Article pubs.acs.org/JPCC
Thermal Collapse of Single-Walled Alumino-Silicate Nanotubes: Transformation Mechanisms and Morphology of the Resulting Lamellar Phases Cristina Zanzottera,† Aurélie Vicente,‡ Marco Armandi,§ Christian Fernandez,‡ Edoardo Garrone,† and Barbara Bonelli*,† †
Department of Applied Science and Technology and INSTM Unit of Torino-Politecnico, C.so Duca degli Abruzzi 24, Politecnico di Torino, I-10129 Torino, Italy ‡ Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 bd du Maréchal Juin, 14050 CAEN, France § Center for Space Human Robotics @ Polito, Istituto Italiano di Tecnologia, Corso Trento, 21, 10129 Torino, Italy ABSTRACT: The thermally induced structural transformations are studied of three imogolite-type nanotube (NT) materials: (i) proper imogolite (IMO, (OH)3Al2O3SiOH) with outer surface covered by Al−OH−Al groups and inner surface lined by silanols); (ii) methyl-imogolite (Me-IMO, (OH)3Al2O3SiCH3), in which the inner surface silanols have been replaced by methyl groups while the outer surface is unchanged, and (iii) the material Me-IMO-NH2, obtained by grafting the outer surface of Me-IMO with 3-aminopropylsilane (3-APS). TG-MS analysis on the parent IMO shows only loss of water (up to ca. 700 K), whereas XRD indicates the formation of a lamellar phase because of the mutual reaction of inner silanols. With both Me-IMO and Me-IMONH2, mass spectrometry and NMR analysis reveal the occurrence of a more complex collapsing mechanism, basically due to the reaction of outer Al-OH groups and inner Si-CH3, following the cleavage of the NTs structure, yielding methane and transient Al-O-CH2-Si species. All three materials show a limited decrease in the interlayer distance caused by collapse as well as a substantial residual porosity. It is concluded that the layered structure can be conceived as consisting of an overall buckled structure, the strong strain within the silico-alumina layer of the single-walled NT providing the driving force against a complete flattening. As a minor feature, decomposition of perchlorate species to chloride anions with the release of molecular oxygen is observed with IMO species that are trapped during the synthesis at the narrow interpores cavities.
1. INTRODUCTION Imogolite, (OH)3Al2O3SiOH, is a naturally occurring aluminosilicate1 with a peculiar structure consisting of micrometers long nanotubes (NTs), with inner and outer diameters equal to ca. 1 and 2 nm, respectively. The outer surface of imogolite (IMO) consists of a gibbsitelike layer, where OH species are bridged over two Al adjacent ions, whereas the inner surface is lined with silanols (Scheme 1). Si and Al atoms are, respectively, in tetrahedral and octahedral coordination: IMO is one of the unique Q3(3Al) aluminosilicates2 where all SiO4 tetrahedra are linked to three Al and a single silanol group.2−4 As shown in Scheme 2, IMO NTs form intertwined bundles, where a pseudoregular arrangement is observed, close to either hexagonal or monoclinic. Three kinds of pores may thus exist: (i) those proper to NTs, ca. 1 nm wide, which become accessible to probe molecules after removal of naturally present water (pores A); (ii) smaller intratubes micropores, ca. 0.3 nm wide, not accessible even to water molecules (pores B); and (iii) larger slit-mesopores among bundles (pores C). Besides occurring ubiquitously as a minor component of soils, IMO can be synthesized via a sol−gel process.5−9 © 2012 American Chemical Society
Functionalization of both inner and outer surface of IMO NTs is actively pursued, owing to possible applications of IMOderived materials, for example, as humidity sensor,13−15 nanofiller in new composites with increased mechanical and optical properties,16−19 inorganic support for biomolecules,20−22 catalyst,9,23,24 and gas25−27 and ion adsorbent.28−32 Recently, a new type of IMO, hereafter referred to as Me-IMO, has been synthesized in which the inner surface is fully methylated, then corresponding to the chemical formula (OH)3Al2O3SiCH3 (Scheme 3a).10,11 This Me-IMO system forms several micrometers long NTs with an inner diameter of ca. 2 nm and an outer diameter of ca. 3 nm, thus significantly larger than in IMO. It is worth noting that such larger NTs probably imply an elliptic rather than circular section, a flattening phenomenon that preludes to the formation of layered structures when the NTs are subjected to thermal treatments.12 As a consequence of the larger outer diameter, the Received: September 12, 2012 Revised: October 18, 2012 Published: October 19, 2012 23577
dx.doi.org/10.1021/jp3090638 | J. Phys. Chem. C 2012, 116, 23577−23584
The Journal of Physical Chemistry C
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Scheme 1. Section of a Natural Imogolite Nanotube (IMO, chemical formula: (OH)3Al2O3SiOH) with an Inner Diameter of ∼1.0 nm1 a
Scheme 3. Section and Lateral View of a Methylated Imogolite Nanotube (Me-IMO, chemical formula (OH)3Al2O3SiCH3) with an Inner Diameter of ∼2.0 nm10 a
a
After functionalization of Me-IMO with 3-APS, the material referred to as Me-IMO-NH2 is obtained12 with an outer surface carrying mono-, di-, and three-legged aminopropyl chains, some of which probably bent on the surface (after ref 12).
