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Selective gas adsorption and unique phase transition
properties in a stable magnesium metal-organic
framework constructed from infinite metal chains†
Yangyang Liu,a Ying-Pin Chen,ab Tian-Fu Liu,a Andrey A. Yakovenko,a
Aaron M. Raiffa and Hong-Cai Zhou*ab
Received 11th June 2013,
Accepted 28th August 2013
A 3D Magnesium MOF PCN-72 has been synthesized from the solvothermal reaction of Mg(NO3)2 and a linear
ligand. This MOF has a unique structure with 1-dimensional (1D) channel as well as infinite metal chains, which
resembles the topology of MIL-53. It is thermally and moisture stable. In situ powder X-ray diffraction studies
DOI: 10.1039/c3ce41106k
reveal its interesting phase transitions under temperature change. Calculation shows that each phase of PCN-72
exhibits unique and unusual thermal expansion properties. After removing coordinated solvent at Mg chains,
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PCN-72 can selectively adsorb CO2 over N2.
Introduction
Research related to metal-organic frameworks (MOFs) has
become one of the hottest areas today.1 MOFs usually have
highly crystalline structures2 and diverse topologies, as well
as tunable functionalities,2,3 which enable them to be potentially applicable for CO2 capture,4 H2 storage,5 gas separation,6 catalysis7 and some other fields.8 MOFs are usually
constructed from organic ligands and inorganic secondary
building units (SBUs, generally metal ions or metal-containing
units).9 So far, the majority of the reported porous MOFs are
based on 3d or 4f elements, while those constructed from s- and
p-elements such as Li, B, Mg, Al and In are relatively rare.10 On
the other hand, it is beneficial to achieve the target of gas storage and separation by obtaining low-density MOFs using lightweight elements. Among the lightweight elements, the use of
Mg is of particular interest due to its close resemblance to 3d
elements (e.g. Zn). In zeolite-type AlPO4, the doping chemistry of
Mg2+ is very similar to that of 3d elements such as Co2+ and
Zn2+.11 However, the number of reported porous Mg MOFs is
still small, even though some of the Mg MOFs are wellknown.12 For example, Mg-MOF-7413 has been thoroughly studied for its excellent CO2 capture and sequestration properties.
Particularly, Mg open metal sites in Mg-MOF-74, which can be
generated by solvent removal, have a high affinity for CO2. A
a
Department of Chemistry, Texas A&M University, College Station, TX 778423012, USA. E-mail: zhou@chem.tamu.edu; Fax: +1 979 845 1595
b
Department of Materials Science and Engineering, Texas A&M University,
College Station, TX 77842, USA
† Electronic supplementary information (ESI) available: Experimental and IR.
CCDC 943806. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ce41106k
9688 | CrystEngComm, 2013, 15, 9688–9693
systematic evaluation of the effects of metal identity in MOFs
on CO2 uptake demonstrated that Mg-MOF-74 outperforms all
other isostructural M-MOF-74 (M: Co, Ni, Zn) materials in low
pressure physisorption of CO2.12e The exceptional CO2 uptake
of Mg-MOF-74 might be attributed to the increased ionic
character of the Mg–O bond beyond simple weight effects.
Mg-MOF-74 is also one of very few reported Mg MOFs
constructed from infinite Mg chains.12b,13
A material usually experiences positive thermal expansion
(PTE) along all three dimensions upon heating due to the
increasing anharmonic vibrational amplitudes of the constituent atoms, ions or molecules.14 In rare cases, the structure
of materials can undergo unusually large PTE, zero thermal
expansion (ZTE), or negative thermal expansion (NTE).10c,15
Emerging as a new class of thermal responsive materials, some
MOFs have shown anomalous thermal expansion behaviour.10c,15b,16
Detailed studies on this can help thermoresponsive frameworks to find applications for sensors or actuators in a singlecrystal device.
Herewereportanew3-dimensional(3D)metal-organicframework(PCN-72)constructedfrominfiniteMgchainsandthelinear
ligand TTTP (2′,3′,5′,6′-tetramethyl-[1,1′:4′,1′′-terphenyl]4,4′′-dicarboxylate). PCN-72 shows good thermal and moisture
stability. It is a thermoresponsive material that exhibits anisotropic thermal expansion properties. Moreover, PCN-72 can
selectively adsorb CO2 gas over N2.
Crystal synthesis and characterization
H4TTTP (2′,3′,5′,6′-tetramethyl-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic
acid) was synthesized by a Suzuki reaction and subsequent
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Scheme 1
Synthesis of H4TTTP, DME = dimethyl ether.
