Adsorption Capacity of a Highly Porous - American Chemical Society

May 7, 2013 - small group of MOFs surpassed the DOE methane target, including PCN-144a [220 v(STP)/v], Ni−Mg-744b [190 v(STP)/v] and NOTT-1075 [185 ...
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High H2 and CH4 Adsorption Capacity of a Highly Porous (2,3,4)Connected Metal−Organic Framework Zhiyong Lu, Liting Du, Kuanzhen Tang, and Junfeng Bai* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: A porous metal−organic framework [Cu6(BDPP)3(H2O)6] (BDPP = 3,5-bis(3,5-dicarboxylphenyl)pyridine) (NJU-Bai10) was synthesized. NJU-Bai10 exhibits a high BET surface area of 2883 m2 g−1, the highest excess H2 volumetric adsorption of 48.0 g L−1 (60 bar, 77 K), and a high methane uptake of 198.6 cm3 cm−3 that surpasses the DOE target.

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spatial obstacle of the bulky central aromatic group in the ligand. Functionalities with the shapelike double bond (−C C−, −NN−, and −NH−CO−) introduced in ligands seldom generated structures with new topologies beyond the NbO type (PCN-10 and PCN-11)7b because zero twisting angles were essentially created between two terminal isophthalate moieties. However, when methylene or some other free-rotative groups were introduced, structures obtained would easily be affected by temperature, such as PCN-12 (85 °C) and PCN-12′ (120 °C), Cu-BBPP-1 (BBPP = 1,3-bis-[3,5bis(carboxy)phenoxy]propane) (85 °C), and Cu-BBPP-2 (105 °C).6e Our group has made some efforts on the investigation of MOFs constructed by isophthalate-containing linkers and dicopper paddlewheel SBUs.8 On the basis of a H6L ligand, 3,3′,3″,5,5′,5″-benzene-1,3,5-triylhexabenzoic acid (H6BHB),9 very recently, we substituted two of the isophthalate moieties with nitrogen atoms and successfully obtained NJU-Bai8, which achieved excellent gas adsorption and separation property.10a As a continuous work, herein, we substituted one of the isophthalate moieties with the nitrogen atom, and the ligand of 3,5-bis(3,5-dicarboxylphenyl)-pyridine (H4BDPP) was synthesized (Scheme 1). Solvothermal reaction of H4BDPP with CuCl2·2H2O in a slightly acidified DMF (N,N-dimethylformamide) afforded blue-green crystals of [Cu6(BDPP)3(H2O)6]∞·xG (G = guest molecules) (NJUBai10, NJU-Bai for Nanjing University Bai’s group). As is expected, NJU-Bai1010d,11 based on a H4L ligand (H4BDPP) exhibits the highest excess volumetric H2 uptake of 48.0 g L−1 at 60 bar and 77 K and a high total H2 uptake capacity of 9.5 wt

etal−organic frameworks (MOFs), due to their intriguing structures and high surface area, have been widely regarded as a promising material for gas separation and storage.1 Particularly, researches focused on using MOFs to store energy carrier gases (e.g., hydrogen and methane) have been actively pursued in the past decade.2 For hydrogen, recently, the U.S. Department of Energy (DOE) revised the new gravimetric (volumetric) system targets of 5.5 wt % (40 g L−1) for the year 2017 and 7.5 wt % (70 g L−1) for the ultimate target at near-ambient temperature (from −40 to 85 °C) and moderate pressure (less than 100 bar).3 In order to promote the vehicular application of methane, DOE has also set the target for methane storage at 180 v(STP)/v (standard temperature and pressure equivalent volume of methane per volume of the adsorbent material) under 35 bar and ambient temperature. However, among the many developments, only a small group of MOFs surpassed the DOE methane target, including PCN-144a [220 v(STP)/v], Ni−Mg-744b [190 v(STP)/v] and NOTT-1075 [185 v(STP)/v]. This small group of MOFs is far from enough to make up a database for further investigation on practical application. Therefore, more endeavors need to be devoted to extending the family of MOFs that surpass the DOE target. MOFs based upon di-isophthalate linkers (H4L) and dicopper paddlewheel secondary building units (SBUs) were widely investigated by several well-known research groups.6 The H4L system is quite preferred by hydrogen and methane, as is exemplified by PCN-127a (a remarkable hydrogen uptake of 3.05 wt % at 1 bar and 77 K), PCN-144a (the record holder for volumetric methane uptake), and NOTT-10x5b (x = 0∼9) (high hydrogen uptake at 1 bar and 77 K). Linear H4Ls generally lead to structures with NbO topology, such as the earliest reported MOF-505,6a as well as PCN-14 and NOTT10X series, except NOTT-109 with PtS topology due to the © 2013 American Chemical Society

