ARTICLE pubs.acs.org/crystal
Syntheses, Structures, and Gas Adsorption Properties of Two Novel CadmiumSodium Organic Frameworks with 1,3,5-Benzenetricarboxylate Ligands Ying Fu,† Jie Su,† Zhibo Zou,‡ Sihai Yang,§ Guobao Li,*,† Fuhui Liao,† and Jianhua Lin*,† †
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China ‡ Beijing Huiwen Middle School, Beijing 100061, People's Republic of China § School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
bS Supporting Information ABSTRACT: Solvothermal reactions of Cd(CH3COO)2 3 2H2O and NaOH with 1,3,5benzenetricarboxylic acid (H3BTC) in a mixture of H2O/DMA (DMA = N,N0 dimethylacetamide) gave rise to two novel 3D porous metalorganic frameworks (MOFs), [Cd3Na6(BTC)4(H2O)12] 3 H2O (1) and [Cd3Na2(BTC)3(H2O)3] 3 [H2N(CH3)2] 3 H2O (2), which were characterized by single crystal and variable temperature powder X-ray diffraction (VTPXRD), IR spectroscopy, elemental analyses, inductively coupled plasma measurements, and coupled thermogravimetricmass spectrometric analyses. Compound 1 crystallizes in the tetragonal space group P42/n and could be described as a 2-fold interpenetrated net having a boracite (bor) topology, bridged by {Na3O12} trimers. Compound 2 crystallizes in the monoclinic space group Cc with 1D channels distributed in an unusual honeycomb fashion along the b-axis. {Na2O10} dimers, (CH3)2NH2+ ions, and guest water are located in these channels. TG-MS and VTPXRD studies revealed that 1 had a high thermal stability up to 340 °C under air, and 2 was stable up to 310 °C. The gas-adsorption investigation disclosed that compound 1 exhibited H2 uptake of 1.11 wt % at 77 K and 1.0 bar, with a high adsorption heat (8.4 kJ mol1).
’ INTRODUCTION Porous metalorganic frameworks (MOFs) have undergone rapid development due to their attractive structures1 and beneficial properties such as storage, separation, catalysis, sensing, magnetism, luminescence, and nonlinear optics.2 The advantages of MOFs for hydrogen storage are based upon their low framework densities, high surface areas, and controllable crystal structures.3 These new materials often display exceptional reversibility and fast kinetics, but very weak H2 adsorption energies (typically 47 kJ mol1) lead to adsorption at cryogenic temperature (often 77 K) and high pressure (typically 2090 bar).4 Thus, an increase in the binding energy of H2 in these materials above 15 kJ mol1 is needed to facilitate H2 adsorption at ambient temperatures and pressures.5 Currently, three main strategies have been used for enhancing H2 binding within MOFs. The first is to construct frameworks with narrow pores and nonlinear channels to increase the affinity for H2 adsorption by the overlap of the potential energy fields of the opposite walls and decrease unused pore space, either by selecting ligands and metal centers or through framework interpenetration.6 The second approach is to pay attention to the creation of open metal sites typically through the removal of coordinated solvent molecules at nodes, which provides stronger direct H2 binding sites to the metal nodes.7 In addition, doping with alkaline metals such as Li, Na, and K in porous materials is appealing, because they are predicted to afford a strong nondissociative interaction with H2 in accordance with theoretical and modeling studies.8 r 2011 American Chemical Society
Following these ideas, the systematic studies on transition and alkali metals coordinated with 1,3,5-benzenetricarboxylate and the assistant ligands have been focused on by us.9 Here, we report the syntheses, crystal structures, and gas adsorption properties of two novel coordination frameworks based on Cd(II) and Na(I) complexes with 1,3,5benzene-tricarboxylate ligands, [Cd3Na6(BTC)4 (H2O)12] 3 H2O (1) and [Cd3Na2(BTC)3(H2O)3] 3 [H2N(CH3)2] 3 H2O (2). These two MOFs display narrow voids, high thermal stabilities, and unsaturated Na sites after removing coordinated solvent molecules. Particularly, compound 1 could be regarded as a success of above approaches to produce a microporous material exhibiting exceptional H2 uptake at ambient pressures and the higher isosteric heat of adsorption for H2.
