Post-synthetic structural processing in a metal-organic framework material as a mechanism for exceptional CO2/N2 selectivity

Article abstract of DOI:10.1021/ja4032049

Here we report the synthesis and ceramic-like processing of a new metal-organic framework (MOF) material, [Cu(bcppm)H₂O], that shows exceptionally selective separation for CO₂ over N₂ (ideal adsorbed solution theory, S_ads = 590). [Cu(bcppm)H ₂O]·xS was synthesized in 82% yield by reaction of Cu(NO ₃)₂·2.5H₂O with the link bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane (H₂bcppm) and shown to have a two-dimensional 4⁴-connected structure with an eclipsed arrangement of the layers. Activation of [Cu(bcppm)H₂O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2D-to-3D transformation (a-axis contraction) to give the adsorbing form, [Cu(bcppm)H ₂O]-ac. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu(bcppm)H ₂O]-ac to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration.

Full text of DOI:10.1021/ja4032049

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Article
Post-synthetic structural processing in a Metal-organic Framework
material as a mechanism for exceptional CO2/N2 selectivity
Witold M. Bloch, Ravichandar Babarao, Matthew R Hill, Christian J Doonan, and Christopher J. Sumby
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4032049 • Publication Date (Web): 12 Jun 2013
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Post-synthetic structural processing in a Metal-organic
Framework material as a mechanism for exceptional CO2/N2
selectivity
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Witold M. Blocha, Ravichandar Babaraob, Matthew R. Hillb, Christian J. Doonana*, Christopher J.
Sumbya*
a School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia
b CSIRO Materials Science and Engineering, Private Bag 33, Clayton South MDC, Victoria 3169, Australia.
KEYWORDS Metal-organic Framework, gas separation, carbon capture, dynamic behavior, post-synthesis processing
ABSTRACT: Here we report the synthesis and ceramic-like processing of a new Metal-organic Framework (MOF) materi-
al, [Cu(bcppm)H2O], that shows exceptional selective separation for CO2 over N2 (ideal adsorbed solution theory, IAST
Sads = 590). [Cu(bcppm)H2O]·xS was synthesized in 82% yield by reaction of Cu(NO3)2·2.5H2O with the link bis(4-(4-
carboxyphenyl)-1H-pyrazolyl)methane (H2bcppm) and shown to have a 2-D 44-connected structure with an eclipsed ar-
rangement of the layers. Activation of [Cu(bcppm)H2O] generates a pore-constricted version of the material through con-
comitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2-D-to-3-D transformation (a-axis
contraction) to give the adsorbing form, [Cu(bcppm)H2O]-ac. The pore contraction process and 2-D-to-3-D transfor-
mation were probed by single crystal and powder X-ray diffraction experiments. The 3-D network and shorter hydrogen
bonding contacts do not allow [Cu(bcppm)H2O]-ac to expand under gas loading, across the pressure ranges examined, or
following re-solvation. This exceptional separation performance associates with a moderate adsorption enthalpy and
therefore an expected low energy cost for regeneration.
strategies have been used to enhance the gas separation per-
formance of MOFs including: generation of exposed metal
Introduction
Metal-organic frameworks (MOFs) are a burgeoning class
sites within the framework, either as part of the metal node12
or appended to the framework via post-synthetic metalla-
tion,13 and by functionalizing the pores with Lewis basic
of crystalline materials that have shown excellent potential
for on-board gas storage,1 molecular separations2,3 and cataly-
sis.4 The widespread interest in developing MOFs for these
applications arises from their robust permanent porosity,
high surface areas and modular synthesis.5 Notably, MOFs
can be readily synthesized from a large range of structurally
diverse organic links and metal clusters which facilitates
careful and systematic tailoring of their pore architectures
and chemistry.5 According to their framework properties up-
on guest removal, Kitagawa has classified MOFs as first, se-
cond and third generation.6 Second generation materials
have rigid permanently porous structures that are not altered
upon the removal or introduction of guest molecules; in con-
trast, third generation materials exhibit a degree of flexibility
or dynamic behavior that allows them to tailor their pore
structures in accordance with the type of guest molecule.7-10
Several types of behavior are possible for dynamic MOFs,
including solid-state expansion/contraction (swelling or
'breathing' processes),6, 11 increases in framework connectivity
(e.g. 2-D-to-3-D)6, 9 and reversible and non-reversible crystal-
to-crystal transformations.6, 11
15
sites.14, Although these approaches have led to materials
with promising performance characteristics, the underlying
separation mechanism relies on increasing the affinity of the
material for CO2. Accordingly, these processes are commonly
associated with higher enthalpies of adsorption for CO2.3d, 15
A
significant drawback of such strategies is that the enthalpy of
adsorption of CO2 is directly proportional to the energy cost
for regeneration of the material subsequent to saturation.16
For example, MOFs with pores functionalized by alkylamines
show enthalpies of adsorption (Qst values) as high as 96
kJmol-1 and typically require temperatures in excess of 100˚C
to fully regenerate.3d, 15, 16
Alternatively, CO2 and N2 can be separated by virtue of
their different kinetic diameters (3.30 and 3.64 Ǻ, respective-
ly). Kinetic separations are not intrinsically associated with a
large ΔH and therefore offer a lower energy penalty for re-
generation of the adsorbent. To effectively separate adsorb-
ates based on size, precise control of the limiting pore diame-
ter is necessary.17, 18 In this context extended metal-organic
materials with rigid, but systematically tuned pore architec-
tures have been utilized to achieve remarkable selectivities
An increasingly topical area in which MOFs have shown
excellent potential is for selective capture of CO2.3 Several
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for CO2 and N2 separations.17 Another approach to carefully
69%) Mp: 350–352°C (decomposes); Found: C 64.2, H 4.0, N
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tune the pore dimensions of MOFs is to utilize third genera-
tion materials that have dynamic or flexible structures as this
provides an opportunity to closely tailor the required pore
diameters.10 In examples of such materials only certain gases
can enter the pores of a material but once inside induce a
selective pore expansion or 'gate opening'.7 A potential down-
side of this ‘gate-opening’ process in gas separations is that
once 'opened' these materials may act as adsorbents for all
components of the gas mixture.
