Structuralization of Ca2+-Based Metal-Organic Frameworks Prepared via Coordination Replication of Calcium Carbonate

Article abstract of DOI:10.1021/acs.inorgchem.6b00397

The emergence of metal-organic frameworks (MOFs) as potential candidates to supplant existing adsorbent types in real-world applications has led to an explosive growth in the number of compounds available to researchers, as well as in the diversity of the metal salts and organic linkers from which they are derived. In this context, the use of carbonate-based precursors as metal sources is of interest due to their abundance in mineral deposits and their reaction chemistry with acids, resulting in just water and carbon dioxide as side products. Here, we have explored the use of calcium carbonate as a metal source and demonstrate its versatility as a precursor to several known frameworks, as well as a new flexible compound based on the 2,5-dihydroxybenzoquinone (H₂dhbq) linker, Ca(dhbq)(H₂O)₂. Furthermore, inspired by the ubiquity and unique structures of biomineralized forms of calcium carbonate, we also present examples of the preparation of superstructures of Ca-based MOFs via the coordination replication technique. In all, the results confirm the suitability of carbonate-based metal sources for the preparation of MOFs and further expand upon the growing scope of coordination replication as a convenient strategy for the preparation of structuralized materials.

Full text of DOI:10.1021/acs.inorgchem.6b00397

Article
pubs.acs.org/IC
2+
Structuralization of Ca -Based Metal−Organic Frameworks Prepared
via Coordination Replication of Calcium Carbonate
†
,‡
†,§
,†
,†
Kenji Sumida, Ming Hu, Shuhei Furukawa,* and Susumu Kitagawa*
†
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Centre for Advanced Nanomaterials, School of Physical Sciences, The University of Adelaide, Adelaide, South Australia 5005,
‡
Australia
*
S Supporting Information
ABSTRACT: The emergence of metal−organic frameworks
(
MOFs) as potential candidates to supplant existing adsorbent
types in real-world applications has led to an explosive growth
in the number of compounds available to researchers, as well
as in the diversity of the metal salts and organic linkers from
which they are derived. In this context, the use of carbonate-
based precursors as metal sources is of interest due to their
abundance in mineral deposits and their reaction chemistry
with acids, resulting in just water and carbon dioxide as side
products. Here, we have explored the use of calcium carbonate
as a metal source and demonstrate its versatility as a precursor to several known frameworks, as well as a new flexible compound
based on the 2,5-dihydroxybenzoquinone (H dhbq) linker, Ca(dhbq)(H O) . Furthermore, inspired by the ubiquity and unique
2
2
2
structures of biomineralized forms of calcium carbonate, we also present examples of the preparation of superstructures of Ca-
based MOFs via the coordination replication technique. In all, the results confirm the suitability of carbonate-based metal sources
for the preparation of MOFs and further expand upon the growing scope of coordination replication as a convenient strategy for
the preparation of structuralized materials.
4
INTRODUCTION
hydroxide have previously been employed in the generation
of monolithic structures of microporous frameworks such as
■
(
1
The design and synthesis of metal−organic frameworks
MOFs) is an area that has experienced tremendous growth
over the past decade, spurred in part by their potential utility in
3−
3g
Cu (btc) (HKUST-1; btc = 1,3,5-benzenetricarboxylate),
3
2
2−
Cu (bdc) (MeOH) (bdc = 1,4-benzenedicarboxylate), and
2
2
2
3i
Cu (bdc) (bpy) (bpy = 4,4′-bipyridine). Note that an
2
2
applications including gas storage, molecular separations2,
additional advantage of such inorganic precursors is the benign
reaction side products (e.g., water) that are generated in
comparison to their halide and nitrate counterparts.
From this perspective, the use of carbonate-based metal
precursors would carry similar advantages in processability and
clean reactivity, although examples of their use have remained
scarce in the literature. Under the acidic conditions employed
for most MOF syntheses, it is anticipated that carbonates would
readily participate in the framework formation, with the
concomitant release of carbon dioxide and water as side
products.
heterogeneous catalysis, drug delivery, and functional devices.