’ EXPERIMENTAL SECTION Materials and Characterizations. All solvents and reagents for the syntheses were of analytical grade and were used as received from commercial sources without further purification. Variable temperature powder X-ray diffraction (VTPXRD) data of the studied samples were collected on a Bruker D8 Discover diffractometer with Cu KR radiation (λ = 1.5418 Å) at 40 kV, 40 mA, and a graphite monochromator at the secondary beam. The heating rate between two temperatures was 0.17 °C s1. Each pattern was recorded after the corresponding temperature retained 600 s, Received: April 10, 2011 Revised: June 18, 2011 Published: June 28, 2011 3529
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Table 1. Crystallographic and Structural Refinement Parameters for 1 and 2 1
2
formula
Cd3Na6C36H38O37
Cd3Na2C29H25O22N
fw crystal system
1537.80 tetragonal
1122.68 monoclinic
space group
P42/n
Cc
a (Å)
13.8730(14)
14.644(3)
b (Å)
13.8730(14)
13.655(3)
c (Å)
13.812(3)
17.683(4)
β (deg)
90.00
111.63(3)
V (Å3)
2658.2(7)
3286.9(11)
Z Dcalcd (g cm3)
8 1.921
4 2.269
λ (Mo KR) (Å)
0.71073
0.71073
μ (mm1)
1.345
2.051
GOF on F2
1.006
1.005
Rint
0.0427
0.0256
R1, wR2 [I > 2σ (I)]
0.0660, 0.1961
0.0425, 0.1616
R1, wR2 (all data)
0.0711, 0.2023
0.0430, 0.1619
within the 540° range (2θ) with a step size of 0.02° and a scanning rate of 10 s step1. IR spectra were recorded on a Magna-IR 750 FTIR spectrophotometer in the region 4000650 cm1. Elemental analyses for C, H, and N were carried out on an Elementar Vario EL III microanalyzer. Inductively coupled plasma (ICP) optical emission spectroscopy for the ratio of Na:Cd was measured by an ESCALAB2000 analyzer. Thermogravimetricmass spectrometric (TG-MS) analyses were performed in air at a heating rate of 10 °C min1 from 50 to 800 °C, using a NETZSCH STA449C instrument. Synthesis of [Cd3Na6(BTC)4(H2O)12] 3 H2O (1). A mixture of Cd(CH3COO)2 3 2H2O (2.0 mmol, 0.5330 g), H3BTC (8.0 mmol, 1.6808 g), NaOH (24.0 mmol, 0.9600 g), DMA (DMA = N,N0 -dimethylacetamide) (7.0 mL), and deionized water (9.0 mL) was sealed in a 23 mL Teflon-lined stainless autoclave and heated at 130 °C for 5 days and then cooled to room temperature. The colorless crystalline product was filtered, washed with DMA and deionized water, and then dried in air to give about 0.92 g of 1 [yield 90% based on Cd(CH3COO)2 3 2H2O]. Anal. calcd for Cd3Na6C36H38O37 (fw 1537.80): C, 28.12; H, 2.49. Found: C, 28.17; H, 2.51. ICP analysis indicated a ratio of Na:Cd = 2.012 (calcd 2.000) for 1.
Synthesis of [Cd3Na2(BTC)3 (H2O)3] 3 [H2N(CH3)2] 3 H2O (2).
