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Energetic Behavior of the Pure Silica ITQ-12 (ITW) Zeolite under High Pressure Water Intrusion Ismail Khay,a Lydie Tzanis,a Jean Daou,* a Habiba Nouali,a Andrey Ryzhikova and Joël Patarin*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Experimental water intrusion–extrusion isotherms were performed at room temperature on pure silica ITW-type zeolites (ITQ-12 zeosil). The water intrusion is obtained by applying a high hydraulic pressure corresponding to the intrusion step. When the pressure is released, the water extrusion occurs at a similar pressure to that of the intrusion one. Therefore, the ‘‘ITW zeosil−water’’ system behaves like a spring and the phenomenon is reproducible over several cycles. Several characterizations have been realized before and after water intrusion–extrusion experiments in order to reveal the presence or the lack of defects after such experiments. Structural modifications at the long range order cannot be observed by XRD analysis after three water intrusion–extrusion cycles. However, solid state NMR spectroscopy get evidence of the presence of Q3 groups revealing the breaking of some siloxane bridges after the intrusion step. The “ITW zeosil−water” system can restore 100% of the stored energy corresponding to about 8 J/g.
1. Introduction
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Zeolites are used in several industrial processes such as petrochemical cracking, ion exchange, and the separation and removal of gases and solvents.1 In last fifteen years, these microporous solids and more particularly pure−silica zeolites (named zeosils) have been studied for applications in the energetic field.2 In these hydrophobic materials, the adsorption of water is extremely weak when the pressure is below the water saturation vapor pressure. To penetrate water in such porous matrixes, a certain pressure must be applied.2 From a general point of view, during this forced penetration (intrusion), the liquid (water in our case) is transformed into a multitude of molecular clusters, which develop a large solid−liquid interface. The mechanical energy, spent by the pressure forces, is converted into interfacial energy. At the microscopic scale, this phenomenon can be explained by the breaking of intermolecular bonds in the liquid to create new bonds with the solid. By reducing the pressure, the system can induce an expulsion of the liquid out of the cavities of the material (extrusion). Depending on zeolite structure, the “zeosil−water” system is able to restore, dissipate, or absorb the supplied mechanical energy during the compression step, and therefore, it displays a spring, shockabsorber, or bumper behavior. Several ‘‘zeosil–water’’ systems have already been studied3-17 and a summary of the energetic performances of these zeosils is reported in our previous papers.15,16 The most studied solid was the MFI-type zeosil (Silicalite-1); which is characterized by a three-dimensional channel system with 10-membered ring (MR) openings (0.55-0.56 nm). The “MFI zeosil– water” system acts as a molecular spring and is able to restore 94% of the stored energy (11 J/g) and this during several water intrusion – extrusion cycles. This journal is © The Royal Society of Chemistry [year]
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Recently, this process was extended to hydrophobic metal organic framework materials (MOFs) such as ZIF-8 which displays a shock absorber behavior at a quite low pressure (27 MPa).18 Our group have also shown that the cage-like zeosils, despite a lower pore opening, display a lower intrusion pressure that the channel like zeosils.15 It was the case for instance of the CHA-type zeosil (cage-like zeosil with 8 MR openings) for which the water intrusion pressure is about three times lower than that of the MFI zeosil (channels with 10 MR openings). However, the dependence of water intrusion pressure on zeolite structure is not clearly understood. Some zeosils with similar channel system show high difference in energetic performances. For example, the intrusion pressure for MTW and AFI zeosils both characterized by 1D channel system with 12 MR openings is 132 and 57 MPa, respectively.13 Recently Bushuev et al. have shown by investigating through molecular dynamic simulations the water intrusion-extrusion in AFI, IFR, MTW and TON pure silica zeolites that intruded water volumes can be correlated with the free volume of the zeosil unit cells.19 More data have to be collected in order to consolidate this theory,that is why the study of water intrusion-extrusion in new zeosils with careful structural and physicochemical characterizations presents a high interest. In this study, the energetic performances of ITW-type zeosil (ITQ-12) using water intrusion−extrusion experiments is presented. This zeosil is characterized by a two dimensional channel system with 8 MR pore openings running along the [001] (3.8 × 4.1 Å) and [100] direction (2.4 × 5.3 Å). The samples were fully characterized before and after water intrusion mainly by powder X-ray diffraction, SEM, thermal analysis, N2 adsorption−desorption, and 29Si NMR spectroscopy. The stored energy was evaluated from the pressure−volume diagrams of the “zeosil−water” system. [journal], [year], [vol], 00–00 | 1
Physical Chemistry Chemical Physics Accepted Manuscript
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DOI: 10.1039/C3CP53570C
using Louër’s DICVOL91 indexing routine21 of the STOE WinXPOW program package.22
2. Experimental section 2.1 Synthesis of Pure Silica ITW-Type Zeolite Samples
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The size and the morphology of the crystals were determined by scanning electron microscopy (SEM) using a Philips XL 30 FEG microscope. 2.5 Nitrogen Adsorption−Desorption Measurements
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Nitrogen adsorption−desorption isotherms were performed using a Micromeritics ASAP 2420 apparatus. Prior to the adsorption measurements, the nonintruded samples were outgassed at 300 °C overnight under a vacuum. The intruded−extruded samples were outgassed at 90 °C overnight to eliminate physisorbed water and to avoid the dehydroxylation process. The specific surface area (SBET) and microporous volume (Vmicro) were calculated using the BET and t-plot methods, respectively. 2.6 Thermal Analysis
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Thermogravimetric (TG) analyses were carried out on a TGLabsys apparatus, under air flow, with a heating rate of 5 °C·min−1 from 20 to 700 °C. Prior to the analysis, the samples were hydrated in a 80% relative humidity atmosphere for 24 h in order to set the hydration state. 2.7 Solid-State NMR Spectroscopy 29
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Si MAS and 1H−29Si CPMAS NMR spectra were recorded on a Bruker Advance II 300 MHz spectrometer. The recording conditions are given in Table 1. Table 1: Recording Conditions of the CPMAS NMR Spectra.
