New Insights in the Formation of Silanol Defects in Silicalite-1 by Water Intrusion Under High Pressure

7/24/2019 New Insights in the Formation of Silanol Defects in Silicalite-1 by Water Intrusion Under High Pressure

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New insights in the formation of silanol defects in silicalite-1 by waterintrusion under high pressure

Thomas Karbowiak,ac Mohamed-Ali Saada,b Se verinne Rigolet,b

Anthony Ballandras,a Guy Weber,a Igor Bezverkhyy,a Michel Soulard,b

Joel Patarin*b and Jean-Pierre Bellat*a

Received 14th January 2010, Accepted 6th May 2010

DOI: 10.1039/c000931h

The watersilicalite-1 system is known to act as a molecular spring. The successive

intrusionextrusion cycles of liquid water in small crystallites (6 3 0.5 mm3) of hydrophobic

silicalite-1 were studied by volumetric and calorimetric techniques. The experiments displayed a

decrease of the intrusion pressure between the first intrusionextrusion cycle and the consecutive

ones, whereas the extrusion pressures remained unchanged. However, neither XRD studies

nor SEM observations revealed any structural and morphological modifications of silicalite-1

at the long-range order. Such a shift in the value of the intrusion pressure after the first water

intrusionextrusion cycle is attributed to the creation of silanol groups during the first water

intrusion. Detailed FTIR and solid-state NMR spectroscopic characterizations provided amolecular evidence of chemical modification of zeolite framework with the formation of local

silanol defects created by the breaking of siloxane bonds.

1. Introduction

Water confinement in nanoscopic and hydrophobic spaces

covers many situations from chemistry (hydrophobic porous

inorganic/organic materials)13 to biology (hydrophobic cavities

of proteins).4,5 In particular, thermodynamic systems consisting

of a hydrophobic porous solid and water as a non-wetting

liquid have been considered as promising devices for energetic

applications.1,6,7 Primary works in this field started in the

middle 90s on silica gels,8,9 and were then extended tofunctionalized organized mesoporous solids1012 and pure

silica zeolites (zeosils), such as *BEA,6 DDR,6 FER,13

CHA14,15 and MFI.1 Investigations were performed either

experimentally to determine the intruded water volume along

with the applied pressure,3,1116,1 or by molecular simulation

(GCMC: Grand Canonical Monte Carlo).13,1721

To penetrate liquid water in a hydrophobic microporous

matrix, a certain pressure must be applied.22 During this forced

penetration (intrusion), mechanical energy can be converted into

interfacial energy. Different behavior, exemplified by isothermal

pressure/volume diagrams, can be observed, which depends on

various physical parameters related to the porous matrix such

as pore size, pore system (cages or channels), dimensionality ofthe channels (1-D, 2-D, 3-D)10,16,23 and on the hydrophobic/

hydrophilic character.13,15 According to the reversible or

irreversible character of the intrusionextrusion cycle, the

waterzeosils systems are able to restore, absorb or dissipate

mechanical energy. Consequently, molecular spring, damper

or shock-absorber behavior can be observed.1,6,21,24 Some of

these systems, such as MFI-type zeolites, displaying an apparent

reversible phenomenon, are able to accumulate and restore

mechanical energy.6,23 On the opposite, *BEA-type zeolite

displays an irreversible phenomenon with no energy restored

during the pressure release.1,6 For other materials (CHA, DDR)

a pronounced hysteresis occurs in the relaxation stage, leading

to only partial restitution of the accumulated energy.6,15

However, thermodynamics of water interaction with hydro-

phobic nanoporous materials is still not well understood. On

the other hand, how the forced penetration of liquid water in

hydrophobic cages or channels can change the structural

properties of the nanoporous solid has never been studied in

detail.

This study is focused on the intrusion and extrusion of water

in silicalite-1. This microporous material is a pure silica

MFI-type zeolite first synthesized in 1978.25 Its structure

displays interconnected channels with 10 membered ring openings.

This three-dimensional open channel system consists of near-

circular straight channels (0.56 0.53 nm2) cross-linked by

elliptical, sinusoidal (zigzag type) channels (0.55 0.51 nm2).26

This porous network together with a high hydrophobicity

(cation-free) and a good thermal and chemical stability makes

silicalite-1 promising membrane material for application in gas

separation by molecular sieving.27,28 Its aluminosilicate form

(ZSM-5 zeolite) is widely used in petrochemical processing, for

improving catalysis or separation stages such as p-xylene

synthesis from toluene (based on shape selectivity) or

ethylbenzene synthesis.29,30

For silicalite-1, water intrusion has been found to occur at a

pressure of about 100 MPa at 298 K with a stored energy of

a Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB),UMR 5209 CNRS, Universitede Bourgogne, 9 Av. A. Savary,BP 47870, F-21078 Dijon, France.E-mail: [email protected]

b Equipe Materiaux a` PorositeControlee (MPC), Institut de Sciencedes Materiaux de Mulhouse (IS2M), LRC 7228 CNRS, Universite de Haute-Alsace, ENSCMu, 3 rue Alfred Werner, F-68093Mulhouse, France. E-mail: [email protected]

c EA 581 EMMA, AgroSup Dijon, 1 esplanade Erasme, UniversitedeBourgogne, F-21078 Dijon, France

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around 10 J per gram of zeolite.31 Water intrusion in this

system was firstly described as a reversible phenomenon from

volumetric measurements,6,23 while a small hysteresis loop

between intrusion and extrusion pressures seems to exist.

GCMC molecular simulations display an intrusion pressure

higher than that obtained by volumetric measurements.13

Nevertheless, the introduction of silanol defects in the model

induces a decrease of the calculated intrusion pressure, which

becomes closer to the measured pressure. The additional

creation of silanol defects in the reference material also leads

to a decrease of the experimental intrusion pressure. Conversely,

it can be noted that its energetic performance can be improved

by increasing its porous volume using carbon black or

organosilane surfactant in the reactant gel to create additional

micropores.23

Besides mechanical effects, thermal effects measured by

high-pressure calorimetry at equilibrium display an intrusion

heat higher than the extrusion heat, during pressure increase

and release, respectively, with a more noticeable hysteresis

between intrusion and extrusion pressures.32 Contrarily to

previous volumetric experiments,6,23 they therefore evidence

water intrusion in silicalite-1 as an irreversible phenomenon.

