CRYSTAL GROWTH & DESIGN
Lithium Based Metal-Organic Framework with Exceptional Stability
2009 VOL. 9, NO. 5 2500–2503
Debasis Banerjee,† Sun Jin Kim,‡ and John B. Parise*,†,§ Department of Chemistry, Stony Brook UniVersity, Stony Brook, New York 11794-3400, Nano-Materials Research Center, Korea, Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, and Department of Geosciences, Stony Brook UniVersity, New York 11794-2100 ReceiVed December 29, 2008; ReVised Manuscript ReceiVed February 13, 2009
ABSTRACT: A three-dimensional lithium based metal-organic framework Li2(2,6-NDC) (1); UL-MOF-1, UL ) ultralight; NDC ) napthalenedicarboxylate; space group P21/c, a ) 10.302(5), b ) 5.345(2), c ) 8.663(4) Å, β ) 98.659(9)°, V ) 471.6(4) Å3 was synthesized. The framework is stable to 610 °C. The synthesis of 1 was achieved using solvothermal methods in N,Ndimethylformamide at 180 °C and was characterized by single crystal X-ray diffraction. The structure consists of layers of twodimensional antifluorite related LiO connected by the NDC linker. Introduction Exploration directed toward the synthesis of novel metalorganic frameworks (MOFs), networks built by an array of metal centers connected through multifunctional ligands, is an expanding field due to potential applications of MOFs in gas storage,1-11 ion-exchange,12-15 catalysis,16,17 and luminescence.18-20 Because of their structural rigidity and the flexibility of the mode of coordination for the carboxylate moiety,21,22 aromatic dicarboxylates [e.g., 1,4-benzene dicarboxylic acid (1,4-BDC), 2,6naphthalene dicarboxylic acid (2,6-NDC)] are the most commonly used multifunctional ligands. The combination of possible metal centers with potential multifunctional ligands provides a vast pool of possible new materials with diverse architectures and properties. One extensively discussed application of MOFs is their use as a gas-storage (H2, CH4) medium. The U.S. Department of Energy has set a target36 for storing H2 at 6 wt% or 45 g L-1 for commercial use in an automobile by year 2010. The MOFs explored thus far as potential H2 storing materials commonly involve first row transition metals (e.g., Ni, Zn).3,23 One strategy for increasing gravimetric storage capacity is with lightweight metal centers (e.g., Mg, Al).2,24,25 Taking this strategy to the limit suggests the need to explore novel frameworks based on the lightest coordinating metal center, lithium. Lithium-based MOFs are rare,26-29 which is especially surprising since recent computational and experimental studies30-34 showed a 2 wt% increase in H2 uptake at room temperature for MOF-5 when it is doped with lithium. This increase in uptake is attributed to the electrostatic interaction of bare lithium centers with hydrogen.31 The possibility of discovery of ultralightweight (UL) frameworks based on lithium rather than Mg, Al,2,23,29 coupled with the high interaction energy of lithium with hydrogen, encouraged us to investigate the range of possible materials forming with lithium and carboxylate ligands. Here we report the solvothermal synthesis and characterization of Li2(2,6-NDC) ULMOF-1 * Corresponding author. Phone: (631) 632-8196. Fax: (631) 632-8240. E-mail:
[email protected]. † Department of Chemistry, Stony Brook University. ‡ Korea Institute of Science and Technology. § Department of Geosciences, Stony Brook University.
