Published: July 07, 2011 r2011 American Chemical Society 12849 dx.doi.org/10.1021/ja2051149 | J. Am. Chem. Soc. 2011, 133, 12849–12857 ARTICLE pubs.acs.org/JACS Understanding the Preferential Adsorption of CO 2 over N 2 in a Flexible MetalÀOrganic Framework Nour Nijem, † Peter Thissen, † Yanpeng Yao, ‡ Roberto C. Longo, † Katy Roodenko, † Haohan Wu, § Yonggang Zhao, § Kyeongjae Cho, † Jing Li, § David C. Langreth, ‡ and Yves J. Chabal* ,† † Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States ‡ Department of Physics and Astronomy and § Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States b S Supporting Information 1. INTRODUCTION MetalÀorganic frameworks (MOFs) have attracted much attention in a variety of fields such as gas separation and storage, catalysis and sensing, and polymers. 1À6 Compared to activated carbon and zeolites, MOFs have much higher surface areas (the largest reported is for MIL-101, with a Langmuir surface area of 5900 m 2 /g), 7 and their structures can be easily tailored. Different pore size/shape and functionalities can be achieved by simply selecting the metal center and/or the ligand. A unique property of some of these frameworks is their flexibility, allowing reversible structural changes to occur as a response to external stimuli such as guest inclusion. 8À17 Such changes have been identified by the appearance of a step (gate opening) and a hysteresis in the adsorption/desorption measurements. 18À20 An in-depth under- standing of the mechanisms involved cannot be derived using solely these physical measurements. Therefore, X-ray diffraction (XRD) techniques have been utilized to understand this gate- opening mechanism. 21,22 Although XRD data have proven valu- able for the characterization of geometrical transformation of MOFs, 23À25 this technique is not sensitive to changes that do not cause geometrical transformation and systems with no long- range periodicity. Moreover, in situ XRD is not readily available, and the analysis of the data is challenging. Vibrational spectroscopy such as infrared (IR) or Raman spectroscopy, on the other hand, is common and offers relatively simple data analyses. Consequently, vibrational techniques have been implemented to study the interactions of guest molecules in MOFs and to characterize MOF films. 18,26À35 However, inves- tigation of the breathing effects in flexible MOFs using IR and Raman spectroscopy has been limited to a few samples, such as MIL-53(Ga,Cr). 18 More work needs to be done to understand the factors that will make it possible to choose the ligands for optimum flexibility and desired selective adsorption. In this work, a novel, flexible, microporous MOF was selected to investigate the mechanisms involved in gate-opening phe- nomena, Zn 2 (bpdc) 2 bpee, where bpdc = 4,4 0 -biphenyl dicarbox- ylate and bpee = 1,2-bis(4-pyridyl)ethylene. This MOF is com- posed of two types of ligands arranged in a three-dimensional interpenetrated structure with one-dimensional parallelogram- shaped micropore channels running along the b-axis (window size ∼5 Â 7 Å; see Figure S1 in the Supporting Information). Each Zn metal center is tetrahedrally coordinated to three carboxylate groups from three bpdc ligands. The two ends of the bpdc ligand bond to the metal center in two different ways, one bidentate and the other monodentate. Two bidentate car- boxylate groups from two centrosymetrically related bpdc ligands are coordinated to two different Zn centers to form the eight-membered ring Zn 2 (COO) 2 2+ secondary building unit (SBU). The bpdc ligands form 2D interpenetrated nets, and Received: June 2, 2011 ABSTRACT: The unusual uptake behavior and preferential adsorption of CO 2 over N 2 are investigated in a flexible metalÀorganic framework system, Zn 2 (bdc) 2 (bpee), where bpdc = 4,4 0 -biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO 2 with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO 2 with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO 2 . DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der WaalsÀ Density Functional Theory method, showing that the more energetically favorable positions for the CO 2 molecules are closer to the CdC bond of the bpee and the CÀC bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guestÀhost interactions and structural changes in the framework.