Received: March 26, 2013 Revised: May 5, 2013 Published: May 7, 2013 2252

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Scheme 1. Ligand Design by Substituting Isophthalate Moieties with Nitrogen Atomsa

a

Two nitrogen atoms (H6BHB to H2PMP); one nitrogen atom (H6BHB to H4BDPP).

% at 100 bar and 77 K. Meanwhile, the total methane uptake at 35 bar and 290 K is 198.6 cm3 cm−3, making it another MOF structure that surpasses the DOE target. Single crystal X-ray diffraction experiments revealed that NJU-Bai10 has a similar structure with Cu-BBPP-1 that was first reported by Zaworotko. 6e It crystallized in the orthorhombic space group Cmcm. There exist two types of BDPP ligands with different dihedral angles between the central and terminal benzene rings, one (BDPP-λ) with about 40° and the other (BDPP-τ) approximately 90°, as shown in Figure 1a. Every BDPP ligand links to four 4-connected dicopper paddlewheel SBUs, and the nitrogen atom of the pyridine ring is uncoordinated. For a better description of the structure, we simplified the ligands and the dicopper paddlewheel SBUs to 2,3-connected nodes and 4-connected nodes, respectively. As a result, the whole structure adopts a (2,3,4)-c 6-nodal net (Figure 1c) and has a new topology with the Schläfli symbol of {6.8.10}4{6.82}2{62.82.10.12}2{62.82.102}{8}3. Two types of polyhedral cages exist in the overall structure as we connect the centers of dicopper paddlewheel SBUs and centers of pyridine rings. One is a cuboctahedral cage (cage-A) which consists of seven dicopper SBUs, one BDPP-τ ligand, and four three-quarter BDPP-λ ligands, as is shown in Figure 1d. The cavity inside this cuboctahedron cage is with a diameter of about 11 Å (excluding van der Waals radii). The other cage (cage-B) consists of eleven dicopper SBUs, one complete and two half BDPP-τ ligands, four three-quarter BDPP-λ ligands, and four one-quarter BDPP-λ ligands. It is actually a combination of a cuboctahedron and two octahedra, once we supplement the edges between centers of pyridine rings by blue dash lines in Figure 1e. The cavities inside the purple cuboctahedron and green octahedron are with diameters of 13 Å and 7 Å, respectively. Cage-A and cage-B are packed through a facet-sharing mode on the ab plane by a ratio of 1:1 (Figure 1f). Along the c direction, cage-A and its inversion align one by one, as well as cage-B (Figure S1 of the Supporting Information). Calculated using PLATON, NJU-Bai10 has a solvent accessible volume of 73.2% in the dehydrated structure (after removal of the coordinated aqua ligands). This high porosity prompts us to investigate its gas adsorption property. The phase purity of the bulk sample was confirmed by powder X-ray diffraction (PXRD), and the framework retains its crystallinity after the removal of guest molecules under vacuum at 120 °C for 12 h (Figure S2 of the Supporting Information). The permanent porosity of NJU-Bai10 was confirmed by the N2

Figure 1. (a) Two types of 2,3-connected ligands (BDPP-τ and BDPP-λ) and 4-connected dicopper paddlewheel SBU. (b) Crystal structure of NJU-Bai10 view along the c direction. (c) Topology of NJU-Bai10, (d) cage-A, (e) cage-B, (f) cages packing on the ab plane in NJU-Bai10. H atoms are omitted for clarity.

