Minoru Miyahara

Kyoto University, Kioto, Kyōto, Japan

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Publications (91)246.37 Total impact

  • Shotaro Hiraide · Hideki Tanaka · Minoru T Miyahara ·
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    ABSTRACT: 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.
    Dalton Transactions 10/2015; DOI:10.1039/c5dt03476k · 4.20 Impact Factor
  • Ryohei Numaguchi · Hideki Tanaka · Shotaro Hiraide · Minoru T. Miyahara ·
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    ABSTRACT: 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, − ΔG host/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, − ΔG host/N and C, which have a temperature dependence.
    Molecular Simulation 07/2015; 41(16):1-10. DOI:10.1080/08927022.2015.1047369 · 1.13 Impact Factor
  • Hideki Tanaka · Shotaro Hiraide · Atsushi Kondo · Minoru T. Miyahara ·
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    ABSTRACT: 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 modelled 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 enables grand canonical Monte Carlo (GCMC) simulations of CO2 adsorption. The free energy analysis of the gate adsorption phenomenon with the resulting GCMC adsorption isotherm of CO2 provides the precise Helmholtz free energy change of the host framework during the structural transition, which is difficult to access experimentally. Finally, we demonstrate that the temperature dependence of the gate adsorption pressure can be predicted using the Helmholtz free energy change of the host.
    The Journal of Physical Chemistry C 04/2015; 119(21):150423155115006. DOI:10.1021/jp512870p · 4.77 Impact Factor
  • Yasushi Mino · Satoshi Watanabe · Minoru T Miyahara ·
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    ABSTRACT: Vertical convective self-assembly is capable of fabricating stripe-patterned structures of colloidal particles with well-ordered periodicity. To unveil the mechanism of the stripe pattern formation, in the present study, we focus on the meniscus shape and conduct in situ observations of shape deformation associated with particulate line evolution. The results reveal that the meniscus is elongated downward in a concave fashion toward the substrate in accordance with solvent evaporation, while the concave deformation is accelerated by solvent flow, resulting in the rupture of the liquid film at the thinnest point of the meniscus. The meniscus rupture triggers the meniscus to slide off from the particulate line, followed by the propagation of the sliding motion of the three-phase contact line, resulting in the formation of stripe spacing.
    Langmuir 04/2015; 31(14). DOI:10.1021/acs.langmuir.5b00467 · 4.46 Impact Factor
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    01/2015; 52(7):382-389. DOI:10.4164/sptj.52.382
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    ABSTRACT: Gold nanoshells with tunable surface plasmon resonances are a promising material for optical and biomedical applications. They are produced through seed-mediated growth, in which gold nanoparticles (AuNPs) are seeded on the core particle surface followed by growth of the gold seeds into a shell. However, synthetic gold nanoshell production is typically a multistep, time-consuming batch-type process, and a simple and scalable process remains a challenge. In the present study, a continuous flow process for the seed-mediated growth of silica–gold nanoshells is established by exploiting the excellent mixing performance of a microreactor. In the AuNP-seeding step, the reduction of gold ions in the presence of core particles in the microreactor enables the one-step flow synthesis of gold-decorated silica particles through heterogeneous nucleation. Flow shell growth is also realized using the microreactor by selecting an appropriate reducing agent. Because self-nucleation in the bulk solution phase is suppressed in the microreactor system, no washing is needed after each step, thus enabling the connection of the microreactors for the seeding and shell growth steps into a sequential flow process to synthesize gold nanoshells. The established system is simple and robust, thus making it a promising technology for producing gold nanoshells in an industrial setting.