As opportunities for their industrial application emerge,
optimization of their method of preparation is paramount in
order to synthesize the materials in a form that not only
maximizes their performance but also considers other aspects,
including cost and environmental sustainability. With regard to
the synthesis of the materials, the use of inexpensive and
abundant precursors is certainly attractive, particularly if they
are readily available within the framework of the current
industrial infrastructure.
The conventional modular synthesis of MOF-based materials
usually entails the complete dissolution of a metal source
MCO + H L → [ML] + CO + H O
3
2
n
2
2
(generally a metal chloride or nitrate salt) and organic ligand
Among the metal carbonates, calcium carbonate is of particular
interest owing to its occurrence in mineral deposits and wide
industrial use. Furthermore, it is one of the most abundant
biominerals, forming the skeletal component of many
organisms, including shellfish and corals. These materials
adopt highly complex, hierarchical structures that present a
within a polar organic solvent, followed by solvothermal
treatment to afford the framework in the form of crystalline
powders or single crystals. Recently, a variety of other inorganic
metal precursors, including oxides and hydroxides, have
received increased attention due to their enhanced process-
ability in the macro- and mesoscopic length scales and ability to
act as sacrificial templates for direct conversion into higher-
order architectures of MOFs via the coordination replication
5
Received: February 17, 2016
3
method. For example, structuralized forms of copper
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00397
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
difficult challenge to mimic using conventional synthetic
platforms. Herein, we explore the potential use of calcium
carbonate as a precursor to MOF materials and look toward
harnessing the unique structuralization of biomineralized forms
of calcium carbonate as templates for the preparation of MOF
superstructures. We first confirm its utility as a precursor by
synthesizing bulk forms of known MOFs, followed by a new
flexible MOF based on the 2,5-dihydroxybenzoquinone
samples were washed by immersion in distilled water and dried in the
air in an oven set to a temperature of 100 °C.
RESULTS AND DISCUSSION
■
2
+
Preparation of Ca -Based Frameworks. In order to
investigate the appropriate reactivity of calcium carbonate as a
precursor for the assembly of metal−organic frameworks,
2+
several ditopic linkers known to form Ca -based frameworks,
(
H dhbq) linker. Then, we demonstrate the conversion of
namely 1,4-benzenedicarboxylate (H
benzenedicarboxylate (H dobdc), and squaric acid (H
2
2
bdc), 2,5-dihydroxy-1,4-
2
biomineralized calcium carbonate into structuralized MOF
architectures via the coordination replication method.
4
sq),
were first tested. Here, the reaction conditions were screened
using a microwave reactor, varying the solvent composition,
reaction temperature, and reaction time. Despite its low
solubility in all of the solvent combinations tested, calcium
carbonate was found to form highly crystalline products,
leading to the successful isolation of the Ca(bdc)(H O)
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise stated, all reagents
were obtained from commercial vendors and used without purification.
All manipulations were carried out in the air, except for the handling of
the activated forms of the frameworks in a nitrogen-filled glovebox.
Solvothermal syntheses were carried out in a Yamato DKN302
constant-temperature oven within glass vials sealed with Teflon-lined
lids. Microwave syntheses were carried out on a Biotage Initiator
microwave reactor equipped with an infrared temperature sensor.
Infrared spectra were collected in attenuated total reflectance (ATR)
mode using a JASCO FT/IR-6100 spectrometer equipped with a TGS
detector.
2
3
6
(
Figure 1A and Figure S1 in the Supporting Information),
Field-Emission Scanning Electron Microscopy (FE-SEM). All
observations were performed using a JEOL JSM-7001F4 microscope
under an emission voltage of 15 kV. Samples were prepared either by
deposition on glass slides by the drop-casting method or by adhesion
on double-sided carbon tape, followed by coating with osmium
nanoparticles to a thickness of 3 nm.
In Situ Powder X-ray Diffraction. All data were collected on a
Rigaku Smartlab X-ray diffractometer equipped with a Cu Kα source.
