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The influence of the wet gels processing on the structure and properties of silica xerogels
Microporous and Mesoporous Materials 84 (2005) 229–235 www.elsevier.com/locate/micromeso
The influence of the wet gels processing on the structure and properties of silica xerogels Alexandra Fidalgo, Laura M. Ilharco
*
Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais, 1, Pt-1049-001 Lisboa, Portugal Received 3 February 2005; received in revised form 13 April 2005; accepted 17 April 2005 Available online 1 July 2005
Abstract The post-synthesis processing of silica alcogels was investigated, with the aim of optimizing the production of monolithic silica xerogels by evaporative drying. The influence of the ageing, washing and drying conditions on the structure and properties of the final xerogels was studied. The initial chemical composition and the remaining processing parameters were pre-determined: the silica precursor (tetraethoxysilane, TEOS), the molar ratios TEOS:H2O:Solvent (1:4:4), the pre-hydrolysis conditions (HCl:TEOS molar ratio of 0.003, stirring at 333 K for 60 min), and the condensation conditions (333 K, with NH4OH:HCl molar ratio of 1). The ageing solvent was varied from the mother liquor to a mixture of TEOS/water/i-propanol with the initial sol composition, the ageing period ranged from 1 to 96 h, the rehearsed washing solvents were n-heptane, n-propanol and i-propanol, and the drying temperatures ranged from 303 to 353 K. The xerogels were characterized by their envelope density, total porosity, specific surface area, pore morphology and molecular structure. It was shown that the best washing solvent to obtain silica monoliths is i-propanol, and that the total porosity and pore morphology of the xerogels are determined by the ageing conditions and by the drying temperature. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Sol–gel; Silica xerogels; Ageing; Solvent; Drying; Structure; Physical properties
1. Introduction It is well known that the molecular structure, pore morphology and physical properties of silica gels prepared by the mild sol–gel process are determined by chemical and processing parameters [1–13]. The applications of these materials in the XXI century are so demanding that to understand and control the influence of those parameters is by no means a concluded task. Besides, the individual effect of each one is not easy to interpret, since most of them are interconnected. Several important papers have stressed the relevance of the chemical parameters, such as the nature of the starting
*
Corresponding author. Tel.: +351 218419220; fax: +351 218464455. E-mail address: [email protected] (L.M. Ilharco).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.04.021
alkoxide [7,11,14], the hydrolysis ratio (water:alkoxide molar ratio) [3,8], the co-solvent [9], and the catalysis conditions that determine the kinetics and mechanisms of the hydrolysis and condensation reactions [1,2,8,13]. Some attention has also been devoted in the literature to the influence of the processing factors, such as the reaction temperature [9], the ageing period and conditions [15–18], the washing solvent [19–21] and the drying conditions [15,19,22]. Once the alcogel (wet gel with the pores filled mostly with alcohol) is formed, it is crucial that an appropriate ageing follows, to stiffen the silica network and favor dissolution/re-precipitation processes. Several phenomena can occur during this period, such as additional hydrolysis and condensation, polymerization, esterification, and Ostwald ripening [9]. The ageing conditions are determinant of the wet gel structure. Namely, if
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ageing is carried out in the mother liquor, the resulting gel is characterized by low porosity. On the contrary, if it proceeds in alcoholic solutions containing the silica precursor, the hydrolysis, condensation and precipitation of the added monomers increase the stiffness of the network without drastic modifications in the pore size [23]. A long ageing eventually results in coarsening, with the collapse of the smaller pores into larger ones, decreasing the gel surface area [24]. This is essential to minimize fracturing upon drying [25], and also to improve the mechanical properties of the wet gel [16,24,26]. The great importance of the solvent used for washing the aged gels derives from the fact that it will be the evaporating fluid upon drying. If the alcogel remains immerse in a residual solution containing water, unreacted TEOS, catalysts and organic solvent (or mixture of solvents), concentration gradients of the more volatile species may imply their diffusion controlled rather than flux controlled transport. Due to different diffusion coefficients of those components, large internal stresses in the network result from their inter-diffusion through the weakly permeable alcogel, leading to cracking. The presence of residual sol in the pores may thus be a source of tensions upon drying. To increase the possibility of obtaining monolithic xerogels with controlled porosity, it must be replaced by an appropriate drying fluid. Drying is the last critical step of a xerogel production. For rationalization purposes, drying may be divided in three stages: the first one occurs while the gel is still immerse in liquid, and corresponds to an approximately constant solvent evaporation rate (usually named the constant rate period or the shrinking period); the second one starts when the gel becomes exposed to atmosphere and drying occurs by solvent flux to the surface, causing a continuous decrease in the rate of mass loss (first falling rate period or opaque period); the last one consists in the evaporation of solvent within the gel, followed by diffusion towards the surface, which accounts for a negligible evaporation rate (second falling rate period). The pressure gradients in the liquid produce differential strain in the solid and hence drying stresses, which are more likely to occur during the opaque period [27]. This simplified description allows to understand the influence of the drying medium and temperature on whether a monolith or a number of pieces is eventually obtained: the temperature controls the viscosity, diffusion coefficient and liquid–vapor tension of the drying medium (the washing solvent) and, therefore, the capillary pressures developed upon drying. However, a major drawback of the sol–gel process is still the difficulty to avoid cracking during evaporative drying of monolithic bodies. In the present paper, we are mostly interested in the optimization of the post-synthesis processing parameters that contribute to reduce the capillary tension created by the liquid–vapor menis-
cus formed during evaporation, eventually allowing to obtain monolithic silica xerogels. With this purpose, several alcogels were prepared with the same composition and in the same conditions. In the post-synthesis processing, the ageing medium and period were varied, different washing solvents were used, and drying occurred at different temperatures. The final xerogels were characterized by their envelope density and total porosity, and their surface area and pore morphology were assessed by analysis of N2 adsorption isotherms, at 77 K. The molecular structure was studied using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.
