Probing the limits of linker substitution in aluminum MOFs through water vapor sorption studies: mixed-MOFs instead of mixed-linker CAU-23 and MIL-160 materials

Article abstract of DOI:10.1039/d0dt01044h

We report a systematic study on the possibility of forming mixed-linker metal-organic frameworks (MOFs) spanning between the aluminum MOFs CAU-23 and MIL-160 with their 2,5-thiophenedicarboxylate (TDC) and 2,5-furandicarboxylate (FDC) linkers, respectively. The planned synthesis of a mixed-linker MOF, combining TDC and FDC in the framework turned out to yield a rather largely intricate mixture of CAU-23 and MIL-160. This is due to the different opening angles of 150° for TDCversus120° for FDC and the concomitantcis-trans versus cis-only OH-bridges in the infinite secondary building unit {Al(μ-OH)(O₂C-)} chains. At the same time, the CAU-23 phase is accompanied by the polymorphic MIL-53-TDC phase withtrans-only OH-bridges. The measurement of water vapor sorption isotherms was the method of choice to confirm the formation of mixed MOFs instead of mixed-linker phases. Thereby, the water sorption isotherms indicate the simultaneous formation of both MOF phases, albeit they do not exclude mixed-linker MOFs which may have formed at low levels of substitution. The differentiationviapowder X-ray diffractometry (PXRD), IR-spectroscopy and nitrogen sorption was either not conclusive enough or impossible, due to similarities of the neat MOF phases. The synthesized MOF mixtures within the TDC?:?FDC ratios of 38?:?62 up to 82?:?18 exhibit two or three uptake steps in the water sorption isotherm, with the first two corresponding to an overlay from the individual water sorption isotherm of CAU-23 and MIL-160 and a third one from the additional MIL-53-TDC.

Full text of DOI:10.1039/d0dt01044h

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DOI: 10.1039/D0DT01044H
ARTICLE
Probing the limits of linker substitution in aluminum MOFs
through water vapor sorption studies: Mixed-MOF instead of
mixed-linker CAU-23 and MIL-160 materials†
Received 00th January 20xx,
Accepted 00th January 20xx
Carsten Schlüsener, Dustin Nils Jordan, Mergime Xhinovci, Tobie J. Matemb Ma Ntep, Alexa Schmitz, Beatriz Giesen,
Christoph Janiak*
DOI: 10.1039/x0xx00000x
We report the systematic study on the possibility of forming mixed-linker MOFs spanning between the aluminum metal-
organic frameworks CAU-23 and MIL-160 with their 2,5-thiophenedicarboxylate (TDC) and 2,5-furandicarboxylate (FDC)
linkers, respectively. The planned synthesis of a mixed-linker MOF, combining TDC and FDC in the framework turned out to
yield rather a largely intricate mixture of CAU-23 and MIL-160. This is due to the different opening angles of 150° for TDC
versus 120° for FDC and the concomitant cis-trans versus cis-only OH-bridges in the infinite secondary building unit
{Al(µ-OH)(O2C-)} chains. At the same time the CAU-23 phase is accompanied by the polymorphic MIL-53-TDC phase with
trans-only OH-bridges. The measurement of water vapor sorption isotherms was the method of choice, to confirm the
formation of mixed MOF instead of mixed-linker phases. Thereby, the water sorption isotherms indicate the simultaneous
formation of both MOF phases, albeit do not exclude mixed-linker MOFs which may have formed at low levels of
substitution. The differentiation via powder X-ray diffractometry (PXRD), IR-spectroscopy and nitrogen sorption was either
not conclusive enough or impossible, due to similarities of the neat MOF phases. The synthesized MOF mixtures within the
TDC:FDC ratios of 38:62 up to 82:18 exhibit two or three uptake steps in the water sorption isotherm, with the first two
corresponding to an overlay from the individual water sorption isotherm of CAU-23 and MIL-160 and a third one from
additional MIL-53-TDC.
O
O
120°
150°
S
180°
Introduction
117°
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Metal-organic frameworks (MOFs), which consists of metal ions
or metal-oxido clusters connected by bridging organic ligands
(“linkers”),1–3 are actively investigated towards potential
application in e.g. catalysis4,5, sensing6, gas separation7,8 and
storage9, drug delivery10, cyclic water adsorption processes for
heat transformation (adsorption chilling)11–14 and water
harvesting from air.15,16 The properties of MOFs are tunable
through the variation of metal ion17 and linker.18–21
A new MOF called CAU-23 [Al(OH)TDC] (CAU = Christian-
Albrechts-Universität), consisting of Al3+ ions and
2,5-thiophenedicarboxylate (TDC) linker molecules (Scheme 1),
was recently published by Lenzen et al. and was stated as an
ideal candidate for ultra-low temperature actuated adsorption-
driven chillers in thermally driven adsorption heat pumps
(AHPs) with an exceptional cycling stability. The most significant
advantage of CAU-23 was the applicable low driving
temperature (60 °C) under retention of high uptake capacities
of 0.37 g(H2O)/g(sorbent) and a low cooling temperature of
10 °C.22
FDC
IPA
TDC
BDC
Al3+
MIL-160
CAU-10-H
CAU-23 or MIL-53-TDC
MIL-53(Al)
cis/trans
cis
trans
Scheme 1 Schematic illustration of linker molecules relevant in this work, their
opening angles and the resulting MOFs with their cis- and/or trans-µ-OH
connectivity of the AlO6 octahedra (cf. Fig. 1 and Fig. S1-S4, ESI†).