The instrument was fitted with a custom-made variable-temperature
sample stage with gas inlet and outlet ports for the flow of controlled
gas compositions. The inlet side was connected to a BELFlow
vaporizer apparatus (BEL Japan, Inc.) using a He carrier gas (flow rate
3
1
00 cm /min) and distilled water as the vapor source. In a typical
Figure 1. Portions of the single-crystal structures of (A) Ca(bdc)-
experiment, a solvated sample was first activated on the sample stage
under a dry He flow, and the humidity of the flow gas was then
sequentially increased in a stepwise fashion via the BELFlow-1 version
6
(
H O) as viewed along the crystallographic c axis, (B) Ca(sq)(H O)
2 3 2
7
as viewed along the crystallographic c axis, and (C) Ca(H dobdc)
2
8
(DMF) as viewed along the crystallographic a axis. Green, gray, blue,
1
.00 software package.
and red spheres represent Ca, C, N, and O atoms, respectively, while
H atoms and extraframework water molecules have been omitted in
panel B for clarity.
[
Ca(dhbq)(H O) ]·H O (1·H O). In a 20 mL glass vial equipped
2 2 2 2
with a magnetic stirrer bar, calcium carbonate (150 mg, 1.50 mmol)
was added to a vigorously stirred suspension of H dhbq (210 mg, 1.50
2
mmol) in water (10 mL). The vial was capped, and the mixture was
stirred overnight at room temperature. After this time, a crimson
suspension was obtained, which was filtered off and washed with water
Ca(sq)(H O) (Figure 1B and Figure S2 in the Supporting
2
7
Information), and Ca(H dobdc)(DMF) (Figure 1C and
2
(
3 × 20 mL) followed by dichloromethane (3 × 20 mL). The resulting
8
Figure S3 in the Supporting Information) frameworks with
powder was dried in vacuo at 80 °C for a period of 12 h to afford 1·
H O as a dark red powder (275 mg, 69%). Anal. Calcd for C H O Ca·
complete consumption of the metal source.
2
6
6
6
0
.1H O: C, 33.36; H, 2.89. Found: C, 33.22; H, 2.74. IR (neat): 3270
Synthesis and Characterization of Ca(dhbq)(H O) .
2
2
2
(
8
m br), 1668 (w), 1641 (w), 1594 (w), 1502 (s), 1387 (s), 1259 (s),
25 (m) cm . Single crystals of the compound could be prepared by
The successful use of calcium carbonate in the synthesis of the
frameworks noted above prompted efforts toward the synthesis
of frameworks possessing greater porosity. Here, the 2,5-
−1
heating the same aqueous reaction mixture to 60 °C under static
conditions for a period of 3 days. Crystallographic data for 1·H O have
2
dihydroxybenzoquinone linker (H dhbq; Figure 2) was
2
been deposited with the Cambridge Crystallographic Data Centre
targeted due to its structural similarity with the squarate
(
CCDC) under accession code 1447832. These data can be obtained
anion employed in the synthesis of Ca(sq)(H O), while
2
free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Coordination Replication Experiments. In a typical experiment,
a biomineralized calcium carbonate sample derived from Baculogypsina
sphaerulata was first immersed in a mixed aqueous solution of sodium
hydroxide and sodium hypochlorite in order to remove trace organic
materials from the structures. After they were rinsed with distilled
water (5 × 100 mL), the samples were further immersed in distilled
water for 24 h. Then, the sample was transferred to an aqueous
solution containing an excess of H sq and heated in a microwave
Figure 2. Molecular structures of squaric acid (H sq) and 2,5-
2
2
reactor at 120 °C for 3, 6, 9, or 12 h. After the replication process, the
dihydroxybenzoquinone (H dhbq) employed in this work.
2
B
DOI: 10.1021/acs.inorgchem.6b00397
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Inorganic Chemistry
Article
2+
potentially providing a greater distance between Ca -based
chains (and consequently a greater pore wall separation). A
number of extended framework materials have previously been
2−
9
reported using the dhbq linker, although these reports have
been limited to compounds based on the transition-metal
elements.
