A New Porous MOF with Two Uncommon Metal–Carboxylate

Article pubs.acs.org/IC

A New Porous MOF with Two Uncommon Metal−Carboxylate− Pyrazolate Clusters and High CO2/N2 Selectivity Hai-Hua Wang,† Li-Na Jia,† Lei Hou,*,† Wen-juan Shi,† Zhonghua Zhu,‡ and Yao-Yu Wang† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China ‡ School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia S Supporting Information *

ABSTRACT: By a less-exploited strategy, a stable framework was constructed by using 4,4′-biphenyldicarboxylic acid (H2bpdc) and methyl-functionalized 3,3′,5,5′-tetramethyl-4,4′bipyrazole (H2bpz) coligands, revealing a new (6,8)-connected net based on two extremely rare trinuclear and tetranuclear metal−carboxylate−pyrazolate clusters. The framework is very porous and possesses not only high CO2 loadings but also very high CO2/N2 selectivities at 308 and 313 K because of the polar pore surface decorated by clusters, pyrazolyl units, and confined cages with methyl groups dangling. Importantly, GCMC simulation identified two favorable CO2 sorption sites located sequentially near Co3(pz)3 and Co3(CO2)2(pz) motifs of the tetranuclear cluster, and the multipoint framework−CO2 interactions were distinguished. The framework also displays remarkable stability toward water and organic solvents.

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INTRODUCTION Metal−organic frameworks (MOFs) as an exceptional class of CO2 capture and separation materials have been intensively explored because of their great advantages over other adsorbents with diverse structures, high surface areas, and modifiable pores.1 Accordingly, numerous MOFs have recently mushroomed as potential CO2 adsorbents. Among those MOFs, polycarboxylate ligands are always dominant research objects owing to simple synthesis and easy functionalization with specific groups.2 Although the structural stability toward moisture in flue gas is one of crucial criteria for MOFs as adsorbents, yet due to weak M−O bonds, most carboxylate-based MOFs have to suffer hydrolysis in water or even humidity.2a,3 The incorporation of an azolate linker offers a very effective strategy to build chemically stable MOFs because of great robustness of M−N bonds arising from stronger basicity of azolates relative to carboxylates.3a,4 Particularly, pyrazolate shows stronger basicity than other azolates, such as imidazolate, triazolate, and tetrazolate, so pyrazolate is more favorable to build robust MOFs.5 The recent advances in pyrazolate-based MOFs validate excellent resistances toward water, acid, and base chemical stimuli.5,6 In contrast to undefined coordination fashions of carboxylate, pyrazolate reveals simple and predictable coordination modes, and its MOF is more designable. Considering the rich diversity of carboxylate ligands, the incorporation of pyrazolate and carboxylate mixed linkers offers a developmental approach to fabricate MOFs. In this regard, a typical example is the partial replacement of terephthalate in MOF-5 by 3,3′,5,5′-tetramethyl4,4′-bipyrazolate (bpz) to yield an unusual Zn4O(CO2)2(pz)4 cluster-based MOF.7 Meanwhile, one Zn2(CO2)2(pz) clusterbased porous framework was also assembled by dicarboxylate and bipyrazolate mixed linkers,8 possessing good chemical and © XXXX American Chemical Society

thermal stability. Surprisingly, the known carboxylate and pyrazolate mixed ligands-based MOFs are extremely scarce,7−9 and their research is urgent. We are interested in developing new metal−carboxylate− pyrazolate systems by combining 4,4′-biphenyldicarboxylic acid (H2bpdc) and methyl-functionalized bipyrazole H2bpz coligands (Figure 1a). The co-coordination of pyrazolate and carboxylate with metal centers easily forms a cluster with condense metal density, facilitating the interactions with CO2, as well as enhancing structural stability. Furthermore, the clusters would be interlinked by a long biphenyl spacer of H2bpdc to form a porous framework. In addition, although considerable efforts have been devoted to increasing CO2 sorption capacity of MOFs by introducing various functional groups (such as −NH2, −NO2, and halogen atoms),1d,2a,10 the methyl group has seldom been concerned even it can improve water stability of a MOF efficiently.11 In H2bpz, the electron-donating methyl groups make pyrazolate more electronegative, which can augment the contacts between pyrazolyl and CO2.12 Meanwhile, the electropositive H atoms of methyl groups dangling in pores are also attractive to the O atoms in CO2. Herein, using these two ligands, a stable MOF, [Co8.5(μ4-O)(bpdc)3(bpz)3(Hbpz)3]·6(DMF)·9(CH3OH)·15(H2O) (1), was constructed by two uncommon metal−carboxylate−pyrazolate clusters. The CO2 sorption was studied by combining experiment and Grand Canonical Monte Carlo (GCMC) simulation.