The origin of this irreversibility probably results from the

creation of silanol defects.15,32 It would also be the

consequence of a metastability of the intruded phase along

with the formation of a vapor phase by a mechanism of

nucleation during extrusion,10,33,34 which could depend on

the rate of pressure variation.

The aim of this work is to focus on this aspect of silanol

defect creation during water intrusion. Therefore, successive

intrusionextrusion cycles were performed using combined

volumetric and calorimetric studies. A further detailed

physicochemical characterization of the material before and

after intrusionextrusion cycles was done using in particular

FTIR and solid state NMR spectroscopies.

2. Experimental

2.1 Material

Silicalite-1 was synthesized in fluoride media according to the

procedure described by Guthet al.35 in the presence of seeds of

silicalite-1 in order to promote crystallization. After the

synthesis, the product was filtered, washed with demineralized

water and dried at 353 K overnight. To liberate completely the

porosity, the solid was then calcined at 823 K under air for 15 hto eliminate the templating agent (tetrapropylammonium

cations). Characterization of the material using FTIR and

NMR spectroscopy showed that these conditions of calcination

preserve the material from silanol defects creation.

2.2 X-Ray diffraction analyses

Powder X ray diffraction patterns were recorded in ambient

conditions with a PANalytical X0pert Pro diffractometer

equipped with the X 0Celerator detector using Cu Karadiation

(l = 0.15418 nm) in the 2y range from 5 to 501 with a step

width of 0.017.

2.3 Scanning and transmission electron microscopy studies

Scanning electron microscopy (SEM) observations of carbon-

metalized silicalite-1 crystals were performed on a JEOL-JSM

6400 F (JEOL, Paris, France), with an acceleration voltage

of 15 kV.

Transmission electron microscopy (TEM) analyses were

carried out using a JEOL 2100 LaB6 model. The samples were

prepared by slow evaporation of a drop of a zeolite ethanolsuspension, deposited on a carbon-coated copper grid and

evaporated.

2.4 Nitrogen adsorptiondesorption isotherms

The nitrogen adsorptiondesorption isotherms were measured

at 77 K using a Micromeritics ASAP 2420 analyzer. Prior to

analyses, the samples were outgassed under vacuum either at

573 K for 15 h (starting material) or at 353 K for 3 h (material

after water intrusion, in order to only remove physisorbed

water). The microporous volume and the external surface were

determined using the t-plot method.36

2.5 N-Hexane or water vapor adsorption isotherms

N-Hexane or water adsorption on silicalite-1 was investigated

at 298 K, under controlled vapor pressure (measured with a

MKS Baratron absolute pressure transducer, MKS, Le Bourget,

France), using a home-made McBain thermobalance. In this

closed system, the sample is hung on a quartz helical spring,

whose elongation indicates the sample mass variation as a

function of the gas pressure at equilibrium. The experimental

accuracy is 0.01 mg for the mass of the adsorbate, 0.5 K

for the temperature, and 1 Pa for the pressure. The

adsorptiondesorption isotherm is measured step by step using

a static method, by increasing (or decreasing) the pressure over

the range 102200 hPa for N-hexane and 10231.5 hPa for

water vapor (0 r p/ps r 1). The mass of the zeolite sample

was around 20 mg. Prior to experiment, the zeolite was out-

gassed under vacuum (105 hPa) for 12 h at 353 K for

N-hexane or for 12 h at 298 K for water. The microporous

volume and the external surface of the samples were determined

using the t-plot method37 fromN-hexane experiments.

2.6 High pressure water intrusionextrusion isotherms

Water intrusionextrusion tests were performed at room

temperature with 0.8 g of water and 0.6 g of silicalite-1 using

a modified mercury porosimeter (Micromeritics Model

Autopore IV) according to the procedure described in a

previous work.14 The values of intrusion (pint) and extrusion

(pext) pressures are defined for the half-volume total variation.

The experimental intrusionextrusion curve is obtained after

subtraction of the curve corresponding to the compressibility

of pure water. Pressure is expressed in MPa, and the volume

variation in mL per gram of anhydrous calcined sample. The

experimental error is estimated to 1% on the pressure and on

the volume.

2.7 High pressure calorimetric studies

The measurement of heat exchange during liquid water

intrusion and extrusion in silicalite-1 was performed using a

home-made equipment composed of a differential calorimeter

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coupled to a high pressure device. An air driven liquid pump

(DSXHF602, Haskel, Villeneuve dAscq, France) was used to

pressurize the system with water within the pressure range

0.1400 MPa. At the end of the circuit, two calorimetric

vessels (designed and manufactured in our laboratory) can

be either connected or isolated from the rest of the circuit.

They are placed in the differential calorimeter (Tian-Calvet

Setaram C80, Setaram, Caluire, France) having a sensitivity of

0.01 mW. Heat flow is recorded for a volume of the calorimetric

vessel of 1 mL with a mass of zeolite of around 0.5 g.

An analogue/digital converter via Test Point interface allows

data acquisition (pressure, temperature, heat flow) during the

time of experiment. Intrusion of the degassed and distilled

liquid water in silicalite-1 was performed under isothermal

conditions at 298 K. The zeolite sample introduced in the

calorimetric vessel was previously outgassed in situ under

primary vacuum for 3 h at 298 K. Pressure measurements

were performed using a precision pressure gauge 0400 MPa

(P105, FGP Sensors, Les Clayes Sous Bois, France). For all

measurements, the same procedure was followed to strictly

obtained equilibrium data. The measurement vessel contained

zeolite with water at a given pressure, whereas the reference

vessel contained water maintained at atmospheric pressure.