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2.3 Powder X-ray Diffraction 50
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X-ray diffraction patterns of the different samples were recorded in a Debye−Scherrer geometry on a STOE STADI-P diffractometer equipped with a curved germanium (111), primary monochromator, and a linear position-sensitive detector (6° 2θ) using Cu Kα1 radiation (λ = 0.15406 nm). Measurements were achieved for 2θ angle values in the 5−50 range, step 0.06° 2θ, and time/step = 900 s. The unit-cell parameters were determined 2 | Journal Name, [year], [vol], 00–00
Si MAS and 1H−29Si 29
Chemical shift standard Frequency (MHz) Pulse width (µs) Flip angle (deg) Contact time (ms) Recycle time (s) Spinning rate (kHz) Scans number
2.2 Water Intrusion−Extrusion Experiments The intrusion−extrusion of water in the zeosil sample (zeosil pellet) was performed at room temperature using a modified mercury porosimeter (Micromeritics Model Autopore IV), as described in our previous works.6 The experimental intrusion−extrusion curves were obtained after subtraction of the curve corresponding to the compressibility of pure water. The values of the intrusion (Pint) and extrusion (Pext) pressures correspond to that of the half volume total variation. Pressure is expressed in megapascals and volume variation in milliliters per gram of anhydrous calcined samples. The experimental error is estimated to 1% on the pressure and on the volume.
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MAS TMSa 59.6 1.7 30 / 80 4 805
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TMS: tetramethylsilane.
3. Results and discussion 85
3.1 Water Intrusion−Extrusion Pressure−Volume Diagrams
Isotherms:
The pressure−volume diagrams of the ITW-type zeosil after one, two and three water intrusion− extrusion cycles are given in Figure 1, and the characteristic data are reported in Table 2.
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The ITW-type zeosil (ITQ-12) was synthesized according to the procedure published by X, Yang and al.20 using 1,3,4trimethylimidazolium hydroxide (TMIOH) as structure-directing agent. The latter was synthesized as follows. First, 34.5 g of 4methylimidazole (Aldrich, 98%) was dissolved in 600 mL of chloroform (Fluka, p.a.). This solution was mixed with 128 g of potassium carbonate hydrate (Fischer, p.a.). A large excess (468 g) of iodomethane (Aldrich, 99%) was then added in two equal portions with an interval time of 3 days. The mixture was then stirred at room temperature for 6 days, the potassium carbonate was filtered out, and the solvent was removed by evaporation. The solid product (ca. 100 g) was washed using diethyl ether. The iodide salt was converted to the hydroxide form (TMIOH) by (1) dissolution in a proper amount of water (50 g of solid: 500 g of H2O) (2) in contact with Dowex 1 anion-exchange resin at room temperature for 14 h. The hydroxide solution was then concentrated under vaccum at 50 °C to a total volume below 100 mL. The concentration of the solution was determined by HCl titration to be 1.16 M. Aerosil 380 Degussa was used as the silica source for the preparation of the zeosil. 2.7 g of fumed silica was blended into 21 g of the concentrated 1,3,4-trimethylimidazolium hydroxide (TMIOH) solution and stirred vigorously for 2 h. A tiny amount of uncalcined ITQ-12 (less than 1 mg) was added into the slurry as seeds for crystallization. To the gel was added 1.1 g of 49 wt % HF solution, and the mixture was further stirred for 1 h. The gel was weighed to determine the amount of water (water content of the fumed silica was neglected). The molar gel composition was 1SiO2: 0.56TMIOH: 0.62HF: 15H2O. This gel was sealed in a Teflon-lined stainless steel autoclave and heated to 175 °C under static conditions for 10 days. The solid products were recovered, washed with large amounts of distilled water, and dried. Calcination was performed in a muffle furnace in air at 740 °C for 6 h.
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Table 2: Characteristics of the Samples: Intrusion (Pint) and Extrusion (Pext) Pressures, Intruded (Vint) and Extruded (Vext) Volumes, Stored (Ea) and Restored (Er) Energies.
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The XRD patterns of the calcined samples before and after water intrusion−extrusion experiments are reported in Figure 2.