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EXPERIMENTAL SECTION

Materials and General Methods. All reagents are commercially available. Elemental analyses (EA) of C, H, and N were determined with Received: November 13, 2014

A

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Inorganic Chemistry Table 1. Crystallographic Data of 1 1 empirical formula formula weight crystal system space group T (K) a (Ǻ ) b (Ǻ ) c (Ǻ ) α (deg) β (deg) γ (deg) V (Ǻ 3) Z Dc/g·cm−3 μ/mm−1 reflns collected reflns unique R(int) GOF R1a, wR2b [I > 2σ(I)] R1, wR2 (all data) Flack parameter

Figure 1. Structures of H2bpdc and H2bpz (a), SBU-1 (b), and SBU-2 (c) in 1. a PerkinElmer 2400C Elemental Analyzer. Infrared spectra (IR) were recorded in the 4000−650 cm−1 region with a PerkinElmer Spectrum 100 instrument. Thermalgravimetric analyses (TGA) were performed in an air stream using a PerkinElmer STA 6000 thermal analyzer at a heating rate of 5 °C/min. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Sorption isotherms were measured using a Micrometrics Tristar 3020 instrument. Samples were immersed in CH2Cl2 for 48 h (2 × 24 h) and then vacuumed at 150 °C for 6 h to remove solvent molecules prior to measurements. Grand Canonical Monte Carlo (GCMC) simulations were performed by the Sorption module of Material Studio (Supporting Information).13 Synthesis of [Co 8.5 (μ 4 -O)(bpdc) 3 (bpz) 3 (Hbpz) 3 ]·6(DMF)· 9(CH3OH)·15(H2O) (1). A mixture of Co(ClO4)2·4H2O (0.073 g, 0.20 mmol), H2bpdc (0.024 g, 0.10 mmol), and H2bpz (0.038 g, 0.20 mmol) in DMF (5 mL) and CH3OH (5 mL) was capped in a Teflon-lined reactor, heated at 130 °C for 3 days, and cooled to room temperature, and purple crystals (21 mg, 26.5%) of 1 were isolated. Anal. Calcd for C129H207Co8.50N30O43: C, 46.01; H, 6.20; N, 12.48%. Found: C, 46.09; H, 6.11; N, 12.39%. IR (KBr, cm−1): 3201w, 2925m, 1662s, 1597s, 1540s, 1495m, 1386vs, 1321m, 1180m, 1091m, 1046s, 847s, 770vs. Caution! Perchlorate salts are potentially explosive, which should be handled with care. X-ray Crystallographic Measurements. Diffraction data were collected with a Cu Kα radiation (λ = 1.54178 Å) at 296(2) K on a Bruker-AXS SMART CCD area detector diffractometer. Absorption corrections were carried out utilizing SADABS routine. The structures were solved by the direct methods and refined by full-matrix leastsquares refinements based on F2.14 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added to their geometrically ideal positions. 1 contains heavily disordered solvent molecules, which cannot be identified from the difference Fourier map due to the weak diffraction intensity of crystal. Thereby, the SQUEEZE routine of PLATON15 was applied to remove the contributions to the scattering from the solvents. The formula was determined by combining singlecrystal structures, EA, and TGA. Relevant crystallographic data are given in Table 1.

a

C102H99Co8.5N24O13 2369.96 cubic P4132 296(2) 33.8846(1) 33.8846(1) 33.8846(1) 90.00 90.00 90.00 38905.2(2) 8 0.809 5.845 118 018 9673 0.1165 1.103 0.0703, 0.0889 0.1527, 0.1920 0.066(8)