Liquid water was compressed step by step in the constant

volumeVof the measurement calorimetric vessel, with successive

equilibrium pressure increments Dp of about 10 MPa. The

calorimetric vessel is isolated from the rest of the hydraulic

circuit between each pressure increase. It took about 1 h for

the system to return to equilibrium before each pressure

increment. The corresponding differential measurement of

heat for eachDpis therefore a true equilibrium measurement.

For each pressure step, the differential heat per gram of zeolite

DQ/mdp is calculated at equilibrium from the integration of

the heat flow as a function of time after subtraction of the

thermal effect of water compression around zeolite.32 Four

successive cycles of liquid water intrusion up to 200 MPa and

extrusion down to 0.1 MPa have been performed.

2.8 FTIR spectroscopic characterization

FTIR spectra were recorded at room temperature on a

BRUKER Equinox 55 spectrometer over the wavenumber

range 4000400 cm1. Spectra were averaged with 200 scans

with a resolution of 2 cm1 and corrected for the background

(spectra collected in the same conditions as those without a

sample). Both KBr diluted and self-supported (compacted

under a uniaxial pressure of 0.1 GPa) preparations were used

for the analyses. The first preparation allows to bettercharacterize strong lattice vibrational bands whereas the

second one is more accurate to well define weak vibrational

bands.38 Moreover, the second preparation allows the

characterization of the sample after in situ outgassing at 298 K

under vacuum in a specific chamber equipped with two KBr

windows. In this case, spectra were recorded until all water

molecules physisorbed on the sample were desorbed.

2.9 Solid-state NMR spectroscopic studies

29Si (I= 1/2) magic angle spinning (MAS) and 1H29Si cross

polarization magic angle spinning (CP-MAS) NMR spectra

were recorded at room temperature, with a Bruker double

channel 7 mm probe with a spinning frequency of 4 kHz, on a

BRUKER AVANCE II 300 spectrometer operating at

B0 = 7.1 T (Larmor frequency n0 (29Si) = 59.63 MHz and

n0 (1H) = 300.13 MHz). 29Si single pulse MAS NMR

experiments were performed with a p/6 pulse duration of 1.9 ms

and a 80 s recycling delay. These recording conditions ensure

the quantitative determination of the proportions of the

different Qn Si species.39 1H29Si CPMAS NMR experiments

were acquired using a ramp for HartmannHahn matching

with a 1Hp/2 pulse duration of 5.7 ms and a contact time of

8 ms. The radiofrequency field strength used for 1Hdecoupling

was set to 62.5 kHz. 1H (I= 1/2) MAS NMR experiments

were performed at room temperature on a BRUKER

AVANCE II 400 spectrometer operating at B0 = 9 . 4 T

(Larmor frequency n0= 400.13 MHz). Single pulse experiments

were recorded with a double channel 2.5 mm BRUKER MAS

probe at room temperature, a spinning frequency of 30 kHz

and ap/2 pulse duration of 5.25 ms. 1Hspin lattice relaxation

times (T1) were measured with the inversion-recovery pulse

sequence for all samples. Typically, 600 scans were recorded.1H double quantum (DQ) MAS NMR experiments were

performed with a p/2 pulse length of 1.9 ms and a spinning

frequency of 30 kHz. The duration of the excitation/reconversion

of the double quantum coherences of the back-to-back

(BABA) pulse sequence40 were adjusted to 2 rotor periods

(66.7ms).

Chemical shifts reported thereafter are relative to tetra-

methylsilane for both 1Hand 29Si nuclei. Deconvolutions of

the spectra were performed using Dmfit software.41

3. Results and discussion

3.1 Mechanical and thermal effects of liquid waterintrusionextrusion cycles in silicalite-1

The water intrusionextrusion isotherms obtained on silicalite-1

at 298 K after one-, two-, three- and four intrusionextrusion

cycles are shown in Fig. 1a, 2a and c. The corresponding

thermal effects related to the successive cycles of water

intrusionextrusion are depicted in Fig. 1b, 2b and d.

The associated experimental data are summarized in Table 1.

The first intrusion of liquid water into the nanopores of the

highly hydrophobic zeolite occurs at relatively high pressure

(around 92 MPa), in agreement with previous volumetric

measurements (Fig. 1).23 The first part of the pressurevolume

curve is quite flat, indicating no intrusion of water or slight

intrusion in the interparticle porosity. Then, a high volume

variation is observed in a rather restricted pressure range when

the capillary pressure is reached. During this step, the water

intrusion in the micropores occurs at a pressure (pint) of 92 MPa.

The amount of intruded water (Vint), which corresponds to the

height of the jump in volume shown in Fig. 1a is 0.094 mL g1.

Lastly, after the complete filling of micropores, a quasi plateau

takes place. This intrusion of water in the porosity produces a

well-defined endothermic effect within the narrow range

9095 MPa (Fig. 1b). This corroborates GCMC molecular

simulations, which predicted that water intrusion is endothermic,19

as well as the first calorimetric measurements performed on


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MFI-type zeolites.12 The thermal energy involved in water

molecules penetrating hydrophobic nanopores is around

7.8 J per gram of zeolite. For one mole of water intruding

into the microporosity, this energy is equal to 1.5 kJ mol1.

This is quite low compared to the adsorption enthalpy of water

vapor on ZSM-5 (around 30 kJ mol1), which is slightly below

the liquefaction enthalpy of water (44 kJ mol1).17,25

Moreover, over 95 MPa, a low exothermicity persists. It could

correspond to the compression of water inside the porosity. The

assumption of an intruded water phase analogous to the liquid

bulk (and correction of its corresponding thermal effect) is

thus untrue. This means that the physical state of the intruded

water phase in hydrophobic nanopores differs from that of the

liquid bulk water surrounding the zeolite crystallites. As

suggested in other materials, a vapor film separating water

from the hydrophobic solid could exist,3 with strong orientation

effects in the interfacial region of water molecule/nanopore

internal surface.42 Or, water molecules confined in silicalite-1

channels become more structured than the liquid bulk and

therefore have a density very different from that of the liquid.

