Article pubs.acs.org/JPCC
Physico-Chemical Properties of Imogolite Nanotubes Functionalized on Both External and Internal Surfaces Cristina Zanzottera,† Aurélie Vicente,‡ Edvige Celasco,† Christian Fernandez,‡ Edoardo Garrone,† and Barbara Bonelli*,† †
Department of Applied Science and Technology, Institute of Chemistry & INSTM Unit of Torino-Politecnico, C.so Duca degli Abruzzi 24, Polytechnic of Turin, I-10129, Turin, Italy ‡ Catalysis and Spectrochemistry Laboratory, ENSICAEN, University of Caen, CNRS, 6 bd du Maréchal Juin, 14050 CAEN, France ABSTRACT: Using a methylated Si precursor instead of tetraethoxysilane (TEOS), methyl-imogolite (Me−IMO), a nanotube material with formula (OH)3Al2O3SiCH3 is obtained in place of the standard imogolite (OH)3Al2O3SiOH (IMO).21 Postsynthesis grafting of the outer surface of Me−IMO with 3aminopropyltriethoxysilane (3-APS) yields a new hybrid material (Me−IMO−NH2), with an entirely hydrophobic inner surface and a largely aminated outer surface. In this paper, the structure and stability of Me−IMO−NH2 are studied in detail and compared with those of Me−IMO by means of (i) X-ray photoelectron spectroscopy (XPS), confirming the surface chemical composition of Me−IMO− NH2; (ii) 1H, 13C, 27Al, 29Si, and heteronuclear correlation (HETCOR) 1H−13C magic angle spinning nuclear magnetic resonance (MAS NMR) experiments, providing evidence for the occurrence of grafting and yielding an estimate of its extent; (iii) infrared spectroscopy, showing that most terminal −NH2 groups are protonated; (iv) X-ray diffraction (XRD) measurements yielding information on the long-range order; and (v) N2 adsorption at −196 °C, yielding specific surface area and pore size distribution. Reaction with 3-APS brings about a limited loss in microporosity, probably caused by functionalization at the mouth of pores, and an increased disorder in the alignment of nanotubes, with neither a big loss of specific surface area nor a sizable change in the distance between nanotubes. As a whole, imogolite-type nanotubes appear to be rather prone to functionalization, which seems to allow the possible tailoring of the properties of both inner and outer surfaces.
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INTRODUCTION Imogolite is a hydrous aluminosilicate with gross chemical composition (OH)3Al2O3SiOH made of hollow nanotubes organized in a porous network of interwoven bundles. Natural imogolite was discovered in the 1960s as an impurity in allophane.1 Its structure consists of a single continuous gibbsite Al(OH)3 sheet, in which, on one side only, three OH groups are substituted by an orthosilicate unit O3SiOH. Si−O bonds are shorter than Al−O bonds, so the gibbsite-like sheet curls up, forming nanotubes with an inner surface lined by silanols and an outer surface covered by bridged Al(OH)Al species (Scheme 1).2 Synthetic imogolite (IMO) was obtained almost simultaneously by Farmer et al.3,5 and Wada et al.,4 making this material available for various applications, such as anion/cation retention from water,6−10 catalysis,11,12 and gas adsorption, separation and storage.13−15 Natural imogolite nanotubes are several micrometers long with external and internal diameters of ca. 2 and 1 nm, respectively.1 The outer diameter of IMO is larger, ca. 2.7 nm. As nanotubes are organized in bundles, three kinds of cavities are present: (i) intratube pores (the voids of constituent © 2012 American Chemical Society
nanotubes, about 1 nm wide); (ii) intertube pores, i.e. spacings between three aligned tubes in a regular packing (0.3−0.4 nm wide); (iii) slit mesopores among bundles.14,15 Infrared (IR) spectroscopic results and catalytic characterization of the acidity of imogolite-based systems have been recently reported:16 as a hydrated aluminosilicate, the behavior of IMO markedly depends on thermal pretreatments. Molecular water is thoroughly removed only above 150 °C, when inner silanols become accessible to probes such as CO and NH3.16 Hydrophilicity of the inner surface is due to a silanol density of 9.1 OH nm−2,16,17 about twice the average value at the surface of hydrated amorphous silicas.18 Adsorption of probe molecules shows that the outer surface displays an amphoteric character, as expected on the basis of the acid/base properties of Aluminum hydroxide. At temperatures around 500 °C, the nanotube structure is lost with the formation of a lamellar phase.16,19 Received: February 6, 2012 Revised: March 5, 2012 Published: March 7, 2012 7499
dx.doi.org/10.1021/jp301177q | J. Phys. Chem. C 2012, 116, 7499−7506
The Journal of Physical Chemistry C
Article
interaction of CO2 with Me−IMO−NH2 will indeed be the subject of a following paper. Functionalization of a surface by means of alkoxy derivatives is a complicated process, especially when the amount of grafting substance is large.20,32,33 We take advantage in the present case of the fact that, contrary to nearly all cases in surface chemistry, one is dealing with an all-surface system (i.e Me−IMO), of which the gross formula is simply (OH)3Al2O3SiCH3. The full functionalization with NH2−(CH2)3−Si(OEt)3 could lead in principle to a substance with a defined gross formula, i.e. NH2− (CH2)3−Si(O)3Al2O3SiCH3. This helps in choosing the amount of 3-APS to use. The most evident choice is a molar ratio 1:1 between (OH)3Al2O3SiCH3 and 3-APS. Instead, a sample with a loading only ca. 1/3 of the APS amount was considered, for the following reasons: (i) inspection of Scheme 1 suggests that not all OH groups possibly react; (ii) the bulky aminopropyl groups are likely to cover the surface even at loadings lower than the theoretical one; (iii) a “diluted” and thus better defined grafted phase is preferable when carrying out structural and spectroscopic studies.