Compound 2 was synthesized from Cd(CH3COO)2 3 2H2O (2.0 mmol, 0.5330 g), H3BTC (2.0 mmol, 0.4202 g), NaOH (1.3 mmol, 0.0532 g), DMA (7.0 mL), and distilled water (3.0 mL). The mixture was sealed in a 23 mL Teflon-lined stainless autoclave and heated at 135 °C for 5 days. After it was cooled down to room temperature, colorless crystals were filtered, washed with DMA and distilled water, and left to air dry to give about 0.43 g of 2 [yield about 57% based on Cd(CH3COO)2 3 2H2O]. The (CH3)2NH2+ was generated via decomposition of DMA. Anal. calcd for Cd3Na2C29H25O22N (fw 1122.68): C, 31.02; H, 2.24; N, 1.25. Found: C, 31.01; H, 2.51; N, 1.34. ICP analysis indicated a ratio of Na: Cd = 0.679 (calcd 0.667) for 2. Crystallographic Studies. Suitable single crystals of 1 and 2 were carefully selected under an optical microscope and glued to thin glass fibers with epoxy resin. X-ray single-crystal diffraction data were collected on a Rigaku Mercury diffractometer equipped with graphite monochromated Mo KR radiation (λ = 0.71073 Å) by using the ω-scan mode at room temperature. The data absorption corrections were applied based on CrystalClear program.10 The structures were solved by the direct method and refined on F2 with full-matrix least-squares methods using the SHELXS-97 and SHELXL-97 programs, respectively.11 All of
Figure 1. (a) Thermal ellipsoid plot (50%) drawing of the coordination environment of Cd(II) and Na(I) ions in 1. Symmetry codes: (i) x, y, z; (ii) x + 1/2, y + 1/2, z; (iii) y, x + 1/2, z + 1/2; (iv) y, x + 1/ 2, z + 1/2; (v) y + 1/2, x, z + 1/2 (color codes: Cd, ciel; Na, green; C, gray; and O, red. The free water molecules and hydrogen atoms are omitted for clarity). (b) Projection of [Cd3(BTC)4] single framework, revealing the existence of channels with a cross-section of 8.2 Å 8.3 Å along the a direction [the cross-sections have been calculated based on van der Waals radii (space-filling representations)]. (c) Projection of [Cd3(BTC)4] single framework, revealing the existence of channels with a cross-section of 6.9 Å 7.0 Å along the c direction. 3530
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the nonhydrogen atoms were refined anisotropically, except that O atoms of uncoordinated water were refined isotropically for 1. Hydrogen atoms were added in the riding model and refined isotropically with OH = 0.82 Å, NH = 0.90 Å, and CH = 0.93 Å (BTC) or CH = 0.96 Å [(CH3)2NH2+]. The crystallographic data and structural refinement parameters are presented in Table 1, and the selected bond lengths are listed in Table S1 of the Supporting Information. CCDC 810694 and 810695 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Gas Adsorption Measurements. N2 and ambient-pressure H2 (0.01.0 bar) isotherms for compound 1 and 2 were recorded on a Micromeritics ASAP 2020 adsorption apparatus. Compounds 1 and 2 were evacuated to 108 bar by heating at 250 °C for 24 h to give fully desolvated samples. H2 and N2 isotherms at 77 K were measured using liquid nitrogen baths; H2 isotherms at 87 K were measured using liquid argon baths. For all isotherms, warm and cold free space correction measurements were performed using ultrahigh-purity (UHP) He gas (99.999%); all measurements were performed using UHP grade (99.999%) N2 and H2.
