Tunable Robust pacs-MOFs: a Platform for Systematic Enhancement

Jan 4, 2018 - research attention in the domain of gas storage and separation. In this study, we ... free amido groups, and charged framework give the ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Tunable Robust pacs-MOFs: a Platform for Systematic Enhancement of the C2H2 Uptake and C2H2/C2H4 Separation Performance Di-Ming Chen, Chun-Xiao Sun, Nan-Nan Zhang, Huan-Huan Si, Chun-Sen Liu,* and Miao Du* Henan Provincial Key Lab of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou, 450002 Henan, China S Supporting Information *

ABSTRACT: As a modulatable class of porous crystalline materials, metal−organic frameworks (MOFs) have gained intensive research attention in the domain of gas storage and separation. In this study, we report on the synthesis and gas adsorption properties of two robust MOFs with the general formula [Co3(μ3-OH)(cpt)3Co3(μ3-OH)(L)3(H2O)9](NO3)4(guests)n [L = 3amino-1,2,4-triazole (1) and 3,5-diamino-1,2,4-triazole (2); Hcpt = 4-(4-carboxyphenyl)-1,2,4-triazole], which show the same pacs topology. Both MOFs are isostructural to each other and show MIL-88-type frameworks whose pore spaces are partitioned by different functionlized trinuclear 1,2,4-triazolate-based clusters. The similar framework components with different amounts of functional groups make them an ideal platform to permit a systematic gas sorption/separation study to evaluate the effects of distinctive parameters on the C2H2 uptake and separation performance. Because of the presence of additional amido groups, the MOF 2 equipped with a datz-based cluster (Hdatz = 3,5-diamino-1,2,4-triazole) shows a much improved C2H2 uptake capacity and separation performance over that of the MOF 1 equipped with atz-based clusters (Hatz = 3-amino-1,2,4-triazole), although the surface area of the MOF 1 is almost twice than that of the MOF 2. Moreover, the high density of open metal sites, abundant free amido groups, and charged framework give the MOF 2 an excellent C2H2 separation performance, with ideal adsorbed solution theory selectivity values reaching up to 11.5 and 13 for C2H2/C2H4 (1:99) and C2H2/CO2 (50:50) at 298 K and 1 bar, showing potential for use in natural gas purification.

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INTRODUCTION Being the simplest unsaturated hydrocarbon with a triple bond, acetylene (C2H2) is used extensively as a fuel and is the main building block of organic chemistry; it is also used as the raw material for the manufacture of various industrial products such as plastics, rubber, and acetic acid.1−3 However, the presence of an impurity such as CO2 would greatly influence the efficiency of acetylene in these processes. On the other hand, the effective removal of trace C2H2 from C2H4 gas is of great importance for the production of polymer-grade C2H4. To date, the most widely used technologies for the removal of C2H2 from a mixture of C2H2/C2H4 include partial hydrogenation and cryogenic distillation, which are high-cost and energy-intensive, so the development of efficient adsorbents for C2H2 capture from C2H4 is of high importance.4−6 As a promising class of porous adsorbent materials, metal− organic frameworks (MOFs) have attracted much research interest because of their designable topological nets and modular compositions.7−9 The endless possibilities for the © XXXX American Chemical Society

selection of building blocks make these kinds of materials ideal platforms for targeted applications such as energy gas storage, greenhouse gas capture, and natural gas purification, by finetuning of their pore size, geometry, and functionality.10−17 However, it is always very difficult for MOFs to possess both high selectivity and adsorption capacity at the same time for the so-called “trade-off” effect.18 From the perspective of material design, gaining an in-depth understanding and control of the C2H2−framework interactions in MOFs is crucial for achieving high effective C2H2 storage and C2H2/C2H4 separation.19−24 Generally speaking, two strategies have been commonly reported to enhance the C2H2−framework interactions in MOFs: (i) the incorporation of open metal sites (OMSs); HKUST-1 presents a good example of using OMSs to bind with the C2H2 molecules to achieve high C2H2 uptake capacity; (ii) functionalization of the organic linkers to provide binding Received: January 4, 2018

