Nanoscale Synthesis of Two Porphyrin-Based MOFs with Gallium and Indium

Article abstract of DOI:10.1021/acs.inorgchem.6b00221

Two porphyrin-based metal-organic frameworks (MOFs) containing gallium or indium, [Ga₂(OH)₂(H₂TCPP)]·3DMF·3H₂O (Ga-PMOF) and [In₂(OH)₂(H₂TCPP)]·3DMF·4H₂O (In-PMOF) (H₆TCPP = 4-tetracarboxyphenylporphyrin), were discovered using high-throughput methods. The structure was refined by the Rietveld-method starting from the structure model of Al-PMOF, [Al₂(OH)₂(H₂TCPP)]. The new PMOFs exhibit BET surface areas between 1150 and 1400 m² g^-1 and are also porous toward CO₂ (Ga-PMOF, 15.2 wt %; In-PMOF, 12.9 wt %). They are thermally stable in air up to 330 °C, but show limited chemical stabilities toward acids and bases. In order to achieve size control, different synthesis routes were investigated, i.e., batch synthesis at different temperatures (yield: In-PMOF-bs-th 96%, Ga-PMOF-bs-th 87%), ultrasound-assisted synthesis (yield: In-PMOF-bs-us 85%), and continuous-flow synthesis (yield: Ga-PMOF-cf 71%). By using these different methods we could control the nucleation rate and the crystal size. The crystal sizes were found to vary about 60 to 160 nm and 70 to 130 nm for Ga- and In-PMOF, respectively, which was proven by dynamic light scattering (DLS), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements.

Full text of DOI:10.1021/acs.inorgchem.6b00221

Article
pubs.acs.org/IC
Nanoscale Synthesis of Two Porphyrin-Based MOFs with Gallium and
Indium
Timo Rhauderwiek,† Steve Waitschat,† Stefan Wuttke,‡ Helge Reinsch,§ Thomas Bein,‡
and Norbert Stock*,†
†Institute of Inorganic Chemistry, Christian-Albrechts-Universitat, Max-Eyth Straße 2, D-24118 Kiel, Germany
̈
‡Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 5-13, D-81377
Munich, Germany
§Department of Chemistry, University of Oslo, N-0371 Oslo, Norway
S
* Supporting Information
ABSTRACT: Two porphyrin-based metal−organic frameworks
(MOFs) containing gallium or indium, [Ga2(OH)2(H2TCPP)]·
3DMF·3H2O (Ga-PMOF) and [In2(OH)2(H2TCPP)]·3DMF·
4H2O (In-PMOF) (H6TCPP = 4-tetracarboxyphenylporphyrin),
were discovered using high-throughput methods. The structure was
refined by the Rietveld-method starting from the structure model of
Al-PMOF, [Al2(OH)2(H2TCPP)]. The new PMOFs exhibit BET
surface areas between 1150 and 1400 m2 g−1 and are also porous
toward CO2 (Ga-PMOF, 15.2 wt %; In-PMOF, 12.9 wt %). They
are thermally stable in air up to 330 °C, but show limited chemical
stabilities toward acids and bases. In order to achieve size control,
different synthesis routes were investigated, i.e., batch synthesis at
different temperatures (yield: In-PMOF-bs-th 96%, Ga-PMOF-bs-th
87%), ultrasound-assisted synthesis (yield: In-PMOF-bs-us 85%), and continuous-flow synthesis (yield: Ga-PMOF-cf 71%). By
using these different methods we could control the nucleation rate and the crystal size. The crystal sizes were found to vary about
60 to 160 nm and 70 to 130 nm for Ga- and In-PMOF, respectively, which was proven by dynamic light scattering (DLS),
powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)
measurements.
for the oxidation of cyclohexane (PCN-221),18 or for the
INTRODUCTION
■
coupling reaction of CO2/propylene oxide (PCN-224).21 To
In the field of crystalline porous materials, metal−organic
frameworks (MOFs) exhibit tunable pore sizes and surface
chemistry, which in turn lead to a wide range of chemical and
date only a few compounds with trivalent metal ions such as
iron (MIL-141, [NaFe(Ni-TCPP)]23 and [Fe2(Fe-TCPP)-
(C4H4N2)(OH2)] (C4H4N2 = pyrazine)24) and aluminum
physical properties.1−3 Hence MOFs are investigated in
(Al-PMOF, [Al2(H2TCPP)(OH)2]·3DMF·2H2O)25 have been
applications such as gas storage and separation, catalysis, and
described with the H6TCPP linker.26 Al-PMOF was inves-
tigated in the photocatalytic methyl viologen assisted H2-
generation on colloidal platinum.25 With the homologues
gallium and indium only In-MOFs containing porphyrin-based
linkers have been reported (MMPF-7, [In1.29O0.57(TCPP)],27
MMPF-8 [In(In-TCBPP)]·12DMF (H6TCBPP = tetrakis-4-
carboxybiphenylporphyrin)27 and UNLPF-10 [In3(TBCPPP)]
(H10TBCPPP = tetrakis-3,5-bis[4-carboxyphenyl]phenyl por-
phyrin)28).
