ZIF-8 framework.

Tuning of gate adsorption: modification of a flexible metal-organic framework by secondary organic ligands.

For realizing selective adsorption of targeted molecules, a flexible metal-organic framework (MOF) was modified with monodentate secondary ligands. Although the modified MOF retains CO2 adsorptivities with a vertical adsorption uptake, the material also shows gate adsorptivities of a specific gas molecule that the pristine MOF does not adsorb.

Collective Interactions of Molecules with an Interfacial Solid

Abstract Confinement of molecules in nanoscale pores of an interfacial solid such as single wall carbon nanotubes of which all component carbon atoms are exposed to the interface with gas phase induces collective phenomena; the confinement effect is interpreted by the interaction potential theory. The nanoconfinement effect gives rise to in-pore phase anomalies for NO, H2O, CCl4, superhigh-pressure effect, hydrophobic to hydrophilic transformation for carbon, and a marked quantum molecular sieving. The confinement of KI in the nanotube space below 0.1 MPa produces high-pressure-phase KI above 1.9 GPa. Water molecules gain hydrophilicity with cluster formation in order to accommodate in the hydrophobic carbon spaces. The subnanometer quantum fluctuation for H2 and D2 gives rise to a marked quantum molecular sieving effect in nanoscale pores. The Cu-based porous coordination polymer crystals can detect the surrounding stimuli such as CO2 pressure change to vary the crystal lattice, accompanying gate adsorption.

Adsorption-Induced Structural Transition of an Interpenetrated Porous Coordination Polymer: Detailed Exploration of Free Energy Profiles

Specific types of coordination polymers show an adsorption-induced structural transition, or so-called "gate adsorption", in which a host framework is said to change its structure from a "closed" nonporous phase to an "open" porous one for guest molecules. To identify the pathway for such a structural transition, we perform grand canonical Monte Carlo simulations for the adsorption of guest molecules in a host interpenetrated framework and calculate the free energy profiles of the structural changes in a complete three-dimensional space. In addition to the open phase found in our previous analyses along a fixed one-dimensional path, we reveal the existence of another open configuration. Each of the two open phases yields the status of global minimum to the other depending on the external pressure, resulting in a two-step isotherm. Moreover, the shape of adsorption hysteresis associated with the structural transition can change depending on the energy barrier between a metastable and a stable state that the system can overcome. Our simulations not only give a comprehensive understanding of stepped isotherms observed empirically but also suggest that isotherms with hysteretic gate adsorption is closely related to the thermal fluctuation of the system.

Gas Adsorption Mechanism and Kinetics of an Elastic Layer-Structured Metal–Organic Framework

The gate adsorption mechanism and kinetics of an elastic layer-structured metal–organic framework (ELM), [Cu(bpy)2(BF4)2]n (ELM-11), that shows typical single-step CO2 gate adsorption/desorption isotherms accompanied with dynamic structural transformation in a wide temperature range were investigated. Adsorption of quite a small amount of CO2 on the external surface of ELM-11 crystals was observed at the pressure just below a gate adsorption pressure and induced a slight structural change in ELM-11. The structural change should start occurring at the outer parts of ELM-11 and transmit to more inner parts with rising pressure. The adsorption provides the stabilization of the framework through the interaction between fluid–solid and fluid–fluid and enables the framework to expand largely along the stacking direction. The CO2 adsorption rate of ELM-11 is almost comparable to that of Zeolite 5A at around ambient temperatures and shows temperature dependence with an anti-Arrhenius trend: higher adsorption rate with lower temperature.

Gate adsorption of CO2 on a flexible one-dimensional copper-based coordination polymer crystal

A flexible one-dimensional copper-based coordination polymer (CP) crystal was synthesized and the crystal structure was solved. This CP crystal shows a single step gate adsorption uptake of CO(2) under moderate conditions (268 K ≤ T ≤ 303 K, P < 1 atm) irrespective of no N(2) uptake at 77 K.

Adsorption of several gases on flexible metal organic framework [Cu(dhbc)2(4,4′-bpy)]·H2O

The adsorption of ten gases on the flexible metal organic framework material [Cu(dhbc)2(4,4′-bpy)]·H2O (Cu(db)) has been measured over a wide range of temperatures and pressures. The gate opening condition and driving force behind gate adsorption for Cu(db) were discussed by examining the adsorption isotherms. The adsorption isotherms for each adsorbate can be generalized to a characteristic curve using the adsorption potential energy (ε) and the adsorption volume. The adsorption potential (εgate) at gate opening is almost constant over a wide range of temperatures; thus, the gate pressure at a desired temperature can be deduced using the εgate evaluated from one adsorption isotherm. The gate opening capacity of the gases was arranged in the order: CO2≒N2O>C2H4≒Xe>CH4>CO>O2>Ar≒N2>H2, which is governed by the interaction energy between the outer surface of Cu(db) and the adsorbate. It is suggested that the gate effect is brought about when the integral interaction energy of adsorbates with the Cu(db) surface exceeds a defined limit correlating with the π−π stacking energy of Cu(db) layers.

Free energy analysis for adsorption-induced lattice transition of flexible coordination framework

We conduct grand canonical Monte Carlo simulations and free energy analysis for a gate adsorption phenomenon, which is experimentally observed in flexible frameworks of porous coordination polymers. Our calculations demonstrate that the stabilization provided by the guest adsorption drives the structural transition, surmounting the energy cost in creating the adsorption space due to the movement of the host framework. Furthermore, the existence of an energy barrier between two local minima in the free-energy landscape is found to result in hysteretic adsorption.

Reversible Structural Change of Cu-MOF on Exposure to Water and Its CO2 Adsorptivity

It is important to study the interaction between water molecules and a host structure for understanding the adsorption mechanism of metal-organic framework (MOF) materials. The evolution of the structure of a flexible Cu-MOF, {[Cu(bpy)(H2O)2(BF4)2](bpy)} (bpy=4,4'-bipyridine), upon dehydration and rehydration was studied by thermogravimetric analysis (TGA), infrared (IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and water adsorption. A nearly reversible structural change was observed upon rehydration. More importantly, a unique CO2 "gate adsorption" phenomenon was observed despite the exposure of the Cu-MOF to water. This shows that the Cu-MOF has relatively good stability after exposure to water.