14.4, C21H16N4O4·0.25H2O requires: C 64.2 H 4.2 N 14.3; νmax
(neat, cm-1): 1701 (C=O), 1613 (C=N), 1568 (C=C), 1508 (C=C);
1H NMR (300 MHz/DMSO): 6.47 (s, 2H, CH2), 7.72 (d, 4H,
H2’/H6’), 7.91 (d, 4H, H3’/H5’), 8.09 (s, 2H, H3 or H5), 8.56
(s, 2H, H3 or H5), 12.8 (s, br, 2H, CO2H); 13C NMR (150
MHz/DMSO): δ 65.0, 122.0, 125.0, 128.4, 128.6, 130.0, 136.3,
138.3, 167.0.
Synthesis of [Cu(bcppm)H2O]·xS (S = solvate). In a
screw cap vial, Cu(NO3)2·2.5H2O (36.3 mg, 0.16 mmol) and
H2bcppm (41.5 mg, 0.11 mmol) were dissolved in a mixture of
DMF (2 mL), distilled water (0.5 mL), ethanol (0.5 mL) and 2
drops of 70% HNO3. The resulting mixture was heated at
85°C for 16 hours, affording blue block-like crystals suitable
for X-ray crystallography. The crystals were soaked in DMF (×
3), acetone (× 8) and heated at 120°C under high vacuum for 3
h to yield a green crystalline powder [Cu(bcppm)H2O]-ac (ac
= activated) (40.8 mg, 82%, based on analysis). νmax(neat, cm-
1): 3125 (w, C-H), 2800-3300 (br, OH), 1608 (m, C=O), 1581 (m,
C=C), 1545 (m, C=C), 1377 (s, C-O) Found C 53.80, H 3.43, N
12.23, C21H16N4O5Cu requires: C, 53.90, H, 3.45, N 11.97.
9
In this paper we present the synthesis and characterization
of a novel MOF material [Cu(bcppm)H2O] that subsequent
to ceramic-like structural processing affords a material dis-
playing exceptional values for CO2/N2 selectivity based on
ideal adsorbed solution theory (IAST) calculations. As-
synthesized [Cu(bcppm)H2O] is a flexible 2-D framework
with an eclipsed 44 net (Figure 1), yet activation generates a
material with a rigid structure and pore sizes tuned for kinet-
ic CO2/N2 separations. Many examples of 2-D MOFs com-
posed of a 44 net are known, however such materials are no-
torious for loss of crystallinity upon desolvation9 or interlayer
sliding10 due to weak interactions between layers. Here, re-
moval of guest molecules from the pores of [Cu(bcppm)H2O]
by heating results in a pore constriction, whilst maintaining
the eclipsed arrangement of the 2-D layers, and ultimately a
2-D-to-3-D transformation at higher temperatures. These
structural changes were monitored by single crystal and
powder X-ray diffraction and gas adsorption measurements.
Combined, these structural changes result in constriction of
the limiting pore diameter to around 3.6 Å, well within the
range that is suitable for kinetic CO2/N2 separations.20
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X-ray Crystallography. Single crystals were mounted in
paratone-N oil on a plastic loop. X-ray diffraction data were
collected with Mo-Kα radiation (λ = 0.7107 Å) using an Ox-
ford Diffraction X-calibur single crystal X-ray diffractometer
at 150(2) K. Data sets were corrected for absorption using a
multi-scan method, and structures were solved by direct
methods using SHELXS-9722 and refined by full-matrix least
squares on F2 by SHELXL-97,23 interfaced through the pro-
gram X-Seed.24 In general, all non-hydrogen atoms were
refined anisotropically and hydrogen atoms were included as
invariants at geometrically estimated positions, unless speci-
fied otherwise in additional details below. Figures were pro-
duced using the program CrystalMaker.25 Publication mate-
rials were prepared using CIFTAB.26 CCDC 930403 and
930404 contain the supplementary crystallographic data for
these structures. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Experimental
General Information. Elemental analyses were performed
by the Campbell Microanalytical Laboratory at the University
of Otago. Thermogravimetric analysis was performed on a
Perkin−Elmer STA-6000 under a constant flow of N2 at a
temperature increase rate of 10°C/min. Infrared (IR) spectra
were recorded on a Perkin−Elmer Fourier−Transform Infra-
red (FT−IR) spectrometer on a zinc−selenide crystal. NMR
spectra were recorded on a Varian Gemini 300 MHz or 600
MHz spectrometer at 23°C using a 5 mm probe. Unless oth-
erwise stated, all chemicals were obtained from commercial
sources and used as received. The compounds di(1H-pyrazol-
1-yl)methane (2)21 and bis(4-iodo-1H-pyrazol-1-yl)methane
(3)21 were prepared by reported procedures.
