Ammonia

Research Article pubs.acs.org/journal/ascecg

Antenna-Protected Metal−Organic Squares for Water/Ammonia Uptake with Excellent Stability and Regenerability Yang Chen, Yong Wang, Chengyin Yang, Shuang Wang, Jiangfeng Yang,* and Jinping Li* Research Institute of Special Chemicals, Taiyuan University of Technology, No. 79, Yingze West Street, Taiyuan 030024, Shanxi, China S Supporting Information *

ABSTRACT: Ammonia is a hazardous gas and the only carbon-free chemical energy carrier that can be largely adsorbed on metal−organic frameworks (MOFs). However, because of the destructive effect of H2O/NH3 on the metal nodes, most MOFs cannot be applied in ammonia capture and uptake. Herein, three Co-4,5-imidazoledicarboxylic series metal−organic squares (MOSs)Co 4 (IDC) 4 (pda) 4 , Co4(IDC)4(phen)4, and Co4(IDC)4(bpy)4were synthesized with a special independent square configuration and zeolitelike supramolecular structures, and their structure and H2O/ NH3 uptake capacity were investigated. Based on the four antennas-protected squares and porous structures, the three MOSs have excellent H2O/NH3 stability, whose structures were not affected by the ad-desorption of H2O, NH3, or H2O/NH3. The three MOFs have a H2O uptake of 17.63, 8.35, and 7.75 mmol/g, respectively, as well as the facile release and repeatable of high ammonia uptakes of 11.5, 5.2, and 3.8 mmol/g, respectively. In addition, the MOFs have good stability and ammonia adsorption (4.73, 2.33, and 1.21 mmol/g, respectively) under humid conditions. Therefore, the three MOSs may be sustainably applied to ammonia uptake applications, because of their high ammonia uptake, ease of release, and the unique structural protection effect of the antenna ligands. KEYWORDS: Metal−organic squares (MOSs), Antenna-protection, Carbon-free chemical energy carrier, Ammonia adsorption, Stability, Regenerability

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INTRODUCTION Ammonia (NH3), a colorless, pungent, and corrosive gas, is the most important gaseous alkaline pollutant, as well as an important chemical gas resource.1,2 Large emissions of NH3 can have a serious impact on public health, the environment, and the economy.3−5 Meanwhile, the global demand for NH3 for use in fertilizer production, the synthesis of chemical products, refrigeration, and CO2 capture is still large.6−8 Besides, NH3 provides the only carbon-free chemical energy carrier solution for the transportation sector, which can be potentially used in generating CO-free H2.9−11 NH3 is transported as a compressed liquid for use in a wide variety of industrial applications, as mentioned above, and is toxic, corrosive, and difficult to handle.12,13 Therefore, an effective method for the uptake and release of NH3 is required. Porous materials have been shown to have good performance in the adsorption and separation of gases (H2, N2, O2, CH4, and CO2).14−17 Research into such materials has gradually addressed their applications in NH3 adsorption and storage.18−20 Conventional adsorbents such as carbon, zeolite, alumina, and silica gels have properties for NH3 adsorption; however, their adsorption capacities are unsatisfactory.21−23 Recently, new sorbents based on porous coordination polymers have received more attention, Yaghi12 has shown that covalent © 2017 American Chemical Society

organic frameworks (COF-10) have a large uptake for NH3 (15 mmol/g at 25 °C and 1 bar). However, it requires high temperature (200 °C) to regenerate and loses 4.5% of its uptake because of the partial disruption of its layered morphology. Metal−organic frameworks (MOFs) have also shown some ability for NH3 adsorption.24−26 It has been recently reported that the uptake of M2Cl2(BTDD) (M = Mn, Co, Ni) was more than 12 mmol/g;27 Prussian Blue even reached 20 mmol/g,28 but their regeneration also needed high temperatures (at least 150 °C). Other typical MOFs (such as MOF-5, MOF-177, and HKUST-1) 29,30 used for NH 3 adsorption can reach more than 10 mmol/g. However, most of their metal nodes are sensitive to H2O and NH3, which results in the destruction of their structures and, thus, the loss of their capacity for regeneration. Even though all of the aforementioned porous materials selectively adsorb NH3, serious problems such as an irreversible adsorption process, difficult regeneration, and structural collapse limit their applications. Received: February 13, 2017 Revised: April 21, 2017 Published: May 5, 2017 5082

