A metal–organic framework functionalized with piperazine exhibiting enhanced CH₄ storage

Abstract

NJU-Bai 19, the first cycloaliphatic ring (piperazine) functionalized MOF-505 analogue, exhibits a notably high methane storage capacity of 246.4 cm³ (STP) cm⁻³ (at room temperature and 65 bar); this is 93.6% of the new volumetric target of the US Department of Energy if the packing density loss is ignored. Compared with NOTT-101, when piperazine groups are inserted, the methane uptake capacity of NJU-Bai 19 at 65 bar and RT can be significantly enhanced with lower Q_st. A relatively low increase was also observed in the uptake at 5 bar. Thus it has a much higher methane storage working capacity (deliverable amount of methane between 65 and 5 bar) of 185 cm³ (STP) cm⁻³ compared with NOTT-101 (174 cm³ (STP) cm⁻³). When the temperature was decreased from 298 to 273 K at working pressures between 65 bar and 5 bar, the volumetric CH₄ working capacity of NJU-Bai 19 is almost unchanged (from 185 to 189 cm³ (STP) cm⁻³), while some of the famous MOFs with high Q_st showed a decrease with a decrease in temperature. The combination of the balanced porosity and framework density and the lipophilic surface is considered to result in the increased methane storage and working capacities.

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Introduction

Natural gas, as a naturally abundant fuel, consisting of nearly 95% CH₄, has long been considered as a possible alternative to conventional gasoline and diesel because of its economic and environmental advantages.¹ However, for future wide utilization of CH₄, safe and efficient storage and transportation is still a great challenge due to its low volumetric energy density (only 0.1% of gasoline) at room temperature.² Adsorbed natural gas (ANG) in porous solid materials has a significant advantage which is that the gas is stored as an adsorbed phase in a porous solid achieves a density competitive with that of compressed natural gas (CNG) at a lower pressure and room temperature, leading to safe and efficient storage with less energy consumption.³

Recently, metal–organic frameworks (MOFs) have appeared as a new class of promising porous materials for CH₄ storage,⁴ which were initially investigated by Kitagawa and Yaghi.⁵ In the context of methane storage, there has been a growing interest in the investigation of MOFs with higher volumetric methane storage capacity and working capacity. For MOFs, the working capacity determines the driving range of vehicles powered by natural gas; thus it is more important than total CH₄ storage capacity. In the past decade, many MOFs with good performance for CH₄ storage have been reported. For example, HKUST-1 (ref. 6) and MAF-38 (ref. 7) with high isosteric heats of CH₄ adsorption (Q_st) exhibit high a volumetric methane uptake of 267 and 263 cm³ (STP) cm⁻³ at 298 K and 65 bar, respectively. Al-soc-MOF-1 (ref. 8) and MOF-905 (ref. 10) with low Q_st show a high gravimetric uptake of 0.5 g g⁻¹ (at 85 bar and 288 K) and a high volume working capacity of 203 cm³ (STP) cm⁻³ (between 80 and 5 bar), respectively. UTSA-76 (ref. 9) and Co(dbp)¹¹ show a record high volume working capacity of 197 cm³ (STP) cm⁻³ (between 65 and 5 bar). In fact, BASF have realized model vehicles equipped with natural gas fuel systems using MOF materials.

The popular method for increasing total methane uptakes is to enhance the interaction between CH₄ and frameworks, mainly presented as higher Q_st. The representative example is Ni-MOF-74,⁶ whose volumetric methane uptake can reach 251 cm³ (STP) cm⁻³ (at 298 K and 65 bar) and the Q_st is 20.6 kJ mol⁻¹. However, this strategy shows no effect on enhancing the working capacity (only 129 cm³ (STP) cm⁻³ for Ni-MOF-74). Moreover, high Q_st may add cost and complexity to manage the exothermic heat of adsorption and endothermic heat of desorption, both of which may result in large reductions of the working capacity.^11,12 In this context, there is an urgent demand to develop MOFs not only with high working capacities but also with low Q_st.^4b

MOFs possess one significant and unique feature which is a finely tunable structure. By inserting carefully selected functional groups, many MOFs could be systematically investigated as platforms, targeting special properties.¹³ We are interested in tuning MOFs with novel functional groups, which play a key role in improving the performances of MOFs. Since our group's pioneering work on introducing C≡C¹⁴ and amide¹⁵ functional groups into MOFs for gas storage study, MOFs based on these two functional groups have made significant progress in gas storage.¹⁶ This motivates us to introduce new functional groups into MOFs for improving CH₄ storage capacities.

