A Mixed-Cluster Approach for Building a Highly Porous Cobalt(II

Feb 20, 2017 - Synopsis. An unusual cobalt(II) isonicotinic acid framework incorporating mixed square-planar Co4(μ2-OH)4(μ4-OH) and cuboidal ...
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A Mixed-Cluster Approach for Building a Highly Porous Cobalt(II) Isonicotinic Acid Framework: Gas Sorption Properties and Computational Analyses Di-Ming Chen, Nan-Nan Zhang, Chun-Sen Liu,* Zhi-Hao Jiang, Xing-Dong Wang, and Miao Du* Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, P. R. China S Supporting Information *

finding of a unique porous cobalt(II) isonicotinic acid framework based on square-planar Co4(μ2-OH)4(μ4-OH) and cubane-type Co4(μ3-OH)4 clusters. To our knowledge, this is probably the first example of an MOF that consists of two such metal clusters in one single framework simultaneously. Notably, this MOF not only shows high specific surface areas, which is the highest observed for all of the cobalt(II) isonicotinic acid frameworks, but also exhibits high H2 storage capacities at cryogenic temperature. The grand canonical Monte Carlo (GCMC) simulation was further carried out to probe the sorption behavior in detail. Red block crystals of {(H2N(CH3)2)[Co8(μ2-OH)4(μ3OH)4(μ4-OH)(Ina)8](H2O)15(DMA)9}n (1; Ina = isonicotinic acid and DMA = N,N-dimethylacetamide) were harvested by a solvothermal treatment of CoCl2·6H2O with HIna in the solvent of DMA with HBF4 as the additive. The X-ray study results reveal that 1 belongs to the tetragonal space group I4̅m2, showing an anionic 3D framework structure with two types of tetranuclear cobalt nodes as the foundational building units. There are two half CoII ions (Co1 and Co2), a half μ2-OH atom, a half μ3-OH atom, one-eighth μ4-OH atom, two half Ina ligands, and oneeighth disordered H2N(CH3) 2 cation derived from the decomposition of DMA solvents in the asymmetric unit.21 The valences of OH atoms are determined from bond-valence-sum (BVS) calculations (Figures S1 and S2).22 Co1 is six-coordinated by two carboxylic O and one pyridine N atoms from three different Ina linkers, and the leaving sites are occupied by two μ2OH and one μ4-OH atoms, resulting in an octahedral [CoNO5] coordination geometry (Figure 1a); Co2 adopts a coordinating mode similar to that of Co1, and the only difference lies in the fact that the μ4-OH atom was replaced by one μ2-OH atom in the coordinated atoms (Figure 1b). Four symmetry-equivalent Co1 atoms join with each other by one μ4-OH and four μ2-OH atoms; this leads to a nearly coplanar Co4(μ4-OH)(μ2-OH)4 plate. Meanwhile, four symmetry-related Co2 atoms are bridged by four μ3-OH atoms to afford a distorted cubane-type Co4(μ3OH)4 cluster. Each planar Co4(μ4-OH)(μ2-OH)4 cluster is connected with six cuboidal Co4(μ3-OH)4 clusters by eight Ina ligands in a single-walled (along the a and b axes) and doublewalled (along the c axis) fashion in the 3D lattice, and each cuboidal Co4(μ3-OH)4 cluster is associated with six planar Co4(μ4-OH)(μ2-OH)4 clusters via the same mode (Figures S3

ABSTRACT: A unique channel-type metal−organic framework (MOF) built up from mixed square-planar Co4(μ2-OH)4(μ4-OH) and cuboidal Co4(μ3-OH)4 clusters with an isonicotinic acid ligand has been successfully fabricated that demonstrates the highest specific surface area and high H2 uptake capacities among all of the cobalt(II) isonicotinic acid frameworks reported so far.