The solvated form of Ca(dhbq)(H O) (1·H O) was
2
2
2
obtained via the addition of calcium carbonate to an aqueous
solution of H dhbq, followed by heating at 60 °C for a period
2
1
0
of 3 days. After this time, red-brown block-shaped single
crystals suitable for diffraction studies were obtained from the
walls of the reaction vessel. X-ray analysis revealed a monoclinic
crystal system (space group C2/m) consisting of an extended
network structure exhibiting narrow, one-dimensional pores
(see Figure 3A). Consistent with its diffuse, oxophilic nature,
Figure 4. Simulated powder X-ray diffraction pattern from the single-
crystal structure of solvated 1 (blue) and experimental data collected
under a dry He flow for an as-synthesized sample at 25 °C (black) and
after the sample temperature was raised to 100 °C (green), 140 °C
(
orange), and 180 °C (red).
Heating of the powder sample under a He flow to 100 °C
resulted in a slight shift of the diffraction peaks to higher angles,
which is consistent with a slight contraction of the unit cell
dimensions. Interestingly, the peak intensities dramatically
diminished at higher temperatures of 140 °C and above,
resulting in a new, high-temperature phase of poorer
crystallinity exhibiting two prominent peaks at 2θ values of
ca. 12.9 and 15.1° with relatively broad peak profiles.
Meanwhile, the thermogravimetric data (Figure S4 in the
Supporting Information) indicated a weight loss attributable to
the gradual loss of one (extraframework) water molecule per
formula unit up to 140 °C, suggesting the structural change
results as a result of the liberation of a water molecule from the
11
pores. The first prominent peak of the high-temperature
phase corresponds to a crystal lattice d spacing of 6.9 Å, which
correlates closely with the distance of 7.1 Å between adjacent
Ca chains bridged by the dhbq ligands, as calculated by an
averaging of the Ca···Ca distances. This suggested that the loss
of long-range order in the sample was due to the collapse of the
one-dimensional pores upon evacuation of the extraframework
water molecules, rather than the decomposition of the
framework via the breaking of the coordination bonds.
In order to test this hypothesis, in situ powder X-ray
diffraction experiments were performed under a He flow
containing variable water concentrations. Figure 5 shows data
stemming from an activated sample of 1 first placed under a
pure He flow at 25 °C (red), which was then subjected a gas
stream containing successively higher concentrations of water
vapor. Relatively minor changes to the diffraction pattern are
observed up to a relative pressure of 0.6, including a slight shift
of the major peaks to lower angles together with a sharpening
of the peaks, presumably due to a slightly enhanced long-range
order of the high-temperature phase. Further increases in the
water composition of the gas flow resulted in a return to the
Figure 3. (A) Portion of the single-crystal structure of Ca(dhbq)-
H O) (1), as viewed along the crystallographic b axis, and (B) a side-
on view of the one-dimensional Ca -based chains within the structure.
Green, gray, and red spheres represent Ca, C, and O atoms,
respectively, while H atoms have been omitted for clarity.
2+
2−
(
2
2
2+
2+
the individual Ca ions reside in an eight-coordinate, distorted-
square-antiprismatic ligand geometry, wherein six coordination
2−
sites are occupied by dhbq oxygen atoms, and a further two
water molecules projecting into the pores to complete the
2−
coordination sphere. A total of three dhbq half-units are
bound to each Ca2 metal center, one of which is coordinated
in a chelating fashion, while the other two are disposed such
+
2
+
that the oxygen atoms bridge adjacent Ca cations to form a
zigzag shaped one-dimensional chain (see Figure 3B).
Neighboring chains are linked via opposite ends of the
dhbq2 linker to form diamond-shaped cavities of ca. 3.7 Å
between opposing walls (based on van der Waals radii) that are
populated by hydrogen-bonded water molecules originating
from the synthesis solvent (four per unit cell).