    Particle and Particle Systems Characterization 08/2014; 32(2). DOI:10.1002/ppsc.201400126 · 3.08 Impact Factor
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    Tatsuya Hanafusa · Yasushi Mino · Satoshi Watanabe · Minoru T. Miyahara ·

    Advanced Powder Technology 05/2014; 25(3). DOI:10.1016/j.apt.2014.04.002 · 2.64 Impact Factor
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    ABSTRACT: Zeolitic imidazolate framework-8 (ZIF-8) has a “gate-opening” framework with narrow pore apertures that swing open by reorientation of 2-methylimidazolate (MeIM) linkers enforced by guest adsorption. The present study aimed to employ free energy analysis to provide insight into the mechanism of the adsorption-induced structural transition that results from the reorientation of the MeIM linkers. We combined experimental Ar adsorption at cryogenic temperatures with grand canonical Monte Carlo simulations to determine the free energy profiles as functions of the rotational angle of the MeIM linker (θIM) and bulk gas pressure. We also estimated the energy fluctuation of the system, which is crucial to discussing the structural transition from a metastable state. The results from the free energy analysis, for example, at 91 K, suggest the following conclusions: A gradual reorientation of the MeIM linkers up to θIM = 10.5° occurs with increasing gas pressure that is followed by a spontaneous structural transition to θIM = 25.5° during the adsorption process (gate opening), and then, during the desorption process, an equilibrium structural transition occurs with the opposite reorientation of the MeIM linkers from θIM = 25.5° to θIM = 10.5° (gate closing).
    The Journal of Physical Chemistry C 04/2014; 118(16):8445–8454. DOI:10.1021/jp500931g · 4.77 Impact Factor
  • Tatsuya Hanafusa · Yasushi Mino · Satoshi Watanabe · Minoru T. Miyahara ·
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    ABSTRACT: Microwire networks composed of noble metal particles are promising for the use of transparent conductive films. Bottom-up approaches can offer a route to establishing a fabrication technique that is robust and cost-effective, and template-assisted self-assembly techniques are widely used. However, they require additional processes to prepare templates and generally suffer from the difficulty in a large-scale fabrication. A template-free technique thus waits to be developed.In the present study, we explore a template free technique to fabricate colloidal networks of Au nanoparticles. We combine the convective self-assembly method with a liquid-level manipulation scheme in which the suspension is periodically pumped out. By using the technique, we successfully fabricate stripe, grid, and triangle patterns with controlled periodicity and examine the relationship between operation parameters and the resultant structures. We then measure the transparency and conductivity of a grid pattern to demonstrate the property as the transparent conductive film.
    Advanced Powder Technology 03/2014; 49(5). DOI:10.1016/j.apt.2014.01.013 · 2.64 Impact Factor
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    ABSTRACT: We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.
    Adsorption 02/2014; 20(2-3). DOI:10.1007/s10450-013-9588-2 · 1.77 Impact Factor
  • Satoshi Watanabe · Kotaro Inada · Minoru T. Miyahara ·

    01/2014; 51(5):355-362. DOI:10.4164/sptj.51.355
  • Ryohei Numaguchi · Hideki Tanaka · Satoshi Watanabe · Minoru T. Miyahara ·
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    ABSTRACT: Porous coordination polymers (PCPs) with soft frameworks show a gate phenomenon consisting of an abrupt structural transition induced by adsorption of guest molecules. To understand the dependence of the gating behavior on the host framework structure, we conduct grand canonical Monte Carlo simulations and a free-energy analysis of a simplified model of a stacked-layer PCP. The interlayer width of the rigid layers composing the simplified model can be changed by guest adsorption and by varying the initial interlayer width h0, which is controlled by the length of pillars between the layers. We introduce three types of gating behavior, one-step gating, filling and gating, and double gating, which depend on three parameters: the initial interlayer width h0; the interaction parameter ∊ss, which determines the host-guest framework interaction as well as the inter-framework interaction; and the elastic modulus of the framework, which depends on the stiffness of the pillars. We show that the one-step gating and the filling and gating behaviors depend strongly on h0 rather than on ∊ss, and thus a transformation from filling and gating to double gating can be achieved by reducing the stiffness of the host framework. This study should be a guideline for controlling the gating pressure of PCPs by modifying their chemical components.