Moreover, the intruded volume (0.094 mL g1) compared to

the pore volume determined by nitrogen adsorption (Table 2),

appears to be lower than the micropore volume of the

material. Therefore, the intruded water would have a lower

density (0.52 g mL1) than the bulk liquid water. This result is

consistent with the water density around 0.6 g mL1

previously reported for MFI zeolite topology,18,43 and confirms

the hypothesis of the intruded phase different from the liquid.

With reference to Fig. 1a, the intrusionextrusion pheno-

menon could first appear quite reversible because the extrusion

pressure is close to the intrusion pressure. Nevertheless,

calorimetric measurements clearly reveal that the intrusion

and extrusion of water in silicalite-1 is irreversible (Fig. 1b).

Extrusion occurs at a lower and over a broader pressure range,

and the consecutive exothermic effect is reduced fivefold when

compared to intrusion (Table 1). It may be noted in Table 1

that the intrusion and extrusion pressures measured by calorimetry

are slightly lower than those measured by volumetry. This

difference probably originates in kinetics, the equilibrium time

used for calorimetry between two consecutive measurements

being very much higher than for volumetry.

Concerning the successive other intrusionextrusion cycles

(second, third and fourth), the pressurevolume intrusion

isotherms (Fig. 2a) are completely superimposed. Similarly,

the pressurevolume extrusion isotherms (Fig. 2c) are also

identical, but slightly shifted towards lower pressures compared

to intrusion. After the first intrusion, intrusionextrusion

experiments are perfectly reproducible. For the three last

cycles water penetrates micropores at a pressure of 90 MPa,

and the extrusion takes place close to 88 MPa. If the intrusion

occurs at a lower pressure than for the first cycle, the extrusion

pressure remains the same. In addition, the intruded and

extruded water volumes are constant for successive cycles.

Such behavior tends to confirm that the watersilicalite-1

system acts as a molecular spring,1 able to store and restore

mechanical energy. The integration of the water intrusion

pressurevolume isotherm (W =R

pdV) gives a mechanical

energy of about 7.8 J per gram of zeolite. The small hysteresis

phenomenon between successive intrusions and extrusions is

particularly well evidenced by the calorimetric measurements

which are more sensitive than the volumetric ones (Fig. 2b and d).

Not only is the maximum intrusion pressure reduced as

compared to the first intrusion, but also it occurs over a

broader pressure range. Surprisingly, the thermal energy

involved is around 3.5 J per gram of zeolite for intrusions

whereas it is reduced to 1.6 for extrusions. In the present state

of this work we are not able to explain this difference. It could

be due to a mechanism of nucleation of gas bubbles within the

confined liquid during extrusion,34,44 which is endothermic.

However we have no real experimental proof to confirm this

hypothesis. After the first intrusion, a certain stability of the

material regarding pressure seems to be established, with

reproducible successive intrusions and extrusions (Scheme 1).

However, a small hysteresis between intrusion and extrusion

curves persists, which could be due to the formation of a vapor

phase by a mechanism of nucleation.10,33,34

Therefore, the difference in calorimetric heats observed

between the first and next intrusions is due to a modification

of the material during the first intrusion of water in the

porosity. This could be ascribed to the existence of tiny

Fig. 1 (a) Pressurevolume diagram of the watersilicalite-1

system for the first intrusionextrusion cycle at 298 K. (b) Correspondingthermal effects related to water intrusionextrusion in silicalite-1

at 298 K.


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hydrophilic defects, which are created throughout the first

intrusion. Such a phenomenon has already been proved by a

molecular simulation study13 and observed for CHA-type

zeolite.14,15

Besides, volumetric and calorimetric experiments were also

performed using larger silicalite-1 crystallites (100 40 40 mm3).

Similar results were obtained. Therefore, the macroscopic

geometry of crystals has no effect on its thermodynamic

Fig. 2 (a) Pressurevolume diagram of the watersilicalite-1 system at 298 K for the second, third and fourth intrusion cycles.

(b) Corresponding thermal effect related to water intrusion in silicalite-1 at 298 K. (c) Pressurevolume diagram of the watersilicalite-1

system at 298 K for the second, third and fourth extrusion cycles. (d) Corresponding thermal effect related to water extrusion in silicalite-1 at

298 K. For each experiment the second-, third- and fourth cycles are completely superimposed.

Table 1 Thermodynamic properties of the watersilicalite-1 system measured during successive intrusionextrusion under mechanical stress atequilibrium at 298 K

Cycle 1 Cycles 2-3-4

Volumea/mL g1 Intrusion 0.094 0.094

Extrusion 0.094 0.094

Pressure/MPa

Intrusion Maximuma 92 90Widthb 9095 5590

Extrusion Maximuma 88 88Widthb 5088 5088

Thermal effectb

Intrusion /J g1 zeolite 7.8 3.5/kJ mol1 intruded water 1.5 0.7

Extrusion /J g1 zeolite 1.5 1.6/kJ mol1 intruded water 0.3 0.3

a Determined from the pressurevolume diagrams. b Determined from the calorimetric measurements.


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properties relative to water intrusion and extrusion under

mechanical stress.

3.2 Structure and porosity modifications

As previously observed by volumetric and calorimetric

experiments, mechanical stress induces modifications in

silicalite-1. At first, it is therefore interesting to know how

the structure and the porosity of the zeolite are modified.

3.2.1 XRD analysis. As reported in Fig. 3, the X-ray

diffraction patterns of silicalite-1 before and after four successive

water intrusionextrusion cycles do not show any significant

difference. Therefore, there is no modification of the material

structure on the long-range scale. No amorphization of the

material is observed. The global crystalline structure is therefore

not affected by the high-pressure intrusion of water.

3.2.2 SEM and TEM analysis.The crystallites of silicalite-1

are relatively small with a crystal size close to 6 3 0.5mm3

(Fig. 4a). Fig. 4b displays some crystallites after four successive

water intrusionextrusion cycles. The overall geometry is

preserved. The crystallites are not so much damaged after four

intrusionextrusion cycles. Only some of them are randomly

broken along the thinner dimension in few pieces.

