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EDGE ARTICLE
Hong-Cai Zhou et al.
A stepwise transition from microporosity to mesoporosity in metal-organic
frameworks by thermal treatment
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Cite this: Chem. Sci., 2011, 2, 103
EDGE ARTICLE
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A stepwise transition from microporosity to mesoporosity in metal–organic
frameworks by thermal treatment†
Daqiang Yuan,a Dan Zhao,a Daren J. Timmonsb and Hong-Cai Zhou*a
Received 28th May 2010, Accepted 2nd October 2010
DOI: 10.1039/c0sc00320d
A Cd MOF with twisted partially augmented the net was synthesized. Activation at increasing
temperatures revealed an unprecedented stepwise transition from microporosity to mesoporosity. This
thermal treatment may serve as an alternative approach for the preparation of mesoporous MOFs.
Introduction
As defined by IUPAC, mesoporous materials have pore sizes in
1 Compared to their microporous
the range of 20–500 A.
mesoporous
counterparts whose pore sizes are less than 20 A,
materials are more desirable in host–guest chemistry, mostly
because they can accommodate larger guest molecules.2 In recent
years, remarkable progress has been made in the design and
synthesis of silica-based mesoporous materials, which were
widely applied in catalysis, sorption, chromatography, and
controlled release of guest molecules.3
As newly emerging porous crystalline materials, metal–organic
frameworks (MOFs) have demonstrated their potency in gas
storage and gas separation.4 However, limited progress has been
made in the discovery of new mesoporous MOFs.5 As part of our
ongoing work towards stable mesoporous MOFs, we reported
mesoMOF-1, whose mesoporosity was retained by turning the
framework into ionic by neutralization treatment.5d Herein we
report another mesoporous MOF (mesoMOF-2), which is
prepared by thermal treatment. This process may serve as an
alternative approach for preparing mesoporous MOFs.
TGA-50 TGA, with a heating rate of 5 C min1. Powder X-ray
diffraction (PXRD) patterns were obtained on a XPert Pro
MPD. Mass spectra were obtained on a VG analytical 70S high
resolution, double focusing, sectored (EB) mass spectrometer at
the center for TAMU Laboratory for Biological Mass Spectrometry. TGA-MS analyses were performed in STA449C-QMS
403 C Thermal Analysis-Quadrupole Mass Spectrometer. IR
spectra (KBr plates) were measured on a Nicolet 740 FTIR
spectrometer.
MesoMOF-2 synthesis
Benzo-(1,2;3,4;5,6)-tris(thiophene-20 -carboxylic acid) (H3BTTC)6 (100 mg, 1.6 104 mol) and Cd(NO3)2$4H2O (300 mg, 1.3
103 mol) were dissolved in 15 mL of dimethylacetamide
(DMA) with 20 drops of HBF4 (40%) in a vial. The vial was
tightly capped and placed in a 100 C oven for 72 h to yield
218 mg of pale yellow cubic crystals (yield: 76% based on
H3BTTC). The crystal has a formula of Cd13(BTTC)8(OH)2(H2O)16$18DMA, which was derived from crystallographic
data,‡ elemental analysis (% calc/found: C 36.28/36.96, H 3.52/
3.65, N 3.97/3.91), and TGA.
Experimental section
Adsorption experiments
General information
N2 and Ar physisorption isotherms were measured up to 1 bar
using a Micromeritics ASAP 2020 surface area and pore size
analyzer. An as-isolated sample of mesoMOF-2 was soaked in
dichloromethane, sonicated for 20 min, and then replaced with
fresh dichloromethane after 12 h. This process was repeated for
up to 60 h to completely wash out any residual unreacted ligand
and DMA in the pores. After the removal of dichloromethane by
decanting, the sample was activated by drying under a dynamic
vacuum at room temperature overnight. Before the measurement, the sample was dried again by using the ‘‘outgas’’ function
of the surface area analyzer for 2 h at a different temperature.
High-purity gases were used (N2: 99.999%, Ar: 99.999%). Pore
size distribution data were calculated from the N2 sorption
isotherms based on DFT model in the Micromeritics ASAP2020
software package (assuming slit pore geometry).
