Water intrusion in mesoporous silicalite-1: An increase of the stored energy

Microporous and Mesoporous Materials 117 (2009) 627–634

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Water intrusion in mesoporous silicalite-1: An increase of the stored energy Mickaël Trzpit, Michel Soulard, Joël Patarin * Laboratoire de Matériaux à Porosité Contrôlée, UMR CNRS 7016, ENSCMu, UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France

a r t i c l e

i n f o

Article history: Received 10 June 2008 Received in revised form 31 July 2008 Accepted 3 August 2008 Available online 9 August 2008 Keywords: Hydrophobic mesoporous zeolite Silicalite-1 Water condensation Water intrusion

a b s t r a c t In silicalite-1 (pure silica MFI-type zeolite), the water adsorption is extremely weak when the pressure is lower than the water saturation vapor pressure. The water condensation (intrusion) is obtained by applying a high hydraulic pressure, approximately of 100 MPa. Extrusion of water occurring at the similar pressure, the ‘‘water–silicalite-1” system constitutes thus a real molecular spring, which can store and restore mechanical energy. In order to increase the stored energy in this system, the porous volume of this zeolite was increased by the creation of additional micro-, meso- and macropores using carbon black or surfactant [3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride, as porogen and templating agents, respectively. The presence of meso- and macropores modifies the water condensation behavior in silicalite-1. Indeed, the water intrusion takes place in two stages. The first one, from 3 to 7 MPa, corresponds to the filling of meso- and macropores but the amount of stored energy is very low. The second one, at 100 MPa, corresponds to the filling of micropores. The increase of the microporous volume leads to an increase of the intruded water volume at high hydraulic pressure. Consequently, the stored energy is greater than for a conventional silicalite-1 zeolite. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Zeolites are crystalline aluminosilicates built from TO4 tetrahedra (T = Si, Al) that are arranged in such a manner that intracrystalline pores and cavities of molecular dimensions are present. In chemical industry, zeolites with pore diameter lower than 14 Å, are among the most important families of materials with a multitude of technical applications as adsorption, catalysis, ion-exchange and membrane technology [1]. Pure silica zeolites (hydrophobic materials) were recently studied as porous matrices for water intrusion and led to a new field of application concerning the energetics [2–5]. The phenomenon was based on the following principle: to spread a drop of a non-wetting liquid on the surface of a solid, a certain pressure must be applied. It is the same to make penetrate this liquid in a porous matrix. During this forced penetration, mechanical energy can be converted into interfacial energy. Indeed, the massive liquid is transformed into a multitude of molecular clusters developing a large solid–liquid interface. At the microscopic scale, this phenomenon results in the breaking of intermolecular bonds in the liquid to create new bonds with the solid. By releasing the pressure, the system can evolve spontaneously by expelling the liquid out of the cavities of the solid (extrusion) with a more or less significant hysteresis [6]. This process allows storing and restoring an important quantity of energy in a small volume.

* Corresponding author. Tel.: +33 3 89 33 6885; fax: +33 3 89 33 6880. E-mail address: [email protected] (J. Patarin). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.08.005

The first works, highlighting these properties, were carried out by Eroshenko. He started to study non-zeolitic porous solids with low melting point metals as non-wetting liquids [7,8]. Then, these studies were extended to systems made up of water and hydrophobic zeolites [2–4] or water and functionalized organized mesoporous solids [9–11]. Depending on the zeolitic structure, and its hydrophobic character different behaviors illustrated by pressure– volume diagrams can be observed. Thus, for a pure silica BEA-type zeolite, whose structure consists in at least two polytypes, the phenomenon is non reversible. The presence of the two polytypes could explain the bumper behavior of such a solid [3]. On the other hand, ‘‘water–zeolite” systems can behave as real molecular springs. A completely reversible process over several water intrusion–extrusion cycles is thus observed. It was the case for strongly hydrophobic silica zeolites such as silicalite-1 (MFI-type) [2] and Si-CHA (CHA-type) [5]. The system is able to accumulate and restore energy. The amount of stored energy (W) is related to the pressure (P) and to the volume (V) according to the following relation:

jWj ¼

Z

V2

PdV V1

where V1 and V2 correspond to the intruded volume of water at the beginning and at the end of the process, respectively. For the silicalite-1, the adsorption of water is extremely weak when the pressure is below the water saturation vapor pressure. The water condensation (intrusion) is obtained by applying 100 MPa [2]. The stored energy in this solid is about 10 J/g of zeolite.

