Langmuir 2008, 24, 949-956
949
Nanostructure of Gel-Derived Aluminosilicate Materials Katalin Sinko´,*,† Nicola Hu¨sing,‡ Gu¨nter Goerigk,§ and Herwig Peterlik| Institute of Chemistry, L. Eo¨tVo¨s UniVersity, Pa´ zma´ ny st 1/a., H-1117, Budapest, Hungary, Institute of Inorganic Chemistry I, UniVersity of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany, Institute of Solid State Physics, Research Centre Ju¨lich, D-52425 Ju¨lich, Germany, and Institute of Material Physics, UniVersity of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria ReceiVed April 6, 2007. In Final Form: October 9, 2007 In the present work, aluminosilicate aerogels prepared under various conditions were compared with respect to their nanostructures and porosity. The purpose of this investigation was to find a suitable way to predict the final product structure and to tailor a required texture. Several Al and Si precursors (Al nitrate, Al isopropoxide, Al acetate, tetraethoxysilane (TEOS), and sodium silicate) were used in our examinations; the solvent content (water and alcohols), surfactants, as well as the catalysts were varied. In addition, the aerogels were subjected to various heat treatments. Hybrid aerogels were synthesized by the addition of different polymers (poly(acrylic acid), polyvinyl acetate, and polydimethylsiloxane). Aluminosilicate and hybrid aerogel structures were investigated by 27Al MAS NMR, SAXS, SEM, and porosity measurements. Loose fractal structures with a good porosity and high Al incorporation can be achieved from TEOS and Al nitrate or isopropoxide via a sol-gel preparation route. The use of Al acetate led to compact aerogel structures independently of the Si precursor, the pH, and the catalyst.
Introduction Aerogel-like materials exhibit very high surface areas and significant nanoporosities. Aluminosilicate aerogels with porous structures reveal very interesting physical properties and have many practical and potential industrial applications, including their use as acid catalysts due to their good chemical resistance1 as membranes for the separation of gas phases,1 purification of water and reactors,2 as adsorbents,3 as humidity control materials,4 and as starting materials for the synthesis of mullite ceramics.5,6 The thorough analysis of the porous structures of these aerogels plays an indispensable role in the specification of their applications. The aim of the present work was to investigate not only the influence of various synthesis conditions on the supramolecular three-dimensional architecture and the porosity of aluminosilicate aerogels but also to develop rule-of-thumb guidelines for the preparation of a material with a deliberate structure. For that, aluminosilicate gels were prepared by a wet chemical sol-gel method from several starting materials such as tetraethoxysilane (TEOS), water glass, inorganic Al salts (Al nitrate), and organic Al salts (Al isopropoxide and Al acetate). To induce elasticity into the network and a less fragile character, organicinorganic hybrid aerogels were also synthesized by the addition of organic polymers (linear and cross-linked poly(acrylic acid), polyvinyl acetate, and polydimethylsiloxane of various molecular weights) to the oligomeric inorganic (aluminosilicate) component. These wet alco- and hydrogels were dried under supercritical * Corresponding author. Tel.: 36 1 2090555; e-mail:
[email protected]. † L. Eo ¨ tvo¨s University. ‡ University of Ulm. § Research Centre Ju ¨ lich. | University of Vienna. (1) Jones, S. D.; Pritchard, T. N.; Lander, D. F. Microporous Mater. 1995, 3, 419-425. (2) Pierre, A. C. Ceram. Int. 1997, 23, 229-238. (3) Gash, A. E.; Tillotson, T. M.; Satcher, J. H.; Hrubesh, L. W.; Simpson, R. L. J. Non-Cryst. Solids 2001, 285, 22-28. (4) Okada, K.; Tomita, T.; Yasumori, A. J. Mater. Chem. 1998, 8, 28632867. (5) Schneider, H.; Okada, K.; Pask, J. A. Mullite and Mullite Ceramics; John Wiley: New York, 1994. (6) Boccaccini, A. R.; Bu¨cker, M.; Kahlil, T. K.; Ponton, C. B. J. Eur. Ceram. Soc. 1999, 19, 2613-2618.
conditions to obtain aerogels. The aerogels were subjected to treatments with several surfactants and to heat treatments. The surfactant molecules were used in this work as structure-directing agents. The aim of their application was to investigate their effect on the pore structure of materials. The amorphous structures of the aerogels were investigated by magic angle spinning nuclear magnetic resonance spectroscopy (27Al MAS NMR), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and porosity measurements. In this work, our experimental results were summarized to rationally relate the synthesis parameters to the final material structure. These results can be used to recognize the interactions between the structures and the synthesis parameters and to predict the final product texture. Experimental Procedures Preparation Methods. Sample 1. In the condensation approach, starting from Al isopropoxide (Al(OCHCH3CH3)3, Aldrich) and TEOS (, Aldrich), TEOS should separately be dissolved and prehydrolyzed because of the very fast hydrolysis of Al isopropoxide in aqueous solutions. The prehydrolysis was performed in ethanol and in a small volume of 2 M HNO3 aqueous solution at 75 °C for 15 min (1 mol of water/Si). Al isopropoxide and H2O were added to this solution of TEOS in ethanol. A homogeneous, optically clear gel formed in a few minutes at 60 °C. The chemical compositions of the sol-gel processes are summarized in Table 1. Sample 2. Using Al salts, aluminosilicate gels were prepared in a one-step procedure from TEOS and the inorganic Al salt precursor (Al(NO3)39H2O, Fluka) in an organic medium (1-propanol) at 75 °C. Because of the acidic hydrolysis of Al(III) ions, it was not necessary to apply any catalyst during the gelation. Several kinds of catalysts are generally used in the sol-gel process of aluminosilicate systems.7,8 TEOS and aluminum nitrate were dissolved in the organic solvent at 75 °C for 1 h to obtain a homogeneous solution. The gelation was performed by refluxing at 80 °C for 10-11 h. Sample 3. The application of Al acetate (Al(OH)(OOCCH3)2, Fluka) in a sol-gel process is a difficult task due to its poor dissolution in water or alcohols. Basic Al acetate can be dissolved only in strongly (7) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, 1990. (8) Miller, J. B.; Tabone, E. R.; Ko, E. I. Langmuir 1996, 12, 2878-2880.
