Communication pubs.acs.org/cm
High CO2 and H2 Uptake in an Anionic Porous Framework with Amino-Decorated Polyhedral Cages Quan-Guo Zhai,†,‡ Qipu Lin,† Tao Wu,† Le Wang,† Shou-Tian Zheng,§ Xianhui Bu,§,* and Pingyun Feng†,* †
Department of Chemistry, University of California, Riverside, California 92521, United States School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, P. R. China § Department of Chemistry and Biochemistry, California State University, Long Beach, California 90840, United States ‡
S Supporting Information *
KEYWORDS: 1,2,4-triazole, metal−organic framework, polyhedral cage, amino decoration, CO2 uptake and BDC in DMF at 90 °C. The triangular DATRZ ligand was selected (i) because its multiple coordination modes allow for the construction of topologically diverse porous materials and (ii) because of the potential of two amine groups to interact with CO2 molecules. The charge- and shape-complementary linear dicarboxylate ligand is introduced to create a chemical system with flexibility in topology and charge density matching (e.g., through dicarboxylate/triazolate ratio). Here, the coassembly between triangular DATRZ, linear BDC ligands, and Zn2 dimers led to the formation of a new anionic porous framework with amino-decorated polyhedral cages. The PXRD pattern of the as-made sample agrees well with the simulated data, confirming the phase purity (Figure S1, Supporting Information (SI)). Single-crystal X-ray analysis shows that CPF-13 crystallizes in the highly symmetric space group P42/mnm. Four crystallographically identical carboxylate groups of BDC ligands coordinate to two Zn(1) centers, forming a [Zn2(CO2)4] paddle-wheel secondary building unit (SBU-1, Figure 1, left). In comparison, one DATRZ ligand
T
he synthesis of novel solid state materials lies at the center of many important applications.1−3 One such application is the sequestration of CO2 emitted from industrial processes to stabilize atmospheric CO2 levels. In this case, low-cost porous materials capable of physisorption of CO2 are ideal candidates because such a process requires less energy for adsorbent regeneration, compared to chemisorption-based processes.4 In addition to zeolites, porous carbon, mesoporous silica,5 and metal−organic frameworks6,7 (MOFs) have all been studied for such applications. One of the most promising strategies to increase the uptake capacity is the deliberate enhancement of the adsorbent− adsorbate interactions. For example, the incorporation of alkylamine functionality within the pores of MOFs offers a significant opportunity to produce highly efficient capture materials by virtue of the affinity of alkylamines for CO2.8 Another factor currently receiving considerable attention is the generation of charged framework, which can enhance electrostatic solid−gas interactions.9 Furthermore, the suitable pore size commensurate with the size of the gas molecule can also be critical because large cavities do not necessarily lead to a high uptake capacity for small gas molecules such as CO2.7b This makes it necessary to generate a pore architecture that allows for efficient use of the pore space, for example, through the formation of metal−organic polyhedral (MOP) cages with suitable cavity size and suitable windows. In this contribution, we report an exceptional anionic porous framework, [(CH 3 ) 2 NH 2 ][Zn 3 (DATRZ)(BDC) 3 ]·xDMF (CPF-13, CPF = crystalline porous framework, DATRZ = 3,5-diamino-1,2,4-triazole, BDC = 1,4-benzenedicarboxylate, DMF = N,N-dimethylformamide), that integrates multiple desirable features such as nanoscopic polyhedral cages, framework functionalization by amino groups, and charged framework. To our knowledge, CPF-13 exhibits the highest CO2 uptake capacity under ambient conditions (81 cm3/g or 3.62 mmol/g at 298 K−1 atm) among known porous materials with the charged (anionic or cationic) framework. It is also shown here that CPF-13 exhibits high H2 uptake and high CO2/N2 and CO2/CH4 selectivity. CPF-13 was obtained in high yield as colorless octahedral crystals via solvothermal reaction between zinc nitrate, DATRZ, © 2012 American Chemical Society
Figure 1. Octahedral paddle wheel SBU-1 (left) and trigonal bipyramidal mixed triazolate−carboxylate paddle wheel SBU-2 (right) in CPF-13.
joins two Zn(2) centers in μ2-N1,N2 bridging mode, together with two carboxylate groups, generating a novel [Zn2(CO2)2(DATRZ)] triazole−carboxylate unit (SBU-2, Figure 1, right). Each SBU-1 links adjacent six SBU-2 through four phenyl groups and two apical coordination of the triazole N4 atoms to serve as an octahedral node while each SBU-2 connects with three SBU-1 and two SBU-2 to form a trigonal bipyramidal node (Figure S3, SI). The octahedral SBU-1 and Received: April 29, 2012 Revised: June 18, 2012 Published: July 10, 2012 2624
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porosity (Figure 3a and Figure S7, SI). The Langmuir and BET surface areas were 1008 and 642 m2/g, respectively. A
trigonal bipyramidal SBU-2 join each other to form a (5,6)connected framework of CPF-13 exhibiting unprecedented {4.58.8}2{42.58.63.82} topology (Figure S3, SI). As shown in Figure 2, one of the most fascinating topological features of CPF-13 is the unique nanoscopic polyhedral cages
Figure 2. (a) Nanoscopic cage enclosed by 4 SBU-1, 10 SBU-2, 8 BDC, and 8 DATRZ ligands. (b) Polyhedral representation of the nanoscopic cage with octahedral SBU-1 and trigonal bipyramidal SBU2. (c) 3D framework of CPF-13. (d) Polyhedral representation of the 3D framework.