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The aim of this paper is to increase the stored energy in this system by increasing the porous volume of silicalite-1 with the creation of additional micro-, meso- and macropores. Indeed, during the past decade, significant efforts have been devoted for developing methods that introduce mesoporosity in zeolite materials by different approaches as reported in two recent reviews [12,13]. In practice, the mesopores can be of two types, i.e., inter- or intracrystalline pores. The first one can be obtained by aggregation of nanosized zeolite crystals and the second one by creating an additional porosity in each individual crystal. Such a mesoporous zeolite can be prepared by several methods. The main nontemplated methods are dealumination [14,15] and desilication [16,17]. In the templating method, the zeolite is grown in the presence of a template, which is removed after synthesis to generate porosity. The first type of templates used were carbon materials [12] such as carbon black, carbon aerogel, carbon nanofibres, carbon nanotubes, mesoporous carbon (CMK) and calcined sugar [18]. It is also possible to use different polymers as noncarbonized resorcinolformaldehyde [19,20], polystyrene beads [21], cationic polymer polydiallyldimethylammonium chloride [22] and silane-functionalized polymers [23]. With the latter template, the silyl groups of the polymer are hydrolyzed and integrated into zeolite crystals. After calcination, homogeneously size of intracrystalline mesopores is highlighted (2.0–3.0 nm). The same phenomenon was previously obtained by Choi et al. [24]. They reported a direct synthesis route to mesoporous zeolites with tunable mesoporous structure using the amphiphilic organosilanes [(CH3O)3SiC3H6N(CH3)2CnH2n + 1]Cl, as a mesopore-directing agent. The mesopore diameters of the mesoporous MFI zeolite crystals could be systematically varied by changing the chain length of the organosilane and/or the hydrothermal synthesis temperature. For n = 12, 16 and 18, pore diameters of 2.1, 3.1 and 3.9 nm were obtained, respectively, without reducing the microporous volume of zeolite. Generally, the templating approaches make it possible a priori to tailor the pore size of the mesopores. These different methodologies have been applied to MFI, MEL, MTW, BEA, CHA, AFI, LTA and FAU framework structures [12,13]. In the present work, intrusion of water was studied in mesoporous silicalite-1 zeolites prepared by using either carbon black or [3(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride. Indeed, these two protocols allow keeping the microporous volume of the zeolitic framework, which is an important characteristic for the experiments of water intrusion. The obtained materials were fully characterized and the pressure–volume diagrams of the ‘‘water–zeolite” systems were established. 2. Experimental 2.1. Preparation of materials The carbon black (CB) from Cabot corresponded to particles of about 20 nm in diameter, forming aggregates of approximately few micrometers. Its BET surface area and total pore volume are about 500 m2 g1 and 0.7 cm3 g1, respectively. The surfactant agent [3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride (Su), was synthesized according to the protocol of Shaojie et al. [25] by mixing 0.1 mol of dimethyloctadecylamine (Accros, 89%) with 0.12 mol of chloropropyltrimethoxylsilane (ABCR, 97%) in absolute methanol. The potassium iodide (0.006 mol, Fluka, 99.5%), acting as a catalyst, is then added. The mixture was then refluxed for 4 days. A solution of [3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride (62% by weight in methanol) is obtained. Mesoporous silicalite-1 samples were synthesized from the starting molar compositions reported in Table 1. Tetraethoxysilan (TEOS, Aldrich, 98%) was used as the silica source. The other

Table 1 Composition of the reaction mixtures of the different silicalite-1 samples Sample

Silicalite-1 Si–CB1 Si–CB2 Si–Su1 Si–Su2

Molar composition TEOS

TPAOH

CB

Su

H2O

1 1 1 1 1

0.2 0.2 0.2 0.2 0.2

0 1.25 1.25 0 0

0 0 0 0.04 0.04

20 20 9 9 9

Time (day)