Scheme 1. Section of a Natural Imogolite Nanotube with an Inner Diameter of about 1.0 nm a
2. EXPERIMENTAL SECTION 2.1. Materials. Me−IMO was obtained as described in refs 21 and 22. The Me−IMO sample was dried at 150 °C in vacuum for 4 h to remove moisture before the treatment with 3-APS. To a stirred suspension of Me−IMO in anhydrous toluene (ca. 60 mL), 3-APS was added in the molar ratios Me− IMO: 3-APS = 1:0.3. The resulting mixture was then refluxed at 100 °C 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. In order to remove adsorbed water, the samples were pretreated at 150 °C under vacuum before most of the characterization experiments reported in the following. 2.2. Methods. XPS (X-ray photoelectron spectroscopy) spectra were recorded on a PHI 5000 Versa Probe equipment using a band-pass energy of 187.85 eV, a 45° take off angle and a X-ray spot size diameter of 100.0 μm. All solid-state MAS NMR (magic angle spinning nuclear magnetic resonance) spectra were recorded on a Bruker Avance-400 (9.4T) 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 recycling delay was used for 29 Si MAS NMR experiments. A two-pulse echo sequence with 1 s recycling delay was used for 1H MAS NMR measurements. The delay between the two pulses was set in order to suppress the broad signal due to water or strongly dipolar coupled hydroxyls. Tetramethylsilane (TMS) was used as chemical shift reference for 1H, 13C, and 29Si nuclei. 27Al MAS NMR measurements were performed using a selective pulse (π/12 flip angle) and 1 s recycling delay. A 0.1 M Al(NO3)3 solution was used as chemical shift reference for the 27Al. 2D heteronuclear correlation spectra (HETCOR) between protons and carbons was recorded using the cross-polarization condition above-mentioned. As the experiment was rotor synchronized, the spectra window in the proton dimension is exactly equal to the spinning frequency (12 kHz). XRD (X-ray diffraction) patterns of powder samples were obtained on a X’Pert Phillips diffractometer using Cu Kα radiation in the 2.5−20° 2θ range (step width = 0.02°).
a
The distance between two adjacent silanols in the same circumference is 0.26 nm, and 0.40 nm is the distance between two silanols of two adjacent circumferences.16
Chemical modification of both the inner and outer surface of imogolite is expected to yield interesting materials. As to the inner surface, functionalization may be achieved by either traditional postsynthesis modification with organosilanes in anhydrous medium20 or by direct synthesis by employing (EtO)3SiCH3 (triethoxymethylsilane, TEMS) as a starting reagent,21,22 which yields a material with the inner surface lined by methyl groups (Me−IMO). The literature also reports several attempts to modify the external surface. Due to the strong interaction between functionalities present there, it is difficult to disperse imogolite in organic solvents or hydrophobic polymer matrices. This notwithstanding, modified imogolite-based materials are expected to play a role in the synthesis of organic/inorganic hybrid materials. For instance, to enhance the blending of poly(methyl methacrylate) with imogolite, the outer surface was functionalized with octadecylphosphonic acid or methacryloyloxyethyl phosphate.23−26 Moreover, polyvinyl-alcohol (PVA)/imogolite hybrid systems can be obtained by a direct in situ synthesis.27 Modification of the imogolite outer surface was also recently attempted for catalytic applications in the dehydroxylation of olefins by means of the complex 3-APS (3-aminopropyltriethoxysilane)−OsO4;28 whereas a few years ago, the nonselective functionalization of both the inner and outer surface of imogolite was attained by grafting 3-APS.29,30 In this work a new hybrid nanotube material is described, hereafter referred to as Me−IMO−NH2, that is obtained by postsynthesis grafting with 3-APS of Me−IMO obtained by direct synthesis. Out of the many possible grafting agents, an amine-carrying group has been chosen, because this type of ligands offers reactive sites for the reversible adsorption of CO2, the selective adsorption of heavy metal ions, and provide grafting points for the deposition of metallic catalysts.28,31,32 The gas-phase 7500
dx.doi.org/10.1021/jp301177q | J. Phys. Chem. C 2012, 116, 7499−7506
The Journal of Physical Chemistry C
Article
BET (Brunauer−Emmett−Teller) specific surface area and pores size distribution were measured by means of N2 adsorption/desorption isotherms at −196 °C on a Quantachrome Autosorb 1C instrument. The non-local-density functional theory (NL-DFT) method was used to evaluate the pore size distribution (PSD) by applying a N2−silica kernel. Thermal stability of the samples was studied by thermo-gravimetric analysis (TGA) under N2 flow on a SDT 2960 instrument for differential thermal analysis (DTA) and thermo-gravimetry− mass spectrometry (TG-MS) analysis. IR spectra were collected on a Bruker Equinox 55 spectrophotometer equipped with a mercury−cadmium− telluride (MCT) cryodetector: self-supporting wafers were studied under a residual pressure