    The Journal of Chemical Physics 12/2013; 140(4). DOI:10.1063/1.4862735 · 2.95 Impact Factor
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    ABSTRACT: ZIF-8 is one member of a family of zeolitic imidazolate frameworks [1,2], and has attracted much attention in many applications because of its high thermal stability and intra-framework flexibility. Recent experimental and simulation studies have revealed that reorientation of the imidazolate linkers of ZIF-8 is induced by molecular adsorption, which lead to increase in the accessible pore volume of ZIF-8. However, the mechanism of the structural transition is still unclear because of the lack of knowledge about free energy change due to the reorientation of the imidazolate linkers. We have therefore focused attention on argon adsorption in ZIF-8, and performed the free energy analysis [3] with the aid of grand canonical Monte Carlo (GCMC) simulations to investigate the adsorption-induced structural transition of ZIF-8. Adsorption isotherms of argon in ZIF-8 were measured at 79 K, 83 K, 87 K, and 91 K. A hysteresis loop was observed in all the adsorption isotherms, and its width increased with increasing temperature, which should provide important insight into the mechanism of the adsorption-induced structural transition of the argon/ZIF-8 system. We constructed atomistic ZIF-8 models by rotating the imidazolate linkers about an axis through two nitrogen atoms from 0 ° to 30 °, and a series of adsorption isotherms of argon in the respective ZIF-8 models were obtained by the GCMC simulations. We used the following force fields with modifications: the universal force field for argon-ZIF-8 interactions and the Amber force field to calculate the potential energy of the ZIF-8 framework. The free energy of the system was calculated as a sum of the Helmholtz free energy of the ZIF-8 framework and the contribution of adsorbed argon obtained by integrating the simulated adsorption isotherm. In the adsorption process, with increasing pressure, the global minimum of the free energy shifts to a larger rotational angle of the imidazolate linker without any activation processes, and a second local minimum appears at around 25.5 º. Further increase in pressure provides a switch of the global minimum, and a spontaneous structural transition takes place when the energy barrier lying between the metastable state at the smaller rotational angle (10.5 º) and the global minimum at the larger rotational angle (25.5 º) becomes less than the energy fluctuation of the system of 0.51 kT per an imidazolate linker. The energy fluctuation of the system determined from the comparison with the experimental data is quite suggestive because its value is comparable in magnitude to the energy of one rotational degree of freedom of the imidazolate linker. Then, in the desorption process, the system stays at the local minimum at 25.5 º regardless of the decrease in pressure, and finally an equilibrium structural transition occurs from 25.5 º to 10.5 º. The fact that the equilibrium structural transition is accomplished in the desorption process differently from the adsorption process is an indication that the energy fluctuation of the system is larger than 0.51 kT/linker. This is reasonable because the energy fluctuation of the system should be proportional to the adsorption amount of argon. Actually, the absorbed amount before the equilibrium structural transition in the desorption process is about 1.2 times larger than that before the spontaneous structural transition in the adsorption process. Finally, we constructed a theoretical adsorption isotherm of argon in ZIF-8 by using the free energy analysis, and obtained a good agreement with the experimental isotherm over the entire range of pressures. This suggests that our scenario for the adsorption and desorption processes obtained from the free energy analysis is quite appropriate to describe the mechanism of the adsorption-induced structural transition of ZIF-8. [1] X. C. Huang et al., Angew. Chem. Int. Ed. 45, 1557 (2006). [2] K. S. Park et al., Proc. Natl. Acad. Sci. U.S.A. 103, 10186 (2006). [3] S. Watanabe et al., J. Chem. Phys. 130 164707 (2009).
    13 AIChE Annual Meeting; 11/2013
  • Satoshi Watanabe · Minoru T. Miyahara ·
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    ABSTRACT: Bottom-up self-organization approaches are promising for fabricating higher-order patterned surfaces composed of colloidal particles. The first example among the patterns that have been extensively studied would be stripes; however, the formation of stripe patterns has so far been confined to partially or fully hydrophobic surfaces. By contrast, we have succeeded in preparing well-defined stripe patterns even on strongly hydrophilic substrates via a convective self-assembly technique. By using this technique, a stripe pattern was produced simply by suspending a substrate in a dilute suspension, without any complicated procedure; the stripes spontaneously aligned parallel to the contact line. Driven by this finding, we further investigate this self-assembly process, and find out that the convective self-assembly is quite promising as a template-free pattern formation technique. In the present paper, we first overview the convective self-assembly technique which is originally developed for uniform film formation, and then present our recent results on the pattern formation of colloidal particles through the convective self-assembly. This technique can produce various patterns including stripes, cluster arrays, and grids in response to macroscopic experimental parameters such as particle concentration and temperature.