10.1021/la702525x CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007
950 Langmuir, Vol. 24, No. 3, 2008
Sinko´ et al. Table 1. Chemical Compositions of the Sol-Gel Processes
aluminum precursors (mol) sample 1 sample 2 sample 3 sample 4 sample 5 + polymer (0.3-3.0 mol)b a
silicon precursors (mol)
organic solvent (mol)
Al(OisoPr)3 1 Al(NO3)3 1 Al(OH)(Ac)2
TEOS 1 TEOS 1 TEOS
ethanol 25 1-propanol 20 ethanol
1 Al(OH)(Ac)2 1 Al(NO3)3
1 Na2SiO3 1 TEOS
20 1-propanol
1
1
20
water (mol)
catalyst (mol) HNO3 0.1
6 9a
H+ (pH ≈ 1) or OH- (pH ≈ 11)
20 NaOH 5.5
180 9a
Crystalline water of Al nitrate. b Polymers: polyvinyl acetate, hydroxyl-terminated polydimethylsiloxane, and poly(acrylic acid).
acidic (pH e 1) or basic (pH g 11) media. The required pH values were obtained by solutions of HNO3, HClO4, or NaOH. At pH ≈ 1, an optically clear, homogeneous gel sample formed in 24 h at 75 °C, after being mixed with an alcoholic solution of TEOS, while at pH ≈ 11, strongly opaque but homogeneous hybrid gels formed in a few minutes even at 25 °C. Sample 4. Using water glass instead of TEOS as the Si precursor, basic conditions had to be applied (pH g 11) to avoid phase separation of (poly)silicic acid. Al(OH)(OOCCH3)2 was dissolved in an aqueous solution of NaOH (4.0 M) at ambient temperature and pressure to obtain a homogeneous solution. After the addition of the diluted (4 mass %) water glass solution to the solution of Al acetate, an aqueous suspension formed. To obtain hydrogels, the main part of the solvent was evaporated at 60 °C in vacuum. Sample 5. The basic principle behind the preparation of composite materials was to react the inorganic and organic components as units of high molecular weight. The inorganic components used in these experiments were prehydrolyzed and -condensed (partly gel state) systems, and the organic components were polymeric systems. The inorganic component having a three-dimensional gel structure cannot react with an organic component. The components were separately dissolved in 1-propanol. TEOS and aluminum nitrate were dissolved in the organic solvent at 75 °C for 1 h to obtain a homogeneous solution and to start the prehydrolysis and condensation procedures. The gelation process was interrupted prior to the gel point. The solutions of the inorganic components were added into the solutions of the organic polymers at 75 °C via a dropping funnel. A homogeneous solution formed, and the gelation took place within 24 h. The mass ratios of polymer/TEOS were varied from 0.3 to 3.0. The organic components of the experiments were polyvinyl acetate (Mowilith 70, Hoechst) with a molecular weight of about 150 000 g mol-1, hydroxyl-terminated polydimethylsiloxane (110 000, 42 500, and 550 g mol-1 puriss Aldrich and 82 000 and 5180 g mol-1 Wacker), and poly(acrylic acid) (Carbopol 907, BF Goodrich). The wet gels were dried under supercritical conditions. After being washed with methanol, the gels were put into a steel autoclave and washed with liquid carbon dioxide for 2-3 days at 284 K and 6.0-7.0 MPa. After completion of the solvent exchange process, the temperature in the autoclave was slowly raised to 313 K, while the pressure was kept at 10 MPa. The supercritical carbon dioxide fluid was slowly released from the autoclave by depressurizing to an ambient pressure for ≈20 h. Opaque aerogel samples could be obtained. The effect of several surfactants and silylation agents were investigated on the structure of aerogels. The wet alcogels were subjected to the solutions of surfactants and silylation agents. Polyoxyethylene(10) cetyl ether, C16H33(OCH2CH2)10OH (Brij 56), and a copolymer of ethylene and propylene oxides, EO20PO70EO20 (P 123), were applied as surfactants, and trimethylchlorosilane, (CH3)3SiCl (TMCS), and hexamehyldisilazane, (CH3)3SiNHSi(CH3)3 (HMDS), were used as surface silylation agents. Characterization Methods. SAXS measurements were conducted using a laboratory instrument. The SAXS equipment was operated with a 12 kW rotating anode X-ray generator and a pinhole X-ray camera with a variable distance (20.5-98.5 cm) from the sample
to the two-dimensional detector (Bruker, AXS). The gels were covered in vacuum tight foil. The two-dimensional spectra were corrected for parasitic pinhole scattering, as well as for the foil scattering. USAXS measurements were performed on the BW4 beamline of HASYLAB at DESY in Hamburg. In the evaluation of SAXS data, the fractal dimensions (d) can be obtained from the slope (µ) of SAXS curves in Porod’s region using a simple power law expression: I(q) ) ∼q-d.9,10 The evaluation method proposed by Freltoft et al.11 was used to interpret the scattering data for samples prepared from Al isopropoxide and Al acetate. The Freltoft et al. formula developed for the evaluation of the scattered intensity to the fractal structure of the aggregates is described in ref 11 I(q) ) υo2(Fs - Fo)2f2(q)S(q)
(1)
where I(q) indicates the scattering function, Fs is the scatteringlength density averaged over a particle of volume υo, Fo is the scattering-length density of the embedding medium, f(q) denotes the single-particle form factor, and S(q) defines the structure factor, obtained from additionally introducing a cutoff length ξ for the fractal objects11 f(q) )
1 υo(Fs - Fo)
∫F(r)e
iqr
dr
(2)
S(q) ) 1 + C(d - 1)Γ(d - 1)ξd (1 + q2ξ2)1/2 sin[(d - 1)arctan(qξ)] (3) qξ d-1 (1 + q2ξ2)d/2 where C is a constant, Γ is the gamma function, d is the fractal dimension, and ξ is the mass fractal correlation length, as it defines the average upper-limit length scale for the mass fractal domain.11 A modified Freltoft et al. approximation could fit most precisely to the SAXS curve for samples of Al nitrate. We modified the Freltoft et al. equation with introducing a new parameter, R, to the particle form factor, which describes the density fluctuations of the particles (i.e., a nonsmooth particle surface).12 In the form factor (f), Freltoft et al. used the Debye approximation for spherical particles f2(q) )
1 x2 2 2 1+ q rF 3
(
)
2
(4)
and the modified form factor (9) Porod, G. General theory. In Small Angle X-Ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: San Diego, 1982. (10) Schaefer, D. W.; Keefer, K. D. Phys. ReV. Lett. 1984, 53, 1383-1386. (11) Freltoft, T.; Kjems, J. K.; Sinha, S. K. Phys. ReV. B.: Condens. Matter Mater. Phys. 1986, 33, 269-275. (12) Sinko´, K.; Torma, V.; Kovacs, A. Mater. Sci. Forum. 2006, 514-516, 1191-1195.