Temperature (°C)

3 3 3 3 8

170 170 170 180 150

CB, carbon black; Su, [3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride.

reactants were tetrapropylammonium hydroxide (TPAOH, Fluka, 20%) and carbon black (CB) or surfactant (Su). After homogenization, the mixtures were introduced in a Teflon-lined stainless-steel autoclave and heated at a given temperature during a variable time. After synthesis, the products were filtered, washed with distilled water and dried at 60 °C overnight. The solid was then calcined at 600 °C under air to completely liberate the porosity. Five solids were selected. Two samples were prepared with the carbon black (Si–CB1 and Si–CB2). According to a previous study (nonpublished results), the carbon/silica ratio was fixed at 1.25 and only the amount of water was changed. With such a carbon/silica ratio, a fully crystallized sample is obtained. Two others samples were prepared with the surfactant (Si–Su1 and Si–Su2). They differed by the temperature or the time of synthesis and these parameters were chosen because the corresponding solids in the paper published by Choi et al. [24] display the best characteristics for the present study (large pore diameter). The last sample is a reference silicalite-1 sample prepared without carbon black and surfactant. 2.2. Instrumentation X-ray diffraction patterns of the different samples were recorded using a PANalytical MPD X’Pert Pro diffractometer operating with Cu Ka radiation (k = 0.15418 nm) in the 2h range 0.5– 50 and equipped with a X’Celerator real-time multiple strip detector. Thermogravimetric (TG) and differential scanning calorimetry analyses (DSC) were carried out on a Setaram TG-DSC 111 apparatus, under nitrogen–argon flow, with a heating rate of 5 °C min1 from 20 to 750 °C. Nitrogen adsorption isotherms were performed using a Micromeritics ASAP 2010 apparatus. Prior to the adsorption measurements, the calcined samples were outgassed at 350 °C overnight under vacuum. The size and the morphology of the crystals were determined by scanning electron microscopy (SEM) using a Philips XL 30 FEG microscope. 29 Si MAS NMR spectra were recorded on a Bruker Advance II 300 MHz spectrometer (B0 = 7.1 T) operating at 59.63 MHz with a spinning frequency of 4 kHz, a p/4 pulse duration of 2.75 ls and a 60 s recycling delay. The intrusion–extrusion of water in silicalite-1 samples was performed at room temperature using a modified mercury porosimeter (Micromeritics Model Autopore IV). Experiments are carried out with 0.5 g of zeolite and 0.5 g of water. The cell containing the ‘‘water–zeolite” system consists in a polypropylene cylinder (diameter: 1.2 cm; volume: 2 cm3) sealed by a mobile piston. This cell is introduced in the 15 cm3 glass cell of the porosimeter which is filled with mercury. The volume variation is determined from the conductivity measurement which depends on the mercury height in the capillary tube of the glass cell. The maximum volume change is about 0.5 cm3. The experi-

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mental intrusion–extrusion curve is obtained after subtraction of the curve corresponding to the compressibility of pure water. The value of the intrusion (Pi) and extrusion (Pe) pressures correspond to that of the half volume total variation. Pressure is expressed in MPa, and volume variation in mL per gram of anhydrous calcined samples. The experimental error is estimated to 1% on the pressure and on the volume.

3. Results and discussion 3.1. XRD and SEM characterizations The X-ray diffraction patterns of calcined samples reported in Fig. 1 are very similar. Whatever the samples, they are well defined and characteristic of silicalite-1. The symmetry is monoclinic

e

d

c

b

a 5

10

15

20

25

30

35

40

45

50

2θ(˚) (CuKα) Fig. 1. X-ray diffraction patterns of calcined materials: (a) silicalite-1, (b) Si–CB1, (c) Si–CB2, (d) Si–Su1 and (e) Si–Su2.