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i.e., 0.12 mL/g). However, Desbiens et al. have shown, in the case of Silicalite-1 that the value of the intruded volume determined for a water density of 1 has to be corrected since average computed water density in the MFI structure is around 0.6 g/mL.5 In our case the density bulk water is lower and close to 0.5. This seems in agreement with the lower pore openings of the ITW zeosil (8 MR instead of 10 MR). At lower pressure (< 0.1 MPa) water molecules are completely expelled from the zeosil. The “ITW zeosil−water” system can restore 100% of the stored energy corresponding to about 8 J/g. This value, compared to the channel-type zeosils (MEL: 6.5 J/g, MFI: 10.6 J/g, MTW: 15 J/g, FER: 15 J/g, TON: 14 J/g,…) shows that the ITW zeolite and the MEL zeolite have the lowest energy storage capacities. For the ITW zeosil, this low stored energy could be explained in part by the lowest intruded volume (accessible porous volume). As already discussed in the introduction, compared to cage-like zeosils (CHA, DDR) and despite a similar pore openings (8 MR), the water intrusion takes place at a higher pressure for this channel zeosil. This result validates that the intrusion pressure for cage-like zeosils depends better on the cage diameter.9
Initially, at low pressure (< 0.1 MPa), the volume variation corresponds to two phenomena; the compactibility of the particle bed (pellet) and the filling of the interparticular porosity of the zeolitic pellet as previously mentioned in references 16 and 18. The intrusion−extrusion of water is reproducible over several cycles. The ‘‘ITW zeosil–water’’ system displays a spring behavior. After 1, 2 and 3 intrusion–extrusion cycles, the extrusion and the extrusion curves are completely superimposable. In all cases, an important spread of the intrusion step is observed. The intrusion of water starts at ~120 MPa with a complete filling of the pores around ~220 MPa. The average value for the intrusion pressure is reached at Pint = 172 MPa. It should be noticed that such a value for the intrusion pressure is the second highest between all studied zeolites (AFI: 58 MPa, MFI: 96 MPa, MTW: 132 MPa, FER: 147 MPa, MEL: 63 MPa, DDR: 60 MPa, STT: 36 MPa, CHA: 37 MPa, TON: 186 MPa,…). These characteristics can be explained by the pore system of this zeosil (channels with small pore openings). The intruded volume, close to 0.047 mL/g, is lower than the one obtained from N2 adsorption-desorption isotherms (see below, This journal is © The Royal Society of Chemistry [year]
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Figure 2: X-ray diffraction patterns of calcined ITQ-12 samples before (a) and after water intrusion−extrusion experiments (b). 55
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They are indexed with the following unit-cell parameters (monoclinic symmetry, Cm space group): Nonintruded sample: a = 10.3260(15) Å, b = 15.002(3) Å, c = 8.8524(15) Å, β = 105.352(14)°, V = 1322.4(3) Å3 Intruded-extruded sample: a = 10.3086(21) Å, b = 14.989(4) Å, c = 8.8474(21) Å, β = 105.274(19) °, V = 1318.8(4) Å3 After three water intrusion–extrusion cycles, no defects were observed at the long range order, only a slight decrease of the unit-cell parameters is observed. As seen below (NMR spectroscopy), after intrusion–extrusion of water, hydrophilic defect sites are created. The interaction of these defects with water molecules might be responsible for the contraction of the unit cell. The crystal morphology of ITQ-12 was examined by scanning electron microscopy. ITQ-12 consists in aggregates of small Journal Name, [year], [vol], 00–00 | 3
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crystals (1 µm × 2 µm) with platelet morphology (Figure 3a). After intrusion-extrusion of water, the ITQ-12 sample displays a similar morphology. However, some crystals seem to be broken (Figure 3b).
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The N2 adsorption−desorption isotherms of the nonintruded sample are shown in Figure 4.
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curve of the nonintruded ITQ-12 sample (Figure 5, solid line) shows clearly a pronounced hydrophobic character. Indeed, the weight loss is very low. Whereas the TG curve of the intrudedextruded sample (Figure 5, dotted line) displays two main weight losses. The first one, from 50 to 135 °C (~2.2 wt %), corresponds to the removal of physisorbed water molecules, and the second one, observed in the temperature range 250-500 °C (~0.8 wt %) is due to water arising from dehydroxylation reactions. Indeed, silanol defects are clearly evidenced by 29Si solid-state NMR spectroscopy (see below).
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29 3.5.1 Si MAS NMR spectroscopy 29 The Si MAS NMR spectra of the calcined ITW-type zeosil before and after water intrusion−extrusion experiments are shown in Figure 6. The spectrum of the nonintruded ITQ-12 sample exhibits six well resolved signals located between −109 and −119 ppm with an intensity ratio close to 1:1:1:1:1:1. (Figure 6, thick solid line). They correspond to Q4 groups (Si(−OSi)4) associated to the 6 crystallographic inequivalent silicon sites of the ITW structure.20
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Figure 5: TG curves of calcined ITQ-12 sample before (solid line) and after water intrusion−extrusion experiments (dotted line).
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Figure 4: N2 adsorption−desorption isotherms at -196 °C of the calcined ITQ-12 sample before (a) and after water intrusion−extrusion experiments (b).
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These isotherms are mainly of type I characteristic of microporous solids. The BET surface area and microporous volume are 298 m2 g−1 and 0.12 cm3 g−1, respectively. After three water intrusion–extrusion cycles, a decrease of the microporous volume (0.10 cm3 g-1) and the BET surface area (255 m2 g-1) is observed. The latter might be due either to the presence of defect sites as show below by solid-state NMR spectroscopy or to the presence of some traces of physisorbed water molecules which might be also responsible of the slight contraction of the unit-cell (see XRD section). Indeed, the intruded-extruded sample was outgassed only at 90 °C overnight (see experimental section) and according to the TG curve (Figure 5) the removal of physisorbed molecules occurs between 50 and 135 °C.
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The thermogravimetric curves of the ITW zeosil before and after intrusion-extrusion of water are reported in Figure 5. The TG 4 | Journal Name, [year], [vol], 00–00
Figure 6: 29Si MAS NMR spectra of the calcined ITQ-12 sample before (thick solid line) and after water intrusion−extrusion This journal is © The Royal Society of Chemistry [year]
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experiments (dotted line).
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After water intrusion-extrusion experiments, new broad resonances located between −98 and −103 ppm (Figure 6, dotted line) appear. They are characteristic of Q3 sites and reveal the presence of silanol (Si-(OSi)3OH) or Si-(OSi)3O- groups. The existence of Q3 sites, mainly silanol groups, which represent 6% of the total Si is in good agreement with the second weight loss (~0.8 wt %) observed on the TG curve. As a result, a significant loss of resolution is observed, illustrated by a shift toward the low field and a broadening of all Q4 sites, which might indicate modifications of bond angles and a decrease of the local structural order of the inorganic framework, respectively. Before and after water intrusion-extrusion experiments, small components with no relationship in intensity with the other components are also detected (see arrows in Figures 6 and 7). They correspond probably to traces of impurities not detected by XRD. 1 3.5.2 H−29Si CPMAS NMR Spectroscopy 1 29 The H− Si CPMAS NMR spectra of the calcined nonintruded and intruded samples are reported in Figure 7. These spectra were performed to enhance the silicon atoms that bear protons and thus to get evidence of the presence of silanol groups.