Gel-DeriVed Aluminosilicate Materials f2(q) )
1 x2 2 2 q rF 1+ 3
(
Langmuir, Vol. 24, No. 3, 2008 951
)
2-R
(5)
where rF is the Freltoft et al. radius, in which the radius of elementary units was calculated from r ) rF2/x0.22. A characteristic parameter for the fractal behavior of the elementary units can be derived from the value of R du ) 2(2 - R)
(6)
where du defines the fractal dimension of elementary units.12 The surface area and porosity of aerogels were characterized by nitrogen sorption analysis at 77 K on an autosorb computer controlled surface analyzer (AUTOSORB-1, Quantachrome or ASAP 2010 Micrometrics). Each sample was degassed at 60 °C for 12 h prior to analysis. The five-point BET surface area was calculated assuming a value of 0.162 nm2 for the cross-sectional area of the nitrogen molecules. The BJH model and the desorption isotherm data were used to calculate the cumulative pore volume as a function of pore diameter. SEM measurements were obtained on a JEOL JSM-5600LV microscope for the observation of morphology of the aerogels. The SEM samples were sputtered with gold. 27Al MAS NMR measurements were performed at room temperature, using a Bruker Avance DRX-500 NMR spectrometer (11.70 T) at 130.3182 MHz. A 52 mm BB{1H}CP/MAS probehead was employed with a 7 mm ZrO2/Kel-F rotor. After a 1 µs X pulse (flip angle was ca. 10° to accommodate the major contribution of the (1/2, -1/2) transition in the observed centerband and the welldetermined signal response to the rf pulse), 4096 data points were acquired during 31 ms, followed by a 1 s relaxation delay. A total of 128 FID was accumulated. The typical spectral width was 65 360 Hz (501 ppm), the line broadening parameter was 30 Hz, and each spectrum was comprised of 2000 data points. All NMR spectra were recorded at MAS conditions to reduce the nuclear quadrupolar interactions to the second-order quadrupole shift level. The spinning speed of 4000 Hz caused spinning sidebands with a periodicity of 30 ppm in the spectra. This spinning rate increased the spectral resolution but could not resolve completely the centerbands of the components. The analysis of the overlapping signals with the computer simulated powder pattern fit was not possible because of the featureless overlapping Gaussian shape of the centerband signals. In the absence of this characteristic second-order quadrupole powder pattern, we could not estimate Cq (the quadrupole coupling constant), ηq (the asymmetry parameter), or the second-order quadrupole shift values. The observed overlapping signals (except the spinning sidebands) were decomposed to Lorentz curves by Bruker WINFIT software on the basis of typical chemical shifts known from the literature. The obtained fitting parameters were the chemical shift, the half peak width, and the integral ratio of the components. The 27Al chemical shifts were recorded with respect to Al(H O) 3+ as 2 6 an external reference. Brinell hardness (HB) measurements were carried out on an automatic penetrometer (Labor MIM). The hardness values were determined by measurement of the depth of the indentation resulting from the load of a body with a precise geometry; the load was 466 mN.
Results Effect of Initial Materials on Porosity and Supramolecular Structure of Aluminosilicate Aerogels. As a first result, it can be seen that the starting materials significantly influence the gel structures. The gel preparations starting from Al nitrate or isopropoxide and TEOS yielded the highest porosity (Table 2). The average BET specific surface area measured in these series varied between 380 and 1050 m2 g-1. The usually published value for aerogels is between 200 and 700 m2 g-1.7,13,14 The (13) Komarneni, S.; Roy, R.; Sevaraj, U.; Malla, P. B.; Breral, E. J. Mater. Res. 1993, 8, 3163-3167.
