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Influence of downsizing of zeolite crystals on the orthorhombic ↔ monoclinic phase transition in pure silica MFI-type
Solid State Sciences 58 (2016) 111e114
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Influence of downsizing of zeolite crystals on the orthorhombic 4 monoclinic phase transition in pure silica MFI-type verinne Rigolet, Claire Marichal, T. Jean Daou, Ihab Kabalan, Laure Michelin, Se ne dicte Lebeau, Jean-Louis Paillaud* Be Porosit ^l Equipe Mat eriaux a e Contro ee (MPC), Institut de Science des Mat eriaux de Mulhouse (IS2M), UMR CNRS 7361, Universit e de Haute-Alsace, 3bis rue Alfred Werner, 68093 Mulhouse Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2016 Received in revised form 2 June 2016 Accepted 23 June 2016 Available online 24 June 2016
The impact of crystal size on the transition orthorhombic 4 monoclinic phase in MFI-type purely silica zeolites is investigated between 293 and 473 K using 29Si MAS NMR and powder X-ray diffraction. Three silicalite-1 zeolites are synthesized: a material constituted of micron-sized crystals, pseudospherical nanometer-sized crystals and hierarchical porous zeolites with a mesoporous network created by the use of a gemini-type diquaternary ammonium surfactant giving nanosheet zeolites. Our results show for the first time that the orthorhombic 4 monoclinic phase transition already known for micron-sized particles also occurs in nanometer-sized zeolite crystals whereas our data suggest that the extreme downsizing of the zeolite crystal to one unit cell in thickness leads to an extinction of the phase transition. © 2016 Elsevier Masson SAS. All rights reserved.
Keywords: MFI Nanosheets Phase transition Powder X-ray diffraction 29 Si MAS NMR
1. Introduction Since the first used of the aluminosilicate ZSM-5 [1] of topology MFI [2,3] as catalyst in methanol to gasoline process (MTG) reported by Meisel et al. [4] and its crystal structure determination [5], the MFI-type crystalline microporous metalosilicates are certainly one of the most studied zeolites both theoretically and experimentally. This is mostly owing to the thermal, mechanical and chemical stability, porous volume and surface area of this framework. The topology MFI is one of the end-members of the pentasil family [3], the other end-member corresponding to zeolite ZSM-11 is of topology MEL [2,3]. A MFI-type framework can be built using eight 5-membered ring units or T12-units which form chains running along [001]. These chains are connected along [010] to forms sheets parallel to the (b,c)-plane. These sheets are linked along a in such a way that neighboring layers are related by a rotation of 180 about a and shifted of ½ b [6]. The obtained 3D microporous structure possesses an interconnected system of two 10-membered rings channels, straight channels parallel to [010] which intersect the zig-zag channels along [001] [5].
* Corresponding author. E-mail address: [email protected] (J.-L. Paillaud). http://dx.doi.org/10.1016/j.solidstatesciences.2016.06.009 1293-2558/© 2016 Elsevier Masson SAS. All rights reserved.
Early after the structure determination, a polymorphic monoclinic to orthorhombic phase transition was observed for calcined ZSM-5 type materials [7]. This transition is detected, below 363 K whatever the chemical composition of the framework [5,8], the number of defect density [9], the temperature [10e14], or the nature and amount of adsorbed species in the pores [15]. This phase transition is reversible and can be explained by a mutual shift of successive (010) pentasil layers along [001], associated with the distortions of 4 out of the 12 non-equivalent crystallographic T sites connecting the pentasil layers [12,16]. Note that this orthorhombic 4 monoclinic phase transition was also observed at low temperature (~175 K) on highly siliceous as-synthesized MFI-type zeolite [17]. Recently, in situ high-temperature synchrotron X-ray powder diffraction data have demonstrated a tricritical phase transition between the ferroelastic [14] (monoclinic) and the paraelastic (orthorhombic) phase in conventional ZSM-5 zeolite [18]. This last experimental fact was connected to the previously discovered strain as an unusual deformation field distribution determined by coherent X-ray diffraction imaging [19]. Conventional MFI-type zeolites are materials whose order of magnitude of the crystals size is in micron scale and, to our knowledge, no crystallographic study has so far reported the orthorhombic 4 monoclinic phase transition in the case of nanosized MFI-type zeolites. In this Communication, in situ high-
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Fig. 1. 1H decoupled 29Si MAS NMR spectra at 293 K, 373 K and after cooling down to 293 K of calcined a) micro-, b) nanosized silicalite-1 and c) pure siliceous MFI-type nanosheets.