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The energetic performances of pure silica ITW-type zeolites were evaluated by water intrusion−extrusion experiments. The average value for the intrusion pressure is reached at Pint = 172 MPa, the intruded volume is 0.047 mL/g. After water intrusion−extrusion experiments, the ITW structure is affected. The breaking of some siloxane bridges was clearly revealed by the presence of Q3 groups on the 1H−29Si CPMAS NMR spectra. These defects have no influence on the intrusion volumes and on the shape of the isotherms. At lower pressure (< 0.1 MPa) water molecules are completely expelled from the zeosil. Therefore, the “ITW zeosil−water” system behaves as a molecular spring. The stored energy is close to 8 J/g of zeolite with a restored energy yield close to 100%.
Notes and references a
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Univ de Haute Alsace (UHA), CNRS, Équipe Matériaux à Porosité Contrôlée (MPC), Institut de Science des Matériaux de Mulhouse (IS2M) UMR 7361, 3,bis rue Alfred Werner, Mulhouse, France. E-mail: [email protected] and joë[email protected]
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Figure 7: H− Si CPMAS NMR spectra of the calcined ITQ-12 sample before (thick solid line) and after water intrusion−extrusion experiments (dotted line). The 1H−29Si CPMAS technique does not provide quantitative results; however, it allows a relative comparison of the spectra if they were registered under the same conditions. As expected, under the NMR experimental conditions (short contact time), the nonintruded ITQ-12 sample provides a barely visible signal (Figure 7, thick solid line) indicating that no or only very few defect sites are present; revealing thus the highly hydrophobic character of this zeosil. Whereas, for the intruded–extruded sample the presence of three main resonances at around -97 and 114 ppm corresponding to Q3 (Si-(OSi)3OH) groups are clearly evidenced (Figure 7, dotted line). It is worthy to note that among the six inequivalent crystallographic sites, some of them seem to be more affected.
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J. Čejka, H. Van Bekkum, A. Corma, and F. Schüth, Stud. Surf. Sci. Catal, 2007, 168, 525. V. Eroshenko, R.C. Regis, M. Soulard and J. Patarin, J. Am. Chem. Soc, 2001, 123, 8129. V. Eroshenko, R.C. Regis, M. Soulard and J. Patarin, C. R. Phys, 2002, 3, 111. M. Soulard, J. Patarin, V. Eroshenko and R. Regis, Stud. Surf. Sci. Catal, 2004, 154B, 1830. N. Desbiens, I. Demachy, A.H. Fuchs, H. Kirsch-Rodeschini, M. Soulard and J. Patarin, Angew. Chem. Int. Ed, 2005, 44, 5310. M. Trzpit, M. Soulard and J. Patarin, Chem. Lett, 2007, 36, 980. M. Trzpit, M. Soulard, J. Patarin, N. Desbiens, F. Cailliez, A. Boutin, I. Demachy and A. Fuchs, Langmuir, 2007, 23, 10131. F. Cailliez, M. Trzpit, M. Soulard, I. Demachy, A. Boutin, J. Patarin, and A. H Fuchs, Phys. Chem. Chem. Phys, 2008, 10, 4817. M. Trzpit, S. Rigolet, J.-L. Paillaud, C. Marichal, M. Soulard and J. Patarin, J. Phys. Chem. B, 2008, 112, 7257. T. Karbowiak, M.-A. Saada, S. Rigolet, A. Ballandras, G. Weber, I. Bezverkhy, M. Soulard, J. Patarin and J.-P. Bellat, Phys. Chem. Chem. Phys, 2010, 12, 11454. M.-A. Saada, S. Rigolet; J.-L. Paillaud, N. Bats, M. Soulard and J. Patarin, J. Phys. Chem. C, 2010, 114, 11650. M.-A. Saada, M. Soulard, B. Marler, H. Gies and J Patarin, J. Phys. Chem. C, 2011, 115, 425. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, Microporous Mesoporous Mater, 2011, 146, 119. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, J. Phys. Chem. C, 2012, 116, 4802. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, J. Phys. Chem. C, 2012, 116, 20389. L. Tzanis, B. Marler, H. Gies, M. Soulard and J Patarin, J. Phys. Chem. C, 2013, 117, 4098. Y.G. Bushuev and G. Sastre, J. Phys. Chem. C, 2011, 115, 21942. G. Ortiz, H. Nouali, C. Marichal-Westrich, G. Chaplais and J. Patarin, Phys. Chem; Chem. Phys, 2013, 15, 4888. Y.G. Bushuev, G. Sastre, J. Vicente de Julian-Ortiz and J. Galvez, J. Phys. Chem. C, 2012, 116, 24916. X.B Yang, M.A. Camblor, Y. Lee, H.M. Liu and D.H. Olson, J. Am. Chem. Soc, 2004, 126, 10403. A. Boultif and D. Louër, J. Appl. Crystallogr, 1991, 24, 987. STOE WinXPOW, version 1.06; STOE and Cie: Darmstadt, Germany, 1999.
4. Conclusions This journal is © The Royal Society of Chemistry [year]
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: I. Khay, L. Tzanis, T. J. Daou, H. Nouali, A. Ryzhikov and J. Patarin, Phys. Chem. Chem. Phys., 2013, DOI: 10.1039/C3CP53570C.
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.