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DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

separation performance. Because of the presence of the additional amido group, the MOF 2 equipped with a datzbased cluster shows a much improved C2H2 uptake capacity and separation performance than that of the MOF 1 equipped with atz-based clusters, although the surface area of the MOF 1 is almost twice that of the MOF 2. Moreover, the high density of OMSs, abundance of amido groups, and charged framework give the MOF 2 excellent C2H2 separation performances, with ideal adsorbed solution theory (IAST) selectivity values reaching up to 11.5 and 13 for C2H2/C2H4 (1:99) and C2H2/CO2 (50:50) at 298 K and 1 bar, showing potential for use in natural gas purification.

sites for guest species; UTSA-100 is an outstanding case with confined pores (pore size 3.3 Å) and amido-decorated pore surroundings, which together enabled excellent C2H2/C2H4 selectivity and high C2H2 uptake capacity at low pressure.6,21 The favorable roles of OMSs and polar donor groups in C2H2 binding have also been widely confirmed by the in situ neutron diffraction experiments.25,26 Compared with the in situ generated OMSs, the effects of polar donor groups are more controllable because chemist can produce various functional organic ligands via organic synthesis. Meanwhile, MOF crystal chemistry (e.g., isoreticular synthesis) allows distinctive structural parameters to be systematically modified and finetuned, and their individual impact on the C2H2 sorption energetics can be pinpointed in order to gain better insight into the structure−property relationship. For instance, a series of highly porous MOFs with an NbO-type network have been synthesized by using various functionalized tetracarboxylic ligands, whose gas sorption/separation performances can be systematically modified to achieve record values.27−31 The pacs-MOF platform, composed of 9-connected trigonalprismatic [Co3(OH)] secondary building units and a 3connected triangular molecular building block, is ideally suited for systematic fine-tuning to discern the relative contribution of each factor toward the overall C2H2 sorption and separation performances.32,33 In fact, we have previously reported that the hydrothermal stability and C2H2 uptake could be fine-tuned by incorporating different molecular blocks in the pores.34,35 In this contribution, we demonstrate the power of this platform by accommodating two rare planar 1,2,4-triazolate-based {Co3} clusters in a bifunctional ligand-based MIL-88-type framework, and the effects of variations on the storage and separation performances were investigated for C2H2, CO2, and C2H4 as adsorbents. The obtained cationic MOFs with the general formula [Co 3 (μ 3 -OH)(cpt) 3 Co 3 (μ 3 -OH)(L) 3 (H 2 O) 9 ](NO3)4(guests)n [L = 3-amino-1,2,4-triazole (1) and 3,5diamino-1,2,4-triazole (2); Hcpt = 4-(4-carboxyphenyl)-1,2,4triazole] are isostructural to each other and show targeted pacstype framework structures whose pore spaces are partitioned by two different trinuclear-functionalized 1,2,4-triazolate-based clusters (Scheme 1). Besides significant improvement of the framework robustness after guest removal, the 1,2,4-triazolatebased clusters give both OMSs and amido groups in the framework. The similar framework components with different amounts of functional groups make them an ideal platform to permit a systematic gas sorption/separation study to evaluate the effects of distinctive parameters on the C2H2 uptake and