DOI: 10.1021/acssuschemeng.7b00460 ACS Sustainable Chem. Eng. 2017, 5, 5082−5089

ACS Sustainable Chemistry & Engineering

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MOFs are an emerging class of porous materials that hold promise to solve many key challenging societal needs over the past 20 years (e.g., H2 storage, CO2 capture, renewable catalysts, and controlled drug delivery).31,32 Meanwhile, metal− organic squares (MOSs), which are a special class of MOFs, are crystalline solids whose square is assembled by the connection of four metal ions or clusters (vertices) through four molecular ligand bridges (sides) to form particular square structures. They are logical targets as rigid and directional square building units to be used for the synthesis of new zeolite-like supramolecular assemblies (ZSAs), which possess potential applications in adsorption, catalysis, and sensing.33 Therefore, we chose three materials: Co4(IDC)4(pda)4·28H2O (ZSA-1),34 Co4(IDC)4(phen)4·17H2O, and Co4(IDC)4(bpy)4·15H2O,35 which represent a series of MOSs assembled using Co(II), 4,5-imidazoledicarboxylate, and dinitrogen ligands (1,2-diaminopropane, 1,10-phenanthroline, and 2,2′-dipyridyl, respectively). These squares-structure-assembled pores contain a large amount of H2O that can be removed and form particular zeolite-like supramolecular pores. In addition, they can adsorb polar molecules with steady structures, because of the protection of the four dinitrogen ligands just like functional ligands, protect the metal or metal cluster center in STU-536 and ZIF-837 to improve their structural stability, and the antenna ligand protection effect on the squares of the three MOSs is relatively rare. These properties suggest that these MOSs may possess a high uptake of H2O and NH3. Their steady structures can ensure their reused application in the uptake and resale of NH3, which are advantages when compared to other MOFs whose structures easily collapse and are difficult to regenerate. The structures of the three MOSs are shown in Figure 1.

Research Article

EXPERIMENTAL SECTION

Characterization of the Samples. The crystallinity and phase purity of the materials were measured by powder X-ray diffraction (PXRD) on a Rigaku Mini Flex II X-ray diffractometer with Cu Kα radiation operated at 30 kV and 15 mA. Scanning was performed over the 2θ range of 5°−40° at 4°/min. Morphological data were acquired by using a SEM system (Hitachi, Model SU8010) operated at 2.0 kV. The TGA of the samples was collected on a thermal analyzer (Netzsch, Model STA 449 F5) at a heating rate of 10 °C/min under an air atmosphere. N2 adsorption/desorption isotherms were obtained using a Micromeritics Tristar II 3020 surface area and pore size analyzer after the samples were activated at 150 °C for 2 h. H2O/NH3 Measurements. Water vapor adsorption on the samples was determined by vapor sorption measurements on an Autosorb-iQ Quantachrome (volumetric technique) with a vapor generator at 25 °C. In the NH3 adsorption measurements, the purity of NH3 was 99.999%. The samples were activated overnight under reduced pressure at 150 °C or until no further weight loss was observed. The adsorption isotherms for NH3 were collected on an Intelligent Gravimetric Analyzer (IGA 001, Hiden, U.K.). Adsorption equilibrium data were collected once a stable pressure (more than 8 adsorption points were recorded from 0 to 1 bar) and weight was maintained for 30 min to reach an adsorption equilibrium at each point along the isotherm. Each curve obtained for NH3 adsorption on the samples was collected at 25 °C, and MOSs were activated under vacuum and 25 °C for 30 min in each cycle. NH3 adsorption under a humid environment using the materials was tested in a 4% NH3 solution steam atmosphere at 25 °C for 30 min and the NH3 adsorption amount was measured via acid solution absorption and titration, which was repeated three times. Monte Carlo Molecular Simulation for NH3 Adsorption on the Three MOSs. Grand Canonical Monte Carlo (GCMC) simulations were performed to study the NH3 adsorption and H2O/ NH3 coadsorption on the MOSs at 25 °C and 1 bar. In this work, the adsorbed molecule density and isosteric heat of adsorption of the MOSs were estimated using a previously reported analysis method.38 For the calculation of the probability distribution density of NH3 adsorption on MOS-1 at infinite dilution, a configurational-bias CBMC simulation in the canonical (NVT) ensemble was performed using the revised Widom’s test particle method.39