drug delivery.2,4−7 An important challenge for the use of MOFs
in drug delivery is to construct highly porous, nanoscaled,
biocompatible materials consisting of nontoxic metals and
linkers. Porphyrin-based linker molecules in combination with
biocompatible metals like Ga and In, which have been shown to
be toxic only in very high concentrations, could lead to MOFs
suitable for applications in drug delivery.4,8−17
4-Tetracarboxyphenylporphyrin (H6TCPP) is the most
intensively investigated linker for the synthesis of porphyrin-
based MOFs, and nearly 30 compounds containing this linker
are known.12 In most cases, these MOFs are based on di- and
tetravalent metal ions like Zn2+ and Zr4+.12 For zirconium the
compounds PCN-221,18 MOF-545,19 MOF-525,19 PCN-223,20
PCN-224,21 and PCN-22522 have been reported. These
porphyrin-based Zr-MOFs exhibit high specific surface areas
and have been tested, for example, as heterogeneous catalysts
For the possible application of porphyrin-based MOFs, for
example in catalysis or drug delivery, the control of particle size
can be very important.3,10,29 Various methods have been
reported for the synthesis of nanoscale MOFs, such as batch
Received: February 3, 2016
© XXXX American Chemical Society
A
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synthesis with thermal heating (bs-th),30−33 ultrasound-assisted
batch synthesis (bs-us),34 and continuous-flow (cf) syn-
thesis.35−37 The latter has also been successfully employed
for the synthesis of CdS nanoparticles.38 In addition, this
synthesis approach has been used for the continuous
production of MOFs such as UiO-66,36 Al-MIL-53,37 or
CAU-1335 with high space−time yields of several hundred
kilograms per cubic meter and day. Although synthetic
strategies providing control of the particle size have been
carried out for tetrapyridylporphyrin-based MOFs like Zn- and
Cu-TPyP,39−41 such studies have not been reported so far for
tetracarboxyphenylporphyrin-based MOFs.
Herein, we present the synthesis and detailed character-
ization of two porphyrin-based MOFs containing Ga and In
(Ga-PMOF and In-PMOF). Three synthesis methods, batch
synthesis with thermal heating (bs-th), ultrasound-assisted
batch synthesis (bs-us), and continuous-flow (cf) synthesis,
were employed to study their influence on the purity and
particle size of the MOFs.
NVision40 microscope was used. Secondary electron images were
acquired using the inlense detector and a low acceleration voltage of
2.5 keV for all samples except for sample Ga-PMOF-cf (120 °C),
which was imaged at an acceleration voltage of 5 keV. TEM studies
were carried out with the In-PMOF-bs-us nanoparticles using a Titan-
Themis 60-300 TEM (FEI Company) operating at 300 kV with a
high-angle annular dark field detector. A droplet of the diluted
nanoparticle dispersion in absolute ethanol was dried on a carbon-
coated copper grid.
Preparation. Three different synthesis methods, i.e., batch
synthesis with thermal heating in HT Teflon inserts = bs-th,
ultrasound-assisted batch synthesis = bs-us, and continuous-flow
synthesis = cf, for the formation of Ga- and In-PMOF were
investigated (Figure 1).
EXPERIMENTAL SECTION
■
Materials. Ga(NO3)3·H2O (99.99%, ABCR), In(NO3)3·H2O
(99.99%, ABCR), DMF (99%, Grussing), acetone (99%, Walther),
̈
Figure 1. Schematic presentation of the different synthesis methods
used in this study. The setups for the batch synthesis with thermal
heating (bs-th), the ultrasound-assisted batch synthesis (bs-us), and
the continuous-flow (cf) synthesis are shown in gray, blue, and red,
respectively. Values in parentheses show the particle size measured by
DLS and the yield of the individual reactions.
and EtOH (99%, Walther) were used without further purification. The
metal salts were dissolved in distilled H2O to achieve different
concentrations. The linker 4,4′,4″,4‴-(5,10,15,20-porphyrintetrayl)-
tetrabenzoic acid (H6TCPP) was synthesized according to literature
procedures42−44 starting with 4-formylbenzoic acid (98%, ABCR) and
pyrrole (98%, ABCR) in propionic acid (99%, Grussing). Details are
̈
given in the Supporting Information.
