An amino-modified Zr-terephthalate metal-organic framework as an acid-base catalyst for cross-aldol condensation

Article abstract of DOI:10.1039/c0cc03038d

After controlled pretreatment, some Zr-terephthalate metal-organic frameworks are highly selective catalysts for the cross-aldol condensation between benzaldehyde and heptanal. The proximity of Lewis acid and base sites in the amino-functionalized UiO-66(NH₂) material further raises the reaction yields.

Full text of DOI:10.1039/c0cc03038d

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An amino-modified Zr-terephthalate metal–organic framework
as an acid–base catalyst for cross-aldol condensationw
Frederik Vermoortele,a Rob Ameloot,a Alexandre Vimont,b Christian Serrec and
Dirk De Vos*a
Received 4th August 2010, Accepted 22nd October 2010
DOI: 10.1039/c0cc03038d
After controlled pretreatment, some Zr-terephthalate metal–
organic frameworks are highly selective catalysts for the cross-
aldol condensation between benzaldehyde and heptanal. The
proximity of Lewis acid and base sites in the amino-functionalized
UiO-66(NH2) material further raises the reaction yields.
that in the amino-substituted analogue UiO-66(NH2), the
cooperative action between the framework Lewis sites and
ligand base groups results in high yields for the cross-aldol
reaction.
UiO-66 was readily prepared under solvothermal conditions
at 393 K in dimethylformamide starting from ZrCl4 and
terephthalic acid;z the material displayed an X-ray powder
diffraction (XRPD) pattern identical to the reported one.15w
Simple replacement of terephthalic acid by 2-aminoterephthalic
acid in the procedure leads to highly crystalline UiO-66(NH2)
in the same conditions of solvent and temperature, even if the
synthesis duration was adapted because of the accelerating
effect of the amine group on the crystallization. UiO-66
exhibits a rigid structure, and we expect the diffraction pattern
of UiO-66(NH2) to be highly similar to that of the non-
substituted material, which was also confirmed by measuring
the XRPD.16 The measured BET surface areas, following the
consistency criteria,20 of UiO-66 and UiO-66(NH2) are 891 m2 gꢀ1
and 1206 m2 gꢀ1, respectively, indicating the highly accessible
surface area of both materials. Based on the SEM observations,
both materials occur as small cubic crystals of B100 nm.w
TGA confirms the high thermal stability of UiO-66, which
collapses only at temperatures above 723 K, while the amino-
substituted variant is stable up to 548 K. Long-term thermal
stability of UiO-66(NH2) was confirmed in a test for 3 months
at 433 K.w Both UiO-66 and UiO-66(NH2) are moisture stable
and as proven by thermogravimetric analysis of DMF-free
Metal–organic frameworks have drawn a lot of attention
because of their tunable composition and structural diversity
combined with a high porosity.1 Many applications have been
studied so far. Among these, gas storage and separation
processes have been investigated most intensively.2 Catalysis
with MOFs is a much less studied field, partly because of the
lower stability of MOFs in comparison to e.g. zeolites, or
because of the lack of accessible active sites.3 Several strategies
have been followed to introduce catalytic activity within the
structure, e.g. use of functionalized ligands,4 post-treatment of
the materials,5 introduction of nanosized metallic clusters,6 or
use of the MOFs as supports for organometallic complexes or
polyoxometallates.7 The use of the true lattice active sites of
MOFs for catalysis has only been described for a limited
number of structural metal ions, including Cr,8 Fe,9 Cu,10
Zn,11 Mn,12 Sc,13 etc.
Recently a few highly stable MOFs were synthesized using
less common metals like Ti and Zr.14,15 The stability of these
MOFs is an incentive for their use in catalysis. For instance
UiO-66 [Zr6O4(OH)4(O2C–C6H4–CO2)6] (UiO for University
of Oslo) shows high thermal stability due to the presence of the
Zr6O4(OH)4 inorganic building blocks.15 The triangular faces
of the Zr6-octahedron in this structure are alternatively capped
with m3-O and m3-OH groups. These building units are bound
to 12 other inorganic subunits through terephthalate ligands.
