Synthesis and Characterization of Nano-Aerogels

7.2. Experimental

In Situ IR Collection. The experiments were carried out in the 100 mL autoclave equipped with ATR-FTIR, as described in Figure 3.5 of Chapter 3. In the case of SiO₂ aerogel synthesis, the temperature and pressure were controlled in the range of 40 – 60 °C and 1300 – 3000 psig, respectively, and the initial concentrations of TEOS and acetic acid were 0.088 M and 0.352 M, respectively. In the case of TiO₂ aerogel synthesis, the temperature and pressure were at 60 °C and 4500 psig, respectively, and the initial concentrations of TIP and acetic acid were 1.10 M and 3.85 M (or 6.05 M), respectively. In the case of ZrO₂ aerogel synthesis, the temperature and pressure were at 40 °C and 4500 psig, respectively, and the initial concentrations of ZBO and acetic acid were 0.547 and 1.76 M, respectively. In situ FTIR spectra were collected automatically using the ASI Applied System ReactIR 4000, at fixed intervals from 600 to 4000 cm^-1 with a resolution of 2 cm^-1.

Modeling. The curve fitting and SIMPLISMA modeling were carried out using the ACD UVIR Processor Version 8.0 software (ACD Inc., Toronto, Ontario). The Gaussian peak profile was used for curve fitting, and the limit of the half-peak width was set at 150 cm^-1 for a better curve fit. The input spectra were not smoothed or pre-processed other than the declared cut-off of the inactive region. During SIMPLISMA modeling, the offset α was set as a default at 20 %.

7.3. Results and Discussion

7.3.1. Conversion of TEOS

Figure 7.1 shows the three-dimensional in situ FTIR spectra collected during TEOS reacting with acetic acid in CO₂, which shows a gradual growth of the water peak at 3343 cm^-1 and oxo bond peak at ~ 1066 cm^-1 during the reaction times of 0-360 minutes.


Figure 7.57. Three-dimensional in situ FTIR spectra: 0.088 M TEOS reacting with 0.362M HOAc in CO₂ at reaction times of 0-360 minutes and at 50 ºC and 3000 psig.

Self-Modeling. In order to decrease the interference from other components, a region of the spectra in Figure 7.1 was selected where the interested component exhibits significant peaks. In the case of TEOS and SiO₂, the most significant peaks of Si-OEt are in the region of 750 -1250 cm^-1. This spectral region is now ready for the self-modeling process (Figure 7.2).



Figure 7.58. In situ FTIR spectra of 0.088 M TEOS reacting with 0.362M HOAc in CO₂ at reaction time of 0-360 minutes and at 50 ºC and 3000 psig.

Using the conventional SIMPLISMA, without second-derivative processing, no reasonable result was obtained, indicating that the pure variables do not exist for all components. By using the second-derivative, however, the SIMPLISMA process generated two resolved pure-component spectra and the concentration profiles (the statistics of the modeling is provided in Appendix 10). The resolved pure-component spectra match those of TEOS and SiO₂ (Figure 7.3 a and b), indicating that the self-modeling was successful. The resulting relative concentration profiles of TEOS and SiO₂ are shown in Figure 7.4. From this figure, it can be noticed that after a reaction time of 280 minutes, the Si-OEt bond concentration remained constant, while the product’s concentration still increased. As described in the introduction section, the Si-OEt bond converts into Si-acetate and Si-OH intermediates (Reactions 7.1 and 7.3) or directly converts into oxo bond (Reaction 7.5) during the polycondensation process. The intermediates can still produce oxo bond when the Si-OEt concentration becomes very low and maintains constant, typical of a condensation type polymerization.

Figure 7.59. Comparison of the self-modeling resolved spectra with (a) TEOS and (b) SiO₂ aerogel spectra. The spectra of TEOS in (a) and SiO₂ in (b) were experimentally collected by means of ATR-FTIR.

Figure 7.60. The relative concentration profiles obtained from the self-modeling method. The source IR data was obtained at 50 °C and 3000 psig, and the initial concentrations of TEOS and acetic acid was 0.088 and 0.362 M, respectively.