Low-temperature cycling heat transformation with AHPs and
MOFs as active material is currently in the focus of research due
to the unsurpassed tunability of MOFs as porous materials (see
Scheme S1, ESI† for a short description of the working
principle).11,23–25. In comparison to the already AHP-application
tested MOFs Al-fumarate (Al-FUM),26–29 and CAU-10-H,30,31 the
new MOF CAU-23 shows similar theoretical coefficient of
performance values (COP) but exhibits a superior S-shaped
water sorption isotherm.22
A MOF sharing the same formula unit to CAU-23, namely the
polymorph MIL-53-TDC [Al(OH)TDC] (MIL = Matériaux de
l’Institute Lavoisier) was reported by Tschense et al. and has
also been investigated for potential applications other than
cycling water sorption.32–35 MIL-53-TDC also exhibits highly
interesting water sorption properties with high hydrothermal
stability and a favorable low isosteric heat of adsorption of only
2.6 kJ g–1 as this heat must be rejected to the environment and
is waste heat. Further, active MIL-53(Al)-TDC can be
regenerated with desorption temperatures below 65 °C, with
also desirable high condenser temperatures of around 40 °C
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität,
D-40204 Düsseldorf, Germany. E-Mail: janiak@uni-duesseldorf.de
† Electronic Supplementary Information (ESI) available: Brief description of cycling
heat transformation with AHPs, the background of the presented MOFs, syntheses
details, PXRD measurements, IR spectroscopic data, nitrogen and water sorption
analyses, SEMs, TGAs and 1H-NMRs. See DOI: 10.1039/x0xx00000x
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and a corresponding water exchange of almost 0.35 g g–1, giving materials, that are synthesized in a direct synthesis with at least
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a working window which renders the material an ideal two differing linker molecules thatDOpI:o1s0s.1e0s3s9/Dt0hDeT01s0a4m4He
candidate for adsorptive cooling applications.32,33
coordinating groups, connectivity and form isostructural MOFs
The opening angle of the TDC linker with 150°, lies between the when applied in a single-linker synthesis (isostructural mixed-
linear benzene-1,4-dicarboxylate (BDC) and V-shaped linker approach).40 Such mixed-linker MOFs have also been
2,5-furandicarboxylate (FDC) and 1,3-benzenedicarboxylate termed MIXMOFs, multivariate MOF (MTV-MOFs) and mixed-
(isophthalate, IPA) linkers (Scheme 1). Hence, the TDC linker component MOFs (MC-MOFs)38,41 and were recently applied for
yielded either trans-connected AlO6 in MIL-53-TDC or cis- and other MOF property tunings in e.g. gas adsorption42, catalysis43,
trans-connected AlO6 in CAU-23 (Fig. 1, Fig. S3, S4 in ESI†).22,32
luminescence44,45 and thermal expansion.46 In general, mixed-
linker MOFs with a random and statistically linker distribution
throughout the whole framework are desired. However, the
generation of such a random distribution is not straightforward
for all linker combinations, due to possible core-shell
composites, linker domains within the framework and
transitions between these heterogeneous linker distribution
arrangements. Even the formation of a MOF structure with a
parent framework and the incorporation of the second linker at
only specific lattice sites can occur. Also, the formation of
physical mixtures of single-linker MOFs is possible and has to be
ruled out.40
a)
b)
Mixed-linker MOFs are most often synthesized for isostructural
single-linker parent MOFs and by employing the linker in a
nonfunctionalized and
a
functionalized version (e.g.
terephthalate and functionalized with -I, -Br, -CH3, -NH3 and -
OH).40,47 The direct synthesis of mixed-linker MOFs may seem to
be the easiest method, employing all linkers and the chosen
metal precursor in a conventional MOF synthesis. However, the
disadvantage of the direct synthesis type is the poor product
control and an unpredictable linker-ratio outcome. Even the
reproducibility of the synthesis and a reliable linker-ratio
characterization can be challenging.40 The synthesis of a core-
shell type MOF-5 reported is a good example, where different
metal-linker solubility properties prevented the random
distribution of both linker types throughout the framework. The
crystallization rates of employed BDC and 2-hydroxybenzene-
1,4-dicarboxylate (BDC-OH) led to the formation of a BDC rich
core, acting as seeding crystals for the formation of BDC-OH rich
shells.48
c)
Fig. 1 Illustration of Al–O chains of a) CAU-23 (cis and trans; helical and straight),
b) MIL-53-TDC (trans; straight) and c) MIL-160 (cis; helical). Graphics produced by
software Diamond36 from cif-files with CSD-Refcodes ZOVHUQ (CAU-23), PIBZOS
(MIL-160) and a cif file provided by Serre and co-workers for MIL-53-TDC.22,37
For Al-based MOFs, we are only aware of that nonfunctionalized
and functionalized versions of the same linker were employed,
e.g. in the synthesis of mixed-linker MOFs of CAU-1049–51 and
MIL-53.43,52–54
We have recently applied a solid-solution mixed-linker
approach,38 where the IPA linker in CAU-10-H was partially
replaced by the FDC linker from MIL-160 (and vice versa) under
retention of the isoreticular structures.39 The mixed-linker
approach allowed to fine-tune the MOF hydrophilicity and
water uptake characteristic between the limits of MIL-160 and
CAU-10-H, e.g. by shifting the steep increase in the S-shaped
water adsorption isotherm in small steps from p/p0 = ~0.05 for
MIL-160 to p/p0 = ~0.18 for CAU-10-H. The mixed-linker
materials also gave a higher coefficient of performance (COPH)
value for the obtained materials over MIL-160 under certain
conditions.39
The term “mixed-linker” lacks a generally valid definition. MOFs
with two (or more) different linker molecules in the framework
can be called mixed-linker MOFs. Here we want to differentiate
the non-stiochiometric substitution of similar linkers in
isostructural MOFs from the stiochiometric linker composition
in pillar-layered MOFs. Herein we refer to mixed-linker MOFs as
CAU-23 and MIL-160 are seen as two of the most promising
adsorbents for heat transformation. Both offer relatively high
uptake values for water vapor (CAU-2322: 375 mg/g, 0.33 p/p0,
298 K; MIL-16055: 360 mg/g, 0.50 p/p0, 303 K) and exhibit
favorable S-shaped isotherms with a steep rise in a small
relative pressure range.22,55-57 The beginning of water uptake
for CAU-23 starts around p/p0 = 0.25 and for MIL-160 at
p/p0 = 0.05.22,55 The pronounced difference is based on the
different hydrophilicity of the respective linker molecule and
the different pore structure. For the application in AHPs, MOFs
should have high water vapor uptake values in the range of
0.05 – 0.40 p/p0.31
Following our mixed IPA/FDC-linker results, we chose TDC and
FDC, that is CAU-23 and MIL-160 with an even higher difference
of 0.25 – 0.05 p/p0 in the beginning of water vapor uptake
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Scanning electron microscopy (SEM) images were taVkieewnAbrtiycleaOJnelinoel
DOI: 10.1039/D0DT01044H
JSM-6510LV QSEM advanced electron microscope (Jeol,
Akishima, Japan) with a LaB6 cathode at 5 – 20 keV. The
microscope was equipped with a Bruker Xflash 410 silicon drift
detector.
compared to CAU-10-H and MIL-160 (difference 0.18 –
0.05 p/p0) to modulate or fine-tune the sorption properties for
AHP.