As far as we know, research on current functional groups for CH₄ storage mainly focuses on the aromatic rings and their derivatives,^8,17 so there are few MOFs constructed with a cycloaliphatic backbone. Moreover, according to previous studies,^9,17a,18 methyl, ethyl and propyl functional groups as well as N atoms in MOFs have a positive effect on methane storage,^9,18 so we consider introducing aliphatic fragments and N atoms in the form of piperazine to take the place of the benzene ring in NOTT-101 and get the first cycloaliphatic ring functionalized MOF [Cu₂(L)(DMF)₂]·3DMF·7H₂O (H₄L, 5,5′-(piperazine-1,4-diyl)diiso-phthalic acid; NJU-Bai 19, NJU-Bai for Nanjing University Bai's group). As expected, compared with NOTT-101,⁹NJU-Bai 19 exhibits a higher CH₄ uptake of 246.4 cm³ g⁻¹ (at 298 K and 65 bar) and a working capacity of 185 cm³ (STP) cm⁻³.

Experimental

Materials

Commercially available reagents were used as received without further purification. Elemental analyses (C, H and N) were carried out on a Perkin-Elmer 240 analyzer. The IR spectra were obtained on a VECTOR TM 22 spectrometer with KBr pellets in the 4000–400 cm⁻¹ region. ¹H NMR spectra were recorded on a Bruker DRX-500 spectrometer with tetramethylsilane as an internal reference. Thermogravimetric analyses (TGA) were performed under a N₂ atmosphere (100 mL min⁻¹) with a heating rate of 5 °C min⁻¹ using a 2960 SDT thermogravimetric analyzer. Powder X-ray diffraction (PXRD) data were collected over the 2θ range 5–45° on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å) with a routine power of 1600 W (40 kV, 40 mA) at a scan speed of 0.1 deg s⁻¹ at room temperature. A Micromeritics ASAP 2020 was used to measure gas adsorption isotherms. To remove all the guest solvents in the framework, the fresh sample of NJU-Bai 19 was guest-exchanged with dry acetone at least 9 times, filtered and degassed at room temperature for 1 day and then at 373 K for another 24 h until the outgas rate was 5 mmHg min⁻¹ before the measurements were made. High-pressure CH₄ sorption isotherms were measured using a Sieverts-type apparatus.¹⁹

Synthesis of H₄L

As shown in Scheme 1, the linker H₄L was synthesized by a two-step reaction procedure. Into a dried 100 mL round bottom flask was added piperazine (10.0 mmol), dimethyl 5-iodoisophthalate (30 mol), CuI (1 mmol), metformin hydrochloride (2 mmol), and Cs₂CO₃ (30 mmol, 3 equiv.). Later on, dry EtOH (25 mL) was added. After the mixture was stirred and refluxed for 24 hours under a N₂ atmosphere, the precipitate was collected by filtration, washed with water and EtOH and dried under vacuum at 353 K to give 1 as white solid. Yield: 74% (3.5 g). The crude 1 was suspended in 150 mL of THF/methanol mixed solvent, to which 100 mL 1 M LiOH aqueous solution was added. After this mixture was stirred at room temperature for 24 hours, the solvents were removed under vacuum, and dilute hydrochloric acid was added to the remaining aqueous solution until it became acidic (pH = 2). The precipitate was collected by filtration, washed with water and dried under vacuum at 353 K to give H₄L as a white solid. IR (KBr, cm⁻¹): 3345 (br, s), 3088 (br, s), 2874 (br, s), 1733 (s), 1673 (m), 1462 (s), 1377 (m), 1341 (m), 1286 (m), 1190 (m), 1090 (m). ¹H NMR (DMSO-d₆, δ ppm): 13.451 (broad peak, COOH), 7.95 (s, 2H, ArH), 7.74 (s, 4H, ArH), 3.43 (s, 8H, CH₂H), anal. calcd (found) for H₄L, C₂₀H₁₈N₂O₈: C, 57.97 (57.88); H, 4.38 (4.50); N, 6.76 (6.61)%. MS (ESI) m/z (M − H⁺)⁻: 414.11.

