Green synthesis of a large series of bimetallic MIL-100(Fe,M) MOFs

Article abstract of DOI:10.1039/d0nj00257g

Here we present a scalable and green methodology to synthesize a large variety of MIL-100(Fe,M), metal-doped iron-based MOFs with high thermal stability and surface areas. Our synthesis is performed at room temperature in aqueous media and can be applied to doping with p-, d- and f-elements in oxidation states from +I to +V, therefore being highly general. The influence of the doping metal nature on the thermal and textural properties is systematically investigated. We show that large differences between the ionic radius of the doping and the templating metals does not lead to phase segregation. The incorporation of Cu(i) drastically lowers the thermal stability of MIL-100(Fe,Cu), while the incorporation of V, Al and Ti induces the formation of mesopores. Finally, we developed a PXRD-based method that can be combined with TGA to easily access the metal ratios in complex mixed-metal MOFs.

Full text of DOI:10.1039/d0nj00257g

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of bimetallic MIL-100(Fe,M) MOFs
Timothy Steenhaut,*a Sophie Hermans*a and Yaroslav Filinchuk*a
Received 00th January 20xx,
Accepted 00th January 20xx
Here we present a scalable and green methodology to synthesize a large variety of MIL-100(Fe,M), metal-doped iron-based
MOFs with high thermal stability and surface areas. Our synthesis is performed at room temperature in aqueous media and
can be applied to doping with p-, d- and f-elements in oxidation states from +I to +V, therefore being highly general. The
influence of the doping metal nature on the thermal and textural properties is systematically investigated. We show that
large differences between the ionic radius of the doping and the templating metals does not lead to phase segregation. The
incorporation of Cu(I) drastically lowers the thermal stability of MIL-100(Fe,Cu), while the incorporation of V, Al and Ti
induces formation of mesopores. Finally, we developed a PXRD-based method that can be combined with TGA to easily
access the metal ratios in complex mixed-metal MOFs.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Green synthesis
of a large series
Iron centers in MIL-100(Fe) have been shown active in catalysis
and gas sorption.21,22,23 It was shown that adding a doping metal
into the structure can impressively improve some properties of
Introduction
Porous materials are essential in several fields such as gas
sorption,1,2 catalysis,3,4,5 chemicals separation,6 sensing,7,8 heat
transformation purposes,9 etc. Metal-organic frameworks
(MOFs) in particular, being porous coordination compounds
made of metallic clusters and organic bridging ligands, are one
of the most recent developments in this field. However, for
many years their applications were limited due to their low
stability and difficulty to synthesize at low cost and on large
scale.10,11 Recent work allowed obtaining several types of stable
MOFs that can withstand high temperatures and large panels of
solvents.12,13 However, the synthesis of such compounds still
largely suffers from inconvenient preparation procedures,14
although some progress has recently been made towards
greener synthetic procedures.15,16,17 The most stable MOFs are
usually obtained from combinations of ligands and metals with
a high affinity, which are most of the time assembled by
solvothermal processes to avoid precipitation of amorphous
compounds. Those procedures are energy and time consuming
and improvement in the synthetic approaches is thus necessary
to render MOFs of industrial importance.18
MOFs and MIL-100 in particular.24,25,26,27 However, until now,
very few combinations of template and doping metals were
investigated and all those materials were synthesized by
solvothermal methods that mostly involve the use of dangerous
acids like HNO3 or HF as modulators. In some cases, the second
metal was introduced by post-synthetic exchange, adding a
second step in the synthesis. Moreover, the possibility of mixing
metals with large differences of ionic radii in the MIL-100
structure has not been investigated yet.
Our goal is to introduce a wide range of metal substitutions into
the Fe-based MIL-100, serving as a low-cost templating
structure; iron having the advantages of being very abundant,
cheap and non-toxic. We started with an improvement of the
synthesis of pure MIL-100(Fe). Apart from solvothermal
procedures (Table 1), it can also be obtained in mild conditions
by mixing solutions of iron (II) chloride and trimesic acid (H3BTC)
deprotonated with NaOH in a basic environment.28,29 We
quickly found that this procedure is reproducible and scalable
but does not allow incorporating any doping metal by simply
adding a second salt along with FeCl2. Inspired by an earlier
work on other MOFs,30 we modified the reported procedure by
using the sodium salt of trimesic acid, Na3BTC∙3H2O, in slightly
acidic environment. We also used iron (II) sulphate as iron
source instead of the corresponding chloride because of its
lower cost and better suitability for industrial needs as it does
not present the corrosion issues occurring in the presence of
chloride ions.31 This allowed us to incorporate many metals in
+II, +III and +IV oxidation states into the structure in a green
single-step synthesis.