By applying the self-modeling method to the IR spectra that were collected at different reaction temperatures and pressures, the Si-OEt concentration profiles were obtained. The resulting Si-OEt concentration profiles were subsequently transferred into the conversion profiles versus the reaction time (Figures 7.5 – 7.6). It should be noted that the absolute concentration of Si-OEt is not necessary for calculation of the Si-OEt conversion. At a reaction time of zero, the relative concentration of Si-OEt is at a maximum. The Si-OEt conversion at the reaction time t can be calculated using Equation 7.19.

Conversion = (C₀ - C_t)/C₀ (7.38)

where, C₀ and C_t are the relative concentrations of Si-OEt when the reaction time is zero and t, respectively.

Figure 7.61. Temperature’s effect on the precursor conversion within a reaction time of 0-360 minutes. The initial concentration of TEOS and acetic acid was 0.088 and 0.362 M, respectively, and the pressure was 3000 psig.


Figure 7.62. Pressure’s effect on the precursor conversion within a reaction time of 0-360 minutes. The initial concentration of TEOS and acetic acid was 0.088 and 0.362 M, respectively, and the temperature was 50 °C.

From the conversion-time curves in Figures 7.5-7.6, some conclusions can be made: the 100 % conversion of Si-OEt was never reached before a reaction time of 360 minutes under the given synthesis conditions, which is similar to the conventional sol-gel process where water was used as a reaction agent and an alcohol as a solvent;^105 the reaction was favored by the higher reaction temperature and the lower reaction pressure. The temperature’s effect on the conversion indicates that the main reaction for Si-OEt consumption (the forward reactions in Equations 7.1, 7.3, and 7.5) is either an irreversible reaction or an endothermal reversible-reaction according to the Arrhenius equation (Equation 1.1), where a higher temperature results in a higher conversion. However, the pressure’s effect is difficult to be explained using transition state theory (Equation 1.2) without knowledge of which step is dominating the consumption of Si-OEt.^{83, 288} Although the effect of temperature and pressure on the oxo bond formation has been studied in Chapter 4, the results in Figure 4.5 and 6 may be complicated due to the overlapping of the IR peaks, as described earlier; on the other hand, the conversion of TEOS in Figures 7.5-7.6 are more reliable, because these results are based on the pure component concentrations.

It should be pointed out that these results were obtained at a relatively low initial concentration of TEOS, when there was no precipitate formed in the view cell, which was crucial to produce SiO₂ nanoparticles.^171 The low concentration resulted in a low absorbance and consequently a higher error for the calculated results.

7.3.2. Concentration Profiles of Ti-Acetate and TiO₂ Aerogels

Figure 7.7 shows the three-dimensional in situ FTIR spectra during polycondensation of TIP with acetic acid at the reaction time of 10–3720 minutes. It can be observed that the product peaks experienced a sudden increase, which was due to the condensation kinetics, as will be explained further later. Figure 7.8 shows the two-dimensional spectra in the region of 600-1880 cm^-1, in which a sudden increase of the product peaks can again be observed. The self-modeling was first carried out in the region of 800 – 1400 cm^-1 where TIP exhibits significant peaks, and no spectrum of TIP was resolved from the mixture spectra. This indicates that the substitution of TIP with acetate was fast, and free TIP disappeared in a reaction time of less than 10 minutes, as the first spectrum was collected at 10 minutes.



Figure 7.63. The three-dimensional in-situ FTIR spectra of 1.10 M TIP reacting with 3.85 M HOAc within a reaction time from 10 to 3720 minutes in CO₂. The reaction temperature and pressure were 60 ºC and 4500 psig, respectively.



Figure 7.64. In situ FTIR spectra of 1.10 M TIP reacting with 3.85 M HOAc in CO₂ within a reaction time from 10 to 3720 minutes, at 60 ºC and 4500 psig. The arrows show the absorbance change trend.
Due to the high initial concentrations of TIP and HOAc, the in situ IR spectra in Figure 7.8 show strong peaks of Ti-acetate bidentates in the region of 1400–1650 cm^-1, where the self-modeling was carried out and two resolved spectra (Figure 7.9-7.10) and the concentration profiles (Figure 7.11) were obtained. Figure 7.9a (the black curve) shows one of the resolved spectra and the curve-fitting results. In this figure, the deconvoluted peaks are labeled as 2 or 3, which match the spectra of Ti₆O₆(OPrⁱ)₆(OAc)₆ (2) and Ti₆O₄(OPrⁱ)₈(OAc)₈ (3) in Figure 7.9b and c, respectively. This result indicates that mixed hexamers were formed during the sol-gel process. The formation of 2 or 3 hexamers from TIP and acetic acid in CO₂ has been described in Chapter 5. The second resolved spectrum is close to that of the TiO₂ aerogel (Figure 7.10), but the resolved peaks in the range of 1350-1600 cm^-1 are wider than the aerogel peaks. The peak broadening in the resolved spectrum is probably due to the Lewis acid-Lewis base interaction between the acetate and the Lewis acids (e.g., acetic acid, isopropanol and CO₂),^{289, 290} which are more fully described in Chapter 8. Saturation of the IR probe may also contribute to the peak broadening, as the peak absorbance was very high (Figure 7.8).