This results in a square-antiprismatic coordination of each Zr
atom with eight oxygen atoms, of which four derive from
carboxylates, and four from m3-O and m3-OH groups. The
framework itself comprises tetrahedral and octahedral cages,
in a 2 : 1 ratio, of free dimensions close to 8 and 11 A
respectively. Access to the cages is provided by triangular
windows with a free diameter of close to 6 A. We here disclose
samples, they exhibit
a high water affinity, adsorbing,
respectively, 16 and 22 wt% water in 79% relative humidity.
This implies that at least a mild dehydration will be necessary
before initiating reactions with adsorbed organic compounds.
The UiO-66 materials were evaluated as catalysts in the
synthesis of jasminaldehyde (JA, a-n-amylcinnamaldehyde).
This fine chemical compound is applied in perfumery for its
violet scent and is commercially synthesized by the condensation
of heptanal (HA) and benzaldehyde (BA) using NaOH or
KOH as homogeneous basic catalysts (reaction (1) in
Scheme 1).17 Sorption experiments confirmed that UiO-66
and UiO-66(NH2) adsorb up to 25 wt% of jasminaldehyde
a Centre for Surface Chemistry and Catalysis, Department of
Microbial and Molecular Systems, K.U.Leuven, Kasteelpark
Arenberg 23, 3001 Leuven, Belgium.
E-mail: dirk.devos@biw.kuleuven.be
b Laboratoire Catalyse et Spectrochimie, UMR CNRS 6506 –
´
ENSICAEN, 6 Bd du Marechal Juin, 14050 Caen Cedex, France
Institut Lavoisier, UMR CNRS 8180, Universite de Versailles
c
´
Saint-Quentin-en-Yvelines, 45 Avenue des Etats-Unis, 78035,
´
Versailles Cedex, France
w Electronic supplementary information (ESI) available: XRD, TGA,
SEM images, filtration test. See DOI: 10.1039/c0cc03038d
Scheme 1
c
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from hydrocarbon solutions, proving that the pores are of
sufficient dimensions.y
Results of the reactions are shown in Table 1. The samples
used were either heated for a short duration at 423 K under air
to remove adsorbed water or were subjected to a more severe
dehydration at higher temperature under deep vacuum
(o10ꢀ6 bar for 6 h). A low heptanal to benzaldehyde ratio
of 1/15 was selected to limit the production of 2-pentyl-2-nonenal
by self-condensation of HA (reaction (2) in Scheme 1).
The activity strongly depends on the catalyst pretreatment.
UiO-66 heated under air after 1 h gives much less conversion
than the same material pretreated under deep vacuum at 573 K
(entries 2 vs. 3). Also, the eventual yield at full HA conversion
varies among the samples, with the best combination of high
yield and high initial rate observed for UiO-66(NH2) (entry 4).
In order to understand the effect of the pretreatments, the
dehydration process was followed in situ using FTIR (Fig. 1).
Self-supporting wafers of the MOFs were mounted in a heated
vacuum FTIR cell, and spectra were recorded at different
temperatures in order to determine the effect of the pretreatment
on the m3-OH groups of the Zr inorganic building block.
The clear nO–H stretching vibration of UiO-66 gradually shifts
upon thermal treatment under deep vacuum, from 3673 cmꢀ1
at 298 K, till 3665 cmꢀ1 at 523 K, until it completely
disappears at 573 K. As proposed by Lillerud et al.,15 this
corresponds to the transformation of the Zr6O4(OH)4 unit to
the Zr6O6 core, with potential access to one open coordination
site per Zr atom. The creation of these Lewis acid sites readily
explains the enhanced catalytic activity of UiO-66 after
dehydroxylation (Table 1, entries 2 vs. 3). Such acid functions
are able to activate either heptanal or benzaldehyde, thus
increasing the reaction rate. UiO-66(NH2) presents similar
spectra. Besides the strong nN–H vibrations, the nO–H is
observed at 3670 cmꢀ1, albeit with a weaker intensity than
in UiO-66. This might be due to the presence of intra-framework
hydrogen bonds between the amino and the hydroxyl groups,
as reported previously for MIL-53(NH2),18 or to a partial
dehydroxylation even at room temperature. In any case,
progressive dehydroxylation of the framework is observed
upon heating to 373 K and full dehydroxylation is already
achieved at 473 K. Note that for both materials, rehydration
fully restores the original spectra, which, together with XRD
follow-up, ensures that the materials survive the pretreatments
intact.