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RESULTS AND DISCUSSION Materials and Morphologies. As can be seen from Figure 1, the three MOSs are square structures with Co−Co separations of 6.002 Å × 6.002 Å × 6.002 Å × 6.002 Å, 5.993 Å × 5.959 Å × 5.993 Å × 5.959 Å, and 5.964 Å × 5.938 Å × 5.964 Å × 5.938 Å, respectively, connected by 4,5imidazoledicarboxylate and at the four corners of each MOSs grew four antennas (1,2-diaminopropane, 1,10-phenanthroline, and 2,2′-dipyridyl, respectively) to obstruct their coordination and separate the individual MOSs. Because the antennas of MOS-1 have rotational symmetry (C4 symmetry) in the square plane, the lengths of the sides are equal. However, the antennas of MOS-2 and MOS-3 have axial symmetry and are separated on two sides, and thus, the length of sides are equal only on their opposite sides. These MOSs assemble into zeolite-like topologies via supramolecular interactions and form specific pores, which afford multifunctional platforms in their application. The three MOSs were synthesized using the methods described previously. Figure S1 in the Supporting Information shows the synthesized and simulated PXRD patterns of the three MOSs. It was found that the three materials were synthesized successfully according to the overlapped PXRD patterns of the synthesized and simulated materials. As can been seen from Figure S2 in the Supporting Information, the structures of the three MOSs synthesized contained a large

Figure 1. Square structures of Co4(IDC)4(pda)4, Co4(IDC)4(phen)4, and Co4(IDC)4(bpy)4.

The objective of this work was to find suitable materials for NH3 uptake, specifically materials with a large adsorption capacity, stable adsorption, and ease of release. For this purpose, three MOSs were synthesized via solvothermal processes, and the samples were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), H2O/NH3 adsorption studies, and Monte Carlo molecular simulations. As these materials exhibited the advantages of high ammonia uptake, ease of release, the unique structural protection by antenna ligands, and their repeated use under dry or humid conditions, they are promising materials for NH3 uptake. For convenience, the three MOSsCo 4 (IDC) 4 (pda) 4 , Co 4 (IDC) 4 (phen) 4 , and Co4(IDC)4(bpy)4were subsequently denoted as MOS-1, MOS-2, and MOS-3, respectively. 5083


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Figure 2. Evolution of the macroscopic morphology and powder X-ray diffraction (PXRD) patterns of the three MOSs with desorbed/adsorbed H2O. (“−” means desorb; “+” means adsorb.)

amount of H2O, which was in accordance with their structural formulas. Twenty-eight (28) H2O molecules form a cluster with a diamond topology in the pore cage in the structure of MOS-1; however, 17 and 15 H2O molecules were scattered in MOS-2 and MOS-3, respectively.34,35 The microscopic morphologies of the three MOSs are shown in Figure S3 in the Supporting Information and the evolution of the macroscopic morphologies and their corresponding PXRD patterns are reflected in Figure 2. MOS-1 is a dark red polyhedral crystal with a size of ∼30 μm, and, after the H2O clusters were removed upon heating at 150 °C for 2 h, the crystals become darker and opaque, which can change to red and transparent crystals upon the adsorption of H2O. The PXRD pattern of MOS-1 was well-retained after the material was subjected to a dehydration and absorption cycle. MOS-2 and MOS-3 are red bulk crystals, which also become darker and opaque after dehydration and can be changed back, similar to that of MOS-1; however, the crystal size became smaller. The PXRD patterns of MOS-2 and MOS-3 remained the same during the transformation. The color and morphological changes of the three MOSs imply that they are sensitive to H2O and have a good H2O stability, which was reflected in the PXRD patterns of the three MOSs being well-retained, no matter if the H2O molecules were removed or adsorbed. TGA and Water Vapor Adsorption. TGA was performed to determine the thermal stability of the three MOSs and to verify the H2O removal temperature and amount. The TGA curves in Figure 3 show that they all underwent two stages of weight loss. A of >20% weight loss of MOS-1 occurred within 80−150 °C, which was due to the removal of the H2O clusters.