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he past 10 years have witnessed the rapid evolution of an emerging class of porous adsorbent materials termed metal−organic frameworks (MOFs) that were constructed by metal nodes and multitopic organic linkers.1 The highly modular nature of these framework materials allows the design of materials tuned with specific pore shapes/surroundings and spaces, which make them ideal candidates for many important energy- and environment-related applications such as energy gas storage (H2, CH4, and C2H2), greenhouse gas capture, hydrocarbon separation, and so on.2−6 Generally speaking, most of these applications of MOFs are largely related to their porous character, which could have an impact on the effective gas-storage performance. This has resulted in extensive research endeavors being devoted to the development of new synthetic strategies for highly porous MOFs, especially those based on metal clusters as secondary building units (SBUs) for their more robust structural nature than those composed of single-metal nodes.7−10 Currently, the most widely used strategy for producing porous MOFs relies on the employment of elongated or mixed organic ligands, while those with one sample linker built on multiclusters have been less explored. One ingenious example is the assembly of three geometrically distinct metal-containing SBUs in one framework to produce a mesoporous MOF with high porosity, in which only one small organic linker was used.11 As one of the most widely used simple organic ligands, isonicotinic acid (Ina) has been widely used to construct MOFs for various applications.12−20 For instance, a highly connected MOF with a windmill-like Co4{Co4(μ4-O)} SBU connected by Ina ligands has been reported by Chen’s group showing a Langmuir surface area of 459 m2/g.17 Despite this, MOFs based on the Ina ligand showing high surface areas (>1000 m2/g) have not been reported so far. As mentioned above, the incorporation of two different metal clusters with a simple organic ligand might be a feasible approach for the construction of MOFs with high specific surface areas and porosities. Bearing this in mind, in this work, we present the © XXXX American Chemical Society

Received: December 31, 2016

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DOI: 10.1021/acs.inorgchem.6b03170 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. (a) 77 K N2 sorption isotherm (inset: pore-size distribution) and (b) H2 sorption isotherms at 77 and 87 K. (c) CO2 and N2 sorption isotherms at 273 and 298 K. (d) CO2 and H2 adsorption enthalpies for 1a based on virial-type fittings.

Figure 1. (a) Co4(μ2-OH)4(μ4-OH) cluster (symmetry codes: A, 1 − y, x, −z; B, 1 − x, 1 − y, z; C, y, 1 − x, −z). (b) Co4(μ3-OH)4 cluster. (c) View of the 1D channels running along the [001] direction. (d) Representation for the pcu α-Po primitive cubic topology.

Ina ligands and is the highest observed for the cobalt(II) isonicotinic acid frameworks (Table S1).12−20 The total pore volume is estimated to be 0.621 cm3/g, which is slightly smaller than the theoretical value of 0.673 cm3/g. This is attributed to the presence of disordered H2N(CH3)2 cations that could not be located via the X-ray study and may block the 1D channels. The pore sizes calculated from analysis of the N2 isotherm at 77 K are distributed around 0.59 Å. Low-pressure H2 adsorption isotherms of 1a were collected at 77 and 87 K and showed rapid kinetics and good reversibility. The maximum H2 uptake capacity for 1a is 1.71 wt % (193 cm3/ g) at 77 K and 1 bar, corresponding to about 27 H2 molecules per unit cell (Figure 2b). Although this value is slightly lower than that of MAF-38 (1.9 wt %) functionalized with a high density of open metal sites, it still outperforms most MOFs based on Ina ligands.12−20 Furthermore, the isosteric heat (Qst) of adsorption for H2 near zero coverage is 9.2 kJ/mol from virial analysis and slightly decreases as the H2 uptake increases (Figure 2d). The isosteric heat of adsorption for 1a near zero coverage is higher than those of many famous MOFs such as MOF-5 (5.2 kJ/mol), NOTT-119 (7.3 kJ/mol), and MOF-74 (8.3 kJ/mol).24−26 The outstanding H2 storage performance of 1a may be attributed to its high pore volume coupled with hydroxyl-group-functionalized 1D channels along the c axis of the crystal structure, which can enhance the interaction between H2 molecules and the framework.27 To gain further insight into the H2 adsorption behavior of 1a, the density distributions of adsorbed H2 molecules at 77 K under a given loading were probed via GCMC simulation. As illustrated in Figure 3a, H2 molecules are primarily adsorbed in the pores along the c axis, and the binding sites are mostly located around the pore walls functionalized with OH atoms. More specifically, it could be observed that the density in 1a preferentially distributes in the regions near the H2N(CH3)2 cations. In addition, the favorable bonding sites H2 within 1a are given in Figure 3b, which showed that the first adsorbed H2 molecule is located at the corner of the 1D channels close to the H2N(CH3)2 cation and μ2OH, with several hydrogen bonds between the open O-atom donors and the H2 molecule. In addition, the sorption isotherms of CO2 and N2 around room temperature were also collected. As shown in Figure 2c, the

