Influence of Terephthalic Acid Substituents on the Catalytic Activity of MIL-101(Cr) in Three Lewis Acid Catalyzed Reactions

Article abstract of DOI:10.1002/cctc.201700236

Six isostructural MIL-101(Cr)-X (X: H, NO₂, SO₃H, Cl, CH₃, and NH₂) materials have been prepared directly by the reaction of Cr^III salts and the corresponding terephthalic acid or by postsynthetic treatments of preformed MIL-101(Cr) following reported procedures. The materials were characterized by using XRD (crystallinity and coincident diffraction pattern), isothermal N₂ adsorption (specific surface areas range from 2740 m² g^?1 for MIL-101(Cr)-H to 1250 m² g^?1 for MIL-101(Cr)-Cl), thermogravimetry (thermal stability up to 400 °C), and IR spectroscopy (detection of the corresponding substituents), and the results were all in agreement with the reported data for these materials. The MIL-101(Cr) materials were tested as heterogeneous catalysts for epoxide ring opening by methanol, benzaldehyde acetalization by methanol, and Prins coupling, observing a clear influence of the substituent that in general follows a linear relationship with the Hammett σ_meta constant of the substituent: the catalytic activity increases as the electron-withdrawing ability of the substituents increases. An up to three orders of magnitude enhancement in the presence of the NO₂ substituent was found for some of these reactions. The present study illustrates the versatility that metal–organic frameworks offer as heterogeneous catalysts that allow the design of actives sites with adequate properties tuned for each reaction.

Full text of DOI:10.1002/cctc.201700236

Accepted Article
Title: Influence of terephthalic acid substituents on the catalytic activity
of MIL-101(Cr) for three Lewis acid catalyzed reactions.
Authors: Andrea Santiago-Portillo, Sergio Navalón, Patricia
Concepción, Mercedes Álvaro, and Hermenegildo García
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To be cited as: ChemCatChem 10.1002/cctc.201700236
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Influence of terephthalic acid substituents on the catalytic activity
of MIL-101(Cr) for three Lewis acid catalyzed reactions.
Andrea Santiago-Portillo,[a] Sergio Navalón,*[a] Patricia Concepción,[b] Mercedes Álvaro,[a] and
Hermenegildo García*[a][b][c]
Abstract: Six isostructural MIL-101(Cr)-X (X: H, NO2, SO3H, Cl, CH3
and NH2) have been prepared directly by reaction of Cr (III) salts and
the corresponding terephthalic acid or by post-synthetic treatments
of preformed MIL-101(Cr), following reported procedures. The
materials have been characterized by XRD (crystallinity and
coincident diffraction pattern), isothermal N2 adsorption (specific
surface area ranging from 2740 for MIL-101(Cr)-H to 1250 m2/g for
MIL-101(Cr)-Cl), Thermogravimetry (thermal stability up to 400ºC)
and IR spectroscopy (detection of the corresponding substituents)
that were all in agreement with the reported date for these materials.
The series of MIL-101(Cr) were tested as heterogeneous catalyst for
epoxide ring opening by methanol, benzaldehyde acetalization by
methanol and Prins coupling, observing a clear influence on the
substituent that in general follows a linear relationship with the
Hammett σm constant of the substituent, the catalytic activity
increases as the electron withdrawing ability of the substituents
increases. Up to three orders of magnitude enhance by the present
of NO2 substituent has been found for some of these reactions. The
present study illustrates the versatility that MOFs offer as
heterogeneous catalyst allowing the design of actives sites of
adequate properties tuned for each reaction.