Similarly, the structure of the particles examined by TEMdoes not reveal any change related to intrusionextrusion

cycles (Fig. 4c and d). Therefore, if silicalite-1 undergoes

modifications, these can only be seen as local defects. It is

noteworthy that silicalite-1 presents a perfect microporous

texture even at the periphery of the particles.

3.2.3 Nitrogen and N-hexane adsorptiondesorption

measurements.Textural properties determined from adsorption

desorption isotherms of nitrogen on the starting material and

on the same material after four successive water intrusion

extrusion cycles are given in Table 2 and Fig. 5. Both

isotherms are of type I, characteristic of microporous solids.

Table 2 Micropore volume (Vm) and external surface (Sext) of silicalite-1 before and after four water intrusionextrusion cycles, determined bynitrogen andN-hexane adsorption isotherms using the t-plot method

Vm/cm3 g1 Sext/m

2 g1

Referencea After 4 cyclesb Referencea After 4 cyclesb

From nitrogen adsorption 0.181 0.146 9 14(Fig. 5)From N-hexane adsorption 0.180 0.170 1 2

(Fig. 6)a Before water intrusion (starting material). b After four water intrusionextrusion cycles.

Scheme 1 Comprehensive representation of the four successive

intrusionextrusion cycles performed on the watersilicalite-1

system at 298 K. (a) Pressurevolume diagrams. (b) Corresponding

thermal effects.

Fig. 3 X-Ray diffraction patterns of calcined silicalite-1 (a) before

and (b) after four water intrusionextrusion cycles.


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The isotherm of the starting material (calcined silicalite-1)

presents a clear step at p/ps close to 0.15, followed by a

plateau. As has been described by Llewellyn et al.,45 this step

corresponds to a density change of the adsorbed phase. It was

ascribed to a phase transition from a lattice fluid-like phase to

a crystalline-like solid phase. The micropore volume (Vm) and

the external surface (Sext) of the starting material, determined

by the t-plot method from the adsorption branch, are

0.181 cm3 g1 and 9 m2 g1, respectively. After four water

intrusionextrusion cycles, the micropore volume decreases to

0.146 cm3 g1 and the external surface increases to 14 m2 g1.

The adsorption isotherm displays a different shape and does

not show any step at p/ps E 0.15. Such a difference between

the starting material and the sample after four successive water

intrusionextrusion cycles could probably be due to a decrease

in the micropore volume consecutive to the creation of defects

in the zeolitic framework. These defects could induce partial

pore blocking which would therefore make less noticeable the

phase transition of nitrogen adsorbed in the channels. In order

to confirm this last hypothesis, adsorption ofN-hexane, a non

specific molecular probe, was performed on both solids, at

room temperature (Table 2 and Fig. 6). As previously noticed

for nitrogen adsorption, the micropore volume significantly

decreases (0.01 cm3 g1) for the material submitted to four

successive water intrusionextrusion cycles. No significant

change in the external surface is however observed. This means

that such high pressure treatment modifies the textural properties

of the material, probably through silanol defects creation

which could induce a slight decrease in the microporous

volume. More surprising is the value of the micropore volume,

calculated from N-hexane data, which is higher than the one

estimated from nitrogen data. In the latter case, the densification

of the adsorbed nitrogen phase seems to be attenuated after

four successive water intrusionextrusion cycles (Fig. 5). The

t-plot from the nitrogen desorption branch actually gives a

value forVmcloser to that obtained from N-hexane adsorption.

In any case, these results suggest a slight decrease in the

micropore volume consecutive to an intrusion of water in

silicalite-1 microporosity.

3.2.4 Water vapor adsorption isotherms. The adsorption

isotherms of water vapor on silicalite-1 obtained at 298 K

before and after four intrusionextrusion cycles are shown in

Fig. 7. First, the adsorption of water on the starting material

displays a very weak affinity for water. The amounts adsorbed

are lower than 1 molec./u.c. below 20 hPa and do not exceed 3

molec./u.c. on approaching saturation. This isotherm of type III

according to the IUPAC classification is characteristic of a gas

adsorption with very weak adsorbateadsorbent inter-

actions.46 It accounts for the hydrophobic character of calcined

silicalite-1, the starting reference material. Once the material

was submitted to an intrusionextrusion cycle, its affinity for

water vapor is significantly modified (Fig. 7). In that case, the

amounts adsorbed at constant pressure are higher than the

ones obtained for the reference sample. This can be a

consequence of chemical modifications of the starting

Fig. 4 Scanning (a and b) and transmission (c and d) electron

microscopy observations of silicalite-1 (a and c) before and (b and d)

after four intrusionextrusion cycles.

Fig. 5 N2 adsorptiondesorption isotherms of silicalite-1 at 77 K

(a) calcined silicalite-1 (reference), and (b) after four successive water

intrusionextrusion cycles (the adsorbed amount is given in cm 3 of

liquid nitrogen per gram of zeolite).

Fig. 6 N-Hexane adsorptiondesorption isotherms of silicalite-1 at

298 K (a) calcined silicalite-1 (reference), and (b) after four successive

water intrusionextrusion cycles.


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hydrophobic material, which becomes slightly hydrophilic.This mechanism probably implies the creation of hydrophilic

silanol defects consecutive to the breaking of siloxane bonds

during intrusion.

3.3 Modifications in the surface chemistry

3.3.1 Infrared spectroscopy. A microscopic investigation

was performed by infrared spectroscopy in order to go deeper

in understanding of modifications, and especially chemical

modifications of the zeolite surface, involved after successive

water intrusionextrusion cycles. Fig. 8 displays the FTIR

spectra of the starting material (calcined silicalite-1) in

comparison with those obtained after silicalite-1 underwent

one water intrusionextrusion cycle and four successive cycles.

After outgassing self-supported materials under high vacuum

at 298 K, the spectra do not show the characteristic HOH

bending vibration at 1640 cm1, indicating that water

physisorbed under room conditions is completely desorbed.