’ RESULTS AND DISCUSSION Infrared (IR) Spectra. IR spectra of 1 and 2 were measured (shown in Figure S1 of the Supporting Information), which confirm the presence of the organic ligands used in the syntheses (through the characteristic bands of aromatic and carboxylate groups). The broad absorption bands of the asymmetric and symmetric stretching vibrations of water appear at 37002700 cm1.12ac The bands at 16161527 and 14441371 cm1 correspond to the asymmetric and symmetric stretching vibrations of the bound carboxylate groups (CO2M), respectively.12 The out-of-plane deformation vibrations of the CH groups in the benzene ring give absorption bands from 762 to 735 cm1.12b,c The absence of the absorption bands from 1680 to 1800 cm1 indicates the complete deprotonation of BTC ligands.12d Crystal Structure of [Cd3Na6(BTC)4 (H2O)12] 3 H2O (1). Compound 1 crystallizes in the tetragonal space group P42/n and consists of a 3D open framework. As illustrated in Figure 1a, the asymmetric unit of compound 1 contains two crystallographic unique Cd(II) cations, two Na(I) ions, one BTC ligand, three coordinated water molecules, and one solvated water molecule. For the three coordinated aqua ligands, two are μ2-H2O bridging ligands, and the other one coordinates to metal centers as terminal water. All atoms are located in general positions except for Cd(1), Cd(2), and Na(2). The Cd(1) atom lies on the 2-fold axis and exhibits the six coordinated configuration to construct a distorted octahedron. Each Cd(1) is chelated by two BTC ligands [O(3), O(4) and O(3)ii, O (4)ii] and further coordinated with two carboxyl oxygen atoms [O(2) and O(2)ii] from two different BTC ligands. The Cd(2) center is situated at a special position with the 4-fold inversion axis. Each Cd(2) atom is eight coordinated with a slightly distorted square antiprism chelated by four BTC ligands [O(5), O(6); O(5)ii, O(6)ii; O(5)iii, O(6)iii; and O(5)v, O(6)v]. This kind of porous MOFs containing the eight coordinated Cd(II) centers is rare.13 The CdO bond lengths range from 2.297(5) to 2.515(6) Å (Table S1 in the Supporting Information). The Na(1) cation is coordinated by six oxygen atoms from four BTC [O(1), O(4)iv, O(5), and O(6)iii] and two μ2-H2O molecules [O(1W), O(2W)i], forming a distorted octahedron. The Na(2) cation is located in an inversion center and also in a six coordinated distorted octahedral fashion. Each of the Na(2) atoms is bound to four oxygen atoms [O(1W),
Figure 2. (a) Two-fold interpenetration of identical [Cd3(BTC)4] frameworks in 1 viewing along the a-axis (the neighboring identical frameworks are represented as blue and purple colors. (b) bor topological representation of the two [Cd3(BTC)4] frameworks with three- and four-connected nodes. (c) Two-fold interpenetration of identical [Cd3(BTC)4] network viewing along [110] direction (the yellow spheres inside the apertures represent the largest van der Waals spheres around 6 Å, which would fit in the apertures without touching the framework). (d) Projection along the a direction of the crystal structure 1 showing {Na3O12} trimers (green-filled bonds and atoms) and hydrogen-bonded network (blue-filled dashed bonds) with the neighboring interpenetrated [Cd3(BTC)4] frameworks (white-filled bonds and atoms). Uncoordinated water molecules are drawn as redfilled atoms.
O(1W)i, O(2W), and O(2W)i] from four μ2-H2O molecules and two oxygen atoms [O(3W) and O(3W)i] from two terminal water molecules. In addition, the distance of NaO bonds is in the range of 2.265(6)2.574(9) Å (Table S1 in the Supporting Information). Removal of the coordinated water molecules from the Na atoms and free water guests may result in partly coordinated Na(1) centers and flexibly uncoordinated Na(2) centers, which are significantly unsaturated Na sites. Moreover, the bridging BTC is completely deprotonated, binding with two separate Cd(1), one Cd(2), and four Na(1) atoms (Figure S2a in the Supporting Information). For each BTC ligand in 1, the first carboxyl group is coordinated with one Cd(2) and two Na(1) atoms adopting the chelating/bridging bidentate fashion, the second carboxyl group links one Cd(1) and one Na(1) atoms with the bidentate mode, while the third carboxyl group uses the chelating/bridging bidentate configuration connecting one Cd(1) and one Na(1) atoms (according to the nomenclature summarized in Figure S3 in the Supporting Information). After omitting the sodium atoms and water molecules, threedimensional network of two distinct Cd(II) results in the formation of a single framework, [Cd3(BTC)4], with intersection 3531