Special Refinement Details for [Cu(bcppm)H2O]·xS:
The structure has large solvent accessible voids. These con-
tained a number of diffuse electron density peaks that could
not be adequately identified and refined as solvent. The
SQUEEZE routine of PLATON27 was applied to the collected
data, which resulted in significant reductions in R1 and wR2
and an improvement in the GOF. R1, wR2 and GOF before
SQUEEZE routine: 9.82%, 35.53% and 1.53; after SQUEEZE
routine: 4.52%, 13.8% and 1.06. There is some disorder in the
structure and the aryl ring and carboxylate group were mod-
eled over two positions.
Synthesis
of
bis(4-(4-carboxyphenyl)-1H-
pyrazolyl)methane (H2bcppm). Bis(4-iodo-1H-pyrazol-1-
yl)methane (1.01 g, 2.53 mmol), 4-carboxyphenylboronic acid
(1.26 g, 7.60 mmol) and an aqueous solution K2CO3 (5.00 g, 30
mL distilled water) were combined in DMF (135 mL). After
degassing the mixture with Ar for 30 minutes, Pd(PPh3)4
(0.442 g, 0.38 mmol, 0.15 equiv.) was added. The resulting
mixture was further degassed for 30 minutes and then heated
at 90oC for 20 hours. Upon cooling to room temperature,
distilled water (200 mL) was added and the reaction mixture
filtered. The resulting solution was washed with dichloro-
methane (5 x 50 mL) and acidified with 20% HNO3 (pH = 3)
to afford a precipitate which was isolated under reduced
pressure. The solid was washed with distilled water, ethanol
and dried at 130°C to give a white solid of H2bcppm (0.68 g,
Special Refinement Details for [Cu(bcppm)H2O]-
heated: The structure has large solvent accessible voids.
These contained a number of diffuse electron density peaks.
The SQUEEZE routine of PLATON27 was applied to the col-
lected data, which resulted in reasonable reductions in R1 and
wR2 and an improvement in the GOF. R1, wR2 and GOF before
SQUEEZE routine: 25.5%, 60.2% and 1.72; after SQUEEZE
routine: 19.4%, 49.5% and 1.56. The data is of relatively poor
quality due to the conditions used to generate the sample
(heating in an oven at 80°C).
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Powder Diffraction. Powder X-ray diffraction data was
under an N2 stream, the sample was activated under vacuum
at 120°C for 3 h to obtain [Cu(bcppm)H2O]-ac. UHP grade
(99.999%) N2 and CO2 were used for all measurements. The
temperatures were maintained at 77 K (liquid nitrogen bath),
195 K (acetone-dry ice bath), 273 K (ice−water bath), or 293 K
(room temperature water), respectively. CO2 enthalpy plots
were obtained with the use of Van’t Hoff plots derived from
isotherms collected at 273 and 293 K respectively.
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collected on a Rigaku Hiflux Homelab system using Cu-Kα
radiation with an R-Axis IV++ image plate detector. Samples
were mounted on plastic loops using paratone-N and data
collected by scanning 90° in phi for 120-300 second expo-
sures. The data was converted into xye format using the pro-
gram DataSqueeze. Simulated powder X-ray diffraction pat-
terns were generated from the single crystal data using Mer-
cury 2.3.28 Variable Temperature Powder X-ray Diffraction
(VT PXRD) was carried out at the Australian Synchrotron on
the Powder Diffraction (PD) beamline. The sample was pre-
pared in a glass capillary (0.3 mm) and exposed to X-rays
with a wavelength of 0.82544 Ǻ. For further details on the
instrument set-up see www.synchrotron.org.au. Further data
on the activated [Cu(bcppm]H2O] were collected on a
Bruker X’Pert with Co Kα radiation.
Table 1. Crystal data and X-ray experimental data for
[Cu(bcppm)H2O]·xS and [Cu(bcppm)H2O]-heated.
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Compound
[Cu(bcppm)H2O]· [Cu(bcppm)H2O]-
xS b
heated
C21H16CuN4O5
C21H16CuN4O5
Empirical
mula
for-
Simulation Methods. Geometric pore size distributions
were calculated based on a Monte Carlo technique using the
procedure described elsewhere.29 The accessible void surface
is determined based on a probe diameter of 3.3 Å which is
equal to the kinetic diameter of CO2 using Atom Volumes
and Surfaces tool within Materials Studio V6.0.0 from Accel-
rys Software Inc. To locate the preferential CO2 adsorption
site in [Cu(bcppm)H2O]-ac, a simulated annealing method
was employed using an in-house code. A UFF force field was
used to describe the [Cu(bcppm)H2O] framework atoms and
a three-site model for CO2 the molecule.30 The total energy of
the system is approximated as the sum of all pair-wise inter-
molecular interactions by a Lennard-Jones (LJ) potential and
coulombic interactions. In the simulated annealing method,
the temperature is lowered in succession allowing the gas
molecule to reach a desirable configuration based on differ-
ent moves such as rotate, translate and re-position with pre-
set probabilities of occurrence. This process of heating and
cooling the system was repeated in several heating cycles to
find the local minima. Forty heating cycles were performed
where the maximum temperature and the final temperature
were 105 K and 100 K, respectively. The NVT ensemble pro-
cess consisted of 107 equilibration steps followed by 107 pro-
duction steps, with the properties calculated from the latter
steps.