2. Experimental section The silica wet gels were synthesized by a two-step sol– gel process, consisting in the catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS), [Si(OC2H5)4], in an organic medium. A schematic diagram of the preparation procedure is shown in Fig. 1. The detailed preparation procedure has been described elsewhere [13]. Very briefly, the initial molar composition was TEOS:H2O:i-PrOH:HCl = 1:4:4:0.003 (equivalent to a TEOS concentration of 40% in weight, hydrolysis ratio of 4, and hydrolysis pH of 2), hydrolysis occurred under stirring at 333 K for 60 min, and condensation proceeded at the same temperature, with NH4OH/HCl molar ratio of 1 (equivalent to a pH of 7). Gelation occurs within minutes at this temperature. All chemicals (TEOS, HCl and NH4OH, i-PrOH, nHep and n-PrOH) were p.a. grade and used without further purification. HCl 1/2 i-PrOH + H2O
1h sealed 140 rpm 333 K
1/2 i-PrOH + TEOS
HYDROLYSIS
NH4OH 333 K Ageing solution
Solvent
Until gelation 333 K
sealed 333 K
333 K
CONDENSATION
AGEING
Saturated atmosphere DRYING
Residual WASHING
Fig. 1. Scheme of the synthesis procedure.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235 Table 1 Series of xerogels prepared and parameters under analysis: ageing period and medium (Ax), washing solvent (Wy) and drying temperature (Dz). x varied between 1 and 96 h; y between n-propanol, i-propanol and n-heptane; z between 303 and 353 K Parameter
Ageing period/h Washing solvent Drying temperature/K
Series Ax
Wy
Dz
x i-PrOH 333
1 y 333
1 i-PrOH z
In order to study the influence of the post-synthesis processing, the alcogels thus obtained were differently aged, washed and dried, partially covered, in a solvent saturated environment, until the weight loss became negligible. The samples with different ageing conditions were named as Ax (where x stands for the ageing period, varied from 1 to 96 h). Those washed with different solvents were named as Wy (where y stands for n-propanol, i-propanol or n-heptane), and those dried at different temperatures were named as Dz (where z stands for 303, 333 and 353 K). Table 1 summarizes the fixed and variable parameters for each of these series of samples. The envelope densities of the xerogels were measured with a GeoPyc 1360 by Micromeritics, using a consolidation force of 50 N for 17 measurement cycles. The samples were outgassed at room temperature, to a residual pressure of 10 4 mbar, prior to weighing. The pore structure of the xerogels was studied by N2 adsorption– desorption isotherms at 77 K, with an equilibration time of 5 s, using an ASAP 2000, from Micromeritics. The samples were slowly outgassed under vacuum for 24 h at 333 K, plus another 24 h at 293 K. The molecular structure was analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, using a Mattson RS1 FTIR spectrometer, with a Specac Selector, in the range 4000–400 cm 1 (wide band MCT detector), at 4 cm 1 resolution. The spectra result from the ratio of 500 added scans for each sample (finely grinded xerogel plus KBr) against the same number of scans for the background (finely grinded KBr).
3. Results and discussion Although all the samples were prepared and processed simultaneously, the results presentation will follow the post-synthesis processing sequence: ageing, washing and drying. 3.1. Ageing medium and period The influence of the ageing conditions on the structure and properties of the final xerogels was studied starting with identical alcogel samples that, once aged, were further processed similarly, i.e., washed with
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i-PrOH and dried at 333 K. These were not arbitrary choices, since all the previous synthesis steps were carried out at this temperature and using i-propanol as the preparation solvent. Different ageing media were used: the residual sol, water/i-PrOH and TEOS/water/i-PrOH mixtures with various compositions. The purpose of adding water to the ageing medium was to favor the dissolution and re-precipitation of silica, which is driven by differences in solubility between surfaces with different curvature radii [16,28]. The addition of TEOS intended to improve the gel stiffness and strength, through its hydrolysis and condensation, followed by precipitation of the monomers/oligomers onto the gel network [28,29]. Simultaneous addition of water also favors this second process, by increasing TEOS reactivity. Attempts to accelerate the process by addition of NH4OH failed, since silica precipitated on the external surface of the alcogels, or a different gel formed. In residual sol, it was observed that ageing periods shorter than 24 h resulted in fracture development during drying. A period of 48 h allowed obtaining monolithic xerogels with increased porosity, but ageing periods longer than 72 h (e.g., 96 h) led to a decrease in the porosity of the dry gels. Eventually, the selected procedure was to leave the gel to age in the residual solution for 24 h at the reaction temperature (333 K), and then plunge it in a mixture of TEOS/water/i-PrOH (in the proportions used to prepare the alcogel), for a variable period, at the same temperature. The effect of the second ageing period followed the same trend as described for the residual sol. This can be observed by comparing xerogels that were soaked in the TEOS/water/i-PrOH mixture for 12, 24 and 48 h, after the initial 24 h period in mother liquor (samples named A12, A24 and A48, respectively). The N2 adsorption/desorption isotherms and the corresponding pore size distributions, obtained by the BJH (Barret–Joyner–Halienda) method [30], are shown in Fig. 2. The isotherms shown in Fig. 2A are type IV, with hysteresis loops type H2, characteristic of capillary condensation in mesopores [30]. The total adsorption volume increases with the ageing period up to a certain point, decreasing from there on, as suggested by the isotherms plateaus. The pore size distributions in Fig. 2B were assessed from the adsorption volumes, assuming cylindrical pore geometry, and using the BJH approximation. It is clear that the average pore size shifts to a higher value with increasing ageing time up to a certain point, reversing afterwards. Concurrently, increasing ageing periods cause an initial broadening of the pore size distribution, followed by a narrowing. Some physical properties of the same xerogels are listed in Table 2. They exhibit the same type of behavior with the ageing period: the specific pore volume, Vp the
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A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235 700
1.8
A12 A24 A48
A
650 600 550
A12 A24 A48
1.4
500
1.2
450
Vp/∆lnΦ
Vads / cm3.g-1
B
1.6
400 350 300
1.0 0.8 0.6
250 200
0.4
150
0.2
100 50 0.0
0.0 0.2
0.4
0.6
0.8
1
1.0
10
100
Φ / nm
p/p0
Fig. 2. (A) N2 adsorption/desorption isotherms of three xerogels that were aged for 24 h in the mother liquor plus 12 h (A12), 24 h (A24) and 48 h (A48) in a mixture of TEOS/water/i-PrOH with the same proportions as in the initial recipe and (B) corresponding pore size distributions using the BJH assumption.