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

site occupancies of 17/6, one μ4-O2− atom, one bpdc2−, one bpz2−, and one Hbpz− linkers (Figure S1, Supporting Information). All Co2+ centers adopt distorted tetrahedral coordination geometries. bpdc2− shows a bididentate syn−syn bridging fashion. Hbpz− and bpz2− exhibit μ3- and μ4-bridging coordinations, respectively. The most interesting structural feature in 1 is the coexistence of two uncommon cluster secondary building units (SBUs), [Co3(CO2)2(pz)4(Hpz)2] (SBU-1) and [Co4(μ4-O)(pz)3(CO2)3] (SBU-2) (Figure 1). SBU-1 contains three linearly-arranged Co2+ ions bridged by four pyrazolates of two bpz2− and two Hbpz−, in which each outer Co1 atom is coordinated by three pyrazolyl N atoms, and one carboxylate O atom of bpdc2−, while the central Co2 atom (1/2 site occupancy) binds four pyrazolyl N atoms. Notably, the similar cluster of SBU-1 was only found in one 1D coordination polymer according to the latest CCDC database (version 5.35).16 The tetranuclear SBU-2 features a 3-fold axis threading Co4 (1/3 site occupancy) and central μ4-O2− atoms, in which Co3 and Co4 atoms display N2O2 and O4 coordination environments, respectively. The Co4O core is fixed by three pyrazolate and three carboxylate groups. Metal− carboxylate−pyrazolate clusters are not commonly observed in complexes; especially, SBU-2 represents a new type of octahedrally configural SBU, being similar to the well-known [Zn4(μ4-O)(CO2)6], [Zn4(μ4-O)(CO2)2(pz)4],7 and [Co4(μ4-O)(pz)6] SBUs.17 In 1, three SBU-1, one SBU-2, three Hbpz−, and three bpz2− interconnect to produce a tetrahedral cage (d = ∼5.3 Å) with methyl groups occupied (Figures 2 and S2a, Supporting Information). The adjacent cages are hinged by co-shared SBU-1 to form 41 helix chains along the a, b, and c axes (Figure S2b). The interlinkages of SBU-1 and SBU-2 with organic linkers generate an intricate 3D open chiral framework (Figure 2). The framework contains a 3D intersected porous system with 54.2% solvent accessible voids,15 including 1D channels along the diagonal and axial directions of cubic cell, with the effective window sizes of ∼7.8 × 7.8 and ∼5.6 × 5.6 Å2,

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RESULT AND DISCUSSION Crystal Structure of 1. 1 crystallizes in the cubic space group P4132, showing an unprecedented (6,8)-connected 3D chiral framework with trinuclear and tetranuclear Co2+ clusters. The asymmetric unit consists of four Co2+ ions with the sum for B

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Figure 2. 3D structure of 1 viewed along the c axis (a) and (111) direction (b), respectively, forming an intersected 3D porous system (yellow ball = tetrahedral cage).

Figure 3. Figures of trinuclear SBU-1 (a) and tetranuclear SBU-2 (b), and unprecedented (6,8)-connected topology net of 1 (c) (green and purple balls represent 6- and 8-connected nodes, respectively).

rapidly, leaving black Co2O3 residue with 20.9% weight (calcd 19.7%). The guest-free phase 1a was obtained by soaking 1 in CH2Cl2 and next heated at 150 °C under vacuum, as evidenced by the FTIR of 1a, wherein the characteristic CO vibration of DMF solvents is absent (Figure S5, Supporting Information). 1a holds the structural integrity, as confirmed by power X-ray diffraction (Figure S3). Sorption Properties. The permanent porosity of 1a is verified by the N2 sorption isotherm at 77 K (Figure 4a), which reveals a type-I isotherm with a BET surface area up to 1815 m 2 g −1 (Langmuir surface area 2020 m 2 g −1 ), demonstrating high porosity. The D-A microporous volume is 0.68 m3 g−1, agreeing well with the calculated (0.67 m3 g−1) from the crystal structure. Using the MP method, the average pore diameter is 6.7 Å. N2 and CO2 sorption isotherms of 1a were also measured at 273 and 298 K; it shows that the N2 and CO2 uptakes at 130 kPa are 12.5/7.9 and 95.9/58.6 cm3 (STP) g−1, respectively (Figure 4). At 298 K and 100 kPa, the CO2 loading (49.1 cm3 (STP) g−1 or

respectively. The porous surface is basically surrounded by methyl groups, phenyl and pyrazolyl rings, and carboxylate motifs. Topologically, SBU-1 and SBU-2 can be simplified as 8- and 6-connected nodes, respectively; thus, 1 is a binodal (6,8)connected net with a point symbol of (334656)2(364751164)3 by Topos18 (Figure 3). The (6,8)-connected net is indeed scarce topology because it requires two kinds of high-connected nodes, and which was reported in very rare complexes.19 To date, even only six (6,8)-connected nets are theoretically speculated in the RCSR database.20 The net in 1 represents a new (6,8)-connected topology that has not yet been reported and speculated. PXRD and TGA. The experimental PXRD pattern of 1 is consistent with the simulated one from the single-crystal structure (Figure S3, Supporting Information), confirming the phase purity. Thermogravimetric analysis of 1 reveals a 29.6% weight loss at 30−170 °C (Figure S4, Supporting Information), attributing to the escapes of all solvent molecules (calcd 29.8%). The framework is stable up to 300 °C; then, it decomposes C

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Figure 4. N2 (a) and CO2 (b) sorption isotherms of 1a at different temperatures.