467.92
orthorhombic
Pnma
467.92
orthorhombic
Pnma
Formula weight
Crystal system
Space group
a (Å)
9.9995(3)
19.2590(4)
15.3449(5)
2955.13(14)
4
8.800(3)
20.734(2)
13.110(5)
2392.0(12)
4
b (Å)
c (Å)
Volume (Å3)
Z
Dcalc (Mg/m3)
Absorption co-
efficient (mm-1)
F(000)
1.052
1.299
0.768
0.949
956
956
0.21 x 0.14 x 0.08
0.33 x 0.16 x 0.12
Crystal
(mm3)
size
2.65 - 29.24
14352
2.79 - 24.99
11882
Theta range for
data (°)
Reflections col-
lected
3605
2159
Independent
reflections
[R(int)]
98.9
2892 [0.0404]
3605/0/205
1.062
99.7
1133 [0.0862]
2159/2/126
1.562
Completeness to
theta full (%)
Observed reflec-
tions [I>2σ(I)]
Data / restraints
/ parameters
Goodness-of-fit
on F2
For predicting gas mixture selectivity Ideal Adsorbed Solu-
tion Theory (IAST) was employed.31 IAST has been widely
used and tested to predict gas mixture selectivity in MOF and
zeolites and showed good agreement with the predicted sim-
ulation selectivity and gas mixture adsorption experiments.17,
32
In this work, a dual-site and a three-site Langmuir-
0.0452
0.1382
0.1937
0.4948
R1 [I>2σ(I)]
wR2 (all data)
Freundlich (LF) equation was used to fit the adsorption iso-
therm of pure CO2 and N2 gas. The fitted parameters (SI Ta-
ble S3) were then used to predict the adsorption of a mixture.
0.587 & -0.325
2.383 & -0.753
Largest
diff.
⁄
ꢅꢇ
Adsorption selectivity was calculated by ꢀꢁꢂꢃ ꢄ ꢅ
where ꢉꢊ
ꢆ
peak and hole
(e.Å-3)
⁄
ꢈꢆ ꢈꢇ
and ꢉꢋ are the amount adsorbed at the partial pressure ꢌꢊ
and ꢌꢋ respectively.
Gas adsorption measurements. Gas sorption isotherm
measurements were performed on an ASAP 2020 Surface
Results and Discussion
Synthesis and structure of [Cu(bcppm)H2O]·xS
Area
and
Pore
Size
Analyzer.
A
sample
of
[Cu(bcppm)H2O]·xS was soaked and washed at least 8 times
with acetone during the course of 2 hours to remove the non-
coordinated solvent molecules. The mixture was transferred
into an ASAP 2020 analysis tube and the solvent was carefully
decanted. After the removal of the residual acetone by drying
H2bcppm was synthesized in one step (69% yield) from
commercially available 4-carboxyphenylboronic acid and
readily synthesized bis(4-iodo-1H-pyrazol-1-yl)methane.21
Reaction of H2bcppm with Cu(NO3)2 under solvothermal
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conditions gave [Cu(bcppm)H2O]·xS after 16 h. It was essen-
During activation of [Cu(bcppm)H2O] a distinct color
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tial to include 0.25 mL of H2O per 1 mL of DMF to the solvent
mixture to obtain the desired material. Although
[Cu(bcppm)H2O]·xS could be prepared in DMF with added
water, addition of EtOH and HNO3 (70%, 2 drops) to the
solvothermal reaction resulted in the formation of large and
regular block-shaped blue crystals suitable for single crystal
X-ray crystallography. X-Ray crystallography revealed a 2-D
structure with a 44 net (Figure 1a) composed of chelating di-
pyrazolyl moieties that bridge to adjacent copper atoms via
the phenyl-carboxylate groups (Figure 1b). The square pyram-
idal copper(II) centre is coordinated by one chelating dipyra-
zolylmethane unit, two monodentate carboxylate donors
from two further bcppm moieties, and a single water mole-
cule.
change of the crystalline material, from blue to green, was
observed. The activated material, [Cu(bcppm)H2O]-ac, was
obtained by heating an acetone solvated sample under vacu-
um at 120°C for 3h. Upon inspection of the PXRD pattern of
[Cu(bcppm)H2O]-ac we noticed a distinct shift in the spac-
ing of the 011 and 020 planes (refer to SI and Figure 3) indicat-
ing a structural modification had occurred. Thermogravimet-
ric analysis (TGA) of [Cu(bcppm)H2O]-ac showed minimal
weight loss and indicated that the material is thermally stable
to 285°C (Figure SI 1). [Cu(bcppm)H2O]-ac could be re-
solvated but did not recover the original unit cell dimensions.
Even after heating [Cu(bcppm)H2O]-ac in methanol at 65°C
the original structure could not be regenerated (Figure SI 5),
indicating a permanent and irreversible transformation dur-
ing the post-synthesis processing.
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As shown in Figure 1b, the 4-connected 2-D layers of
[Cu(bcppm)H2O]·xS are eclipsed and give rise to regular
diamond-shaped channels that propagate along the a-axis.