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Crystal Growth & Design channels running in three directions. In the single framework, each Cd(1) is connected to four Cd(1) and four Cd(2) centers via four BTC ligands, while each Cd(2) is linked to eight adjacent Cd(1) centers via four BTC bridges. The Cd(1) 3 3 3 Cd(2), Cd(1)iii 3 3 3 Cd(2) and Cd(1) 3 3 3 Cd(1)iii distances separated by the BTC ligands are 9.749, 9.828, and 9.810 Å, respectively. The most prominent cavities are 8.2 Å 8.3 Å in cross-section and run along the a-axis (or b-axis), whose rhombic windows are constructed by two Cd(1) and two Cd(2) atoms connected by four BTC bridges (Figure 1b). The other smaller rhombicshaped channels with a cross-section of 6.9 Å 7.0 Å are parallel to the c direction, and their windows are formed by alternately linked four Cd(1) atoms and four BTC ligands (Figure 1c). As it is usually found in MOFs including large cavities, the potential channels of [Cd3(BTC)4] are filled by the another identical network in a typical 2-fold interpenetration fashion, to minimize the big void cavities and stabilize the framework (Figure 2a). Therefore, the size of the largest window is distributed around 6.7 Å 3.0 Å along [110] direction, and the internal diameter is around 6 Å (Figure 2c). The effective free volume of the framework was calculated by PLATON/SOLV14 to be 35.2% of the crystal volume (936.4 Å3 out of the 2658.3 Å3 unit cell volume), using a default probe size of 1.2 Å. A better insight into the structure can be accessed by the topological analysis. Taking Cd(II) cations and BTC anions as nodes for this structure, two different types of 4-connected nodes and one kind of 3-connected node are observed. The conjoint node distances are in the range of 5.6235.717 Å. The resulting structure is a binodal (3,4)-connected three-dimensional network with boracite (bor in RCSR) topology and the Schl€afli symbol of (62.84)3(63)4, according to the nomenclature defined by Yaghi and O'Keeffe.12a Figure 2b shows the 2-fold interpenetrated framework of bor topology. However, the corresponding complex connectivity results in the lower symmetry of the framework (P42/n) as compared to an ideal bor topology (P43m) (Figure S4 in the Supporting Information). The two interpenetrated [Cd3(BTC)4] open frameworks are bridged together by the {Na3O12} trimers to form a threedimensional structure of 1 (Figure 2d). The {Na3O12} trimer consists of Na(1)O6, Na(2)O6, and Na(1)iO6 polyhedra, in which Na(1)O6 and Na(2)O6 are linked by two μ2-H2O bridging ligands and the adjacent Na(1) 3 3 3 Na(2) separation is 3.742 Å. The volume fraction of the dehydrated framework was estimated by PLATON/SOLV to be 17.2% of the total volume (455.9 Å3 out of the 2658.3 Å3 unit cell volume). It is worth noting that the {Na3O12} bridges reduce the flexibility of the 2-fold interpenetrated net. To the best of our knowledge, this kind of MOF including alkali-bridged interpenetration merely has limited known examples.15 The connection between the open frameworks is further established by the hydrogen-bonded network from water molecules [O(1W)O(4W)] and carboxylate groups, which confers additional stability to the structure (Figure 2d and Table S2 in the Supporting Information). Crystal Structure of [Cd3Na2(BTC)3 (H2O)3] 3 [H2N(CH3)2] 3 H2O (2). The single crystal X-ray diffraction analysis suggests that compound 2 is in the monoclinic space group Cc. In the asymmetric unit of 2, there are three crystallographic independent Cd(II) cations, two Na(I) ions, three BTC ligands, three coordinated aqua molecules, one free water molecule, and one (CH3)2NH2+ ion (see Figure 3a). Of the three coordinated aqua ligands, one is the μ2-H2O bridging ligand, and the others link to metal centers as terminal water. The distances of CdO bonds range from 2.239(8) to 2.621(9) Å, and the NaO