Therefore, the vibrational bands observed in the hydroxyl

stretching region can be attributed to silanol defects (SiOH)

created on the external and/or on the internal surface of the

material (Fig. 8a). The starting material contains very few

silanol defects as shown by the very small contributions of the

SiOH stretching vibrations in the wavenumber range

40003000 cm1 (Fig. 8a, reference). It is noteworthy that

after the first water intrusionextrusion cycle, significant

changes occur in the FTIR spectrum of silicalite-1 (Fig. 8a,

1 cycle; Table 3). Firstly, a sharp and intense band appears at

3731 cm1, which can be unambiguously assigned to the

OH symmetric stretching vibration of isolated silanol groups,

similar to those of all silica-based materials. Isolated silanols

located on the external surface of the zeolite are mostly

reported to generate a vibration band at around 3740 cm1,4750

and especially geminal species, which are formed by two

OH groups linked to an external Si atom.51 In the present

study, as will be confirmed by NMR spectroscopy (see below),

there are no geminal silanol groups, and therefore we consider

that the band at 3731 cm1 may be due to isolated silanols

located into the silicalite-1 pores.51,52 If the formation of

silanol defects results from breaking of siloxane bonds

(RSiOSiR), this brings an additional evidence of the

impossibility to generate geminal species in this way. Secondly,

the FTIR spectrum after water intrusion also shows a broader

band at lower frequency (36503700 cm1), characteristic of

stretching vibration of terminal silanols, whose oxygen atoms

are only involved in hydrogen-bonding with the nearer hydroxyls,

into zeolite pores.48,49,52 Thirdly, at lower frequency, in the

32003650 cm1 domain, a very large band can be attributed

Fig. 7 Water adsorption isotherms of silicalite-1 samples (a) reference,

before water intrusion, and (b) after four successive water intrusion

extrusion cycles.

Fig. 8 FTIR spectra of silicalite-1 before (reference) and after one

and four water intrusionextrusion cycles at 298 K. From bottom to

top: spectra collected (a) from self-supported samples after complete

outgassing of the sample, and (b) from KBr diluted samples (only

windows of interest are shown).

Table 3 Characteristic modifications in FTIR spectrum of silicalite-1after intrusionextrusion cycles with attributions for vibrations

implied

Wavenumber/cm1Modificationafter intrusion Attribution

3731 Apparition n(OH)(isolated silanols)4750,52

36503700 Apparition n(OH)(terminal silanols)48,49,52

32003650 Apparition n(OH)(vicinal hydrogen-bonded silanols)48,49,52

1240 Shift- 1238 nas (SiOSi)5658

960 Apparition n(SiO)(silanols)5355

631 Shift- 628 d (OSiO) andd (SiOSi)5861


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to stretching vibration of vicinal hydrogen-bonded silanols,48,49,52

whose hydrogen atoms are involved in weak hydrogen-bonding

with the nearer hydroxyls. These silanols interacting via

hydrogen bonds could correspond mainly to dimers but also

to higher oligomers.

Additional information can be obtained by analyzing the

fingerprint region. Considering the corresponding FTIR spectra

obtained with KBr-diluted samples (Fig. 8b; Table 3),

characteristic vibrations of the MFI zeolite structure were

observed, with fundamental, harmonic and complex bands,

as described by Bernardet et al.38 As previously detailed for

the hydroxyl region, the creation of silanol defects is also

confirmed by the appearance of a new band at 960 cm1,

which can be assigned to the stretching vibration of SiO in

silanol groups, both pure and hydrogen bonded, mainly

located in the micropores.5355 Two other striking differences

appear in the infrared spectra after water intrusion in silicalite-1.

A significant shift to lower wavenumber (2 cm1) is observed

for the asymmetric stretching vibrations nas(SiOSi), initially

located at 1240 cm1 (see insert in Fig. 8b).5658 Another shift

is noticeable for the weak vibration band located at 631 cm1.

This band undergoes a shift of around 3 cm1 to a lower

wavenumber and a small decrease in intensity. This band

could be attributed to bending d(OSiO) and d(SiOSi)

vibrations58 and is assumed to be characteristic of vibrations

of double five-rings subunits of the framework.5961 Such

modification of the vibrational modes of the zeolitic framework

shows clearly that the chemical properties of the inner and/or

outer surfaces are changed after water intrusion. The creation

of silanol defects consecutive to water intrusion in the

microporosity probably induces structural changes in the local

environment within the framework.

To go further in the interpretation of the results, an

additional water intrusionextrusion experiment was also

performed on the non calcined silicalite-1 sample, therefore

still containing the structure-directing agent. The infrared

spectrum of the treated sample does not show any peaks

characteristic of a silanol defect. Therefore, we can reasonably

conclude that in the case of the template-free material, the silanol

defects are created during water intrusion and are localized within

the internal pore surface. This tends to confirm the small fraction

of silanol defects created in the microporosity. It is also

exemplified by the relative intensity of isolated and terminal

silanols compared to the vicinal hydrogen-bonded species. In

such a confined space this last contribution is not so great. We can

assume that the silanol groups are randomly distributed from the

isolated state to the hydrogen-bonded state with adjacent groups,

from the breaking of siloxane bonds (RSiOSiR) to give two

silanol groups (RSiOH). The different possible configurations

previously discussed are summarized in Scheme 2.

After the first intrusionextrusion of water in silicalite-1

pores, additional cycles do not affect anymore the surface

chemistry of the material, as shown in Fig. 8. This leads to the

conclusion that silicalite-1 becomes chemically modified

during the first water intrusion in micropores. The creation

of silanol defects consecutive to the breaking of siloxane bonds

renders the material slightly hydrophilic. Thus it explains why

the second intrusion pressure is lower than the first intrusion

pressure because the material becomes slightly less hydrophobic.

This also implies that the thermal energy involved during the

first intrusion process does not correspond exclusively to the

internal energy of the phase transition from the bulk phase to

the intruded phase. It also includes the energy involved in the

formation of silanol defects and the interaction of water with

these defects. From the second cycle, stability in intrusion and

extrusion is achieved. Silanol defects were created during the

first intrusion and the thermal energy measured for the second

intrusion (around 3.5 J g1 zeolite) truly corresponds to the

internal energy of the phase transition of water from the bulk

phase to the intruded phase.