Experimental
Thermogravimetric analyses (TGA) were carried out on a Netzsch
TG209 F3 Tarsus (Netzsch, Selb, Germany) device under a
synthetic air atmosphere, ramping at 5 K min−1 to 650 °C.
Materials and methods
All chemicals were commercially obtained from various sources
and used as received without further purification. 2,5-
Furandicarboxylic acid (H2FDC; Ox Chem: 98 %, 95 %; Apollo
Scientific: 97 %), 2,5-thiophenedicarboxylic acid (H2TDC; Sigma
Aldrich: 99 %), Al(OH)(CH3COO)2 (Alfa Aesar: not specified),
AlCl3 · 6H2O (Janssen Chimica: 99 %), NaAlO2 (VWR Chemicals:
50 – 56 % based on Al2O3), NaOH (micro granules; Chem Solute:
>98.8 % and NaOH (pellets; Merck: 99 %). The used water was
de-ionized via ion exchange.
FT-IR spectra were measured in ATR-mode (Platinum ATR-QL,
Diamond) on a Bruker TENSOR 37 IR spectrometer in the range
of 4000 – 550 cm–1.
Elemental (CHN) analyses were performed on a PerkinElmer
CHN 2400 Analyzer (PerkinElmer, Waltham, US).
Powder X-ray diffractograms (PXRD) were obtained at ambient
1H-NMR spectra were recorded on Bruker Avance III – 300 (1H:
300 MHz) and –600 (1H: 600 MHz) instruments. Approximately
10 – 20 mg of each MOF sample was dissolved in 5 % NaOD in
D2O.
temperature on a Bruker D2 Phaser (300 W, 30 kV, 10 mA;
Bruker. Billerica, US) using Cu-Kα radiation (λ = 1.54182 Å)
between 5° < 2Θ < 50° with a scanning rate of 0.135° s−1 or
0.025° s–1. The diffractograms were obtained on a flat “low
background sample holder”, in which at a low angle the beam
spot is strongly broadened so that only a fraction of the
reflected radiation reaches the detector, hence the low relative
intensities measured at 2Θ < 7°. The analyses of the
diffractograms were carried out with the “Match! 3.5.3.109”
software.
High-angle annular darkfield scanning transmission electron
microscopy (HAADF-STEM) imaging was performed on a FEI
Tecnai G2 F20 electron microscope (Gemeinschaftslabor für
Elektronenmikroskopie RWTH-Aachen, Ernst Ruska-Centrum
für Mikroskopie und Spektroskopie mit Elektronen, Jülich)59
operated at 200kV accelerating voltage and equipped with a
Gatan UltraScan 1000P detector. The MOF-sample was
dispersed in acetonitrile and one drop of the dispersion was
placed on a 200 μm carbon-coated copper grid and dried under
air.
Nitrogen (purity 99.9990 %) physisorption isotherms were carried
out on a Nova 4200e or Autosorb-6 from Quantachrome at 77 K.
Water vapor sorption isotherms were measured volumetrically
on a Quantachrome VSTARTM vapor sorption analyzer with four
parallel stations at 293 K. Before each sorption measurement,
the samples were activated under vacuum (1x10–3 mbar) at
423 K for 2 h or until a pressure of 5 · 10–2 mbar was achieved.
EDX spectroscopy and elemental mapping acquired in STEM
mode were also performed on the FEI Tecnai G2 F20. For point
EDX measurements the exposure time of the individual EDX
spectrum was 3 min. All STEM measurements were carried out
using the Gatan Digital Micrograph software.
Syntheses of MOF materials
Brunauer-Emmett-Teller (BET) surface areas were calculated from
four or five adsorption points in the pressure range p/p0 =
0.006 – 0.08 by applying Rouquerol plots (r > 0.998), unless
Lenzen et al. reported the green synthesis route for CAU-23
with a relative large scale approach using 4.3 g 2,5-
otherwise stated. Total pore volumes were calculated from the thiophenedicarboxylic acid (H2TDC; 25 mmol), 2.0 g sodium
hydroxide (NaOH; 50 mmol), two aqueous solutions of
aluminum chloride (AlCl3·6H2O;18.75 mL; 1.0 mol/L;
N2 sorption isotherms at p/p0 = 0.90. Density functional theory
(DFT) calculations for the pore size distribution curves were
done with the native NovaWin 11.03 software using the ‘N2 at
77 K on carbon, slit pore, non-local density functional theory
(NLDFT) equilibrium’ model for nitrogen. Brunauer-Emmett-
Teller (BET) surface areas for microporous materials with Type I
isotherms should be seen and designated as 'apparent S(BET)'
as 'the BET-area derived from a Type I isotherm must not be
treated as a realistic probe accessible surface area' but
'represents an apparent surface area, which may be regarded
as a useful adsorbent “fingerprint”.58 In the following, we refer
to ‘BET’ for simplicity but imply ‘apparent BET’.
18.75 mmol) and sodium aluminate (NaAlO2·0.2H2O; 12.5 mL;
0.5 mol/L; 6.25 mmol) and 100 mL of distilled water.22 Here we
followed a modified protocol of Lenzen et al.,22 with reduced
batch sizes, for example, only one-fourth of the reported molar
amounts were used.