Scheme 1 The synthesis procedure for the organic linker H₄L used to construct NJU-Bai 19.

Synthesis of NJU-Bai 19

H₄L (10.0 mg) and Cu(NO₃)₂·3H₂O (20 mg, 0.1 mmol) were dissolved in 2 mL solvent of DMF in a vessel, to which 20 drops HBF₄ were added. The vessel was sealed and heated to 75 °C for 1 day and then cooled to room temperature at a rate of 5 °C per hour. Blue block crystals of NJU-Bai 19 were filtered and washed with DMF. Yield 11.2 mg. Selected IR (cm⁻¹): 3418 (br, s), 1655 (m), 1585 (vs), 1449 (m), 1378 (m), 1243 (w), 1001 (m), 779 (m), 730 (m). Anal. calcd (found) for NJU-Bai 19, C₃₅H₆₃–Cu₂N₇O₂₀: C, 40.85 (40.77); H, 6.17 (6.35); N, 9.51 (9.26)%; calcd (found) for activated NJU-Bai 19, C₂₀H₁₄Cu₂N₂O₈: C, 44.70 (37.10); H, 2.63 (4.35); N, 5.21 (4.36)%.

Single-crystal X-ray crystallography

Single-crystal X-ray diffraction data were measured on a Bruker Apex II CCD diffractometer using graphite monochromated Mo/Kα radiation (λ = 0.71073 Å). Data reduction was made with the Bruker SAINT program. The structures were solved by direct methods and refined with the full-matrix least squares technique using the SHELXTL package. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2 × U_eq of the attached atom. The unit cell includes a large region of disordered solvent molecules, which could not be modelled as discrete atomic sites. We employed PLATON/SQUEEZE to calculate the diffraction contribution of the solvent molecules and, therefore, to produce a set of solvent-free diffraction intensities; the structures were then refined again using the data generated. Crystal data are summarized in Table S1.†

Results and discussion

Solvothermal reaction of Cu(NO₃)₂·3H₂O with H₄L in a solvent mixture of DMF, H₂O and fluoroboric acid gave pale-blue block-shaped crystals of NJU-Bai 19 in high yield. Single-crystal X-ray diffraction analysis reveals that like other isoreticular NbO-type MOFs, NJU-Bai 19 is also constructed from binuclear Cu(II) paddlewheel nodes, further bridged by L⁴⁻ to form a 3D (4,4)-connected net. Typically, there exist two types of cages which are alternately stacked in a 1 : 1 ratio with the formation of a 3D framework. The spherical cage is about 9.7 Å in diameter and the large shuttle-shaped cage is about 9.3 × 22.2 Å. Viewing along the c axes, there exist triangular channels whose diameters are approximately 5 Å, while along the a or b axis, isosceles triangular windows can be observed, in which the size is controlled by the length of the spaces. Obviously, the chair conformations of piperazine project into the pore, narrowing the isosceles triangular windows. The chair conformations also make the ligand L⁴⁻ slightly shorter than its counterpart in NOTT-101, which further contracts this window. Calculated from the PLATON program, NJU-Bai 19 has an accessible pore volume of 71.5% (7985 ų out of the 11 170 ų per unit cell volume) (Fig. 1).

Fig. 1 (a) Each ligand connected to four Cu(II) paddlewheel nodes; (b) the spherical-like cage of about 9.7 Å in diameter; (c) the large shuttle-shaped cage of about 9.3 × 22.2 Å.