Sample
Vp/cm3 g
A12 A24 A48
0.62 1.18 0.73
1
SBET/m2 g 877 1046 671
1
/BJH,ads/nm
qe/g cm
3.57 4.79 4.32
0.89 0.68 0.79
3
surface area, SBET and the average pore diameter, /BJH firstly increase and then decrease. This is accompanied by an opposite variation of the envelope density, qe. The variations observed by increasing the second ageing period up to 24 h (increase in Vp, SBET and /BJH and decrease in qe) are caused by a coarsening of the silica network, resulting from two processes: (i) the deposition of silica dissolved from the particle surfaces (positive curvature) onto necks between primary particles (negative solubility); (ii) the reaction of the fresh monomers (added TEOS) with the original gel network. Even if the wet gel is gaining mass due to the later, the xerogelÕs network becomes stiffer, and consequently, shrinkage and pore collapse are reduced during drying. Beyond 24 h, the only active process is the dissolution of the smaller silica particles, with re-precipitation onto the larger ones. Therefore, longer ageing is not responsible for further stiffening, contributing only to a reduction of the pore size, and reversing the variation of the other xerogels physical properties. These results suggest that the most effective ageing is obtained when coarsening has occurred only to a small degree, adequate to guarantee an increase in the network stiffness. The structural evolution caused by the two ageing processes referred above can be further elucidated by
the DRIFT spectra of the differently aged xerogels, shown in Fig. 3. The broad band at 3400 cm 1 is assigned to the mO–H mode of residual silanol (Si–OH) groups and of adsorbed water, involved in a variety of hydrogen bonds. The presence of water can also be observed from the band at 1640 cm 1, assigned to the dH–O–H mode. The most intense band, with maximum at 1080 cm 1, is assigned to the masSi–O–Si mode. This mode is usually split, by long range Coulomb interactions, into a transverse optical (TO) and a longitudinal optical (LO) components. The latter is responsible for the high wavenumber shoulder, at 1100 cm 1. The position and shape of this band are highly dependent on the average Si–O–Si angles and bond lengths, thus reflecting the distribution of primary silica network units (mostly
νasSi-O-Si
A12 A24 A48 Kubelka-Munk / a.u.
Table 2 Specific pore volume, Vp, surface area, SBET, average pore diameter, /BJH, and envelope density, qe, of silica xerogels that were aged for 24 h in the mother liquor plus 12 h (A12), 24 h (A24) and 48 h (A48) in a mixture of TEOS/water/i-PrOH with the same proportions as in the initial recipe
ν Si-O d
ν O-H
νsSi-O-Si δ H-O-H
4000
3600
3200
2800
1600 1400 1200 1000 800 600
Wavenumber / cm-1 Fig. 3. DRIFT spectra of the xerogel samples A12, A24 and A48. The spectra were normalized to the masSi–O–Si maximum, to allow comparing band intensities relative to the silica network.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235
cyclic siloxane units containing four or six Si atoms) [31]. Also relevant are the bands at 950 and 800 cm 1, assigned to the mSi–Od (dangling oxygen atoms in the silica network, including silanol groups and broken Si–O– Si bridges) and to the msSi–O–Si modes, respectively. The effect of the dissolution/re-precipitation process in the silica network can be inferred from the evolution of the silanol related bands. With increasing ageing time, there is a gradual decrease in the relative intensities of the bands at 3400 and 950 cm 1, indicating an increase in the degree of condensation, while silica dissolves from particle surfaces (containing the Si–OH groups) and redeposits onto the necks as SiO2. This also indicates that the xerogels become more hydrophobic with increasing ageing time, which may be confirmed by the simultaneous decrease in the relative intensity of the band related only to adsorbed water (at 1640 cm 1). The effect of the stiffening process in the structure of the resulting xerogel can be inferred from the evolution of the masSi–O–Si band. By increasing the additional ageing period from 12 to 24 h, there is an evolution of the silica backbone structure, as indicated by the decrease of the relative intensity of the high wavenumber shoulder (at 1100 cm 1). Such decrease is usually associated with a more glass-like structure [32], reflecting the increase in the silica network stiffness upon ageing. However, this evolution is no longer observed when the additional ageing period is increased from 24 to 48 h, confirming that increased ageing times involve only the dissolution/re-precipitation process. As a whole, these results suggest that there is an optimum ageing period for each wet gel. In the present study, the advisable ageing should consist in a 24 h period (in mother liquor) plus 24 h (in water/TEOS/ i-PrOH). However, the period required to attain the same ageing degree depends obviously on the dimensions and shape of the wet gel.
1.0
3.2. Washing solvent In order to analyze the influence of the washing solvent, the samples of series Wy were dried in the same conditions, i.e., in a solvent saturated environment at 333 K, in a partially covered container. The washing solvent effect is well described by the drying profiles of the samples, shown in Fig. 4A, in terms of relative mass (mass at instant t/initial mass, m/m0) versus time, and Fig. 4B, in terms of mass loss rate (dm/dt) versus time. Fig. 4A only shows two of the three drying stages referred above: the shrinking period, where large mass variations occur, and the final stage, where the sample mass stabilizes and the gel may be considered dry. Fig. 4B is more informative, as it allows identifying also the intermediate drying stage. The solvent evaporation rate is approximately constant during the shrinking period, drastically decreasing throughout the opaque period, and eventually becoming approximately zero at the last stage. At first sight, alcogels dry faster in n-heptane (the last stage is reached at 48 h), followed by n-propanol (144 h) and i-propanol (192 h). The large differences between the three solvents examined rely on the drying behavior during the first two stages: the evaporation rate in the constant rate period is much higher for n-heptane and this period is much shorter than for the alcohols; on the other hand, the opaque period is a lot smoother for i-PrOH than for the other two solvents. During the constant rate period, evaporation occurs from the liquid surface, or from liquid filled pores. Thus, for washed wet gels, the evaporation rate is governed by the difference between the vapor pressure of the solvent and the partial vapor pressure in the drying chamber atmosphere. During this stage, a small shrinkage of the wet gel is driven by capillary pressure (DP = 2c/r, c being the surface tension of the liquid and r the meniscus radius), when menisci start to appear at the gel surface.
-0.7
Wn-PrOH Wi-PrOH Wn-Hep
A 0.9
Wn-PrOH Wi-PrOH Wn-Hep
B -0.6
0.8
(dm/dt) / g.hr-1
-0.5
0.7
m/m0
233
0.6 0.5 0.4
-0.4 -0.3 -0.2 -0.1
0.3
0.0 0
48
96 144 192 240 288 336
t/h
0
48
96 144 192 240 288 336
t/h
Fig. 4. Drying profiles of wet gels washed with n-PrOH, i-PrOH, and n-Hep: (A) relative mass (m/m0) and (B) mass loss rate (dm/dt) as a function of time (t).