enhanced by substitution in the aromatic ring of terephthalate
linker.[10] It was demonstrated that the electron donor or acceptor
ability of substituents influences through inductive effects the
electron density of the metal nodes and in this way the activity
as Lewis acid centers can be altered.[10] Considering the interest
of MOFs as catalysts, it is necessary to expand this initial study
to other MOFs particularly that have exhibited large stability and
activity, to determine in which extent and for which reaction
types this methodology based on the use of substituted aromatic
linkers can be useful in catalysis.[2]
In this context, in the present work, we have selected MIL-
101(Cr) and studied the influence of the presence of
terephthalate substituents on three different types of Lewis acid
catalyzed organic reactions. MIL-101 is constituted by trimeric
Cr3-µ3O building units acting as nodal centers that are
connected to six terephthalate linkers.[15-18] Each Cr3+ ion is
octahedrally coordinated by four terephthalate linkers, the µ3O
atom and a sixth position that is occupied by fluoride ions in two
of the three Cr atoms and by a solvent molecule in the third
one.[15-18] Figure 1 illustrates the structure of MIL-101(Cr), the
nodes and linkers defining supertetrahedral units that arrange
forming two types of cavities of 1.6 and 1.2 nm dimension,[18] the
smallest ones accessible through pentagonal windows (5.5 A)
and the largest (8.6 A) ones through a hexagonal openings.[18]
Introduction
Due to the large percentage of transition metal ions having
frequently coordinative exchangeable position, their large
porosity and accessibility of the intracrystalline voids, metal
organic frameworks (MOFs) have become amount the most
widely studied heterogeneous catalyst for the liquid phase
reactions.[1-8] While actives sites can also be located at the
linkers or hosted on the empty spaces of MOFs,[9] the most
frequent case in the use of MOFs as heterogeneous catalyst is
when metal nodes act as Lewis acid sites.[2, 10-14] In a pioneer
study de Vos and coworkers have shown for UiO-66 that the
catalytic activity of the material can be tuned and further
Figure 1. Schematic 3D representation of the MIL-101 structure made of
smaller (green) and larger (red) cages connected through super tetrahedra
(ST). The illustration shows in the left the ensemble of the two types of cages
and in the right isolated cages of each type of cavity. Reprinted with
permission from ref.[18]
[a]
Andrea Santiago-Portillo, Dr.S.Navalón, Dr Patricia Concepción,
Prof.M.Álvaro Prof. H.García
Departamento de Química and Instituto de Tecnología Química
CSIC-UPV, Universidad Politécnica de Valencia, Consejo Superior
de Investigaciones Científicas, Av. de los Naranjos s/n, 46022
Valencia, Spain.
E-mail: hgarcia@qim.upv.es
Center of Excellence for Advanced Materials Research, King
Abdulaziz University, Jeddah, Saudi Arabia
MIL-101 has a very large surface area and pore volume with
reported values of 2,700 m2/g and 2.1 cm3/g, respectively.[16, 18]
One of the most important features of MIL-101 is the reliability
of its preparation procedure and the high crystallinity of the
samples obtained.[16, 18] This situation contrasts with the initial
[b]
case for which the influence of substituents on catalysis was
[19, 20]
reported, namely, UiO-66.
It is known that UiO-66 can
present in its structure a large proportion of defects, depending
on the synthetic protocol used has been frequently reported.
The presence of defects with metal ions having unsaturated
positions would, certainly, overlay with the influence of
Supporting information for this article is given via a link at the end of
the document.
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Figure 2 XRD patterns of MIL-101(Cr)-NO2 (a), MIL-101(Cr)-SO3H (b), MIL-
101(Cr)-Cl (c), MIL-101(Cr)-H (d), MIL-101(Cr)-CH3 (e) and MIL-101(Cr)-NH2
(f).
substituents on the aromatic ring when determining the catalytic
activity of the materials, thus, making uncertain the exact
influence of substituents on the catalytic activity.[20] In contrast,
by selecting MIL-101 with low structural defects a more accurate
confirmation of the importance of the inductive effects played by
the substituent tuning the acid strength of the metal centers can
be obtained.[16, 21] In addition, MIL-101(Cr) also enjoys a high
structural, thermal and chemical stability that makes this material
among the most robust and stable MOFs, a prerequisite
necessary for its use as heterogeneous catalyst.[3, 14, 22-24]
Isothermal nitrogen adsorption measurements (Figures S2-S7)
revealed that MIL-101(Cr)-H is the one that exhibits the largest
specific surface area (2,750 m2 g-1) and pore volume (2.2 cm3 g-
1) (Table S1). The rest of the samples have lower specific
surface area and porosity volume ranging between 1,950 and
1,300 m2 g-1 and from 2.0 to 1.1 cm3 g-1 (Table S1).[24, 26] The
decrease in surface area and porosity when substituents are
present on the aromatic ring was already noticed in the literature
and attributed to the intracrystalline space required to
accommodate the substituents.[26]
Results and Discussion
Catalyst characterization
The presence of substituents and their nature can be
determined by infrared spectroscopy[26] wherein the specific
vibration peaks expected for each substituent were recorded.
