Communication pubs.acs.org/crystal
Fusing High Symmetric Coordination Polyhedrons of Cu6(PIP)4, Cu12(PIP)8, and Cu12(PIP)24 into an Unprecedented Porous MOF: Synthesis, Structure, and Its Remarkable CO2 Selectivity Ruirui Yun, Jingui Duan, Junfeng Bai,* and Yizhi Li State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *
ABSTRACT: An unprecedented metal−organic framework, NJU-Bai6, formed by piling up the nanocages Cu6(PIP)4, Cu12(PIP)8, and Cu12(PIP)24 through sharing faces of the “opened” triangular and square windows, has been designed and structurally characterized. Very interestingly, it exhibits the third highest selectivity of CO2 over N2 with the value about 60 at room temperature.
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metal−organic framework, [Cu1.5PIP2][(CH3)2NH2]·xGuest [PIP = 5-(pyridin-4-yl-)isophthalic acid], NJU-Bai6 (Nanjing University Bai group)], a charged framework constructed by piling up the octahedron of Cu6(PIP)4, and two different types of cuboctahedrons of Cu12(PIP)8 and Cu12(PIP)24 (Figure 1). Quite interestingly, to the best of our knowledge, NJU-Bai6 exhibits the third highest selectivity of CO2 over N2 (S = 60, S means selectivity) at the near practical conditions. Solvothermal reaction of Cu(NO3)2·3H2O with PIP in N,Ndimethylformamide containing nitric acid afforded a high yield of polyhedral blue crystals of NJU-Bai6,11 which crystallizes in centrosymmetric tetragonal space group P4(2)/n. The overall structure of NJU-Bai6 consists of three kinds of nanocages (one type of octahedron and two types of cuboctahedrons): Cu6(PIP)4, Cu12(PIP)8, and Cu12(PIP)24, which are based upon four-coordinated Cu(II) ions and a tridentate ligand of PIP. The diameters of these nanocages are ca. 0.9, 1.6, and 1.8 nm, respectively, and they further share the “opened” triangular and square windows, respectively, to build this extended structure. It has been assigned as RCSR symbol tbo, which is defined as twisted boracites (Supporting Information Figure S1). Although Fujita, Stang, and others have reported a series of M6(L)4 and M12(L)24 (M = Pt or Pd, L = ligand) nanocages based upon the three connected nitrogen-containing ligands,4c,12 the cuboctahedral cage Cu12(PIP)8, which is formed
hese two decades have witnessed the rapid development of metal−organic frameworks (MOFs) due to their fantastic structures and potential applications, such as strategic gas storage, capture, etc.1 Very recently, MOFs based on MOPs (metal−organic polyhedra) have attracted great attention which was initially reported by Zaworotko’s groups.2 Meanwhile, coordination polyhedrons (CPs) are another intriguing topic in current supramolecular chemistry,3 which were first revealed by the pioneering chemist Fujita and, later on, became one of the research focuses because of their fascinating structures and relevance to self-assembly in nature,4 as well as their intriguing potential applications in materials science, host−guest chemistry, and related issues.5 However, metal−organic frameworks based on these high symmetric coordination polyhedrons have been little investigated, except for MOPs, but to directly fuse MOPs into extended MOFs remains a higher challenge. In addition, with global warming, CO2 capture has been one of the most urgent issues. Since the report of CO2 sorption in metal−organic frameworks (MOFs) by Yaghi and co-workers,6 it has become apparent that porous MOFs offer great potential in carbon capture and sequestration technologies (CCSTs).7 However, capture of low-concentration CO2 (∼15%) from nitrogen-rich streams, which reflect those in a flue gas mixture, remains a challenging task at present.1d,8 We are interested in construction of novel coordination nanostructures9 and metal−organic frameworks from high symmetric multidentate ligands with attractive properties.2b,10 Herein, we deliberately selected a three connected ligand which is the geometric requested building block for coordination nanocages with one pyridyl and two carboxylate groups (Figure 1a) and Cu(II) ions to successfully design an unprecedented © XXXX American Chemical Society
Received: October 29, 2012 Revised: November 28, 2012
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Figure 2. Sorption isotherms of CO2 and N2 on NJU-Bai6 at 298 K (green: CO2; blue: N2).
enthalpy for NJU-Bai6 is ca. 29.7 kJ/mol at zero loading, which reflects a strong adsorbent−adsorbate interaction. The higher Qst value in NJU-Bai6 may be attributed to the existence of an ion framework and (CH3)2NH2+ countercations in the framework. In conclusion, we have successfully designed and structurally characterized an unprecedented metal−organic framework, NJU-Bai6 formed by piling up the nanocages Cu6(PIP)4, Cu12(PIP)8, and Cu12(PIP)24 through sharing faces of the “opened” triangular and square windows, demonstrating first that these high symmetric coordination nanocages are also excellent building blocks toward MOFs. In addition, very interestingly, NJU-Bai6 exhibits the third highest selectivity of CO2 over N2, with the value of 60 at the near practical conditions. Our work will enrich and facilitate current MOFs chemistry in terms of both new types of structures and excellent properties.
Figure 1. Forming process of the structure of NJU-Bai6, which is constructed by facing MnLm nanocages; underneath: space at microscale vs macroscale piling up.
by eight intact ligands viewed as enclosed triangular facets, has not been reported previously. Furthermore, construction of the extended structures based upon these high symmetric coordination nanocages to create new types of MOFs is still a big challenge. This is the first example formed by piling up nanocages Cu6(PIP)4, Cu12(PIP)8, and Cu12(PIP)24 through sharing faces of the “opened” triangular and square windows. To confirm the permanent porosity of NJU-Bai6, the SCD (supercritical carbon dioxide) activated sample was degassed under high vacuum at room temperature. N2 adsorption for NJU-Bai6 at 77 K exhibits a quasi-reversible isotherm (Supporting Information Figure S4). The estimated apparent Brunauer−Emmett−Teller (BET) surface area is ca. 1051 m2/g (Langmuir surface area ca. 1717 m2/g) (Figure S5). Furthermore, CO2 and N2 low-pressure adsorption− desorption isotherms were measured at 273 and 298 K (0− 1.0 atm). At 298 K, the uptake amount of CO2 is 6.13 cm3 g−1 (STP; 1.17 wt %) at 0.15 atm, which is substantially higher than those of the well-known MOFs, such as MOF-177 (0.6 wt %, at 298 K and 0.15 atm), and ZIF-8 (0.6 wt %, at 298 K and 0.15 atm).8,13 In contrast, its N2 adsorption is almost zero (0.5218 cm3 g−1) at 298 K and 0.75 atm. As shown in Figure 2, the selectivity of NJU-Bai6 is 60 at 298 K, which is calculated from the pure-component isotherms by dividing the mass of CO2 adsorbed at 0.15 atm and N2 adsorbed at 0.75 atm according to the equation in Figure S5. As far as we know, it is the third highest record for MOF materials,8,14 indicating that NJU-Bai6 may be an excellent candidate for the postcombustion capture of CO2. To better understand the observations and evaluate the extent of CO2−framework interactions, the isosteric heat (Qst) of CO2 adsorption was calculated by the virial method using experimental isotherm data at 273 and 298 K. The adsorption
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Experimental details, additional figures, X-ray crystallogrophic information file (CIF), TGA and PXRD data, gases sorption data, and details of the isosteric adsorption enthalpy calculations. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the support of this work by the Major State Basic Research Development Programs (2011CB808704), the NSFC (20931004), the Fundamental Research Funds for the Central Universities (1114020501), and the Science Foundation of Innovative Research Team of NSFC (2101062).
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REFERENCES
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