CAU-23. H2TDC (6.26 mmol) was mixed with 12.75 mmol of
NaOH and stirred in 25 mL de-ionized H2O till a clear solution of
Na2TDC was achieved. After complete dissolution of H2TDC, 1
eq. of Al3+ from two aqueous solutions of aluminum chloride
hexahydrate (AlCl3·6H2O; 1.0 mol/L; 4.69 mL; 4.69 mmol) and
sodium aluminate (NaAlO2·0.2H2O; 0.5 mol/L; 3.1 mL;
1.56 mmol) was added to the linker solution (Table S1 in ESI†).
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The resulting suspension was then stirred under reflux FDC were determined postsynthetically by digestion NMR in 5 %
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1
conditions for 6 h.
NaOD in D2O (see H NMR spectra in DSOeIc: t1i0o.1n03S91/D00,DETS0I1†04f4oHr
The cooled down suspension was centrifuged to recover a white details).
product. The precipitate was dispersed three times in 45 mL
each of de-ionized water and afterwards centrifuged. In a final
Results and Discussion
step, the product was dispersed in 45 mL de-ionized water and
stirred for at least 16 h before being centrifuged again. The
resulting white solid was dried in a vacuum oven at 60 °C (ca.
10 mbar) for not less than 24 h. The yield is listed in Table S1 in
ESI†.
We further investigated the synthesis of CAU-23 with varying
synthesis conditions (e.g. reaction scale, reaction time and
washing procedure). We note that we could only reproduce
CAU-23, despite various attempts, with a BET-surface area of at
best 1056 m2/g compared to the literature value of 1250 m2/g
reported by Lenzen et al.22 The introduction of small amounts
≤ 5% FDC to the synthesized materials enhanced the porosity.
The BET-surface areas of respective materials were comparable
to the published values of CAU-23 by Lenzen et al.22
With the amount of 10% H2FDC offered during the CAU-23
synthesis, a mixed MOF with 1 mol% of FDC or a molar TF ratio
of 99:01 was obtained, which matched the CAU-23 porosity
characteristics. The BET-surface area and total pore volume of
TF 99:01 were comparable to the published values of CAU-23 by
Lenzen et al.22 Hence, in the following water and gas sorption
studies CAU-23 synthesized in this work is a material with a TF
ratio of 99:01, unless noted otherwise.
MIL-160. The synthesis was carried out as described for CAU-23
with 6.25 mmol H2FDC. Yield is listed in Table S1, ESI†.
MIL-53-TDC. The synthesis of MIL-53-TDC was performed
following a protocol of Tannert et al., albeit with lower amounts
of the starting materials.33 H2TDC (0.270 g, 1.57 mmol) was
dissolved in 2.5 mL DMF and a solution of 1.19 g (1.79 mmol)
aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O) in 10 mL
de-ionized water was added. The resulting suspension was then
stirred under reflux conditions at 135 °C for 24 h (Table S3 in
ESI†). The workup was as described for CAU-23. Yield is listed in
Table S3, ESI†.
Mixed Materials. For mixed 2,5-thiophenedicarboxylate/2,5-
furandicarboxylate (TF) MOF materials different amounts of
H2TDC and H2FDC (taken together as 1 eq.; typically, 1.57 mmol)
were mixed with 2 eq. of NaOH and stirred in 10 mL de-ionized
H2O till a clear solution of Na2TDC and Na2FDC was achieved.
After complete dissolution of H2TDC and H2FDC, 1 eq. of Al3+
from two aqueous solutions of aluminum chloride hexahydrate
(AlCl3·6H2O; 1.17 mL of a 1 mol/L storage solution) and sodium
aluminate ((NaAlO2·0.2H2O; 0.78 mL of a 0.5 mol/L storage
solution) was added to the linker solution. The specific molar
amounts are given in Table S1 in ESI†. The resulting suspension
was then stirred under reflux conditions for 24 h. We note that
the extended reaction time for mixed MOFs over the 6 h given
for neat CAU-23 and MIL-160 led to a higher crystallinity and
porosity. The workup was as described before. Yield is listed in
Table S1, ESI†.
From the at first sight apparent near identity of
2,5-thiophenedicarboxylate (TDC) and 2,5-furandicarboxylate
(FDC) the synthesis of mixed-ligand aluminum MOFs stretching
from the single-linker MOF CAU-23 (TDC only) to MIL-160 (FDC
only) seemed straightforward in order to fine-tune the
hydrophilicity.
TDC as a V-shaped linker with an opening angle of 150° lies
directly in between typical V-shaped (e.g. FDC, 117°;
isophthalate, IPA 120°) and linear linkers (e.g. terephthalate,
BDC, 180°; fumarate, FUM, 180°) (Scheme 1). Whether the
linker TDC would lead to a combined cis-trans-connected MOF
as in CAU-23 was uncertain at the beginning. With the discovery
of MIL-53-TDC a trans-only-connected phase was described.32
Likewise, a cis-only-connected structure of TDC with Al3+ may
also be possible.
Aside from the stereochemistry, the MOFs CAU-23 and MIL-160
have the same infinite SBU of formula {Al(OH)(O2C-)2},
composed of corner-sharing {Al(OH)2(O-C-O)4} octahedra. The
difference lies “only” in the combined cis- and trans- (CAU-23)
or solely cis-positioning (MIL-160) of the bridging µ-OH-vertices.
The structure of CAU-23 consists of alternating units of four
consecutive cis- and four trans-corner-sharing {Al(OH)2(O-C-
O)4} polyhedra in the infinite SBU. These different sections
resemble a combination of MIL-160 (cis) and MIL-53-TDC
(trans). It did not seem impossible that the cis-sections could be
bridged by the FDC ligand and the trans-sections by the TDC
ligand to arrive at a mixed-linker MOF.
Nine materials with varying fractions of TDC and FDC and the
three single-linker MOFs CAU-23, MIL-53-TDC and MIL-160
were synthesized (Scheme 1). Different amounts of H2TDC and
H2FDC were dissolved under complete deprotonation with
NaOH in water. Aqueous solutions of AlCl3·6 H2O and NaAlO2
were consecutively added to the linker solution and the
reaction mixture was refluxed for 24 h. CAU-2322, MIL-16055,60
and MIL-53-TDC33 were obtained from modified literature
procedures (for further syntheses details see Exp. Section,
Scheme 2 and Section S3 in ESI†).
