Mingxing
Zhang
*ab,
Cong
Chen
b,
Qian
Wang
b,
Wensheng
Fu
a,
Kunlin
Huang
*a and
Wei
Zhou
*c
aDepartment of Chemistry, Chongqing Normal University, Chongqing, 401331, China. E-mail: zmx0102@hotmail.com; kunlin@cqnu.edu.cn
bState Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
cNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, USA. E-mail: wzhou@nist.gov
First published on 21st November 2016
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 Qst. 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 Qst 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.
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 (Qst) 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 Qst 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 Qst. 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 Qst 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 Qst 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 Qst.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 CC14 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.
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.
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.
MOFs | Total uptakea at 65 bar (35 bar) | Working capacityb at 65 bar | Ref. | ||
---|---|---|---|---|---|
cm3 (STP) cm−3 | g g−1 | cm3 (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-101c | 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 |
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 Qst 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 Qst 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 Qst than NOTT-101. However, the Qst 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 Qst 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 Qst 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 Qst. However, we found that the Qst 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 CO2Qst 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 CH4Qst 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 Qst. 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.
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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