Among remarkably stable MOFs, MIL-100 derivatives show
quite interesting properties (see Table 1),19 like high surface
areas up to more than 2000 m²/g, thermal stability and
resistance to solvents.20 Their structures are composed of
[M3O(X)(H2O)2]6+ clusters (M = Fe, Al, Cr, Sc, In, V or mixed
metal, see M1 and M2 in Table 1; X = OH-, Cl-, F-) linked by
benzenetricarboxylate (BTC3-) ligands.
a. Université catholique de Louvain, MOST, Place Louis Pasteur 1, 1348 Louvain-la-
Neuve.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
We also demonstrate that we can control the amount of doping
metal in the structure and that metals with large ionic radii (Fig.
1),32 such as lanthanides, can also be incorporated into the
MIL-100(Fe) structure. Finally, we present a new convenient
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way to determine the percentage of a doping metal in MOFs by
powder X-ray diffraction (PXRD).
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DOI: 10.1039/D0NJ00257G
Table 1 Mono- and bimetallic MIL-100(M1,M2) MOFs described in the literature to this date, their synthesis methods and applications.
6
M1
M2
Synthesis method
Application
Ref
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8
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33
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Cr, V or Sc
-
Solvothermal
Hydrothermal and solvothermal
Solvothermal
CO2 adsorption
Sc
Al, Cr, Sc or V
V, Al, Fe or Cr
Al, Cr, Fe, In, Sc
or V
Al, Cr or Fe
Friedel-Crafts, oxidation and tandem catalysis
Catalytic tetrahydropyranylation
Catalytic condensation of glycerol with acetone
Prins condensation of -pinene and
formaldehyde
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-
-
-
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Solvothermal
Solvothermal
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Fe
Sc
Mn
Ti
Hydrothermal (with HF)
Solvothermal, post-synthetic exchange; high
valence metal metathesis and oxidation
Solvothermal
Catalytic reduction of NOx by NH3
Photodegradation of methylene blue
40
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Ti
Fe
-
Ni
Photocatalysis
Prins reaction of -pinene and
paraformaldehyde
Hydrothermal, HNO3 assisted
addition of FeSO4 solution, a dark green precipitate forms
immediately and the pH drops to 7.25. The precipitate turns to
orange after a few minutes due to the Fe oxidation by air (Fig.
2). Our new procedure uses a solution of sodium trimesate (see
its prior synthesis in the experimental part), showing a
practically neutral pH of 7.07. Upon the addition of FeSO4
solution, it yields initially a colourless transparent solution with
pH 6.08, which slowly produces a yellow precipitate turning to
orange upon air oxidation (Fig. 2). The appearance and the
colour of the obtained MIL-100(Fe) powders are significantly
different as well as the specific surface areas and pore size
distributions, despite the samples displaying the same crystal
structure by PXRD (Fig. S1 and Fig. S2).
Results and discussion
New green synthesis of MIL-100(Fe) and incorporation of Co
The synthesis of MIL-100(Fe) in aqueous media at room
temperature is possible both in slightly acidic and in slightly
basic conditions. While the reaction products are both MIL-
100(Fe), the reaction mechanisms and the intermediates
appear to be very different, and as we will show below allows
for effective metal substitution only in the acidic conditions. In
Fig. 1 Ionic radii of iron and the doping metals that were incorporated in MIL-
100(Fe,M) in this work (lower part) and their position in the periodic table
(upper part).
the known synthesis procedures,28,29 a mixture of trimesic acid
and NaOH typically shows high pH of about 12.28,29 Upon the
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To investigate the possibility of synthesizing mixed-metal MIL-
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DOI: 10.1039/D0NJ00257G
100(Fe,M) compounds in aqueous media, we decided to first
test M = cobalt. MIL-100(Fe,Co) was obtained by mixing a
solution of iron and cobalt sulphates with a solution of sodium
trimesate in air. This mixture allowed the reaction to proceed in
an acidic media and consequently allowed the incorporation of
the second metal. Synchrotron PXRD shows a single phase
MIL-100(Fe,Co) (Fig. S3) whatever the Fe/Co ratio (in the 5-
35mol% Co range). The ATR-IR spectra are all very similar to the
undoped MIL-100(Fe) (Fig. S4). The incorporation of Co into the
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Fig. 2 Solutions before mixing (left), colour change during the reaction
(middle) and the final product on a watch glass (right) for the synthesis of
MIL-100(Fe) in (A) acidic and (B) basic conditions. The differences in
coloration of the precipitates during the reaction indicate the existence of
different reaction intermediates.
structure was confirmed and quantified by inductively coupled
plasma-optical emission spectroscopy (ICP-OES) (see Fig. 3).