temperature powder X-ray diffraction (PXRD) data collection and 29 Si MAS NMR have been performed on conventional calcined micro- and nanosized silicalite-1, pure silica form of ZSM-5 zeolite, as well as on pure silica MFI-type nanosheets to determine the lattice parameter variations and the local order as a function of the particles size in the temperature range 293e473 K. 2. Experimental In the present work, the micro-sized silicalite-1 sample was prepared in a classical fluoride medium [20] and nanosized silicalite-1 was prepared in a non fluorinated media as described by Lew et al. [21]. The pure silica MFI-type nanosheets were synthesized with a diquaternary ammonium surfactant as structure directing agent following the method recently reported by Kabalan et al. [22,23] based on the recipe reported by Ryoo et al. after the discovery of aluminum-containing MFI-type nanosheets [24]. Before the PXRD and solid state NMR experiments, the samples were calcined in a muffle furnace at 823 K during 8 h in air to remove all organic moieties occluded in the pores. The evolutions of the PXRD patterns with temperature were followed on a PANanalytical X’Pert PRO MPD diffractometer (CuKa1 radiation) equipped with an X’Celerator real-time multiple strip detector and an Anton Paar HTK1200 heating chamber in flat plate reflection geometry (sample cup made of Al2O3). Unit cell €r algorithm (DICVOL) parameters were determined from the Loue [25] implemented in the Stoe Win XPOW software [26]. For the solid state NMR study, samples were packed in a 7 mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 4 kHz. 1H decoupled 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectra were recorded at ambient temperature (293 K) on a Bruker Avance II 300 spectrometer (B0 ¼ 7.1T) at 59.6 MHz. These experiments were performed with a p/6 pulse duration of 2 ms and a recycle delay of 80 s. The spectra recorded at 373 K were performed with the same acquisition conditions on a Bruker Avance II 400 spectrometer (B0 ¼ 9.4 T) at 79.5 MHz equipped with a variable temperature control unit BVT3000 (278e473 ± 0.2 K). 29Si chemical shifts are relative to TetraMethylSilane (TMS, 0 ppm). 3. Results and discussion Proton decoupled 29Si MAS NMR spectra of the calcined silicalite-1 zeosil prepared with different particles sizes and morphologies are presented in Fig. 1. The spectra recorded at 293 K of the calcined silicalite-1 zeosil prepared by the conventional
Fig. 2. PXRD patterns waterfall plots showing the monoclinic 4 orthorhombic phase transition for calcined pure silica a) nano- and b) microsized silicalite-1 in the 22.5e25.0 2q angular array and collected between 299 and 473 K. Black Miller indices are in space group P21/n, the blue ones are in space group Pnma. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
I. Kabalan et al. / Solid State Sciences 58 (2016) 111e114
method, leading to 15e25 mm length crystals (Fig. A1a), display at least 10 resolved resonances of variable intensity, detected between 108 and 118 ppm corresponding to Q4 sites (Si(eOSi)4) (Fig. 1a). They correspond to the 24 non equivalent crystallographic T sites expected for a monoclinic framework structure of the calcined silicalite-1 zeolite described in space group P21/n [7] and in agreement with already published 29Si MAS NMR data [10,27]. The sharpness of the resonances indicates a well crystallized sample. Decreasing the crystal size of silicalite-1 to about 70 nm (Fig. A1b) of silicalite-1 significantly broadened the resonances leading to, at least, only 8 resolved resonances in the same chemical shift range (Fig. 1b). Indeed, decreasing the crystal size induces a distribution of SieO bonds and SieOeSi angles that broadens the lines. The same observation was recently reported for nanosized silicalite-1 prepared in fluoride media [28]. Further huge broadening is noted upon changing the morphology to lamellar as we already stated it (Fig. 1c, Fig. A1c) [22]. In the latter case, a signal is detected