Scheme 2 Schematic representation of silanol defects formation in the microporosity of silicalite-1 under high water pressure (p > 100 MPa),

involving the breaking of aRSiOSiR bond to give twoRSiOH groups, as identified by infrared spectroscopy.


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3.3.2 Solid state NMR29Si MAS NMR. 29Si MAS NMR spectra of the different

samples are shown in Fig. 9. The spectrum of the starting

silicalite-1 material (calcined sample, Fig. 9a) is highly

resolved. It exhibits 17 very sharp resonance lines between

109 and 117 ppm assigned to the 24 crystallographically

inequivalent silicon sites and associated to the Q4 groups

(Si[(OSi)4]). This feature implies an excellent homogeneity

of the silica environments and thus a tiny amount of defects in

the material. After one or four water intrusionextrusion

cycles, both spectra are very similar in shape but display only

8 broad components, indicating significant loss of resolution

compared to the initial spectrum (Fig. 9b and c). The broadening

of the resonances is ascribed to a decrease in the local

structural order of the silicalite-1 framework resulting from

the creation of defects. However, the observed disorder is not

as marked as in the case of the silicalite-1 synthesized in

alkaline medium, whose spectrum (Fig. 9d) shows a resolution

even worse. It is worthy of note that the defects created during

intrusion of water disappear when the material is again

calcined at 823 K; the spectrum (not reported) of silicalite-1

after four successive intrusionextrusion cycles followed by

calcination showing a resolution similar to that of the starting

material (Fig. 9a). Noticeably, whatever the spectrum, no

signal around 100 ppm characteristic of Q3 species

(HOSi[(OSi)3]) is detected. This indicates that the number

of defects is not so high and probably less than 3%. Indeed,

for a pure silica chabazite, which contains about 3.5% of

silanol defects a component at 102 ppm is clearly observed in

the 29Si MAS NMR spectrum.15

1H29Si CPMAS NMR. In order to get evidence of the

presence of silanol groups 1H29Si CPMAS NMR experiments

were performed. Such a technique allowed an amplification ofthe signals of silicon sites near or in interaction with protons.

The 1H29Si CPMAS NMR spectra of the calcined silicalite-1

samples before and after one and four water intrusionextrusioncycles are shown in Fig. 10. As expected, the calcined reference

sample provides a barely visible signal (Fig. 10a), whereas for

the intrudedextruded samples the presence of two resonances

around102 and114 ppm corresponding to Q3 (HOSi[(OSi)3])

and Q4(Si[(OSi)4]) groups,39 respectively, are clearly evidenced.

The component at 102 ppm reveals thus, in agreement with

the FTIR results, the presence of silanol groups. However,

contrary to what is observed by FTIR, NMR studies indicate

that the amount of Q3defects increases significantly (Fig. 10c)

between one and four cycles.

It is noteworthy that there is no resonance around 90 ppm,

assigned to the geminal Q2 groups ([(HO)2]Si[(OSi)2]).

As evidenced by FTIR spectroscopy, this result unambiguouslyconfirmed that the structural defects correspond to vicinal

OH silanol issued from the breaking of siloxane bonds

(RSiOSiR) under high water pressure.

1H-MAS NMR. As shown in Fig. 11, the 1H-MAS NMR

spectra of the different silicalite-1 samples are well resolved. In

agreement with the results discussed above, the signal recorded

for the calcined reference material is very low (Fig. 11a),

indicating that the defect sites are few and that silicalite-1 is

highly hydrophobic. It should be noted that the total amount

of physisorbed water determined by thermogravimetry is close

to 1.0 wt%. After deconvolution of the signal, 6 resonances at

0.95, 1.2, 1.35, 1.5, 3.9 and a low field broad shoulder at

around 68 ppm can be detected in this calcined material.

According to their chemical shifts, these resonances can be

assigned to silanol groups (RSiOH) usually detected

between 0 and 2 ppm,62 hydrogen bonded water at 3.9 ppm15

and physisorbed water involved in a very strong hydrogen

bonds at around 68 ppm.63,64

The signal obtained for silicalite-1 after one and four water

intrusionextrusion cycles (Fig. 11b and c) is more intense and

both spectra are dominated by three resonances characteristic

of water (3.9 and 7.1 ppm) or of water interaction (1.35 ppm).

Interestingly, these spectra reveal two additional relatively

sharp resonances at 0.15 and 2.1 ppm which could correspond

Fig. 9 29Si MAS NMR spectra of silicalite-1: (a) starting calcined

material, (b) after one water intrusionextrusion cycle, (c) after four

successive water intrusionextrusion cycles, (d) calcined sample

prepared in alkaline medium.

Fig. 10 1H29Si CP-MAS NMR spectra of silicalite-1: (a) starting

calcined material, (b) after one water intrusionextrusion cycle,

(c) after four successive water intrusionextrusion cycles.


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to silanol groups (RSiOH) formed during water intrusion.

In contrast to the case of chabazite,15 the signal at 2.1 ppm

cannot be attributed to geminal silanol species since no signal

corresponding to Q2 groups is observed in the 1H29Si

CPMAS NMR spectra. However, due to the large number

of crystallographic silicon sites in the MFI-type structure

(24 sites), the chemical shift, exceeding 2 ppm, might also be

attributed to silanol groups. In addition, the intensity of the

component at 2.1 ppm increases with the number of intrusion

extrusion cycles, indicating that, in agreement with the 1H29Si

CPMAS NMR results, some additional defects are created

after the first intrusion of water (Fig. 11b and c).

In order to get further insight into the proton resonances

assignment and the H H proximities, 1H single quantum/

double quantum (DQ) MAS NMR spectrum was performed

on silicalite-1 after four water intrusionextrusion cycles

(Fig. 12). This experiment aims to characterize pairs of dipolar

coupled protons. The presence of a signal in the DQ spectrum

indicates that two protons are in close proximity (o0.5 nm).65

As reported in Fig. 12, the 1Hresonances at 0.95, 1.2, 1.35,

1.5 ppm display a strong autocorrelation peak represented by

a diagonal dotted line in the 2D spectrum, indicating that

there are at least two silanol groups in close proximity.

