Journal of Sol-Gel Science and Technology 28, 175–184, 2003c© 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.

Synthesis of Silica Aerogels: Influence of the Supercritical CO2on the Sol-Gel Process

IRINA SMIRNOVA AND WOLFGANG ARLTFachgebiet Thermodynamik und thermische Verfahrenstechnik, TK 7, Technische Universitat Berlin,

Strasse des 17. Juni 135, 10623 Berlin, GermanyI.Smirnova@vt.tu-berlin.de

Received February 10, 2003; Accepted May 14, 2003

Abstract. The synthesis of silica aerogels was modified by addition of supercritical CO2 during the sol-gel process.It was shown, that CO2 acts as a catalyst and accelerates the gelation significantly. This effect was studied under amultitude of experimental conditions. The influence of the precursor concentration, temperature and the nature ofthe catalysts and solvent on the gel formation in presence of CO2 was studied. Several gels obtained by this methodwere dried and transparent silica aerogels were produced.

Keywords: silica aerogels, supercritical CO2, reaction enhancement, sol-gel process

1. Introduction

Silica aerogels are usually synthesized by hydrolysisand following condensation of tetraalkylorthosilicate.The reaction can be summarized as follows:

Si(O(CH2)nCH3)4 + 2H2O

−→Cat.

SiO2 + 4HO(CH2)nCH3 (1)

Both acids and bases can be used as catalysts. Thereare two different ways of performing the reaction:1-step and 2-step processes. In the 1-step process allsubstances are mixed together with the catalysts and themixture is stored up to gelation. This method providessilica aerogels with the density ρ ≥ 0.02 g/cm3 [1, 2].In 2-step process two different catalysts (acid and base)are used for the hydrolysis and gelation [3]. This meth-ods allows to produce aerogels with the densities as lowas 0.003 g/cm3 [4]. Numerous investigations reportedthe effect of the synthesis conditions on the gelationprocess and properties of the resulting aerogels [e.g. 5–8]. An important factor in the synthesis of low densityaerogels is the gelation time. In the one-step process,the gelation of a sample having a density of 0.02 g/cm3

requires a relatively long time (approximately 14 days).

In the two-step process, the gelation time of such sam-ples can be as short as 72 hours, but this process in-cludes several tedious steps, e.g., special purificationof the reagents and distillation of the alcohols after thefirst step [4]. Different types of catalysts have been uti-lized in order to accelerate the gelation process [8]. Inthe present work, it is suggested to use CO2 as a cat-alyst for the gelation process. CO2 has been used asa co-solvent for different reaction over several years.Loy et al. [9] conducted the gelation directly in super-critical CO2 using formic acid as condensation agent.However, the influence of CO2 on the gelation time wasnever reported. In this work the 2-step sol-gel processin presence of CO2 has been studied. A special atten-tion was paid to the influence of various parameters(e.g. temperature, component ratio, solvent nature) onthis process.

2. Experimental

2.1. Apparatus

A high pressure autoclave was used in this work.The autoclave consists of a stainless steel cylinder,

176 Smirnova and Arlt

Figure 1. Flow sheet of the apparatus for aerogel production.

2 viewing glass windows (stainless steel 1.4462 andborosilicate glasses DIN 7080, Herbert Ind., Wupper-tal), and 2 flanges. The volume of the autoclave is249.5 ml. The maximal working pressure of the au-toclave is 20 MPa, maximal working temperature is100◦C. For the extraction of the solvent, the autoclavehas 3 inlets and 2 outlets with the diameter of 1 mm.The temperature is regulated with an accuracy of ±1◦C.For the main part of the experiments, CO2 support wasneeded. The flow sheet of the apparatus is shown inFig. 1. The gas originating from the tank (1) is com-pressed in the gas pump (3) and heated up to the tar-get temperature in the heat exchanger (5). The pumpprovides a constant flow rate, which can be regulatedadditionally by the back pressure regulator (14). Thesolvent eventually present in the outlet gas stream canbe condensed in a heat exchanger (15).