To confirm the permanent porosity of NJU-Bai 19, the acetone solvent-exchanged sample was degassed under high vacuum at 373 K for 24 hours to obtain the evacuated framework. As shown in Fig. 2a, the activated NJU-Bai 19 can take up 687 cm³ g⁻¹ N₂ at 77 K. The N₂ isotherm measurements were repeated on several batches of samples using different apparatus and essentially identical results were obtained. The BET surface area and Langmuir surface area of NJU-Bai 19 are calculated to be around 2803 and 3007 m² g⁻¹, which are comparable to those of NOTT-101 (2805 and 3045 m² g⁻¹, respectively). On the basis of the maximum amount of N₂ adsorbed, the calculated total pore volume is 1.063 cm³ g⁻¹. A pore size distribution analysis by the non-local density functional theory (NLDFT) utilizing N₂ gas at 77 K shows that NJU-Bai 19 exhibits a narrow distribution of micro-pores around 14 Å (Fig. 2a). Compared with NOTT-101, those values are a little lower due to the introduction of piperazine.

Fig. 2 (a) N₂ sorption isotherms at 77 K (filled symbols: adsorption; open symbols: desorption); inset: pore size distribution analysed by the NLDFT method. (b) The total high-pressure CH₄ adsorption isotherms for NJU-Bai 19 at 273 K and 298 K (filled and open symbols represent adsorption and desorption, respectively). Data of pure CH₄ stored in the high pressure gas tank are presented as a black line.

The unique pore functionalization and comparable porosity of NJU-Bai 19 to that of NOTT-101 prompted us to examine its methane storage capacities. Temperature-dependent high-pressure adsorption measurements were performed at the Center for Neutron Research, National Institute of Standards and Technology (NIST), using a computer-controlled Sieverts apparatus. The total volumetric methane sorption isotherms of NJU-Bai 19 at 273 K and 298 K are shown in Fig. 2. At 298 K and 35 bar, NJU-Bai 19 has a high total volumetric methane storage capacity of 200.4 cm³ (STP) cm⁻³, far exceeding the DOE's previous target of 180 cm³ (STP) cm⁻³ assuming that there is no packing loss. There are only very few MOFs whose methane storage capacities can reach 200 cm³ (STP) cm⁻³. With the pressure increasing to 65 bar, the volumetric methane uptake of NJU-Bai 19 can be further increased to 246.4 cm³ (STP) cm⁻³, which corresponds to the amount of methane stored in a CNG tank at 249 bar. Thus, NJU-Bai 19 could effectively reduce the pressure required for useful loading (246 v/v) by a factor of 3.8, down to 65 bar. Comparatively, compared to NOTT-101, although those two MOFs got almost the same pore volumes, NJU-Bai 19 shows a notably higher volumetric methane storage capacity (246.4 cm³ (STP) cm⁻³vs. 228.4 cm³ (STP) cm⁻³). In fact, the value of NJU-Bai 19 is comparable to and even higher than that of famous MOFs for CH₄ storage such as NU-125,²⁰ NU-135,²¹ PCN-14,⁶ UTSA-20 (ref. 6) and ZJNU-50 (ref. 22) (Table 1). Furthermore, at 65 bar and RT, a tank filled with NJU-Bai 19 would deliver almost four times more methane than an empty tank.