Figure 3. TGA data for the three MOSs.

A plateau formed after the first weight loss in MOS-1 and the structure remained steady at temperatures up to 280 °C. The structure collapsed beyond 300 °C with a huge weight loss. MOS-2 and MOS-3 also had a weight loss of ∼16% at 80−150 °C to remove H2O and the dehydrated structures remained steady to 350 °C. The materials were destroyed at temperatures between 400 °C and 500 °C. The amount of the first weight loss for the three MOSs was in accordance to the H2O content in these structures; the activation process of these materials can occur at 150 °C. In order to verify the ability for H2O uptake, the water vapor adsorption of the three MOSs were investigated. Because many H2O molecules exist in the original structures of the three MOSs, they exhibit a good water vapor adsorption ability, 5084


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also fits the dual-site Langmuir−Freundlich equation, which is similar to that of H2O adsorption (see Figure S5 in the Supportiing Information); when the pressure was decreased, the NH3 was totally desorbed. Therefore, the NH3 uptake in MOS-1 was physical adsorption and the gas can be easily released by only decreasing the pressure. In addition, after vacuuming at 25 °C for 30 min, the second adsorption isotherm was tested in which the adsorption capacity was almost coincident with the first one, which implied that MOS-1 was reusable in NH3 uptake without performance loss. MOS-1 was assembled from directional square building units and formed zeolite-like topologies via supramolecular interactions, which has a big pore channel and square windows (Figure S6 in the Supporting Information). So, MOS-1 exhibited an excellent NH3 uptake capacity and easy gas release due to its huge pore channel, pore size, and large BET specific area of 1112 m2/g (Figure S7 in the Supporting Information). The NH 3 adsorption capacities of MOS-2 are shown in Figure 5b; because of its smaller pore channel, pore size, and specific area, it has a smaller adsorption of 5.2 mmol/g and the second recycle adsorption capacity also reached its initial value, which demonstrates its reusability. In Figure 5c, the NH3 adsorption of MOS-3 shows a two-step adsorption isotherm. The first step adsorption of NH3 was small at pressures of MOS-2 > MOS-3) was in accordance to how many H2O molecules they can contain in their synthesized structures. Generally, these MOSs have advantages, in regard to their H2O adsorption capacities and H2O stability, over many other MOFs.40 NH3 Adsorption and Structural Stability. The NH3 adsorption capacity and regenerability of the three MOSs are shown in Figure 5. The two recycling NH3 adsorption capacity tests using the three MOSs were determined using a thermogravimetric analyzer. The adsorption−desorption isotherm behavior of MOS-1 (Figure 5a) shows that the adsorption capacities of NH3 increase upon increasing the pressure. When the pressure was increased to 1 bar, the gas adsorption isotherm shows an absorption capacity for NH3 at 25 °C of 11.5 mmol/g, and the NH3 adsorption equilibrium

Figure 5. Two recycling NH3 adsorption−desorption isotherms and five cycles of ammonia uptake for the three MOSs at 25 °C. 5085


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Figure 6. Adsorbed NH3 molecule density (red points) in the three MOS samples from GCMC simulations at 1 bar and 25 °C.

Most MOFs cannot sustain the destruction of NH3 under humid conditions, because the cooperative interactions of H2O and NH3 have irreversible effects on the metal nodes, which cause their structures to be destroyed.48,49 Therefore, the H2O/ NH3 coadsorption on the three MOSs was tested three times to testify their NH3 adsorption capacity and structural stability. According to the test method described in the Supporting Information (Figures S10 and S11), the NH3 adsorption capacity under humid conditions was obtained. MOS-1, MOS2, and MOS-3 had an adsorption capacity of 4.73, 2.33, and 1.21 mmol/g, respectively, which correlate with their H2O and NH3 uptake ability. On the other hand, the NH3 adsorption capacity of the three MOSs under humid conditions was less than half that found under dry conditions, that is to say, H2O is competitive in H2O/NH3 coadsorption, and this was also reflected in Figure S12 in the Supporting Information, in which the H2O/NH3 coadsorbed MOSs also showed a strong peak for H2O absorption at 3200−3600 cm−1. Furthermore, the coadsorbed H2O and NH3 molecule density obtained from the GCMC simulations for MOS-1, as a typical material, is shown in Figure S13 in the Supporting Information. The density of H2O is more than that of NH3, and the H2O and NH3 molecules are irregularly dispersed in the pores. The reusability of the materials is an important index in their NH3 uptake applications; considering that many MOFs are sensitive to H2O and NH3, the materials should possess good H2O and NH3 structural stability.50 A comparison of the PXRD patterns of the three MOSs adsorbing H2O or NH3 are shown in Figure 7. We can see that the patterns of the three MOSs were well-retained when compared to the newly synthesized and activated samples when adsorbing H2O, NH3, or both. This reflects that the three MOSs can be used with excellent structural stability under both dry NH3 conditions and humid

