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Live encapsulation of a Keggin polyanion in NH2-MIL-101(Al) observed
by in situ time resolved X-ray scatteringw
Jana Juan-Alcañiz,a Maarten Goesten,a Alberto Martinez-Joaristi,a Eli Stavitski,*b
Andrei V. Petukhov,c Jorge Gascon*a and Freek Kapteijna
Received 17th April 2011, Accepted 6th June 2011
DOI: 10.1039/c1cc12213d
The templating effect of the Keggin polyanion derived from
phosphotungstic acid (PTA) during the synthesis of NH2-MIL101(Al) has been investigated by means of in situ SAXS/WAXS.
Kinetic analysis and structural observations demonstrate that
PTA acts as a nucleation site and that it stabilizes the precursor
phase NH2-MOF-235(Al). Surprisingly kinetics of formation
are little changed.
During the last decade, Metal–Organic Frameworks (MOFs)
have attracted a great deal of attention in the field of nanostructured materials.1 The combination of organic and
inorganic subunits in these crystalline porous materials has
led to vast chemical versatility. In spite of initial scepticism
owing to poor stability of the first MOF generation, impressive
progress has been made during the last few years, yielding
promising results in very different technological disciplines,
such as adsorption and heterogeneous catalysis.2
One of the most promising approaches to catalytic applications of MOFs is by the encapsulation of active species during
the synthesis of the porous solid via the so-called ‘‘ship in a
bottle’’ approach. In fact, the great topological richness of
MOFs combined with relatively mild synthesis conditions
offers excellent opportunities for hosting large catalytically
active molecules. Among the various possibilities,3 the
stabilization and immobilization of polyoxometalates (POMs)
through the formation of POM-containing coordination
polymers have attracted a lot of attention.4 Due to their rich
structural and chemical variety,5 these materials possess
tunable shape, size and high negative charge, and are remarkably
versatile building blocks in the construction of coordination
supramolecules.6 Frequently, POMs have been shown to act as
anionic templates to build three-dimensional metal–organic
frameworks, while the host MOF structure is not altered by
this templating effect. Sun et al.7 showed the encapsulation of
POMs of the Keggin structure in the cavities of the wellknown HKUST-1 MOF. Around the same time, we reported
the successful encapsulation of one specific POM, phosphotungstic acid (PTA), in the large and medium cavities of the
mesoporous MIL-101(Cr).8 Canioni et al.,9 following a similar
hydrothermal, one-pot approach, introduced various POMs
into the cavities of MIL-100(Fe) and Bajpe et al.10,11 reported
the room temperature synthesis of different POM-HKUST-1
composites. In the latter work, the templating effect of
the Keggin units was demonstrated by means of ex situ
NMR/NIR/SAXS. Even though the use of void-filling
templates for synthesis of MOFs had been reported before,
Bajpe et al.10 presented the first molecular-level mechanism of
such a templating effect.
Understanding how these materials are assembled will
ultimately enable the rational design of new generations of
MOFs and MOF composites targeting specific morphology
and properties. However little is known still about the
mechanism that governs their crystallization: only a few
ex situ12–14 and in situ15–18 studies on the crystallization of
different prototypical MOFs have been reported up to date,
while only one publication addresses templating effects.10
In this work, we report an in situ combined small- and wideangle X-ray scattering (SAXS/WAXS) study on the crystallization of NH2-MIL-101(Al)19 in the presence of Keggin units
of phosphotungstic acid (PTA), a heteropoly acid (HPA).
Experimental details are described in ESIw and by Juanhuix et al.20
The scattering patterns recorded during the course of the
formation of NH2-MIL-101(Al) and HPA-NH2-MIL-101(Al)
at 403 K are shown in Fig. 1. In both cases the Bragg
reflections appear after an induction time, with positions of
a
Catalysis Engineering–Chemical Engineering Dept, Delft University
of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
E-mail: j.gascon@tudelft.nl
b
National Synchrotron Light Source, Brookhaven National
Laboratory, Upton NY 11973, USA. E-mail: istavitski@bnl.gov
c
Van’t Hoff Laboratory for Physical and Colloid Chemistry,
Debye Institute for Nanomaterials Science, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands
w Electronic supplementary information (ESI) available: Experimental
details and additional results. See DOI: 10.1039/c1cc12213d
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Chem. Commun., 2011, 47, 8578–8580
Fig. 1 3D X-ray scattering intensity plots recorded during crystallization of NH2-MIL-101(Al) (a) and HPA-NH2-MIL-101(Al) (b).