referred to as Me-IMO-NH2.12 The inner surface of this material contains only Si-CH3 groups, whereas the outer surface is more complex due to the presence of mono-, di-, and threelegged aminopropyl species, some of them doubly bound to the surface (Scheme 3b), and to the presence of residual aluminols. The structure of IMO shown in Scheme 1 is unstable above ca. 570 K because a transition to a lamellar structure occurs at this temperature. In a seminal paper, MacKenzie et al.4 envisaged two possible mechanisms for such a transition, which lead, in principle, to different lamellar structures (Scheme 4 a,b). In both cases, the starting point is the cleavage of the NTs probably triggered by the loss of some external Allinked OH groups.33 In the first mechanism, condensation of silanols takes place across the NTs diameter, causing the tubes to flatten with the formation of the repeating sequence: Al−O− (Si−O−Si)−O−Al (Scheme 4 a). In the second mechanism, the cleavage occurs along the NTs, which then unfold. As a consequence, hydroxyl elimination takes place between silanol and aluminol groups of two adjacent NTs, which then eventually condense into a lamellar structure featuring the repeating sequence: O−(Si−O−Al)−O (Scheme 4b). This latter mechanism was considered to be unlikely on the basis of both theoretical calculations and structural considerations.4 Recent data about dehydration, dehydroxylation, and partial rehydroxylation of IMO up to 673 K by Kang and coworkers support this view.34 Note that, currently, the layers in the lamellar structure are assumed to be fully flat, as described in Scheme 4. In the present article, the subject of structural collapse due to thermal treatment is extended to the materials that recently became available, that is, Me-IMO and Me-IMO-NH2. We report, on the basis of experimental results concerning solidstate NMR (nuclear magnetic resonance), powders XRD (Xray diffraction), TG-MS (thermogravimetry−mass spectrometry), and FT-IR (Fourier transform infrared) spectroscopy that these materials do show an irreversible collapse phenomenon, just as IMO does. The question addressed is whether the mechanism proposed for IMO also holds for surfaces with a different nature. Besides, some morphological features of the lamellar phases, created from Me-IMO, Me-IMO-NH2, and
a
Green circles: Al atoms; grey circles: OH groups; magenta circles: Si atoms; blue circles: O atoms.
Scheme 2. Sketch of the “Ideal” Hexagonal Packing of Imogolite Nanotubes and of the Three Kind of Pores Occurring in Imogolite Bundlesa
a
Pores A: ca. 1 nm wide, proper to NTs; pores B: smaller inter-tubes micropores, ca. 0.3 nm wide; and pores C: larger slit-mesopores among bundles.
intertube B pores are also larger and may become accessible to small molecules. Other modifications of the inner surface through classical postsynthesis functionalization have also been proposed.13 Moreover, the outer surface of Me-IMO has been recently functionalized by means of 3-aminopropylsilane (3-APS), leading to a new, basically hydrophobic, material, hereafter 23578
dx.doi.org/10.1021/jp3090638 | J. Phys. Chem. C 2012, 116, 23577−23584
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Scheme 4. Two Collapsing Mechanisms Proposed for Imogolite by MacKenzie et al. (after ref 4)
IMO, are discussed. A future paper will deal with the acidity of the novel species of silico-alumina given rise by collapse.