To further study the mechanism of NH3 adsorption, we calculated the NH3 distributions in the three MOSs, using GCMC simulations at 25 °C and 1 bar. As shown in Figure 6, the NH3 molecule adsorption sites in MOS-1 and MOS-2 were found in the corners of the channels and in the center of MOS3, which was because MOS-1 and MOS-2 had large pore sizes and MOS-3 had a smaller and flat pores. Because of the NH3 adsorption in MOSs being via pore adsorption and not vacant metal coordination adsorption, the NH3 molecules did not destroy the metal nodes, which are the sensitive positions in their structure. The isosteric heat of adsorption (Q0st) for NH3 was also obtained from the GCMC for MOS-1, MOS-2, and MOS-3; values of 37.1, 43.6, and 45.6 kJ/mol, respectively, were observed. Because of the smaller Q0st value for NH3 in the three MOSs than that found in other MOFs,43,44 the adsorbent can be easily released by only decreasing the pressure, in accordance to the desorption curves shown in Figure 5; however, we found that the desorption of MOS-2 and MOS-3 were lagged, because of their relatively high Q0st. The probability distribution density of NH3 adsorption on MOS-1 at infinite dilution is shown in Figure S9 in the Supporting Information, and we can find the NH3 molecules distribute in the pores of the structure, which were away from metal nodes and did not influence the structure. According to the NH3 adsorption properties of the three MOS samples, Table 1 lists a summary and comparison of some of the reported NH3 adsorption porous materials and MOFs. It can be seen from the comparison that MOS-1 has a high NH3 uptake capacity and, most importantly, the three MOS samples are superior to most porous materials, which have steady structures and repeatable adsorption ability without performance loss after several recycles, and the adsorbates can be desorbed easily by a decrease in pressure. 5086


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Table 1. Ammonia Adsorption and Desorption Properties under Specific Conditions for Various Porous Materials Surface Area (m2 g−1) Langmuir

pressure, P (bar)

temperature, T (°C)

0.987 0.967 BCb BCb 1.066

25.15 25.15

Recycle Conditions pressure, P (bar)

temperature, T (°C)

porous materials

BET

5A zeolite 13X zeolite ACa 12NN-ACc MOF-5

368 462 1073 926 2449

3917

7.43 9.03 0.13 2.45 12.2

MOF-177

3275

5994

12.2

1.066

25

COF-10

1148

15

1

25

0.1 Torr

200

Mn2Cl2(BTDD)

1917

15.47

1

25

vacuum

200

Co2Cl2(BTDD)

1912

12

1

25

vacuum

200

Ni2Cl2(BTDD)

1762

12.02

1

25

vacuum

200

946 164

9.84 5.69 12.5

1 BCb 1

25 20 25

vacuum

150

25

vacuum

120

vacuum

60

UiO-66-NH2 UiO-66-OH Cu(INA)2

a

Adsorption Conditions ammonia uptake (mmol g−1)

Zn(INA)2

6

1

ELM-12

6.1

1

25

MOS-1

1112

1549

11.5

1

25

vacuum

25

MOS-2

76

115

5.2

1

25

vacuum

25

MOS-3

27

96

3.8

1

25

vacuum

25

Cu3(BTC)2

1460

8.8

BCb

25

IRMOF-3 MOF-199 IRMOF-62 Cu-MOF-74 Co-MOF-74 Mg-MOF-74 Ni-MOF-74 Zn-MOF-74

1568 1264 1814 1170 835 1206 599 496

6.2 5.1 1.4 2.6 6.7 7.6 2.3 3.7

BCb BCb BCb BCb BCb BCb BCb BCb

25 25 25 20 20 20 20

adsorption loss

structural collapse structural collapse 4.5% (3 cycles) no loss (3 cycles) no loss (3 cycles) no loss (3 cycles) 50% (3 cycles) no loss (3 cycles) no loss (3 cycles) no loss (2 cycles) no loss (5 cycles) no loss (5 cycles) no loss (5 cycles) structural collapse

ref 21 21 22 22 29 29 12 27 27 27 27 45 2 46 44 this work this work this work 47 20 20 20 13 26 26 26 26

