A unique magnesium-based 3D MOF with nanoscale cages and temperature dependent selective gas sorption properties

Article abstract of DOI:10.1039/c3cc38730e

A porous Mg-based 3D metal-organic framework with unique nanoscale cages and two-fold interpenetrating pcu nets has been synthesized and characterized. It shows gas-uptake capacities for N₂, H₂, O₂ and CO₂ at low temperatures and selective adsorption of CO ₂ over O₂ and N₂ at room temperature. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38730e

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A unique magnesium-based 3D MOF with nanoscale
cages and temperature dependent selective gas
sorption properties†
Cite this: Chem. Commun., 2013,
49, 1753
Received 5th December 2012,
Accepted 14th January 2013
Yong-Liang Huang, Yun-Nan Gong, Long Jiang and Tong-Bu Lu*
DOI: 10.1039/c3cc38730e
www.rsc.org/chemcomm
A porous Mg-based 3D metal–organic framework with unique was constructed by the coordination of a tetracarboxylate
nanoscale cages and two-fold interpenetrating pcu nets has been ligand (pyrene-1,3,6,8-tetracarboxylic acid, H4PTCA) with
synthesized and characterized. It shows gas-uptake capacities for Mg2+. Interestingly, this 3D porous MOF has nanoscale poly-
N2, H2, O2 and CO2 at low temperatures and selective adsorption of hedral cages and shows temperature dependent selective gas
CO2 over O2 and N2 at room temperature.
sorption properties.
The hydrothermal reaction of MgCl2ꢀ6H2O, H4PTCA and HCl
In the past decade, the design and synthesis of porous metal– at 90 1C in a mixture solution of DMF–dioxane–H2O (2 : 1 : 1)
organic frameworks (MOFs) have attracted widespread attention produced yellow block-shaped crystals of 1 in high yield (ESI†).
due to their intriguing structural diversities of architecture and The result of X-ray crystal structural analysis‡ reveals that 1
potential applications such as gas separation and storage,1 ionic/ crystallizes in the cubic space group Im3m, in which there are two
%
molecular recognition,2 heterogeneous catalysis,3 luminescence half crystallographically independent Mg(II) ions in one asymmetric
tuning,4 fabrication of metal nanoparticles,5 and so on. To date, unit (Fig. S2, ESI†), and Mg1 and Mg2 show similar distorted
a large number of porous MOFs have been constructed using octahedral geometries. In addition, the PTCA ligand is located in
transition metals and lanthanide ions as framework nodes.6 a 4-fold axis, and the dioxane molecule is located both in a 2-fold
Recently, the coordination chemistry of MOFs based on light main axis and a mirror plane. In 1, the six oxygen atoms coordinated to
group metal cations such as Li+, Mg2+, and Al3+ has been revealed Mg1 are from four PTCA ligands and two water molecules, while six
and studied. With these lightweight main group elements, the oxygen atoms around Mg2 come from two PTCA ligands, three
MOFs have an advantage of low-density, which will be beneficial water molecules and one dioxane molecule. In 1, the inorganic brick
for on-board hydrogen storage application.7 Up to now, a number consists of octahedral Mg dimers with a common corner corres-
of research groups have done the previewing work on magnesium- ponding to a m2-H2O aquo ligand as a secondary building unit
based MOFs.8 For example, Long et al. have found that a porous (SBU). As illustrated in Fig. 1, twelve PTCA ligands bridge a total of
Mg-based MOF, Mg3(ndc)3(DEF)4 (H2ndc = 2,6-naphthalene 24 Mg2 SBUs to generate a large nanoscale cage with a diameter of
dicarboxylic acid), shows strong hydrogen gas affinity. Matzger 2.3 nm, and its calculated pore volume is 6.3 nm3. To the best of our
et al. have reported that MOF-74 (Mg2(dobdc), H4dobdc = knowledge, such a large cage based on main group metal ions has
2,5-dihydroxyterephthalic acid) exhibits a high uptake capacity not been reported so far. On the surface of each cage, there are
for CO2 at room temperature. However, compared with large two types of windows: six square windows with dimensions of 7.9 ꢁ
numbers of porous MOFs constructed using transition metals 7.9 Å (Mg to Mg distances from SBUs) and eight triangular windows
and lanthanide ions, Mg-based porous MOFs are still rare.
with dimensions of 7.4 ꢁ 7.4 ꢁ 7.4 Å (Mg to Mg distances from
Herein, we report a novel Mg-based 3D porous MOF, SBUs) (Fig. S3, ESI†). Furthermore, four PTCA ligands link four SBUs
[Mg16(PTCA)8(m2-H2O)8(H2O)16(dioxane)8]ꢀ(H2O)13ꢀ(DMF)26 (1), which to form a tube with dimensions of 10.7 ꢁ 10.7 Å (centroid–centroid
distances from PTCA). The tubes connect the nanoscale cages to
form a three-dimensional (3D) metal–organic framework of 1, in