The formation of metal-BTC intermediates containing Fe(II) and
Co(II) that are subsequently oxidized into MIL-100(Fe,Co) can
explain why the incorporation only succeeds in acidic media,
whereas in basic media it is prevented by the formation of
hydroxide species. The overall reaction of the synthesis
proceeds in two separate steps. At first, iron (II) trimesate
precipitates, followed by oxidation in air, leading to the desired
MIL-100(Fe) structure (equations 1 and 2).
Fig. 3 (a) Incorporated quantity of cobalt into MIL-100(Fe) in function of the
quantity used for the synthesis. Indicated percentages correspond to mol%). (b)
PXRD patterns of compounds obtained using different Co/Fe ratios for the
synthesis, showing pure MIL-100 phase for Co/Fe = 0/100 to 40/60 and the
apparition of a second crystalline phase for higher Co/Fe ratios.
into the structure increases with the engaged Co/Mtot
.
However, there is a limit to the amount of Co that can be
incorporated into the structure. When the used Co/Mtot ratio is
higher than 40mol% a second crystalline phase is present in the
powder diffraction patterns (Fig. 3b). When the reaction time
is increased, the amount of incorporated Co also increases. For
instance, when 20mol% of Co is used, ICP-OES shows in the
resulting MIL-100(Fe,Co) a Co/Mtot value of 7.9mol% after 4 h
while it is 11mol% after 24 h of reaction. For comparison
purposes, we performed a post-synthetic exchange by soaking
pure MIL-100(Fe) into a concentrated aqueous solution of Co2+.
This led to a compound with very poor substitution compared
to the direct synthesis, as shown by ICP (for more details, see
Experiment S6 in the SI).
3 Fe2+ + 2 BTC3- + x H2O Fe3BTC2∙xH2O
(1)
(2)
→
→
4 Fe3BTC2∙xH2O + 3 O2 4 Fe3O(OH)(H2O)2BTC2 + (4x-10) H2O
Indeed, in the slightly acidic medium, we have successfully
isolated the intermediate Fe3BTC2∙12H2O, as well as its Co
analogue (Fig. S5).
Variation of the amount of doping Co
In order to verify whether we can control the amount of
incorporated doping metal, we varied the starting ratio of Fe(II)
and Co(II) in the synthesis solution. The reaction time (4 h), the
volume of the reaction mixture and the stirring rate were kept
constant to ensure that the oxidation would proceed in the
same manner. The obtained MIL-100(Fe,Co) samples were
calcined and digested in aqua regia followed by ICP-OES
measurements. The Co/Mtot ratios in the obtained MOFs were
plotted against the Co/Mtot ratios used in the synthesis (Fig. 3a).
A clear trend is observed: the amount of cobalt incorporated
Incorporation of other doping metals
A large variety of other doping metals was used to evaluate the
limits of our synthesis strategy. One would expect that at a
certain size of the doping element a substitution will not occur.
For this reason, we investigated the incorporation of several
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transition, p-block and lanthanide metals, with a particular results and potentially a predictive power for the iVniecwoArrptioclreaOtniloinne
attention on the differences in size and the possible oxidation of other metals.
states of the doping elements.
DOI: 10.1039/D0NJ00257G
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A
O
O
O
O
OH
MIII
OH
MIII
O O
MIII
O
O O
MIII
O
H2O
O
heat, vacuum
O
O
O
O
O
O
- 2 H2O
O
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O
O
O
O
MIII
MIII
O
O
O
O
OH2
MII = Ni, Co or Zn
O
B
O
O
O
SBET
OH2
O O
MIII
O
O O
MIII
O
MII
MII
H2O
O
O
heat, vacuum
O
O
O
O
O
- 3 H2O
O
O
O
O
MIII
MIII
O
O
O
O
O
OH2
Scheme 1 (A) Only two water molecules can be desorbed from Fe2MIII-
µ3-oxo clusters whereas (B) three water molecules can be desorbed
from Fe2MII-µ3-oxo clusters, leading to more higher surface areas.
Fig. 4 Illustration of the preferential incorporation process. Metals in the +III
oxidation state are incorporated preferentially over iron in MIL-100(Fe,M),
leading to higher bulk than surface concentrations of M. Similarly, Fe is
incorporated preferentially over +II metals. The blue and pink areas of the graph
indicate the zones expected for preferential incorporation of Fe over M and of
M over Fe respectively. The doping metals undergoing redox during the
formation of the MOF are all located in the green areas, indicating the existence
of more complex processes during the incorporation of those metals. The M/Fe
ratio used for all syntheses was 20/80. Indicated percentages correspond to
mol%.