Paper
a linear ligand. This encouraged us to compare the structure
of PCN-72 with MIL-53. From Fig. 1 and Fig. 2, it is very
straightforward that PCN-72 and MIL-53 have the same metal
chains, except that PCN-72 has a coordinated DMSO molecule
between two metals while MIL-53 has the –OH group connecting
two metals. They both have a 1D channel structure. More interestingly, we found that PCN-72 and MIL-53 share the same
topology (Fig. 2c). Their topological type is seh-4,6-Imma. It is a
4,6-connected net with point (Schläfli) symbol of
{32·62·72}{34·42·64·75} upon considering ligands as 4-connected
nodes and Mg as 6-connected nodes.
Thermogravimetric analysis (TGA) (Fig. 3) indicates that
PCN-72 decomposes at around 410 °C. Uncoordinated solvent
molecules in the channels of the MOF can be totally removed
at 90 °C. The weight loss from about 285 to 360 °C corresponds to the loss of coordinated DMSO molecules to Mg
metals. In order to further understand the thermal properties
of PCN-72, temperature dependent in situ PXRD was
performed in Argonne National Lab (details of the experiments can be found in ESI†). In the experiment, PCN-72 was
Fig. 1 Single crystal X-ray structure of PCN-72. (a) View along the b-axis, (b) view
along the c-axis, (c) the infinite Mg chain SBU and (d) open Mg sites after the
removal of coordinated DMSO (hydrogen atoms have been omitted for clarity).
hydrolysis with KOH in an overall yield of 70% (Scheme 1,
details see ESI).†
The solvothermal reaction of Mg(NO3)2·6H2O and H4TTTP
in a mixed solvent of DMA (N,N-dimethylacetamide), ethanol
and DMSO (dimethyl sulfoxide) at 100 °C for 3 days yielded
colorless needle shaped crystals (PCN-72, Fig. 1). X-Ray structural analysis revealed that PCN-72 crystallizes in the orthorhombic Imma space group with a = 36.85(2), b = 7.469(4), c =
8.756(5) Å. As shown in Fig. 1, the structure of PCN-72 has
infinite 1D Mg chain SBUs (Fig. 1c). These chains are linked
via carboxylate groups of the TTTP ligand and coordinated
DMSO molecules, forming a 3D framework. 1D channels in
PCN-72 accommodate uncoordinated solvent residues, which
can be removed by evacuation at high temperature. The coordinated DMSO molecules can also be removed at higher temperature to generate the open metal sites in the Mg chains
(Fig. 1d).‡
Fig. 2 (a) Single crystal X-ray structure of MIL-53 (Cr), view along the c-axis, (b)
infinite metal chains in MIL-53 (Cr) and (c) topology of MIL-53 and PCN-72 ((a) and
(b) are generated from the cif file of ref. 14, hydrogen atoms have been omitted
for clarity).
Results and discussion
The Mg chains in PCN-72 resemble the metal chains in
MIL-53 MOFs17 (Fig. 2) and both of them are constructed from
‡ Crystal data for PCN-72: C26H26MgO5S; Mr = 474.85; orthorhombic; a = 36.85(2),
3
b = 7.469(4), c = 8.756(5) Å; V = 2410(2) Å ; T = 110 K; space group Imma; Z = 4;
Rint = 0.0676, R1 = 0.1139, wR2 = 0.2291; GOF = 1.053.
This journal is © The Royal Society of Chemistry 2013
Fig. 3
TGA plot of PCN-72.
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To study the details of the phase transformation of
PCN-72, a fresh sample was heated to 285 °C at a rate of
0.1 °C s−1 and held at that temperature for 60 min. It was then
cooled down to 25 °C (in situ PXRD shown in Fig. 4b). The Le
Bail calculated curves fit their corresponding observed PXRD
patterns collected at 25 °C and 200 °C, as approved by the
residuals in the final whole pattern decomposition plots
(Fig. S1a, b†), and acceptable R-factors shown in Table S1.†
This means that these materials are pure phase with similar
structures, which was found in the single crystal experiment.
Also, from the same table, it is noted that this orthorhombic
crystal exhibits a positive volumetric thermal expansion;
oppositely, the final unit cell parameter of a shrinks with a
temperature increase, exhibiting negative linear thermal expansion. This thermal response keeps going until an unexplained
peak, 2θ = 5.8°, grows at 285 °C, in which framework transformation occurred. Further peaks from the new phase emerge
after maintaining a temperature plateau for one hour.
Direct attempts to index the patterns collected at 285 °C
failed, suggesting a multiphase exists. Comparing the pattern
difference between the original phase and the mixed phase,
unassigned peaks, such as 2θ = 1.96°, 3.01°, were selected for
second phase indexing. By using TOPAS,18 it was proposed
that monoclinic C2 with a = 35.3575, b = 12.1105, c = 6.0708
and β = 82.5690° is the most promising possible solution.