2.2. Sol-Gel Process

The sol-gel process was conducted following the prin-ciple scheme of the 2-step method [4].

In the first step, TMOS was mixed with water,methanol, and hydrochloric acid. Methanol was usedin order to get a homogeneous mixture. The molar ratio

of the components was:

1 mol TMOS : 2.4 mol MeOH

: 1.3 mol H2O : 10−5 mol HCl

The reaction mixture was stirred at room temperaturefor 30 minutes. Lee et al. showed [7], that the distilla-tion of alcohol after the first step can be eliminated forthe aerogel with a target density higher than 0.01 g/cm3.The same approach was employed in this work.

In the second step acetonitrile, water, and ammoniasolution were consequently added to the initial mixtureunder constant stirring. No distillation of alcohol wasconducted. Different mole ratios of the ammonia basecatalyst to TMOS were used at the second step:

1 mol TMOS : 10−n mol NH4OH; 1 < n < 4.

The final molar ratio of the components after the secondstep was:

1 mol TMOS : 2.4 mol MeOH : 4 mol H2O

: 10−5 mol HCl : 10−n mol NH4OH;

1 < n < 4.

Synthesis of Silica Aerogels 177

Water contained in the catalysts solutions was takeninto account. The amount of acetonitrile was calculatedto get the desirable concentration of TMOS (target den-sity of the gel, ρtarget), as defined in Eq. (1),

ρtarget = mSiO2

Vacetonitrile + VMixture(1)

where Vacetonitrile is the volume of acetonitrile, mSiO2

the amount of the silicon oxide in TMOS in gram,VMixture the sum of the volumes of all compounds in themixture, excluding acetonitrile (excess volume beingneglected).

After all compounds were added, the mixture wasstirred additionally for 5 minutes at room temperature.A small amount of the resulting solution (∼5 ml) waspoured into a glass vial and stored separately (controlsample). The rest (∼120 ml) was weighed and poureddirectly into the autoclave which was then closed andheated to the target temperature (25◦C < T < 70◦C).While heating, the autoclave was rocked to allow abetter heat transfer. When the target temperature wasreached, CO2 could be added to the solution. The CO2

vessel was weighed, then CO2 was allowed to passthrough the heat exchanger and enter the rocking auto-clave. Then the CO2 tank was weighed to determine theamount of CO2 introduced in the autoclave. Typically2–3 g of CO2 were added at once. After the addition ofeach portion of the gas, the pressure in the autoclave in-creased immediately and after few minutes decreaseddue to the dissolution of CO2 in the liquid phase. Afterthe pressure stopped to change, the next portion of CO2

was added into the autoclave. The viscosity of the liquidphase increased continuously with increasing CO2 con-centration, this was seen through the window of the cell(the autoclave was rocked with a constant velocity andthe movement of the liquid could be observed). Whena certain concentration of CO2 (called further Copt.

CO2)

was reached, the solution lost its fluidity and became agel. At this point, no more CO2 was added. The wholeprocedure took 10 to 30 minutes. All experiments wererepeated at least twice.

To determine the gelation time the experimentalprocedure was slightly different to the one describedabove. The amount of CO2 needed to reach the pre-determined concentration in the liquid phase was cal-culated and added to the sol. This was done not insmall portions, as described previously, but at once.The gelation time (tCO2

gel ) was determined as a time in-terval from the moment of the CO2 addition to themoment when the solution lost its fluidity. The time

period needed for the CO2 addition (normally around 5minutes) was subtracted from the results. The controlsamples were stored at the same temperature, whichwas maintained in the autoclave during the gelationof the main sample, and their gelation time (tgel) wasdetermined.