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DOI: 10.1039/C3CP53570C
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Energetic Behavior of the Pure Silica ITQ-12 (ITW) Zeolite under High Pressure Water Intrusion Ismail Khay,a Lydie Tzanis,a Jean Daou,* a Habiba Nouali,a Andrey Ryzhikova and Joël Patarin*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Experimental water intrusion–extrusion isotherms were performed at room temperature on pure silica ITW-type zeolites (ITQ-12 zeosil). The water intrusion is obtained by applying a high hydraulic pressure corresponding to the intrusion step. When the pressure is released, the water extrusion occurs at a similar pressure to that of the intrusion one. Therefore, the ‘‘ITW zeosil−water’’ system behaves like a spring and the phenomenon is reproducible over several cycles. Several characterizations have been realized before and after water intrusion–extrusion experiments in order to reveal the presence or the lack of defects after such experiments. Structural modifications at the long range order cannot be observed by XRD analysis after three water intrusion–extrusion cycles. However, solid state NMR spectroscopy get evidence of the presence of Q3 groups revealing the breaking of some siloxane bridges after the intrusion step. The “ITW zeosil−water” system can restore 100% of the stored energy corresponding to about 8 J/g.
1. Introduction
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Zeolites are used in several industrial processes such as petrochemical cracking, ion exchange, and the separation and removal of gases and solvents.1 In last fifteen years, these microporous solids and more particularly pure−silica zeolites (named zeosils) have been studied for applications in the energetic field.2 In these hydrophobic materials, the adsorption of water is extremely weak when the pressure is below the water saturation vapor pressure. To penetrate water in such porous matrixes, a certain pressure must be applied.2 From a general point of view, during this forced penetration (intrusion), the liquid (water in our case) is transformed into a multitude of molecular clusters, which develop a large solid−liquid interface. The mechanical energy, spent by the pressure forces, is converted into interfacial energy. At the microscopic scale, this phenomenon can be explained by the breaking of intermolecular bonds in the liquid to create new bonds with the solid. By reducing the pressure, the system can induce an expulsion of the liquid out of the cavities of the material (extrusion). Depending on zeolite structure, the “zeosil−water” system is able to restore, dissipate, or absorb the supplied mechanical energy during the compression step, and therefore, it displays a spring, shockabsorber, or bumper behavior. Several ‘‘zeosil–water’’ systems have already been studied3-17 and a summary of the energetic performances of these zeosils is reported in our previous papers.15,16 The most studied solid was the MFI-type zeosil (Silicalite-1); which is characterized by a three-dimensional channel system with 10-membered ring (MR) openings (0.55-0.56 nm). The “MFI zeosil– water” system acts as a molecular spring and is able to restore 94% of the stored energy (11 J/g) and this during several water intrusion – extrusion cycles. This journal is © The Royal Society of Chemistry [year]
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Recently, this process was extended to hydrophobic metal organic framework materials (MOFs) such as ZIF-8 which displays a shock absorber behavior at a quite low pressure (27 MPa).18 Our group have also shown that the cage-like zeosils, despite a lower pore opening, display a lower intrusion pressure that the channel like zeosils.15 It was the case for instance of the CHA-type zeosil (cage-like zeosil with 8 MR openings) for which the water intrusion pressure is about three times lower than that of the MFI zeosil (channels with 10 MR openings). However, the dependence of water intrusion pressure on zeolite structure is not clearly understood. Some zeosils with similar channel system show high difference in energetic performances. For example, the intrusion pressure for MTW and AFI zeosils both characterized by 1D channel system with 12 MR openings is 132 and 57 MPa, respectively.13 Recently Bushuev et al. have shown by investigating through molecular dynamic simulations the water intrusion-extrusion in AFI, IFR, MTW and TON pure silica zeolites that intruded water volumes can be correlated with the free volume of the zeosil unit cells.19 More data have to be collected in order to consolidate this theory,that is why the study of water intrusion-extrusion in new zeosils with careful structural and physicochemical characterizations presents a high interest. In this study, the energetic performances of ITW-type zeosil (ITQ-12) using water intrusion−extrusion experiments is presented. This zeosil is characterized by a two dimensional channel system with 8 MR pore openings running along the [001] (3.8 × 4.1 Å) and [100] direction (2.4 × 5.3 Å). The samples were fully characterized before and after water intrusion mainly by powder X-ray diffraction, SEM, thermal analysis, N2 adsorption−desorption, and 29Si NMR spectroscopy. The stored energy was evaluated from the pressure−volume diagrams of the “zeosil−water” system. [journal], [year], [vol], 00–00 | 1
Physical Chemistry Chemical Physics Accepted Manuscript
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Physical Chemistry Chemical Physics
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DOI: 10.1039/C3CP53570C
using Louër’s DICVOL91 indexing routine21 of the STOE WinXPOW program package.22
2. Experimental section 2.1 Synthesis of Pure Silica ITW-Type Zeolite Samples
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The size and the morphology of the crystals were determined by scanning electron microscopy (SEM) using a Philips XL 30 FEG microscope. 2.5 Nitrogen Adsorption−Desorption Measurements
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Nitrogen adsorption−desorption isotherms were performed using a Micromeritics ASAP 2420 apparatus. Prior to the adsorption measurements, the nonintruded samples were outgassed at 300 °C overnight under a vacuum. The intruded−extruded samples were outgassed at 90 °C overnight to eliminate physisorbed water and to avoid the dehydroxylation process. The specific surface area (SBET) and microporous volume (Vmicro) were calculated using the BET and t-plot methods, respectively. 2.6 Thermal Analysis
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Thermogravimetric (TG) analyses were carried out on a TGLabsys apparatus, under air flow, with a heating rate of 5 °C·min−1 from 20 to 700 °C. Prior to the analysis, the samples were hydrated in a 80% relative humidity atmosphere for 24 h in order to set the hydration state. 2.7 Solid-State NMR Spectroscopy 29
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Si MAS and 1H−29Si CPMAS NMR spectra were recorded on a Bruker Advance II 300 MHz spectrometer. The recording conditions are given in Table 1. Table 1: Recording Conditions of the CPMAS NMR Spectra.