The alignment of the windows in [Cu(bcppm)H2O]·xS is
driven by strong interlayer hydrogen bonding between the
hydrogen atoms of the water ligands in one layer and the
carbonyl oxygen atoms from an adjacent layer (Figure 1c and
Figure 2, left, O-H···O: dH···O = 2.00 and 2.06 Å, DO-H···O = 2.69
and 2.71 Å, angle= 162.9 and 153.1°). The structure has solvent
accessible channels along the a-axis (pore diameter of ca. 6.9
Å), yet in the b and c-axes [Cu(bcppm)H2O]·xS is essentially
close-packed (Figure 1c). Powder X-ray Diffraction (PXRD)
on a bulk sample [Cu(bcppm)H2O]·xS indicated that the
material was phase pure (Figure 3).
Figure 2. Enlargements of the hydrogen bonding in
[Cu(bcppm)H2O]·xS (between the 2-D layers) and
[Cu(bcppm)H2O]·heated (left and right respectively).
Post-synthetic processing
Figure 1. (a) A schematic representation of the synthesis of [Cu(bcppm)H2O]·xS showing a simplified representation of the
structure. (b) A view of the pores of [Cu(bcppm)H2O]·xS along the a axis, and (c) a view along the c axis.
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We further probed the post synthetic processing of
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[Cu(bcppm)H2O] by variable temperature synchrotron pow-
der X-ray diffraction experiments. These experiments were
undertaken in sealed capillaries and were used to continu-
ously monitor the structural changes of two solvated samples:
[Cu(bcppm)H2O]·xS and [Cu(bcppm)H2O]-acetone (Figure
4). Depending on the boiling point of the solvates included in
the framework, PXRD confirms that [Cu(bcppm)H2O] un-
dergoes the trellis-like structural change, namely elongation
and contraction of the b and c-axis, respectively; a gradual
contraction is observed for the as-synthesized form, while an
acetone exchanged form displays a rapid transition. The
PXRD data was indexed up to 140°C, as after this point the
crystallinity diminished and the activated 3-D form
[Cu(bcppm)H2O]-ac was obtained. At 140°C the a-axis was
reduced by only ~0.33 Ǻ indicating that the water remains
coordinated to only one copper centre and the material has
yet to undergo the 2-D-to-3-D transition. This transition
takes place above 140°C under the experimental conditions
(sealed capillary) and is accompanied by reductions of long-
range order and a further shift of the 011 and 020 miller
planes. We note that peak broadening and a decrease in ob-
served crystallinity is known for layered materials and results
from shifts in the interlayer packing6, 9 and, in this case, the 2-
D-to-3-D transformation. Finally, we observed that the im-
portant Miller indices (011 and 020) of the activated material
closely match data simulated from the single crystal structure
of [Cu(bcppm)H2O]-heated (Figure 3, Figure SI 4).
9
10
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19
Figure 3. PXRD patterns of (a) [Cu(bcppm)H2O]·xS (blue,
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simulated);
[Cu(bcppm)H2O]-heated
(b)
[Cu(bcppm)H2O]·xS
(red);
(c)
(d)
(green,
simulated);
[Cu(bcppm)H2O]-ac (yellow); and (e) [Cu(bcppm)H2O]-ac
exposed to 1 bar of CO2 (khaki).
Despite considerable effort to obtain a structure for
[Cu(bcppm)H2O]-ac, the decrease in the observed crystallini-
ty thwarted all approaches to characterise this particular
phase by X-ray diffraction methods. Further understanding of
the structural changes during post-synthetic processing that
lead to [Cu(bcppm)H2O]-ac was provided when single crys-
To probe the 'locked' nature of [Cu(bcppm)H2O]-ac we in-
vestigated whether the material responds structurally to gas
adsorption, at pressures relevant for post-combustion CO2
capture; i.e. whether adsorption of CO2 gas is accompanied
by an undesirable expansion of the structure. This would
amount to 'gate opening', thus increasing the pore diameter
and essentially reducing the selectivity of this material in a
mixed gas stream. Therefore we performed PXRD experi-
ments on [Cu(bcppm)H2O]-ac in the presence of CO2. Satis-
fyingly no change was observed in the PXRD pattern at 1 bar
of CO2 at room temperature (Figure 3e), indicating that the
material remains contracted, or in a ‘locked’ state and retains
its constricted pore diameter upon adsorption of CO2 gas.
Furthermore, the material also displays reasonable stability
to water, due to its narrow pore dimensions and relatively
hydrophobic channel structure, and can be soaked in water
for 24 hours without appreciable loss of crystallinity.
tals were obtained of
a
sample heated to 80°C
([Cu(bcppm)H2O]-heated). This crystal structure revealed
that the material has undergone a prominent trellis-like con-
traction, narrowing along the c-axis and elongation of the b-
axis (Figure 4(a) versus 4(b)), and a 2-D-to-3-D transfor-
mation. 2-D-to-3-D transformations are commonly encoun-
tered in 2-D materials as a result of changes in the coordina-
tion sphere of the metal due to desolvation, or through ligand
insertion or substitution reactions.33 In [Cu(bcppm)H2O]-
heated the a-axis has been noticeably shortened from
9.9995(3) Å in the as-synthesized material to 8.800(3) Å.