[ultralight MOF] forming a three-dimensional (3-D) network with two-dimensional (2-D) corner and edge sharing Li-O tetrahedra. Experimental Section Synthesis. ULMOF-1 was synthesized under solvothermal conditions in Teflon-lined 23-mL Parr stainless steel autoclaves. Starting materials include lithium nitrate (LiNO3, 99+%, Acros-organic), 2,6naphthalenedicarboxylic acid (2,6-NDC, 95+%, Sigma-Aldrich), ammonium fluoride (NH4F, 98%, Sigma-Aldrich), N,N-dimethylformamide (DMF, 99%, Sigma-Aldrich), and ethanol (99%, SigmaAldrich). The synthesis was achieved by using a mixture of 0.005 mol of 2,6-NDC (0.565 g), 0.01 mol of LiNO3 (0.345 g), and 0.002 mol of NH4F (O.038 g). The mixture was dissolved in 15 g of DMF. The resultant gel was stirred for 4 h, transferred to a Teflon lined vessel, and heated at 180 °C for 5 days. The product was obtained as yellowish, needle-shaped crystals (yield: 60%, 0.347 g, calculated based on the amount of metal used) that were recovered by filtration and washed with ethanol. Crystal Structure Determination. The structure of ULMOF-1 was determined from single-crystal X-ray diffraction data collected from a crystal (Table 1) selected under a polarizing binocular microscope. It was glued on the tip of a glass fiber mounted on a Bruker P4 diffractometer equipped with a SMART CCD detector. The sampleto-detector distance was 5.135 cm. A total of 1650 frames was collected with an exposure time of 30 s, combining φ and ω-scans with a step width of 0.30°. The raw intensity data were integrated with software packages SMART37 and SAINT.38 The absorption correction was applied using Psi-scan method on the strongest reflections (I/σ: 15). The structure was solved using Direct methods and the model refinement was carried out using full matrix least-squares on F2 using SHEXLTL.39 Oxygen atoms were located first, followed by the determination of the other atom positions (C, Li) from the Fourier difference map. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added to the naphthalene carbon atoms using geometrical constraints (HFIX command). Bulk sample identitification and phase purity were determined using powder X-ray diffraction. The data was collected using a Scintag PAD-X diffractometer equipped with Cu KR radiation within a 2-theta range of 5°-40° (step size of 0.04° with a counting time of 0.75 s/step). The powder data collected was consistent with the powder pattern simulated based on the structure determined from single crystal diffraction data (Figure S4, Supporting Information). Combined TGA-DSC analysis was performed using a Netzsch 449C Jupiter instrument. The sample was heated from room temperature to 750 °C under a N2 atmosphere with a heating rate of 10 °C/min (Figure 5). The product recovered was a poorly crystalline
10.1021/cg8014157 CCC: $40.75 2009 American Chemical Society Published on Web 03/24/2009
Lithium Based Metal-Organic Framework Table 1. Crystallographic Data and structure Refinement Detailsa empirical formula formula weight collection temperature wavelength (Å) space group unit cell dimension
volume (Å3) Z calculated density (g/cm3) absorption coefficient F(000) crystal size (mm) θ range of data collection index range total reflection independent reflection goodness of fit absorption correction refinement method data/restraints/parameter final R [I > 2σ(I) R indices largest difference peak and hole
C6H3O2Li 114.02 298(2) 0.71073 P21/c a ) 10.302(5) b ) 5.345(2) c ) 8.662(4) β ) 98.659(9) 471.6(4) 4 1.606 0.117 232 0.22 × 0.12 × 0.07 2.00-26.73 -13 e h e 11 -6 e k e 6 -10 e l e 10 2931 997 [R (int) ) 0.0725] 1.062 psi-scan full-matrix least-squares on F2 997/0/82 R1 ) 0.0405, wR2 ) 0.1060 R1 ) 0.0557, wR2 ) 0.1132 0.263 and -0.196
a w ) 1/[ σ2(F02) + (0.0538P)2 + (0.0847P)], where P ) (F02 + 2FC2)/3.
Crystal Growth & Design, Vol. 9, No. 5, 2009 2501 Table 2. Selected Bond Distance (Å) and Angle (Deg) for ULMOF-1a Li(1)-O(1) Li(1)#2-O(2) Li(1)#3-O(2) Li(1)#1-O(1) C1-C2 O(2)#4-Li(1)-O(1) #1 O(1)#1-Li(1)-O(1) O(2)#4-Li(1)-O(2) #3 O(1)#1-Li(1)-O(2) #3 O(1)-Li(1)-O(2) #3 O(2)#4-Li(1)-O(1)
2.016(3) 1.965(3) 2.029(3) 1.968(3) 1.508(2) 103.40(15) 91.71(14) 114.33(15) 129.53(17) 99.33(14) 116.92(16)
a Symmetry code: #1 -x + 2, -y, -z + 2, #2 x, -y + 1/2, z - 1/2, #3 x + 2, -y + 1, -z + 2, #4 x, -y + 1/2, z + 1/2.
Figure 2. Local environment of lithium in ULMOF-1.
Figure 1. View of Li2(2,6 NDC) from [010] direction showing the connectivity of the organic layer with the alternating antifluorite type LiO layer. black powder. Subsequent powder diffraction pattern (Figure S1, Supporting Information) is evident of the destruction of the framework.