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2.3 Powder X-ray Diffraction 50
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X-ray diffraction patterns of the different samples were recorded in a Debye−Scherrer geometry on a STOE STADI-P diffractometer equipped with a curved germanium (111), primary monochromator, and a linear position-sensitive detector (6° 2θ) using Cu Kα1 radiation (λ = 0.15406 nm). Measurements were achieved for 2θ angle values in the 5−50 range, step 0.06° 2θ, and time/step = 900 s. The unit-cell parameters were determined 2 | Journal Name, [year], [vol], 00–00
Si MAS and 1H−29Si 29
Chemical shift standard Frequency (MHz) Pulse width (µs) Flip angle (deg) Contact time (ms) Recycle time (s) Spinning rate (kHz) Scans number
2.2 Water Intrusion−Extrusion Experiments The intrusion−extrusion of water in the zeosil sample (zeosil pellet) was performed at room temperature using a modified mercury porosimeter (Micromeritics Model Autopore IV), as described in our previous works.6 The experimental intrusion−extrusion curves were obtained after subtraction of the curve corresponding to the compressibility of pure water. The values of the intrusion (Pint) and extrusion (Pext) pressures correspond to that of the half volume total variation. Pressure is expressed in megapascals and volume variation in milliliters per gram of anhydrous calcined samples. The experimental error is estimated to 1% on the pressure and on the volume.
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MAS TMSa 59.6 1.7 30 / 80 4 805
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TMS: tetramethylsilane.
3. Results and discussion 85
3.1 Water Intrusion−Extrusion Pressure−Volume Diagrams
Isotherms:
The pressure−volume diagrams of the ITW-type zeosil after one, two and three water intrusion− extrusion cycles are given in Figure 1, and the characteristic data are reported in Table 2.
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The ITW-type zeosil (ITQ-12) was synthesized according to the procedure published by X, Yang and al.20 using 1,3,4trimethylimidazolium hydroxide (TMIOH) as structure-directing agent. The latter was synthesized as follows. First, 34.5 g of 4methylimidazole (Aldrich, 98%) was dissolved in 600 mL of chloroform (Fluka, p.a.). This solution was mixed with 128 g of potassium carbonate hydrate (Fischer, p.a.). A large excess (468 g) of iodomethane (Aldrich, 99%) was then added in two equal portions with an interval time of 3 days. The mixture was then stirred at room temperature for 6 days, the potassium carbonate was filtered out, and the solvent was removed by evaporation. The solid product (ca. 100 g) was washed using diethyl ether. The iodide salt was converted to the hydroxide form (TMIOH) by (1) dissolution in a proper amount of water (50 g of solid: 500 g of H2O) (2) in contact with Dowex 1 anion-exchange resin at room temperature for 14 h. The hydroxide solution was then concentrated under vaccum at 50 °C to a total volume below 100 mL. The concentration of the solution was determined by HCl titration to be 1.16 M. Aerosil 380 Degussa was used as the silica source for the preparation of the zeosil. 2.7 g of fumed silica was blended into 21 g of the concentrated 1,3,4-trimethylimidazolium hydroxide (TMIOH) solution and stirred vigorously for 2 h. A tiny amount of uncalcined ITQ-12 (less than 1 mg) was added into the slurry as seeds for crystallization. To the gel was added 1.1 g of 49 wt % HF solution, and the mixture was further stirred for 1 h. The gel was weighed to determine the amount of water (water content of the fumed silica was neglected). The molar gel composition was 1SiO2: 0.56TMIOH: 0.62HF: 15H2O. This gel was sealed in a Teflon-lined stainless steel autoclave and heated to 175 °C under static conditions for 10 days. The solid products were recovered, washed with large amounts of distilled water, and dried. Calcination was performed in a muffle furnace in air at 740 °C for 6 h.
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Table 2: Characteristics of the Samples: Intrusion (Pint) and Extrusion (Pext) Pressures, Intruded (Vint) and Extruded (Vext) Volumes, Stored (Ea) and Restored (Er) Energies.
Cycle Ring Pint Pext Vint Vext Ea (J/g)b Er % a a a (J/g)c restored Size (MPa) (MPa) (mL/g) (mL/g)a (T)* energy
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The XRD patterns of the calcined samples before and after water intrusion−extrusion experiments are reported in Figure 2.