around 103 ppm corresponding to Q3 (HOeSi(eOSi)3) groups i.e., principally, in this case, to silanol groups on the surface of the nanosheets. No Q3 were detected for the two others samples indicating the absence of defects. Upon heating at 373 K, the corresponding spectra are displayed on Fig. 1. At 373 K, the 29Si MAS NMR spectrum of the calcined conventional sample presents at least 9 resonances in the 110 to 118 ppm range (Fig. 1a). The spectrum is significantly different from the one recorded at ambient temperature. The well resolved resonance at 109 ppm has disappeared. Indeed, for ZSM-5 zeolites, a temperature induced structural phase transformation from monoclinic to orthorhombic is mentioned several times in the literature upon heating [13,16,27], the critical temperature depends on the Al contents and explains the difference between 29Si MAS NMR spectra below and above the phase transition. Despite a clear broadening of the lines (Fig. 1b), this transformation also occurs for MFI-type nanocrystals. After cooling down the samples to 293 K, the 29Si MAS NMR spectra are superimposed to the one recorded
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before heating. Consequently, this phase transition is perfectly reversible as already known for conventional silicalite-1 even in the case of nanocrystals. It is worthy to note that from the 29Si MAS NMR spectra of the siliceous MFI-type nanosheets sample before and after heating at 373 K, despite the apparent similarity of the two spectra, it is not possible to conclude about the existence of a phase transition since the resolution is particularly low (Fig. 1c). In order to check this point, we undertook a X-ray study of the three samples at variable temperatures. Monitoring PXRD patterns as a function of temperature allowed us to follow the variation of the unit cell parameters for micro- and nanosized silicalite-1 (Table A1 and Fig. A2). On Fig. 2, due to the crystal size effect, an enlargement of the reflections width for the nanosized crystals is observed. The monoclinic 4 orthorhombic phase transition is less precise (Fig. 2a) in comparison with the conventional micro-sized silicalite-1 (Fig. 2b) but in both cases, it occurs at about 363 K ± 10 K. Accordingly, under warming, the doublet of reflections of indices 1 13 þ 1 33 and 133 þ 313 in space group P21/n progressively move into a single visible reflection of indices 133 þ 313 indexable in space groups Pnma (12 silicon T sites) or P212121 (24 silicon T sites) (Fig. 2 and Table A1). The difference in crystal size for these two samples from 15e25 mm to 70 nm does not seem modify the shift of the successive pentasil layers as the phase transition occurs at approximately the same temperature. Aside less marked change in the case of nanosized due to a greater inaccuracy on the location of the reflections, the evolution of the different unit cell parameters follows the same tendency for micro- and nanosized silicalite-1 (Table A1 and Fig. A2). This result is in agreement with those obtained previously in the case of conventional silica-rich ZSM-5 and silicalite-1 zeolites [13,14,16,27,29]. In the case of the nanosheets the evolution of the powder XRD patterns with temperature is different. From Fig. 3, no change of the lines shape and width is observable over the entire temperature range, i.e. 298e773 K. The temperature dependence of the PXRD
Fig. 3. Evolution of the powder XRD patterns as a function of temperature for pure siliceous MFI-type nanosheets. An offset has been added to the intensities at each step for clarity. The insets show magnifications of the 22.5e25.0 (left) and 25.4e25.7 2q (right) angular arrays showing the negative thermal expansion of the lattice without change of the line width for the nanosheets. The * indicate the position of the 012 reflection of a/Al2O3 (sample holder) for which the evolution of the position is in accordance with an expected weak positive thermal expansion in this temperature range [32].
