Physical Chemistry Chemical Physics

This paper is published as part of a PCCP Themed Issue on:

"Molecules in Confined Spaces: The Interplay between Spectroscopy and Theory to develop

Structure-Activity Relationships in the fields of Heterogeneous Catalysis, Sorption, Sensing and Separation Technology"

Guest Editor: Bert Weckhuysen

Editorial Highlight

Editorial Highlight: Molecules in confined spaces Robert A. Schoonheydt and Bert M. Weckhuysen, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b905015a

Perspectives

Nanoporous oxidic solids: the confluence of heterogeneous and homogeneous catalysis John Meurig Thomas, Juan Carlos Hernandez-Garrido, Robert Raja and Robert G. Bell, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819249a

Supported vanadium oxide in heterogeneous catalysis: elucidating the structure–activity relationship with spectroscopy Ilke Muylaert and Pascal Van Der Voort, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819808j

Correlating phase behaviour and diffusion in mesopores: perspectives revealed by pulsed field gradient NMR Rustem Valiullin, Jörg Kärger and Roger Gläser, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b822939b

Communications

Viscosity sensing in heated alkaline zeolite synthesis media Lana R. A. Follens, Erwin K. Reichel, Christian Riesch, Jan Vermant, Johan A. Martens, Christine E. A. Kirschhock and Bernhard Jakoby, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b816040f

A small molecule in metal cluster cages: H2@Mgn (n = 8 to 10) Phillip McNelles and Fedor Y. Naumkin, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819479c

Papers

Confinement effects on excitation energies and regioselectivity as probed by the Fukui function and the molecular electrostatic potential Alex Borgoo, David J. Tozer, Paul Geerlings and Frank De Proft, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b820114e

Rotation dynamics of 2-methyl butane and n-pentane in MCM-22 zeolite: a molecular dynamics simulation study Shiping Huang, Vincent Finsy, Jeroen Persoons, Mark T.F. Telling, Gino V. Baron and Joeri F.M. Denayer, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819334g

Reactivity in the confined spaces of zeolites: the interplay between spectroscopy and theory to develop structure–activity relationships for catalysis Mercedes Boronat, Patricia Concepción, Avelino Corma, María Teresa Navarro, Michael Renz and Susana Valencia, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b821297j

The influence of the chemical compression on the electric properties of molecular systems within the supermolecular approximation: the LiH molecule as a case study Anna Kaczmarek and Wojciech Bartkowiak, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819346k

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View Article Online / Journal Homepage / Table of Contents for this issue

Multinuclear gallium-oxide cations in high-silica zeolites Evgeny A. Pidko, Rutger A. van Santen and Emiel J. M. Hensen, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b815943b

Metal–organic frameworks as high-potential adsorbents for liquid-phase separations of olefins, alkylnaphthalenes and dichlorobenzenes Luc Alaerts, Michael Maes, Monique A. van der Veen, Pierre A. Jacobs and Dirk E. De Vos, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b823233d

Tetramethyl ammonium as masking agent for molecular stencil patterning in the confined space of the nano-channels of 2D hexagonal-templated porous silicas Kun Zhang, Belén Albela, Ming-Yuan He, Yimeng Wang and Laurent Bonneviot, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819872c

Photovoltaic activity of layered zirconium phosphates containing covalently grafted ruthenium tris(bipyridyl) and diquat phosphonates as electron donor/acceptor sites Laura Teruel, Marina Alonso, M. Carmen Quintana, Álvaro Salvador, Olga Juanes, Juan Carlos Rodriguez-Ubis, Ernesto Brunet and Hermenegildo García, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b816698f

The characterisation and catalytic properties of biomimetic metal–peptide complexes immobilised on mesoporous silica Gerhard D. Pirngruber, Lukas Frunz and Marco Lüchinger, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819678h

Physisorption and chemisorption of alkanes and alkenes in H-FAU: a combined ab initio–statistical thermodynamics study Bart A. De Moor, Marie-Françoise Reyniers and Guy B. Marin, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819435c

Accelerated generation of intracrystalline mesoporosity in zeolites by microwave-mediated desilication Sònia Abelló and Javier Pérez-Ramírez, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b819543a

Regio- and stereoselective terpene epoxidation using tungstate-exchanged takovites: a study of phase purity, takovite composition and stable catalytic activity Pieter Levecque, Hilde Poelman, Pierre Jacobs, Dirk De Vos and Bert Sels, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b820336a

Probing the microscopic hydrophobicity of smectite surfaces. A vibrational spectroscopic study of dibenzo-p-dioxin sorption to smectite Kiran Rana, Stephen A. Boyd, Brian J. Teppen, Hui Li, Cun Liu and Cliff. T. Johnston, Phys. Chem. Chem. Phys., 2009 DOI: 10.1039/b822635k

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Direct spectroscopic detection of framework-incorporated vanadium in

mesoporous silica materialswzySepideh Zamani,a Vera Meynen,b Alina-Mihaela Hanu,ad Myrjam Mertens,c

Eveline Popovici,dSabine Van Doorslaer*

aand Pegie Cool*

b

Received 31st October 2008, Accepted 27th March 2009

First published as an Advance Article on the web 8th May 2009

DOI: 10.1039/b819442b

Framework-incorporated vanadium mesoporous silica materials with different contents in

vanadium were obtained by a facile, direct synthesis at room temperature, using VOSO4�5H2O as

the vanadium precursor. The porous characteristics of the samples and the coordination

environment of the vanadia in the structure were studied by a combination of techniques: X-ray

diffraction, N2-adsorption/desorption, FT-Raman, FTIR-PAS and UV-Vis-DR, electron

paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy.

A structural comparison is made using pulsed EPR and ENDOR spectroscopic techniques

between vanadia deposited on the surface of MCM-41 by the Molecular Designed Dispersion

method and as-synthesised samples of vanadia incorporated in the mesoporous silica framework

using the above-mentioned synthesis method. The EPR study on the non-calcined samples proves

the incorporation of a high amount of vanadium in the silica framework by the observation of a

strong hyperfine coupling of the unpaired electron with 29Si. It demonstrates the feasibility for

EPR to reveal structural information on true incorporation of metal ions in framework positions

leading to metal oxides.