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To release this pressure, the gel shrinks and its compressive strength increases. The time length for this drying period (initial plateau in Fig. 4B) decreases in the order n-propanol (96 h), i-propanol (48 h) and n-heptane (24 h), indicating that the meniscus radius must increase conversely, since all the alcogels have nearly the same compressive strength. The second drying period (the opaque period) begins when the menisci start receding into the gel body. During this stage, drying occurs by evaporation of the solvent once it diffuses to the surface, and is therefore controlled by its adhesion to the pore walls, which in turn depends on the interactions between the silanol groups at the inner pore surfaces and the solvent molecules. In this regard, the solvent that clearly looses is n-heptane, which is unable to form hydrogen bonds and thus evaporates faster. Comparing the two alcohols, the OH group of i-PrOH is much more accessible than that of n-PrOH, since the latter has rather hindered rotational conformers. We believe that this is the main reason for the second drying period to be so slow when the solvent is i-PrOH. This behavior may explain why i-PrOH is the most adequate solvent to obtain monolithic xerogels, since fissuring occurs with higher probability during that drying period. 3.3. Drying temperature To evaluate the influence of the drying temperature, samples Dz were prepared, aged and washed in similar conditions and further dried at different temperatures (303, 333 and 353 K), in a saturated environment at ambient pressure. The N2 adsorption/desorption isotherms of the thus dried xerogels are shown in Fig. 5A. Fig. 5B shows the corresponding t-plots, obtained by conversion of the partial pressures, p/p0, to the average statistical thickness, t, of the N2 film adsorbed on a
400
A
non-porous hydroxilated silica, assuming that the thickness of a single molecular layer is of about 0.354 nm. The effect of increasing the drying temperature on the pore morphology of the xerogels can be clearly seen in the evolution of the isotherms: from type I (characteristic of essentially microporous materials) to type IV (typical of mesoporous materials) [30]. Nevertheless, all the isotherms present hysteresis loops, which are consistent with the presence of mesopores. On the other hand, from the analysis of the corresponding t-plots, one may conclude that all the samples contain micropores as well (intercept of the low t linear region, after conversion to liquid volume). This drying temperature effect is particularly clear up to 333 K. An increase above 333 K imparts only small changes in the pore morphology, being responsible for just a small increase in the total pore volume (related to the plateau of the isotherms). The specific pore volume and surface area (obtained from the N2 adsorption isotherms), the specific micropore volume (obtained from the t-plots), and the envelope densities of the same xerogel samples are listed in Table 3. The average pore diameter obtained from the BJH analysis upon adsorption is also included, as in Section 3.1. The changes observed in the pore morphology when increasing the drying temperature from 303 to 333 K
Table 3 Specific pore volume, Vp, surface area, SBET, micropore volume, Vlp, envelope density, qe, and average pore diameter, /BJH, of silica xerogels dried at different temperatures
D303 D333 D353
1
SBET/m2 g 1
Vlp/ cm3 g
577 744 788
0.060 0.049 0.034
Vads / cm3g-1
250 200
qe/ g cm 1.50 1.04 1.00
/BJH,ads/nm 3
2.58 3.28 3.20
B
200
100
150
D303 D333 D353
100
0 0.0
1
300
300
Vads / cm3g-1
0.30 0.56 0.59
400
D303 D333 D353
350
Vp/cm3 g
Sample
0.2
0.4
p/p0
0.6
0.8
1.0
0
2
4
6
8 10 12 14 16 18 20
t/Å
Fig. 5. (A) N2 adsorption/desorption isotherms of three xerogels that were dried at 303 (D303), 333 (D333) and 353 K (D353) and (B) corresponding t-plots.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235
are accompanied by a significant decrease in the envelope density of the xerogel (qe), and correspond to an increase in the specific pore volume and surface area of the samples. Concurrently, an increase in the average pore diameter and a slight decrease in the micropore volume are observed. In conclusion, by increasing the drying temperature from 303 to 333 K, more porous xerogels may be obtained, due to a better preservation of the mesopore structure upon drying, with little loss of the micropore structure. A further increase of the drying temperature to 353 K imparts no significant changes in the physical properties of the samples. By changing the drying temperature from 303 to 353 K, the xerogel evolves from a micro/mesoporous material (20% micropores) to an essentially mesoporous one (6% micropores). Although the evaporation rate is higher at higher temperatures, the pressure gradient within the alcogel becomes lower, probably due to a prevailing effect of the decrease in the solvent viscosity. Thus, the gel shrinks less during the first drying stage. That explains the increase in the average pore diameter for samples dried above 303 K. Higher average pore diameters and lower interfacial tensions have the common effect of lowering the capillary pressure felt by the pore walls at the critical point, thus contributing to preserve a higher amount of mesopores in these samples.
4. Conclusions The possibility of obtaining monolithic silica xerogels by evaporative drying at ambient pressure is achieved by optimization of the ageing procedure and choice of the drying medium (washing solvent). By increasing the ageing period up to a certain limit, the silica network stiffens, with an increase of the average pore dimensions. However, further ageing leads to a decrease in the xerogel porosity, without any gain in stiffness. The critical ageing period depends on the geometry and dimensions of the wet gel samples. In the present study, for cylindrical rod shaped wet gels of 20 cm3, the optimum ageing procedure proved to consist of two 24 h periods. A smooth transition from the shrinking to the opaque stage of drying, to avoid fissuring, is accomplished by using i-propanol as the washing solvent. The possibility of tailoring the xerogelÕs porosity and pore morphology is further achieved by optimization of the drying temperature: an increase from 303 to 333 K allows raising the porosity, retaining a higher mesopore/micropore proportion. Above 333 K, no major changes are observed.
Acknowledgements This work was supported by Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT)—Project POCTI/CTM/33487/
235
2000. The authors wish to express their gratitude to Prof. Rui Almeida for the use of the ASAP 2000. Alexandra Fidalgo acknowledges FCT for the post-doc grant PRAXIS XXI/BPD/20234/2004.