AC ≡ activated carbon. bBC ≡ breakthrough capacity. c12NN-AC ≡ acid-modified activated carbon.

Figure 7. Comparison of the powder X-ray diffraction (PXRD) patterns of the three metal−organic squares (MOSs) adsorbing H2O/NH3.

NH3 conditions. What’s more, their two recycle adsorption capacities were very close, which also indicated that their

structures were retained. This was attributed to the Co(II) center being saturated at its six coordination sites by two 4,55087


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Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from 15N-Stable Isotope in Size-Resolved Aerosol Ammonium. Environ. Sci. Technol. 2016, 50, 8049−8056. (2) Chen, Y.; Li, L.; Li, J.; Ouyang, K.; Yang, J. Ammonia capture and flexible transformation of M-2(INA) (M = Cu, Co, Ni, Cd) series materials. J. Hazard. Mater. 2016, 306, 340−347. (3) Paulot, F.; Jacob, D. J.; Pinder, R. W.; Bash, J. O.; Travis, K.; Henze, D. K. Ammonia emissions in the United States, European Union, and China derived by high-resolution inversion of ammonium wet deposition data: Interpretation with a new agricultural emissions inventory (MASAGE_NH3). J. Geophys. Res. 2014, 119, 4343−4364. (4) Huang, W.; Yuan, T.; Zhao, Z.; Yang, X.; Huang, W.; Zhang, Z.; Lei, Z. Coupling Hydrothermal Treatment with Stripping Technology for Fast Ammonia Release and Effective Nitrogen Recovery from Chicken Manure. ACS Sustainable Chem. Eng. 2016, 4, 3704−3711. (5) Zhou, F.; Ciais, P.; Hayashi, K.; Galloway, J.; Kim, D. G.; Yang, C.; Li, S.; Liu, B.; Shang, Z.; Gao, S. Re-estimating NH3 Emissions from Chinese Cropland by a New Nonlinear Model. Environ. Sci. Technol. 2016, 50, 564−572. (6) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of ammonia synthesis changed the world. Nat. Geosci. 2008, 1, 636−639. (7) Flórez-Orrego, D.; de Oliveira Junior, S. On the efficiency, exergy costs and CO2 emission cost allocation for an integrated syngas and ammonia production plant. Energy 2016, 117, 341−360. (8) Makhlouf, A.; Serradj, T.; Cheniti, H. Life cycle impact assessment of ammonia production in Algeria: A comparison with previous studies. Environ. Impact Assess. Rev. 2015, 50, 35−41. (9) Klerke, A.; Christensen, C. H.; Nørskov, J. K.; Vegge, T. Ammonia for hydrogen storage: challenges and opportunities. J. Mater. Chem. 2008, 18, 2304−2310. (10) Little, D. J.; Smith, M. R., III; Hamann, T. W. Electrolysis of liquid ammonia for hydrogen generation. Energy Environ. Sci. 2015, 8, 2775−2781. (11) Wang, L.; Yi, Y.; Zhao, Y.; Zhang, R.; Zhang, J.; Guo, H. NH3 Decomposition for H2 Generation: Effects of Cheap Metals and Supports on Plasma−Catalyst Synergy. ACS Catal. 2015, 5, 4167− 4174. (12) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.; Hunt, J. R.; Yaghi, O. M. Exceptional ammonia uptake by a covalent organic framework. Nat. Chem. 2010, 2, 235−238. (13) Katz, M. J.; Howarth, A. J.; Moghadam, P. Z.; DeCoste, J. B.; Snurr, R. Q.; Hupp, J. T.; Farha, O. K. High volumetric uptake of ammonia using Cu-MOF-74/Cu-CPO-27. Dalton Trans. 2016, 45, 4150−4153. (14) Huang, H.; Zhang, W.; Yang, F.; Wang, B.; Yang, Q.; Xie, Y.; Zhong, C.; Li, J.-R. Enhancing CO2 adsorption and separation ability of Zr(IV)-based metal−organic frameworks through ligand functionalization under the guidance of the quantitative structure−property relationship model. Chem. Eng. J. 2016, 289, 247−253. (15) Kumar, D. P.; Choi, J.; Hong, S.; Reddy, D. A.; Lee, S.; Kim, T. K. Rational Synthesis of Metal−Organic Framework-Derived Noble Metal-Free Nickel Phosphide Nanoparticles as a Highly Efficient Cocatalyst for Photocatalytic Hydrogen Evolution. ACS Sustainable Chem. Eng. 2016, 4, 7158−7166. (16) Yu, J.; Wu, Y.; Balbuena, P. B. Response of Metal Sites toward Water Effects on Postcombustion CO2 Capture in Metal−Organic Frameworks. ACS Sustainable Chem. Eng. 2016, 4, 2387−2394. (17) Li, L.; Yang, J.; Li, J.; Chen, Y.; Li, J. Separation of CO2/CH4 and CH 4 /N 2 mixtures by M/DOBDC: A detailed dynamic comparison with MIL-100(Cr) and activated carbon. Microporous Mesoporous Mater. 2014, 198, 236−246. (18) Van Humbeck, J. F.; McDonald, T. M.; Jing, X.; Wiers, B. M.; Zhu, G.; Long, J. R. Ammonia capture in porous organic polymers densely functionalized with Bronsted acid groups. J. Am. Chem. Soc. 2014, 136, 2432−2340. (19) Kumar, P.; Kim, K.-H.; Kwon, E. E.; Szulejko, J. E. Metal− organic frameworks for the control and management of air quality: Advances and future direction. J. Mater. Chem. A 2016, 4, 345−361.