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EXPERIMENTAL SECTION

Materials and Methods. All of the reagents were commercially acquirable and were used directly. The 4-(4-carboxyphenyl)-1,2,4triazole (Hcpt) ligand was synthesized according to a previous approach.36−38 Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku Ultima IV diffractometer. Thermogravimetric analyses (TGA) were performed using a Labsys NETZSCH TG 209 Setaram apparatus with a heating rate of 10 °C/min. C, H, and N analyses were performed on a Vario EL III elemental analyzer. N2, CH4, C2H2, C2H4, C2H6, and CO2 sorption isotherms were collected on a Belsorp MAX volumetric sorption equipment. Synthesis of {[Co6(μ3-OH)2(atz)3(H2O)9(cpt)3](NO3)4(DMA)3(H2O)4}n (1). A mixture of Hcpt (45 mg, 0.225 mmol), Co(NO3)2·6H2O (45 mg, 0.15 mmol), atz (16 mg, 0.2 mmol), dimethylacetamide (DMA; 2.5 mL), H2O (1 mL), and HBF4 (0.5 mL, 40%, aqueous) was sealed in a glass vial, which was kept at 393 K for 36 h under autogenous pressure. After cooling to room temperature, purple crystals for 1 were recovered by filtration and washed with DMA. Yield: 60% [based on Co(NO3)2·6H2O]. Elem anal. Found (calcd) for C45H82Co6N28O36: C, 27.62 (27.79); H, 4.56 (4.25); N, 20.49 (20.17). Synthesis of {[Co6(μ3-OH)2(datz)3(H2O)9(cpt)3](NO3)4(DMA)5}n (2). A mixture of Hcpt (40 mg, 0.2 mmol), Co(NO3)2·6H2O (45 mg, 0.15 mmol), Hdatz (15 mg, 0.15 mmol), DMA (2.5 mL), ethanol (1 mL), dioxane (1 mL), and H2O (1 mL) was sealed in a glass vial. After the addition of 6 drops of HBF4 (40%, aqueous), the vial was kept at 393 K for 36 h under autogenous pressure. After cooling to room temperature, pink crystals for 2 were collected by filtration and washed with DMA [48% yield based on Co(NO3)2·6H2O]. Elem anal. Found (calcd) for C53H95Co6N33O34: C, 30.66 (30.43); H, 4.72 (4.58); N, 22.65 (22.09). X-ray Structural Determination and Refinement. The crystallographic data for 1 and 2 were collected on an Oxford SuperNova diffractometer using Cu Kα radiation (λ = 1.54184 Å) at ambient temperature. The structures of 1 and 2 were determined by direct methods and refined on F2 by a full-matrix least-squares technique using SHELXTL-2014.39 All non-H atoms were treated anisotropically, and the H atoms of the organic ligands were treated isotropically using riding models. For the highly disordered nature of the lattice guests, they could not be well located in the refinements, so the SQUEEZE option embedded in PLATON was used to remove the contribution of their charges.40 The chemical formulas of 1 and 2 were determined by a combination of elemental analysis, crystallographic data, and TGA. Details of data collection and refinements are summarized in Table 1.

Scheme 1. Structural Representation of Two Isostructural MOFs Investigated in This Work

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RESULTS AND DISCUSSION Crystal Structure and Topological Analysis. Solvothermal reactions of Hcpt and different triazole ligands with Co(NO3)2·6H2O in a mildly acidic solution yield two isostructural MOFs with the general formula [Co3(μ3-OH)(cpt)3Co3(μ3-OH)(L)3(H2O)9](NO3)4(guests)n [L = atz (1) and datz (2)], where Co3(μ3-OH)(cpt)3 represents the formula of the parent MIL-88-type framework and Co3(μ3-OH)B

DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1 and 2 compound empirical formula fw temperature/K cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z ρcalc/(g/cm3) μ/mm−1 radiation GOF on F2 final R indexes [I ≥ 2σ (I)] final R indexes (all data) largest diff peak/hole/ (e/Å3)

1 C33H24Co6N21O17 1340.36 293(2) hexagonal P63/mmc 15.4611(8) 15.4611(8) 20.0365(9) 90 90 120 4148.0(5) 1.99992 1.073 9.652 Cu Kα (λ = 1.54184) 1.084 R1 = 0.0809, wR2 = 0.2346 R1 = 0.0838, wR2 = 0.2376 1.42/−1.28

2 C33H30Co6N24O17 1388.28 293(2) hexagonal P63/mmc 15.5928(5) 15.5928(5) 19.9410(6) 90 90 120 4198.8(3) 2.00016 1.098 9.560 Cu Kα (λ = 1.54184) 1.255 R1 = 0.0914, wR2 = 0.2645 R1 = 0.0992, wR2 = 0.2761 2.14/−2.28

Figure 2. pacs topological network for 2.

(L)3(H2O)9 represents the formula of the trinuclear 1,2,4triazolate-based clusters, which space the pore of the parent acs nets to afford the targeted pacs networks (Figure S1). Because the two MOFs have similar frameworks, only the structure of 2 is described here. The X-ray studies show that complex 2 belongs to the hexagonal space group P63/mmc and consists of two crystallographically independent CoII ions (Co1 and Co2) in the asymmetric unit. As shown in Figure 1a, three symmetryrelated Co1 ions are bridged by three carboxylic groups, three triazole groups, and one μ3-OH atom to afford the trigonalprismatic Co3 cluster, which is further connected by the cpt− ligands to give rise to the MIL-88-type framework; the planar Co3 cluster is formed by the connection of three symmetry-

Figure 3. N2 sorption isotherms for the two MOFs measured at 77 K.