Introduction

After many years of intensive research on the development

of inorganic molecular sieves, mesoporous materials with

well-defined pore systems are still very attractive for scientists.

The discovery of the M41S family, in 1992, by researchers

from Mobil Oil Corporation1,2 has induced an explosive

growth in the development of synthetic methods to produce

these materials. One of the most intensively studied M41S

materials is MCM-41. This mesoporous material possesses

hexagonally packed (space group p6mm) uni-dimensional

cylindrical pores with uniform pore diameters between 2 and

10 nm, high specific surface areas up to 1000 m2/g and pore

volumes up to 1.3 ml/g.

Extensive research has been carried out to stabilizeMCM-41,3

to expand the pore sizes4 and to extend the framework

composition in order to induce catalytic properties. It is well

known, that the siliceous MCM-41 does not have any catalytic

activity. However, this material can be used as a suitable

porous matrix for the design of high surface area catalysts

by introducing metal ions, metal oxides, metal or organo-

metallic complexes or other types of active species. The most

studied method to introduce metal ions in the MCM-41 or

mesoporous silica network is the direct hydrothermal syn-

thesis, where the metal-ion precursor is directly added to the

synthesis gel. Many heteroatoms, such as Al,5 Ti,6,7 B,8 Zr9

and V,10–16 have been incorporated into mesoporous silica by

this method. One of the main objectives of this work is to

incorporate vanadium in the silica framework in order to

produce more stable materials towards leaching.

Vanadium is a key component in many catalytic redox

processes such as the oxidation of methanol,17 oxidation and

ammoxidation of aromatics,18 oxidative dehydrogenation of

alkanes,19,20 epoxidation reactions of olefins16 and selective

catalytic reduction of NOx with ammonia.21 The catalytic

activity and selectivity of vanadium-containing catalysts

strongly depends on the nature, coordination,22,23 location

and content of vanadia in the catalyst. Generally, three forms

of vanadium species can be present: isolated tetrahedral,

two-dimensional octahedral species and crystalline V2O5.

This work is focused on the facile synthesis of mesoporous

silica with different vanadium contents and their spectroscopic

characterization. The V-incorporated mesoporous silica

aUniversity of Antwerp, Department of Physics, Universiteitsplein 1,B-2610 Wilrijk, Belgium. E-mail: sabine.vandoorslaer@ua.ac.be;Fax: +32 3 265 2470; Tel: +32 3 265 2461

bUniversity of Antwerp, Department of Chemistry, Universiteitsplein1, B-2610 Wilrijk, Belgium. E-mail: pegie.cool@ua.ac.be;Fax: +32 3 265 2374; Tel: +32 3 265 2355

c Flemish Institute For Technological Research, VITO NV,Boeretang 200, B-2400 Mol, Belgium

dAl. I. Cuza University Iasi, B-dul Carol, nr. 11, 700506, Iasi,Romania

w This article was submitted as part of a Themed Issue on molecules inconfined spaces. Other papers on this topic can be found in issue 16 ofvol. 11 (2009). This issue can be found from the PCCP homepage[http://www.rsc.org/pccp].z Electronic supplementary information (ESI) available: Fig. S1:FTIR-PAS spectra of the as-synthesised (non-calcined) TCV samples;S2: Raman spectra of (a) TC, (b) TCV3.4 and (c) TCV4.0; S3: CW-EPRspectra showing effect of ageing on TCV and VO(acac)2/MCM-41samples; S4: W-band ESE-detected EPR spectra of TCV3.4; S5:W-band ELDOR-detected NMR spectra of TCV3.4; S6: X-bandMims-ENDOR spectra of TCV3.4. See DOI: 10.1039/b819442by In honour of the retirement of Professor Robert A. Schoonheydt ofthe Catholic University of Leuven (Belgium).

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 5823–5832 | 5823

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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catalysts were synthesized at room temperature using

VOSO4�5H2O as the vanadium precursor. A detailed characteriza-

tion of the samples was performed using X-ray diffraction,

N2-adsorption/desorption, continuous-wave (CW) and pulsed

electron paramagnetic resonance (EPR), pulsed electron nuclear

double resonance (ENDOR), Fourier transform infrared spectro-

scopy with photo acoustic detection (FTIR-PAS), UV-Vis

diffuse reflectance spectroscopy (UV-Vis-DR) and electron

probe microanalysis (EPMA). The vanadium content in the

samples was varied in the range of 0.8–4% (w/w). We observed

that the insertion of vanadium inside the framework leads to

materials with poor ordering. In the same time the presence of

vanadium in the structure has no influence on the meso-

porosity of the synthesized catalysts.

Although different studies deal with framework incorpora-

tion of metal ions in mesoporous systems, proof of such

incorporation has often been indirect and in many cases metal

incorporation has been contested because of lack of direct

structural proof. We show here that in the case of incorpora-

tion of paramagnetic transition-metal ions, pulsed EPR

provides a powerful means to prove such incorporation.

Materials and methods

Material synthesis

MCM-41 and TCV (mesoporous silica materials with

incorporated vanadium) were synthesized using tetraethyl-

orthosilicate (TEOS) as a silica source, cetyltrimethylammonium

bromide as a template agent, NaOH (2 M) as mineralizing

agent and VOSO4�5H2O as the vanadium source. All the

chemical reagents were produced by Acros Organics. The

molar gel composition of the synthesis mixture is as follows:

SiO2/surfactant/Na2O/VO2/H2O= 1/0.12/0.25/(0.00–0.05)/136.

The surfactant was first dissolved in water under vigorous

stirring. After 10 minutes, NaOH (2 M) was added to the

dissolved surfactant solution and the mixture was stirred for

half an hour. The silica source was added and the mixture

(pH = 12) was stirred for 1 hour at room temperature. To

synthesize TCV, an aqueous solution of VOSO4�5H2O was

added 10 minutes after the addition of TEOS and the mixture

(pH = 12) was stirred for 50 minutes at room temperature.

The obtained gel was kept at room temperature for 72 hours.