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* T : number of tetrahedral Si (a) determined from the water intrusion−extrusion isotherms (b) stored energy Ea = Vint x Pint (c) restored energy Er = Vext x Pext (d) % restored energy = Er/Ea x 100
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i.e., 0.12 mL/g). However, Desbiens et al. have shown, in the case of Silicalite-1 that the value of the intruded volume determined for a water density of 1 has to be corrected since average computed water density in the MFI structure is around 0.6 g/mL.5 In our case the density bulk water is lower and close to 0.5. This seems in agreement with the lower pore openings of the ITW zeosil (8 MR instead of 10 MR). At lower pressure (< 0.1 MPa) water molecules are completely expelled from the zeosil. The “ITW zeosil−water” system can restore 100% of the stored energy corresponding to about 8 J/g. This value, compared to the channel-type zeosils (MEL: 6.5 J/g, MFI: 10.6 J/g, MTW: 15 J/g, FER: 15 J/g, TON: 14 J/g,…) shows that the ITW zeolite and the MEL zeolite have the lowest energy storage capacities. For the ITW zeosil, this low stored energy could be explained in part by the lowest intruded volume (accessible porous volume). As already discussed in the introduction, compared to cage-like zeosils (CHA, DDR) and despite a similar pore openings (8 MR), the water intrusion takes place at a higher pressure for this channel zeosil. This result validates that the intrusion pressure for cage-like zeosils depends better on the cage diameter.9
Initially, at low pressure (< 0.1 MPa), the volume variation corresponds to two phenomena; the compactibility of the particle bed (pellet) and the filling of the interparticular porosity of the zeolitic pellet as previously mentioned in references 16 and 18. The intrusion−extrusion of water is reproducible over several cycles. The ‘‘ITW zeosil–water’’ system displays a spring behavior. After 1, 2 and 3 intrusion–extrusion cycles, the extrusion and the extrusion curves are completely superimposable. In all cases, an important spread of the intrusion step is observed. The intrusion of water starts at ~120 MPa with a complete filling of the pores around ~220 MPa. The average value for the intrusion pressure is reached at Pint = 172 MPa. It should be noticed that such a value for the intrusion pressure is the second highest between all studied zeolites (AFI: 58 MPa, MFI: 96 MPa, MTW: 132 MPa, FER: 147 MPa, MEL: 63 MPa, DDR: 60 MPa, STT: 36 MPa, CHA: 37 MPa, TON: 186 MPa,…). These characteristics can be explained by the pore system of this zeosil (channels with small pore openings). The intruded volume, close to 0.047 mL/g, is lower than the one obtained from N2 adsorption-desorption isotherms (see below, This journal is © The Royal Society of Chemistry [year]
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Figure 2: X-ray diffraction patterns of calcined ITQ-12 samples before (a) and after water intrusion−extrusion experiments (b). 55
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They are indexed with the following unit-cell parameters (monoclinic symmetry, Cm space group): Nonintruded sample: a = 10.3260(15) Å, b = 15.002(3) Å, c = 8.8524(15) Å, β = 105.352(14)°, V = 1322.4(3) Å3 Intruded-extruded sample: a = 10.3086(21) Å, b = 14.989(4) Å, c = 8.8474(21) Å, β = 105.274(19) °, V = 1318.8(4) Å3 After three water intrusion–extrusion cycles, no defects were observed at the long range order, only a slight decrease of the unit-cell parameters is observed. As seen below (NMR spectroscopy), after intrusion–extrusion of water, hydrophilic defect sites are created. The interaction of these defects with water molecules might be responsible for the contraction of the unit cell. The crystal morphology of ITQ-12 was examined by scanning electron microscopy. ITQ-12 consists in aggregates of small Journal Name, [year], [vol], 00–00 | 3
Physical Chemistry Chemical Physics Accepted Manuscript
DOI: 10.1039/C3CP53570C
Physical Chemistry Chemical Physics
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crystals (1 µm × 2 µm) with platelet morphology (Figure 3a). After intrusion-extrusion of water, the ITQ-12 sample displays a similar morphology. However, some crystals seem to be broken (Figure 3b).
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Figure 3: SEM micrographs of the calcined ITQ-12 samples before (a) and after water intrusion-extrusion experiments (b). 3.3 N2 Adsorption−Desorption Isotherms 10
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The N2 adsorption−desorption isotherms of the nonintruded sample are shown in Figure 4.
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curve of the nonintruded ITQ-12 sample (Figure 5, solid line) shows clearly a pronounced hydrophobic character. Indeed, the weight loss is very low. Whereas the TG curve of the intrudedextruded sample (Figure 5, dotted line) displays two main weight losses. The first one, from 50 to 135 °C (~2.2 wt %), corresponds to the removal of physisorbed water molecules, and the second one, observed in the temperature range 250-500 °C (~0.8 wt %) is due to water arising from dehydroxylation reactions. Indeed, silanol defects are clearly evidenced by 29Si solid-state NMR spectroscopy (see below).
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29 3.5.1 Si MAS NMR spectroscopy 29 The Si MAS NMR spectra of the calcined ITW-type zeosil before and after water intrusion−extrusion experiments are shown in Figure 6. The spectrum of the nonintruded ITQ-12 sample exhibits six well resolved signals located between −109 and −119 ppm with an intensity ratio close to 1:1:1:1:1:1. (Figure 6, thick solid line). They correspond to Q4 groups (Si(−OSi)4) associated to the 6 crystallographic inequivalent silicon sites of the ITW structure.20
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Figure 5: TG curves of calcined ITQ-12 sample before (solid line) and after water intrusion−extrusion experiments (dotted line).
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Figure 4: N2 adsorption−desorption isotherms at -196 °C of the calcined ITQ-12 sample before (a) and after water intrusion−extrusion experiments (b).
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These isotherms are mainly of type I characteristic of microporous solids. The BET surface area and microporous volume are 298 m2 g−1 and 0.12 cm3 g−1, respectively. After three water intrusion–extrusion cycles, a decrease of the microporous volume (0.10 cm3 g-1) and the BET surface area (255 m2 g-1) is observed. The latter might be due either to the presence of defect sites as show below by solid-state NMR spectroscopy or to the presence of some traces of physisorbed water molecules which might be also responsible of the slight contraction of the unit-cell (see XRD section). Indeed, the intruded-extruded sample was outgassed only at 90 °C overnight (see experimental section) and according to the TG curve (Figure 5) the removal of physisorbed molecules occurs between 50 and 135 °C.
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The thermogravimetric curves of the ITW zeosil before and after intrusion-extrusion of water are reported in Figure 5. The TG 4 | Journal Name, [year], [vol], 00–00
Figure 6: 29Si MAS NMR spectra of the calcined ITQ-12 sample before (thick solid line) and after water intrusion−extrusion This journal is © The Royal Society of Chemistry [year]
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experiments (dotted line).