Al incorporation into the silicate network did not decrease the porosity in these series, contrary to several published results.14-16 The fractal structure and the small elementary units resulted in a high porosity. The SAXS curves (Figure 1) and the values of the mass fractal dimension (d ) 2.2-2.3, Table 2) prove the fractal characters of these aluminosilicate structures. The SAXS data evaluated by the modified Freltoft et al. approximation (du ) 2.1-2.2) indicate a fractal character even for the elementary units of aluminosilicate aerogel systems prepared from Al nitrate.12 The primary units of aluminosilicate aerogels are generally dense particles.17,18 The primary building units of the Al isopropoxide gels are much smaller and more compact than those of the Al nitrate gel. The compact structure has been verified by the accuracy of the modified Freltoft et al. evaluation (du ) 4.0) and Al MAS NMR spectroscopy. For the latter method, the strong peak at 55 ppm indicates a large content of tetrahedrally coordinated Al7,13,19 (25-35% of ∑Al content) (Table 3 and Figure 2, curve 1) for the sample prepared from Al isopropoxide. The tetrahedral Al atoms are on the inside of the particles; the octahedrally coordinated Al atoms detected at 4 ppm are bonded to the surface of particles by shared OH groups or oxygen bridges.20 The partly hydrolyzed Al(III) ions (at 0 ppm) are not connected by chemical bonds to the gel networksthey form weak associates. The highest Al incorporation with 70-90% of the total Al content could be achieved by the application of Al isopropoxide (Table 3 and Figure 2). Also, a very high Al incorporation (50-70% of the total Al content) formed in the gel samples of Al nitrate.19 For comparison, the aluminosilicate systems produced by traditional melting processes contain a maximum of 10% bonded Al content. Immiscibility gaps in silicate melts are known to have both stable and metastable upper critical solution temperatures.21 The gel systems obtained from Al acetate and TEOS or water glass solutions have compact structures with very small porosities. The reaction of Al acetate and TEOS leads to the formation of a randomly branched gel network in acidic medium (Table 2 and Figure 1). The numbers of Al(III) ions connected to the silica network are negligible in this aerogel systems (Table 3). The partly hydrolyzed Al(III) ions are inserted in the amorphous silica network as small weak associates. The gel structure derived from Al acetate and water glass is a composite system; it consists of a fractal-like branched aluminosilicate network and a nanosized Al-containing phase (Figure 1). Some ordering can be observed on its SAXS curve; on the basis of Al MAS NMR and XRD measurements, this ordering can be associated with a formation of crystalline zeolite-like Al-containing phase. In addition, this compact structure has been verified by the porosity data and the Al MAS NMR measurements, indicating only tetrahedrally coordinated Al atoms in both wet and dried gel systems prepared from Al acetate and water glass (Table 2 and Figure 2). Effect of Solvent Content on Porosity and Supramolecular Structure of Aluminosilicate Aerogels. The porosity (i.e., the specific surface area and the pore volume) are strongly influenced by solvent contents (Table 4). For example, the rise of solvent (14) Aravind, P. R.; Mukundan, P.; Pillai, P. K.; Warrier, K. G. K. Microporous Mesoporous Mater. 2006, 96, 14-20. (15) Hernandez, C.; Pierre, A. C. Langmuir 2000, 16, 530-536. (16) Dunphy, D. R.; Singer, S.; Cook, A. W.; Smarsly, B.; Doshi, D. A.; Brinker, C. J. Langmuir, 2003, 19, 10403-10408. (17) Schaefer, D. W.; Keefer, K. D. Phys. ReV. Lett. 1986, 56, 2199-2202. (18) Chaput, F.; Lecombe, A.; Dauger, A.; Boilot, J. P. Chem. Mater. 1989, 1, 199-201. (19) Lartiges, B. S.; Bottero, J. Y.; Derrendinger, L. S.; Humbert, B.; Tekely, P.; Suty, H. Langmuir 1997, 13, 147-152. (20) Sinko´, K.; Mezei, R.; Rohonczy, J.; Fratzl, P. Langmuir 1999, 15, 66316636. (21) Doremus, R.H. Glass Science; John Wiley: New York, 1994.
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Sinko´ et al.
Table 2. Effect of Starting Materials on Porosity and SAXS Data of Aluminosilicate Aerogelsa
starting materials
specific surface area (BET) (m2 g-1)
Al nitrate + TEOS Al isopropoxide + TEOS Al acetate + TEOS Al acetate + water glass
630 ( 1.6 820 ( 4.0 84 ( 2.0 10 ( 1.0
av pore diameterb (nm)
pore volumec (cm3 g-1)
mass fractal dimension (d)
primary particle radius (r) (nm)
12.0 ( 0.2 9.2 ( 0.05 5.0 ( 0.2
1.6 ( 0.1 2.0 ( 0.4 0.6 ( 0.1
2.27 ( 0.07d 2.20 ( 0.07e 1.70 ( 0.05e 2.55 ( 0.8e
3.2 ( 0.1d 1.1 ( 0.2e 1.2 ( 0.5e 1.8 ( 0.6e
fractal dimension of primary particles (du) 2.17 ( 0.1d 4.0 ( 0.7d
a Samples were prepared under acidic conditions with similar solvent contents and 1 mol ratio of Al/Si. b BJH adsorption average pore diameter (4V/A). c BJH adsorption cumulative pore volume of pores between 1.7 and 300.0 nm diameter. d Calculated by modified Freltoft et al. approximation. e Calculated by original Freltoft et al. approach.
Figure 1. SAXS curves for aluminosilicate aerogels prepared from various initial materials.