This occurs with a change in the coordination environment
of the copper(II) centre from five coordinate to a Jahn-Teller
distorted octahedral six coordinate (Figure 2, right) and, as a
consequence of a now bridging water ligand, to a 3-D struc-
ture. This form of the structure is also reinforced by noticea-
bly shorter hydrogen bonding between the bridging water
ligand and a carboxylate in the adjacent layer(O-H···O: dH···O
= 1.73 Å, DO-H···O = 2.59 Å, angle= 143.3°). These combined
structural changes explain why the expanded material cannot
be regenerated upon re-solvation and the material is ‘locked’
in a contracted state. Calculations on an energy minimized
form of [Cu(bcppm)H2O]-heated reveal the material has a
limiting pore diameter of 3.6 Å (pore size maxima of 4.5 Å),
perfect for kinetic separation of gases like CO2 and N2. It is
also reasonable to suggest that this behavior, along with sec-
ondary sphere effects (hydrogen bonding and loss of pore
solvent), account for the distinct color change of this material
during activation.
Gas adsorption
[Cu(bcppm)H2O] was shown to be essentially non porous
to N2 gas at both 77, 273 and 293 K (Figure SI 6 and 7) but
porous to CO2 at 273 and 293 K (Figure 5a). Furthermore, we
saw no appreciable uptake of CO2 at 195 K that confirms that
the limiting pore diameters of the material are very close to
the kinetic diameter of CO2 at low temperatures (a likely ex-
planation is that CO2 molecules cannot enter the pores at 195
K due to large diffusional resistances, but at 273 and 293 K
the additional thermal energy allows the molecules to over-
come these resistances).32, 34 A BET surface area was calculat-
ed using the adsorption isotherm for CO2 at 273 K using crite-
ria detailed elsewhere.35 A surface area of 155 m2/g was ob-
tained and this compares favorably with the geometric sur-
face area calculated for [Cu(bcppm)H2O]-heated of 175 m2/g.
A pore size distribution obtained from the 273 K CO2 iso-
therm (Figure SI 9) provides a limiting pore size of 3.6 Å that
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Page 6 of 10
matches that calculated from the single crystal structure of
[Cu(bcppm)H2O]-heated.
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Figure 4. Representations of the crystal structures of (a) [Cu(bcppm)H2O]·xS and (b) [Cu(bcppm)H2O]-heated showing the
solvent accessible pore surface with a probe of diameter 3.30 Å. (c) Temperature dependant changes of the unit cell axes deter-
mined from indexing the PXRD data, open circles: [Cu(bcppm)H2O]·xS, filled circles: [Cu(bcppm)H2O]-acetone. Open and filled
triangles represent the unit cell dimensions from the crystals structures of [Cu(bcppm)H2O]·xS and [Cu(bcppm)H2O]-heated
respectively. (d) Temperature dependant changes of the unit cell volume. Open black circles: [Cu(bcppm)H2O]·xS, red circles:
[Cu(bcppm)H2O]-acetone. Black triangle: cell volume from the crystal structure of [Cu(bcppm)H2O]·xS, red triangle: cell volume
of [Cu(bcppm)H2O]-heated.
Figure 5a shows that the uptake of N2 at 293 K reaches a
maximum of only 0.09 mmol/g at 1200 mbar. In contrast,
CO2 gas adsorption isotherms at 273 K and 293 K clearly
show that the pore structure of [Cu(bcppm)H2O]-ac is
readily accessible to the smaller adsorbate at these tempera-
tures. The total CO2 uptakes at 293 and 273 K are 1.70 and
1.85 mmol/g, respectively. Inspection of the CO2 isotherms
(Figure 5a) shows steep uptake in the low pressure region
with an initial calculated enthalpy of adsorption for CO2 of
29 kJ/mol. The moderate heat of adsorption (Table 2) is
consistent with an absence of strong adsorption sites and
can be rationalized by a constricted pore space facilitating
interactions with the pore walls. NVT simulated annealing
suggests that CO2 weakly interacts with multiple oxygen
and hydrogen atoms that line the pore walls (Figure 5b).
The predicted heat of adsorption from NVT simulation is
around 29.5 kJ/mol, which is in excellent agreement with
the experimental value. Similar multi-point CO2-framework
interactions were recently observed in a MOF material
where the pores acted as single molecule traps.18 The ob-
served hysteresis in the CO2 isotherms can be attributed to
the small pore windows in the MOF. The relatively high
CO2 and marginal N2 uptake at ambient temperature
prompted
us
to
investigate
the
capacity
of
[Cu(bcppm)H2O] to selectively adsorb CO2 over N2. From
the single component isotherms the wt% of CO2 (0.15 atm)
and N2 (0.85 atm) was found to be 4.64% and 0.22%, respec-
tively. The potential for a material to perform separations
from a mixed gas stream can be estimated using IAST. In
terms of prediction, IAST gives the closest approximation of
binary gas selective adsorption performance from single
component isotherms.17, 32 IAST calculations performed on
[Cu(bcppm)H2O] yield a selectivity for CO2/N2 (15/85 wt%
mixture) of 590 at 293 K and 1 atm. Table 2 shows that
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Journal of the American Chemical Society
[Cu(bcppm)H2O] outperforms all but one state-of-the art
As outlined elsewhere,16 for MOFs to be economically vi-
1
2
3
4
5
6
7
8
MOF and zeolite materials. This is remarkable given that
the material lacks both unsaturated metal centers and
amine groups. However, we note that the total CO2 uptake
capacity of [Cu(bcppm)H2O] is moderate compared to high
surface area MOFs such as MOF-74. During the course of
this work a material, SIFSIX-3-Zn, that also displays excep-
tional CO2/N2 selectivity was reported. SIFSIX-3-Zn has
more regular square-shaped channels with dimensions of
3.84 Å and lined with Lewis basic groups on the SiF6 anions
that notably enhance the uptake of CO2 into the material,
giving a Qst of 45 kJmol-1 at low loading.