Unfortunately, the Le Bail fittings did not decompose the pattern with tolerable accuracy on account of the first split peak
(Fig. S1 c†). On the other hand, the TGA curve dropped a hint
of the remaining unresolved peaks. Since coordinated DMSO
molecules were gradually extracted from 285 °C to 360 °C,
the crystal dimension might be slightly changed. This hypothetical phase 1′ was constructed with Imma, a = 36.051, b =
7.308, c = 9.168 Å and V = 2415 Å3, simulating by Materials
Studio 5.5.19 As expected, the Bragg positions corresponding
to phase 1′ fit with those yet unresolved peaks successfully.
Fig. 4 Temperature dependent in situ PXRD for PCN-72. (a) Decomposition scan,
(b) phase transformation details study and (c) PXRD at different temperatures
generated from synchrotron data.
heated under a helium atmosphere from room temperature
until sample decomposition. As shown in Fig. 4a, the sample
decomposed at around 400 °C, which is in a good agreement
with the TGA data. Interestingly, new reflections started to
appear from about 285 °C in the in situ PXRD (Fig. 4a). This
is an indication of the presence of a new phase. Based on
TGA data, this occurs at the temperature when the removal
of coordinated DMSO molecules starts.
9690 | CrystEngComm, 2013, 15, 9688–9693
Fig. 5 Final Le Bail whole pattern decomposition plots for PCN-72 at 285 °C for
30 min: observed data (black spots) and calculated profiles (red solid line), relative
intensity (y-axis) and 2θ (x-axis); the difference is drawn as black curves below
the profiles. Tick marks indicate the calculated peak positions of the corresponding compound.
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Table 1 The summary of the cell and refinement parameters from the synchrotron
powder X-ray diffraction after phase transition
Compound
PCN-72 powder suffering phase transition
Temperature
Phase
285 °C for 30 min
Phase 1
Phase 1′
(Original)
(nonsolvent)
Orthorhombic
Orthorhombic
Phase 2
(New)
Monoclinic
Imma
37.00(1)
7.68(1)
8.90(1)
90.00
90.00
90.00
2529
C2
35.58(1)
12.16(1)
6.09(1)
90.00
82.02
90.00
2635
Crystal
system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
3
V (Å )
Rp
wRp
GoF
Imma
35.91(1)
7.35(1)
9.05(1)
90.00
90.00
90.00
2389
3.21
5.67
1.30
Consequently, the final Le Bail whole pattern decomposition
plot was described well with original phase 1, nonsolvent
phase 1′, as well as emerging phase 2 (Fig. 5, Table 1).
As shown in Fig. 4c (inset), the (2 0 0) peak (2θ = 1.89°),
which is assigned to phase 1, decreases its intensity dramatically when the temperature reaches 285 °C. In contrast, the
(2 0 0) peaks assigned to phase 1′ (2θ = 1.96°) increases in
intensity gradually after the removal of DMSO molecules at
the 285 °C plateau region. This observation supports our previous assumption, which was developed by simulation.
Noticeably, phase 1′ undergoes unusual negative volumetric
thermal expansion, whereas phase 1 reveals totally positive
elongation in all directions. The unit cell volume of phase 1′
appears to be 2389 Å3 at 285 °C, which is not only smaller
than the predicted value of 2415 Å3, but also smaller than the
unit cell of phase 1 at room temperature (Table 1). Supposedly, the unit cell compression of phase 1′ is caused by the
volumetric expansion of phase 1. After removal of DMSO
molecules, this infinite Mg chain turns to be more flexible,
as can be regarded as a “thermal compensator”, which
is responsible for the extremely stable thermal stability
of PCN-72.
It is also worth noting that the (2 0 0) peak (2θ = 2.03°)
assigned to the new phase 2 does not give obvious change
with temperature, while non-solvent phase 1′ collapses. In
spite of phase 2 growing as monoclinic C2, which is different from the original orthorhombic phase, the unit cell
parameters and β value do not change much. This phase
transition could be attributed to stress release for thermal
expansion. Since the lattice torsion is relaxed, the new phase
is no longer forming.
Based on the thermal properties of PCN-72, different activation conditions (including heating the sample under vacuum at 50, 100, 250, 285 and 360 °C) have been carried out
for PCN-72. Gas sorption measurements showed that PCN-72
was not porous until the activation temperature reached
285 °C. The calculated free volume in PCN-72 is only 7.6% by
PLATON. This explains the non-porosity of PCN-72 at a lower
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activation temperature, since lower temperature activation
can only remove the uncoordinated solvent molecules in the
channels of PCN-72. However, after activation at 285 °C or
higher, which removes the coordinated DMSO solvent and
generates open metal sites, PCN-72 can selectively adsorb
CO2 over N2 (Fig. 6a). Gas sorption isotherms for the sample
activated at 360 °C (fully desolvated) demonstrated that
PCN-72 can barely adsorb N2 even at 77 K, but reaches about
30 cm3 g−1 (5.9 wt%) of CO2 uptake at room temperature and
1 bar. CO2 sorption isotherms at lower temperature (273 K
and 195 K) were also measured (Fig. 6a). Room temperature
N2 adsorption for PCN-72 was measured, however, no adsorption data can be obtained due to the very small amount of
uptake, which is even under the detection limit of the instrument. This showed the good selectivity of CO2 over N2 in
PCN-72 at room temperature.