The resulting solids were filtered, washed with 300 ml distilled

water, and dried at room temperature for 2 days. Finally the

products were calcined in a programmable oven at 550 1C,

for 6 h, with a heating rate of 1 1C/min, under ambient

atmosphere.

A reference siliceous MCM-41 material was made in the

same way without the addition of VOSO4�5H2O and will be

denoted as TC. The various vanadium-containing TCV

samples are denoted as TCVx in which x represents the

percentage of vanadium in the final material.

The MDD (Molecular Designed Dispersion) method was

applied to deposit isolated tetrahedral vanadium on the

surface of MCM-41 via a post-synthetic treatment. The

MDD method makes use of vanadyl acetylacetonate com-

plexes that are deposited on the surface of the porous support.

The large complexes assure steric hindrance and a sufficient

distance between the vanadyl centers to avoid cluster

formation.24,25 The MDD deposition was performed in a dry

air glovebox under constant flow. A calculated amount of

vanadyl acetylacetonate (VO(acac)2) was added to 150 ml of

zeolite-dried toluene and stirred. The MCM-41 support (1.0 g)

was dried overnight at 200 1C and cooled down in the

glovebox under dry air. The dried MCM-41 support was

added to the VO(acac)2 solution and stirred for 1 hour at

room temperature. Afterwards, the modified support was

filtered off, washed 5 times with 25 ml of dried toluene and

vacuum dried (high vacuum below 0.01 mbar). Calcination of

the samples was performed in a programmable oven at 550 1C

for 6 hours with a heating rate of 1 1C/min in ambient

atmosphere.

Methods

X-Ray diffraction patterns (XRD) were collected on a Panalytical

X’Pert PRO MPD diffractometer using Ni-filtered Cu Karadiation. Measurements were done in the 2y mode using a

bracket sample holder with a scanning speed of 0.041/4 s

continuous mode.

N2-sorption measurements were performed on a

QUANTACHROMEAutosorb 1-MP automated gas adsorption

system using nitrogen as the adsorbate at �196 1C. Prior to

adsorption, the calcined samples were outgassed under high

vacuum for 16 hours at 200 1C. The pore diameter was

calculated from the adsorption branch using the BJH method.

The specific surface area was calculated by the BET method in

the range of relative pressure 0.05–0.35. The total pore volume

was taken at the relative pressure of 0.95.

FTIR-PAS spectra were recorded on a Nicolet 20SX

spectrometer, equipped with a McClelland photo acoustic cell.

About 500 scans were taken with a resolution of 8 cm�1. The

PAS spectrometer was placed in an isolated bench, which was

constantly purged with dry air to ensure IR measurements

under dry conditions.

UV-Vis-DR spectra were obtained at room temperature on

a Nicolet Evolution 500 UV-Vis spectrometer, with a diffuse

reflectance accessory (Thermo-electron RSA-UC40 integrat-

ing sphere) using BaSO4 standard as a reference. Samples were

diluted with KBr to 2 wt%.

The vanadium loading was determined by elemental analysis

carried out on a JEOL JCXA 733 superprobe (electron probe

microanalysis, EPMA).

X-band CW-EPR measurements were performed on a

Bruker ESP 300E instrument, equipped with a liquid helium

cryostat (Oxford Inc.), at a microwave (mw) frequency of

about 9.5 GHz. A mw power (10 mW), modulation frequency

(100 kHz) and modulation amplitude (0.5 mT) were applied.

The EasySpin program26 was utilized to simulate the CW-EPR

spectra.

X-band pulsed EPR measurements were performed on a

Bruker ESP380E equipped with a helium-gas flow cryostat

(Oxford Inc.). The measurements were done at 4 K, with a

repetition rate of 1 kHz. W-band pulsed EPR experiments

were performed on a Bruker E680 spectrometer (94 GHz)

equipped with a helium-gas flow cryostat. All measurements

were done at 6 K using a repetition rate of 667 Hz.

5824 | Phys. Chem. Chem. Phys., 2009, 11, 5823–5832 This journal is �c the Owner Societies 2009

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X-band and W-band electron-spin-echo (ESE) detected

EPR experiments were performed with the p/2–t–p–t–echosequence and mw pulse lengths tp/2 = 16 ns, tp = 32 ns

(X-band) and tp/2 = 140 ns, tp = 280 ns (W-band). Different

t values were used as mentioned in the appropriate figure

captions.

X-bandHyperfine Sublevel Correlation (HYSCORE) experi-

ments27 were done using the pulse sequence p/2–t–p/2–t1–p–t2–p/2–t–echo and mw pulse length (tp/2 = 16 ns,

tp = 16 ns). The time intervals t1 and t2 were varied in steps

of 16 ns starting from 96 ns to 6480 ns. The measurements

were done at two different t values (t = 96 ns and 176 ns). In

order to get rid of unwanted echoes, an eight-step phase cycle

experiment was applied. The time-domain HYSCORE spectra

were baseline corrected with a third-order polynomial,

apodized with a Hamming window and zero filled. The

absolute-value spectra, obtained by two-dimensional Fourier

transformation, were added for the different t values to reduce

the blind-spot effect. The HYSCORE spectra were simulated

using a program developed at ETH Zurich.28

X-band Mims ENDOR experiments29 were performed using

the mw pulse sequence: p/2–t–p/2–t–p/2–t–echo with pulse

lengths tp/2 = 16 ns and a radio-frequency (rf) p pulse of 11 msapplied during time t, t=13.8 ms. Each experimental ENDOR

spectrum is the sum of four experiments with t = 120, 160,

200 and 240 ns. The ENDOR spectra are simulated using the

EasySpin program.26

W-band ELDOR-detected NMR experiments30 were per-

formed using the mw sequence: (HTA)mw2–T–

(p/2)mw1–t–(p)mw1–t–echo sequence, with tHTA = 7 ms,tp/2 = 160 ns and tp = 320 ns. T = 1.2 ms and t = 1 ms.The second microwave frequency, mw2, was varied in steps of

100 kHz in the mw range 1 � 50 MHz.