Characterization. For the optimization of synthesis conditions,
our high-throughput (HT) methodology employing a 24-high-
throughput reactor system was used.33,45,46 The high-throughput
PXRD measurements were carried out with a STOE HT
diffractometer equipped with an xy-stage and an IPDS system (Cu
Kα1 radiation) in transmission geometry. High resolution PXRD
patterns were measured on a STOE Stadi-P powder diffractometer
equipped with a Mythen detector (Cu Kα1 radiation). For the
ultrasound-assisted syntheses the ultrasonication bath (Sonorex RK
The synthesis of Ga- and In-PMOF-bs-th was studied using high-
throughput methods (Vmax = 2.0 mL per Teflon insert, details see
Figure S2). Different reaction temperatures between 80 and 160 °C in
20 °C steps, different molar ratios of reactants, additives like benzoic,
formic, and acetic acid, and different volume ratios of H2O:DMF
between 1:6 and 1:38 were tested. The results of the HT experiments
leading to highly crystalline Ga- and In-PMOF-bs-th are shown in
Figure S2, and the optimized synthesis conditions are described in the
following paragraphs.
100, 40 kHz, 80/160 W, Bandelin) and glass vials from Duran (Vmax
=
Optimized batch reactor synthesis of Ga-PMOF-bs-th: H6TCPP
(15 mg, 1.9 × 10−2 mmol), DMF (724 μL), and 1 M aqueous
Ga(NO3)3·H2O solution (37.8 μL, 3.8 × 10−2 mmol) were added to
the 2.0 mL reactor. The reactor was heated for 48 h at 120 °C and
cooled down to rt in 6 h. The resulting product was washed two times
each with DMF and acetone. A yield of 220 mg, 87% (based on
H6TCPP) was obtained for Ga-PMOF-bs-th, [Ga2(H2TCPP)(OH)2]·
3DMF·3H2O.
Optimized batch reactor synthesis of In-PMOF-bs-th: H6TCPP (15
mg, 1.9 × 10−2 mmol), DMF (724 μL), and 0.5 M aqueous In(NO3)3·
H2O solution (75.6 μL, 3.8 × 10−2 mmol) were added to the 2.0 mL
reactor. The reactor was heated for 48 h at 120 °C and cooled down to
rt in 6 h. The resulting product was washed two times each with DMF
and acetone. A yield of 292 mg, 96% (based on H6TCPP) was
obtained for In-PMOF-bs-th [In2(H2TCPP)(OH)2]·3DMF·4H2O.
Both MOF syntheses can be scaled up in 30 mL Teflon reactors by
using the 12-fold amount of all reactants, the same molar ratio of metal
to linker (2:1), the same temperature−time program, and the same
washing procedures.
7.0 mL, Pyrex) were used. IR spectra were recorded using a Bruker
ALPHA-FT-IR A220/D-01 spectrometer equipped with an ATR unit.
NMR data were measured on a Bruker DRX 500 spectrometer. For
the quantitative analysis of occluded guest molecules, 1H NMR
spectroscopy was employed. The samples were dissolved in a mixture
of 10% deuterochloric acid (DCl) in D2O and deuterated dimethyl
sulfoxide (d6-DMSO) (molar ratio = 1:7). Sorption experiments were
performed with a BEL Japan Inc. BELSORP-max. Before sorption
measurements all samples were acitivated at 140 °C under vacuum
(10−2 kPa) for 16 h. Thermogravimetric measurements were
performed on a NETZSCH STA 409 CD analyzer under a flow of
air (75 mL min−1) with a heating rate of 4 °C min−1 over a
temperature range between 25 and 700 °C in Al2O3 crucibles. The
data were corrected for buoyancy and current effects. Dynamic light
scattering measurements were performed on a Delsa Nano C particle
analyzer from Beckman Coulter with ethanol as solvent. Prior to the
measurements, the samples which were stored in ethanol were
redispersed. In the case of the reaction products Ga- and In-PMOF-bs-
th as well as Ga-PMOF-cf the redispersion was accomplished by
ultrasonication for 30 min. In-PMOF-bs-us formed a stable dispersion
after washing with ethanol. For the measurement, 20 μL of the
respective dispersion were added to a cuvette containing 4 mL of
ethanol and measured subsequently. The particle sizes of all obtained
samples were also determined from PXRD data using the program
suite TOPAS academics V4.1. A LaB6 standard was used to determine
the instrumental line broadening, and the particle size was calculated
employing the Scherrer equation.47,48 For SEM imaging a Zeiss
Elemental analysis for Ga-PMOF-bs-th and In-PMOF-bs-th:
[Ga2(H2TCPP)(OH)2]·3DMF·3H2O
calcd (%) C 55.5, H 4.5, N 7.9; found (%) C 55.1, H 4.4, N 8.0
[In2(H2TCPP)(OH)2]·3DMF·4H2O
calcd (%) C 51.0, H 4.3, N 7.3; found (%) C 51.2, H 4.3, N 7.8
B
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Ultrasound-assisted synthesis could only be achieved for the In-
PMOF. In-PMOF-bs-us was synthesized in glass-vials (Vmax = 7.0 mL)
by mixing H6TCPP (51 mg, 6.5 × 10−2 mmol), DMF (4 mL), and 0.5
M aqueous In(NO3)3·H2O solution (257.0 μL, 12.9 × 10−2 mmol).