content by about 6-fold led controllably to a 3-fold increase of the specific surface area in the aluminosilicate fractal systems. The solvent content affects less the nanostructure (i.e., the mass fractal dimensions of network and the size of primary particles). Only the upper limit of the fractal domain size (ξ) increased significantly with the larger solvent content (Table 4). Very similar results were obtained in the investigation of both aerogel samples of Al nitrate and Al isopropoxide. Scanning electron micrographs displayed the growing porosity as a function of solvent content (Figure 3). The samples displayed in the SEM images were produced from Al nitrate and TEOS. Effect of Heat and Surfactant Treatment on Porosity and Structure of Aluminosilicate Aerogels. The heat treatment (200-500 °C, 2-4 h) condensed considerably the aerogel samples derived from Al nitrate and TEOS (Table 5 and Figure 4). The stronger the heat shock (i.e., the higher the temperature), the more compact the sample is. The mass fractal dimension increased, the porosity decreased, and the elementary units became smaller and more compact: 3-D particles have formed by the effect of heat treatment. The application of surfactants and surface silylation agents (TMCS, HMDS, Brij 56, and P 123) diminished significantly the porosity (e.g., the specific surface area is generally reduced from 630 to 110-150 m2 g-1 in the samples of Al nitrate and TEOS (Table 5)). On the basis of evaluated SAXS data, the surfactants and silylation agents affected in a like manner the fractal dimensions as the heat treatment; the structures became more compact and built up generally from small 3-D units. With respect to the SAXS curves, the surfactant treatments retained the fractal character of the samples better than the heat treatments (Figure 4). The SAXS curves for aerogels treated by surfactants allow for larger fractal domains. After heating, the aerogel samples treated with surfactants revealed very similar supramolecular structures as the heated samples without surfactants. The
surfactants and heat treatments of aerogel samples derived from Al isopropoxide resulted in the same changes. The heat treatments under ambient pressure and the treatment with surfactants or silylation agents did not issue any remarkable growth in the mechanical strength of aerogels. Porosity and SAXS Data of Hybrid (Aluminosilicate and Polymer) Aerogels. The addition of polymers and thus the formation of hybrid structures plays an important role in attempts to increase the mechanical strength of aerogels. To reduce the fragility of aluminosilicate aerogels, we prepared hybrid gels with organic polymers (polyvinyl acetate, PVAc; hydroxylterminated polydimethylsiloxane, PDMS; and poly(acrylic acid), PA). The SAXS curves for pure aluminosilicate and hybrid aerogels of cross-linked PA show similar trends (Figure 5). With respect to the value of the mass fractal dimensions and the high octahedrally bonded Al content, the hybrid structures of PDMS (82 000 g mol-1) and cross-linked PA kept the fractal characters (Tables 3 and 6). By the effect of PDMS and PA, the primary units became a little smaller and more compact. The highest bonded Al content could be detected by Al MAS NMR in the hybrid systems of PDMS, although the Al ions react as Lewis acids not only with TEOS molecules but with hydroxyl-terminated PDMS as well.22 The hybrid system of linear PA can be characterized by a randomly branched structure. The size of primary units became larger by the addition of linear PA, which is also indicated by the higher tetrahedrally bonded Al content. The addition of PVAc to the aluminosilicate system yielded an aggregate structure built up from 3-D primary particles with a rough surface. The bonded Al content is negligible in this system. Using PDMS, PA, and PVAc polymers, the porosity of hybrid aerogels diminished considerably; the specific surface area reduced by half as compared to that of pure aluminosilicate aerogel prepared with the same solvent content. With increasing the amount of polymer, the reduction of porosity was stronger. The polymers were not removed prior to surface area analysis. Effect of Molecular Weight of Polymer on Porosity and SAXS Data of PDMS Hybrid Aerogels. The molecular weight of PDMS influences weakly the bond systems; on the other hand, it has a strong effect on the structure in the supramolecular range (Figure 6).22 PDMS of smaller molecular weights (550 and 5180 g mol-1) destroyed the fractal structure of aluminosilicate oligomers. (The fits resulted in large errors, indicating that the conventional approach for fractal structures has to be taken with caution in this case, Table 7.) These gel structures proved to be weak associates of aggregates of larger sizes (>100 Å); the slope of the SAXS curve in a log-log plot is close to -4. By the effect of treatment with TEOS molecules (0.1 mol of TEOS/Si) prior to the gel point, the PDMS units were incorporated into the gel networks (Figure 6). Because of the destroyed fractal structures, the porosity of the hybrid materials prepared with PDMS of 550 or 5180 g mol-1 is negligible. The scattering of the nonreacted PDMS molecules can be observed in the q-range of >0.05 Å-1. (22) Sinko´, K.; Fe´l, K.; Zrı´nyi, M. Polym. AdV. Technol. 2003, 14, 776-783.
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Langmuir, Vol. 24, No. 3, 2008 953
Table 3. Al Incorporation Measured by 27Al MAS NMR in Aerogelsa aerogels
nonbonded Al content (%)
octahedrally bonded Al content (%)
tetrahedrally bonded Al content (%)
5-20 25-50 98-100
45-60 40-65 0-2 0-2 45-65 40-60 70-85 1-5
25-35 2-10
aluminosilicate prepared from Al isopropoxide + TEOS aluminosilicate prepared from Al nitrate + TEOS aluminosilicate prepared from Al acetate + TEOS aluminosilicate prepared from Al acetate + water glass hybrid of linear PA hybrid of cross-linked PA hybrid of PDMS hybrid of PVAc a
30-45 30-55 15-30 95-99
98-100 5-20 1-6
Samples were prepared from 1 mol of Si and 1 mol of Al precursors. Table 4. Effect of Solvent Contents on Porosity and SAXS Data of Aluminosilicate Aerogelsa
mass ratios propanol/(Si + Al) water/(Si + Al)
3.0 10.0 20.0 4.5 14.5 24.5
specific surface area (BET) (m2 g-1)
av pore diameterb (nm)
pore volumec (m3 g-1)
mass fractal dimensiond (d)
primary particle radiusd (r) (nm)
fractal correlation lengthd (ξ) (nm)
fractal dimension of primary particlesd (du)
380 ( 1.0 630 ( 1.6 1035 ( 12 380 ( 1.0 916 ( 5.0 1010 ( 8.0
11.3 ( 0.2 12.0 ( 0.2 8.5 ( 0.05 11.3 ( 0.2 8.8 ( 0.05 9.5 ( 0.05
1.1 ( 0.1 1.6 ( 0.1 3.61 ( 0.7 1.1 ( 0.1 3.16 ( 0.4 3.98 ( 0.7
2.46 ( 0.18 2.27 ( 0.07 2.40 ( 0.10 2.46 ( 0.18 2.05 ( 0.07 2.40 ( 0.02
2.4 ( 0.2 3.2 ( 0.1 3.4 ( 0.1 2.4 ( 0.2 3.1 ( 0.2 1.25 ( 0.3
6.2 (5.8e) f (29e) f (36e) 6.2 (5.8e) f (30e) 26.3 (28e)
2.52 ( 0.2 2.17 ( 0.1 2.05 ( 0.1 2.52 ( 0.1 2.14 ( 0.2 2.70 ( 0.2
a Aerogel samples were prepared from TEOS and Al nitrate. b BJH adsorption average pore diameter (4V/A). c BJH adsorption cumulative pore volume of pores between 1.7 and 300.0 nm diameter. d Calculated by the modified Freltoft et al. approach. e Size was calculated by the modified Freltoft et al. fit from USAXS data. f Size was >100-200 nm.