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shows only a gradual and regular shift of all the diffraction peaks towards lower angle values when the temperature increases (Fig. 3). This is characteristic of a negative thermal expansion of the lattice as observed for conventional silicalite-1 above 373 K [29]. Even if the lines width is wider in the case of the nanosheets, one would expect an evolution of the line broadening for some lines during a monoclinic 4 orthorhombic or orthorhombic 4 monoclinic symmetry change but this is not observed even in the 22.5 e25.0 2q range whatever should be the direction of the transformation. An explanation may be given by the nanosheets thickness. Indeed, the overall thickness of the lamellar stacking was measured between 10 and 40 nm (Fig. A1c), the thickness of each individual nanosheet being of the same order of magnitude as the unit cell parameter b of the MFI topology, i.e. less than 2 nm [22e24,30]. Due to the particularly low resolution of the PXRD patterns, it is not possible to distinguish here a monoclinic or orthorhombic lattice. In conventional zeolites (micro- or nanosized) for which the crystal thickness contains tens, hundreds or thousands of unit cells, monoclinic and orthorhombic domains may coexist. It has been shown from single crystals that, upon cooling, the orthorhombic form of HZSM-5 obtained above the critical temperature switches into a combination of twin domains with a monoclinic symmetry [18,31]. In the case of nanosheets the presence of multidomains is not possible, monoclinic, orthorhombic or other, and could explain the absence of transition in the temperature range used in this study. Acknowledgements We thank Ludovic Josien and Loic Vidal from IS2M for their assistance with electronic microscopy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2016.06.009. References [1] R.J. Argauer, G.R. Landolt, Crystalline zeolite ZSM-5 and method of preparing the same, in: Mobil Oil Corp, USA, 1972. [2] C. Baerlocher, L.B. McCusker, B. Olson, W.M. Meier, Atlas of Zeolite Framework Types, Elsevier, Boston; Amsterdam, 2007. [3] C. Baerlocher, L.B. McCusker, Database of Zeolite Structures, http://www.iza-
structure.org/databases/). [4] S.L. Meisel, J.P. McCullough, C.H. Lechthaler, P.B. Weisz, Chem. Tech. Leipz. 6 (1976) 86e89. [5] G.T. Kokotailo, S.L. Lawton, D.H. Olson, W.M. Meier, Nature 272 (1978) 437e438. [6] C. Baerlocher, L.B. McCusker, Building Scheme for MEL and MFI in Database of Zeolite Structures, http://www.iza-structure.org/databases/ModelBuilding/ MFI.pdf). [7] E.L. Wu, S.L. Lawton, D.H. Olson, A.C. Rohrman, G.T. Kokotailo, J. Phys. Chem. 83 (1979) 2777e2781. [8] H. van Koningsveld, J.C. Jansen, H. van Bekkum, Zeolites 10 (1990) 235e242. [9] G.L. Marra, G. Tozzola, G. Leofanti, M. Padovan, G. Petrini, F. Genoni, B. Venturelli, A. Zecchina, S. Bordiga, G. Ricchiardi, Orthorhombic and monoclinic silicalites: structure, morphology, vibrational properties and crystal € lderich (Eds.), Stud. Surf. defects, in: J. Weitkamp, H. Karge, G.H. Pfeifer, W. Ho Sci. Catal, Elsevier, 1994, pp. 559e566. [10] D.G. Hay, H. Jaeger, G.W. West, J. Phys. Chem. 89 (1985) 1070e1072. [11] A. Endoh, Zeolites 8 (1988) 250e251. [12] H. van Koningsveld, F. Tuinstra, J.C. Jansen, H. van Bekkum, Zeolites 9 (1989) 253e256. [13] B.F. Mentzen, M. Sacerdote-Peronnet, Mater. Res. Bull. 28 (1993) 1017e1024. [14] B.F. Mentzen, J.M. Letoffe, P. Claudy, Thermochim. Acta 288 (1996) 1e7. [15] H. van Koningsveld, F. Tuinstra, H. van Bekkum, J.C. Jansen, Acta Crystallogr. Sect. B Struct. Sci. 45 (1989) 423e431. [16] H. van Koningsveld, J.C. Jansen, H. van Bekkum, Zeolites 7 (1987) 564e568. [17] J.M. Chezeau, L. Delmotte, T. Hasebe, N.B. Chanh, Zeolites 11 (1991) 729e731. [18] M. Ardit, A. Martucci, G. Cruciani, J. Phys. Chem. C 119 (2015) 7351e7359. [19] W. Cha, N.C. Jeong, S. Song, H.