Consequently, these resonances correspond to clusters of

silanols. An autocorrelation is also observed for the broad

resonance at around 7 ppm in agreement with the previous

assignment of the 1H resonances to physisorbed hydrogen-

bonded water molecules. Since this signal does not correlate

with any others, it can be concluded that these water molecules

are in strong hydrogen bonding with themselves. Another

autocorrelation concerns the peak at 3.9 ppm, which confirms

its attribution to water molecules. More interestingly, a

correlation between this resonance (3.9 ppm) and the one at

1.35 ppm is clearly evidenced by DQ MAS NMR technique.

This confirms the assignment to hydrogen bonded water and

specifies that this type of water molecule is involved in a

hydrogen bond in a specific manner with only those particular

silanols.

After water intrusion, we already mentioned the emergence

of two new resonances at 0.15 and 2.1 ppm that each presents

an autocorrelation on the DQ MAS NMR spectrum of the

four times intrudedextruded sample shown in Fig. 12. The

case of the 1

Hresonance at around 0.1 ppm has already beenobserved and discussed for hydrophobic pure silica CHA-type

zeolite.15 The autocorrelation peak in the DQ MAS NMR

spectrum at this chemical shift implies the proximity of at least

two protons but could correspond to neighboring silanols

obtained after the breaking of one siloxane bond (first hypothesis)

or to an unique water molecule in a very hydrophobic

environment (second hypothesis). In the present state of our

investigations, it is currently not possible to decide in favor of

one of these two hypotheses. At least, the autocorrelation

observed at 2.1 ppm proves that defects created during the

intrusion of water under high pressure are of neighbor SiOH

regarding NMR spectroscopy. From this result, it can therefore

be concluded that the defects are created after the breaking ofsiloxane bonds (RSiOSiR) to yield vicinal silanol sites

(QSi(OH)OSi(OH)Q). This fact is in good agreement with

FTIR analyses and the mechanism proposed in Scheme 2.

4. Conclusions

This work represents a successful attempt to go deeper in the

understanding of the phenomena involved during the water

intrusionextrusion process in a pure silica MFI-type zeolite.

Combined volumetric and calorimetric experiments reveal a

shift in the water intrusion isotherm of the first cycle compared

Fig. 11 1H(I = 1/2) MAS NMR spectra of silicalite-1: (a) starting

calcined material, (b) after one water intrusionextrusion cycle,

(c) after four successive water intrusionextrusion cycles.

Fig. 12 1H Single quantum/double quantum (DQ) MAS NMR

spectrum of silicalite-1 sample after four water intrusionextrusioncycles. Black line (TT) corresponds to the correlation between two

different proton sites.


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to the other cycles, whereas the extrusion isotherms displayed

similar shapes. No significant decrease of the intruded volume

is observed. From high pressure calorimetry, the corresponding

thermal effects of water intrusion and extrusion in silicalite-1

are found to be endothermic and exothermic, respectively. The

first intrusion involves the higher thermal energy compared to

extrusion or other successive intrusions and extrusions. Under

high water pressure, structural and surface defects are generated

in silicalite-1. XRD and SEM analyses do not show any

modification of the global structure. However, FTIR and

solid-state NMR spectroscopic investigations give new insight

concerning the creation of defects in the short range order.

Looking down to the molecular scale, based on the various

vibration modes of silanol groups by FTIR spectroscopy and

on the 29Si-MAS, 1H29Si-CPMAS, 1H-MAS and DQ MAS

NMR spectra, both techniques confirm that the defects

correspond to vicinal silanol groups (RSiOH) resulting

from the breaking of siloxane bonds (RSiOSiR). The

created silanol defects are at the origin of the hysteresis

which is observed between the first and the other water

intrusionextrusion cycles. A certain stability of the material

regarding pressure seems to be established after the first cycle

and no significant difference was detected between the second

and other successive cycles by high pressure calorimetry and

FTIR spectroscopy. However, solid-state NMR spectroscopic

data suggest that the amount of SiOH slightly increases with

the number of cycles. Nevertheless, the techniques we have

used cannot give more precise information on the location of

these defects in the structure in order to determine whether the

defect creation is random or not. The evolution of both

mechanical work and heat exchanged when successive

intrusionextrusion cycles are performed in this system is

summarized in Fig. 13. The thermal energy involved during

the first water intrusion includes at least four phenomena:

obviously the energy of the phase transition from the bulk

phase to the intruded phase (endothermic), but also the energy

involved in breaking of siloxane bonds (endothermic), the

energy of formation of silanol defects (exothermic) and

the energy of water adsorption on these hydrophilic sites

(exothermic). For the extrusion and subsequent intrusion

extrusion cycles, only the energy of the phase transition from

the bulk phase to the intruded phase gives rise to the measured

thermal effects, however with persistent irreversibility between

intrusion and extrusion. It may be noted that the internal

energy (DU= Q + W) of intrusion is in any case equal to the

internal energy of extrusion as could be expected since the

internal energy is a function of states. However, the difference

between these two values is low (about 2 J g1). We are not

able to explain such a difference. It could be due to the fact

that it is very difficult to accurately subtract the mechanical

and thermal effects due to the compression of liquid water. It

could also be the result of slight modifications of the material

which could occur at each intrusionextrusion cycle as

suggested by the NMR experiments. Finally, only a rather

small amount of thermal energy is dissipated along the

intrusionextrusion cycles. This is thus in favor of good

mechanical energy storage as the variation of the internal

energy of the system is essentially converted in mechanical

work without much heat exchange.

Acknowledgements

This work was supported by the French Agence Nationale de

la Recherche through the ANR program Heter-eau, under

Contract No. BLAN 06-3_144027. Thanks are due to

Anne-Catherine Faust for her technical assistance. The

authors also thank Christian Paulin, Laboratoire Inter-

disciplinaire Carnot de Bourgogne, for assistance with high

pressure calorimetry.

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