The gel was aged in the autoclave during 12 hours.Then the supercritical drying procedure was carriedout. At first, the solvent was extracted from the gel byCO2 at 40◦C. During this process, some cracks wereseen to appear within the gel body due to the sudden gasflow. The flow rate was maintained at 100 NL/h duringthe extraction. After the extraction was completed, CO2

was slowly vented out from the autoclave.The resulted gels and aerogels were studied by IR

spectroscopy. For this measurements, the silica aero-gels were powdered and than compressed with waxand KBr. The adsorption spectra were measured inthe region of 600–4000 cm−1 by the IR Spectrometer“Magna System 750”.

Densities of the liquid solutions were measured us-ing a glass pycnometer with the volume of 25 cm3.A sol was filled into the pycnometer, thermostatizedduring 20 minutes and weighed immediately. Everyexperiment was repeated at least 3 times and the aver-age value was calculated. The experimental error wasestimated to be ±0.002 cm3.

3. Results and Discussion

3.1. Influence of CO2 on the Gelation Process

In order to characterize the influence of CO2 on thegelation process, gelation times were measured forthe samples prepared by CO2 addition (tCO2

gel ) and forthe control samples, that were allowed to form a gelwithout CO2 addition (tgel). In Table 1, the results forthe gels with a target density of 0.03 g/cm3 are pre-sented. One can see that the gelation time decreasesdrastically with CO2 addition. For example, the gela-tion time at 25◦C is 161 hours for the control sample,

Table 1. Gelation times at 25 and 40˚C. Componentmolar ratio: 1 mol TMOS : 2.4 mol MeOH : 4 mol H2O :10−5 mol HCl: 10−2 mol NH4OH.

tgel without CO2 tgel with CO2

25◦C 161 hour 60 min

40◦C 21 hour 53 min

178 Smirnova and Arlt

Figure 2. Dependence of the gelation time on the amount of CO2

added to the sol at 40◦C. Component ratio : 1 mol TMOS : 2.4 molMeOH : 4 mol H2O : 10−5 mol HCl: 2·10−3 mol NH4OH. Solvent:acetonitrile; ρtarget = 0.04 g/cm3.

but only 1 hour in the case of CO2 addition. At highertemperature the difference in gelation times is notas large, but still significant (21 hours compared to53 minutes). So, the CO2 accelerates the gelation pro-cess. This fact has never been reported in the literature.

The influence of the CO2 concentration on the gela-tion time was measured. The experimental results arepresented in Fig. 2, where the solid line represents thetrend line fitted to the experimental data. It can be seenthat the gelation time decreases rapidly with increas-ing CO2 content and approaches nearly constant valuewhen the total CO2 concentration reaches 20 wt%. Fur-ther addition of CO2 does not enhance the reactionsignificantly.

Considering that the reaction takes place in the liq-uid phase, we can expect that the reaction rate dependson the concentration of CO2 in the liquid phase ratherthan on the total CO2 concentration in the system.Solubility of CO2 in the liquid phase is characterizedby the pressure over the reacting mixture. The depen-dence of the pressure over the solution on the totalCO2 concentration in the system is shown in Fig. 3.All experiments were conducted in very diluted solu-tions (ρtarget < 0.04 g/cm3, Cacetonitrile > 75 wt%), sothe partitioning of CO2 between the liquid and vapourphases might be close to that in the binary mixtureCO2—solvent . It was shown [10, 11], that in the bi-nary mixture acetonitrile—CO2 the concentration ofacetonitrile in the vapor phase is very low (mole frac-tion of CO2 is close to unity). So we can assume, that inthe system sol—CO2 the concentration of CO2 in thevapour phase is also close to 100%. A reliable mea-surement of the CO2 concentration in the liquid phaseis impossible in our case, because the composition ofthe liquid phase changes continuously due to the fast

Figure 3. Pressure–concentration dependence in the system sol–CO2. Component ratio : 1 mol TMOS : 2.4 mol MeOH : 4 molH2O : 10−5 mol HCl: 2·10−3 mol NH4OH; Solvent : acetonitrile;ρtarget = 0.04 g/cm3.