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(space group P21/n). No significant changes were observed after the intrusion–extrusion experiments (not reported), which means that no modifications occur at the long-range order. Addition of carbon black or surfactant in the synthesis mixture does not prevent the growth of the MFI structure. To get an idea of the creation of organized mesopores in silicalite-1 samples prepared with the method of Choi et al., the X-ray diffraction patterns, between 0.5° and 5° (2h), have been carried out and reported in Fig. 2. The small-angle XRD patterns show the presence of organized mesoporosity. Indeed, two peaks are observed at 2° and 1.9° (2h) for samples Si– Su1 and Si–Su2, respectively. However, Choi et al. have shown that the formation of mesoporous structure [24] (without MFI zeolite structure) exhibits an XRD peak at 2h = 2.3°, which decreases in intensity when the MFI structure appears. Therefore, it is difficult to know if the solids are composed of a mixture of MFI zeolite and mesoporous silica or correspond to a mesoporous silicalite-1.

The formation of the latter solid will be confirmed by SEM and water intrusion experiments (see below). The morphology of the crystals of five samples was examined by scanning electron microscopy (Fig. 3). For the reference sample (a), the crystals are characteristic of silicalite-1 with sizes close to 40  20  10 lm3. For samples prepared with carbon black (Fig. 3b and c), aggregates of few micrometers, composed of small crystals of silicalite-1 with dimensions ranging from 100 nm to 3 lm, are observed. Samples prepared with surfactant are characterized too by aggregates of nanocrystals. However, in that case the nanocrystals display a narrow crystal size distribution with dimensions close to 400  350  250 nm3 and 240  220  120 nm3 for samples Si–Su1 (Fig. 3d) and Si–Su2 (Fig. 3e), respectively. From these pictures, it appears clearly that the particles are very homogeneous which is in favor with the formation of mesoporous silicalite-1 rather than a mixture of silicalite-1 and mesoporous silica. 3.2. N2 adsorption–desorption measurements

Fig. 2. X-ray diffraction patterns of calcined materials in the 2h range 0.5°–5°: (a) Si–Su1 and (b) Si–Su2.

N2 adsorption–desorption isotherms and textural characteristics of the five calcined samples are reported in Fig. 4 and Table 2, respectively. The isotherm of the reference silicalite-1 sample (a) is of type I with the presence of a clear step for relative pressures slightly above 0.15. This step was described by Llewellyn et al. [26] and corresponds to a density change of the adsorbed phase. It was attributed to a phase transition from a lattice fluidlike phase to a crystalline-like solid phase. After this step, a plateau is observed. The BET surface area is about 400 m2 g1 and the microporous volume of 0.185 cm3 g1. N2 adsorption–desorption isotherms of samples Si–CB1 (b) and Si–CB2 (c) are similar to that of the reference sample. However, the microporous volume of these two materials is larger (0.205 cm3 g1) and the condensation step which appears at P/P0 close to 1 reveals the presence of macropores. The macropore volume is equal to 0.143 cm3 g1 for sample Si–CB1 and 0.065 cm3 g1 for sample Si–CB2. These macropores correspond to an intercrystalline porosity which results of the aggregation of nanocrystals. (Fig. 3b and c). N2 adsorption–desorption isotherms of Si–Su1 (d) and Si–Su2 (e) are essentially of type I with a slight contribution of type IV. They can be broken down into several parts: the filling of micropores with the increase of the adsorbed nitrogen volume at relative pressure lower than 0.1, the filling of mesopores in the P/P0 range 0.25–0.85 and finally the

Fig. 3. Micrographs of calcined materials: (a) silicalite-1, (b) Si–CB1, (c) Si–CB2, (d) Si–Su1 and (e) Si–Su2.

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450

e

3 Adsorbed amount (cm /g) STP

400 350

d 300

c

250

b

200 150

a

100

Adsorption Desorption

50 0 0.0

0 .2

0.4

0 .6

0.8

1.0

Relative pressure (P/P0) Fig. 4. N2 adsorption–desorption isotherms of calcined materials: (a) silicalite-1 starting to 0 cm3 g1, (b) Si–CB1 starting to 50 cm3 g1, (c) Si–CB2 starting to 100 cm3 g1, (d) Si–Su1 starting to 150 cm3 g1 and (e) Si–Su2 starting to 200 cm3 g1.