Figures S8-S14 show the relevant region of the IR spectra
where the characteristic vibration bands corresponding to each
substituent have been indicated. IR spectra are particularly
relevant to follow post-synthetic modification of MIL-101(Cr)-H
and MIL-101(Cr)-NO2 leading to the formation of MIL-101(Cr)-
SO3H and MIL-101(Cr)-NH2, respectively, by monitoring the
disappearance of the NO2 bands (1500-1380 cm-1) and the
appearance of the characteristic vibrations of SO3H (1,160 cm-1)
and NH2 (3,450 and 1,625 cm-1).
Thermogravimetric analyses in air of MIL-101(Cr) materials
show their stability up to about 300 ºC (Figures S15-S20). In
general, a good relationship between the theoretical and the
experimental chromium content for all the MIL-101(Cr) materials
prepared in the present work was determined (Table S1).[26] In
the case of MIL-101(Cr)-Cl, the slightly higher metal content
experimentally observed (~1.65 %) respect to the theoretical
value may indicate the presence of some amount of chromium
oxide inside the cavities of the material. In addition, the FT-IR
spectrum of MIL-101(Cr)-Cl (Figures S8 and S12) presents a
weak band at around 1,710 cm-1 that could be ascribed to the
carbonyl stretching band of the non-coordinated free terephthalic
acid. Thus, in MIL-101(Cr)-Cl either the presence of chromium
oxide or the presence of free terephthalic acid inside the cavities,
may be the reason for their lower than expected BET surface
area and pore volume. In addition, these impurities could be
responsible for the lower intensity of the X-ray diffraction peaks
at low 2Ɵ angles characteristic of the pore symmetry in the MOF.
In the present study we have prepared six MIL-101(Cr)-X (X: H,
NO2, NH2, Cl, SO3H, CH3) having different substituents on the
terephthalate linker. Scheme S1 summarizes the preparation of
each of the samples under study, while the experimental section
contains the details, amounts of reagents, temperatures and
times of the hydrothermal synthesis used in the preparation of
the six MIL-101(Cr)-X. Four of them, including the parent MIL-
101(Cr)-H, were obtained directly by reaction of chromium salts
with the corresponding terephthalate under hydrothermal
condition as previously reported.[16, 25-27] The MIL-101(Cr)-SO3H
was obtained by reaction of MIL-101(Cr)-H with HSO3Cl,[27] while
MIL-101(Cr)-NH2 was prepared from MIL-101(Cr)-NO2 chemical
reduction by of the nitro groups with SnCl2 in acid media.[25] The
series of MIL-101(Cr)-X used in the present study have been
already reported in the literature and in our case characterization
data agrees with those already reported for these samples. [16, 25-
27]
Specifically, PXRD diffraction of the samples exhibits the
same pattern indicating that all of them are isoestructural (Figure
2). The broad X-ray diffraction peaks observed for the MIL-
101(Cr)-X materials are probably due to their small crystal sizes
ranging from 100 to 400 nm, as revealed by SEM images
(Figure S1). In the particular case of MIL-101(Cr)-CH3, two types
of crystals of submicrometric and micrometric sizes were also
observed in SEM.
Catalytic activity
As commented earlier the purpose of this study was to assess
the influence of substituents on the terephthalate linker on the
catalytic activity of the MIL-101(Cr)-X for three different acid
catalyzed reactions (Scheme 1). Aimed at this purpose, the ring
opening of epoxide by methanol (Figure 3), benzaldehyde
acetalization by methanol (Figure 4) and the Prins coupling of β-
pinene and formaldehyde (Figure 5) and the reactions carried
out in the presence of the set of MIL-101(Cr)-X as
heterogeneous catalysts.
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(■), MIL-101(Cr)-SO3H (○), MIL-101(Cr)-Cl (▲), MIL-101(Cr)-CH3 (∆), MIL-
101(Cr)-NH2 (●) and blank control in the absence of catalyst (♦). Reaction
conditions: Catalyst (0.04 mmol of Cr), benzaldehyde (1 mmol), methanol (3
mL), 30 ºC. b) Plot of the logarithm of the relative initial reaction rate vs the σm
Hammett constant of each substituent on the terephthalate linker.
