Communication pubs.acs.org/IC
Novel azo-Metal−Organic Framework Showing a 10-Connected bct Net, Breathing Behavior, and Unique Photoswitching Behavior toward CO2 Le Le Gong, Xue Feng Feng, and Feng Luo* School of Biology, Chemistry and Material Science, East China University of Technology, Fuzhou, Jiangxi 344000, P. R. China S Supporting Information *
technique is most likely applied in low-energy CO2 capture and release because the current adsorbent technologies based on a MOF adsorbent, a zeolite, or other porous materials mainly rely on pressure, temperature, or vacuum,3−5 and this, in nature, is inevitably high energy consumption. Furthermore, we adopted a distinct approach through the construction of a diarylethene MOF and achieved a 75% dynamic change of CO2 adsorption.14 In this work, we report another very exciting result of 75% dynamic change and 78% instantaneous release of CO2 in an azoMOF, namely, ECUT-15 with the framework of Co3(L)2 (AzDC)3 (L = N1,N3-dipyridin-4-ylisophthalamide; H2AzDC = 4,4′-diazene-1,2-diyldibenzoate acid). The crystal of ECUT-15 is determined by single-crystal X-ray diffraction at 113 K, giving the monoclinic space group P21/C. The solvents within in it could not exactly be assigned, and then they are treated by the PLATON Squeeze program.18 There are two crystallography-independent CoII ions (Figure 1). Co1 is sixcoordinated in an octahedral geometry finished by two N atoms from two L ligands [Co−N = 2.047(4)−2.162(4) Å] and four O atoms from three AzDC2− carboxylate ligands [Co−O =
ABSTRACT: Herein, we report a robust azo-metal− organic framework (MOF), namely, ECUT-15, which can be described as a 10-connected bct net built on trinuclear Co3 subunits. The activated samples of it perform a somewhat breathing behavior. Most importantly, under UV irradiation, this MOF performs outstanding photoswitching behavior toward CO2, giving great variation in the CO2 capture/release performance, for example, 45% under static conditions and 75% under dynamic measurements, as well as instantaneous release of up to 78%.
M
etal−organic frameworks (MOFs), as a renovated material platform, are now receiving growing interest because of not only their intriguing topology architectures but also a large number of applications in many fields such as gas adsorption/separation, sensors, catalysis, etc.1,2 Among them, the performance of CO2 capture and separation is very outstanding, and MOF materials are even viewed as one of the promising porous sorbents for such an application.3−5 Especially, sometimes we can apply some approaches, based on the designable nature of MOFs,10−16 such as presetting the openmetal and Lewis basic sites and strongly polarizing functional groups in the channel of MOFs, to enhance the CO2 adsorption capacity and selectivity.6−10 On the other hand, researchers recently paid extensive attention to a new subclass, viz., photosensitive MOFs, which feature certain light-responsive organic groups on the internal surface of MOFs. Recent advances disclosed that the merit of such MOFs can facilitate the host−guest interactions and adsorption/release of cargo and even sometimes modulate the performance.11−17 For example, Zhou et al. designed the MOF of PCN-13 with the pore wall decorated by free-standing photosensitive azobenzene units and disclosed that the transto-cis transformation of an azobenzene unit in PCN-13 under both UV and heating treatment can significantly affect the CO2 adsorption.11 Kitagawa et al. proposed a distinct approach by doping an azobenzene unit inside a MOF, and a big change of N2 adsorption under UV trigger was observed.12 Moreover, Hill et al. explored a more facile method by employing an azobenzene unit as a building block to construct a MOF and disclosed that the local C−C−N bending movement of azobenzene ligands under UV rather than the familiar trans-to-cis transformation of the azobenzene unit would also cause a very sharp decrease of CO2 adsorption, resulting in a 64% dynamic change of CO2 adsorption.13 In this regard, they thought that this kind of © XXXX American Chemical Society
Figure 1. Views of (a) the Co3 subunit and the 10-connecting mode for each Co3 node, (b) the 3D framework with a 1D channel along the a axis, (c) the 10-connected bct net, (d) the polyhedron of the Co3 node, and (e) the coordination surrounding the CoII ions. Received: September 5, 2015
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DOI: 10.1021/acs.inorgchem.5b02037 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
N2 or CO2 at low temperature.22 As expected, ECUT-15 displays a stepwise N2 sorption profile at 77 K (Ptrans = 0.35 P/P0) and the desorption branch does not follow the trace of adsorption branch, giving a big hysteresis, indicative of a transformation from narrow pore (np) to large pore (lp) (Figure S3). A similar trend is also observed in CO2 adsorption at 195 K within a pressure limit of P < 1 bar (Figure S4). However, the CO2 isotherm at 195 K manifests much more pronounced steps, a higher transition pressure (P/P0 ≈ 0.5), and smaller hysteresis. A similar feature of gas-specific isothermal sorption profiles has been well established in most dynamic MOFs like that of MIL53, DMOF-1-AM, [Zn2(2,5-BME-bdc)2(dabco)]n, and YOMOF, where the breathing process featuring structural transitions between the np and lp forms has been well realized.23 Recent advances have revealed that the acylamide group owns high affinity toward the CO2 molecule, and in some acylamideMOFs, high selective CO2 adsorption over other gases including N2, CH4, CO, and O2 has been observed.24 This arouses us to further explore the adsorption selectivity of this MOF material. As shown in Figure S4, ECUT-15 barely adsorb N2, CH4, CO, and O2 (