    Advanced Powder Technology 11/2013; 24(6):897–907. DOI:10.1016/j.apt.2013.06.001 · 2.64 Impact Factor
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    ABSTRACT: Zeolite‐templated carbon is extraordinarily flexible and behaves as a kind of elastic microporous material, unlike other microporous materials, such as activated carbons, zeolites, and metal–organic frameworks. Its uniform micropores (1.2 nm) can be contracted/recovered reversibly just by loading mechanical force, and such pore‐size change induces noticeable increase in gas‐physisorption amount. For more details see the Full Paper by H. Nishihara et al. on page 13009 ff.
    Chemistry 09/2013; 19(39). DOI:10.1002/chem.201390153 · 5.73 Impact Factor
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    ABSTRACT: Nanoporous materials, such as zeolites, activated carbons, and metal-organic frameworks (MOFs), are peculiar platforms in which a variety of guest molecules are stored, reacted, and/or separated. The size of the nanopores is essential to realize advanced functions. In this work, we demonstrate a very simple but innovative method for the control of nanopore size, that is, reversible and continuous control by mechanical force loaded to soft nanoporous materials. The elastic properties of several microporous materials, including zeolites, zeolite-templated carbon (ZTC), activated carbon, and MOFs (e.g., ZIF-8), are examined and it is found that ZTC is a material that is suitable for the aforementioned idea thanks to its extraordinary soft properties compared to the others. The original pore size of ZTC (1.2 nm) can be contracted to 0.85 nm by using a relatively weak loading force of 135 MPa, whereas the other microporous materials barely contracted. To demonstrate the change in the physical properties induced by such artificial deformation, in situ gas adsorption measurements were performed on ZTC with and without loading mechanical force, by using CO2 , CH4 , and H2 , as adsorbates. Upon the contraction by loading 69 or 135 MPa, CO2 adsorption amount is increased, due to the deepening of the physisorption potential well inside the micropores, as proved by the increase of the heat of adsorption. Moreover, the adsorption amount is completely restored to the original one after releasing the mechanical force, indicating the fully reversible contraction/recovery of the ZTC framework against mechanical force. The experimental results are theoretically supported by a simulation using Grand Canonical Monte Carlo method. The similar adsorption enhancement is observed also on CH4 , whereas H2 is found as an exception due to the weak interaction potential.
    Chemistry - A European Journal 09/2013; 19(39). DOI:10.1002/chem.201301806 · 5.73 Impact Factor
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    ABSTRACT: We synthesized size-controlled zeolitic imidazolate framework (ZIF-8) nanoparticles using a T-type micromixer. The nanocrystallization of ZIF-8 proceeds by a rapid coordination reaction of Zn2+ ions and 2-methylimidazole (2-MeIM) in a highly concentrated solution. Therefore, a rapid mixing of the raw materials is required to control the size and morphology of ZIF-8 nanoparticles. First, we prepared ZIF-8 nanoparticles at various flow rates of raw materials to investigate the mixing performance of a T-type micromixer. The size of the ZIF-8 nanoparticles decreased with an increase in flow rates at Reynolds number (Re) < 2000 (regarded as laminar flow). However, the size and shape of the resultant nanoparticles did not depend on the flow rates at Re > 2000 (transition state between laminar flow and turbulent flow), demonstrating the high mixing performance. Further, we systematically examined the effects of temperatures and [2-MeIM]/[Zn2+] ratios on the resultant ZIF-8 nanoparticles to elucidate the formation mechanism and to optimize the conditions for the synthesis of smaller ZIF-8 nanoparticles. Through a detailed analysis, we concluded that the requisite conditions for the preparation of smaller ZIF-8 nanoparticles are lower temperatures and higher [2-MeIM]/[Zn2+] ratios, which affect the nucleation process and the particle growth process, respectively. Finally, we examined the adsorption properties of the resultant smaller ZIF-8 nanoparticles. We found that the amount of adsorbed N2 gas for the ZIF-8 nanoparticles synthesized by our method is higher than the amount for a conventional ZIF-8 sample, and the adsorption rate is faster because of the nanocrystallization.