Discussion
Figure 2. 27Al MAS NMR spectra for aerogels. The x-axis is shifted by 50 ppm. Aluminosilicate aerogel prepared (1) from Al isopropoxide + TEOS; (2) from Al nitrate + TEOS; (3) from Al acetate + TEOS; (4) from Al acetate + waterglass; (5) PDMS hybrid aerogel; and (6) cross-linked PA hybrid.
In the case of 550 g mol-1, the nonreacted PDMS molecules scatter in the range of 0.05-0.5 Å-1, at larger molecular weights, this range shifts toward smaller q-ranges (0.01-0.05 Å-1). The fractal structures remaining in the hybrid systems were obtained from PDMS of larger molecular weights (42 500 and 82 000 g mol-1) (Figure 6 and Table 7). The PDMS molecules of 42 500 g mol-1 modified least the supramolecular structure of the aluminosilicate system; its porosity, its fractal characters, and the elementary units measured by SAXS and nitrogen sorption were altered slightly. The PDMS of 82 000 g mol-1 changed the porosity rather than the fractal properties. On the basis of Brinell hardness measurements, the strength of the aluminosilicate aerogels increased about 2-3-fold by the addition of various polymers.22 The growth of the hardness was 7-8-fold after a washing step with organic solvents.
Effect of Initial Materials. The type of initial materials determines essentially both atomic structures and nanostructures in the sol-gel procedure of aluminosilicate systems. With varying the precursors, extremely different gel structures can be prepared by similar ways. The application of aluminum nitrate (Al(NO3)39‚H2O) as inorganic Al precursors has been motivated by its low price, good chemical resistance in air, solubility in alcohols, and controllable hydrolysis. The extensive hydrolysis of Al nitrate produces an acidic medium required for the hydrolysis of TEOS. Because of the spontaneous decomposition of the nitrate in organic media at 80 °C, the pH is continuously increasing during the gelation, supporting the condensation reactions. The hydrolysis rate of Al nitrate is very similar to that of TEOS, which is advantageous for Al incorporation into the silica network. The sol-gel preparation of the aluminosilicate system starting from Al nitrate and TEOS produces one of the highest Al incorporations into the silica network (Table 3).23 The high Al incorporation belongs strictly to the fractal structure; in a fused bulk aluminosilicate system, the highest bonded Al content is about 10%. The octahedrally coordinated Al ions on the surface of elementary units balance the charge of AlO4- ions in the inside of particles (Figure 7). The function of octahedron Al atoms is a network modifier to compensate for the negative charge of Al ions inserted as network formers in the tetrahedral sites.24,25 The (23) Sinko´, K.; Fe´l, K.; Rohonczy, J.; Hu¨sing, N. Smart Mater. Struct. 2001, 10, 1078-1084. (24) Schmu¨cker, M.; MacKenzie, K. J. D.; Schneider, H.; Meinhold, R. J. Non-Cryst. Solids 1997, 217, 99-105. (25) da Silva, M. G. F. J. Non-Cryst. Solids 2006, 352, 807-820. (26) Dennington, R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. Gauss View, version 3.09; Semichem, Inc.: Shawnee Mission, KS, 2003. (27) Loewenstein, W. Am. Mineral. 1954, 39, 92-96. (28) Liu, Y.; Nekvasil, H.; Tossell, J. J. Phys. Chem. A 2005, 109, 30603066. (29) Lee, S. K. J. Phys. Chem. B 2004, 108, 18228-18233. (30) Criscenti, L. J.; Brantley, S. L.; Mueller, K. T.; Tsomaia, N.; Kubucki, J. D. Geochim. Cosmochim. Acta 2005, 69, 2205-2210. (31) Swainson, I. P.; Dove, M. T. Phys. Chem. Mineral. 1995, 22, 61-65.
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Figure 4. SAXS curves for aluminosilicate aerogels prepared from Al nitrate and TEOS. The samples were treated with a surfactant, TMCS, or heated.
Figure 5. SAXS curves for hybrid aerogels prepared from Al nitrate, TEOS, and various polymers.
Figure 3. Scanning electron micrographs of aluminosilicate aerogels prepared with different solvent contents displaying porosity (A ) 3, B ) 10, and C ) 20 mol of propanol/(Si + Al); magnification 10 000× and 28 000×).
primary units of the aluminosilicate network have fractal characters, and the resulting aerogels are very porous (Table 2). The gelation of Al isopropoxide and TEOS also leads to the formation of very porous fractal structures (Table 2). Owing to the fast hydrolysis of Al isopropoxide, TEOS should be prehydrolyzed if a homogeneous distribution is desired. The prehydrolysis time of TEOS determines the degree of the Al incorporation. Without the prehydrolysis of TEOS, a homogeneous gel could not be obtained because Al isopropoxide forms a separate phase. With a longer prehydrolysis time, Si-containing units formed, and the Al atoms were incorporated on the surface of elementary units rather than on the inside of them. These facts (32) Liu, Y.; Pinnavaia, T. J. J. Mater. Chem. 2004, 14, 3416-3420.