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Figure 3. (a) Thermal ellipsoid plot (50%) drawing of the coordination environment of Cd(II) and Na(I) ions in 2. Symmetry codes: (i) x + 1/ 2, y + 1/2, z; (ii) x + 1/2, y + 1/2, z + 1/2 (color codes: Cd, ciel; Na, green; C, gray; and O, red. The guest molecules and hydrogen atoms are omitted for clarity). (b) Projection of [Cd3(BTC)3] framework along the b direction. The channels have a cross-section of 4.5 Å 3.0 Å based on van der Waals radii (space-filling representations). (c) Crystal packing of structure 2 along the b-axis. The [Cd3(BTC)3] framework and {Na2O10} dimers are drawn by white- and green-filled bonds, respectively. Hydrogen bonds are represented with blue dashed lines, the uncoordinated water molecules with red spheres, and (CH3)2NH2+ ions with purple bonds. 3532
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Crystal Growth & Design bond lengths are in the range of 2.293(12)2.665(10) Å (Table S1 in the Supporting Information). In the structure of 2, both Cd(1) and Cd(2) centers are in heptacoordinated and exhibit distorted pentagonal bipyramidal geometries. Cd(1) and Cd(2) centers have the same arrangement of ligands: Each of them is chelated by three BTC ligands through carboxyl oxygen atoms [Cd(1): O(3), O(4), O(6), O(7), O(11), and O(13); Cd(2): O(1), O(2), O(5), O(9), O(12), and O1(14)] and bound to a separate carboxyl oxygen atom [Cd(1): O(15); Cd(2): O(17)] from one BTC ligand. Each six-coordinated Cd(3) center connects six oxygen atoms [O(1), O(8), O(10), O(16), O(17), and O(18)] to construct a distorted octahedral geometry. The O(1) and O(17) atoms come from two separate BTC ligands, whereas the O(8), O(10), O(16), and O(18) atoms come from two chelated carboxylates from two different BTC ligands. Especially, Cd(2) and Cd(3) are bridged by the C(13) and C(24) carboxylate groups to form a edge-shared dimer, in which the Cd(2) 3 3 3 Cd(3) distance is 3.882 Å. The Na(1) cation is also six-coordinated by six oxygen atoms from four BTC [O(4)i, O(8), O(11)i, and O(15)i], one μ2-H2O molecule [O(1W)], and one terminal water [O(2W)] to construct a distorted octahedron. The Na(2) ion is five-coordinated to form a distorted trigonal bipyramidal geometry by binding with three BTC [O(2), O(6), and O(14)], one μ2-H2O molecule [O(1W)ii], and a terminal aqua molecule [O(3W)]. In addition, three independent BTC ligands, which are all completely deprotonated, have different arrangements of metal cations (Figure S2bd in the Supporting Information). For the first type of BTC ligand (Figure S2b in the Supporting Information), the first carboxyl group uses the chelating bidentate configuration to connect one Cd(3) atom, the second one is coordinated with Cd(1), Na(1) and Cd(2), Cd(3) atoms adopting the bridging unidentate fashion, while the third one gives the similar mode as the first one by linking one Cd(2) atom. For the second type of BTC linker (Figure S2c in the Supporting Information), the first carboxyl group is of the chelating/bridging bidentate mode linking one Cd(2), one Cd(3), and one Na(2) atoms, either the second or the third one uses the chelating/bridging bidentate fashion to connect with one Cd and one Na atoms [Cd(2), Na(2) or Cd(3), Na(1)]. For the third kind of BTC ligand (Figure S2d in the Supporting Information), all of the three carboxyl groups adopt the chelating/bridging bidentate fashion through ligating one Cd and one Na centers, and those are Cd(1) and Na(2), Cd(1) and Na(1), and Cd(1) and Na(1), respectively. In the structure of 2, after omitting the sodium atoms and guest molecules, the Cd(II) atoms are combined by BTC ligands to construct a 3D network, [Cd3(BTC)3], with unusual 1D honeycomb channel of ca. 4.5 Å 3.0 Å along the b-axis (Figure 3b). By PLATON/SOLV analysis, we calculated the effective free volume of the [Cd3(BTC)3] framework as being 22.6% of the crystal volume (743.9 Å3 out of the 3287.0 Å3 unit cell volume). Furthermore, the channels of [Cd3(BTC)3] are filled with Na(I) cations, water molecules, and (CH3)2NH2+ ions to build up the whole structure of 2 as shown in Figure 3c. The two Na(I) centers share a vertex to form a {Na2O10} dimer by one μ2-H2O linker with a short Na(1) 3 3 3 Na(2) separation of 4.511 Å. The cooperative OH 3 3 3 O and NH 3 3 3 O interactions connect with coordinated water molecules [O(1W)O(3W)], free guests [O(1W) and N(1)], and carboxylate groups to form hydrogen-bonded network and increase stability of the structure (Figure 3c and Table S2 in the Supporting Information). Many attempts to exchange (CH3)2NH2+ for alkali cations were all unsuccessful. In spite of removing H2O molecules, the framework merely possesses 3.5% of solvent accessible void in the