Results

Fig. 1 shows X-ray diffraction patterns of calcined MCM-41

(TC) and calcined TCV materials (TCV0.8, TCV1.6, TCV3.4

and TCV4.0) with increasing vanadium concentration (%) in

the catalyst (Table 1).

The XRD patterns reveal the typical hexagonal ordering of

mesoporous MCM-41 materials for the reference TC sample.

However, all vanadium-incorporated TCV samples have lost

clear structural ordering in comparison to the TC sample.

From Fig. 1, it is obvious that the first diffraction peak

shifts to lower 2y degrees with the addition of vanadium to

the synthesis mixture. Therefore, it can be concluded that the

presence of vanadium in the synthesis mixture results in the

increase of the d-spacing of the TCV material. An enlargement

of the d-spacing can be rationalised by taking into account

that the bond length of V–O (1.8 A) is larger compared to the

bond distance in Si–O (1.6 A) which results in an increase of

the pore diameter and the pore wall thickness.16 Therefore, the

incorporation of vanadium into the framework of TCV could

give rise to an increase in the d-spacing calculated from the

position of the first diffraction peak. The enlargement of the

d-spacing of the vanadium-containing TCV materials in com-

parison to the siliceous TC could be an indication for the

incorporation of vanadium in the walls of the mesoporous

material rather than on the surface. The obtained structural

changes upon incorporation of vanadium into the framework

is in good agreement with the published literature data.10,16

The d-spacing for TCV samples slightly decreases for the

sample with the highest vanadium loading (Table 1). The

results suggest that in case of a high quantity of vanadium

present in the initial synthesis gel, some vanadia species will be

deposited on the pore walls rather than being incorporated in

the framework.

The framework-incorporated vanadium distorts to a great

extent the hexagonal ordering of the material resulting in

poorly ordered materials. Indeed, the longer bond lengths of

the V–O bonds could result in some strain in the structure.

This is also supported by an increased XRD-peak broadening

when vanadium is present. However, it should be noted that

changes in the peak ratios of the diffraction pattern can also be

observed due to the deposition of some vanadium on the

surface.31 Although this peak broadening and loss in struc-

tural characteristics could be due to the incorporation and

deposition of vanadium, it must be mentioned that also the

presence of SO42� ions in the synthesis mixture, originating

from the vanadyl salt, could influence the micellar structure

during synthesis and therefore lead to poorly ordered or even

disordered structures. Therefore, conclusions made on the

framework incorporation of vanadium based on XRD diffrac-

tion patterns and changes in d-spacings or lattice parameters

should be taken as highly speculative and could lead to

misinterpretation of the data.

The nitrogen adsorption/desorption isotherms at �196 1C

for calcined TCV0.8, TCV1.6, TCV3.4 and TCV4.0 (Fig. 2) are of

type IV according to the IUPAC classification, showing a

capillary condensation at a relative pressure (P/P0) between

0.2 and 0.4. The samples TCV0.8, TCV1.6 and TCV3.4 exhibit a

steep capillary condensation step due to their narrow pore-size

Fig. 1 X-Ray diffraction patterns of calcined TC and TCV samples.

The diffraction patterns were 20 times enlarged.

This journal is �c the Owner Societies 2009 Phys. Chem. Chem. Phys., 2009, 11, 5823–5832 | 5825

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distributions. All materials show similar pore sizes, independent

of the amount of vanadium that was added to the synthesis

mixture. For TCV4.0 the capillary condensation becomes

smaller and the pore-size distribution broadens slightly. This

could suggest some loss in the pore structure due to the high

amount of vanadium in the TCV structure (Fig. 2). The

porous characteristics of the different materials do not change

with the vanadium content except for the highest loadings.

The small decrease in pore volume and surface area for the

TCV4.0 sample can be attributed to some structural loss.

FTIR-PAS measurements were performed on the calcined

TCV samples in order to confirm the presence of vanadium in

the framework of the mesoporous silica materials (Fig. 3).

FTIR-PAS spectra of the non-calcined materials can be found

in the supplementary informationz (Fig. S1). In the spectral

range 4000–3000 cm�1 of the FTIR-PAS spectra of the

calcined materials, a dominant band can be observed at

3745 cm�1 (Fig. 3A) assigned to the O–H stretching of isolated

free silanols.32 The intensity decreases when higher vanadium

contents are obtained, which is in agreement with previous

studies on the incorporation of vanadium in various structures

(e.g. zeolite beta).33 The lower spectral range (1500–400 cm�1)

shows the contributions from Si–O bondings and V–O

bondings. When vanadium is added to the synthesis mixture,

a band around 930 cm�1 starts to appear. This band is

attributed to the Si–O–V bonds of VOx species in the structure

of TCV. High vanadia contents result in a blueshift and

broadening of the band at 930 cm�1 up to 945 cm�1 for the

material with the highest loading (TCV4.0). This peak at

930 cm�1 is similar to the one reported by Zhang et al. who

appointed this to a tetrahedrally coordinated vanadyl (V5+),

Table 1 Characteristics of the TCV materials

Sample Vanadium content (%) Specific surface area (m2/g) Pore volume (cm3/g) Pore diameter (nm) d (nm)

TC — 975 0.72 2.50 3.2TCV0.8 0.8 1070 0.85 2.66 4.3TCV1.6 1.6 1090 0.86 2.66 4.4TCV3.4 3.4 1060 0.87 2.66 4.3TCV4.0 4.0 938 0.77 2.66 4.0

Fig. 2 Nitrogen adsorption/desorption isotherms at �196 1C (top)

and derived pore-size distribution (bottom) of the vanadium-modified

TCV samples studied in this work.

Fig. 3 FTIR-PAS spectra of calcined TC and TCV samples. (A)

Enlargement of 4000–2500 cm�1 region, (B) 400–4000 cm�1 region.

5826 | Phys. Chem. Chem. Phys., 2009, 11, 5823–5832 This journal is �c the Owner Societies 2009

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incorporated in a HZSM-5 zeolite framework.34 The appear-

ance of this band is a good indication of vanadium incorpora-

tion in the framework of the mesoporous silica. Nevertheless,

framework incorporation of transition metals is often

also claimed when IR bands at 960–970 cm�1 are present.

