Langmuir 2004, 20, 10389-10393
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Preparation and Surface Properties of Low-Density Gels Synthesized Using Prepolymerized Silica Precursors Julita Mrowiec-Białon´,*,† Andrzej B. Jarze¸ bski,†,‡ Lucjan Paja¸ k,§ Zbigniew Olejniczak,| and Mirosław Gibas‡ Institute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland, Faculty of Chemistry, Silesian University of Technology, M. Strzody 7, 44-101 Gliwice, Poland, Institute of Information and Materials Science, Silesian University, Bankowa 12, 40-007 Katowice, Poland, and Institute of Nuclear Physics, Radzikowskiego 152, 31-342 Krako´ w, Poland Received April 26, 2004. In Final Form: August 25, 2004 Properties of silica xerogels and aerogels synthesized using a number of prepolymerized silica precursors were probed by 29Si magic-angle spinning (MAS) NMR spectroscopy, the small-angle X-ray scattering (SAXS) method, the nitrogen adsorption method, and transmission electron microscopy (TEM) to show that xerogels with attractive textural properties can easily be prepared using this type of precursors and the conventional one-step, base procedure. Pore sizes and overall pore volumes in these materials can be notably larger than those in the corresponding materials synthesized using tetraethoxysilane. This positive effect stems from the stronger structure of the polymeric network due to a higher degree of silica condensation on one side and a larger thickness of polymeric chains on the other. The thorough investigations of the fine silica structure demonstrate, however, that the relationship between the microstructure of the silica precursor and the micro- and macrostructures of dry gels is complex and the use of more condensed precursors favors, but does not necessarily ensure, more porous dry materials, under the same reaction conditions. Ethyl silicate 40 may be recommended as a low-cost precursor suitable for applications in this situation.
Introduction The exciting discovery in the early 1990s of new mesoporous materials with ordered structures1 markedly overshadowed interests in the conventional gels with disordered structures produced by the sol-gel method. However, the practical need for materials with an open structure, similar to that of aerogels, hardly available using the template approach, fueled the development of synthetic routes aimed to afford low-density xerogels (LDXs), that is, aerogel-like materials prepared using safe techniques involving non-supercritical drying. As the extent of shrinkage of the wet gel during drying is controlled by the balance between capillary pressure and the bulk modulus of the solid network, then lowdensity xerogels can be obtained either by minimizing the capillary pressure, by decreasing surface tension or increasing pore diameter, or by improving the strength and stiffness of the gel structure.2 Haereid et al.3,4 first devised an effective strategy to strengthen the skeleton of the wet gel by aging the pristine alcogel in the siloxane solution. Another effective procedure to obtain LDXs proposed by Desphande, Smith, and Brinker5,6 consisted * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +48322310811. Fax: +48322310318. † Polish Academy of Sciences. ‡ Silesian University of Technology. § Silesian University. | Institute of Nuclear Physics. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Smith, D. M.; Scherer, G. W.; Anderson, J. M. J. Non-Cryst. Solids 1995, 188, 191. (3) Haereid, S.; Dahle, M.; Lima, S.; Einarsrud, M.-A. J. Non-Cryst. Solids 1995, 186, 96. (4) Haereid, S.; Nilsen, E.; Einarsrud, M.-A. J. Non-Cryst. Solids 1996, 204, 228. (5) Desphande, R.; Smith, D. M.; Brinker, C. J. US Patent 5,565,142, 1996.