The mixed materials are denoted as TF xx:yy (T = TDC, F = FDC;
xx = fraction of TDC, yy = fraction of FDC. The values for the
MOF-incorporated fractions of TDC and FDC were confirmed
post-synthetically through solution H NMR-spectroscopy with
the MOFs dissolved in 5 % NaOD in D2O (see 1H NMR spectra in
Section S10 in ESI† for details). Based on the integral ratios of
1
1
TDC and FDC protons in the H NMR spectra, the actual linker
ratios were calculated (see Table S1 in ESI†). The molar starting
and the postsynthetically determined ligand ratio were within a
few percent for synthesis ratios of T:F  0.5:0.5. For synthesis
ratios of T:F  0.6:0.4 up to 20% more TDC than offered was
found in the mixed materials with reciprocally less FDC (Table
S1, ESI†). CHNS-elemental analyses support the post-synthetic
NMR determination and show a consistent decrease of sulfur
Materials synthesized with both linkers (TDC and FDC) are
designated as TF xx:yy (T = TDC, F = FDC; xx = molar fraction of
TDC, yy = molar fraction of FDC). The molar fractions of TDC and
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content with a decreasing fraction of TDC (when starting from
CAU-23) (Table S2, ESI†).
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MIL-53-TDC simulated
DOI: 10.1039/D0DT01044H
MIL-53-TDC
CAU-23 simulated
CAU-23 (TF 99:01)
TF 95:05
O
O
O
O
S
O
–O
O– –O
O–
TF 82:18
TDC
(T)
FDC
(F)
TF 72:28
AlCl3·6H2O / NaAlO2
TF 53:47
H2O/NaOH
100 °C
TF 38:62
TF 30:70
TF 18:82
O
O
E = S:  CAU-23 or MIL-53-TDC
E = O:  MIL-160
TF 07:93
Al
O
MIL-160
O
MIL-160 simulated
E
E
E
E
5
10
15
20
25
30
35
40
45
50
O
O
O
2 Theta [°]
O
Fig. 2 PXRD patterns of mixed TF material, CAU-23, MIL-53-TDC and MIL-160
together with simulated diffractograms of CAU-23, MIL-53-TDC and MIL-160. The
theoretical powder patterns were calculated from the CSD-deposited cif-files of
CAU-23 (CSD-Refcode ZOVHUQ22), MIL-160 (CSD-Refcode PIBZOS37) and for MIL-
53-TDC from a cif-file that was generously provided by Serre and co-workers. For
an enlarged version of the figure with full-page width see Fig. S5, ESI†.
Al
Al
E = O, S
O
O
O
O
O
O
gives a first indication that it is not mixed-linker MOFs which are
formed but a mixed-MOF material of CAU-23/MIL-53-TDC with
MIL-160. For mixed-linker MOFs one would expect a single
phase as determined by the higher linker fraction.
To the best of our knowledge, the aspect of mixed-linker versus
mixed-MOFs has not been elucidated before for structurally
very similar MOFs where the PXRD patterns do not allow for a
distinction.
Al
O
O
mixed-MOFs: CAU-23 / MIL-53-TDC / MIL-160
T
:
F
with T:F = 07:93 up to 95:05
Scheme 2 Schematic formation of the MOF materials with varying fractions of TDC
and FDC. The given ratio 2,5-thiophene- to 2,5-furandicarboxylate (T:F) was
determined postsynthetically by digestion NMR.
Selected scanning electron microscopy (SEM) images indicate
that the obtained materials have no characteristic microcrystal
All materials were obtained as highly microcrystalline powders.
Powder X-ray diffraction (PXRD) patterns of all MOF products
are shown in Fig. 2. The diffractograms of the single-linker MOFs
match their respective simulated diffractograms. Deviations in
the relative intensities of the reflections between the simulated
and experimental diffractograms are due to the used flat-
surface sample holder (for further details see Experimental
Section).22,60 Characteristic reflections at 17.1 and 17.4 °2 Theta
for CAU-23 (space group P21212) and at 15.3 and 22.8 °2 Theta
for MIL-160 (space group I41md) allow to differentiate each of
the two MOFs. The characteristic reflections of CAU-23 and MIL-
160 can also be observed in the intended mixed-linker
materials. In these mixed materials, the reflection intensities for
CAU-23 and MIL-160 vary depending on the respective linker
fractions.
morphology and show
a tendency to form particle
agglomerates (Fig. 3 and Section S8, ESI†). Microcrystallinity can
partially be recognized by the facetted primary particles. The
mixed-MOF materials show varying particle and agglomerate
sizes, in some cases rod-like and smaller spherical agglomerated
particles were observed. The occurrence of two different
morphologies is another indicator for the formation of two
different phases next to each other in a mixed-MOF instead of
the intended mixed-linker material.
Thermogravimetric (TG) analysis measurements under
synthetic air atmosphere to a final temperature of 650 °C show
for most samples a two-step weight loss (Fig. 4 and Section S9,
ESI†). From 25 °C to 200 °C adsorbed H2O molecules are
removed. The second weight loss starts around 350 °C and is
related to the decomposition of the linkers. Above 500 °C, the
framework is decomposed and the residual amorphous mass
consists of an undefined mixture of Al2O3 and Al2O2S.
A possible MIL-53-TDC phase cannot be ruled out, due to the
smaller number of reflections for MIL-53-TDC (space group
Pmna) which in addition overlap with CAU-23. Overall, the PXRD
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DOI: 10.1039/D0DT01044H
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b)
a)
c)
Fig. 3 SEM images of a) CAU-23 (TF 99:01), b) TF 53:47, c) MIL-160. Magnification in all images is 10 000; the scale bar is 1 µm; see Section S10 in ESI† for additional
SEM images.
micropore volume of TF 53:47 is only slightly higher than those
of the parent MOFs, which indicates a sizable fraction of
additional mesopores. A closer inspection of total and micro-
pore volume also reveals that this increased mesopore volume
100
90
is found from about TF 82:18 to TF 30:70. The partial
mesoporosity is also reflected in the hysteresis of the N2
sorption isotherms (Fig. S8, ESI†).