Table 2 N2 adsorption data of the different silicalite-1 samples Sample

SBET (m2/g)

Microporous volume (cm3/g)

Mesoporous volume (cm3/g)

Total porous volume (cm3/g)a

Meso- and macropores diameter (nm)b

Silicalite-1 Si–CB1 Si–CB2 Si–Su1 Si–Su2

388 477 438 476 467

0.185c 0.205c 0.206c 0.200d 0.198d

0 0 0 0.054e 0.083e

0.196 0.348 0.271 0.276 0.366

– >20 >50 2.6 2.7

a b c d e

Determined Determined Determined Determined Determined

at P/P0 = 0.99. by the BJH method on the adsorption curve. at P/P0 = 0.45 (on the plateau). at P/P0 = 0.25. between P/P0 = 0.25–0.85.

filling of macropores (P/P0 > 0.9) corresponding to the intercrystalline porosity. Due to the presence of the phase transition discussed above, the values of the micro and mesopore volumes are difficult to discriminate and the corresponding values reported in Table 2 are probably not accurate. However, since the amount of adsorbed nitrogen is higher to that of the reference sample, N2 adsorption isotherms put clearly brought out the existence of mesopores. The mesoporous volume is about 0.054 and 0.083 cm3 g1 for Si–Su1 and Si–Su2, respectively. The microporous volumes seem also to be more important. The diameter of mesopores is about 2.6 nm. The Si–Su2 sample also presents a significant macroporous volume (0.1 cm3 g1). Therefore, using either CB or surfactant as templating agent, it is possible to significantly increase the micropore volume of silicalite-1 and create meso- and macropores. 3.3.

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Si MAS NMR spectroscopy

The 29Si MAS NMR spectrum of the calcined reference silicalite1 sample (Fig. 5a) exhibits seven resonances between 108 and 118 ppm corresponding to the Q4 groups (Si–[(OSi)4]) and ascribed to the 24 crystallographically inequivalent silicon sites. No

signals assigned to Q3 groups corresponding to nonbonded defects, such as „Si–OH groups (silanol) and expected at about 100 ppm/ TMS, are observed. The 29Si MAS NMR spectra of the calcined Si– CB2, Si–Su1 and Si–Su2 samples (Fig. 5c–e) are completely similar to that of the reference sample. All these samples correspond to silicalite-1 without significant silanol defects. By contrast, the spectrum of the calcined Si–CB1 sample (Fig. 5b) displays only three broad components between 108 and 118 ppm, revealing thus a lower resolution and a structural disorder at the local level. Moreover, a very small peak accounting for 5.5% of the total 29Si NMR signal is also detected at 103 ppm and can be assigned to Q3 groups HO–Si–(OSi)3. This sample displays silanol defects. 3.4. Intrusion–extrusion of water: pressure–volume diagrams Experimental water intrusion–extrusion diagrams of the five samples performed at room temperature are reported in Fig. 6. Characteristics data are given in Table 3. In the case of the reference silicalite-1 sample (Fig. 6a), initially, the curves exhibit a linear part due to the slight water intrusion in the zeolitic porosity. When the pressure increases and the capillary pressure is reached (Pint = 89 MPa), an important variation of volume is observed (Vint). Water molecules penetrate into the microporous volume of the zeolitic framework. A complete filling of the pores takes place at higher pressure. When the pressure is released, the phenomenon is reversible. However, the extrusion of water occurs at a lower pressure (Pext = 86 MPa) showing a very small hysteresis and the extruded volume reaches the initial one. The ‘‘water–silicalite-1” system constitutes a real molecular spring which is able to store and restore an important quantity of energy in a small volume (about 9.6 J/g of zeolite). The intruded volume calculated with a water density of 1 is close to 0.108 cm3 g1 which is far away from the one determined from N2 adsorption measurements (0.185 cm3 g1, Table 2). However, this result is in agreement with a water density in the MFI structure of 0.6 as it was shown by Desbiens et al. [27,28]. The water extrusion curves of Si–CB1 and Si–CB2 samples (Fig. 6b and c, respectively) are similar to the intrusion curves. In

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e

d

c

b a

Fig. 5.