related Co2 ions with three datz− ligands and one μ3-OH atom. To our knowledge, such a type of planar trinuclear triazolate cluster has usually been reported for the metal CuI/CuII and AgI ions, while the planar trinuclear CoII cluster has not been reported so far.41−43 The in situ generated Co3(μ3-OH)(datz)3

Figure 1. (a) Two different types of Co3 units in 2. (b) View of the pore space partition through the planar Co3 units in 2. (c) Two types of intersecting free spaces in 2. (d) View of the solvent-accessible volume in 2 composed of the two types of intersecting free spaces. C

DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Gas sorption isotherms for 1a at 298 K. (b) Gas sorption isotherms for 2a at 298 K. (c) Adsorption selectivity predicted by IAST for 1a. (d) Adsorption selectivity predicted by IAST for 2a.

of two robust MOFs, N2 adsorption measurements were carried out on their activated samples at 77 K, which showed reversible type I adsorption behaviors of the microporous materials (Figure 3). The Brunauer−Emmett−Teller surface areas derived from the N2 sorption isotherms are 573 and 196 m2/ g, and the pore volumes were 0.957 and 0.246 cm3/g for 1a and 2a, respectively. The pore-size distribution, calculated using the Horvath−Kawazoe method, reveals two sharp peaks centered at 6.6 and 8.3 Å for 1a and 5.8 and 6.6 Å for 2a (Figure S6). Gas Sorption Properties. To probe the effect of additional −NH2 groups on the gas sorption properties, we examined the C2H2, CO2, C2H4, C2H6, and CH4 gas adsorption isotherms of both 1a and 2a at temperatures of 273 and 298 K and 1 bar. As shown in Figure 4a,b, both 1a and 2a show obviously different adsorption amounts for these gases, especially for C2H2. For instance, the uptake amounts of C2H4 and CO2 are 42 and 46 cm3/g for 1a and 63 and 66 cm3/g for 2a at 298 K and 1 bar. Surprisingly, the adsorbed C2H2 amounts for both compounds are quite different; 2a shows an uptake capacity of 121 cm3/g, which is nearly twice that of 1a (63 cm3/g). Compared with 1a, the adsorption amount of 2a for C2H2 is obviously improved. To understand such a discriminatory sorption behavior, the adsorption enthalpies of 1a and 2a for C2H2 were calculated based on the viral equation. As shown in Figure S7, the zerocoverage C2H2 adsorption enthalpies of 1a and 2a were then calculated as 29.1 and 34.2 kJ/mol, respectively. Such obvious enhancement in the C2H2 adsorption enthalpy could be attributed to the synergistic effect of the incorporated additional −NH2 groups and modification of the pore environment. The obvious difference in the C2H2, C2H4 and CO2 adsorption behaviors illuminates that 2a might be potentially applied in the removal of C2H2 from C2H2/C2H4 and C2H2/CO2 mixtures. To predict the C2H2/C2H4 and C2H2/CO2 separation performances of 2a and compare them

molecular building block uses its three uncoordinated N atoms for pore space partition although holding the three binding sites on the trigonal-prismatic Co3 cluster (Figure 1b). Apart from the abundance of free −NH2 groups, the planar Co3 clusters also have a lot of coordinated H2O molecules, which could generate OMSs for gas binding after activation. Notably, there are two types of intersecting free spaces having different shapes that exist in the structure. The first one is the trigonalbipyrimidal cages with an inner hole of 5.8 Å shaped by six cpt− ligands and five trigonal-prismatic Co3 clusters; the other one is the finite segments that formed through spacing of the finite 1D channels by the planar Co3 clusters (Figure 1c). Thus, the porous structure of 2 can also be viewed as a combination of trigonal-bipyrimidal cages and finite segments, which accounts for 62% of the effective solvent-accessible voids of the overall crystal volume without considering the lattice guests, as calculated by PLATON software (Figures 1d and S2).44 Because the Co3(μ3-OH)(cpt)3 and Co3(μ3-OH)(L)3(H2O)9 building blocks are bivalent, the overall charge renders a cationic framework (4+ per formula unit). The framework charge is tentatively balanced by disordered NO3− anions that located in the lattice (Figure S3). From the topological view of point, the trigonal-prismatic Co3 and planar Co3 clusters could be viewed as 9- and 3-connected nodes, respectively, so the whole network of 2 could be viewed as a three-periodic 3,9connected MOF with the targeted pacs topology (Figure 2). PXRD and TGA. The phase purities of the two assynthesized MOFs were confirmed by a comparison of the experimental and calculated PXRD patterns (Figure S4). Activated 1a and 2a were prepared by solvent exchange with CH2Cl2, followed by heating at 120 °C under dynamic vacuum for 48 h. The full activation and structural integrity of 1a and 2a have been confirmed by both TGA data and PXRD measurements (Figure S5). To evaluate the permanent porosity D

DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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with 1a as well as other MOF materials, IAST calculations based on the dual-site Langmuir−Freundlich (DSLF) equation were conducted on the C2H2/C2H4 (1:99) and C2H2/CO2 (50:50) mixtures.45 Parts c and d of Figure 4 show a comparison of the IAST selectivities calculated for 1a and 2a. The C2H2/C2H4 and C2H2/CO2 adsorption selectivities of 2a were calculated to be 11.5 and 13 at 298 K and 1 bar, respectively, which are much higher than those of 1a under the same conditions. Because both frameworks possess similar networks but different amounts of free −NH2 groups, the improved C2H2 uptake capacity and much higher C2H2/C2H4 and C2H2/CO2 demonstrate the positive effect of free −NH2 groups in these processes.46−48 The C2H2/C2H4 selectivity of 2a is also higher than that of many MOFs such as CoMOF-74 (1.7), NOTT-300 (2.17), and UTSA-60a (∼5.5).26,49 It should be pointed out that many reported MOFs show high C2H2 uptake but low C2H2/C2H4 selectivity, while those with high C2H2/C2H4 selectivity usually feature low C2H2 uptake. To our knowledge, 2a represents a rare case of porous MOFs possessing both high C2H2 uptake capacity and C2H2/C2H4 selectivity.

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 grants from the National Natural Science Foundation of China (21471134, 21571158, and 21601160), Program for Science & Technology Innovative Research Team in University of Henan Province (15IRTSTHN-002), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (152101510003), Plan for Scientific Innovation Talent of Henan Province (154200510011), and the doctoral program of Zhengzhou University of Light Industry (2015BSJJ042).

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CONCLUSION In conclusion, two analogous MOFs (1 and 2) with the targeted pacs topological nets have been designed and synthesized by utilizing the molecular building block strategy. Because of the presence of an additional −NH2 group, 2 with a datz-based cluster shows an improved C2H2 uptake capacity and separation performance over that of 1 with atz-based clusters, although the surface area of 1 is almost twice that of 2. Moreover, the high density of OMSs, abundance of free amido groups, and charged framework give 2 excellent C 2 H 2 separation performances, with IAST selectivity values reaching up to 11.5 and 13 for C2H2/C2H4 (1:99) and C2H2/CO2 (50:50) at 298 K and 1 bar, showing potential for use in natural gas purification.

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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/acs.inorgchem.7b03278. Additional crystal structure diagrams (Figures S1−S3), TGA curves (Figure S4), PXRD patterns (Figure S5), pore-size distribution (Figure S6), calculation of the sorption heat for C2H2 uptake using a virial expression, C2H2 adsorption enthalpies (Figure S7), calculations of C2H2/C2H4 selectivities based on IAST, DSLF fitting (Figure S8), and FT-IR spectra (Figure S9) (PDF) Accession Codes

CCDC 1814038−1814039 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (48) Wang, L.; Ye, Y.; Li, Z.; Lin, Q.; Ouyang, J.; Liu, L.; Zhang, Z.; Xiang, S. Highly Selective Adsorption of C2/C1 Mixtures and SolventDependent Thermochromic Properties in Metal−Organic Frameworks Containing Infinite Copper-Halogen Chains. Cryst. Growth Des. 2017, 17, 2081−2089. (49) Wen, H.-M.; Li, B.; Wang, H.; Wu, C.; Alfooty, K.; Krishna, R.; Chen, B. A Microporous Metal−organic Framework with Rare Lvt Topology for Highly Selective C2H2/C2H4 Separation at Room Temperature. Chem. Commun. 2015, 51, 5610−5613.

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DOI: 10.1021/acs.inorgchem.7b03278 Inorg. Chem. XXXX, XXX, XXX−XXX