References [1] T.W. Zerda, I. Artaki, J. Jonas, J. Non-Cryst. Solids 81 (1986) 365. [2] E.J. Pope, J.D. Mackenzie, J. Non-Cryst. Solids 87 (1986) 185. [3] R.A. Assink, B.D. Kay, J. Non-Cryst. Solids 99 (1988) 359. [4] C.J. Brinker, J. Non-Cryst. Solids 100 (1988) 31. [5] H. Schmidt, J. Non-Cryst. Solids 100 (1988) 51. [6] D.R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [7] I. Hasegawa, S. Sakka, J. Non-Cryst. Solids 100 (1988) 201. [8] H. Yang, Z. Ding, Z. Jiang, X. Xu, J. Non-Cryst. Solids 112 (1989) 449. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science and Technology— the Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, MA, 1990. [10] L.L. Hench, J.K. West, Chem. Rev. 90 (1994) 33. [11] E. Nielsen, M.-A. Einarsrud, G.W. Scherer, J. Non-Cryst. Solids 221 (1997) 135. [12] D.C.L. Vasconcelos, W.R. Campos, V. Vasconcelos, Mater. Sci. Eng. A 334 (2002) 53. [13] A. Fidalgo, M.E. Rosa, L.M. Ilharco, Chem. Mater. 15 (2003) 2186. [14] C. Alie´, R. Pirard, J.P. Pirard, J. Non-Cryst. Solids 311 (2002) 304. [15] J. Zarzycki, M. Prassas, J. Phalippou, J. Mater. Sci. 17 (1982) 3371. [16] T. Mizuno, H. Nagata, S. Manabe, J. Non-Cryst. Solids 100 (1988) 236. [17] P.J. Davis, C.J. Brinker, D.M. Smith, J. Non-Cryst. Solids 142 (1992) 189. [18] S. Haereid, J. Anderson, M.-A. Einarsrud, D.W. Hua, D.M. Smith, J. Non-Cryst. Solids 185 (1995) 221. [19] R. Deshpande, D.W. Hua, D.M. Smith, C.J. Brinker, J. NonCryst. Solids 144 (1992) 32. [20] D.M. Smith, D. Stein, J.M. Anderson, W. Ackerman, J. NonCryst. Solids 186 (1995) 104. [21] J.H. Harreld, T. Ebina, N. Tsubo, G. Stucky, J. Non-Cryst. Solids 298 (2002) 241. [22] W.L. Huang, S.H. Cui, K.M. Liang, S.R. Gu, Z.F. Yuan, J. Colloid Interface Sci. 246 (2002) 129. [23] M.-A. Einarsrud, J. Non-Cryst. Solids 225 (1998) 1. [24] S. Haereid, M. Dahle, S. Lima, M.-A. Einarsrud, J. Non-Cryst. Solids 186 (1995) 96. [25] G.M. Pajonk, A.V. Rao, B.M. Sawant, N.N. Parvathy, J. NonCryst. Solids 209 (1997) 40. [26] J.W. Scherer, S.A. Pardenek, R.M. Swiatek, J. Non-Cryst. Solids 107 (1988) 14. [27] G.W. Scherer, J. Non-Cryst. Solids 109 (1989) 171. [28] S. Haereid, E. Nilsen, V. Ranum, M.-A. Einarsrud, J. Sol–Gel Sci. Technol. 8 (1997) 153. [29] S. Haereid, E. Nilsen, M.-A. Einarsrud, J. Non-Cryst. Solids 204 (1996) 228. [30] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [31] A. Fidalgo, L.M. Ilharco, Chem. Eur. J. 10 (2004) 392. [32] R.M. Almeida, C.G. Pantano, J. Appl. Phys 68 (1990) 4225.
The influence of the wet gels processing on the structure and properties of silica xerogels Alexandra Fidalgo, Laura M. Ilharco
*
Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´cnico, Av. Rovisco Pais, 1, Pt-1049-001 Lisboa, Portugal Received 3 February 2005; received in revised form 13 April 2005; accepted 17 April 2005 Available online 1 July 2005
Abstract The post-synthesis processing of silica alcogels was investigated, with the aim of optimizing the production of monolithic silica xerogels by evaporative drying. The influence of the ageing, washing and drying conditions on the structure and properties of the final xerogels was studied. The initial chemical composition and the remaining processing parameters were pre-determined: the silica precursor (tetraethoxysilane, TEOS), the molar ratios TEOS:H2O:Solvent (1:4:4), the pre-hydrolysis conditions (HCl:TEOS molar ratio of 0.003, stirring at 333 K for 60 min), and the condensation conditions (333 K, with NH4OH:HCl molar ratio of 1). The ageing solvent was varied from the mother liquor to a mixture of TEOS/water/i-propanol with the initial sol composition, the ageing period ranged from 1 to 96 h, the rehearsed washing solvents were n-heptane, n-propanol and i-propanol, and the drying temperatures ranged from 303 to 353 K. The xerogels were characterized by their envelope density, total porosity, specific surface area, pore morphology and molecular structure. It was shown that the best washing solvent to obtain silica monoliths is i-propanol, and that the total porosity and pore morphology of the xerogels are determined by the ageing conditions and by the drying temperature. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Sol–gel; Silica xerogels; Ageing; Solvent; Drying; Structure; Physical properties
1. Introduction It is well known that the molecular structure, pore morphology and physical properties of silica gels prepared by the mild sol–gel process are determined by chemical and processing parameters [1–13]. The applications of these materials in the XXI century are so demanding that to understand and control the influence of those parameters is by no means a concluded task. Besides, the individual effect of each one is not easy to interpret, since most of them are interconnected. Several important papers have stressed the relevance of the chemical parameters, such as the nature of the starting
*
Corresponding author. Tel.: +351 218419220; fax: +351 218464455. E-mail address: [email protected] (L.M. Ilharco).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.04.021
alkoxide [7,11,14], the hydrolysis ratio (water:alkoxide molar ratio) [3,8], the co-solvent [9], and the catalysis conditions that determine the kinetics and mechanisms of the hydrolysis and condensation reactions [1,2,8,13]. Some attention has also been devoted in the literature to the influence of the processing factors, such as the reaction temperature [9], the ageing period and conditions [15–18], the washing solvent [19–21] and the drying conditions [15,19,22]. Once the alcogel (wet gel with the pores filled mostly with alcohol) is formed, it is crucial that an appropriate ageing follows, to stiffen the silica network and favor dissolution/re-precipitation processes. Several phenomena can occur during this period, such as additional hydrolysis and condensation, polymerization, esterification, and Ostwald ripening [9]. The ageing conditions are determinant of the wet gel structure. Namely, if
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ageing is carried out in the mother liquor, the resulting gel is characterized by low porosity. On the contrary, if it proceeds in alcoholic solutions containing the silica precursor, the hydrolysis, condensation and precipitation of the added monomers increase the stiffness of the network without drastic modifications in the pore size [23]. A long ageing eventually results in coarsening, with the collapse of the smaller pores into larger ones, decreasing the gel surface area [24]. This is essential to minimize fracturing upon drying [25], and also to improve the mechanical properties of the wet gel [16,24,26]. The great importance of the solvent used for washing the aged gels derives from the fact that it will be the evaporating fluid upon drying. If the alcogel remains immerse in a residual solution containing water, unreacted TEOS, catalysts and organic solvent (or mixture of solvents), concentration gradients of the more volatile species may imply their diffusion controlled rather than flux controlled transport. Due to different diffusion coefficients of those components, large internal stresses in the network result from their inter-diffusion through the weakly permeable alcogel, leading to cracking. The presence of residual sol in the pores may thus be a source of tensions upon drying. To increase the possibility of obtaining monolithic xerogels with controlled porosity, it must be replaced by an appropriate drying fluid. Drying is the last critical step of a xerogel production. For rationalization purposes, drying may be divided in three stages: the first one occurs while the gel is still immerse in liquid, and corresponds to an approximately constant solvent evaporation rate (usually named the constant rate period or the shrinking period); the second one starts when the gel becomes exposed to atmosphere and drying occurs by solvent flux to the surface, causing a continuous decrease in the rate of mass loss (first falling rate period or opaque period); the last one consists in the evaporation of solvent within the gel, followed by diffusion towards the surface, which accounts for a negligible evaporation rate (second falling rate period). The pressure gradients in the liquid produce differential strain in the solid and hence drying stresses, which are more likely to occur during the opaque period [27]. This simplified description allows to understand the influence of the drying medium and temperature on whether a monolith or a number of pieces is eventually obtained: the temperature controls the viscosity, diffusion coefficient and liquid–vapor tension of the drying medium (the washing solvent) and, therefore, the capillary pressures developed upon drying. However, a major drawback of the sol–gel process is still the difficulty to avoid cracking during evaporative drying of monolithic bodies. In the present paper, we are mostly interested in the optimization of the post-synthesis processing parameters that contribute to reduce the capillary tension created by the liquid–vapor menis-
cus formed during evaporation, eventually allowing to obtain monolithic silica xerogels. With this purpose, several alcogels were prepared with the same composition and in the same conditions. In the post-synthesis processing, the ageing medium and period were varied, different washing solvents were used, and drying occurred at different temperatures. The final xerogels were characterized by their envelope density and total porosity, and their surface area and pore morphology were assessed by analysis of N2 adsorption isotherms, at 77 K. The molecular structure was studied using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.