Figure 5. IAST adsorption selectivities and isotherms of 1a for CO2 over N2 at different compositions.

in 1a, which make the framework highly polar, causing specific interactions with CO2 due to its large quadrupole moment. Meanwhile, the methyl groups of H2bpz narrow the pores in 1a, and restrict the diffusion of N2 into pores owing to its larger dynamic diameter (3.64 Å) than CO2 (3.3 Å). The isosteric heat (Qst) of CO2 adsorption was calculated by the virial equation from the sorption isotherms at 273 and 293 K (Figure S7, Supporting Information). 1a shows an initial Qst of 27.0 kJ mol−1, and it declines slowly to 25.4 kJ mol−1 at 58.6 cm3 (STP) g−1 coverage. These values are moderately high and comparable with those in some amine-functionalized MOFs,26 reflecting strong framework−CO2 interactions. It is worth noting that the reported gas sorption measurements in MOFs were majorly made at or below 298 K, and very few were performed at higher temperatures, which are more practical to evaluate postcombustion CO2 capture performance in realistic working temperatures of about 313−333 K.1a,2a,27 Strikingly, at 308 K and 130 kPa, the CO2 uptakes in 1a reached

8.8 wt %) is comparable with those (6.2−9.1 wt %) in some zinc− imidazolate zeolite frameworks,2a whereas it exceeds the values in some Zn4O-containing MOFs with nanopores (9.0−32.0 Å), such as SNU-70 (3.5 wt %),21 UMCM-1 (3.8 wt %),22 and MOF-5 (4.5 wt %),23 indicating the advantage of CO2 capture in small pores owing to overlap of the potential fields of pore walls. The selectivities for CO2−N2 mixtures in 1a were estimated using the ideal adsorbed solution theory (IAST) (Figure S6, Supporting Information). For an equimolar mixture, the selectivities at 298 K are in the range of 28−12 from 0.1−130 kPa (Figure 5a), being approximate to those values in MOFs [Zn(BDC-R)(TED)0.5] (R = −H, −OH, and −NH2) (3−17),24 PCN-61 (13−15),25 and [Cu24(TPBTM)8] (24−22).25 Even though the CO2 content reduces a 15% ratio, a typical flue gas composition in a postcombustion process, the CO 2 /N 2 selectivities still reach to 13−32 (Figure 5b). The significant CO2/N2 selectivity is mainly related to the existence of metal− carboxylate−pyrazole clusters, pyrazol motifs, and methyl groups D

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Figure 6. Simulated favorable CO2 sorption sites in 1a: (a) site I, OCO2···Cpz = 3.606−3.917 Å, Co···OCO2 = 3.460−3.509 Å, OCO2···H = 3.069−3.378 Å; (b) site II, CCO2···πcentroid = 3.459 Å, OCO2···Ccarboxylate = 3.324 and 3.449 Å, Co···OCO2 = 3.541−3.627 Å, OCO2···H = 2.883 Å.