-j. Park, T.C. Thanh Pham, R. Harder, B. Lim, G. Xiong, D. Ahn, I. McNulty, J. Kim, K.B. Yoon, I.K. Robinson, H. Kim, Nat. Mater. 12 (2013) 729e734. [20] J.L. Guth, H. Kessler, R. Wey, New route to pentasil-type zeolites using a non alkaline medium in the presence of fluoride ions, in: Y. Murakami, A. Iijima, J.W. Ward (Eds.), New Developments in Zeolite Science and Technology, Proceedings of the 7th International Zeolite Conference, Elsevier, 1986, pp. 121e128. [21] C.M. Lew, Z. Li, S.I. Zones, M. Sun, Y. Yan, Microporous Mesoporous Mater. 105 (2007) 10e14. [22] I. Kabalan, G. Rioland, H. Nouali, B. Lebeau, S. Rigolet, M.B. Fadlallah, J. Toufaily, T. Hamiyeh, T.J. Daou, RSC Adv. 4 (2014) 37353e37358. [23] I. Kabalan, I. Khay, H. Nouali, A. Ryzhikov, B. Lebeau, S. Albrecht, S. Rigolet, M.B. Fadlallah, J. Toufaily, T. Hamieh, J. Patarin, T.J. Daou, J. Phys. Chem. C 119 (2015) 18074e18083. [24] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Nature 461 (2009) 246e249. [25] A. Boultif, D. Louer, J. Appl. Crystallogr. 24 (1991) 987e993. [26] STOE Win XPOW, STOE & Cie GmbH, Darmstadt, 2006. [27] J. Klinowski, T.A. Carpenter, L.F. Gladden, Zeolites 7 (1987) 73e78. [28] H. Zhou, J. Mouzon, A. Farzaneh, O.N. Antzutkin, M. Grahn, J. Hedlund, Langmuir 31 (2015) 8488e8494. [29] D.S. Bhange, V. Ramaswamy, Mater. Res. Bull. 41 (2006) 1392e1402. [30] J. Jung, C. Jo, K. Cho, R. Ryoo, J. Mater. Chem. 22 (2012) 4637e4640. [31] H. van Koningsveld, J.M. Bennett, Zeolite Structure Determination from X-Ray Diffraction, in: H.G. Karge, J. Weitkamp (Eds.), Structures and Structure Determination, Springer-Verlag, Berlin; New York, 1999, pp. 1e29. [32] H. Chikh, F. Si Ahmed, A. Afir, A. Pialoux, J. Alloys Compd. 654 (2016) 509e513.
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Influence of downsizing of zeolite crystals on the orthorhombic 4 monoclinic phase transition in pure silica MFI-type verinne Rigolet, Claire Marichal, T. Jean Daou, Ihab Kabalan, Laure Michelin, Se ne dicte Lebeau, Jean-Louis Paillaud* Be Porosit ^l Equipe Mat eriaux a e Contro ee (MPC), Institut de Science des Mat eriaux de Mulhouse (IS2M), UMR CNRS 7361, Universit e de Haute-Alsace, 3bis rue Alfred Werner, 68093 Mulhouse Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2016 Received in revised form 2 June 2016 Accepted 23 June 2016 Available online 24 June 2016
The impact of crystal size on the transition orthorhombic 4 monoclinic phase in MFI-type purely silica zeolites is investigated between 293 and 473 K using 29Si MAS NMR and powder X-ray diffraction. Three silicalite-1 zeolites are synthesized: a material constituted of micron-sized crystals, pseudospherical nanometer-sized crystals and hierarchical porous zeolites with a mesoporous network created by the use of a gemini-type diquaternary ammonium surfactant giving nanosheet zeolites. Our results show for the first time that the orthorhombic 4 monoclinic phase transition already known for micron-sized particles also occurs in nanometer-sized zeolite crystals whereas our data suggest that the extreme downsizing of the zeolite crystal to one unit cell in thickness leads to an extinction of the phase transition. © 2016 Elsevier Masson SAS. All rights reserved.