Figure 5. a) Time-conversion plot for the Prins coupling of β-pinene using
each MIL-101(Cr)-X as catalyst. Legend: MIL-101(Cr)-H (□), MIL-101(Cr)-NO2
(■), MIL-101(Cr)-SO3H (○), MIL-101(Cr)-Cl (▲), MIL-101(Cr)-CH3 (∆), MIL-
101(Cr)-NH2 (●) and blank control in the absence of catalyst (♦). Reaction
conditions: Catalyst (0.08 mmol of Cr), β-pinene (1 mmol), formaldehyde (1
mmol), acetonitrile (2.5 mL), 80 ºC. b) Plot of the logarithm of the relative initial
reaction rate vs the σm Hammett constant of each substituent on the
terephthalate linker.
Scheme 1. Three acid-catalyzed reactions that were studied using the series
of MIL-101(Cr) solids as heterogeneous catalysts. The temporal profiles of
substrate conversion depending on the substituent on the terephthalate linker
are presented in Figures 3-5. As it can be seen there, the three reaction types
follow exactly the same relative activity order for the set of catalysts, MIL-
101(Cr)-NO2 being far more active than the rest of catalyst. Figures 3-5
include also the corresponding plot of the relative initial reaction rate in
logarithmic scale versus the σm Hammett constant of the substituent.
In the precedent in the literature,[10] the σmeta Hammett constant
of the substituent on the aromatic ring was selected as the
representative parameter to correlate the relative initial reaction
rates due to the better linearity respect to σortho. This was also
the parameter selected in the present study. As it can be seen in
these Figures, epoxide ring opening (Figure 3) and Prins
coupling (Figure 5) follow a relatively good linear relationship
between catalytic activity to the material and σm Hammett
constant, while in the case of benzaldehyde acetalization, MIL-
101(Cr)-X having electron donor substituents (CH3 and NH2)
have higher catalytic activity than expected according to σm
values (Figure 4). In the case of epoxide ring opening the initial
reaction rate changes four orders of magnitude from NH2 (the
less active catalyst; 0.00058 mmol h-1) to the NO2 substituent
(5.49 mmol h-1) and the catalytic activity of the NO2 substituted
catalyst increases more than 300 times respect to the parent
MIL-101(Cr) (0,018 mmol h-1). Although similar trend is also
observed for the Prins reaction, in this case, the influence of the
substituent is not so remarkable, the initial relative reaction rate
increasing from the NH2 (0.00078 mmol h-1) to the NO2 (0.166
mmol h-1) functionalized material by 200 times, the NO2 catalyst
being about 8 times more active than the parent MIL-101(Cr)
(0.021 mmol h-1).
Figure 3. a) Time-conversion plot for the ring opening of styrene oxide by
methanol using each MIL-101(Cr)-X as catalyst. Legend: MIL-101(Cr)-H (□),
MIL-101(Cr)-NO2 (■), MIL-101(Cr)-SO3H (○), MIL-101(Cr)-Cl (▲), MIL-
101(Cr)-CH3 (∆), MIL-101(Cr)-NH2 (●) and blank control in the absence of
catalyst (♦). Reaction conditions: Catalyst (0.04 mmol of Cr), styrene oxide (1
mmol), methanol (2.5 mL), 50 ºC. b) Plot of the logarithm of the relative initial
reaction rate vs the σm Hammett constant of each substituent on the
terephthalate linker.
To put these values in context, it has to be indicated that in the
case of substituted UiO-66-X previously reported the enhanced
of catalytic activity for the cyclization of citronellal was 56 times.
For the case of benzaldehyde acetalization, also a remarkable
three orders of magnitude enhanced of the catalytic activity by
introduction of NO2 group (3.105 mmol h-1) versus the parent
MIL-101(Cr) (0.0113 mmol h-1) was observed, although as
commented above the NH2 substituted material was significantly
more efficient (about two orders of magnitude) than expected by
considering exclusively the σm Hammett constant. Since in the
Figure 4. a) Time-conversion plot for the benzaldehyde acetalization using
each MIL-101(Cr)-X as catalyst. Legend: MIL-101(Cr)-H (□), MIL-101(Cr)-NO2
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previous two reactions a good linearity has been observed, it is
unlikely that this deviation of the linearity could be due to some
defects on the MOF structure or some other structural factor.[10,
coincident for MIL-101(Cr)-NO2 and MIL-101(Cr)-NH2 (Figure
S1). This characterization data reinforces that the increase of
catalytic activity of MIL-101(Cr) materials tested in the present
work for three acid catalyzed reactions should be attributed to
the higher Lewis acidity induced by the presence of electron
withdrawing groups.