2. Experimental section The silica wet gels were synthesized by a two-step sol– gel process, consisting in the catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS), [Si(OC2H5)4], in an organic medium. A schematic diagram of the preparation procedure is shown in Fig. 1. The detailed preparation procedure has been described elsewhere [13]. Very briefly, the initial molar composition was TEOS:H2O:i-PrOH:HCl = 1:4:4:0.003 (equivalent to a TEOS concentration of 40% in weight, hydrolysis ratio of 4, and hydrolysis pH of 2), hydrolysis occurred under stirring at 333 K for 60 min, and condensation proceeded at the same temperature, with NH4OH/HCl molar ratio of 1 (equivalent to a pH of 7). Gelation occurs within minutes at this temperature. All chemicals (TEOS, HCl and NH4OH, i-PrOH, nHep and n-PrOH) were p.a. grade and used without further purification. HCl 1/2 i-PrOH + H2O
1h sealed 140 rpm 333 K
1/2 i-PrOH + TEOS
HYDROLYSIS
NH4OH 333 K Ageing solution
Solvent
Until gelation 333 K
sealed 333 K
333 K
CONDENSATION
AGEING
Saturated atmosphere DRYING
Residual WASHING
Fig. 1. Scheme of the synthesis procedure.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235 Table 1 Series of xerogels prepared and parameters under analysis: ageing period and medium (Ax), washing solvent (Wy) and drying temperature (Dz). x varied between 1 and 96 h; y between n-propanol, i-propanol and n-heptane; z between 303 and 353 K Parameter
Ageing period/h Washing solvent Drying temperature/K
Series Ax
Wy
Dz
x i-PrOH 333
1 y 333
1 i-PrOH z
In order to study the influence of the post-synthesis processing, the alcogels thus obtained were differently aged, washed and dried, partially covered, in a solvent saturated environment, until the weight loss became negligible. The samples with different ageing conditions were named as Ax (where x stands for the ageing period, varied from 1 to 96 h). Those washed with different solvents were named as Wy (where y stands for n-propanol, i-propanol or n-heptane), and those dried at different temperatures were named as Dz (where z stands for 303, 333 and 353 K). Table 1 summarizes the fixed and variable parameters for each of these series of samples. The envelope densities of the xerogels were measured with a GeoPyc 1360 by Micromeritics, using a consolidation force of 50 N for 17 measurement cycles. The samples were outgassed at room temperature, to a residual pressure of 10 4 mbar, prior to weighing. The pore structure of the xerogels was studied by N2 adsorption– desorption isotherms at 77 K, with an equilibration time of 5 s, using an ASAP 2000, from Micromeritics. The samples were slowly outgassed under vacuum for 24 h at 333 K, plus another 24 h at 293 K. The molecular structure was analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, using a Mattson RS1 FTIR spectrometer, with a Specac Selector, in the range 4000–400 cm 1 (wide band MCT detector), at 4 cm 1 resolution. The spectra result from the ratio of 500 added scans for each sample (finely grinded xerogel plus KBr) against the same number of scans for the background (finely grinded KBr).
3. Results and discussion Although all the samples were prepared and processed simultaneously, the results presentation will follow the post-synthesis processing sequence: ageing, washing and drying. 3.1. Ageing medium and period The influence of the ageing conditions on the structure and properties of the final xerogels was studied starting with identical alcogel samples that, once aged, were further processed similarly, i.e., washed with
231
i-PrOH and dried at 333 K. These were not arbitrary choices, since all the previous synthesis steps were carried out at this temperature and using i-propanol as the preparation solvent. Different ageing media were used: the residual sol, water/i-PrOH and TEOS/water/i-PrOH mixtures with various compositions. The purpose of adding water to the ageing medium was to favor the dissolution and re-precipitation of silica, which is driven by differences in solubility between surfaces with different curvature radii [16,28]. The addition of TEOS intended to improve the gel stiffness and strength, through its hydrolysis and condensation, followed by precipitation of the monomers/oligomers onto the gel network [28,29]. Simultaneous addition of water also favors this second process, by increasing TEOS reactivity. Attempts to accelerate the process by addition of NH4OH failed, since silica precipitated on the external surface of the alcogels, or a different gel formed. In residual sol, it was observed that ageing periods shorter than 24 h resulted in fracture development during drying. A period of 48 h allowed obtaining monolithic xerogels with increased porosity, but ageing periods longer than 72 h (e.g., 96 h) led to a decrease in the porosity of the dry gels. Eventually, the selected procedure was to leave the gel to age in the residual solution for 24 h at the reaction temperature (333 K), and then plunge it in a mixture of TEOS/water/i-PrOH (in the proportions used to prepare the alcogel), for a variable period, at the same temperature. The effect of the second ageing period followed the same trend as described for the residual sol. This can be observed by comparing xerogels that were soaked in the TEOS/water/i-PrOH mixture for 12, 24 and 48 h, after the initial 24 h period in mother liquor (samples named A12, A24 and A48, respectively). The N2 adsorption/desorption isotherms and the corresponding pore size distributions, obtained by the BJH (Barret–Joyner–Halienda) method [30], are shown in Fig. 2. The isotherms shown in Fig. 2A are type IV, with hysteresis loops type H2, characteristic of capillary condensation in mesopores [30]. The total adsorption volume increases with the ageing period up to a certain point, decreasing from there on, as suggested by the isotherms plateaus. The pore size distributions in Fig. 2B were assessed from the adsorption volumes, assuming cylindrical pore geometry, and using the BJH approximation. It is clear that the average pore size shifts to a higher value with increasing ageing time up to a certain point, reversing afterwards. Concurrently, increasing ageing periods cause an initial broadening of the pore size distribution, followed by a narrowing. Some physical properties of the same xerogels are listed in Table 2. They exhibit the same type of behavior with the ageing period: the specific pore volume, Vp the
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A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235 700
1.8
A12 A24 A48
A
650 600 550
A12 A24 A48
1.4
500
1.2
450
Vp/∆lnΦ
Vads / cm3.g-1
B
1.6
400 350 300
1.0 0.8 0.6
250 200
0.4
150
0.2
100 50 0.0
0.0 0.2
0.4
0.6
0.8
1
1.0
10
100
Φ / nm
p/p0
Fig. 2. (A) N2 adsorption/desorption isotherms of three xerogels that were aged for 24 h in the mother liquor plus 12 h (A12), 24 h (A24) and 48 h (A48) in a mixture of TEOS/water/i-PrOH with the same proportions as in the initial recipe and (B) corresponding pore size distributions using the BJH assumption.