−
original crystalline phase of 1·H O, confirming that the
2
transition between the low- and high-temperature phases is
12
The stability of 1·H O toward evacuation of both the bound
reversible. A water adsorption isotherm collected at 298 K
2
and unbound water molecules within the pores was probed
using a combination of powder X-ray diffraction studies and
thermogravimetric analysis. A comparison of the diffraction
data collected for an as-synthesized sample revealed an
excellent match with that of the simulated diffraction pattern
(Figure 6) further confirmed the affinity of the pores of
activated 1 toward water, wherein the step observed at P/P =
0
0.7 is in good agreement with the observations from the in situ
powder X-ray diffraction experiments. Note that low-pressure
adsorption data collected for N (77 K) and CO (298 K)
2
2
from the single-crystal structure of 1·H O (see Figure 4).
resulted in no uptake, suggesting that the initial population of
2
C
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Inorganic Chemistry
Article
periodically structured macroporous copper via a two-step
13
procedure. This involved the initial generation of a polymer
14
cast representing an exact negative of the template and
dissolution of the calcium carbonate under acidic conditions,
followed by electrochemical deposition of the copper metal into
the voids of the polymer scaffold. In our work, we focused on
the direct use of the biomineral template as a sacrificial material
and conversion into the MOF architectures via the coordina-
tion replication method.
The Foraminera, which are organisms that mostly reside on
or within the sediment bed of the seafloor, feature tests (shells)
constructed from calcium carbonate that in many cases can
possess tremendously elaborate structures. Here, we obtained a
15
sample of Baculogypsina sphaerulata (Figure 7A; “star sand”)
Figure 5. Evolution of the powder X-ray diffraction pattern collected
for an activated sample of 1 upon exposure to increasing levels of
humidity. The fraction associated with each plot represents the relative
pressure of water (P/P ) in the gas mixture.
0
Figure 6. H O adsorption isotherm collected at 298 K for an activated
2
sample of 1.
the pores with water molecules is required for the observed
structural transition. Further, the exposure of activated 1 to
larger coordinating solvent molecules (e.g., methanol, ethanol,
acetone, and acetonitrile) did not induce a return to the original
crystalline phase, which is a result of the small cavity size (ca.
Figure 7. Field-emission SEM images of (A) a biomineralized calcium
carbonate sample derived from Baculogypsina sphaerulata (“star
sand”) (the inset gives an enlarged view of the macroporous features
on the surface) and enlarged views of the macropores (B) before and
3
.7 Å) in comparison to the kinetic diameter of these solvent
molecules.
Coordination Replication of Biomineralized Calcium
15
Carbonate. The low solubility of calcium carbonate in water
and organic solvents and high reactivity toward the organic
linkers tested above suggested that structuralized forms of the
metal precursor would be ideal for the fabrication of
(
C) after microwave treatment for 12 h at 120 °C in an aqueous
solution containing H sq. The scale bar within the inset corresponds
to 10 μm.
2
2
+
superstructures of Ca -based frameworks. In this context,
biomineralized forms of calcium carbonate are ubiquitous in
nature, and many feature extraordinary, hierarchically porou5s
architectures that are optimized for their particular functions.
While examples of the fabrication of materials that replicate
such complex structures have remained rare, one elegant study
utilized the ordered skeletal plate of a sea urchin to template
from Hoshizuna-no-hama (“Star Sand Beach”), Iriomote Island,
Okinawa, Japan, which possesses a star-shaped shell approx-
imately 1.5 mm in size. Closer inspection of the surface of the
structure indicated the presence of higher order patterning at
the surface and the presence of a high density of macropores
approximately 1 μm in diameter (Figure 7B). Analysis of the
powder X-ray diffraction pattern of a ground form of the
D
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Inorganic Chemistry
Article
sample revealed its composition to be a moderately crystalline
form of calcite (see Figure S8 in the Supporting Information),
which corresponds to the form of calcium carbonate used
successfully in the bulk MOF syntheses.