Commercially available reagents were used as received.
Elemental analyses (C, H, and N) were obtained from Canadian
Microanalytical Service, Ltd. 1H NMR data were collected on
a Mercury 300 MHz NMR spectrometer. Thermogravimetry
analyses (TGA) were performed under N2 on a SHIMADZU
a
Department of Chemistry, Texas A&M University, College Station, TX,
77843, USA. E-mail: zhou@mail.chem.tamu.edu; Fax: +1 979 845 4719;
Tel: +1 979 845 4034
b
Department of Chemistry, Virginia Military Institute, Lexington, VA,
24450, USA. E-mail: TimmonsDJ@vmi.edu; Fax: +1 540 464 7261
† Electronic supplementary information (ESI) available: Extra figures,
N2 & Ar sorption isotherms, pore volume data, TGA-MS. CCDC
reference number 762237. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0sc00320d
This journal is ª The Royal Society of Chemistry 2011
Chem. Sci., 2011, 2, 103–106 | 103
Single crystal X-ray study
Single crystal X-ray data of mesoMOF-2 were collected on
a Bruker AXS Smart III X-ray diffractometer outfitted with
a Mo X-ray source and an APEX II CCD detector equipped with
an Oxford Cryostream low temperature device. The APEX-II
software suite was used for data collection, cell refinement,
reduction, and absorption correction. Structures of mesoMOF-2
were solved by direct method and refined by full-matrix leastsquares on F2 using SHELXTL.7 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 Ueq of
the attached atom. The solvent molecules of mesoMOF-2 are
highly disordered, and attempts to locate and refine the solvent
peaks were unsuccessful. Contributions to scattering due to these
solvent molecules were removed using the SQUEEZE routine of
PLATON structures were then refined again using the data
generated.8
Result and discussion
X-ray diffraction studies reveal that mesoMOF-2 crystallizes in
cubic space group Fm-3c, a rare space group for MOFs.9 As can
be seen in Fig. 1b, four Cd atoms are bridged by eight carboxylates to form the rare square-planar tetranuclear cadmium
secondary building unit (SBU). All four Cd atoms in the SBU are
seven-coordinated with capped triangular prism geometry. Each
SBU connects eight BTTC ligands, and each BTTC binds three
SBUs to form a twisted truncated-octahedral cage with
[Cd(OH)2(H2O)4] situated inside the cage (Fig. 1c). In this cage,
the six vertices are occupied by SBUs, and all eight faces are
spanned by the BTTC ligands, leaving almost no entrance for
small molecules. An icosahedral-type cavity (Fig. 1d) is formed
by simple cubic packing of eight cages (Fig. 1e), giving rise to
a zeolite-like (ITQ-21) structure.10 The internal surface can be
Fig. 1 (a) The BTTC ligand (Red: O; Grey: C; Yellow: S). (b) The
tetranuclear cadmium SBU (Green: Cd). (c) The twisted truncatedoctahedral cage [4668] with [Cd(OH)2(H2O)4] situated inside. (d) The
icosahedral-type cavity [320] (the distance between opposite vertices is
(e) Polyhedra 3D packing in mesoMOF-2. (f) The window and
25.8 A).
cavity in mesoMOF-2 simplified by the Schwarz P surface model.
104 | Chem. Sci., 2011, 2, 103–106
simplified by the Schwarz P model, in which large cavities
measured O/O) are interconnected through
(diameter 18.4 A,
measured O/O) (Fig. 1f).
small windows (diameter 12.2 A,
Topology analysis by the Topos 4.0 program suggests the
(43;62;8)3(63) topology symbol, which belongs to a twisted
partially augmented the net (Figure S1†).11
In order to confirm its permanent porosity, N2 and Ar sorption isotherms were collected (Figure S2 and Figure S3†). After
activation at 30 C, the N2 sorption isotherm of mesoMOF-2
shows typical Type I behavior, indicating a microporous structure, with a BET surface area of 842 m2 g1 (Langmuir surface
area 1022 m2 g1) and a total pore volume of 0.364 cm3 g1.