Result and Discussion The structure of ULMOF-1 consists of a combination of alternating layers of LiO and the aromatic bridging unit (Figure 1). Lithium atom is present in distorted tetrahedral coordination with carboxylate oxygen in the framework (Table 2), which has been observed before with a recently reported lithium-based framework.24 Each lithium atom is bonded to four carboxylate moieties associated with four different naphthalene rings (Figure 2). The bond length of Li1 with carboxylate oxygen ranges from 1.96(3) to 2.02(3) Å, similar to previously reported values.28,29 The bond valence sum35 of lithium matches the expected value of +1 (calculated: 0.97). The tetrahedrally coordinated lithium forms 2-D Li-O-Li layers (Figure 3). Basic unit of the layer is made with a pair
Figure 3. Ellipsoidal view of the 2-D Li-O-Li layer from the [100] direction.
of edge-shared lithium tetrahedra connected to each other in a corner-shared arrangement (Figure S3, Supporting Information). Distances between the lithium atoms in each edgeshared tetrahedron are 2.77(6) Å, while the distance between the lithium atoms of the corner-shared tetrahedra is 3.26(4) Å. The oxygen atoms, connecting the lithium atoms of the two edge shared tetrahedra, are joined with the neighboring oxygen atoms of the corner shared tetrahedra through the carboxylate carbon atom. In total, each carboxylate group is coordinated with four lithium centers. The orientation of the
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Figure 4. Time-dependent heating effect on ULMOF-1.
carboxylate group in the 2-D layer is very well organized; that is, two-carboxylate groups connect the basic unit of the 2-D layer, with one of the carboxylate groups connecting to the lower LiO layer and the other one connecting to the upper layer (Figure S3, Supporting Information). The 2-D LiO layer forms an antifluorite type structural motif, which is common in lithium oxide based compounds. The basic difference between the antifluorite motif and the observed 2-D LiO layer in ULMOF-1 is the presence of tetrahedral vacancies in the layer of ULMOF-1. The antifluorite motif consists of edgeshared tetrahedra, but, in the case of ULMOF-1, 1/2 of the lithium tetrahedra are missing. Carboxylate groups use that free space for connecting adjacent edge shared tetrahedra. Each of these antifluoroite types LiO layers are on average 10.3 Å apart from each other with the carboxylate oxygen atoms forming an abab type layer. Naphthalene dicarboxylates connect each of the LiO layers. The naphthalene rings are stacked in layers along the [010] direction. The distance between two consecutive naphthalene rings along the [010] direction is 3.7-3.9 Å, while the distance in the [001] direction is between 4.28-4.5 Å. The close contacts between the naphthalene rings along the [010] direction indicates possible π-π* interactions between the adjacent rings. Because of the close packed nature of naphthalene rings connecting the adjacent layers, the 3-D network has no solvent accessible volume.40 No subsequent gas-adsorption study has been performed on the compound due to its nonporous nature. ULMOF-1 is not soluble in any of the common organic solvent (e.g., acetone, methanol, ethanol, ethyl acetate, DMF). ULMOF-1 showed extremely high thermal stability under N2 atmosphere. Two samples of ULMOF-1 were heated at 500 °C for 4 and 20 h respectively to check the thermal stability of the compound. The diffraction pattern (Figure 4) taken after 4 h of heating showed a diffraction pattern similar to ULMOF-1, showing the retention of the original structural stability. Heating of the sample for 20 h resulted in a slightly different diffraction pattern with an increase in intensity for the peaks corresponding to the [002] and [112] plane. The change in diffraction maxima due to prolonged heating could be attributed to the change in the stacking pattern of the naphthalene rings. The TGA-DSC data (Figure 5) showed that there is no change of phase until 610 °C, which indicates that the framework maintains stability up to that temperature. The structure subsequently collapses to
Figure 5. TGA-DSC plot of ULMOF-1.
a poorly crystalline black powder (Figure S1, Supporting Information). Conclusion We have synthesized a new 3-D MOF using lithium as a metal center and 2,6-NDC as an organic linker. The framework contains a novel 2-D LiO antifluorite layer connected by an organic linker. ULMOF-1 showed unprecedented thermal stability in comparison to other members of the MOF family. The detailed structural characterization of the framework through single crystal X-ray diffraction enables us to understand the underlying chemistry of this potentially important class of material. Currently, we are working on ligands of more diverse nature and exploring different synthetic conditions, which will enable us to synthesize frameworks with sustainable porosity. Acknowledgment. This work is supported by the National Science Foundation (DMR-0800415). D.B. would like to thank Dr. Lauren Borkowski for the assistance with the single crystal diffraction studies. S.J.K. is grateful for the support from Korea Institute of Science and Technology (KIST). Supporting Information Available: Figures S1–S5 and X-ray crystallographic information file (CIF) is available for ULMOF-1. This material is available free of charge via the Internet at http://pubs.acs.org.
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