Encouraged by the composition of the materials, residual
organic matter, such as proteins, was first removed by
dissolution under basic conditions. After washing and drying
of the samples, the sample was placed in an aqueous solution of
Crystallographic data for 1·H O (CIF)
2
AUTHOR INFORMATION
Corresponding Authors
E-mail for S.F.: shuhei.furukawa@icems.kyoto-u.ac.jp.
E-mail for S.K.: kitagawa@icems.kyoto-u.ac.jp.
Present Address
■
*
*
§
Department of Physics, East China Normal University,
Shanghai 200241, People’s Republic of China.
H sq (15 mM) heated to 120 °C for 12 h. After this time, SEM
2
revealed a significantly rougher macropore morphology and the
presence of rodlike crystallites embedded on the surface of the
material (Figure 7C). Powder X-ray diffraction data (Figure S8
in the Supporting Information) confirmed the partial
conversion of the calcium carbonate to a highly crystalline
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Ca(sq)(H O) phase, with the original calcite reflections still
K.S. is grateful to the JSPS Postdoctoral Fellowship Program for
Foreign Researchers and the Australian Research Council for
funding (DE160100306). This work was supported by a Grant-
in-Aid for Scientific Research (15H03785 (S.F.)) from the
MEXT of Japan. iCeMS is supported by the World Premier
International Research Initiative (WPI), MEXT of Japan. The
authors thank CeMI for assistance with electron microscopy,
Dr. M. Nakahama for experimental assistance, and Dr. C. J.
Sumby and Ms. N. Shimanaka for helpful discussions.
2
present following the replication process. Interestingly, longer
reaction times did not lead to full conversion but resulted in the
formation of large, rodlike single crystals of Ca(sq)(H O) up to
2
500 μm in length (Figure S9 in the Supporting Information).
The propensity for bulk crystallization to occur suggests the
relatively fast dissolution of the calcium carbonate phase
relative to the formation of the Ca(sq)(H O) phase.
2
Furthermore, a large volume increase of approximately 2.9
2
+
times (based on the density of Ca ions in single-crystal
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2
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2
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R. Cryst. Growth Des. 2011, 11, 2717−2720. (b) Darago, L. E.; Aubrey,
E
DOI: 10.1021/acs.inorgchem.6b00397
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Inorganic Chemistry
Article
M. L.; Yu, C. J.; Gonzalez, M. I.; Long, J. R. J. Am. Chem. Soc. 2015,
1
(
37, 15703−15711.
10) The reaction of other metal salts, such as CaCl ·4H O and
2
2
Ca(NO ) ·4H O, using similar reaction conditions resulted in
3
2
2
amorphous and low-crystallinity samples, and these solids were not
characterized further.
(
11) Further heating of the sample above 450 °C resulted in a large
2
−
transition whose magnitude is consistent with the loss of the dhbq
linker from the solid, resulting in the decomposition of the framework.
12) Note that the alternative rehydration pathway of dissolution and
(
recrystallization would require the presence of a solid−fluid interface,
which is not present in the case of a framework placed under a
hydrated gas stream. Furthermore, while immersion of a similarly
activated material in liquid water also immediately restored the original
1
·H O phase (see Figure S7 in the Supporting Information), this could
2
not be achieved in other solvents such as methanol. This further
suggests that the restoration of the low-temperature phase occurs due
to water migrating back into the pores.
(
13) Lai, M.; Kulak, A. N.; Law, D.; Zhang, Z.; Meldrum, F. C.; Riley,
D. J. Chem. Commun. 2007, 3547−3549.
14) Yue, W.; Park, R. J.; Kulak, A. N.; Meldrum, F. C. J. Cryst.
Growth 2006, 294, 69−77.
15) World Forminera Database: Baculogypsina sphaerulata; http://
www.marinespecies.org/foraminifera/aphia.php?p=taxdetails&id=
84702 (accessed January 30, 2016).
(
(
4
F
DOI: 10.1021/acs.inorgchem.6b00397
Inorg. Chem. XXXX, XXX, XXX−XXX