reaction. However, this value can be estimated fromthe experimental results on the basis of some assump-tions. The estimation procedure is based on the iterationmethod. The volume of the liquid phase is calculatedusing the known mass of the solution in the autoclaveand the experimentally determined density of the liquidphase, presented in Table 2. The CO2 mass in the gasphase is determined using its density under the experi-mental conditions [12]. Subtracting this value from thetotal CO2 amount added to the system one can get theamount of CO2 in the liquid phase. A new liquid phasevolume is calculated under the assumption that the den-sity of the liquid phase does not change significantly.This iteration procedure is repeated until the concen-tration of CO2 in the liquid phase remained constant.

In Fig. 4 the dependence of the pressure over theliquid on the concentration of CO2 in the liquid phase,calculated by this procedure is presented in compari-son with the experimental data for the binary systemCO2—acetonitrile, reported by Kordikowski et al. [11].From the plots, it can be seen that the solubility of CO2

in the systems studied in this work is very close to thatin acetonitrile. Thus, the data on CO2 solubility in ace-tonitrile can be used to make the first rough estimationof the CO2 content in the liquid phase.

The dependence of the gelation time on the CO2

concentration in the liquid phase, calculated using the

Table 2. Densities of the sols. Experimental error±0.002 g/cm3.

ρsolution (g/cm3) ρsolution (g/cm3)ρtarget (g/cm3) (23◦C) (33◦C)

0.03 0.808 0.793

0.04 0.818 0.809

0.05 0.827 0.817

Synthesis of Silica Aerogels 179

Figure 4. Solubility of CO2 in the mixture studied at 40◦C.Component ratio: 1 mol TMOS : 2.4 mol MeOH : 4 mol H2O :10−5 mol HCl: 2·10−3 mol NH4OH. ρtarget = 0.04 g/cm3; Solvent :acetonitrile.

Figure 5. Dependence of the gelation time on the CO2 concentra-tion in the liquid phase. Component ratio : 1 mol TMOS : 2.4 molMeOH : 4 mol H2O : 10−5 mol HCl: 2·10−3 mol NH4OH. Solvent :acetonitrile; ρtarget = 0.04 g/cm3.

iteration procedure described above is presented inFig. 5. It can be seen that this dependence exhibits asimilar trend to that presented in Fig. 2. This indicatesthat even the total concentration of CO2 in the sys-tem is a suitable parameter for the characterization ofthe gelation enhancement due to the CO2 addition. Thegelation time always decreases with the increasing CO2

concentration in the liquid phase. So CO2 can be con-sidered as an alternative catalyst for the 2-step sol-gelprocess. In order to clarify the catalytic effect of CO2,gelation process in presence of CO2 was studied undera multitude of experimental conditions. The influenceof the precursor concentration, temperature, pH valueand the nature of the catalyst on the gel formation inpresence of CO2 was investigated.

3.2. Influence of the Process Parameteron the Gelation in Presence of CO2

As CO2 is added to the system as described inSection 2.2, gelation takes place after a certain concen-

Figure 6. Dependence of Copt.CO2

in the liquid phase on the targetdensity of the gel at different temperatures. Solvent–acetonitrile.Component ratio : 1 mol TMOS : 2.4 mol MeOH : 4 mol H2O :10−5 mol HCl: 2·10 3 mol NH4OH.

tration of CO2 is achieved. The concentration reachedin the liquid phase at this point (Copt.

CO2) seems to be char-

acteristic for the solution of a given target density. Thevalues of Copt.

CO2, experimentally determined for the gels

of different target densities are presented in Fig. 6. Onecan see, that Copt.