Materials porosities
Before nitrogen sorption experiments, all materials were
activated by heating under vacuum at 200°C for 10 hours.
Importantly, all the obtained materials sustained this activation
step without collapsing of the structures. The BET surface areas
of the MIL-100(Fe,M) materials were very high for most doping
metals. (Fig. S8) Nearly all the obtained compounds show
nitrogen absorption-desorption curves with a very similar
shape; of the I(a) type according to the IUPAC classification of
physisorption isotherms,45 characteristic of purely microporous
materials (Fig. S8). However, for M = Cr the surface area is
significantly lower than with other first-row transition metals.
Second-row transition metals show smaller, although still large,
surface areas (SBET = 1350 m²/g for M = Y and 1462 m²/g for M
= Cd). The rare-earth elements, Sm and Tb, also show similar
surface areas. However, La and especially Pr and Ce show
reduced surface areas; going below 1000 m²/g. Those samples
contain secondary phases revealed by PXRD. These phases
disappear at high temperatures and reappear only after
rehydration (Fig. S9 D). Noticeably, the temperature at which
those secondary phases disappear is dependent on the ionic
radius of the lanthanide metal (the larger the doping metal, the
higher the temperature needed for eliminating the secondary
phase) (Fig. S9 A and B). XPS measurements on those samples
show that some sodium is present on the surface (Fig. S10). The
reduced surface areas can thus be explained by the presence of
sodium salts, undergoing crystal to amorphous dehydration. A
similar secondary phase is present in MIL-100(Fe,Y) (Fig. S9 C).
More importantly, MIL-100(Fe,M) with doping metals having
typical oxidation state +II, namely Ni, Co and Zn, show about
10% higher surface areas than MIL-100(Fe) likely due to the
improved dehydration of the metal sites, see Scheme 1. The
surface area of the nickel-doped MOF (2274 m²/g) is
significantly larger than the one of MIL-100(Fe,Ni) materials
previously reported (1525-1570 m²/g).41
To our surprise, all the doping ions that can be solubilized in
water substituted well into MIL-100(Fe), despite some large
differences in ionic radii with Fe(III), see Fig. 1. However, a few
elements are not suitable for incorporation into MIL-100(Fe) by
our synthetic procedure. This is the case of Pb2+ that
2-
precipitates in the presence of the SO4 ions of the iron (II)
sulphate precursor, or of Bi3+ that undergoes oxolation in water.
S-PXRD patterns (Fig. S6) and ATR-IR spectra (Fig. S7) of the
different samples showed that they are all single phase,
isostructural and that all the tested metals can be incorporated
into the structure. The Mdopant/Mtot ratios obtained after 16h
reaction using 20mol% of doping metal were determined by
ICP-OES and appear to be significant (7.5 to 29.9mol%) but
dependent on the nature of the used metals (Table S1).
Comparison of ICP and X-ray photoelectron spectroscopy (XPS)
results shows that the bulk and surface contents in a given
doping metal are not always identical. This could be due to a
preferential incorporation of either the doping (M) or template
metal (Fe) and the subsequent variation of the relative
concentrations of the metals in solution during the crystal
growth.42,43,44 Metals that are preferentially incorporated are
then more concentrated in the core of the crystal than on its
surface. This theory is supported for the +II and +III doping
metals that do not undergo oxidation neither reduction during
the incorporation process. Indeed, the iron being oxidized from
+II to +III during the formation of the MOF, is incorporated
preferentially over +II metals (Co2+, Ni 2+ Zn2+ and Cd2+) but not
over +III metals (Al3+, Cr3+, Y3+ and Ln3+). However, when the
doping metal undergoes redox during the incorporation (i.e. Cu,
Ti, V), more complex processes are at work, as shown in Fig. 4.
The clear-cut regions of the preferential substitution with
respect to the oxidation states of the doping metal, illustrated
in Fig. 4, suggest satisfactory rationalization of the obtained
Interestingly, the MIL-100(Fe,M) samples with M = Cu, Al, V and
Ti show a slight hysteresis in their sorption-desorption curves.