The glass vial was ultrasonicated in a water bath for 7 h. The final
temperature of the water bath was 50 °C. The resulting product was
centrifuged for 30 min at 20000 rpm and washed three times with
ethanol by subsequent centrifugation, each time for 30 min at 20000
rpm. A yield of 68 mg, 85% (based on H6TCPP) was obtained for In-
PMOF-bs-us, [In2(H2TCPP)(OH)2]·3DMF·4H2O.
good agreement with the cell parameters of Al-PMOF,
corresponding to the different metal ion.25 Using the structure
of Al-PMOF25 with the respective metal ion gallium or indium
as starting model, their crystal structures were successfully
refined by Rietveld methods with TOPAS academic V4.1.48
The crystallographic data and the results of the Rietveld
refinement are shown in Table 1 and Figures 2 and 3. Pawley
Table 1. Crystallographic Data of Al-, Ga-, and In-PMOF
Continuous-flow synthesis of Ga-PMOF-cf and In-PMOF-cf was
carried out using a two-syringe-pump flow reactor setup as previously
described.35 The MOFs were obtained using concentrations of 0.1 mol
L−1 of Ga(NO3)3 and H6TCPP in a mixture of 94 vol % DMF and 6
vol % water. For the synthesis of In-PMOF-cf, corresponding solutions
with a concentration of 0.05 mol L−1 were used. The solutions were
injected into a flow reactor (V = 15 mL) with flow rates of 0.167 mL
min−1 for metal and 0.084 mL min−1 for the linker solution
respectively, giving a total reaction time of 60 min. The different
flow rates for metal and linker solution led to a molar metal-to-linker
ratio of 2 to 1. During the reaction the flow reactor was kept at a
temperature of 100 °C. In a second experiment a flow rate of 0.125 mL
min−1 for both starting materials was used (molar ratio metal-to-linker
1:1) and the reaction temperature was raised to 120 °C. The products
were collected and isolated by centrifugation for 15 min at 15000 rpm
and washed three times with ethanol by subsequent centrifugation,
each time for 15 min at 15000 rpm. The formation of Ga-PMOF-cf at
100 °C yielded 397 mg, 71% (based on H6TCPP) with a space−time
yield of 648 kg m−3 d−1. The corresponding synthesis of In-PMOF-cf
at 100 °C was also carried out, but the reaction product contains X-ray
amorphous impurities. Hence a detailed characterization of this
product was not carried out. To confirm that In-PMOF-cf was
obtained a Pawley fit was performed (Figure S5).
Al-PMOF25
Ga-PMOF-bs-th
In-PMOF-bs-th
SG
Cmmm
Cmmm
32.950(4)
6.700(2)
16.556(2)
2.4
Cmmm
33.432(2)
7.152(1)
16.718(1)
1.6
a [Å]
b [Å]
c [Å]
GOF
Rwp
31.978(3)
6.5812(4)
16.862(2)
4.0
6.5
5.7
RBragg
0.75
0.76
RESULTS AND DISCUSSION
■
Synthesis. Three different synthesis settings (batch syn-
thesis with thermal heating = bs-th, ultrasound-assisted batch
synthesis = bs-us, continuous-flow synthesis = cf) were used to
synthesize the two new porphyrin-based MOFs Ga- and In-
PMOF. The two MOFs were investigated by high-throughput
methods, and the resulting high-throughput experiment that led
to the highly crystalline Ga- and In-PMOF-bs-th is shown in
Figure S2. The optimized synthesis time and temperature was
48 h at 120 °C to achieve highly crystalline products with
different particle sizes (92 (25) for Ga-PMOF and 132 (25) nm
for In-PMOF). Using ultrasound-assisted and continuous-flow
synthesis, it was possible to vary the resulting particle size of
both MOFs in the range of 100 nm. In case of the ultrasound-
assisted batch synthesis (bs-us) only In-PMOF-bs-us could be
obtained. The synthesis was established in an ultrasonication
bath within 7 h, which led to an increase of the reaction
temperature to 50 °C. This synthesis led to particles sized at
the order of 60 nm. Continuous-flow synthesis (cf) led to both
MOFs, but In-PMOF-cf was only obtained with X-ray
amorphous impurities. For the synthesis of Ga-PMOF-cf two
different temperatures, 120 and 100 °C, were employed, which
led to products with different particle sizes of about 65 nm (120
°C) and 156 nm (100 °C), respectively. Details regarding the
particle size are given in the corresponding section below.