29

-100

-95

-90

-105 -110 δppm/TMS

-115

-120

-125

Si MAS NMR spectrum of calcined samples: (a) silicalite-1, (b) Si–CB1, (c) Si–CB2, (d) Si–Su1 and (e) Si–Su2.

Zone 2

Zone 1

Zone 1

Zone 2

0.14

a

0.12 0.10 0.08 0.06

Intrusion Extrusion

0.04 0.02 0.00 0

Volume (mL/g)

0.14

30

60

90

120

150 180

210

d

b

0.12 0.10 0.08 0.06

Intrusion Extrusion

0.04

Intrusion E x trusion

0.02 0.00 0

30

60

90

120

150 180

210

30

60

90

120

150

180

210

e

c

0.14

0

0.12 0.10 0.08 0.06

Intrusion Extrusion

0.04

Intrusion Extrusion

0.02 0.00 0

30

60

90

120

150 180

Pressure (MPa)

210

0

30

60

90

120

150

180

210

Pressure (MPa)

Fig. 6. Pressure–volume diagrams of ‘‘water-sample” systems at room temperature: (a) silicalite-1, (b) Si–CB1, (c) Si–CB2, (d) Si–Su1 and (e) Si–Su2.

M. Trzpit et al. / Microporous and Mesoporous Materials 117 (2009) 627–634 Table 3 Characteristics of the ‘‘water–zeolite” systems Samples

Silicalite-1 Si–CB1 Si–CB2 Si–Su1 Si–Su2

Intrusion pressure (MPa)

Extrusion pressure (MPa)

Intruded volume (cm3 g1)

Zone 1

Zone 2

Zone 1

Zone 2

Zone 1

Zone 2

– 3 5 7 4

89 80 99 90 89

– 2.8 4.8 –b –b

86 70 91 87 86

– 0.048 0.012 0.025 0.019

0.108 0.086 0.117 0.110 0.109

Microporous volumea (cm3/g)

0.185 0.205 0.206 0.200 0.198

The experimental error is estimated to 1% on the pressure and on the volume. a Determined by N2 adsorption. b No extrusion in zone 1.

that case too, the system is completely reversible. For these samples, the intrusion–extrusion curves are clearly composed of two zones (zones 1 and 2). By comparing the PV-diagrams (mainly zone 2) with that of the reference sample a significant difference was observed. For the Si–CB1 sample, although the microporous volume determined by N2 adsorption measurement is higher, the intrusion pressure (80 MPa) and the intruded volume (0.086 cm3 g1) in zone 2 are lower. Moreover, an extra shift is observed, and it is accompanied by a pronounced rounding of the second transition. This phenomenon can be explained by the existence of silanol defects in the structure (Fig. 5b) which lead to a decrease of the intrusion pressure and the intruded volume, as shown in our previous work [29]. Compared to the reference sample, the amount of stored energy in zone 2 is lower (about 6.9 J/g of zeolite). In the case of Si–CB2 sample, which is characterized by a higher microporous volume than the reference, an increase of the intrusion pressure and intruded volume are observed in zone 2 (Pint = 99 MPa and Vint = 0.117 cm3 g1). Therefore, the ‘‘water–Si–CB2” system stores more energy than the ‘‘water–silicalite-1” system (+20%; 11.6 J/g of zeolite). For Si–CB1 and Si–CB2 samples, the zone 2 of the PV-diagrams corresponds thus to the filling of micropores. The presence of another step at a lower pressure, about 4 MPa (zone 1), which is more pronounced for Si–CB1 sample, can be attributed to the existence of an additional porosity. However, the amount of stored energy in this zone (about 0.15 J/g of zeolite) is negligible and does not increase significantly the storage capacity of the ‘‘water–Si-CB” systems. The water intrusion–extrusion isotherms of Si–Su1 and Si–Su2 samples (Fig. 6d and e, respectively) are also composed of two zones (zones 1 and 2). The behavior in zone 2 is very similar to that observed for the reference sample. No significant increase of the intruded volume is observed, although microporous volumes seem to be more important than the reference sample. Therefore, the amount of stored energy in zone 2 does not increase. It is worthy to recall that the accuracy on the micropore volume determined by N2 adsorption–desorption measurement can be questionable because of the presence of the phase transition at P/P0 = 0.15 (see Section 3.2). However, the fact that the intruded volume of the Si–Su samples in zone 2 is similar to the one of the reference silicalite-1 sample is in favor with the formation of a pure mesoporous silicalite-1 and not to a mixture of silicalite-1 and mesoporous silica. The water intrusion experiments can therefore be a good tool to check the quality of the hydrophobic mesoporous zeolites. As observed for the calcined Si-CB samples, another intrusion step is observed at a lower pressure for Si–Su samples (zone 1). It can be attributed to the existence of mesoporosity. The intrusion pressures are 7 and 4 MPa for Si–Su1 and Si–Su2 samples, respectively. The corresponding intruded volumes (0.025 cm3 g1 for Si–Su1 and 0.019 cm3 g1for Si–Su2) are lower than those determined from N2 adsorption–desorption measurements, i.e., 0.054 and