of surface modification of the wet gel. In particular, surface tension was suppressed by exchanging the pore fluid with an aprotic solvent and residual surface silanol groups were eliminated by methylation of the surface using chloromethylsilane. The latter not only modified the contact angle between the surface and the pore fluid but, more importantly, reduced the formation of siloxane bridges across the pores and hence induced the spring-back of the shrunken silica network to the original structure. However, both methods involved additional time-consuming postgelation operations. More recently, Alie´ et al.7,8 observed that strong gel structures can readily be prepared by incorporating an additive, for example, 3-(2-aminoethylamino)propyltrimethoxysilane, during the synthesis of the gel to induce the nucleation mechanism related to the difference in reactivity between the additive and main silica precursors, tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). Also, more recently, we have observed that pores (and pore volumes) in hybrid xerogels and aerogels synthesized using ethyl silicate 40 (ES) were notably larger than those in the corresponding materials synthesized using TEOS alone.9-11 The ES is a common commercial form of ethoxypolysiloxane.12 As it is environmentally benign and available at prices competi(6) Smith, D. M.; Stein, D.; Anderson, J. M.; Ackerman, W. J. NonCryst. Solids 1995, 186, 104. (7) Alie´, C.; Pirard, R.; Lecloux, A. J.; Pirard, J. P. J. Non-Cryst. Solids 1999, 246, 216. (8) Alie´, C.; Pirard, R.; Lecloux, A. J.; Pirard. J. P. J. Non-Cryst. Solids 2001, 285, 135. (9) Mrowiec-Białon´, J.; Lachowski, A. I.; Kargol, M.; Malinowski, J. J.; Jarze¸ bski, A. B. Stud. Surf. Sci. Catal. 2000, 129, 859. (10) Mrowiec-Białon´, J.; Jarze¸ bski, A. B. Langmuir 2001, 17, 626. (11) Kholdeeva, O. A.; Trukhan, N. N.; Vanina, M. P.; Rommannikov, V. N.; Parmon, V. N.; Mrowiec-Białon´, J.; Jarze¸ bski, A. B. Catal. Today 2002, 75, 203. (12) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990; p 113.
10.1021/la0400656 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/20/2004
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Table 1. Characteristic Parameters of Silica Xerogels (X) and Aerogels (A) preNH3/Si sample cursor molar ratio X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 A1 A2 A3 A5 A5 A6 A7 A8 A9 A10 A11 A12
P1
P2 ES TEOS P1
P2 ES TEOS
0.008 0.016 0.024 0.032 0.040 0.008 0.016 0.024 0.008 0.032 0.008 0.032 0.008 0.016 0.024 0.032 0.040 0.008 0.016 0.024 0.008 0.032 0.008 0.032
SBET (m2/g) 411 ( 5.8 671 ( 3.3 781 ( 1.4 763 ( 1.7 733 ( 2.1 797 ( 5.4 1128 ( 6.3 1115 ( 6.8 734 ( 2.3 898 ( 1.9 1004 ( 2.8 1146 ( 8.4 349 ( 2.6 332 ( 2.4 321 ( 2.0 299 ( 2.5 303 ( 2.3 401 ( 3.6 411 ( 3.5 420 ( 4.0 234 ( 3.4 400 ( 3.1 514 ( 6.7 514 ( 6.0
VpN2 dBJH Fa Vt (cm3/g) (nm) (g/cm3) (cm3/g) 0.916 1.384 1.587 1.688 1.675 0.705 1.075 1.138 1.20 1.631 0.60 1.177 0.846 0.965 2.580 0.57 0.58 4.27 4.32 4.74 1.28 1.26 4.45 4.00
6.69 7.53 8.53 9.69 9.85 3.74 3.90 4.04 6.36 7.37 2.94 3.88 11.74 10.76 28.94 9.4 9.7 24.9 23.6 24.3 14.0 14.0 21.7 20.2
0.5756 0.5068 0.4239 0.4003 0.4098 0.6307 0.5877 0.5823
1.28 1.52 1.90 2.04 1.99 1.13 1.25 1.26
0.4067
2.00
0.5332 0.0483 0.0774 0.0840 0.0740 0.0714 0.1307
1.42 20.2 12.5 11.4 13.0 13.6 7.2
0.1252 7.5 0.0809 11.9 0.0723 13.4 0.1179 9.0 0.1567 5.9
tive to those of most common silica precursors (i.e., TEOS and TMOS), it seems rational to carry out more systematic studies of the potentials of the application of ES and prepolymerized silica precursors, in general, in the synthesis of low-density silica xerogels. In the following, we address this issue. As the aerogel structure replicates the skeleton of wet gels, normally more difficult to examine, to get a deeper understanding of the preparation-structure relationship, we deemed it necessary to prepare also a set of the corresponding aerogels and to probe the nanostructure using 29Si magic-angle spinning (MAS) NMR spectroscopy, the small-angle X-ray scattering (SAXS) method, nitrogen adsorption-desorption at 77 K, and transmission electron microscopy (TEM). Experimental Section
Figure 1.
29Si
NMR spectra for silica precursors.