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crystal volume (113.6 Å3 out of the 3287.0 Å3 unit cell volume), based on a PLATON/SOLV calculation. Thermal Properties. The thermal stabilities of compound 1 and 2 were measured by TG-MS and VTPXRD (shown in Figure S5 in the Supporting Information). Compound 1 lost the free and coordinated water molecules in the temperature range of 50180 °C (found, 15.23 wt %; calcd, 15.23 wt %), and the fully dehydrated sample (1a) was stable up to 340 °C without further weight loss (Figure S5a in the Supporting Information), which was confirmed by VTPXRD patterns (Figure S5b in the Supporting Information). The ion currents showed peaks at these temperatures only for ions with m/z = 17 and 18, which may be assigned to HO+ and H2O+, respectively. The destruction of the material occurred from 340 to 520 °C (Found 38.65 wt %, Calc. 39.04 wt %), leading to the formation of CdO and Na2CO3 as the residue (found, 46.12 wt %; calcd, 45.72 wt %) indicated by the TG and VTPXRD results. The MS peaks of ions with m/z = 17, 18, 28, and 44 were observed during the above decomposition, which may correspond to HO+, H2O+, CO+, and CO2+, respectively. The TG-MS analysis of compound 2 was demonstrated in Figure S5c in the Supporting Information, and the weight loss attributed to the departure of water molecules (found, 6.14 wt %; calcd, 6.42 wt %) was observed in the range of 80310 °C. The destruction of the material occurred from 310 to 480 °C (found, 50.15 wt %; calcd, 49.83 wt %). The ion currents of m/z = 17 (HO+) and 18 (H2O+) appeared in both above steps, whereas the MS peaks of ions with m/z = 30 (NO+ or CH3NH+), 44 [CO2+ or (CH3)2N+], and 46 [NO2+ or (CH3)2NH2+] were observed in the second process. VTPXRD studies (Figure S5d in the Supporting Information) revealed that compound 2 was stable up to 310 °C, and crystallinity was retained upon the loss of all water molecules. The diffraction patterns measured between 25 and 180 °C showed no evident changes, which could correspond to the release of guest water molecules. Above 180 °C, the patterns showed an apparent shift toward the higher angle 2θ of the peaks due to shrinkage of the framework with the removal of coordinated water molecules, and the resulting phases at the higher temperatures (above 310 °C) were CdO and Na2CO3, corresponding to the TG residue (found, 43.71 wt %; calcd, 43.75 wt %). N2 and H2 Adsorption Properties. Compounds 1 and 2 were pretreated at 250 °C and 108 bar for 24 h to generate dehydrated form 1a and 2a. For 1a, N2 and H2 adsorption curves were shown in Figure 4a,b. However, N2 and H2 adsorption of 2a appeared to fail since attempts of (CH3)2NH2+ ions exchange were unsuccessful. Thus, we merely discuss the details of adsorption properties for 1a as follows. The N2 isotherm tested at 77 K revealed that 1a exhibited typical type I adsorption behavior: The adsorption and desorption branches were closed without hysteresis. Compound 1a adsorbed 142 cm3 g1 of N2 at 77 K, corresponding to Brunauer EmmettTeller (BET) surface area of 473 m2 g1 (within the range 0.06 < p/p0 < 0.2) and the Langmuir surface area of 618 m2 g1. The pore volume calculated from the maximum N2 adsorption was 0.220 cm3 g1. Furthermore, the N2 adsorption showed very good reversibility. The surface area for N2 adsorption is not too impressive, comparing with the values of very well studied microporous MOFs such as MOF-177 (4630 m2 g1 for BET model, 5250 m2 g1 for Langmuir model) and MOF-5 (3800 m2 g1 for BET model, 4400 m2 g1 for Langmuir model).16 By analyzing the crystal structure of 1a, interpenetration decreases the internal diameter of aperture to 6 Å without blocking 3533