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After water intrusion-extrusion experiments, new broad resonances located between −98 and −103 ppm (Figure 6, dotted line) appear. They are characteristic of Q3 sites and reveal the presence of silanol (Si-(OSi)3OH) or Si-(OSi)3O- groups. The existence of Q3 sites, mainly silanol groups, which represent 6% of the total Si is in good agreement with the second weight loss (~0.8 wt %) observed on the TG curve. As a result, a significant loss of resolution is observed, illustrated by a shift toward the low field and a broadening of all Q4 sites, which might indicate modifications of bond angles and a decrease of the local structural order of the inorganic framework, respectively. Before and after water intrusion-extrusion experiments, small components with no relationship in intensity with the other components are also detected (see arrows in Figures 6 and 7). They correspond probably to traces of impurities not detected by XRD. 1 3.5.2 H−29Si CPMAS NMR Spectroscopy 1 29 The H− Si CPMAS NMR spectra of the calcined nonintruded and intruded samples are reported in Figure 7. These spectra were performed to enhance the silicon atoms that bear protons and thus to get evidence of the presence of silanol groups.
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The energetic performances of pure silica ITW-type zeolites were evaluated by water intrusion−extrusion experiments. The average value for the intrusion pressure is reached at Pint = 172 MPa, the intruded volume is 0.047 mL/g. After water intrusion−extrusion experiments, the ITW structure is affected. The breaking of some siloxane bridges was clearly revealed by the presence of Q3 groups on the 1H−29Si CPMAS NMR spectra. These defects have no influence on the intrusion volumes and on the shape of the isotherms. At lower pressure (< 0.1 MPa) water molecules are completely expelled from the zeosil. Therefore, the “ITW zeosil−water” system behaves as a molecular spring. The stored energy is close to 8 J/g of zeolite with a restored energy yield close to 100%.
Notes and references a
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Univ de Haute Alsace (UHA), CNRS, Équipe Matériaux à Porosité Contrôlée (MPC), Institut de Science des Matériaux de Mulhouse (IS2M) UMR 7361, 3,bis rue Alfred Werner, Mulhouse, France. E-mail: [email protected] and joë[email protected]
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Figure 7: H− Si CPMAS NMR spectra of the calcined ITQ-12 sample before (thick solid line) and after water intrusion−extrusion experiments (dotted line). The 1H−29Si CPMAS technique does not provide quantitative results; however, it allows a relative comparison of the spectra if they were registered under the same conditions. As expected, under the NMR experimental conditions (short contact time), the nonintruded ITQ-12 sample provides a barely visible signal (Figure 7, thick solid line) indicating that no or only very few defect sites are present; revealing thus the highly hydrophobic character of this zeosil. Whereas, for the intruded–extruded sample the presence of three main resonances at around -97 and 114 ppm corresponding to Q3 (Si-(OSi)3OH) groups are clearly evidenced (Figure 7, dotted line). It is worthy to note that among the six inequivalent crystallographic sites, some of them seem to be more affected.
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J. Čejka, H. Van Bekkum, A. Corma, and F. Schüth, Stud. Surf. Sci. Catal, 2007, 168, 525. V. Eroshenko, R.C. Regis, M. Soulard and J. Patarin, J. Am. Chem. Soc, 2001, 123, 8129. V. Eroshenko, R.C. Regis, M. Soulard and J. Patarin, C. R. Phys, 2002, 3, 111. M. Soulard, J. Patarin, V. Eroshenko and R. Regis, Stud. Surf. Sci. Catal, 2004, 154B, 1830. N. Desbiens, I. Demachy, A.H. Fuchs, H. Kirsch-Rodeschini, M. Soulard and J. Patarin, Angew. Chem. Int. Ed, 2005, 44, 5310. M. Trzpit, M. Soulard and J. Patarin, Chem. Lett, 2007, 36, 980. M. Trzpit, M. Soulard, J. Patarin, N. Desbiens, F. Cailliez, A. Boutin, I. Demachy and A. Fuchs, Langmuir, 2007, 23, 10131. F. Cailliez, M. Trzpit, M. Soulard, I. Demachy, A. Boutin, J. Patarin, and A. H Fuchs, Phys. Chem. Chem. Phys, 2008, 10, 4817. M. Trzpit, S. Rigolet, J.-L. Paillaud, C. Marichal, M. Soulard and J. Patarin, J. Phys. Chem. B, 2008, 112, 7257. T. Karbowiak, M.-A. Saada, S. Rigolet, A. Ballandras, G. Weber, I. Bezverkhy, M. Soulard, J. Patarin and J.-P. Bellat, Phys. Chem. Chem. Phys, 2010, 12, 11454. M.-A. Saada, S. Rigolet; J.-L. Paillaud, N. Bats, M. Soulard and J. Patarin, J. Phys. Chem. C, 2010, 114, 11650. M.-A. Saada, M. Soulard, B. Marler, H. Gies and J Patarin, J. Phys. Chem. C, 2011, 115, 425. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, Microporous Mesoporous Mater, 2011, 146, 119. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, J. Phys. Chem. C, 2012, 116, 4802. L. Tzanis, M. Trzpit, M. Soulard and J. Patarin, J. Phys. Chem. C, 2012, 116, 20389. L. Tzanis, B. Marler, H. Gies, M. Soulard and J Patarin, J. Phys. Chem. C, 2013, 117, 4098. Y.G. Bushuev and G. Sastre, J. Phys. Chem. C, 2011, 115, 21942. G. Ortiz, H. Nouali, C. Marichal-Westrich, G. Chaplais and J. Patarin, Phys. Chem; Chem. Phys, 2013, 15, 4888. Y.G. Bushuev, G. Sastre, J. Vicente de Julian-Ortiz and J. Galvez, J. Phys. Chem. C, 2012, 116, 24916. X.B Yang, M.A. Camblor, Y. Lee, H.M. Liu and D.H. Olson, J. Am. Chem. Soc, 2004, 126, 10403. A. Boultif and D. Louër, J. Appl. Crystallogr, 1991, 24, 987. STOE WinXPOW, version 1.06; STOE and Cie: Darmstadt, Germany, 1999.
4. Conclusions This journal is © The Royal Society of Chemistry [year]
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