CO2decreases with increasing target den-

sity. A higher value of target density implies a higherinitial concentration of TMOS in the solution (see thedefinition of the target density, Eq. (1)). It is known,that solutions having a higher initial concentration ofTMOS gelify faster [13]. In this case, the solution issimply more concentrated and so more monomers par-ticipate in the condensation reaction. The condensationreaction is believed to be of second order with respect tothe TMOS concentration [14]. As soon as the reactionrate itself increases, less CO2 is needed to complete thegelation in the case of a higher TMOS concentration.

Increasing temperature also accelerates the gelation[13]. So it could be expected, that the Copt.

CO2value de-

creases at higher temperatures. The influence of tem-perature on the CO2 concentration reached at the gelpoint, was studied for the gels with different target den-sities. The target density region of the studied sampleswas chosen following several criteria: in the case ofrelatively high target densities (ρtarget > 0.1 g/cm3) thegelation time achieved by common processes is alreadylow (<1 hour), so no additional reaction enhancementis needed. Samples of lower densities require longergelation time, so the method suggested in this workmight be advantageous. In the case of very low den-sity (ρtarget < 0.01 g/cm3), rather large amount of CO2

would be needed (see Fig. 2). In this case, it is useful

180 Smirnova and Arlt

to increase the amount of the base catalyst before CO2

addition is conducted. The dependence of the Copt.CO2

val-ues on temperature is shown in Fig. 6. One can see thatfor the gels of all target densities, Copt.

CO2decreases with

increasing temperature. It can be explained by the fact,that both hydrolysis and condensation take place in afaster rate at a higher temperature, less CO2 is neededfor the gelation enhancement.

The gels produced in presence of CO2 were dried anda number of aerogels with densities from 0.032 g/cm3

to 0.1 g/cm3 were produced. Due to the fact, that the gelis formed directly in the autoclave, its form and dimen-sions are defined by the construction of the apparatus.In our case the aerogel of half-cylindrical form with fol-lowing dimensions were obtained: diameter—50 mm,length—85 mm, height 25 mm. The average shrinkage,defined as the ratio ρtarget/ρ, reached 10–20%.

3.2.1. Combination of CO2 with Other Catalysts.Since CO2 accelerates the gelation process, it may beassumed to act in some way as a catalyst. In order toprove whether CO2 can replace the commonly usedcatalysts, different catalysts were used in combinationwith CO2 during the gelation process. At first, the pos-sible combinations of CO2 with both catalysts com-monly used for the gel synthesis (NH4OH and HCl)were studied.

The corresponding catalyst concentrations as well asfinal state after CO2 addition are summarized in Table 3.

Sample A1 was made using both HCl and NH4OH inthe first and second steps respectively, without anyCO2 addition and dried to obtain the conventionalaerogel (control sample).

Sample A2 was made using the same procedure, butCO2 was added to the solution. The gel was aged inthe autoclave for 12 hours and dried. A transparent

Table 3. Dependence of the gelation process with CO2 addition on the combination of catalysts used. 1 mol TMOS: 2.4 mol MeOH : 4 mol H2O : a mol HCl: b mol NH4OH; ρtarget = 0.03 g/cm3.

Catalyst used(mol catalyst per mol TMOS)

Sample HCl (a) NH4OH (b)CO2 mol ratio

(mol CO2: mol TMOS) Final state

A1 10−5 2 ∗ 10−2 0 “Slow” gelation resulted in a transparent gel

A2 10−5 2 ∗ 10−2 16.72 “Fast” gelation resulted in a transparent gel

A3 0 0 0.1 Precipitation of colloid particles

A4 10−5 0 0.1 Precipitation of colloid particles

A5 0 2 ∗ 10−2 16.72 Gelation resulted in slightly opaque gel

aerogel having the density 0.034 g/cm3 was ob-tained.

Sample A3 was made in the absence of catalysts inboth the first and second step. The mixture was filledin the autoclave and CO2 was slowly added. When5 wt% of CO2 was added to the system, a precipitateappeared in the autoclave.