14]
It is suggested that the higher than expected activity of MIL-
101(Cr)-NH2 should reflect the operation of an alternative
reaction mechanism in which besides metal centers with tunable
electron density by the terephthalate linker, the amino groups
play active role, probably interacting with the carbonyl group of
benzaldehyde and opening new pathways for acetalization,
likely involving an imine.[14, 28] It is also possible that other factors
like different adsorption ability could contribute to the observed
deviation of the linearity.
In order to correlate the observed catalytic activity with the Lewis
acidity of the sample, CO adsorption monitored by IR
spectroscopy has been used for Lewis acid titration (Figure 6).[29]
Figure 6 shows the IR spectra of CO adsorbed on MIL-101(Cr)-
NO2, MIL-101(Cr)-NH2 and MIL-101(Cr)-H catalysts, observing a
shift in the value of the ν(CO) frequency that appears at 2198
cm-1 for the most active MIL-101(Cr)-NO2 sample, 2194 cm-1 for
the parent MIL-101 (Cr)-H sample, and 2191 cm-1 for the less
active MIL-101(Cr)-NH2 sample. The observed CO frequencies
are due to the interaction of CO with Cr3+,[30] where a shift to
higher frequencies should correspond to a higher Lewis acidity
of the Cr3+ site. Accordingly, a quantitative scale of Lewis acidity
can be determined for the samples MIL-101(Cr)-X, following the
order MIL-101(Cr)-NO2> MIL-101(Cr)-H> MIL-101(Cr)-NH2,
which correlate with the catalytic activity of the samples.
The absence of metal leaching and the occurrence of truly
heterogeneous reaction mechanisms were demonstrated in the
cases of MIL-101(Cr)-NO2 by analysis of the liquid phase after
the reactions and by hot filtration tests (Figures S21-S23).
Chemical analysis shows that the amount of Cr at the end of the
reactions is lower than 0.5 % of the total chromium present in
the initial catalyst. Control experiments under the same
conditions using the amount of chromium determined by
chemical analysis show no activity for the reactions under
consideration under the conditions employed in the presence
study (Figure 7a). In the case of the hot filtration test, the solid
catalyst was removed at the reaction temperature, once the
reaction has been initiated and conversion reached about 50%.
It can be seen in Figures S21-S23 that the reactions stop once
the solid catalyst is removed from the reaction.
Figure 7. Time-conversion plot for the Prins coupling of β-pinene using MIL-
101(Cr)-NO2 and Chromium acetate as catalyst. Legend: MIL-101(Cr)-NO2 (■),
chromium acetate in the same amount of MIL-101(Cr)-NO2 (□), chromium
acetate in the same amount of the leaching measured in MIL-101(Cr)-NO2 (○)
and chromium acetate in double amount than the leaching occurring in MIL-
101(Cr)-NO2 (∆). Reaction conditions: Catalyst (0.08 mmol of Cr), β-pinene (1
mmol), formaldehyde (1 mmol), acetonitrile (2.5 mL), 80 ºC. The inset of panel
b) shows the XRD of fresh MIL-101(Cr)-NO2 (A), three times used MIL-
101(Cr)-NO2 (B) and four times used MIL-101(Cr)-NO2 (C).
Figure 6. FTIR spectra of CO adsorption at 6mbar and -176 ºC on MIL-101
(Cr)-NO2, MIL-101 (Cr)-NH2 and MIL-101 (Cr)-H samples.
Also in the case of MIL-101(Cr)-NO2 reusability was confirmed
by performing a series of consecutive runs using the same
sample for Prins reaction (Figure 7b). At the end of each
reaction cycle, the catalyst was separated by filtration, washed
twice with hot ethanol and dried before the next use. As it can be
seen in Figure 7b the catalytic activity as well as the crystallinity
of the solid material is maintained upon reuse.