Characterization Methods. Precursor samples kept in the closed standard tubes were analyzed using a UNITY/INOVA 300 NMR spectrometer (Varian). The 29Si spectra were recorded at 59.614 MHz with the application of gated decoupling and 60100 s relaxation delay. 29Si MAS NMR spectra from selected xerogel samples were measured at 59.517 MHz using a Bruker HP-WB high-speed MAS probe equipped with a 4 mm zirconia rotor to record the NMR spectra at a spinning speed of 4 kHz. The acquisition delay used in accumulation was 120 s, and the number of acquisitions ranged from 400 to 600. In deconvolution of the Q2, Q3, and Q4 MAS signals, both the peak positions and widths of the peaks were not fixed; the starting values of the former were -92, -100, and 110 ppm, respectively. The intensity sum of Q2, Q3, and Q4 was normalized to 100% for each spectrum. Nitrogen adsorption-desorption isotherms measured at 77 K with a Micromeritics ASAP 2000 instrument were used to obtain values of the specific surface area, SBET, estimated from a linear section of the adsorption isotherm, taking five points in the range 0.05-0.2 p/p0, and the pore volume, VpN2, determined by means of the BJH method.14 Apparent density, Fa, was determined from mercury porosimetry data afforded by a Micromeritics Auto Pore 9220 instrument, whereas the skeletal densities, Fs, were obtained from the standard helium pycnometry measurements. The latter two quantities were used to calculate the total pore volume, Vt (VpN2 + Vmacropores). SAXS measurements were performed using a Kratky camera with a line-collimated primary beam system and Cu KR radiation. The samples investigated were granules loosely packed between two thin foils. The TEM images of selected samples were obtained on a JEOL 2000SX instrument operating at 160 kV using a standard holey carbon technique.
Precursor and Sample Preparation. At first, we prepared in duplicate two prepolymerized precursors, labeled P1 and P2, one of which (P1) aimed to reproduce ethyl silicate 40. They were synthesized under acidic conditions by the partial hydrolysis of TEOS (Aldrich) using 40 or 60% of the theoretical water needed, that is, with a Si/H2O molar ratio of 0.8 or 1.2 in P1 and P2, respectively. The reaction was carried out at 50 °C for 5 h under reflux with a constant removal of the ethanol released by distillation. A part of each precursor was transferred into a standard tube and analyzed to obtain 29Si NMR spectra. Alcogels were prepared using four different silica precursors: commercial ethyl silicate 40 (Unisil-Tarnow, Poland), P1, P2, and TEOS. The synthesis of those prepared with ES, P1, or P2 was carried out at 50 °C, and the molar ratio of the compounds in all reaction mixtures was Si/EtOH/H2O/NH3 ) 1:8:3:x, where x took the values of 0.008, 0.016, 0.024, 0.032, and 0.040. Alcogels synthesized using TEOS were prepared in a conventional twostep, acid-base procedure at 50 °C. In the acidic step lasting 0.5 h, the molar ratio of the compounds involved was Si/EtOH/H2O/ HCl ) 1:4:2:0.0008, whereas, in the basic step which followed, the compounds met the overall molar ratio Si/EtOH/H2O/NH3 ) 1:8:4:x. All alcogels were aged for 7 days and then dried at 50 °C for 100 h to give xerogels or by using a high-temperature supercritical drying procedure (T ) 270 °C; p ) 20 MPa), described in detail earlier,13 to afford aerogels. After synthesis, all xerogels were additionally dried at 100 °C for 5 h. Table 1 gives the conditions of xerogel (X) and aerogel (A) preparation.
As can be seen from the 29Si NMR spectra of polysilicate precursors given in Figure 1, the content of specific Qn species assigned to Si(OSi)n(OX)4-n entities (X ) R or H), and being a direct indicator of silica cross-linkage, appeared to be noticeably different in P1 and P2, unlike
(13) Mrowiec-Białon´, J.; Paja¸ k, L.; Jarze¸ bski, A. B.; Lachowski, A. I.; Malinowski, J. J. Langmuir 1997, 13, 6310.
(14) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
Results and Discussion
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Figure 3. Nitrogen adsorption isotherms from TEOS and P1 xerogels; Si/NH3 molar ratio of 1:0.032.
Figure 2.
29
Si MAS NMR spectra for silica xerogels.