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the diffraction maxima closely matching those predicted for
the NH2-MIL-101 structure (fd3% m cubic, a = 88.87 Å).
Although some characteristic peaks of the MIL-101 structure
are still present when HPA is added to the synthesis mixture,
the relative intensity of the Bragg peaks changes dramatically.
In addition, the temporal evolution of small angle scattering
patterns prior to the onset of crystallization is clearly different
for these two cases.
The intensity of the diffraction peaks at B2 nm 1 is
considerably lower when HPA is added: while the 111 Bragg
reflection (Q = 1.1 nm 1) is hardly affected, reflections 022
(Q = 1.9 nm 1), 113 (Q = 2.05 nm 1) and 222 (Q = 2.1 nm 1)
almost disappear. We attribute this intensity change to the
successful encapsulation of the Keggin unit in both middle and
large cavities, in line with results presented earlier by Férey
et al. after impregnation of similar moieties in MIL-101(Cr)21
and by Canioni et al. after encapsulation of other HPAs in
MIL-100(Fe).9 These results point to a very efficient encapsulation of the HPA8 and suggest that the deprotonated,
negatively charged, Keggin units act as nucleation sites for
the formation of the MOF.10
SAXS patterns taken at early times deserve special attention
(Fig. 1a and b). Notably, the development of the scattering
intensity with time passes through a maximum over the whole
Q range when HPA is added to the synthesis mixture (Fig. 1b).
A more conspicuous view of changes in the scattering profile
vs. time is presented in Fig. 2, which shows a selection of the
log Q–log I(Q) plots measured in the beginning of the crystallization experiments at 313 K for both NH2-MIL-101(Al) and
HPA-NH2-MIL-101(Al) before the appearance of the Bragg
peaks. In both cases the SAXS intensity closely follows a
power-law decay Q a with a between 3.5 and 3.4 for the
NH2-MIL-101(Al) and between 2.2 and 2.8 for the HPA
containing system. The same trend is observed at lower
temperatures with a between 2.9 and 3 and from 2.3 to 3 for
NH2-MIL-101(Al) and HPA-NH2-MIL-101(Al) respectively
(see ESIw). In both cases the decay is slower than the asymptotic behaviour of a = 4 predicted by the Porod law for
compact particles with sharp interfaces,22 indicating that
MOF systems have more complex multiscale structures.
SAXS studies of the crystallization of different zeolites
yielded a values of B3,23–26 suggesting that the formation of
NH2-MIL-101(Al) proceeds through a similar precursor gel
formation mechanism. When HPA is added to the synthesis
mixture, a clear change in the slope of the log Q–log I(Q) plot
Fig. 2 Selected I(Q) profiles starting at time 0 until the beginning of
the crystallization in log–log representation. Black lines illustrate the
Q a decay. T = 413 K. Left: NH2-MIL-101(Al), right: HPA-NH2MIL-101(Al).
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Fig. 3 Development of the X-ray scattering at different Q values
during the MOF synthesis at 413 K. Intensities are normalized. Top:
NH2-MIL-101(Al), bottom: HPA-NH2-MIL-101(Al).
can be observed, with a values varying from 2 (fractal growth
regime)22,27,28 at the beginning of the experiment to almost 3
(precursor gel formation mechanism) just before the onset of
crystallization.
The development of the scattering intensity vs. time is
presented for different Q values in Fig. 3. Values were selected
in such a way that they do not coincide with any Bragg
reflection. Scattering intensity corresponding to Q values of
1, 1.5 and 2.25 nm 1 passes through a maximum for
HPA-NH2-MIL-101(Al) syntheses, in contrast to NH2-MIL101(Al), where an asymptotic evolution is observed for all Q
values except for Q = 1.0, where a small decay takes place.
In both cases, the decay in the slope of these scattering
intensities coincides with the inflection in the scattering at
Q = 0.25 nm 1 and with the onset of the development of
MIL-101 Bragg reflections. In terms of local density fluctuations, these findings indicate formation and dissolution of
clusters that differ in size distribution and shape in the
presence of HPA.