method, and their porous volume (Table 1) (Quantachrome Autosorb 1C; the accuracy of the pressure transducer is 0.11%
2. EXPERIMENTAL 2.1. Materials. All ACS grade reagents from Sigma-Aldrich were used. A reference sample of IMO was synthesized as described in ref 8. Me-IMO was synthesized using Al-secbutoxide as source of aluminum and TEMS (triethoxymethylsilane) as a source of silicon, with molar ratio Al/Si 2:1. The obtained sol was dialyzed against deionized water for 4 days.10,11 For the synthesis of Me-IMO-NH2, Me-IMO was first dried at 423 K in vacuum for 4 h to remove moisture before grafting with 3-APS. 3-APS was then added to a stirred suspension of Me-IMO in anhydrous toluene (ca. 60 mL) in the molar ratios Me-IMO/3-APS 1:0.3. The resulting mixture was then refluxed at 373 K for 12 h under a nitrogen atmosphere. The slurry was cooled to room temperature and washed with toluene, and the product was then filtered and dried at room temperature.12 The morphological and textural features of the samples studied are reported in the original papers, to which the reader is referred.8,10,12 All samples were subjected to two different treatments: (i) under vacuum outgassing at 423 K, yielding dehydrated samples with “standard” NTs structure, indicated hereafter as IMO-s, Me-IMO-s, and Me-IMO-NH2-s and (ii) under argon flow at 773 K, yielding collapsed (lamellar) samples, indicated with an ending -c. 2.2. Methods. XRD patterns of powder samples were obtained by a X’Pert Phillips diffractometer using Cu Kα radiation in the 2.5−20° (2θ angle) range (step width = 0.02°). Samples thermal stability under Ar flow was studied by thermogravimetric (TG) analysis coupled to mass spectrometry (MS) on an SDT 2960 DTA/TGA-MS apparatus (heating rate: 10 degrees min−1). N2 isotherms were measured at 77 K on dehydrated samples before and after collapsing to determine their specific surface area (SSA), according to the Brunauer−Emmett−Teller (BET)
Table 1. Some Textural Features of the Samples As Derived from N2 Isotherms at 77 K and Powder XRD Patterns SSA (m2 g−1)a
Vtot (cm3 g−1)b
Vmicro (cm3 g−1)c
d100 (nm)d
IMO-s IMO-c Me-IMO-s
362 197 665
0.26 0.16 0.39
0.04 0.02 0.09
2.31 2.14 3.02
Me-IMO-c
530
0.30
0.13
2.62
Me-IMONH2-s Me-IMONH2-c
518
0.35
0.04
3.05
450
0.26
0.12
2.64
sample
ref 8 8 this work this work this work this work
a Specific surface area as calculated according to the BET (Brunauer− Emmett−Teller) method through multipoint calculation by choosing the result given by the best linear-fit in the 0.1 to 0.2 P/P0 range; the resulting estimated error is 3% of the obtained value. bAs derived from N2 adsorption isotherm at 77 K. cAs derived by applying the αs method. dInter-reticular distance, as derived from XRD patterns; the estimated error is ±0.01 nm.
in the whole pressure range). To check reproducibility, we performed two consecutive adsorption/desorption cycles with IMO-s. Solid-state MAS NMR spectra were recorded on a Bruker Avance-400 (9.4 T) spectrometer using 4 mm-OD zirconia rotors and a spinning frequency of 12 kHz. {1H}-13C crosspolarization (CP) MAS experiments were performed using a contact time of 2 ms and a recycle time of 1 s. Single-pulse excitation (30° flip angle) and 30 s of recycling delay were used for 29Si MAS NMR experiments. Tetramethylsilane (TMS) was used as chemical shift reference for 1H, 13C, and 29Si nuclei. 27 Al MAS NMR measurements were performed using a selective pulse (π/12 flip angle) and 1 s recycling delay. A 23579
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0.1 M Al(NO3)3 solution was used as chemical shift reference for the 27Al. IR spectra of self-supporting wafers were collected on an Equinox 55 spectrophotometer equipped with MCT (mercury cadmium telluride) cryodetector. Wafers were treated under high vacuum (residual pressure