Crystal Structure. All compounds were obtained as
microcrystalline powders. Initial powder X-ray diffraction
(PXRD) measurements confirmed that Ga- and In-PMOF are
isostructural to Al-PMOF.25 For the structure refinement high
resolution PXRD patterns of the batch synthesis products were
used. Pawley fits of the high resolution data of Ga- and In-
PMOF-bs-th with the program TOPAS academic V4.148 are in
Figure 2. Result of the Rietveld refinement of Ga-PMOF-bs-th.
Measured data are shown as a black line and calculated data as a red
line, and the blue line gives the difference plot. Predicted peak
positions are marked as vertical bars.
fits were also carried out for the products In-PMOF-bs-us, Ga-
PMOF-cf, and In-PMOF-cf. The results of the Pawley fits and
Figure 3. Result of the Rietveld-refinement of In-PMOF-bs-th.
Measured data are shown as a black line, calculated data as a red
line and the blue line gives the difference plot. Predicted peak
positions are marked as vertical bars.
C
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Figure 4. Chains of trans corner-sharing GaO6 and InO6 polyhedra in Ga- and In-PMOF with a few added linker molecules.
the refined cell parameters are shown in Figures S3, S4, and S5
and Table S1.48
The title compounds of framework composition
[Ga2(H2TCPP)(OH)2] and [In2(H2TCPP)(OH)2] crystallize
in the space group Cmmm. They are isostructural to the
corresponding Al-PMOF25 which contains the same linker, and
therefore the structure is only briefly described. GaO6 and InO6
polyhedra are connected by μ-OH groups and form chains of
trans corner-sharing polyhedra (Figure 4). The other
coordinating O atoms are from deprotonated carboxylate
groups of the linker molecules. The chains are connected by
H2TCPP4− molecules to form a three-dimensional framework.
Two types of pores are formed each along [010] and [001],
with diameters of 5.0/11.8 Å and 3.4/11.8 Å respectively for
Ga-PMOF (Figure 5) and 5.0/11.8 Å and 4.0/12.6 Å
respectively for In-PMOF. The diameters were determined
using DIAMOND V.3 taking the van der Waals radii of guest
molecules into account.49,50
Sorption Properties. Sorption experiments using N2 and
Figure 5. Space filling model of the crystal structure of Ga-PMOF;
view along [010] (top) and along [001] (bottom). The pores along
[010] have diameters of 5.0 and 11.8 Å, and along [001] pores with
diameters of 3.4 and 11.8 Å are observed. Carbon is shown in gray,
oxygen in red, nitrogen in blue, and gallium in turquoise. In the crystal
structure of In-PMOF the pores along [010] have the same size as Ga-
PMOF, and along [001] pores with diameters of 4.0 and 12.6 Å are
observed.
CO2 as adsorptives were performed. All samples were activated
at 140 °C under vacuum (10−2 kPa) for 16 h. N2 sorption
measurements at 77 K were measured for all the different
reaction products obtained using the three synthesis methods
(Table 2, Figure 6, and Figures S7 and S8). The micropore
volumes Vm were determined by using the amount of adsorbed
N2 at the relative pressure p/p0 = 0.5. CO2 sorption
measurements at 298 K were only performed for the samples
obtained from the scale-up reaction using the batch synthesis,
i.e., Ga- and In-PMOF-bs-th (Table 2, Figure S6). To verify the
stability of all the investigated products, PXRD measurements
were carried out after every sorption experiment. The data are
shown in Figure S9 and confirm the stability of the compounds.
Depending on the synthesis method employed, the specific
surface areas are in the range of 1150 to 1400 m2 g−1 and 1200
to 1250 m2 g−1 for the various Ga- and In-PMOFs, respectively.