Very recently, we identified the MOF-235(Al) structure as a
precursor of the MIL-101 structure in the competitive
formation of NH2-MIL-101(Al) and NH2-MIL-53(Al),29 in
agreement with Millange et al.,17 who observed the formation
of such MOF-235(Fe) phase prior to the formation of
MIL-53(Fe). MOF-235(Al) is composed of trimeric Al(III)
clusters linked by terephthalate in a similar fashion as
MIL-101.30 The main Bragg reflection of this structure
appears at Q = 6.3 nm 1 and can be clearly identified at the
beginning of every crystallization experiment (see Fig. S4,
ESIw); notably, this reflection appears immediately at the onset
of the crystallization and is the only observable reflection when
HPA-MIL-101(Al) is synthesized at 393 K. Along these lines,
we attribute the evolution of the scattering shown in Fig. 3 to
the early formation of MOF-235(Al) clusters with sizes in the
range of 4–6 nm and 1–6 nm for NH2-MIL-101(Al), and
HPA-NH2-MIL-101(Al), respectively. When no HPA is
present, such clusters reconstruct into a more ordered
NH2-MIL-101 phase (Q = 0.25 nm 1).
When HPA is present, a large number of the MOF-235(Al)
clusters at smaller typical length scales are formed, then
redissolved and further reassembled into the NH2-MIL101(Al) phase. This can be deduced from the correlation
between intensity decrease at small scales––NH2-MOF-235(Al)
breakdown––and at the same time, growth of crystals at the
larger scale––NH2-MIL-101(Al) formation. Remarkably, the
stability of both phases, NH2-MIL-101(Al) and MOF-235(Al),
Chem. Commun., 2011, 47, 8578–8580
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seems to be enhanced in the presence of HPA, as inferred by
the absence of decay in the Q = 0.25 nm 1 scattering and by
the fact that experiments at lower temperature (493 K, see
ESIw) resulted in the selective formation of the MOF-235(Al)
phase, while NH2-MIL-101(Al) was formed at the same
temperature in the absence of HPA.
The crystallization was carried out at different temperatures
to quantify the kinetics of the process and the possible
synthesis accelerating effects of the HPA. Analysis of the
kinetic profiles was performed using the model developed by
Gualtieri31 and applied by Millange et al. for the formation of
several prototypical MOFs.16 This model, described in the
ESIw, allows decoupling the nucleation and crystal growth
processes. The fitting of the kinetic profiles yielded nucleation
and growth rate constants, kn and kg, which are given in
Table S1 (ESIw). The Arrhenius relation activation energies
for nucleation and growth were found to be 82 4 and
94 kJ mol 1, respectively, for the host MOF, whereas values
of 75 and 102 kJ mol 1 are found for the encapsulated Keggin
unit–MOF composite.
It has been suggested that together with a molecular
templating effect, the addition of PTA to the synthesis mixture
of CuBTC accelerates the rate of formation. In contrast,
for NH2-MIL-101, this is clearly not the case: both preexponential factors and energies of activation are hardly
affected upon addition of HPA to the synthesis mixture,
suggesting that once the primary units are formed, synthesis
occurs at similar rates.
Based on this kinetic information the major events taking
place during the encapsulation of HPA in NH2-MIL-101
could be identified. Assembly of the disordered MOF-235
phase rapidly occurs in the intermediate temperature regime.
The presence of HPA not only stabilizes the MOF-235 phase,
but also promotes the fractal growth of this structure. We infer
that the high concentration of negatively charged HPAs in
solution provides a large number of nucleation sites that
promote fast formation of MOF-235 subunits that rapidly
aggregate, giving rise to fractal-like structures. Once the
crystallization of the MIL-101 phase begins, such agglomerates fall apart, and finally the HPA nuclei are encapsulated
in the MIL-101 matrix. One of the future challenges is to
determine whether the HPA is already encapsulated in the
MOF-235 in the early stage of the synthesis. In this light it is
emphasized that other methods such as vibrational spectroscopy, X-ray absorption techniques and NMR should be
combined with SAXS/WAXS in order to obtain complete
chemical information of the different units assembled during
crystallization.
We thank the European Synchrotron Radiation Facility,
ESRF, for provision of beamtime and we are grateful to
Dr Francois Fauth for his assistance during the experiments at BM16. J.G. gratefully acknowledges the Dutch
National Science Foundation (NWO-CW VENI) for financial
support.
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Chem. Commun., 2011, 47, 8578–8580
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