able for CO2 capture both the selectivity, capacity and en-
thalpy of adsorption need to be carefully considered. This
has led to the employment of various strategies to increase
the affinity of a material for CO2, including Lewis bases,14, 15
13
interpenetration3b and unsaturated metal sites.12, Lewis
bases are known to imbue materials with exceptional selec-
tivity, however this is commonly associated with a high
enthalpy of adsorption.14, 15 Coordinatively unsaturated met-
al sites are another strategy for enhancing CO2 capture that
has been studied and has resulted in excellent selectivities
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13
and CO2 uptakes.12, A limitation of this strategy is that
other polar molecules, such as H2O, can competitively bind
to these sites and potentially mitigate their influence.37 Fur-
ther, ‘soft’ porous materials that display gated adsorption
behavior with increased CO2 pressures have also been in-
vestigated,7 but additional studies are required to assess the
capacity of these materials to separate CO2 in mixed gas
streams.
Recent work has examined the mechanism of CO2 cap-
ture in MOFs where limiting the degrees of freedom in a
flexible structure has led to increased selectivity for CO2 by
restricting the limiting pore size of the MOF to the ultra-
microporous region.17, 18, 32 Due to the larger kinetic diame-
ter of N2 with respect to CO2 excellent selectivity can be
achieved at relatively moderate adsorption enthalpies. Zhou
and co-workers described a MOF material with a dynamic
structure that acts as a single molecule trap for CO2.18 Simi-
larly, investigations reticulating the chemistry of SIFSIX
materials has discovered a framework displaying unprece-
dented levels of selectivity for CO2 over N2.17a In this pre-
sent work we have used post-synthesis processing of a 2-D
layered material with a dynamic framework to generate a
rigid, contracted framework with constricted pores pos-
sessing diameters of 3.6 Å. Controlled heating facilitates a
rearrangement of the co-ordination sphere and changes in
framework connectivity to allow a constricted pore envi-
ronment to be generated. While in [Cu(bcppm)H2O] the
capacity is somewhat lower than some benchmark materi-
als, the capacity is dictated by need to maintain limiting
pore dimensions and is in keeping with other high perform-
ing MOF materials that utilize this approach.17, 18 Thus, the
principle of pore size constriction through
a post-
Figure 5. (a) CO2 and N2 isotherms at 293 K. (b) A view
showing the location of CO2 in [Cu(bcppm)H2O] identified
from NVT simulated annealing. Selected distances between
the CO2 molecule and the framework atoms are shown in
angstroms.
processing mechanism demonstrates the potential to pro-
vide MOF materials with desirable characteristics for CO2
capture.
Table 2. Comparison of IAST calculated adsorption capacity and selectivity for 15/85 CO2/N2 mixture in different porous materi-
als.
c
Materials
CO2 loading
(mmol/g)b
1.04a
N2 loading
(mmol/g)b
0.010a
Sads
Initial enthalpy of ad-
Reference
sorption Qst (kJ/mol)
[Cu(bcppm)H2O]
MgMOF-74
590a
182b
146b
329b
442b
315b
29
39-47
31-37
96
52
33
This work
Ref. 12a, 12c, 12e
Ref. 36
6.20
0.193
NaX zeolite
2.72
0.106
mmen-CuBTTri
PPN-6-CH2DETA
UTSA-16
2.22
3.04
2.37
0.038
0.039
0.043
Ref. 15a
Ref. 14f
Ref. 17c
Ref. 17a
SIFSIX-3-Zn
a. Data collected at 293 K.
1818d
45
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Journal of the American Chemical Society
Page 8 of 10
b. 296 K and 1 atm. From Ref. 17c.
c. 15/85 wt% mixture at 296 K and 1 atm.
d. 10/90 wt% mixture at 298 K and 1 atm.
1
2
3
2) Li, J.-R.; Sculley, J.; Zhou, H.-C., Chem. Rev. 2012, 112, 869-
932.
4
5
Conclusion
Here
we
have
reported
the
synthesis
of
6
7
8
9
3) a) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C., Chem. Soc. Rev.,
2009, 38, 1477-1504. b) D'Alessandro, D. M.; Smit, B.; Long, J.
R., Angew. Chem. Int. Ed., 2010, 49, 6058-6082. c) Li, J.-R.;
Ma, Y.; McCarthy, M. C.; Sculley, J.; Yu, J.; Jeong, H.-K.;
Balbuena, P. B.; Zhou, H.-C., Coord. Chem. Rev. 2011, 255,
1791-1823. d) Sumida, K.; Rogow, D. L.; Mason, J. A.;
McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long,
J. R., Chem. Rev. 2012, 112, 724-781.
[Cu(bcppm)H2O]·xS from a link capable of restricted struc-
tural flexibility and copper(II) nitrate. The ceramic-like post-
synthesis processing that leads to the exceptional selectivity
of CO2 over N2 for [Cu(bcppm)H2O] originates from a trellis-
like movement of the 2-D structure and a 2-D-to-3-D trans-
formation which results in an activated material with a rigid,
‘locked’ conformation. The rigid activated form of
[Cu(bcppm)H2O] has a limiting pore size of 3.6 Å that is
perfectly suited for CO2/N2 separations and results in an IAST
selectivity that places this compound in the top echelon of
materials for carbon capture. Due to a low enthalpy of ad-
sorption for CO2 the material is also likely to have a notably
low energy penalty for regeneration. Our current investiga-
tions are centered on further tuning the structure metrics of
this material and exploiting this link design in the synthesis
of other MOF materials that display dynamic structural
changes on post-processing. The outstanding features of
[Cu(bcppm)H2O] have led us to commence investigations
regarding the inclusion of this material into mixed-matrix
membranes.