These values compare well with the reported one for Al-PMOF
(1400 m2 g−1). Taking into account the different atomic masses
of Al, Ga, and In, very similar specific surface areas in m2
mmol−1 are found. The theoretical specific surface areas of Al-,
Ga-, and In-PMOF were simulated using the program suite
Materials Studio (v. 5.0)51 and employing the strategy reported
from the center of a sphere with a diameter of 3.60 Å that rolls
across the pore surface. The observed surface area values are
slightly smaller by about 100−200 m2 g−1 compared to the
calculated ones. The CO2 sorption measurements, which were
only performed for the scaled-up products of Ga- and In-
PMOF-bs-th, show an uptake of 15.2 and 12.9 wt % at 1 bar
and 298.15 K for Ga- and In-PMOF-bs-th, respectively.
Thermal and Chemical Stability. To remove nonreacted
linker molecules as well as solvent molecules and to investigate
the chemical stability of the PMOFs, the samples were treated
in different solvents (H2O, EtOH, MeOH, acetone) at rt for 24
h under stirring followed by thermal activation between 120
and 180 °C for 12 h under vacuum (10−2 kPa). The results of
these experiments (Figure 7) show that the crystallinity of the
Ga- and In-PMOF correlates strongly with the presence of
by Duren et al.52 With this strategy the surface area is calculated
̈
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Table 2. Results of the Sorption Experiments (N2 at 77 K and CO2 at 298.15 K) of the Ga- and In-PMOFs Obtained Using
a
Different Synthesis Methods: Ga-PMOF-bs-th (scaled up), In-PMOF-bs-th (scaled up), In-PMOF-bs-us, and Ga-PMOF-cf
a
b
These results are compared to the literature values of Al-PMOF.25 The micropore volume was calculated at p/p0 = 0.5. The theoretically accessible
surface areas were calculated using the strategy reported by Duren et al.52
̈
washing with EtOH, MeOH, or acetone and activation at 140
°C under vacuum (10−2 kPa) for 16 h.
Samples treated with H2O and HCl could be easily
regenerated within a few seconds by impregnation with DMF
at room temperature. Thermal activation above 145 °C at 10−2
kPa leads to the removal of all DMF molecules and a strong
decrease in crystallinity (Figure 7). These samples could also be
regenerated in all cases by impregnation with DMF at room
temperature within a few seconds. This indicates that loss of
the long-range order is a reversible process.
The thermal stability was also investigated with thermogravi-
metric measurements (TG). For Ga- and In-PMOF-bs-th, TG
measurements were carried out for the as-synthesized and the
activated form, to determine the amount of solvent molecules
in the pores. For all other compounds the TG data of the as-
synthesized products are shown. The results of the TG
investigations are summarized in Table 3, and the TG curves
are presented in Figure 8 and Figures S10−S14. In the as-
synthesized samples three characteristic steps of weight loss are
observed. The loss up to 100 °C is assigned to the release of
physisorbed water molecules and the one between 100 and 300
°C to the evaporation of DMF. Above 330 °C the
decomposition of the PMOFs take place and the final products
are In2O3 and Ga2O3 as confirmed by PXRD measurements
Figure 6. N2-sorption isotherms of Ga- and In-PMOF-bs-th measured
at 77 K. Filled squares represent adsorption and empty squares
desorption, respectively.
1
DMF molecules in the pores, which was determined by H
NMR spectroscopy. Washing with water, 2 M HCl, or 2 M
NaOH leads to a decrease or loss of long-range order (Figure
7). Corresponding stability tests at pH = 7 using a Na2HPO4/
NaH2PO4 buffer system were also carried out and led to the
formation of X-ray amorphous products. Due to these
experiments, the best activation procedure was determined as
Figure 7. Thermal (left) and chemical stability (right) shown for In-PMOF-bs-th at activation temperatures between 120 and 180 °C and in different
solvents (24 h, stirring at room temperature). The values in parentheses denote the number of DMF molecules per H2TCPP4− ion (as = as
synthesized).
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a
Table 3. Results of the Thermogravimetric Measurements of All Obtained Compounds
mass loss linker decompn [%]
mass loss physisorbed [%]
H2O DMF
residual mass [%]
measd
calcd
Ga-PMOF-bs-th (as)
Ga-PMOF-bs-th (act)
In-PMOF-bs-th (as)
In-PMOF-bs-th (act)
In-PMOF-bs-us (as)
Ga-PMOF-cf (as)
14.4
19.1
19.4
24.0
19.3
14.2
68.0 (1:1.8)
76.4 (1:2.1)
53.8 (1:2.1)
73.5 (1:1.9)
57.9 (1:2.0)
59.9 (1:2.0)
63.0 (1:2.0)
80.6 (1:2.0)
57.3 (1:2.0)
70.9 (1:2.0)
57.1 (1:2.0)
62.2 (1:2.0)
2.9 (1.9 H2O)
4.6 (2.6 H2O)
3.0 (2.6 H2O)
3.0 (1.9 H2O)
3.4 (2.6 H2O)
5.5 (4.1 H2O)
13.5 (2.2 DMF)
24.9 (5.2 DMF)
20.0 (3.9 DMF)
10.3 (1.9 DMF)
a
The number of solvent molecules per formula unit and resulting metal to linker ratios are shown in parentheses. The abbreviations “as” and “act” in
parentheses denote as-synthesized and activated samples (solvent exchange and thermal activation at 140 °C and 10−2 kPa for 16 h).
crystallographic details such as lattice parameters can be
obtained for the PMOFs.