Samples A4 and A5 were prepared similarly to sampleA2, but in both cases only one of the catalysts wasadded during the synthesis. When HCl was used asthe only catalyst (sample A4), a white precipitatewas formed after the CO2 addition. If only NH4OHwas added (sample A5), the following CO2 additionleads to a gelation within 30 minute. The resultinggel was slightly opaque.

The gels formed by samples A1 and A5 as well as thepowders obtained from samples A3 and A4 were driedat ambient pressure at 60◦C and analyzed by IR spec-troscopy. The spectra were compared with the spec-trum of the silica aerogel obtained from the sample A2and the major peaks were assigned as recommended by[15–17]. The corresponding spectra are shown in Fig. 7.By comparison of the corresponding peaks, it can beconcluded that all samples consist predominantly of sil-icon oxide. The characteristic absorption at 1090 and806 cm−1 (SiO2) is very strong. Some CH3 groups(2856 cm−1) were identified in the samples A2–A5.They were assigned to the Si OCH3 groups, presentin the gel due to the uncompleted hydrolysis of TMOS.Water peaks (3429, 1655 cm−1) were identified in sam-ples A2–A5, although it is difficult to say whether thecorresponding peaks represent the free unreacted wateror water adsorbed onto the gel surface, as in the case ofsilica aerogel. It should be mentioned that IR spectragive only qualitative (and not quantitative) informationabout a sample.

Synthesis of Silica Aerogels 181

Figure 7. IR spectra of samples A1–A5. The spectra are shifted in the height by adding of constant values to the transmittance.

Based on the spectra, we conclude that CO2 additionto the sol, in the case of no additional catalysts (sampleA3), leads to a rapid precipitation of colloid silicon ox-ide particles. This proves that CO2 is not suitable as theonly catalyst for the gel formation. The combination ofCO2 and acid catalyst used in the first step (sample A4)shows the same effect. On the contrary, combination ofCO2 with only base catalyst (NH4OH) leads to rapidgelation (sample A5). A transparent gel can be obtainedhowever only if both HCl and NH4OH are used.

3.2.2. Replacement of CO2 by Formic Acid. It isknown [18], that the acid catalyst mostly acceleratesthe hydrolysis, and the base one accelerates condensa-tion. Since CO2 addition to the sols without HCl resultsin a fast gelation, we can suppose that CO2 acts to someextent as an acid catalyst. CO2 dissolves in the sol andreacts with water present in the system to carbonic acid.Huang et al. [19], reported that the addition of strongacids during the second step can decrease the gelationtime. It was suggested, that this effect is based on thevariation of the pH value and the shift of the isoelec-tric point of the solution [19]. Nevertheless, H2CO3 is aweak acid, so in our case, the pH of the solution changesonly slightly with CO2 addition (pH = 8 before CO2

addition, pH = 7.5 after CO2 addition). The pH valueswere measured in the autoclave directly by means of pHpaper. However, we proved the effect of another weakacid on the gelation time. Formic acid was used for thispurpose. The starting solutions were prepared using thesame component ratio as in the case of CO2 addition (1mol TMOS : 2.4 mol MeOH : 4 mol H2O : 10−5 mol

HCl : 10−2 mol NH4OH). Formic acid solutions hav-ing different concentrations (0.01–10%) were slowlyadded to the sol until the pH value of 3 was reached.No gelation enhancement comparable with that due tothe CO2 addition was observed. So it was proved thatthe gelation enhancement can not be due to the simplechanging of the pH value because of the carbonic acidformation.

3.2.3. Replacement of CO2 by Buffer Solutions. It issupposed that the carbonic acid resulting from the CO2

addition might react with other components present inthe solution. One possibility may be that H2CO3 reactswith NH4OH, as depicted in Eq. (2).