80
70
60
50
40
30
20
500
TF 53:47
TF 95:05
MIL-160
TF 38:62
TF 18:82
TF 30:70
CAU-23 (TF 99:01)
TF 72:28
TF 82:18
450
400
350
300
250
200
150
100
CAU-23 (TF 99:01)
TF 72:28
TF 38:62
TF 07:93
MIL-160
100
200
300
400
500
600
Temperature [°C]
Fig. 4 Selected TGA curves of single-linker MOFs CAU-23 (TF 99:01) and MIL-160
and selected mixed MOFs TF 72:28 and TF 38:62 under synthetic air atmosphere,
heating rate 5 K/min. See Section S9 in ESI† for additional details.
Sorption properties
Brunauer-Emmett-Teller, BET-surface areas were determined
for all synthesized materials from N2 sorption isotherms at 77 K.
All mixed materials have high BET-surface areas between 834 –
1272 m2/g, which have to be compared with the single-linker
BET-surface areas for CAU-23 of 1250 m2/g22 and MIL-160
1162 m2/g (Lit.:55,60 1070 – 1150 m2/g).
Compared to the surface areas of the single-linker MOFs
CAU-23 and MIL-160, the mixed-MOF BET-surfaces and pore
volumes mostly lie somewhat under the calculated values based
on the linker fractions (Fig. 5, Table 1) but are still within the
experimental error range of ±40 m2/g for S(BET) and ±80 m2/g
for S(BET)calc (see footnote a in Table 1) as well as ±0.02 cm3/g
for Vpore and ±0.04 cm3/g for calc Vpore (see footnote b in Table
1). A noteworthily different sample is TF 53:47 with a higher
surface area than its neighboring TF samples and especially a
high total pore volume of 0.58 cm3/g, which is much higher than
the total pore volume of CAU-23 (0.49 cm3/g) and MIL-160
(0.45 cm3/g) and consequently significantly exceeds the
calculated pore volume (Table 1). At the same time, the
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure p/p0 [–]
1400
1200
1000
800
600
400
200
0
BET-Surface
Pore volume
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fraction of TDC
Fig. 5 Top: N2 adsorption isotherms of mixed MOFs with different ratios linker
fractions. For clarity, only the adsorption isotherms are shown. See Fig. S8, ESI†
for full N2 sorption isotherms (including desorption branches). Bottom: Results of
mixed MOF materials: BET-surface areas and pore volumes.
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Table 1 Results of N2 sorption of TF materials.
d
e
% of calc.
S(BET)
[%]
calc.
Vpore(total)
[cm3/g]
% of calc. Vpore (micro;NLDFT) Vpore(micro) Vtotal – Vmicro Vtotal
–
a
b
S(BET) a
Compound
S(BET)calc.
[m2/g]
Vpore(total)
[cm3/g]
b,c
f
Y
Vpore(total)
[%]
[cm3/g]
[cm3/g]
Vmicro
[cm3/g]
[m2/g]
[cm3/g]
CAU-23 Lit.22
CAU-23 (TF 99:01)
TF 95:05
1250
1232
–
–
100
98
82
93
106
87
70
80
83
100
–
–
–
100
102
93
102
123
97
98
83
82
100
–
0.48i
0.46
0.45
0.38
0.42
0.48
0.37
0.32
0.34
0.35
0.43
0.40-0.48j
0.39
–
–
1231
1229
1219
1212
1199
1189
1180
1175
1167
1162
–
0.49
0.50
0.45
0.49
0.58
0.45
0.45
0.38
0.37
0.45
0.49
0.49
0.48
0.48
0.47
0.47
0.46
0.46
0.45
0.45
–
0.43
0.42
0.33
0.40
0.44
0.34
0.28
0.32
0.34
0.41
0.03
0.05
0.07
0.07
0.10
0.08
0.13
0.04
0.02
0.02
0.06
0.08
0.12
0.09
0.14
0.11
0.17
0.06
0.03
0.04
1203
TF 82:18
1005
TF 72:28
1129
TF 53:47
1272
TF 38:62
1040
TF 30:70
830
TF 18:82
934h
TF 07:93
964
MIL-160
1162h
1070-1150
1029i
MIL-160 Lit.55,60
MIL-53-TDC
MIL-53-TDC Lit.32,33
–
–
0.44
–
–
–
0.36
–
1150-1102
–
–
0.48k-0.46
–
a From repeated (up to four times) N2 sorption studies with BET-surface area determination of samples from the same batch we can note that the deviation of surface
area is ±40 m2/g in the range of 1000-1200 m2/g or about 4%. In S(BET)calc two fractional surface areas are added, doubling the error to ±80 m2/g (~8%). S(BET) calculated
as the sum of the linker-fraction-weighted surface areas of the neat MOFs (CAU-23 = 1232 m2/g; MIL-160 = 1162 m2/g) from the following formula S(BET)calc
=
Fraction(TDC)  1232 m2/g + Fraction(FDC)  1162 m2/g. b Total pore volume (Vpore(total)) at p/p0 = 0.90 for pores ≤20 nm. From repeated (up to four times) N2 sorption
studies with Vpore determination of samples from the same batch we can note that the deviation in Vpore is ±0.02 cm3/g in the range of 0.50 cm3/g. In calc Vpore two
fractional surface areas are added, doubling the error to ±0.04 cm3/g. c Calc Vpore(total) calculated as the sum of the linker-fraction-weighted pore volumes of the neat
MOFs (CAU-23 = 0.49 cm3/g; MIL-160 = 0.45 cm3/g;) from the following formula calc.Vpore(total) = Fraction(TDC)  0.49 cm3/g + Fraction(FDC)  0.45 cm3/g. d Micropore
volume (Vpore(micro;NLDFT)) calculated from N2 adsorption isotherm at p/p0 = 0.1 for pores with d ≤ 2 nm (20 Å). e Micropore volume (Vpore(micro)) refers to the volume that
originates only from micropores, obtained by the V-t-method with thickness method ‘DeBoer’ in the range of 0.15 < p/p0 < 0.55. All correlation coefficients (r) within
calculations were ≥0.998. f Difference of Vpore (total) and Vpore (micro;NLDFT) g Difference of Vpore (total) and Vpore(micro) by the V-t-method. h Only a 2-point BET- with a positive C-
.
constant possible. i Only a 3-point BET with a positive C-constant possible. j Micropore volume with undisclosed conditions. k Micropore volume determined at p/p0 = 0.5.