10
11
12
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4) a) Corma, A.; Garcia, H.; Llabres, F. X.; Xamena, I.; Chem.
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M.; O'Keeffe, M.; Yaghi, O. M., Acc. Chem. Res., 2001, 34, 319-
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K.; Eddaoudi, M.; Kim, J., Nature, 2003, 423, 705-714. c) Férey,
G., Chem. Soc. Rev., 2008, 37, 191-214. d) Zhou, H.-C.; Long, J.
R.; Yaghi, O. M., Chem. Rev., 2012, 112, 673-674. e) Cook, T. R.;
Zheng, Y.-R.; Stang, P. J., Chem. Rev., 2013, 113, 734-777.
ASSOCIATED CONTENT
6) a) Kitagawa, S.; Kitaura, R.; Noro, S., Angew. Chem. Int.
Ed., 2004, 43, 2334-2375. b) Kitagawa, S.; Uemura, K., J. Solid
State Chem., 2005, 178, 2420-2429.
Thermogravimetric analysis, infrared spectra, further details
of the PXRD experiments, gas adsorption data, and details of
the IAST calculations for [Cu(bcppm)H2O], and single crys-
tal data for [Cu(bcppm)H2O]-pdes. This material is available
free of charge via the Internet at http://pubs.acs.org
7) a) Suh, M. P.; Cheon, Y. E.; Lee, E. Y., Coord. Chem. Rev.,
2008, 252, 1007-1026. b) Horike, S.; Shimomura, S.; Kitagawa,
Nat. Chem., 2009, 1, 695-704. c) Férey, G.; Serre, C., Chem.
Soc. Rev., 2009, 38, 1380-1399. d) Keene, T. D.; Rankine, D.;
Evans, J. D.; Southon, P. D.; Kepert, C. J.; Aitken, J. B.; Sumby,
C. J.; Doonan, C. J., Dalton Trans., 2013, 42, 7871-7879. e)
Lyndon, R.; Konstas, K.; Ladewig, B. P.; Southon, P. D.;
Kepert, P. C. J.; Hill, M. R.,. Angew. Chem. Int. Ed. 2013, 52,
3695-3698. f) Bloch, W. M.; Sumby, C. J., Chem. Commun.,
2012, 48, 2534-2536.
AUTHOR INFORMATION
Corresponding Authors
* School of Chemistry and Physics, The University of Ade-
laide, SA 5005, Australia. CJD: Phone +61 8 8313 5770. Fax. +61
8 8313 4358. Email: christian.doonan@adelaide.edu.au; CJS:
Phone +61 8 8313 7406. Fax. +61 8 8313 4358. Email: christo-
pher.sumby@adelaide.edu.au
Author Contributions
8) a) Jia, Q. X.; Wang, Y.-Q., Yue, Q.; Wang, Q.-L.; Gao, E.-Q.,
Chem. Commun., 2008, 4894-4896. b) Kitagawa, S.; Uemura,
K., Chem. Soc. Rev., 2005, 34, 109-119. c) Serre, C.; Mellot-
Draznieks, C.; Surblé, S.; Audebrand, N.; Filinchuk, Y.; Férey,
G., Science, 2007, 315, 1828-1831. d) Maji, T. K.; Uemura, K.;
Chang, H.-C.; Matsuda, R.; Kitagawa, S., Angew. Chem. Int.
Ed., 2004, 43, 3269-3272.
The manuscript was written through contributions of all au-
thors.
Funding Sources
ARC FT0991910 and FT100100400
ACKNOWLEDGMENT
CJS and CJD gratefully acknowledge the Australian Research
Council for funding (FT0991910 and FT100100400). This re-
search was supported by the Science and Industry Endow-
ment Fund. Aspects of this research were undertaken on the
Powder Diffraction beamline at the Australian Synchrotron,
Victoria, Australia.
9) a) Uemura, K.; Kitagawa, S.; Fukui, K.; Saito, K., J. Am.
Chem. Soc., 2004, 126, 3817-3828. b) Uemura, K.; Kitagawa, S.;
Kondo, M.; Fukui, K.; Kitaura, R.; Chang, H.-C.; Mizutani, T.,
Chem. Eur. J., 2002, 8, 3586-3600.
10) a) Biradha, K.; Fujita, M., Angew. Chem. Int. Ed., 2002, 41,
3392-3395. b) Lim, K. S.; Ryu, D. W.; Lee, W. R.; Koh, E. K.;
Kim, H. C.; Hong, C. S., Chem. Eur. J., 2012, 18, 11541-11544. c)
Biradha, K.; Hongo, Y.; Fujita, M., Angew. Chem. Int. Ed.,
2002, 41, 3395-3398. c) Nakagawa, K.; Tanaka, D.; Horike, S.;
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