Interestingly, gradually increasing the activation temperature to
230 C reveals a step-by-step transition from Type I to Type IV
isotherm with an increasingly obvious H2 hysteresis loop
(Fig. 2a). The same trend is also observed in Ar isotherms.
Because hysteresis loops are normally associated with mesopores, it appears that the pore size increases as the activation
temperature becomes higher. This pore size increase is supported
by the pore size distribution data obtained by nonlocal density
functional theory (NLDFT) (Fig. 2b). At low activation
the
temperature (30 C), the smallest pore size detected is 5.9 A,
largest one is 25.2 A, with the highest pore size distributes at
These data correspond well with the crystal model. At
11.8 A.
a higher activation temperature (50 C), the largest pore size
Further increase of the activation temperature to
reaches 37.0 A.
but the most abundant
220 C enlarges the largest pore (63.4 A),
Fig. 2 (a) Nitrogen isotherms measured at 77 K for mesoMOF-2 activated at different temperature. (b) Pore size distribution of mesoMOF-2
activated at different temperature.
This journal is ª The Royal Society of Chemistry 2011
It is evident that there is
pore still remains the same size (11.8 A).
a stepwise transition from microporosity to mesoporosity in this
framework upon increased activation temperature.
It is tempting to attribute this porosity transition to the
framework deterioration at higher temperature. Such a conclusion, however, can not explain the following experimental data
convincingly. First of all, the deteriorated framework should
have weakened X-ray diffraction due to the loss of periodicity.
Nevertheless, the twisted partially augmented the net in mesoMOF-2 is very robust and is maintained even after heating at 250
C, which is proved by powder X-ray diffraction studies (Fig. 3).
Additionally, single crystal X-ray scattering spots can still be
harvested on a sample crystal activated at 220 C (Figure S4†).
The crystal structure can be solved using these data, unveiling
a similar structure with the as-synthesized crystal. This unexpected framework stability may result from the cage packing
geometry (Fig. 1e).5m Besides, framework deterioration normally
leads to lower pore volumes. In mesoMOF-2, however, there is
a steady increase in pore volume as the activation temperature
elevates to 220 C (Figure S5†). Such pore volume enhancement
can not be simply attributed to framework deterioration.
Another possible reason for this porosity transition is
a gradual removal of unreacted H3BTTC and solvent molecules
in the channels. Once again, this reasoning is not convincing.
First of all, without any guest molecule, the largest cavity in
(measured Cd/Cd). This
mesoMOF-2 has a diameter of 24.1 A
is much smaller than the experimental pore size distribution data.
Secondly, we used a much larger metal/ligand ratio (8.125 : 1) to
ensure the complete depletion of ligand during the reaction (the
theoretical metal/ligand ratio is only 1.625 : 1). Before being
activated under vacuum, the fresh sample was subjected to
rigorous solvent exchange to completely wash out any residual
unreacted ligand and DMA in the pores. As can be seen from the
IR spectrum of the activated sample (Fig. 4), the absorption peak
corresponding to carboxyl group vibration from the uncomplexed H3BTTC (1650 cm1) is absent. In addition, the peak
corresponding to carbonyl group vibration (1610 cm1) from
DMA in the fresh sample is almost invisible in the activated
sample, indicating near complete removal of solvent in the
Fig. 3 Powder X-ray diffraction patterns of mesoMOF-2 heated at
different temperature.
This journal is ª The Royal Society of Chemistry 2011
Fig. 4 IR spectra of uncomplexed ligand (H3BTTC), terthienobenzene,
as-synthesized sample, and samples activated at 30 C and 220 C.
activated sample. Additional evidence comes from the elemental
analyses. The theoretical sulfur content in a completely activated
mesoMOF-2 framework is 15.11 wt%, assuming no loss of terthienobenzene, and 23.21 wt% in pure ligand (H3BTTC$2H2O).
Therefore, if the framework has unreacted ligand within, the
sulfur content should be between these two values. However, the
sulfur content in the sample activated at 30 C is only 11.56 wt%,
indicating no residual ligand within channels and the partial loss
of terthienobenzene.