We note that the N2 sorption isotherms, BET surface area and Thus, a CAU-23 type isotherm with the increase at ~0.25 p/p0 is
porosity do not indicate if a mixed-linker or mixed-MOF obtained for TF 82:18 and 95:05 (Fig. 6a), whereas a MIL-160
material has been formed. Nitrogen sorption isotherms only type isotherm with the increase at ~0.05 p/p0 is given by
reflect and yield the changes in porosity, but water vapor TF 07:93, TF 18:82 and TF 30:70(Fig. 6c). The uptake amount is
sorption isotherms should be able to assess the aspect of mixed- modulated in line with the pore volume (Fig. 7). Mixed materials
linker39 versus mixed-MOF material. MOFs are typically less with fractions from TF 72:28 to TF 38:62 exhibit water isotherms
hydrophilic than zeolites and feature S-shaped Type V sorption with uptake steps at ~0.05 p/p0 and ~0.25 p/p0, which are
isotherms for water with a steep increase at higher p/p0.14,58 For evidently a superposition of the isotherms of the single-linker
MOFs with one type of pore structure there will be a single step MOFs MIL-160 and CAU-23 (Fig. 6b). The water sorption
in this S-shaped isotherm. For mixed-linker MOFs the steep isotherms of these mixed materials, thus, correspond to a
increase should gradually shift with the linker ratio between the physical ex-situ mixture of the corresponding amounts of CAU-
steep increase of the two single-linker parent MOFs.39 For 23 and MIL-160 (Fig. S10, ESI†).
mixed-MOF materials, the water sorption isotherm should be a For the materials with a predominant major phase and the
superposition of the individual isotherms weighted by the linker minor phase below ~30 wt% as in Fig. 6a and 6c, the water
fraction.
uptake behavior can be dominated by the major phase and the
The water sorption isotherms in Fig. 6 show that mixed minor phase may not exhibit its footprint. This seems to be
materials with a larger excess of one linker exhibit S-shaped especially the case when the minor phase is the more
isotherms with the steep increase similar to the single-linker hydrophobic one. While the more hydrophilic MIL-160 phase
MOFs (see Section S7, ESI† for an explanation of the S-shape). already shows up at ~18% in the TF 82:18 material with an
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a)
b)
c)
d)
Fig. 6 Water vapor adsorption isotherms at 293 K for mixed TF-MOFs and single-linker MOFs (MIL-160 and MIL-53-TDC). a) Excess of TDC. b) Nearly equal
amounts of both linkers. c) Excess of FDC. d) CAU-23 and MIL-53-TDC comparison. For clarity, only the adsorption isotherm parts are depicted. See Section S7,
ESI† for full water vapor sorption isotherms (including desorption branches).
increasing water uptake between 0.05 < p/p0 < 0.25 (Fig. 6a), MIL-101, MIL-100 and DUT-67 show several water adsorption
the more hydrophobic CAU-23 phase is first seen with its step steps, due to their pore structures with two different pore sizes
at ~0.25 p/p0 in the TF 38:62 material, that is, at ~38% (MIL-101: 29, 34 Å; MIL-100: 25, 29 Å; DUT-67: 8.8,
contribution (Fig. 6c).
16.6 Å).24,25,61–63 We note that water vapor sorption isotherms
For our CAU-23(TF 99:01) material we note that there is an provide an interesting means to discern phases of different
uptake step around 0.3 p/p0 which was not observed in the hydrophilicity/~phobicitiy, which are otherwise structurally too
work by Lenzen et al.22 Also, TF 53:47 with its exceptional high similar to be differentiated by PXRD. Here we show that water
uptake shows a three-stepped sorption isotherm (Fig. 6d). We sorption isotherms can unequivocally differentiate the case of
explain this third uptake step through the formation of a third mixed-MOF materials from mixed-linker MOFs. The case mixed-
phase, namely MIL-53-TDC which has its uptake step at MOF materials is clearly revealed if the fraction of both phases
~0.35 p/p0 (Fig 7d).32,33 MIL-53-TDC was first published by is high enough to have their distinct isotherm curvature show
Tschense et al. in 201732 and was evaluated in our group for up in the full isotherm (Fig. 6c). Nevertheless, there is a certain
heat transformation applications.33 MIL-53-TDC, which is probability that the introduction of both linkers in one MOF
difficult to separately detect by PXRD, may form together with phase, i.e. a mixed-linker MOF may have formed at low levels of
the CAU-23 phase, either through the small changes in synthesis substitution. Unfortunately, due to the high similarity of the
conditions or through the admixture of the tiny amounts of FDC. PXRD patterns, the verification of a minor mixed-linker MOF in
The possible formation of a MIL-53-TDC phase in a purely the presence of the major mixed-MOF phases seems virtually
aqueous synthesis is intriguing and has not been reported yet, impossible.
so far MIL-53-TDC has only been synthesized in a water-DMF We ascribe the formation of a mixture of single-linker Al-MOF
mixture.
phases rather than to mixed-linker Al-MOFs to the small but
To the best of our knowledge, multi-stepped water sorption important cis-trans structural differences between CAU-23 and
isotherms are unknown for the neat single-linker MOFs with an MIL-160 (Fig. 1). CAU-23 has a V-shaped linker with a relatively
identical pore structure (e.g. CAU-23 and MIL-160). The MOFs large opening angle (Scheme 1) and consists of alternating units
8 | J. Name., 2012, 00, 1-3
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500
of four consecutive trans- and four cis-corner-sharing AlO6
polyhedra in the infinite SBU. The Al-MOFs MIL-160 and its
structural parent CAU-10-H have V-shaped linkers FDC and IPA
linkers with small opening angles of  120° (Scheme 1) and have
both cis-connected AlO6 polyhedra (Fig. S1-S4 in ESI†).