However, others have assigned this band to vacant T-sites in a

silica/zeolite framework or Si–O� groups. Therefore, the band

assignment for framework-incorporated vanadium is not

unambiguous and is subject of a lot of controversy.33,35,36

Hence, IR of the calcined as well as the non-calcined materials

will not give clear, unmistakable indications of framework

incorporation, extra-framework positions or surface deposi-

tion of vanadium as isolated tetrahedral vanadyl sites. As will

be shown further, pulsed EPR offers a better way of assigning

these sites.

The Raman spectra of the samples TCV3.4 and TCV4.0 with

the highest vanadium content show only a band at 1042 cm�1,

characteristic for the stretching vibration of the terminal VQO

group of tetrahedral vanadium species (Fig. S2z). This band

could not be detected for the samples TCV0.8 and TCV1.6 due

to the low vanadium content in these samples (not shown).

No bands characteristic for V2O5 clusters (994, 697, 286

and 147 cm�1) could be observed in any of the samples37,38

(Fig. S2z).UV-Vis-DR spectroscopy gives additional information on

the coordination of the vanadia species present in the calcined

TCV samples (Fig. 4). All samples were measured in the

dehydrated state after drying the samples at 200 1C. Sub-

sequently, the materials were re-measured after 24 hours of

exposure to air. The dry vanadium-incorporated TCV

materials were white immediately after calcination. The

UV-Vis-DR spectra of the calcined, dry materials show a

broad absorption band between 220 and 350 nm, in case of

low vanadium content. These bands are assigned to the

charge-transfer transitions of V5+ species in an isolated tetra-

hedral environment.39–41 For a sample with higher vanadium

content (TCV4.0) a similar spectrum is obtained. However, a

shoulder can be observed between 350 and 420 nm. Therefore,

next to the tetrahedral isolated vanadium species, also a small

amount of oligomeric and polymeric tetrahedral VO4

(350–400 nm) can be observed.39

After 24 hours of exposure to air, all the samples changed

color and became yellow due to the coordination of water to

the vanadia centers. The UV-Vis-DR spectra for hydrated

samples show two broad bands, the first in the 220–320 nm

region and the second in the region 320–520 nm. These

spectral changes suggest a coordination of one water molecule

to the O3/2V5+QO centers resulting in a square-pyramidal

O3/2V5+QO� � �H2O coordination (410 nm). When two water

molecules are adsorbed, an octahedral O3/2V5+QO� � �2H2O

coordination occurs with a maximum around 470 nm.39,42,43

A comparison with the dried sample TCV1.6 and TCV4.0

reveals that the hydration process gives rise to the formation

of an additional band at 370–470 nm, indicating the adsorp-

tion of generally one water molecule.44 These observations

show the accessibility of the vanadia species for water.

The results of XRD, IR and UV-DR only give indirect

proof for the incorporation of the vanadia species in frame-

work positions. Moreover, they do not conclude on the

possibility whether incorporation has diminished the possibi-

lity to leaching, mobility and clustering of the active vanadium

centers resulting in a more stable material. Therefore, an EPR

study has been performed to elucidate these findings on

stability and true incorporation of the active vanadia centers.

Since EPR can only detect paramagnetic species and since V(V)

is diamagnetic, the EPR experiments were performed on the

non-calcined precursors of the final catalysts. Before calcina-

tion, vanadium is still in its EPR-active vanadyl form.

Comparative measurements are performed on an MCM-41

sample in which VO(acac)2 is deposited via the MDD method.

Fig. 5 depicts the normalized X-band CW-EPR spectra

of different non-calcined TCV and VO(acac)2/MCM-41

samples under varying conditions. The comparison with the

VO(acac)2/MCM-41 samples is made to analyze the differ-

ences of direct incorporation and post-synthesis deposition of

vanadium centers in MCM-41. The CW-EPR spectra of all

TCV samples under study (Fig. 5a–d) are characteristic of

VO(II) complexes with vanadium in a square-pyramidal or

distorted octahedral coordination.45–47 The normalized

CW-EPR spectra of TCV1.6 (Fig. 5a) and TCV3.4 (Fig. 5b)

taken at room temperature are identical, indicating that the

vanadyl loading is not strongly perturbing the local structure

of the vanadyl sites in the concentration range chosen here.

Furthermore, the CW-EPR spectrum of TCV3.4 does not

change upon lowering of the temperature (Fig. 5b and c),

showing that the vanadyl sites are immobilized in or on

the siliceous material. This contrasts the observation for

VO(acac)2/MCM-41, for which the EPR spectra taken at

room and low temperature differ clearly (Fig. 5e and f). At

room temperature, the axial spectrum shows motional

averaging (total spectral width decreases) indicating a large

mobility of the VO(II) centers. As the temperature is lowered,

the mobility of the vanadyl complex is thermally decreased.

This is a clear proof that the VO(II) species are not fixed

on the walls of the support after this post-synthesis deposi-

tion. The simulation parameters of the VO(II) center

in VO(acac)2/MCM-41 given in Table 2 are typical for a

Fig. 4 UV-Vis-DR spectra of dry TCV1.6 (grey —) and TCV1.6 after

air exposure for 24 hours (black —); dry TCV4.0 (grey –m–) and

TCV4.0 after air exposure for 24 hours (black –m–).

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pentaquovanadyl [VO(H2O)52+].48 This shows that the acac

ligands of the vanadyl complex have been fully replaced by

water. An analogous observation was already made for the

deposition of VO(acac)2 on SBA-15.46

The CW-EPR spectrum of TCV3.4 dehydrated under a

dynamic vacuum of 10�6 mbar (Fig. 5d) shows only minor

changes in comparison with the non-dehydrated one (Fig. 5c,

Table 2). This proves once more that the vanadyl species are

fixed in/on the mesopore walls of the TCV samples. In

contrast, the CW-EPR spectrum taken after dehydration of

non-calcined VO(acac)2/MCM-41 under the same conditions

consists of two components (Fig. 5g): a broad signal centered

around gE 1.98, usually ascribed to high local concentrations

of vanadyl complexes,49 and a resolved spectrum due to

square-pyramidal VO(II) sites. The EPR simulation para-

meters of this latter contribution are clearly different from

the ones of pentaquovanadyl [VO(H2O)52+] (Table 2),

indicating a major change in the VO(II) environment upon

evacuation. The appearance of the dipolar-broadened signal

around g E 1.98 indicates that a large fraction of the initially

mobile, but isolated, VO(II) complexes have started to cluster

during the dehydration process. A similar effect was observed

when the non-calcined VO(acac)2/MCM-41 samples were aged

for 10 months under dry air (see ESI, Fig. S3z). The same

ageing treatment did not induce a change in the CW-EPR

spectrum of non-calcined TCV3.4.