Fig. 1 Infrared spectra of (A) UiO-66 and (B) UiO-66(NH2), as such,
or upon dehydration in deep vacuum (bottom).
Two hypotheses can be forwarded to explain the reactivity
of samples that were not subjected to a deep dehydroxylation
pretreatment. First, the (weak) Bronsted acidity of the O–H
¨
groups may play a role. Alternatively, the temperature
dependent spectra (Fig. 1) show that dehydroxylation starts
at temperatures around 373 K for UiO-66, or even below
373 K for UiO-66(NH2). As samples were dehydrated at 423 K
to remove physisorbed water from the pores, and as the
reaction itself takes place at 383 K, even these mild conditions
are sufficient to induce partial dehydroxylation in reaction
conditions. As expected, fully dehydroxylated UiO-66 is more
active but the selectivity somewhat suffers from the production
of the HA self-condensation product (B20% of products
formed), which can be explained by the presence of too many
strong Lewis sites in the material.
By contrast, UiO-66(NH2) combines high activity with high
selectivity, resulting in yields that are eventually B10% higher
than for UiO-66. It was shown previously, e.g. for alumino-
phosphate materials,19 that the combination of acid and base
sites results in superior performance for a cross-aldol reaction:
a Lewis acid site can interact with the carbonyl group of
benzaldehyde, increasing its polarization and facilitating the
attack of heptanal, which is activated on a nearby base site.
In UiO-66(NH2), the Zr sites can activate benzaldehyde, while
the aminogroups in close proximity can activate the methylenic
group in the aliphatic aldehyde, leading to the superior
selectivity reported in Table 1.
Table 1 Conversions and selectivities after 1 h of reaction, and
jasminaldehyde yield at full HA conversion for the solventless reaction
of BA (1.4 g) and HA (0.1 g) in the presence of 10 wt% catalyst
(393 K)
Catalyst
Conversiona (%) Selectivityb (%) Yieldc (%)
1
2
3
4
5
—
UiO-66d
UiO-66e
1
30
42
—
82
81
91
90
—
85
80
92
92
Some further reactions were undertaken to optimize the
reaction for UiO-66(NH2). Increasing the temperature to 433 K
raised the yield to 90% after 1 h. Hot filtration tests proved
that the catalytic activity was associated with the solid exclusively,
and not with the liquid phase.w The cubic lattice is intact after
the catalytic experiments. By gradual addition of the heptanal
through a syringe pump to a mixture of benzaldehyde and
UiO-66(NH2)d 67
UiO-66(NH2)f 38
a
b
Conversion of HA after 1 h. Selectivity for JA, based on HA after
c
1 h. Yield of JA, at full HA conversion after 24 h. Pretreated under
d
e
air (423K). Pretreated under deep vacuum at high temperatures
f
(573 K). Pretreated under deep vacuum at high temperatures (473 K).
c
1522 Chem. Commun., 2011, 47, 1521–1523
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UiO-66(NH2), the reaction yield was eventually maximized at
93%, for a moderate overall benzaldehyde/heptanal ratio of 5/1.
In conclusion we report the highly selective synthesis
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DDV is grateful for funding in the Metusalem project
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z Synthesis of UiO-66(NH2): 13.5 mmol ZrCl4 (Acros), 13.5 mmol
2-aminoterephthalic acid (Sigma-Aldrich) and 13.5 mmol H2O were
dissolved in 6.7 mol N,N-dimethylformamide (Acros) at room
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1400 mg benzaldehyde was injected in a vial containing 150 mg
catalyst. The reaction mixture was stirred at 700 rpm and heated in
an aluminium heating block (373 K–433 K). Reaction samples were
filtered through a 0.45 mm filter and analyzed with a Shimadzu 2014
GC equipped with a FID detector and an apolar CP-Sil 5 CB column.
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Chem. Commun., 2011, 47, 1521–1523 1523