Sample
Vp/cm3 g
A12 A24 A48
0.62 1.18 0.73
1
SBET/m2 g 877 1046 671
1
/BJH,ads/nm
qe/g cm
3.57 4.79 4.32
0.89 0.68 0.79
3
surface area, SBET and the average pore diameter, /BJH firstly increase and then decrease. This is accompanied by an opposite variation of the envelope density, qe. The variations observed by increasing the second ageing period up to 24 h (increase in Vp, SBET and /BJH and decrease in qe) are caused by a coarsening of the silica network, resulting from two processes: (i) the deposition of silica dissolved from the particle surfaces (positive curvature) onto necks between primary particles (negative solubility); (ii) the reaction of the fresh monomers (added TEOS) with the original gel network. Even if the wet gel is gaining mass due to the later, the xerogelÕs network becomes stiffer, and consequently, shrinkage and pore collapse are reduced during drying. Beyond 24 h, the only active process is the dissolution of the smaller silica particles, with re-precipitation onto the larger ones. Therefore, longer ageing is not responsible for further stiffening, contributing only to a reduction of the pore size, and reversing the variation of the other xerogels physical properties. These results suggest that the most effective ageing is obtained when coarsening has occurred only to a small degree, adequate to guarantee an increase in the network stiffness. The structural evolution caused by the two ageing processes referred above can be further elucidated by
the DRIFT spectra of the differently aged xerogels, shown in Fig. 3. The broad band at 3400 cm 1 is assigned to the mO–H mode of residual silanol (Si–OH) groups and of adsorbed water, involved in a variety of hydrogen bonds. The presence of water can also be observed from the band at 1640 cm 1, assigned to the dH–O–H mode. The most intense band, with maximum at 1080 cm 1, is assigned to the masSi–O–Si mode. This mode is usually split, by long range Coulomb interactions, into a transverse optical (TO) and a longitudinal optical (LO) components. The latter is responsible for the high wavenumber shoulder, at 1100 cm 1. The position and shape of this band are highly dependent on the average Si–O–Si angles and bond lengths, thus reflecting the distribution of primary silica network units (mostly
νasSi-O-Si
A12 A24 A48 Kubelka-Munk / a.u.
Table 2 Specific pore volume, Vp, surface area, SBET, average pore diameter, /BJH, and envelope density, qe, of silica xerogels that were aged for 24 h in the mother liquor plus 12 h (A12), 24 h (A24) and 48 h (A48) in a mixture of TEOS/water/i-PrOH with the same proportions as in the initial recipe
ν Si-O d
ν O-H
νsSi-O-Si δ H-O-H
4000
3600
3200
2800
1600 1400 1200 1000 800 600
Wavenumber / cm-1 Fig. 3. DRIFT spectra of the xerogel samples A12, A24 and A48. The spectra were normalized to the masSi–O–Si maximum, to allow comparing band intensities relative to the silica network.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235
cyclic siloxane units containing four or six Si atoms) [31]. Also relevant are the bands at 950 and 800 cm 1, assigned to the mSi–Od (dangling oxygen atoms in the silica network, including silanol groups and broken Si–O– Si bridges) and to the msSi–O–Si modes, respectively. The effect of the dissolution/re-precipitation process in the silica network can be inferred from the evolution of the silanol related bands. With increasing ageing time, there is a gradual decrease in the relative intensities of the bands at 3400 and 950 cm 1, indicating an increase in the degree of condensation, while silica dissolves from particle surfaces (containing the Si–OH groups) and redeposits onto the necks as SiO2. This also indicates that the xerogels become more hydrophobic with increasing ageing time, which may be confirmed by the simultaneous decrease in the relative intensity of the band related only to adsorbed water (at 1640 cm 1). The effect of the stiffening process in the structure of the resulting xerogel can be inferred from the evolution of the masSi–O–Si band. By increasing the additional ageing period from 12 to 24 h, there is an evolution of the silica backbone structure, as indicated by the decrease of the relative intensity of the high wavenumber shoulder (at 1100 cm 1). Such decrease is usually associated with a more glass-like structure [32], reflecting the increase in the silica network stiffness upon ageing. However, this evolution is no longer observed when the additional ageing period is increased from 24 to 48 h, confirming that increased ageing times involve only the dissolution/re-precipitation process. As a whole, these results suggest that there is an optimum ageing period for each wet gel. In the present study, the advisable ageing should consist in a 24 h period (in mother liquor) plus 24 h (in water/TEOS/ i-PrOH). However, the period required to attain the same ageing degree depends obviously on the dimensions and shape of the wet gel.
1.0
3.2. Washing solvent In order to analyze the influence of the washing solvent, the samples of series Wy were dried in the same conditions, i.e., in a solvent saturated environment at 333 K, in a partially covered container. The washing solvent effect is well described by the drying profiles of the samples, shown in Fig. 4A, in terms of relative mass (mass at instant t/initial mass, m/m0) versus time, and Fig. 4B, in terms of mass loss rate (dm/dt) versus time. Fig. 4A only shows two of the three drying stages referred above: the shrinking period, where large mass variations occur, and the final stage, where the sample mass stabilizes and the gel may be considered dry. Fig. 4B is more informative, as it allows identifying also the intermediate drying stage. The solvent evaporation rate is approximately constant during the shrinking period, drastically decreasing throughout the opaque period, and eventually becoming approximately zero at the last stage. At first sight, alcogels dry faster in n-heptane (the last stage is reached at 48 h), followed by n-propanol (144 h) and i-propanol (192 h). The large differences between the three solvents examined rely on the drying behavior during the first two stages: the evaporation rate in the constant rate period is much higher for n-heptane and this period is much shorter than for the alcohols; on the other hand, the opaque period is a lot smoother for i-PrOH than for the other two solvents. During the constant rate period, evaporation occurs from the liquid surface, or from liquid filled pores. Thus, for washed wet gels, the evaporation rate is governed by the difference between the vapor pressure of the solvent and the partial vapor pressure in the drying chamber atmosphere. During this stage, a small shrinkage of the wet gel is driven by capillary pressure (DP = 2c/r, c being the surface tension of the liquid and r the meniscus radius), when menisci start to appear at the gel surface.