In Fig. 6 the X-band ESE-detected EPR spectra of the non-

calcined TCV and VO(acac)2/MCM-41 samples taken at 4 K

are shown. Besides the contribution due to the isolated VO(II)

sites that was also found in the CW-EPR spectra (Fig. 5), all

ESE-detected EPR spectra of the as-received TCV samples

show a broad underlying signal (see amplifications of spectra

6b and 6c). This signal was not dominantly present in the

CW-EPR spectra, which is due to the fact that the CW-EPR

spectra represent the derivative of the absorption signal. The

observation of the broad signal in the ESE-detected EPR

spectra proves that this signal cannot be ascribed to the

occurrence of VO(II) clusters. Indeed, although VO(II) clusters

Fig. 5 Normalized experimental (solid line) and simulated (dashed

line) X-band CW-EPR spectra of non-calcined (a) TCV1.6 and

(b) TCV3.4 at 298 K; (c) TCV3.4 at 15 K; (d) dehydrated TCV3.4 at

15 K; (e) VO(acac)2/MCM-41 at 298 K; (f) VO(acac)2/MCM-41 at

15 K; (g) dehydrated VO(acac)2/MCM-41 at 298 K. The simulation

parameters are shown in Table 2.

Table 2 g and 51V hyperfine principal values of the VO2+ centers in the TCV samples and in VO(acac)2/MCM-41

Samples gx,y gz Ax,y/MHz Az/MHz

TCV(0.8,1.6,3.4) (non-calcined) 1.974 (� 0.001) 1.935 (� 0.001) 182 (� 2) 522 (� 2)VO(acac)2/MCM-41 (non-calcined) 1.977 (� 0.002) 1.932 (� 0.001) 204 (� 3) 547 (� 2)TCV3.4 (non-calcined+dehydrated) 1.975 (� 0.001) 1.935 (� 0.001) 184 (� 2) 521 (� 2)VO(acac)2/MCM-41 (non-calcined+dehydrated) 1.976 (� 0.002) 1.934 (� 0.001) 193 (� 3) 521 (� 2)

Fig. 6 X-band ESE-detected EPR spectra taken at 4 K of the

as-received samples: (a) TCV0.8, (b) TCV1.6, (c) TCV3.4 and

(e) VO(acac)2/MCM-41. As a comparison, the ESE-detected EPR

spectrum of dehydrated TCV3.4 is given in (d). * and m indicate the

positions where HYSCORE experiments were performed (see text). The

arrow marks the small spectral changes upon dehydration of TCV3.4.

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are known to give rise to broad, dipolar-broadened back-

ground signals in the CW-EPR spectra,49 the dipolar inter-

action will shorten extremely the phase-memory time, Tm, so

that no spin echo can in general be observed for these

clusters.46,47 Instead, for TCV3.4, the broad background signal

was found to have a Tm E 1 ms, whereby the Tm of the

well-resolved VO(II) component had field-dependent Tm values

in the order of 700–800 ns. A similar trend was observed

earlier for VO(acac)2 on TiOx and for VO(acac)2 and

TiO(acac)2 deposited on SBA-15.47 The broad signal may

stem from a multitude of isolated VO(II) sites with different

ligation and/or surrounding (and hence different EPR para-

meters) or from localized d-electrons in crystalline vanadium

pentoxide.46,50 W-band ESE-detected EPR experiments

revealed that the broad signal has average g and maximum

g values that are larger than the ones observed for the isolated

species (see ESI, Fig. S4z). The average g values are similar to

the ones of the broad signals found in orthorhombic vanadium

pentoxide bronze powders.50

Fig. 6c and d show the X-band ESE-detected EPR spectra

before and after dehydration of TCV3.4, respectively. Only

small differences are observed in the lower field region (see

arrow Fig. 6, spectra normalized to highest peak). This

indicates that some changes have occurred upon dehydration.

The X-band ESE-detected EPR spectrum of both as-

received non-calcined VO(acac)2/MCM-41 (Fig. 6e) and the

sample after evacuation (figure not shown) contains only the

contribution of the isolated VO(II) species and no broad

background. This proves that the broad signal observed

around g E 1.98 in the CW-EPR spectrum of dehydrated

non-calcined VO(acac)2/MCM-41 (Fig. 5g) is indeed due to

VO(II) clusters.

To obtain more detailed information about the struc-

ture and local coordination environment of vanadyl ions,

HYSCORE, pulsed ENDOR and ELDOR-detected NMR

experiments were carried out.

Fig. 7(a) shows the HYSCORE spectrum of the non-

calcined VO(acac)2/MCM-41 sample taken at an observer

position where all the molecular orientations are selected. We

observe a broad ridge centered around the 1H Larmor

frequency (B15 MHz). This signal stems from the interactions

with the surrounding 1H nuclei. The maximum observed

coupling is 13 MHz, typical for equatorially coupled OH of

pentaquovanadyl complexes [VO(H2O)5]2+,51 confirming again

that the acac ligands are fully replaced by water. Fig. 7(b) shows

the simulation of the HYSCORE spectrum in Fig. 7(a) using

the proton hyperfine values mentioned in Table 3.

The HYSCORE spectrum of TCV3.4 (Fig. 7(c)) taken at the

same observer position shows a different pattern. The maximal

observed proton coupling is now 6.0 MHz. This agrees with

what is typically observed for the hyperfine coupling of axial

OH protons.51 The proton hyperfine coupling used in the

simulation in Fig. 7(d) is given in Table 3. The coupling is

clearly different from the one observed for VO(acac)2/MCM-41

sample (Fig. 7(a)), where the vanadyl ion is fully water-

coordinated, indicating that no equatorial water ligation is

happening in the present case.