DLS. The results of the DLS measurements are shown in
Figures S17 and S18. The observed particle sizes are between
about 60 and 160 nm for Ga-PMOF and 70 and 130 nm for In-
PMOF (Table 4). The synthesis of Ga-PMOF in the
Table 4. Particle Sizes of Ga-PMOF-bs-th, In-PMOF-bs-th,
In-PMOF-bs-us, and Ga-PMOF-cf as Obtained from DLS
Measurements and the Evaluation of the PXRD Data Using
a
the Scherrer Equation
b
c
rH [nm]
PI
particle size (PXRD) [nm]
Ga-PMOF-cf (120 °C)
Ga-PMOF-bs-th
65 (18)
92 (25)
156 (28)
74 (21)
132 (25)
0.19
0.12
0.30
0.23
0.20
38 (11)
64 (22)
Ga-PMOF-cf (100 °C)
In-PMOF-bs-us
Figure 8. Thermogravimetric curve of an as-synthesized sample of In-
PMOF-bs-th. Evaluation of the measurement reveals a molar ratio of
linker:metal:H2O:DMF of 1:2.1:2.6:5.2 (Table 3).
92 (30)
In-PMOF-bs-th
105 (70)
a
b
Values in parentheses are the standard deviations. rH = hydro-
c
dynamic radius. PI = polydispersity index.
(Figure S15). The results of the thermogravimetric analyses fit
well with the elemental analyses given in the Experimental
Section. Only the amounts of occluded solvent molecules differ
slightly due to the storage conditions of the samples prior to
the measurements. The TG curves of the activated samples of
Ga- and In-PMOF-bs-th show a small mass loss of a few
percent up to 100 °C, which is due to the release of 2 to 3
physisorbed H2O molecules per formula unit (Table 3).
IR Spectroscopy. IR spectra of as-synthesized samples of
Ga- and In-PMOF-bs-th are shown in Figure S16, and the
assignment of the bands is given in Table S2. Since the bands
for the title compounds are observed at very similar wave
numbers, only the IR spectrum of Ga-PMOF is briefly
discussed. The stretching vibration of the N−H group is
observed at 3320 cm−1. The asymmetric (νas) and symmetric
(νs) stretching vibrations of the coordinating carboxylate
groups are observed at 1590 and 1415 cm−1, respectively.
The characteristic ν(CC) valence vibration of the phenyl
rings is observed at 1545 cm−1. Further characteristic vibrations
such as the ν(C−N) valence vibration and δ(C−H, N−H) and
γ(C−H) deformation vibrations of the porphyrin moiety are
found at 1362, 961, and 796 cm−1, respectively (Table S2). The
vibration at 1710 cm−1 is due to the presence of DMF
molecules and can be assigned to the ν(CO) stretching
vibration.53,54
continuous-flow reactor using synthesis temperatures of 100
and 120 °C and molar ratios of reactants of metal-to-linker of
2:1 and 1:1 leads to different particle sizes of 65 nm (120 °C)
and 156 nm (100 °C), respectively.
PXRD. The particle sizes of all obtained samples were also
determined from PXRD data. They are in most cases consistent
with the DLS measurements taking into account that the
hydrodynamic radius is larger than the particle size (Table 4).55
In one case the particle size determined from PXRD and DLS
data is in the same range, but in this case high standard
deviations are found.
SEM. The samples were also characterized by SEM
measurements (Figures S19−S23). The micrographs reveal
plate-shaped morphologies for all samples, and the edges of the
plates are not very well-defined. Therefore, and because of
particle agglomeration, the determination of a particle size
distribution of the samples was not possible.55 From the
micrographs one can deduce that thermal heating leads to
larger particles than the ones obtained under ultrasonication
and that in the flow reactor at 120 °C the smallest particles are
obtained. This is in agreement with the other characterization
measurements.
The particle size and morphology of In-PMOF-bs-us
nanoparticles were also investigated by TEM measurements
(Figure 9). This sample was chosen due to its high crystallinity
and the smallest particle size of the synthesized In-PMOFs, as
shown with DLS measurements (Figure S18).