MIL-53-TDC possesses the MIL-53-type structure with an
infinite SBU of entirely trans-connected AlO6 polyhedra, cis-
connected polyhedra are leading to helical chains, whereas
trans-sharing polyhedra are forming infinite straight chains.
Hence, in CAU-23 there are alternating helical and straight
sections. The occurrence of two different Al–O-chain
connectivities in one crystal structure is surprising and unique,
as the different sections resemble a combination of either MIL-
53-TDC (trans) and MIL-160 or CAU-10-H (cis) (Fig. 1, Fig. S1-
S4).22,60,64,65
The opening angle between the carboxylate groups of the used
linker molecule determines the cis-trans generated structure
for Al-MOFs.22 Linear linkers with an opening angle of 180° (e.g.
terephthalate and fumarate) lead to trans-connected AlO6
polyhedra (MIL-53 and Al-FUM). V-shaped linker with an
opening angle around 120° (FDC 116°; IPA, 120°) gives cis-
connected AlO6 polyhedra (MIL-160, CAU-10-H). 30,37,64,66 Due to
the differing linker opening angles and resulting (cis-trans) SBU
topologies, the side-by-side formation of both individual MOF
phases seems to be clearly favored over the formation of a
mixed-linker phase.
View Article Online
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TF 53:47
450
400
350
300
250
200
150
CAU-23 (TF 99:01)
MIL-53-TDC
TF 72:28
TF 95:05
TF 38:62
TF 30:70
MIL-160
TF 18:82
TF 07:93
TF 82:18
Total uptake
0.5 0.6 0.7
0.0
0.1
0.2
0.3
0.4
Vpore(total) [cm3/g]
Fig. 7 Water uptake vs pore volume of all materials. Top: Uptake after uptake steps
vs micropore volume ((Vpore(micro;NLDFT)) calculated from N2 adsorption isotherm at
p/p0 = 0.1). Bottom: Total uptake vs total pore volume ((Vpore(total) calculated from
N2 adsorption isotherm at p/p0 = 0.9). See Table S4, ESI† for the respective water
uptake values.
To support the existence of different phases in the obtained
TF 53:47 material, point EDX measurements of diffuse and rod-
like particles in the STEM image (Fig. 8, cf. SEM images Fig. 3b
and Section S8 in ESI†) revealed a different elemental
composition regarding aluminum and sulfur content such that
the more crystalline rod-like particles show a significantly
higher content of aluminum compared to sulfur. The diffuse
particle regions instead had an about equal aluminum to sulfur
content of 55 to 45 atom%. The formula of MIL-53-TDC and
CAU-23 of [Al(OH)TDC] requires one sulfur per aluminum atom,
associating their phase with the diffuse particles. These findings
correspond with the simultaneous formation of two MOF
phases from a mixed-linker approach.
For TF 53:47 the high-water uptake can be understood from its
increased total pore volume (Table 1, Fig. 7). In Fig. 7, the water
uptake values are plotted versus the pore volume. Both the
water uptake directly after the steep rise of the last uptake step
versus micropore volume (from NLDFT; at 0.1 p/p0) as well as
the total water uptake versus total pore volume; both at
0.9 p/p0 show a reasonable linear correlation.
500
450
TF 53:47
400
CAU-23 (TF 99:01)
TF 72:28
350
MIL-53-TDC
TF 38:62
TF 95:05
300
250
200
150
MIL-160
TF 82:18
TF 07:93
TF 30:70
TF 18:82
Uptake after uptake steps
0.3 0.4 0.5 0.6
Vpore(micro;NLDFT) [cm3/g]
0.0
0.1
0.2
Fig. 8 STEM images of TF 53:47 with point EDX measurements of rod (Al 85, S 15
atom%) and diffuse particle regions (Al 55, S 45 atom%).
Conclusion
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Journal Name
We have investigated the mixed-linker solid solution approach
for the preparation of mixed-linker MOFs spanning the
hydrophilicity region between the less hydrophilic single-linker
CAU-23 and the more hydrophilic MIL-160. The structural
differences of both MOFs, in particular the cis or trans
connectivity of the infinite {Al(µ-OH)(O2C-)} SBU prevented the
formation of clearly identifiable mixed-linker MOFs. Employing
nearly equal fractions of 2,5-thiophenedicarboxylate (TDC) and
2,5-furandicarboxylate (FDC) led to the formation of mixed-
MOF phases rather than mixed-linker MOFs. This could only be
proven from water vapor sorption experiments which turned
out as the method of choice to differentiate between mixed-
linker and mixed-MOF phases materials. The water sorption
isotherms showed water uptake steps at 0.05 and 0.25 p/p0,
which correspond to the neat parent MOFs MIL-160 and CAU-
23, respectively. Thereby, the water sorption isotherms indicate
the simultaneous formation of both MOF phases, albeit do not
rule out that mixed-linker MOF may have formed at low levels
of substitution. Some mixed-MOF samples even exhibited an
unexpected third water uptake step after 0.30 p/p0, which
could be assigned to the water vapor isotherm of MIL-53-TDC.
Hence, the particular formation of a MIL-53-TDC phase, hitherto
unknown from purely aqueous synthesis conditions, became
evident. Furthermore, the addition of small amounts of 2,5-
furandicarboxylate to the synthesis of CAU-23 improved the
reproducible formation of this MOF together with its
crystallinity and BET-surface area.
View Article Online
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Conflicts of interest
There are no conflicts to declare.
26 M. F. de Lange, B. L. van Velzen, C. P. Ottevanger, K. J. F. M.
Verouden, L.-C. Lin, T. J. H. Vlugt, J. Gascon and F. Kapteijn,
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28 S. Gökpinar, S.-J. Ernst, E. Hastürk, M. Möllers, I. El Aita, R.
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The authors gratefully acknowledge the financial support of the
Federal German Ministry of Education and Research (BMBF) in
the project Optimat under grant no. 03SF0492A/C.
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