ARTICLE pubs.acs.org/Langmuir
Tuning of Gate Opening of an Elastic Layered Structure MOF in CO2 Sorption with a Trace of Alcohol Molecules Yan Cheng,† Hiroshi Kajiro,‡ Hiroshi Noguchi,† Atsushi Kondo,§ Tomonori Ohba,† Yoshiyuki Hattori,|| Katsumi Kaneko,^ and Hirofumi Kanoh*,† †
Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Nippon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba, 293-8511, Japan § Tokyo University of Agriculture and Technology, 3-8-1 Harumi, Fuchu, Tokyo 183-8538, Japan Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan ^ Exotic Nanocarbon Research Center, Shinshu University, 4-17-1, Wakasato, Nagano 380-8553, Japan
)
‡
ABSTRACT: It is important to tune the sorption behavior of metalorganic framework (MOF) materials. Ethanol treatment on the hydrated form of [Cu(bpy)2(BF4)2], which is a representative flexible MOF showing the fascinating gate phenomenon on CO2 sorption, induces an easier dehydration and a significant decrease in the CO2 gate pressure. The results of IR, X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) measurements indicated that water molecules in the lattice of the hydrated form can be removed even at room temperature after the ethanol treatment and the basic 2D layered structure remains with a slight interlayer expansion. The results of thermogravimetric (TG) and gas chromatograph/mass spectrometry (GC/MS) analyses and of CO2 sorptions indicated that the change of the gate phenomenon was caused by a trace of residual ethanol molecules included in the structure. Similar phenomena were observed on alcohols with different polarity and molecular size.
1. INTRODUCTION Flexible porous metalorganic frameworks (PMOFs) with dynamic structure transitions, attributed to soft interactions in the architectures, have been expected to show sorption properties different from traditional porous materials.13 One of the most interesting phenomena in flexible PMOFs is gate sorption caused by the guest-induced framework transition, which shows an abrupt increase and decrease of the sorption amount at a definite pressure with almost nil sorption below the threshold pressure.46 With this kind of novel properties, these materials have been expected to develop into a unique class of materials for gas separation and molecular sensor technologies.4,711 On the other hand, though studies about the synthesis and the structure of flexible PMOFs have been very prosperous, a next challenge is to control the functional aspects resulting from the dynamic nature. Sorption ability, a basic function of most MOFs, is generally mentioned in most of the studies, but few studies have been reported on how to tune the sorption behavior through a post-treatment after synthesis. A gate phenomenon of an elastic layered structure MOF, [Cu(4,40 -bpy)2(BF4)2] (ELM-11), is a repesentertive example of novel sorption behaviors. This material shows unique gate sorption on CO2, N2, and CH4 through an expansive modulation of the layered structure.4,1218 It shows a great advantage in the r 2011 American Chemical Society
capacity and the recovery efficiency of CO2 sorption, compared with other nanoporous materials.15 The structure is obtained through a removal of the coordinated water molecules in its hydrated form by heating; the stacking of the layers is stabilized by soft interactions such as ππ interaction and hydrogen bonds of HF interaction.12,19 Generally, it has been understood that the guesthost interactions in a flexible MOF structure can significantly influence its property. So, we wonder whether introducing guest molecules to the host structure can influence the sorption behavior, such as the gate pressure. If the gate pressure can be lowered, the requirement on the initial concentration of the adsorbate can be lowered and its application can be widely broadened; it could also lead to safer and energy-favored gas storage. Herein, we present results obtained for control of sorption behavior of ELM-11 through alcohol treatment on its hydrated form. We show that the treatment can result in easier dehydration and cause a reduction in the gate pressure of CO2 sorption by a trace of residual solvent molecules.
Received: March 17, 2011 Revised: April 18, 2011 Published: May 02, 2011 6905
dx.doi.org/10.1021/la201008v | Langmuir 2011, 27, 6905–6909
Langmuir
ARTICLE
2. EXPERIMENTAL SECTION 2.1. Synthesis. Based on the method described in literature,19 an improved method was used for the synthesis of the hydrated form, [Cu(bpy)2(BF4)2(H2O)2], named as pre-ELM-11. Cu(II) tetrafluoroborate aqueous solution (0.04 M) was heated at 343 K for 2 h and then slowly mixed with a methanol solution containing 0.08 M 4,40 -bipyridine at 343 K. After the subsequent reflux for 2 h at 343 K, the mixed solution was settled for 15 h. The powder materials of [Cu(bpy)2(BF4)2(H2O)2] were obtained as blue crystals after evaporation, filtration, and drying. The dehydrated form [Cu(bpy)2(BF4)2], referred as ELM-11, was obtained from the precursor through heating at 373 K for 3 h (