and S4). As a result, an extended 3D network with 1D nanosized channels running along the [001] direction was formed, showing a large solvent-accessible void of 59.3% that was occupied by guest molecules and disordered countercations (Figure 1c). The calculated pore-diameter distribution according to the Poreblazer software is 7.24−10.95 Å, and its calculated accessible surface area is 1472 m2/g.23 Theoretically, MOFs with large free volume tend to interpenetrate, and the present result implies that the incorporation of two mixed-metal clusters in one framework might be helpful to inhibit framework interpenetration. Topologically, both the planar Co4(μ4-OH)(μ2-OH)4 and cuboidal Co4(μ3-OH)4 clusters could be simplified into 6connected nodes, so that the whole network of 1 could be described as an 8-connected pcu α-Po primitive cubic net with a point symbol of (412.63) (Figures 1d and S5). The powder X-ray diffraction (PXRD) patterns of the assynthesized 1 and its solvent-exchanged samples were collected at ambient temperature and are consistent with the simulated data, showing its phase purity and framework rigidity (Figure S6). The thermodynamic property of 1 was studied by thermogravimetric analysis (TGA). The TGA curve showed a two-step weight loss in the temperature range of 52−304 °C. The first step from 52 to 121 °C with approximately 10.3% weight loss may be due to the escape of lattice water molecules (ca. 10.1%), and the second 30.3% weight decrease from 121 to 292 °C might due to the continuous release of the lattice DMA and HN(CH3)2 molecules (calcd 30.7%). 1 showed a short plateau from 293 to 362 °C, and then the framework began to decompose (Figure S7). The guest-free samples of 1 (1a) could be generated by pumping CH2Cl2-exchanged samples of 1 at 30 °C for 24 h under high vacuum. Its framework integrity was confirmed by the PXRD measurement, and full activation was verified by the TGA data (Figures S6 and S7). The N2 sorption isotherm of 1a at 77 K clearly shows a reversible type I adsorption behavior characteristic of microporous material with a saturated uptake of 402 cm3/ g and a calculated Brunauer−Emmett−Teller (Langmuir) surface area of 1500 (1667) m2/g, which is close to the value calculated from the Poreblazer software (1472 m2/g; Figure 2a). The surface area of 1a surpasses that of the most MOFs based on B

DOI: 10.1021/acs.inorgchem.6b03170 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry ORCID

Chun-Sen Liu: 0000-0002-5095-7359 Miao Du: 0000-0002-1029-1820 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21471134, 21571158, and 21601160), Plan for Scientific Innovation Talent of Henan Province (Grant 154200510011), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant 152101510003), Program for Science & Technology Innovative Research Team in University of Henan Province (Grant 15IRTSTHN-002), and doctoral plan of of Zhengzhou University of Light Industry (Grant 2015BSJJ042).

Figure 3. (a) Simulated H2 adsorption density contours in 1a. (b) Favorable bonding sites between H2 and the MOF.

CO2 sorption isotherm of 1a showed a steep rise with uptakes of 86.8 cm3/g (273 K) and 39.1 cm3/g (298 K) at 1 bar without reaching saturated adsorption, respectively, indicating a moderate binding ability between CO2 and the framework. Compared to the CO2 adsorption, the N2 uptakes are only 4.8 cm3/g at 273 K and 3.1 cm3/g at 298 K, which are much lower than those of CO2 under the same conditions. The high uptake capacity for CO2 gas over N2 in 1a may be associated with the quadruple moment of CO2 (−1.34 × 10−39 Cm2), which induces efficient interaction with the charged framework.28−30 Using the initial slopes of the CO2 and N2 isotherms, the Henry’s law CO2/N2 selectivity was calculated to be 38 at 273 K and 40 at 298 K, respectively. The observed selectivity is above the average value reported for nonfunctionalized MOFs under the same conditions, including ZIF-100, Cu-BTTri, Fe-BTT, MOF508b, and so on.31−35 The adsorption enthalpies (Qst) of CO2 are counted from the adsorption data at 273 and 298 K by using a virial-type fitting method to quantitatively assess the binding strength between CO2 and the framework, which reveals a value of 24.1 kJ/mol, and this value is comparable to that of an aminofunctionalized Ina-based MOF (23.58 kJ/mol), suggesting moderately strong interaction between CO2 and the framework.36 In summary, an unusual 3D 6-connected channel-type cobalt(II) isonicotinic acid framework has been obtained, which represents the first case of MOFs that incorporate both planar Co4(μ4 -OH)(μ2 -OH) 4 and cuboidal Co4 (μ3 -OH) 4 clusters in one single framework. Because of its high porosity, this MOF exhibits a high H2 uptake capacity at 77 K and a moderately high CO2 uptake capacity at 273 K. In addition, the above results also give new insight into the fabrication of noninterpenetrated porous MOFs by taking advantage of mixedcluster strategies.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03170. BVS calculations, additional structures, PXRD data, TGA curve, and sorption data fittings (PDF) CIF file (CIF)

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*E-mail: [email protected]. *E-mail: [email protected]. C

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DOI: 10.1021/acs.inorgchem.6b03170 Inorg. Chem. XXXX, XXX, XXX−XXX