imidazoledicarboxylates and one antenna ligand in the structures, so contact with H2O or NH3 was difficult and did not destroy the Co(II) center,46,51 and the adsorbed H2O/NH3 exists in the pores of materials so that they are away from metal nodes and do not influence the structures. Thus, their stable structures are a significant advantage in NH 3 uptake applications, when compared to other MOFs, which have poor structural stability.

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CONCLUSIONS Metal-organic framworks (MOFs) have been shown to have good performance in the adsorption of ammonia, because of their excellent adsorption surfaces, uniform pore structure, and diverse characteristics; however, their poor structural stability limits their applications. We have demonstrated three metalorganic squares (MOSs)Co4(IDC)4(pda)4, Co4(IDC)4(phen)4, and Co4(IDC)4(bpy)4, which were synthesized via solvothermal processesand the samples were characterized using XRD, TGA, SEM, and water vapor/NH3 adsorption studies. Because of their particular square configuration and the protection of the four antennas, the three MOSs display good H2O uptake (17.63, 8.35, and 7.75 mmol/g, respectively) and repeatable ammonia uptake in which MOS-1, MOS-2, and MOS-3 showed dry ammonia adsorption capacities of 11.5, 5.2, and 3.8 mmol/g, respectively, and wet ammonia adsorption capacities of 4.73, 2.33, and 1.21 mmol/g, respectively. The structures of the three MOSs were not destroyed by H2O, NH3, and their cooperative interactions. Therefore, with the advantages of good structural stability and reusable high ammonia uptake, the MOSs have potential application in ammonia adsorption and storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00460. Synthesis method, PXRD patterns, structures with water, SEM, adsorption model, pore size, specific area, H2O/ NH3 coadsorption method, five cycles, probability distribution density, IR spectra, and coadsorbed molecules density (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 351 6010550. E-mail: [email protected] (J. Yang). *Tel.: +86 351 6010550. E-mail: [email protected] (J. Li). ORCID

Yang Chen: 0000-0001-5743-4182 Jinping Li: 0000-0002-2628-0376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We show great gratitude to the National Natural Science Foundation of China (No. 21676175). REFERENCES

(1) Pan, Y.; Tian, S.; Liu, D.; Fang, Y.; Zhu, X.; Zhang, Q.; Zheng, B.; Michalski, G.; Wang, Y. Fossil Fuel Combustion-Related Emissions 5088


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