In the low-frequency area of the HYSCORE spectrum

(Fig. 7(c)), there are signals stemming from interactions with

29Si and 51V nuclei. The signal strength of these contribu-

tions is rather low (relative to the proton HYSCORE signal)

and long signal accumulation was needed to obtain the29Si HYSCORE. This is expected, because of the low natural

abundance of 29Si (4.67%).

The vanadium signal stems from nearby vanadium nuclei

(minimum distance between the VO(II) center and the 51V

nucleus is 3.15 � 0.15 A, when interpreting the hyperfine

coupling in terms of an isotropic coupling (0.27 MHz) and a

point-dipolar contribution52). Since no strong dipolar effects

were observed in the CW-EPR spectra, the ridge must stem

from V5+ centers near the vanadyl center. Oxidation of V(IV)

to V(V) is likely to happen under oxygen atmosphere. Note

that the relative 51V HYSCORE signal intensity is small

(see ESIz), which indicates that only a small fraction of the

VO(II) centers have nearby 51V nuclei. The latter interpretation

is corroborated by the fact that the intensity of the 51V signal

was batch dependent, reflecting the small heterogeneity in

the spread of the vanadium atoms. A similar batch-dependent51V signal was observed in W-band ELDOR-detected NMR

spectra (ESI, Fig. S5z).Two different sets of 29Si interactions are observed in the

HYSCORE spectrum in Fig. 7(c). The ridge in the (+,+)

quadrant stems from weakly coupled Si nuclei at a distance of

2.9 � 0.15 A from the VO(II) center (distance derived from

the hyperfine values in Table 3 using the point-dipolar

approximation52). The Fermi-contact term of �0.3 � 0.3 MHz

agrees with a spin density of B0.01% in the s-orbital. In the

(�,+) quadrant two cross-peaks centered at about (�0.92,6.62) MHz and (�6.62, 0.92) MHz are observed, stemming

from a strong interaction with a surrounding 29Si nucleus

(Table 3, simulations in Fig. 7(d)). The hyperfine coupling is

dominated by the Fermi-contact contribution (�7.8 MHz �0.5 MHz) corresponding to a spin density of B0.23% on the29Si nucleus (gn(

29Si) o 0). Because of the significant error on

the A2 value (Table 3), a detailed analysis of the anisotropic

part of the hyperfine contribution is difficult. Based on the

difference between the A1 and A3 value and assuming a simple

point-dipolar approximation, the distance of the 29Si nucleus

to the VO(II) center can be estimated as being 2.5 A (� 0.2 A).

However, the HYSCORE simulations point to a rhombicity of

the anisotropic hyperfine contribution, indicating that the

simple point-dipolar approximation does not hold and

that a small spin density in the p-orbital(s) will have to be

considered. The above derived distances should therefore only

be considered as rough estimates.

It should be noted that the absolute sign of the hyperfine

couplings in Table 3 cannot be derived from the HYSCORE

experiments. The sign of the 1H couplings was chosen as in the

pentaquovanadyl case and the sign of the 29Si and 51V couplings

were chosen based on the point-dipolar approximation.

In order to interpret the 29Si signals, a comparison can

be made with what is known about pentaquovanadyl. Earlier1H ENDOR experiments revealed information on the proton

spin density and V–H distances for this system (Fig. 8a). The

largest spin density is found for the equatorial protons. By

similarity, we can interpret the 1H and 29Si data of TCV in

terms of a framework-incorporated vanadyl center (Fig. 8b).

Since the spin density and V–Si distance derived from the

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strong 29Si coupling is similar to what is observed for equator-

ial protons in the pentaquovanadyl complex, we can ascribe

these interactions to equatorial 29Si nuclei. Similarly, the weak29Si interaction can be related to axial Si nuclei. The observed

proton coupling may be linked to an axial silanol binding or to

a proton linked via a hydrogen bond to the vanadyl oxygen.

In order to elucidate the nature of the broad background

signal in the ESE-detected EPR spectra of TCV samples

(Fig. 7a–d), X-band HYSCORE and X-band pulsed ENDOR

spectra were taken at magnetic field settings where predo-

minantly the broad signal is contributing to the spin echo and

at a nearby positions where also the isolated VO(II) species are

strongly contributing (marked with * and m, respectively in

Fig. 6). No 29Si signals could be detected in the HYSCORE

spectra of the broad background signal (not shown). The

X-band 1H Mims-ENDOR revealed a predominant sharp

signal at the proton Larmor frequency, where a broader

feature stemming from axial OH protons was observed at

position m (see ESI, Fig. S6z). The W-band ELDOR-detected

NMR spectra showed similar 51V contributions for the two

magnetic field settings (ESI, Fig. S5z). The broad ESE-

detected EPR signal thus results from a paramagnetic center

in a vanadium-rich and 1H- and 29Si-poor surrounding,

corroborating our earlier suggestion that the signal stems from

crystalline vanadium pentoxide.

Discussion

The nitrogen sorption (Fig. 2) and XRD (Fig. 1) experiments

performed on TCV samples, in which framework

Fig. 7 (a) HYSCORE spectrum of the VO(acac)2/MCM-41 sample, showing only a proton signal (no other signal was observed); (b) computer

simulation of (a); (c) HYSCORE spectrum of the as-prepared TCV3.4 sample, showing contributions from 1H, 51V and 29Si signals in the (+,+)

quadrant and also strong 29Si coupling in the (�,+) quadrant indicated by arrow; (d) computer simulation of (c). Both spectra were taken at 4 K at

the observer position known to excite all the orientations (347.4 mT) with t = 176 ns.

Table 3 Comparison of the 1H, 51V and 29Si hyperfine couplings derived from the HYSCORE spectra for the samples under study in comparisonwith known data from pentaquovanadyl

Samples Nucleus A1/MHz A2/MHz A3/MHz Ref.