Finally, the catalytic activity of MIL-101(Cr)-NO2 was compared
with the activity of chromium acetate as homogeneous catalyst
under the same conditions. The results are presented in Figure
The higher catalytic activity of MIL-101(Cr)-NO2 respect to MIL-
101(Cr)-NH2 is mainly attributed to electronegative inductive
effects, therefore, increasing the Lewis acidity of the solid
material as the inductive effect of the substituent in the aromatic
ring increases. As commented before, the MIL-101(Cr)-NH2
material was obtained by chemical reduction of preformed MIL-
101(Cr)-NO2.This chemical reaction does not apparently alters
the morphology of the MOF particles and SEM images reveal
that the shape and particle size distribution are almost
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were introduced into a Teflon autoclave containing demineralized water
(8 mL). Subsequently, in the case of MIL-101(Cr)-H, HF (10 µL) was
added. Then, the autoclave was heated at the corresponding
temperature for a specific period of time (see Table 1 for synthesis
conditions for each MIL-101(Cr)-X). Once the autoclave was cooled at
room temperature, the formed precipitate was washed several times with
dimethylformamide (DMF) at 120 ºC and, then, with ethanol at 80 ºC.
MIL-101(Cr)-NH2 material was prepared by post-synthetic reduction of
MIL-101(Cr)-NO2 with SnCl2·H2O according to reported procedures.[25] In
the case of MIL-101(Cr)-SO3H, the material was obtained by post-
synthetic sulfonation of MIL-101(Cr)-H with chlorosulfonic acid (HSO3Cl)
as previously reported in the literature.[27]
7. As it can be seen there, chromium acetate is also active to
promote the Prins coupling, but MIL-101(Cr)-NO2 exhibits higher
initial rate and also higher conversion and higher final
conversion at 48 h than the soluble chromium salt, this showing
the advantage of introducing an adequate substituent to
enhance the catalytic activity of active sites. When comparing
homogeneous and heterogeneous catalyst, it is a general
observation that immobilization of the active sites on a solid
results in lower activity due to diffusion restrictions of substrates
and reagents to reach the sites that are less accessible than
when they are dissolved in a liquid phase. Although this could be
the case when comparing the catalytic activity of chromium
acetate in CH3CN solution with MIL-101(Cr)-H, the possibility to
introduce an electron withdrawing substituent on the
terephthalate linker renders the solid catalyst far more active
than the homogeneous Cr(III) salt.
Table 1. Temperature and time employed in the preparation of MIL-101(Cr)-X
Conclusions
materials.
In the present work it has been shown that, in agreement with
previous reports, the catalytic activity of MOFs as solids Lewis
acids can be increased by the presence of electron withdrawing
substituents on the organic linker that by inductive effects can
increase the electronic deficiency on the nodal metal ions acting
as catalytic centers. In the present case, up to three orders of
magnitude enhancement has been determined when comparing
the activity of the amino and nitro substituted MIL-101(Cr)-X.
With respect to the studies reported in the literature, the present
study has the advantage of being based on MIL-101 that is
known to be one of the most crystalline MOFs structure is less
prone to the presence of structural defects, thus, providing
evidence in support of the influence of the substituent on firm
ground. Two of the reactions studied epoxide ring opening and
Prins coupling, exhibit a high linearity between the relative initial
reaction rates in logarithm scale and the σm Hammett constant,
suggesting that no other factors besides acidity are influencing
in the process.
Temperature (ºC)
Time (h)
MIL101(Cr)-H
200
180
180
180
8
MIL101(Cr)-NO2
MIL101(Cr)-Cl
MIL101(Cr) -CH3
120
96
120
Catalyst characterization. Powder X-ray diffraction (PXRD) patterns of
MIL-101(Cr)-X materials were recorded on a Philips XPert diffractometer
equipped with a graphite monochromator (40 kV and 45 mA) employing
Ni filtered CuKα radiation. ATR-FTIR spectra of MIL-101(Cr)-X materials
were collected using a Bruker Tensor27 instrument after heating the
samples were heated in an oven (100 ºC for 16 h) to remove physisorbed
water. CO-FTIR spectra were recorded in
a Nexus 8700 FTIR
spectrophotometer using an IR cell allowing in situ treatments at
controlled temperature, from -176 ºC to 500 ºC, and connected to a high
vacuum system with gas dosing facility. For CO adsorption
measurements the samples were pressed into self-supported wafers and
treated in vacuum (10-6 mbar) at 150 ºC for 2 h. After activation, the
wafers were cooled down to -176 ºC under dynamic vacuum followed by
CO dosing at increasing pressure (0.4-6 mbar). IR spectra were collected
after each dosage. All IR spectra corresponding to CO adsorption
measurement have been normalized to the weight of the IR wafer. N2
Experimental Section
Materials. All the reagents and solvents used in this work were of
analytical or HPLC grade and supplied by Sigma-Aldrich.