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory
which each cage is surrounded by six neighboring cages connected
by six tubes to generate a pcu topological net (Fig. 2). In the
framework, large octagonal channels (2.4 ꢁ 2.4 nm) are formed
along a, b and c directions and are size-matchable with the
nanoscale cages (2.3 nm in diameter), leading to a two-fold
interpenetrating net (Fig. 3). As far as we know, analogous
MOFs with a two-fold interpenetrating pcu net are still rare, and
of Optoelectronic Materials and Technologies, School of Chemistry and Chemical
Engineering, Sun Yat-Sen University, Guangzhou 510275, China.
E-mail: lutongbu@mail.sysu.edu.cn; Fax: +86-20-84112921
† Electronic supplementary information (ESI) available: Synthesis, single and
Rietveld XRD refinement crystallographic data for 1, gas sorption measurements
and analysis, TG, and variable temperature XPRD patterns (PDF). CCDC 913523.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc38730e
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Fig. 1 A schematic illustration of the nanoscale cage containing 12 PTCA
ligands and 24 Mg2 dimer SBUs.
Fig. 4 Gas adsorption isotherms of 1d for (a) H2, N2 and O2 at 77 K; CO2 at
195 K, (b) CO2, N2 and O2 at 273 K and 298 K. Filled shapes = adsorption, open
shapes = desorption.
from 30 to 550 1C, while 10 shows plateaus from 60 to 400 1C
(Fig. S4, ESI†). Unfortunately, the results of variable temperature
PXRD measurements of 10 show that the powder diffraction
peaks disappeared after 70 1C (Fig. S5, ESI†), indicating the
collapse of the framework structure. Fortunately, the methanol
molecules in 10 can be removed under high vacuum at room
temperature to obtain a desolvated sample 1d. The gas adsorp-
tion measurements of 1d show that it can adsorb N2, H2, O2 and
CO2 at low temperatures (Fig. 4a). At 77 K, the sorption isotherm
of N2 shows a typical Type-I curve, with the amounts of N2
uptake increasing abruptly at the beginning and then gradually
reaching a plateau of 123.7 cm3 (STP) gꢂ1 at 1.0 atm. Fitting
the Langmuir and BET equations to the N2 adsorption isotherm
gave estimated surface areas of 532.9 and 438.1 m2 gꢂ1, respec-
tively. The H2 and O2 adsorption isotherms reveal that 1d can
store up to 92.2 cm3 (STP) gꢂ1 hydrogen at 77 K/1.0 atm and
200.1 cm3 (STP) gꢂ1 oxygen at 77 K/0.19 atm. It is worth noting
that the adsorption capacity for O2 is higher than that for N2,
which may be due to the smaller kinetic diameter and lower
critical temperature of O2 (3.47 Å, 33.2 K) than those of N2
(3.64 Å, 126.2 K), which allows the O2 adsorption in a more
condensable manner in the cages with smaller windows.1e In
addition, the CO2 adsorption isotherm of 1d measured at 195 K
and 1.0 atm shows two-step sorption. From 0 to 0.1 atm, 1d can
adsorb 97.2 cm3 (STP) gꢂ1 of CO2, and then it adsorbs more CO2
Fig. 2 Polyhedral representation of the pcu net in 1.
only two analogous MOFs have been reported so far.9 The pores
of 1 are filled with disordered DMF and H2O molecules, and the
solvent-accessible volume calculated using PLATON10 is 40.2%.
Afterwards, soaking 1 in methanol leads to the exchange of DMF
and H2O guest molecules with methanol to give 10.
The results of thermogravimetric analysis (TGA) indicate
that 1 shows continuous weight loss in the temperature range
and reaches a plateau of 160.5 cm3 (STP) gꢂ1
.
Fig. 3 The two-fold interpenetration of 1 observed along the a axis.
1754 Chem. Commun., 2013, 49, 1753--1755
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In conclusion, we have successfully constructed a novel two-
fold interpenetrating Mg-based 3D MOF containing large nano-
scale cages with a diameter of 2.3 nm. The desolvated solid 1d
exhibits selective gas adsorption for CO2 over N2 and O2 around
room temperature, indicating that the present material can be
potentially applied in a CO2 capture process.
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This work was supported by 973 Program of China
(2012CB821705), NSFC (grant no. 20831005, 21121061 and
91127002), and NSF of Guangdong Province (S2012030006240).
Notes and references
%
‡ Crystal data for 1 (C270H368N26O143Mg16): cubic, space group Im3m, 12 S. Coriani, A. Halkier, A. Rizzo and K. Ruud, Chem. Phys. Lett., 2000,
a = b = c = 35.1938(1) Å, V = 43591.2(2) Å3, Mr = 6654.86, Z = 6, rcalcd
=
326, 269.
1.521 g cmꢂ3, m = 1.352 mmꢂ1, T = 150(2) K, R1 = 0.1487, wR2 = 0.2497, 13 D. Q. Yuan, D. Zhao, D. F. Sun and H. C. Zhou, Angew. Chem.,
GOF = 1.010 for 3203 reflections with I > 2s(I). CCDC 913523 (1).
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1753--1755 1755