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S-PXRD patterns of those three compounds also show peak observed on the nitrogen sorption isotherm, peak broadening
broadening as compared to the other materials. These in the diffraction patterns, as well as the DloOwI:e1r0.t1h0e39rm/Da0lNsJt0a0b2i5li7tGy
observations suggest the presence of mesopores and/or of MIL-100(Fe,Cu). For MIL-100(Fe,V), V+5 was detected along
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intergrain porosities into the materials.
with V(IV) and V(III). The +V oxidation state of the doping metal
is also incompatible with a perfect cluster structure (unless one
of the iron atoms is present in the +II oxidation state, but this is
again incompatible with the strong oxidation potential of V5+),
see Fig. 6, lower part. Defects around the V5+ centres must
therefore be considered in this case too, correlated again with
the hysteresis observed on the nitrogen sorption isotherm of
MIL-100(Fe,V) and peak broadening in the diffraction profiles.
Thermal behaviour
The thermal stability of the obtained compounds was evaluated
by TGA measurements under both nitrogen and air. All the
compounds, with the notable exception of MIL-100(Fe,Cu)
show very similar decomposition temperatures under nitrogen
(around 420°C), as well as under air (around 355°C) (Fig. S11 and
Fig. 5). The presence of copper in MIL-100(Fe,Cu) has an
important impact on the thermal stability of the compound as
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Using PXRD for doping metal quantification
the decomposition temperature drops from ~355°C to 327°C Finally, a methodological development made in the course of
under air and from ~420°C to 354°C under nitrogen. this work deserves a discussion. The most usual methods for
Heating the samples at a rate of 10 K∙min-1 did not allow to get dosing the metals in MOFs, and more specifically bimetallic
rid of all the coordinated solvent molecules before the MOFs, include ICP and Atomic Absorption Spectroscopy (AAS).
decomposition, even if this last event occurs at temperatures However, these methods need digestion of the sample and a
above 400°C (354°C in the case of MIL-100(Fe-Cu)). This can be dilution prior to analysis. Also, a standard curve must be
explained by the strong bonds formed between the coordinated established for both methods. Moreover, a different hollow
solvent molecules and the metal centres.
cathode lamp must be used for quantifying each different metal
by AAS. For this reason, it would be convenient to have a valid
method for simultaneously determining the metal content and
the Mdopant/Mtot ratio without need for sample digestion and
dilution. PXRD would be a convenient way to determine this by
Rietveld refinement as most laboratories working on MOFs
have access to this method. However, the complex structure of
MIL-100(Fe,M) compounds and the difficulty to attain complete
Determination of oxidation states
OH- and O2-
can compensate
charges
OH
MII
OH
MIII
O
2O
O
O
O
O
O
O
O
O
O
O
MIV
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
FeIII
FeIII
FeIII
FeIII
OH2
FeIII
H2O
OH2
FeIII
OH2
H2O
H2O
O
O
O
O
O
O
O
O
O
O
O
Fig. 5 Decomposition temperature of the MIL-100(Fe,M) compounds
determined by TGA under air and nitrogen.
Defects
OH
XPS was used to determine the oxidation states of the
incorporated doping metals. The analysis revealed that
oxidation states depend highly on the nature of the doping
metal, ranging from +I to +V (Fig. S12). The incorporation of
metals with the oxidation states +II, +III and +IV can easily be
accommodated by varying the nature of the oxygenated ligand
on the doping metal site (Fig. 6, upper part). If we consider that
only one doping metal atom is present in each M3-µ3-oxo cluster
(i.e. Fe2M-µ3-oxo clusters) at the maximum, and that the iron
centres are in the +III oxidation state, then M(II) does not need
any counter ion. Similarly, M(III) is balanced by OH- (the
O
MV
2O
O
O
O
O
O
Incompatibility
of oxidation
states
O
MI
O
HO
O
O
O
O
O
O
O
O
O
FeIII
FeIII
FeIII
OH2
O
FeII
OH2
O
H2O
H2O
O
O
O
O
O
O
Fig. 6 Illustration of neutral Fe2M-µ3-oxo clusters that can be formed with M in
oxidation states +II, +III and +IV with charge balance by OH- or O2- units (upper
part) and problems arising in the charge balance in the presence of doping
metals in the +I or +V oxidation states.
removal of solvents and guests from the pores does not allow
to accomplish metal content determination using this method
directly on the MIL-100 compounds. However, we tested a
strategy that consists in calcination of the sample in air followed
by measuring the PXRD pattern of the resulting oxides (Fig. S13)
and subsequent Rietveld refinement, allowing to determine
Mdopant/Mtot. Here we show that this determination is quite
precise in the specific cases where two oxides having different
2-
presence of Cl- of SO4 was not detected by XPS) and M(IV) is
balanced by µ1-O2- ions.