formed from 4 [Zn2(CO2)4] paddle-wheel SBU-1 and 10 [Zn2(CO2)2-(DATRZ)] mixed triazole−carboxylate SBU-2. Four SBU-1 are bridged by four SBU-2 to form an extended hexagonal equator plane of the cage, which are further capped by three trigonal pyramids at the top and bottom, respectively. Although a number of metal−organic polyhedral frameworks have been constructed by utilizing paddle-wheel SBUs with angular 120° isophthalate ligands,10 the 180° BDC ligands are rarely found to link such SBUs to form metal−organic polyhedra due to the limitation of its linear coordination geometry. In this work, the presence of the mixed triazole− carboxylate trigonal bipyramidal SBU-2 helps overcome the shape limitation and leads to the formation of this unique polyhedral cage in CPF-13. In addition to 14 binuclear SBUs, there exist 8 BDC and 8 DATRZ ligands in the cage. The cage, about 28.5 × 16.3 × 14.0 Å3 in dimension, contains 8 distorted pentagonal windows. Each window is formed by 2 [Zn2(CO2)4] paddle-wheel SBUs and 3 [Zn2(CO2)2(DATRZ)] SBUs with dimensions of about 8.1 × 5.4 Å2. The amine functionalities from the triazole ligands just locate in the windows. These polyhedral cages serve as supramolecular building block (SBB) to extend in the ab plane and then pack along c via “ABAB” mode to form a 3D framework (Figures S4 and S5, SI). The PLATON calculation indicates this framework has a total solvent-accessible volume of 4300 Ǻ 3, which occupies approximately 64.7% of the volume of the whole crystal. Thermogravimetric analysis shows that the removal of solvent molecules occurs in the temperature range of 40−220 °C and there is no further weight loss up to ≈300 °C (Figure S6, SI). The architectural stability and permanent porosity of CPF-13 were also confirmed by gas adsorption measurements (N2, H2, and CO2) on a Micromeritics ASAP 2020 surface area and pore size analyzer. The activated sample was prepared by exchanging the solvent in the as-synthesized CPF-13 with dry CH2Cl2 for 2 days, followed by evacuation at 80 °C for 12 h, and the stability of the guest-free material is confirmed by the PXRD patterns (Figure S1, SI). CPF-13 exhibits a type I adsorption isotherm typical of materials of permanent micro-
Figure 3. (a) N2 and H2 isotherms of CPF-13 at 77 K. (b) CO2, CH4, and N2 isotherms at 273 or 298 K. Inset shows the enthalpy of adsorption (Qst) as a function of CO2 uptake (Figure S10, SI).
micropore volume of 0.350 cm3/g (using Horvath−Kawazoe method) and the median pore size of 4.62 Å were also calculated. Further measurements show that the uptake of H2 at 1 atm and 77 K reaches 2.0 wt % (223.9 cm3/g) (Figure 3a and Figure S8, SI), which is significantly higher than many well-known metal−organic frameworks with larger surface area, such as MOF-177,11 MOF-74-Zn,12 ZIF-8,13 SNU-1,14 and PCN-6.15 Impressively, CPF-13 exhibits a very high CO2 uptake capacity of 116 cm3/g (5.2 mmol/g) at 273 K and 1 atm (Figure 3b and Figure S9, SI). At 298 K and 1 atm, CPF-13 has a CO2 uptake of 81 cm3/g (3.62 mmol/g). Although CPF-8 has lower CO2 uptake than the neutral Mg-MOF-747a (8.48 mmol/g at 298 K), few neutral MOF structures exhibit a CO2 uptake of more than 3.6 mmol/g at 298 K and 1 atm.6,7 To our knowledge, CPF-13 has the highest CO2 uptake among charged MOFs under ambient conditions (Table S1, SI). For example, although the anionic framework H3[(Cu4Cl)3(BTTri)8]16,17 with large 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene (BTTri) ligand shows a 1770 m2/g BET surface area, its CO2 uptake is 3.24 mmol/g at 298 K at 1 atm. Similarly, Bio-MOF-1, Zn8(ad)4(BPDC)6O·2Me2NH2,18 another well-known anionic framework, has a 1680 m2/g BET surface area and exhibits 3.41 mmol/g CO2 adsorption at 273 K and 1 atm. CPF-13 also has a CO2 uptake capacity higher than CPM-6, even though the latter exhibits a fasciating cage-in-cage framework with charged framework and well-partitioned pore space.7b It is worth noting that the use of 2-amino-1,4-benzenedicarboxylic acid (in place of 1,4-benzene−dicarboxylic acid in CPF-13) in our experiments led to the formation of an isoreticular structure, NH2−CPF-13 (Figure S11, SI). However, the gas uptakes of N2 (77 K, 1 atm), CO2 (273 K, 1 atm), or H2 (77 K, 1 atm) are dramatically reduced, likely due to the 2625