Catalyst preparation. Isoestructural MIL-101(Cr)-X (X: H, NO2, SO3H,
Cl, CH3 and NH2) materials were prepared following already reported
procedures.[16, 25-27] Briefly, the corresponding substituted terephtalic acid
(1.5 mmol) and Cr(NO3)3·9H2O (for the synthesis of MIL-101(Cr)-H) or
CrCl3 (for the synthesis of MIL-101(Cr)-X (X: NO2, Cl, CH3) (1 mmol)
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adsorption isotherms at 77 K were recorded using a Micromeritics ASAP
ionization detector. Quantification was carried out by using calibration
curves of authentic samples against nitrobenzene.
2010 device. Termogravimetric analyses were performed on
a
TGA/SDTA851e METTLER TOLEDO station. Scanning electron
microscopy (SEM) images were collected using a Zeiss instrument.
Leaching experiments. At the end of the Prins reaction, the most active
MIL-101(Cr)-NO2 sample was removed by filtration (Nylon filter, 0.2 µm).
Then, the organic phase was stirred with HNO3 aqueous solution (3 M),
and the system heated at 80 ºC for 24 h to extract Cr3+ ions. The
presence of chromium in the aqueous phase was analyzed by chemical
analysis using an ICP-AES instrument.
Catalytic experiments. Briefly, the required amount of MIL-101(Cr)-X
(X: H, NO2, NH2, Cl, CH3) employed as catalyst (0.04 mmol of Cr for ring
opening of epoxides and acetalization or 0.08 for the Prins reaction) was
introduced into a two-necked round-bottom flask (25 mL). Then, the MIL-
101(Cr) materials were pretreated at 150 ºC under vacuum for 16 h to
remove solvent molecules and generate coordinatively unsaturated sites
at the metal clusters. Subsequently, the system was cool down to the
required reaction temperature. In the case of the ring opening epoxides,
styrene oxide (1 mmol) dissolved in methanol (2.5 mL) was added to the
round-bottom flask containing the activated catalyst previously stabilized
at 50 ºC. In the acetalization of aldehydes, benzaldehyde (1 mmol)
dissolved in methanol (3 mL) was introduced at 30 ºC in the round-
bottom flask containing the activated catalyst. In the Prins reaction, β-
pinene (1 mmol) and formaldehyde (1 mmol) dissolved in acetonitrile (2.5
mL) were added to the round-bottom flask containing the activated
catalyst that was equilibrated at 80 ºC. The reactions were magnetically
stirred and reaction aliquots sampled at the corresponding reaction times.
Acknowledgements
Financial support by the Spanish Ministry of Economy and
Competitiveness (Severo Ochoa and CTQ2015-69153 and
CTQ2014-53292-R and Generalitat Valenciana (Prometeo
2013/14) and is grateful acknowledged. SN thanks financial
support by the Fundación Ramón Areces (XVIII Concurso
Nacional para la Adjudicación de Ayudas a la Investigación en
Ciencias de la Vida y de la Materia, 2016).
Keywords: Heterogeneous catalysis • Lewis acid catalysis •
metal-organic frameworks • effect of ligand substituent • tuning
of catalytic activity
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Entry for the Table of Contents (Please choose one layout)
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Electron withdrawing substituents on
terephthalate ligand of MIL-101(Cr)-X
enhance the catalytic activity of the
material for Lewis acid catalyzed
reactions
Andrea Santiago-Portillo, Sergio
Navalón, Patricia Concepción,
Mercedes Álvaro, Hermenegildo
García*
Page No. – Page No.
Title
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