Figure 7.65. FTIR. (a) The black curve is one of the resolved spectra from in situ IR and the curve-fitting results, the residual sum of squares: 4.02E-8; (b) and (c): the spectra of single crystals of 2 and 3, respectively. Note: the black curves = experimental or resolved IR spectra; the pink curve = the fitting curve; blue curves = the individual Gaussian function curves; and the red curve = the residual curve. The number 2 and 3 denote Ti₆O₆(OPrⁱ)₆(OAc)₆ and Ti₆O₄(OPrⁱ)₈(OAc)₈, respectively.



Figure 7.66. (a) The other resolved spectrum from the in situ IR spectra, which is attributed to the aerogel product, and (b) TiO₂ aerogel spectrum.

The relative concentration profiles of the hexamer mixture and the TiO₂ aerogel are shown in Figure 7.11. In the initial stage of the reaction, the hexamer concentration increased until the maximum was reached and then decreased gradually; after a critical point at 1800 minutes, a significant reduction in the concentration occurred at the reaction time of 1800–2040 minutes, followed by gradual decreasing. The product concentration experienced the identical, but opposite trend.

Figure 7.67. The relative concentration profiles during TIP reacting with HOAc in CO₂ within a reaction time from 10 to 3720 minutes, at 4500 psig and 60 ºC; the initial concentration of TIP and HOAc was 1.10 and 3.85 M, respectively.

Similar concentration profiles can be observed in Figure 7.12 where a higher acid/TIP ratio was applied, and the difference between Figure 7.12 and 7.11 is a shorter time for the sudden concentration change. In this case, the critical point was at a reaction time of 230 minutes, likely due to the higher acid/alkoxide ratio increasing the rate of reactions.

Figure 7.68. The relative concentration profiles during TIP reacting with HOAc in CO₂ within a reaction time from 10 to 610 minutes, at 4500 psig and 60 ºC; the initial concentration of TIP and HOAc was 1.10 and 6.05 M, respectively.

According to Figures 7.11 and 7.12, the formation of the product was accompanied with the consumption of the hexamers.

The phenomenon of a sudden change of concentration profiles was consistent with the phase observations obtained using the view cell reactor. The phase change can be described as four separate stages: (1) the initial phase was transparent with a light amber color during the first few hours to more than one day depending on the synthesis conditions; (2) the solution became red and translucent likely due to the growing and agglomeration of the colloidal particles,^291 with this stage being relatively short, i.e. from 40 minutes to two hours; (3) it became opaque with white particles appearing, and the stirrer bar being still movable; and (4) the stirrer bar becoming immobile. The stages 1 to 4 correspond to the formation of hexamers, colloidal particles, larger particles and gel, respectively, which agrees with the in situ IR observations. According to the concentration profiles, the colloidal particle formation stage was abrupt, which likely happened in stage (2).

As the condensation of the metal complexes (in this case they are a mixture of complexes 2 and 3) can only take place on the OR group, the reactions for formation of the macromolecules can be written as:

Hydrolysis:

M_xO_y(OAc)_z(OR)_n+ H₂O → M_xO_y(OAc)_z (OR)_n-1(OH) + ROH (7.39)

Condensation 1:

M_xO_y(OAc)_z (OR)_n-1(OH) + M_xO_y(OAc)_z(OR)_n → M_2xO_2y+1(OAc)_z(OR)_2n-2 + ROH

(7.40)

Condensation 2:

2 M_xO_y(OAc)_z (OR)_n-1(OH) → M_2xO_2y+1(OAc)_z(OR)_2n-2 + H₂O (7.41)

Water is known to react with titanium alkoxides very quickly to form titanium oxide.^292 From Equations 7.20-7.22, it can be anticipated that the process will be dominated by the reactions that generate water, since water was absent at the initial stages of the process. A hydroxylated metal complex M_xO_y(OAc)_z(OR)_n-1(OH) can react with either a nonhydroxylated M_xO_y(OAc)_z(OR)_n or another hydroxylated complex. Two cases will be discussed, as described below.