CO2 + H2O ↔ H2CO3 (2)2 NH4OH + H2CO3 ↔ (NH4)HCO3 + 2H2O

This system is known to be a buffer solution, whichmight provide a favorable setting for the condensationreaction. In order to model this effect and prove thesuggestions, the influence of ammonia hydrocarbon-ate (NH4HCO3) itself on the gelation rate was studied.The experiments were carried out similary to the case offormic acid. Aqueous ammonium hydrogen carbonatewas slowly added to the sol. The results are summarizedin Table 4. When relatively large concentration of am-monium hydrogen carbonate was used (samples B1 andB2), precipitation of silicon oxide particles took place.This effect was also observed with other base catalysts,when a certain catalyst concentration was exceeded. In

182 Smirnova and Arlt

Table 4. Influence of ammonium hydrogen carbonate on the gelation at 20◦C.

NH4 HCO3 concentration Final NH4 HCO3

Sample of the initial solution (wt%) concentration in sol (wt%) tgel Final state

B1 5 0.43 – Precipitation of colloid particles

B2 2.5 0.123 – Precipitation of colloid particles

B3 0.5 0.0043 4 days “Slow” gelation resulted in a transparent gel

Table 5. Effect of the CO2 addition on the gelation process in different solvents.

Solvent Final pressure (MPa) Total CO2 concentration (wt%) CCO2 in the liquid phase (wt%) Final state

Methanol 7.36 64.9 36.09 Precipitation of colloid particles

Ethanol 7.55 46.8 38.3 Precipitation of colloid particles

Acetonitrile 3.21 37.2 28.08 Gelation

the case of a lower ammonium hydrogen carbonateconcentration (sample B3), the normal “slow” gela-tion took place, as in the case of the common two-step procedure. No gelation enhancement comparableto CO2 addition was observed. So the formation ofNH4HCO3 does not seem to be the reason for gelationenhancement.

3.2.4. Effect of the Nature of Base Catalyst on theGelation Enhancement. In this case one can use an-other base catalysts instead of ammonium hydroxideat the second step. In order to prove this suggestionNaOH was used instead of ammonium hydroxide withthe molar ratio 1 mol TMOS: 0.002 mol NaOH·CO2

was added to the solution in small portions at 40◦C upto the concentration of 33.8 wt%. At this point the so-lutions formed gels, whereas the corresponding controlsamples gelified only within ∼20 hours. It was the sameeffect of gelation enhancement as in the case when am-monium hydroxide was used as a catalyst at the secondstep. It may, therefore, be concluded that the gelationenhancement by CO2 does not depend on the nature ofthe base catalyst.

3.2.5. Effect of Solvents on the Gelation Enhance-ment. It is known, that interactions between the sol-vent and reaction products (dimers, oligomers etc.) pre-sented in the solution have a strong influence on theprocess itself and, in particular, on the gelation time[20]. So, we proved whether the effect of the gela-tion enhancement can be obtained in the alcoholic sol-vent commonly used for the aerogel production. Thereaction was conducted alternatively in methanol andethanol and the results were compared with that for

the case when acetonitrile was used as a solvent. Theamount of each solvent was calculated so that a tar-get density of 0.03 g/cm3 could be obtained. In bothsteps, the same solvent was used. The results are sum-marized in Table 5. If acetonitrile was used as a sol-vent, the mixture gelified as the CO2 concentration inthe liquid phase achieved 28.08%. The resulting gelwas transparent, in agreement with our previous re-sults (Section 4.1). In the case of methanol and ethanol,no gelation was achieved and precipitation of colloidparticles took place after a certain concentration ofCO2 was reached in the solution. In order to provethe chemical nature of the particles, the precipitatewas collected, dried at 60◦C and analyzed by IR spec-troscopy. The corresponding spectra are presented inFig. 8. The spectra of the colloidal particles obtainedin these experiments are similar to those presented inFig. 7. So if ethanol and methanol are used as solvents,CO2 addition leads to the precipitation of the colloidal

Figure 8. IR Spectra of the colloidal particles obtained from ethanoland methanol solutions after CO2 addition. The spectra are shiftedin the height by adding of constant values to the transmittance.