Keywords: MFI Nanosheets Phase transition Powder X-ray diffraction 29 Si MAS NMR
1. Introduction Since the first used of the aluminosilicate ZSM-5 [1] of topology MFI [2,3] as catalyst in methanol to gasoline process (MTG) reported by Meisel et al. [4] and its crystal structure determination [5], the MFI-type crystalline microporous metalosilicates are certainly one of the most studied zeolites both theoretically and experimentally. This is mostly owing to the thermal, mechanical and chemical stability, porous volume and surface area of this framework. The topology MFI is one of the end-members of the pentasil family [3], the other end-member corresponding to zeolite ZSM-11 is of topology MEL [2,3]. A MFI-type framework can be built using eight 5-membered ring units or T12-units which form chains running along [001]. These chains are connected along [010] to forms sheets parallel to the (b,c)-plane. These sheets are linked along a in such a way that neighboring layers are related by a rotation of 180 about a and shifted of ½ b [6]. The obtained 3D microporous structure possesses an interconnected system of two 10-membered rings channels, straight channels parallel to [010] which intersect the zig-zag channels along [001] [5].
* Corresponding author. E-mail address: [email protected] (J.-L. Paillaud). http://dx.doi.org/10.1016/j.solidstatesciences.2016.06.009 1293-2558/© 2016 Elsevier Masson SAS. All rights reserved.
Early after the structure determination, a polymorphic monoclinic to orthorhombic phase transition was observed for calcined ZSM-5 type materials [7]. This transition is detected, below 363 K whatever the chemical composition of the framework [5,8], the number of defect density [9], the temperature [10e14], or the nature and amount of adsorbed species in the pores [15]. This phase transition is reversible and can be explained by a mutual shift of successive (010) pentasil layers along [001], associated with the distortions of 4 out of the 12 non-equivalent crystallographic T sites connecting the pentasil layers [12,16]. Note that this orthorhombic 4 monoclinic phase transition was also observed at low temperature (~175 K) on highly siliceous as-synthesized MFI-type zeolite [17]. Recently, in situ high-temperature synchrotron X-ray powder diffraction data have demonstrated a tricritical phase transition between the ferroelastic [14] (monoclinic) and the paraelastic (orthorhombic) phase in conventional ZSM-5 zeolite [18]. This last experimental fact was connected to the previously discovered strain as an unusual deformation field distribution determined by coherent X-ray diffraction imaging [19]. Conventional MFI-type zeolites are materials whose order of magnitude of the crystals size is in micron scale and, to our knowledge, no crystallographic study has so far reported the orthorhombic 4 monoclinic phase transition in the case of nanosized MFI-type zeolites. In this Communication, in situ high-
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Fig. 1. 1H decoupled 29Si MAS NMR spectra at 293 K, 373 K and after cooling down to 293 K of calcined a) micro-, b) nanosized silicalite-1 and c) pure siliceous MFI-type nanosheets.