MIL-53 MOFs are a series of unique flexible MOFs that
can “breathe” during the adsorption and desorption of CO2,
which is associated with a phase transition of the MOF structure. The phase transformation of MIL-53 was tracked by gassorption-coupled X-ray diffraction.20 Although bearing the
Fig. 6 (a) Gas adsorption isotherms of PCN-72 after being activated at 360 °C and
(b) PXRD of PCN-72 (activated at 360 °C: blue, activated at 250 °C: green, assynthesized: black, simulated: red).
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CrystEngComm
same topology as MIL-53 and with longer linkers, PCN-72
does not show any “breathing” effect on CO2 gas adsorption.
In fact, it does not have a “breathing” effect on any gas measured (N2, H2, CO2, H2 sorption data can be found in ESI,†
Fig S2) The “breathing” behaviour of MIL-53 cannot only be
initiated by guest molecules (e.g. CO2) but also by temperature change.21 The phase transformation in PCN-72, however,
can only be caused by temperature change, which was confirmed by in situ PXRD. Compared to the –OH groups in
MIL-53, the DMSO molecules in PCN-72 result in more steric
hindrance for it to “breathe”. But these DMSO molecules can
be removed at high temperature (285 °C), which explains the
phase transformation in PCN-72 was only induced by temperature change, but not by guest molecules.
The CO2 adsorption capacities of Mg-MOF-74, MIL-53 (Al),
MIL-53 (Cr) and PCN-72 have been summarised in Table 2.
Although the CO2 adsorption capacity of PCN-72 is much
lower than the best performing MOF Mg-MOF-74, which has
the highest reported CO2 adsorption capacity (27.5 wt%) at
room temperature and 1 bar,22 PCN-72 is still a good material
for selective adsorption of CO2 over N2.
PXRD for activated samples at different temperatures
(Fig. 6b) indicates that PCN-72 maintains its framework
structure after removing free solvent molecules in the channels. New reflections appeared (e.g. 2θ = 7.95°) when the sample is activated at 360 °C. This is the evidence of transformation
into a new phase (Phase 2) by the removal of coordinated
DMSO molecules. This new phase is the activated phase with
open Mg sites that has selective adsorption of CO2 over N2.
To investigate how moisture affects PCN-72, the moisture
stability was studied. The as-synthesized sample was dried
under vacuum for 12 h at 100 °C to remove free solvent
molecules. Powder X-ray diffraction was measured after
exposing the desolvated sample in the open air for one
month. PXRD in Fig. 7 shows that the PCN-72 framework
maintained the structure after being exposed to humid air
for one month, confirming that PCN-72 is stable to moisture. Due to its good thermal and moisture stability, PCN-72
might be a promising porous material for CO2 capture from
flue gas.
Table 2
Acknowledgements
CO2 adsorption capacities of MOFs at 1 bar
MOF
Mg-MOF-74
MIL-53 (Al)
MIL-53 (Cr)
PCN-72
BET Surface
2 −1
Area (m g )
1174
1300
CO2 capacity
(wt%)
Temp (K)
Ref.
27.5
10.6
8.5
5.9
298
298
304
295
22
23
20
This work
Conclusions
A 3D magnesium MOF PCN-72 with unique 1D magnesium
chains was synthesized. It is a topological analog of MIL-53.
PCN-72 does not show a “breathing” effect with gas molecules. However, calculation based on in situ PXRD experiment
showed that PCN-72 undergoes phase transition with temperature change. More interestingly, different phases of PCN-72
exhibit different thermal expansion properties owing to the
unique structure of PCN-72. This special feature enables
PCN-72 to be a potential tunable thermoresponsive material
that can find applications in sensors or actuators devices.
After removing coordinated DMSO at 360 °C, PCN-72 can
selectively adsorb CO2 over N2. It is a stable material that
might also be promising for CO2 capture from flue gas.
This work was supported by the U.S. Department of Energy
(DOE DE-SC0001015 and DE-FC36-07GO17033), the National
Science Foundation (NSF CBET-0930079), and the Welch
Foundation (A-1725). Use of the Advanced Photon Source, an
Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National
Laboratory, was supported by the US DOE under Contract
No. DE-AC02-06CH11357.
Notes and references
Fig. 7 Moisture stability of PCN-72 (blue: in open air for 1 month, black: assynthesized, red: simulated).
9692 | CrystEngComm, 2013, 15, 9688–9693
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