Table 1 Comparison of some MOFs for CH₄ storage

MOFs	Total uptake^a at 65 bar (35 bar)		Working capacity^b at 65 bar		Ref.
	cm^3 (STP) cm^−3	g g^−1	cm^3 (STP) cm^−3	Q st
a At 298 K. b The working capacity is defined as the difference in CH4 uptake between 65 and 5 bar. c The data (0.704 g cm^−3) were calculated based on the crystal densities collected at 298 K; the framework density based on the He adsorption is about 0.680 g cm^−3.
NJU-Bai 19	246 (205)	0.249	185	14.8	This work
NOTT-101^c	228 (194)	0.246	174	15.5	9
HKUST-1	267 (227)	0.216	190	17.0	6
UTSA-76	257 (211)	0.263	197	15.4	9
Co(dbp)	203 (161)	NA	197	13.0	11
PCN-14	230 (195)	0.197	157	17.6	6
MAF-38	263 (226)	0.246	187	21.6	7
Ni-MOF 74	260 (230)	0.197	145	21.4	6
UTSA-20	230 (184)	0.181	170	18.2	17e
UTSA-88	248 (204)	0.206	185	15.1	17f
NU-125	232 (182)	0.287	183	15.5	20
NU-135	230 (187)	0.219	170	16.6	21
ZJNU-50	229 (178)	0.274	184	15.0	22

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NJU-Bai 19 shows lower gravimetric capacity than some famous microporous MOFs with a higher pore volume, such as Al-soc-MOF-1,⁸ NU-111,²³ MOF-205 (ref. 16e) and UTSA-76.⁹ Although NJU-Bai 19 has a lower volumetric methane storage capacity than HKUST-1 and Ni-MOF 74, it has a higher gravimetric methane storage capacity of 0.249 g g⁻¹ than HKUST-1 (0.216 g g⁻¹) and Ni-MOF-74 (0.197 g g⁻¹) due to a larger pore volume and a lower framework density. This is really remarkable as NJU-Bai 19 adsorbing natural gas to such an extent may make it become commercially attractive.

Apart from its high methane storage capacities, NJU-Bai 18 also exhibits high methane working capacity, which is defined here as the difference in uptake between 5 bar and 65 bar. As discussed above, when piperazine groups are inserted, the methane uptake capacity of NJU-Bai 19 at 65 bar and RT can be significantly enhanced. However, a relatively low increase was observed in the uptake at 5 bar. As a result, the methane storage working capacity of NJU-Bai 19 can be improved from 174 cm³ (STP) cm³ in NOTT-101 to 185 cm³ (STP) cm³. Most importantly, this capacity is only slightly lower than that of some of the best MOFs, HKUST-1, UTSA-76 and Co(bdp), comparable to or higher than that of most of the other promising MOFs: NU-125, UTSA-20, NU-111,²³ NU-135, PCN-14 and ZJNU-50 (Table 1). According to recent research,^8,17b when temperature was decreased from 298 to 273 K at working pressures between 65 bar and 5 bar, some of the famous MOFs with high Q_st showed a decrease in the volumetric CH₄ working capacity with a decrease in temperature (Table S2†), the volumetric CH₄ working capacity of NJU-Bai 19 is almost unchanged (from 185 to 189 cm³ (STP) cm⁻³). This phenomenon may be caused by the relatively low CH₄ heat of adsorption of NJU-Bai 19.

To better understand the obvious improvement of the methane storage capacity and working capacity from NOTT-101 to NJU-Bai 19, we calculated the Q_st from the temperature-dependent isotherms using the virial method. Considering the similar concentrations of open Cu sites²⁴ (2.67 mmol cm⁻³ in NJU-Bai 19vs. 2.60 mmol cm⁻³ in NOTT-101), the similar binding energy of CH₄ with ligands in NJU-Bai 19 and NOTT-101 (10.31 kJ mol⁻¹vs. 10.06 kJ mol⁻¹, Fig. S14†) and the smaller cage windows of NJU-Bai 19, we supposed it should have a higher initial Q_st than NOTT-101. However, the Q_st value of NJU-Bai 19 for CH₄ adsorption is a little smaller than that of NOTT-101 (14.8 kJ mol⁻¹vs. 15.5 kJ mol⁻¹) and falls into the range of 12 kJ mol⁻¹ to 15 kJ mol⁻¹ which belongs to MOFs without strong CH₄ binding sites.^4c,6 The Q_st is also lower than that of other analogues of NOTT-101 which are decorated with –CF₃, F, –NO₂ and –CH₃. This indicates that the initial Q_st value is not purely controlled by the open Cu sites, and there should be some contribution from the organic linkers in MOFs. With CH₄ loading, the CH₄–CH₄ interactions become important, thus leading to an increase in Q_st. However, we found that the Q_st in NJU-Bai 19 is still smaller than that in NOTT-101 and UTSA-76 (Fig. S12†) even after high-concentration methane loadings. We also checked other probe molecules and found that the initial CO₂Q_st of NJU-Bai 19 is also lower than that of NOTT-101 (23.6 kJ mol⁻¹vs. 26.6 kJ mol⁻¹). Considering the lower CH₄Q_st during the process of adsorption and unchanged volumetric CH₄ working capacity with a decrease in temperature, the introduction of a cycloaliphatic moiety into the organic linker may weaken the interactions between the gas and framework. This indicates that the piperazine in the organic linker can facilitate heat management during the adsorption and desorption process. This point needs further understanding although much work has been done on it as it is unusual and very interesting.