temperature powder X-ray diffraction (PXRD) data collection and 29 Si MAS NMR have been performed on conventional calcined micro- and nanosized silicalite-1, pure silica form of ZSM-5 zeolite, as well as on pure silica MFI-type nanosheets to determine the lattice parameter variations and the local order as a function of the particles size in the temperature range 293e473 K. 2. Experimental In the present work, the micro-sized silicalite-1 sample was prepared in a classical fluoride medium [20] and nanosized silicalite-1 was prepared in a non fluorinated media as described by Lew et al. [21]. The pure silica MFI-type nanosheets were synthesized with a diquaternary ammonium surfactant as structure directing agent following the method recently reported by Kabalan et al. [22,23] based on the recipe reported by Ryoo et al. after the discovery of aluminum-containing MFI-type nanosheets [24]. Before the PXRD and solid state NMR experiments, the samples were calcined in a muffle furnace at 823 K during 8 h in air to remove all organic moieties occluded in the pores. The evolutions of the PXRD patterns with temperature were followed on a PANanalytical X’Pert PRO MPD diffractometer (CuKa1 radiation) equipped with an X’Celerator real-time multiple strip detector and an Anton Paar HTK1200 heating chamber in flat plate reflection geometry (sample cup made of Al2O3). Unit cell €r algorithm (DICVOL) parameters were determined from the Loue [25] implemented in the Stoe Win XPOW software [26]. For the solid state NMR study, samples were packed in a 7 mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 4 kHz. 1H decoupled 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) spectra were recorded at ambient temperature (293 K) on a Bruker Avance II 300 spectrometer (B0 ¼ 7.1T) at 59.6 MHz. These experiments were performed with a p/6 pulse duration of 2 ms and a recycle delay of 80 s. The spectra recorded at 373 K were performed with the same acquisition conditions on a Bruker Avance II 400 spectrometer (B0 ¼ 9.4 T) at 79.5 MHz equipped with a variable temperature control unit BVT3000 (278e473 ± 0.2 K). 29Si chemical shifts are relative to TetraMethylSilane (TMS, 0 ppm). 3. Results and discussion Proton decoupled 29Si MAS NMR spectra of the calcined silicalite-1 zeosil prepared with different particles sizes and morphologies are presented in Fig. 1. The spectra recorded at 293 K of the calcined silicalite-1 zeosil prepared by the conventional
Fig. 2. PXRD patterns waterfall plots showing the monoclinic 4 orthorhombic phase transition for calcined pure silica a) nano- and b) microsized silicalite-1 in the 22.5e25.0 2q angular array and collected between 299 and 473 K. Black Miller indices are in space group P21/n, the blue ones are in space group Pnma. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
I. Kabalan et al. / Solid State Sciences 58 (2016) 111e114
method, leading to 15e25 mm length crystals (Fig. A1a), display at least 10 resolved resonances of variable intensity, detected between 108 and 118 ppm corresponding to Q4 sites (Si(eOSi)4) (Fig. 1a). They correspond to the 24 non equivalent crystallographic T sites expected for a monoclinic framework structure of the calcined silicalite-1 zeolite described in space group P21/n [7] and in agreement with already published 29Si MAS NMR data [10,27]. The sharpness of the resonances indicates a well crystallized sample. Decreasing the crystal size of silicalite-1 to about 70 nm (Fig. A1b) of silicalite-1 significantly broadened the resonances leading to, at least, only 8 resolved resonances in the same chemical shift range (Fig. 1b). Indeed, decreasing the crystal size induces a distribution of SieO bonds and SieOeSi angles that broadens the lines. The same observation was recently reported for nanosized silicalite-1 prepared in fluoride media [28]. Further huge broadening is noted upon changing the morphology to lamellar as we already stated it (Fig. 1c, Fig. A1c) [22]. In the latter case, a signal is detected around 103 ppm corresponding to Q3 (HOeSi(eOSi)3) groups i.e., principally, in this case, to silanol groups on the surface of the nanosheets. No Q3 were detected for the two others samples indicating the absence of defects. Upon heating at 373 K, the corresponding spectra are displayed on Fig. 1. At 373 K, the 29Si MAS NMR spectrum of the calcined conventional sample presents at least 9 resonances in the 110 to 118 ppm range (Fig. 1a). The spectrum is significantly different from the one recorded at ambient temperature. The well resolved resonance at 109 ppm has disappeared. Indeed, for ZSM-5 zeolites, a temperature induced structural phase transformation from monoclinic to orthorhombic is mentioned several times in the literature upon heating [13,16,27], the critical temperature depends on the Al contents and explains the difference between 29Si MAS NMR spectra below and above the phase transition. Despite a clear broadening of the lines (Fig. 1b), this transformation also occurs for MFI-type nanocrystals. After cooling down the samples to 293 K, the 29Si MAS NMR spectra are superimposed to the one recorded