have been proven by Al MAS NMR measurements. At a shorter time, the number of tetrahedrally bonded Al atoms increased on the inside of the primary building blocks. Fifteen minutes proved to be the optimal prehydrolysis time. The highest Al incorporation could be detected in the samples prepared from Al isopropoxide and TEOS (Table 3). The amorphous network is built up from very small 3-D particles. The remarkable number of tetrahedral Al atoms confirms the compact primary particles. Regarding environmental protection, the most important advantage of the use of Al acetate is that the acetate molecules can very easily escape as CO2 and H2O molecules during the heat treatment. Optically clear, weak, compact randomly branched gel structures were obtained from Al acetate and TEOS in a strongly acidic medium; the samples have a negligible bonded Al content. A strong acidic medium is required for the dissolution of Al acetate. The gel structure is dominated by an amorphous silica network. The nonbonded Al ions form weak associates inserted in the silica network. In replacing TEOS with an inexpensive water glass solution (sodium metasilicate) a hard, compact composite system developed in basic medium; Al-containing, crystalline nanoparticles were inserted in an amorphous aluminosilicate network. The compact structure also has been verified by a ≈100% tetrahedrally bonded Al content (Table 3 and Figure 8). The fast condensation reactions of Al acetate molecules are dominated in the gelation process yielding a compact structure; their hydrolysis processes and reactions with various Si precursors
Gel-DeriVed Aluminosilicate Materials
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Table 5. Effect of Heat and Tenside Treatments on Porosity and SAXS Data of Aluminosilicate Aerogelsa
treatments untreated heat treatment (200 °C, 2 h) heat treatment (500 °C, 2 h) treated by surfactants (TMCS)f treated by surfactants (Brij 56)g treated by surfactants (P 123)h treated by Brij 56g + heat treatment (500 °C, 4 h) treated by P 123h + heat treatment (500 °C, 4 h)
specific surface area (BET) (m2 g-1)
av pore diameterb (nm)
pore volumec (m3 g-1)
mass fractal dimension (d)
primary particle radius (r) (nm)
fractal dimension of primary particlesd (du)
630 ( 1.8 440 ( 2.0 441 ( 2.0 135 ( 1.4 116 ( 1.1 356 ( 3.0 292 ( 1.5
12.0 ( 0.2 12.8 ( 0.1 13.8 ( 0.1 6.7 ( 0.1 13.3 ( 0.1 13.7 ( 0.1 18.0 ( 0.2
1.6 ( 0.12 1.5 ( 0.15 1.5 ( 0.15 0.1 ( 0.05 0.39 ( 0.1 1.2 ( 0.12 0.93 ( 0.1
2.25 ( 0.07d 2.50 ( 0.04d 2.70 ( 0.05e 2.80 ( 0.003d 2.87 ( 0.005e 2.76 ( 0.004d 2.77 ( 0.02e
3.2 ( 0.1d 1.6 ( 0.2d 1.1 ( 0.05e 2.2 ( 0.5d 1.80 ( 0.05e 0.92 ( 0.08d 0.94 ( 0.07e
2.31 ( 0.1 3.1 ( 0.6
435 ( 1.7
14.5 ( 0.1
1.3 ( 0.15
2.70 ( 0.01e
0.85 ( 0.07e
2.55 ( 0.05 3.02 ( 0.4
a Aerogel samples were prepared from TEOS and Al nitrate. b BJH adsorption average pore diameter (4V/A). c BJH adsorption cumulative pore volume of pores between 1.7 and 300.0 nm diameter. d Calculated by the modified Freltoft et al. approximation. e Calculated by the Freltoft et al. approach. f Trimethylchlorosilane. g Polyoxyethylene(10) cetyl ether. h EO20PO70EO20.
Table 6. Porosity and SAXS Data of Hybrid (Aluminosilicate and Polymer) Aerogels
c
hybrid aerogels
specific surface area (BET) (m2 g-1)
pore volumea (m3 g-1)
mass fractal dimensionb (d)
primary particle radiusb (r) (nm)
fractal dimension of primary particlesb (du)
fractal correlation length (ξ) (nm)
nonhybrid aluminosilicate linear PA cross-linked PA PDMS PVAc
630 ( 1.8 294 ( 2.4 357 ( 2.6 344 ( 2.4 223 ( 2.5
1.60 ( 0.12 0.54 ( 0.1 0.68 ( 0.1 0.78 ( 0.1 0.80 ( 0.1
2.27 ( 0.07 1.75d ( 0.13 2.63 ( 0.4 2.53 ( 0.01 2.98d ( 0.15
3.2 ( 0.1 3.9 ( 0.7 2.0 ( 1.0 2.3 ( 0.04 0.85 ( 0.2
2.17 ( 0.1 2.36 ( 0.1 2.91 ( 0.14 2.82 ( 0.03 3.52 ( 0.1
c c c c 4.8 ( 1.4
a BJH adsorption cumulative pore volume of pores between 1.7 and 300.0 nm diameter. b Calculated by the modified Freltoft et al. approximation. Size calculated by the modified Freltoft et al. approach was >100-200 nm. d Not a fractal structure.
Table 7. Effect of Molecular Weight of Polymer on Porosity and SAXS Data of Aluminosilicate Hybrid Aerogels molecular weight of PDMS (mol g-1)
specific surface area (BET) (m2 g-1)
av pore diametera (nm)
pore volumeb (m3 g-1)
mass fractal dimensionc (d)
primary particle radiusc (r) (nm)
fractal dimension of primary particlesc (du)
550 550 (treated by TEOS) 42500 82000
7.3 ( 0.4 52.2 ( 0.6 660 ( 2.5 304 ( 2.4
6.0 ( 0.1 10.0 ( 0.2 8.5 ( 0.1 9.1 ( 0.2
0.1 ( 0.2 0.23 ( 0.2 1.30 ( 0.1 0.70 ( 0.1
2.98d ( 0.3 2.53 ( 0.06 2.27 ( 0.1 2.79 ( 0.1
43.7 ( 40 37.9 ( 38 2.85 ( 0.7 3.2 ( 0.6
1.57d ( 0.3 1.38d ( 0.05 2.85 ( 0.02 2.88 ( 0.02
a BJH adsorption average pore diameter (4V/A). b BJH adsorption cumulative pore volume of pores between 1.7 and 300.0 nm diameter. c Calculated by modified Freltoft et al. approach. d Not fractal structure.