Table 2. Qn Distributions in the Silica Precursors precursor
SiO2 content (wt %)
Q0
Q1
Q2
Q3
TEOSa
28.3 40.0 40.1 46.3
98.39 7.17 0.78 0
0 23.66 33.93 6.96
0 33.90 40.94 52.50
0 35.26 24.35 40.55
ES P1 P2 a
1.61% Si(EtO)3(OH). Table 3. Qn Distributions in Silica Xerogels
sample
Q2
Q3
Q4
Q4/Q2
Q4/Q3
X1 X4 X10 X11
7.8 3.1 1.3 5.4
33.9 34.3 29.8 41.3
58.3 62.6 68.9 53.3
7.47 20.5 51.8 9.87
1.72 1.82 2.31 1.29
in P1 and ES where they were fairly similar. In P2, the total population of Q2 and Q3 sites amounted to ∼90%, whereas, in P1 and especially ES, the populations of Q1, Q2, and Q3 sites were roughly similar and ranged from 23 to 35% (cf. Table 2). As expected, after drying, the Q4 and Q3 sites predominated the silica structure, but highly uncondensed Q2 sites were also present in appreciable amounts (Table 3). The population of Q4 exceeded 60% in xerogels prepared using ES and P1, and it decreased in the order ES > P1 > P2 > TEOS, clearly showing a strong, but very complex, effect of the structure of the silica precursor on the resulting structure of the dry gel. Indeed, the most condensed P2 precursor appeared to yield somewhat less condensed silica gels, in terms of the population of Q4 sites, than the less condensed P1 and ES precursors, under the same reaction conditions. Interestingly, the content of ammonia used as a catalyst to facilitate the condensation reaction had a minor effect on the final degree of condensation, as can be inferred from the distributions of Qn sites in dry materials (Figure 2 and Table 3). Indeed, the population of Q3 and Q4 sites increased by only a few percent with the rise in ammonia content, regardless of the type of precursor. As expected, the microstructural properties of silica making up the skeleton appeared to bear on those of the porous texture of dry gels. The nitrogen adsorption isotherms were notably different for the TEOS and P1
Figure 4. Pore size distributions in xerogels prepared using TEOS, ethyl silicate, and P1 precursors; Si/NH3 molar ratio of 1:0.032.
xerogels (Figure 3), whereas they were quite similar for the ES and P1 samples. The larger degree of silica condensation in P1 and ES xerogels well correlates with the larger size of pores in the same materials. In this respect, the choice of the silica precursor has been found to be of considerable importance (cf. Figure 4 and the dBJH values given in Table 1). Contrary to the microstructural properties of silica, the xerogel texture appeared to be much more sensitive to the ammonia content, especially in the NH3/Si molar ratio range 0-0.032 (Table 1). The pore volumes in xerogels synthesized with considerable ammonia content using P1 and ES exceeded 1.6 cm3/g, whereas in their counterparts synthesized using P2 and TEOS this quantity was not larger than ∼1.1 cm3/g, that is, the value often observed in xerogels prepared under strongly basic conditions.15 Quite surprising may be the opposite trend seen in the corresponding aerogels (cf. Table 1), even with all reservation concerning the reliability of predictions resulting (15) Fremery, E.; Mutin, P. H. J. Sol.-Gel Sci. Technol. 2002, 24, 191.
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Figure 5. Scattering spectra from aerogels prepared using TEOS and P1 precursors.
from the nitrogen adsorption method.16,17 Indeed, the values of VpN2 from the ES and P1 families of aerogels synthesized in the same way are very low in aerogel standards and considerably lower than those observed in TEOS and P2 aerogels, irrespective of ammonia concentration. Moreover, the range of the latter, around 4 cm3/g, agrees with the values reported before.13 To elucidate these seemingly contradictory trends, we have to consider the values of the apparent density and especially of the corresponding total pore volume, Vt, given in Table 1. The latter are considerably larger in the ES and P1 families of aerogels than in the other samples, corroborating the trend seen in xerogels and earlier expectations. Thus, bearing in mind the polymeric structure of aerogels, the lower values of VpN2 in P1 and ES aerogels can be explained by the prevailing presence of very bulky voids in these samples (owing to their very low shrinkage), markedly larger than those in P2 and TEOS aerogels, and not by the poor pore volume itself. The large portion of these macropores is not detectable by the nitrogen adsorption technique, as Scherer18 pointed out in an excellent analysis of the similar situation. Analysis of the scattering curves (SAXS) is often used to obtain averaged values of the width of chains making up the silica skeleton of aerogels.19 As can be seen from Figure 5, the locus of maximum of the Jq3 versus q curves (J, scattering intensity) drifts toward lower values of wave vector q with the rise in ammonia concentration. For the TEOS aerogel, the value of qc is roughly twice as large as that for the P1 counterpart. Thus, from the approximate expression for chain thickness l ≈ π/qc,19,20 we can deduce that, in the P1 aerogel, silica chains are thicker than those in the TEOS counterpart by a factor of ∼2. This implies a more robust structure and hence larger values of (total) pore volume in the former material, in perfect agreement (16) Reichenauer, G.; Scherer, G. W. J. Non-Cryst. Solids 2000, 277, 162. (17) Reichenauer, G.; Scherer, G. W. J. Non-Cryst. Solids 2001, 285, 167. (18) Scherer, G. W. J. Non-Cryst. Solids 1998, 225, 192. (19) Jarze¸ bski, A. B.; Lorenc, J.; Paja¸ k, L. Langmuir 1997, 13, 1280. (20) Hasmy, A.; Vacher, R.; Jullien, R. Phys. Rev. B 1994, 50, 1305.
