Article pubs.acs.org/Langmuir
Double-Step Gate Phenomenon in CO2 Sorption of an Elastic LayerStructured MOF Manami Ichikawa,† Atsushi Kondo,‡ Hiroshi Noguchi,† Natsuko Kojima,† Tomonori Ohba,† Hiroshi Kajiro,§ Yoshiyuki Hattori,∥ and Hirofumi Kanoh*,† †
Graduate School of Science, Chiba University, Chiba 263-8522, Japan Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan § Nippon Steel & Sumitomo Metal Corporation, Shintomi, Futtsu, Chiba 293-8511, Japan ∥ Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan ‡
S Supporting Information *
ABSTRACT: A double-step CO2 sorption by [Cu(4,4′bpy)2(BF4)2] (ELM-11) was observed during isothermal measurements at 195, 253, 273, and 298 K and was accompanied by interlayer expansion in the layered structure of ELM-11. The first step occurred in the range of the relative pressure (P/P0) from 10−3 to 10−2. The second step was observed at P/P0 ≈ 0.3 at the four temperatures. Structural changes in ELM-11 during the CO2 sorption process were examined by X-ray diffraction (XRD) measurements. The structural change for the first step was well understood from a detailed structural analysis, as reported previously. The XRD results showed further expansion of the layers during the second step as compared to the already expanded structure in the first step, and both steps were found to be caused by the gate phenomenon. The energy for the expansion of the layer structure was estimated from experimental and simulated data.
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INTRODUCTION Porous metal−organic frameworks (PMOFs) and porous coordination polymers, which exhibit dynamic structural transitions attributed to soft interactions in their architectures, are expected to have sorption properties different from those of traditional porous materials.1−3 One of the most interesting phenomena in flexible MOFs is a guest-induced structural transition, which typically occurs at a threshold gas pressure and leads to an abrupt increase in the sorption isotherm, a phenomenon referred to as “breathing” and “gate sorption”.4−10 The breathing of MIL-53 consists of micropore filling accompanied by structural shrinkage and swelling with volume expansion.7,8 The gate sorption of layer-structured MOFs is accompanied by an abrupt increase and decrease in the sorption amount at a definite pressure with almost no sorption below the threshold pressure.4−6,11−15 Such guest-induced framework transitions have also been studied using theoretical and computational methods.8,16−18 With such novel properties, these materials are expected to be developed into a unique class of materials for gas separation and molecular sensor technologies.4,19−23 While there have been numerous studies on the synthesis and structures of flexible PMOFs, the next challenge is to control the functional aspects that result from the dynamic nature of PMOFs. Sorption ability, a basic function of most MOFs, is generally mentioned in most studies; however, only a few studies have reported on multistep gate phenomena.7,13,14,21,22 The gate phenomenon of an elastic layered-structure MOF, [Cu(4,4′-bpy)2(BF4)2] (ELM-11), is a representative example © XXXX American Chemical Society
of novel sorption behavior. This material shows unique gate sorption with CO2, N2, and CH4 through an expansive modulation of the layered structure.4,24−27 ELM-11 also exhibits a better capacity and recovery efficiency for CO2 sorption as compared to other nanoporous materials.9,26 The structure is obtained via the removal of coordinated water molecules from its hydrated form by heating. The stacking of the layers is stabilized by soft interactions such as π−π interactions and H−F hydrogen bonds.24 Very recently, detailed structures of ELM-11 and CO2-sorbed ELM-11 were reported.11,12 Generally, it is understood that the guest−host interactions in a flexible MOF structure can significantly influence its properties. Therefore, we considered whether introducing guest molecules such as CO2 molecules could induce a multistep gate phenomenon via molecular sorption. Multiple stages accompanied by structural changes in the crystal structures of MOFs have been reported.7 MOFs exhibit complicated structural changes that are dependent on gas pressure. We have reported the double-step sorption of [Cu(4,4′-bpy) 2 (OSO 2 CF 3 ) 2 ] (ELM-12) and [Co(4,4′bpy)2(OSO2CF3)2] (ELM-22); the first step consists of the micropore filling of the original open pores, and the second step is the gate phenomenon accompanied by an interlayer expansion.25 The first step does not occur due to an expansion of the layer structure, but due to adsorption on micropores. Received: July 10, 2016 Revised: September 1, 2016
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DOI: 10.1021/acs.langmuir.6b02551 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Double-step sorption behavior was also observed in ELM11;13,14 however, it is unclear whether the second sorption is a gate phenomenon accompanied by a structural change because the crystal structure was not examined for the second step. In the present study, a double-step gate phenomenon for CO2 sorption in ELM-11 was found at 195 K by measuring the X-ray diffraction (XRD) patterns of ELM-11 before and after both steps. In both of the steps, the interlayer distance increased due to the incorporation of CO2 molecules. Thus, this is the first report to show that the second sorption-step is due to gate opening.
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EXPERIMENTAL SECTION
On the basis of the method described in the literature,27 an aqueous solution of Cu(II) tetrafluoroborate (0.04 M) was heated at 343 K and then slowly mixed with a methanol solution containing 0.08 M 4,4′bipyridine (4,4′-bpy) at 343 K. After being refluxed for 2 h at 343 K, the mixed solution was allowed to settle for 15 h. Pre-ELM-11, {[Cu(BF4)2(4,4′-bpy)(H2O)2](4,4′-bpy)}, was obtained as a blue powder after evaporation, filtration, and drying. ELM-11 was obtained from Pre-ELM-11 by heating it at 373 K under a vacuum below 5 mPa for 2 h. After the pretreatment under vacuum (