Figure 6. SAXS curves for hybrid aerogels prepared from aluminosilicate oligomers and PDMS of different molecular weights.
are not so intensive. The connection of Al-containing, crystalline nanoparticles to the 3-D silica network obtained from TEOS is much weaker than to the sodium ion-containing silicate phase. The sodium ions may compensate for the charge of the tetrahedron AlO4- units. Porosity. The highly bonded Al content, the loose fractal structure, and the very small elementary units derived from the
Figure 7. Bonds system of the fractal aluminosilicate network prepared from Al nitrate and TEOS. All geometry optimizations were performed using Gauss View26 and considering the Loewenstein rule,27 the results of Al MAS NMR, and the published values for Al-O-Si and Si-O-Si bond angles28-30 and bond lengths.29,30
gelation of Al nitrate or isopropoxide and TEOS guarantee good porosity properties. The octahedrally incorporated Al atoms on the surface of elementary units are mainly coordinated by water molecules and in fewer rates even by organic compounds
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PDMS, the nitrate ions turned into HNO3 molecules in the less polar medium, and they escaped as nitrous gases as the pH rose. Thus, the condensation reactions of TEOS became more intensive, yielding a less porous structure. The gel structures obtained from PDMS of lower molecular weight, without any treatment, proved to be weak associates of aggregates. Treatment with TEOS before the gel point increased the mechanical strength and the porosity by decreasing the number of silanol groups and making the PDMS chains longer. In this manner, the Al atoms reacted with PDMS in a smaller ratio.22 The fractal structures provided the aerogels with new special properties, which are different from those of fused aluminosilicate glass or ceramics. The controllable, variable (from compact to extreme low) density and porosity, a high Al content with the homogeneous structure, and the piezoelectricity are new properties observed in our research.23
Conclusion
Figure 8. Bond system of amorphous aluminosilicate network obtained from Al acetate and water glass. All geometry optimizations were performed using Gauss View26 and considering the Loewenstein rule,27 the results of Al MAS NMR, and the published values for Al-O-Si and Si-O-Si bond angles30,31 and bond lengths.30-32
supporting the development of small elementary units and fractal structures. The porosity of pure aluminosilicate aerogels depends strongly on the solvent content of the initial wet gels. The degree of porosity can be very well-controlled by the solvent content. The most suitable solvent is propanol to adjust the porosity regarding Al incorporation. The number of bonded Al atoms decreased drastically in large water contents.23 Using Al acetate as an Al precursor led to the formation of more compact structures. The surfactant and heat treatments of aerogels did not really improve the mechanical strength, reduce the porosity, or condense the fractal structures and elementary units. Heating removes the shared OH ions and the water or alcoholic molecules from the surfaces of hydrated, solvated elementary units. The surfactants cover the surface of particles displacing the other molecules, ions, which can be attracted to the surface of the particles. The addition of polymers also diminished the porosity; on the other hand, they served the purpose of their application and decreased the fragility of the aerogels. The polymers grew in mechanical strength by 2-3-fold. The porosity of the hybrid aerogels depends rather on the concentration of the organic polymer content than on the solvent content. The best result can be achieved by the use of PDMS; these hybrid systems kept the fractal system of the aluminosilicate component and revealed a good homogeneity. The molecular weight of PDMS strongly affected the porosity; PDMS of lower molecular weights (e.g., 550 g/mol) drastically reduced the porosity, and the influence of PDMS on larger molecular weights was much weaker (Table 7). The drastic reduction has been the result of reactions of the OH-terminated PDMS. The Al and Si MAS NMR data verify the reactions between PDMS and Al ions as well as PDMS and TEOS.22 The Al ions react with the OH terminal groups of PDMS as a Lewis acid.22 The intensity of the reaction of PDMS with the Al(III) ions depends on the molecular weight of PDMS (i.e., on the number of OH groups). Because of the reaction of Al ions with PDMS, the Al incorporation into the silica network weakened. Owing to the fast reactions of the Al(III) ions with
The initial materials essentially influence the structures of the sol-gel-derived aluminosilicate aerogels. Fractal structures were produced by gelations of TEOS and Al nitrate or Al isopropoxide; however, the primary particles of gel networks were significantly different. The elementary units of the aluminosilicate structure prepared from Al nitrate have fractal characters; the units of systems derived from Al isopropoxide are very small 3-D particles. The gelation process of TEOS and Al acetate produces a weak and compact structure; in acidic medium, a randomly branched structure with a negligible bonded Al content is formed. Using water glass and Al acetate solutions, a hard, compact composite system forms; the composite structure is built up from Alcontaining crystalline nanoparticles and an amorphous aluminosilicate network. The fractal structures and the small sizes of the primary particles provide the aluminosilicate aerogel systems with a high porosity. The large number of incorporated Al atoms in the silica network loosens significantly the fractal system by the molecules attracted to the surface of particles growing on the specific surface area. The porosity of the fractal-like aluminosilicate aerogels depends strongly on the solvent content of the initial wet gels. With changing the amount of organic solvent in the gels, the specific surface area is varied between 300 and 1100 m2 g-1. The gelation of Al acetate and different Si precursors led to the formation of compact aerogel structures with specific surface areas of 10-60 m2 g-1. The surfactant and heat treatments of aerogels only slightly increased the mechanical strength, reduced the porosity by 3080%, and condensed the fractal structures and elementary units. Hybrid structures were obtained by polymer additions to aluminosilicate oligomers. The hardness of the hybrid aerogels displayed a 2-3-fold increase as compared to that of pure aluminosilicate aerogels; however, the porosity diminished by 45-65%. The specific surface area of the hybrid aerogels was altered in this series between 200 and 700 m2 g-1. The hybrid aerogels of PDMS have the best material properties; out of those, fractal framework, respectable mesoporosity, good homogeneity, and mechanical strength can be mentioned. The molecular weight of PDMS changed the porosity significantly; the lower the molecular weight, the more drastic the reduction of porosity was. Treatment with TEOS before the gel point increased the homogeneity, the porosity, and the hardness of the aerogels. Acknowledgment. This study was supported by OTKA T 043636 funds and I-04-009 EU in HASYLAB. LA702525X