Mrowiec-Białon´ et al.
Figure 6. Scattering spectra from xerogels prepared using TEOS and P1 precursors.
with the earlier observations (Table 1). Similar analyses performed for xerogels were less conclusive, due to the lack of a maximum identified with the presence of well formed spherical particles of specific size. Inspection of Figure 6 revealed, however, a gradual evolution of the shape of scattering curves in very short scales, from a clear slope, observed in the P1 xerogel with low ammonia content, toward a plateau at higher ammonia concentration. Most likely, this is a sign of higher microstructural homogeneity, due to higher condensation indicated by 29Si NMR spectra. Interestingly, in the corresponding TEOS xerogel, the level of homogeneity in the same scales appeared to be much lower, again in agreement with the results given in Table 3. The measurements of skeletal density afforded by helium pycnometry confirmed the positive effect of higher ammonia content on silica condensation. The values of Fs in the P1 xerogels rose from 1.87 to 2.00 g/cm3 with the increase in ammonia content from 0.008 to 0.040 in NH3/Si ratio, whereas in the P1 aerogels this growth was less significant and Fs was ∼1.93 g/cm3 in the aerogel at high ammonia concentration. While the examination of TEM images of LDXs failed to give precise quantitative data, it has confirmed the presence of a highly porous structure in the xerogels obtained, with larger pores more clearly seen in those prepared using P1 and ES rather than TEOS (Figure 7). To summarize, the results obtained consistently indicate that a judicious use of prepolymerized silica precursors in the conventional preparation procedure results in dry gels with more condensed silica and a thicker and stronger polymeric network, showing notably larger voids and a larger overall pore volume due to reduced shrinkage. They also demonstrate that the relationship between the microstructure of the silica precursor and the micro- and macrostructures of dry gels is complex and the use of more condensed precursors favors, but does not necessarily give, the more porous dry materials, under the same reaction conditions (cf. the P2 and P1 samples). However, the effective application of prepolymerized silica precursors affords new possibilities in the synthesis of low-density
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been applied to obtain Co-containing polyoxometalatebased siliceous catalysts with attractive texture.21 Conclusions Silica xerogels with attractive textural properties can easily be prepared using prepolymerized silica precursors and the conventional one-step, base procedure. Pore sizes and overall pore volumes in these materials are notably larger than those in the corresponding materials synthesized with the use of TEOS due to reduced shrinkage in drying. This positive effect stems from the stronger structure of the polymeric network owing to a higher degree of silica condensation on one side and a larger thickness of polymeric chains on the other. The potential created by a suitable choice of precursor seems to be larger than those afforded by the optimum concentration of basic catalyst, and ethyl silicate 40 may be recommended for use in this situation. Figure 7. TEM image of xerogels prepared using TEOS (left) and P1 (right) precursors; Si/NH3 molar ratio of 1:0.032.
LA0400656
xerogels at low cost and ethyl silicate 40 may be worth consideration in this situation. This concept has recently
(21) Kholdeeva, O. A.; Vanina, M. P.; Timofeeva, M. N.; Maksimovskaya, R. I.; Trubitsina, T. A.; Melgunov, M. S.; Burgina, E. B.; MrowiecBiałon´, J.; Jarze¸ bski, A. B.; Hill, C. L. J. Catal. 2004, 226, 363.