Synthesis of Silica Aerogels 183

particles and not to gel formation. The formation ofSiO2 microparticles in supercritical CO2 was recentlydescribed by Moner-Girona et al. [21]. However theyhave used formic acid as a condensation solution andconducted the process without any additional solvent.

Artaki et al. [22] have shown, that the gelation in apolar protic solvent, such as methanol, ethanol etc. isfaster than in polar aprotic solvents, i.e. acetonitrile. Formethanol and ethanol the gelation time is already veryshort, so the additional enhancement of the reaction byCO2 might make the reaction so fast, that silicon oxideparticles instead of a gel are formed. If acetonitrile isused, the gelation time is longer. CO2 also enhancesthe reaction in acetonitrile, but this reaction is still slowenough to allow the formation of a uniform cluster (agel). This suggestion would explain the fact, that gelformation takes place preferably in acetonitrile.

Ferrieri et al. [23] studied several reactions in themixtures of supercritical CO2 and acetonitrile. Theyhave shown that in such solutions acetonitrile formsclusters around the dilute solutes, so that their localconcentration increases resulting in the faster reaction[22]. We suppose that a similar effect takes place dur-ing the sol-gel process in the presence of CO2. WhenCO2 is added to the sol, it dilutes the mixture, but thelocal concentration of the partly hydrolyzed precursorincreases, resulting in a faster gelation. This suggestionmight be proved by the spectroscopic measurements,what is the purpose of our future work.

4. Conclusions

The 2-step sol-gel process was conducted in presenceof supercritical CO2. It has been found that the additionof CO2 during the second step leads to a fast gelationof the system. Corresponding gelation time is up to10 time shorter than that of the control sample (preparedwithout CO2 addition). The resulting gels were driedand transparent silica aerogels were obtained.

It was found that the gelation time decreases with in-creasing CO2 concentration in the liquid phase. If CO2

is added to the system consequently, a certain concen-tration (Copt.

CO2) should be reached before the gelation

occurs. The value of Copt.CO2

decreases with the increas-ing target density of the gel and temperature.

Although the CO2 addition enhances the reaction,the final product depends on its combination with othercatalysts. It was shown that the gel formation only takesplace if CO2 is combined with both catalysts usedin the common two-step process (HCl and NH4OH),

otherwise CO2 addition leads to opaque gels or to arapid formation of colloidal particles. Such particlesare also obtained if the reaction is carried out in alco-holic solvents. The nature of the base catalyst does notplay an important role. The gelation enhancement isalso observed in cases when another catalyst (NaOH)instead of ammonium hydroxide was used in the secondstep.

It was shown that the addition of formic acid to a solinstead of CO2 does not result in a fast gelation. Basedon these experiments, we conclude that the enhance-ment of the reaction by CO2 is not a simple effect of thechange of pH. Both carbonate buffer and ammoniumhydrogen carbonate solution do not result in a fast gela-tion, when they are used instead of CO2. So CO2 actsnot only as a catalyst itself, but provides some optimalconditions for the condensation reaction, acceleratedby a common base catalysts (NH4OH or NaOH).

The strong dependence of the gelation enhancementon the solvent nature allows one to conclude that someinteraction between CO2 and acetonitrile might takeplace. It can be supposed that due to the addition ofCO2 the local concentration of the partly hydrolyzedTMOS increases leading to a faster gelation.

The nature of the gelation enhancement by CO2 hasnot been fully resolved by the conducted work. Furtherinvestigation is needed for the understanding of thiseffect.

Acknowledgments

This work was supported by DFG (Project Ar236/10-1)and Alexander von Humboldt Society (Program “Co-operation with Asia”). The authors are thankful to JozoMamic, Susanne Hoffmann, and Mihaela Dragan fortheir help with the experiments and S. Imme for the IRmeasurements.

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