Case 1: if the nonhydroxylated complexes are more active, Condensation 1 (7.21) is close to or faster than condensation 2 (7.22). In this case, at the initial stage of the reaction when the concentration of the hydroxylated metal complexes is still low, they will have a higher possibility to react with the nonhydroxylated complexes, which generates alcohol. The alcohol slowly reacts with acetic acid and generates both water and an ester, and the water reacts with another nonhydroxylated complex. In this case, the kinetics is controlled by the slow esterification reaction that generates water, and a gradual reaction process can be anticipated.

Case 2: if the nonhydroxylated complexes are inactive, Condensation 1 (7.21) is much slower than condensation 2 (7.22). In this case, at the initial stage when the concentration of the hydroxylated metal complexes is still low, they do not have many possibilities to react with one another. Thus, a very slow start of the reactions can be expected. However, after a certain reaction time when the concentration of the hydroxylated complexes reaches a certain level, they are able to react with one another significantly and generate a large amount of water. Subsequently, the water will react quickly with more nonhydroxylated complexes and accelerate the reaction, leading to the concentration profiles shown in Figures 7.11 and 7.12, which are similar to self-catalysis reaction profiles described in the literature.^293

Based on the above discussion, it can be rationalized that, during hydrolysis and condensation of the Ti complexes 2 and 3 into the colloidal particles, Condensation 1 should be much slower than Condensation 2.

7.3.3. Concentration Profiles of Zr-Acetate and ZrO₂ Aerogels

Similar to the sol-gel chemistry of titanium alkoxides, when zirconium alkoxides react with acetic acid, they form zirconium acetate complex structures, in which there is unreacted ending alkoxide groups (-OR). Then, the alkoxide group is hydrolyzed and condensated with other metal-OH or metal-OR to form oxo bond, as shown in Equations 7.20-7.22.

The three-dimensional in situ FTIR spectra collected during ZBO reaction with acetic acid in CO₂ are shown in Figure 7.13, from which a gradual increase of water and product peaks can be observed. Figure 7.14 shows the two-dimensional spectra in the region 1300-1650 cm^-1, where a gradual formation of product peaks can be observed.

Figure 7.69. Three-dimensional in situ FTIR spectra during the direct sol-gel process of ZBO with acetic acid within a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively.



Figure 7.70. Two-dimensional in situ FTIR spectra during the direct sol-gel processing of ZBO with acetic acid within a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively. The arrows indicate the absorbance change trend.

Using the SIMPLISMA technique, similar to the cases of TIP as described earlier, no spectrum of ZBO was resolved from the mixture spectra in the region of 800 – 1300 cm^-1 where ZBO exhibits significant peaks. This indicates the substitution of ZBO with acetate was also very fast. In the region of 1300–1650 cm^-1, both the resolved component spectra and the concentration profiles were obtained (Figures 7.15 and 7.16).

Figure 7.71. Two resolved spectra from the in situ IR spectra by means of SIMPLISMA modeling. One spectrum is attributed to the Zr-acetate complex and the other to the ZrO₂ aerogel.



Figure 7.72. The relative concentration profiles of Zr-acetate complex and the condensation product during the direct sol-gel processing of ZBO with acetic acid in scCO₂ with a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively.

From the concentration profiles, one can see that the consumption of the Zr-acetate complex and formation of product were gradual, which was different from the case of TIP reacting with acetic acid. The kinetics of the process can be explained by case 1 (described above), where Condensation 1 is close to or faster than Condensation 2. The different kinetics between the Ti complexes and the Zr complexes can be explained by the different activity of the metal complexes. As described in Chapter 5 and 6, when the acid/Ti alkoxide ratio was 5.5, well-defined nanostructures were synthesized; on the other hand, when the acid/Zr alkoxide ratio was above 3.22, precipitate was produced. This indicates that both the Zr alkoxides and complexes are more active than the Ti alkoxides and complexes. As discussed earlier, the more active the complexes, the more likely the kinetics are similar to Case 1, i.e. controlled by the esterification reaction.