40.2 cm3 (STP) g−1, but only a trace of N2 (0.8 cm3 (STP) g−1) was observed (Figure 4). Especially for CO2:N2 = 50:50 and 15:85 mixtures, they show very high IAST selectivities of 58−188 and 49−148 from 0.1 to 130 kPa (Figure 5c,d), respectively. Most importantly, at 313 K, 1a is nonadsorptive for N2 but shows excellent CO2 loading of 30.2 cm3 (STP) g−1 at 100 kPa, which is among the best values of CO2 uptakes in MOFs at 313 K (Table S1, Supporting Information), and are comparable with those in zinc−adeninate bio-MOFs-1 with multipoint sorption sites,27b but exceeding greatly those in MOF-5 (1.13 mmol g−1) and MOF-177 (0.80 mmol g−1). GCMC Simulation. Furthermore, GCMC simulation (see the Supporting Information) was applied to identify the framework−CO2 binding details at 298 K. Below 100 kPa, two independent CO2 binding sites in 1a were found: CO2-I and CO2-II. CO2-I is located in the vicinity of the Co3(pz)3 motif of SBU-2 in the tetrahedral cage with the molecular axis being vertical to the plane defined by three Co1 atoms (Figure 6a). For CO2-I, one electronegative O atom contacts with three Co atoms (Co···O = 3.460−3.509 Å) and six electropositive pyrazolyl C atoms (O···C = 3.606−3.917 Å). The Co···O distances are shorter than the sum of van der Waals radii of cobalt (2.03 Å) and oxygen (1.52 Å), indicating strong interactions. The other O atom of CO2-I is involved in multiple C−H···O contacts with methyl H atoms of Hbpz− in the cage (O···H = 3.069−3.378 Å). When increasing the loading to four CO2 molecules per unit cell, a second favorable site CO2-II near the Co3(CO2)2(pz) motif of SBU-2 is accessible (Figure 6b). Differing from the situation in CO2-I, in CO2-II, two O atoms form moderate interactions with three Co atoms (Co···O = 3.541−3.627 Å), two carboxylate C atoms (O···C = 3.324 and 3.449 Å) of two bpdc2−, and one methyl H atom of Hbpz− (C···H = 2.883 Å). Furthermore, CO2-II also forms a strong CCO2···πpz interaction (C···πcentroid = 3.459 Å) with one pyrazolate in SBU-2 due to almost parallel arrangement between them,12 whereas, for site CO2-I, only the weak CCO2···πpz contacts exist with long C···πcentroid separations (3.968−4.117 Å) (Figure S8, Supporting Information).

The above two CO2 favorable sites near SBU-2 are very similar to the first sorption site of α(CO2)3 triangular regions around the Zn4O(CO2)6 cluster in MOF-5.28 The preferential sites of CO2-I other than CO2-II could be attributed to the overlap of potential field from pocket walls in the confined cages with densely occupied methyl groups, as well as abundant C−Hmethyl···OCO2 and CCO2···πpz interactions. This result is further confirmed by higher binding energies of the framework with CO2-I (−35.0 kJ mol−1) than CO2-II (−21.8 kJ mol−1). Notably, below 1 atm, the region around SBU-2 dominates CO2 adsorption (Figure S9, Supporting Information), no obvious binding sites near SBU-1 because it is enclosed by methyl groups of pyrazolyls. The multipoint contacts in 1a lead to an overall high simulated adsorption heat of 24.9−27.2 kJ mol−1 from 0−1 atm, matching well with the experimental values (25.6− 27.0 kJ mol−1). At higher pressure, the additional CO2 sorption sites including phenylene faces and edges of the bpdc2− linker are accessible in 1a (Figure S9). As known to all, the stability toward water or humidity is a crucial indicator of a MOF for CO2 capture from postcombustion flue gas that contains a certain amount of moisture. 1 is not only stable in common organic solvents but also maintains framework integrity in different environments, such as in boiling benzene or methanol for 24 h, in a DMF−H2O (1:3) mixture for 24 h, or even in water for 12 h at room temperature, as well as in ambient air beyond 30 days, as confirmed by PXRD (Figure S3). This advantage can be ascribed to robust Co−Npyrazolate bonds in 1, distinguishing it from the rapid collapse or hydrolysis of MOF-5 in air or DMF−H2O (1:1) solvents.6c,29

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CONCLUSIONS In summary, this work demonstrates that the incorporation of pyrazolate and carboxylate mixed linkers is a practicable approach to construct unique MOFs for CO2 capture. 1 embodies the following impressive novelties: (1) 1 shows remarkable chemical stability. (2) 1 possesses a new (6,8)connected topology based on two unusual metal−carboxylate− pyrazolate clusters. (3) The polar pore surface decorated by E

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clusters, pyrazolyl motifs, and confined cages with dangling methyl groups affords 1 high CO2 loading and highly selective capture for CO2 over N2 at 313 K; especially, this temperature is a more practical working temperature in the postcombustion CO2 capture process. (4) More importantly, GCMC simulation confirmed the multipoint framework−CO2 interactions, and directly identified the preferential CO2 sorption site in Co3(pz)3 motifs rather than Co3(CO2)2(pz). This contribution may spark a broad spectrum of interest in the fabrication of stable MOF materials by combining carboxylate and pyrazolate mixed linkers.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format, additional structural figures, FTIR, TGA, PXRD patterns, the detailed calculations on sorption, and bond length/angle, and GCMC simulation methodlody. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21471124, 21371142, and 21001088), NSF of Shannxi province (2013KJXX-26, 13JS114, and 2014JQ2049), the Australian Research Council Discovery project DP1096948, and the Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (338080060).

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DOI: 10.1021/ic502733v Inorg. Chem. XXXX, XXX, XXX−XXX