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prediction, on the basis of which the H2 storage capacity was estimated to be 2.27 wt % per 1000 m2 g1.18 By fitting the LangmuirFreundlich equation, the maximum H2 uptake of 1a was predicted to be 1.12 wt %, which was in accordance with the experimental value. Under ambient pressures (1.0 bar), the H2 uptake amount of 1a is typical comparing to MOF-177 (1.25 wt %) and MOF-5 (1.32 wt %), although these famous MOFs have large pore dimensions (MOF-5, 15 Å; MOF-177, 10.8 Å).16 The result suggests that many reported MOFs have spaces between their walls that are too large for the effective adsorption of H2 (the kinetic diameter for H2, 2.89 Å17), on the basis of the confirmed monolayer prediction for H2 adsorption. However, in 1a, the rolling surface of the pores and the nonlinearity of the channels may be appropriate for accommodate more H2 as several reported references.19 To investigate the interaction between H2 and the framework, the isosteric heats of the adsorption were calculated by fitting the ClausiusClapeyron equation to the H2 adsorption isotherms measured at 77 and 87 K (Figure 4b). The equation is as follows: ðln PÞn ¼ Qst =RT þ C where P is the pressure, n is the amount adsorbed, T is the temperature, R is the universal gas constant, and C is a constant. By calculating the slope of the ln P (1/RT) curve, the coverage dependency of Qst was presented graphically in Figure 4c. Interestingly, the isosteric heats of adsorption for 1a increased initially with H2 loading until a maximum of 8.4 kJ mol1, higher than MOF-177 (4.4 kJ mol1) and MOF-5 (5.2 kJ mol1).16 These values suggest that there may be effects due to H2H2 interactions at low coverage.7c,16c,20 The reason of high adsorption heat of 1a may be that its convoluted narrow pores and unsaturated Na sites afford strong binding affinities for H2 molecules.
Figure 4. (a) N2 sorption isotherms of 1a at 77 K. (b) H2 adsorption isotherms of 1a at 77 and 87 K up to 1.0 bar. (c) Coverage dependency of the isosteric heat of adsorption for H2 in 1a.
any adsorptive sites and leads to a more convoluted void region. These narrow voids are too small to incorporate more N2 molecules (the kinetic diameter for N2, 3.64 Å17). H2 adsorption isotherm was recorded at 77 K under atmospheric pressures, which also showed type I behavior with good reversibility and no hysteresis. The H2 adsorption kinetic data confirmed that equilibrium was achieved rapidly within approximately 3 min of the isotherm pressure step. Compound 1a could adsorb nearly 1.11 wt % H2 at 1.0 bar and 77 K. The saturation value of H2 uptake on a unit specific surface area was calculated to 2.35 wt % per 1000 m2 g1. The result confirmed the monolayer
’ CONCLUSIONS The combination of transition metals [Cd(II)] and alkali metals [Na(I)] to react with the aromatic BTC ligands afforded two novel 3D MOFs through solvothermal syntheses. Compound 1 constructs a {Na3O12}-bridged 2-fold interpenetrated framework with bor topology, while 2 forms a {Na2O10}, (CH3)2NH2+, and free water-filled 1D honeycomb channel network. In compound 1, the exceptional H2 storage capacity of 1.11 wt % at 77 K and 1.0 bar has been observed with the higher isosteric heat of 8.4 kJ mol1. This illustration demonstrates that metal (transition alkali) organic frameworks with very narrow voids and exposed alkali sites may provide a useful platform for further new MOFs with enhanced H2 storage performance. We are currently investigating the possibilities of gas adsorption for compound 2, through any method of replacing (CH3)2NH2+ by alkali cations. ’ ASSOCIATED CONTENT
bS
Supporting Information. Crystallographic data for 1 and 2 in CIF format; IR, TG-MS, VTPXRD, and details about selected bond lengths and hydrogen bonds; connectivity modes of 1,3,5-benzenetricarboxylate ligands; and the ideal model of bor net. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Tel: (8610)62750342. Fax: (8610)62753541. E-mail: liguobao@ pku.edu.cn (G.L.) or
[email protected] (J.L.).
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