On the other hand, the presence of Cu(I), that is detected in
MIL-100(Fe,Cu), cannot be explained by a perfect cluster
structure. It is likely that in this case defects are present in
significant amounts in the cluster or in the framework structure
(Fig. 6, lower part). This nicely correlates with the hysteresis
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crystal structures are obtained. For example, calcination of For AAS, samples were digested in aqua regiaVifeowllAortwiclee dOnlbinye
MIL-100(Fe,Co) leads to a mixture of Fe2O3 and CoFe2O4. Once filtration to remove the reprotoDnOatI:e1d0.103H93/DB0TNCJ.002A5A7GS
Rietveld refinement is performed and the weight fractions of measurements were performed using a Perkin Elmer 3110
Fe2O3 and CoFe2O4 are determined, the metal ratio can easily atomic absorption spectrometer. For Fe and Co analysis, an
be calculated (see supporting info). This method does not apply oxidizing air-acetylene flame was used to atomize the samples.
in case solid solutions form or isostructural oxides with similar IntensitronTM hollow cathode lamps (Perkin Elmer) were used
cell dimensions are obtained. For example, MIL-100(Fe,Cr) is for the measurements (pure Co cathode for cobalt
oxidized into (Fe,Cr)2O3, Rietveld refinement is thus of no use determination and mixed FeNiCu lamp for Fe determination).
for metal quantification. When the main and the doping metals Lab PXRD analyses of burned MIL-100(Fe,M) samples were
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have a large difference in number of electrons (i.e. Al and Fe) or performed using
a MAR345 diffractometer with X-rays
when absorption coefficients are too high, the method also generated by a Rigaku UltraX 18S X-ray generator (molybdenum
gives uncertain results (although absorption correction in anode, 0.71073 Å), with a monochromated beam (Xenocs FOX
Rietveld refinement can be applied to a certain extent). The 3D mirror).
main advantage of this method is that the analysis can be TGA/DSC measurements under air and nitrogen were recorded
coupled to TGA measurements, by measuring a PXRD pattern of on a Mettler Toledo TGA/DSC 3+ STARe System. Gas flows of 100
the combustion residue. Characterization by TGA and by ICP (or ml/min and heating rates of 10 °C/min were used.
AAS) measurements via conventional methods would require NMR spectra were recorded at room temperature (296 K) on a
the destruction of a larger amount of sample than the method Bruker Avance II 300 spectrometer operating at 300.1 MHz
we propose. The precision of our method was tested against ICP for 1H and 75.76 MHz for 13C. Experiments were run under
and AAS for MIL-100(Fe,Co) and against ICP for MIL-100(Fe,M) TopSpin program (3.2 version, Bruker) using a BBO or a BBFO
(M = Cr, Co, Ni, Cu and Zn), showing its validity (see Fig. S13).
{1H,X} probeheads equipped with a z-gradient coil.
Chemicals
Trimesic acid (98%) and sulphuric acid (96%) were purchased
from Acros Organics. Denaturated ethanol (Technisolv, 99%),
diethyl ether (GPR Rectapur), hydrochloric acid (37%) and
sodium hydroxide pellets were purchased from VWR Chemicals.
Iron sulphate (technical grade) was supplied from Forever
products. Cobalt(II) sulphate heptahydrate (99+%) was
purchased from Janssen Chimica. Copper(II) chloride dihydrate
was purchased from Riedel-deHaën. Cadmium nitrate
hexahydrate (99.9%), aluminium chloride (99.9%), vanadium(III)
chloride (97%) and titanium(III) chloride (99.999%) were
purchased from Sigma-Aldrich. Cobalt(II) chloride hexahydrate
(98%) was purchased from Fluka. Nickel(II) chloride
hexahydrate, yttrium(III) nitrate pentahydrate and potassium
dichromate (99.8%) were purchased from Merck.
Praseodymium(III,IV) oxide (99.9%) was purchased from Acros.
Lanthanum(III) carbonate was purchased from K&K
Laboratories, Inc. Samarium(III) oxide (99.9%) was purchased
from Koch-Light Laboratories Ltd. Manganese(II) chloride (98%),
cerium(III) acetate hydrate (99.9%) and terbium(III) nitrate
hexahydrate (99.999%) was purchased from Aldrich. Zinc
chloride (98%) was purchased from Alfa Aesar.