adsorption isotherm at 77 K, as is shown in Figure S3 of the Supporting Information. The type-I isotherm indicates that NJU-Bai10 is a microporous material. On the basis of the N2 adsorption isotherm, the Brunauer−Emmett−Teller (BET) surface area and Langmuir surface area of NJU-Bai10 are calculated to be 2883 m2 g−1 and 3107.9 m2 g−1, respectively. Notably, the experimental surface area of NJU-Bai10 agrees 2253

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saturated excess volumetric H2 uptake is calculated to be 48.0 g L−1. To the best of our knowledge, this is the highest value ever reported among MOFs for saturated excess adsorption at 77 K (Table S2 of the Supporting Information). Nevertheless, the total uptake amount is more relevant for gas storage and delivery purposes. Thus, taking the gaseous H2 compressed within the void pore at 77 K into consideration, the total gravimetric H2 uptake can reach as high as 9.5 wt % (104.9 mg g−1, 70.1 g L−1) at 100 bar, which positions NJU-Bai10 within the range of the U.S. Department of Energy’s revised long-term systems target for onboard H2 storage, 7.5 wt % (=81 mg g−1) or 70 g L−1, albeit at cryogenic rather than ambient temperature. To evaluate the methane adsorption property of NJU-Bai10, methane isotherms were measured in the pressure range of 0− 50 bar at 290 K. As shown in Figure 2b, the CH4 isotherm is not fully saturated when the pressure reaches up to 50 bar. At 50 bar, NJU-Bai10 shows an excess uptake of 277.8 cm3 g−1 and a total uptake of 334.4 cm3 g−1. Meanwhile, at 35 bar, the total methane uptake amount of NJU-Bai10 is 297.3 cm3 g−1, which is far larger than that of the volumetric-uptake record holder PCN-14 (264 cm3 g−1). However, when the crystal density (dehydrated NJU-Bai10: 0.668 g cm−3 < PCN-14: 0.871 g cm−3) is considered, the volumetric methane total uptake of NJU-Bai10 is 198.6 cm3 cm−3, much smaller than that of PCN144a (230 cm3 cm−3). Nevertheless, it still surpasses the DOE target (180 cm3 cm−3) of porous materials for methane storage at ambient temperature and 35 bar. Moreover, based on the isotherms measured at 273 and 298 K at a low pressure range (0−1 bar), the zero coverage CH4 adsorption enthalpy of NJUBai10 is calculated to be 14.9 kJ mol−1 (Figure S9 of the Supporting Information). The uncoordinated nitrogen atoms in NJU-Bai10 also prompt us to measure its CO2 adsorption capacity in the high-pressure range, as is shown in Figure S11 of the Supporting Information. The CO2 uptake of NJU-Bai10 reaches saturation when the pressure is up to 35−40 bar, with the value of 307.4 cm3 cm−3 at 40 bar and 298 K. It is quite comparable to those of MOFs with much larger surface area, such as MIL-101 (300 cm3 cm−3, with a surface area of 5900 m2 g−1) and MOF-177 (323 cm3 cm−3, with a surface area of 5640 m2 g−1). In summary, based on a H4L ligand of H4BDPP, we successfully synthesized a porous MOF, NJU-Bai10. It exhibits a high apparent BET surface area of 2883 m2 g−1. Just as expected, this new H4L series’ MOF shows the highest excess volumetric H2 uptake of 48.0 g L−1 among MOFs and a high total H2 uptake capacity of 9.5 wt %. Furthermore, the total methane uptake at 290 K and 35 bar is 198.6 cm3 cm−3, making it surpass the DOE target. Thus, NJU-Bai10 may be an excellent candidate for hydrogen and methane storage. On the basis of this structure, further modification for property improvement is on the way.