Particle Size. The influence of the synthesis method on the
particle sizes of the resulting PMOFs was studied using
dynamic light scattering (DLS), SEM, and TEM measurements.
While the first method leads to quantitative information on the
bulk properties and yields the hydrodynamic radius, the latter is
used to get information about the particle shape. In addition
Figure 9 shows a typical TEM image of the In-PMOF-bs-us
sample, in which the morphology and size of the crystalline
F
DOI: 10.1021/acs.inorgchem.6b00221
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 9. Left: TEM image of the as-synthesized In-PMOF-bs-us on a carbon support film. The inset shows the Fourier transform of the TEM
image, in which the radial distance represents the lattice fringe distance. Right: Details of the TEM image shown on the left side with the lattice
planes corresponding to the [001] reflection of In-PMOF-bs-us (1.65 nm).
particles are visible, in addition to individual sets of lattice
planes. The particles exhibit different types of morphology and
sizes and tend to agglomerate. Most of the particles appear
almost circular in the projection, hinting at a spherical shape.
Other particles are bounded by straight lines, indicating the
formation of facets. The lateral size of the particles ranges from
25 to about 80 nm.
In order to determine the lattice parameters of the crystalline
phase, the interplanar distances between the various sets of
lattice planes were measured by means of digital image analysis.
The inset in Figure 9 shows a Fourier transform (FT) of the
TEM image. The radial distance of the apparent spots indicates
the lattice distance in reciprocal space. A comparison with
calculated values for the In-PMOF-bs-us sample shows very
good agreement with the measured ones (Table 5).
yield of 648 kg m−3 d−1. Both MOFs show a high BET surface
area between 1100 and 1400 m2 g−1 as determined from N2
physisorption measurements at 77 K, and they are also porous
toward CO2 (uptake in Ga-PMOF is 15.2 and In-PMOF it is
12.9 wt % at 298.15 K and 1 bar).
Both MOFs show thermal stability up to about 145 °C and
chemical stability in all tested organic solvents. The observed
loss of long-range order after treatment with H2O or activation
temperatures above 145 °C is reversible; impregnation with
DMF at room temperature for a short time restores highly
crystalline materials. The small particle size and the high
crystallinity of the title compounds in combination with their
porosity could make these porphyrin-based MOFs possible
candidates in drug delivery applications or in catalysis.
ASSOCIATED CONTENT
■
Table 5. Measured Lattice Distances from TEM Micrographs
Compared with the Calculated Ones from the Crystal
Structure of In-PMOF
S
* Supporting Information
a
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00221.
measd lattice
distance [nm]
calcd lattice distance from
PXRD [nm]
corresponding lattice
plane [hkl]
Synthesis and characterization of H6TCPP and other
experimental details (PDF)
Crystallographic data of Ga-PMOF-bs-th (CIF)
1.65
11.6
0.74
1.67
11.8
0.75
[001]
[201]
[202]
a
The corresponding lattice planes are also shown.
Crystallographic data of In-PMOF-bs-th (CIF)
CCDC 1438646 and 1438647 also contain 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/getstructures.
CONCLUSION
■
We demonstrated the successful synthesis and characterization
of two new porphyrin based MOFs containing gallium and
indium, i.e., Ga-PMOF [Ga2(H2TCPP)(OH)2]·3DMF·3H2O
and In-PMOF [In2(H2TCPP)(OH)2]·3DMF·4H2O, respec-
tively. The synthesis of both MOFs was investigated by high-
throughput methods,33 and the structures were successfully
refined by the Rietveld method. Three synthesis methods, i.e.,
batch synthesis, ultrasound-assisted, and continuous-flow syn-
thesis were employed to vary the particle size of the resulting
PMOFs. The resulting nanoparticles had diameters between
about 60 and 160 nm for Ga-PMOF and 70 and 130 nm for In-
PMOF as demonstrated by DLS, PXRD, SEM, and TEM
measurements (for In-PMOF-bs-us). The TEM measurements
show that In-PMOF-bs-us is obtained as nanoparticles of high
crystallinity. The use of the continuous flow synthesis allows us
to synthesize Ga-PMOF-cf at gram scale with a space−time
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: stock@ac.uni-kiel.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We appreciate support from the Deutsche Forschungsgemein-
schaft (SPP-1362; STO 643/5-2), and we thank Achim Folster
for help in designing Figure 1. The authors thank Dr. Steffen
Schmidt for assistance with TEM data acquisition and Ramona
Hoffmann for measuring the SEM images.
̈
G
DOI: 10.1021/acs.inorgchem.6b00221
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