As established before,²⁵ the methane uptake of MOFs at high pressure is mainly dominated by the second adsorption surface (mainly organic ligands). Piperazine provides a high-efficiency strategy to simultaneously introduce two kinds of CH₄-favoured sites (N atoms and –CH₂– groups) into MOFs. Considering the similar binding energy of CH₄ with N atoms in NJU-Bai 19 and benzene ring in NOTT-101, the –CH₂– groups which form the lipophilic surface may play a more important role at high CH₄ loading. Moreover, to obtain high volumetric methane storage capacities, ideal MOFs should have balanced porosities and framework densities. NJU-Bai 19 has a more balanced porosity and framework density than NOTT-101 (Fig. S13†), which may be another important reason for the improved methane storage and working capacities. The combined characters indicate that NJU-Bai 19 is a potential stepping stone to MOFs which can achieve the objective of a high CH₄ uptake and working capacities with smaller Q_st. The piperazine functional group might open the door to constructing MOFs with a high performance using a cycloaliphatic backbone. This also motivates us to further design new MOFs with other aliphatic groups to improve their methane storage performance.

Conclusions

In summary, we have developed the first cycloaliphatic ring functionalized MOF (NJU-Bai 19) with piperazine groups. Compared with widely used aromatic ring functional groups, piperazine exhibits the following advantages. (1) Piperazine provides a high-efficiency strategy to introduce –CH₂– groups into MOFs for improving CH₄ storage; (2) piperazine brings a more balanced porosity and framework density than other aromatic rings; (3) piperazine may bring smaller CH₄Q_st, which may facilitate the heat management during the CH₄ storage process; (4) compared with aromatic rings, it is easier and cheaper to introduce piperazine into MOFs. Above all, NJU-Bai 19 shows a significantly enhanced volumetric methane storage capacity of 246.4 cm³ (STP) cm⁻³ (at 298 K and 65 bar) and a working capacity of 185 cm³ (STP) cm⁻³. The combination of the balanced porosity and framework density and the lipophilic surface is considered to result in the increased methane storage and working capacities. Continuous work to develop MOFs with other new functional groups targeting higher methane storage performance is still ongoing.

Acknowledgements

We thank the support of this work by the National Natural Science Foundation of China (21371091 and 21271192), Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ120632), International Science and Technology Cooperation Program of Chongqing (cstc2014-gjhz20002), and Chongqing Normal University Scientific Research Foundation Project (16XLB016). In addition, we thank Mr Yunzhi Li and Mr Fei Meng of Nanjing University for the theoretical calculations. We also gratefully acknowledge Prof. Junfeng Bai (Nanjing University) for his help and guidance.