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0.083 cm3 g1. Such a difference could be explained by the presence of adsorbed water molecules at the beginning of the intrusion experiment. Indeed, by thermogravimetric analysis, the amount of adsorbed water in the Si–Su samples is 2.5 wt% whereas it is only 1 wt% in the reference silicalite-1 sample. If a similar amount of water is observed for sample Si-CB-2 (2.5 wt%), for the Si–CB1 sample the water content is larger and close to 4.5 wt%. Such a result is in agreement with the solid-state NMR experiments described above (presence of silanol groups). Contrary to what is observed for the Si-CB samples, the release of the pressure in ‘‘water–Si– Su” systems does not expel water molecules intruded in zone 1, out of the cavities of the solid. Therefore, the stored energy during intrusion is not restored. The trapping of water in the mesopores in zone 1 observed only for the Si–Su samples might be explained by the nature of the mesopores. They are probably hydrophilic and similar to those observed for some functionalized MCM-type materials characterized by large pores and for which the intrusion of water is not reversible [10]. 4. Conclusion The porous volume of silicalite-1 zeolite was successfully increased by the creation of additional porosity. The formation of additional micropores was obtained by the use of carbon black in the reactant gel. These new micropores lead to an increase of the intruded volume at high pressure (100 MPa) and thus an increase of the amount of stored energy compared to a classical silicalite-1 (+20%). This work highlights that the creation of additional micropores is an excellent solution to increase the energetic performance of ‘‘water–zeolite” systems. The use of [3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride, a amphiphilic organosilane surfactant, allowed the formation of organized mesopores in silicalite-1 while maintaining a perfect crystallinity of the zeolite. The water intrusion– extrusion isotherms display two steps. The first one takes place at high pressure (100 MPa) and is characteristic of silicalite-1. The second one occurs at a lower pressure (4–7 MPa) and is attributed to mesopores. However by releasing the pressure, the water contained in the mesopores does not leach out. The important point of this part of the work is the possibility of adding a second water intrusion, while maintaining the same behavior of the zeolite with water. This method is very encouraging to increase the amount of stored energy but still work has to be performed to decrease the diameter of mesopores or make them more hydrophobic. Moreover, water intrusion experiment seems to be an excellent tool to clearly check if the synthesized material corresponds to a pure zeolitic phase with mesopores or to a mixture of zeolite and mesoporous silica. Acknowledgments This work was supported by the French Ministry of Education and Research (doctoral grant to M. T.), the Agence National de la Recherche under contract # BLANC06-3_144027 and the ACI program ‘‘Nanothermomécanique”. The authors would like to thank Dr. Jean-Louis Paillaud for fruitful discussions. References [1] J. Cˇejka, H. van Bekkum, A. Corma, Introduction to zeolite science and practice, in: Studies in Surface Science and Catalysis 168, third revised ed., Elsevier B.V., Amsterdam, 2007, pp. 525–1035. [2] V. Eroshenko, R.C. Regis, M. Soulard, J. Patarin, J. Am. Chem. Soc. 123 (2001) 8129. [3] M. Soulard, J. Patarin, V. Eroshenko, R.C. Regis, Recent advances in the science and technology of zeolites and related materials, in: E. Van Steen, L. Callanan,

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