Experimental
Materials and methods
Synchrotron Powder X-ray diffraction (S-PXRD) patterns of
MIL-100(Fe,M) samples were recorded at the Swiss-Norwegian
beamlines (SNBL) at the European Synchrotron Radiation
Facility (ESRF, Grenoble, France) at a wavelength of 0.68683 Å
or 0.79800 Å using a Pilatus 2M detector. The samples were
loaded into glass capillaries of 0.5 mm diameter. For
temperature dependent measurements, a calibrated heating
gas blower was used and the samples were heated from room
temperature to 500°C.
ATR-IR spectra were recorded using
a Bruker Alpha
spectrometer equipped with a Platinum ATR module (diamond
crystal) housed in an MBraun argon-filled glovebox. Spectra
were recorded in the range of 4000 – 370 cm-1 with a resolution
of 4 cm1.
Nitrogen sorption measurements at 77 K were performed on a
Micromeritics ASAP 2020 instrument. All samples were
degassed at 200°C during 10h prior to analysis.
XPS analyses were carried out with a SSI-X-probe (SSX 100/206)
photoelectron spectrometer from Surface Science Instruments,
equipped with a monochromatized microfocus Al X-ray source.
The samples were prepared by sticking on a double-face
adhesive tape mounted onto small brass samples holders that
were placed on an insulating ceramic carousel (Macor). An
electron flood gun combined with a nickel grid were used to
avoid charge effects. The CasaXPS software (Casa Software Ltd.)
was used to perform the data treatment.
All chemicals were used as received except for iron sulphate,
which was purified by heating a saturated solution in boiling
water in the presence of iron metal and sulphuric acid for one
hour. The obtained hot solution was filtered and allowed to
cool. Ethanol was added to the solution to crystallize the iron
sulphate, which was isolated by filtration on a glass frit. The
purified FeSO4 heptahydrate was washed with ethanol and
dried under vacuum (using a Schlenk line).
ICP-AES measurements were performed after sample
calcination at 500°C followed by digestion in aqua regia. An
ICAP-6500 apparatus from ThermoFisher Scientific was used to
carry out the analyses.
Synthesis of MIL-100(Fe) in basic conditions (old procedure)
H3BTC (1.42 g) and NaOH (0.80 g) were added to 125 ml of
deionized water and the solution was sonicated until all the
H3BTC was dissolved (pH = 12). A second solution was made by
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dissolving 2.25 g of FeSO4∙7H2O in 125 ml of water (pH = 4.87). filtered using a glass frit and acetone was usedVtieow AwrtaicslehOntlhinee
Both solutions were mixed and allowed to react in the presence resulting crystals. The salt was then dried under vacuum (using
DOI: 10.1039/D0NJ00257G
of air (in a 500 ml erlenmeyer flask, 500 rpm agitation) for 15 a Schlenk line) and stored in a desiccator. FTIR LaCl3∙xH2O: 3356
hours. Immediately after addition, the measured pH was 7.25, cm-1 (broad), 2117 cm-1, 1616 cm-1 (sharp, intense), 582 cm-1,
and the obtained precipitate dark green, then the colour turned 415 cm-1 ; SmCl3∙xH2O: 3330 cm-1, 3226 cm-1 (broad), 2235 cm-
to orange upon oxidation. The precipitate was separated from 1, 1624 cm-1 (sharp, intense), 1084 cm-1 (broad), 586 cm-1, 448
the supernatant liquid by centrifugation and washed using 3 x cm-1 ; PrCl3∙xH2O 3330 cm-1, 3222 cm-1 (broad), 2226 cm-1, 1628
50 ml of deionized water and 3 x 50 ml of denatured ethanol. cm-1 (sharp, intense), 1080 cm-1 (broad), 574 cm-1, 440 cm-1.
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The obtained solid was then dried using a rotary evaporator at
Synthesis of MIL-100(Fe,M) in acidic conditions (new procedure)
70°C and once the powder was dry, the evaporation flask was
The syntheses of the doped MIL-100(Fe,M) MOFs were realized
allowed to cool under vacuum, using a Schlenk line.
by following the identical procedure to that for the synthesis of
MIL-100(Fe) in acidic conditions. The only difference is that the
Synthesis of MIL-100(Fe) in acidic conditions (new procedure)
Na3BTC trihydrate was obtained by adding 1 eq. of H3BTC to 3 2.25 g of iron sulphate were replaced by a mixture of
eqs. NaOH in a minimum amount of water. The obtained FeSO4∙7H2O and another metal salt in different ratios. Table S2
solution was filtered to remove any unreacted trimesic acid. A lists the used salts and their respective quantities. The pH and
Ethanol was added to the filtrate to precipitate the sodium the colours of the different precipitates is also given in Table S2
trimesate. The obtained compound was washed with ethanol to B.
remove any unreacted NaOH (the pH was tested using a paper
indicator) and then with diethylether. The obtained white fluffy
Conclusions
very light powder was then dried in a large beaker in an oven at
1
45°C overnight. H NMR (300.1 MHz, D2O) δ 8.42 (s, 3H). 13C
We developed a green method for synthesizing bimetallic
NMR (75.76 MHz, D2O) δ 174.75, 136.53, 131.43. TGA weight
loss upon dehydration: exp. 17.8 %, calc. 16.4 %.