It has been reported that H3BTTC can undergo decarboxylation to liberate terthienobenzene by heating.6 Indeed, we
observed trace amount of white crystalline powder sublimed to
the upper inner wall of the sample tube after gas sorption
measurements. Analysis of the white powder by 1H-NMR and
EI-MS confirmed the presence of terthienobenzene (Fig. 5).
TGA-MS of mesoMOF-2 fully exchanged with dichloromethane
and dried was measured. The signal of trivalent anion of
the terthienobenzene was clearly identified upon heating
(Figure S6†). Since there is no free H3BTTC within the channels,
the detected terthienobenzene should come from the decarboxylated BTTC ligand within the framework. In mesoMOF-2,
the BTTC ligand serves as the pore wall. Their decarboxylation
will open up the twisted truncated-octahedral cages (Fig. 1c)
which are inaccessible at first. The opening of these cages leads to
the incremental increase of pore diameter and stepwise transition
Fig. 5 1H-NMR and EI-MS data of sublimed terthienobenzene after gas
sorption measurements.
Chem. Sci., 2011, 2, 103–106 | 105
from microporosity to mesoporosity in the framework. It is likely
that the framework remains intact during the initial BTTC
decarboxylation allowing the pore volume to increase steadily
with increasing activation temperature until 220 C. When the
temperature is increased further (230 C), the framework begins
to collapse accounting for the reduced pore volume.
Further evidence comes from the sample mass change after
each activation. According to Figure S8,† the sample mass
reduction before 50 C is contributed by the residual solvent.
From 50 C to 120 C, the sample mass barely changes. When the
temperature went above 120 C, a substantial mass reduction
was observed, which is contributed by the loss of externally
coordinated water (theoretical weight percent: 4.51 wt%) and
terthienobenzene unit (theoretical weight percent: 41.20 wt%). As
can be seen from Figure S9,† further activation of the sample at
230 C over a period of time would reveal a further mass
reduction up to 54.26 wt%, which corresponds well with the
theoretical mass loss of all the coordinated water and terthienobenzene unit (48.01 wt%).
Since activation conditions and activation time are both
involved in this process, it is hard to determine how much
terthienobenzene the framework can lose before it degrades.
Further activation at 230 C definitely leads to additional
terthienobenzene loss (Figure S9†). However, this does not
necessarily give rise to an increase in mesoporosity, because the
framework starts to collapse once a certain amount of terthienobenzene is lost (Figure S10†). This process is irreversible
because of the covalent bond breaking during the thermal
treatment.
Conclusions
In this report, a thermally controlled stepwise transition from
microporosity to mesoporosity in MOFs was found. This
thermal treatment may serve as an alternative approach towards
mesoporous MOFs with three prerequisites: (1) ligand prone to
decomposition; (2) volatile fragments can be liberated during
thermal activation; and (3) robust framework to prevent framework disintegration. Further studies will be focused on applying
this approach to generate more mesoporous MOFs and their
subsequent applications in host–guest chemistry.
Acknowledgements
This material is based upon work supported as part of the Center
for Gas Separations Relevant to Clean Energy Technologies, an
Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-SC0001015.
Notes and references
‡ Crystal data for MesoMOF-2: C120H60Cd13O66S24, Mr ¼ 4788.32, 0.16
0.15 0.14 mm3, cubic, space group Fm-3c (No. 226), a ¼ 39.2048(18),
3, Z ¼ 8, Dc ¼ 1.056 g cm3, F000 ¼ 18528, Mo-Ka
V ¼ 60258(5) A
T ¼ 173(2) K, 2q ¼ 55.8 , 102342 reflections
radiation, l ¼ 0.71073 A,
collected, 3156 unique (Rint ¼ 0.0984). Final GooF ¼ 1.152, R1 ¼ 0.0648,
wR2 ¼ 0.1561, R indices based on 2957 reflections with I > 2s(I), 88
parameters, 6 restraints. CCDC 762237 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.
106 | Chem. Sci., 2011, 2, 103–106
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