-0.7
Wn-PrOH Wi-PrOH Wn-Hep
A 0.9
Wn-PrOH Wi-PrOH Wn-Hep
B -0.6
0.8
(dm/dt) / g.hr-1
-0.5
0.7
m/m0
233
0.6 0.5 0.4
-0.4 -0.3 -0.2 -0.1
0.3
0.0 0
48
96 144 192 240 288 336
t/h
0
48
96 144 192 240 288 336
t/h
Fig. 4. Drying profiles of wet gels washed with n-PrOH, i-PrOH, and n-Hep: (A) relative mass (m/m0) and (B) mass loss rate (dm/dt) as a function of time (t).
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A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235
To release this pressure, the gel shrinks and its compressive strength increases. The time length for this drying period (initial plateau in Fig. 4B) decreases in the order n-propanol (96 h), i-propanol (48 h) and n-heptane (24 h), indicating that the meniscus radius must increase conversely, since all the alcogels have nearly the same compressive strength. The second drying period (the opaque period) begins when the menisci start receding into the gel body. During this stage, drying occurs by evaporation of the solvent once it diffuses to the surface, and is therefore controlled by its adhesion to the pore walls, which in turn depends on the interactions between the silanol groups at the inner pore surfaces and the solvent molecules. In this regard, the solvent that clearly looses is n-heptane, which is unable to form hydrogen bonds and thus evaporates faster. Comparing the two alcohols, the OH group of i-PrOH is much more accessible than that of n-PrOH, since the latter has rather hindered rotational conformers. We believe that this is the main reason for the second drying period to be so slow when the solvent is i-PrOH. This behavior may explain why i-PrOH is the most adequate solvent to obtain monolithic xerogels, since fissuring occurs with higher probability during that drying period. 3.3. Drying temperature To evaluate the influence of the drying temperature, samples Dz were prepared, aged and washed in similar conditions and further dried at different temperatures (303, 333 and 353 K), in a saturated environment at ambient pressure. The N2 adsorption/desorption isotherms of the thus dried xerogels are shown in Fig. 5A. Fig. 5B shows the corresponding t-plots, obtained by conversion of the partial pressures, p/p0, to the average statistical thickness, t, of the N2 film adsorbed on a
400
A
non-porous hydroxilated silica, assuming that the thickness of a single molecular layer is of about 0.354 nm. The effect of increasing the drying temperature on the pore morphology of the xerogels can be clearly seen in the evolution of the isotherms: from type I (characteristic of essentially microporous materials) to type IV (typical of mesoporous materials) [30]. Nevertheless, all the isotherms present hysteresis loops, which are consistent with the presence of mesopores. On the other hand, from the analysis of the corresponding t-plots, one may conclude that all the samples contain micropores as well (intercept of the low t linear region, after conversion to liquid volume). This drying temperature effect is particularly clear up to 333 K. An increase above 333 K imparts only small changes in the pore morphology, being responsible for just a small increase in the total pore volume (related to the plateau of the isotherms). The specific pore volume and surface area (obtained from the N2 adsorption isotherms), the specific micropore volume (obtained from the t-plots), and the envelope densities of the same xerogel samples are listed in Table 3. The average pore diameter obtained from the BJH analysis upon adsorption is also included, as in Section 3.1. The changes observed in the pore morphology when increasing the drying temperature from 303 to 333 K
Table 3 Specific pore volume, Vp, surface area, SBET, micropore volume, Vlp, envelope density, qe, and average pore diameter, /BJH, of silica xerogels dried at different temperatures
D303 D333 D353
1
SBET/m2 g 1
Vlp/ cm3 g
577 744 788
0.060 0.049 0.034
Vads / cm3g-1
250 200
qe/ g cm 1.50 1.04 1.00
/BJH,ads/nm 3
2.58 3.28 3.20
B
200
100
150
D303 D333 D353
100
0 0.0
1
300
300
Vads / cm3g-1
0.30 0.56 0.59
400
D303 D333 D353
350
Vp/cm3 g
Sample
0.2
0.4
p/p0
0.6
0.8
1.0
0
2
4
6
8 10 12 14 16 18 20
t/Å
Fig. 5. (A) N2 adsorption/desorption isotherms of three xerogels that were dried at 303 (D303), 333 (D333) and 353 K (D353) and (B) corresponding t-plots.
A. Fidalgo, L.M. Ilharco / Microporous and Mesoporous Materials 84 (2005) 229–235
are accompanied by a significant decrease in the envelope density of the xerogel (qe), and correspond to an increase in the specific pore volume and surface area of the samples. Concurrently, an increase in the average pore diameter and a slight decrease in the micropore volume are observed. In conclusion, by increasing the drying temperature from 303 to 333 K, more porous xerogels may be obtained, due to a better preservation of the mesopore structure upon drying, with little loss of the micropore structure. A further increase of the drying temperature to 353 K imparts no significant changes in the physical properties of the samples. By changing the drying temperature from 303 to 353 K, the xerogel evolves from a micro/mesoporous material (20% micropores) to an essentially mesoporous one (6% micropores). Although the evaporation rate is higher at higher temperatures, the pressure gradient within the alcogel becomes lower, probably due to a prevailing effect of the decrease in the solvent viscosity. Thus, the gel shrinks less during the first drying stage. That explains the increase in the average pore diameter for samples dried above 303 K. Higher average pore diameters and lower interfacial tensions have the common effect of lowering the capillary pressure felt by the pore walls at the critical point, thus contributing to preserve a higher amount of mesopores in these samples.
4. Conclusions The possibility of obtaining monolithic silica xerogels by evaporative drying at ambient pressure is achieved by optimization of the ageing procedure and choice of the drying medium (washing solvent). By increasing the ageing period up to a certain limit, the silica network stiffens, with an increase of the average pore dimensions. However, further ageing leads to a decrease in the xerogel porosity, without any gain in stiffness. The critical ageing period depends on the geometry and dimensions of the wet gel samples. In the present study, for cylindrical rod shaped wet gels of 20 cm3, the optimum ageing procedure proved to consist of two 24 h periods. A smooth transition from the shrinking to the opaque stage of drying, to avoid fissuring, is accomplished by using i-propanol as the washing solvent. The possibility of tailoring the xerogelÕs porosity and pore morphology is further achieved by optimization of the drying temperature: an increase from 303 to 333 K allows raising the porosity, retaining a higher mesopore/micropore proportion. Above 333 K, no major changes are observed.
Acknowledgements This work was supported by Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT)—Project POCTI/CTM/33487/
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2000. The authors wish to express their gratitude to Prof. Rui Almeida for the use of the ASAP 2000. Alexandra Fidalgo acknowledges FCT for the post-doc grant PRAXIS XXI/BPD/20234/2004.
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