Notes and references

1. (a) S. Yeh, Energy Policy, 2007, 35, 5865–5875; (b) R. F. Service, Science, 2014, 346, 538–541.
2. G. Whyatt, Issues affecting adoption of natural gas fuel in light- and heavy-duty vehicles, US Department of Energy, Pacific Northwest National Laboratory, Washington, 2010.
3. (a) A. Schoedel, Z. Ji and O. M. Yaghi, Nat. Energy, 2016, 1, 16034; (b) H. Furukawa, K. E. Cordova, M. O'Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444.
4. (a) T. A. Makal, J.-R. Li, W. Lu and H.-C. Zhou, Chem. Soc. Rev., 2012, 41, 7761–7779; (b) Y. He, W. Zhou, G. Qian and B. Chen, Chem. Soc. Rev., 2014, 43, 5657–5678; (c) J. A. Mason, M. Veenstra and J. R. Long, Chem. Sci., 2014, 5, 32–51.
5. (a) M. Kondo, T. Yoshitomi, H. Matsuzaka, S. Kitagawa and K. Seki, Angew. Chem., Int. Ed., 1997, 36, 1725–1727; (b) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469–472.
6. Y. Peng, V. Krungleviciute, I. Eryazici, J. T. Hupp, O. K. Farha and T. Yildirim, J. Am. Chem. Soc., 2013, 135, 11887–11894.
7. J.-M. Lin, C.-T. He, Y. Liu, P.-Q. Liao, D.-D. Zhou, J.-P. Zhang and X.-M. Chen, Angew. Chem., Int. Ed., 2016, 55, 4674–4678.
8. D. Alezi, Y. Belmabkhout, M. Suyetin, P. M. Bhatt, Ł. J. Weseliński, V. Solovyeva, K. Adil, I. Spanopoulos, P. N. Trikalitis, A.-H. Emwas and M. Eddaoudi, J. Am. Chem. Soc., 2015, 137, 13308–13318.
9. B. Li, H.-M. Wen, H. Wang, H. Wu, M. Tyagi, T. Yildirim, W. Zhou and B. Chen, J. Am. Chem. Soc., 2014, 136, 6207–6210.
10. J. Jiang, H. Furukawa, Y.-B. Zhang and O. M. Yaghi, J. Am. Chem. Soc., 2016, 138, 10244–10251.
11. J. A. Mason, J. Oktawiec, M. K. Taylor, M. R. Hudson, J. Rodriguez, J. E. Bachman, M. I. Gonzalez, A. Cervellino, A. Guagliardi and C. M. Brown, Nature, 2015, 527, 357–361.
12. (a) J. B. Mota, A. Rodrigues, E. Saatdjian and D. Tondeur, Carbon, 1997, 35, 1259–1270; (b) K. S. Walton and M. D. LeVan, Adsorption, 2006, 12, 227–235.
13. (a) X. L. Cui, K. J. Chen, H. B. Xing, Q. W. Yang, R. Krishna, Z. B. Bao, H. Wu, W. Zhou, X. L. Dong, Y. Han, B. Li, Q. L. Ren, M. J. Zaworotko and B. L. Chen, Science, 2016, 353, 141–144; (b) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Nature, 2013, 495, 80–84; (c) L. Du, Z. Lu, K. Zheng, J. Wang, X. Zheng, Y. Pan, X. You and J. Bai, J. Am. Chem. Soc., 2012, 135, 562–565; (d) S. Ma, D. Sun, J. M. Simmons, C. D. Collier, D. Yuan and H.-C. Zhou, J. Am. Chem. Soc., 2008, 130, 1012–1016; (e) A. H. Assen, Y. Belmabkhout, K. Adil, P. M. Bhatt, D. X. Xue, H. Jiang and M. Eddaoudi, Angew. Chem., Int. Ed., 2015, 54, 14353–14358.
14. Y. Hu, S. Xiang, W. Zhang, Z. Zhang, L. Wang, J. Bai and B. Chen, Chem. Commun., 2009, 7551–7553.
15. (a) R. Sun, Y.-Z. Li, J. Bai and Y. Pan, Cryst. Growth Des., 2007, 7, 890–894; (b) B. Zheng, J. Bai, J. Duan, L. Wojtas and M. J. Zaworotko, J. Am. Chem. Soc., 2011, 133, 748–751.