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before heating. Consequently, this phase transition is perfectly reversible as already known for conventional silicalite-1 even in the case of nanocrystals. It is worthy to note that from the 29Si MAS NMR spectra of the siliceous MFI-type nanosheets sample before and after heating at 373 K, despite the apparent similarity of the two spectra, it is not possible to conclude about the existence of a phase transition since the resolution is particularly low (Fig. 1c). In order to check this point, we undertook a X-ray study of the three samples at variable temperatures. Monitoring PXRD patterns as a function of temperature allowed us to follow the variation of the unit cell parameters for micro- and nanosized silicalite-1 (Table A1 and Fig. A2). On Fig. 2, due to the crystal size effect, an enlargement of the reflections width for the nanosized crystals is observed. The monoclinic 4 orthorhombic phase transition is less precise (Fig. 2a) in comparison with the conventional micro-sized silicalite-1 (Fig. 2b) but in both cases, it occurs at about 363 K ± 10 K. Accordingly, under warming, the doublet of reflections of indices 1 13 þ 1 33 and 133 þ 313 in space group P21/n progressively move into a single visible reflection of indices 133 þ 313 indexable in space groups Pnma (12 silicon T sites) or P212121 (24 silicon T sites) (Fig. 2 and Table A1). The difference in crystal size for these two samples from 15e25 mm to 70 nm does not seem modify the shift of the successive pentasil layers as the phase transition occurs at approximately the same temperature. Aside less marked change in the case of nanosized due to a greater inaccuracy on the location of the reflections, the evolution of the different unit cell parameters follows the same tendency for micro- and nanosized silicalite-1 (Table A1 and Fig. A2). This result is in agreement with those obtained previously in the case of conventional silica-rich ZSM-5 and silicalite-1 zeolites [13,14,16,27,29]. In the case of the nanosheets the evolution of the powder XRD patterns with temperature is different. From Fig. 3, no change of the lines shape and width is observable over the entire temperature range, i.e. 298e773 K. The temperature dependence of the PXRD
Fig. 3. Evolution of the powder XRD patterns as a function of temperature for pure siliceous MFI-type nanosheets. An offset has been added to the intensities at each step for clarity. The insets show magnifications of the 22.5e25.0 (left) and 25.4e25.7 2q (right) angular arrays showing the negative thermal expansion of the lattice without change of the line width for the nanosheets. The * indicate the position of the 012 reflection of a/Al2O3 (sample holder) for which the evolution of the position is in accordance with an expected weak positive thermal expansion in this temperature range [32].
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shows only a gradual and regular shift of all the diffraction peaks towards lower angle values when the temperature increases (Fig. 3). This is characteristic of a negative thermal expansion of the lattice as observed for conventional silicalite-1 above 373 K [29]. Even if the lines width is wider in the case of the nanosheets, one would expect an evolution of the line broadening for some lines during a monoclinic 4 orthorhombic or orthorhombic 4 monoclinic symmetry change but this is not observed even in the 22.5 e25.0 2q range whatever should be the direction of the transformation. An explanation may be given by the nanosheets thickness. Indeed, the overall thickness of the lamellar stacking was measured between 10 and 40 nm (Fig. A1c), the thickness of each individual nanosheet being of the same order of magnitude as the unit cell parameter b of the MFI topology, i.e. less than 2 nm [22e24,30]. Due to the particularly low resolution of the PXRD patterns, it is not possible to distinguish here a monoclinic or orthorhombic lattice. In conventional zeolites (micro- or nanosized) for which the crystal thickness contains tens, hundreds or thousands of unit cells, monoclinic and orthorhombic domains may coexist. It has been shown from single crystals that, upon cooling, the orthorhombic form of HZSM-5 obtained above the critical temperature switches into a combination of twin domains with a monoclinic symmetry [18,31]. In the case of nanosheets the presence of multidomains is not possible, monoclinic, orthorhombic or other, and could explain the absence of transition in the temperature range used in this study. Acknowledgements We thank Ludovic Josien and Loic Vidal from IS2M for their assistance with electronic microscopy. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2016.06.009. References [1] R.J. Argauer, G.R. Landolt, Crystalline zeolite ZSM-5 and method of preparing the same, in: Mobil Oil Corp, USA, 1972. [2] C. Baerlocher, L.B. McCusker, B. Olson, W.M. Meier, Atlas of Zeolite Framework Types, Elsevier, Boston; Amsterdam, 2007. [3] C. Baerlocher, L.B. McCusker, Database of Zeolite Structures, http://www.iza-
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