8578 Chem. Commun., 2011, 47, 8578–8580 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 8578–8580

Live encapsulation of a Keggin polyanion in NH2-MIL-101(Al) observed

by in situ time resolved X-ray scatteringw

Jana Juan-Alcaniz,aMaarten Goesten,

aAlberto Martinez-Joaristi,

aEli Stavitski,*

b

Andrei V. Petukhov,cJorge Gascon*

aand Freek Kapteijn

a

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-MIL-

101(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 nano-

structured 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 applica-

tions 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 well-

known HKUST-1 MOF. Around the same time, we reported

the successful encapsulation of one specific POM, phospho-

tungstic 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 wide-

angle X-ray scattering (SAXS/WAXS) study on the crystalli-

zation 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

Fig. 1 3D X-ray scattering intensity plots recorded during crystalli-

zation of NH2-MIL-101(Al) (a) and HPA-NH2-MIL-101(Al) (b).

a Catalysis Engineering–Chemical Engineering Dept, Delft Universityof Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.E-mail: j.gascon@tudelft.nl

bNational Synchrotron Light Source, Brookhaven NationalLaboratory, 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 Netherlandsw Electronic supplementary information (ESI) available: Experimentaldetails and additional results. See DOI: 10.1039/c1cc12213d

ChemComm Dynamic Article Links

www.rsc.org/chemcomm COMMUNICATION

This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8578–8580 8579

the diffraction maxima closely matching those predicted for

the NH2-MIL-101 structure (fd%3m cubic, a = 88.87 A).

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 Ferey

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 encapsu-

lation 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 crystalli-

zation 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 asymp-

totic 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

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-MIL-

101(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 fluctua-

tions, 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-MIL-

101(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),

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-NH2-

MIL-101(Al).

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).

8580 Chem. Commun., 2011, 47, 8578–8580 This journal is c The Royal Society of Chemistry 2011

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 pre-

exponential 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 agglo-

merates 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 spectro-

scopy, 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 experi-

ments at BM16. J.G. gratefully acknowledges the Dutch

National Science Foundation (NWO-CW VENI) for financial

support.

Notes and references

1 J. R. Long and O. M. Yaghi, Chem. Soc. Rev., 2009, 38, 1213–1214.2 A. Corma, H. Garcia and F. X. L. Xamena, Chem. Rev., 2010, 110,4606–4655.

3 M. H. Alkordi, Y. Liu, R. W. Larsen, J. F. Eubank andM. Eddaoudi, J. Am. Chem. Soc., 2008, 130, 12639–12641.

4 R. Yu, X.-F. Kuang, X.-Y. Wu, C.-Z. Lu and J. P. Donahue,Coord. Chem. Rev., 2009, 253, 2872–2890.

5 I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171–198.6 X. Y. Zhao, D. D. Liang, S. X. Liu, C. Y. Sun, R. G. Cao, C. Y. Gao,Y. H. Ren and Z. M. Su, Inorg. Chem., 2008, 47, 7133–7138.

7 C.-Y. Sun, S.-X. Liu, D.-D. Liang, K.-Z. Shao, Y.-H. Ren andZ.-M. Su, J. Am. Chem. Soc., 2009, 131, 1883–1888.

8 J. Juan-Alcaniz, E. V. Ramos-Fernandez, U. Lafont, J. Gasconand F. Kapteijn, J. Catal., 2010, 269, 229–241.

9 R. Canioni, C. Roch-Marchal, F. Secheresse, P. Horcajada,C. Serre, M. Hardi-Dan, G. Ferey, J.-M. Greneche, F. Lefebvre,J.-S. Chang, Y.-K. Hwang, O. Lebedev, S. Turner and G. VanTendeloo, J. Mater. Chem., 2011, 21, 1226–1233.

10 S. R. Bajpe, C. E. A. Kirschhock, A. Aerts, E. Breynaert,G. Absillis, T. N. Parac-Vogt, L. Giebeler and J. A. Martens,Chem.–Eur. J., 2010, 16, 3926–3932.

11 L. H. Wee, S. R. Bajpe, N. Janssens, I. Hermans, K. Houthoofd,C. E. A. Kirschhock and J. A. Martens, Chem. Commun., 2010, 46,8186–8188.

12 N. A. Khan and S. H. Jhung,Cryst. Growth Des., 2010, 10, 1860–1865.13 S. Surble, F. Millange, C. Serre, G. Ferey and R. I. Walton, Chem.

Commun., 2006, 1518–1520.14 M. Shoaee, M. W. Anderson and M. P. Attfield, Angew. Chem.,

Int. Ed., 2008, 47, 8525–8528.15 S. Hermes, T. Witte, T. Hikov, D. Zacher, S. Bahnmuller,

G. Langstein, K. Huber and R. A. Fischer, J. Am. Chem. Soc.,2007, 129, 5324.

16 F. Millange, R. El Osta, M. E. Medina and R. I. Walton,CrystEngComm, 2011, 13, 103–108.

17 F. Millange, M. Medina, N. Guillou, G. Ferey, K. M. Golden andR. I. Walton, Angew. Chem., Int. Ed., 2010, 49, 763–766.

18 G. Seeber, G. J. T. Cooper, G. N. Newton, M. H. Rosnes, D.-L.Long, B. M. Kariuki, P. Kogerler and L. Cronin, Chem. Sci., 2010,1, 62–67.

19 P. Serra-Crespo, E. V. Ramos-Fernandez, J. Gascon and F. Kapteijn,Chem. Mater., 2011, 23, 2565–2572.

20 J. Juanhuix, A. Labrador, D. Beltran, J. F. Herranz, P. Carpentierand J. Bordas, Rev. Sci. Instrum., 2005, 76, 086103–086104.

21 G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour,S. Surble and I. Margiolaki, Science, 2005, 309, 2040–2042.

22 O. Glatter and O. Kratky, Small Angle X-ray Scattering, AcademicPress Inc. (London) LTD, 1982.

23 D. Grandjean, A. M. Beale, A. V. Petukhov and B. M. Weckhuysen,J. Am. Chem. Soc., 2005, 127, 14454–14465.

24 A. M. Beale, A. M. J. van der Eerden, S. D. M. Jacques,O. Leynaud, M. G. O’Brien, F. Meneau, S. Nikitenko, W. Brasand B.M.Weckhuysen, J. Am. Chem. Soc., 2006, 128, 12386–12387.

25 P. de Moor, T. P. M. Beelen, R. A. van Santen, L. W. Beck andM. E. Davis, J. Phys. Chem. B, 2000, 104, 7600–7611.

26 C. J. Y. Houssin, C. E. A. Kirschhock, P. Magusin, B. L. Mojet,P. J. Grobet, P. A. Jacobs, J. A. Martens and R. A. van Santen,Phys. Chem. Chem. Phys., 2003, 5, 3518–3524.

27 V. Boffa, H. L. Castricum, R. Garcia, R. Schmuhl, A. V. Petukhov,D.H.A. Blank and J. E. ten Elshof,Chem.Mater., 2009, 21, 1822–1828.

28 M. Sztucki, T. Narayanana and G. Beaucage, J. Appl. Phys., 2007,101, 114304.

29 E. Stavitski, M. Goesten, J. Juan-Alcaniz, A. Martinez-Joaristi,P. Serra-Crespo, A. V. Petukhov, J. Gascon and F. Kapteijn, 2011,Submitted.

30 A. C. Sudik, A. P. Cote and O. M. Yaghi, Inorg. Chem., 2005, 44,2998–3000.

31 A. F. Gualtieri, Phys. Chem. Miner., 2001, 28, 719–728.