MIL-100(Fe,M) compounds at room-temperature in water,
avoiding energy demanding solvothermal processes and
avoiding the use of strong mineral acids, all these conditions are
good prerequisites for an easy scale up. This synthesis is
performed in a single step, and therefore is less time-consuming
and more efficient than post-synthetic exchange approaches.
We showed that the incorporation of doping metals is only
possible at pH values below 7 and that Na3BTC∙3H2O is a useful
ligand source. Importantly, we demonstrated that the amount
of the incorporated doping metal can be tuned in a controlled
manner. Metals in the +I, +II as well as +V oxidation states and
lanthanides were incorporated as dopants into the structure of
MIL-100(Fe) for the first time. We showed that large differences
between the ionic radius of the doping and the templating
metals does not lead to phase segregation. We also showed that
the incorporation of Cu(I) drastically lowers the thermal stability
of MIL-100(Fe,Cu) and that the incorporation of V, Al and Ti
induces formation of mesopores. Finally, we developed a PXRD-
based method that can be combined with TGA to easily access
the metal ratios in complex mixed-metal MOFs with less sample
required than the currently used ICP and AAS based methods.
The obtained MIL-100(Fe,M) compounds could serve in fields
like gas sorption and catalysis. On the one hand, the presence
of metals in the +II oxidation state can strongly influence the
absorption of CO2 or could serve in redox catalysis. On the other
hand, the presence of lanthanide and other high oxidation state
metals could serve as efficient Lewis acid sites for catalysis.
Research in these directions is currently underway in our
laboratory.
1.86 g of Na3BTC trihydrate was dissolved in 125 ml of deionized
water. A second solution was made by dissolving 2.25 g of
FeSO4∙7H2O in 125 ml of water (pH = 4.87). Both solutions were
mixed together and allowed to react in the presence of air (in a
500 ml erlenmeyer flask, 500 rpm agitation) for 15 hours.
Immediately after addition, the measured pH was 6.08, and a
yellow precipitate appeared slowly, then the colour turned to
orange upon oxidation. The precipitate was separated from the
supernatant liquid by centrifugation and washed using 3 x 50 ml
of deionized water and 3 x 50 ml of denaturated ethanol. The
obtained solid was then dried using a rotary evaporator at 70°C
and once the powder was dry, the evaporation flask was
allowed to cool under vacuum, using a Schlenk line.
Synthesis of CrCl3 hexahydrate
Chromium(III) chloride was synthesized by gently adding 5.92 g
of K2Cr2O7 with 20 ml of methanol in 17 ml of concentrated HCl
(37%). This reaction was carried out in a very large erlenmeyer
flask placed in an ice bath (the reaction is very exothermic). The
obtained solution was then boiled to dryness and the obtained
green solid was washed with ethanol and ether followed by
drying under vacuum. FTIR 3526 cm-1 (sharp), 2986 cm-1
(broad), 1584 cm-1, 888 cm-1, 722 cm-1, 612 cm-1, 494 cm-1, 408
cm-1, the spectrum corresponds to reference data.46
Synthesis of LaCl3, SmCl3 and PrCl3 hydrates
To 3 eqs. of concentrated (37%) aqueous HCl was added 1 eq.
Ln in the form of the corresponding oxide (La2O3, Sm2O3 or
Pr6O11). The oxide was reacted with the acid until a clear
solution was obtained. Then excess acetone was added to
remove the excess water and the organic and aqueous layers
were shaken vigorously. The organic layer was then eliminated
by decantation. This operation was repeated until a solid
started to precipitate in the aqueous layer. The solid was then
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
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DOI: 10.1039/D0NJ00257G
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The authors acknowledge financial support through the FRIA
funding of F.R.S./FNRS for T.S. and the FNRS for grants EQP
U.N038.13 and CdR J.0164.17. Dr. Dmitry Chernyshov and Dr.
Iurii Dovgaliuk are acknowledged for their help during
synchrotron measurements at the SNBL of the ESRF in
Grenoble. We thank Dr. Pierre Eloy for help with the XPS
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800x400mm (96 x 96 DPI)