    The Chemical Engineering Journal 07/2013; 227:145-150. DOI:10.1016/j.cej.2012.08.065 · 4.32 Impact Factor
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    ABSTRACT: We construct an atomistic silica pore model mimicking templated mesoporous silica MCM-41, which has molecular-level surface roughness, with the aid of the electron density profile (EDP) of MCM-41 obtained from X-ray diffraction data. Then, we present the GCMC simulations of argon adsorption on our atomistic silica pore models for two different MCM-41 samples at 75, 80, and 87 K, and the results are compared with the experimental adsorption data. We demonstrate that accurate molecular modeling of the pore structure of MCM-41 by using the experimental EDP allows the prediction of experimental capillary evaporation pressures at all investigated temperatures. The experimental desorption branches of the two MCM-41 samples are in good agreement with equilibrium vapor–liquid transition pressures from the simulations, which suggests that the experimental desorption branch for the open-ended cylindrical pores is in thermodynamic equilibrium.
    Adsorption 04/2013; 19(2-4). DOI:10.1007/s10450-013-9486-7 · 1.77 Impact Factor
  • Minoru T Miyahara · Hideki Tanaka ·
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    ABSTRACT: We present a modification of the molecular dynamics simulation method with a unit pore cell with imaginary gas phase [M. Miyahara, T. Yoshioka, and M. Okazaki, J. Chem. Phys. 106, 8124 (1997)] designed for determination of phase equilibria in nanopores. This new method is based on a Monte Carlo technique and it combines the pore cell, opened to the imaginary gas phase (open pore cell), with a gas cell to measure the equilibrium chemical potential of the confined system. The most striking feature of our new method is that the confined system is steadily led to a thermodynamically stable state by forming concave menisci in the open pore cell. This feature of the open pore cell makes it possible to obtain the equilibrium chemical potential with only a single simulation run, unlike existing simulation methods, which need a number of additional runs. We apply the method to evaluate the equilibrium chemical potentials of confined nitrogen in carbon slit pores and silica cylindrical pores at 77 K, and show that the results are in good agreement with those obtained by two conventional thermodynamic integration methods. Moreover, we also show that the proposed method can be particularly useful for determining vapor-liquid and vapor-solid coexistence curves and the triple point of the confined system.
    The Journal of Chemical Physics 02/2013; 138(8):084709. DOI:10.1063/1.4792715 · 2.95 Impact Factor
  • Ryohei Numaguchi · Hideki Tanaka · Satoshi Watanabe · Minoru T Miyahara ·
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    ABSTRACT: We conduct grand canonical Monte Carlo simulations and a free-energy analysis for a simplified model of a stacked-layer porous coordination polymer to understand the gate phenomenon, which is a structural transition of a host framework induced by the adsorption of guest particles. Our calculations demonstrate that stabilization of the system due to the guest adsorption causes host deformation under thermodynamic equilibrium. We also investigate spontaneous transition behaviors (gate opening and closing under metastable conditions). The structural transition should occur when the required activation energy, which is determined using the free-energy analysis, becomes equal to the system energy fluctuation. To estimate the system energy fluctuation, we construct a kinetic transition model based on the transition state theory. In this model, the system energy fluctuation can be calculated by setting the adsorption time and transition domain size of the host framework. The model demonstrates that a smaller domain size results in a gate-opening transition at lower pressure. Furthermore, we reveal that the slope of the logarithm of the equilibrium structural transition pressure versus reciprocal temperature shows transition enthalpy, and that slopes of the gate-opening and -closing transition pressures versus reciprocal temperature show activation enthalpies.
    The Journal of Chemical Physics 02/2013; 138(5):054708. DOI:10.1063/1.4789810 · 2.95 Impact Factor