16. (a) R. Yun, Z. Lu, Y. Pan, X. You and J. Bai, Angew. Chem., Int. Ed., 2013, 52, 11282–11285; (b) M. Zhang, B. Li, Y. Li, Q. Wang, W. Zhang, B. Chen, S. Li, Y. Pan, X. You and J. Bai, Chem. Commun., 2016, 52, 7241–7244; (c) O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen, A. Ö. Yazaydın and J. T. Hupp, J. Am. Chem. Soc., 2012, 134, 15016–15021; (d) O. K. Farha, A. Özgür Yazaydın, I. Eryazici, C. D. Malliakas, B. G. Hauser, M. G. Kanatzidis, S. T. Nguyen, R. Q. Snurr and J. T. Hupp, Nat. Chem., 2010, 2, 944–948; (e) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. Ö. Yazaydin, R. Q. Snurr, M. O'Keeffe and J. Kim, Science, 2010, 329, 424–428.
17. (a) I. Spanopoulos, C. Tsangarakis, E. Klontzas, E. Tylianakis, G. Froudakis, K. Adil, Y. Belmabkhout, M. Eddaoudi and P. N. Trikalitis, J. Am. Chem. Soc., 2016, 138, 1568–1574; (b) J. Cai, X. Rao, Y. He, J. Yu, C. Wu, W. Zhou, T. Yildirim, B. Chen and G. Qian, Chem. Commun., 2014, 50, 1552–1554; (c) Y. He, W. Zhou, G. Qian and B. Chen, Chem. Soc. Rev., 2014, 43, 5657–5678; (d) H.-M. Wen, B. Li, D. Yuan, H. Wang, T. Yildirim, W. Zhou and B. Chen, J. Mater. Chem., 2014, 2, 11516–11522; (e) Z. Guo, H. Wu, G. Srinivas, Y. Zhou, S. Xiang, Z. Chen, Y. Yang, W. Zhou, M. O'Keeffe and B. Chen, Angew. Chem., Int. Ed., 2011, 50, 3178–3181; (f) G. Chang, B. Li, H. Wang, Z. Bao, T. Yildirim, Z. Yao, S. Xiang, W. Zhou and B. Chen, Chem. Commun., 2015, 51, 14789–14792.
18. C. E. Wilmer, M. Leaf, C. Y. Lee, O. K. Farha, B. G. Hauser, J. T. Hupp and R. Q. Snurr, Nat. Chem., 2012, 4, 83–89.
19. W. Zhou, H. Wu, M. R. Hartman and T. Yildirim, J. Phys. Chem. C, 2007, 111, 16131–16137.
20. C. E. Wilmer, O. K. Farha, T. Yildirim, I. Eryazici, V. Krungleviciute, A. A. Sarjeant, R. Q. Snurr and J. T. Hupp, Energy Environ. Sci., 2013, 6, 1158–1163.
21. R. D. Kennedy, V. Krungleviciute, D. J. Clingerman, J. E. Mondloch, Y. Peng, C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, J. T. Hupp, T. Yildirim, O. K. Farha and C. A. Mirkin, Chem. Mater., 2013, 25, 3539–3543.
22. C. Song, Y. Ling, Y. Feng, W. Zhou, T. Yildirim and Y. He, Chem. Commun., 2015, 51, 8508–8511.
23. Y. Peng, G. Srinivas, C. E. Wilmer, I. Eryazici, R. Q. Snurr, J. T. Hupp, T. Yildirim and O. K. Farha, Chem. Commun., 2013, 49, 2992–2994.
24. H. Wu, J. M. Simmons, Y. Liu, C. M. Brown, X.-S. Wang, S. Ma, V. K. Peterson, P. D. Southon, C. J. Kepert, H.-C. Zhou, T. Yildirim and W. Zhou, Chem.–Eur. J., 2010, 16, 5205–5214.
25. H. Wu, W. Zhou and T. Yildirim, J. Am. Chem. Soc., 2009, 131, 4995–5000.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, first-principle calculations, TGA, PXRD and heat of adsorption of CH₄. CCDC 1491433. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ta06037d

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