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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: jean-pierre.bellat@u-bourgogne.fr
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: joel.patarin@uha.fr
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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