quite well with that calculated using Materials Studio 5.5 (3221.7 m2 g−1), indicating a full activation. The total pore volume obtained from N2 isotherm is 1.11 cm3 g−1. Low-pressure hydrogen sorption measurements of NJUBai10 were also performed at 77 and 87 K to investigate the H2 uptake. The adsorption isotherms were fully reversible and exhibit 2.49 wt % H2 uptake under the conditions of 77 K and 1 bar (Inset in Figure 2a), which is among the highest at low

Figure 2. (a) H2 adsorption isotherms at 77 K in the high-pressure range (0−100 bar), total (red ●), excess (blue ●). Inset of (a) is the H2 adsorption isotherms in the low pressure range (0−1 bar), 77 K (orange ●), 87 K (purple ●). (b) CH4 adsorption isotherms at 290 K, total (purple ■), excess (green ■).

pressure. This high H2 uptake amount at low pressure indicates a good hydrogen affinity of framework. Thus, based on the isotherms measured at 77 and 87 K, the zero coverage H2 adsorption enthalpy of NJU-Bai10 is calculated to be 6.3 kJ mol−1, higher than those of MOFs with a similar surface area and also assembled by H4L and dicopper SBUs, such as NOTT-1025b (5.70 kJ mol−1), NOTT-11010b (5.68 kJ mol−1), and NOTT-11110b (6.21 kJ mol−1). Besides the open metal sites on dicopper paddlewheels, the uncoordinated nitrogen atoms of the ligands may have some contributions to the hydrogen adsorption, just as what theoretical studies predicted.10c We also measured the H2 adsorption capacity of NJU-Bai10 at 77 K under a high-pressure range (Figure 2a). The saturated excess gravimetric H2 uptake is 6.7 wt % (71.9 mg g−1) at 60 bar, which makes it among the top ten highest excess gravimetric H2 uptake MOFs (Table S2 of the Supporting Information). Notably, when the crystal density (dehydrated, 0.668 g cm−3) is taken into consideration, the

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, crystallographic data, additional gas adsorption isotherms of NJU-Bai10. CCDC 925715. This material is available free of charge via the Internet at http:// pubs.acs.org. 2254

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(9) Guo, Z.; Wu, H.; Srinivas, G.; Zhou, Y.; Xiang, S.; Chen, Z.; Yang, Y.; Zhou, W.; O’Keeffe, M.; Chen, B. Angew. Chem., Int. Ed. 2011, 50, 3178. (10) (a) Du, L.; Lu, Z.; Zheng, K.; Wang, J.; Zheng, X.; Pan, Y.; You, X.; Bai, J. J. Am. Chem. Soc. 2013, 135, 562. (b) Yang, S.; Lin, X.; Dailly, A.; Blake, A. J.; Hubberstey, P.; Champness, N. R.; Schröder, M. Chem.Eur. J. 2009, 15, 4829. (c) Negri, F.; Saendig, N. Theor. Chem. Acc. 2007, 118, 149. (d) Liu, Y.; Li, J.-R.; Verdegaal, W. M.; Liu, T.-F.; Zhou, H.-C. Chem.−Eur. J. 2013, DOI: 10.1002/ chem.201203297. NJU-Bai10 has also been termed PCN-305. (11) Crystal data for NJU-Bai10: C63H39N3O30Cu6 (guest molecules removed with PLATON SQUEEZE), Mr = 1699.27, orthorhombic, Cmcm, a = 25.2913 (14) Å, b = 33.5090 (16) Å, c = 18.6810 (13) Å, α = β = γ = 90°, V = 15831.9 (16) Å3, Z = 4, Dc = 0.713 g cm−3, F000 = 3408, T = 291 (2) K, 47169 reflections collected, 8222 independent reflections (Rint = 0.0416), R1 = 0.0485, wR2 = 0.1190 for [I > 2s(I)]. CCDC 925715 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Major State Basic Research Development Programs (Grant 2011CB808704), the NSFC (Grant 20931004), and the Postdoctoral Research Funding of the Jiangsu Province (Grant 1102023C). We are grateful to the kind help of Prof. Wenlong Liu of Yangzhou University for high-pressure gas adsorption measurements.

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dx.doi.org/10.1021/cg400449c | Cryst. Growth Des. 2013, 13, 2252−2255