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blockage of the pentagonal windows (about 3.2 × 1.5 Å2, SI) by the extra amino group. We have also succeeded in enlarging the polyhedral cages by using larger 2,6-naphthalenedicarboxylic acid ligand (to replace 1,4-BDC in CPF-13), but the resulting material is highly unstable toward the solvent loss, perhaps due to its large porosity. These results suggest that CPF-13 appears to have an optimum pore size to sustain the stability of the porous framework while exhibiting a high porosity for the gas uptake. Either larger or smaller pore size has a detrimental effect on its potential application in gas sorption. To further understand the adsorption properties of CPF-13, the isosteric heat of adsorption (Qst) of CO2 was determined by fitting the adsorption data collected at 273 and 298 K to the virial model. At zero loading, indicative of the interaction of CO2 with the most energetically favored sites in the framework, Qst was determined to be ∼28.2 kJ/mol. The Qst remained relatively constant and reached ∼26.5 kJ/mol at the maximum measured loading. The Qst value is higher than other anionic frameworks, such as the 21 kJ/mol in H3[(Cu4Cl)3(BTTri)8] and 25.38 kJ/mol in [(CH 3 ) 2 NH 2 ] 3 [(Cu 4 Cl) 3 (btc) 8 ]· 9DMA.16,17 This suggests an enhanced interaction of CO2 with the framework, likely due to the presence of the amino groups from the DATRZ ligands. It is worth noting that while the Qst value at zero loading of CPF-13 is lower than two amino-functionalized anionic MOFs, H3[(Cu4Cl)3(BTTri)8]·ethylenediamine (90 kJ/mol)16 and bio-MOF-1 (55 KJ/mol), 26.5 kJ/mol at maximum CO2 loading for CPF-13 is significant because the Qst values of the other two anionic MOFs both decrease rapidly to about 22 kJ/ mol with increasing pressure. In our opinion, the maintainable high heat of adsorption Qst for CPF-13 maybe due to the available reactive surface sites (−NH2 groups) does not decrease rapidly with increasing pressure. The adsorption selectivity for CO2 over N2 is a prerequisite for the application in CO2 sequestration. Figure 3b also shows the N2 isotherms obtained for CPF-13 at 273 K and up to 1 atm. The near-linear adsorption profile for N2 is indicative of its low affinity for the amino groups and the charged framework, as expected from its relatively low polarizability. The selectivity of CO2/N2 was calculated from the single-component isotherm data by using the nonlinear fitting.19 The CO2/N2 selectivity for CPF-13 at 273 K is 35:1 at 0.1 bar and 31:1 at 1 bar, which decreases to 9.5:1 at 0.15 bar and room temperature. In addition, single-component gas sorption experiments were also carried out for CH4 at 273 K and up to 1 atm (Figure 3) to examine the separation capability of CPF-13 for CO2 and CH4. The CH4 uptakes are only 0.11 mmol/g at 0.1 bar and 0.98 mmol/g at 1 bar. This leads to high uptake ratios for CO2 over CH4, which reaches 18:1 at 0.1 bar and 5:1 at 1 bar. Such results show that CPF-13 has better selectivity than two other anionic MOFs and their lithium−exchanged products, [Zn2(NDC) 2(diPyNI)]·Li and [Zn2 (TCPB)(DPG)]·Li.20 Again, the amino functionality, together with other features such as polyhedral cages and charged frameworks, is likely responsible for the higher CO2/N2 and CO2/CH4 selectivity by CPF-13 over the entire pressure range measured. In conclusion, by using readily accessible 3,5-diamino-1,2,4triazole and 1,4-BDC as complementary ligands to generate two types of supramolecular building units, [Zn2(CO2)4] paddle-wheel SBU-1 and mixed [Zn2(CO2)2(triazole)] triazole−carboxylate SBU-2, we have synthesized an exceptional porous anionic framework material. Its integrated features such as charged frameworks and amine functionalized nanoscopic
polyhedral cages contribute to a very high CO2 and H2 uptake capacity, as well as high CO2/N2 and CO2/CH4 selectivity.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Experimental details, TGA, XRPD, additional figures, and CIF file. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0002235.
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