TCV3.41H �0.3 � 0.2 �0.3 � 0.2 6.3 � 0.2 This work51V �0.4 � 0.4 �0.4 � 0.4 1.6 � 0.229Si (weak) 0.3 � 0.4 0.3 � 0.4 �1.6 � 0.229Si (strong) �6.0 � 0.3 �8.0 � 1.0 �9.4 � 0.3

VO(acac)2/MCM-41 1H 1.3 � 0.2 1.3 � 0.2 13.0 � 0.2 This work�0.8 � 0.2 �0.8 � 0.2 6.0 � 0.2

VO(H2O)52+ in frozen methanol 1H (axial) 3.21 3.21 6.12 51

1H (VQO� � �H) 1.40 1.40 4.341H (in-plane) 0.7 0.7 15.671H (out-of-plane)a 1.03 1.03 13.38

a Equatorial proton.

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incorporation was aimed at via a direct synthesis at room

temperature, suggest that a high quantity of vanadium is

indeed incorporated in the pore wall. The appearance of a

band around 930 cm�1 in the FTIR-PAS spectra of the

calcined TCV samples (Fig. 3) indicates the formation of a

Si–O–V bond formation.

Direct evidence for the incorporation of vanadium during

synthesis is obtained from the HYSCORE study of the vanadyl

precursors of the final catalyst. The observation of two types of29Si hyperfine interactions can be directly linked to the presence of

axial and equatorial Si–O–V entities, which can only be explained

in terms of framework incorporation of the VO(II) unit (Fig. 8b).

To our knowledge, this is the first time that framework incorpora-

tion of a transition metal could be so unambiguously proven in a

mesoporous silica support. It is interesting to note that vanadyl

centers with similar principal g and 51V hyperfine values as

observed here for TCV have been reported in V-silicalites.53 This

supports the assignment of the 930 cm�1 band in the FTIR-PAS

spectra to silicalite-type V5+ centers in TCV.

Despite the large percentage of vanadium-incorporation in

the TCV wall, the UV-Vis-DR experiments prove that the

vanadium sites are still accessible and susceptible to changes in

coordination (Fig. 4). The proof that vanadium can be

immobilized in the mesoporous silica (TCV) walls, while still

being accessible to guest molecules in order to remain its

catalytic activity in chemical reactions, is very important

towards the further application of these materials. Certainly,

these framework-incorporated TCV catalysts exhibit a signifi-

cant advantage compared to deposited vanadia MCM-41

supports, when it comes to leaching of the active centers.

Moreover, the coordination of metal oxides in the framework

of mesoporous materials is known to be of great importance

since differences in coordination of incorporated metal oxides

result in low catalytic activities.54,55

Therefore, the presented synthesis of the TCV materials is

thought to be a good way to achieve mesoporous materials with

framework-incorporated vanadium sites with a similar coordi-

nation as in vanadium-containing zeolites. In this way, a

material is obtained that combines large pores with frame-

work-incorporated vanadium. They will allow good mass trans-

fers for bulky molecules and viscous liquids while maintaining

reduced leaching behaviour and good accessibility towards the

active sites. The XRD experiments show that the d-spacing of

the TCV samples with high vanadium loadings decreases

slightly, pointing to some vanadia species not being incorpo-

rated in the pore walls. This is in agreement with the observation

of the broad background signal in the ESE-detected EPR

spectra, which has similar characteristics as that observed earlier

for localized d-electrons in crystalline vanadium pentoxide.50

This interpretation is corroborated by the pulsed EPR and

ENDOR data that show that we are dealing with a vanadium-

rich and Si- and H-poor species. Therefore, good framework

incorporation of tetrahedral vanadium will only be possible up

to about 3.5 wt% in mesoporous silica materials.

It is interesting to note that Shylesh and Singh44 did not

observe an EPR signal from non-calcined V-MCM-41 syn-

thesized hydrothermally at high pH from VOSO4 and TEOS

(termed VMT), whereas a spectrum with similar parameters as

found for our TCV samples was observed after a hydrothermal

synthesis using fumed silica as the silica source (referred to as

VMS). Disappearance of the EPR spectrum can be due to aerial

oxidation of V4+ to the EPR-silent V5+ as has been found also

for other V-containing MCM catalysts56 or is also suggested to

stem from the presence of V4+ ions in highly symmetrical lattice

positions for which very short relaxation times are expected.57

Shylesh and Singh44 suggested that the better stability/capping

of the V4+ species in the VMS matrix versus the VMT matrix

may help to retain the tetravalent state. Our current observation

indicates that one retains the V4+ state when the synthesis is

done at room temperature, indicating that both the synthesis

temperature and the silica source play a role. Furthermore, the

fact that we observe clear EPR signals from vanadyl species

shows that the disappearance of the EPR signal in VMT

materials is unlikely to be due to V4+ ions in highly symmetrical

lattice positions. It can probably be ascribed to a more

rapid oxidation of the vanadyl centers due to the elevated

temperature.

Conclusions

A series of framework-incorporated vanadium mesoporous

silica (TCV) samples, with different vanadium contents

Fig. 8 (a) Overview of known spin density and H–V distances for pentaquovanadyl.51 (b) Model for the framework-incorporated vanadyl in TCV

based on the present HYSCORE and ENDOR data.

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(0.8–4%) were obtained by direct synthesis at room temperature,

using VOSO4�5H2O as the vanadium precursor.

A profound characterization of the final catalysts by XRD,

FTIR-PAS and FT-Raman spectroscopy and of the VO(II)

precursor by pulsed EPR/ENDOR spectroscopy proves

unambiguously that a high quantity of vanadium could be

immobilized in the mesoporous silica framework. Due to the

incorporation of vanadium in the porous structure, the order-

ing of the TCV materials was lost. Moreover, the incorporated

vanadium species were found to be still accessible for guest

molecules, such as the hydrating water.

Acknowledgements

V. M., P. C. and S. V. D. acknowledge the FWO – Flanders

for financial support. This work is part of the European NOE

project (EU-FP6) ‘Inside Pores’ and a research project funded

by the Special Fund of Research GOA-BOF by the University

of Antwerp. Sepideh Zamani thanks the University of

Antwerp for PhD funding (UA-BOF NOI project).

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